The lightning impulse generator usually adopts multi-stage impulse voltage generators. The working principle of multi-stage impulse voltage generators can be simply summarized as multiple capacitors being charged in parallel, and then automatically connected in series for discharge, forming a very high amplitude impulse voltage wave. People usually use impulse energy (W) to represent the load capacity of the impulse generator.
When the single-stage charging voltage of a multi-stage impact generator is U, the nominal energy of the impact generator is: W = NCU²/2, where N is the number of stages of the impact generator, C is the capacitance per stage (in units of F), U is the voltage per stage (in units of V), and W is in units of J. Generally, considering the efficiency test capability of the impact generator and the economic performance of the equipment, it is usually necessary to select the ratio of the main capacitor of the impact voltage generator to the test specimen capacitor to satisfy the relationship 5 < C1/C2 < 10.
The measurement system for lightning impulse voltage generally employs a resistive-capacitive divider. The accuracy of this divider is less than 1%. With the development of computer technology, measurement instruments commonly adopt digital memory oscilloscopes, whose measurement error is no more than 1%. The total uncertainty of the full wave amplitude of the impulse voltage measurement system is within 3%. For the impulse test, another point to note is that once the product undergoes breakdown, especially in the chopping wave test, the impulse current flowing through the grounding resistance will have a significant voltage drop. If the voltage drop is too large, it will cause the instrument to break down. Therefore, it is necessary to pay attention that the grounding resistance value should not be high. Generally, it is stipulated to be below 0.5 ohms. If necessary, the grounding of the product can be separated from that of the instrument.
When the transformer capacity is large and the waveform cannot be satisfied due to the large capacitance, several stages of the impulse voltage generator should be operated in parallel. When conducting the impulse test at the midpoint of the transformer, since it is a three-phase input wave and the capacitance is large, but the test voltage is generally not high, the impulse voltage generator stages should be connected in parallel and then pressurized. The full-wave voltage and the chopping wave voltage are not uniformly distributed along the winding.
During the test, all the terminals except the one applying the voltage should be grounded, including the midpoint of the high-voltage winding and the end of the low-voltage winding. During the impulse test, there is an impulse-induced voltage between the middle part of the low-voltage winding and the middle part of the iron core column if the field strength is too high. During the impulse test on the high-voltage winding, the low-voltage winding may break down against the iron core column. When using the H-L-L-H or L-H-L structure, there will also be an impulse-induced voltage at the connection point of the two low-voltage windings. When designing, it is necessary to pay attention to the impulse withstand voltage at these parts.
During the transformer impact test, whether the test passes or a fault occurs is mainly determined by the recorded waveform diagrams. Monitoring the discharge sound and conducting power frequency re-tests can be used as auxiliary methods. The transformer impact test mainly detects the terminal voltage and the currents in each part of the circuit. By analyzing the obtained voltage and current, the insulation condition of the product can be determined. This method is currently the main application method.
During the transformer test, if there is no internal fault, only the discharge sound of the transformer will be heard. When there is a fault, there will be a dull sound of varying intensity inside the product, which can easily be distinguished from the discharge sound of the equipment. From the impact faults of our factory, basically every time there is a fault, an abnormal sound can be heard inside the transformer. Paying attention to listening can help in diagnosing the faults.
The determination of whether the lightning impulse test transformer is qualified mainly relies on the impulse voltage waveform entering the device and the current waveform at the neutral point. Record the waveforms at 50% (reduced voltage) and 100% of the voltage, and directly compare the waveforms to determine whether the transformer has passed the test. To better assess the insulation condition of the transformer, it is generally required to record three values: the entering voltage, the current at the neutral point, and the capacitance transfer current. The entering voltage has a relatively low sensitivity. It mainly indicates the local electrical field of the main insulation (such as the presence of a cutoff wave in the wave voltage waveform). If it is a local fault, it may show a weaker reflection or fail to indicate. The neutral point current and the capacitance transfer current can reflect local discharge faults. Depending on the nature of the fault, these two current detections will have different sensitivities and different waveform changes.
Analysis of several typical fault waveforms:
When the lightning impulse coil’s leading end discharges to ground, the waveform of the incoming voltage shows a truncation point voltage that can reach zero potential, and the neutral point current decreases. Discharge along the support bars from the leading end to the trailing end causes breakdown at the trailing end. The incoming wave forms a truncated wave but the truncation point is not at zero potential, and the neutral point current oscillates and increases. Inter-turn breakdown discharge of the coil causes the incoming wave to change little, while the neutral point current shows a significant increase. The discharge spectrum can be referred to in the “Transformer Lightning Impulse and Operational Impulse Tests” for an example analysis of electrical fault detection. However, in lightning impulse tests, sometimes there is sparking on the ground wire and poor contact between the injured wire and the ground, resulting in inconsistent injured waveforms at 50% of the test voltage and 100% of the test voltage. Therefore, the analysis of the lightning impulse test waveforms must be careful and thorough.
Post time: Feb-07-2026