E/F element of CDG36 trip

[cristhmart]

Hi everyone. I have a 7.5 MW power plant with 3 old generators units of 2.5 MW working in parallel directly attached to a 4160 V bus bar. One of the units is having Earth Fault trips as it is read in a CDG36 relay. The CDG36 coil current is the sum of the secondary three phase currents of the line CT’s in a wye configuration.
A Standby Earth Fault and Differential relays are present too, but none of them is giving trip signal. The Standby Earth Fault Relay is connected through a CT to the neutral wire of the generator and the Differential Relays have six independent CT’s.
Having checked the generator feeder and power connections, it was decided to mount a three phase Power Quality Analyzer on the very CT’s of the CDG36. All the measured variables in the three phases are each other consistent except for the power factor and the VAR in one of them. Mean while in the other two phases there is a lag power factor, the inconsistent one has a lead power factor with equal magnitude of current related to the other ones.
Provided there is a Standby Earth Fault protection, it was decided to lock out the e/f element of the CDG36 and keep running the unit until a suitable solution was found.
Any feedback from you would be very helpful. Thanks in advance.

 

[RAgrawal]

My reply is based on assumption that neutral of the said generator is earthed, where as neutral of other 2 generators is not earthed. Please confirm on what elecment of CDG 36 it tripped - i mean on time lag / inst. element?

CDG36 relay is a non directional relay. If its is put on a Generator with residual connection, it will see fault of down stream also and hence can mal operate. In that case neither stand by earth fault nor differential will operate.

You just check, most probably, at the time of tripping, there should be a fault in down stream system causing Generator to feed earth fault and during fault feeding generator tripped.

If this is the case, you should coordinate Generator relay for down stream fault.

 

[RalphChristie]

Residual earth fault relay (CDG36)
Have to see any earth fault downstream due to the fact that an earth fault will cause an unbalance condition.

Standby earth fault Relay
Will see any earth fault on the system due to the fact that the fault current has to return through the neutral point.

Differential Relay
Will just see an fault in the zone (between the diff-CTs)

The Standby Earth Fault Relay has to see any earth faults seen by the CDG36. (earth fault current have to return through neutral) However, the setting (delay) is normally longer than the residual (CDG36) relay's delay, for coordination purposes. Thus, the CDG36 have to trip before the Standby relay for a downstream earth fault.
If you are saying that with the CDG36 disconnected there are no Standby Relay trips, and you can assure there are no downstream earth faults, I would suspect a damaged line-CT or a line-CT with a changed polarity.

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[cristhmart]

In fact, all the generators are low resistance grounded. Time-lag element of CDG36 is tripping and instant stage is not implemented.

The generator is running since last monday without any other fault trip and e/f element of CDG36 out of order. It was first suspected of the line CT polarity, although no work has been made in this circuit since long time ago, and was inverted momentarily since the all power factor phases meassured became distorted. I am agree on suspect about the line CT but I wonder wether a magnetization issue may be. How could it be happened? Is it a natural CT fault? Is it reversible? How could it be prevented? I really appreciate your comments.

 

[RalphChristie]

cristmart:

What is your CT-ratio's and your settings for the Residual scheme (CDG36) and the Standby scheme?
Have you measured the current in the CT of the standby scheme? How does it compare to the residual current in the line CTs?
Have you anything connected to your line CTs? Things like O/C relays, ammeters, combination of ammeter and selector switch, test-blocks, etc?
Have you any big motors downstream?
Have you done any changes on your system?
To what value do you limit your earth fault current?



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[cristhmart]

Thanks Ralph for your valuable questions and sorry for taking so long to respond. Everything you mentioned earlier was verified: protection coordination, appropriate CT’s burden and right instruments connections. The current tap position in the CDG36 is 2 and the line CT ratio is 600/5 for the following explanation purposes.

After locking out the E.F. Relay trip, was making evident a fault in the voltage regulator was developing too, requiring more increments in voltage adjust potentiometer than usual in order to avoid leading power factor. At first glance it was assume two faults were arising, and was decided to change the voltage regulator. Our voltage regulators are BASLER SSR 120-12 model. During dismounting of the faulty one was found an abnormal electrical contact between terminals 21-22 (1? sensing) and 30 (signal ground), originated for the shield grounding of the remote potentiometer.

Afterward, during synchronization (from 19:40:10 to 19:40:35) with another generator the following data was collected:

HOUR I1 I2 I3 PF1 PF2 PF3 In
19:40:09 213 210 213 0,76 0,76 0,77 1,64
19:40:10 211 209 212 0,76 0,76 0,77 2,52
19:40:11 194 192 195 0,75 0,75 0,76 2,33
19:40:12 165 162 166 0,77 0,78 0,79 1,42
19:40:13 143 140 142 0,83 0,83 0,84 0,48
19:40:14 114 111 113 0,89 0,89 0,9 0,48
19:40:15 105 102 105 0,92 0,92 0,93 1,47
19:40:16 105 102 105 0,89 0,89 0,9 1,49
19:40:17 104 102 105 -0,91 -0,9 -0,91 210,08
19:40:18 99,2 96,7 99,6 -0,9 -0,89 -0,89 199,47
19:40:19 99,8 97,4 100 -0,91 -0,91 -0,89 198,61
19:40:20 97,6 95,5 99,9 -0,88 -0,87 -0,86 198,19
19:40:21 98,1 96 101 -0,88 -0,88 -0,89 198,03
19:40:22 91,2 88,7 91,5 -0,91 0,91 -0,91 232,52
19:40:23 91,5 89 91,8 0,94 0,94 0,94 2,66
19:40:24 93,8 91 93,7 0,92 0,92 0,93 1,27
19:40:25 93,6 90,8 93,5 0,92 0,92 0,93 1,27
19:40:26 93,6 90,8 93,5 0,92 0,92 0,93 1,27
19:40:27 95,3 92,6 95,5 0,92 0,92 0,93 1,53
19:40:28 95,2 92,6 95,4 0,92 0,92 0,92 2,71
19:40:29 94,8 92,2 93,6 0,91 0,92 0,93 2,41
19:40:30 96,1 92,4 94,1 0,93 0,93 0,94 0,58
19:40:31 95,6 90,6 93,2 0,9 0,92 0,92 4,30
19:40:32 98,9 96 98,4 0,89 0,9 0,91 1,36
19:40:33 98,4 95,5 97,9 0,89 0,89 0,9 0,98
19:40:34 98,8 95,8 98,2 0,88 0,88 0,89 1,00
19:40:35 98,9 95,9 98,3 0,87 0,88 0,89 1,04

Therefore, it was inferred the E.F. trips were developed during generator underexcitation periods.