1. Overview

GXLD alumina, electrolytic aluminum plants, etc., are energy-consuming, large power consumption, only rely on the national grid power supply, one expensive, two secondary steam power plants and aluminum oxide plants limited output, so the aluminum power base usually has its own power plants.

The existing steam generator set of the power plant is 3155 MW (steam), 3 boilers and 1 standby unit 4520 t. Each steam engine has two circulating pumps (a, b) and the power station has a total of six circulating pumps. Usually, only one of the pumps is turned on, and the other is standby.The replacement cycle of the circulating pump is three months. Circulating pump motor with two taps, speed of 495 RPM, can output 1600 kW; Low speed 425 RPM, output 1250 kW /1120 kW. Speed adjustment: summer water volume, motor speed is fast, winter water volume is small, motor speed is low.

In order to reduce power consumptionand reduce power generation costs, our factory has studied and used 3 1600 kW/6 kvFD5000 high voltage frequency inverters of FGI Company for 6 circulating pumps of 1#, 2# and 3# units.

2. Implementation plans

For the operation of the frequency inverter, there are two modes: remote control and on-site to improve the safety of the system. The operation status of the FD5000 high voltage frequency inverter is realized through the field DCS, which can realize the remote control of the inverter and the field control.

In order to ensure the safe operation of the unit, when the high pressure switch of the frequency inverter and the pump fails, the backup circulating pump must be automatically put into operation under the variable frequency working state.

In order to ensure the safety of the unit, when the high pressure switch of the inverter and the pump fails, the standby circulating pump should be automatically started according to the frequency conversion speed regulation.The frequency inverter is required to supply switching output to the DCS including: fault alarm, ready, operation display, high voltage off permit, connector jump voltage signal, pump bypass switch off signal, frequency conversion KM1 off signal.DCS needs to provide input signals to the frequency inverter are: frequency conversion start (dry junction, effective when closed), frequency conversion stop (dry junction, effective when closed), frequency conversion emergency stop (effective when closed);DCS needs to input two current sources of 4~20 ma to the inverter, one of which is the frequency of the circulating pump, and the other is the pressure at the main pipe of the circulating pump.

The analog quantity required by DCS is: two constant current sources of 4~20 ma, of which the analog quantity is corresponding to the output frequency, current and other physical quantities; The analog input to the inverter is: 1 constant current source, 4-20 ma, representing the output pressure of the inverter.

The frequency inverter is available in manual/automatic modes. When selecting the manual control mode, the frequency inverter is not controlled by the PID controller, but by the local or remote control of the speed control button, which can make the flow of the pump change and realize the manual adjustment of the condenser vacuum. When the automatic control mode is used, the condenser vacuum degree is set by the operator with CRT, and the measured result of the vacuum transmitter installed on the condenser is used as the feedback of the controlled quantity and compared with the set value.When the data measured by the vacuum sensor is lower than the specified range, the output of the PID controller is used to increase the speed of the inverter, so that the flow of the pump is increased, so as to achieve the purpose of cooling and speeding up. On the contrary, when the PID control output is reduced by frequency conversion, the flow rate of the pump is reduced, and the cooling is slowed down until the measurement results are consistent with the given value, and the speed of the motor is stable within a certain range. The speed of circulating pump is controlled automatically to adjust the pump.

3.Application advantage

July 2010 compared with July 2011, after using FD5000 high voltage frequency inverter.The average load of Unit 1 is basically about 118MW, and the power frequency operation data of the circulating pump is as follows: the grid voltage is 6.2kV, the motor current is about 163A, the power factor is 0.82, the pipeline pressure is 0.2MPa, and the average vacuum degree of the unit is about 90.5%. According to the operation data, it can be seen that after the conversion transformation of the 1 # unit, the input current is significantly reduced, the vacuum degree of the unit is improved, and the power saving effect is obvious.