www.siemens.

com/energy/hvdc

High Voltage
Direct Current Transmission –
Proven Technology for Power Exchange

Answers for energy.

2
Contents

Chapter Theme Page

1 Why High Voltage Direct Current? 4

2 Main Types of HVDC Schemes 6

3 Converter Theory 8

4 Principle Arrangement
of an HVDC Transmission Project  11

5 Main Components 14

5.1 Thyristor Valves 14

5.2 Converter Transformer 18

5.3 Smoothing Reactor 20

5.4 Harmonic Filters 22

5.4.1 AC Harmonic Filter 22

5.4.2 DC Harmonic Filter 25

5.4.3 Active Harmonic Filter 26

5.5 Surge Arrester 28

5.6 DC Transmission Circuit 31

5.6.1 DC Transmission Line 31

5.6.2 DC Cable 32

5.6.3 High Speed DC Switches 34

5.6.4 Earth Electrode 36

5.7 Control & Protection 38

6 System Studies, Digital Models,

Design Specifications  45

7 Project Management 46

3
 1 Why High Voltage
Direct Current?
1.1 Highlights from the High Voltage Direct In 1941, the first contract for a commercial HVDC
­Current (HVDC) History system was signed in Germany: 60 MW were to be
supplied to the city of Berlin via an underground
The transmission and distribution of electrical energy cable of 115 km length. The system with ±200 kV
started with direct current. In 1882, a 50-km-long and 150 A was ready for energizing in 1945. It was
2-kV DC transmission line was built between Miesbach never put into operation.
and Munich in Germany. At that time, conversion
between reasonable consumer voltages and higher Since then, several large HVDC systems have been
DC transmission voltages could only be realized by realized with mercury arc valves.
means of rotating DC machines. The replacement of mercury arc valves by thyristor
valves was the next major development. The first
In an AC system, voltage conversion is simple. thyristor valves were put into operation in the late
An AC transformer allows high power levels and nineteen-seventies.
high insulation levels within one unit, and has The outdoor valves for Cahora Bassa were designed
low losses. It is a relatively simple device, which with oil-immersed thyristors with parallel/series
requires little maintenance. Further, a three-phase connection of thyristors and an electromagnetic
synchronous generator is superior to a DC generator firing system.
in every respect. For these reasons, AC technology Further development went via air-insulated air-
was introduced at a very early stage in the develop- cooled valves to the air- insulated water-cooled
ment of electrical power systems. It was soon design, which is still state of the art in HVDC valve
accepted as the only feasible technology for genera- design.
tion, transmission and distribution of electrical The development of thyristors with higher current
energy. and voltage ratings has eliminated the need for
parallel connection and reduced the number of
However, high-voltage AC transmission links have series-connected thyristors per valve. The develop-
disadvantages, which may compel a change to ment of light-triggered thyristors has further reduced
DC technology: the overall number of components and thus contrib-
■■ Inductive and capacitive elements of overhead lines uted to increased reliability.
and cables put limits to the transmission capacity Innovations in almost every other area of HVDC have
and the transmission distance of AC transmission been constantly adding to the reliability of this tech-
links. nology with economic benefits for users throughout
■■ This limitation is of particular significance for the world.
cables. Depending on the required transmission
capacity, the system frequency and the loss evalua- Self-Commutated Voltage Sourced Converters
tion, the achievable transmission distance for Voltage sourced converters require semiconductor
an AC cable will be in the range of 40 to 100 km. devices with turn-off capability. The development
It will mainly be limited by the charging current. of Insulated Gate Bipolar Transistors (IGBT) with high
■■ Direct connection between two AC systems with voltage ratings have accelerated the development
different frequencies is not possible. of voltage sourced converters for HVDC applications
■■ Direct connection between two AC systems with in the lower power range.
the same frequency or a new connection within a The main characteristics of the voltage sourced
meshed grid may be impossible because of system converters are a compact design, four-quadrant
instability, too high short-circuit levels or undesir- operation capability and high losses.
able power flow scenarios. Siemens is offering voltage sourced converters for
HVDC applications with ratings up to 250 MW under
Engineers were therefore engaged over generations the trade name HVDCplus Power Link Universal
in the development of a technology for DC transmis- Systems.
sions as a supplement to the AC transmissions. This paper focuses upon HVDC transmission systems
with high ratings, i.e. with line-commutated current
Line-Commutated Current Sourced Converters sourced converters.
The invention of mercury arc rectifiers in the nine-
teen-thirties made the design of line-commutated HVDC = high voltage direct current
current sourced converters possible. DC = direct current
AC = alternating current
4 IGBT = insulated gate bipolar transistor
 1.2 Technical Merits of HVDC 1.4 Environmental Issues
The advantages of a DC link over an AC link are: An HVDC transmission system is basically environment-
■■ A DC link allows power transmission between friendly because improved energy transmission possi-
AC networks with different frequencies or networks, bilities contribute to a more efficient utilization of
which can not be synchronized, for other reasons. existing power plants.
■■ Inductive and capacitive parameters do not limit The land coverage and the associated right-of-way
the transmission capacity or the maximum length cost for an HVDC overhead transmission line is not
of a DC overhead line or cable. The conductor cross as high as that of an AC line. This reduces the visual
section is fully utilized because there is no skin effect. impact and saves land compensation for new projects.
It is also possible to increase the power transmission
For a long cable connection, e.g. beyond 40 km, capacity for existing rights of way. A comparison
HVDC will in most cases offer the only technical between a DC and an AC overhead line is shown
solution because of the high charging current of in Fig. 1-2.
an AC cable. This is of particular interest for trans-
mission across open sea or into large cities where
a DC cable may provide the only possible solution.
■■ A digital control system provides accurate and
fast control of the active power flow.
■■ Fast modulation of DC transmission power can be
used to damp power oscillations in an AC grid and
thus improve the system stability.

1.3 Economic Considerations

For a given transmission task, feasibility studies
are carried out before the final decision on imple-
mentation of an HVAC or HVDC system can be taken.
Fig.1-1 shows a typical cost comparison curve AC-tower DC-tower
between AC and DC transmission considering:
■■ AC vs. DC station terminal costs
■■ AC vs. DC line costs
■■ AC vs. DC capitalised value of losses
Fig. 1-2: Typical transmission line structures
The DC curve is not as steep as the AC curve because for approx. 1000 MW
of considerably lower line costs per kilometre. For
long AC lines the cost of intermediate reactive power There are, however, some environmental issues
compensation has to be taken into account. which must be considered for the converter stations.
The break-even distance is in the range of 500 to The most important ones are:
■■ Audible noise
800 km depending on a number of other factors, like
■■ Visual impact
country-specific cost elements, interest rates for project
■■ Electromagnetic compatibility
financing, loss evaluation, cost of right of way etc.
■■ Use of ground or sea return path
in monopolar operation
Costs
In general, it can be said that an HVDC system is
Total AC Cost
highly compatible with any environment and can
Total DC Cost be integrated into it without the need to compromise
on any environmentally important issues of today.
DC Losses
AC Losses

DC Line

AC Line
DC Terminals

AC Terminals

Break-Even Transmission
Distance Distance

Fig. 1-1: Total cost/distance

5
 2 Main Types of HVDC Schemes

2.1 DC Circuit
The main types of HVDC converters are distinguished
by their DC circuit arrange-ments. The following
equivalent circuit is a simplified representation of
the DC circuit of an HVDC pole.

Ud

Ud1 Ud2

Fig. 2-3: Back-to-back converter Station Vienna Southeast

Fig. 2-1: Equivalent DC circuit
2.3 Monopolar Long-Distance Transmissions
The current, and thus the power flow, is controlled For very long distances and in particular for very
by means of the difference between the controlled long sea cable transmissions, a return path with
voltages. The current direction is fixed and the power ground/sea electrodes will be the most feasible
direction is controlled by means of the voltage polarity. solution.
The converter is described in the next section.

2.2 Back-to-Back Converters

The expression Back-to-back indicates that the recti- HVDC

AC System 2
AC System 1

fier and inverter are located in the same station. Cable/OHL

Back-to-back converters are mainly used for power
transmission between adjacent AC grids which can
not be synchronized. They can also be used within Electrodes
a meshed grid in order to achieve a defined power flow.

HVDC

Fig. 2-4: Monopole with ground return path

AC System 1

AC System 2

In many cases, existing infrastructure or environmental

constraints prevent the use of electrodes. In such cases,
a metallic return path is used in spite of increased
cost and losses.

HVDC
AC System 1

AC System 2

Fig. 2-2: Back-to-back converter

Cable/OHL

HVDC = high voltage direct current

LVDC
DC = direct current
AC = alternating current
Ud = DC voltage 12-pulse
Id = DC current
OHL = overhead line
LVDC = low voltage direct current
Fig. 2-5: Monopole with metallic return path

6
2.4 Bipolar Long-Distance Transmissions
A bipole is a combination of two poles in such a way
HVDC
that a common low voltage return path, if available,
Cable/OHL
will only carry a small unbalance current during normal

AC System 1

AC System 2
Electrodes
operation.
This configuration is used if the required transmission
capacity exceeds that of a single pole. It is also used
HVDC
if requirement to higher energy availability or lower Cable/OHL
load rejection power makes it necessary to split the
capacity on two poles.
During maintenance or outages of one pole, it is still Fig. 2-8: in monopolar metallic return
possible to transmit part of the power. More than 50 % operation (converter pole outage)
of the transmission capacity can be utilized, limited
by the actual overload capacity of the remaining pole. 2.4.2 Bipole with Dedicated Metallic Return Path
The advantages of a bipolar solution over a solution for Monopolar Operation
with two monopoles are reduced cost due to one If there are restrictions even to temporary use of
common or no return path and lower losses. The main electrodes, or if the transmission distance is relatively
disadvantage is that unavailability of the return path short, a dedicated LVDC metallic return conductor
with adjacent components will affect both poles. can be considered as an alternative to a ground
return path with electrodes.
2.4.1 Bipole with Ground Return Path
This is a commonly used configuration for a bipolar
transmission system. The solution provides a high HVDC
degree of flexibility with respect to operation with Cable/OHL
AC System 1

AC System 2
reduced capacity during contingencies or maintenance.
LVDC
Cable/OHL

HVDC
HVDC Cable/OHL
Cable/OHL
AC System 1

AC System 2

Electrodes
Fig. 2-9: in bipolar balanced operation (normal)

HVDC 2.4.3 Bipole without Dedicated Return Path

Cable/OHL for Monopolar Operation
A scheme without electrodes or a dedicated metallic
return path for monopolar operation will give the
Fig. 2-6: in bipolar balanced operation (normal)
lowest initial cost.

Upon a single-pole fault, the current of the sound

pole will be taken over by the ground return path HVDC
and the faulty pole will be isolated. Cable/OHL
AC System 1

AC System 2

HVDC HVDC
Cable/OHL Cable/OHL
AC System 1

AC System 2

Electrodes

Fig. 2-10: in bipolar balanced operation (normal)

HVDC
Cable/OHL Monopolar operation is possible by means of bypass
switches during a converter pole outage, but not
during an HVDC conductor outage.
Fig. 2-7: in monopolar ground return
A short bipolar outage will follow a converter pole
operation (converter pole or OHL outage) outage before the bypass operation can be established.

