TRANSMISSION
The world’s biggest FACTS project
with series compensation
by K Braun, A Krummhoiz, D Retzmann, U Rohr, G Thumm, Cigré
Progressive worldwide urbanisation and the trend towards megacities with more than 10-million inhabitants pose new challenges. According
to UN statistics more than half of the gross domestic product of every country in the world is produced in the large cities.
One of the most important factors for
successful economic dynamics in megacities
is an effective infrastructure. It goes without
saying that the basis for this infrastructure is
constituted by a reliable and efficient power
supply.
An important development in the power
supply of megacities is the relocation, for
environmental reasons, of power generation
from city centres to more distant surrounding
regions. However, sufficient power to cover the
ever increasing demand must be delivered
from additional sources which are normally
located very far from the load centres. For
this reason transmission of large power blocks
over long distances is becoming increasingly
important. Furthermore, efficiency and
reliability of supply play an important role
in planning electrical systems, particularly
in the face of increasing energy prices and
almost incalculable safety risks during power
blackouts.
power generated in the Tala hydroelectric
power plant (located in Bhutan) is transmitted
via a newly built 400 kV double circuit
transmission line to Gorakhpur substation.
There the line is connected to the existing
400 kV network and feeds into the northern
industrialised region around the capital New
Delhi. Due to this interconnection the north
east of India can also profit from economical
hydro power.
Benefits of series compensation
Series compensation is the ideal “low cost”
solution for bulk power long distance AC
transmission. Series compensation can
also be applied very effectively in meshed
systems for balancing the load flow by means
of ‘load displacement’.
The principle of series compensation
consists of using a series capacitor which
compensates the for the line’s reactance,
so the line becomes virtually shorter. Due to
this, the transmission angle is reduced and
the system stability is increased. This enables
a higher transmission capacity of the existing
line without having to install new lines. The
simplest form of series compensation is “fixed
series compensation” (FSC).
The systems at Purnea and Gorakhpur
substations use a combination of FSC
and TCSC (thyristor controlled series
compensation).
TCSC is used if fast control of the line
impedance is required, for load flow control,
and for damping power oscillations.
The main features of series compensation
schemes are that they increase the
transmission capacity and, in the case of
controlled series compensation, they also
This is also the case in India. An increasing
demand for electricity and a growing
awareness of the need for reliable power
supply, for their fast developing megacities
in particular, have prompted Powergrid (the
utility company which operates India’s high
voltage transmission networks) not only to
continue with its transmission plan, but also
to strengthen its networks with flexible AC
transmission systems (FACTS).
This promising transmission enhancement
plan was developed and implemented by
Powergrid in order to establish an integrated
national power grid and to support an
additional program for new generation
capacities. Today’s program of 9500 MW
interregional power transfer capacity is
expected to be enhanced to 30 000 MW
by 2012.
With the help of its 400 kV series compensation
systems, which are the preferred FACTS
solutions for Purnea and Gorakhpur substations
projects (see Fig. 1), Powergrid has taken steps
to enhance its east to west power transfer. The
Fig. 1: Purnea substation.
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Basically, the thyristor valve could have
been used to provide the bypass function
alone, without the spark gap. For this project,
however, it was decided to use a separate
triggered spark gap as a fast protecting
device, because of very high thermal stress
for the thyristors caused by extremely high fault
currents. This enables immediate operation of
the thyrister valve in the full dynamic range,
e.g. for power oscillations damping which is
normally required after fault clearing in order
to stabilise the system.
The capacitor discharge current is limited by
means of the bypass damping circuit in the
same way as in the case of the FSC.
Fig. 2: Single line diagram of Purnea / Gorakhpur substation for FSC and TCSC.
provide damping for power oscillations,
load flow control and mitigate SSR (subsynchronous resonances)
Design aspects of series compensation
At Purnea and Gorakhpur substations in
the northern part of India both fixed series
compensation (FSC) and thyristor controlled
series compensation (TCSC) are used. Fig. 2
shows the single line diagram.
FSC
A segment of a fixed series capacitor bank
(FSC) consists of a capacitor bank and a MOV
(metal oxide varistor) assembly, connected in
parallel. It also includes a spark gap which is
used to protect capacitors and MOV against
over-voltages which occur during and after
faults in the transmission system.
protected against over-voltages by the MOV.
