05-07-2012, 12:50 PM
Inrush Current Control in Transformers
Inrush Current Control.pdf (Size: 94.84 KB / Downloads: 143)
Abstract
The paper deals with the problem of magnetizing inrush current phenomenon in
transformers and describes the design of a robust time delay relay which can be used to
control the inrush current into transformers. Test results are presented on the
performance of the time delay relay to demonstrate the effectiveness of the relay in
reducing the switch on inrush current into a small laboratory transformer.
Introduction
When a transformer is switched on to
line, at times the circuit breaker trips or their
fuse blows. This happens even if the
transformer is on no load with its secondary
open. This is due to the heavy magnetizing
current drawn by the transformer. This current
may reach a level exceeding the full load
current. However, this heavy inrush current
magnitude depends on the instant on the a.c
wave at which the transformer is switched on.
If the instant happens to be such that the a.c
voltage wave is going through its peak value,
then there will be no inrush current drawn by
the transformer. The magnitude of the current
in this case will be at normal no load value.
Time Delay Relay Circuit
Refer to Fig. 2 wherein the time delay
relay circuit and its connection to a load such
as a transformer is shown. As soon as the
transformer is switched on the TDR comes into
action. The series resistor R limits the initial
current drawn by the transformer. After a
predetermined time the relay cuts out the
resistor R by short circuiting it through its NO
contact - thus allowing the normal transformer
operation. The design and working of the TDR
is as described below.
Test Results
The test results are presented in two parts:
(a) The TDR is tested for its pick up and drop
out performance as shown in the recordings in
Figs. 3 and 4. It was found that the pick up
voltage is 6 V and took 330 ms to close the NO
contact. This is the time duration in which the
series resistor is in the circuit. The drop out
voltage was 4.5 V and took 500 ms to open the
contact to insert the resistance R in the circuit
again. (b) The recording in Fig. 5 shows the
current drawn by the transformer without the
TDR and series resistor in place. The
transformer was rated at 1Kva at 220volts. The
switch on in rush current was seen to be 32
amperes while its full load current was 4.5 amp.
The recording in fig.6 is with a resistance of 50
ohm connected in series with the primary and
with TDR in place. The maximum switch on
current in this case is limited to about 4 ampere.
Clearly with TDR the inrush current is contained.
Conclusion
The inrush current drawn by a transformer
may reach 5 to 6 times its full load current and
will last for a few cycles time. This may result in
nuisance tripping of circuit breaker. A resistor
inserted in series with primary will reduce the
inrush current during switch on. This series
resistor has to be cut out. Otherwise the normal
operation of the transformer will be impaired. A
time delay relay as described in this paper can be
used advantageously for this purpose. The inrush
current can be reduced to tolerable limits by
selecting a suitable value for the series resistor.