1 Principle and effect of current transformer
The operation principle of current transformer is similar to that of transformer. The difference is that in the iron core of the transformer, the alternating main flux is the current of the communication voltage applied at both ends of the primary winding. The alternating main flux in the iron core induces the corresponding secondary electromotive force and secondary current in the secondary winding of the current transformer. Since the primary winding and secondary winding are wound on the same iron core, they are connected by the same alternating main flux. Therefore, numerically, the current and turn product of the primary winding and secondary winding should be equal, that is, i1n1 = i2n2, so I1 / I2 = N2 / N1 = k, K is called the transformation ratio of the current transformer.
Current transformer is mainly used to change the high voltage and large current in the power system into low voltage and small current according to a certain share, that is, no matter how large the primary side current is, the secondary side (usually) is 5A. Supply appearance, instrument, relay, etc. for measurement and maintenance.2 Precautions for current transformer in operation:
1) The secondary side of the current transformer during operation shall not be open circuit, and the equipment fuse and knife switch shall not be installed on the secondary side. This is because the number of turns of the secondary winding of the current transformer is far greater than that of the primary winding. Under the condition of open circuit, the current transformer is suitable for a step-up transformer. According to the flash of relevant materials, the voltage value can reach 100 v. Endanger the safety of operators. Therefore, when there is current at the primary side, the secondary side shall not be constructed except for selecting the corresponding short-circuit method (the current transformer is approximately in short-circuit condition during operation, so K1 and K2 can be directly short circuited and grounded). Remember.
2) For current transformer equipment, one end (usually K2), iron core and shell of the secondary side of the current transformer shall be firmly grounded. In order to prevent the primary side voltage from cascading to the secondary side due to insulation damage of primary and secondary side windings, endangering the safety of operators.3) When installing current transformer equipment, the accuracy level shall be considered. High precision measurement surface, low precision for maintenance. Be careful when choosing.
4) When using current transformer equipment, pay attention to the polarity (same name end). The terminals at the primary side are L1 and L2 (or P1 and P2). The current at the primary side flows in from L1 and out from L2. The terminals on the secondary side are K1 and K2 (or S1 and S2), that is, the terminals on the secondary side flow out from K1 and in from K2. L1 and K1, L2 and K2 are of the same polarity (same name), and no mistake is allowed. Otherwise, if the watt hour meter is connected, the watt hour meter will turn.
5) The primary winding of current transformer can be divided into single turn and multi turn, and LQG type is single turn. When using LMZ type (through core type), pay attention to whether there is through core data on the nameplate. If so, wear the required number of turns as required. Note: the number of turns through the center is subject to the number of turns through the hollow, not the number of turns around the periphery (otherwise, one turn will be wrong).6) The secondary winding of current transformer is divided into one winding and two windings. If there are two windings, one winding with high precision (small error value) is usually used for measurement, and the other with low precision (large error value) is usually used for maintenance.7) It is necessary to select 2.5mm2 copper core insulated wire for the connection line of current transformer. It is necessary to select 4mm2 copper core insulated wire according to some rules of some power industries, but my concept is not necessary.
3 Application of current transformer1) Measurement of communication currentIf the equipment is not operating at additional current value, setting current measuring equipment is a necessary skill method. For equipment above 40kW, it is necessary to install ammeter for monitoring.
The measurement of communication current has the methods of direct measurement and widening measurement through current transformer. Direct measurement is to connect the appropriate ammeter in series to the current circuit. The circuits shown in the figure below are connected through current transformer.
(a) Select a current transformer to measure the current in the three-phase balance line. The selection of current transformer is based on the range of ammeter, and the range of ammeter is based on the practical value of load current, which should account for 2 / 3 of the total range of ammeter to the full scale value. For example, if the additional current of a device is 100A, select an ammeter with a full-scale value of 150A. At this moment, the mutual inductance ratio of current transformer shall be 150 / 5A, and the ammeter with the ratio of 150 / 5A shall also be selected accordingly. When reading, read directly with the indicated value of the meter.
(b) Two current transformers are selected and connected into an incomplete star to measure the three-phase current in the three-phase balanced or unbalanced line.(c) Three current transformers are selected and connected into a complete star connection method to measure the three-phase current in the three-phase balanced or unbalanced line.2) Measurement of communication electricity
In order to broaden the range of communication watt hour meter, the most commonly used method in the factory is to use current transformer to expand many ranges.(1) The top of the list is the measurement method of single-phase watt hour meter and current transformer. It can be used to measure three-phase balance or single-phase electricity metering.(2) The second is the measurement method of selecting two element three-phase three wire active watt hour meter and adding two current transformers. It can be used to measure the electricity in three-phase balanced or unbalanced lines.
(3) The third is the measurement method of selecting three-phase four wire active watt hour meter and adding three current transformers. It can measure the electricity metering in three-phase balanced or unbalanced lines.3. Maintenance in electric drive lineFor overload maintenance of high-power motor, it is usually impossible to buy corresponding thermal relay due to high current. Under such circumstances, it is usually handled by adding current transformer. The essence is to replace large current with small current, and the thermal relay within 5A can meet the requirements of overload maintenance.
(1) Current transformer is used for high-power self coupling step-down protection and overload maintenance in motor.(2) Current transformer is used for high-power frequency sensitive rheostat and overload maintenance in motor.(3) Current transformer is used for high-power star angle step-down protection and overload maintenance in motor.
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