The MOV is designed to withstand stresses
such as these. The operation strategy for the
bypass devices is the same as for the FSC.
The thyristor valve branch with the reactor is
connected in parallel to the capacitor. The
impedance of the parallel L C circuit can
be varied by means of firing the thyristor
valve. In case of AC line faults the thyristor
valve can be blocked in order to reduce the
capacitive impedance and consequently
the fault current.
A damping circuit is connected in series
with the triggered spark gap and the bypass
switch in order to reduce component stress
during transient discharge of the capacitor.
The bypass switch is connected in parallel
with the spark gap to provide current
commutation for the time when the current
capability of the MOV or the spark gap is
exceeded during AC faults.
In case of high short circuit currents caused
by faults on the AC line, to which the
capacitor bank is connected, the spark
gap and the bypass switch will be triggered
in order to protect the capacitor and the
MOV from overload. A fault on other lines,
for example behind the neighboring buses,
usually produces lower currents in the series
capacitor and the bypass devices do not
need to be operated.
TCSC
The main components of the TCSC segment
are also shown in Fig. 2. The capacitor is
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The TCSC offers the opportunity of power
oscillation damping and power-flow control
via the controllable impedance. The thyristor
controlled reactor branch is formed by a
reactor and a thyristor valve. The thyristor
valve is made up of a number of series
connected thyristors, two branches of which
are connected in anti parallel. The valve is
located in a housing on the platform.
For FACTS applications, thyristors are a key
element in control of passive components.
Siemens is the only company which supplies
the LTT (direct light triggered thyristor) valves
with wafer integrated over-voltage protection
for HVDC and FACTS. The system of direct light
triggering fires the thyristors with a light pulse.
TRANSMISSION
A TCSC was found to be the optimal solution
to damp these oscillations.
For power oscillation damping the full
capacitive operating range can be used.
The benefits of the damping function are
depicted in Fig. 4.
Sub-synchronous resonances (SSR) studies
Fig. 3: Light triggered thyristor (LTT) technology.
The light pulse generated at ground potential
is guided by fiber optics directly from the
valve control to the thyristor gate.
After fault clearing, the TCSC is ready to fulfill
its main purpose - power oscillation damping
with its full capacitive operating range.
Light triggered thyristors offer significant
benefits over conventional electrically
triggered thyristors - less electronic
components on high potential and increased
reliability (see Fig. 3).
Studies
The thyristor valve is fired at an angle which is
calculated according to control requirements
- impedance control or current control. With
impedance control the transmission angle of
the line can be influenced and current control
determines the load flow. In normal steady
state operation the TCSC impedance is set
to a value which is 1,2 times the capacitor
impedance. The dynamic range covers
1,3 p.u. During remote faults in the system
the thyristor valve continues firing. With severe
disturbances the thyristor valve is blocked.
The rated nominal technical data of the
world’s biggest FSC/TCSC project are, for
Purnea substation, 420 kV, 743 Mvar FSC and
112 Mvar TCSC and, for Gorakhpur substation,
420 kV, 716 Mvar FSC and 108 Mvar TCSC.
In the run up to the basic design specifications,
oscillation damping (POD) and subsynchronous resonance (SSR) studies were
carried out, as well as calculations necessary
for defining the control parameters.
Power oscillation damping studies
Studies showed that power oscillations occur
between the two interconnected systems
after severe disturbances, for example due to
a loss of generation in one of the two systems.
The compensation degree of a line has
to be designed according to the system
conditions - the line length and stability
requirements. However, the design also has
to take into account the fact that series
compensated lines can excite torsional
oscillations in generators with long shafts.
These sub-synchronous resonances (SSR), as
they are termed, can pose a severe problem
for large thermal units such as steam turbines,
combined cycle turbines, large gas turbines,
and especially for nuclear units. Under certain
conditions generator shafts as long as these
can be damaged or even destroyed.
