1、 Abstract The volatility and intermittency of wind speed lead to voltage stability changing of wind farm connecting power system.This paper studies the influence of local wind farm power fluctuation and the improvement strategies on grid voltage stability.In simulation model,the influences on grid v
2、oltage stability improving are researched of different reactive power compensator.Finally,with different operation sections of wind power,the grid voltage fluctuations are analyzed to evaluate the improvement effects of grid voltage stability with different reactive power compensation devices.Index
3、Terms wind farm power fluctuation;voltage stability improvement strategy;simulation model;I.INTRODUCTION Recently,with the development of wind power technology,large-scale wind farms are connected in power systems.The connected operating of wind farm will cause voltage quality and voltage stability
4、problems as the randomness and volatility of wind power 1.The reactive compensation is important for voltage regulation in local power network with long distance wind farm connecting 2.Reference 3 shows that the types of wind turbines,the control system and the grid condition can impact the voltage
5、stability of wind power connecting system.Reference 4 shows the short-circuit ratio of wind generator connecting node and impedance ratio of trans line are important reasons of the voltage fluctuation and flicker with wind power connecting.In this paper,the stability of power grid voltage affected b
6、y the power fluctuation of wind farm and the improvement effects produced by reactive power compensation device with the wind farm connected to the grid is researched.Firstly,this paper analyses the reactive power problem of wind power with a simple wind farm as example.Secondly,the reactive power c
7、ompensation equipment of wind farm is introduced.Finally,the PSCAD software for wind farms is used for analyzing the different effects to grid voltage with different reactive power compensation devices and with no reactive power compensation.II.MAIN TYPES OF REACTIVE POWER COMPENSATION EQUIPMENT OF
8、WIND FARM At present,the dynamic reactive power compensation device used in wind farm is mainly composed of shunt capacitor bank,static VAR compensator(SVC)and static synchronous compensator(STATCOM).A.Shunt Capacitor Bank The shunt capacitor bank is presently widely used.The shunt capacitor bank ca
9、n compensate the reactive power of the system by the switching of the capacitors with a control mode based on its local power factor.B.Static VAR Compensator(SVC)SVC is currently the most widely used reactive power compensation device which is based on flexible AC transmission system(FACTS)technolog
10、y.The utility model is the parallel connection of a controllable reactor and a power capacitor(fixed or group switching).The capacitor can emit capacitive reactive power,and the controllable reactor can absorb inductive reactive power.According to the different structure principle,SVC technology can
11、 be divided into SSR-Self-saturable reactor,TCR-Thyristor controlled reactor,TSC-Thyristor switched capacitor,TCT,AR and so on 5.With the development of high power electronic device manufacturing technology,TCR/TSC mode has become the mainstream technology of SVC which is used in this paper.C.STATCO
12、M The main circuit of the STATCOM is voltage type bridge and current type bridge.In practical,the voltage type bridge is widely used.Therefore,the working principle of the voltage type bridge is mainly introduced in this article.The voltage type bridge based on the inverter of voltage source is a re
13、active power regulating device.The whole device is equivalent to a voltage source,whose voltage can be controlled.This paper assumes its voltage is U 1,the system voltage is U S,the reactor reactance is X and the output current is Analysis of Influence with Connected Wind Farm Power Changing and Imp
14、rovement Strategies on Grid V oltage Stability Bai Qinyu1,Yuan Lin2,Ma Jiayi2,Lu Tianqi3,Zhang Xiaotian3,Wang Youyin3(1.Electric Power Research Institute of State Grid Liaoning Electric Power Co.,Ltd.2.STATE GRID SHENYANG ELECTRIC POWER SUPPLY COMPANY 3.State Grid Liaoning Electric Power Company Lim
15、ited Economic Research Institute)2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804210000003 Page1/52029 as(1).1 sUUIjX=(1)Therefore,the complex power absorbed by the device is given in(2)which named S.Generally,STATCOM cannot absorb activ
16、e power and just makes the active power balanced among the three-phase power.The phase of it is same with system voltage,and the reactive power absorbed by the device is given in(2)which named Q.*11,sss s sU U U US U I U Q UjX X=(2)If the voltage generated by the device is different from the system
17、voltage,the device can emit or absorb reactive power to effectively suppress the voltage fluctuation.When the system voltage drops a lot,STATCOM outputs its maximum instantaneous reactive current.If the voltage cannot be restored,it maintains its maximum reactive current output which can be converte
