& Transient Voltage Recovery Capacitors Grading Capacitors & Transient Voltage Recovery Capacitors

What Are Grading Capacitors Type are high voltage capacitors installed on circuit breakers by two methods Line to ground Across multi-break contacts They may be installed internally or externally

What Do They Do? Grading Capacitors are used in combination with High Voltage Circuit Breakers with different purposes. Single break Circuit Breakers: Grading Capacitors can be used to increase the switching capacity of the breaker

Capacitors Connected Line-Gnd. Externally Dead Tank Capacitors Connected Line-Gnd. Externally Breaker Contact Capacitor Ground Connection

Dead Tank Capacitors Connected Line-Gnd. Internally

Multi-break Circuit Breakers Installed across the contacts Grading Capacitors ensure uniform voltage distribution across all contact points during normal and switching system operation 

Live Tank Capacitors Connected Across Contats Internally

Capacitor Specifications 90 up to 300 kV High customization range to match CB mechanical and electrical parameters Sealed construction, maintenance free High mechanical resistance to shock and vibration due CB‘s switching operation Available in compliance with all international standards

Issues With Grading Capacitors Background: A number of disturbing incidents (violent failures, comprehensive leak problems, etc.) in recent years Little qualitative knowledge concerning the stresses grading capacitors are subjected to in service exists No international standards for testing grading capacitors are available

Leak is the most common problem type Oil leaking out (both AIS and GIS) SF6leaking in (in GIS) Moisture ingress (in AIS)

Pre-Insertion Resistors PIR is pre-insertion-resistor. This is a resistor of about 200-400 ohms which gets temporarily closed before closing the circuit breaker. The sequence is (close order)->closing of PIR->10-12milliseconds->closing of main break.

But while opening, PIR is first disconnected by the Breaker operating mechanism and then i.e. after 10 ms, main contacts of Breaker are opened. The main purpose of PIR is to limit the initial charging current of line. As the charging current of long line will be much more, it is advisable to use Breaker having PIR in long transmission line.

When electrical circuit having capacitance or transformer which can have inrush. Also transformers may have large inrush current. Transmission line (due to its capacitance) is another such circuit. For closing in such circuit first the PIR comes in circuit to limit the inrush current. When the main contacts close, the current through PIR becomes zero as the shunt path is bridged by main contact.