FAQ: HIGH VOLTAGE SYSTEM

LIST OF FREQUENTLY ASKED QUESTION ON HIGH VOLTAGE SYSTEM BY ELECTRICAL SURVEYOR

Hey,

I'm sharing few questions asked by electrical surveyor on High voltage systems during ETO oral examination.

Q. Why are Neutral Ground Resistor's (NER) rated for 10sec?
A. NGR are placed in the neutral circuit & hence will energized only in fault conditions thus their continuos loading is not expected. Hence they are selected for intermittent rating. Similarly when we place a transformer in neutral grounding circuit the KVA rating obtained after calculation is multiplied by diversity factor to obtain smaller rating cause the therefore it will not be continuosly rated.

Q. What is the acceptable minimum limit for insulation resistance on HV system?
A. 1MegaOhm per KV plus one test result should be compared with previous readings and readings taken for similar equipment. Any values below the NETA(National Electrical Testing Association) standards minimums or sudden departures from previous values should be investigated.

Q. How do you use a HV testing probe to prove a circuit is isolated?
A. HV probe is used to prove dead after disconnection and isolation, the tester must be tried out before and after such test by checking it with proven supply. Testing has a long insulated handle with a HV indicator , Finger guard and handle, and has a brass tip.

Q. Why Humming sound in occurs in HV transmission lines?
A. The HT lines causes humming because of corona effect. When the voltage of transmission lines increases too much to produce electric field required to break the inter molecular bond of the air molecules. Hence the air particle around the lines get ionized and start moving freely and during this movement several collisions occur and resulting a humming sound.

Q. What is the pressure of gas in SF6 circuit breaker?
A. 

In the closed position of the breaker, the contacts remain surrounded by sulphur hexafluoride gas (SF6) gas at a pressure of about 2.8 kg/cm2.The movement of the moving contact is synchronized with the opening of a valve. The valve permits sulphur hexafluoride gas (SF6) gas at 14 kg/cm2 pressure from the reservoir to the arc interruption chamber. It varies with different makers.

Q. Type of cables used in HV system?
A. XLPE(Cross-Linked PolyEthylene) is suitable for voltage ranges from low to extra high voltage.