Analysis of the local discharge in the insulator of the generator set

[Overview] The insulation system of the generator set is a measure to protect users. Risk factors such as leakage and insulator damage are partly affected by the customer's wrong operations or the external environment. Local discharges in the generator set insulator that are unknown to everyone will also cause. Jiangsu Haixing Power Systematically explains the mystery to everyone.

1. What is local discharge of generator sets? What are the main forms?
Under the action of an electric field, the electric field strength of the local area of ​​the insulator in the insulator system reaches the breakdown field strength, and discharge occurs in some areas. This phenomenon is called Partial Discharge. Partial discharge occurs only in the insulation part and does not penetrate the entire insulation.
The local discharge in the generator set mainly includes internal discharge of the main insulation of the winding, end corona discharge and tank discharge (including tank corona). In addition, there is also a hazardous discharge in the generator set, which is an arc discharge caused by the fracture of the strands or joint of the stator coil. The mechanism of this discharge is different from that of local discharge.

2. What is the reason for the partial discharge in the insulation of the generator set? What are the dangers?
During the production process of large generator set stator wire rods, due to process reasons, there may be air gaps or impurities between the insulating layer or the insulating layer and the strand; during operation, under the combined action of electricity, heat and mechanical forces, insulation deterioration will also directly or indirectly, causing new air gaps between the insulating layers. Due to the different dielectric coefficients of air gaps and solid insulation, the electric field distribution of this interlayer medium composed of air gaps (impacts) and insulation is uneven. Under the action of the electric field, when the working voltage reaches the starting discharge voltage of the air gap, a local discharge occurs. The local discharge start voltage is closely related to the dielectric constant of the insulating material and the thickness of the air gap.
The partial discharge of gas in the air gap is a flow-injected high-pressure glow discharge, and a large number of charged particles (electrons and ions) collide with the main insulation at high speed, thereby destroying the molecular structure of the insulation. In the air gap where the main insulation is partially discharged, the local temperature can reach 1000℃, which deteriorates the adhesive and strand insulation in the insulation, causing the strand to be loose and short-circuit, causing the main insulation to be partially overheated and thermally cracked, and damage the main insulation.
The further development of partial discharge is to cause dendritic discharge to occur inside the insulation, causing further deterioration of the main insulation, forming a discharge channel and destroying the insulation.

3. What are the main methods for online monitoring and electrical measurement methods for local discharge of generator sets?
The local discharge of generator sets is monitored online, and the current pulse current method (ERA) is the mainstream method. According to the response bandwidth of the detection device, the local discharge devices insulated by generators can be divided into narrowband detection devices and broadband detection devices. Currently, broadband devices are generally used in detection devices.
One of the key technologies for online local discharge monitoring of generator sets is how to obtain fault signals, that is, detection technology corresponding to the sensor. According to the type and arrangement of the online detection sensor of the generator, there are mainly the following monitoring methods:

(1) The theory principle of the generator neutral point coupling radio frequency monitoring method
is: When local discharge is generated in any part of the generator, electromagnetic waves with a very wide frequency will be generated, and the corresponding radio frequency generated anywhere in the generator (Radio) Frequency) current flows 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 the partial discharge. The partial discharge on the main insulation of the generator can be regarded as a point signal source. The electromagnetic waves generated by the electromagnetic disturbance caused by the local discharge in the space. Since the electromagnetic coupling between different slots of the generator is relatively weak, the transmission line theory can be used to analyze the propagation of the pulse in the winding, that is, the discharge pulse in the winding propagates along the winding at the speed. According to this theory, if a broadband current transformer is installed at the neutral point of the generator, the high-frequency discharge waveform of the partial discharge can be monitored to monitor the internal discharge and discharge changes 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 discharge phenomenon inside the stator coil. The advantage of this monitoring method is that the neutral line has low potential to ground, and the high-frequency CT sensor is relatively easy to make and install; the disadvantage is that it has strong signal attenuation and high-signal processing technology requirements. In addition, the electromagnetic coupling between the slots of generators of different sizes is large, and not all of them can be ignored. Therefore, there are great errors in the theoretical analysis of transmission lines, especially for large water-wheel generators with large numbers of slots.

(2) Portable capacitive coupling monitoring method
A local discharge online monitoring device developed in Canada in the 1970s. When monitoring the discharge signal, three capacitors (such as 375pF, 25kV each) are overlapped on the three-phase outlet of the generator, and the signal is introduced into the oscilloscope through a bandpass filter (such as 30kHz to 1MHz) and the time domain waveform of the discharge signal is displayed. This method is still being used in some power plants in Canada. Its disadvantage is that it depends on experienced operators to distinguish external interference signals from internal discharge signals.