Following a pole outage caused by the converter,

the current can be commutated from the ground
return path into a metallic return path provided
by the HVDC conductor of the faulty pole.
7
 3 Converter Theory

3.1 Bridge Circuit Function The angle between the time at which the valve voltage
Current flows through the valves when the voltage becomes positive and the firing time (start of commu-
between the anode and cathode is positive. For the tation) is referred to as the firing delay. Fig. 3-2 shows
valve to commutate the current, there must be a that for a firing delay of 90°, the average voltage equals
positive potential (voltage), and the thyristor must zero. i.e. the positive and negative areas of the curve –
have firing pulses. In the reverse direction, i.e. when voltage against time – cancel each other out. No active
the potential between anode and cathode is negative, power flows through the converter. When the firing
a firing pulse has no effect. The flow of current in delay is greater than 90°, the negative voltage/time
a valve ends when the voltage between anode and areas dominate, and the polarity of the average direct
cathode becomes negative. The instant when current voltage changes. Due to physical reasons, the direction
begins to flow through a valve, or to commutate of the current does not change. (The thyristor valves
from one valve to another, can be delayed by post- conduct current only in one direction.) When the direc-
poning the firing. This method permits the average tion of energy flow is reversed, the delivery changes
value of the outgoing voltage of the rectifier to be to the supply side. The rectifier becomes an inverter
changed. The firing pulses are generated by synchro- which delivers energy to the AC network.
nizing the network using an elec-tronic control device.
These pulses can be displaced from their ”natural The average value of the direct voltage as a function
firing“ point, which is the point where the two phase of the firing delay is given by:
voltages intersect. The method of firing-pulse dis- Udiα = 1.35 * UL * cos α
placement is called phase control. UL = secondary side line voltage
α = firing angle
γ = extinction angle
DC current in each valve and phase
1 3 5 1
L1 L2 L3
60° 0°
0° 120° Ud Udi

6 2 4 6 2 ωt
ld = 0°
0°
i1
i2

i3
i4 ωt
= 60°
60°
i5
i6
ωt = 90°
iL 90 °
1 = 90°
iL
2
iL
3
150° ωt = 150°
ld = 30°
1 3 5

L1

L2 Ud 180°
ωt = 180°
L3 = 0°

4 6 2

Fig. 3-1: Six-pulse converter bridge Fig. 3-2: DC voltage of bridge converter as a function of α

8
 3.2 12-Pulse Group and Converter Transformer 3.3 Reactive Power as a Function of Load
HVDC converters are usually built as 12-pulse circuits. The curve of reactive power demand of an
This is a serial connection of two fully controlled HVDC station with changing active power P
6-pulse converter bridges and requires two 3-phase can be calculated from equation:
systems which are spaced apart from each other by
30 electrical degrees. The phase difference effected Q = P * tan [ arc cos ( cos α - dx)]
to cancel out the 6-pulse harmonics on the AC and
DC side. In Fig. 3-5, the reactive power demand of a converter
is presented under three different control methods.

If the terminal DC voltage Ud and the firing angle

3 1 2 α (or the extinction angle γ of an inverter) are held
constant, curve (1) will be obtained. If, however,
Uv is held constant (Udi = const regulation), a linear
curve such as (2) is obtained. The power of a con-
4
verter can also be changed when the (nominal)
current is held constant by varying the DC voltage.
Curve (3) shows the reactive power demand for this
control method. It is important to note that the entire
1 Valve Branch area between curves (1) and (3) is available for reactive
2 Double Valve
3 Valve Tower power control. Each point within this area can be set
4 6-pulse Bridge by the selection of firing angles α and β (or γ).

Fig. 3-3: Arrangement of the valve branches in a 12-pulse bridge

= 60°

Q/PN
Udi 1.2
3
1.0

ωt 0.8

0.6
Secondary Voltage
of the Transformer 0.4 2

0.2 1
Basic AC Current
P/PN

0.5 1.0

1 Ud = const; = const ( = const)

2 Ud = const; Uv = const
3 Id = const

Fig. 3-4:Current displacement with angle control Fig. 3-5:Reactive power demand of an HVDC converter

HVDC DC Circuit dx = relative inductive voltage drop

UdN = PdN Rec/IdN Uv = valve voltage
UdN => nominal DC voltage 12-pulse Ud = DC voltage 12-pulse
IdN => nominal DC current α = firing angle
PdNRec => nominal DC active power β = 180°-α
at the rectifier γ = extinction angle

9
 3.4 Reactive Power Control
The possibility of electronic reactive power control
420 kV 50 Hz 420 kV 50 Hz as demonstrated in the preceding section is used
only to a very limited degree in HVDC technology.
This is due to economic reasons. Both control reac-
tive power and commutation reactive power are
increased by the reduction of the DC voltage and
the corresponding increase of current. However,
load losses increase with the square of the current.
For this reason, application is limited to the light
loads where the necessary filter circuits produce
a considerable overcompensation for the reactive
power required by the converter.
Q = 103 Mvar Q = 103 Mvar
Fig. 3-6 depicts the reactive power control of the
Dürnrohr HVDC link. In this system, a compensation
to ± 60 Mvar was specified. Compliance with the
Q = 103 Mvar Q = 103 Mvar
Q limit is achieved by load-dependent switching of
a capacitor bank and one of the two high-pass filters.
Q = 103 Mvar Q = 103 Mvar
Electronic reactive power is used only in the light
load range. Normally, there is a difference between
the connect and disconnect points of the reactive
power elements. This provides a ”switching hysteresis”
Fig. 3-6: Reactive-power compensation and control which prevents too many switching operations or
of an HVDC back-to-back link even a ”pumping”.

Q (Mvar)
100

80
Over load
Reduced minimum load
Normal minimum load

Normal load

60

40

20

100 200 300 400 500 600 700 P (MW)

0.2 0.4 0.6 0.8 1.0 1.2 P
PN
–20

–40

–60
Electronic Capacitor
reactive bank
power
regulation High–pass filter 2
–80
High–pass filter 1

–100

Reactive-Power Balance
UAC in p.u. (AC bus voltage)
– cap. reactive-power reactive-power reactive-power reactive-power
+ ind converter AC filters reactors capacitors
QNetwork = + QConv – QFK*UAC2 + QL*UAC2 – QC*UAC2

10
 4 Principle Arrangement of
an HVDC Transmission Project
The Principle Arrangement of an HVDC Transmission
Project is reflected on the Moyle Interconnector
project. The HVDC stations between Northern Ireland
and Scotland are operating with the following
highlights:
■■ Direct light triggered thyristor valves for the
complete HVDC system, with 1872 thyristors
in total, with 20 % better reliability and all valve
components free from oil.
■■ Triple tuned AC filter in both stations.
■■ Unmanned stations, fully automatic remote
operation and automatic load schedule operation.
■■ Hybrid optical ohmic shunt for DC current
measuring unit.
■■ Low noise station design for:
– AC filter capacitor and reactors
– Converter transformer
– Converter valve water cooling system
– DC hall with smoothing reactor
■■ Station design for DC see/land cable with
integrated return conductor and fibre optic
cable for control and communication.

Date of contract 09/99

Delivery period 27 months

System Data
Transmission capacity 2 x 250 MW
System voltages 250 kV DC
275 kV AC
Rated current 1000 A
Transmission distance 63.5 km

Moyle Interconnector
Ballycronan
More Auchencrosh Coylton

Overhead Line

Converter Station Converter Station AC Substation

Undersea Cables
Existing 275-kV Existing 275-kV
Transmission System Transmission System

Alternating Current Direct Current Alternating Current

Northern Ireland Scotland

11
250 DC Power Cable. HVDC Station Auchencrosh L L
63.5 km to HVDC Station HA
DC
Ballycronan More
Northern Ireland L
VA
Smoothing Reactor L
AC-Filter O G
DC R
Pole 1, 250 MW
kV NT DIN
AC-Filter O
C UIL
Thyristor
Valves 2 50 B
C-Shunt

Thyristor
Valves
AC-Filter 1

S
AN
Pole 2, 250 MW

TR
AC-Filter
2

ER
Smoothing Reactor 16
3

RT
AC-Filter
14

N VE
CO
AC Bus

SH
6
14

12
 L L
HA
E 14
LV

4
EA
AR
ER
M
OR
SF

NK

11
BA
R
TO
CI
PA
CA
NT

9
8 10
HU

15
27
SWI 5 kV AC
5 15 TCH
YAR 5 6
D 7 5 6 5 6
10
8 9

9
10 10 275 kV
8 9 12 13 OHL

14

1 Quadruple Thyristor Valve 9 Disconnector

2 Converter Transformer 10 Current Transformer
3 Air Core Smoothing Reactor 11 Voltage Transformer
4 Control Room and Control Cubicle 12 Combined Current-Voltage Transformer
5 AC Filter Capacitor 13 Capacitive Voltage Transformer
6 AC Filter Reactor 14 Surge Arrester
7 AC Filter Resistor 15 Earthing Switch
8 Circuit Breaker 16 AC PLC Filter

13
 5 Main Components

5.1 Thyristor Valves 5.1.1 Introduction

The thyristor valves make the conversion from AC
Components into DC and thus are the central component of any
Siemens is a leading supplier of HVDC systems all HVDC converter station. The thyristor valves are of
over the world. Our components are exceeding the indoor type and air-insulated. Siemens has more
the usual quality standards and are system-tailored than 30 years experience in the development and
to the needs of the grid. manufacturing of thyristor valves and has maintained
the technical leadership by introducing new innovative
concepts such as the corrosion-free water cooling and
the self-protecting direct-light-triggered thyristor.
This directly reflects in the high reliability of these
valves.

valve valve valve

valve valve valve

valve valve valve

valve valve valve

12-pulse group

Multiple valve unit –

quadruple valve Valve
branch

Fig. 5.1-2: General arrangement of a 500 kV MVU (valve tower)

Fig. 5.1-1: Principle circuit diagram of a 12-pulse group
consisting of three quadruple valves

14
5.1.2 Valve Design
The modular concept of the Siemens thyristor
valves permits different mechanical setups to best
suit each application: single, double, quadruple
valves or complete six-pulse bridges – either
free – standing or suspended from the building
structure.

For seismic requirement reasons which exist in

some regions of the world, the standard Siemens
valves for long distance transmission are suspended
from the ceiling of the valve hall. The suspension
insulators are designed to carry the weight and
additional loads originating for example from an
unbalanced weight distribution due to insulator
failure, an earthquake or during maintenance.
Connections between modules (piping of cooling
circuit, fibre optic ducts, buswork, and suspension
insulator fixtures) are flexible in order to allow
stress-free deflections of the modules inside an
MVU (multiple valve unit) structure. Figure 5.1-2
shows a typical quadruple valve tower for a 500 kV DC
system. Each valve is made up of three modules.
Four arresters, each related to one valve, are
located on one side of the valve tower. Ease of
access for maintenance purposes, if required,
is another benefit of the Siemens valve design.

By varying the number of thyristors per module

and the number of modules per valve, the same
design can be used for all transmission voltages
that may be required.

