How to judge the polarity of transformer

Transformer is an indispensable and important equipment in power system, and its main function is to realize voltage rise and fall through electromagnetic induction principle. In practical application, the connection mode of transformer, parallel operation and relay protection setting are closely related to its polarity. Therefore, it is of great significance to accurately determine the homonym end (i.e. polarity) of transformer winding to ensure the safe operation and correct wiring of transformer.

First, the basic concept of transformer polarity

The polarity of transformer refers to the directional relationship of induced electromotive force between primary winding and secondary winding at a certain moment. According to whether the directions of induced electromotive force are consistent, the corresponding terminals of transformer windings can be divided into homonym terminals or heteronym terminals.

Usually, the polarity of transformer label is "minus polarity". That is, when the primary current flows in from the head end marked with "*", the secondary current also flows out from the head end marked with "*", and their instantaneous directions are the same. This polarity relationship is called polarity reduction; On the contrary, it is polarity (less used).

Second, the common methods of transformer polarity determination

# 1. DC method (impact method)

This is the simplest and most intuitive method, which is suitable for polarity detection of small transformers. The specific operation is as follows:

-Connect a DC power supply (such as a dry battery) to both ends of the primary winding temporarily;
-Connect a pointer ammeter or voltmeter at both ends of the secondary winding;
-When the power supply is switched on, observe the deflection direction of instrument pointer;
-If the terminal connected to the positive electrode on the primary side deflects in the same way as the positive end of the instrument on the secondary side, the two terminals are the same name.

This method is based on the basic principle of Lenz's law in electromagnetic induction, and is suitable for rapid on-site judgment.

# 2. Communication method

Ac method is to judge polarity by applying AC voltage, which is suitable for transformers or large capacity transformers in field operation. The operation steps are as follows:

-Apply low-voltage AC power to the primary winding;
-Measure the voltage between the primary and secondary windings with an AC voltmeter;
-If the measured total voltage is the difference between the potentials of the two windings, it indicates that the wiring is reduced in polarity;
-If it is the sum of two potentials, it is added polarity.

This method is relatively safe, and it is suitable for situations where DC power supply cannot be used for testing.

# 3. Polarity discriminator method

With the development of science and technology, there are special devices for judging the polarity of transformers in modern power testing instruments. This kind of instrument is easy to operate and has high precision. It can quickly judge the polarity of transformers and is widely used in substations and distribution systems.

Third, the importance of polarity judgment

1. Ensure the normal operation of transformers in parallel: When multiple transformers operate in parallel, if the polarity is wrong, the circulating current will be too large and even the equipment will be damaged.
2. Ensure the correct action of the relay protection: the polarity error will affect the phase of the current transformer (CT) signal, resulting in misoperation or refusal of the protection.
3. Improve the measurement accuracy: In the measurement system, the polarity error of CT and PT will lead to inaccurate electric energy measurement.

IV. Conclusion

Transformer polarity is one of the key parameters in transformer operation and maintenance. Whether in the design, installation or maintenance stage, the polarity relationship between windings should be accurately judged. Reasonable selection of DC method, AC method or using professional instruments can effectively improve the accuracy and efficiency of judgment, thus ensuring the safe and stable operation of power system.

Mastering the method of judging transformer polarity is a basic skill that every electric worker should have, and it is also an important guarantee to ensure the correct operation of power equipment.