Appendix B - High Voltage Direct Current Transmission

106 Pages·2008·10.44 MB·English

High Voltage Direct Current
Transmission –
Proven Technology for Power Exchange
Answers for energy.
1
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 11
HVDC Transmission Project
5 Main Components 14
5.1 Thyristor Valves 15
5.2 Converter Transformer 18
5.3 Smoothing Reactor 21
5.4 Harmonic Filters 22
5.4.1 AC Harmonic Filter 23
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 33
5.6.3 High Speed DC Switches 34
5.6.4 Earth Electrode 36
5.7 Control & Protection 38
6 System Studies, Digital Models, 45
Design Specifications
7 Project Management 46
3
1 Why High Voltage Direct Current ?
1.1 Highlights from the High Line-Commutated Current Sourced Self-Commutated Voltage Sourced
Voltage Direct Current (HVDC) History Converters Converters
The transmission and distribution of The invention of mercury arc rectifiers in Voltage sourced converters require
electrical energy started with direct the nineteen-thirties made the design of semiconductor devices with turn-off
current. In 1882, a 50-km-long 2-kV DC line-commutated current sourced capability. The development of Insulated
transmission line was built between converters possible. Gate Bipolar Transistors (IGBT) with high
Miesbach and Munich in Germany. voltage ratings have accelerated the
In 1941, the first contract for a commer-
At that time, conversion between development of voltage sourced
cial HVDC system was signed in
reasonable consumer voltages and converters for HVDC applications in the
Germany: 60MW were to be supplied
higher DC transmission voltages could lower power range.
to the city of Berlin via an underground
only be realized by means of rotating
cable of 115 km length. The system The main characteristics of the voltage
DC machines.
with±200 kV and 150 A was ready for sourced converters are a compact design,
In an AC system, voltage conversion is energizing in 1945. It was never put four-quadrant operation capability and
simple. An AC transformer allows high into operation. high losses.
power levels and high insulation levels
Since then, several large HVDC systems Siemens is offering voltage sourced
within one unit, and has low losses. It is
have been realized with mercury arc converters for HVDC applications with
a relatively simple device, which requires
valves. ratings up to 250 MW under the trade
little maintenance. Further, a three-phase
name HVDC plus Power Link Universal
synchronous generator is superior to a The replacement of mercury arc valves
Systems.
DC generator in every respect. For these by thyristor valves was the next major
reasons, AC technology was introduced development. The first thyristor valves This paper focuses upon HVDC trans-
at a very early stage in the development were put into operation in the late mission systems with high ratings, i.e.
of electrical power systems. It was soon nineteen-seventies. with line-commutated current sourced
accepted as the only feasible technology converters.
for generation, transmission and distri- The outdoor valves for Cahora Bassa
bution of electrical energy. were designed with oil-immersed
thyristors with parallel/series connection
However, high-voltage AC transmission of thyristors and an electromagnetic firing
links have disadvantages, which may system.
compel a change to DC technology:
Further development went via air-
• Inductive and capacitive elements of insulated air-cooled valves to the air-
overhead lines and cables put limits insulated water-cooled design, which is
to the transmission capacity and the still state of the art in HVDC valve design.
transmission distance of AC trans-
mission links. The development of thyristors with higher
• This limitation is of particular signi- current and voltage ratings has eliminated
ficance for cables. Depending on the the need for parallel connection and
required transmission capacity, the reduced the number of series-connected
system frequency and the loss eva- thyristors per valve. The development of
luation, the achievable transmission light-triggered thyristors has further
distance for an AC cable will be in the reduced the overall number of
range of 40 to 100km. It will mainly components and thus contributed to
be limited by the charging current. increased reliability.
• Direct connection between two AC Innovations in almost every other area
systems with different frequencies is of HVDC have been constantly adding
not possible. to the reliability of this technology with
• Direct connection between two AC economic benefits for users throughout
systems with the same frequency or the world.
a new connection within a meshed
grid may be impossible because of
system instability, too high short-circuit
levels or undesirable power flow
scenarios. HVDC= high voltage direct current
DC = direct current
Engineers were therefore engaged over
AC = alternating current
generations in the development of a
IGBT = insulated gate bipolar
technology for DC transmissions as a
transistor
supplement to the AC transmissions.
