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Key problems about current transformer

Key problems about current transformer

2021-12-21

1. If the secondary load impedance of current transformer exceeds its allowable secondary load impedance, why does the accuracy decrease?

The size of secondary load impedance of current transformer does great harm to the accuracy of voltage transformer. This is because if the secondary load impedance of the current transformer increases a lot and exceeds the allowable secondary load impedance, the standard value of the excitation regulator current will be greatly increased and the transformer core will be saturated. Under such conditions, a large part of the primary current will be used to produce the excitation regulator current, so as to greatly increase the deviation of the voltage transformer, Its accuracy decreases with time.


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 also be increased. Why?

In case of normal operation of system software or external short-circuit fault in the scope of differential protection, the current scale and position of current transformers on both sides of differential protection are the same, and there should be no current injected into differential protection automobile relay, but in fact, the characteristics of these two current transformers are unlikely to be completely consistent, the current of excitation regulator is different, and the secondary current is not easy to be the same, The vehicle relay will pass through the unbalanced current. 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 the special ferrite core with small loss to make the transformer core, and increase the cross section of the transformer core.


3. What are the types of wiring methods for secondary winding resistance of current transformer?

According to the different provisions of relay protection device and protection device, the secondary winding resistance of current transformer generally has the following types of wiring methods:

(1) complete (three-phase) star wiring;

(2) incomplete (two-phase) star wiring;

(3) triangular connection;

(4) connect three phases in parallel to obtain zero sequence current wiring;

(5) wiring between two;

(6) one phase is connected in series with two current transformers;

(7) one phase is connected in series with two current transformers.


4. What is the wiring index of current transformer? What does the wiring index do?

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, KC = IK / I2

IK -- current flowing into automobile relay;

12 -- secondary current flowing into current transformer;

The wiring index is a key parameter in the setting calculation of relay protection devices. The wiring index must be taken into account in the calculation of the posture value of various current maintenance accurately measured components.


5. What is reverse battery charging of voltage transformer? What are the hazards to the protector?

According to the bus duct battery charging with no inductive power on the secondary side of the voltage transformer, it is called reverse battery charging. For example, if the transformation ratio of 225kv voltage transformer is 2200, even if the primary bus duct without power failure is not grounded, although its impedance (including bus duct capacitor and grounding resistance) is large, it is assumed to be 1m Ω, the impedance seen from the secondary measurement of voltage transformer can only be 1000000 / (2200) 2 = 0.2 Ω, which is almost a short-circuit fault, Therefore, the reverse battery charging current is very large (the key decision of the reverse battery charging current depends on the leakage reactance of the cable resistor and two voltage transformers), which will lead to the tripping of the small switch at the secondary side of the voltage transformer or the melting of the circuit breaker during operation, resulting in the loss of working voltage of the protector during operation, which will lead to misoperation or refusal of operation of the protector.


6. Installation of zero sequence current transformer

(1) The cable joint and the support frame of zero sequence current transformer shall be protected by reliable insulation.

(2) In case of single-phase grounding, the current of the grounding device will not only pass through the grounding, but also pass through the cable skin. In order to avoid wrong operation of equipment in case of common faults of single-phase grounding outside the area, the cable connector and wire connector shall cross the zero sequence current transformer and then the grounding device. (the wire connector shall cross the zero sequence current transformer and then the grounding device.)


Common wiring of voltage transformer. There are four common wiring methods of voltage transformer in three-phase circuit: as shown in the figure below

1. The wiring of a single-phase electric voltage transformer is used for symmetrical three-phase circuit. The secondary side can be equipped with instrument panel and automobile relay, as shown in Figure 1 (a).

2. The V / V wiring of two single-phase electric voltage transformers can accurately measure the two-color phase voltage, but not the phase voltage, as shown in Figure 1 (b).

3. Three single-phase electric voltage transformers are connected into Y0 / Y0 shape, as shown in Figure 1 (c). The instrument panel and automobile relay that can accurately measure the phase voltage in supply-demand balance, and the insulation supervision ammeter that can specify the phase voltage in supply-demand balance.

4. A three-phase five core voltage transformer is connected into Y0 / Y0/ Δ (open triangle), as shown in Figure 1 (d) Diagram. The secondary electromagnetic coil connected in Y0 shape is used to supply power to the instrument panel, automobile relay and insulation layer supervision ammeter. The auxiliary secondary electromagnetic coil is connected in an open triangle, and the power supply system supplies the insulation layer supervision voltage relay. When the three-phase system software is working normally, the three-phase voltage is balanced, and the voltage at both ends of the open triangle is zero. When one is connected to ground Zero sequence voltage appears on both sides of the open triangle, so that the insulating layer monitors the posture of the voltage relay and sends a signal.

Deeply analyze the following wiring diagram of Pt connected to V / V: the figure on the left shows proper wiring and balanced working voltage; The figure below shows incorrect wiring and unbalanced working voltage.


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