How to measure transformer

Transformer is an important equipment for measurement and protection in power system, which is widely used in voltage and current measurement, relay protection and electric energy measurement. It converts high voltage or large current into standard low voltage or small current according to a certain proportion through the principle of electromagnetic induction, so as to facilitate the safe and accurate work of measuring instruments, protective devices or control systems. This paper will briefly introduce the measuring principle and application of mutual inductor.

A, the basic types of transformer

Transformer is mainly divided into voltage transformer (PT) and current transformer (CT).

1. Voltage transformer (PT or VT): used to convert high voltage into standard low voltage (usually 100V or 110V), which is convenient for voltmeter, power meter, electric energy meter and other measuring equipment.
2. Current transformer (CT): used to convert large current into standard small current (usually 5A or 1A) to meet the needs of ammeters, relay protection devices and measuring instruments.

Second, the working principle of transformer

Transformer works on the principle of electromagnetic induction. It consists of a primary winding, a secondary winding and an iron core;

-The primary winding is connected in series (current transformer) or in parallel (voltage transformer) with the tested circuit.
-When a current flows in the primary winding or a voltage is applied, an alternating magnetic flux is generated in the iron core.
When the magnetic flux passes through the secondary winding, it induces electromotive force, thus generating corresponding current or voltage on the secondary side.

According to the mutual inductance ratio (that is, the ratio of the turns of the primary winding to the secondary winding), the high voltage or high current can be scaled down, so that the measured parameters can be measured.

Third, the measuring method of transformer

1. Direct measurement method:
-Compare the measured transformer with the standard transformer as a reference, and determine its ratio error and phase error.
-Common test equipment includes comparator and transformer calibrator.

2. Indirect measurement method:
-In actual operation, the actual value of the primary side is calculated by measuring the voltage or current output from the secondary side of the transformer and combining with the known transformation ratio.
-For example, if the rated transformation ratio of a current transformer is 400:5, when the measured current at the secondary side is 2.5A, the current at the primary side can be calculated as: (400/5) × 2.5 = 200A.

3. Error test:
-In order to ensure the measurement accuracy, it is necessary to test the ratio difference (ratio error) and angle difference (phase error) of the transformer.
-Testing is usually carried out in a laboratory environment, with standard loads and standard transformers.

Fourth, the application of mutual inductor

Transformer is widely used in power system;

-Power metering: used in electric energy meters to realize accurate electric energy metering.
-Relay protection: provide reliable current and voltage signals for the protection device, which can be used to judge the line fault and trigger the action of the circuit breaker.
-Automatic monitoring: providing input signals for substation integrated automation system to realize remote monitoring and control.

V. Matters needing attention in measurement

1. It is forbidden to open the secondary side of current transformer, otherwise high voltage may be generated, endangering equipment and personal safety.
2. It is forbidden to short-circuit the secondary side of the voltage transformer, so as to prevent over-current from burning the equipment.
3. When selecting transformer, factors such as its accurate grade, rated capacity and installation environment should be considered.
4. Regularly check the accuracy of transformer to ensure the reliability of system measurement and protection.

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In a word, as an indispensable measurement and protection equipment in power system, the measurement accuracy of transformer is directly related to the safe and stable operation of the system. Understanding the basic principle, measuring method and application of mutual inductor is helpful to better play its role in modern power system. With the development of smart grid, the digitalization and intelligence of transformers will also become the future development trend.