Transformer 10

1. What is the purpose of the current transformer?

Answer: The current transformer changes the large current into a small current according to a certain ratio, provides various currents for the instrument and relay protection, and isolates the secondary system from the high voltage. It not only ensures the safety of people and equipment, but also simplifies and standardizes the manufacture of instruments and relays, and improves economic efficiency.

2. Why is it not allowed to run the current transformer for a long time?

Answer: When the current transformer is overloaded for a long time, the error will increase and the meter indication will be incorrect. In addition, due to the increase of the primary and secondary currents, the iron core and the windings are overheated, the insulation aging is fast, and even the current transformer is damaged.

3. What causes an abnormal sound in the running current transformer?

Answer: The current transformer overload, the secondary side open circuit and the internal insulation damage discharge, etc., will cause abnormal sound. In addition, the internal corona formed by the uneven coating of the semiconductor lacquer and the looseness of the clamped iron may cause the current transformer to generate a large sound.

4. Why is there no fuse on the primary side of the 110kV and above voltage transformers?

Answer: Because the structure of voltage transformers of 110kV and above adopts single-phase cascade type, the insulation strength is large, and because the 110kV system is a neutral point direct grounding system, the phases of the voltage transformer cannot withstand the line voltage for a long time, so The fuse is not installed on the primary side.

5. After the primary side fuse of the voltage transformer is blown, why is it not allowed to replace it with a normal fuse?

Answer: For example, a 10kV voltage transformer is used, and the rated current of the primary fuse is 0.5A. Fuses filled with quartz sand have better arc extinguishing performance and larger current interrupting capacity, and have the effect of limiting short-circuit current. Or ordinary fuses can not meet the current capacity requirements.

6. What fault will cause the primary and secondary fuses of voltage transformers of 35kV and below to be blown?

Answer: Internal faults of the voltage transformer, including phase-to-phase short-circuit, winding insulation damage, and short-circuit faults and resonant overvoltages at the voltage transformer outlet and the grid connection wires will cause the primary side fuse to blow. A short circuit fault in the secondary circuit will cause the secondary side fuse of the voltage transformer to blow.

7. What should you watch out for when deactivating the voltage transformer?

answer:

(1) Do not lose the voltage of the protection automatic device;

(2) Voltage switching must be performed;

(3) Prevent reverse charging and remove secondary lead wires (including capacitors);

(4) After the secondary load is completely disconnected, disconnect the primary side power supply of the transformer.

8. What are the phenomena and hazards of the secondary short circuit of the voltage transformer? Why?

Answer: The secondary short circuit of the voltage transformer will cause a large short-circuit current to the secondary coil, which will burn the voltage transformer winding, which will cause one or two breakdowns, so that the relevant protection will malfunction, and the instrument has no indication. Because the impedance of the voltage transformer itself is very small, the primary side is a constant voltage power supply. If the secondary short circuit occurs, a large short-circuit current will be generated in the secondary winding under the action of the constant voltage power supply, and the transformer is burned to damage the insulation. Second breakdown. Loss of the voltage transformer will cause the distance protection and voltage-related protection to malfunction, affecting the safety of the system, so the voltage transformer can not be short-circuited twice.

9. What is the impact of voltage transformer faults on relay protection?

Answer: The faults frequently occurring in the secondary circuit of the voltage transformer include: the fuse is blown, the contact of the isolation switch is poor, and the secondary wiring is loose. The result of the failure is to reduce or eliminate the voltage of the relay protection device. For the protection relay that reflects the voltage drop and the protection device that reflects the phase relationship between the voltage and the current, such as directional protection, impedance relay, etc., it may cause malfunction and rejection.

10. What protection should be stopped when the voltage loop is broken or the voltage transformer is faulty?

Answer: Should stop protection: current and voltage protection controlled by directional components; low voltage or composite voltage blocking overcurrent protection; current balance protection; distance protection; lateral direction protection; protection of trip voltage reduction and tripping; low frequency protection. Because the above protection is related to voltage changes, in order to prevent voltage loop disconnection or voltage transformer failure, voltage-related protection should be removed immediately.