Analysis of Partial Discharge in the Insulator of Generator Sets

【Overview】Generator setThe insulation system is a measure to protect users. Risk factors such as leakage and damage to the insulation are partly caused by the customer's incorrect operation or the influence of the external environment, which is not known to everyone.Generator setPartial discharges within the insulator can also cause it. Jiangsu Haixing Power systematically explains the secrets to everyone.

1. What is partial discharge of generator set? What are the main forms?
Under the action of the electric field, the electric field intensity in local areas of the insulator in the insulation system reaches the breakdown field strength, and discharge occurs in some areas. This phenomenon is called partial discharge (Partial Discharge). Partial discharge only occurs locally in the insulation and does not penetrate the entire insulation.
There are three main types of partial discharge in the generator set: internal discharge of the winding main insulation, end corona discharge and slot discharge (including slot corona). In addition, there is a kind of hazardous discharge in the generator set, which is arc discharge caused by the breakage of stator coil strands or joints. The mechanism of this discharge is different from partial discharge.

2. What is the cause of partial discharge in the insulation of the generator set? What's the harm?
During the production process of large generator set stator bar, due to process reasons, there may be air gaps or impurities between the insulation layer or between the insulation layer and the strands; during operation, under the combined action of electricity, heat and mechanical force , will also directly or indirectly lead to insulation degradation, causing new air gaps between insulation layers. Due to the different dielectric coefficients of the air gap and solid insulation, the electric field distribution of this interlayer medium composed of air gap (impurities) and insulation is uneven. Under the action of the electric field, when the operating voltage reaches the initial discharge of the air gap When the voltage is applied, partial discharge occurs. The starting voltage of partial discharge is closely related to the dielectric constant of the insulating material and the thickness of the air gap.
The partial discharge of the gas in the air gap is a streamer-like high-pressure glow discharge. A large number of charged particles (electrons and ions) collide with the main insulation at high speed, thus destroying the molecular structure of the insulation. In the air gap where partial discharge occurs in the main insulation, the local temperature can reach 1000°C, which causes the adhesive in the insulation and the insulation of the strands to deteriorate, causing the strands to loosen and short-circuit the strands, causing local overheating and thermal cracking of the main insulation, damaging the main insulation. .
The further development of partial discharge will cause dendritic discharge inside the insulation, causing further deterioration of the main insulation, forming a discharge channel and causing insulation damage.

3. What are the main electrical measurement methods for online monitoring of partial discharge in generating units?
On-line monitoring of partial discharge of generator sets is currently based on the electrical pulse current method (ERA) as the mainstream method. According to the response bandwidth of the detection device, the partial discharge device of the generator insulation can be divided into a narrow-band detection device and a broadband detection device. The current detection device Equipment generally uses broadband devices.
One of the key technologies for online partial discharge monitoring of generator sets is how to obtain fault signals, that is, detection technology based on sensors. According to the type and arrangement of generator partial discharge online detection sensors, there are mainly the following monitoring methods:

(1) Generator neutral point coupling radio frequency monitoring method
The theoretical principle is: when partial discharge occurs anywhere in the generator, electromagnetic waves with a very wide frequency will be generated, and the corresponding radio frequency (Radio) waves generated anywhere in the generator will Frequency) current will flow through the neutral point grounding wire, so the partial discharge sensor can be selected on the neutral point grounding wire to extract the electromagnetic signal of partial discharge. The partial discharge on the main insulation of the generator can be regarded as a point signal source, electromagnetic waves generated in space by electromagnetic disturbances caused by partial discharges, because the generator does not The electromagnetic coupling between the same slots is relatively weak, so the transmission line theory can be used to analyze the propagation of pulses in the windings, that is, the discharge pulses in the windings propagate along the windings at a speed of . According to this theory, a broadband current is installed at the neutral point of the generator With the transformer, the high-frequency partial discharge waveform can be monitored to monitor the internal discharge amount and changes in the discharge amount of the generator.
The radio frequency monitoring method uses a wide-band high-frequency current sensor to pick up high-frequency discharge signals from the neutral line of the generator stator winding to reflect the internal discharge phenomenon of the stator coil. The advantage of this monitoring method is that the neutral line has a low potential to ground and a high Frequency CT sensors are relatively easy to make and install; The disadvantage is that due to the severe signal attenuation, it requires high signal processing technology. In addition, the electromagnetic coupling between generators of different sizes is greatly different, and not all can be ignored, so the theoretical analysis of the transmission line has a large error. , especially for large hydrogenerators with many slots.

(2) Portable capacitive coupling monitoring method
An online partial discharge monitoring device developed in Canada in the 1970s. When monitoring the discharge signal, three capacitors (such as each 375pF, 25kV) are connected to the three-phase outlet line of the generator, and the signal passes through a band-pass filter (such as 30k Hz to 1MHz) is introduced into the oscilloscope and displays the time domain waveform of the discharge signal. This method is still used in some power plants in Canada. Its disadvantage is that it relies on experienced operators to distinguish external interference signals and internal discharge signals. .

