Systemsfor600MWTurbo-generator Tianjin Datang Panshan Power Generation Co., Ltd. (Tianjin 301900) Cai Jidong Wei Lifeng is essential for stable operation. Therefore, taking the domestic 600MW large-scale steam turbine generator plant power system as an example, combined with the actual operation, The installation method of the large-scale steam turbine generator set outlet circuit breaker and the principle of switching the power consumption of the plant are introduced for the relevant personnel.
Tianjin Datang Panshan Power Generation Co., Ltd. is equipped with two 600MW steam turbine generator sets, which are connected to the 5(1)kV Beijing-Tianjin-Tang Power Grid by generator-transformer unit wiring. The machine is equipped with an outlet circuit breaker. The power consumption systems of the two units are independent of each other. Each machine has a high-voltage factory working transformer with a high-voltage side capacity of 63MVA and a low-voltage side single-winding capacity of 35MVA. Each unit plant system has two 6kV plants. With the busbar, there are 1 high-voltage factory standby transformer for 2 units (see).
1 The influence of the generator outlet circuit breaker on the power consumption mode of the plant Since the generator outlet of our factory is equipped with a circuit breaker, the power consumption mode of the plant is more flexible than that of the conventional power plant, and it also has new features. The working power is introduced from the generator end through the factory working transformer, and the standby power is introduced by the 220kV system through standby. In normal operation, the factory bus is supplied by the working power supply. When the unit is shut down, it only needs to disconnect the generator outlet 03 circuit breaker. The power consumption of the factory is reversed and supplied by the main transformer from the 500kV system. The factory bus is still powered by the working power supply. When the working power supply side fails, the quick-cutting device automatically switches to the standby power supply, and the power consumption of the plant is changed from standby to standby. The advantage of this scheme is that the unit does not need to switch the power consumption of the plant normally, and only needs to operate the generator breaker. The plant has high reliability.
When the unit fails within the generator breaker (such as generator, turbine, boiler failure), it only needs to jump to develop the motor breaker, the power supply of the plant will not disappear, and there is no need to switch, which improves the reliability of the plant power. At the same time, the workload of the operator is reduced.
(4) The internal fault of the generator circuit breaker only needs to jump to develop the motor circuit breaker, and does not need to jump the high-voltage side 500kV circuit breaker of the main transformer, which has less influence on the power grid structure of the system and is beneficial to the power grid.
Taking into account the above advantages, after the correct operation of the technical personnel of all parties, it proves that its performance is good; the wiring mode has passed the test of actual operation) 2 about the 6kV power consumption switching 2.1 switching principle when the plant is powered off, due to With inertia and stored magnetic field energy, the motor will continue to rotate in a short time and convert the magnetic field energy into electrical energy. Due to the inconsistent capacity and parameters of the motors, there will be exchange of electromagnetic energy and kinetic energy between the motors. At this time, some of the asynchronous motors have actually been transferred to the asynchronous generator operating conditions, so the voltage of the factory bus is multiple asynchronous power generation. The synthesis of the feedback voltage emitted by the machine is called the bus residual voltage. Since there is no motive force and excitation, the amplitude and frequency of the residual voltage will gradually decay with time, and the phase difference between the residual voltage and the standby power supply voltage will gradually increase. In order to more clearly show the residual pressure phase, the residual pressure phasor diagram is drawn in polar coordinates (see). Before the standby power is put into operation, the standby voltage is generally the system voltage, and the voltage amplitude and frequency are rated values.
The trajectory of the residual voltage phasor of the 6kV busbar in the form of polar coordinates (slower residual voltage decay). With the rapid development of large-scale units, the capacity of the high-voltage motor is increased. The capacity of the 600MW unit to start the boiler feed pump is 6 300kW or more. After the large-capacity motor is powered off, the voltage attenuation is slow, and the amplitude of the residual voltage is also large, which brings many problems to the automatic switching of the power supply for the factory. If the power is turned back on when the residual voltage is large, the motor may be damaged by the impact, which has a great influence on the thermal parameters of the furnace operation, which may cause the furnace to operate unstable. The process of the motor switching process is now analyzed. The equivalent circuit for motor switching is shown.
The equivalent circuit and the phasor diagram standby power supply voltage when the motor is re-powered; AU is the beat voltage between the standby power supply voltage and the residual voltage of the bus; s is the equivalent of the power supply; the motor group and the low voltage load on the bus are converted to The equivalent reactance after the voltage of the high voltage plant; the angle between the power supply voltage and the residual voltage of the motor. After the standby power supply is closed, the voltage Um of the motor is: Let Um be equal to the allowable voltage when the motor starts, that is, 1 times the rated voltage of the motor Ude: 67 is calculated as Au (%) 64 (unit length is 1) as the radius The A'A" arc is on the right side of the safety zone of the power plant backup power supply. In the AB section of the residual pressure curve, the power switching achieved is called fast switching, ie before point B (0.3s) in the figure. Switching is safe; switching after delay to point C is called delay switching, ie switching after C point (0.47s) in the figure is safe for the motor; wait until the residual voltage is attenuated to 20% ~ 40 The switching realized at %, that is, the low voltage verification switching or the residual voltage switching is also safe for the motor. Both the delay switching and the residual voltage switching are slow switching.
