Dalian Developing Zone Thermal Power Plant is the only coal-burning power plant combined heat and mainly for heat supply. Capacity is 144MW. Here are parameters:

Recently, power generating cost is increasing with the price of power and coal rising. In order to reduce power consumption rate, after full investigations, we decide to reform with technology of variable frequency to heighten competitive power.

There are three sets of 220t/h boilers equipped with four medium voltage water supply pumps. Pumps’ inlet fetches water from the public pipe of deaerator, and the outlet water flows into the same pipe. The supply water is adjusted by the governing valve. Some water might flow back deaerator by opening the recycling valve if there is much deviation of water supply. However, the self-circulation would result in waste of water and power so that there is great potential for energy-saving.

Before the reform, the water flow is adjusted by opening of valve. Actually it is adjusted by the pressure loss, not the output of motor. After the reform, in full-opening state of valve, flow is controlled by adjusting motors’ speed with great potential of energy saving. In 2005, we made a lot of field surveys and researches, finally we choose LEADER HAERVEST which takes the largest market share in Chinese medium voltage VFD field

1. Main circuit

The system is type of one drive one. VFD directly connects with the 6kV power cable to drive the motor. To improve the reliability, a bypass circuit might be needed.

The bypass circuit is composed of 3 medium voltage isolator switches QS1, QS2 and QS3 (QF is the original medium voltage breaker ). It requires that QS2 and QS3 can not be closed together and they are interlocked mechanically. When the motor is driven by medium voltage VFD, QS1 and QS2 are closed and QS3 is open; when the motor is in direct on line, QS3 is closed and QS1 and QS2 are open.
For the purpose of protecting, medium voltage VFD is link locked with the switch QF. Once medium voltage VFD faults, QF will switch off. In direct on line state, medium voltage VFD allows QF switch-on and cancels the switch-off signal to enable the motor to start.

2. Parameters of water supply pump

parameters of motors

parameters of pumps

3. Parameters of circulating pump

4. Type-choosing

According to operating experience, the max work current of water supply pump’s motor is less than 150A, we choose HARSVERT-A06/170. The rated output current is 170A which fully meets the need of site. In a similar way, we choose HARSVERT-A06/050 for motor of circulating pump.

5.Site control of VFD

The output frequency of VFD is controlled by a 4~20mA standard industrial signal which is from a LU-900M intelligent monitoring instrument with PI controller. The instrument input a 4-20mA pressure signal of pipe. Adjust set value through the meter panel which can implement non-disturbance switch for open/close loop control. The system figure as below:

6. Energy saving effect

The motor’s current of water supply pump is about 120~130A before reform, and 90~110A after the reform.

Flow is basically same. Current of heat supply network circulating pump is about 55A, After the reform, it comes to 45A.

7.Other effects of the application

(1) After the reform, it only needs to adjust the speed of the motor in the control room so that the valve is not need to be changed. By VFD, speed starts from zero and rises gradually. Thus, there is no large starting current which could protect the motors.

(2) Reduction of the maintenance. After the reform, it could avoid vibration caused by pumps’ deviation from working area in valve control mode. Generally, the application of VFD is for reducing the speed of pump. Because of slow start-up and low-speed running, it correspondingly prolongs the lifetime of the components, such as the hermetic seal, shaft.

(3) Reduction of working strength. For the interlocks between the running devices and back-up devices, system implements protection and malfunction alarm automatically, so it greatly reduces the working strength, improves productivity and provides reliable guarantee for the optimizing operating.

(4) Reducing the impact of grid side. In direct on line state, the starting current is about 6~7 times of rated. After the reform, soft-start becomes a reality. Starting current is about rated. Starting time is prolonged correspondingly so that there is not too much impact on the grid side. At the same time it could prolong the lifetime of the motors.

8.Conclusions

Since the VFD was used, it has been running stably and greatly. It improves the site operating state, and reduces the working strength of the operators. Most of all, the station service labour decreases obviously. In 2006, for better effect we purchased four sets of VFD for the boilers’ fans again. We do believe more and more VFDs would be used in thermal- power corporations for building a more conservation-minded society.