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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 （kW） Rated output current Accelerating/decelerating 220 0.1～3000s （A） time Extended according to user Input frequency （Hz） 45～55 Digital output/input demand （ Rated input voltage V） 6000V±10% Ambience temperature 0～40degree Storage/transportation Power factor 0.95（>20% load） -40～70 degree temperature Efficiency >0.96 cooling Air cooling Output frequency（Hz） 0～120 Humidity <90%, without condensation Resolution （Hz） 0.01 Altitude <1000m 120%1min， Over load IP IP20 150%3s, 200%immediately 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 A Motor current（A） 168.5 191 182.4 185.7 Opening （%） 52 85 69.3 72.2 B 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 frequency（kW） 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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