In China's power system, domestic generators occupy a large proportion, and how to make these units operate safely and efficiently is an important task for power workers. Years of work experience has realized that there are some common problems in the operation of domestic generator sets, and these problems involve many aspects such as design, construction and operation.
1 generator grounding brush problem
Due to the generator stator magnetic field can not be absolutely uniform and other reasons, the rotor will produce a few volts or more potential difference. Due to the generator rotor and bearings, the earth formed by the loop impedance is very small, it may form a large shaft current. In order to prevent this current from being formed, the manufacturer places an insulating sheet under all the bearings on the exciter side of the generator to cut off the shaft current path. At the same time, in order to ensure the same axis with the potential, it should be installed in the generator turbine shaft carbon brush. Chinese manufacturers have provided a grounding brush device (some foreign manufacturers to provide two), and the design department in the design drawings according to the requirements of the rotor ground protection, the secondary protection wiring connected by carbon brush shaft, so many construction units Use the supplied carbon brush supplied by the manufacturer as a "spindles" for rotor earthing protection that is not grounded.
It is detrimental to have the generator shaft not grounded at the turbine side. Due to the potential difference between the generator shaft and the ground, it may cause the shaft to discharge through the oil gap of the bearing pad, resulting in electrical burn of the bearing bush. In some accident situations it is more damaging and if the insulation of the generator exciter side bearing to the ground (including the tubing connecting the bearing to the ground) is damaged, it may cause a great axial current, especially if the stator coil In the event of a turn-to-turn short circuit, the shaft current will be larger, not only damaging the bearing pad, but it may also cause magnetization of the relevant components of the turbine due to the large axial current flowing through the turbine shaft. For this reason, the generator set must be grounded (or grounded via a small resistor) during the installation phase, and should be taken into consideration during operation to maintain the insulation of the bearings at the exciter terminals of the generator, with special attention to prevent the generator exciter The side of the temperature, vibration thermodynamic cable metal jacket to the bearing insulation short circuit.
2 high-voltage backup power plant automatic switching problems
High-voltage power plant with automatic standby power backup (standby power automatically), China has two ways to design: one for fast switching without time limit, two low-voltage lockout (ie, slow switching).
(1) The voltage analysis of the high-voltage plant motor when it is put into standby power. After the high-voltage power plant has been tripped for any reason, all the motors on the bus will decelerate and generate a residual voltage on the bus due to the back EMF of the motor. The residual voltage will decay over time, ie the magnitude of the residual voltage is gradually decreasing and the phase difference from the system will change over time.
1) when the standby power switch to the bus, the motor will suddenly increase a voltage K â–³ U, such as â–³ U over the General Assembly on the motor caused a great impact on the current, if the switching time is shorter, â–³ U smaller, Switching the more favorable. Section 22-1 of the Electrical Design Handbook for Electric Engineering states (2) that when K = 0.67 it is safe to switch before 0.3 s.
2) The variation of theta angle with time is mainly due to the capacity and characteristics of the large motor carried on the busbar. Therefore, for different capacity of the generator angle Î¸ changes with time is not the same, that is, with the same capacity of the generator set is also different due to the selection of the main auxiliary.
3) When the angle Î¸ is about 120 Â° ~ 240 Â°, switching is the most dangerous. The motor will withstand the maximum impact voltage, which may cause motor damage, damage or overcurrent protection trips.
4) If the switching time is too long, on the one hand, the auxiliary engine speed of the unit is reduced too much, which will cause the deterioration of the operating conditions of the boiler. In addition, the bus bar residual pressure is too low, the motor switch after the start-up difficulties, may cause the unit operating conditions of large disturbances or even outage, it is not desirable.
(2) quickly switch the existing problems. According to the unit selected by the auxiliary motor situation, and some units of high-voltage power plant switching design for a time-free switch. As the selected circuit breaker is a domestic less oil switch, the closing time of about 0.2 s, plus the relay action time, switching time will be up to about 0.3 s, so the switch is not safe. Has been put into operation of the power plant is the switching method, the high-pressure plant should be on the bus as soon as possible to switch the power switch to close the vacuum circuit breaker or SF6 circuit breaker fast (closing time: 0.08 ~ 0.2 s) this way Switching is more secure. Some import units will be limited to fast switching within 150 ms, if the switch fails to automatically switch to slow switching, this design is more reasonable.
(3) slow switching problems. Domestic units of high-voltage power plant automatic switching with a lot of slow switching, the design uses low-voltage detection plus time switching mode, that is, switching conditions 2: â‘ bus voltage below a certain value to allow switching; â‘¡ switching limited to A set time to complete. The question now is how to set the low voltage test relay and the time lock relay. It is necessary to ensure a successful switch, but also to ensure that high-voltage factory motor in the switch is not over-voltage impact, and the motor to withstand the voltage to achieve the switch as soon as possible.
Low-voltage test relay setting must consider the worst case, that is, the system voltage and plant bus bar residual voltage just reverse switching. If the standby power switch instantaneous system voltage in the high-pressure plant transformer voltage drop of about 30 V (specific pressure drop value can be obtained by calculation or test), in order to ensure that switch to the bus when the high-voltage plant motor voltage does not exceed 1.1 Ue , The low voltage check relay setting can be set to 45 V, but generally should not exceed 45 V, given that the factory bus voltage is still decreasing for the brief time the low voltage check relay contacts are closed to standby power.
Time blocking relay setting, to ensure that the switching time can be reliably completed, but also can not be set for too long, which may result in low voltage switching, so that the factory can not start the motor. The correct setting value is best determined experimentally; for example, the curve of the residual voltage of the busbars with time when the high-voltage power supply tripping the recording unit in normal operation mode is changed, so as to obtain the lockout at the setting of a low voltage checking relay Time relay setting.
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