Key Challenges for Current Transformers

1. If the secondary load impedance of the current transformer exceeds its allowable secondary load impedance. Why is the accuracy reduced? The size of the secondary load impedance of the current transformer has a great influence on the accuracy of the voltage transformer. This is because, if the secondary load impedance of the current transformer is increased a lot, and exceeds the allowable secondary load impedance, the standard value of the excitation regulator current will increase greatly, and the transformer core will be saturated. Under these conditions, a significant portion of the primary current will be used to produce the excitation regulator current, which will greatly increase the deviation of the voltage transformer, and its accuracy will decrease accordingly.

2. For the current transformer used for differential protection, it is stipulated that the transformer core is good, and the cross section of the transformer core should be increased. Why? When the system software is in normal operation or the external short-circuit fault in the differential protection category, the current value and position of the current transformers on both sides of the differential protection are the same, and there should be no current injected into the differential protection automotive relay, but in fact the two current transformers The characteristics are unlikely to be exactly the same, the excitation regulator current is different, the secondary current is not easy to be the same, and the unbalanced current will pass through the automotive relay. In order to reduce the unbalanced current, it is necessary to improve the structure of the current transformer so that it will not be saturated, or use a special ferrite core with low loss to make the transformer core, and increase the cross section of the transformer core.

3. What kinds of methods are there for the wiring of the secondary winding of the current transformer? According to the different regulations of relay protection devices and protection devices, the secondary windings of current transformers generally have the following types of wiring methods: ⑴, complete (three-phase) star connection; ⑵, incomplete (two-phase) star connection ; ⑶, delta wiring; ⑷, three-phase parallel connection to obtain zero-sequence current wiring; ⑸, two-phase distance wiring; ⑹, one phase with two current transformers connected in series; ⑺, one phase with two current transformers Wiring in series.

4. What is the wiring index of the current transformer? What is the function of the wiring index? According to the ratio of the current of the automobile relay to the secondary current of the current transformer, it is called the wiring index of the current transformer, that is, Ik in the formula Kcu003dIk/I2-- The current flowing into the automobile relay; 12-- The secondary current flowing into the current transformer; the wiring index is a key main parameter in the setting and calculation of the relay protection device. For the measurement of the posture value of various current maintenance and accurate measurement components, the wiring index must be considered.

5. What is the voltage transformer reverse battery charging? What are the hazards to the protector? According to the secondary side of the voltage transformer, the battery charging of the busbar without induction charging is called reverse battery charging. For example, for a 225kV voltage transformer, the transformation ratio is 2200. Even if the primary busbar is not grounded, the impedance (including the busbar capacitor and grounding resistance) is very large. It is assumed to be 1MΩ, but from the secondary measurement of the voltage transformer. The impedance obtained can only be 1000000/(2200)2u003d0.2Ω, which is almost a short-circuit fault, so the reverse battery charging current is very large (the reverse battery charging current is mainly determined by the leakage reactance of the cable resistor and the two voltage transformers) , it will cause the small switch on the secondary side of the voltage transformer in operation to trip or the circuit breaker to melt, so that the protector in operation loses its working voltage, which will cause the protector to malfunction or refuse to operate.

6. Installation of zero-sequence current transformers (1) Cable joints and support frames of zero-sequence current transformers are reliably protected by insulating materials. (2) When single-phase grounding occurs, the current of the grounding device will not only pass through the ground, but will also pass through the cable skin. In order to avoid the wrong operation of the equipment when the common fault of single-phase grounding outside the area, the cable joint and the wire joint should cross the zero-sequence current transformer and then the grounding device. (The wire connector crosses the zero-sequence current transformer and then the grounding device) The wiring of common voltage transformers, there are four common wiring methods of voltage transformers in three-phase circuits
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