1. Preface

Medium Voltage frequency control drive is a new electric drive and it has rapid development in twentieth century, mainly used in AC motor speed control. Its technology and performance are better than any other kind of speed control mode (such as: Voltage Step-down speed control, pole changing speed control, slip speed control, internal feedback cascade speed control and hydraulic coupling). The VFD speed control has a great acceptance in broad users as its remarkable energy saving，high-precision speed control, wide adjustment range，faultless power electronics protection, as well as easy implementation of the automatic communication function. VFD also brings great convenience to user in safe operation, reliable installation, maintenance etc, which makes domestic and foreign enterprises adopt best choice of energy-saving method for motor.

2. Method and principle

2.1 Principle

As we know, the relationships of speed(N), flow(Q), pressure(H) and power(P) is as follows：

Q1/ Q2＝n1/n2                        （2）

H1/ H2＝（n1/n2）2              （3）

P1/ P2＝（n1/n2）3                （4）

 From above, if other operating conditions remain unchanged, through reducing motor speed, energy-saving effect is very remarkable. For example, working condition only demands 50% flow, personnel can adjust speed to half of rated. Power consumption is only 12.5% of rated value, that is, theoretically, energy saving rate can reach to 87.5%.

3. The comparison between VFD and hydraulic coupling

In order to test the energy efficiency of medium-voltage devices, power plant makes comparison test for VFD and hydraulic coupling on fans. After VFD reform, the fan operating efficiency is significantly improved. VFD saves power 26.17% more than  hydraulic coupling.

4 VFD energy-saving analysis

The power consumption of thermal power plant, pumps /fans account for 75% or more. These pumps and fans are adjusted by damper, which causes not only poor quality and  slow  response but also a waste of energy.

4.1  Energy saving analysis on fans

The unit makes a comparison test in 50%, 75%, 100% load of three kinds on the # 4 furnace blower, as the following sheet 1。

                                                             Sheet 1 

From the sheet 1 we can see，the efficiency is 75％～80％ in variable frequency state and about 55％(figure2) in direct on line state；the power-saved is 750kWh, 602.5kWh, 733.6kWh when the load is in 100MW, 150MW, 200MW. After the reform, blower efficiency is markedly improved. Energy-saving effect is remarkable.     

After the reform of 2 sets blowers, in the case of operation time 7000h, power saved is 4928770kWh. According to price 0.30 yuan/(kWh), calculate direct economic benefit, approximate 1.48 million(USD 217647) is saved.

4.2 Pump energy-saving analysis.

4.2.1 Pump flow control

1——Input power controlled by  damper

2——Input power controlled  hydraulic coupling
3——Input power controlled  by VFD

4——Shaft Power when in the speed

In VFD control，full load power factor is close to l in 50Hz，the operating current is much lower than rated current. Filter capacitors in the VFD improve the power factor, saving around 20% capacity for the plant

4.2.2 Condenser Pump Energy Saving Analysis

Unit makes a comparison test in 350MW, 315MW, 280MW, 240MW, 210MW, 175MW load for the # 1 furnace blower, as the following sheet 2.

sheet 2       # 1 Condensation Pump Test conditions and test results

After the reform，On the one hand it reduces the throttle loss during operation. Current of condenser pump lowers and energy is saved. On the other, beacuse pressure declines, the working condition of low-pressure heaters is greatly improved, which reduces the leakage of low-pressure heater and the maintenance. VFD has a significant safety and economic benefits.

Energy calculation：

Annual  electricity saving：ΔC= Cd－Cb = 403.9- 207.0 = 1969000kW?h

Energy-saving efficiency：（ΔC/Cd）×100% =（196.9/ 403.9）×100% = 48.75%

According to power price 0.334 yuan/(kWh), yearly direct economic benefit is 1969000× 0.334 = 657646yuan (USD 96712)

5.  Conclusion

This application proves that HARSVERT-A Medium-voltage VFD has good performance, high reliability. Its energy-saving effect is better than any other form of speed control mode, especially in the low load. After the auxiliary equipments use frequency control technology, the stability of automatic devices is greatly improved. Startup performance of the motor is heightened. It extends the lifetime of the motor. VFD achieves VF contorl in fans and water pumps of the old power plant. It is an ideal energy-saving project. Generally, the cost of  investment can be recovered within 1~3 years.