Application of high voltage frequency converter in circulating water pump of power plant

1. Introduction

The alumina plant and electrolytic aluminum plant of Guangxi Xinfa Aluminum and Power Co., Ltd. are large electricity consumers and require a large amount of electricity. If they only use electricity from the national grid, the cost will be high and production will be restricted when electricity is tight. In addition, the secondary steam of the power plant is required in large quantities by the alumina plant, so aluminum power bases generally have their own self-contained power plants.

The company's power plant has three 155MW generator sets (steam turbines) and four 520t/h boilers (three in use and one in reserve). Each steam turbine has two circulating water pumps (A and B circulating pumps), and the entire power plant has six circulating water pumps. Under normal circumstances, only one of the A and B circulating pumps is used, and the other is used as a reserve. The circulating water pump is switched every three months. The circulating water pump motor has two taps, high speed 495rpm, power 1600kW; low speed 425rpm, power 1250kW/1120kW. The speed regulation method is to use the motor at high speed in summer when the water supply is large; and use the motor at low speed in winter when the water supply is small.

In the circulating water system, the circulating water pump is used to realize the recycling of water resources. The hot water at the outlet of the condenser enters the cooling water tower. The heat of the circulating water is transferred to the atmosphere to reduce the temperature. Then, it is pressurized by the circulating water pump and enters the condenser to cool the low-pressure cylinder. Since the water level in the system is basically stable, the head of the circulating water pump is also basically stable. In other words, the amount of circulating water determines the power consumption of the circulating water pump.

As the load and external environment of the unit are constantly changing, the vacuum is also constantly changing, so it is necessary to adjust the circulating water volume in time to ensure the safe and economical operation of the unit. In normal operation in winter, a circulating water pump running at a low speed is sufficient to meet the cooling needs of the unit, but in seasons with large temperature differences, large load changes, and spring and autumn, although the circulating water pump has a high and low speed dual-speed adjustment mode, it cannot ensure that the unit operates in an economical operation mode, resulting in high power consumption and high power generation costs. Therefore, it is necessary to perform frequency conversion transformation on the circulating water pump.

By using a high-voltage frequency converter to adjust the speed of the circulating water pump motor according to actual needs, and then adjust the cooling water volume of the water pump, it can not only reduce the power consumption of the motor, but also achieve a more favorable vacuum control purpose, thereby ensuring and improving the operating conditions of the generator set and achieving the purpose and effect of energy saving and consumption reduction.

In order to reduce the power consumption rate of the plant and reduce the cost of power generation, the company's power plant leaders decided to use three sets of 1600kW/6 kV high-voltage inverters produced by Shandong New Wind Photovoltaic Technology Development Co., Ltd. to transform the circulating water pumps of the 1#, 2# and 3# generator sets.

2. On-site parameters of transformation equipment

The four circulating water pumps of units 1# and 2# are of the same model, and the circulating water pump A of unit 1# is taken as an example.

The circulating water pumps of Unit 3# A and Unit 3# B are of the same model, so the 3# A circulating water pump is taken as an example.

3 Fengguang JD-BP37 series high voltage frequency conversion system technical parameters

The Fengguang brand JD-BP37 series high-voltage inverter produced by Shandong Xinfeng Photoelectric Co., Ltd. uses high-speed DSP as the control core, adopts speed-free vector control technology and power unit series multi-level technology, and is a high-high voltage source inverter. Its harmonic index is less than the national harmonic standard of IEE519-1992, with high input power factor and good output waveform quality. It does not need to use input harmonic filters, power factor compensation devices and output filters; there is no additional heating and torque pulsation, noise, output dv/dt, common mode voltage and other problems caused by harmonics, and ordinary asynchronous motors can be used.

The technical parameters of JD-BP37-1600F high voltage inverter are shown in Table 3.

