Field Testing of Vacuum Circuit Breakers

Introduction

Vacuum circuit breaker uses vacuum as arc quenching medium inside a vacuum bottle. Vacuum circuit breakers are a dependable current interruption component suitable for medium voltage switch gear. It can function seamlessly with minimum maintenance and is vital for reliable and safe power. They are exceptionally durable and are appropriate for fault clearing. Vacuum circuit breakers are used in porcelain housed circuit breakers and metal-clad switch gear.

Circuit breakers are tested on annual basis or after major repair work of the breaker. Following tests are generally conducted in field as routine test -

1. Insulation Resistance Measurement

2. Vacuum Bottle Integrity Check

3. Contact Resistance measurement

4. Breaker operation time measurement

5. Checking of Close Coil / Trip Coil / Spring Charging motor

6. Mechanical Checks

Lets dive into each of the tests in details

Insulation Resistance Measurement

Insulation measurement test is most basic test generally carried out on all sorts of electrical equipment's.

As shown in figure below, Insulation resistance is measured in between poles, across poles, and pole to earth. Since vacuum is involved, IR value between poles and Across poles is generally in "TΩ" or at least in "GΩ". IR is measured between poles and pole to earth with breaker in close condition.

Vacuum Bottle Integrity Check

Vacuum bottle integrity is tested at site with the help of High voltage test set. With breaker in open condition. Voltage is applied to one end of the pole with other end connected to return of the test set. Voltage is selected based on last test voltage applied and generally kept 80% of the last test voltage.

In case AC test set is not available, DC voltage of equivalent magnitude, i.e. peak value of AC Test voltage, is applied.

For 6.6kV AC rated breakers, 6.6kV x 2 + 1kV = 14.2kV AC voltage (or 14.2kV x 1.414 = 20kV DC voltage) is applied during commissioning phase. For second such test, 14.2kV x 0.8 = 11.36kV AC test voltage (or 16kV DC Test voltage) can be applied. Afterwards, test are conducted at constant voltage of 10kV AC (or 14kV DC) voltage since there is no meaning of testing below this voltage.

VIDAR Test set is one such kit for testing Vacuum Bottles manufactured by Megger. Voltage selections are available for DC Test voltages on test set. Its very easy and safe handling such test sets. Direct indication of pass or fail are provided for convenient identification of vacuum bottles integrity.

Contact Resistance Measurement

To assess the condition of the breaker contacts, the power contact resistance measurement is usually performed. Contact resistance is measured of each phase with the help of Ohm's law. High magnitude DC current is passed through contact (generally in the range of 100Amp DC). Voltage drop across terminals is measured, which in turn gives resistance through simple V/I formula.

Breaker operation time measurement

Following operating times can be tested on vacuum circuit breakers.

Closing Time (C) : It is duration from initiating close command till breaker contacts closed.
Open Time (O) : Duration from initiating trip command till breaker contact opened and arc is quenched.
Close - Open Time (CO) : C-O time (also called dwell time) is the amount of time the breaker is in the closed position during a close-open operation.

Generally, these timings are observed as follows based on experience of 6.6kV VCB

Close Time - 2 Cycles

Open Time - 1 Cycle

C-O Time - 1.5 cycles

Misc Electrical Checks

Breaker close and trip coil resistance, spring charging motor winding resistance is measured for ensuring healthiness.

Close Coil Resistance - _______ kΩ

Open Coil Resistance - _______ kΩ

Spring Charging Motor Winding Resistance - ________ kΩ

Mechanical Checks

Under mechanical Checks, following points are covered

A. Breaker tripping when racking in or out in close condition

B. Breaker mechanical closing and tripping operations

C. Manual Spring charging

D. Support insulator inspection

E. Operation of auxiliary contact mechanism

All above tests are carried out at field as routine test at least once a year. Same can be done after repairing of breakers and replacement of major spare.

For VCB Test report format, click here.

That's all for this article. See you in next one . . . .