15
 5.1.3 Thyristor Development system faults without any limitations. In case of elec-
Thyristors are used as switches and thus the valve trically triggered thyristors (ETT), this is only possible
becomes controllable. The thyristors are made of if enough firing energy is stored long enough on the
highly pure mono-crystalline silicon. The high speed thyristor electronics.
of innovation in power electronics technology is
directly reflected in the development of the thyristor. Direct light-triggered thyristors with integrated over-
voltage protection (LTT) is now a proven technology
and the Siemens standard. It was implemented success-
Thyristor Current Thyristor Blocking
(IdN) Voltage (UDRM) fully for the first time in 1997 (Celilo Converter Station
LTT 8 of the Pacific Intertie). It shows excellent performance
and no thyristor failures or malfunction of the gating
kA kV
system have been recorded. BPA has emphasized its
Thyristor Blocking confidence in this technology in 2001 by awarding
6 Voltage 6 Siemens the contract to replace all mercury arc valves
with direct-light-triggered thyristor valves. Further-
more, this valve technology is used for the Moyle
4 4 Interconnector (2 x 250 MW), which went into
service in 2001 and is on contract for the 3000-MW,
Thyristor Current for
Long-Distance Transmission ± 500-kV Guizhou-Guangdong system.
2 2
Monitoring of the thyristor performance is achieved
by a simple voltage divider circuit made from standard
off-the-shelf resistors and capacitors; monitoring
signals are transmitted to ground potential through
1970 1980 1990 2000 2003
a dedicated set of fibre optic cables as for the ETT.
However, all electronic circuits needed for the evalu-
Fig. 5.1-3: Thyristor development ation of performance are now located at ground
potential in a protected environment, further simpli-
The high performance thyristors installed in HVDC fying the system. The extent of monitoring is the
plants today are characterized by silicon wafer same as for the ETT.
diameters of up to 5’’ (125 mm), blocking voltages
up to 8 kV and current carrying capacities up to It can be expected that this technology will become
4 kA DC. Thus no parallel thyristors need to be the industry standard in HVDC thyristor valves of the
installed in today’s HVDC systems for handling 21st century, paving the way towards maintenance-
the DC current. The required DC system voltages free thyristor valves.
are achieved by a series connection of a sufficient
number of thyristors.
Light Pipe

5.1.4 LTT (Light-Triggered Thyristor)

It has long been known that thyristors can be turned Cu Si
on by injecting photons into the gate instead of elec-
trons. The use of this new technology reduces the
number of components in the thyristor valve by up to Cu Mo
80 %. This simplification results in increased reliability
and availability of the transmission system. With LTT
technology, the gating light pulse is transmitted via Fig. 5.1-4: The optical gate pulse is transmitted directly
a fibre optic cable through the thyristor housing to the thyristor wafer
directly to the thyristor wafer and thus no elaborate
electronic circuits and auxiliary power supplies are
needed at high potential. The required gate power
is just 40 mW. The forward overvoltage protection is
integrated in the wafer. Further benefits of the direct
light triggering are the unlimited black start capability
and the operation during system undervoltage or

Cu = Copper
Si = Silicon
Mo = Molybdenum
LTT = Light-triggered thyristors
ETT = Electrically triggered thyristors
Fig. 5.1-5: Valve module with direct-light-triggered thyristor

16
 5.1.6 Flame Resistance
A lot of effort has been invested by Siemens
to minimize the fire risk:
■■ All oil has been eliminated from the valve and
its components. Snubber capacitors and grading
capacitors use SF6 as a replacement for
impregnating oil.
■■ Only flame-retardant and self-extinguishing
plastic materials are used.
■■ A wide separation between the modular units
ensures that any local overheating will not affect
neighbouring components.
■■ Careful design of the electrical connections avoids
Fig. 5.1-6: Silicon wafer and housing loose contacts.
of a direct-light-triggered thyristor

The past has shown that Siemens HVDC installations

5.1.5 Valve Cooling
have never been exposed to a hazardous valve fire.
Siemens has used the parallel water cooling principle
The tests performed on actual components and
for more than 25 years. No corrosion problems have
samples in the actual configuration as used in the
ever been encountered.
valve indicate that the improved design is indeed
flame-retardant and the risk of a major fire following
The thyristors are stacked in the module with a heat
a fault is extremely low or even non-existent.
sink on either side. The water connection to the heat
sinks can be designed in parallel or series as shown in
figure 5.1-7. The parallel cooling circuit provides all
thyristors with the same cooling water temperature.
This allows a better utilization of the thyristor capa-
bility. Siemens makes use of this principle, which offers
the additional advantage that electrolytic currents
through the heat sinks – the cause for electrolytic
corrosion – can be avoided by placing grading
­electrodes at strategic locations in the water circuit.
­Siemens water cooling also does not require any
de‑oxygenizing equipment.

b a c

a c
b

Fig. 5.1-7: Piping of module cooling circuit –

parallel flow (top); series flow (bottom)
a) thyristor; b) heat sink; c) connection piping; d) manifold

Fig. 5.1-8: Converter valves Sylmar HVDC station,

Los Angeles, USA

17
 5.2 Converter Transformer

Siemens supplies transformers which meet all

requirements concerning power, voltage, mode
of operation, low noise level, connection tech-
niques, type of cooling, transport and installation.
They also comply with special national design
requirements.

All over the world, power transformers from

Nuremberg enjoy a great reputation. What the
Nuremberg plant manufactures reflects today’s
state of the art and testifies to the highest levels
of quality and reliability. Our quality management
system is certified to DIN 9001, the world’s most
stringent standard. Our accredited test laboratories
likewise meet the latest specifications.
Project: Tian Guang
HVDC bipolar
long-distance transmission Fig. 5.2-1: Converter transformer for the Tian Guang
PN = 2 x 900 MW HVDC project during type test
Ud = ± 500 kV
Transformers: SN = 354/177/177 MVA
1-ph/3-w unit
UAC = 220 kV

5.2.1 Functions of the HVDC Converter

Transformer
The converter transformers transform the voltage
of the AC busbar to the required entry voltage of
the converter.

The 12-pulse converter requires two 3-phase systems

which are spaced apart from each other by 30 or
150 electrical degrees. This is achieved by installing
a transformer on each network side in the vector
groups Yy0 and Yd5.

At the same time, they ensure the voltage insulation

necessary in order to make it possible to connect
converter bridges in series on the DC side, as is
necessary for HVDC technology. The transformer
main insulation, therefore, is stressed by both the
AC voltage and the direct voltage potential between
valve-side winding and ground. The converter trans-
formers are equipped with on-load tap-changers
in order to provide the correct valve voltage.

Transformer Rating

STrafo Rec (6-pulse) = √2 * IdN * Usec Rec

IdN nominal DC current

Usec Rec Transformer-voltage

valve side (Rectifier)

STrafo Inv (6-pulse) = √2 * IdN * Usec Inv

Usec Inv Transformer voltage

valve side (inverter) Converter transformer for the
Three Gorges HVDC project 284 MVA, 1-ph/2-w unit
18
5.2.2 Transformer Design Variations 5.2.3 HVDC Makes Special Demands on
There are several aspects which play a role in Transformers
selecting the transformer design: HVDC transformers are subject to operating conditions
that set them apart from conventional system or
Transportation Weight and Dimensions power transformers. These conditions include:
In systems of high power, weight can be an important ■■ Combined voltage stresses
consideration, in particular where transportation is ■■ High harmonics content of the operating current
difficult. The relative transportation weights of the ■■ DC premagnetization of the core
4 major design types are approximately as follows:
The valve windings which are connected to the rectifier
and the converter circuit are subject to the combined
Single-phase – two-winding transformer 1 load stress of DC and AC voltage. Added to this stress
are transient voltages from outside caused by lightning
Single-phase – three-winding transformer 1.6 strikes or switching operations.

Three-phase – two-winding transformer 2.2 The high harmonics content of the operating current
results from the virtually quadratic current blocks of the
Three-phase – three-winding transformer 3.6 power converter. The odd-numbered harmonics with
the ordinal numbers of 5, 7, 11, 13, 17 … cause addi-
tional losses in the windings and other structural parts.
The transport dimension and the weight of the
converter transformer depends on the limitations
for street, railway and shipping, especially in the
case of bridges, subways and tunnels.

19
 5.2.4 Main Components of the thyristor valve towers. This frequently leads
of the Converter Transformer to very high connection heights and the need to
mount the oil expansion tank at a significant height.
Core In close cooperation with the equipment design
HVDC transformers are normally single-phase department, the engineering specialists at the
transformers, whereby the valve windings for Nuremberg Transformer Plant have always been
the star and delta connection are configured able to find a design suited to every customer
either for one core with at least two main limbs requirement.
or separately for two cores with at least one main
limb, depending on the rated power and the Bushings
system voltage. Appropriately sized return limbs Compared to porcelain, composite bushings provide
ensure good decoupling for a combined arrange- better protection against dust and debris. A 15%
ment of windings. higher DC voltage testing level compared to the
windings underscores the particular safety aspect
The quality of the core sheets, the lamination of of these components.
the sheets, and the nominal induction must all
conform to special requirements covering losses, Special Tests for HVDC Transformers
noise level, over-excitation, etc. Special attention Special tests for verifying operating functionality
must be paid to the DC premagnetization of the are required for HVDC transformers. The applicable
core due to small asymmetries during operation international standards are subject to constant
and stray DC currents from the AC voltage network. further development. Separate tests with DC voltage,
The effects of DC premagnetization must be compen- switching and lightning impulse voltages cover the
sated by appropriate design and manufacturing range of different voltage loads. The 2-MV DC volt-
efforts (e.g. additional core cooling ducts, avoidance age generator in the Nuremberg Transformer Plant
of flux pinching in the core sheet). is well-suited for all required DC voltage and reverse
poling tests. The most important criterion is partial
Windings discharge. A maximum of 10 discharges over 2000 pC
The large number of parameters concerning during the last 10 minutes of the test is permitted.
transport limitations, rated power, transformer
ratio, short-circuit voltage, and guaranteed losses 5.3 Smoothing Reactor
require significant flexibility in the design of
windings. 5.3.1 Functions of the Smoothing Reactor
■■ Prevention of intermittent current
In concentric winding arrangements, star or delta ■■ Limitation of the DC fault currents
valve windings lying directly on the core have ■■ Prevention of resonance in the DC circuit
proven optimal in many cases. The line winding, ■■ Reducing harmonic currents including
normally with a tapped winding, is then mounted limitation of telephone interference
radial outside this core configuration.
Prevention of intermittent current
The valve windings with high insulation levels and The intermittent current due to the current ripple
a large portion of current harmonics make particular can cause high over-volt-ages in the transformer and
demands on the design and the quality of the wind- the smoothing reactor. The smoothing reactor is used
ing manufacturing. Together with its pressboard to prevent the current interruption at minimum load.
barriers, each limb set, including a valve, an over­
voltage and a tapped winding, forms a compact Limitation of the DC fault current
unit, which is able to cope with the demand made The smoothing reactor can reduce the fault current
by voltage stress, loss dissipation, and short-circuit and its rate of rise for commutation failures and
withstand capability. DC line faults.
This is of primary importance if a long DC cable is
Tank used for the transmission. For an overhead line trans-
The unconventional tank design in HVDC transformers mission, the current stress in valves is lower than the
result from the following requirements: stress which will occur during valve short circuit.
■■ The valve-side bushing should extend
into the valve hall Prevention of resonance in the DC circuit
■■ The cooling system is mounted on the opposite The smoothing reactor is selected to avoid resonance
side to facilitate rapid transformer exchange in the DC circuit at low order harmonic frequencies like
100 or 150 Hz. This is important to avoid the amplifi-
For HVDC transformers with delta and star valve cation effect for harmonics originally from the AC
winding in one tank, the valve bushing must be system, like negative sequence and transformer
arranged so that their ends conform to the geometry saturation.
20
Reducing harmonic currents including limitation The wall bushing in composite design is the
of telephone interference state-of-the-art technology which provides
Limitation of interference coming from the DC over- superior insulation performance.
head line is an essential function of the DC filter
circuits. However, the smoothing reactor also plays
an important role to reduce harmonic currents acting
as a series impedance.