4
1.2 Technical Merits of HVDC 1.3 Economic Considerations 1.4 Environmental Issues
The advantages of a DC link over an AC For a given transmission task, feasibility An HVDC transmission system is basi-
link are: studies are carried out before the final cally environment-friendly because
decision on implementation of an HVAC improved energy transmission possi-
• A DC link allows power transmission
or HVDC system can be taken. Fig.1-1 bilities contribute to a more efficient
between AC networks with different
shows a typical cost comparison curve utilization of existing power plants.
frequencies or networks, which can
between AC and DC transmission
not be synchronized, for other reasons. The land coverage and the associated
considering:
• Inductive and capacitive parameters right-of-way cost for an HVDC overhead
do not limit the transmission capacity • AC vs. DC station terminal costs transmission line is not as high as that
or the maximum length of a DC • AC vs. DC line costs of an AC line. This reduces the visual
overhead line or cable. The conductor • AC vs. DC capitalised value of losses impact and saves land compensation for
cross section is fully utilized because new projects. It is also possible to in-
The DC curve is not as steep as the AC
there is no skin effect. crease the power transmission capacity
curve because of considerably lower line
for existing rights of way. A comparison
For a long cable connection, e.g. beyond costs per kilometre. For long AC lines
between a DC and an AC overhead line
40 km, HVDC will in most cases offer the cost of intermediate reactive power
is shown in Fig. 1-2.
the only technical solution because of compensation has to be taken into
the high charging current of an AC cable. account.
This is of particular interest for trans-
The break-even distance is in the range
mission across open sea or into large
of 500 to 800km depending on a number
cities where a DC cable may provide the
of other factors, like country-specific cost
only possible solution.
elements, interest rates for project
• A digital control system provides financing, loss evaluation, cost of right
accurate and fast control of the active of way etc.
power flow.
• Fast modulation of DC transmission
AC- DC-
power can be used to damp power
tower tower
oscillations in an AC grid and thus
improve the system stability.
Fig. 1-2: Typical transmission line structures
for approx. 1000 MW
There are, however, some environmental
issues which must be considered for the
converter stations. The most important
Costs Total AC Cost
ones are:
• Audible noise
Total • Visual impact
DC Cost • Electromagnetic compatibility
• Use of ground or sea return path
in monopolar operation
AC Losses DC Losses
In general, it can be said that an HVDC
system is highly compatible with any
environment and can be integrated into
it without the need to compromise on
DC Line
any environmentally important issues of
today.
AC Line
DC
Terminals
AC
Terminals
Break-Even Transmission
Fig. 1-1: Distance Distance
Total cost/distance
5
2 Main Types of HVDC Schemes
2.1 DC Circuit 2.2 Back-to-Back Converters 2.3 Monopolar Long-Distance
Transmissions
The main types of HVDC converters are The expression Back-to-back indicates
distinguished by their DC circuit arrange- that the rectifier and inverter are located For very long distances and in particular
ments. The following equivalent circuit in the same station. for very long sea cable transmissions, a
is a simplified representation of the return path with ground/sea electrodes
Back-to-back converters are mainly used
DC circuit of an HVDC pole. will be the most feasible solution.
for power transmission between adjacent
AC grids which can not be synchronized.
They can also be used within a meshed
grid in order to achieve a defined power 1 HVDC 2
± Id ± flow. m Cable/OHL m
e e
Ud1 Ud2 yst yst
S Electrodes S
C C
A A
HVDC
Fig. 2-1: Equivalent DC circuit 1 2
m m
e e Fig. 2-4: Monopole with ground return path
The current, and thus the power flow, is st st
y y
controlled by means of the difference C S C S In many cases, existing infrastructure or
between the controlled voltages. The A A environmental constraints prevent the
current direction is fixed and the power use of electrodes. In such cases, a
direction is controlled by means of the metallic return path is used in spite of
voltage polarity. The converter is de- Fig. 2-2: Back-to-back converter increased cost and losses.
scribed in the next section.
1 HVDC 2
m m
Cable/OHL
e e
st st
y y
C S LVDC C S
A A
Fig. 2-5: Monopole with metallic return path
HVDC= high voltage direct current
DC = direct current
AC = alternating current
U = DC voltage 12-pulse
d
Fig. 2-3: Back-to-back converter I = DC current
d
Station Vienna Southeast OHL = overhead line
LVDC = low voltage direct current
6
2.4 Bipolar Long-Distance 2.4.1 Bipole with Ground Return 2.4.2 Bipole with Dedicated Metallic
Transmissions Path Return Path for Monopolar Operation
A bipole is a combination of two poles This is a commonly used configuration If there are restrictions even to temporary
in such a way that a common low voltage for a bipolar transmission system. The use of electrodes, or if the transmission
return path, if available, will only carry a solution provides a high degree of distance is relatively short, a dedicated
small unbalance current during normal flexibility with respect to operation with LVDC metallic return conductor can be
operation. reduced capacity during contingencies considered as an alternative to a ground
or maintenance. return path with electrodes.
