High Resistance Grounding

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steve66

steve66

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I have an existing transformer 7.5MVA, 34.5KV to 4160V, delta to wye. The neutral of the Wye is grounded through a transformer. The secondary of this transformer is connected to a 0.75 ohm resistor. The turns ratio of the transformer is 2400:120.

How do I calculate the impedence between the neutral and ground to enter into SKM's software?

Thanks in advance:
Steve
 
Re: High Resistance Grounding

Turns ratio squared. </font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">2400/120 = 20.</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">20 x 20 = 400.</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">400 x 0.75 = 300 ohms</font>
<font size="2" face="Verdana, Helvetica, sans-serif">This can be derived from:</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">V2 / V1 = N2 / N1</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">I2 / I1 = N1 / N2</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">R2 / R1 = (V2 / I2) / (V1 / I1)</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">R2 / R1 = (N2 / N1) ^ 2</font>
<font size="2" face="Verdana, Helvetica, sans-serif">I hope I typed all that correctly.
 
Re: High Resistance Grounding

Thanks Charlie B.

To add one other thing I left out: there is also a coil across the transformer secondary. It is labeled 59G?? And there is a contact to a tap on the resistor labeled "pulsing contactor".

Any idea what this is, and does it affect the impedence to ground??

Steve
 
Re: High Resistance Grounding

Device type "59" is an overvoltage detection relay. Device type "59G" detects an overvoltage condition in the ground wire. I infer that this device senses a ground fault from the fact that current flowing in the ground wire will create a larger than normal voltage.

I do not know what that "pulsing contactor" might be.

Here is a link to device types and their numbers.
 
Re: High Resistance Grounding

This is basically what it does:
"The Pulsing Ground-Fault System provides a quick way to locate faults, eliminating the need to sequentially shut down equipment. A ground-fault can be located in a resistance-grounded system by tracing a pulsed ground-fault current."
"Pulsing contactor provides a method for tracing a ground fault to its source ? the contactor is used to bypass a portion of the grounding resistor, so the current, proportional to resistance in the circuit, will rise, and the resulting current pulse is detectable with a handheld, clamp-on ammeter."
 
Re: High Resistance Grounding

Cool. It all makes sense now. I see there is a "pulse" switch on the grounding control panel. And the 59G would set off the ground fault light.

For some reason I thought the 59G controlled the pulsing contactor.


Steve
 
Re: High Resistance Grounding

Since both Charlie and templdl's replies are sound, I originally "passed" on making a comment.

But I again looked at the size of the system and the implied ground fault current limitation (8 amps) - this is a questionable, although not necessarily a bad design.

7.5 MVA is good sized for 4160V. Without knowing the entire system I imagine it has a fair capacitance - this will especially be true if there is power factor correction at the 4160V level or surge capacitors on any 4kV motors.

IEEE 142 recommends that the resistor?s resistsnce be no more than the system?s zero sequence capacitive impedance; otherwise, the system can have most of the problems of an ungrounded system. Determining the ?zero sequence capacitive impedance? is usually pretty difficult. Often it can only be measured.
 
Re: High Resistance Grounding

Its and existing system. I'm not sure why the 300ohm grounding resistance was chosen. I don't think there is any power factor correction or surge caps.

By the way, what is a surge cap? Are they common on 4160 V motors? The site does have a 900HP and 3 600HP motors.

Steve
 
Re: High Resistance Grounding

Surge capacitors are relatively common with vacuum contactor based systems; although the vacuum contactor technology has also improved considerably over the years. Essentially they dampen the voltage surge created when a vacuum contactor opens. With old air-blast and air-magnetic type MV interrupters the actual current interruption was relatively gradual on a current-decay basis. Old style vacuum interrupters were far more abrupt and the voltages created by Lenz?s law were very high.

The voltage surge can be degrading to MV motor insulation systems. These insulation systems are more marginally designed than LV motors. With the lower physical tolerances of the insulation, the motor designer can reduce relative air-gap dimensions and improve both power factor and efficiency in the motor.

There have been entire papers written on the subject ? so this is a VERY condensed summary. Many manufactures of vacuum based motor controllers and switchgear will tell you surge capacitors are no longer necessary. Many motor manufactures will tell you they still strongly recommend them. Manufactures that make both ? are ambivalent.

At 4160V, the last really good independent motor applications engineer I worked with recommends using them at 750hp and above although he does doesn?t start insisting until you get to 1250.
 
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