QnA: HIGH VOLTAGE SYSTEM
Hello, my dear users, METO shares a list of FAQ with answers about High Voltage Safety and Switchgear. It might help you while giving the exam, working onboard.
Q. Why HV system alternator neutral is required to be earthed but not in Low voltage alternator(440V) or Simply why Earthed Neutral System in HV system?
A. In all electrical equipment is tested by a test voltage which is twice the system voltage plus one. For Example: If system voltage is 6.6KV the test voltage will be 2 X 6.6 + 1 in KV, i.e., 14.2KV approximately say 15KV.
If there is an Inductive or Capacitive fault the transient surge voltage occurs which is 4 times system voltage to earth. So for 6.6KV system surge voltage becomes 4 X 6.6KV = 26.4KV if neutral is insulated.
In order to Restrict the transient surge voltage arising out of Inductive or Capacitive fault to earth and to keep within test voltage the neutral of the system has to be earthed in order to keep insulation of the winding healthy. The earthing is done through NER so that the transient surge voltage becomes 26.4KV ÷ √3 (26.4K divided by square root of 3) = 15KV which is within the test voltage range.
Q. What must be the value of NER/NGR?
A. For Shore :
Earth fault current not to exceed Full load current. Therefore Value of NER is calculated by R = V ÷ (√3 x I ) say for 6.6KV system with full load current(I) of 500A, then the value of NER will be 7.6Ω(Ohms) approximately 8Ω. Its not continuos rated type.
For HV Ship :
Earth fault current will be in several amperes. Say 400Amps full load fault current flowing through ships hull is not safe can cause spark and major fire accidents, injury. Single earth fault means it damages so NER is placed in series with a neutral earth line whose typical value is 1 Ohm per Volt (1Ω/V) to restrict the earth fault current from 0 to 5 Amps only. For example, For a 6.6KV system 6.6KΩ NER must be used. A special type of NER of continuos rated type must be used else fault current will overheat NER so cooling arrangements are also provided.
Q. Type of Megger used in HV insulation test?
A. Motorized megger. It is recommended to carry out PI(Polarization Index) or DAR(Dielectric Absorption Ratio) test in HV system so battery-operated megger fluctuates giving erratic readings if operated for long time duration because voltage not steady hence motorized megger is preferable.
Q. What is Polarization Index test?
A. The polarization index is the ratio of megger values taken for 10 minutes (R10) to the megger value taken for 1 minute (R1). It is always desired to have a polarization index of an electrical insulator more than 2. Its hazardous to have a polarization index less than 1.5. Formula : PI = R10 / R1
Q. What is Dielectric Absorption Ratio test ?
A. A dielectric absorption ratio is the ratio of two time-resistances readings. For example, a 60-second reading divided by a 30-second reading, A Megger insulation tester makes a dielectric absorption ratio test easier and is known for giving the best results. For example, a Megger insulation tester can run for 60 seconds; however, it allows you to record readings at the 30-second interval and at the 60-second interval. Formula :
Q. What is procedure for racking out Generator VCB?
A. Once Generator is shut off fully i.e., Air, Fuel valves shut, Operation mode set to manual. Follow the procedure given above:-
- Turn the knob in MSBD to De-excitation
- Put the Lock-Out relay to trip position: this is a safety relay has interlock for operation of equipment.
- Control mode is set to Local
- Using HV line detector(hot stick) test on both incoming and outgoing side of VCB to ensure that circuit is dead at one side and live at other side since busbar is powered by other generators.
- Now open VCB toolbox, Rack out VCB.
- Once racked out lock VCB using locking lever so that it ll not move from its position.
- Operate CME(circuit main earth) for earthing down residual voltage. Once operated GROUND ON light lits up.
- Lock CME lever using KEY provided and keep the Key safe.
- Check again with HV detector (hot stick) id found earth it using earth stick.
- Place play cards and prepare PTW(permit to work). Racking in procedure is descending of this.
Stringent safety precautions must be taken in order to work in HV system. Proper PPE must be used such as HV gloves, Arc Flash suites, etc.
Q. Why key is used in high voltage equipment maintenance?
A. Key is used lock CME (Circuit Main Earth) lever to avoid unnecessary operation of equipment during maintenance. Key is kept in Key safe panel. Once CME is locked it will not allow the VCB to rack-in and rack-out.
Q. What is the acceptable minimum limit for insulation resistance on HV system?
A. 1 MegaOhm 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 an 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 cause humming because of the corona effect. When the voltage of transmission lines increases too much to produce the electric field required to break the intermolecular bond of the air molecules. Hence the air particle around the lines gets ionized and starts moving freely and during this movement, several collisions occur and resulting in 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 sulfur hexafluoride gas (SF6) gas at a pressure of about 5 bars. Pressure varies from 2-14 bars depending upon the type of SF6 breaker.
Q. Why SF6 circuit breaker is not used onboard?
A. SF6 is colorless, odorless, non-toxic and inert gas. If it leaks it settles at the bottom level may cause a health risk, if used must be kept in separate compartment with all necessary safety equipment. VCB has minimal health hazard or no hazard can be used up to the voltage of 33KV, onboard HV ship voltage is within this range so VCB is preferable.
Q. Type of insulation used in HV system?
A. XLPE(Cross-Linked Polyethylene) is suitable for voltage ranges from low to extra-high voltage.
Q. What is Earthing down in HV system?
A. Earthing down ensures that isolated equipment remains safe. There are two types earthing down in a high voltage switchboard.
- Circuit Earthing: an incoming feeder cable is connected by a heavy earth connection from earth to all three conductors. This is done at circuit breaker using special key. This key is then locked in the key safe. The circuit breaker cannot be racked in until the circuit earth has been removed.
- Busbar Earthing: When necessary to work on the busbar they must be completely isolated from all possible electrical sources. This will include generator incoming cables, section or bus-tie breakers, and transformer section. The busbar are connected together and earthed down using portable leads which gives visible confirmation of the earthing arrangement.
Q. Why are Neutral Ground Resistor (NER) rated for 10sec?
A. NGR is placed in the neutral circuit and hence will be energized only in fault conditions thus their continuous loading is not expected. Hence they are selected for intermittent rating. Similarly, when we place a transformer in a neutral grounding circuit the KVA rating obtained after the calculation is multiplied by a diversity factor to obtain a smaller rating cause the, therefore, it will not be continuously rated.
Q. What are the disadvantages of high voltage system?
A. A high voltage system requires a higher degree of insulation for its control devices, transmission systems as well as motors and generators.
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