This configuration is used if the required
transmission capacity exceeds that of a
single pole. It is also used if requirement
to higher energy availability or lower load HHVVDDCC HVDC
rejection power makes it necessary to CCaabbllee//OOHHLL Cable/OHL
1 2 1 2
split the capacity on two poles. m EElleeccttrrooddeess m m m
e e e LVDC e
Dpoulrein, git mis asitniltle pnoasnscibe leo rt oo uttraagnesms oitf poanret C Syst C Syst C Syst Cable/OHL C Syst
of the power. More than 50% of the A HHVVDDCC A A HVDC A
transmission capacity can be utilized, CCaabbllee//OOHHLL Cable/OHL
limited by the actual overload capacity
of the remaining pole.
Fig. 2-6: in bipolar balanced operation (normal) Fig. 2-9: in bipolar balanced operation (normal)
The advantages of a bipolar solution over
a solution with two monopoles are
Upon a single-pole fault, the current of
reduced cost due to one common or no the sound pole will be taken over by the 2.4.3 Bipole without Dedicated
return path and lower losses. The main ground return path and the faulty pole Return Path for Monopolar Operation
disadvantage is that unavailability of the
will be isolated.
return path with adjacent components A scheme without electrodes or a
will affect both poles. dedicated metallic return path for
monopolar operation will give the lowest
HVDC initial cost.
Cable/OHL
1 2
m Electrodes m
e e
st st HVDC
y y
S S Cable/OHL
C C 1 2
A HVDC A m m
Cable/OHL ste ste
y y
S S
Fig. 2-7: in monopolar ground return C C
operation (converter pole or OHL outage) A HVDC A
Cable/OHL
Following a pole outage caused by the
converter, the current can be commutated Fig. 2-10: in bipolar balanced operation (normal)
from the ground return path into a
metallic return path provided by the
Monopolar operation is possible by
HVDC conductor of the faulty pole.
means of bypass switches during a
converter pole outage, but not during an
HVDC conductor outage.
HVDC
Cable/OHL A short bipolar outage will follow a
1 2 converter pole outage before the bypass
m Electrodes m operation can be established.
e e
st st
y y
S S
C C
A HVDC A
Cable/OHL
Fig. 2-8: in monopolar metallic return
operation (converter pole outage)
7
3 Converter Theory
3.1 Bridge Circuit Function The angle between the time at which 3.2 12-Pulse Group and Converter
the valve voltage becomes positive and Transformer
Current flows through the valves when
the firing time (start of commutation) is
the voltage between the anode and HVDC converters are usually built as 12-
referred to as the firing delay. Fig. 3-2
cathode is positive. For the valve to pulse circuits. This is a serial connection
shows that for a firing delay of 90°, the
commutate the current, there must be of two fully controlled 6-pulse converter
average voltage equals zero. i.e. the
a positive potential (voltage), and the bridges and requires two 3-phase sys-
positive and negative areas of the curve
thyristor must have firing pulses. In the tems which are spaced apart from each
– voltage against time – cancel each
reverse direction, i.e. when the potential other by 30 electrical degrees. The phase
other out. No active power flows through
between anode and cathode is negative, difference effected to cancel out the
the converter.
a firing pulse has no effect. The flow of 6-pulse harmonics on the AC and DC
current in a valve ends when the voltage When the firing delay is greater than side.
between anode and cathode becomes 90°, the negative voltage/time areas
negative. The instant when current begins dominate, and the polarity of the average
to flow through a valve, or to commutate direct voltage changes. Due to physical
from one valve to another, can be delayed reasons, the direction of the current does
by postponing the firing. This method not change. (The thyristor valves conduct 3 1
2
permits the average value of the outgoing current only in one direction.) When the
voltage of the rectifier to be changed. direction of energy flow is reversed, the
The firing pulses are generated by syn- delivery changes to the supply side. The
chronizing the network using an elec- rectifier becomes an inverter which
tronic control device. These pulses can be delivers energy to the AC network.
