Thyristorised Static VAR Compensators - Types of Various Static VAR Compensators

Thyristorised Static VAR Compensators:

A fast stepless variable compensation can be provided by thyristorised control of shunt reactors and capacitors. Static VAR System (SVS) acts within a few seconds and provides transient voltage control and improves voltages stability.

Electric loads generate and absorb reactive power since the transmitted load varies considerably form one hour to another the reactive power balance in a grid varies as well.

The result can be unacceptable voltage amplitude variations or even a voltage depression, at the extreme or a voltage collapse.

A rapidly operating Static VAR Compensator (SVC) continuously provide the reactive power required to control dynamic voltage oscillations under various system conditions and there by improve the power system transmission and distribution stability. 

By installing an SVC at one or more suitable joints in the network, we can increase transfer capability and reduce losses while maintaining a smooth.

SVC consists of :

1. Thyristor Switched Capacitor (TSC)

2. Thyristor Switched or Controlled Reactors (TSRTCR)

3. Fixed Capacitor Thyristor Control Reactors (FC-TCR)

4. Thyristor Switched Capacitor or Thyristor Controlled Reactor (TSC-TCR)

Advantages of SVC are :

(1) Reduction of unwanted reactive power flows and therefore reduced network losses.

(2) Keeping of apparent power exchanges with balanced reactive power,

(3) Improvement of power quality especially with huge demand fluctuations like industrial machines, railway, under ground trains system etc.

(4) Compensation of thyristor converters in conventional HVDC lines.

(5) Improvement of static or transient stability,

Types of Various SVCs :

1. Thyristor Switched Capacitor (TSC):

A thyristor switched capacitor scheme consists of a capacitor bank split up into appropriately sized units, each of which is switched on and off using thyristors switches.

Each single phase unit consists of a capacitor (C) in series with a bidirectional thyristors switch and a small inductor (L) as shown in figure. Where S1, S2, S3 are three different thyristor switches. 

The purpose of the inductor is :

1. To limit switching transients

2. To prevent reasonance

3. To damp inrush current

The thyristors firing controls are designed to minimize to switching transients.

2. Thyristor Controlled Reactor (TCR)

A TCR is one of the most important building blocks of thyristor-based SVCs. Although it can be used alone. 

It is more often employed in conjunction with fixed or thyristor switched capacitors to provide rapid continuous control to reactive power over the entire selected lagging to the leading range.

3. Fixed Capacitor Thyristor Controlled Reactor (FC- TCR ):

The FC-TCR is continuously controllable lagging to leading VARS through thyristor control of reactive current. 

The TCR provides continuously controllable reactive power only in the lagging power factor. 

To extend the dynamic controllable range to the leading power factor domain a fixed capacitor bank is connected in shunt with TCR.

The TCR is rated larger than the fixed capacitor to compensate the capacitive MVA and to provide net inductive reactive power. 

The fixed capacitor banks usually connected in a star configuration are split into more than one 3-phase group. Each capacitor contains a small tuning inductor that is connected in series and tunes the branch to act as a filter for a specific harmonics order.

4. Thyristor Switched Capacitor - Thyristor Controlled Reactor (TSC-TCR) :

The TSC-TCR compensator usually comprises in TSC banks and a single TCR that are connected in parallel. The rating of the TCR is chosen to be an input of the total SVC rating. 

The capacitor can be switched in discrete steps whereas continuous control within the reactive power span of each step is provided by the TCR.

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