The core components of the partial discharge testing system include the test power supply, coupling capacitor, detection impedance, partial discharge measuring instrument, calibrator, as well as the shielding and grounding system. The functions and requirements of each part are as follows.
I. System Composition
(1) Test Power Supply
The test power supply should be a non-discharge power supply with medium frequency (usually 100-400Hz) or industrial frequency (50/60Hz), providing sufficient high test voltage to avoid the partial discharge signal being submerged by background interference. Its main types are as follows:
Medium Frequency Generator Set (MFC): As a traditional and reliable power supply form, it is driven by an electric motor to generate a sine wave, having advantages of large capacity and good waveform, but it has the characteristics of large volume and high cost.
Electronic Variable Frequency Power Supply (AC/DC/AC Inverter): It belongs to the modern mainstream power supply type, converting industrial frequency into medium frequency through power electronic technology, having the advantages of small volume, light weight, and adjustable frequency. However, it has higher requirements for design and manufacturing, and it is necessary to ensure that the power supply itself has no partial discharge.
Series Resonance System: For large-capacity transformer test scenarios, it can be used in conjunction with an inductor to effectively reduce the requirement for power supply capacity.
(2) Coupling Capacitor
The coupling capacitor is connected in parallel to the two ends of the test sample, serving to provide a low-impedance channel for the partial discharge pulse and coupling the pulse signal to the detection impedance. The partial discharge level of this device must be far lower than the partial discharge requirement value of the transformer.
(3) Detection Impedance (Detection Unit)
The detection impedance is connected to the grounding end of the coupling capacitor, usually an inductive resistor or an RLC circuit, with the main function of picking up the partial discharge pulse signal and converting it into a voltage signal.
(4) Partial Discharge Measuring Instrument (PD Analyzer)
The PD Analyzer is the core device of the entire testing system, responsible for receiving the voltage signal from the detection impedance. Its key functions are as follows:
Filtering and Amplification: Precisely extracting weak partial discharge signals from strong background noise.
Display and Measurement: Real-time display of key parameters such as the waveform, amplitude (pC), phase (φ), and discharge frequency (N) of the partial discharge pulse.
Graphical Analysis: Modern digital partial discharge analyzers can generate φ-q-n graphs (phase – discharge quantity – discharge frequency), through which the type of discharge can be determined, such as internal discharge, surface discharge, corona discharge, etc.
Regular Calibration: The PD Analyzer needs to be calibrated according to specifications on a regular basis to ensure measurement accuracy.
(5) Calibrator
The application scenarios of the calibrator are before and after the test. The operation method is to inject a known charge quantity of standard pulses into the transformer terminals, thereby achieving the calibration of the apparent discharge quantity (pC) of the entire measurement system. This step is a key link to ensure the accuracy and comparability of the measurement results.
(6) Shielding and Grounding System
Shielding Room / Shielding Tent: To eliminate the influence of space electromagnetic interference on the test, the entire test circuit, including the transformer, power supply, and measurement lines, should be conducted in a metal shielding room.
Single-point Grounding: All test equipment must adopt single-point grounding, forming an equipotential environment to prevent ground circulation from introducing additional interference.
II. Test Methods (Steps and Key Points)
Partial discharge tests are usually applied as routine tests after the completion of transformer manufacturing, as well as for handover tests after equipment installation, or for fault diagnosis of equipment. The basic circuit used is the parallel connection method, that is, the coupling capacitor is connected in parallel to the test sample, and the detection impedance is connected in series to the grounding end of the coupling capacitor.
(1) Preparation Work
Clean and dry the transformer body to ensure that the test environment humidity meets the requirements, usually the environmental humidity should be lower than 85%.
Complete the arrangement of all test equipment, the connection lines of the equipment should follow the principle of short and firm, and use coaxial shielded cables. Set up a shielding environment that meets the requirements and ensure that the system achieves reliable single-point grounding.
Set up safety barriers and warning signs in the test area to ensure the safety of the test.
(II) System Calibration
Connect the calibrator in parallel between the high-voltage terminal of the transformer and the ground.
Inject a standard square wave pulse with a known charge quantity (such as 100 pC) into the test system.
Adjust the scale coefficient of the partial discharge instrument until the screen display reading is consistent with the injected charge quantity, and record this scale coefficient.
(III) Background Noise Measurement
Measure and record the background noise level without applying the test voltage, or at a low voltage much lower than the partial discharge initiation voltage. Generally, the background noise should be lower than 5 pC, or less than 50% of the allowable partial discharge quantity.
(IV) Voltage Boost and Pre-boost
Boost the test voltage to 1.8 times the rated phase voltage (Um), or boost to the pre-boost voltage value specified in the technical agreement at a stable speed.
Maintain the pre-boost voltage for 60 seconds. The purpose of this process is to “activate” any insulation defects that may exist in the transformer, making it easier to detect in the subsequent measurement voltage.
(V) Reduce Voltage to Measurement Voltage and Maintain
Reduce the voltage to 1.5 times the rated phase voltage (Um), or the partial discharge measurement voltage specified in the technical agreement (usually 1.3-1.7 times the rated voltage).
Maintain the measurement voltage for at least 30 seconds, while conducting the partial discharge measurement. This stage is the core measurement stage of the test.
Key data recording: During the stage of maintaining the measurement voltage, read and record the larger apparent discharge quantity (pC) in the stable state.
(VI) Reduce Voltage and Re-calibration
Reduce the test voltage to zero and disconnect the test power supply.
Re-calibrate the system to verify whether the scale coefficient change of the measurement circuit is within ±10%. If it exceeds this range, the test result of this trial is invalid.
(VII) Data Recording and Spectrum Analysis
Completely record all test data such as test voltage, background noise level, partial discharge quantity under the measurement voltage, etc.
Analyze with the φ-q-n spectrum to determine the discharge type of the transformer and the possible discharge location.
Post time: Jan-17-2026