Current Transformers

Key Issues of Current Transformers

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

The size of the secondary load impedance of the current transformer has a great influence on the accuracy of the transformer. This is because if the secondary load impedance of the current transformer increases a lot and exceeds the allowable secondary load impedance, the value of the excitation current will increase greatly, causing the core to enter a saturated state. In this case , a large part of the primary current will be used to provide the excitation current, which will greatly increase the error of the transformer and its accuracy will decrease.

2. The current transformer used for differential protection requires a good core and an enlarged core cross-section. Why?

When the system is operating normally or when there is an external short circuit within the differential protection range, the current values and phases of the current transformers at both ends of the differential protection are the same, and there should be no current flowing into the differential relay. However, in fact, the characteristics of the two sets of current transformers cannot be completely If they are the same, the excitation current will be different, the secondary current will not be equal, and an unbalanced current will flow in the relay.

In order to reduce the unbalanced current, the structure of the current transformer must be improved so that it does not saturate, or special silicon steel sheets with small losses must be used to make the core and the core cross-section must be enlarged.

3. What are the ways to connect the secondary winding of current transformer?

According to the different requirements of relay protection and automatic devices, the secondary winding of current transformer usually has the following wiring methods:
⑴. Complete (three-phase) star connection;
⑵. Incomplete (two-phase) star connection;
⑶. Delta connection;
⑷. Connect three phases in parallel to obtain zero sequence current wiring;
⑸. Two-phase difference wiring;
⑹. One phase uses two current transformers connected in series;
⑺. One phase uses two current transformers connected in parallel.

Current Transformers

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

The ratio of the current passing through the relay to the secondary current of the current transformer is called the wiring coefficient of the current transformer, that is, Kc=Ik/I2 where;

Ik–the current flowing in the relay;

12–The secondary current flowing into the current transformer;

The wiring coefficient is an important parameter in the calculation of relay protection settings. The calculation of the action values of various current protection measuring components must take the wiring coefficient into consideration.

5. What is voltage transformer reverse charging? What impact will it have on protective devices?

Charging the dead busbar through the secondary side of the voltage transformer is called reverse charging. For example, for a 220kV voltage transformer, the transformation ratio is 2200. Even if the primary busbar is not grounded during a power outage, its impedance (including busbar capacitance and insulation resistance) is relatively large, assuming it is 1MΩ. However, the impedance seen from the secondary measurement of the voltage transformer is only 1000000/(2200)2=0.2Ω, which is almost a short circuit, so the reverse charging current is large (the reverse charging current is mainly determined by the cable resistance and the leakage reactance of the two voltage transformers), which will cause the secondary side of the voltage transformer to be in operation. The small switch trips or the fuse blows, causing the operating protective device to lose voltage, which may cause the protective device to malfunction or refuse to operate.

6. Installation of zero sequence current transformer

(1) The cable head and the bracket of the zero-sequence current transformer should be reliably isolated with insulators.

(2) When a single-phase grounding occurs, the grounding current not only flows in the ground, but may also flow along the cable sheath. In order to prevent malfunction of the device when a single-phase ground fault occurs outside the area, the ground wire of the cable head should pass through the zero-sequence current transformer and then be grounded. (The ground wire passes through the zero sequence current transformer and then is grounded)

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