Generation of test voltage for communication
The method of generating the test voltage for communication
The industrial frequency high voltage is usually generated by using high-voltage test transformers; for test samples with large capacitance, a series resonant circuit can be adopted to generate high voltage; for test samples with windings such as power transformers and voltage transformers, a 100-300Hz medium-frequency power supply can be used to excite the low-voltage winding and induce high voltage in the high-voltage winding.
High-voltage test transformer circuit
The wiring for the AC withstand voltage test should be determined based on the voltage, capacity of the tested item, and the actual on-site test equipment conditions. Usually, the test transformer is a complete set of equipment.
When conducting AC withstand voltage tests on electrical components with large capacitance such as transformers and capacitors, the capacity of the test transformer often fails to meet the test requirements. In the field, reactors are often used for parallel compensation. When the parameters are appropriately selected and the reactance of the two parallel branches is equal to the inductance, the circuit enters a parallel resonance state. At this time, the load on the test transformer is the smallest. When using a parallel resonant circuit, special attention should be paid. The test transformer should be equipped with an overcurrent instantaneous protection device because when the tested component breaks down, the resonance disappears, and the test transformer is at risk of overcurrent.
Series resonant circuit
For electrical equipment with large capacitance and high test voltage, such as GIS, generator and transformer cross-linked cables, and high-voltage circuit breakers, which require large-capacity test equipment, a series resonance test device can be used. It can conduct withstand voltage tests on larger capacitance and higher test voltage equipment with a smaller power supply capacity. The circuit consists of the load capacitance of the tested equipment, the inductor in series with it, and the power supply.
Since the resistance in the test circuit is very small, the quality factor of the test circuit is very high. In most normal cases, it can reach around 50, that is, the output voltage is 50 times the excitation voltage. Therefore, this method can obtain a higher test voltage using a lower-voltage test transformer. Because the circuit is in resonance during the test, the circuit itself has a good filtering effect, and the harmonic components in the power supply waveform are greatly reduced at both ends of the test equipment. Usually, a good sine wave voltage is output.
When the test sample breaks down, the circuit loses its resonant condition, and the output current of the power supply automatically decreases. The voltage across the test sample suddenly drops, thereby limiting the degree of damage to the test sample.
According to the different adjustment methods, series resonant devices can be classified into two major categories: power frequency series resonant devices (with adjustable reactors or with fixed reactors and tuning capacitor banks, operating at a frequency of 50Hz) and variable-frequency series resonant devices (with fixed reactors, operating at a frequency generally ranging from 10Hz to 300Hz).
The inductance of the reactor used in the power frequency series resonance device can be adjusted continuously. When the test voltage is high, several reactors can be connected in series.
The variable-frequency series resonance device regulates the power supply frequency through a high-power variable-frequency power supply, causing the circuit to resonate. The inductance of the used reactor is fixed (unadjustable). The test frequency changes according to the capacitance of the tested item. Since the test frequency of the variable-frequency series resonance device varies with different capacitances of the tested items, its application range is limited.
When using the series resonant device in practice, the tuning of the test circuit must be carried out at a very low excitation voltage. By adjusting the inductance of the reactor or changing the power supply frequency, the voltage at the test specimen end should be made as high as possible. At this point, the circuit reaches the resonant state. Then, increase the excitation voltage at the specified rising speed to make the high-voltage side reach the test voltage. After the withstand test is completed, reduce the voltage uniformly and quickly, and then cut off the power supply.
Medium-frequency power supply device
The induction withstand voltage test and partial discharge test of transformers require medium-frequency power supplies. The main ways to obtain medium-frequency power supplies on-site are: medium-frequency power supply unit sets, three-times-frequency power supply devices, medium-frequency synchronous generator sets, and electronic frequency conversion devices. For large transformer tests, the medium-frequency power supply unit sets are more commonly used on-site.
Double-frequency power supply unit
An asynchronous motor with a wound-rotor design, when three-phase alternating current is passed through the rotor (or stator), is driven by another asynchronous motor to achieve synchronous addition of the mechanical speed to the rotating magnetic field. This induces a higher-frequency sinusoidal alternating current on the stator (or rotor). The alternating magnetic field is adjusted by a three-phase voltage regulator.
Tripler frequency power supply device
The tripler power supply device is composed of three single-phase transformers. The primary windings of these transformers are connected in a star configuration, while the secondary windings are connected in an open delta configuration, thereby generating a three-times-frequency voltage.
Medium-frequency synchronous generator set
The medium-frequency generator set is composed of one motor driving one medium-frequency synchronous generator. By changing the resistance value of the excitation resistor in the excitation circuit of the generator, the excitation machine can alter the excitation on the generator rotor, thereby enabling the stator of the generator to output a smooth and adjustable voltage. The use of brushless excitation generators can completely avoid the interference of carbon brush sparks, which is very beneficial for the measurement of local discharge.
Electronic frequency conversion device
The electronic frequency conversion device is an electronic device that uses high-power electronic technology to generate AC sine wave or square wave voltages. During practical application, it is necessary to ensure that the voltage applied to the tested item meets the requirements of a sine wave.
Post time: Mar-05-2026