The partial discharge test of transformers is an important and sensitive test item for detecting and evaluating the insulation condition inside the transformer. The core purpose is to identify local insulation weaknesses such as bubbles, sharp corners, and impurities that do not penetrate the electrodes, in order to avoid potential failure risks in advance.
Test equipment
The partial discharge test system is a comprehensive measurement system. Its core components and related requirements, functions are as follows:
Test power supply
As the core power supply part of the test, it is required to be a pure, non-discharge medium-frequency power supply to avoid interference from the power frequency power supply itself, which may cause confusion between the discharge of the test sample and the power frequency interference signal. It mainly includes two types:
Power frequency resonant device: The most widely used and suitable type, divided into frequency-adjustable series resonant device and fixed-frequency series resonant device. It can compensate for the capacitive current of the transformer through the reactor, significantly reducing the power supply capacity requirements, and generating high-quality sine waves.
Medium-frequency generator set: The traditional power supply method, generated by an electric motor – generator set at around 400Hz, then reduced to the required voltage for the test. Due to its large size and high noise, it has been gradually replaced by resonant devices.
Coupling capacitor
Its core function is to provide a low-impedance transmission channel for the partial discharge pulse signal, while blocking the high-frequency voltage from the power frequency from entering the measurement circuit. The equipment itself must meet the requirements of no discharge.
Detection impedance
As a key sensor connected in series between the coupling capacitor and the measuring instrument, it can extract voltage signals from the flowing partial discharge pulse current and transfer the signals to the measuring instrument.
Partial discharge detector
It is the core control and analysis unit of the entire test system, responsible for receiving, processing, displaying and recording the partial discharge signals. Its core functions include:
Signal amplification and filtering: Amplifying and filtering weak partial discharge signals, while filtering out specific frequency bands of interference signals to ensure the validity of the signals.
Signal display: Supporting various display forms such as ellipses and straight lines, presenting the characteristics of partial discharge pulses intuitively.
Parameter measurement: Precisely measuring core parameters such as apparent discharge quantity, discharge repetition rate, and average current, among which the apparent discharge quantity is the main measurement indicator.
Location and identification: High-end instruments are equipped with multi-channel measurement functions. Through comparing the signal arrival times at different winding terminals, the discharge source can be roughly located; at the same time, by analyzing the discharge spectrum, different types of discharge such as internal discharge, surface discharge, and corona discharge can be identified.
Calibrator
The key calibration equipment before the test, it can inject a known standard pulse of charged quantity to the two ends of the test sample, thereby calibrating the scale coefficient of the entire measurement system, which is a necessary step to ensure the accuracy and comparability of the measurement results.
Test method
The partial discharge test of transformers is mainly carried out during the equipment factory stage or after major maintenance. The test process follows the requirements of standardized specifications, and is divided into two core links: test wiring and test procedure:
1. Test wiring
It includes two basic wiring methods: direct method and balanced method. The direct method is the most commonly used and highly sensitive method, which can be further divided into parallel method and series method; multi-terminal measurement is the conventional operation form for transformer tests:
Direct method – Parallel method: The detection impedance is connected in parallel with the test sample, suitable for test scenarios where one end of the test sample is grounded.
Direct method – Series method: The detection impedance is connected in series with the coupling capacitor, suitable for test scenarios where both ends of the test sample are not grounded.
Balanced method: Using two similar test samples, or an un-discharged coupling capacitor and the test sample forming a bridge, with excellent anti-interference ability, but the on-site operation process is complex, mostly used in laboratory environments.
Multi-terminal measurement: Installing detection impedance on multiple terminals of the transformer’s high-voltage winding, neutral point, and low-voltage winding, facilitating the subsequent location analysis of the discharge source.
2. Test procedure The test procedure is divided into five steps according to the operation sequence: preparation work, system calibration, pre-pressurization, measurement, judgment and recording. Each step has clear operation requirements and standards:
Preparation work
The transformer oil needs to be tested and the results should be qualified. It should be left to stand for more than 48 hours to fully eliminate the bubbles in the oil.
Short-circuit and ground all the secondary windings of the bushing current transformers, and take protective measures.
Connect the test circuit strictly according to the wiring diagram to ensure that all connection parts are firm and reliable.
System calibration
Connect the calibrator in parallel between the high-voltage terminal of the transformer and the ground.
Inject standard charge into the system, adjust the gain of the detector to make the instrument display value consistent with the injected value, and record the scale coefficient at this time.
Pre-pressurization
Steadily increase the voltage to the specified pre-pressurization voltage and maintain for the specified duration (such as 60 seconds). Through this process, activate or eliminate some unstable insulation weaknesses. Measurement
Reduce the voltage to 1.1 times Um/√3 and maintain it for 5 minutes. Read the background interference level.
Increase the voltage to the measurement voltage, keep it for the specified duration (such as 30 seconds or more) in accordance with the standard, simultaneously measure and record the large amounts of partial discharge.
Again reduce the voltage to 1.1 times Um/√3 and maintain it for the specified duration (such as 30 minutes). Conduct a long-term measurement and continuous observation of the partial discharge situation.
Judgment and Recording
Throughout the test, observe the starting voltage and extinction voltage of the partial discharge signal, as well as the stable discharge amount under the measurement voltage and the growth trend of discharge amount over time, etc. as key information.
Completely record all the data during the test process, and at the same time save typical partial discharge spectra to provide a basis for insulation condition assessment.
Post time: Jan-22-2026