Capacitors and Condensors

Capacitors and Condensors

Capacitors – Advances in Series-Compensated Line Protection Series capacitors are used to compensate the inductance of transmission line. Series capac...

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Capacitors – Advances in

Series-Compensated Line Protection

Series capacitors are used to compensate the inductance of transmission line. Series capacitorswill increase the transmission capacity and the stability of the line. Series capacitors are also used to share the load between parallel lines. Depending on the size of the bank, each phase consists of one or two segments.

 

Series capacitors require spark gaps or metal oxide varistors (MOVs) to reduce or eliminate over voltages across the capacitors. Spark gaps flash over to remove the capacitor when the voltage exceeds a given value, but they may not fire for low-current faults. capacitor still in operation.

 

Conduct transient studies and tests to ensure secure and dependable operation of relays in series compensated line applications.

 

The mho element with the dashed-line characteristic overreaches for this condition. The traditional solution is to set the mho element to cover 90 percent of the line when the capacitor is in service.

 

Series capacitors are used to compensate the inductance of transmission line. Series capacitorswill increase the transmission capacity and the stability of the line. Series capacitors are also used to share the load between parallel lines. Depending on the size of the bank, each phase consists of one or two segments.

After an overview of transmission line series compensation, we review series-compensated line protection challenges, that include voltage inversion, current inversion, and distance estimation errors. We then present modern solutions to improve directional, distance, and differential element operation on series-compensated lines. Later we provide relay setting guidelines. Finally, we present and discuss several cases of protection scheme operation for actual faults.

Transmission line series compensation increases power transfer capability and improves power system stability. However, series compensation increases the fault current level and may also cause generator sub synchronous resonance. The capacitive reactance XC is typically

from 25 percent to 75 percent of the line inductive reactance XL.

The authors have also seen lines compensated to 100 percent. Series capacitors may be installed at one or both line ends. Line ends are typical capacitor locations, because it is generally possible to use space available in the substation. In turn, this reduces installation cost. Another possibility is to install the series capacitors at some central location on the line. Series capacitors located at the line ends create more complex protection problems than those installed at the center of the line.

Series capacitors require spark gaps or metal oxide varistors (MOVs) to reduce or

eliminate over voltages across the capacitors. Spark gaps flash over to remove the capacitor when the voltage exceeds a given value, but they may not fire for low-current faults.

Thus, the line protection scheme must also perform properly with the series capacitor still in operation.

Series-compensated lines present unique challenges for directional, distance, and differential elements because the transient response of the series capacitor is not readily predictable. Conduct transient studies and tests to ensure secure and dependable operation of relays in series compensated line applications. This paper reviews series-compensated line protection challenges and presents modern solutions to improve directional, distance, and differential element operation on series-compensated lines.

The paper also provides relay setting recommendations and presents several cases of protection scheme operation for actual faults. II. SERIES-COMPENSATED LINE PROTECTION CHALLENGES The line reactance change and the sub harmonic-frequency oscillations caused by the series capacitors may affect line protective relays.

Series capacitors can also generate high frequency transients. The analog and digital filters in microprocessor-based relays attenuate all high-frequency components [1]. Therefore, high-frequency transients have very little effect on most modern relays. A. Voltage Inversion Affects Directional Discrimination A voltage inversion is a change of 180 degrees in the voltage phase angle. For elements responding to phase quantities, voltage inversion can occur for a fault near a series capacitor if the impedance from the relay to the fault is capacitive rather than inductive. Voltage inversion may affect directional and distance elements.

Series Capacitors Affect Distance Estimation

Series compensation introduces errors in the impedance that distance elements estimate. The series capacitor modifies the line impedance that the relay measures. Furthermore, sub harmonic-frequency oscillations cause the impedance estimation to oscillate. The basic problem is that the impedance estimation depends on the state of the capacitor protection.  For high current faults, the capacitor spark gap flashes and removes the capacitor from service. The relay measures the correct line impedance for a line-end fault . The dashed circle is the characteristic of a Zone 1 mho element set to cover 90 percent of the line when the capacitor is out of service. For low-current faults, the spark gap does not flash and the capacitor remains in service. The capacitive reactance modifies the impedance estimate . The mho element with the dashed-line characteristic overreaches for this condition. The traditional solution is to set the mho element to cover 90 percent of the line when the capacitor is in service. However, the Zone 1 instantaneous coverage drops to approximately 50 percent of the line length for high-current faults (capacitor out of service).

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