CAPACITOR INRUSH/ OUTRUSH REACTORS BASIC AND TUTORIALS

CAPACITOR INRUSH/ OUTRUSH REACTORS BASIC INFORMATION

What Are Capacitor Inrush/ Outrush Reactors?

Capacitor switching can cause significant transients at both the switched capacitor and remote locations.

The most common transients are:

• Overvoltage on the switched capacitor during energization

• Voltage magnification at lower-voltage capacitors

• Transformer phase-to-phase overvoltages at line termination

• Inrush current from another capacitor during back-to-back switching

• Current outrush from a capacitor into a nearby fault

• Dynamic overvoltage when switching a capacitor and transformer simultaneously

Capacitor inrush/outrush reactors (Figure 2.9.15) are used to reduce the severity of some of the transients listed above in order to minimize dielectric stresses on breakers, capacitors, transformers, surge arresters, and associated station electrical equipment.

High-frequency-transient interference in nearby control and communication equipment is also reduced. Reactors are effective in reducing all transients associated with capacitor switching, since they limit the magnitude of the transient current (Equation 2.9.5), in kA, and significantly reduce the transient frequency (Equation 2.9.6), in Hz.

where

Ceq = equivalent capacitance of the circuit, F

Leq = equivalent inductance of the circuit, H

VLL = system line-to-line voltage, kV

Therefore, reflecting the information presented in the preceding discussion, IEEE Std. 1036-1992, Guide for Application of Shunt Power Capacitors, calls for the installation of reactors in series with each capacitor bank, especially when switching back-to-back capacitor banks.

Figure 2.9.16 shows a typical EHV shunt-capacitor installation utilizing reactors rated at 550 kV/1550 kV BIL, 600 A, and 3.0 mH

550-kV capacitor inrush/outrush reactors.

CONSTANT VOLTAGE TRANSFORMER BASICS AND TUTORIALS

CONSTANT VOLTAGE TRANSFORMER BASIC INFORMATION

What Is a Constant-Voltage Transformer?

A well-known solution for electrical “noise” in industrial plants has been the constant-voltage transformer, or CVT.

The typical components of a CVT are shown in Figure 2.8.2. The magnetic shunt on the central core has the following effects on the core’s reluctance. It reduces the reluctance of the core.

 This can be thought of as introducing more resistance in parallel to an existing resistance. The magnetic shunt in the CVT design allows the portion of the core below the magnetic shunt to become saturated while the upper portion of the core remains unsaturated.

This condition occurs because of the presence of the air-gap between the magnetic shunt and the core limbs. Air has a much higher reluctance than the iron core.

Therefore, most of the flux passes through the lower portion of the core, as shown by the thick lines in Figure 2.8.2.

In terms of an electrical analogy, this configuration can be thought of as two resistances of unequal values in parallel.

The smaller resistance carries the larger current, and the larger resistance carries the smaller current.

The CVT is designed such that:

• The lower portion of the central limb is saturated under normal operating conditions, and the secondary and the resonating windings operate in the nonlinear portion of the flux-current curve.

• Because of saturation in the central limb, the voltage in the secondary winding is not linearly related to the voltage in the primary winding.

There is consonance between the resonating winding on the saturated core and the capacitor. This arrangement acts as a tank circuit, drawing power from the primary. This results in sustained, regulated

oscillations at the secondary with the applied line frequency.