1 Overview Voltage transformer is an important equipment in the operation of power system. It is the contact element between primary system and secondary system in AC circuit. It is used to transmit information to supply measuring instruments, instruments, protection and control devices. It belongs to special transformers and works. The principle is basically the same as the transformer. The basic structure is mainly composed of a primary winding, a secondary winding, and an iron core. Matching insulation measures are provided between the primary winding and the secondary winding and the iron core. Under normal circumstances, the secondary loop voltage is proportional to the primary loop voltage.
It is not difficult to see from the principle characteristics of the voltage transformer that the secondary winding cannot be short-circuited or grounded. The magnitude of the secondary voltage, related to the primary voltage, the magnetic potential produced by the secondary voltage, balances the primary voltage magnetic potential. If a secondary circuit short-circuit fault occurs, the impedance is infinity, the secondary voltage is equal to zero, and the magnetic potential is also equal to zero. A primary voltage will act on the excitation, causing the core to be heavily saturated and the sinusoidal alternating magnetic flux to become a trapezoidal wave. The winding will induce a large current, magnetic saturation will increase the iron loss and generate heat, and when the duration is long, the insulating performance of the winding will be reduced or burned out. At the same time, it will also cause the secondary side fuse fuse to blow, affect the meter: In severe cases, it may cause the protection device to malfunction and burn the voltage transformer. To this end, the national standard DL408-91 "Working Safety Regulations" Chapter 10 "relay protection, instrumentation and other secondary circuit work requirements" emphasizes "strictly prevent the secondary circuit of live voltage transformers short circuit or grounding."
2 Common Causes There are many causes of short-circuit faults in the secondary circuit of the voltage transformer. Several common causes are briefly described below.
(1) The short circuit of the connecting cable in the loop.
(2) The secondary loop conductors are grounded by moisture, corrosion, and damage, and they develop a two-phase ground fault.
(3) There is a metal short defect inside, causing a short circuit in the secondary circuit.
(4) The outdoor terminal box is heavily dampened, and the terminal joints are rusted.
(5) Potential problems in voltage transformer wiring.
(6) Forgetting during pre-test and maintenance.
3 Regular inspection of short-circuit faults can be judged from the following phenomena through inspections to identify defects:
(1) In the operation of the voltage transformer, the body has large non-uniform noise.
(2) When the voltage transformer is running, the body has a higher temperature rise and has a greater odor.
(3) The meter indication is abnormal and the protection device malfunctions.
(4) Voltage transformer burnout, secondary winding burnout.
4 On-line monitoring and protection Since voltage transformers are used in a large number of power system operations and there are multiple sets of windings on the secondary side of each device, it is difficult to detect fault defects in a timely manner during inspections by operating personnel, and the defects are long-standing. May eventually cause major personal and equipment accidents. Here is a method,
The working principle of Fig. 1: The monitoring unit monitors the current in the secondary loops of the PT separately, and when the loop current rises to a certain value for a certain period of time [Note: The critical values ​​of protection, alarm current and protection action time can be set ( Initial value: current ≥8A, 20ms)], considered as a short circuit of the secondary circuit, at this time, the signal of the protection unit is issued, so that the protection unit is actuated and disconnected (the impedance matching function of the unit is turned on) and issued simultaneously. The fault point instructs the alarm unit (installed in the main control room) to sound and light alarms and indicate the loop where the fault point is located, so that the maintenance personnel can find the fault circuit in time.
It can not only perform on-line monitoring, but also has the function of eliminating the automatic short-circuit defect of the secondary circuit of the voltage transformer and indicating the fault point.
5 Conclusion (1) Voltage transformer secondary circuit short-circuit fault can be used to strengthen the inspection method, through the phenomenon of faults found defects, can also be used online monitoring methods, timely detection of hidden faults, timely replacement, so as to avoid the fault on the human body And equipment safety hazards.
(2) The method of real-time monitoring of the secondary circuit short-circuit fault of the voltage transformer is also in compliance with the provisions of the DL408-91 guideline.
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