Partial discharge testing instruments commonly employ detection methods including the direct method, bridge balance method, and current pulse method. The basic principle is as follows: When a partial discharge occurs in the test sample, an instantaneous voltage change ΔU is generated across the sample Cx. This voltage change is coupled to the detection impedance Zm through a coupling capacitor Ck, generating a pulse current I in the circuit. This pulse current I is sampled, amplified, and displayed to determine parameters such as the apparent discharge quantity of the partial discharge. The pulse current method primarily utilizes the lower frequency portion of the partial discharge signal spectrum, avoiding radio interference.
Partial discharge testing instruments are equipped with RLC-type detection impedances to measure the pulse signal generated by partial discharge and suppress power frequency and other low-frequency interference signals from the experimental power supply.
Partial discharge testing instruments are pulse peak indication testing systems with computer-controlled filtering, amplification, sampling, storage, and display functions. Data acquisition is performed synchronously with the test voltage. This system can measure the discharge repetition rate (n), average discharge current (I), and square law (D) using direct methods and bridge balance methods, and can generate various graphs such as q-n, q-φ, and q-φ-t. The entire measurement system can detect ultrasonic signals with partial discharge quantities less than 1000 pC. Its software features statistical evaluation of test results and automatic fault identification based on partial discharge data, and can automatically generate reports; the virtual oscilloscope offers linear, elliptical, 2D, and 3D graphical display modes.
This device is suitable for partial discharge testing of power equipment such as GIS, cables, power transformers with voltage levels of 110kV and above, power transformers with voltage levels of 35kV and above, bushings, surge arresters, and coupling capacitors.
In addition to the direct method based on pulse current, modern partial discharge testing instruments also widely employ various non-invasive or detection methods based on different physical principles. The transient ground voltage method primarily detects transient ground voltage signals in the 3-100MHz frequency band. When partial discharge occurs in high-voltage electrical equipment, the discharge charge first accumulates in the grounded metal part adjacent to the discharge point, forming electromagnetic waves that propagate in all directions, simultaneously generating a transient ground voltage that travels to the ground through the outer surface of the equipment's metal casing. The ultrasonic method uses an ultrasonic sensor to receive ultrasonic signals generated by partial discharge. Power equipment generates sound waves during discharge, with a wide frequency spectrum; the intensity of the discharge is inferred from changes in sound pressure. The ultra-high frequency (UHF) method uses a sensor to detect UHF electromagnetic wave signals in the 300MHz-3GHz frequency band. The pulse current generated by partial discharge has an extremely short rise time, exciting electromagnetic waves with frequencies up to several GHz, effectively avoiding low-frequency interference such as corona discharge. The high-frequency current method uses a Rogowski coil-type high-frequency current sensor connected to the equipment's grounding wire to detect the high-frequency pulse current signal generated by partial discharge; this is a non-invasive detection method. These methods each have their own characteristics and are often integrated into the same detection instrument to achieve comprehensive detection of partial discharge in various types of power equipment.
