HIGH VOLTAGE DC TRANSMISSION (HVDC)

Presented by:

Avanish Kumar
Sanskar Chourasia
Akshay Chaudhary

Engineering (HVDC)
(2023-2024)
HISTORY :
❑ First HVDC System Commissioned in 1954, Gotland, Sweden
• ±100kV 20MW 97 kilometers of submarine cable
❑ Longest Distance in Operation 1983, DR Congo
• ±500kV 560MW 1709 kilometers overhead-line
❑ Longest Submarine Cable 2008,
❑ Norway to Netherlands
• ±450kV, 700MW 583 km submarine cable
• Connection of asynchronous systems
❑ Highest Voltage in Operation 2010,
❑ Yunnan-Guangdong, China
• ±800kV, 5000 MW
❑ First Multi-Terminal HVDC System 1992,
❑ Quebec-New England
• ±450kV 2000MW
METHODS AND WORKING
Basics of HVDC Operation

Taken from a 3-phase AC network

Converted to DC in a converter station

Transmitted by DC line or cable

(underground or submarine)

Converted back to AC in another converter

station

Injected into AC network

CONVERTER STATION
Converter station is normally split into two areas:
AC switchyard which incorporates the AC harmonic filters and HF filters
“Converter island” which incorporates
the valve hall(s), control and
services building, converter
transformers and DC switchyard.
VALVE HALL

Valves associated with each twelve-pulse bridge are normally contained within
a purpose built building
This enclosure provides a clean, controlled environment in which the thyristor
valves can safely operate without the risk of exposure to pollution or outdoor
conditions.
Within the valve hall, the thyristor valves are typically suspended from the roof
of the building
low voltage being closest to the roof
high voltage being at the lowest point
on the valve.
An air gap between the bottom of the valve and the valve hall floor provides
high voltage insulation.
AC FILTERS

AC side current waveform of a HVDC converter, is highly non-sinusoidal,

and, if allowed to flow in the connected AC network, might produce
unacceptable levels of distortion
AC side filters are therefore required as part of the converter station in
order to reduce the harmonic distortion of the AC side current and voltage
to acceptably low levels
Shunt-connected AC filters appear as capacitive sources of reactive power
at fundamental frequency, and normally are used to compensate most or
all of the reactive consumption of the converter
Design of the AC filters, therefore, normally has to satisfy these two
requirements of harmonic filtering and reactive power compensation.
Design influenced by a number of factors
Specified harmonic limits
AC system voltage conditions
Switched filter size (dictated by voltage step limit, reactive power
balance…)
Two main filter types:
Tuned filter or band-pass filter which is sharply tuned to one or several
harmonic frequencies (single (e.g. 11th) double (e.g. 11/13th) and
triple (e.g. 3/11/13th) tuned types)
Damped filter or high-pass filter offering a low impedance over a broad
band of frequencies.
band-pass filters for the 11th and 13th harmonic
high-pass filters for the higher harmonics.
CONVERTER TRANSFORMERS
It is important that the converter transformer be
thermally designed to take into consideration
both the fundamental frequency load and the AC
harmonic currents that will flow from the
converter through the converter transformer to
the AC harmonic filters.
EARTH ELECTRODES
Essential component of the monopolar HVDC transmission system, since
they carry the operating current on a continuous basis
Contribute to lower cost costs for the earth electrodes are significantly
lower than the costs for a second conductor (with half the nominal
voltage)
Earth electrodes are also found in all bipolar HVDC systems
Since the direct currents in the two poles of the HVDC are never
absolutely equal, in spite of current balancing control, a differential
current flows continuously from the station neutral point to ground.
It is common practice to locate the grounding of the station neutral point
at some distance (10 to 50 kilometers) from the HVDC station by means of
special earth electrodes.
DC SMOOTHING REACTOR
Normally required for power transmission schemes; they are not required
for back-to-back schemes.
In general it is used to
Reduce the DC current ripple on the overhead transmission line or
cable
Limitation of the DC fault currents
Protect the thyristor valve from fast front transients originating on the
DC transmission line (for example a lightning strike)
DC smoothing reactor is normally a large air-cored air-insulated reactor
DC SWITCHGEAR
Switchgear on the DC side of the converter is typically limited to
disconnector-switches and earth switches for scheme reconfiguration and
safe maintenance operation.
DC FILTER

