1. Preface

  11# and 12# unit started to generate power in 1990, 1991. Condenser pump produced by Shanghai Pump Co., Ltd is 4 level centrifugal pump. Because of pump characteristic, there is safe redundancy for flow and water head in design. Machinery units join peak adjustment. Load varies frequently. For conventional valve adjustment, there is much throttle loss. If this adjustment method is changed to variable speed adjustment, pump would have a great potential in energy saving. At present, the most mature and effective method is variable frequency adjustment. On base of research and demonstration, we finally decide to use HARSVERT-A VFD.

Pump data

2. Reform scheme

   Because one pump works and the other is spare, we adopt one VFD to control two motors. In normal state, VFD drives one pump and the other pump is spare. When VFD or working pump is broken, the spare pump starts.

In figure, MA and MB are two condenser pumps. KM1 and KM2 are power line switch of motor. KM3 is variable frequency switch of motor. QS1 and QS2 are variable frequency output switch of two pumps.

QS1 and QS2 are vacuum switch.

KM1 and QS1 are electrical interlock, KM2 and QS2 are electrical interlock, QS1 and QS2 are electrical interlock.

3. Problems and solution

3.1 Condenser water dissolved oxygen exceeds in variable frequency operation.

It is caused by floating ring of spare pump and low cooling water pressure of lower guide bearing. The floating ring and cooling pipe connect to outlet of first level impeller and steady pressure watertank. Cooling pipes of A and B pump connect to downcomer of watertank. When pump runs in direct on line state, its outlet gate is in adjusting state. Outlet pressure is about1.7Mpa. Condensing water from first level impeller enters lower guide bearing and floating ring seal, then enters cooling water system of spare pump (about 0.4Mpa). But when pump runs in low load in variable frequency state, outlet pressure is 0.8～1.2Mpa, cooling water pressure from first impeller is much lower than that in direct on line state (about 0.2～0.3 Mpa). It can not satisfy requirement of spare pump seal system, causing increasing dissolved oxygen.

This problem is solved by adding a pipe to seal water system to parallel connect with original seal water of condenser pump.

3.2 Safety

3.2.1 Two pumps share one VFD. When pump runs in direct on line state, deaerator level is adjusted by valve. If working pump trips off, spare pump link-starts, little influence on operation. But, when pump runs in variable frequency state, with valve full open, there will be great influence on system if the spare pump link-starts. Therefore, we add following logic: in the case of load more than 200MW, when variable frequency pump trips off and spare pump link-starts, valve opening automatically closes to 60% and closes to 30% at load of 100MW down.

3.3.2 During start/stop, pump must run in direct on line state to keep low pressure bypass demand.

3.4 Resonance frequency 

  User can set parameter in VFD to jump resonance points. Motor has been running stably and no obvious variety of indexes for 7 months.

4. Reform effect

After reform, 11# condenser pump started to run in 21 March. It run stably in several days and energy-saving effect is obvious. From below table we can see less load it is, more energy VFD saves.

#11unit:  200MW accounts for 20% of total annual time;         

  170MW accounts for 65% of total annual time;

        150MW accounts for 15% of total annual time.

#11unit annual power consumption is 3100000kWh, after reform, energy saved is

310×(20%×27% 65%×44% 15%×50%)=310×41.3%=1287000KWh

Price: 0.31yuan/kWh

So 400000yuan (USD 58823) is saved.

Power consumption statistic

This project cost is 1200000yuan and the investment is recovered in 3 years.

After reform with VFD, water level is adjusted smoothly. Soft start of motor relieves vibration effect. Economic benefit is considerable and reform achieves expected effect.

Reference:

Yang Xingyao    motor speed adjusting principal and system  Beijing water conservancy&power publishing company  1995