Master student Tsyplakova D.D., Ph.D. Dyakov I.A.
Tambov State Technical
University, Russia
The use of differential logic protection
reverse order.
Recently
increased the number of cases of failure of power transformers due to the
natural aging of the windings and unauthorized discharge of oil from the
transformer tank, resulting in irreversible processes occur in the windings.
Periodic
maintenance and repair, diagnostics, testing of transformers on their
suitability for further use do not exclude the possibility of damage. To
minimize damage, you must apply a special method of relay protection of
transformer differential protection logic reverse sequence without time-delay
(DZT).
Operating principle: to improve reliability
and sensitivity of protection, it seems appropriate to apply differential logic protection reverse order, which operates under
unbalanced short circuits (KZ) in the transformer. For its implementation it is
necessary to use the device to filter negative-sequence current three-phase
electrical installation, made with one differential relay series PHT [2]. The
filter captures the negative phase-sequence current and not responding to
three-phase short-circuit and overload currents.
The application
of the DZT to the device to filter negative-sequence current, is made with series relay RNT is a power relay reverse order, makes it possible to significantly
reduce the current protection operation by detuning from the magnetizing
current when the transformer is under voltage, and also during recovery after
short-circuit clearance, as the negative phase-sequence current is
significantly less than the current direct sequence (Fig.
1).
Fig.1 - direct
Currents (a), inverse (b) zero (C) sequences when the transformer
TMN-10000/35 at idle. Fig.2 - logical Differential protection negative
sequence transformer.
The magnetizing current is of
asymmetric shape of the curve and are therefore poorly transformed through
bystronasyschayuschiesya transformer relay PHT. Three-phase fault inside the
transformer is unlikely because of the strength of interphase insulation. KAZ
differential relay is connected to the current transformers TA1 and TA2 from
the high voltage (HV) and low voltage (LV) transformer. Power relay KW1 and
current relay KA1 and KA2 are connected to the current transformer TA3 by NN
(Fig.2). In the absence of a reserve winding of the transformer with the LV
these relays are connected in series with the differential relay KAZ.
Autotransformers ÂÓ1–ÂÓ3 serve to align
the secondary currents of the current transformers.
With an unbalanced short circuit in
the transformer to trigger the relay KAZ and KW1. Relay KW1 is triggered when
feeding the fault location motors. After the closing of relay contact KAZ and
KW1 KL1 relay whose contacts operate to disconnect the transformer. In the
absence of current feeding the relay contacts are closed KA1 and KA2 in the
circuit of relay KL1. To ensure protection from the initial value of current in
asymmetric fault conditions (which can turn into an unlikely three-phase fault)
relay KL1 works to slow down when you return.
At three-phase short-circuit damage to gas off protection and
current protection.
When the
transformer is in idle mode on the damage in the transformer relay ÊW1 does not work. However, in the closed
condition of the relay contacts KA1 and KA2, as well as the contacts of the
switch Q2 on the LV side. After the closing of relay contact ÊAZ relay KL1 will work with the action to disconnect
the transformer. Unsymmetrical external fault on the LV side protection has no
effect because the relay KW1 is not working. Unsymmetrical external fault on
the HV side protection is also not valid, since the relay KAZ does not work.
The primary
current of the negative sequence relay KAZ is chosen on the basis of the
detuning from negative sequence current due to unbalance when the transformer is idling and when the feeding motor short circuit on
the HV side:
I2ÑÇ = ÊÎÒÑ · IÍÅÑ ,
(1)
whereÊÎÒÑ – factor detuning (ÊÎÒÑ = 1,2);
IÍÅÑ – current unbalance
negative sequence.
The current of
the relay is determined by the expression:
where ÊÑÕ – factor of the circuit connections of
current transformers (it is assumed that ÊÑÕ = 1,73);
KTT –
the transformation ratio of current transformers with the HV.
The number of
turns of the equalizing winding and the differential apply (2) and is defined
by the expression (3):
The primary current of the relay KW1
is equal to the current ²2ÑÇ. Relay KA1 and KA2 must be thermally stable to
prolonged load currents of the transformer.
Electromechanical protective relays
of the transformer provide a solution to all the problems that should be solved
by devices of relay protection. Sensitivity to electromagnetic interference
devices of relay protection microprocessor element base is several orders of
magnitude higher than conventional. Therefore, you should specify that with a
simple parallel connection of the output contacts of the relay KL1 and MP
devices the likelihood of incorrect-positives increases, not decreases.
To ensure
effective protection from excessive triggering of the MP devices to
dramatically increase the level of their electromagnetic immunity. Only in this
case it is possible to apply redundancy by including on the parallel output
pins of microprocessor and Electromechanical relays.
References:
1.Ermilov, A. A.
Foundations of industrial power.- 4-e Izd., Rev.
and extra-M.: Energoatomizdat, 1983
2.Zasypkin A. S. relay protection of transformer. M.:
Energoatomizdat, 1989
3.Guide to electricity and electrical equipment / ed. by
A. A. Fedorov.
In 2 V. – M., Energoatomizdat, 1986