Statcom Documentation
-
Upload
balu33babu -
Category
Documents
-
view
81 -
download
0
description
Transcript of Statcom Documentation
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.