Electrical debugging is an important part of electric power work. In electrical commissioning work, secondary circuit inspection is an important debugging content. It is related to whether the measurement, protection, communication and other functions of the power system can play a role. premise. In the secondary circuit, whether the wiring of the current transformer is correct is the correct basis for the current secondary circuit, so the correctness of the wiring of the current transformer is very important. Many electrical commissioning personnel do not have a deep understanding of it, and often make mistakes, such as differential protection malfunction, electricity meter reversal and so on. Let's make a comprehensive and detailed discussion of this issue.

1.the principle of current transformer structure

The structure of the current transformer is relatively simple, and is composed of a primary winding, a secondary winding, a core, and a frame, a casing, a terminal, and the like insulated from each other. The working principle is basically the same as that of the transformer. The number of turns (N1) of the primary winding is less, and it is directly connected in series in the power line. When the primary load current () passes through the primary winding, the alternating magnetic flux generated produces a proportionally reduced two. Secondary current (); the number of turns (N2) of the secondary winding is more, and the closed circuit is formed in series with the secondary load (Z) of the current coil of the instrument, relay, transmitter, etc., as shown in Fig. 1.

Since the primary winding has the same ampere-turns as the secondary winding, I1N1=I2N2, the current transformer rated current ratio: In the actual operation of the current transformer, the load impedance is small, and the secondary winding is close to the short-circuit state, which is equivalent to a short-circuiting transformer.

2. Wiring principle of current transformer

(1) The secondary side of the current transformer is not allowed to open. The secondary open circuit may have serious consequences. First, the iron core overheats and even burns the transformer. Second, because the secondary winding has a large number of turns, it will induce dangerous high voltage and endanger the safety of people and equipment.

(2) The secondary side of the high-voltage current transformer must be grounded at all. Since the primary side of the high-voltage current transformer is high voltage, when the high-voltage breakdown of the primary and secondary coils due to insulation damage, the high voltage will enter the low voltage. If the secondary coil is grounded at one point, the high voltage is introduced into the earth, which ensures Safety of people and equipment. However, it should be noted that the secondary circuit of the current transformer only allows one point to be grounded, and no grounding is allowed. Otherwise, it may cause shunting and affect the use.

The secondary coil of the low voltage current transformer should not be grounded. Since the voltage of the low-voltage transformer is low, the insulation between the primary and secondary coils is large, and the possibility of breakdown of the primary and secondary coils is small. In addition, the ungrounding of the secondary coils will cause a second return.

The insulation capacity of roads and meters is improved, and the number of accidents caused by lightning strikes can be reduced. In addition, the differential protection is formed by a differential relay (such as BCH-2). The current transformer on both sides of the differential protection can only be grounded at one point. Generally, the grounding point is set at the protection screen, and when the differential protection is used. When the microcomputer protection device is used, the current transformers on both sides should be grounded separately.

(3) The measurement level and protection level of the current transformer cannot be connected incorrectly. Because the thickness of the design and protection winding core is different, if the connection is wrong, the accuracy of the measurement in normal operation is reduced, and the energy measurement is not accurate. Second, in the event of a short-circuit fault, the design of the core of the metering winding is guaranteed. When the short-circuit current exceeds a certain multiple of the rated current, the core is saturated, which limits the increase of the secondary current to protect the meter. The core of the relay protection winding is not saturated, and the secondary current increases correspondingly with the short-circuit current, so that the relay protection operates accurately. If the connection is wrong, the relay protection action is not sensitive and the meter may burn out.

(4) Since the secondary winding of the current transformer cannot be opened, the windings that are not used by the current transformer need to be shorted. However, there are multiple tapped current transformers. The unused taps should be empty and cannot be shorted. For example, a current transformer has taps 1S1, 1S2, and 1S3 twice, of which 1S1 and 1S2 are 300/5A, and 1S1 and 1S3 are 600. /5A, when you need to use 300/5A, use 1S1, 1S2, you should not short 1S1, 1S3, otherwise it will affect the measurement accuracy of the tap.

(5) When the metering winding of the current transformer and the relay protection winding involved in the direction are connected, master the two points to determine the wiring. First, look at the installation position of the current transformer, that is, determine which side of the current transformer L1 is installed; Look at the winding function or relay protection type. The above two points can determine the secondary wiring of the current transformer.

3.current transformer use precautions

(1) The polarity connection should be correct. Current transformers are generally marked with reduced polarity. If the polarity is not connected correctly, it will affect the metering. Even when multiple current transformers are connected in parallel on the same line, all short-circuit accidents will occur.

(2) The secondary circuit should be provided with a protective grounding point and connected reliably. In order to prevent the high voltage from entering the low voltage side after the insulation breakdown between the primary and secondary windings, the personal and instrument safety is endangered. The secondary side of the current transformer should be provided with a protective grounding point. Only one grounding point is allowed, and the current mutual inductance is generally close. The cabinet terminal of the unit is grounded.

(3) The secondary winding is not allowed to open during operation. Otherwise, it will lead to the following serious consequences: high voltage on the secondary side, endangering personal and instrument safety; overheating may burn the winding; increase measurement error.

(4) The secondary circuit of the current transformer used for electric energy metering should not be connected with relay protection devices and automatic devices to prevent mutual influence.