MODULE 2

PRINCIPLE OF OPERATION, CONFIGURATION AND CONTROL CHARACTERISTICS OF STATIC VAR COMPENSATOR(SVC)- TCR TSC AND FC +TCR

COMPENSATORS BASED ON TCR

TCR:

A shunt-connected, thyristor-controlled inductor whose effective reactance is varied in a continuous manner by partial-conduction control of the thyristor value.

An elementary single-phase thyristor-controlled reactor (TCR) is shown in Fig The current in the reactor can be controlled from maximum to zero by the method of firing delay angle control. That is the duration of the current conduction intervals is controlled by delaying the closure of the thyristor valve with respect to the peak of the applied voltage in each half-cycle For firing angle = 0◦ the amplitude is at its maximum and for firing angle = 90◦ the amplitude is zero and no current is flowing during the corresponding half-cycle. Like this the same effect is provided as with an inductance of changing value.

Thyristor-Switched Capacitor (TSC)

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

In Fig single-phase thyristor-switched capacitor (TSC) is shown. The TSC branch can be switched out at a zero crossing of the current. At this time instance the capacitor value has reached its peak value. The disconnected capacitor ideally stays charged at this peak value and the voltage across the nonconducting thyristor varies in phase with the applied ac voltage. Normally, the voltage across the capacitor does not remain constant during the time when the thyristor is switched out, but it is discharged after disconnection. To minimize transient disturbances when switching the TSC on, the reconnection has to take place at an instance where the AC voltage and the voltage across the conductor are equal, that is when the voltage across the thyristor valve is zero. However, there will still be transients caused by the nonzero duS/dt at the instant of switching, which, without the reactor, would result an instant current in the capacitor The interaction between the capacitor and the current limiting reactor produces oscillatory transients on current and voltage. From these elaborations it follows that firing delay angle control is not applicable to capacitors; the capacitor switching must take place at that specific instant in each cycle at which the conditions for minimum transients are satisfied. For this reason, a TSC branch can provide only a step-like change in the reactive current it draws (maximum or zero). Thus, the TSC is a single capacitive admittance which is either connected to or disconnected from the AC system. The current through the capacitor varies with the applied voltage. To approximate continuous current variations, several TSC branches in parallel may be used.

Static Var compensator (SVC)

SVC is a shunt connected static Var generator/load whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific power system variable. Typically, the power system control variable is the terminal bus voltage. Static Var Compensator (SVC): A shunt-connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system (typically bus voltage). SVC is an umbrella term for several devices. The SVC devices discussed in the following sections are the TCR, TSR and TSC.

There are two popular configurations of SVC.

· fixed capacitor (FC) and thyristor controlled reactor (TCR) configuration and

· thyristor switched capacitor (TSC) and TCR configuration.

In the limit of minimum or maximum susceptance, SVC behaves like a fixed capacitor or an inductor. Choosing appropriate size is one of the important issues in SVC applications in voltage stability enhancement.

•Static var compensators (SVCs) are shunt-connected static generators and/or absorbers whose outputs are varied so as tocontrol specific parameters of the electric power system.

Static var systems are applied by utilities in transmission applications for several purposes. The primary purpose is usually for rapid control of voltage at weak points in a network. Installations may be at the midpoint of transmission interconnections or at the line ends. Static Var Compensators are shunt connected static generators / absorbers whose outputs are varied so as to control voltage of the electric power systems. In its simple form, SVC is connected as Fixed Capacitor-Thyristor Controlled Reactor (FC-TCR) configuration as shown in Fig. 1. The SVC is connected to a coupling transformer that is connected directly to the ac bus whose voltage is to be regulated. The effective reactance of the FC-TCR is varied by firing angle control of the antiparallel thyristors. The firing angle can be controlled through a PI (Proportional + Integral) controller in such a way that the voltage of the bus, where the SVC is connected, is maintained at the reference value.

An SVC is based on thyristor controlled reactors (TCR), thyristor switched capacitors (TSC), and/or Fixed Capacitors (FC) tuned to Filters. A TCR consists of a fixed reactor in series with a bi-directional thyristor valve. TCR reactors are as a rule of air core type, glass fibre insulated, epoxy resin impregnated

A TSC consists of a capacitor bank in series with a bi-directional thyristor valve and a damping reactor which also serves to de-tune the circuit to avoid parallel resonance with the network. The thyristor switch acts to connect or disconnect the capacitor bank for an integral number of half-cycles of the applied voltage. A complete

SVC based on TCR and TSC may be designed in a variety of ways, to satisfy a number of criteria and requirements in its operation in the grid. Two very common design types, both having each their specific merits,

are shown in Fig. 3a and 3b.

In order to investigate the impact of SVC on power systems, appropriate SVC model is very important. In this section, SVC and its mathematical model will be introduced. SVC is built up with reactors and capacitors, controlled by thyristor valves which are in parallel with a fixed capacitor bank. It is connected in shunt with the transmission line through a shunt transformer and thus, represented in Figure 1 [1]. Figure 2 shows the equivalent circuit at which SVC is modeled