1、 1 Abstract This paper proposes the dynamic modelling and the coordinating control strategy of the thermal energy storage(TES)based photovoltaic and concentrating solar power(PV/CSP)hybrid power system which is employed in the initial stage of power system restoration.Firstly,a coordinating control
2、strategy of the hybrid system is established in consideration of the dynamic balance of system power and power quality problems during the restoration.Secondly,a CSP station with TES system and the dynamic control system is investigated to diminish system active power fluctuations from illumination
3、changes.And the initial storage capacity of the TES is reasonably evaluated to complete the severest system restoration.Finally,the PV system with STATCOM is designed for the stability of voltage and frequency at the point of PV/CSP integration.In Matlab/Simulink,a model is built in to simulate the
4、transient process of PV/CSP self-start,no-load transformer and transmission line charging,and the start-up of the long-distance auxiliary machine.Result shows that the proposed methods can effectively decrease the voltage and frequency fluctuations and achieve the purpose of smoothing power system r
5、estoration.Index Terms PV/CSP hybrid power system,coordinating control strategy,TES system,power system restoration I.INTRODUCTION he expansion of power system brings challenges to the restoration process after the blackout.The vulnerability of power system structure and the complexity of restoratio
6、n strategy further increase the possibility of restoration failure.Therefore,in order to minimize the accident losses in the power outage,the study on the power system restoration at initial stage is of great significance 1.Photovoltaic(PV)and concentrated solar power(CSP)station,which are two types
7、 of technology with rapid integration in power system and self-starting capability,have great potential for the application in the power system restoration 2.However,the intermittent solar power penetration brings about uncertainty in the power system operation and control This work is part of the p
8、roject titled”Studies on the Key Technologies of Optimal Allocation and Coordinate Operation of Concentrating Solar Power Generations”,which is supported by State Grid Cooperation Company Science and Technology Project under Grant NY71-17-005.system.The actual power output of PV station is significa
9、ntly influenced by environmental factors,which result in the failure of the power system restoration.Reference 3 proposed a control system of the two-level structure battery in PV system to realize voltage regulation.The modeled performance and cost viability of a hybrid grid that utilizes the combi
10、nation of solar photovoltaic(PV),batteries,and fuel cell(FC)systems were explored in 4.Whereas,the small capacity and high costs of electricity storage block the rapid development of the multi-type renewable energy.Literature 5 explored the technical comparison between electricity storage and therma
11、l storage.Thermal energy storage provides broader application perspectives.A combination of different types of energy storage was proposed to achieve the required efficiency of power plant with low cost 6.Literature 7 demonstrates that CSP station with TES system has the self-starting capability and
12、 the flexibility in power system restoration even under the state of complete darkness.Therefore,the hybrid system has the feasibility of being a black start power source,although its cost is relatively higher than PV station 8.The collaboration operation of PV and CSP station,not only enables black
13、-start procedure,but also has economic value.Reference 9 shows that hybrid CSP and PV plants could achieve capacity factors above 70%and provide lower energy costs than CSP-only plants,by reducing the size of the storage and solar field size.Reference 10 presented an approach to obtain the best conf
14、iguration according to market constraints or its willingness to achieve a certain level of capacity factor.This paper is initiated to recover the first auxiliary machine and consider the dynamic power balance and power quality during the restoration.The rest of the paper is organized as follows:Sect
15、ion II presents the configuration of the hybrid power system.In section III,a coordinating control strategy and the separate PV and CSP control systems are proposed.Furthermore,the initial capacity of the TES in the restoration is evaluated.In section IV numerous results and comparison on the typica
16、l hybrid system demonstrate the effectiveness of the proposed control strategies.Finally,conclusions are drawn in section V.Modelling and Control Strategy for Hybrid PV and Central Receiver CSP in Power System Restoration YIQING ZHANG1*,LIANG ZHAO2,MIAO MIAO3,JUNXIAN LI3,TAO SHI2 1.School of Electri
17、cal Engineering,Southeast University,Nanjing 210096,China;2.China Electric Power Research Institute,Nanjing 210003,China;3.State Grid Qinghai Electric Power Company Economic Research Institute,Xining 810008,China T 2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.201
