Application of Partial Discharge Detector in Large Motor

Since the 1950s, a large amount of research work has been carried out abroad on this technology, and this technology has made great progress. According to reports, tens of thousands of foreign motors have adopted on-line monitoring technology and have achieved very good results. Its technology has gradually transitioned from narrowband systems to broadband systems and to ultra-wideband systems.

In foreign countries, a variety of technologies have been successfully applied to the local detection of motors. These methods mainly include the following:

(1) Neutral point coupling monitoring method

Since the arc discharge in any part of the motor will generate a corresponding RF current in the neutral point grounding line, the partial discharge monitoring point can be selected at the neutral point grounding line. In the 50's, Westinghouse developed an on-line monitoring system for generator partial discharge. The working principle is that the discharge signal is drawn from the neutral point of the stator winding and sent to the oscilloscope through a band-pass filter to display the time domain waveform of the signal on the oscilloscope screen. In practical applications, noise signals and discharge signals must be identified by experienced operators, and it is difficult to promote them.

(2) Portable capacitive coupling monitoring method

In the 1970s Ontaio Hydro of Canada developed an on-line monitoring device for partial discharges. To monitor the discharge signal, three capacitors (375pF each, 25kV each) were lapped on the generator 3-phase outgoing line, introduced into the oscilloscope through (30kHz to 1MHz), and the time-domain waveform of the discharge signal was displayed. This method was applied at some Canadian power plants and achieved good results. Its drawback is still relying on experienced operators to distinguish between external interference signals and internal discharge signals, resulting in the promotion of this monitoring method has been limited.

(3) Radio frequency monitoring

The radio frequency monitoring (Radio Frequency Monitoring) method is actually a method improvement proposed by Johnson. This method uses a high-frequency current sensor, Rokowski coil RC capacitive-capacitive high-pass filter to pick up high-frequency discharge signals from the generator stator winding neutral line to discover the internal discharge phenomenon of the stator coil. At present, it has been used in many generators and large-scale high-speed AC motors, and has achieved good results. In 1980, Westinghouse developed a commercial RF monitor with an amplifier center frequency of 1 MHz, a bandwidth of 5 kHz, and an alarm circuit.

(4) PDA monitoring method

PDA is the abbreviation of Partial Discharge Analyzer. The PAD monitoring method was introduced by Ootaio Hydro in Canada in the 1970s. It is mainly used for on-line monitoring of partial discharges inside a hydroelectric generator. It uses the characteristics of the discharge signal in the winding and the external noise signal to suppress the noise when it propagates in the winding and extracts the discharge signal. The PAD test method has been adopted in the on-line monitoring of foreign hydroelectric generators.

(5) Slot coupler (SSC) monitoring method

Because the structure of the stator winding of the turbine generator is different from that of a hydroelectric generator, the PDA test cannot be satisfactorily applied to on-line monitoring of the turbo generator. In 1991, Ontaio Hydro and Iris Power Enginrrring used TGA (Turbine Generator Analyzer) for on-line monitoring of partial discharge signals of turbine generators. This method is relatively costly and requires a special device—Stator Slot Coupler (SSC)—to be buried under the slot wedges of each slot of the stator. The SSC is used to detect the discharge pulse of each slot. For generators that have been put into operation, stator windings must be modified before implementation, which is often unacceptable for power plants.

(6) Resistance temperature measuring element monitoring method

The use of resistive RTD conductors embedded in certain slots in the stator as discharge sensors to measure partial discharge pulses has no effect on the primary circuit of the motor. This PD sensor has a wide frequency characteristic (3 to 30 MHz), which makes it easy to distinguish between PD pulses and noise. However, since the dual-sensor ratio method is used to distinguish between signal and noise, it is impossible to distinguish the partial discharge signal and noise in some parts of the motor.

1.2 Domestic research

Domestic studies on line monitoring have started late and development has been slow. In particular, due to the complexity of the partial discharge process, the current theory is still imperfect and the research on this area has brought great difficulties. In 1990, Shanghai ranked "Radio Frequency Monitor" as a scientific and technological research project, and was awarded the second prize by Shanghai Second Polytechnic University and Shanghai University of Science and Technology at that time. It was completed in May 1992 and passed the appraisal in October. The radio frequency monitor of Gongda University has been applied to many domestic motors, and the radio frequency monitor of HKUST has good application practice in Shanghai Power Plant. The two systems fully met Westinghouse’s product standards in the early 1980s. Its amplifier center frequency is 1MHZ, wideband 5kHz, device selectivity is good, antijamming ability is strong, but because the passband is too narrow, the information obtained is less, the sensitivity is low and does not have the partial discharge comprehensive characteristic analysis ability.

The DJYC-1 type electric motor bureau developed by Tsinghua University used a single-chip microcomputer to control the sampling system and used a combination of low-speed sampling and high-speed sampling. The low-speed sampling results were used to collect the amplitude and corresponding information of the discharge signal. The high-speed sampling results were used to analyze The subtle characteristics of the discharge waveform have achieved certain results.

The electric motor bureau developed by Xi'an Jiaotong University puts a wideband system on the line monitoring system, which can measure various characteristic quantities of partial discharge. Combined with modern digital processing technology, it has made some attempts in the automatic recognition of discharge types.

The on-line monitoring system developed by Shanghai Jiaotong University contains a narrowband system and a broadband processing system. The wideband system uses high-speed and long-time sampling. DSP and computer are used to process large amounts of data. The characteristic values ​​of the extracted signals are stored in the database. , Through the long-term monitoring of the motor to determine the insulation damage of the motor. Narrowband systems are used for comparative studies.

Technically speaking, domestically developed equipment can be divided into narrowband systems and broadband systems. For narrow-band systems, it is only sensitive to severe spark discharges at the late stage of partial discharge, and it cannot distinguish between on-board and off-board discharge signals as well as fault classification and positioning. For a broadband system, the wider the bandwidth, the richer the PD information collected, but the higher the system requirements. On the one hand requires a higher sampling rate and digital processing capabilities, on the other hand the identification of interference is also the key.

1.3 Signal propagation characteristics and impact of the test system

(1) The partial discharge signal is a high frequency signal. Since the stator coil has a large inductive reactance, it is a complicated LCR network.

The arrival of the sensor from the point of release to the sensor will cause great distortion and oscillation, which will seriously affect the test results. For general samples, BG7354-87 specifies partial discharge measurement and calibration methods, but the current partial discharge calibration method is not suitable for the calibration of the discharge capacity of large generator stators. A working group of the IEEE Institute of Electrical Engineering is drafting a guideline (P1434) on this issue, reminding people to pay attention to the different interpretations of the results of the motor stator partial discharge test.

(2) Different test systems are involved except that the attenuation and oscillation of the PD signal during propagation have a serious impact on the test results.

The test result of the PD signal also has a great influence. According to the report [3], in a partial discharge inspection of a 4kV motor, four commercial broadband partial discharge test systems perform apparent charge calibration according to the recommended method of ASTM-D1868, but they test the same partial discharge signal. The results were expressed as PC (pico) from 60 PC to 1000 PC, and the test results for the same motor differed by 17 times. The impact of the test system on the test results is mainly due to the wideband, input impedance, input sensor, and sensor installation location of the different test systems.

Based on the above reasons, when analyzing the results of the line test, the test system and test methods should also be considered together. The test results of different test systems and the test results of different motors are not comparable. In this sense, the interpretation of the results of the PD test results tends to be more valuable over time.