5.3.2 Sizing of the smoothing Reactor

While the current and voltage rating of the smooth-
ing reactor can be specified based on the data of the
DC circuit, the inductance is the determining factor
in sizing the reactor.Taking all design aspects above
into account, the size of smoothing reactors is often
selected in the range of 100 to 300 mH for long-
distance DC links and 30 to 80 mH for back-to-back
stations.

5.3.3 Arrangement of the Smoothing Reactor

In an HVDC long-distance transmission system, it
seems quite logical that the smoothing reactor will
be connected in series with the DC line of the station
pole. This is the normal arrangement. However in
Fig. 5.3-1: Oil-insulated smoothing reactor –
back-to-back schemes, the smoothing reactor can
Three Gorges project
also be connected to the low-voltage terminal. ■ Inductance: 270 mH
■ Rated voltage: 500 kV DC
5.3.4 Reactor Design Alternatives ■ Rated current: 3000 A DC
There are basically two types of reactor design:
■■ Air-insulated dry-type reactors
■■ Oil-insulated reactors in a tank

The reactor type should be selected taking the

following aspects into consideration:
■■ Inductance
■■ Costs
■■ Maintenance and location of spare units
■■ Seismic requirements

An advantage of the dry-type reactor is that main-

taining spare units (to the extent necessary) is not
very expensive because they usually consist of several
partial coils. However for very large inductances it
is possible to have more than one unit and it could
be a problem if much space is not available.

In high seismic regions, setting them on post-insulators

or on an insulating platform is a possible problem.
Oil-insulated smoothing reactors are then the preferred
solution.

The oil-insulated reactor is economical for very

high power (Id2 * Ldr). It is the best option for
regions with high seismic requirements.

One bushing of the oil-insulated smoothing reactor

penetrates usually into the valve hall, while the Fig. 5.3-2: Air-insulated smoothing reactor –
other bushing is normally in a vertical position. Tian Guang project
For the air-insulated dry-type smoothing reactor, ■ Inductance: 150 mH
a wall bushing is needed to connect with the valves. ■ Rated voltage: 500 kV DC
■ Rated current: 1800 A DC

21
 5.4 Harmonic Filters
The filter arrangements on the AC side of an
HVDC converter station have two main duties:
■■ to absorb harmonic currents generated
by the HVDC converter and thus to reduce
the impact of the harmonics on the connected
AC systems, like AC voltage distortion and
telephone interference
■■ to supply reactive power for compensating
the demand of the converterstation

Each filter branch can have one to three tuning

frequencies. Figure 5.4.1-1 shows different
harmonic filter types with their impedance
frequency characteristics.

5.4.1 AC Harmonic Filter

5.4.1.1 Design Criteria for AC Filters

Reactive Power Requirements

Fig. 5.4.1-2: AC filters and capacitor banks of Gezhouba/Shanghai
The reactive power consumption of an HVDC converter
depends on the active power, the transformer reactance
Single-tuned
and the control angle. It increases with increasing
active power. A common requirement to a converter
Harmonic Order station is full compensation or overcompensation
1000
Filter Impedance

at rated load. In addition, a reactive band for the

800
load and voltage range and the permitted voltage
600
400
step during bank switching must be determined.
200 These factors will determine the size and number
0 of filter and shunt capacitor banks.
0 10 20 30 40 50
Harmonic Performance Requirements
HVDC converter stations generate characteristic
Double-tuned
and non-characteristic harmonic currents. For a
Harmonic Order twelve-pulse converter, the characteristic harmonics
1000
are of the order n = (12 * k) ± 1 (k = 1,2,3 ...). These
Filter Impedance

800 are the harmonic components that are generated

600 even during ideal conditions, i.e. ideal smoothing
400 of the direct current, symmetrical AC voltages, trans-
200 former impedance and firing angles. The characteristic
0 harmonic components are the ones with the highest
0 10 20 30 40 50 current level, but other components may also be
of importance. The third harmonic, which is mainly
caused by the negative sequence component of the
Triple-tuned
AC system, will in many cases require filtering.
Harmonic Order
800 An equivalent circuit for determination of harmonic
Filter Impedance

600
performance is given in figure 5.4.1-3. The most
commonly used criteria for harmonic performance
400
are related to the harmonic voltage on the converter
200 station busbar. The purpose of the filter circuit is to
0 provide sufficiently low impedances for the relevant
0 10 20 30 40 50
harmonic components in order to reduce the harmonic
voltages to an acceptable level.
Fig. 5.4.1-1: Different harmonic filter types

22
 There are basically two methods to include the
network impedance in the filter calculations:
■■ to calculate impedance vectors for all relevant
harmonics and grid conditions,
Ih ZF Uh ZN ■■ to assume locus area for the impedance vectors.

The modelling of a complete AC network with all its

components is very complex and time-consuming.
For this reason, the locus method is very often used.
Harmonic Filter Network It is based on a limited number of measurements
Current Impedance Impedance or calculations. Different locus areas for different
Source
harmonics or bands are often determined to give
a more precise base for the harmonic performance
Fig. 5.4.1-3: Equivalent circuit for calculation
calculation.
of harmonic voltages and currents in the AC system
A typical locus area is shown in fig. 5.4.1-4. It is
The acceptance criteria for the harmonic distortion assumed that the impedance vector will be some-
depend on local conditions and regulations. A com- where inside the perimeter of the coloured area.
monly used criterion for all harmonic components
up to the 49th order is as follows: The impedance vector of the filter is transformed
into the Y plane for each harmonic frequency.

With both the network and the filter impedances

Dn Individual harmonic voltage
plotted in the admittance plane, the shortest vector
distortion of order n in percent
between the filter admittance point and the network
of the fundamental AC busbar
admittance boundary gives the lowest possible
voltage (typical limit 1%)
admittance value for the parallel combination of
the network and the filter. This value is used to
Drms Total geometric sum of
determine the highest possible harmonic voltage.
individual voltage distortion
Dn (typical limit 2 %)

The BTS Telephone Interference Factor (TIF) and the

CCITT Telephone Harmonic Form Factor (THFF) are
determined with weighted factors in order to evaluate
the voltage distortion level on the AC busbar with
respect to the expected interference level in nearby
analogue telephone systems. The IT product is a
criterion for harmonic current injected into AC over- Ymin = 1/Rmax Ymax = 1/Rmin
head lines. The criteria based on telephone inter-
ference are in many cases irrelevant, because modern
digital telephone systems are insensitive to harmonic
interference. Yf

Network Impedance Yres

The distortion level on the AC busbar depends on
the grid impedance as well as the filter impedance.
An open circuit model of the grid for all harmonics
is not on the safe side. Parallel resonance between
the filter impedance and the grid impedance may
create unacceptable amplification of harmonic com- Fig. 5.4.1-4 Circle of network admittance
ponents for which the filters are not tuned. For this and the resonance conditions
reason, an adequate impedance model of the grid
for all relevant harmonics is required in order to
optimize the filter design.
Ih = Harmonic source current
Zf = Filter impedance
ZN = Network impedance
Uh = Harmonic voltage

23
 Fig. 5.4.2-1 DC filter of Guangzhou/China

The selective resonance method represents a reason- Requirements to Ratings

able compromise. It takes into consideration the fact
that the highest voltage distortion (highest harmonic Steady-State Calculation
voltage) occurs with a parallel resonance between The voltage and current stresses of AC filters consist of
filter and AC network. It is unrealistic however, to the fundamental frequency and harmonic components.
assume that such a parallel resonance takes place Their magnitudes depend on the AC system voltage,
at all frequencies. Normally it is sufficient to consider harmonic currents, operating conditions and AC system
in the calculation of total distortion and TIF value impedances. The rating calculations are carried out
only two maximum individual distortions from the in the whole range of operation to determine the
resonance calculation. The AC network is assumed highest steady-state current and voltage stresses
to be open for the remaining harmonic currents. for each individual filter component.

The filter calculations must reflect detuning caused Transient Calculation

by AC network frequency deviations and component The objective of the transient rating calculation
parameter deviations. Production tolerances, tempe- is to determine the highest transient stresses for
rature drift and failure of capacitor elements are the each component of the designed filter arrangement.
main contributors to parameter deviations. The results of the transient calculation should
contain the voltage and current stresses for each
component, energy duty for filter resistors and
arresters, and the insulation levels for each filter
component.
24
To calculate the highest stresses of both lightning 5.4.2.2 Design Criteria for DC Filter Circuits
and switching surge type, different circuit The interference voltage induced on the telephone
configurations and fault cases should be studied: line can be characterized by the following equation:
■■ Single-Phase Ground Fault
The fault is applied on the converter AC bus
next to the AC filter. It is assumed that the
filter capacitor is charged to a voltage level Ieq = (Hµ * Cµ * Iµ(x))2
corresponding to the switching impulse
protective level of the AC bus arrester. Vin(x) = Z * Ieq
■■ Switching Surge
For the calculation of switching surge stresses,
a standard wave of 250/2500 µs with a crest value
where
equal to the switching impulse protective level
of the AC bus arrester is applied at the AC converter
bus.
Vin(x) = Interference voltage on the
■■ Filter Energization
telephone line at point x
The AC filter is assumed to be energized at the
(in mV/km)
moment for the maximum AC bus peak voltage.
This case is decisive for the inrush currents
Hµ = Weighting factors which reflect
of AC filters.
the frequency dependence of
■■ Fault Recovery after Three-Phase
the coupling between telephone
Ground Fault
and HVDC lines
Various fault-clearing parameters should be
investigated to determine the maximum energy
Cµ = “C message“ – weighting factors
stresses for AC filter arresters and resistors.
The worst-case stresses are achieved if the
Iµ(x) = Resulting harmonic current
HVDC converters are blocked after fault initiation,
of the ordinal number µ in
while the AC filters remain connected to the
the HVDC line at point x as the
AC bus after fault clearing and recovery of the
vector sum of the currents
AC system voltage. In this case, a temporary
caused by the two HVDC stations
overvoltage with high contents of non-characteristic
harmonics will occur at the AC bus due to the
Ieq = Psophometric weighted equivalent
effects of load rejection, transformer saturation
disturbing current
and resonance between filter and AC network at
low frequency.
Z = Mutual coupling impedance
between the telephone and
5.4.2 DC Harmonic Filter
HVDC lines
5.4.2.1 DC Filter Circuits
Harmonic voltages which occur on the DC side of
a converter station cause AC currents which are The equivalent disturbing current combines
superimposed on the direct current in the trans- all harmonic currents with the aid of weighting
mission line. These alternating currents of higher factors to a single interference current. With
frequencies can create interference in neighbouring respect to telephone interference, it is the
telephone systems despite limitation by smoothing equivalent to the sum of all harmonic currents.
reactors. It also encompasses the factors which determine
the coupling between the HVDC and telephone
DC filter circuits, which are connected in parallel lines:
to the station poles, are an effective tool for com- ■■ Operating mode of the HVDC system
bating these problems. The configuration of the (bipolar or monopolar with metallic or
DC filters very strongly resembles the filters on ground return)
the AC side of the HVDC station. There are several ■■ Specific ground resistance at point x
types of filter design. Single and multiple-tuned
filters with or without the high-pass feature are The intensity of interference currents is strongly
common. One or several types of DC filter can dependent on the operating condition of the
be utilized in a converter station. HVDC. In monopolar operation, telephone
interference is significantly stronger than in
bipolar operation.

