Partial discharge detection is changing from an optional question to a mandatory question. When planned maintenance shifts towards predictive maintenance, the first multiple-choice question facing power operation and maintenance personnel is often not "whether to test", but "what method to use for testing". The three mainstream detection methods of ultra-high frequency (UHF), high-frequency current transformer (HFCT), and transient earth voltage (TEV) each have their own emphasis. Choosing the wrong method not only wastes resources, but also may miss the real insulation hazard.
The fundamental difference between the three methods can be traced back to an engineering problem: what path does the partial discharge signal take when it propagates from the defect to the sensor? TEV relies on the transient potential changes of discharge pulses on the metal casing and grounding connection, UHF relies on the propagation of electromagnetic waves in the internal cavity of the device, and HFCT relies on the flow of high-frequency current along the grounding conductor or shielding layer. Different propagation paths result in vastly different equipment, anti-interference capabilities, and positioning methods.
Let's take a look at TEV first. When partial discharge occurs inside the switchgear, rapidly changing current pulses will cause short-term potential changes at the joints, flanges, and grounding connections of the metal casing. TEV sensors obtain this signal through capacitive coupling on the surface of the cabinet. Its biggest advantage is that it is non-invasive and does not require power outages. Testing personnel can scan by cabinet number and measuring point in sequence, making it suitable for rapid inspection of medium voltage switchgear with a large number and limited power outage windows. However, the TEV signal strength is closely related to the defect distance, cabinet structure, and grounding method. The readings of the same partial discharge source on different cabinets may vary, and cross device comparisons cannot be made solely based on amplitude.
HFCT is taking a different path. It is clamped on the cable grounding wire or shielding circuit, and identifies anomalies by inducing high-frequency pulse currents in the conductor. Partial discharge pulses propagate along the grounding wire, and HFCT can achieve high sensitivity online monitoring in scenarios such as cable joint defects and insulation aging. Studies have shown that HFCT has good response performance in the range of 1MHz to 25MHz, with a maximum transmission impedance value of 16.4mV/mA, and can measure an apparent discharge of 5pC. Its frequency range usually covers 100kHz to 20MHz or even higher, which is particularly suitable for equipment with clear grounding circuits such as cables and motors.
The detection frequency band of UHF is the highest, generally between 300MHz and 3GHz. The nanosecond level steep pulse current generated by partial discharge will radiate ultra-high frequency electromagnetic waves, and UHF sensors couple these signals in an antenna manner. Due to its distance from power frequency noise and most radio interference, this frequency band has an extremely high signal-to-noise ratio and is the most mainstream solution for partial discharge detection in GIS equipment. The propagation attenuation of UHF signals in GIS metal cavities is small, and the shell can effectively shield external interference, making UHF method have significant advantages in sensitivity and anti-interference ability.
The differences between the three methods determine that they cannot be replaced by each other, and the core logic of selection is actually very clear: it depends on the device type. UHF is preferred for GIS, TEV is preferred for switchgear and ring main unit, and HFCT is preferred for cables and motors. However, in practical engineering, a single sensor is difficult to meet the detection needs of complex on-site environments, and multi-sensor fusion is becoming an industry consensus.
Wuhan UHV Power Technology Co., Ltd., rooted in the Wuhan Optics Valley high-voltage testing equipment industry circle, provides a practical path worth referencing in this regard. Its partial discharge detection system supports synchronous operation of three sensing modes: TEV, UHF, and HFCT, with a pulse resolution of ≤ 2ns. It has a built-in database of 23 typical discharge modes and can automatically generate PRPD phase distribution maps. Handheld devices integrate ultrasound TEV、 The four detection functions of ultra-high frequency and high-frequency current can adapt to different scenarios from factory testing, handover acceptance to live inspection.
Actual cases can better illustrate the problem. In the pre operation live detection of the converter transformer in a ± 800kV ultra-high voltage converter station, technicians used a multi-sensor synchronous detection scheme to effectively strip fixed interference signals in the strong electromagnetic environment of the valve hall, successfully capturing weak discharge of a certain phase winding with specific phase characteristics, and assisting in the localization of abnormal points through a UHF sensor array. In another case, a converter station in East China deployed 16 UHF monitoring points and used time difference positioning method to continuously monitor for 72 hours. Eventually, a 0.3mm air gap defect inside the bowl insulator was located, avoiding unplanned power outages and losses of about 1.2 million yuan.
It is worth noting that the standardization construction of partial discharge testing in the industry is also accelerating. The detection system of Wuhan UHV has set up multiple digital filtering mechanisms in the signal processing stage, such as frequency domain windowing, pulse waveform recognition, wavelet transform denoising, etc., to distinguish periodic interference, random white noise, and real partial discharge pulses from both the time domain and frequency domain dimensions. Its desktop partial discharge detection system has a sampling rate of 10M/s, a minimum measurable discharge capacity of 0.1pC, and a measurement range from 0.1pC to 100000pC. It can maintain sensitivity at weak signal ends without saturation distortion when the discharge capacity increases.
Returning to the topic of selection, instead of dwelling on which method is "best", it is better to answer three questions first: What is the tested device? How is the electromagnetic environment on site? Do you need rapid screening or precise positioning? By clarifying these three points, the positions of UHF, HFCT, and TEV will naturally become clear. In the current trend of multi-sensor fusion, choosing a partial discharge detection system that can synchronously access multiple sensing methods and has intelligent diagnostic capabilities may be more practical than repeatedly weighing on a single method.