18、d to be a constant current source until the voltage is restored.III.THE ANALYSIS ON THE POWER FLUCTUATIONS OF WIND POWER FARM TO VOLTAGE OF THE GRID INFLUENCES In this paper,a double-fed wind power farm of a region is used as an example for studying its interaction with the grid.The line power flow
19、distribution and the interaction of wind turbine group in the wind power farm interior are not studied in this paper.In actual operation currently,because the double-fed wind turbine almost doesnt have the reactive power adjustment ability,the converter of the double-fed wind turbine group doesnt pr
20、oceed the setting of the reactive power control.The number of the exports of wind power farm step-up switchyard make reactive power exchange with system is not controlled in this simulation.The reactive power compensation device in wind power farm is only regarded as the solution when the voltage of
21、 control bus causes accidents.A.Simulation Conditions Simplifier model with 93 parallel running 1.5MVA double-fed wind turbine groups is used in wind power farm simulating.The output changing rate at the same time of all wind turbines in the wind power farm is 1p.u.The shunt capacitor group as SVC a
22、nd STATCOM are used as the reactive power compensation device in wind power farm.The energy of electric is transmitted form the wind turbines in the wind power farm to 66kV step-up switchyard by a plenty of 10kV transmission lines.The energy of electric is transmitted to 220kV step-up switchyard by
23、220kV transmission lines,and it is transmitted to a 220kV substation named W by 66kV transmission lines.All transformer capacity is 150MVA.It is assumed that all transformer tap lead is not regulated.The diagram of wind power farm that accessed to the system of a 220kV substation named W is shown in
24、 Fig.1.VGSVCMain grid220kV side of transformer W 66kV side of 66kV station transformer10kV side of wind farmGrid side converter93 parallel running 1.5MVA double-fed wind turbine groupsThe SVCThe STACOMThe capacitance690V side of wind farm220kV side of 220kV station transformer Fig.1.The power system
25、 with wind power farm connecting The simulation of reactive power compensation device is set groups and parameters as follows.1.Manual and mechanical switching capacitors are 3 groups with 8Mvar.2.TSC+TCR is used with 0.5Mvar 24 groups of TSC and 12Mvar 1 group of TCR.3.STATCOM capacity is 100Mvar.T
26、he control method of Group 1 is bus voltage of 66kV side set from 0.99 to 1.03 p.u.in 66kV step-up switchyard.The control method of Group 2 and Group 3 is bus voltage of 66kV side set 1.0p.u in 66kV step-up switchyard.B.The Simulation of The Grid Voltage Because the present method of the wind power
27、generation that accessed to the grid mainly is 220kV and 66kV transmission,there are two main simulations of wind power farm which are 220kV transmission and 66kV transmission.The buses that are needed to note the voltage which include the wind turbines 690V,66kV side of 66kV step-up switchyard,220k
28、V side of 220kV step-up switchyard and 220kV side of substation named Transformer W.Each bus voltage fluctuation curve is from simulating this wind power farm output from 0 to 100%gradually.Without compensation device,the curve of each bus voltage fluctuation with the output of the wind power farm i
29、s shown in Fig.2.From Fig.2,in the 4-18s period,of the wind farm output stage changes from 0 to the rated power.The voltage of wind turbine increases with the wind power farm output increases and the consumption of reactive power increases when 12s it changes lower.The voltages of the rest buses are
30、nt 2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804210000003 Page2/52030 dramatically change.This is due to the absorption of many reactive powers from the system side to maintain the voltage.4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 0.00 0.2
31、0 0.40 0.60 0.80 1.00 1.20 voltagens of wind generator 690V side,66kV side of 66kV station transformer,220kV side of 220kV station transformer and 220kV side of transformer W Wind farm output power Pg(a)The fluctuation curves with power changing from 0 to 1.2 p.u 0.940 0.960 0.980 1.000 1.020 1.040
32、4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 Voltage of wind generator 690V sideVoltage of 220kV side of transformer WVoltage of 66kV side of 66kV station transformerVoltage of 220kV side of 220kV station transformerWind farm output power Pg(b)The fluctuation curves with power changing from 0.94 to 1.05 p.u
33、 Fig.2.The wind power farm output and the fluctuation curves of bus voltages with no compensation device used.The curve of when the shunt capacitor bank is used as compensation device in the wind power farm each bus voltage fluctuation with the wind power farm output changing is shown in Fig.3.4.0 6