18、8CICED2018 Paper No.201804270000509 Page1/52109 II.POWER SYSTEM RESTORATION PLANNING As shown in Fig.1,PV plants and CSP station are integrated into the power system through a 35kV/330kV transformer.The PV plants mainly include 10 photovoltaic array with a rated capacity of 0.94 MVA each.Each unit i
19、s connected in parallel to a 35kV bus through a 0.48kV/35kV transformer.A static synchronous compensator(STATCOM)is used to provide voltage support.The large-capacity TES is installed in the CSP station.The TES based PV and CSP integrated power system employed in the paper to start the auxiliary mac
20、hine will accelerate power system restoration after power outage.Fig.1 Model of power system restoration considering PV/CSP hybrid system III.MODELLING AND CONTROL STRATEGY During the power system restoration,the frequency is constrained between 49.5 and 50.5Hz,and the voltage is limited between 0.9
21、 and 1.1p.u.The over voltage protection regulation stipulates that the maximum multiple of switching overvoltage is no more than four,and the maximum voltage sag is restricted over 0.7p.u.11.A.Coordinating Control Strategy The coordinated controller operates the real-time collection of PV/CSP and po
22、wer system.It is also responsible for the power distribution.The control system is illustrated as Fig.2.Fig.2 Two Stage coordinating control strategy Where Ppv t and Pcsp t represents actual power output of PV and CSP respectively;Ppv,max t and Pcsp,max t are maximum power output;P*pv,P*csp and P*TE
23、S are the reference values of each unit.N parallel and N series is parallel and series quantity of PV module;P*t,U*t,I*t and f*t are measured statistics of power system.The restoration based on coordinated controller in the hybrid system mainly includes the following steps:Step 1:Based on the data c
24、ollection of PV/CSP system and the power system,the coordinated controller implements power distribution in principle of PV operating in full capacity and CSP compensating power vacancy.Step 2:According to P*pv,P*CSP and P*TES,PV and CSP system modify power output under the respective control strate
25、gy.Step 3:If the voltage at the 35kV grid connection point is out of limit,the STATCOM and the excitation system are adjusted accordingly.The TES system is used to regulate frequency.Step 4:After the transmission line and the load restoration,if the voltage and frequency are modified within the limi
26、t and the overall power of hybrid system satisfies the power requirement of the auxiliary machine start-up,the auxiliary machine starts gradually and then the thermal power units,which perform the restoration process of the subsequent conventional restoration.B.Control Strategy of PV System 1)V/f Co
27、ntrol Strategy V/f control strategy uses a virtual phase-locked loop with a voltage reference value to guarantee the stability of voltage and frequency.This control feature must ensure that the sensitive load nearby continues to work,and guarantee that during the grid reconstruction process,the adde
28、d load will not affect the system operating status.Therefore,in order to facilitate rapid response to load switching and reduce the impact on the system,V/f control has a good tracking response performance 12.2)Modified MPPT Control In order to enable the PV array to track the load fluctuation and p
29、rovide the needed active power.The modified MPPT(Maximum Power Point Tracking)control is as follows:If the maximum power output of the PV system is more than the active power required by the load,the PV adopts the limited power output mode and the power output is equal to the load.If the maximum pow
30、er of PV is less than the active power required by the load,the PV system adopts MPPT mode,and the system power vacancy is supplemented by CSP/TES.C.Modelling and Dynamic Control of CSP System 1)Model Formulation Based on the operation mechanism of CSP,its structure is demonstrated as Fig.3.Solar Fi
31、eldTES systemPower BlocksolartPpbtPTES,ctPTES,dtPS-PtPSF,losstPTES,losstPPB,losstP Fig.3 Structure of CSP station=p p pp p pp p p35kV10.5kV 35kVSTATCOM35kV35kV 330kV330kV 10.5kV380VTESTEGMMRLoadS2S3 S5S6S7MS1S4T1T2T3T4T5T6T7T8T9T10M1M2M3PV System 10MWCSP System 10MWP Photovoltaic ModuleR Central Rec
32、eiver SystemT TurbineE Excitation SystemG GeneratorM Auxiliary MachineL1 L2Coordinated controllerPV station with parallel-series PV arrayCSP station with large-capacity TES system*t t t tP,U,I,f*pv parallel seriesP(N,N)*csp TESP,Ppv pv,maxttP,Pcsp csp,maxttP,PPower SystemControl Signal2018 China Int
33、ernational Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804270000509 Page2/52110 The solar energy received by solar field is expressed as(1).,(1)solart SF SF b nP A I Where SF represents conversion efficiency of solar energy to thermal power;A SF is heliostat mi
