Fuse Sizing for PMH-9 switchgear

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Thread Starter

CuriousOne

I have a old detroit diesel. 4160 VAC, 2500 kVA output through 15 kV stepup transformer. Inspectors recently found problems with the pad mounted switchgear. S&C PHM-12. Must replace switchgear with new PMH-9.

PMH-9 arrived with no fuses. I have been tasked with sizing the proper fuses. I read everything I can find, yet I am still confused.
I started the diesel and synchronized and proceed to full load. I recorded 4160 Volts and 220 amps. I then applied the math. 4160 X 220 X square root of 3. This math matched generator output on gauges.
If I do a ratio compare to the 15 kV, the a 66 amp fuse is minimum. If I do 125 percent of that then 82 fuse is needed.

My problem is the fuse will be on the primary side of the transformer and I am having problems reversing the math to protect the transformer and underground cabling.

Any 3 phase for dummys stuff would be appreciated.
 
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The diesel is late 60's GM 2500kVA, 4160VAC, 0.85PF.

Transformer nameplate is missing but I was able to obtain some information from drawings.

Transformer is early 70's GE 2000kVA. Input is 4160VAC and output is 13800VAC. My understanding is it's original purpose was as a step down transformer for the starting motors on GE turbine.

Note: The KW limiter for the diesel genset is set to 1.6MW because of the transformer limitations in regards to kVA of transformer.
 
CuriousOne... as long as kVA through the transformer (in either direction) never exceeds 2,000 kVA, then, size the HV-side fuse as follows:

Amp = [1.25 x 2,000kVA]/[Sqrt(3) x 13.8kV] = 105 A.

Caveat: If the transformer is used to power the 4.16kV bus when the DG-set is unavailable, then I strongly recommend the addition of HV-side, Phase-Loss protection, to the system. Do you require additional information?

Regards, Phil Corso
 
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Mr. Corso

Before we proceed with additional info. Let me be sure I understand the existing info.

Amp = 125% of 2000000VA / sqrt(3) X 13800 Volts.

Could you be more descriptive of the 125% of the 2000kVA? Just to further my understanding.

"Caveat:" You were "spot on" correct in your assumptions regarding the 2000kVA transformer still having the ability to start the (2)turbines.
Is this a common or acceptable?

I believe the transformer protection for the 2000kVA xfrm HV side is utilizing the the transformer protection relays associated with the larger low side 104MVA 13.8 to 161kV transformer connected to the GE Generators.

The larger GE generators may be retired very soon along with the larger transformer. This changes the aspect of the diesel. The diesel may be required to power the starting transformers of other turbines on site. That may change the way the relay protection works.

Yes, I am very curious about transformer protection. It might be necessary to install high side protection on the 2000kVA transformer to address it's new utilization.
 
CuriousOne... following are answers to your questions:

1) The 125% multiplier comes from the National Electric Code, NEC. It permits "primary" fusing at 125% of the transformer’s rating.

2) There are no standards I'm aware of that are against use of smaller fuses.

3) I can't visualize how protection applied to the 104MVA xfmr, can detect HV-side "Phase-Loss" associated with the 2,000kVA one. Actually, my recommendation wasn't for protection of the transformer. Instead, I was concerned about motors connected to the 4.16kV bus or additional 4.16kV/LV transformation, should a fuse inadvertently "open!"

4) Unless there is "Negative Sequence Relaying" to detect current-unbalance, then, HV-side "Phase-Loss" could cause serious damage!

Regards, Phil Corso
 
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Mr Corso,

Thank you for your assistance.

The original size was "NO FUSES". It became a safety issue.
 
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