20W resistor on PT Broken delta 1

[vreijb]

I have 2 generator plant grounded at low resistance (200A) at 13.8KV generating voltage. the PTs are 13.8/1.73:110/1.73-110/3
110/3 is in broken delta connected to a resistor 63 Ohm, 20W
Can any one tell me what this resistor will do and whether it is damping resistor. The switchgear manufacturer confirms that the resistor is sized correct.

 

[dpc]

It prevents ferroresonance between the system capacitance and the inductance of the PTs and helps keep the neutral point from shifting too far.

The 20 watts seems a little light to me. If I understand your PT nomenclature, during a ground fault you would have 110 volts across the relay and resistor. If you are tripping immediately, it might be OK.

But broken delta ground fault detection is not typical for a low-resistance grounded system, since the 200 A of neutral current will be the simplest fault detection.

Are there conditions where the system is operated ungrounded for periods of time?

 

[vreijb]

the system always operates with the ground as stated. in case of earth fault the protection system will trip the breaker.
I asked the swg manufacturor to verify the wattage of the damping resistor and he confirmed it is adequate

 

[dpc]

Well, with 200 A of primary ground fault current, I'm not sure what the broken delta protection is buying you. But the ground fault needs to be cleared quickly, or you're going to need a new resistor.

 

[slavag]

Yep.
Im agree with Dave. 20W seems is very low value.
But isn't ungrounded or high impedance system, maybe it's enough. I hope mnf know what need do.
Best Regards.
Slava

 

[NCTHAI]

It is opined th t ferroresonance phenomenon is only in CVT - capacitive voltage transaformers, used in EHV systems. Resitsance in CVT are self destructing device for protection purpose.

 

[cranky108]

I don't think the resistor is for grounding. It sounds more like across the open in the broken delta. This would be to supress harmonics.

Is the broken delta used for ground fault detection?

 

[Kiribanda]

vreijb,

As DPC pointed out, there is something wrong in sizing the resister.For me this is a wrong application.

1)If 63 Ohms is used with a ratio of 13800-110 V then you have introduced an almost ungrounded system at 13.8 kV side.It will not allow 200 A ground fault current to flow during a ground fault. As a result your ground fault protection system will remain without response.

2)So to control the ground fault current at 200 A as intended,you need a very low resister(0.0025 Ohm)across the broken delta.

3)In that case why you introduced broken delta method whereas you can control the ground fault current at 200 A with a simple 40 Ohm resister at 13.8 kV.

That is why I say it is a wrong application.

Any comments are welcome

 

[stevenal]

Kirabanda,

I don't think the OP is using a resistor on the voltage transformer secondary to limit primary ground faults. A primary ground impedance is required. If a low resistance is used on the VT secondary, the delta is no longer broken, and V0 will not be sensed properly.

 

[burnt2x]

Agree with stevenal and cranky108.
BTW, by inserting a resistor in the delta secondary, ferroresonance is averted. The magnetizing inductance of the PT and the line to groujd capacitance of the system makes an L-C circuit that will possibly resonate at a given condition. With a resistor in series, the ripples will die out. The broken delta resistance also tends to damp out possible overvoltages caused during intermittent arcing faults to ground. Another benefit of resistance on secondary delta of the PT is the tendency of the resistance to stabilize the neutral of the system under normal conditions equalizing the magnitude of the three phase-to-ground voltages.
The only problem I see is that the rating of the resistor should be such that it will limit the burden rating of the PT's and have a wattage enough to ride through during SLG faults. As previously stated bu others, 20W is too low.
If my computations are correct, at 63 ohms, you should specify 200 watts.

 

[stevenal]

Question for the group:

Resistors of this type come with a continuous rating, but the application is momentary. 200W continuous dissipation is not needed here. Is there a rule of thumb that can be used for underrating a resistor that is used momentarily, or at some duty cycle other than continuous?

 

[dpc]

This is an interesting question when dealing with grounding resistors because the standard 10-sec resistor really has no official continuous rating, at least in the ANSI world.

Of course, there must be some continuous current capability. Most references I have been able to find suggest using 8% to 10% of the 10 sec rating for a continuous rating.

So looking at the inverse, a 20 W continuous rating MIGHT imply a 10 sec rating of 200 W or so, assuming the temperature was not too high before the fault occurred.

It might be possible to obtain a short-time resistor rating from the supplier.

 

[burnt2x]

Good point!
Perhaps a further analysis on the heat dissipation and ambient temperature considerations could help to arrive at a 10-sec resistor rating!
However, on other scenarios, some ungrounded systems do not employ tripping; just indication/ alarms! We need to assume continuous application (maybe hours of troubleshooting/locating and correcting the fault), so the safe resistor wattage should be the continuous rating of the resistor.
Still, trying to derate on grounded systems seems logical. Any word from the electrical device manufacturers' side could enlighten the group.