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Working Principle and Performance Characteristics of High Voltage Generator Set

Nov. 30, 2018

The generator set is a power generation device that is composed of a synchronous generator, a drive machine, a control, and an accessory device. The uninteruptible power supply system of modern data and communication enters widely uses medium and high power diesel generator sets as backup power sources for utility power. Their rated output line voltage is 400V, phase voltage is 230V, which belongs to the secondary distribution network voltage category, that is the low voltage section in Chinas transmission network system. These diesel generator sets can also be referred to as low voltage power generation groups.

Working Principle and Performance Characteristics of High Voltage Generator Set.jpg


Basic working principle


The basic working principle of a high voltage generator set is the same as that of low voltage generator set. Generally, the diesel engine drives the synchronous generator to work together under the cooperation of accessory device and the controller to generate electricity. The output power of low-voltage generator sets is more than ten to several thousand kVA, and the output power of high-voltage generator sets is more than one thousand kVA. There are also a small number of high-voltage generator sets that use gas turbines to drive synchronous generators. However, regardless of the type of engine used as the drive machine, their spindle speed should be set at 1500 rpm. This is to ensure that the three-phase synchronous generators are capable of emitting 50 Hz AC per second in accordance with national regulations.


The winding coil on the synchronous generator rotor generates a DC rotor magnetic field after passing a direct current, and its strength is represented by a magnetic flux Φ. When the rotor is rotated by the engine, the magnetic field of the rotor cuts the winding embedded in the stator of the synchronous generator during the rotation (the wire is called a turn around one turn. There is an N turns coil in each winding), and the two ends of the winding are included. Electromotive force E. The stronger the rotor flux Φ, the more the number of turns of the stator winding coil N, the higher the voltage generated. They synchronous generator has three (three-phase) stator windings, and the effective value of the induced electromotive force E of each winding is: E=4.44ΦfNK.


Where f is the frequency of the synchronous generator output voltage 50Hz, K is the ratio of the number of the full-circle coil to the short-distance coil in the stator winding, called the pitch factor, usually K ≤ 1. It can be seen that in order to make the synchronous generator emit a higher voltage, the magnetic flux Φ of the rotor magnetic field can be increased in a two-pronged manner and the number of turns N of the stator winding can be increased. High-voltage synchronous generators are manufactured according to this technical concept. With the gradual improvement of motor manufacturing technology and the continuous emergence of new material, the technical performance of modern high-voltage synchronous generators has become more and more perfect.


Performance and characteristics


For more than half a century, high-voltage generator sets have gradually spread from the military industry to other sectors of the national economy, mainly due to the following performance characteristics of its high voltage synchronous generators (hereinafter referred to as high voltage generators or generators).


1. High output voltage

According to the relevant national standards, the line voltage of the rated output of the high voltage generator set is actually the line voltage of the high voltage generator output of 10KV. The rated output line voltage of the low-voltage generator set. Actually the line voltage of the low voltage generator output is 400V. the output voltage of the high voltage generator is 25 times higher than that of the low voltage generator.


2. Small output current

The power supply capability of the generator is characterized by its rated capacity SH=1.73UHIH. It defines the rated apparent power SH available to the generator with a rated power factor of 0.8 when the rated line current of the nominal line voltage UH is fed to the load is IH.


The rated apparent power that the low voltage generator can output is SHO, and the rated apparent power output that the other high-voltage generator can output is also SHO. Then there is (high-voltage power generation) 1.73UHGIHG = 1.73UHDIHD (low voltage generator). At this time, the ratio of the line current IHG output by the high voltage generator to the output line current IHD of the low voltage generator is



It can be seen that the same rated apparent power is output, and the line current supplied by the high-voltage generator is only one-fifth of that of the low-voltage generator. The current is small, the loss of electric energy on the transmission line is small, and the cross-sectional area of the transmission and distribution cable can be much smaller, which is beneficial to long-distance transmission and can save copper.


3. Strong rotor field

The high-voltage generator can obtain a rotor magnetic field and a magnetic flux Φ that are several to several times stronger than the low voltage generator by increasing the number of turns of the rotor winding coil and then direct current of the input coil (referred to as the excitation current). the excitation current comes from the generator automatic voltage regulation system consisting of a rotary rectifier, a brushless excitation generator and an automatic voltage regulator. The system links the rotor magnetic field flux Φ and the high voltage generator output voltage U with a negative feedback logic. If the decreases due to load fluctuations, the control system increases Φ and causes U to rise. It not only provides a large excitation current to the rotor winding to generate strong magnetic field and magnetic flux Φ, which creates conditions for the generator to output high voltage, and also automatically stabilizes the output voltage of the high voltage generator.


4. The number of turns in the stator winding is large

The more the number of turns of the coil, the stronger the induced electromotive force generated when the magnetic field cuts its motion. This is the basic principle of the generator. The high voltage generator has small current, and the wires used in the stator winding coil are much thinner than the low voltage generators. This facilities the increase of the number of turns in the startor windings. According to the design requirements, the parameters of the stator winding coil of the high voltage generator with different rated output power, the wire diameter, the size and shape of each turn are not the same. However, in general, the number of turns of the coils in the winding of the high voltage generator stator is generally several times to ten times more than that of the low voltage generator.


The number, shape, length or diameter of the grooves in the stator embedded in these windings are also manufactured according to the performance requirements of high voltage generators. Usually, the stator is also provded with a ventilation and heat dissipation groove and a temperature sensor to ensure that the temperature rise of the stator during operation meets the design requirements.


5. Neutral grounding is not normal

10KV high voltage generators are usually load-feed line voltages for delta-connected loads. Generally, the neutral line is not grounded. However, in order to protect the generator or to meet some special needs, the neutral line needs to be grounded through the resistor cabinet and the contactor cabinet. This is because leakage or short circuit in any of the windings theselves or their transmission lines causes the currents of the three windings to be unbalanced to generate a neutral current. The resistor cabinet not only prevents the surge of current, but also provides real-time varying voltage data for the detection control device. The measurement and control device studies the neutral voltage and automatically regulates the high voltage generator to operate outside the damage curve, although there is a neutral current but does not exceed the tolerance. Otherwise the command will be stopped to protect the high voltage generators.


6. High maintenance requirements

The high voltage generator belongs to the high voltage electrical equipment above 10KV. Its generator room, power distribution room, control cabinet, feeder cabinet, measuring cabinet, transmission and distribution cable, etc. Should meet the relevant national standards, in order to lay a good job for safety production. Because the use and maintenance of high voltage equipment is strict, personnel in management or control should be trained in high voltage electrical professional and have high voltage electrical operation qualifications, such as high voltage electrical. On the work, especially when handling and testing high voltage equipment on site, it is necessary to wear protective tooling, strictly in accordance with the process and specifications.


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