High Voltage DC (HVDC)
Transmission
• Basic structure and operation.
• Commutation.
• Harmonics.
• DC links.
• Controls.
• Fundamental frequency, reduced model.
• Steady state model.
• Faults and protections.
1
Basics
• AC voltages are first converted into dc, the
power is then transmitted on a high
voltage dc line, and finally converted again
into ac at the other side of the HVDC link.
2
Basics
3
Basics
• This results on the Vdc voltage:
• Each “valve” is on for 120o.
4
Basics
• Using thyristor valves in the converter:
5
Basics
• For a firing delay
angle 0 < α < 90o
(rectifier, i.e. Vdc > 0 ):
6
Basics
• For α = 90o:
7
Basics
• For α =180o (inverter,
i.e. Vdc < 0):
8
Basics
• Hence, a basic 6-pulse HVDC link is basically structure as
follows:
• Large inductors Ld are used to make the current Id constant,
i.e. it is based on current-sourced converters.
9
Commutation
• The transformer reactance Xc and the
constant current lead to commutation
problems:
10
Commutation
11
Commutation
• Definitions:
– α → Firing angle
– µ → Overlap angle
– β → “On” angle
– γ → Extinction angle:
• The value of angle γ is associated with the valve “extinction”
angle , i.e. the time these valves have to turn off.
• Commutation problems or “failures” may occur in inverters
when the value of α is large, i.e. γ is small, as there is not
enough time for the valves to turn off.
• Hence, inverter controls are designed to keep γ > γmin ≈ 10o.
12
Harmonics
• As Id is constant, the
ac current has large
harmonic content, as
previously shown.
• This can be reduced
by connecting in
series two phase-
shifter 6-pulse
converters:
13
Harmonics
14
Harmonics
• The harmonic content in this signal is
reduced by eliminating the 5th and 7th
harmonics:
15
Harmonics
• Harmonics in the ac and dc side are reduced by
introducing filters:
DC
AC
16
DC Links
• Bipolar (e.g. Nelson River, Manitoba, Bipole 1, ±450 kV, 1620 MW, 1972 &
1977; Bipole 2, ±500 kV, 1800 MW, 1978 & 1985):
• More terminals can be added leading to Multi-terminal HVDC links (e.g.
Quebec-New England, ±450kV, 2000 MW, at least 3 terminals, 1990)
17
DC Links
• Monopolar underwater connections (e.g. Sardinia-Mainland,
Italy, 200 kV, 200 MW, 1967):
• Bipolar connections are preferred nowadays for technical and
environmental reasons.
18
DC Links
• Back-to-back (e.g. Eel River, New Brunswick-Quebec,
80 kV, 350 MW, 1972):
• List of HVDC links:
http://en.wikipedia.org/wiki/List_of_HVDC_projects
19
DC Links
20
DC Links
• In Europe
[http://en.wikipedia.or
g/wiki/List_of_HVDC_
projects]:
21
Controls
22
Controls
• Valve Group Control with constant Id control:
23
Controls
24
Controls
• There are two distinct typical control modes:
– Normal operating conditions (“slow” controls):
• Rectifier:
– Controls Id and αr through its LTC transformer taps ar.
– If the ac voltage Vr goes down, ar may eventually reach its
max. limit; the current is then controlled through αr.
– If Vr goes down further, αr eventually reaches its min. limit (~
5o), at which point the control changes from Id to αr, while
the inverter takes over Id control with a margin reduction ∆Id
(~ 10%) in its reference value.
• Inverter:
– Controls γi and Vdi through its LTC transformer taps ai.
– If the ac voltage Vi goes down, ai eventually reaches a max.
limit, at which point the inverter losses Vdi control.
25
Controls
– For fast controls around the operating point, the
taps ar and ai remain fixed:
• Rectifier:
– Controls Id through its firing angle αr.
