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Id
In
2
1
Id1
1
Fig.1. Theoretical characteristic curve of a biased differential relay.
The effect of the bias function on the operation
of the relay is graphically illustrated by the
characteristic curve presented in Fig. 2.
The differential relay has two adjustable
parameters, the basic setting p of the differential
current and the starting ratio setting s. Using
the designations in Fig. 2 the settings p and s
can be specified as follows:
p = I
/ I
d1
n
s = I
/ I
d2
b2
The slope of the characteristic curve varies in
the different sections:
Partial range 1
In the range I
= 0...0.5 x I
b
n
required for the relay to operate is constant.
The value of the differential current is equal to
the basic setting value p of the relay. The basic
setting primarily allows for differential currents
caused by a no-load situation of the power
transformer, but it may also affect the whole
level of the characteristic curve. At rated voltage
the no-load losses of the power transformer
can be expressed as tenths of a per cent only.
But if the voltage of the power transformer
suddenly rises due to an operational disturb-
ance, the magnetizing current of the transformer
consequently rises. Normally, the magnetic
flux density of the power transformer is rather
high at rated voltage, and then a voltage rise of
a couple per cents causes the magnetizing
current to rise by several tens of units of per
cent. This fact is considered in the basic setting.
Ib2
2
1
2
Partial range 2
The range between 0.5 x I
the influence area of the starting ratio s. By
setting the starting ratio, it is possible to
influence the slope of the characteristic curve
in this area, i.e. how big a change in the
differential current, in relation to the load current
change, is required for tripping. The starting
ratio makes allowance for errors of the current
(3)
transformers and changes of the tap-changer
(4)
position. When using current transformers of
the 10P accuracy class, the current error in the
rated current range does not exceed 3 %.
Hence the errors caused by the current
transformers may be in the class of 3+3 % =
6 % at the most.
the differential current
The starting ratio must not be too high, because
the sensitivity of the differential relay to interturn
faults of the power transformer depends
primarily on the starting ratio.
Partial range 3
At high bias current values, I
characteristic curve has a constant slope,
approx. 100 %. This means that the increase
in the differential current required for relay to
tripping equals to the increase in the corre-
sponding bias current.
The actual characteristic curve and the setting
range of the biased differential relay are
illustrated in Fig. 2. The setting values can be
adjusted within the shaded area.
Ib3
Id2
4
3
3
n
B
Id3
5
Ib
In
< I
< 2.5 x I
is called
b
n
> 2.5 x I
, the
b
n
3
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