Distribution Transformer Test 1

[steelerfan28655]

Is there a way to test distribution transformers for winding faults using a meter or a thumper(Hi-pot) device? Our linemen typically test for winding faults by inserting a smaller sized fuse, re-energizing the transformer, and waiting to see if the fuse will hold. The problem is, with our emphasis on power quality these days, If the fuse does not hold there is a draw down of voltage on our system. So we are wondering what is done on other systems to verify the windings of a transformer before energizing.

Any help or suggetsions are greatly appreciated.

 

[busbar]

(1) DC-insulation resistance between windings and each winding to core/can, and (2) turns-ratio test between the windings will give a reasonable assay of transformer condition. Some TTR sets will also display relative excitation current.

 

[jghrist]

Another problem with testing by re-energizing is that sometimes there is an incipient high side fault that will result in a fiery catastrophic failure when re-energized. I remember a few years back seeing a video of linemen engulfed in flames while re-fusing a polemount transformer.

A good reason to use a 30' hotstick to close the cutout while re-fusing.

A smaller size fuse may also blow on inrush even if there is no fault. One of the criteria for sizing transformer fuses is to make sure it is large enough to withstand inrush.

 

[steelerfan28655]

Thanks for the replies. What brought this about was a failure of a 750 kVA, delta- wye, 12,470 to 480, station type transformer. The transformer was in parallell with one other transformer. The guys isolated the secondary of each transformer, then (using a hot stick) threw in an 8K fuse. This has been the usual practice for years around here. The problem is, the other industrial customers on the same circuit see a draw down in their voltage when we "intentionally" cause a fault.

Thanks Again.

Happiness is a way of travel, not a destination.

 

[busbar]

Given the equipment value and the cost of lost production/revenue in the case of 750kVA, a better “checklist” may be §7.2.2 NETA 2001 Maintenance Testing Specs for liquid-filled transformers, or ANSI/NETA Standard MTS 7.2.2 Standard for Maintenance Testing of Liquid-Filled Transformers. from netaworld.org.

 

[GamezBeCJ]

I agree with busbar.
A falut large enough to blow a fuse must be detectable trough:
- Insulation resistance (usually refered as megger)
- Turn ratio (usually refered as TTR)
- Winding resistance
- Also you may test the oil dielectric strength.

A good guide for performing test in the field is IEEE Std 62-1995 "IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus - Part 1: Oil Filled Power Transformers, Regulators and Reactors."

 

[Dopler]

Factory Mutual Global in their specification 5-20, also recommends power factor (dissipation factor) for bushings and windings, DGA for oil.

 

[Rocketir]

Quench the arc to minimize the voltage dip. Try to test using the S&C Fault tamer fuse.

http://www.sandc.com/products/faulttamer/default.asp

Jes

 

[edison123]

A surge tester will be helpful in detecting shorted turns in the windings. This is an off-line test requiring non-tested winding to be shorted.

 

[AussieHV]

If you use a ttr you may get results which may indicate that everything is OK. I recently did one with an open circuit in the tap changer and it was only picked up with the resistance test.