(3) Coupling capacitor method on the outlet bus of the generator set
The sensor adopts a fixed installation form. A capacitive coupler is installed on each phase of the generator outlet bus and a capacitive coupler or high-frequency current sensor is installed on the neutral point of the generator. The principle is that the capacitive coupler installed at the bus outlet is used for The partial discharge pulse signal from inside the stator winding of the generator is measured. The capacitive coupler installed at the neutral point is used to monitor the spatial noise on site. The corresponding test instrument is a 4-channel monitoring instrument. The tester corresponding to this method uses Hardware and software methods have a major impact on the site. The noise from the external discharge measurement is eliminated. For example, the noise generated by the excitation brush is eliminated through the system analysis software; the noise from the space is received through the antenna and eliminated using the comparison method. Some do not use the neutral point position. sensor, and the software method is used to eliminate noise. One of the disadvantages is that the coupling capacitor is located on the high-voltage side of the generator unit, and its own reliability affects the reliability of the unit. This is a method that is currently widely used in hydro-generator units and It can be used in steam turbine generator sets and is widely used in Europe.

(4) Paired coupling capacitor method on the outlet bus of the generator set
The partial discharge signal of this method is obtained through high-voltage coupling capacitors installed on the bus rings of each phase of the generator stator winding or on the generator outlet bus. Each phase has a pair of coupling capacitors, and the installation position of each pair of couplers has a certain spatial distance to eliminate interference from outside the motor.
Since each phase is equipped with dual sensors with a certain spatial distance, the difference in the time delay between the discharge pulse signal and the external interference signal reaching the two sensors is used to eliminate the random pulse interference signal, and the discharge signal and external noise signal in the winding are used to It has different characteristics during propagation to suppress noise and extract discharge signals. At the same time, digital filtering, amplitude identification, dynamic threshold and other software processing methods are used to filter out other interference. After the 6-channel signals coupled to the sensor enter the signal conditioning unit, one of the two signals corresponding to one of them is selected through the multi-channel switch for amplification processing, and then enters the acquisition card, which is then converted into a digital signal by the acquisition card for monitoring and data processing.
This monitoring method is suitable for hydro-generator units, which are relatively large in size and easy to install the coupler. This method eliminates interference based on the symmetry of the two parallel branches on the paired coupler. In fact, it is difficult to make the parameters of the two branches symmetrical. Therefore, this asymmetry should be reduced as much as possible or the line should be used for compensation to improve the efficiency. The ability to suppress interference. Another shortcoming is the same as above, that is, the reliability of the coupling capacitor affects the reliability of the unit. Companies in North America have adopted this monitoring method more often.

(5) Generator set stator slot coupler method
This method is to install the coupling sensor SSC (Stator SlotCoupler) directly in the stator slot. This stator slot coupler is an 'antenna' used to detect partial discharge signals. It is installed in the slot of the stator slot near the exit end. wedge below. Each SSC is about 50cm long, 1.7mm thick, and the same width as the stator slot. The stator slot coupler has a very good frequency response in the frequency range from 10 to 1000MHz, so it can detect relatively realistic pulse waveforms of high-frequency signals along the stator slot.
The stator slot coupler is proposed to detect partial discharge pulses in large turbine generators. Its important feature is that it can produce different impulse responses to partial discharge and electrical noise. Theoretical research and actual measurements show that the partial discharge pulse generated by the stator winding can be detected by the SSC in the form of pulses with a width of 1 to 5 ns, while all various internal and external noises are detected in the form of pulses with a width greater than 20 ns. This is This is because when noise propagates through the windings, the stator windings act as natural filters. This obvious difference in pulse width makes it easy to distinguish stator partial discharge from other interfering noise.
This method is suitable for large turbine generators. Its advantage is that it has strong ability to distinguish partial discharge signals from noise signals, and its sensitivity is also high among these methods; however, this method requires SCC to be buried under the slot wedge of the generator winding. , so the cost of manufacturing and embedding the coupler is very high, and it is limited in the application of hydro-generators with multiple branches and multiple slots.

(6) Monitoring method using resistive temperature measuring element wires embedded in the stator slot as a sensor
This method uses some resistive temperature measuring device (RTD) wires embedded in the stator slot as partial discharge sensors without the need to install other sensors. This method is theoretically similar to the SSC method, and uses resistive temperature measuring device (RTD) wires pre-embedded in certain slots of the stator as discharge sensors to measure partial discharge pulses, which will not cause any harm to the generator circuit. No impact, low additional cost. This kind of partial discharge sensor also has a wide frequency characteristic (about 3~30MHz), which makes it easy to distinguish partial discharge pulses from noise pulses. This method is still in the exploratory and experimental stage, and it should be said that this is a promising monitoring method.

Our country has not yet promulgated relevant standards for online detection of partial discharges in generator sets. IEEE mainly recommended the use of trial standards for partial discharge monitoring of motors (IEEE Trial—Use Guide t0 the Measurement of Partial Discharges in RotatingMachinery) promulgated in 2000. Capacitive coupling method and stator slot coupling method are used for online monitoring of generator partial discharge.