What is 100%
protection of
generator? Why
95% protection
is used not 100%?
- 9 Answers
- 37388 Views
- NTPC, I also Faced
- E-Mail Answers
Answer Posted / aswin joseph
95% stator earth fault protection uses an overvoltage relay at the neutral transformer of a generator. Because there is always a minimum voltage at the neutral because of harmonics and zero sequence, a little margin is necessary and only 95% of the generator windings can be covered with this method.
100% stator earth fault protection uses current injection at low frequency or minimum third harmonic voltage to detect earth faults. It does not have the limitation at the earth side of the winding, so using this method and the one described for the 95% you can cover 100% of the stator.
Both detect earth faults at generator stator windings when generators neutrals are connected to earth using neutral transformers or reactance, which make neutral currents too small to use overcurrent relays.
Injection method protects 100% stator windings without the need of a 95% relay. It works on its own.
The principle is the following: a signal generator injects voltage pulses in the windings of the machine at a frequency different from the rate frequency of the system or any harmonic (typically 10 Hz to 25 Hz, 87 Hz...). These pulses are transmitted to ground through the winding insulation and back to the signal generator through the earthing resistor.
Together with the signal generator, the relay also measures the current in the signal injected. In normal conditions, total impedance is high and low currents are measured. If there is a phase to earth fault in any phase, impedance to ground decreases and current increases rapidly, indicating the fault.
This method works even with the machine out of service if the relay is connected (careful with this during maintenance).
This system is normally implemented at the generator neutral transformer, but if it is not available (for example, because a resistance or reactance is used instead of a transformer, or the generator is delta connected), voltage transformers can be connected at the generator terminals with the secondary connected in broken delta, since this allows the injection of the same signal to all windings.
| Is This Answer Correct ? | 32 Yes | 7 No |
Post New Answer View All Answers
how i chosse capacitor bank if my energy meter is 48 kvr? tell me capacitor bank chossen formula?
how to find the resistance and gap volatage of the diode??
How can we design a HVAC Plant room which includes AHU,FCU,Chillers,Cooling towers, Pumps etc & how will they work? any Drawing if possible?
The size and construction of bushings in a transformer depend upon?
why we dont prefer ac voltage controller instead dimmerstat [if both have same rating] for conducting experiments in laboratories?
what questions are asked in the interview for the post of an electrical maintenance engineer in FMCG companies?
How can we find out LT UG cable fault (size,from-4*25mm sqr to 4*300mm sqr)? How mani types UG cable faults?
WHAT KIND OF LIGHTENING PROTECTION SCHEMS CAN BE ADOPTED FOR THE FLAT ROOF OF BIG DIMENSION.
Where should the lighting arrestor be placed in distribution lines?
following are the questions asked in interviews: 1) Considering the short circuit fault, which switchgear is preferable, SFU OR MCCB? Which operates faster? whicg one is popular in Indian Market and why? 2)Define SCPD. What is type2 coodination? 3)Why the contactor rating is higher for the fuseless selection than for fuse selection? whether the ratinf of SFU or MCCB remains the same in both the cases? 4)If the HP rating is known then can you calculate the ratings of L.V Switchgear(SFU/MCCB, contactor, OLR) without using the selection chart?
what is the difference between fault current ant short circuit current?
We have 110Vac ,10A ac supply which is converted into 110VDC & How much DC current? which factor is reqired for dc current calulation?
H.t motor 6600 volt to same motor lt supply 440volt supply applyed to h.t motor to effect....
sir, i am 3 years diploma course pass out candidate.what type of certificate can i got in marine service.
HOW TO CALCULATE HV TEST VOLTAGE FOR ELECTRICAL EQPT.
