There are many problems in the operation of generators. There are often two situations in the operation of generators: "main shaft grounding" and "rotor grounding". What does this mean? Shaanxi Wanpeng came to popularize this important relevant knowledge for everyone. 1. Why does the generator shaft grounding carbon brush not cause a rotor grounding alarm?
First of all, we need to distinguish between the two concepts of "main shaft grounding" and "rotor grounding".
The so-called large-shaft grounding means that the "metal surface" of the entire rotor cannot accumulate static charges. Excessive accumulation of static charges will cause the potential to the ground to rise, which will break down the lubricating oil film of the rotor bearing and lose its lubricating effect., and then direct friction occurs between the rotor journal and the bearing bush, so we must connect the metal surface of the rotor to the earth and release the accumulated static charges in time. This is also one of the functions of the large-shaft grounding carbon brush, which is to eliminate the shaft voltage and prevent shaft current from being generated.
Rotor grounding refers to the grounding of the electrical winding bars inside the rotor. This is not allowed, because the internal winding circuit of the rotor is DC. Under the condition of good insulation, the voltage to ground between the positive and negative poles of the winding is evenly distributed. When a point grounding occurs, the voltage to ground of the other pole that is not grounded will increase to full voltage. At this time, it is easy to break down the insulation of the ungrounded pole, so a point grounding fault will quickly develop into a two-point grounding. Therefore, we need to set up one-point grounding protection. When a one-point grounding occurs, an alarm will be issued in time or tripped to stop the generator from operation.
2. Why does the generator need to install a grounding brush on the shaft?
Install a grounding brush on the shaft so that the generated shaft current can be grounded through this brush to reduce harm to the bearing. The effect is better than using insulating bearing blocks.
Since the air gap is always not so uniform during generator installation, and the impedance is not nearly the same during coil installation, shaft voltage will be induced on the generator rotor during generator operation. Due to the existence of shaft voltage, during operation, this voltage can cause shaft current to penetrate the oil film through the bearings at both ends of the rotor, burning the bearing bushes. In order to prevent this phenomenon, add insulation to the bearing bush seat at the excitation end of the generator (the entire bearing is insulated from the ground), and use a grounded carbon brush to ground the main shaft at the steam end. In addition, various protections for generator excitation and rotor measurement are insulated from the ground is actually the main shaft of the carbon brushes.
3. The main shaft grounding carbon brush has the function of measuring the voltage between the positive and negative poles of the rotor and the ground. How can the voltage be measured?
The main shaft is not grounded, but the bearing housing is insulated with thick insulating pads. When the main shaft rotates, it is also suspended and contacts the bearing housing through an oil film. During operation, the rotor coil of the generator is not allowed to be grounded at two points (otherwise a serious accident will occur), and one point of grounding must be dealt with in time.
The carbon brush of the main shaft is grounded through a "rotor grounding relay", which is connected to the anode and cathode of the rotor at the same time. Once any leakage occurs to the rotor's anode, cathode or any point between the positive and negative electrodes, the potential of the main shaft will change, and the "rotor grounding relay" will detect it and remind the operator to handle it. Under normal circumstances, since the excitation loop of the generator is not grounded, neither the positive nor negative pole has a voltage to the main shaft. Only when a certain point in the loop is grounded or the insulation drops to unqualified will the positive and negative poles generate corresponding voltages.
4. What is the working principle of the grounded carbon brush?
Motor installation errors cause uniform air gaps or difference in coil impedance, and the induced shaft voltage forms a loop through the carbon brush and housing, bypassing the bearings. Shorts the cross-voltage that breaks through the oil film at both ends of the bearing to avoid the formation of shaft currents that ablate the bearing or bush. Application examples of grounding carbon brushes In large generators, generally, the excitation end bush seat is insulated (the entire bearing is insulated from the ground), and the steam end shaft is grounded with a grounded carbon brush. Because of various protections for generator excitation and rotor measurement, the ground is actually the steam end shaft connected to the carbon brushes. When the rotor coil leaks electricity to the rotor core, there will be a ground voltage on the excitation positive or negative pole to the ground, indicating that the rotor has failed and should be shut down for maintenance. The grounding brush is generally connected to the rotor insulation monitoring loop and sends an alarm signal when current flows through it.
Shaanxi Wanpeng Power Technology Development Co., Ltd. was established in October 2013. The company is located in the ancient city of Xi'an, the national central city. At present, the main business projects include: sales, installation, leasing and after-sales service of generators, generator sets, electrical equipment, industrial and mining equipment power transmission and transformation supporting equipment; sales and after-sales service of mechanical parts; sales, installation and after-sales service of mechanical and electrical equipment; Design and construction of noise and pollution control projects in computer rooms.
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