Residual VT

[electrolitic]

Condition: single phase fault on an unearthed neutral 3-phase system
Question: Can I obtain 59N protection using just 2 single phase VTs connected in open delta configuration?
Thanks

 

[davidbeach]

No. All you'll see is phase-phase voltage and that will not be impacted by the ground fault.

 

[waross]

NOT RECOMMENDED,
Very short term use, while waiting for delivery of spares.
Not for major systems but possibly for some temporary protection of small, isolated, islanded systems.
Use only if suitable VTs and over-voltage relays are on hand. Don't buy equipment for this kludge.
If the VT primaries are connected line to ground Then on a phase to ground fault at least one of the VTs will show 1.73 times normal voltage.
NOT RECOMMENDED.

(While this scheme may be possible, I prefer davidbeach's answer.)

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

 

[davidbeach]

If it is a true open delta, not two of three transformers in a broken delta, none of the primary windings are connected to ground and therefore there will be no zero-sequence voltage on the secondary.

 

[electrolitic]

Mr Davidbeach
The question is considering only two primary windings connected phase to ground (simply 2/3 of a conventional wye connection). So, zero sequence voltage at tertiary is feasible, I suppose.
When the phase not connected to VT is grounded the summation voltage at tertiary windings would increase. If a phase connected to the VTs is grounded the summation voltage at teriary windings also would increase. It was a theoretical question just to evaluate the risks, the overvoltages levels involved. The open delta(also called "V" connection) normally grounds the center point of "V" at secondary side.Here, we are grounding the "V" at primary side, as the wye connection does. Probably there is a mistake/limitations involved in my considerations. That is what I would want to know. Thanks.

 

[davidbeach]

I'm having a hard time trying to picture what you're doing, could you post a diagram? If you're saying that an open delta has primary windings grounded you must be in IEC land and the terminology used is different from what I'm used to.

 

[waross]

Hi David. This is what I believe that he means:
Consider three PTs connected Wyeelta with the wye point grounded. Now remove one of the CTs and leave the other two.
The PTs must have a high impedance compared to the system impedance to ground so as to maintain a reasonable neutral position.
Now, if either of the phases with the PTs is grounded, the other PT secondary voltage will rise to reflect line to line voltage.
If the unmetered phase is faulted to ground then the voltage of both PTs will rise to reflect line to line voltage.
You have to detect the grounded phase condition by looking at the over-voltage on at least one of the unfaulted phases.
As I hinted, probably not ready for prime time.
I can't see you ever considering this solution on your system, David.
Remember the ground detector lights on the old 440 Volt Delta systems?
When a phase went to ground, one light went out and the other two got brighter.
If a one lamp burned out, It looked like a grounded phase BUT the other two lamps did not increase in brightness.
A similar situation here. Look for the voltage Rise.



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

 

[davidbeach]

At that point just go with the old 1 VT method and use a single 27/59 relay. Undervoltage is a fault on that phase, overvoltage is a fault on one of the other two phase. Not sure what to do with the second VT.

 

[waross]

I like that David.

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

 

[GroovyGuy]

Hi electrolitic
I agree with david & waross; you mean broken-delta, and not open-delta, correct?
ie Broken-delta implies 3 PTs, with grounded wye primary and broken delta secondary.
I don't understand how you could detect 59N with two open-delta PTs?
Curious: How big is this machine (MVA & voltage)? Grid-connected, or isolated? Step-up xfmr?

Regards

 

[waross]

2 single phase VTs connected in open delta configuration

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

 

[electrolitic]

Even out of prime time: 1.The residual 59N from the 60’s is well known: 3Ph VTs with 1ry,2ry and 3ry windings. 1ry windings wye connected and 3ry windings broken delta connected including V0 voltmeter and damping resistor at delta terminals and an incandescent lamp paralleling each delta winding. Some designers called it as ANSI 64. A reliable and selective ground protection was achieved combining it and residual 50/51G at feeders (no core balance CTs available). I had started up hundreds. 2. Presently I had rejected a design where voltage measurements and 27, 59 and 59N protections were connected to a V-V-open delta - please confirm terminology at Google) - arrangement of VTs (simply doesn’t work for 59N) at ungrounded 440VAC 3 Ph system.
Just for fun I had started a thread to understand the hipothetical results of a “2/3 of a neutral grounded wye VT transformers”- (V-V connection grounded at 1ry side ) trying to perform a 59N protection. A simple detection of phase overvoltage to ground. David answered why not a single VT phase to ground detecting “27N” and “59N”? 1/3 instead of 2/3... I will never apply it but it was interesting to learn it. Thanks David.