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Questions and answers about current transformers -

by:Transmart     2023-01-15

If the secondary load impedance of the current transformer exceeds its allowable secondary load impedance, why does 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, and the iron core will enter a saturated state. In this case , a large part of the primary current will be used to provide the excitation current, so that the error of the transformer will be greatly increased, and its accuracy will decrease accordingly. The current transformer used for differential protection requires a good iron core, and the cross section of the iron core must be enlarged. Why? When the system is operating normally or the differential protection range is short-circuited externally, the current value of the current transformer at both ends of the differential protection Same as the phase, there should be no current flowing into the differential relay, but in fact the characteristics of the two sets of current transformers are impossible
are exactly the same, the excitation current will be different, the secondary current will not be equal, and the relay will flow
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 special silicon steel sheets with low loss to make the iron core, and increase the cross section of the iron core. What are the wiring methods for the secondary winding of the current transformer? According to the different requirements of relay protection and automatic devices, the secondary winding of the current transformer usually has the following wiring methods: ⑴Complete (three-phase) star connection; ⑵ Incomplete (two-phase) star connection; (3) Delta connection; (4) Three-phase parallel connection to obtain zero-sequence current connection; (5) Two-phase differential connection; (6) One-phase connection with two current transformers in series; Parallel connection of current transformers. What is the connection coefficient of the current transformer? What is the function of the connection coefficient? The ratio of the current through the relay to the secondary current of the current transformer is called the connection coefficient of the current transformer, that is, Kc=Ik/I2 where Ik--flow into the relay 12--The secondary current flowing into the current transformer; The connection coefficient is an important parameter in the calculation of relay protection setting. The calculation of the action value of various current protection measuring components must consider the connection coefficient. What is voltage transformer reverse charging? What impact does it have on the protection device? Charging the uncharged bus 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 bus is not grounded, the impedance (including bus capacitance and insulation resistance) is relatively large, assuming 1MΩ, but the impedance seen from the secondary measurement of the voltage transformer is only 1000000/(2200)2=0.2Ω, almost a short circuit, so the reverse charging current is relatively 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 small during operation. The switch trips or the fuse blows, causing the protection device in operation to lose voltage, which may cause the protection device to malfunction or refuse to operate. Installation of zero-sequence current transformer (1) The cable head and the bracket of zero-sequence current transformer should be reliably isolated by insulators. (2) When single-phase grounding occurs, the grounding current not only flows in the ground, but also flows along the cable sheath. In order to prevent the device from malfunctioning 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 ground.
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