Discussion on short circuit of secondary circuit o

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Brief discussion on short circuit of secondary circuit of voltage transformer Abstract: This paper introduces the short circuit of secondary circuit of voltage transformer, common fault causes, routine inspection and patrol inspection methods, and describes the monitoring and preventive measures for short circuit fault of PT secondary circuit

1 overview

voltage transformer is an important equipment component in the operation of power system. It is a connecting element between primary system and secondary system in AC circuit. It is used to transmit information to supply measuring instruments, meters, protection, control devices, etc. it is a special transformer, and its working principle is basically the same as that of transformer. The basic structure is mainly composed of primary winding, secondary winding and iron core. There are matching insulation measures between the primary winding, secondary winding and iron core. Under normal conditions, the secondary circuit voltage is directly proportional to the primary circuit 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 is related to the primary voltage. The magnetic potential generated by the secondary voltage balances the magnetic potential of the primary voltage. In case of secondary circuit short-circuit fault, the impedance is infinite, the secondary voltage is equal to zero, and the magnetic potential is also equal to zero. The primary voltage will all act on the excitation, so that the iron core is seriously saturated, and the sinusoidal alternating magnetic flux becomes trapezoidal wave. The secondary winding will induce large current. The magnetic saturation will increase the iron loss and generate heat. If it lasts for a long time, the insulation performance of the winding will be reduced or burned out. At the same time, it will also cause the fuse of the secondary side fuse to blow, affecting the meter: the shore hardness is seriously characterized by the value of L. the larger the value of L, the misoperation of the protection device and the burning of the voltage mutual may be caused. 2. Precautions: the visual sensor. For this reason, the national standard dl408-91 "safety work regulations for electric power industry" emphasizes strict prevention of short circuit or grounding in the secondary circuit of live voltage transformer in Chapter 10 of the work requirements for relay protection, instruments and other secondary circuits

2 common causes

there are many causes of short circuit fault in the secondary circuit of voltage transformer. The following is a brief description of several common causes

(1) the connecting cable in the circuit is short circuited

(2) one phase grounding occurs due to moisture, corrosion and damage of secondary circuit conductor, and then develops into two-phase grounding short circuit

(3) there is metal short-circuit defect inside, resulting in short circuit of secondary circuit

(4) the outdoor terminal box is seriously damp and the terminal connection is rusted

(5) hidden dangers in voltage transformer wiring

(6) forgotten during pre-test and maintenance

3 routine inspection

short circuit fault can be judged from the following phenomena through patrol inspection and defects are found:

(1) during the operation of the voltage transformer, the main body has large uneven noise

(2) when the voltage transformer operates, the body has a high temperature rise and has a large peculiar smell

(3) the indication of the connected meter is abnormal, and the protection device malfunctions

(4) voltage transformer and secondary winding are burnt out

4 monitoring and protection

since there are a large number of voltage transformers in the operation of the power system, and there are multiple groups of windings at the secondary side of each equipment, it is difficult for the operators to find fault defects in time during the inspection process, making the defects exist for a long time, which may eventually cause major personal and equipment accidents. The following is a method. The working principle of Figure 1: the monitoring unit monitors the current in each circuit of PT secondary circuit respectively. When the circuit current rises to a certain value and lasts for a certain time [Note: the critical value of protection and alarm current and the protection action time can be set (initial value: current 8a, 20ms)] pressure cancellation. 2: the greater the elastic recovery after removing the air from the experimental oil pump, the greater the secondary short circuit in the same circuit, At this time, a signal is sent to the protection unit to make the protection unit act and disconnect (the unit shall be connected with impedance matching function), and an audible and visual alarm is sent to the fault point indication alarm unit (installed in the main control room) to indicate the circuit of the fault point, so that the maintenance personnel can find the circuit of the fault in time

it can not only monitor, but also automatically eliminate the defects of secondary same circuit short circuit of voltage transformer and indicate the fault point

5 conclusion

(1) the short-circuit fault of the secondary circuit of the voltage transformer can be detected through the phenomenon of the fault by strengthening the patrol inspection, and the hidden trouble can also be found and eliminated in time by monitoring, so as to avoid the hidden trouble of the fault to the safety of personnel and equipment

(2) the method of real-time monitoring the short-circuit fault of the secondary circuit of voltage transformer also conforms to the provisions in DL guidelines

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