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The principle of operation of the device for differential
protection, type DT will be demonstrated on an example of the device for
differential protection of two-winding transformers. The single-line block
diagram is indicated In fig. 1.
The device comprises in each phase:
-differential transformer (DT)
-restrain ing transformer (ST)
-locking assembly
-electronic assembly
Under normal operating conditions, current I1 -I2 passes through the differential
transformer circuit, and current (I1 + I2 )/2 through the restraining
transformer circuit. Current (I1) is proportional to the primary current,
and current (I2) to the secondary current of the protected transformer.
The proportionality coefficients have been so selected that under normal
operating conditions I1 = I2 , so there will be no current running through
the differential transformer. The current running hereby through the restraining
transformer is I1; at the comparator circuit in- put in the electronic
assembly, there exists a signal which prevents comparator activation,
thus the differential protection device will not operate.
The device for differential protection is so designed that no operation
shall take place in case of short circuits outside the protected zone.
However, any fault occuring within the protected zone reduces the effect
of the restraining current, and thus i mproves the device sensitivity
to such types of faults.
In the case of short circuits within the protected zone (and in the case
that the transformer lower voltage side is connected onto other power
sources in the network) , there runs a sum of currents I1 and I2 through
the differential transformer circuit, and through the restraining transformer
circuit their difference; at the comparator circuit input there exists
only the signal which activates the comparator, which results in practically
instantaneous operation of the differential protection device. Due to
magnetizing currents and errors in the current and interposed transformers
ratios (specially if tapchanging equipment is present) a certain amount
of debalance current usually flows through the differential transformer.
Consequently, the lowest differential protection setting is so selected
as to allow for the protection of power transformers with tap-changing
facilltys within the limits of ± 15% Un.
When a power transformer is energized, particularly if its secondary is
at no-load, a very high magnetizing inrush current (in an amount up to
10 In) can flow through its primary winding, which causes current I1 in
the input circuit of the differential protection device. Considering that
current I2 is hereby zero, it follovvs that the entire input current I1
is differential current, thus operation of the differential protection
device would result. In order to prevent such unwonted operation, a locking
assembly is included in the structure of the differential protection device,
which determines on the basis of the differential current harmonic contents
(second harmonic) whether magnetizing current or fault current is in question,
and cansequently forbids or permits the differential protection device
to respond.
The operating characteristics of the differential protection device at
20% In setting is indicated in fig. 2,. where Id stands for dIfferential
current, and Is for restraining current.
The area above knee-shaped operating cahacteristic represents the operating
zone, and the area below it the in sensitivity zone of the differential
protection device.
According to figs. 1. and 3. it follows that each phase
of the differential protection device operates quite indepen- dently,
and that the only blocks common for all three phases are the electromechanical
actuating relay and the comparator of high differential currents Id >>
.
Information on restraining and differential current are fed to the summing
block, and if their proportion is such as to activate the comparator circuit,
the differential protection device will be operated (the actuating relay
contacts close). In that case the comparator circuit also generates an
electric signal required to operate the signalling module, if signalling
of faulty phase is required. The signalling module, when operated by the
comparator output signal, energizes an indicating lamp in the appropriate
phase, indicating thus that the device has operated and identifies tile
faulty phase. Resetting of the signalling module (to turn out indicating
lamp) is performed by depressing button K. By removing a jumper across
the terminals 26 and 27 (fig. 3) it is possible to effect remote resetting
of the signalling on the differential protection device. However, no signalling
resetting is possible if, for any reason, the protected object has not
been cut-out. If high differential current occurs as a consequence of
magnetizing current at the power transformer energisation, the locking
assembly locks the comparator circuit operation, thus-preventing faulty
operation of the device. In case of very high differential currents occuring
(in excess of the amount set on the comparator Id >>, the device
will be activated, without regard to the amount of the restraining current
and differential current harmonic contents.
Inputs A1, C1, A2, C2, A3, C3 represent connections of the block strcture
indicated in fig. 1 (for phase B only) with corresponding blocks of the
phases A and C.
The devices for differential protection are sol id state units (static
construction). Internal current transformers at the input and auxiliary
relays of fairly high breaking capacity at the output, provide for a galvanic
isolation of the electronic circuitry of the device from the power circuits.
The device is housed in a standard case for surface mounting
(see the dimensions i n fig. 6) .The front plate (under a glazed cover)
bears the functional diagram with the connection diagram (fig. 3), operation
setting elements (jumpers) and indicating lamps with associated resetting
button. The top terminals (arranged also under the cover) are equipped
with jumpers (across the terminals 15 and 17; 19 and 21; 23 and 25) which
enable simple checking of proper connection of the interposed transformers
and measurement of current through the differential transformer in operation.
Standard interposed transformers are used with the DT type relays for
differential protection. The choice of interposed transformers and the
type of their connection are determined by the type of connection of the
protected object, and can be made according to table 1 and fig. 5.
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