25
 5.4.3 Active Harmonic Filter 400 kV AC busbars of the station. This has the advan-
tage that the harmonic control requires just one
Active filters can be a supplement to passive filters measurement point, compared to a current measuring
due to their superior performance. They can be scheme, which would require to measure the current
installed on the DC side or on the AC side of the at several points and combining the measured signals.
converter. The connection to the high-voltage system The other advantage is that the active filter works
is achieved by means of a passive filter, forming a just like a passive filter ideally should do, i.e. elimi-
so-called hybrid filter. This arrangement limits the nating the voltage in the bus, thus representing no
voltage level and the transient stresses on the active change in philosophy.
part, so that comparatively low equipment ratings
can be used. Appropriate design allows the exploita-
tion of the positive characteristics of both passive
and active filters. Additionally, the passive part
can be used as a conventional passive filter if the
active part is by-passed for maintenance purposes.

Main Components

No. Component

1 IGBT converter

2 Reactor for inductivity adapting

3 Thyristor switch for converter

overvoltage and overcurrent
protection

4 Transformer

5 Low-pass filter
Fig. 5.4.3-1: Active DC filter on site (Tian Guang HVDC project)
6 Vacuum switch
The active filter is fully assembled in a transportable
7 ZnO arrester container and is tested at the factory as a complete
system before shipping. Fig. 5.4.3-5 shows the
8 Isolators and grounding switches installed active AC filter (in the container) at the
9 LC branch for deviating the Tjele substation.
50-Hz current component

400 kV Bus
The Siemens active filters use voltage-sourced
IGBT converters with a high switching frequency Existing
Capacitive
to produce an output voltage up to approximately Potential Optical Passive
700 Vpeak, containing harmonics up to the 50th Transformer
Interface
From the Filter
Other Phases ZF2
as required. A powerful high-speed control and
protection system processes the currents and/or Underground
Active AC Filter Cable
voltages measured at the network by appropriate To/From Main
sensors and produces the control pulses for the Common Simadyn D Components
cubicle of the Other
IGBT’s. Control and Auxiliary (control and
Phases (in
Equipment protection)
the Container)

A transformer matches the voltage and current levels 2

at the converter output and provides the required 1 3 4 5 6 7
IGBT
insulation level. The goal of the scheme is to inject conv. LP 9
8
harmonics in the network with the same amplitude
and the opposite phase of the harmonics at the
measurement point in order to cancel them.
Main Components
The filter for AC application comprises three single- (one Set for Each Phase)
phase systems controlled by a common digital con-
trol system. A major difference is the measurement:
instead of measuring the line current, the active filter Fig. 5.4.3-2: Single-line diagram of the active AC filter.
at Tjele measures and eliminates harmonics at the All phases have the same topology.

26
One harmonic controller is dedicated to each harmonic
selected for elimination by the action of the active
filter. In these harmonic controllers, the particular
Voltage on the To the IGBT
harmonic is isolated and expressed by a complex 400-kV Busbar Converter
signal in the frequency domain.

This is done through multiplication by sin (hωt)

and cos (hωt), where h is the order of the harmonic, Optical Self-Tuning IGBT
ω the network angular frequency and t the time. Input system control

These two orthogonal signals are produced by a

module synchronized by the fundamental compo-
nent of the filter current. The signal pair obtained LP PI
∑
Filter Controller
after the mentioned multiplication and filtering feeds
a complex controller with PI characteristic. The out-
put of the controller is then shifted back to the time To Other From Other
domain by multiplication by cos (hωt) and sin (hωt). Harmonic Harmonic
The process is essentially linear, so that all harmonic Controllers cos (hωt) Controllers
Synchronization sin (hωt)
controllers can operate simultaneously and the sum
of all harmonic controller outputs gives the wave- Harmonic Controller
form required by the active filter. This signal is then
given to the IGBT control module, which includes
a pulse width modulator besides functions for pro-
tection and supervision of the converter. Fig. 5.4.3-3: Principle block diagram of the harmonic control

Harmonic voltages at the 400-kV bus … with active filter control (23th, 25th, 35th,
(L1) without and … 47th and 49th harmonics)

Fig. 5.4.3-4: Plots from measurement: left without, right with active filter control

Fig. 5.4.3-5: Installation of the active AC filter, 400-kV substation Tjele (Denmark)

27
 5.5 Surge Arrester ■■ Low resistance during surges so that overvoltages
are limited
Siemens surge arresters are designed optimally ■■ High resistance during normal operation in order
to the following requirements: to avoid negative effects on the power system and
■■ Excellent pollution performance for coastal and ■■ Sufficient energy absorption capability for stable
desert regions or in areas with extreme industrial operation
air pollution.
■■ High mechanical stability, e.g. for use in seismic zones. MO (Metal Oxide) arresters are used in medium-,
■■ Extremely reliable pressure relief behaviour high- and extra-high-voltage power systems.
for use in areas requiring special protection.
Here, the very low protection level and the
What is more, all Siemens surge arresters are high energy absorption capability provided
sized for decades and the material used provides during switching surges are especially important.
a contribution towards the protection of the For high voltage levels, the simple construction
environment. of MO arresters is always an advantage.

The main task of an arrester is to protect the equip- Arresters with Polymer Housings
ment from the effects of overvoltages. During Fig. 5.5-2 shows two Siemens MO arresters
normal operation, it should have no negative effect with different types of housing. In addition
on the power system. Moreover, the arrester must to what has been usual up to now – the porcelain
be able to withstand typical surges without incurring housing – Siemens offers also the latest generation
any damage. Non-linear resistors with the following of high-voltage surge arresters with polymer
properties fulfil these requirements: housing.

Arrester Voltage Referred Rated Voltage ÛR

to Continuous Operating Continuous Operating
Voltage Û/ÛC Voltage ÛC

1
20 °C
115 °C
150 °C

0
10-4 10-3 10-2 10-1 1 10 102 103 104
Current through Arrester Ia [A]

Fig. 5.5-1: Current/voltage characteristics of a non-linear

MO arrester

Fig. 5.5-2: Measurement of residual voltage on porcelain-housed

(foreground) and polymer-housed (background) arresters

28
 Flange with Gas Diverter Nozzle
Seal

Pressure Relief Diaphragm

Compressing Spring

Metal Oxide Resistors

Composite Polymer Housing

FRP Tube/Silicon Sheds

Fig. 5.5-3: Cross-section of a polymer-housed arrester

Fig. 5.5-3 shows the sectional view of such an The polymer-housed high-voltage arrester design
arrester. The housing consists of a fibre-glass- chosen by Siemens and the high-quality materials
reinforced plastic tube with insulating sheds used by Siemens provide a whole series of advan-
made of silicon rubber. The advantages of this tages including long life and suitability for outdoor
design which has the same pressure relief device use, high mechanical stability and ease of disposal.
as an arrester with porcelain housing are absolutely
safe and reliable pressure relief characteristic, For terminal voltage lower than the permissible
high mechanical strength even after pressure maximum operating voltage (MCOV), the arrester
relief and excellent pollution-resistant properties. is capacitive and carries only few milli-amps. Due
The very good mechanical features mean that to its extreme non-linear characteristics, the arrester
Siemens arresters with polymer housing (type behaves at higher voltages as low-ohmic resistor
3EQ/R) can serve as post insulators as well. and is able to discharge high current surges.
The pollution-resistant properties are the result Through parallel combination of two or more
of the water-repellent effect (hydrophobicity matched arrester columns, higher energy absorption
of the silicon rubber). capability of the ZnO arrester can be achieved.

Routine and type tests have been determined

in accordance with the international standards:
■■ IEC 60060 High-voltage test techniques
■■ IEC 60071 Insulation coordination
■■ IEC 60099 Surge arresters

29
 AC Filter Bank 9 10 DC to DC line 10
DC
1
Aa
AC V D D
2 Filter 11
4 AC Bus 5

A V
DC 12
3 7 C Filter
V
Fdc2
Fac1 Fac2 6
A V Fdc1
8 Neutral

Valve Hall Boundary Cn

E E
Neutral 8

Arrester Type Location Main Task

AC bus arrester ’A’ The ZnO arrester will be installed close to the Limit the overvoltages on the primary and
converter transformer line side bushing secondary side of the converter transformer

AC filter bus The ZnO arrester will be installed Protect the AC filters busbar
arrester ‘Aa’ at the busbar of the AC filter banks against lightning surges

Valve-arrester ‘V’ 3-pulse commutation group The main events to be considered with respect
to arrester discharge currents and energies are:
a) Switching surges from the AC system
through converter transformer
b) Ground fault between valve and HV bushing
of converter transformer during rectifier
operation

Converter group 12-pulse converter group Protection against overvoltages

arrester ‘C’ from the AC and DC side

DC bus arrester ‘D’ At the HV smoothing reactor They will protect the smoothing reactor and
and at the DC lines the converter station (e.g. DC switchyard)
against overvoltages coming from the DC side

Neutral DC bus Neutral DC bus The neutral bus arresters protect the LV
arrester ‘E’ terminal of the12-pulse group and the neutral
bus equipment

AC filter arrester AC filter The operating voltage for the AC filter arresters
‘Fac‘ consists of low fundamental frequency and
harmonic voltages. Overvoltages can occur
transiently during faults

DC filter arrester DC filter The operating voltage for the DC filter arresters
‘Fdc‘ consists of low DC component and harmonic
voltages. Overstresses may occur transiently
during DC bus fault to ground

30
5.6 DC Transmission Circuit

5.6.1 DC Transmission Line

DC transmission lines could be part of overall HVDC
transmission contract either within a turnkey package
or as separately contracted stand-alone item, later
integrated into an HVDC link.

As an example of such a transmission line design,

an existing bipolar tower for the 300-kV link
between Thailand and Malaysia is shown in
Fig. 5.6.1-1.

