Statcom

STATCOM(Static Synchronous Compensator)
By: Flamin chacko
Electrical Engineer
Introduction
This s a small note discussing about one of compensators that is mostly used in
power system for improving quality of power transmitted form sourse to load.here in this paper
its concentrated on the very basic of this compensator telling answer for why? What? When?
How? etc.. As an electrical engineer in our day to day life we are dealing with real and reactive
power which can determine the quality of power transmitted from source to load.

STATCOM

A static synchronous compensator (STATCOM), also known as a "static synchronous
condenser" ("STATCON"), is a regulating device used on alternating current electricity
transmission networks. It is based on a power electronics voltage-source converter and can act as
either a source or sink of reactive AC power to an electricity network. If connected to a source of
power it can also provide active AC power. It is a member of the FACTS family of devices.

The figure shows the equivalent circuit of a STATCOM system. The GTO converter with
a dc voltage source and the power system are illustrated as variable ac voltages in this figure.
These two voltages are connected by a reactance representing the transformer leakage
inductance.
Using the classical equations that describe the active and reactive power flow in a line in terms
of Vi and Vs, the transformer impedance (which can be assumed as ideal) and the angle
difference between both bars, we can defined P and Q. The angle between the Vs and Vi in the
system is d. When the STATCOM operates with d=0 we can see how the active power send to
the system device becomes zero while the reactive power will mainly depend on the voltage
module. This operation condition means that the current that goes through the transformer must
have a +/-90º phase difference to Vs. In other words, if Vi is bigger than Vs, the reactive will be
send to the STATCOM of the system (capacitive operation), originating a current flow in this
direction. In the contrary case, the reactive will be absorbed from the system through the
STATCOM (inductive operation) and the current will flow in the opposite direction. Finally if
the modules of Vs and Vi are equal, there won´t be nor current nor reactive flow in the system.

Thus, we can say that in a stationary state Q only depends on the module difference between Vs
and Vi voltages. The amount of the reactive power is proportional to the voltage difference
between Vs and Vi.

There can be a little active power exchange between the STATCOM and the EPS. The
exchange between the inverter and the AC system can be controlled adjusting the output
voltage angle from the inverter to the voltage angle of the AC system. This means that the
inverter can not provide active power to the AC system form the DC accumulated energy if the
output voltage of the inverter goes before the voltage of the AC system. On the other hand, the
inverter can absorb the active power of the AC system if its voltage is delayed in respect to
the AC system voltage.
The STATCOM smoothly and continuously controls voltage from V1 to V2. However, if the
system voltage exceeds a low-voltage (V1) or high-voltage limit (V2), the STATCOM acts as a
constant current source by controlling the converter voltage (Vi) appropriately.

V-I characteristic of a STATCOM
Thus, when operating at its voltage limits, the amount of reactive power compensation provided
by the STATCOM is more than the most-common competing FACTS controller, namely the
Static Var Compensator (SVC). This is because at a low voltage limit, the reactive power drops
off as the square of the voltage for the SVC, where Mvar=f(BV2), but drops off linearly with the
STATCOM, where Mvar=f(VI). This makes the reactive power controllability of the STATCOM
superior to that of the SVC, particularly during times of system distress.

Application of the Three-Phase STATCOM in Voltage Stability
Voltage stability is one of the biggest problems in power systems. Engineers and
researchers have met with the purpose of discussing and trying to consolidate a definition
regarding to voltage stability, besides proposing techniques and methodologies for their analysis.
Most of these techniques are based on the search of the point in which the system’s Jacobian
becomes singular; this point is referred as the point of voltage collapse or maximum load ability
point. The series and shunt compensation are able to increase the maximum transfer capabilities
of power network .Concerning to voltage stability, such compensation has the purpose of
injecting reactive power to maintain the voltage magnitude in the nodes close to the nominal
values, besides, to reduce line currents and therefore the total system losses. At the present time,
thanks to the development in the power electronics devices, the voltage magnitude in some node
of the system can be adjusted through sophisticated and versatile devices named FACTS. One of
them is the static synchronous compensator (STATCOM).

USES
Usually a STATCOM is installed to support electricity networks that have a poor power
factor and often poor voltage regulation. There are however, other uses, the most common use is
for voltage stability. A STATCOM is a voltage source converter (VSC)-based device, with the
voltage source behind a reactor. The voltage source is created from a DC capacitor and therefore
a STATCOM has very little active power capability. However, its active power capability can be
increased if a suitable energy storage device is connected across the DC capacitor. The reactive
power at the terminals of the STATCOM depends on the amplitude of the voltage source. For
example, if the terminal voltage of the VSC is higher than the AC voltage at the point of
connection, the STATCOM generates reactive current; on the other hand, when the amplitude of
the voltage source is lower than the AC voltage, it absorbs reactive power. The response time of
a STATCOM is shorter than that of an SVC, mainly due to the fast switching times provided by
the IGBTs of the voltage source converter. The STATCOM also provides better reactive power
support at low AC voltages than an SVC, since the reactive power from a STATCOM decreases
linearly with the AC voltage (as the current can be maintained at the rated value even down to
low AC voltage).

Other Compensators
A static VAR compensator (SVC) can also be used for voltage stability. However, a
STATCOM has better characteristics than a SVC. When the system voltage drops sufficiently to
force the STATCOM output current to its ceiling, its maximum reactive output current will not
be affected by the voltage magnitude. Therefore, it exhibits constant current characteristics when
the voltage is low under the limit. In contrast the SVC's reactive output is proportional to the
square of the voltage magnitude. This makes the provided reactive power decrease rapidly when
voltage decreases, thus reducing its stability. In addition, the speed of response of a STATCOM
is faster than that of an SVC and the harmonic emission is lower. On the other hand STATCOMs
typically exhibit higher losses and may be more expensive than SVCs, so the (older) SVC
technology is still widespread.

Statcom

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