50kva Single Phase Transformer Question

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WattsJr

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Had a service call recently in which a customer asked if I could replace a 50kva single phase transformer and diagnose why this particular one has failed twice. The scenario is as follows:


Customer has a 50kva single phase transformer set up next to the meter base boosting 120/240v to 240/480v for a below ground feed of a little over 1000' to a PumpHouse with two 10hp irrigation pumps as the only load. There is a second 50kva transformer in the PumpHouse that steps the voltage back down to 120/240v single phase to feed the pump motors.
The first transformer failure was after they said a lightning strike hit a tree close to the PumpHouse. The pumps were run for a while the next day before tripping the 200a main breaker feeding the first transformer. The second failure occurred after a tree branch fell on the utilities high voltage line feeding the utilities pole mounted transformer. At the time of the second incident the main breaker feeding the transformer was on as well as the disconnect to the underground feeds but the disconnect to the second transformer was off so there was no load on the system. There is no surge suppression on the system.


Connections and equipment are as follows:


200a main breaker feeds (1) 50kva single phase transformer with 120/240v primary, and 240/480v secondary. Primary Conductors are (3) 4/0cu and a #2cu ground. Secondary conductors are the same. Primary ungrounded conductors are terminated to X1 and X4. X2 and X3 are tied together with no neutral connecting to it. The #2 ground is connected to the chassis.
Secondary ungrounded conductors are terminated on H1 and H2. The #2 ground is also connected to the chassis.
The neutrals are joined together with a Polaris lug and do not connect to anything in this transformer.
Next to the transformer is a disconnect on the secondary fused at 90 amps.


Inside the PumpHouse there is a disconnect fused at 90 amps before the step down transformer. The Transformer connections are: 240/480V primary ungrounded conductors connect to H1 and H2. 120/240v secondary are connected to X1 and X4. X2 and X3 are tied together and the neutral to the panel is connected to it.


Diagnostics under full load are as follows:
At the main disconnect each phase is drawing 76.4 amps on the primary 120/240v. On the secondary 240/480v we are drawing 37.4 amps per phase. Voltage readings on the secondary of the second transformer in the PumpHouse is 226volts. Both pump motor current readings are steady and within the fla ratings. The system has been run for over 8 days straight at a time without issue.


From first glance it would appear as though we are in need of a robust surge suppression system. Both occurrences happened after storms. However, I am not 100% sure of the transformer sizing, grounding, and the connections.
Both transformers are 50kva with the intention of taking a single phase 120/240v 200amp service, converting it to a 240/480v 100amps for the 1000' of underground travel to accommodate (3) 4/0 cu and a #2cu ground through an existing 2" conduit, and then stepping the voltage back down for the two 120/240v pump motors. Does the first transformer need to be larger?
Basically with the grounding we are bringing everything back over 1000' on a #2 cu ground wire connected to the services ground rod. Should another ground rod be added at the PumpHouse and should it bond to the neutral derived off the second transformers X2-X3 connection?
The neutral coming from the service travels the length of the system but does not connect to anything. Should it connect to anything or is it just basically a spare wire?


I will be turning in a quote to the customer to install a robust surge suppression system and change out the 50kva step up transformer. Before doing this I would like to get some insight from others who have more knowledge with transformers than yours truly.


So, what say you? Thank you in advance.


WattsJr.
 

Smart $

Esteemed Member
Location
Ohio
IMO if the transformer is still working, it did not fail twice. When transformers fail, they mostly let the smoke out and no longer deliver any power. In cases where they partially fail, they no longer output voltage within nominal rating tolerance. Tripping of the OCPD is not an indication of transformer failure. Not even sure that happened with your second event description.

A 200A breaker is a little small for a 50kVA 120/240 1Ø transformer primary, especially if it is a backfed step-down transformer?. Power up surge can more easily trip it. Not sure what tripped it if after running pumps a while.
 
Alternative test?

Alternative test?

I would like to suggest an alternative test. I may have missed it but I don’t see which of the two Transformers has 'failed' twice. I’m assuming it is the Step Up Transformer that is failing not the Pump House Step Down. In your description you mention that the failures occurred after Storms. The first failure was after running the system for a day before failure (after the Storm).
I suspect the 1,000’ Underground Cabling has a compromised Insulation point or points. Storms often have Rain so the one day of the System working properly before failure would give moisture from the Storm time to soak into the soil and draw extra Current. At one point you mention that the 200 Amp Main Breaker tripped. With a normal under power Amp draw of 76.4 Amps, where did the additional draw come from to trip a 200 Amp Breaker?
I don’t see a practical way to ‘fault’ the Underground Cabling by waiting until a day or two after a good rain without disconnecting both ends of the Underground Cabling and Megging the Cables individually. If Megging with a good Earth Ground to each Underground Cable shows that one cable has a lower Ohm reading than the others under normal weather conditions would tell me that the Underground Cabling is also suspect. You could also Meg from each conductor to the others to see if they are conducting to each other with additional moisture.
10 HP Single Phase Motors have lots of Inrush and Capacitors. Are the Motors Short Cycling or repeated Start / Stop? Are the Motors both Starting at the same time or do they have a Staggered or Sequenced Start? On the Pump House Transformer, are the Primary Taps being utilized fully to keep the voltage up? 226 Volts at the Pump House Secondary would make it harder for the Motors to Start. Just a thought.
JimO
 
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