The Second Part:The Reason of Generator Generates Shaft Voltage

Do you know why the generator generates shaft voltage? Next, let Dingbo Power continue to introduce the causes of generator Generates shaft voltage in the previous article.

2. Shaft voltage caused by electrostatic charge.

The DC voltage between the shaft and the grounding plate is generated by the electrostatic charge generated by the friction between the high-speed wet steam and the blades of the low pressure cylinder of the steam turbine under certain conditions. This electrostatic effect can only occur occasionally under certain steam conditions, not often. With the different operating conditions, the shaft voltage of this nature will sometimes be very high, and the potential will reach hundreds of volts. When people touch it, they will feel numb. It is not easy to conduct to the exciter side, but if no measures are taken to guide the electrostatic charge to the ground, it will gather on the bearing oil film at the turbine side of the diesel generator and eventually discharge on the oil film, resulting in bearing damage.

3. Shaft voltage caused by static excitation system.

At present, static excitation system is widely used for large steam turbine generator set. A new shaft voltage source is introduced into the static excitation system due to the influence of thyristor arc exchange. The static excitation system supplies the AC voltage to the generator excitation winding through the static silicon controlled rectifier output DC voltage, which is a pulsating voltage. For the static excitation system with three-phase fully controlled bridge, the excitation output voltage waveform has 6 pulses in one cycle. This rapidly changing pulse voltage generates AC voltage between the shaft and the ground through the capacitive coupling between the excitation winding of the generator and the rotor body. This kind of shaft voltage is in the form of pulsating peak, and its frequency is 300Hz (when the AC side voltage frequency of excitation system is 50Hz). It is superimposed on the shaft voltage caused by magnetic asymmetry, so that the oil film can bear higher peak voltage. When it is increased to a certain extent, it will break through the oil film and form electric current, which will cause burns and damage to mechanical parts.

4. Shaft voltage caused by remanence.

When the generator is seriously short-circuited or under other abnormal conditions, it will often magnetize the large shaft, bearing bush, casing and other components and retain certain residual magnetism. The magnetic line of force generates a longitudinal branch at the bearing bush. When the main shaft of the unit rotates, it will generate electric potential, called monopole electric potential. Under normal conditions, the monopole potential generated by weak remanence is only millivolts. However, when the rotor winding is short-circuited between turns or grounded at two points, the monopole potential will reach several volts to more than ten volts, which will generate a large shaft current, which will flow along the axis through the shaft, bearing and foundation plate circuit, not only burning the large shaft, bearing bush and other components, but also making these components seriously magnetized, which will bring difficulties to the unit maintenance.

Dingbo Power hope the above information could help you. Dingbo Power is a diesel generator manufacturer integrating the design, supply, commissioning and maintenance of diesel generator sets. We can customize 30KW-3000KW diesel generator sets of various specifications according to customer requirements. If you would like to know more information, please contact us via email dingbo@dieselgeneratortech.com.