5.6.1.1 Towers
Such DC transmission lines are mechanically
designed as it is practice for normal AC
transmission lines; the main differences are:
■■ The conductor configuration
■■ The electric field requirements
■■ The insulation design

5.6.1.2 Insulation
The most critical aspect is the insulation design and
therefore this topic is described more detailed below:
For DC transmission lines, the correct insulation design
is the most essential subject for an undisturbed oper-
ation during the lifetime of the DC plant. Fig. 5.6.1-1: DC transmission line (bipolar tower 300-kV link)

Design Basics Types of Insulators

■■ The general layout of insulation is based There are 3 different types of insulators applicable
on the recommendations of IEC 60815 which for DC transmission lines:
­provides 4 pollution classes. ■■ Cap and pin type
■■ This IEC is a standard for AC lines. It has to be ■■ Long-rod porcelain type
observed that the creepage distances recom- ■■ Composite long-rod type
mended are based on the phase-to-phase voltage
(UL-L). When transferring these creepage distances In detail:
recommended by IEC 60815 to a DC line, it has Cap and Pin Type
to be observed that the DC voltage is a peak volt- Positive Aspects:
age pole to ground value (UL-G). Therefore, these ■■ Long-term experience/track record
creepage distances have to be multiplied by the ■■ Good mechanical strength
factor √3. ■■ Vandalism-proof
■■ Insulators under DC voltage operation are subjected ■■ Flexibility within the insulator string
to more unfavourable conditions than under AC
due to higher collection of surface contamination Negative Aspects:
caused by constant unidirectional electric field. ■■ Very heavy strings
Therefore, a DC pollution factor as per recommen- ■■ Insulator not puncture-proof
dation of CIGRE (CIGRE-Report WG04 of Cigre SC33, ■■ Poor self-cleaning ability
Mexico City 1989) has to be applied. ■■ Loss of strength/reliability due to corrosion
of pin in polluted areas caused by high track
The correction factors are valid for porcelain insulators current density (this is extremely important
only. When taking composite insulators into consider- for DC lines)
ation, additional reduction factors based on the FGH ■■ Many intermediate metal parts
report 291 “Oberflächenverhalten von Freiluftgeräten ■■ High RIV and corona level
mit Kunststoffgehäusen“ must be applied. ■■ For DC applications, special shed design
and porcelain material necessary
■■ Very expensive

31
 Long-Rod Porcelain Type Composite Long-Rod Type
Positive Aspects: Positive Aspects:
■■ Long-term experience/track record ■■ Small number of insulators in one string
■■ Good mechanical strength ■■ Up to 400 kV per unit possible
■■ Puncture-proof ■■ Good mechanical strength, no chipping
■■ Good self-cleaning ability of sheds possible
■■ Less intermediate metal parts ■■ Very light – easy handling during construction
■■ Due to caps on both insulator ends and maintenance, logistical advantages in areas
not subjected to pin corrosion because with poor access
of low track current density ■■ Puncture-proof
■■ Moderate price ■■ Good self-cleaning behaviour – hydrophobicity
of surface which offers advantages of less
Negative Aspects: creepage distance up to pollution class II
■■ Heavy strings ■■ Very good RIV and corona behaviour
■■ String not very flexible ■■ Good resistance against vandalism
■■ Under extreme vandalism failure of string ■■ Shorter insulator string length
possible ■■ Very competitive price

Negative Aspects:
■■ Relatively short track record in DC application
(since 1985 first major application in the USA)
■■ Less tracking resistance against flash-over
(can be improved by means of corona rings)

Example/Comparison of Insulator Application for a 400 kV Transmission Line

Cap and Pin Porcelain Long-Rod Composite Long-Rod

Insulator string 5270 mm 5418 mm 4450 mm

length 31 insulators 4 insulators 1 insulator

Creepage per unit 570 mm 4402 mm 17640 mm

Weight of string 332 kg 200 kg 28 kg

Breaking load 160 kN 160 kN 160 kN

5.6.2 DC Cable 5.6.2.2 Different Cable Types

For HVDC submarine cables there are different
5.6.2.1 General Application for DC Cables types available.
An important application for HVDC are transmission
systems crossing the sea. Here, HVDC is the preferred 1) Mass-Impregnated Cable
technology to overcome distances > 70 km and trans- This cable type is used in most of the HVDC
mission capacities from several hundred to more than applications. It consists of different layers
a thousand MW (for bipolar systems). For the submarine as shown in Fig. 5.6.2.2-1.
transmission part, a special cable suitable for DC current
and voltage is required.

32
The conductor is built of stranding copper layers of 1) XLPE
segments around a central circular rod. The conductor To overcome the disadvantages of the above men-
is covered by oil and resin-impregnated papers. The tioned cable types, extensive R&D was conducted
inner layers are of carbon-loaded papers whereas by the cable suppliers. The result is the XLPE cable.
the outer layer consists of copper-woven fabrics. XLPE means ‘cross-linked polyethylene‘ and forms the
The fully impregnated cable is then lead-sheathed insulation material. The conductor is the segmented
to keep the outside environment away from the copper conductor insulated by extruded XLPE layers.
insulation. The next layer is the anti-corrosion The insulation material is suitable for a conductor
protection which consists of extruded polyethylene. temperature of 90 °C and a short-circuit temperature
Around the polyethylene layer galvanized steel of 250 °C. Although the main application for XLPE
tapes are applied to prevent the cable from perma- cables is the land installation and the offshore industry,
nent deformation during cable loading. Over the steel XLPE with extruded insulation material for HVDC
tapes a polypropylene string is applied followed systems of lower transmission capacities are under
by galvanized steel wire armour. development.
The technology is available for voltages up to 500 kV
and a transmission capacity of up to 800 MW in one 2) Lapped Thin Film Insulation
cable with installation depths of up to 1000 m under As insulating material a lapped non-impregnated thin
sea level and nearly unlimited transmission lengths. PP film is used instead of the impregnated materials.
The capacity of mass-impregnated cables is limited The tests for the cable itself are completed. Now the
by the conductor temperature which results in low tests for the accessories such as joints are under
overload capabilities. process.
This type of cable can sustain up to 60 % higher
2) Oil-Filled Cable electrical stresses in operation, making it suitable
In comparison to mass-impregnated cables, for very long and deep submarine cables.
the conductor is insulated by paper impregnated Another area of development are the cable arrange-
with a low-viscosity oil and incorporates a longi- ments. For monopolar transmission systems, either
tudinal duct to permit oil flow along the cable. the return path was the ground (’ground return‘)
Oil-filled cables are suitable for both AC and DC or a second cable. The first solution always provokes
voltages with DC voltages up to 600 kV DC and environmental concerns whereas the second one
great sea depths. Due to the required oil flow has excessive impact on the costs for the overall
along the cable, the transmission line lengths transmission scheme.
are however limited to <100 km and the risk Therefore, a new cable was developed with an inte-
of oil leakage into the environment is always grated return conductor. The cable core is the tradi-
subject to discussions. tional design for a mass-impregnated cable and the
return conductor is wound outside the lead sheath.
5.6.2.3 Future Developments for HVDC Cables The conductor forms also part of the balanced armour,
Most of the research and development activities together with the flat steel wire layer on the outside
for new cable types are done with the insulation of the return conductor insulation.
material. These include: This cable type was installed between Scotland and
Northern Ireland for 250 kV and 250 MW. R&D is on-
going to increase the voltage as well as the capacity
of the cable with integrated return conductor.

■ 1 Conductor of copper-shaped wires 

■ 2 Insulation material
■ 3 Core screen
■ 4 Lead alloy sheath
■ 5 Polyethylene jacket
■ 6 Reinforcement of steel tapes
■ 7 Bedding
■ 8 Armour of steel flat wires

Fig. 5.6.2.2-1: Mass-impregnated cable

33
 5.6.3 High Speed DC Switches

5.6.3.1 General
Like in AC substations, switching devices are also
needed in the DC yard of HVDC stations. One group
of such devices can be characterized as switches
with direct current commutation capabilities,
commonly called ”high-speed DC switches”.

Siemens standard SF6 AC circuit-breakers of proven

design are able to meet the requirements of high-
speed DC switches.

MRTB at Tian Guang/China DC switchyard

Bipolar Monopolar metallic return

HVDC HVDC
Overhead Line Overhead Line
AC System 1

AC System 2

AC System 1

AC System 2
Electrodes Electrodes

HVDC HVDC
Overhead Line Overhead Line

Monopolar ground return

HVDC
Overhead Line
AC System 1

AC System 2

Electrodes

HVDC
Overhead Line

Fig. 5.6.3-1: HVDC system configurations

5.6.3.2 Types and Duties of the High-Speed DC Switches

Type Duties

HSNBS The HSNBS must commutate some direct current into the ground
(High-Speed Neutral Bus Switch) ­electrode path in case of faults to ground at the station neutral.

HSGS The HSGS is needed to connect the station neutral to the station
(High-Speed Ground Switch) ground grid if the ground electrode path becomes isolated.

MRTB If one pole of a bipolar system has to be blocked, monopolar operation

(Metallic Return Transfer of the second pole is achieved automatically, but with return current
Breaker) through ground (refer to Fig. 5.6.3-1). If the duration of ground return
operation is restricted, an alternate mode of monopolar operation is
possible if the line of the blocked pole can be used for current return.
This mode is called metallic return (refer to Fig. 5.6.3-1). The MRTB is
required for the transfer from ground to metallic return without inter-
ruption of power flow.

GRTS The GRTS is needed for the retransfer from metallic return to bipolar
(Ground Return Transfer Switch) operation via ground return, also without interruption of power flow.

34
5.6.3.3 Design Considerations

Details regarding the duties of ”HSNBS” and ”HSGS” Lp Cp

are not discussed here but the more severe require-
ments for “MRTB“ and ”GRTS“ are explained.

Fig. 5.6.3-2 shows the disposition of MRTB and GRTS. l3 l1 Uarc

Rm and Lm represent resistance and inductance of the
transmission line. Re and Le comprise resistances and
inductances of the ground return path.
l1

Rm Lm t0 t1 t
l4
tc
Uabsorber
GRTS

l3 Re Le
l0 Uarc
l3
MRTB

Fig. 5.6.3-4: Equivalent circuit relevant to MRTB and GRTS operation

With reference to Fig 5.6.3-4, the principle of com-

Fig. 5.6.3-2: Equivalent circuit relevant to MRTB and GRTS operation mutation is as follows: At t0, the contacts of the breaker
separate, thereby introducing an arc into the circuit.
The ground resistance Re is normally much lower than The characteristic of this arc sets up an oscillatory
the metallic resistance Rm. Therefore, during the transi- current (frequency determined by Lp Cp) which is
tional steady-state condition with both MRTB and superimposed on the current I1. As Rp is very small, the
GRTS closed, most of the current is flowing through oscillation is not damped but increases. As soon as the
ground which determines the commutation require- current I1 passes through zero (refer to t1 in Fig. 5.6.3-4),
ments for MRTB and GRTS. I3 may reach values of up the breaker current is interrupted. I3, however, remains
to 90 % of the total current I0 and I4 values of up to unchanged now charging the capacitor Cp until it
25 % of I0. reaches a voltage limited by the energy absorber. This
voltage acts as a counter voltage to reduce the current
The following considerations refer to MRTB only. From I3 and to increase the current I4 (refer to Fig. 5.6.3-4
the above it can be concluded that the commutation and Fig. 5.6.3-3). When the absorber limiting voltage
duties for transfer from ground to metallic return (MRTB) has been reached, the current I3 flows into the absorber
are much heavier than from metallic to ground return which dissipates an amount of energy determined by
(GRTS). the counter voltage to bring I3 to zero. When I3 has
dropped to zero, I4 equals I0 and the current commu-
Fig. 5.6.3-3 shows the basic MRTB circuit. An energy tation from ground to metallic return has been com-
absorber and the LpCp resonant circuit (Rp represents pleted. It should be noted that the current I0 of the
the ohmic resistance of that branch only) are connected system (refer to Fig. 5.6.3-2) did not change, i.e. the
in parallel to the main switch (MRTB) which is a con- power transmission was never interrupted.
ventional SF6-type AC breaker.
There are also MRTB principles other than the explained
one which are based on complex resonant circuits,
l4 Rm Lm externally excited with additional auxiliary power
sources. With respect to reliability and availability, the
advantage of the above principle with passive resonant
GRTS Rp Lp Cp circuit which is used by Siemens is quite evident. The
l3 Re Le nozzle system and specifically the flow of SF6 gas in the
l0 Siemens standard SF6 AC breakers result in an arc char-
acteristic which ensures reliable operation of the passive
MRTB resonant circuit. One unit of a standard three-phase
AC breaker is used. Extensive series of laboratory tests
Energy Absorber have shown the capabilities of Siemens SF6 breakers
for this application. Furthermore, such switches are
successfully in operation in various HVDC schemes.
Fig. 5.6.3-3: Details of the MRTB circuit

