Ground Fault Detection of a Delta/Delta transformer

[7carisfast]

Look for some advice on adding secondary GF protection on a 4000KVA 13.8/4.16KV transformer. Does anyone have a link which explains my options or can anyone give some advice?

 

[busbar]

 
What is the 4kV system-grounding method—ungrounded/solid/high-resistance/low-resistance?

Start with IEEE Std 142 §1.

 

[PWR]

A 4.16kV delta transformer connection offers no neutral point on which to establish a solid, low, or high resistance ground connection. Is it your intent to operate ungrounded(rarely practical at 4.16kV) or do you wish to utilize a zig-zag grounding transformer to establish a neutral? What type of loads do you have?

 

[7carisfast]

The method is an ungrounded 4.16KV Delta. The loads are all motors.

 

[dpc]

The most commonly used ground detection scheme involves three voltage transformers connected grounded wye - broken delta. One corner of the secondary delta is left open and a voltage relay is connected across the open corner (along with a resistor to prevent ferro-resonance.)

The voltage across the corner will be 3Vo.

See any relaying text, such as Blackburn's "Protective Relaying".

 

[PWR]

Based on industrial practice in the US, you are fighting history to operate motors at 4.16kV ungrounded. I can cite plants that "gave up" trying to operate such systems in the 1950s and 60s. Since then, more, not fewer problems have been identified with ungrounded systems. I'm not saying it can't be done, but the question is why?

 

[busbar]

To add to dpc’s comments, the transformer primaries, although normally exposed to ø-n voltage, must be rated for full ø-ø voltage—id est, 35:1 [not 20:1] for this application.

Chapter 6 of Beeman’s IPSH (circa 1955) has a very lucid description of the potential problem. It is very real—with tragically expensive results at 480/600V, murder at 4160V, and mass suicide at 12kV.

 

[dpc]

I used to think this type of system had faded away in the US, but I am currently working on two system studies at east coast power plants that both have extensive 4160 V ungrounded systems, including a lot of big motors. The plants have been operating this way for about 40 years.

So they are still around, in spite of the conventional wisdom that they are a problem.

 

[rbulsara]

Look up 59G relay application. Best suited for ground fault detection on ungrounded systems (and high resistance grounded).

Here is one of the links..

http://www.byramlabs.com/pdf/basler/ubg7bull.pdf

 

[busbar]

If switchgear/motor/cable-run/transformer conditions have been kept consistently sanitary, and winding insulation has no 'weak spots,' it is possible for ungrounded systems to operate successfully, but practically, that is the exception to the general case.

Surge capacitors can be their own double-edged swords.

 

[JoeBoardman]

I would recommend using a Bender Ground Fault Monitor, e.g. IRDH275 with a voltage coupler, e.g. AGH520S. It measures the resistance level of the ungrounded system and alarms when the resistance to ground drops below the alarm setpoint. Also may want to look at RCM470DY Directional Ground Fault Relay too. See

http://www.groundfault.com.