what is current transformer？

Transformers include voltage transformers and current transformers, which are special transformers that are widely used in power supply systems to supply voltage coils or current coils of measuring instruments and relays.

A voltage transformer is a voltage conversion device that has dual functions of voltage conversion and isolation. It converts the high voltage of the high-voltage circuit or low-voltage circuit into a low voltage (generally 100V), which is supplied to instruments and relay protection devices to achieve measurement and metering. , protection and so on.

The current transformer is a current conversion device with dual functions of current conversion and isolation. It converts the large current of the high-voltage circuit or low-voltage circuit into a low-voltage small current (generally 5A), which is supplied to the instrument and relay protection device to realize measurement, Measurement, protection and other functions. In actual measurement, in order to better match the range of the instrument, there are also transformers that convert small currents below 5A into large currents (5A). In this article, we mainly introduce current transformers to you.

The current transformer is a conversion device with a rated current of 1A or 5A that converts the large current on the primary side into a proportional one that is suitable for use through a meter or a relay.

1. How does a current transformer work？

Electromagnetic current transformers (hereinafter referred to as current transformers) are widely used in power systems. It works similar to a transformer. The ratio of the primary and secondary currents of a current transformer is called the rated mutual inductance ratio of the current transformer.

In the formula, IN1—the rated current of the primary coil; IN2—the rated current of the secondary coil.

2. Characteristics of current transformers

a.The primary winding is connected in series in the circuit, and the number of turns is very small; therefore, the current in the primary winding depends entirely on the load current of the circuit under test, and has nothing to do with the magnitude of the secondary current;

b.The current coil impedance of the instrument connected to the secondary winding of the current transformer is very small, so under normal circumstances, the current transformer operates in a state close to short circuit.

3. Current Transformer Error

The current error is the difference between the measured value of the secondary current multiplied by the rated mutual inductance ratio and the actual primary current, expressed as a percentage

4. Wiring form of current transformer

The wiring form of the current transformer refers to the connection form between the current transformer and the measuring instrument or the protective relay.

1.Three-phase three-complete star connection can accurately reflect the real current of each phase in the three phases. This wiring method is applied in the large current grounding system to protect the three-phase short circuit, two-phase short circuit and single-phase grounding short circuit of the line.

2.The incomplete star connection of two phases and two relays can accurately reflect the real current of the two phases. This wiring method is applied in the small current grounding system with ungrounded neutral point of 6~10kV to protect the three-phase short circuit and two-phase short circuit of the line.

3.The two-phase differential connection reflects the difference current between the two phases. The wiring feature is that the U and W phase current transformers are connected into a current difference type, and the current passing through the relay is the current difference between the U and W phase current transformers on the secondary side. This wiring method is applied in the small current grounding system with 6~10kV two-phase differential wiring and 10kV neutral point not grounding, to protect the three-phase short circuit, two-phase short circuit, small-capacity motor protection and small-capacity transformer protection of the line.

4.When the three-phase load is balanced in the single-phase connection, the single-phase current can be used to reflect the three-phase current value, which is mainly used for measuring circuits.

5.The current flowing into the third relay in the two-phase three complete star connection is:

This wiring method is applied in the large current grounding system to protect the three-phase short circuit and two-phase short circuit of the line.

5. Pay attention to the following when wiring the current transformer:

1.The secondary side of the current transformer must not be open-circuited during use. When disassembling the instrument or relay during use, the secondary side should be short-circuited in advance. During installation, the wiring should be reliable, and the secondary side is not allowed to install fuses.

2.One end of the secondary side must be grounded. Prevent primary and secondary side insulation from being damaged, and high voltage from entering the secondary side, endangering personal and equipment safety.

3.Pay attention to polarity when wiring. The polarity terminals of the primary and secondary sides of the current transformer are indicated by letters.

4.The primary side is connected in series in the line, and the relay or measuring instrument on the secondary side is connected in series.

High-voltage current transformers are mostly made of two iron cores and two auxiliary windings, which are respectively connected to measuring instruments and relays to meet the different requirements of measuring instruments and relay protection. The iron core of the current transformer for measurement should be easily saturated when the primary side is short-circuited, so as to limit the multiple of the secondary side current increase; the iron core for relay protection should not be saturated when the primary side is short-circuited, so that the secondary side The current increases proportionally to the primary current.

6. Load requirements for current transformers

1.Accuracy level of current transformer：

Current transformers are divided into different accuracy levels according to the size of the error during measurement. The accuracy level refers to the maximum error when the primary current is the rated value within the specified secondary load range.

2.Current transformer 10% error curve：

The 10% error curve of the current transformer is the relationship curve between the multiple n of the primary current and the allowable maximum secondary load impedance of the current transformer under the condition that the error of the current transformer does not exceed ±10%.

3.Rated capacity of current transformer：

The rated capacity of the current transformer refers to the output capacity of the secondary coil when the current transformer operates under the rated secondary current and rated secondary impedance.

Since the secondary current of the current transformer is the standard value (5A or 1A), its capacity is often expressed by the rated secondary impedance. Because the error of the current transformer is related to the secondary load, the same current transformer will have different rated capacity when used in different accuracy levels.

The requirement of the current transformer for the load is that the sum of the load impedances cannot exceed the rated secondary impedance value of the transformer.