Variable speed drive principal for condenser pump

2 sets of 100MW turbine generators started to run in 1993 and 1997. The plant reformed with Gozuk variable speed drive on condenser pumps in March, 2005. Variable speed drive has been running stably. Its energy saving is obvious.

Work state of condenser pump
After being cooled and condensed in condenser, steam is collected in hot-well. Condenser pump transfers condensing water to desecrator in time. Keeping pump stability and safety is key to ensure whole plant safety. So monitoring and adjusting water level in condenser is important.

In normal operation, water level cannot be too low or too high. When load increases, condensing water increases, water level goes up. When load is low, water level is down.

Power of motors is 6kV/1000kW with redundancy. One motor works and the other is spare. Before reforming with variable speed drive, water level is controlled by adjusting outlet valve opening. Adjusting quality is bad and great energy is wasted on valve. Frequent adjustments cause valve reliability down.

After reforming with variable speed drive, outlet valve opens fully. Outlet flow is controlled by adjusting motor speed.

Principal of Gozuk variable speed drive
Gozuk variable speed drive adopts technology of multi-level PWM topology. Several LV PWM power cells series connect to output medium voltage. Optical fiber connects MV circuit and controller.

These are advantages of variable speed drive: soft start, soft brake, auto speed adjusting, small start current, continuous adjustable start-up time. Gozuk variable speed drive has self-copyright operation & monitor software. Local /remote start/stop, function, parameter, fault, record are shown in WINDOWS interface of colorful LED touch screen. The interface can show voltage/current waveform, frequency and speed at any time. Integrated fault monitor circuit and precise fault orientation detect each part of variable speed drive. All power cells are intellectually designed. When there is fault, the fault information is sent back to main controller. Controller closes IGBT in time to protect main circuit. Fault position, type are shown on HMI. This makes fault point clear at a glance and it is applicable for common personnel.
When external analogue signal is broken or short circuit, variable speed drive supplies alarm signal and keeps output frequency same. Control power source of variable speed drive accepts AC220V and DC 220V. UPS is equipped in variable speed drive to ensure continuous operation if control power is broken.

Precaution
To prevent air from entering condenser and avoid breaking vacuum, pressure of condenser main pipe cannot be too low. When deaerator pressure is more than pressure of main pipe, steam and water of deaerator inside come back to condenser, which causes water impact in main pipe. Thus outlet valve cannot open fully in variable frequency state.

Energy saving effect
We made comparison test for 1# condenser pump on 17 May, 2005. Variable speed drive saved energy 470kW, 611kW, 631kW in state of 100MW, 75MW, 50MW. Energy saving rate was 47.4%, 70.8%, 78.4%.

Yearly energy saved is 4639MW•h. Price: 310yuan/1MW•h, so 1438000yuan can be saved. The cost was recovered in 1.5 years. The variable speed drive also reduced impact on valve, kept constant pressure.

Conclusion
Variable frequency technology is a new drive technology which adopts advanced electronic technique, computer control technique and communication technique. It goes ahead of any other speed adjusting methods. Variable speed drive is bound to be widely used in more and more plants in future.