The low-pressure heater drainage system of the 125MW unit in the power plant adopts the combined operation mode of stepwise self-flow and low-pressure drainage pump. When the drainage pump is running at a fixed speed, there are common problems such as high pump failure rate, large pipeline vibration, and low commissioning rate. Using frequency conversion technology to change the low feed water level control mode from valve adjustment to variable frequency speed control of the drain pump, practice shows that the drain pump is stable, safe and reliable, and has significant energy-saving effects. The heat recovery system of the steam turbine of the power plant consists of a seven-stage steam extraction, a condenser, two high-pressure heaters (hereinafter referred to as high heater), four low-pressure heaters (hereinafter referred to as low heater) and a deaerator. The high pressure part of the drainage system is drained from high pressure to low pressure step by step to the deaerator, and the low pressure is drained from No. 4 to No. 3 and then to No. 2 low pressure. Then it is pumped into the #3 low pressure condensate inlet main pipe through the drainage pump. No. 1 low water heater drains to the condenser. The drain pump is 150NW-78×2, with a head of 160m, a power of 55kW, and a speed of 2970r/min. The outlet flow of the drain pump is adjusted by an electric regulating valve.

1. The problems existing before the frequency conversion transformation of the low-pressure drain pump
(1) Due to the constant speed of the low-heat drain pump, the throttle valve of the regulating valve is large during operation, and the outlet pressure is high. A large amount of water leakage at the pump shaft seal packing often causes the loss of working fluid. At the same time, it is easy to cause water in the bearing, which not only affects the normal pump The operation even damages the bearings and pumps, and the energy loss is large.
(2) The linearity of the pump outlet regulating valve is not high, which affects the stable operation of the automatic regulation.
(3) Due to poor regulation performance, low and low drainage water level or empty pump operation without water level often occur, causing drainage pump cavitation, drainage pipeline vibration, large current change range, pump impeller and guide vane, pump shaft, bearing , Packing and other components are damaged, which affects the service life of the pump, and the maintenance workload is large and the cost is high; when the water level is too high in serious cases, the low water level and high water level protection action will affect the safe operation of the unit.
In recent years, with the improvement and maturity of frequency conversion technology, our company adopted frequency conversion speed regulation technology to carry out frequency conversion transformation of the 2B low-pressure drain pump during the overhaul of unit #2 in May 2006. After more than a year of operation, it has been shown that the low-pressure addition drainage system after the transformation is stable, safe and reliable, and energy-saving is significant.
 The control principle of the variable frequency speed regulation of the low-heat drain pump is to output a signal to the frequency converter through its water level automatic control unit (located in the DCS system), and the frequency converter controls the frequency converter according to this signal to change the speed of the motor, thereby changing the pump's speed. When the water level is low, the output frequency of the inverter is increased to achieve the purpose of controlling the low water level.
2. Composition of frequency conversion speed control system
The system is mainly composed of an automatic control unit (located in the DCS system for self-improvement), a frequency converter and a drain pump. The frequency converter adopts Danfoss VLT5000 series, which is mainly divided into two parts: the main loop and the control loop: the main loop is composed of a rectifier, a filter link and an inverter, which is used to provide a voltage and frequency regulation power supply for the motor, and the rectifier converts the power frequency power into a direct current The power supply, the filtering link absorbs the voltage fluctuation between the rectifier and the inverter, and the inverter converts the DC power supply to the AC power supply. The control circuit provides signals to the main circuit, including the frequency and voltage calculation circuit, the voltage/power detection circuit of the main circuit, the speed detection circuit of the motor, the drive circuit that amplifies the control signal of the calculation circuit, and the inverter and the motor. Composition of protection circuit.
3. Modification method
(1) The original manual start-stop, interlocking and interlocking are retained to control the working power switches of the two sets of devices.
(2) The inverter control cabinet is installed in the low-voltage switch room on the 5m floor of the main factory building. There are three control modes: full-automatic, manual and constant-speed operation. Set up monitoring and remote operation devices in the centralized control room, and keep the circuit breaker and control switch of the original drain pump. The wiring method is changed from the original drain pump power plate cable to the frequency conversion control cabinet, and then the power cable from the frequency conversion control cabinet to the motor.
(3) The normal operation mode is the automatic frequency conversion operation of the 2B drain pump, and the 2A pump as the power frequency standby mode. When the 2B drain pump system fails and trips, the 2A drain pump starts to run automatically to ensure the energy-saving effect of the low-heat drain system.
(4) After the inverter fails, the DCS interlock will automatically start another drain pump for power frequency operation, and the water level control will be switched to control by the pump outlet regulating valve.
Effect after transformation
(1) After the low-heat drain pump adopts the frequency conversion control mode, the automatic control performance is excellent, the low-feed water level is basically stable, and the general fluctuation range is within 50mm; the drain pump runs stably, with low noise, low current, and low current fluctuation value; The system runs stably and reliably, and there is no excessively low water level or high water level operation.
(2) The drainage pump adopts frequency conversion control mode, which avoids the phenomenon of low and no water level operation, cavitation of the drainage pump blades, emptying of the pump, and shaft channeling. It reduces the heating and wear of the pump bearing and prolongs the life of the drainage pump. One year after the renovation, there has been no occurrence of pump outlet pipeline vibration and damage to pump impellers, guide vanes, bearings and other components, and there is basically no maintenance.
(3) After the frequency conversion control of the drain pump, the energy loss caused by the throttling of the pump outlet regulating valve is eliminated, and the no-load loss of the pump and the motor is effectively reduced.