Controlled Switching — Buyer´s Guide
F-3
Edition 2, 2006-09
Switching of No-load Power Transformers
Power Transformers
Application
Opening Operations
No-load transformer currents are relatively
small, and current chopping effects are
therefore more pronounced than for corre-
sponding interruption of shunt reactors. The
natural oscillations of the transformer wind-
ings, however, are much less pronounced
and more strongly damped. Therefore, the
overvoltages generated during opening op-
erations are low in amplitude and normally
considered quite harmless. For limiting over-
voltages, controlled opening operations are
thus not needed.
After interruption, a residual flux may remain
in the transformer core and influence the
subsequent making conditions. Interruption
at natural current zero will lead to the low-
est residual flux. However, since the no-load
current is so small, it may quite easily be
chopped by the circuit breaker relatively far
from natural current zero, and this may lead
to high residual flux, see Figure 4.
In certain cases, when the connecting leads
to the transformer have sufficient capaci-
tance, interruption may be associated with
a damped oscillation (related to the trans-
former inductance and the surrounding ca-
pacitance). Such oscillation will decrease the
residual flux towards zero.
Figure 5.
Inrush current at not controlled energizing of un-
loaded power transformer
Figure 4.
Magnetic flux in transformer core at rated voltage
Se
ve
ra
lk
A
1 cycle
Magnetizing current
Peak flux at rated voltage
Flux
Residual flux level
Control of Opening Operations
In situations where there may be residual
flux, controlled opening operations may be
utilized to control its magnitude and polar-
ity. By using controlled opening operations
as a support for the subsequent controlled
closing operations, the inrush current can be
limited even further.
Closing Operations: Inrush Current
The magnetic flux in the transformer core
is proportional to the integral of the voltage
across a winding. Under steady state condi-
tions, both voltage and flux are symmetrical
and sinusoidal, as exemplified in Figure .
The situation may be quite different when en-
ergizing the circuit at an unfavorable instanta-
neous voltage value. In a single-phase case,
and disregarding residual flux, the worst
situation would be when making occurs at
voltage zero, see Figure 2. In this case, the
flux is initially forced up to a peak value twice
as high as in stationary operation, it will be
highly asymmetrical and only slowly attain its
normal symmetrical shape. Consequently,
the core will be driven far into saturation,
and the corresponding current – the inrush
current – will be extremely asymmetrical and
non-sinusoidal, see Figure 5. Depending on
the polarity, any residual flux in the core at
the instant of energizing may add to the flux
related to the voltage, and make the situation
even more severe.
A typical inrush current may have peak val-
ues reaching several kA. The result is me-
chanical stress on the transformer windings,
interference on secondary circuits from high
zero-sequence current, and network distur-
bances by current harmonics.
