I. Capacity and Sizing Calculation
Q: What parameter is most often overlooked during equipment selection, yet most likely to cause test failure?
A: The equivalent capacitance (C) of the test specimen. Most testers focus only on voltage parameters and tend to overlook the impact of capacitance. For high-capacitance specimens such as long cables or large transformers, significant capacitive current may be generated during operation. If the testing equipment lacks sufficient capacity, the test voltage may fail to reach the set value, directly resulting in test failure.
Q: How can one estimate the required test current and capacity?
A: Under 50 Hz power frequency conditions, the test current and equipment capacity can be directly estimated using the following core formulas:
Test current (I) ≈ 2π × 50 × C × U
Required capacity (P) = U × I
If the specimen’s capacitance is not clearly specified, typical industry values can be used for estimation:
1. Power transformers: For capacities <1000 kVA, equivalent capacitance is approximately 1000 pF; for capacities >1000 kVA, it ranges from 1000 to 10,000 pF.
2. High-voltage power cables: Oil-paper insulated cables have 250–300 pF/m; gas-insulated cables have about 60 pF/m.
3. Voltage transformers: Equivalent capacitance ranges from 200 to 500 pF.
Q: After calculating the required capacity, how much margin should be allowed in actual equipment selection?
A: To ensure test voltage waveform quality and avoid risks associated with operating at full load, it is recommended that the rated capacity of the test equipment be selected as 1.2 to 1.5 times the theoretical calculated value, providing ample operational margin.
Q: If the specimen’s capacitive current is too high, is there a way to reduce the demand on power supply capacity?
A: Yes—by connecting a compensation reactor in parallel on the high-voltage side, the inductive current can cancel out the capacitive current produced by the specimen, significantly reducing the power supply capacity requirement. This method is highly adaptable and easy to operate, making it a commonly used optimization strategy in field testing.
II. Voltage Regulation and Measurement
Q: What is the best way to regulate voltage, and how should the voltage regulator be selected?
A: Select a suitable voltage regulator based on the test capacity to ensure stable regulation and efficient equipment operation:
1. For low-capacity tests (<100 kVA): Preferably use an autotransformer, which offers superior waveform quality and low operating losses.
2. For high-capacity tests (>100 kVA): Use an induction-type voltage regulator, capable of meeting wide-range voltage adjustment requirements.
Q: Is it reliable to read measurements directly from instruments on the low-voltage side when measuring high voltage?
A: No, it is unreliable. During testing, due to the combined effects of transformer leakage reactance and specimen capacitance, a capacitive rise effect occurs, causing a noticeable deviation between the actual high-voltage side voltage and the readings from low-voltage instruments. To ensure measurement accuracy, dedicated measuring devices such as capacitive voltage dividers must be installed on the high-voltage side to directly capture voltage data.
Q: When configuring protective resistors, besides resistance value, what other parameters must be considered?
A: Resistance value is the fundamental parameter, but thermal capacity, withstand voltage rating, and physical dimensions must also be carefully considered:
1. Thermal capacity and withstand voltage: At the moment of specimen breakdown, the protective resistor bears the full high voltage and high current. Insufficient thermal capacity may lead to resistor burnout or explosion, posing safety hazards.
2. Resistor length: Adequate length must be provided to prevent surface flashover. Industry standards generally require a minimum length of 0.5 meters per 100 kV of test voltage.
III. Key Points for AC Test Transformer Configuration
Q: How should the “low-voltage arm” and “high-voltage arm” of a capacitive voltage divider be matched?
A: Stability and accuracy of the voltage division ratio are critical requirements in voltage divider configuration. The high-voltage arm should use high-quality capacitors with low loss and minimal temperature coefficient to ensure parameter stability; the low-voltage arm can be paired with precision resistors or precision capacitors. After the equipment is assembled, the entire voltage divider set must be calibrated together with the test transformer. Without calibration, the actual measurement error of the uncalibrated equipment may exceed 5%, failing to meet the required test accuracy.
Q: What is the purpose of the “zero-position protection” function in the voltage control console?
A: Zero-position protection is a core safety feature that must be forcibly included in test equipment. Before starting the voltage increase, the voltage regulator must return to zero position; otherwise, the equipment cannot close and start. This function effectively prevents erroneous operations such as starting under pressure or sudden voltage rise, avoiding damage to test specimens and ensuring personnel and equipment safety. It serves as the first line of defense for test safety.
Q: What special requirements are there for long-duration withstand voltage tests (e.g., 1 hour) regarding configuration?
A: Long-duration withstand voltage tests primarily evaluate the equipment’s long-term thermal stability and system operational reliability. The configuration must meet three key requirements:
1. Transformers and voltage regulators must have sufficient rated capacity margin to prevent overheating and equipment failure during prolonged operation;
2. Protective resistors should be optimized with heat dissipation design to ensure stable performance under continuous load without overheating or failure;
3. The measurement system must maintain stable performance, with voltage fluctuations during testing controlled within ±1%.
Q: What should be done if on-site conditions are limited and power supply capacity is insufficient?
A: Prioritize using parallel compensation reactors on the high-voltage side. Calculate appropriate reactor specifications based on parameters to cancel capacitive current and reduce power supply capacity requirements. If this solution does not meet needs, consider setting up an independent test power source using high-capacity generator sets. However, this approach significantly increases configuration costs and should only be used as a backup option.
Q: What additional considerations are needed when configuring “AC/DC dual-use” functionality?
A: The equipment must be equipped with rectifier silicon stacks and filter capacitors, while strictly adhering to two core configuration requirements:
1. Rectifier silicon stack selection: The reverse breakdown voltage must be at least twice the peak test voltage to prevent breakdown and damage;
2. Measurement system compatibility: Under DC test conditions, standard capacitive voltage dividers create short circuits and must not be used. Replace them with resistive voltage dividers or dedicated DC high-voltage meters with isolation functions.
Q: Are there any special considerations when testing generators or motors?
A: Motor-type test specimens typically have relatively small equivalent capacitance but complex insulation structures and high test voltage levels. Two key aspects require special attention in configuration:
1. Test frequency: Confirm whether the test standard allows slight deviation from power frequency; most applications can accommodate frequencies between 45–55 Hz;
2. Anti-interference configuration: Install anti-corona devices as needed to eliminate surface corona discharge on equipment, preventing interference with measurements and test result evaluation.
Q: When purchasing a complete test set, what is the relationship between the equipment’s “rated capacity” and its “maximum load capacitance”?
A: These two parameters are equivalent and can be converted into each other. Given the equipment’s rated output voltage U and rated capacity P, the maximum allowable load capacitance can be calculated using the formula: Cmax = P / (2πf U²). Before test configuration, verify that the specimen’s capacitance falls within the equipment’s capacity range using this formula to avoid test failures and disputes caused by parameter mismatches.
Post time: Jun-29-2026