34、rror area;I b,n is direct hourly beam solar radiation.Other equality constraints is in(2)and(3).,-,(2)solar SF loss S P TES ct t t t tP P P b P-,(1)(3)S P TES d PB loss PBt t t t tP P P b P Where PSF,loss t and PPB,loss t represents solar power loss and the thermal losses of power block respectively
35、;PSP t is the power flow from solar field to power block through heat-transfer fluid.PTES,c t and PTES,d t represent discharging and charging power of TES;b t is 0-1 variable and it determines the operating state of TES.For the TES,the thermal loss is non-negligible 13 and it is included in the ener
36、gy storage of TES.,-1-1,(-)+(1-)(4)TES TES TES TES ct t t t tTES dttE E tE b P tb P t Where represents the dissipation factor;t is the time interval.The maximum capacity of TES is determined by full-load hours(FLH),and the minimum capacity is set to guarantee the stability of system 14.max max(5)TES
37、 TES TES PBtE E H P Where is the ratio;H TES is FLH;PPB max represents the maximum generated power.The charging and discharging power are adjustable in the limitation.,max0(6)TES c TES ctPP,max0(7)TES d TES dtPP,max max0(8)TES c TES dPP,0(9)SF losstP,0(10)TES losstP,0(11)PB losstP Where PTES,c max a
38、nd PTES,d max represents the maximum charging and discharging power of TES respectively.2)Dynamic Control Strategy of TES System The control principle is that when the output power of PV and CSP exceeds the local load power,the TES mode enters the charging state until the TES is fully charged 15.If
39、the output power of the hybrid system is less than the load power demand,the TES module enters the discharging state to supplement the shortage of power.The control strategy achieves the dynamic conversion of the charging,discharging,and standing by statement of the TES,and determines the reference
40、output value of PV and CSP hybrid system P*,which is used for the optimal distribution of PV and CSP station.The detailed flow chart is demonstrated in Fig.4.Measurement:TES discharging TES standing byOutputNoNo NoYesYesYes*LOADttPP*PV CSPt t tP P P*LOADttPP Optimal distribution maxTES TEStEE maxTES
41、 TES PBtE H P PVtPCSPtPLOADtP+PV CSP LOADt t tP P P*PV CSPt t tP P P TES charging*tP*pv csp TESP P P、Fig.4 Dynamic control strategy of TES system 3)Initial Capacity Design of TES system Under the situation of complete darkness,the initial active power is offered only by the TES after connected to th
42、e grid.In this paper,the TES is considered paralleled to the system at the initial period.After connected to the system,the effective and continuous power supply of TES needs to be considered.The power system restoration mainly includes three stages 16 as shown in Fig.5.In the power system restorati
43、on,the TES system provides active power mainly in the first two stages.For the third stage,it can be considered as a normal restoration process,and the TES system is in the state of power balance.In this paper,according to the traditional restoration stage and the TES dissipation coefficient,the dur
44、ation of the active power supply is set to be at least 5h.The lower limit of TES storage is supposed to be 10%of the total TES capacity.PREPARETION1-2 HoursSYSTEMRESTORATION3-4 HoursLOADRESTORATION8-10 Hours Fig.5 Typical power system restoration Considering the line loss Pline,loss t and the active
45、 power need of the load Pload t,the power output of the TES system and the thermal energy storage of the TES is indicated in Fig.6 and Fig.7.2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804270000509 Page3/52111-,TES,dt0(12)+(13)P(14)sola
46、r S P TES ct t tload line loss PBt t tPBtpbI P P PP P PP Where,PPB tis the output of the power block;pb is the efficiency of the power block;Fig.6 Output power of the TES system Fig.7 illustrates the changing curve of the TES energy under different initial capacity.The reference value of energy stor
47、age is 200MJ.Fig.7 TES energy curve under different initial storage According to Fig.7,the TES initial storage of 60.9%(121.8MJ)and 70%(140MJ)supply power for 3.5h and 4h respectively.The supply hours are less than 5 hours and could not guarantee the power balance,which leads to the failure of the p
48、ower system restoration.Through the simulation in MATLAB,the initial heat storage of TES is at least 77.93%(155.86 MJ)to ensure the 5 hours power supply.IV.CASE STUDY The model is shown as Fig.1.At the nominal environmental condition(1000W/m2,25 C),PV station and CSP station connect to the 330kV tra
49、nsmission line with the load and auxiliary machine linked at the terminal side.The reference value of the power:50MVA,the voltage reference value is the line nominal voltage.The detailed parameter is illustrated in TABLE I.The restoration process and the time arrangement is as follows:Stage 1:self-s
50、tarting of the hybrid system:PV station and STATCOM starting,simultaneously CSP station starting;Stage 2:charging of the transmission line:t=0.2s the no-load transmission line L1,L2 and transformer T8 charging;Stage 3:load restoration and start of the auxiliary machine:t=0.5s the long-distance loads
