AC drives energy saving on condensing water pump

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

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

Equipment	Motor model	Power	Rated voltage	Rated current	Rated speed	Pump model	Flow	Lift
Condensing pump	YLKK500-4	900kW	6kV	103.3A	1480rpm	NLT350-400*6	847m3/h	288m

Variable frequency system
The system is composed of three medium voltage isolators QS1, QS2 and QS3. QS2 and QS3 are mechanical interlock. They can not 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.


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

Condensing pump operation state
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 AC drive, water level of condenser is adjusted by changing outlet valve opening. Linearity is inferior and lots of energy wastes on valve. In the mean time, frequent operation on valve leads to valve reliability down, affecting stable operation of system.

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



Benefit analysis

1. 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

2. 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

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