Application of Feedwater Frequency Conversion Control System in 100MW Units

1. Introduction

The renovation project is for the 380t/h pulverized coal boiler feed water system of a 100MW unit in a thermal power plant. The system has three 1600kW/6kV feed water pumps, operating in two-in-one mode.

Through the demonstration of the frequency conversion transformation plan of the water supply system, combined with the actual situation on site; the power system plan for the water supply system transformation of the 100MW unit is finally formulated as a two-to-three automatic switching plan, and the system electrical schematic diagram is shown below. Among them, #3, #4, and #5 are the original high-voltage switches of the water supply pump, and QF01 and QF02 are the spare high-voltage switch intervals of the I and II sections of the 6kV busbar room. Two HARSVERT-A06/200 inverters are installed in the high-voltage inverter room on the steam turbine side, and two KYN28-12Z high-voltage switches are installed respectively, namely QF21~QF24.

In order to achieve the design goal after the frequency conversion transformation of the water supply system, a water supply frequency conversion control system cabinet was added in the electronic room of the centralized control room. It is used to realize the frequency conversion, operation, switching, interlocking and other logical processing functions of the water supply system under the power frequency mode, and realize the automatic adjustment function of water supply under different operation modes to meet the actual operation needs of the unit.

After the system was transformed by frequency conversion, the actual operation for nearly a year proved that the transformation was successful and achieved good production efficiency and economic benefits.

2. System Function Implementation

After adopting the two-to-three automatic switching scheme and independent water supply control system, the water supply system mainly realizes the following functions:
1. Realize the four operation modes of "one variable or one working", "two variable and one standby", "one working, one variable and one standby", and "two working and one standby".
2. The three water supply pumps 3#, 4#, and 5# can all be in variable frequency/working frequency operation state.
3. In any operation mode of each water supply pump, the system has complete electrical, logical interlocking and interlocking dual relationships. That is: there are effective interlocking relationships between the high-voltage switch QF3 (#3 supply) and QF24, between QF4 (#4 supply) and QF21, between QF5 (#5 supply) and QF22, QF23, between QF21 and QF22, and between QF23 and QF24. If a single running pump trips, the standby pump is put into the system interlock to start the standby pump.
4. When the feedwater pump is in the power frequency or variable frequency operation mode, the interlock of the lubricating oil pump and the protection circuits of the shaft temperature, vibration, etc. of the feedwater pump operate accurately and effectively.
5. When the feedwater pump is in power frequency/variable frequency operation and switching, the system accurately links the pump outlet electric door to operate and the opening event is automatically selected, and the operation is stable.
6. The system can realize remote operation and local operation functions.
7. It has remote alarm, fault indication and fault self-diagnosis processing capabilities.
8. The system can realize automatic adjustment of the drum water level and continuous and smooth switching control under various operating modes between 30 and 100MW load of the unit.

3. System Optimization

According to the principle of "minimum modification" in design concept, the system only added three water supply pump operation mode selection switches in the existing water supply system, and carried out electrical interlocking transformation on the secondary control circuit of the industrial/variable frequency high-voltage switch. Four high-voltage switches, one system control cabinet and two frequency converters were added and put into use after construction and system joint debugging; the system has a high degree of integration, compact and reasonable design structure, which greatly reduces the system transformation cycle and cost.

During the design and demonstration of the system, it was estimated that the load response capacity of the unit using the one-power-one-variable operation mode was only 74-100MW. This conclusion was confirmed during the actual commissioning process. When the unit load was reduced to 76MW, the displacement of the variable frequency feedwater pump was reduced to 70-80t/h; the displacement of the power frequency pump was 200 t/h. At this time, in order to prevent the variable frequency feedwater pump from entering the unsafe working area, the system opened the feedwater pump recirculation door to maintain the minimum flow value of the feedwater pump.

The one-work-one-change operation mode is an intermediate operation mode during the system operation process, and is only used as a backup mode in the case of reverse pump operation of the feedwater pump or inverter fault tripping. However, the smooth operation and actual controllability under the "one-work-one-change" operation mode greatly reduce the threat to the safety of the unit operation during the system operation mode transition process.

Practice has proved that under certain load conditions, two water supply pumps can achieve "one working and one variable" operation, which is of great significance to the water supply system with two variable frequencies.

4. Actual operation effect

1. Control quality

By comparing the actual operation of two industrial frequency feed water regulating valve controls and two variable frequency feed water pumps before and after the frequency conversion transformation, the following aspects are mainly reflected:
① By adopting the frequency conversion control method, the control quality of the boiler water level is changed from ±30mm fluctuation before the transformation to ±10mm fluctuation after the transformation, and the control quality is significantly improved.

② Before the frequency conversion transformation, when the unit increases or decreases the load, the feedwater adjustment gate cannot achieve timely response of the system, which often leads to the automatic release of the drum water level. Human intervention is required to maintain the system operation, and the automatic input rate is low. After the frequency conversion transformation, the system response speed has increased, which can fully meet the needs of increasing or decreasing the load of the unit. It ensures that the drum water level does not overshoot, improves the level of automatic operation, and ensures the safety of unit operation.
③ According to the changes in the unit load, especially during the unit startup process, the system operation structure is reasonably configured, which greatly reduces the workload of operation. The water level can be fully automatically controlled in different operation modes and during the switching process.

2. Mode switching

When a single device fails in the system or a regular pump reverse operation is required, a smooth transition can be achieved under automatic water level conditions, and the system automatically switches the control strategies under different internal modes smoothly and without disturbance. The operation mode switching test under the load of 80MW unit proves that the system has good response speed and control quality, which can fully meet the actual operation needs.

3. Actual power saving effect

According to the actual operation in the past year, the unit consumption of the feed water pump of the boiler under different load conditions has been statistically analyzed. The unit consumption of the feed water pump has dropped significantly before and after the transformation. Please refer to the following chart for details.

The actual electricity saving effect statistics in the first quarter of 2006 are shown in the following table:

If the average load is 84% ​​and the operation hours are 8,000 per year, 5.846 million kWh of electricity can be saved. Based on the on-grid electricity price of 0.25 yuan per kWh, 1.4615 million yuan can be saved annually, and the investment can be recovered in about 3 years.

V. Conclusion

The above actual operation data and practice fully demonstrate that the use of a complete water supply frequency conversion control system solution in the boiler water supply system can effectively solve a series of problems caused by the application of high-voltage frequency conversion technology in the water supply system, and can be effectively handled through a variety of comprehensive means. The actual trouble-free operation record for more than a year shows that the system has the characteristics of good operation quality, safety and stability, high reliability, and maintenance-free. It has a high value for promotion and application.