displaced from their ”natural firing“ point, 4
The average value of the direct voltage
which is the point where the two phase
as a function of the firing delay is given
voltages intersect. The method of firing-
by: 1 Valve Branch
pulse displacement is called phase
control. Udiα = 1.35 * UL * cos α 2 Double Valve
U = secondary side line voltage 3 Valve Tower
L
4 6-pulse Bridge
α = firing angle
γ = extinction angle
Fig. 3-3: Arrangement of the valve branches
in a 12-pulse bridge
DC current in each valve and phase
1 3 5 1
0°
L L L
1 2 3 Udi
60°
0° 120° Ud ωt α = 0°
6 2 4 6 2 0°
I
d
i1
i
2
ωt
α = 60°
i
3
60°
i
4
i
5
i ωt α = 90°
6 90° γ = 90°
i
L
1
i
L
2
i 150° ωt
L
3 α = 150°
Id γ = 30°
1 3 5
L
1
180° ωt
L2 Ud α = 180°
γ = 0°
L
3
4 6 2
Fig. 3-1: Six-pulse converter bridge Fig. 3-2: DC voltage of bridge converter
as a function of α
8
3.3 Reactive Power as a Function
α= 60° of Load
The curve of reactive power demand of
an HVDC station with changing active
power P can be calculated from equation:
Udiα
Q = P * tan [ arc cos ( cos α - dx)]
In Fig. 3-5, the reactive power demand
of a converter is presented under three
ωt
different control methods.
If the terminal DC voltage U and the
d
firing angle α (or the extinction angle γ
of an inverter) are held constant, curve
(1) will be obtained. If, however, U is
v
Secondary Voltage held constant (Udi = const regulation), a
of the Transformer linear curve such as (2) is obtained. The
power of a converter can also be changed
α Basic AC Current
when the (nominal) current is held con-
stant by varying the DC voltage. Curve (3)
shows the reactive power demand for
this control method. It is important to
note that the entire area between curves
ϕ
(1) and (3) is available for reactive power
control. Each point within this area can
be set by the selection of firing angles
αand ß (orγ).
Fig. 3-4:
Current displacement with angle control
Q/PN
HVDC DC Circuit
1.2
U = P /I
dN dN Rec dN 3
1.0
U => nominal DC voltage 12-pulse
dN
I => nominal DC current 0.8
dN
P => nominal DC active power
dNRec 0.6
at the rectifier
0.4 2
0.2 1
P/PN
0.5 1.0
1 Ud= const; α= const( = const)
2 Ud= const; Uv= const
3 Id = const
Fig. 3-5:
Reactive power demand of an HVDC converter
d = relative inductive voltage
x
drop
U = valve voltage
v
U = DC voltage 12-pulse
d
α = firing angle
ß = 180°-α
γ = extinction angle
9
3 Converter Theory
3.4 Reactive Power Control
420 kV 50 Hz 420 kV 50 Hz
The possibility of electronic reactive
power control 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
reactive 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 Q = 103 Mvar Q = 103 Mvar
the light loads where the necessary filter
circuits produce a considerable overcom-
Q = 103 Mvar Q = 103 Mvar
pensation for the reactive power required
by the converter.
Fig. 3-6 depicts the reactive power control Q = 103 Mvar Q = 103 Mvar
of the Dürnrohr HVDC link. In this system,
Fig. 3-6: Reactive-power compensation and
a compensation to ± 60 Mvar was spe-
control of an HVDC back-to-back link
cified. Compliance with the Q limit is
achieved by load-dependent switching
Q(Mvar)
of a capacitor bank and one of the two 100
high-pass filters. Electronic reactive
power is used only in the light load range.
Normally, there is a difference between 80
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or even a ”pumping”. 40 mi mi
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20 edu Nor
R
100 200 300 400 500 600 700 P(MW)
_P___
0.2 0.4 0.6 0.8 1.0 1.2 PN
-20
-40
-60
Electronic Capacitorbank
reactive-power
regulation High-passfilter2
-80
High-passfilter1
-100
Reactive-Power Balance
U in p.u. (AC bus voltage)
AC
– cap. reactive-power reactive-power reactive-power reactive-power
+ ind converter AC filters reactors capacitors
Q = + Q – Q *U 2 + Q *U 2 – Q *U 2
Network Conv FK AC L AC C AC
10