What Are the High Voltage Test Transformers_ Use of High Voltage Test Transformer

High voltage tester refers to a kind of transformer that generates high voltage for high voltage test. High voltage test transformer has the characteristics of high voltage, small capacity, short continuous working time, thick insulation layer and usually one end of high voltage winding is grounded.

High voltage test transformer is called test transformer for short. The test transformer is generally a step-up transformer. The test transformer uses the power frequency high voltage induced by the secondary side to test the insulation performance of various electrical products and insulating materials. Test transformer is an indispensable electrical equipment for withstand voltage test in the repair of motor, transformer and electrical products. The test transformer is mainly used for power frequency withstand voltage test of electrical products, partial discharge measurement, thermal stability test of insulating medium, etc; For medium and high frequency electrical equipment, special frequency test transformers can also be used for withstand voltage test and measurement of relevant electrical parameters.

The primary and secondary windings of the test transformer have a large voltage ratio. The primary voltage is 0.4kV, 3KV, 6kV and 10kV. The secondary voltage can be made into 100 2000kv or higher, and the capacity can be 3 9000kva. For the field test of power equipment, the light test transformer is generally used; 50kV step-up test transformer is used for withstand voltage test of 6kV, 10kV transformers and other power equipment; For the withstand voltage test of high voltage, ultra-high voltage transformers and other power equipment with voltage level above 35kV, ultra-high voltage test transformers greater than 5 times the primary rated voltage of the tested equipment, such as step-up test transformers of 500kV, 700kv, 800kV and certain capacity level.

Characteristics of high voltage test transformer

1) High voltage: the primary voltage of the test transformer is 220V or 380V, but the secondary voltage of a single test transformer often reaches thousands to tens of thousands of volts. The test transformer with secondary voltage exceeding 750kV usually adopts multiple cascade structures.

2) Small current: the rated current of the test transformer is actually the capacitive current of the tested object, so it is generally less than 1a. However, the secondary current of the test transformer used for cable and large motor test, external insulation pollution test, line corona test and other projects can reach several amps.

3) Short working time: due to the short withstand voltage time of the tested product, the test transformer is generally 0.5h or 1H short-time working system except for external insulation pollution test, line corona test and cable test.

4) The test transformers are generally single-phase and indoor devices (some are also made into outdoor devices): they are oil immersed self cooling structure, the winding insulation layer is thick, and one end of the high-voltage winding is usually grounded.

5) The insulation level of the head and end of the secondary winding of the test transformer is different: the head end is high potential, and the end is directly grounded or grounded through an ammeter.

6) High requirements for design and manufacturing process: due to the high working voltage, the insulation structure has a decisive impact on the overall size of the test transformer. Vacuum drying is generally required for the body, and vacuum oil injection is required for above 100kV.

Classification of high voltage test transformers

1) In terms of structure, there are iron shell type and insulated shell type test transformers.

2) In terms of power frequency, there are several frequency level test transformers such as power frequency, frequency doubling, intermediate frequency and high frequency.

3) According to the voltage level, it can be divided into low voltage, high voltage and ultra-high voltage test transformers.

4) According to the use form, it can be divided into light mobile type and fixed type; There are also single use and two or more cascade use test transformers.

The test transformer is generally single-phase and indoor devices, which are oil immersed self cooling structure. Because of its high working voltage, its winding insulation layer is thick, and one end of the high-voltage winding is grounded.

The specific structural types of the test transformer generally include single bushing type, double bushing type and insulating cylinder type.

Technical requirements for high voltage test transformer

â‘  Output voltage waveform. The output voltage waveform of the test transformer shall be close to the sine wave as far as possible. In order to reduce the voltage waveform distortion caused by the harmonic voltage generated by the harmonic component of no-load current passing through the impedance of voltage regulator and transformer, the magnetic flux density should be selected in the straight line section of core magnetization curve; At the same time, voltage regulating equipment with small waveform distortion shall be selected, and filter device can be added if necessary.

â‘¡ Impedance voltage. The secondary current of the test transformer is generally capacitive current. When the secondary current flows through the impedance of the voltage regulator and the test transformer, the output voltage will exceed the value determined by the voltage ratio. Therefore, the impedance voltage of the test transformer should not be too large, otherwise it may affect the accuracy of the test results and reduce the short-circuit capacity of the test equipment. However, if the impedance voltage is too small, the short-circuit current may increase when the test object breaks down or flashover. The impedance voltage of a single test transformer is generally 4.5 10%, and when multiple series transformers are formed, it can reach 30 40%.

What are the high voltage test transformers

1） DJ oil immersed high voltage test transformer

Oil immersed test transformer: large volume, heavy weight, large capacity, convenient maintenance and high maintenance cost in the later stage; Mainstream products.

The oil immersed type is filled with 25# transformer oil, and the insulation cooling effect is the best

2） GTB dry type high voltage test transformer

Dry type test transformer: maintenance free, light weight, small volume, inverted and high cost.

3） Inflatable high voltage test transformer

Inflatable test transformer: it needs later maintenance, with good arc extinguishing performance, light weight and danger.

The oil immersed type is filled with 25# transformer oil, and the insulation cooling effect is the best; Dry type has small volume and lightest weight, but the voltage is difficult to be high; The inflatable type is filled with SF6 gas.

Application and operation method of AC / DC high voltage test transformer

During the step-up or withstand voltage test of AC / DC high-voltage test transformer, if the following unusual conditions are found, the voltage shall be reduced immediately and the power supply shall be cut off. As of the experiment, the reasons have been found out: â‘  the pointer of the voltmeter swings greatly; â‘¡ The insulation of the invention burns or smokes; â‘¢ There is unusual sound in the test object.

Utilization and operation methods of AC / DC high voltage experimental transformer:

1. Connect the power supply of the control box (console) and turn on the indicator light.

2. Press the start button and the operation indicator light is on (otherwise, the voltage regulator is not in the zero position, please rotate the hand wheel of the voltage regulator to the zero position counterclockwise) indicating that the experimental operation can be stopped.

3. Move the hand wheel of the voltage regulator clockwise to make the conditioning rate 2KV / s better, and watch the high-voltage meter rise to the required voltage value. For example, in the process of boosting, when the test object breaks down, short circuit and overcurrent occurs, the overcurrent relay acts to automatically cut off the power supply.

4. If the experiment requests overcurrent control, adjust the overcurrent relay to the power supply current value required by the experiment in advance before operation, and the current relay will stop overcurrent protection according to the experiment request.

5. While monitoring the high-voltage meter, an external 0.5-level voltmeter can be accepted at the instrument end for monitoring.

6. Measure the insulation resistance and check the insulation condition before and after the withstand voltage test.