As stand-alone capacity gradually increases, shaft voltage becomes a serious problem for large generators with static self-excitation system. Shaft voltage waveform with complex harmonic pulse components, the oil film insulation is particularly harmful. When the shaft voltage does not exceed the damage value of the oil film, the shaft current is very small. If the shaft voltage exceeds the bearing layer breakdown voltage, a large shaft current is formed on the bearing, the so-called EDM current, which ablates the bearing components and causes a great hazard. Magnetic circuit asymmetry, unipolar effects, capacitor current, electrostatic effects, static excitation system, shell, shaft and other permanent magnetization may cause shaft voltage.

 

Shaft voltage refers to the voltage generated between the two bearing ends of the motor or the shaft and the bearing of the motor when the motor is running. Under normal circumstances, the shaft voltage is low, the generator shaft and bearing oil film between the existing insulation can play a better role. However, if for some reason the shaft voltage is raised to a certain value, the membrane will break down and form a loop of shaft current. Shaft currents not only destroy the stability of the oil film but also gradually deteriorate the oil quality of the lubricating cooling. At the same time, as the shaft current passes through the metal contact point between the bearing and the shaft, the metal contact point is small and the current density is high, , So that bearing local burning. Burn the bearing alloy splashing under the action of grinding force, the inner surface of the bearing will burn small pits. Eventually, the bearing will be damaged due to mechanical wear and tear, in the worst case, the bearing will be burned, resulting in the accident being forced to shut down.

 

Generator shaft voltage is always there, but generally not high,

 

1, the generator shaft voltage causes

(1) Axial voltage due to magnetic asymmetry

It is AC voltage present at both ends of the turbine generator shaft. As the stator core fan-shaped stamping, rotor eccentricity, the permeability of the sector-shaped pieces, as well as cooling and clamping of the axial guide slots and other generators caused by the manufacture and operation of the magnetic asymmetry, resulting in a shaft, bearings and The basis of the board, including alternating flux loop. This produces a voltage difference across the generator shaft. Each kind of magnetic asymmetry will cause the corresponding magnitude and frequency axis voltage component, the voltage components of all axes are superposed together, make the frequency component of this axis voltage very complicated, among them the amplitude of the fundamental wave component is maximum, 3 times and 5 The subharmonic amplitude is slightly smaller, and the higher harmonic components have a small amplitude. This AC shaft voltage is generally 1 ~ 10 V, it has a greater energy. If you do not take effective measures, such shaft voltage through the shaft --- bearing --- base plate and other places to form a loop, resulting in a large shaft current. The arcing caused by the shaft current is applied between the bearing and the shaft surface. The main consequence is the wear of the tungsten and shaft surfaces on the bearing and the rapid deterioration of the lubricant. This will speed up the bearing mechanical wear and tear, the bearing will be burned in serious cases.

 

(2) Electrostatic charge induced shaft voltage

This DC voltage between the shaft and the grounded platen is generated by the electrostatic charge that is rubbed by the high-speed flow of wet steam and the turbine low-pressure cylinder blades under certain conditions. This electrostatic effect is only occasionally under certain steam conditions can occur, not often exist. With different operating conditions, the shaft voltage of this nature is sometimes very high, the potential reaches hundreds of volts, when people touch will feel hemp. It is not easily conducted to the exciter side, but if no action is taken to direct the electrostatic charge into the ground, it will accumulate on the generator turbine side bearing oil film and eventually discharge on the oil film resulting in bearing damage.

 

(3) Shaft voltage caused by static excitation system

Currently, large-scale turbogenerators commonly used static excitation system. Static excitation system due to SCR arc change, the introduction of a new shaft voltage source. The static excitation system supplies the AC voltage to the excitation winding of the generator through a static thyristor rectifier output DC voltage. The DC voltage is a pulsating voltage. For a static excitation system with a three-phase fully controlled bridge, the waveform of the excitation output voltage has six pulses in one cycle. This rapidly changing pulsating voltage generates an alternating voltage between the shaft and the ground through capacitive coupling between the field winding of the generator and the rotor body. The shaft voltage is pulsating and spikes at a frequency of 300 Hz (when the excitation system AC voltage frequency of 50Hz), it superimposed on the shaft voltage caused by magnetic asymmetry, so that the oil film to withstand higher spikes. When increased to a certain extent, the breakdown of oil film, the formation of current caused by mechanical parts burn and damage.

