The Hazards Caused by Diesel Gensets Shaft Voltage and Preventive Measures

The size of shaft voltage varies with the conditions of each generator set. Generally speaking, the larger the generator set capacity, the greater the asymmetry of its air gap magnetic flux and structure. The higher the harmonic component in the magnetic field, the saturation degree of the iron core and the unevenness of the stator, the higher the peak value of the shaft voltage. The waveform of the shaft voltage has complex harmonic components. For the unit with static controllable rectifier excitation, the high pulse component in the shaft voltage waveform is particularly harmful to the oil film insulation. When the shaft voltage reaches a certain value, if appropriate measures are not taken, the oil film will be broken down and the shaft current will be generated.

If the shaft current of the steam turbine generator set is large, the shaft journal, bearing bush and other relevant parts through which the shaft current passes will be burned, the drive worm and worm gear of the main oil pump of the steam turbine will be damaged, the arc caused by the shaft current will burn the bearing parts and make the bearing lubricating oil aging, thus accelerating the mechanical wear of the bearing, and the shaft current will make the steam turbine parts, generator end cover, bearing and other parts around the shaft strongly magnetized, The unipolar potential is generated at the journal and impeller.

When the high shaft voltage is enough to break through the oil film between the shaft and the bearing, the discharge occurs. The discharge circuit is the generator shaft - journal - bearing bush - bearing bracket - generator set  base. Although the shaft voltage is not high, and the 300MW generator is about 6V, the circuit resistance is very small, so the shaft current generated may be large, sometimes hundreds of amperes. The shaft current will gradually degrade the quality of lubricating and cooling oil, and in serious cases, the bearing bush will be burnt out and forced to shut down and cause accidents. Therefore, during installation and operation, measure and check the voltage between the shaft and bearing of the generator set.

Prevention and elimination measures of generator shaft voltage.

The following precautions are usually taken:

(1) During design and installation, insulating pads are usually installed between the bearing bracket and the base at the excitation end of the generator, and insulation measures are taken for all oil pipes, screws, screws, etc.

(2) The grounding brush of the main shaft at the steam turbine side of the generator is designed to release the electrostatic charge at the low pressure section of the turbine and ensure that the electric potential between the shaft and the ground is the same.

In addition to eliminating the large shaft voltage, the large shaft grounding carbon brush also has the following functions to protect the motor: a Measure the positive and negative voltage of the rotor to the ground. B. As the protection of rotor one-point grounding.

(3)In order to reduce the shaft voltage caused by the asymmetry of the magnetic circuit of the steam turbine generator set, the measures to eliminate or reduce the third or fifth harmonic components in the shaft voltage are considered in the design of the generator. A new generator structure is adopted. During installation, the manufacturer's process and design requirements are strictly followed to prevent rotor eccentricity.

(4) In order to prevent shaft voltage caused by one-point grounding short circuit of rotor winding, the two-point grounding protection device of excitation circuit is put into operation.

(5) In order to cut off the shaft current, the excitation end includes the generator bearing, the oil seal of the hydrogen-cooled generator, the water inlet and outlet support and the water inlet and outlet pipe flange of the water-cooled generator rotor, and the insulation pad is installed between the tail bearing and the base plate of the base. The fasteners of the bearing block and the oil pipes connected to the bearing block shall also be insulated from the bearing. Double insulation measures can be adopted.

(6) When designing the motor, avoid magnetic circuit asymmetry.

(7) The axial magnetic flux shall be avoided during the design, manufacture and operation of the motor.

(8) Insulate the bearing pedestal from the ground.

(9) Install the grounding brush on the shaft.

(10) Use non-magnetic bearing seat or additional coil.

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

The high shaft voltage of the generator has a great impact on the normal operation of the generator, so it should be measured regularly. For some large generator sets, it is necessary to install online protection devices to monitor whether the generator is in normal operation. China's power industry is in the development period of large power grid and large diesel gensets. With the increase of the generator set capacity of generator sets and the wide use of static excitation system, it is very important to take effective protective measures to suppress the generation of shaft voltage and harmful shaft current.

The above is the harm and preventive measures of generator generates shaft voltage introduced by Dingbo Power. If you are interested in diesel generators and want to know more about diesel generators, please contact us via email: dingbo@dieselgeneratortech.com.