34、.0 8.0 10.0 12.0 14.0 16.0 18.0 0.00 0.20 0.40 0.60 0.80 1.00 1.20 Voltage fluctuations of wind generator 690V side,66kV side of 66kV station transformer,220kV side of 220kV station transformer,220kV side of transformer WWind farm output power Pg(a)The fluctuation curves with power changing from 0 t
35、o 1.2 p.u.0.920 0.940 0.960 0.980 1.000 1.020 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 Voltage of 220kV side of transformer WVoltage of 66kV side of 66kV station transformerVoltage of 220kV side of 220kV station transformerWind farm output power Pg Voltage of wind generator 690V side(b)The fluctuation c
36、urves with power changing from 0.92 to 1.03 p.u.Fig.3.The wind power farm output and the fluctuation curves of bus voltages with shunt capacitor bank used.Before the wind farm output reaches 60%,the line reactive loss is small,the manual switching capacitor cannot move.Then 1 groups of capacitors we
37、re put into with the increase of the wind turbine output and the amount of reactive power of lines and transformers.All the capacitor banks were put into while the wind turbine terminal voltage increases up to max 1.02p.u.with the input of the capacitor which can be seen from Fig.3.Under the action
38、of the shunt capacitor,the voltage of the 66kV bus is slightly increased,while the other two 220kV bus voltage is stable.The curves of each bus voltage fluctuation with the wind power farm output changing is shown in Fig.4 while the static VAR compensator(SVC)is used as compensation device in the wi
39、nd farm.4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 0.00 0.20 0.40 0.60 0.80 1.00 1.20 Voltage fluctuations of wind generator 690V side,66kV side of 66kV station transformer,220kV side of 220kV station transformer,220kV side of transformer WWind farm output power Pg(a)The fluctuation curves with power chan
40、ging from 0 to 1.2 p.u.0.940 0.960 0.980 1.000 1.020 1.040 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 Voltage of wind generator 690V sideVoltage of 220kV side of transformer WVoltage of 220kV side of 220kV station transformerVoltage of 66kV side of 66kV station transformer(b)The fluctuation curves with po
41、wer changing from 0.94 to 1.05 p.u.Fig.4.The wind power farm output and the fluctuation curves of bus voltages with static VAR compensator(SVC)used 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 0.00 0.20 0.40 0.60 0.80 1.00 1.20 Voltage fluctuations of wind generator 690V side,66kV side of 66kV station trans
42、former,220kV side of 220kV station transformer,220kV side of transformer W Wind farm output power Pg(a)The fluctuation curves with power changing from 0 to 1.2 p.u.0.920 0.940 0.960 0.980 1.000 1.020 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 Wind farm output power Pg Voltage of 220kV side of transformer
43、WVoltage of 220kV side of 220kV station transformerVoltage of 66kV side of 66kV station transformerVoltage of wind generator 690V side(b)The fluctuation curves with power changing from 0.92 to 1.03 p.u.Fig.5.The wind power farm output and the fluctuation curves of bus voltages with synchronous compe
44、nsator(STATCOM)used 2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804210000003 Page3/52031 It can be seen from Fig.4 that when SVC is used as compensator there is no significant change in the voltage of bus with the changing of wind power
45、 farm output.Compared with the without compensation device,it raises the outlet voltage level of the wind turbine.Compared with the shunt capacitor bank,it makes exit voltage of the wind turbine and the 66kV bus voltage are more stable.However,the wind turbine exit voltage and 66kV voltage is still
46、slightly reduced when it reached 12s.The curves of each bus voltage fluctuation with the wind power farm output changing is shown in Fig.5 while the synchronous compensator(STATCOM)is used as compensation device in the wind power farm.It can be seen from Fig.5,when STATCOM is used as compensator,wit
47、h the changing of wind power farm output,the bus voltage is almost unchanged,and each bus voltage level is more stable compared with SVC.IV.CONCLUSION In this paper,the power grid voltage stability caused by the power fluctuation of the double-fed wind power farm is analyzed,and the improvement effe
48、ct of different reactive power compensation devices on the grid voltage of stability is analyzed by simulation.The following conclusions can be obtained while the control of the wind power farm and the reactive power have no exchange:1 The double-fed wind power farm has negligible effect on the stab
49、ility of the system voltage when the wind power farms occupy small proportion of whole grid.2 With the condition of manual and mechanical switched capacitor,static VAR compensator(SVC),static synchronous compensator(STATCOM)configuration,when the double-fed wind power farm output increased from 0 to
50、 100%,STATCOM more obviously improve the voltage stability of the grid effect compared with the other two kinds of compensation device.REFERENCES 1 M.G.Gracia,M.P.Comech,J.Sallan,“Modeling wind farms for grid disturbance studies,”Renew-able Energy,vol.33,pp.2109-2121,2016.2 V.Akhmatov,H.Knudsen,“An