 

(4) Residual magnetic induced shaft voltage

When the generator is severely short-circuited or under other abnormal conditions, the large shaft, the bearing bush, the casing and other components are often magnetized and a certain residual magnetization is retained. Magnetic line in the bearing at the longitudinal branch, when the unit axis rotation, it will have a potential, known as the monopolar potential. Under normal circumstances, the monopolar potential generated by weak residual magnetism is only millivolt level. However, when the rotor winding inter-turn short circuit or two-point grounding, the unipolar potential will reach a few volts to ten dozen volts, will have a great axial current, along the axial warp beam, bearing and base plate circuit circulation, not only burnout Shaft, bushings and other components, but also make these parts seriously magnetized, to the maintenance of the unit to bring difficulties.

 

2, the generator shaft voltage hazards

The size of the shaft voltage varies with the condition of each unit. In general, the larger the unit capacity, the larger the asymmetry of the air gap flux and structure. The harmonic components and core saturation in the magnetic field and the stator is also the greater the degree of unevenness, the higher the peak voltage axis, the shaft voltage waveform with complex harmonic components, the use of stationary controllable rectifier excitation unit, the axis The voltage waveform has a high pulse component, which is particularly harmful to the oil film insulation. When the shaft voltage reaches a certain value, the oil film will be punctured to generate the shaft current if proper measures are not taken.

 

If the turbine shaft current is large, the shaft current through the journal, bushings and other related components will burn out, the main steam turbine turbine worm and worm gear will be damaged, the shaft current caused by the arc will ablate the bearing components and make Bearing wear-out accelerates the mechanical wear of the bearing. Shaft currents strongly magnetize turbine components, generator caps, bearings and other components around the shaft and create unipolar potentials at the journals and impellers.

 

The shaft voltage is too high enough to penetrate the oil film between the shaft and the bearing, the discharge occurs, the discharge circuit for the generator shaft ------- journal ------ bearing ---- Bearing ----- Unit base. Although the shaft voltage is not high, a 300MW generator is about 6V, but the loop resistance is small, so the resulting shaft current can be large, sometimes up to hundreds of amps. Shaft current will make lubrication of the oil gradually deteriorate, in serious cases will burn the bearing, was forced to stop causing accidents. Therefore, during installation and operation, check the voltage between the shaft of the genset and the bearing.

 

3, generator shaft voltage prevention and elimination measures

Usually use the following precautions

(1) During the design and installation, an insulation pad is usually installed between the bearing bracket and the base located at the excitation end of the generator, meanwhile all the tubing, screws, screws and the like are to be insulated.

 

(2) The design of a generator brushless motor grounding brush, used to release the low voltage section of the turbine electrostatic charge to ensure that the shaft and the same potential.

 

(3) In order to reduce the shaft voltage caused by the asymmetry of the magnetic circuit of the steam turbine generator, the design of the generator was taken into consideration to eliminate or reduce the third or fifth harmonic components in the shaft voltage. The new generator structure was adopted to install In strict accordance with the manufacturers of technology, design requirements, to prevent eccentric rotor.

 

(4) In order to prevent the rotor winding a little grounding short circuit and generate shaft voltage, run into the excitation loop two point grounding protection device.

 

(5) In order to cut off the shaft current, at the excitation end, including generator bearings, hydrogen-cooled generator oil seal, water inlet and outlet water cooled rotor bearings and water inlet and outlet pipe flanges, rear bearings and chassis base plate Install insulation pad. The bearing housings and the housings connected to the housings must also be insulated from the bearings using double insulation.

 

(6) In the motor design, to avoid the magnetic asymmetry.

 

(7) Avoid generating axial flux in the design, manufacture and operation of the motor.

 

(8) Insulate the bearing housing to the ground.

 

(9) Install a grounding brush on the shaft.

 

(10) non-magnetic bearing or additional coil.

 

(11) Add a bypass capacitor to ground at the armature output of the DC motor.

 

4, the shaft voltage measurement

The insulation of the rotor grounding brush and the bearing is very important for the protection of the shaft voltage against the safe operation of the generator. In actual operation, due to the installation, deterioration of the operating environment, wear, etc., will make the rotor ground or bearing insulation decline, leading to shaft voltage increases, the shaft current increases, the final may damage the generator. Therefore, the regular measurement of shaft voltage, to improve the operation of the generator is very necessary