Voltage and Reactive Power Compensation

 

Static VAR Compensator | Source: GE Grid Solution

The voltage drop in an A.C. electric power supply system, caused by problem loads which are large compared with the short circuit level of the system, is mainly due to the reactive component of the load flowing through the system reactance. The variations in loads can cause voltage fluctuations and consequent objectionable or irritating light flickers. 

These troublesome loads sometimes produce harmonic currents, which are large enough to cause distortion problems to other consumers whose electricity supply is provided from the same busbar or at the point of common coupling.

To provide reactive VAr control in order to support the power supply system voltage and to filter the harmonic currents in accordance with Electricity Authority recommendations, which prescribe the permissible voltage fluctuations and harmonic distortions, reactive power (VAr) compensators are required.

These compensators can be grouped into two major groups these are synchronous compensators (condensers) and static VAr compensators. The static VAr compensators have no moving parts. The speed of response of the synchronous compensator is low and the cost is high when compared with the static VAr compensators and hence the latter is the preferred solution.

Static VAR Compensators

Static var compensator (SVC) is a shunt connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current to maintain or control specific parameters of the electrical power system (typically bus voltage). 

At least four different types of static Var compensator (SVC) are available. These are the following: 

1. saturated reactor type compensators
2. thyristor controlled reactor compensator
3. thyristor switched capacitor compensator
4. STATCOM (Static Compensator)

Unlike a synchronous machine the saturated reactor has no rotating parts, no inertia and inherently remains in synchronism with the supply. A saturated reactor can only absorb reactive power. It does not need any external control to force it to absorb reactive power. 

It does so by the nature of the saturation feature of the magnetising characteristic of its core iron as it operates normally in saturated flux region. The saturated reactor is inherent in its response and the speed of response is fast. The reactive power required for compensation is generated by parallel connected shunt capacitance (often in the form of tuned or damped harmonic filters). The order of harmonic filters depends primarily on the harmonic (number) currents generated by the troublesome loads. 

Related Article: Substation Control

Thyristor Controlled Reactor Compensator

Thyristor controlled reactor (TCR) is a shunt-connected thyristor-controlled inductor whose effective reactance is varied in a continuous manner by partial conduction of the thyristor valve. The thyristor-controlled reactor comprises a linear reactor, connected in series with a ‘thyristor valve’ made up of inverse-parallel (back-to-back) connected pairs of high power, high voltage thyristors. 

Thyristor Controlled Reactor

Unlike saturated reactor (which is inherent, and does not need any external control), the variation of current in the linear VAr reactor (for VAr absorption) is obtained by control of the thyristor conduction duration in each half-cycle. The firing angle delay is 90 degrees as measured from the applied voltage zero for full conduction, and can be varied up to 180 degrees delays for no conduction.

Thyristor Switched Capacitor

A thyristor switched capacitor (TSC) is a shunt-connected thyristor switched capacitor whose effective reactance is varied in a stepwise manner by full or zero conduction operation of the thyristor valve.

Thyristor Controlled Capacitor

Thyristor valves consisting of inverse-parallel connected thyristors, generally similar to those used for the TCR, are used to give rapid switching of blocks of capacitors. The capacitors are switched in block only, i.e. step-by-step, as they cannot be firing angle controlled, unlike the TCR, as excessive capacitor inrush currents would result. After the capacitor current through the thyristor ceases at current zero, unless re-gating occurs, the capacitors remain charged at peak voltage while the supply voltage peaks in the opposite polarity after a half cycle.

Static VAR Compensator

The general arrangement of the Static VAr Compensator is shown on the next page. It consists of thyristor-controlled reactors (TCR) in parallel with thyristor switched capacitors (TSC). The reactive types of equipment of the compensator are connected to the transmission line, through a transformer to prevent the equipment from having to withstand full system voltage. A control system determines the exact gating instants of reactors according to a predetermined strategy. The strategy usually aims to maintain the transmission line voltage at a fixed level.

Static VAR Compensator | Source: GE Grid Solutions

Static VAr compensators are used to help power transmission over long A.C. transmission lines by injecting reactive power at points down the line to maintain voltage levels. 

StatCom (Static Compensators)

A static synchronous generator operated as a shunt-connected static VAr compensator (SVC) whose capacitive or inductive output current can be controlled independently of the A.C. system voltage. STATCOM is based on a voltage source converter. STATCOM has superior dynamic reactive power compensation ability and a wider operating voltage range, than a normal SVC. The phases of the STATCOM are independently controlled during system disturbances. 

General Diagram of STATCOM

Source:

• Substation Design Application guide | pp. 63-66
• Author: V. Ayudarai
• Download Here