35
 5.6.4 Earth Electrode As shown in figure 5.6.4-1, the electrode conductor
itself, which is generally made of iron, is laid horizon-
5.6.4.1 Function of the Earth Electrode tally at a depth of approximately 2 m. It is embedded
in the HVDC System in coke which fills a trench having a cross section of
Earth electrodes are an essential component of approximately 0.5 x 0.5 m2.
the monopolar HVDC transmission system, since
they carry the operating current on a continuous The advantage of this design becomes apparent in
basis. They contribute decisively to the profitability anodic operation. The passage of the current from
of low-power HVDC systems, since the costs for a the electrode conductor into the coke bed is carried
second conductor (with half the nominal voltage) primarily by electrons, and is thus not associated
are significantly higher, even for transmission over with loss of the material.
short distances, than the costs for the earth electrodes.
Several typical patterns of horizontal land electrodes
Earth electrodes are also found in all bipolar HVDC are illustrated in figure 5.6.4-2
systems and in HVDC multi-point systems. As in any
high-voltage system, the power circuit of the HVDC
system requires a reference point for the definition a) b)
of the system voltage as the basis for the insulation
coordination and overvoltage protection. In a bipolar
c) d) e) f)
HVDC system, it would conceivably be possible to
connect the station neutral point to the ground mat a) Line Electrode
b) Multi-Line Electrode
of the HVDC station to which the line-side star points
c) Ring-Shaped Electrode
of the converter transformers are also connected. But d) Ring-Shaped Electrode with second ring
since the direct currents in the two poles of the HVDC e) Star-Shaped Electrode
are never absolutely equal, in spite of current balanc- f) Forked Star Electrode
ing control, a differential current flows continuously
Fig. 5.6.4-2: Plan view of a typical design of
from the station neutral point to ground. It is com- horizontal land electrodes
mon practice to locate the grounding of the station
neutral point at some distance (10 to 50 kilometres)
5.6.4.2.2 Vertical Land Electrode
from the HVDC station by means of special earth
If the ground strata near the surface have a high
electrodes.
specific resistance, but underneath, there is a con-
ductive and sufficiently thick stratum at a depth
5.6.4.2 Design of Earth Electrodes
of several tens of meters, the vertical deep electrode
Earth electrodes for HVDC systems may be land, coastal
is one possible solution.
or submarine electrodes. In monopolar HVDC systems,
which exist almost exclusively in the form of sub-
Figure 5.6.4-3 shows, as an example, one of the
marine cable transmission systems, there are funda-
four deep electrodes at Apollo, the southern station
mental differences between the design of anode
of the Cahora Bassa HVDC system.
and cathode electrodes.

5.6.4.2.1 The Horizontal Land Electrode Manhole

If a sufficiently large area of flat land with relatively Flexible
homogeneous ground characteristics is available, the Connection
Feed Cable
horizontal ground electrode is the most economical
form of a land electrode. Concrete Cover
80 m

Graphite Rod
Fill Crushed
Stone
1.5 – 2.5m Borehole ø:
10
m

Crushed Stone 0.57 m

Coarse Grain
Conductor (Iron) Graphite
40 m

Conductive
Coke Bed
Layer
(5 x 0.5m2)

Fig. 5.6.4-1: Cross section through a horizontal land electrode Fig. 5.6.4-3: Vertical electrode at Apollo, the Southern
Cahora Bassa HVDC station
36
 Electrode
Module

Feed
Cable

Concrete Cover Coke Bed Graphite Rod

0.5 – 1 m Cable

2–5m
0.5 – 1 m

Fig. 5.6.4-4: Linear submarine electrode (anodic operation)

5.6.4.2.3 Cathodic Submarine Electrodes 5.6.4.2.5 Anodic Coastal Electrode

The design and construction of the cathodic submarine The conventional design of a coastal electrode is
electrodes of a monopolar HVDC system with sub- similar to that of a vertical land electrode. Graphite
marine power transmission cable do not present rods surrounded by a coke bed are installed in bore-
any particular problems. Since there is no material holes which are sunk along the coastline.
corrosion, a copper cable laid on the bottom should
theoretically suffice. The length of the cable must The advantage of the coastal electrodes is easy
be designed so that the current density on its surface accessibility for inspection, maintenance and
causes an electrical field of < 3 V/m in the surrounding regeneration, if necessary.
water, which is also safe for swimmers and divers.
A coastal electrode can also be configured in the
5.6.4.2.4 Anodic Submarine Electrodes form of a horizontal land electrode if the ground
Figure 5.6.4-4 shows an example of a linear submarine has the necessary conductivity or if the necessary
electrode for anodic operation. The prefabricated conductivity can be achieved by irrigating the trench
electrode modules are lowered to the ocean floor and with salt water. In either case, it is assumed that
then connected to the feed cable. When the submarine even with a coastal electrode, the current flow to
electrodes are divided into sections which are connected the opposite electrode takes place almost exclusively
to the HVDC station by means of separate feed cables, through the water.
the electrode can be monitored from the land.

37
 5.7 Control & Protection

5.7.1 General
The WIN-TDC Control and Protection System plays an All WIN-TDC components from the Human Machine
important role in the successful implementation of HVDC Interface (HMI) workstations, the control and
transmission systems. High reliability is guaranteed protection systems down to the state of the art
with a redundant and fault tolerant design. Flexibility measuring equipment for DC current and voltage
(through choice of optional control centres) and high quantities have been upgraded to take advantage
dynamic performance were the prerequisites for the of the latest software and hardware developments.
development of our control and protection system. These control and protection systems are based
Knowledge gained from over 30 years of operational on standard products with a product life cycle for
experience and parallel use of similar technology the next 25 years.
in related fields has been built into the sophisticated
technology we can offer today. The control is divided into the following
hierarchical levels:
Main objectives for the implementation of the HVDC ■■ Operator control level (WIN CC)
control system are reliable energy transmission ■■ Control and protection level (Simatic TDC)
which operates highly efficient and flexible energy ■■ Field level (I/Os, time tagging, interlocking)
flow that responds to sudden changes in demand
thus contributing to network stability.

Dispatch Centres

Operator Control Level

WinCC WinCC Printer Printer GPS

Opposite Converter
System 1 System 2

Telecommunication
Telecommunication Equipment

Equipment

Station
Remote Control Remote Control
Router Router Interface Interface Router Router
System 1 System 2

LAN 1

LAN 2

Common Pole 1 Pole 2

Control and
Protection Level
Pole Control and Pole Control and Pole Control and Pole Control and
Station Control Station Control DC Protection DC Protection DC Protection DC Protection
System 1 System 2 System 1 System 2 System 1 System 2
Simatic TDC Simatic TDC Simatic TDC Simatic TDC Simatic TDC Simatic TDC

TDM Bus TDM Bus

Measuring Measuring Measuring Measuring

System 1 System 2 System 1 System 2

FB1 FB2 FB1 FB2 FB1 FB2

Field Level

I I I I I I

O O O O O O

TDM Bus: Time Division Multiplexing Bus (Optical Measuring Bus)

Fig. 5.7.1-1: HVDC control hierarchy, one station (bipolar HVDC transmission scheme)

38
 Internet

Fig. 5.7.1-2: Remote access connection

In the following section, functions, tasks and 5.7.1.4 Best Support – Remote Access
­components are described to provide an overview. As an optional feature, the control system can be
accessed remotely via point-to-point telephone
5.7.1.1 High Availability connection or via Internet. This allows remote plant
The main design criteria for Siemens HVDC systems monitoring and fault detection including diagnostics.
is to achieve maximum energy availability. This applies To ensure the data security, a VPN (Virtual Private
to the design of the control and protection systems Network) encrypted connection is used. Furthermore,
as well. A single fault of any piece of equipment in a password protected access ensures that only autho-
the control and protection systems may not lead to rized personnel have access.
a loss of power. Therefore, the primary control and
protection components are configured as redundant With the use of a standard web browser, main
systems. diagnosis data can be monitored. Expert access
to the control components is also possible. This
5.7.1.2 Self-Testing Features remote access feature provides flexible support
All control and protection systems are equipped for the commissioning and maintenance personnel
with self-diagnostic features that allow the by our design engineers.
operator to quickly identify and replace the
defective part to recover redundancy as soon 5.7.1.5 Modular Design
as possible. The control and protection systems use multiprocessor
hardware. This means that the computing capacity
5.7.1.3 Low Maintenance can be scaled according to the requirements.
With today’s digital systems there is no requirement
for routine maintenance. However, should it be Therefore, the most economic solution can be found
necessary to replace single modules, the design at the start. Additional computing capacity can be
is such that there is no operational impact on the added at any time later, if required.
HVDC system. This is achieved by designing all
primary components as redundant systems, where 5.7.1.6 Communication Interfaces
one system can be switched off without impact The control and protection systems as well as the
on the other system. operator control system communicate via Ethernet
or Profibus. For remote control interfacing, a number
of standard protocols are available. Custom protocols
can be implemented as an option.
39
 5.7.2 Control Components

5.7.2.1 Operator Control System

The tasks of a modern operation and monitoring
system within the HVDC control system include
the following:
■■ Status information of the system
■■ Operator guidance to prevent maloperation
and explain conditions
■■ Monitoring of the entire installation and
auxiliary equipment
■■ Graphic display providing structural overview
of the entire system

Fig. 5.7.1-3: Operator workstation, typical screen layout

for a monopolar HVDC system overview

■■ Troubleshooting support with clear messages

to quickly resume operation
■■ Display and sorting of time tagged events
(time is synchronised via GPS clock)
■■ Display and archiving of messages
■■ Automatic generation of process reports

Fig. 5.7.1-4: Sequence of events recording (SER),

report layout for SER information

40
■■ Analysisof operating mode based on user-
defined and archived data (trend system)
■■ Generation of process data reports

5.7.2.2 Control and Protection System Level

The primary tasks in this level are:
■■ Measuring
■■ Control of Power Transmission
■■ Protection

Measuring
DC values are measured by means of the hybrid optical
DC measuring system. This system measures the
voltage drop over a shunt or a voltage divider,
converts this voltage into a telegram and transfers
it to the measurement cubicle via fibre optics.

The scheme is designed to be completely redundant,

therefore loss of a signal does not lead to an impact
on power transmission. This measuring principle
contributes to an increased availability of the control
and protection scheme.