– If αr reaches its min. limit, the current control is transferred
to the inverter with a margin reduction in its reference value.
• Inverter:
– Controls γi.
– Voltage Dependent Current Order Limiter
(VDCOL):
• Is a control mode used to reduced the reactive power demand of the
converter when ac voltages are too low, and thus reduced the risk of a
commutation failure.
• When V is too low, the VDCOL reduces Id, which lead to a in a reduction
in P, and as a result in a Q reduction as well.
26
Controls
27
Controls
• Nelson River controls:
28
Controls
– The FBCC (frequency-based capability control) provides protection against
overloading by reducing the power order if the frequency drops below 59 Hz.
– The EPOT (excess power order transfer) fully uses the combined available capability
by transferring excess power capacity of one bipole to the other. It is activated only
when a major outage occurs such as a valve group or pole block.
– The power reduction for tie-line trips ensures system stability upon loss of
interconnection lines by reducing power by an amount equal to the tie line loading
prior to tripping.
– The undervoltage reduction controller reduces power when the Dorsey voltage starts
to collapse, releasing MVARs from both the dc link and the ac system; it does not
react to faults.
– The allocator allocates the power to each bipole according to a preset power order.
– The SEFC (sending-end frequency control) and RFEC (receiving EFC) minimize ac
system oscillations.
– The REDC (receiving-end damping control) prevents changes in the angle of the
Dorsey 230 kV bus voltage.
29
Simplified Model
• The fundamental frequency, balanced
model is represented using the following
per unit dynamic equations:
30
Simplified Model
31
Simplified Model
• Simple control model (similarly for the
inverter angle γi):
32
Simplified Model
which represents the following control modes:
33
Steady State Model
• The previous
equations lead to the
following p.u. steady
state equations:
34
Steady State Model
• Plus the “slow” control equations, which define four
additional variables depending on the control mode:
– Typical operating conditions :
• Rectifier → αr (cos αr) and Id (or Pr for power control)
• Inverter → γi (cos γ i) and Vdi
– Rectifier tap limits:
• Rectifier → armax and Id (or Pr for power control)
• Inverter → γ i (cos γ i) and Vdi
– Inverter control:
• Rectifier → αrmin (cos αrmin) and armax
• Inverter → γ i and Id - ∆I (or Pi for power control)
• Taps and converter angles must be kept within their
control limits.
35
Example
• For a bipolar 12-pulse HDVC link:
Determine the ratings and taps for all transformers.
36
Example
37
Example
– Rectifier:
38
Example
– Inverter:
39
Faults and Protections
• AC-side short-circuits:
– TNA simulations for
short-circuit near the
inverter:
40
Faults and Protections
– Fault severity is less than an equivalent ac link,
since the fast voltage controller quickly reduces
current (power) to zero.
– Faults near the rectifier side do not require any
control actions, since as long as there is a small
voltage, commutation is guaranteed and the
system quickly recovers after the fault clearance.
– Faults near the inverter side can lead to
commutation failures leading to large current
transients; this can be avoided by reducing the
valve’s firing angles.
41
Faults and Protections
• DC line faults:
– Cannot be extinguished until the dc current is brought down to
zero (unless special dc breakers are used to drive the current to
zero before opening).
– Under normal converter controls, the rectified current rises and
the inverter current falls, forcing the inverter into rectification to
increase the current.
– To reduce the current to zero:
• The rectifier voltage is reversed by ramping up the firing angle,
converting it into an inverter to force the energy out of the dc system
quickly.
• The inverter firing angle is clamped to avoid rectifier operation.
– Fault recovery:
• The rectifier angle is ramped out until nominal conditions are attained.
• The inverter angle remains clamped.
42
Faults and Protections
– Actual HVDC dc line fault response:
43
Faults and Protections
• Converter valves need be protected from
overcurrents to avoid damage; the following
coordination strategy may be used:
44
Faults and Protections
• Typical ac and dc protections:
45
