Comparison of voltage Transformers and current Transformers

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Comparison of voltage Transformers and current Transformers

Voltage Transformers (PT) and current Transformers (CT) are important electrical equipment in power systems. They undertake the functions of isolation between high and low voltage systems and the conversion of high voltage to low voltage. The correctness of its wiring is of great significance to the normal work of the system's protection, measurement, and monitoring equipment. When new installation of PT, CT is put into operation or replacement of PT, CT secondary cables, using the polarity test method to verify the correctness of PT and CT wiring is already an essential work procedure for relay protection workers.

To avoid reverse polarity, it is necessary to find the "same name end" of the Transformer input and output. The specific method is "point polarity". Here take the current Transformer as an example to explain how to point the polarity. The specific method is to connect the pointer multimeter to the secondary output winding of the Transformer, and the multimeter to be in the DC voltage range; then fix the negative pole of a dry battery on the primary output wire of the current Transformer; then use the positive pole of the dry battery to "point" "The primary input lead of the current Transformer, so that a (positive) pulse current will be generated in the primary circuit of the Transformer; at the same time, observe which direction the pointer of the pointer multimeter" offsets ". If the needle of the multimeter is offset from 0 to left Shift, j means that the needle is “positive start”, indicating that the “input terminal of the current Transformer” and the “second output of the current Transformer connected to the positive terminal of the pointer multimeter” have the same name. Called "positive polarity" or "minus polarity"; if the multimeter's hands are shifted from 0 to right, that is, the hands are "reversely turned on", indicating that the "current Transformer primary input terminal" and "pointer multimeter" The secondary output terminal of the current Transformer connected to the positive terminal is not the terminal with the same name, and this wiring is called "reverse polarity" or "plus polarity".

Each product has its own precautions, and the following aspects should be noted when applying a Transformer:

1. The rated primary current of the current Transformer is generally selected according to 1.2 to 1.4 times the current of the line. This is mainly to consider that the line Transformer will not burn down the current Transformer and current meters or electrical meters and other electrical equipment when the line is overloaded.

2. The rated primary current of the current Transformer cannot be selected too much different from the actual working current of the line, which will affect the measurement accuracy of the current Transformer.

3. The accuracy of the Transformer can only be guaranteed if the Transformer is within the rated secondary output load range. Therefore, including the secondary line load and the load of the metering device are the actual working load of the Transformer. When the secondary actual output load of the Transformer is greater than the secondary rated output load of the Transformer, the accuracy of the Transformer will be reduced, and the mutual inductance will be destroyed when the overload is severely overloaded. Device.

4. When the secondary actual output load of the Transformer is lower than the rated secondary output load of the Transformer, the accuracy of the Transformer will decrease.

5. Select appropriate Transformer products for different applications.

6, outdoor Transformers and indoor Transformers are not mixed.