I2 Install Transformers Flashcards
Advantages of a single, Three-phase transformer
Operating efficiency is higher
Weighs less and requires less space
Cost of a single 3 phase transformer is less
Advantages of a bank of three single-phase transformers
Can be operated at a reduced kVA if one goes down
The cost of a spare unit is less
Wye to delta and delta to wye connections must always have
a phase displacement or 30 degrees withe the high side leading the low side.
Wye to Wye transformer connections should be made so that
phase shift is 0 degrees
In theory if the connections are made correct then the closure voltage should be
Zero volts.
If a mistake is made on the connections what will the closure voltage read?
Double the line-voltage value
Open delta transformer bank can only supply
57.7% of a balanced three-phase closed-delta bank.
How are the high-voltage and low-voltage terminals of a three-phase transformer normally marked?
High H1, H2, H3
Low X1, X2, X3, X0
Before connecting the load, what test should be performed on the secondary of a three-phase transformer band with a wye-connected secondary.
a voltage test to determine that all line-to-line voltages are equal
what test must be performed on the secondary of a three-phase transformer bank with a delta-connected secondary?
Mesh test or delta closure test
what advantages does a autotransformer have over isolted-winding transformer
costs less to manufacture
smaller
coil and core losses decrease, more efficient
lower percent impedance which gives good voltage regulation from no-load to full-load
the much lower impedance of autotransformer means
it has a higher available fault current
Disadvantages of an autotransformer
no electrical isolation
and open in the coil may cause the primary voltage to be impressed across the load
high fault current
when is a zig-zag autotransformer used?
to filter out third harmonics
Why must we never open the secondary cct of a live CT!!
risk of electric shock
