Transformrer Voltage Ratio Test

The voltage ratio of the transformer refers to the ratio of the primary side voltage U1 to the secondary side voltage U2 when the transformer is running at no load, which is referred to as the voltage ratio (or ratio), that is,

K=

The purpose of measuring the voltage ratio:

1. Check the correctness of the turns ratio of the transformer windings;

2. Check the condition of the tap changer;

3. After the transformer fails, the commonly used measured voltage ratio is used to check whether the transformer has a turn-to-turn short circuit;

4. Determine if the transformer can run in parallel.

When the voltage difference between the secondary side of the two parallel transformers is 1% of the rated voltage, the circulating current in the two transformers will reach about 10% of the rated current, which increases the loss of the transformer and occupies the capacity of the transformer. . Therefore, the difference in voltage ratio should be limited to a certain range. For transformers with a voltage ratio of less than 3, the allowable deviation is ±1%, and all other transformers (rated tap positions) are ±0.5%.

The measurement method of the voltage ratio generally has a dual voltmeter method and a variable ratio bridge method.

Dual voltmeter method

When measuring the voltage ratio, the applied voltage is preferably close to the rated voltage (generally not less than 1/3 of the rated voltage) and should be applied to the power supply side. For the Step-Up Transformer to be applied to the low voltage side, the step-down transformer is applied to the high voltage side.

The voltage ratio of a three-phase transformer can be measured with a three-phase or single-supply power supply. It is relatively easy to measure with three-phase voltage, and it is easy to find faults with a single-phase power supply than with a three-phase power supply. When measuring the voltage ratio of the transformer connected by Y, d or D, y with a single-phase power supply, the non-test of the delta connection winding is short-circuited accordingly, so that there is no magnetic flux in the non-test phase, and the magnetic phase of the pressurized phase is uniform. .

First, direct dual voltmeter method

A voltage is applied to one side of the transformer, and a voltage is measured on both sides of the primary and secondary windings by a voltmeter (line voltage or phase voltage is converted into a line voltage), and the ratio of the line voltages on both sides is the measured voltage ratio.

When measuring the voltage ratio, the power supply voltage is required to be stable. If necessary, a voltage regulator device should be added. The secondary side voltmeter lead should be as short as possible and the contact should be good to avoid errors. The accuracy of the measuring voltmeter should not be lower than 0.5, and the primary and secondary voltages must be read simultaneously.

Second, the voltage voltmeter double voltmeter method

When measuring the voltage ratio under the rated voltage of the transformer under test, there is generally no accurate high-voltage AC voltmeter, which must be measured by a voltage transformer. The accuracy of the voltmeter used is not less than 0.5, and the accuracy of the voltage transformer is 0.2.

When a large Power Transformer is instantaneously full-voltage excited, an inrush current may be generated in the transformer, so that an overvoltage is generated on the secondary side, and the generator can be used for voltage regulation, which is easy to implement in a power plant, and the substation only uses a new input from the transformer. The impact closing test after operation or overhaul is performed together.

For 110/10kV high voltage transformers. If 380V excitation is used on the low voltage side, the voltage transformer should be used to measure the voltage on the high voltage side. The accuracy of the voltage transformer should be one level higher than the meter, the voltmeter is 0.5, and the voltage transformer should be 0.2.

Variable ratio bridge method

The variable ratio bridge can be used to easily measure the voltage ratio of the transformer under test.

Domestic and international partial voltage ratio automatic measuring instrument technical parameters

The basic principle of the voltage ratio automatic measuring instrument is the voltage measurement method and the bridge method described above. It generally uses a single-chip microcomputer as a microprocessor, accepts the input of the panel keyboard and the switch quantity, automatically tracks and controls the range and bridge balance, and performs data processing on the measurement results. Finally, the measurement result is stored and printed, and the voltage is quickly completed. Ratio measurement.

The key to the high accuracy of the voltage ratio automatic measuring instrument is:

1. High accuracy standard voltage transformer. Whether it is the voltage transformer on the high-voltage side or the low-voltage side of the transformer under test, the accuracy must be high enough to get accurate Un and Ux.

2. The AC/DC converter must have high precision and high input impedance to reduce the shunt of the standard voltage transformer and ensure that the converted DC voltage is exactly at the rms value of the AC voltage.

3. The microcontroller used by the microprocessor should have sufficient memory and processing power.

4. Equipped with well-functioning software to control the whole machine and perform data processing.

The functional characteristics of the voltage ratio automatic measuring instrument are as follows:

1. During the measurement process, the sampling of the primary and secondary winding signals of the tested transformer is synchronized, which can avoid the influence of power supply voltage fluctuation.

2. The digital processing function of the CPU is very strong. Generally, the noise elimination algorithm and the mean value algorithm can be added to the software to improve the stability and anti-interference performance of the data.

3, generally have instrument working status and error information display.

4. The voltage ratio automatic measuring instrument adopts the CPU, and can install the IEEE488 general instrument control interface into the measuring instrument machine. After connecting with the PC, it can realize remote control and data exchange, and can form an automatic measuring system of multiple instruments.

The voltage ratio automatic measuring instrument has the unparalleled function of the general voltage ratio measuring instrument, and their appearance changes the current status of the voltage ratio measurement and improves the efficiency.