600V Generator with HRG + Surge Arrestors

[mbk2k3]

Do 600V generator suppliers/packagers typically include surge arrestors with HRG grounded units?

I have a system where: generator is 600V, delta, with the neutral point grounded via a 5A NGR.

2 days ago the generator tripped offline: Phase B / C PTs failed and Phase C surge arrestor failed/operated.

I suspected a ground fault on phase A, since this would cause phase B and C voltages to rise from 347V to 600V (PTs are rated for 347 only), and cause the PTs to fail.

But I can't explain why the arrestor operated/failed......

The NGR isn't operated or switched in/out, elimination any striking transients.

What could cause this type of failure.....

 

[xnuke]

Everything in a HRG system has to be rated for line-to-line voltage to prevent failure during a SLG fault when the neutral point shifts. The surge arresters need to be rated for use at 600 V; perhaps they too were rated for use at 347 V.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dpc]

I'm not clear how you have HRG on a delta system. Is there a separate grounding transformer? Agree with xnuke, arresters must be rated for full line-to-line voltage. MCOV should be at least 110% of maximum expected voltage.

Also, with HRG, the voltage on the unfaulted phases can exceed rated voltage due to repetitive restrike of the ground fault. This is similar to ungrounded delta systems. The HRG just limits the maximum voltage excursion. The resistor lowers the time constant and the voltage should max out at maybe 2.0 pu, depending on resistor sizing and system capacitance. With ungrounded systems, it can go much higher.

 

[waross]

Is this a four wire, 300:600 Volt system similar to the 120:240 four wire deltas?

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

 

[mbk2k3]

to answer people's questions:

PTs were ordered to be replaced since we should have 600V rated, not 347V.

The surge arrestors are correctly rated for 600V.

3 wire delta system that is grounded through generator neutral via NGR. if generator breaker is open, system is ungrounded (where the PTs and surge arrestors failed).

 

[waross]

3 wire delta system that is grounded through generator neutral via NGR.

Maybe a three wire wye system? The absence of a neutral carried through does not make a wye a delta.

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

 

[mbk2k3]

@waross

attached a sketch of the system...

I call this 3 wire delta on the 600V side of the transformer, since the transformer is wye-delta, and 600V is on the delta side.

ive also marked where the PTs and surge arrestors were that failed.

 

[xnuke]

What is the MCOV of your surge arresters?

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mbk2k3]

surge arrestor datasheet says:

voltage protection level = 2.5kV
Voltage protection lebel at 5kA = 2kV
suppressed votlage rating SVR = 1800V

 

[waross]

Most of us would call that an NGR wye system fed by a Wye generator.
The load is a delta transformer.
Calling the generator a delta has caused some confusion.
But back to the issue at hand, you should be good to go with 600 V rated PTs.


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

 

[edison123]

@OP

"3 wire delta on the 600V side of the transformer" is correct.

"generator is 600V, delta, with the neutral point grounded via a 5A NGR" is not.

Muthu

http://www.edison.co.in

 

[mbk2k3]

@edison123

I can see the mistake I made. Thanks for clarifying.

I guess I was referring to the delta ungrounded system when the generator breaker is open.

 

[mbk2k3]

so an update:

after some troubleshooting we have found some interesting observations.

reminder about the system:
[ul]
[li]12.47kV (utility) (wye on transformer primary)[/li]
[li]stepped down to 600V in our plant (3 wire/delta on transformer secondary)[/li]
[li]600V ungrounded bus feeds some aux. loads[/li]
[li]600V generator breaker, when closed, gives the 600V bus a ground reference through an NGR.[/li]
[/ul]

observations:
[ul]
[li]we opened all the aux. loads, so we were looking at just the secondary voltage on the transformer.[/li]
[li]L-L voltages were all 600V.[/li]
[li]L-G voltages were hovering between 312-360V (should be 347V)[/li]
[li]Vg or 3V0 hovering around 24V with all aux. loads disconnected, and hovers between 90-160V when auxiliaries are connected.[/li]
[li]The neutral current on HV side of transformer (CT in the neutral leg of the WYE side) is constantly picking up 8A or so.[/li]
[li]SEL relay events show some messed up voltages when the generator breaker is open and aux. are connected (see attachment):[/li]

Va L-G = 416V
Vb L-G = 642V
Vc L-G = 947V
Vg / 3Vo = 1820V​

[/ul]

Some thoughts about troubleshooting:
Engineering consultants state that the high Vc and Vb resulted in those PTs failing (rated for 347V). The high Vc is probably why surge arrestor (rated for 600V) failed.

The million dollar questions:
[ol 1]
[li]Why does the voltage fluctuate and neutral point shift so much when the system is ungrounded?[/li]
[li]Is it a utility disturbance?[/li]
[li]Transformer X2-X3 winding resistance was a tad bit higher than X1-X3 and X1-X2. Could that mean something?[/li]
[li]Is there some weird capacitance/filter/resonance effect going on? Sounds like some weird ferroresonance effect[/li]
[/ol]