STATCOM

1.1 INTRODUCTION

Static Synchronous Compensator (STATCOM) is defined as a self commutated switching power

converter supplied from an appropriate electric energy source and operated to produce a set of

adjustable multiphase voltages, which may be coupled to an ac power system for the purpose of

exchanging independently controllable real and reactive power. The STATCOM has been

defined as per CIGRE/IEEE with following three operating structural components. First

component is

Static: based on solid state switching devices with no rotating components; second component is

Synchronous: analogous to an ideal synchronous machine with 3 sinusoidal phase voltages at

fundamental frequency; third component is

Compensator: provided with reactive compensation [17].

(Or)

A static synchronous generator operated as a shunt connected static VAR compensator whose

capacitive or inductive current can be controlled independent of the AC system voltage[18].

Fig. 1.1 (a) Single line diagram of STATCOM

STATCOM is one of the key Converter-based Compensators which are usually based on the

VSI or CSI.

Fig.1.1(b) VSC based Statcom Fig.1.1(c) CSC based Statcom

VSC are often preferred over current sourced converters for FACTS applications.

The STATCOM is composed of a VSC with a dc capacitor, coupling transformer, and signal

generator and control circuit. Due to its versatile nature and speedy response, STATCOM finds a

wide application in the field, both as a reactive power compensating device and harmonic

absorber.

1.2 PRINCIPLE OF OPERATION:

The Basic principle of reactive power generation by a voltage-sourced converter is akin

to that of the conventional rotating synchronous machine shown schematically in fig.1.2 (a). For

purely reactive power flow, the three phase induced EMFs Ea , Eb and Ec of the synchronous

rotating machine are in phase with the system voltages Va , Vb and Vc . The reactive current

I drawn by the synchronous compensator is determined by the magnitude of the system voltage

V,

that of the internal voltage E, and the total circuit reactance (synchronous machine

reactance plus transformer leakage reactance plus system short-circuit reactance) X.

Fig. 1.2 (a) Synchronous Condenser

(1-1)

The corresponding reactive power Q exchanged can be expressed as follows:

(1-2)

The STATCOM is basically a DC-AC voltage source converter with an energy storage unit,

usually a DC capacitor. It operates as a controlled SVS connected to the line through a coupling

transformer. Fig.1.2 (b) shows the schematic configuration of STATCOM. The controlled output

voltage is maintained in phase with the line voltage, and can be controlled to draw either

capacitive or inductive current from the line in a similar manner of a synchronous condenser, but

much more rapidly. STATCOM is a primary shunt device of the FACTS family, which uses

power electronics to control power flow and improve transient stability on power grids.

The STATCOM regulates voltage at its terminals by controlling the amount of reactive

Power injected into or absorbed from the power system. The variation of reactive power is

performed by means of a VSC connected on the secondary side of a coupling transformer. The

VSC uses forced commuted power electronics devices (GTO’s or IGBT’s) to synthesize the

voltage from a dc voltage source. The operating principle of STATCOM is explained in Fig.1.2

(b) .For computation purposes, we assume that the active and reactive power is transferred

between two sources V1 and V2, where V1 represents the system voltage to be controlled and

V2 is the voltage generated by the VSC. In steady state operation, the voltage V2 generated by

the VSC is in phase with V1 (∂=0, angle of V1 with respect to V2) so that only the reactive

power is flowing from V1 to V2; i.e. STATCOM is observing reactive power. It can be seen that

if V2 > V1 then current Iq flows from the counter to ac system through reactance and

converter generates capacitive reactive power for ac system. On the other hand, if V2 < V1 then

current Iq flows from ac system to the converter and converter absorbs inductive reactive power

from ac system.

The amount of reactive power is given by:

A capacitor connected on the DC side of the VSC acts as a dc voltage source.

1.3 V-I AND V-Q CHARACTERISTICS:

(1-3)

The STATCOM is essentially an alternating voltage source behind a coupling reactance with the

corresponding V-I and V-Q characteristics shown in fig.1.3 (a) and fig.1.3 (b). These show that

STATCOM can be operated over its full output current range even at very low (theoretically

zero), typically about 0.2 p.u. system voltage levels. In others words, the maximum capacitive or

inductive output current of the STATCOM can be maintained independently of the ac system

voltage, and the maximum VAR generation or absorption changes linearly with the AC system

voltage.

Fig :-1.3(a) V-I Characteristics Fig :-1.3(b) V-Q Characteristics

The STATCOM provides voltage support under large system disturbances during which the

voltage excursions would be well outside of the linear operating range of the compensator.

The capability of providing maximum compensating current at reduced system voltage

enables the Statcom to perform in variety of applications.

The VI and VQ characteristics illustrate, the Statcom may depending on the power

semiconductors used have increased transient rating in both inductive and capacitive

regions. The maximum attainable transient over current of the Statcom in the capacitive region

is determined by the maximum current turn of capability of the power semiconductors (GTO)

employed. The transient current rating of the Statcom in the inductive range is theoretically

limited only by the maximum permissible e GTO junction temperature, which would in

principle allow the realization of a higher transient rating in this range than that attainable in

the capacitive range. However, it could be pointed out that this possibility would generally not

exist if the converter poles were operated to produce a pulse width modulated waveform when

the current conduction between the upper and lower values is transferred several times during

each fundamental half cycle. Even with the non-PWM converters abnormal operating

conditions should be carefully considered in the implementation of transient ratings above the

peak turn off current capability of the semiconductors employed, because if an expected

natural commutation would be missed for any reason converter failure requiring a forced

shut down would likely occur.

APPLICATIONS OF STATCOM

2.1 APPLICATIONS OF STATCOM

The STATCOM has the following applications in controlling power system dynamics.

1. Damping of power system oscillations

2. Damping of subsynchronous oscillations

3. Balanced loading of individual phases

4. Reactive compensation of AC-DC converters and HVDC links

5. Improvement of transient stability margin

6. Improvement of steady-state power transfer capacity

7. Reduction of temporary over-voltages

8. Effective voltages regulation and control

9. Reduction of rapid voltages fluctuations (flicker control)[4].

TYPES OF CONTROL OF STATCOM

3.2 Open loop characteristics of STATCOM Model

Having the linearized system state space model as equation (3.9), we can use the linear

system method to analyze the characteristics of STATCOM model. There are several open loop

system properties we need to know before we design the controller.

Although system observability and controllability are the properties of the system presentation,

these are two important criterions that must be established before any attempt in controller

design is done. These two characteristics of a system depend on the state space presentations of

the system. For a given system, different state space presentations have different effects on the

controller design algorithm.

Another important property of a system is its open loop dynamic response characteristic that

gives us not only the background information about the system performance but also the

guideline for controller design.

1. Obsevability and Controllability

The state space model of STATCOM has three state variables - i, , V. All of them can be measured

from the power system, which means the system is observable.