4 Principle of frequency conversion control of circulating water pump

From the operating principle of the steam turbine, we know that the pressure of the condenser in operation mainly depends on the steam load, the cooling water inlet temperature and the cooling water volume. The cooling water temperature mainly depends on natural conditions. Therefore, under the condition of a certain steam load, the vacuum of the condenser can only be increased by increasing the cooling water volume. However, the vacuum of the condenser is not the higher the better. It is most economical only when the difference between the additional power generation of the steam turbine due to the increase in vacuum and the additional power consumption for increasing the circulating water volume is the largest. When the variable frequency circulating water pump is running, the DCS system of the unit determines the optimal vacuum of the unit to adjust the operating speed of the circulating water pump, that is, controls the circulating water volume to keep the vacuum of the unit in the best state, ensuring that the unit operates in an economical state.

5. Frequency conversion control scheme

5.1 Solution Introduction

There are two control modes for variable frequency speed regulation system operation: remote control and local control. These two control modes can improve the safety performance of the system. The operation status of the high-voltage inverter is monitored through the existing DCS in the power plant, and the inverter is controlled through remote control and local control.

In order to ensure the safe operation of the generator set, in the variable frequency operation mode, when the inverter or water pump fails and trips the high-voltage circuit breaker, the spare circulating water pump needs to be automatically put into operation.

The variable frequency speed regulation system is connected to the existing DCS system of the generator set. The DCS automatically controls the speed of the boiler circulating water pump according to the set program based on the load of the unit. The switch outputs that the inverter needs to provide to the DCS include fault alarm, ready indication, operation indication, high-voltage closing permission, high-voltage interlocking signal, water pump bypass switch closing signal, and frequency conversion KM1 closing signal; the switch quantities that the DCS needs to provide to the inverter include: frequency conversion start (dry node, valid when closed), frequency conversion stop (dry node, valid when closed), and frequency conversion emergency stop (dry node, valid when closed); the analog quantities that the DCS needs to provide to the inverter include: 2 4~20mA current source outputs, one signal is the circulating water pump frequency setting, which is the speed setting value of the inverter, and the other signal is the circulating water pump main pipe pressure setting; the analog quantities that the inverter needs to provide to the DCS include: 2 4~20mA current source outputs, and the physical quantities corresponding to the analog output are output frequency and output current; the analog quantities provided to the inverter on site include: 1 4~20mA current source output, indicating the outlet pressure of the frequency conversion pump.

The inverter has manual/automatic control modes. When the manual control mode is selected, the inverter speed regulation is not through the PID controller, but through the local and remote control speed control buttons, thereby changing the flow of the water pump and achieving the purpose of manually adjusting the condenser vacuum. When the automatic control mode is selected, the circulating water pump speed is regulated by the operator through the analog operator on the CRT of the DCS system to set the condenser vacuum set value, and the measured value of the vacuum transmitter installed on the condenser is used as the feedback value of the process control variable and compared with the set value. When the measured value of the vacuum transmitter is less than the set value, the output of the PID controller increases the inverter speed, and the water pump flow increases and the cooling is accelerated. Conversely, the output of the PID controller reduces the inverter speed, and the water pump flow decreases and the cooling is decelerated until the measured value is equal to the set value, and the motor speed stabilizes at a certain value. Automatic control of the circulating water pump speed is achieved, thereby achieving the purpose of water pump regulation.

5.2 On-site signal connection

Since the 1#, 2#, and 3# circulating water pumps are too far away from the high-voltage circuit breaker and the road surface they pass through is all cement, it is impossible to directly lay the interlocking control signal line between the high-voltage inverter and the on-site high-voltage circuit breaker.

There are three interlocking signals between the high-voltage circuit breaker and the high-voltage frequency converter. The first is the high-voltage closing position signal (i.e. the circuit breaker is normally open auxiliary), the second is the high-voltage closing permission signal (referring to the industrial and frequency conversion high-voltage closing permission signal, i.e. the industrial and frequency conversion circuits do not have the closing conditions, and the high-voltage circuit breaker is not allowed to be closed), and the third is the high-voltage switch signal (when the frequency converter has a serious fault and cannot operate normally, or when the equipment needs to stop in an emergency, the circuit breaker is separated to protect the frequency converter and the equipment). In order to meet the requirements of the interlocking control of the two, we decided to transfer through the DCS control system to achieve the same control purpose.