(3) The coupling capacitor method
sensor on the outlet bus of the generator set adopts a fixed installation form. A capacitive coupler is installed on each phase of the outlet bus of the generator and a capacitive coupler or high-frequency current sensor is installed at the neutral point of the generator. The principle is that the capacitive coupler installed at the outlet of the bus is used to measure the local discharge pulse signal from the inside of the generator stator winding, and the capacitive coupler installed at the neutral point is used to monitor the spatial noise at the site. The corresponding test instrument is a 4-channel monitoring instrument. The corresponding tester for this method uses hardware and software, etc. The method eliminates the noise that mainly affects local discharge measurement on site. For example, the noise generated by the excitation brush is eliminated through system analysis software; the noise from the space is received by the antenna and is eliminated by comparison. Some do not use sensors at neutral point parts, but use software to eliminate noise. One of the disadvantages is that the coupling capacitor is located on the high voltage side of the generator set, and its own reliability affects the reliability of the unit. This is a method that is currently used more frequently. Both the water turbine generator set and the steam turbine generator set can be used, and it is more commonly used in Europe.

(4) The local discharge signal of this method is obtained by a high-voltage coupling capacitor
installed on each phase bus of the generator stator winding or a high-voltage coupling capacitor on the generator outlet bus. Each phase has a pair of coupling capacitors, and each pair of couplers has a certain spatial distance to eliminate interference from the outside of the motor.
Since each phase is equipped with a dual sensor with a certain spatial distance, the discharge pulse signal and external interference signal arrive at the different delays of the two sensors are used to eliminate random pulse interference signals, and the discharge signal in the winding and external noise signal have different characteristics when propagating in the winding to suppress noise and extract discharge signals. At the same time, other interferences are removed by using software processing methods such as digital filtering, amplitude identification, and dynamic thresholds. After the 6 signals coupled to the sensor enter the signal conditioning unit, the two corresponding signals are gated through multiple switches for amplification processing, and then enter the acquisition card, which is converted into a digital signal for monitoring and data processing.
This monitoring method is suitable for hydrowheel generator sets, because the hydrowheel generator set is relatively large, making it easier to install the coupler. This method eliminates interference by symmetry between two parallel branches on paired couplers. In fact, it is difficult to make the parameters of the two branches symmetric. Therefore, such asymmetry should be minimized or compensation should be used as lines to improve the ability to suppress interference. Another disadvantage is the same as above, that is, the reliability of the coupling capacitor affects the reliability of the unit. More companies in North America use this monitoring method.

(5) Generator set stator slot coupler method
This method is to directly install a coupling sensor SSC (Stator SlotCoupler) in the stator slot. This stator slot coupler is an 'antenna' used to detect locally released signals, which is installed under the groove wedge of the stator slot near the outlet end. Each SSC is about 50cm long and 1 or 7mm thick, and is as wide as the stator groove. The stator slot coupler has a very good frequency response in the frequency range from 10 to 1000 MHz, so it can detect a relatively real pulse waveform of the high-frequency signal along the stator slot.
The stator slot coupler is proposed to detect partial discharge pulses in large steam turbine generators. Its important feature is that it produces different impulse responses to partial discharge and electrical noise. Theoretical research and practical measurements show that the local discharge pulses generated by the stator winding can be detected by the SSC in about 1 to 5 ns wide pulses, while all various internal and external noises are detected in pulses greater than 20 ns wide pulses. This is because when the noise propagates through the winding, the stator winding plays a natural filtering role. This obvious difference in pulse width makes it easy to distinguish local discharge of the stator from other disturbance noises.
This method is suitable for use in large steam turbine generators. Its advantage is that the local discharge signal and noise signal have strong distinction ability and high sensitivity among these methods; however, this method requires that SCC be buried under the groove wedge of the generator winding, so the cost is very high in the production and burial of the coupler, and is limited in the application of water turbine generators with multiple branches and multiple grooves.

(6) Monitoring method of using resistive temperature measuring element wires buried in the stator slot as sensors.
This method is to use certain resistive temperature measuring element (RTD) wires buried in the stator slot as partial discharge sensors without installing other sensors. This method is theoretically similar to the SSC method, and the use of resistance temperature measuring element (RTD) wires pre-buried in certain grooves of the stator as a discharge sensor to measure local discharge pulses will not have any impact on the generator circuit and are low in additional costs. This local discharge sensor frequency characteristics are also wider (about 3 to 30 MHz), which facilitates the distinction between local discharge pulse and noise pulse. This method is still in the exploration and experiment stage, and it should be said that this is a monitoring method with great development prospects.

my country has not yet issued relevant standards for online detection of generator units. In its IEEE Trial—Use Guide t0 the Measurement of Partial Discharges in Rotating Machinery, IEEE mainly recommends the use of capacitive coupling method and stator slot coupling method for online monitoring of local discharge of generators.