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Frequency inverter energy savings for Thermal Power Plant

This application analyzes technology characters of frequency inverter and its energy saving effect. Combining technics characters of condensing pump, it makes energy saving estimation.

The power plant has 8 machines with capacity of 1280MW. In order to abide by state policy of energy saving and emission reduction, the plant planned to make reconstruction with frequency inverter for condensing pumps of two 300MW machines.

Variable frequency system
The system is composed of three medium voltage isolators QS1, QS2 and QS3. QS2 and QS3 are mechanical interlock. They cannot close simultaneously. When motor runs in variable frequency, QS1 and QS2 close, QS3 opens. When motor runs in direct on line, QS3 closes, QS1 and QS2 open.
variable frequency system

Advantages: There are obvious points of power off for frequency inverter maintenance, which can ensure personnel safety. Bypass circuit can switch load to direct on line. Price is low.

Operation state of condensing pump
After condensating, vapour which does work in low pressure cylinder of steam turbine is collected in condensing water tank. Condensing pump delivers condensing water to deaerator in time to maintain level balance. So continuous and stable operation of condensing pump is one of important factors which ensure generator safety and economized production.

Before using medium voltage frequency inverter, water level of condenser is adjusted by changing outlet valve opening. Linearity is inferior and lots of energy wastes on valve. In the meantime, frequent operation on valve leads to valve reliability down, affecting stable operation of system.

After using medium voltage frequency inverter, no need to adjust outlet valve. Valve opens at a large scope. Adjusting frequency inverter output frequency changes motor speed, reaching purpose of outlet flow adjustment. Following is condensing water system:

condensing water pump system

Benefit analysis
Power saving is evident. Following is operation data. Compare before and after currents, we find current lowers greatly.
Load (MW) 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300
Pump current (before) 82 82 81 82 82 82 84 84 85 86 87 88 88 90 91 93
Pump current (after) 19 19 24 24 25 31 32 29 32 38 40 42 50 52 55 60
Current down A 63 63 57 58 57 51 52 55 53 48 47 46 38 38 36 33

Following is statistic of power consumption in a month.
Generator Power generating kW·h Power consumption of pump kW·h Power consumption/power generating Power saved kW·h
#7 line frequency 191166000 646200 0.34%
#7 variable frequency 193716000 411300 0.21% 234900

Through analyzing condensing water pump system of Thermal Power Plant, we think it is feasible to reconstruct with frequency inverter for two pumps. Changing valve control to variable speed adjustment can reach purpose of lowering power consumption rate. Frequency inverter also brings benefits in terms of safety, reliability and equipment maintenance.

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