Main feature
chapter IiI
Making energy available to all, today and tomorrow
Static VAr Compensators: not just for
Alstom Grid has recently
delivered an SVC that is operated
mainly in power oscillation
damping control mode. When
system transfer conditions allow,
the SVC may also be used as a
reactive power reserve.
38
Alstom Grid///Spring-Summer 2011
The VAr in Static VAr Compensators (SVC)
stands for Volt-Ampere reactive (VAr) and is
the unit used to measure reactive power in
an AC power system. Typically the SVC is
used as a voltage or a reactive power controller at the point of common coupling. Power
damping control methods are often provided
too, but they are usually operated in parallel
with voltage or reactive power control, and
only part of the reactive capacity of the SVC
is allocated for damping controls.
The Kangasala SVC (KA SVC) built by Alstom
Grid in Finland for the Scandinavian network
The Kangasala
SVC installation.
RTDS testing of
control system.
ments were specified.” These included at
least 98 percent annual availability for forced
outages and at most three forced outages
per year.
Meeting availability targets
reactive power
is unusual, as Power Systems Engineer Pauli
Halonen explains. “The customer, Fingrid
Oyj, needed its main control mode to be
power oscillation damping (POD) control.
However, the SVC also has the traditional
voltage control and reactive power control
modes that can be activated as a backup
source for reactive power or to support the
system voltage. Since the KA SVC is applied
to enhance operational reliability, and in
future may be applied to relieve limitations
in transfer capacity during network congestions, high availability and reliability require-
To achieve the availability requirement, the
KA SVC was equipped with duplicated and
fully redundant control, protection and DC
auxiliary power systems. Separate cooling
systems were implemented for each Thyristor Controlled Reactor (TCR) and Thyristor
Switched Capacitor (TSC)
branch. The design allows
KA SVC to be operated
with any combination of
SVC components as long
as the fifth harmonic filter
bank remains in service.
The structure of the 20 kV
bus and the placement
of the components on the
20 kV yard allows maintenance and repairs to be carried out on the
components on one side of the building while
the other components are still in operation.
The redundancy controller monitors the
integrity of both control systems and decides
which control system is active.
The POD control mode is specified to damp
the inter-area power oscillations in the frequency range of 0.2 Hz to 0.6 Hz. It consists
of six first-order, fully adjustable transfer
functions whose output is dead-band
filtered. The output of the dead-band function is multiplied by output gain and converted to the SVC reactive power output
request value. The whole POD control
branch is therefore a sixth-order transfer
function. “The purpose is to provide a fully
adjustable POD control base to satisfy
present and future user demands,” says
Halonen. “The POD control parameters
are specified by the customer; they can
also be changed later on by the user.”
Parameters can be
changed through the
remote or local control of
the SVC.
The SVC
has shown that
it increases
the operational
safety margin.
Successful testing
The SVC controls were
extensively tested during
the commissioning. In addition to typical step response,
control mode transition and
reactor switching tests, the response of power
oscillation control was tested by modulating
the input of the POD controller using a numerical signal generator implemented as part of
the SVC controls. The results were compared
to results achieved from PSCAD transient
simulation software showing a high
correlation between results, even though the
network model used in PSCAD was
a simplified version.
Alstom Grid///Spring-Summer 2011
39
Main feature
chapter IiI
Making energy available to all, today and tomorrow
M o r e
The effect of the SVC on the system
oscillation damping was lastly tested by
carrying out the same change in system
transfer conditions with different SVC control modes. The change in transfer conditions invoked inter-area oscillation at
0.3 Hz frequency, which was sufficient for
damping verification. In each test the peak
of the first swing was approximately 20 mHz.
Comparing the results showed that the
amplitude of the first swing was very
similar, but differences could be observed
already during the second half of the first
period of the oscillations. The lower peak
amplitude of oscillation was only -10 mHz
with POD controls whereas while operating in the reactive power or the voltage
control modes, the corresponding amplitude was -15 mHz.
In the POD control mode, the level of 0.3 Hz
oscillations decreased below the peakto-peak amplitude of 10 mHz after approximately two periods of 0.3 Hz oscillations.
The same level of oscillation was reached
in voltage control mode within four periods
of oscillation and in reactive power control
mode within seven periods of oscillations.
A future-proof SVC
Based on these tests, the KA SVC has a
significant positive effect on the damping
of 0.3 Hz in inter-area oscillations. The test
results can be applied as a basis to modify
the controls in case there is a need to retune
them to adapt to possible future structure
changes in the transmission network.
The SVC has shown that it increases the
operational safety margin, thus ensuring
high attenuation of inter-area oscillations
even under heavy power transfer conditions.
This is likely to become even more important
in the coming decades. As networks expand,
for instance with the installation of new,
large generating units and large-scale wind
power, they will become more complicated
to manage and the potential for disruption
will increase.
To maintain high operational reliability and
to enhance transmission capacity, including
cross-border flows after the addition of new
generating capacity, the Finnish transmission network has been reinforced with new
transmission lines, series compensation,
HVDC connections and SVC. Halonen is
confident that Alstom Grid’s SVC solution
can efficiently support the network. “Thanks
to its high degree of redundancy and the
robustness of the design, the SVC will
improve the operational safety margin even
in the most difficult system operating conditions and support system stability under
system disturbances.”
The cooling system
and thyristor valve.­
40
Alstom Grid///Spring-Summer 2011
Pauli Halonen
Versatile SVCs
Static VAr Compensators are used
in both industrial and electric utility
applications. Industrial SVCs are
designed to compensate reactive
power, stabilise voltage and filter
harmonic currents from industrial
loads. Typical applications can be
found in the steel industry. As they
improve the power quality, the
end-user production increases,
the total power losses are reduced
and reactive power penalties are
avoided. Utility SVCs are typically
installed in the utility networks
to support system voltage and to
provide reactive power locally.
This results in increased power
transfer capacity and voltage stability.
Increasingly, utility SVCs will also
contribute to power oscillation
damping in the electric networks.
A prime example is the location
of the Kangasala SVC right in the
middle of the generation area
of southern Finland. Allocating the
whole 200 MVAr (ind)/240 MVAr (cap)
capacity for power oscillation damping
means that the effect of the SVC on
damping inter-area electromechanical
oscillations is quite significant.