###
**What
is SATACOM?**

STATCOM or Static
Synchronous Compensator is a power electronic device using force commutated
devices like IGBT, GTO etc. to control the reactive power flow through a power
network and thereby increasing the stability of power network. STATCOM is a
shunt device i.e. it is connected in shunt with the line. A Static Synchronous
Compensator (STATCOM) is also known as a Static Synchronous Condenser (STATCON).
It is a member of the Flexible AC Transmission System (FACTS) family of
devices.

The terms Synchronous in
STATCOM mean that it can either absorb or generate reactive power in
synchronization with the demand to stabilize the voltage of the power network.

###
**Working
Principle of STATCOM:**

To understand the working
principle of STATCOM, we will first have a look at the reactive power transfer
equation. Let us consider two sources V1 and V2 are connected through an
impedance Z = Ra + jX as shown in figure below.

In the above reactive power
flow equation, angle δ is the angle between V1 and V2. Thus if we maintain
angle δ = 0 then Reactive power flow will become

Q = (V2/X)[V1-V2]

and active power flow will
become

P = V1V2Sinδ / X =0

To summarize, we can say
that if the angle between V1 and V2 is zero, the flow of active power becomes
zero and the flow of reactive power depends on (V1 – V2). Thus for flow of
reactive power there are two possibilities.

1)
If the magnitude of V1 is more than V2, then
reactive power will flow from source V1 to V2.

2)
If the magnitude of V2 is more than V1,
reactive power will flow from source V2 to V1.

This principle is used in
STATCOM for reactive power control. Now we will discuss about the design of
STATCOM for better correlation of working principle and design.

You may like to read Reactive Power and Voltage Control of a Transmission Line

####
**Design
of STATCOM:**

STATCOM has the following
components:

1)
A Voltage Source Converter, VSC

The
voltage-source converter is used to convert the DC input voltage to an AC
output voltage. Two of the common VSC types are as below.

a) Square-wave Inverters using Gate
Turn-Off Thyristors: In this type of VSC, output AC voltage is controlled by
changing the DC capacitor input voltage, as the fundamental component of the
converter output voltage is proportional to the DC voltage.

b)
PWM Inverters using Insulated Gate
Bipolar Transistors (IGBT): It uses Pulse Width Modulation (PWM) technique to
create a sinusoidal waveform from a DC voltage source with a typical chopping
frequency of a few kHz. In contrast to the GTO-based type, the IGBT-based VSC
utilizes a fixed DC voltage and varies its output AC voltage by changing the
modulation index of the PWM modulator.

2)
DC Capacitor

DC
Capacitor is used to supply constant DC voltage to the voltage source
converter, VSC.

3)
Inductive Reactance

A
Transformer is connected between the output of VSC and Power System.
Transformer basically acts as a coupling medium. In addition, Tranformer neutralize
harmonics contained in the square waves produced by VSC.

4)
Harmonic Filter

Harmonic
Filter attenuates the harmonics and other high frequency components due to the VSC.

A simplified diagram along
with equivalent electrical circuit of STATCOM is shown in figure below.

Now we will understand how
the STATCOM works though we have discussed the basic operating principle of
STATCOM. As can be seen from the figure above, source V1 represents the output
voltage of the STATCOM. In case of reactive power demand increases in the power
system, STATCOM increases its output voltage V1 while maintain the phase
difference between V1 and V2 to zero (it shall be noted here that there will
always exists small phase angle between V1 and V2 to cater for the leakage impedance
drop in the interconnecting Transformer ). As V1 > V2, reactive power will
flow from STATCOM to the power system. Thus STATCOM, supplies reactive power
and acts as reactive power generator.

Again, if the voltage of
power system increase due to load throw off, STATCOM will reduce its output
voltage V1 and therefore will absorb reactive power to stabilize the voltage to
normal value.

The above mode of operation
of STATCOM is called Voltage Regulation Mode.

But as we know every
equipment has got their own limitations, so STATCOM must also have some
limitation of supplying or absorbing reactive power. Yes, there exists a
limitation and this limitation is imposed by the current carrying capacity of force
commutated devices like IGBT, GTO etc. Therefore, if the operation of STATCOM
reaches their limitation, it does not further increases or decreases its output
voltage V1 rather it supplies or absorbs fixed reactive power equal to its
limiting value at a fixed voltage and current and acts like constant current
source. This mode of operation of STATCOM is called VAR Control Mode.

Thus form the above
discussion, the operation of STATCOM can be classified into two modes:

1)
Voltage Regulation Mode

2)
VAR Control Mode

The figure below well
explains the above two modes of operation of STATCOM.

The figure above is the
Voltage Current Characteristics of STATCOM. As can be seen, voltage regulation
capability of STATCOM is from V1 (in lower side) to V2 in upper side of power
system. If the voltage of power system goes below V1 or above V2, STATCOM acts
in VAR Control mode. Here V1 and V2 are just taken for example, it should not
be confused with the V1 (used for output voltage of STATCOM) and V2 (Voltage of
power system) used in the discussion above.

**Application of STATCOM:**

Voltage stability is one of
the biggest problems in power systems. Engineers and researchers are trying to
consolidate a definition regarding to voltage stability, besides proposing
techniques and methodologies for their analysis.

**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. Today due 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).***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. (we will discuss point of voltage collapse in next post)*
Usually a STATCOM is
installed to support electrical networks that have a poor power factor and
often poor voltage regulation. The most common use of STATCOM 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.

## 1 comment:

good information on STATCOM

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