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									Medium voltage VFD application in ID fan of power plant

Abstract: this article states HARSVERT-A VFD application in Shentou Second Power Plant and its energy saving effect.
Key words: medium voltage VFD system, ID fan, energy saving and emission reduction

Technology of medium voltage variable frequency speed adjustment is a new type driving technology which emerged in 1990s.
It is mainly used for AC motor speed adjustment. Its technology and performance are superior to any other speed adjustment
mode (such as voltage drop adjustment, pole-changing control, slip difference control, cascaded control and hydraulic coupling).
Variable frequency adjustment is accepted by most users as its evident energy saving, high precision, wide adjustment range,
completed protections and auto communication. It also brings most convenient and fastest service in terms of reliable operation,
easy installation and free maintaince. It has been first choice product of energy saving.
Phase I generator of Shentou Second Power Plant is 500MW steam turbine generator. Boilers of two generators are equipped
with two ID fans which are all axis flow fans. Air flow is controlled by damper. During operation, max opening of damper is
not more than 85%. The adjustment just changes flow resistance. Output power does not change. Throttle loss is big and much
power is wasted. In the mean time, during starting of motor, it creates impact current which is 5~7 times of rated current.
Automatization level of ID fan is low. Adjustment is not timely and operation efficiency is low. Thereby, plant decides to
reconstruct with variable frequency adjustment to reduce flooding loss and throttle loss.
I. Constitution of medium voltage variable frequency speed adjustment system
1. Characters of ID fan
Air flow needs to vary with load variety. Because model of fan was selected in early term, air flow is adjusted just by damper
opening, which causes much energy wasted. In the mean time, generator does not cooperate with DCS well due to slow
response and nonlinear ity. Automatization level lowers greatly.
2. Variable frequency speed adjustment system of boiler
Four VFDs adopt scheme of “one drive one”.
Parameters of HARSVERT-A06/220

     Capacity (kVA)              2250                          Analogue input               0~10V/4~20mA

     Matching motor power
                                 2200                          Analogue output              0~10V/4~20mA
     Rated    output current                                   Accelerating/decelerating
                                 220                                                        0.1~3000s
     (A)                                                       time
                                                                                            Extended      according   to user
     Input frequency (Hz) 45~55                                Digital output/input

     Rated input voltage V) 6000V±10%                          Ambience temperature         0~40degree
     Power factor                0.95(>20% load)                                            -40~70 degree
     Efficiency                  >0.96                         cooling                      Air cooling

     Output frequency(Hz) 0~120                                Humidity                     <90%, without condensation
     Resolution (Hz)             0.01                          Altitude                     <1000m
     Over load                                                 IP                           IP20

3. Characters of variable frequency speed adjustment system
Personnel adjusts output value of DCS according to temperature of flue gas, temperature of boiler vapour and negative pressur e.
This output value is given value of frequency (speed) which is 4~20mA signal feeding back to VFD. VFD compares speed
output and given value, then adjusts motor speed automatically.
On above base, draw curve of given value in different loads and temperatures. Fix safe up limitation and safe low limitation,
make special arithmetic of fan speed adjustment. Send samples of load, temperature and negative pressure variety to DCS. DCS
makes control operation, feeds back the outcome to VFD in form of 4~20mA signal. VFD adjusts speed automatically through
comparing speed output and given value from DCS, thereby adjusting motor speed and implementing auto control of fan speed.
Here are characters of variable frequency speed adjustment system: speed adjustment system can run in variable frequency, and
also can switch to direct on line. When control power of VFD is off, inside UPS can supply power to maintain continuous
operation. When speed given from DCS faults, VFD alarms, meanwhile, VFD continues to run in original speed and keeps
working conditon unchangeable. Preserve original motor, no need to change original equipmen.

II. Energy saving analysis
1. Parameters
                Equipment                                         ID fan
                Fan model                                         ARB4500-2A/90

                Flow (105Nm3 /h)                                  20.376
                Full pressure(pa)                                 1256
                Motor model                                       YKK2200-16

                Motor power Pdn (kW)                              2200
                Motor speed N0(r/min)                             372
                Motor voltage U0 (kV)                             6

                Motor current I 0 (A)                             292
                Power factor of motor                             0.84

2. Fan parameters in each load
                 ID fan                 Load (MW)                     302      350          457       499

                                        Opening (%)                   53       85           69.7      74
                                        Motor current(A)          168.5       191         182.4     185.7

                                        Opening (%)                   52       85           69.3      72.2
                                        Motor current(A)              168.2    189          177.6     180

3. Calculation
Power calculation in direct on line state
During operation, fan A and fan B should be adjusted simultaneous ly according to load variety. Preformance indexes of fan A
and fan B are same. So we make consolidation of data of A and B. Infer power consumption in variable frequency state
according to curve of flow percentage and damper opening.
Pd :motor power;U:input voltage of motor;I:input current of motor;COSφ:power factor.
Formula: Pd =1.732×U×I×COSφ
We can calculate COSφ according to above formula.
Get following table after processing ID fan data
Load (MW)                             302                   350                   457                   499
Average opening(%)                    52.5                  85                    69.5                  73.1

Motor current(A)                      336.7                 380                   360                   365.7
Total power(kW)                       2939.15               3317.13               3142.54               3192.30

We get following linearity relation of damper opening and total power according to above data.

Note: Bule line: damper opening
Pink line: total power

We infer total power at 100% opening:
P100 =(3192.30-3142.54)/(73.1-69.5)×(100-73.1)+3192.73=3935.89kW
That is, total power is 3935.89kW when opening is 100%

Power calculation in variable frequency state
Fan is square torque load. Its speed N, flow Q, pressure H and shaft power P have following relations: Q∝N, H∝N2 ,P∝N3
That is, flow is in direct rate to speed. Pressure is in direct rate to square of speed. Shaft powe is in direct rate to cube of speed.
As per formula: P / /Pdn=( N / / N0 )3 =(Q/ /Q0 )3 , we can calculate total power consumption in each load.
Pdn is power of 100% opening, equal to P 100 .
For load of fan, opening rate is regarded as air flow rate approximately.
Energy saving rate:
л= (Pd- Pb )/ Pd ×100%
we get following result according to above formula
Load (MW)                           302              350           457               499
Average opening(%)                  52.5             85            69.5              73.1

Total   power     in     variable
                                    569.5351         2417.127      1321.287          1537.428
Energy saving rate(%)               80.62            27.13         57.95             51.84

III. Concluding remarks
How to lower cost, heighten competitive power, strengthen management and explore energy saving is an important event that
power plant must research. Making reconstruction with smedium voltage variable frequency speed adjustment for equipments,
such as ID fan, FD fan, feed pump and circulating pump, not only lowers power consumption and coal consumption, but also
heightens safety and reliability, reduces system faults.

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