WattsJr
New member
- Location
- Huntsville, Al. US
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.
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.