Voltage Regulators used with a resistance grounded system

[podobing]

All,
I work for the Mining Industry. All of the three phase power that is delivered to the underground area of the mine is resistance grounded. The grounded side of the neutral grounding resistor (15 ampere limit) is connected to a safety ground bed that must be located 25 feet (minimum) away from the substation ground bed. The only other connection that I am accustomed to seeing at the safety ground bed is the ground conductor(s) of the cable(s) that go to the underground area of the coal mine. I believe that the idea of the 25 foot separation distance is to prevent lightning from possibly being directly coupled to the frames of all of the equipment that is used underground.

As you would expect, the other side of the neutral grounding resistor (NGR) is connected to the X0 bushing of the substation transformer.

Here comes the curve ball. One customer has installed three single phase voltage regulating transformers (auto-transformers) to the cable going underground The voltage regulators are connected in a wye configuration. I question where the neutral point of the voltage regulators is connected. It is currently connected to the safety ground bed. My question is: Where do I connect the neutral point of the voltage regulators? I feel that I have three choices: 1) The X0 of the supply transformer; 2) The safety ground bed; 3) Floating. I am leaning toward the X0 bushing. The system must still function as a resistance grounded system. It currently has 59G and 51G protection across the NGR.

Not that it matters, but just for your information: The mine supply voltage is 12470 VAC, and the transformer size is 10 MVA.

Try to Stay Warm.
Regards,
Podobing

 

[waross]

First, I may be wrong but I suspect that another reason for the ground location separation may be to prevent a ground potential rise on the substation ground grid being seen by the safety ground grid in the event of ground fault passing through the substation ground grid.

The normal voltage to ground and the voltage that your regulators will see is 7200 Volts.
Were the regulators to be connected to ground then two of them will see 12470 Volts in the event of a ground fault. A 7200 Volt regulator may not last long with 12470 Volts applied.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I think Bill has a valid concern regarding voltage (neutral) shift during faults. In general, I don't think you can use three wye connected regulators on an ungrounded system, which is essentially what you have. Even if you ground the regulator neutral, the zero sequence impedance will still be high - the regulators cannot source zero sequence current.

The regulators are susceptible to surge voltage damage and always come with a series arrester. I'd probably consult with the regulator manufacturer to get their advice.

You might be able to connect the regulators in delta since you have no single-phase loads (at 12.47 kV). But then you'd need 12.47 kV regulators, not 7.2 kV.

 

[waross]

If you have no neutral on the primary side you have a problem.
You may be able to derive a primary neutral with three 7200 Volt distribution transformers.
The transformers are connected in wye:delta. The secondary voltage is not important as long as all three have the same secondary voltage.
The secondaries are connected in delta. The primaries are connected in wye and the wye point will provide a stable neutral point relative to the 12470 Volt phases. This may be used for the neutral connection of the regulators.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[odlanor]

podobing,
Is The 15 ampere limit for fault-to-ground provide ALARME for this kind of fault(instead of trip)?

 

[podobing]

Waross,
The substation transformer is actually a 10 MVA: 69 kV primary (delta); 12470 secondary (wye), with the X0 bushing on the secondary being connected to the neutral grounding resistor. The voltage regulating transformers are there to compensate for the voltage drop when large motors located underground are started and ran. Some longwall mining applications have over 7000 connected horsepower. While not all of the horsepower is used all of the time, it is there if (when) it is needed.

Odlanor,
By law, ground fault tripping occurs at less than half of the rating of the NGR. Typical tripping current is 5 amperes, and typical tripping times are on the order of 5 seconds to allow coordination with circuit breakers located underground.

Dave

 

[waross]

In that instance the X0 terminal is the correct connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[odlanor]

podobing,
I think more appropriate to open a new thread on this subject.
thank you