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3. OPERATION PRINCIPLES
3.2.
DIFFERENTIAL UNIT
Figures 1 and 2 represent the simplified connection diagram of the differential protection and its
behaviour with internal and external faults respectively, without any saturation on C.T. cores.
Auxiliary intermediate current transformers are aimed at equalising the currents received by the relay for
every input position, since the main transformers may have a different transformation ratio. They have been
specially designed to provide a homogeneous response (same saturation characteristic) for all the inputs to
the measure unit, thus allowing the use of main transformers with different characteristics.
The V
current is the operation magnitude and it is proportional to the differential current. The V
D
is the restraint magnitude and it is proportional to the sum of the currents of all the positions associated to
the bus to be protected.
In ideal conditions, for an external fault, current flows through the input circuits of the different positions
without differential current; thus, V
an internal fault, all the fault current goes through the differential circuit which makes V
Figure 15 shows the block diagram of the percentage restraint differential unit and the supervision
differential unit.
For the main measure unit, V
value part of the V
restraint voltage value obtaining thus a combined signal which is applied to a level
F
detector. The restraint current ratio K subtracted from the differential voltage is called restraint percentage
and it determines the operation characteristic of the unit as well as its sensitivity.
The level detector is a fixed V
1.5 milliseconds and a reset time of 40 milliseconds in order to ensure a constant signal in the output relay.
The V
level of the detector is calculated so that the unit may produce an output when the I
o
magnitude is over 0.1 Rms. Figure 3 shows the operation characteristic corresponding to this equation.
In the case of internal faults, we assume that no current transformer is saturated and therefore the
equivalent circuit with its corresponding current distribution is that of
Note that in these conditions all the fault current will pass through the differential unit. From the design of the
circuit we have:
N
= N
= N (1) Input transformer's ratio
ED
EF
R
= R
= R (2) Load resistance of the restraint and differential transformers.
D
F
By analysing the behaviour of the differential unit in the first half cycle of the current at a 50 Hz rated
frequency in the network we will have:
3-2
is zero and V
D
and V
voltages are applied to a sum circuit which subtracts from the V
D
F
threshold level comparator (factory adjusted), with an operation time of
O
V
0
18T
90 -9T
0
90
BUS1000 Busbar Protection
is equal to twice the value of the fault current, whereas for
F
3.2.1. BEHAVIOUR WITH INTERNAL FAULTS
fig. 1.
V
-KV
D
F
180
voltage
F
and V
equal.
D
F
D
- K I
D
F
GEK-98514B
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