poles fuse link coordination with substation transformer internal bayonet fuse

[jaraujo1]

I was asked to review a water pump electrical substation transformer internal fuse rating and to specify the time coordination between the transformer internal fuse and the fuse link at the pole. Let me provide the specifications and the engineering approach I used for a possible recommendation. I am just looking for a sanity check.
Tansformer voltage secondary: 488/277
Transformer voltage primary: 13.8 KV
Transformer VI: 500 KVA
Transformer manufacturer: Cooper Power system
Internal bayonet primary fuse provided: 50 amps
Basis for the rating :ANSI/IEEE XXXX: transformer hold 160% of full load for 6 hours without blowing out. It also monitors transformer oil temperature.
Time interval for the fuse to blow .1 -.4 sec.
The client would like to see .4 seconds coordination between the internal fuse and the pole fuse link.
After reviewing the current - time curve for the bayonet fuse at .1 sec, I came up with 1500 amps.
Next, I talked to the Cooper and Bussman application engineers to get current curve for fuse link of K ant T type. After I reviewed those curve the only way I could get the .4 second coordination is if I use 150 amps fuse link K class. My belief is that .4 second coordination is too long of a time interval.
a .1 second or 100 millisecond coordination could be obtained with a fuse link of the K type at 80 amps and should be sufficient for this application.
I am in the process of reviewing the internal bayonet fuse for time delay to handle the motor locked rotor and start current ( both are about the same ).






Thanks for any feedback.

 

[beanland]

Coordination

Coordination

You need to identify the specific type of Cooper bayonet fuse, there are three types with different TCC. The 50A appears to be the dual-sensing version.

I assume you are talking about a riser fused cutout feeding the transformer and coordination between the curout fuse and the bayonet. Correct?

I recommend using S&C Positrol "T" type fuses. They are of a better design and widely used by utilities.

In addition, if the available fault current exceeds the rating of the bayonet, you need partial range current limiting fuses in series with the bayonet.

The user also needs to verify upstream coordination with the feeder breaker or line recloser.

Are you looking for coordination only for secondary faults or for all faults including primary winding faults?

Is the transformer wye-wye or delta-wye? What is impedance? 5.75 is normal at 500kVA.

 

[jaraujo1]

Thanks!

Thanks!

Beanland,

Thanks for your recommendation:
Dual sensing? it does sense the oil temp and the secondary load, the one used is the C12 bayonet.

You are correct I am talking about coordination between a riser fused cutout feeding the transformer and the bayonet fuse in the primary side of the transformer.

Thanks for this recommendation. Do you have info available on the S&C Positrol T type of fuse?

I am aware of the current limiting fuse in series with the bayonet. I am
talking to Cooper because the transformer has been delivered and is on site.

Great feedback on the upstream coordination.

I am doing coordination all the way down to the motor load.

delta-wye configuration, 5.75 ohms impedance is a good estimate.

 

[wirenut1980]

I agree that you need to look at coordination between the pole riser fuse and the upstream protective device. That also determines what you can use at the riser pole, especially if you do not have control of that upstream overcurrent device. T link fuses at the riser pole will have a longer delay than K links, although I have not seen those used typically at riser poles...I am wondering if they might not adequately protect the underground primary cable. It sounds like the OP is trying to achieve coordination on a fault located between the main OCPD and the transformer.

I'll check more on this on Tuesday.

 

[beanland]

Coordination

Coordination

S&C Positrol TCC ==> http://www.sandc.com/support/tccs_positrol.asp

Typical impedance is 5.75% in per unit based on rated kVA and voltage.

Delta primary changes things for coordination between 13.8kV primary devices and secondary devices. Make sure to take that into consideration.

It is unlikely you will even come close to the conductor damage curve with any of the fuses recommended.