The advantages of such a scheme are:

■■ Reduced weight (100 kg)
■■ Linear response (passive system)
■■ Improved EMC (due to fibre optics)
■■ Integrated harmonic measurement (Rogowsky coil)
for use in active filters or harmonic monitoring
schemes.

duplicated sensor head redundant channel

Shunt

Analog / Digital Optical

Digital
Control/
Protection
Digital / Optical Signal fibre Digital System

Id Optical
Electrical Energy
Power fibre Energy Power
Electrical supply
Optical Energy Energy

Sensor Head

Sensor Head Box at high voltage level Measuring Cubicle

Fibre optic cable

Fig. 5.7.1-5: Trend system, example for trend display

41
 Control of Power Transmission The HVDC-related protection functions are referred
The pole control system is responsible for firing to as DC protection. These include converter protection,
the thyristor valves so that the requested power is DC busbar protection, DC filter protection, electrode
transmitted. The pole controls on each side of the line protection and DC line protection.
transmission link therefore have to fulfill different The AC protection scheme consists mainly of the
tasks. The pole control system on the rectifier side AC busbar, the AC line and the AC grid transformer
controls the current so that the requested power protection as well as the AC filter protection and
is achieved. The pole control system on the inverter converter transformer protection.
side controls the DC voltage so that rated DC voltage The task of the protective equipment is to prevent
is achieved. damage of individual components caused by faults
or overstresses.
The pole control is implemented redundantly.
A failure in one system thus has no impact on Each protection zone is covered by at least two inde-
power transmission. pendent protective units – the primary protective
unit and the secondary (or back-up) protective unit.
This system can be repaired while the other system
remains in operation. In bipolar schemes a redundant Comprehensive system monitoring and measurement
pole control system is assigned to each pole. Failures plausibility functions are implemented in the protection
in one pole will not have any impact on the remaining systems. This serves to prevent false trips due to
pole. singular equipment failure.

Protection The protection functions of the various protective

The DC protection system has the task of protecting relays are executed reliably for all operating condi-
equipment and personnel. The protection systems can tions. The selected protective systems ensure that
be divided into two areas, which are subsequently all possible faults are detected, annunciated and
divided into different protection zones. cleared selectively.

Redundant Local Area Network

1 AC Busbar Protection
2 AC Line Protection
Red. Field Bus
3 AC Filter Protection
4 Converter Transformer Protection
Pole System Pole
Selection 5 Converter Protection
Control Control
System 1

System 2

Control 6 DC Busbar Protection

7 DC Filter Protection
3
Converter Converter 8 Electrode Line Protection
Control Control
9 DC Line Protection
System 1

System 2

2
Valve Base Valve Base
Electronic Electronic 9

7 6

Fibre Optic 8

4
5
1
12-Pulse Group

Fig. 5.7.1-7: Redundant pole control system Fig. 5.7.1-8: Protection zones, one pole/one station
structure (for one 12-pulse group)

42
Fig. 5.7.1-9: Real-time simulator

All protective equipment in the HVDC converter 5.7.4 Testing and Quality Assurance
station is implemented either with digital multi- The design process has a number of defined review
microprocessor systems or with digital Siemens steps. These allow verification of the control and
standard protective relays. ”The DC protection is protection system functionality and performance
designed to be fully redundant. Additionally both before delivery to site (see figure 5.7.1-10).
protection systems incorporate main and back-up
protection functions using different principles. Already along with the tender, the use of accurate
The AC protection con-sists of a main and back-up simulation tools allows to answer specific perfor-
system using different principles. Each protective mance issues that are vital to the customer’s grid.
system is assigned its own measuring devices as
well as power supplies.” 5.7.4.1 Offline Simulation EMTDC
Siemens uses a simulation model that includes
5.7.3 Control Aspects all details of control and protection functionality
in detail. Thus forecast of real system behaviour
5.7.3.1 Redundancy is reliable. Therefore it is possible to optimize
All control and protection systems that contribute the application to find the best economic solution
to the energy availability are configured redundantly. while providing the optimum performance.
This covers any single faults in the control and pro-
tection equipment without loss of power. 5.7.4.2 Dynamic Performance Test
The offline simulation with EMTDC is already an
5.7.3.2 Operator Training extremely accurate forecast of the real system
For Siemens HVDC application, an operator training behaviour. To verify the findings and optimize
simulator is optionally available. The simulator allows the controller settings, the control and protection
the operator to train with the same hardware and systems are additionally tested during the dynamic
software as in the real process. This simulator consists performance test with a real-time simulator. During
of the original operator workstation and a simulation that phase, the customer may witness these perfor-
PC. The simulation PC runs the HVDC process and mance tests of the final control and protection
feeds the relevant data to the workstation. software.

43
 5.7.4.4 On-Site Tests
On-site tests are basically divided into test steps
regarding the related station (station A, station B)
and into the test steps related to the whole HVDC
system.

At the precommissioning stage, the base work for

commissioning the control system and protection
system is required. The main task is preparation
and individual testing of any single system.

This is required to assure the systems are free of

transportation damage. The next station-related tests
are the subsystem tests. Subsystems consist of equip-
ment items which are grouped according to common
Fig. 5.7.1-11: Example of a functional performance test setup functions like AC filter banks or thyristor valve systems.
The main task is testing the proper function of inter-
5.7.4.3 Functional Performance Test connected systems before switching on high voltage.
In the functional performance test, the dedicated Following this, station tests with high voltage but
control and protection hardware is installed and no energy transfer will take place. Finally, system
tested with a real-time simulator. The purpose of and acceptance tests with several operating points
the FPT is to test the proper signal exchange between of energy transfer will be used for fine tuning and
the various control components as well as the verifi- verification of system performance.
cation of the specified control sequences. This allows
optimized commissioning time. Furthermore, customer
personnel can participate in this test for operator
training and become familiar with the control system.

Hardware Cubicle Real time

design manufact. off-site tests:
Functional and
Dynamic
Design Software design On-site
Contract Performance Operation
specification and pre-testing tests

EMTDC* Control and

Protection study

*) EMTDC: Electro Magnetic Transient Program for DC Application

Fig. 5.7.1-10: The main steps for the HVDC control and protection versus the time starting from the contract award
up to commercial operation

44
6 System Studies, Digital Models,
Design Specifications
6.1 System Studies 6.2 Digital Models
During the planning stage of a HVDC project, prelimi- Digital models of HVDC system can be developed
nary studies are carried out in order to establish the according to the specified requirements. Typically
basic design of the whole HVDC transmission project. a digital model of dc system is needed for a specific
This includes the co-ordination of all relevant technical load flow and stability simulation program, while
parts of the transmission system like HVDC converters, another digital model is required for simulation in
AC and DC overhead lines as well as the submarine a typical electromagnetic transients program such
cable, if applicable. All specified requirements will be as EMTDC. The functionality and settings of HVDC
taken into account and are the basis for the preliminary control and protection system will be represented
design of the HVDC transmission link. In addition, in a proper manner in such models, which allow
special attention is paid to improving the stability of suitable simulation of steady state and transient
both connected AC systems. Several additional control behavior of HVDC system in the corresponding digi-
functions like power modulation, frequency control tal programs. Digital models consistent with the
and AC voltage limiter can be included in order to actual dc control and protection system are benefi-
provide excellent dynamic behaviour and to assist cial both for the operation of the HVDC scheme and
the AC systems if the studies show it necessary. Sub- for the network studies including DC link. Typically
synchronous oscillation will be avoided by special such models can be developed on request in the
control functions, if required. All the AC system con- detailed project design stage when all major design
ditions and the environmental conditions as given works of control and protection functions are
in the relevant documents will be considered in the completed.
design calculations. The final design of the HVDC
transmission system, including the operation charac- 6.3 Control and Protection Design Specifications
teristics, will be defined during the detailed system Design Specifications are written for the control,
studies. All necessary studies are carried out to protection and communication hardware and soft-
confirm the appropriate performance requirements ware. The control panels are then designed, manu-
and ratings of all the equipment. Due consideration factured inspected and tested in accordance to the
is given to the interaction with the AC systems on design specification. The software for the control
both sides, the generation of reactive power, system and protection is also written in accordance to the
frequency variations, overvoltages, short circuit design specification. It is tested using real time
levels and system inertia during all system simulators in the dynamic performance test and
configurations. functional performance test.

Typically the following studies are carried out: Specifications for the topics below are typically
a) Main Circuit Parameters written:
b) Power Circuit Arrangements a) General Control and Protection
c) Thermal Rating of Key Equipment b) Interface Systems
d) Reactive Power Management c) Station Control
e) Temporary Overvoltages d) Diagnosis Systems
and Ferro-Resonance Overvoltages e) Pole Control
f) Overvoltage Protection f) HVDC Protection
and Insulation Coordination g) AC Protection
g) Transient Current Requirements h) Metering and Measuring
h) AC Filter Performance and Rating i) Operator Control
i) DC Filter Performance and Rating j) Communication
j) AC Breaker and DC High-Speed
Switch Requirements
k) Electromagnetic Interference
l) Reliability and Availability calculations
m) Loss Calculation
n) Subsynchronous Resonance
o) Load Flow, Stability and Interaction
between different HVDC Systems
p) Audible Noise
45
 7 Project Management

7.1 Project Management in HVDC Projects 7.1.2 Transparency

The success and functional completion of large A clear process structure plan (PSP) standardised
projects depends on the structuring of the project for HVDC projects makes the project contents and
team in accordance with the related work and sequences transparent in their commercial and
manpower coordination. Periodical updates and technical aspects. Associations and interactions
adaptation of design guarantee the execution of are clarified according to procedure of work.
the project with constant high quality within the
target time frame. Throughout all production, work- 7.1.3 Risk reduction
ing process and on-site activities, health, safety and Any risks that could arise due to incorrect deadlines,
environmental protection (HSE) measures as well unclear technical concepts or excessive costs will be
as application of commonly agreed quality standards recognised early enough by a monitoring system so
such as DIN EN ISO 9001 are of prime importance that counter measures can be taken. This increases
to Siemens. contract quality and creates the basis for clear design
criteria.
7.1.1 Division Responsibilities
The overall project is divided and organised according 7.1.4 Progress Report
to design activities and technical component groups. Periodical meetings with subcontractors, in-house
These features make it possible to define clear function control working teams and customer are recorded
packages which are to a great extent homogeneous in progress reports which form an integral part of
within themselves and can be processed with mini- the quality insurance system.
mised interfaces.

Quality Project Management

TechnicalCommercial Procurement
Assurance
Project Manager Project Manager

Control & Protection

Basic Design Station Design
Engineering Logistic
Documentation
Training

Transient Network
Analyser
Civil Engineering Commercial
Functional
Main Performance Test
Civil Construction
Components
Commissioning
Installation
Communication
Systems Financing

Fig.7-1: Project organisation plan

46
7.1.5 Scheduling Deadlines for project decisions – especially those of
The hierarchically structured bar-chart schedule the critical path – can easily be identified enabling the
is a high-level control tool in project management. project manager to make up-to date pre-estimates
The clear structure of sequential processes and and initiate suitable measures in due time.
parallel activities is crucial for execution of a 24
to 36 month duration, according to the project
requirements.

Activity Time

Award of Contract

Engineering/System
Studies

Manufacturing

Transportation

Civil Works &

Buildings

Erection &
Precommissioning

Station Tests

System Tests

Commercial
Operation
Fig.7-2: Structured bar-chart timeschedule

47
Published by and copyright © 2011:
Siemens AG
Energy Sector
Freyeslebenstrasse 1
91058 Erlangen, Germany

Siemens AG
Energy Sector
Power Transmission Division
Power Transmission Solutions
Freyeslebenstrasse 1
91058 Erlangen, Germany
www.siemens.com/energy/hvdc

For more information, please contact

our Customer Support Center.
Phone: +49 180/524 70 00
Fax: +49 180/524 24 71
(Charges depending on provider)
E-mail: support.energy@siemens.com

Power Transmission Division

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descriptions of the technical options available, which
Trademarks mentioned in this document
may not apply in all cases. The required technical
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options should therefore be specified in the contract.
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