Depending on the construction of the
generator, the turbine and the excitation
system, typical resonance frequencies of
generator turbo set shafts are between 10
and 45 Hz with very low damping. Using
only fixed series compensation without
the TCSC, high levels of compensation on
transmission lines can produce interaction
of the electric resonances of the system
with the mechanical torsional frequencies
of generator shafts. A small disturbance
in the electric torque can excite relatively
high amplitudes of torsional oscillations
when the corresponding electric frequency
matches one of the mechanical oscillation
modes of the generator shaft. An electric
torque, produced by the LC circuit of line
impedance and series capacitor, may act as
a negative damping to torsional oscillations
when the LC resonance frequency interacts
with the generator shaft. The LC resonance
frequency is of sub-synchronous nature, as
the compensation degree is always less
than 100%.
A much better effect is produced using the
TCSC for SSR mitigation by changing the
apparent impedance of the TCSC according
to torsional oscillation with a suitable phase
to the oscillating electric torque. The passive
mitigation effect of the TCSC depends on the
resonance frequency, the firing angle, the
line configuration, as well as on other system
parameters, such as the short circuit power of
the system. All these influencing parameters
are covered in the SSR study.
TCSC control
Fig. 4: Power oscillation damping.
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The closed loop control (CLC), the open
loop control (OLC) and the redundant
protection system are fully digitalised with
TRANSMISSION
interconnected systems can be avoided.
Resynchronisation of the two systems would
be of great technical complexity, that is
why avoiding separation is of vital interest
for transmission.
Project overview
Siemens received the order for the FSC/
TCSC in the end of May 2004, and a project
team with members from both India and
Germany worked on it. The responsibilities
in this Facts turnkey project, named TALA
High Capacity East North Interconnector
II, were shared between the head office
in Germany and the Dehli office. Reactors,
capacitor banks, MOV, circuit breakers,
spark gaps, thyristor valves and the control
system for this project, financed by World
Bank, were supplied by Siemens Germany.
All other equipment and activities which
included platforms, civil works, installation
and commissioning were provided by
Siemens India.
Erection and commissioning
Fig. 5: Gorakhpur substation.
Siemens’ Win TDC signal processor system.
A number of signals are measured on the
platform at high voltage. After conversion
into optical pulses the signals are sent to
ground potential via fiber optic links, and
then reconverted into values in a digital
format. The auxiliary power supply for the
conversion on platform is sent via a second
fibre optic link.
The main task of the closed loop
control system is to determine the firing
angle (alpha) which defines the TCSC
impedance. Normally, the TCSC operates
in the impedance mode, where the TCSC
provides an impedance as required
(reference value). As soon as power
oscillation along the line is detected by
means of line current measurement, the
control system switches over to the power
oscillation damping mode. The trigger set
generates the firing pulses according to
the firing angle which is the input value.
The correct timing of the firing pulses is
continuously synchronised with the AC
system frequency through the line current.
Digital filtering is used to determine the
reference point for each firing pulse. It is
also the case under system conditions
with non sinusoidal waveforms of line
current. The control and protection of the
TCSC detects eventual overload of all
components, and it takes into account the
overload requirements. The protection can
respond by blocking the thyristor valves, by
triggering the spark gap (fast bypass) and by
closing the bypass switch, depending on the
type, severity and duration of the fault.
After a single phase fault in the system,
it is foreseen that only the faulty phase is
opened by the line protection, and it will
be re closed after about one second. In this
case, only one phase element of the TCSC
is bypassed to provide the highest possible
stabilisation of the system. Only when the line
is opened in all three phases, will the TCSC
be bypassed in all three phases as well.
The use of a configuration with the FSC
plus the TCSC offers significant benefits for
the transmission of power. The fixed series
capacitors (FSC) compensate 40% of
the line impedance, reduce transmission
losses and, consequently, increase power
transfer. They also improve steady state and
dynamic stability of the system.
Thyristor controlled series capacitors (TCSC)
constitute the solution which provides
transient stability of the increasing power
transfer. Each of the TCSC is designed to
compensate 6% of the line impedance
during steady state operation, and to vary
the impedance in transient conditions
in a range between 1 and 3 p.u. This
enables damping power oscillations
which may occur during and after system
disturbances.
This means that, with the advanced control
features of the TCSC, separation of the two
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Early in 2006 the transmission lines and
FACTS equipment were put into operation.
Since then they have been providing a
reliable transmission corridor for delivering
power from the eastern part of India to the
load centers in the north of the country.
Fig. 5 shows a view of the installation at
Gorakhpur substation.
Acknowledgement
This article was first published in the December
2006 issue of Cigré’s Electra magazine and
is republished with permission. 