1. Introduction

SHANXI YANGGUANG POWER GENERATION CO., LTD  1# unit (coal-fired) is 300MWgenerator with two sets of static blade axial-flow ID fan and YKK800-8-W motor. There is no speed-adjusting device for motor. Air flow is controlled by changing angle of blade. The parameter table is as follows:

In general, the load of the unit varies in the range of 50%~100%. Generator output power varies. Forced air flow and induced air flow of boiler vary correspondingly. ID fan is adjusted by changing angle of blade. Although there is energy-saving to some extent contrast to damper adjustment. The loss is still considerable, especially in the situation of low load. Secondly the slow adjustment of static blade may lead to the load lag correspondingly. Generally, the starting current of asynchronous motor is 8～10 times of the rated current, which may surge the power grid. Big torque also affects lifetime of motor and fan badly.

When the speed of the fan varies, the efficiency varies a little. The flow is directly proportional to speed. Pressure is directly proportional to the square of the speed, and shaft power is directly proportional to the cube of the speed. Furthermore, the shaft power would reduce in cube relation when the speed is down, so does the motor power. Then we know that speed adjusting is the key of energy-saving for fans. It could achieve linear adjustment for ID fan with VFD.

 VFD can optimize the operation of the motor and enhance efficiency greatly to save energy. There were some restrictions before, such as price, reliability, capacity etc. VFD was not widely used. Recently, with the rapid development of power devices, control theory and computer science,declining price of VFD and increasing reliability, MV VFD has been widely used in auxiliary equipments of power plants.

2 VFDs in ID fan of 1# unit adjust motor speed to control air flow and pressure. According to the practical situation, operation current of ID fan is just around 140A in full load. We choose model of VFD in conformity with practical current rather than rated power for the consideration of energy-saving. Finally we choose HARSVEST-A06/220 which is made by Beijing Leader & Harvest Electrical Technologies Co.,Ltd.. It is voltage source digital control VFD, high-high type, H bridge series connection. Rescontruction was from May to July in 2005, total 40 days. VFD started to run on 10 July in 2005 with 1# unit together.

2. Application of MV VFD

2.1 Structure of VFD

         Figure1 structure of multi-level voltage source VFD

Multi-level voltage source VFD is composed of phase-shifting transformer, power cells and controller. ( fig.1 )

2.2 Ports between VFD and the field

Controller is composed of high speed SCM, HMI and PLC. SCM realizes PWM control and power cell protection. HMI based on Windows CE has friendly touch screen interface, and it also realizes remote control and net control. Inside PLC treats with the logic of digital signals and communicates with the controller system of user. The PLC is Siemens S7-200 which has perfect interface to DCS.

(1) The interface between DCS and VFD

There are 11 signals between DCS and VFD. Among the rest, there are 9 digital signals and 2 analog signals.

(2) The materials and cables

There are 14 cores in the cable which is used to transport digital signals from VFD to the MC cabinet of the DCS, 4 cores for digital signals from RC cabinet of the DCS to VFD, and another 4 cores for the analog signals between DCS and VFD . 800m cable is needed in DCS inside.

Module used: DCS control module  6DS1412-8RR 2 pieces   6DS1717-8RR 2 pieces  6DS1723-8BB 1 piece

③the following content is added to the DCS interface

2.3 Running mode and control logic

Generally, two fans run in variable frequency state. Considering the situation of VFD fault, we prepare two modes:

l        One fan works in direct on line and the other works in variable frequency.

l        All two fans works in direct on line.

VFD has two operation modes: local/remote. In remote control, speed signal from DCS can trace speed-feedback of VFD. In local control, remote action to VFD is invalid.

VFD is controlled by DCS in manual or auto. In manual mode, operator could adjust pressure by controlling speed of VFD through DCS interface.

(1) Allowable conditions of VFD start

Because preconditions of VFD start-up are closing MV switch and start feedback is1. Original conditions of fan start-up are preserved for VFD start-up.

Ready signal from VFD is another condition.

In commissioning of remote control of VFD, we find there is a considerable impulse on combustion chamber because of high speed set in VFD. Thus when VFD starts, limitation of speed-set must be less than 30% .

In short, there must be three conditions of VFD start:

l        For ID fan A /B, MV parts S07-1, S08-1 feedback are 1.

l        The ready signals S0701-RD, S0801-RD from PLC are 1.

l        The speed-set value of VFD C0701TRS, C0801TRS output is less than 30%.

②Protection of trip-off

When VFD of single side fan trips off, it link-closes FD fan of side. Logic of damper and blade keep unchanged.

When VFDs of double side fan trip off, because MV switch link colse, original boiler link trip keeps unchanged.

The boiler’s safe operation is guarantee of plant power. Although the VFD is very reliable, it must make sure boiler can run safely once VFD is in failure. Thus it requires the switch between DOL(direct on line) and VFD. Once a VFD is in failure and it can not recovery immediately, original static blade adjustment is needed. In this case, we must stop one VFD, turn up the other, put the blade into use, and then adjust and modify some parameters by commissioning.

3 . Comprehensive Evaluation

3.1 Energy efficiency significantly

Below there are some data for comparing.

By comparing the data, at the same circumstances, the average daily consumption of unit 1# is 16431kW.h, others are 32450kW.h. Power saved is 16019kW.h. Power-saving rate is 49.37%.

3.2 energy-saving calculation

If every fan works 7200 hours in a year and two fans are driven by VFDs, the power saved might be 4805700 kW.h. Power price of power is 0.2 yuan/kW.h. cost saved should be 961140yuan(USD141344)

4. Concluding remarks

To sum up, VFD widely application in power plants is the trend in future. It not only  saves erergy significantly, but also has good adjustment ability. VFD prolongs the lifespan of fans and motors. With the development of high-technology, cost-decreasing, and new design, it simplifies the structure of VFD, reduces components and makes VFD more reliable.