The no-discharge withstand voltage test is the core method for inspecting the insulation performance of high-voltage power equipment. Its main objective is to detect the latent insulation defects that may exist in the equipment during manufacturing, transportation, or installation. The local discharge level of the test device itself, its voltage output capacity, and system stability directly determine the accuracy and reliability of the test results. Based on engineering practice, this article systematically elaborates on the selection logic, typical application scenarios, and current standard system of the no-discharge withstand voltage test device, providing operational reference for technicians.
Core selection elements
A reasonable selection must be based on three key parameters, which are mutually restrictive and require comprehensive consideration.
1. Rated Voltage
The rated voltage of the device should be no less than the test voltage value specified by the tested equipment and it is recommended to reserve 10% to 20% of engineering margin to cope with the voltage increase requirements during equipment aging or special conditions.
Example: For a 110kV power transformer, the AC withstand voltage value is usually 160kV, then the device rated voltage should not be lower than 176kV, and a 200kV-rated product is recommended.
2. Rated Capacity
The rated capacity (kVA) is closely related to the equivalent capacitance (C) of the tested equipment and the test voltage (U). The calculation formula is: $$P=2πfCU^2×10^{−3$$
Where, f = 50Hz. In practice, special attention should be paid: the unit length capacitance of cables is extremely large. Under the same voltage level, the required device capacity for long-distance cables is often several times that of a transformer. For example, a 220kV-level long cable may require a capacity of 1500kVA or more, while a transformer of the same voltage level only requires several hundred kVA.
3. Background Local Discharge Level
This is the key indicator that determines the validity of the test results. The background local discharge level of the device must be much lower than the allowable discharge level of the tested product. Current mainstream products can control the background local discharge within 3 pC, while high-quality equipment can reach 1 pC or below. The lower the background noise, the higher the recognition degree of small defects, and the stronger the confidence of the test results.
Typical application scenarios
Different tested equipment have different requirements for the form and topology of the device due to their structures, capacitance characteristics, and test sites.
Power transformers (oil-immersed / SF6): Medium capacity, choose a split-type power frequency device, focus on the stability of output voltage and the fineness of voltage regulation.
GIS combined electrical equipment: Small capacity, choose a variable-frequency resonance device, focus on the frequency adjustment range and waveform distortion rate.
Long-distance power cables: Extremely large capacity, choose a large-capacity variable-frequency resonance device, focus on the matching of device capacity and the configuration of excitation transformers.
Generators / motors: Large capacity, select a vehicle-mounted or dedicated power frequency device, focus on the background local discharge of the entire machine and the anti-interference ability.
The specific applicable scenarios of different device forms are as follows:
Split-type (fixed): Composed of a no-discharge transformer, voltage regulator, filter, and measurement unit independently, suitable for laboratories or fixed test sites, which can be flexibly combined according to the test object and has strong scalability.
Vehicle-mounted (mobile laboratory): Integrate the entire system into a shielded vehicle, suitable for on-site tests in substations, wind farms, and hydropower plants that need frequent relocation, with rapid deployment capabilities and self-contained power supply, effectively avoiding errors and interference introduced by on-site assembly.
Variable-frequency resonance type: Designed for on-site tests of large-capacity capacitive loads (GIS, long cables). By adjusting the frequency to make the circuit resonate, a small power supply capacity outputs high voltage while the waveform is close to sine, with strong resistance to on-site electromagnetic interference.
Test basis standard system
The design, manufacture, inspection, and use of no-discharge test devices should follow the following current national and industry standards:
DL/T 848.3‑2019 “General Technical Conditions for High Voltage Test Equipment – Part 3: Transformer without Arcing Discharge” This standard is a core specialized standard in this field, replacing the previous version DL/T 848.3‑2004. It clearly stipulates the technical requirements such as arc suppression performance, insulation level, temperature rise limit, and working conditions of the device, and is applicable to both oil-immersed and SF6 gas-insulated mainstream structural types.
GB/T 7354 / IEC 60270 “Partial Discharge Measurement” This is a basic method standard for partial discharge detection, uniformly stipulating the measurement principle, calibration procedures, and coupling methods. It serves as the basis for determining the discharge quantity in both factory inspection and on-site measurement of the device.
GB 50150 “Standard for Electrical Equipment Commissioning Tests” This standard specifies the test voltage values and judgment criteria for various power equipment during on-site commissioning, providing direct design input for determining the rated voltage of the device.
Practical Suggestions for Selection and Reminder
Based on extensive engineering experience, the following selection and usage points are proposed:
Preliminary assessment of test objects: Clearly identify the types of main tested equipment (transformer, GIS, cable, or generator), and thereby initially define the voltage level and capacity range.
Verification of voltage according to standards: Refer to GB 50150 and corresponding equipment-specific standards (such as GB/T 6451, GB/T 7674), to obtain the precise AC withstand voltage value of the tested item, which serves as the hard lower limit of the rated voltage.
Emphasis on background noise control: In addition to the device’s own arc suppression level, the background interference of the entire test circuit should also be evaluated. If the electromagnetic interference in the on-site environment is severe, a device with an all-enclosed integrated design or a dedicated shielding room (cover) for partial discharge tests should be considered first.
Review completeness of the complete set configuration: When purchasing or leasing, not only the main unit should be considered, but also confirm whether the supporting accessories are complete, at least including: no-arc coupling capacitor, capacitance divider, protection resistor, high-voltage connection wire, and calibration pulse generator.
Plan the test site in advance: The absence of partial discharge tests has clear requirements for the site environment. Factors such as grounding resistance, environmental humidity, and the distribution of surrounding interference sources should be considered. If necessary, site pre-treatment should be carried out.
Post time: Aug-21-2026