Converter operation results in voltage harmonics being generated at the

DC terminals
This AC harmonic component of voltage will result in AC harmonic current
flow in the DC circuit
The field generated by this AC harmonic current flow can induce harmonic
current flow in open-wire telecommunication systems
• In a back-to-back scheme, these harmonics are contained within the
valve hall with adequate shielding
• With a cable scheme, the cable screen typically provides adequate
shielding
• With open-wire DC transmission it may be necessary to provide DC
filters to limit the amount of harmonic current flowing in the DC line
CONVERTERS
Ways of achieving AC/DC/AC conversion in HVDC system:
Natural Commutated Converters:
Most used in the HVDC systems as of today
The component that enables this conversion process is the thyristor, which is a controllable
semiconductor
Known as CSC – Classic or LCC – Line Commutated Converters
Producers
SIEMENS – HVDC CSC CLASSIC
ABB – HVDC CSC CLASSIC
ALSTOM – HVDC LCC
 Forced Commutated Converters:
The valves of these converters are built up with semiconductors with
the ability turn-on but also to turn-off.
Two types of semiconductors are normally used GTO (Gate Turn-Off
Thyristor) or the IGBT (Insulated Gate Bipolar Transistor)
Known as VSC – Voltage Source Converters
Introduced a spectrum of advantages, e.g. feed of passive networks
(without generation), independent control of active and reactive
power, power quality…
Producers:
SIEMENS – HVDC PLUS
ABB – HVDC LIGHT
ALSTOM – HVDC VSC
LCC vs VSC
Layout and Footprint of the converter
station.
VSC converters are also considerably
more compact than line-commutated
converters (mainly because much less
harmonic filtering is needed) e.g.
600MW LCC converter station requires
about 14000 m2 whereas a VSC
HVDC needs only 3000m2. This
requirement is very important on
offshore platforms.

Transformers
The VSC controller allows the use of standard two-winding transformers
This gives more flexibility to build and design the offshore station
Harmonics
LCC require harmonic filters, VSC only simple high‐pass filter for high
order harmonics
 LCC vs VSC
LCC HVDC VSC HVDC
Size single range converter 150 - 1500MW 50 – 1100MW
Semiconductor technology Thyristor IGBT
DC voltage ±800kV ±320kV
Converter technology Line commutated Self commutated
Control of reactive power No, only switching regulation yes, continuous control
Voltage control Limited Extensive
Fault ride through No Yes
Black start capability No Yes
Power reversal without interruption No Yes
Minimum ESCR 2 No required
Minimum DC power flow 5-10% of rated power No minimum required
Typical losses per convertor 0,80 % 2%
Operating experience >20 years 8 years
Operating experience offshore No Yes
Construction time 3 (2)* years 1 year
LCC vs VSC

Long distance transmission Interconnections of WPP connection to Feed of isolated

over land/sea asynchronous networks network loads

LCC or CCC HVDC with

OHL/Cables √ √

CCC Converters in
Back-to-Back √

VSC converters in
Back-to-Back √ √

VSC Converters with land/sea

cables √ √ √ √
ADVANTAGES OF HVDC SYSTEMS
Major advantage of flexibility in power exchange in comparison with HVAC
Fast control of power flow – practically independently from frequency, voltage or angle at
terminal buses
Fast change of direction of transmitted power – due to inherent properties of the electronic
equipment in converters
Controllable – power injected where needed, supplemental control, frequency control
Bypass congested circuits – no inadvertent flow
Lower losses
Reactive power demand limited to terminals independent of distances
Narrow Right-of-Way (ROW) –
land coverage and the associated right-of-way cost for an HVDC overhead transmission line is
smaller,
reduced visual impact
higher power transmission capacity for same RoW
no Electromagnetic field (EMF) constraints
Cost Comparison HVDC vs. HVAC
HVDC has a higher installation cost due to the converter stations and filtering requirement
The cost of an HVDC line is less than the cost of an AC line. Long AC lines are more expensive
due to shunt and series compensation requirements
DISADVANTAGES
❑ (expensive)
Converter stations needed to connect to AC power grids are very expensive.
Converter substations generate current and voltage harmonics, while the conversion process is accompanied by reactive power
consumption. As a result, it is necessary to install expensive filter-compensation units and reactive power compensation units.
❑ (complex)
In contrast to AC systems, designing and operating multi-terminal HVDC systems is complex.
❑ (capacities)
The number of substations within a modern multi-terminal HVDC transmission system can be no larger than six to eight, and large
differences in their capacities are not allowed.
it is practically impossible to construct an HVDC transmission system with more than five substations.
❑ (difficult grounding)
Grounding HVDC transmission involves a complex and difficult installation, as it is necessary to construct a reliable and permanent
contact to the Earth for proper operation and to eliminate the possible creation of a dangerous “step voltage.”
❑ (power faults)
During short-circuits in the AC power systems close to connected HVDC substations, power faults also occur in the HVDC
transmission system for the duration of the short-circuit. Inverter substations are most affected.
FUTURE RECOMMENDATION:

❑ The need for a faster, more efficient and more reliable deployment of
offshore HVDC transmission systems for connection of wind farms, oil
and gas platforms, multi terminal interconnectors as well as a future
HVDC grid.
CONCLUSION

❑ Increasing demand of electrical power and need for bulk efficient electrical
power transmission system lead to the development of HVDC
transmission system. HVDC transmission system today become one of
the best alternative for transmitting bulk power over long distance with
very less losses.
Thank You !