Current Transformer

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The important role of the current transformer

1) It is beneficial to expand the range of the measuring instrument, and the power consumption is small, because the current transformer reduces the large current or high voltage to a small current or low voltage.

2) It is beneficial to the standardization and miniaturization of measuring instruments, because it is not necessary to design instruments of different ranges according to the magnitude of the measuring current or the level of the measuring voltage after using the current transformer.

3) It is beneficial to ensure the safety of measurement staff and instrumentation equipment, because the current transformer isolates the large current or high voltage of the circuit under test. In addition, when overvoltage or overcurrent occurs in the power line failure, since the iron core of the current transformer tends to bubble, its output will not increase proportionally, which can protect the measuring personnel and the instrument.

4) It is beneficial to reduce the insulation requirements of secondary equipment such as measuring instruments, because it is no longer necessary to design measuring instruments according to the actual measured current or voltage after using the current transformer, thereby simplifying the instrument process, reducing production costs, and facilitating installation and use.

5) It is beneficial to carry out long-distance measurement, because after using the current transformer, long-distance small-section wires can be used to carry out long-distance measurement conveniently.

In addition, the zero-sequence current or zero-sequence voltage component can be taken out through the current transformer for use by the relay protection device reflecting the ground fault; the wiring method can also be changed through the current transformer to meet various measurement and protection requirements without being affected by the primary circuit. limit.

Current transformers classification

1) According to the voltage level: it can be divided into high voltage and low voltage. Low-voltage current transformers can be divided into claw type and through-core type according to their shapes, which can be selected according to actual needs. For low-voltage current transformers with large transformation ratios, the claw type is used, and the joints should be handled properly, otherwise it will be easy to burn; while the low-voltage current transformers with small transformation ratios adopt the through-core type, due to the multi-turn installation method, it is necessary to prevent the generation of metering magnification. error. In the current power system, it is not recommended to use multi-turn feedthrough low-voltage current transformers.

2) According to the installation location: it can be divided into indoor and outdoor current transformers.

3) According to the type of insulation: it can be divided into oil insulation, casting insulation, dry type, porcelain insulation and gas insulation and other current transformers.

4) According to the purpose: it can be divided into measurement and protection current transformers.

5) According to the accuracy level: it can be divided into 0.1, 0.2S, 0.2, 0.5S, 0.5, 1.0, 2.0, 3.0, 5.0 measurement current transformers and 5P, 10P protection current transformers (P means protection ), and there are 0.01, 0.02, 0.05 grades for precise measurement in the test. Electric energy metering devices usually use 0.2S, 0.5S class current transformers for measurement.

Troubleshooting Method for Secondary Open Circuit of Current Transformer

1) If the secondary open circuit of the current is found, it is necessary to first distinguish which group of current circuits the fault belongs to, and whether the phase of the open circuit has any impact on the protection, report the scheduling, and remove the protection that may malfunction.

2) Minimize the primary load current. If it is seriously damaged, the load should be transferred, and the power failure should be checked and dealt with. If there is a state-owned bypass, the bypass power supply can be used to ensure the power supply.

3) Try to short-circuit the current transformer on the nearest test terminal as much as possible, and then check and deal with the open point. When short-circuiting, use a special short-circuit wire for short-circuit. It needs to be done according to the drawing.

4) Pay attention to the phenomenon during short-circuit. If there is spark during short-circuit, it means that the short-circuit is valid, and the fault point is in the circuit below the short-circuit point, which can be further searched. If there is no spark during short-circuit, it may be that the short-circuit is invalid. The fault point may be in the loop before the short contact, and the short contact can be changed forward point by point to narrow the range to clarify the open point.

5) Within the fault range, check the terminals and components that are prone to failure, and check the parts of the circuit that have been touched during operation. For the detected faults, those that can be dealt with by themselves can be dealt with immediately, and then the protection that has been withdrawn can be put into operation. If there is a fault that cannot be handled by itself or cannot be identified by itself, it should be reported to the superior and professional maintenance personnel will be sent to check and deal with it. At this time, the current transformer should be short-circuited for the second time, or the load should be transferred, and the power failure should be dealt with.

Operation and Maintenance of Voltage Transformer

1) Inspection items before voltage operation
Check the casing to see if there is any crack or damage;
Check the oil filling, its appearance should be clean, the amount of oil is sufficient, and there is no leakage of oil;
Check the lead wires, wire clips and the connection parts of the secondary circuit, they should be in good contact and not loose;
Check the grounding condition of the casing and the primary and secondary sides, the grounding should be correct and good, and the grounding wire should be firm and reliable;
According to the test procedures, conduct a comprehensive test and be qualified.

2) Inspection items of the inspection
Check whether the porcelain casing is clean, whether there are defects, cracks and discharges, and whether the sound is normal;
Check whether the oil level of the oil-filled current transformer is normal and whether there is leakage;
Check whether the joints are overheated and sparked, whether the bolts are loose, and whether there is any abnormal smell;
Check whether the secondary winding has an open circuit, whether the grounding wire is in good condition, and whether there is looseness or breakage;
Check whether the three-phase indication of the ammeter is within the allowable range, and whether the current transformer is overloaded.

3) When the voltage transformer in operation has the following faults, it should immediately quit operation:
(1) The porcelain bushing is broken and seriously discharged.
(2) The insulation of the high-voltage coil breaks down, emits smoke, and emits a burnt smell.
(3) There are discharge sounds and other noises inside the voltage transformer, and there is spark discharge between the coil and the casing or between the lead and the casing.
(4) The oil leakage is serious, and the oil surface cannot be seen in the oil standard pipe.
(5) The case temperature exceeds the allowable temperature rise and continues to rise.
(6) The high-voltage melt is blown twice in a row.


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