1、 AbstractWhen the power grid is operating in high loading and the load increasing of the nearby units is difficult in grid abnormal condition,the overpower capability of the generation units in grid abnormal area will be very important for the grid.A unit overpowering method is proposed under power
2、grid abnormal conditions.The proposed method is based on the change of the operation mode of the turbine high pressure feedwater heater system.A theoretical analysis method for evaluation of overpowering of the existing generation unit using the proposed method is put forward.Based on the design par
3、ameter of the full load condition,an overpower factor of the unit under high pressure feedwater heater bypass condition can be calculated.This overpower factor is illustrated as a function of the feedwater heater outlet mixed temperature drop or feedwater heater bypass flow fraction.An example of th
4、e unit overpowering scheme is provided for a 1000MW super-super-critical generation unit.It is shown that under the condition of grid abnormal operation condition the high pressure feedwater heater bypass is opened partly and 5%-35%of the feedwater is bypassed through the heater bypass line,and and
5、a 0.77%-5.09%of the unit rated output increase can be expected under full load condition.Typically the high pressure feedwater heater bypass is opened and 20%of the feedwater is bypassed,and nearly 3%of the unit output can be increased under full load condition.It is also shown that under the HP hea
6、ter bypass operation condition the final temperature dropping is nearly linear to the bypass line flow fraction.Therefore,controlling of the final feedwater temperature can be taken as an indirect indication of the heater bypass flow.The theoretical analysis results also give some operation limitati
7、on for future site application.Index TermsPower Grid Abnormal Condition;Overpowering;Generation Unit;Turbine Cycle;Feedwater Heater Bypass.Manuscript received July 10,2018.Haisheng Yang is with the Hebei Electric Power Testing and Research Institute,No.238 Ti Yu Nan Da Jie,Shijiazhuang,Hebei PRC.,ph
8、one:86-311-66672341;fax:86-311-66672119;email:.Shengyang Lu is with the Hebei Electric Power Testing and Research Institute,No.238 Ti Yu Nan Da Jie,Shijiazhuang,Hebei PRC.I.INTRODUCTION OR the safe operation of the power system,all the generation units require certain standby generation capacity dur
9、ing certain power grid abnormal conditions.From the operation point of view of the power grid,if the standby generation capacity is close or at the same geographical position to the abnormal fault generation units,the impact on the power grid will be controlled to a minimum level.When the power grid
10、 is operating in high loading and the load increasing of the nearby units is difficult in grid abnormal condition,the overpower capability of the generation units in grid abnormal area will be very important for the grid.The proposed method of unit overpowering is described under power grid abnormal
11、 conditions.The proposed method is based on the change of the operation mode of the turbine high pressure feedwater heater system,especially the heater bypass lines.Under power grid abnormal condition,the high pressure feedwater heater bypass valve is opened,and the feedwater flow through the high p
12、ressure feedwater heater and the resulting extraction steam flow required to the heaters are decreased,thus the steam flow through the steam turbine and power output of the unit are increased.This concept of unit overpowering has been used in Japan due to the large-scale shutdown of the nuclear powe
13、r plants after the tsunami disaster in 2011.During normal operation,for the efficiency of the power plant,the high pressure feedwater heater bypass line is closed,and opening of the bypass line is triggered only in feedwater heater abnormal conditions,such as heater tube leakage conditions.Reference
14、 1 analyzed the effect of the high pressure feedwater heater bypass line leakage due to bypass valve untightness.The analysis in 1 is based on the 200MW generation units.The three high pressure heaters shared one common bypass line.The methodology is based on the commonly used concept of Unit Overpo
15、wering Capability Potential Analysis of Conventional Generation Units using HP Feedwater Heater Bypass Haisheng Yang,Shengyang Lu Thermal Technology Depart.Hebei Electric Power Testing and Research Institute No.238 Ti Yu Nan Da Jie,Shijiazhuang,Hebei PRC F2018 China International Conference on Elect
16、ricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201805280000198 Page1/5 922Equivalent Enthalpy Drop.After analyzing the change of the Equivalent Enthalpy Drop of the live steam,and the heat change of the turbine cycle,the effect on the unit performance can be analyzed.It is shown by the
17、 calculation results that with heater bypass line leakage of 5.4%,the unit power output increases 1.23MW(or 0.615%)and the unit performance decreases 0.137%.No more details are provided in the paper.Reference 2 provided more details for analyzing the unit performance change due to the leakage of the
