Voltage transformer is an important equipment component in the operation of power system. It is a communication element between the primary system and the secondary system in the AC circuit. It is used to transmit information to measuring instruments, meters, protection and control devices, etc. It belongs to special transformers and its working principle is basically the same as that of transformers. The basic structure is mainly composed of primary winding, secondary winding and iron core. There are matching insulation measures between the primary and secondary windings and the iron core. Under normal circumstances, the secondary circuit voltage is proportional to the primary circuit voltage.
It is not difficult to see from the principle characteristics of the voltage transformer that its secondary winding cannot be short-circuited or grounded. The size of the secondary voltage is related to the primary voltage, and the magnetic potential generated by the secondary voltage balances the magnetic potential of the primary voltage. If a short-circuit fault occurs in the secondary circuit, the impedance is infinite, the secondary voltage is equal to zero, and the magnetic potential is also equal to zero. The primary voltage will act entirely on the excitation, causing the iron core to be severely saturated, and the sinusoidal alternating magnetic flux to become a trapezoidal wave. The secondary winding will induce a large current, and the magnetic saturation will increase the iron loss and heat. If it lasts for a long time, the insulation performance of the winding will decrease or burn out. At the same time, it will also cause the fuse of the secondary side fuse to melt, affecting the meter: in severe cases, it may cause the protection device to malfunction and burn the voltage transformer. For this reason, the national standard DL408-91 "Electric Power Safety Work Regulations" Chapter 10 "Working Requirements on Secondary Circuits such as Relay Protection and Instruments" emphasizes "strictly prevent the secondary circuit of the live voltage transformer from being short-circuited or grounded."
1. Common causes
There are many reasons for the short circuit of the secondary circuit of the voltage transformer. Here are some common reasons!
(1) The connecting cable in the circuit is short-circuited.
(2) The secondary circuit wire is damp, corroded and damaged, causing one phase to be grounded, and then developing into a two-phase ground short circuit.
(3) There is a metal short circuit defect inside, causing a short circuit in the secondary circuit.
(4) The outdoor terminal box is seriously damp, and rust occurs at the terminal connection.
(5) Hidden dangers in the voltage transformer wiring.
(6) Forgotten during the pre-test and maintenance process.
2. Routine inspection
Short circuit faults can be diagnosed through inspections and the following phenomena can be used to identify defects:
(1) The connected meter indication is abnormal and the protection device malfunctions.
(2) The voltage transformer is burned out or the secondary winding is burned out.
(3) During the operation of the voltage transformer, the body has a large uneven noise.
(4) During the operation of the voltage transformer, the body has a high temperature rise and a strong odor.
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