How to judge the quality of current transformers

Update date:2018-05-12 Source:MAXGE

How to judge the quality of current transformers

1, the principle of current transformer selection and testing

1) The rated voltage of the current transformer is not less than the rated voltage of the line of the installation point

2) Current transformer selection based on one load calculation current transformer change

3) Select the accuracy of the current transformer according to the requirements of the secondary circuit and verify the accuracy

4) Verify dynamic stability and thermal stability.

 current transformer

2, current transformer ratio selection

The ratio of the rated current I1n and the secondary rated current I2n of the current transformer is called the rated current ratio of the current transformer, Ki=I1n/I2n≈N2/N1. In the formula, N1 and N2 are the number of turns of the primary and secondary windings of the current transformer.

Current transformer primary side rated current ratio (such as 20.30,40,50,75.100,150 (A), 2Xa/C) and other specifications, the secondary rated current is usually 1A or 5A.

Among them, 2Xa/C means that the same product has two kinds of current ratios, which is achieved by changing the connection mode of the connecting piece outside the oil storage cabinet at the top of the product. When connected in series, the current ratio is a/c, and the current ratio in parallel is 2Xa/C.

Under normal circumstances, the current ratio of the current transformer for measurement shall be selected so that the rated current I1n at one time is not less than the load current in the line (that is, the calculation IC).

If the load calculation current in the line is 350A, then the current transformer ratio should be selected as 400/5. The current transformer used for protection can be selected to be larger in order to ensure its accuracy.

3, good and bad measuring methods

If you have a clamp meter and ammeter is very easy to do, first use the universal meter to measure whether the coil has a short circuit, access the circuit work, the current transformer output is directly connected to the 5A current meter, use the clamp meter to measure the actual current of the bus, compare the current of the current meter, Knowing that the multiple of the transformer multiplied by the current meter should be similar to the clamp meter, the error is too large to be a problem.

Common answers: (Which kind is more appropriate?)

 current transformer

How to identify current transformers?

The first answer

For high-voltage current transformers: the current transformer can only have a small impedance in the secondary circuit, so it tends to short-circuit in normal operation, the sound is very small, and the sound is louder in the event of a fault. The grounding should be good, and the terminal lead of the primary winding of the capacitive current transformer should be reliably grounded. Oil-immersed transformers should not see oil leakage. In addition, when the secondary winding or loop of the current transformer is short-circuited, the ammeter and power meter indicate zero; when the current transformer secondary circuit is open, the fault point terminal will break through the fire.

Make the following measurements:

1. Winding insulation resistance;

2. Media loss; measured values should comply with relevant regulations

 current transformer

The second answer

1, appearance: current transformer shell, insulation, coils, etc. are aging, electric shock, burnt and other traces. The following refers to the current transformer below 500V, with 500V megohmmeter inspection.

2, primary coil path, can not be broken.

3, secondary coil path, can not be broken.

4, the primary coil and secondary coil can not be accessed.

5. Check the insulation resistance with 500V megohmmeter. The insulation of the primary coil and the secondary coil to the enclosure is greater than 0.38 MΩ.

6, using the current clamp meter to monitor the current ratio is correct. (Mainly to check whether there is a local short circuit within the coil).

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