Transformer connections
True
A lighter transformer can be found in a bank with a Delta secondary (T/F)
Wye
A service voltage of 120/208 is provided by what transformer connection?
4 WYE-WYE WYE-Delta Delta-WYE Delta-Delta
A three phase transformer can be connected in how many different combinations?
Service voltage of the customer
The secondary connection is determined by the ....
Than the neural to hot leg voltage
The voltage between the neutral and the power leg is 1.73 times greater than what?
Power leg
This can be known as the high leg, wild leg, or stinger leg.
Power transformers
Transformers that share somewhat equal portions of single phase and three phase load and smaller than the lighter transformer
WYE- Delta bank
Transformers with two primary bushings must be used for this type of connection because the Wye point is floated.
- all three transformers can share the single phase load equally - they can be built using single or dual primary bushing transformers
WYE connected banks have two main advantages...
If the primary coil voltage ratings of the transformers match the phase to phase voltage of the system, connect Delta
What is the Delta primary connection rule?
Grounding strap must be removed, x2 must be insulated
When banking pad mounted transformers, what needs to be done?
False
When paralleling the coils connect the AC and BD pair to x1 and x3? (T/F)
Alley Cay Bad Dog The A and C ends connect to one secondary bushing, and the B and D ends connect to another secondary bushing
When paralleling the coils internally, how should it be connected?
Three phase
When supplying three-phase power from underground systems, the transformers used are ____________ transformers
Wye-Delta
Which connection requires the primary WYE point to be floated?
Because it is not grounded
Why is floating the WYE hazardous to the lineworker?
X3
On a WYE-WYE bank which secondary lug is usually left idle?
If the primary coil voltage ratings of the transformer match the phase to neutral voltage of the system, connect WYE
What is the WYE primary connection rule?
Delta
What type of bank is this used for?
False
A lighter transformer can be found in a bank with a Wye secondary (T/F)
Open WYE-Open Delta
Although banking transformers in an underground system is rare, some utilities install __________ -__________ banks using two single phase pad-mounted transformers
Yes With a wye transformer connection Remember: secondary coil arrangement must be parallel
Can a bank of 120/240 volt transformers provide 120/208 volt three phase service? How?
False
Delta secondaries allow all three transformers to share the single load equally (T/F)
True one transformer must provide the majority of the lighting load
Delta secondaries do not allow all three transformers to share the single load equally (T/F)
By comparing the primary coil voltage rating of the transformer with the system voltage
How is the primary connection determined?
Delta only
If a transformer has only one number for a primary coil voltage, how should it be connected?
Yes
If one transformer is damaged in a WYE delta bank can this be connected in a open bank to still provide 3 phase service?
Connected WYE
If the phase to phase voltage of the service needed is 1.73 times the phase to neutral voltage of the service needed, how should the secondary be connected?
Connected Delta
If the phase to phase voltage of the service needed is 2 times the phase to neutral voltage of the service needed, how should the secondary be connected?
Either a turns ratio test or a physical check by taking the lid off
If you don't know the configuration of your coils ( parallel or series) how can you check?
True
Lineworkers who understand transformer banking know that determining the primary connection is a separate consideration that determining the secondary connection (T/F)
120 volts
One a typical 120/240 volt single phase transformer, When the two secondary coils are connected together in parallel inside the tank, the the secondary voltage will be ...
120/240 volts
One a typical 120/240 volt single phase transformer, When the two secondary coils are connected together in series inside the tank, the the secondary voltage will be ...
If there is a primary system disturbance or load imbalance this will subject the transformer to SEVER overloading
What is the danger of grounding the WYE point of a WYE-Delta?
If one transformer fails, the customer is down to one phase until the defective transformer is fixed or replaced
What is the disadvantage of a WYE connected bank?
Corner ground
What is used when customers need only three-phase three-wire services
Corner ground
What is used when the NESC still requires a ground reference on a three phase system without needing to install a neutral conductor
WYE
What type of bank is this used for?
The proper voltage would still be delivered, but the kVA rating of the bank would be cut in half
What would happen if WYE secondaries were connected but the coils were series?
Floated (not grounded)
With a WYE - Delta bank, each transformer is connected to the system and the Wye point is...
One hot leg on the secondary side to deliver three phase power
With a WYE-WYE bank, each of the three transformers provide....
No
With a WYE-WYE bank, will an open bank provide three phase power if the secondary were connected open WYE?
Parallel
_________ the secondary coil when making WYE secondary connections
A ground is attached to the winding midpoint of one of the transformers
A power leg only exists on Delta and open Delta secondaries when .....