The Principle of Current Transformer Connection

Wire connection of current transformer shall follow a series principle: namely the primary winding should be connected in series with the circuit under test, while the secondary winding should be connected in series with all instruments and loads.

Choose the right change according to the magnitude of measured current, otherwise the error will increase. Meanwhile, one end of the secondary side must be grounded, so as to avoid personnel and equipment accidents caused by high voltage at the primary side when the insulation is damaged.

The secondary side is not allowed for open circuit. Once it's open circuited, current I1 at the primary side would fully turn to magnetizing current, φm and E2 would increase sharply. As a result, the iron core would occur supersaturation magnetization, heat seriously and even burn out coils. Meanwhile, supersaturation magnetization of the magnetic circuit would increase errors. Under normal operation, secondary side of the current transformer is nearly short circuited, if it's broken suddenly, the excitation electromotive force increases sharply, the magnetic flow in the iron core shows seriously saturated flat waves. As a result, the secondary side winding would induce high spike waves when the magnetic flow passes through zero, it can be thousands or even tens of thousands of volts, thus endangering the personnel safety and insulation performance of instruments.

Moreover, open circuit at the secondary side makes the E2 to be hundreds of volts, it may lead to electric shock accident when touching on. Therefore, secondary side of the current transformer is equipped with short-circuiting switch, so as to prevent open circuit at the primary side. Once the secondary side is open circuited in use, remove the circuit loads immediately and then stop to treat. Use it again after all are treated.

In order to meet the needs of malfunction judgement and fault recorder of measuring instruments, relay protection and circuit breaker, 2～8 current transformers of secondary winding are set in the generators, transformers, outlets, busbar sectional circuit breakers, busbar circuit breakers and bypass circuit breakers. As for large current grounding system, it's generally set by three phases; For small current grounding system, it’s generally set by two phases or three phases.

For protective purpose current transformer, its installation positions shall be set to eliminate the non protection zone of main protection device. For example, if there are two groups of current transformer, they should be set at two sides of the breaker if possible, so as to ensure the breaker in the cross protection range.

In order to prevent busbar fault caused by flashover of pillar type current transformer, the current transformer is usually placed on the outlet or transformer side of the circuit breaker.

In order to reduce the generator damage when internal fault, the current transformer used for automatically regulating excitation device should be placed on the outlet side of generator stator winding. In order to facilitate analysis and detect internal faults before the generator is incorporated into the system, the current transformer for measuring instruments should be installed on the neutral side of generator.

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