18、 high pressure feedwater heater bypass lines.Various calculation models considering different kind of bypass line configuration are provided based on the Equivalent Enthalpy Drop Methodology.The example analysis in 2 is based on the 300MW generation unit.It is shown by the calculation results that w
19、ith heater bypass line leakage of 10%,the unit performance decreases 0.04%to 0.05%for heater bypass lines separately arranged for each heater,and 0.3%for heater bypass line commonly used for all high pressure heaters.All the above references discussed the economic impact of the high pressure heaters
20、 bypass line leakage,but no attention is paid to the overpower possibility of the unit under bypass line operation condition,which is very helpful for the power grid abnormal conditions.The objectives of the related work including:(1).to study the feasibility of unit overpowering using the high pres
21、sure feedwater heater bypass system,under abnormal grid condition;(2).to analyze the potential of the overpowering capacity for a certain existing generation unit.II.THEORETICAL ANALYSIS OF THE UNIT OVERPOWERING CAPABILITY USING HP HEATER BYPASS LINE Improving the unit overpowering capability using
22、high pressure heater bypass is useful especially for the following occasions:The overpowering capability of the unit is limited by the status of the auxiliary equipment,such as feedwater pump.It is not possible to increase the power output of the unit with normal technical measures,such as increasin
23、g the inlet live steam flow with higher opening of the turbine main control valves.When the generation unit is operating in high load and the unit overpower is limited by the capacity of the boiler equipment such as boiler fans and pulverizers,the overpowering capability of the generation units will
24、 be restrained for the power grid.A.Principle of the Unit Overpowering Using High Pressure Feedwater Heater Bypass Line The proposed unit overpowering method is based on the changing the feedwater flow using the existing high pressure feedwater heater bypass line.As an example,under power grid abnor
25、mal condition,the high pressure feedwater heater bypass valve is opened,and the feedwater flow through the high pressure feedwater heater and the resulting extraction steam flow required to the heaters are decreased,thus the steam flow through the steam turbine and power output of the unit are incre
26、ased.This process will increase the power output of the unit to support the grid abnormal situation.Using the proposed technique,the unit overpowering of the unit is directly related to the change of the feedwater flow or the temperature dropping the final feedwater temperature.Under same operation
27、load,higher extent of the feedwater flow change or temperature dropping the final feedwater temperature will lead to higher load change.On the other hand,the high pressure heater feedwater flow change is also limited by the magnitude of the final feedwater temperature dropping and the adaptation abi
28、lity of the boiler,especially the steam temperature control.Therefore,the unit overpowering capability should be balanced with the capability of the existing systems.B.Procedures for Evaluation of Unit Overpowering Capability Using HP Heater Bypass Line When considering using of the proposed techniq
29、ue on certain generation unit,one has to evaluate the potential of the unit overpowering capability of the unit first,that is,what is the maximum power output increase under certain unit load condition and how that condition can be achieved.A theoretical analysis method for evaluation of the unit ov
30、erpowering under above situation is put forward.Based on the design parameter of the unit full load condition,the unit overpowering capability of the unit can be calculated.This capability can be illustrated as a function of the high pressure feedwater heater bypass flow or final feedwater temperatu
31、re drop.The theoretical results can also be used for evaluation of the unit for overpowering purposes under other operation conditions.During feasibility study phase,the following procedures are proposed for evaluating the potential of the unit overpowering capability using HP heaters bypass line:Ga
32、thering the detailed design data of the turbine rated conditions,including:feedwater flow,final feedwater temperature,inlet temperature of the HP heaters,feedwater pressure,extraction steam flow and steam inlet enthalpy for each HP heaters,high pressure turbine exhaust steam enthalpy,IP turbine inle
33、t steam enthalpy,and LP turbine exhaust steam enthalpy.Calculate the the feedwater flow through feedwater heaters under bypass line partly opening condition.Under HP heater bypass operation condition,the feedwater flow through the HP feedwater heaters can be calculated using the equation below:),(),