1.1 Fast switching Assume that the working power supply is in phase with the standby power supply during normal operation, and the voltage phasor end point is A. After the bus is de-energized, the residual voltage phasor end point will move along the residual pressure curve from A to B. If it can be in A-B When the backup power supply is closed in the segment, the safety of the motor can be ensured without causing the motor speed to drop too much. This is called fast switching. In the middle, the fast switching time should be less than 0.2s. In practical application, point B is usually defined by the phase angle. For example, considering the inherent time of closing, the setting angle when the closing command is issued should be less than 60 and the closing time, such as the average frequency. When the difference is 1 Hz, the closing time is 100 ms, and the advance amount is about 36. The setting value should be set to 24°.
2.1.2 Delay switching After the B point, the BC segment is not safe and the switching is not allowed. The switching implemented after the C point to the CD segment is commonly referred to as delayed switching. 2.1.3 The method of capturing the fixed delay is not reliable. The best way is to define the closing area by frequency difference and angular difference and try to close the angle when the angle difference is zero. This is called synchronous capture. Switch. For example, the synchronous capture switching time is about 0.6 s. For the case where the residual voltage decays faster, the time is much shorter. If the synchronous capture switching can be realized, especially the in-phase point closing, it is also very advantageous for the self-starting of the motor. Therefore, the bus voltage of the factory is attenuated to 65% ~ 70%, the motor speed does not drop greatly, and when the standby is closed The impact is minimal. It should be noted that the "synchronous" of the synchronous capture switching is different from the "synchronous" of the simultaneous grid connection of the generator. During the synchronous capture switching, the motor is equivalent to an asynchronous generator, and the stator winding magnetic field has been converted from an synchronous magnetic field to an asynchronous magnetic field, and the rotor has no external motive force and an external excitation current. Therefore, if the standby power supply is closed, if the phase angle difference is small Even if there are some frequency differences and pressure differences, the stator magnetic field will quickly return to synchronization, and the motor will quickly resume normal asynchronous operation.
2.1.4 The switching realized after 40% residual voltage switching is called residual voltage switching. Although the residual voltage switching can ensure the safety of the motor, due to the long power outage time, the self-starting success of the motor and the self-starting time will be greatly increased. limits.
2.1.5 Factors affecting the residual voltage characteristic curve of the busbar Due to the large difference in the characteristics of the motors on the factory bus, the difference between the synthesized busbar residual voltage characteristic curve and the classified motor phase angle and residual voltage curve is also large. The definition of the safety zone is strictly determined by calculation based on various motor parameters, characteristics, load and other factors. In actual operation, the residual voltage characteristics of the busbar can be determined according to the test of a typical unit. Tests have shown that the time and speed of the bus voltage and frequency decay and the time to reach the initial inversion are determined by the load on the bus before the test. According to the residual voltage characteristic, the maximum phase angle difference that allows the standby power supply to be closed can be determined. Considering the closing time of the circuit breaker, the maximum phase angle difference and frequency difference before the closing can be determined.
It is assumed that the pre-accident working power supply is in phase with the standby power supply, and it is assumed that the two power sources are still in phase from the accident to the moment when the working circuit breaker is tripped. If the simultaneous mode switching is adopted, the switching off time (power-off time) is set to be small. (For example, 10ms), the standby power supply has a small time angle difference, and the inrush current and the self-starting current are very high. It is assumed to be 100ms for the 600MW unit, and the phase angle difference is 20~30°. The inrush current when the standby power is turned on is not too large, and generally does not cause equipment damage or fast cutting failure. When the data sheet is used, it must be adjusted according to the residual pressure curve after field test, usually a few seconds, and the self-starting current is limited to 4-6 times of the rated current. It can be seen that the capture switching in the same period has obvious advantages compared with the residual voltage switching and the long delay switching. The circuit breaker used in our factory is the VD4 vacuum circuit breaker manufactured by ABB Company. The closing time of the closing is very short, which provides the necessary conditions for fast switching.
2.2 Plant power switching mode The power switching mode of our factory is divided into three situations: normal switching, accident switching, and abnormal switching. The details are as follows.
1 Normal switching Normal switching is initiated manually and can be performed on the DCS system or device panel. The normal switching is bidirectional, and can be switched from the working power source to the standby power source or from the standby power source to the working power source.
The normal switching of the plant power is switched in parallel, that is, the uninterruptible power switching mode is adopted, and the power-off switching mode is not adopted. The operating personnel have concerns about power-off switching: First, they are afraid that the circuit breaker will lose power when the unit fails to meet the power; second, if the power-off time is long, it will affect the stable operation of the furnace. However, parallel switching is not perfect, and there is also a downside. In the parallel switching process, the short circuit capacity of the power system of the plant is large. For example, in the process of parallel switching, it happens that the power system of the plant is faulty. The consequences of the circuit breaker rejection caused by fast switching are more serious (the probability of failure of the plant power system during the parallel switching process is very low). After the trade-offs, the parallel switching method was chosen.
~7 is the single-phase voltage and current recording of the normal switching of the plant power (cutting power supply from the standby power supply).
The system wiring and operation mode have a great influence on the normal parallel switching. The system wiring mode and operation mode determine the initial phase angle between the factory bus voltage and the standby power supply voltage during normal operation. If the initial phase angle is large (for example, greater than 20) Normal parallel switching may fail due to too large circulation or cause equipment damage.
2 The accident switch is small. If you turn through the series switch. When the power is off, Liang Shikang, Xu Guangyi. Factory power system protection. Beijing: Water Conservancy and Electric Power
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