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Fig. 2. Setting range of the characteristic curve of the biased differential relay SPAD 330 C.
In power transformer protection the stabilization
of the operation with regard to the restraint
current is not enough to ensure a correct
operation of the relay under all operation
conditions .
The connection of the power transformer to the
network causes a connection inrush current,
the amplitude of which may be many times the
magnitude of rated current and which may
have a half-time of up to several seconds. To
the differential relay this connection inrush
current represents pure differential current,
and would make the relay operate almost
always when the transformer is connected to
the network. To eliminate the effect of the
connection inrush current the differential relay
is provided with a blocking function, the
operation of which is based on the occurrence
of a second harmonic component of the
differential current. The second harmonic
component in the connection inrush current
may vary in the range 12...100 %. The operation
of the differential relay is blocked when the
second harmonic component of the current on
one of the phases exceeds the set blocking
level. The blocking level is adjustable within
the range 10...20 % of the differential current.
The unintended operations caused by partial
saturation of the current transformers are
prevented not only through stabilization. The
unintended operations are also prevented in
4
4.0
Id
In
Curve p/%
1
50
3.0
2
20
2.0
1.0
0.5
0.0
0.0
0.5
1.0
s/%
50
10
1.
2.0
2.5
3.0
4.0
such a way that the measurement of the
differential current and a possible tripping
decision are based only on current samples
measured during the quarter-cycle following
the zero crossing of the HV and LV side phase
currents. This lowers the requirements on the
current reproduction capability of the current
transformers.
The differential relay is also provided with a
high-set instantaneous stage. The setting range
of the high-set stage is 20...30 x I
selected so that the relay does not operate on
the high-set stage until high above the highest
unsymmetrical connection inrush current
levels. As the bias current goes below approx.
33 % of the differential current the setting value
of the high-set stage is automatically halved,
cause under such circumstances a fault must
exist within the protected area. The load current
has no stabilizing effect on the operation of the
high-set stage.
In transformer protection it has previously been
necessary to use intermediate current trans-
formers in association with differential relays,
in order to match the vector group and the sec-
ondary currents of the main current transform-
ers. In most cases, when using SPAD 330C,
the intermediate transformers can be omitted,
because the matching can be made by using
the switches and the setting knobs on the front
panel of the relay modules.
2.
5.0
6.0
Ib
In
. It has been
n
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