Electrical Machine - IES Academy

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Transformer
Chapter-1
Load test on three-phase transformers
The primary and secondary windings are
connected in delta. The lv winding is excited
at normal voltage and frequency. W1 and W2
record the total core loss. The voltage injected
in hv winding in open-delta is adjusted till full
load current flows in it (hence, in lv winding
also). W3 records ohmic losses in all three
phases. The magnitude of the low-injected
voltage in the open delta is equal to (full load
current) (3 zeH).
Parallel Operation
Parallel operation increases load sharing capacity by sharing more current.
Advantages of parallel operation:
1. Improved reliability.
2. Operation becomes more economic and efficient.
Conditions for Parallel Operation:
1. The transformers must have the same voltage
ratios otherwise a circulating current will flow.
2. The per unit leakage impedances of the
transformers based on their own kVA ratings
must be equal otherwise transformer with
smaller leakage impedance will share more
load.
3. The ratio of equivalent leakage reactance to
equivalent resistance, i.e. xe/re should be same
for all the transformers.
4. The transformers should be connected in correct
polarity, otherwise large circulating currents
will flow. (condition to be followed strictly).
No – load operation
If no-load secondary voltages Ea
and Eb
are unequal or are out of phase, then the resultant
Ea
− Eb
voltage Ea
- Eb
will circulate a current Ic given by I c =
zea
+ zeb
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