34、(),(),(1 21 2t p h t p ht p h t t p hFR=(1)2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201805280000198 Page2/5 923Where FR refers to the feedwater flow fraction through the HP feedwater heaters compared with the total feedwater flow;p refe
35、rs to the feedwater pressure,MPa;1t refers to the inlet temperature of the HP heaters,;2t refers to the final feedwater temperature under baseline condition(without heater bypass operation),;t refers to the final feedwater temperature drop under heater bypass operation compared with the baseline con
36、dition,;),(1t p h refers to the enthalpy calculation formula based on medium pressure and temperature from the IAPWS.Calculate the unit overpowering under bypass line partly opening condition.For the HP turbine,the overpower can be calculated using:)()1(1=niHPeiextiext HPh h F FR P(2)Where HPP refer
37、s to the HP turbine overpower due to the heater bypass operation,kW;n refers to the number of HP feedwater heaters using extraction steam from HP turbine;iextFrefers to extraction steam flow to HP feedwater heater i,kg/s;iexthrefers to extraction steam enthalpy to HP feedwater heater i,kJ/kg;HPeh re
38、fers to the HP turbine exhaust steam enthalpy,kJ/kg.For the IP and LP turbine,the overpower can be calculated using:=1rhm1LP-IP)()()1(ieiexte ext exth h Fh h F FR P(3)Where LP-IPP refers to the IP and LP turbine overpower due to the heater bypass operation,kW;m refers to the number of HP feedwater h
39、eaters using extraction steam from IP turbine;jextF refers to IP turbine extraction steam flow to HP feedwater heater j,kg/s;jexth refers to IP turbine extraction steam enthalpy to HP feedwater heater j,kJ/kg;rhh refers to the IP turbine inlet steam enthalpy,kJ/kg;eh refers to the LP turbine exhaust
40、 steam enthalpy,kJ/kg.The total overpowering of the unit can be calculated using:LP-IP HPP P P+=(4)Where P refers to the unit total overpower due to the heater bypass operation,kW.For achieving expected unit overpowering capability,the following procedures are proposed for operation of HP heaters by
41、pass mode:Unit control mode is adjusted to keep the constant opening of the turbine main control valves,so that the turbine inlet steam is stable.The HP feedwater heater bypass line valves are opened slowly to feed the bypass line with part feedwater flow.Monitor the dropping gradient of the final f
42、eedwater temperature,and control the total temperature dropping within 25.Keep the stable operation of the boiler,and control the outlet parameters such as main steam pressure,main steam temperature and reheat temperature.Monitor the extraction pressure of the turbine within limits.III.EXAMPLE ANALY
43、SIS FOR AN ACTUAL GENERATION UNIT An example of the proposed unit overpowering scheme is provided for a 1000MW generation unit.The 1000MW steam turbine is the super-super-critical regenerative condensing turbine with 5 casings and 6 exhaust end.Two 50%capacity feedwater pump are provided.For the tur
44、bine cycle,three high pressure feedwater heaters,one deaerator,and six low pressure feedwater heaters are arranged.The design data of the steam turbine unit is listed in Tab.1.TABLE I.DESIGN DATA OF THE STEAM TURBINE Items Unit Data Unit Output under TRL MW 1050 Unit Heat Rate THA kJ/kWh 7146 Main S
45、team Pressure MPa(a)28 Reheat Steam Pressure MPa(a)5.52 Main Steam Temperature 600 Reheat Steam Temperature 620 Main Steam Flow THA t/h 2804.29 Condensate Flow THA t/h 2116.46 Turbine Back Pressure kPa 3.3 Potential evaluation of the unit overpowering capability using the HP feedwater heater bypass
46、line is made based on the procedures provided in the Section II.2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201805280000198 Page3/5 924TABLE II.CALCULATED RESULTS OF THE POWER OUTPUT CHANGES Final Feedwater Temperature()Final Feedwater Tem
47、perature Dropping()Bypass Flow Fraction(%)Power Output Increase(kW)Power Output Increase(%)298.41 0.01 0.00-16.82 0.00 294.95-3.45 5.00 8071.86 0.77 291.52-6.88 10.00 16113.46 1.53 288.11-10.29 15.00 24066.93 2.29 284.72-13.68 20.00 31869.77 3.04 281.35-17.05 25.00 39440.00 3.76 278.00-20.40 30.00 4
48、6678.10 4.45 274.67-23.73 35.00 53468.94 5.09 Operation analysis is made based on the THA rated load condition.Various final feedwater temperature dropping under HP heater bypass operation are tried and the corresponding unit power output changes are calculated.The results are listed in Tab.2.Relati
49、ve change of the power output corresponding to the HP feedwater bypass flow fraction is shown in Fig.1.The function of the bypass feedwater flow fraction corresponding to final temperature dropping is shown in Fig.2.Based on the analysis results,it is shown that under the condition of grid abnormal
50、operation condition the feedwater flow to the HP heater bypass line can be increased about 5%-35%,and a 0.77%-5.09%of the unit rated output increase can be expected under full load condition.The theoretical analysis results also give some operation limitation for future site application.S=0.00271613
