PROVIDING ANCILLARY SERVICES IN DISTRIBUTION NETWORKS WITH VANADIUM REDOX FLOW BATTERIES: ALPSTORE PROJECT

Transcription

1 PROVIDING ANCILLARY SERVICES IN DISTRIBTION NETWORKS WITH VANADIM REDOX FLOW BATTERIES: ALPSTORE PROJECT Leoold HERMAN Boštjan BLAŽIČ Igor PAČ Faculty of Electrical Engineering, Faculty of Electrical Engineering, Faculty of Electrical Engineering, niversity of Ljubljana Slovenia niversity of Ljubljana Slovenia niversity of Ljubljana Slovenia ABSTRACT This aer deals with voltage regulation in low voltage networks with converter connected hotovoltaic and storage units (Vanadium Redox Flow Batteries VRB). It resents a decouled current control method alied to the roduction of reactive ower of the PV-VRB converter. The aim of this control strategy is to ensure aroriate voltage rofile and increase the integration of renewable distributed generation units into the distribution networks. INTRODCTION In the develoment of electric ower system, countries must achieve ambitious targets to meet key environmental commitments. This will increase the reliability of energy suly, reduce the imact on the environment, and also rovide economic growth and the develoment of jobs and emloyment. Today it is generally acceted that Smart Grids are key element of the future ower systems and an enabling factor to meet the above commitments. The concet of Smart Grids is an ugrade of the existing concet of oeration and design of the ower system. It involves the individual elements of the system, both classical and new elements, such as the distributed generation (DG) resources, advanced measurement systems, flexible rosumers, virtual ower lants, electric cars and energy storage systems. Electricity storage technologies are art of the Smart Grid concet. Since these technologies are at different stages of develoment, the rimary aim of this aer will be to show ossibilities of emloying VRB storage technology that is available on the marker to rovide ancillary services in distribution networks. Some of the functionalities that can be realized using VRB technology are: - Island oeration (IO) caabilities, - Peak ower shaving functionality; - Estimation of savings in the construction (develoment) of the electricity network (reinforcements), if storage units are added to the large consumers; - Transfer of nocturnal energy in use during the day; - The balancing of schedules; - Voltage suort. In this aer, voltage suort functionalities of the VRB technology together with the PV system will be demonstrated by means of numerical simulations. LOT PROJECT Within the AlStore research roject framework, an exerimental rototye has been imlemented in the Slovenian remote Aline Sace area. Main goal of the ilot roject is to show benefits of emloying storage technologies in such areas, where small villages are often sulied by a long (radial) overhead distribution lines. As these lines are heavily exosed to the external influences (trees, snow...) that are causing short-circuits on the line, short- and long-term ower suly interrution are more often than average. Also, connecting DG unit at the end of such lines, can cause unaccetable voltage deviations. First results obtained by numerical simulations show romising caabilities of the roosed control strategies for the future design of such systems. Simlified scheme of the ilot roject (simulation case) is shown in Tab. 1 and Fig. 1. Tab. 1: Basic technical characteristics of the ilot roject. Module Power Other VRB 10 kw 48 V dc PV 10 kw 70 m 2 Electronics 10 kw V dc Fig. 1: Simlified scheme of the ilot roject: flow battery, PV generation unit and electric car charging station. VOLTAGE PROFILE IN LOW VOLTAGE DISTRIBTION NETWORKS Imact of DG units on voltage rofile Fig. 2 shows an examle of a distribution feeder and its voltage rofile. Nowadays voltage regulation in CIRED /5

2 HV MV Z MV a) LV1 LV Z LV LV2 P L, L P G G G -Δ LV1 max 1 -δ min 1) Low load, DG connected 2) High load, DG disconnected b) Fig.2. a) Simle model of a distribution network, b) voltage rofile. distribution networks is carried out mainly by automatic ta changers at the HV/MV transformers. The MV/LV transformers are usually without automatic ta changers, however, deending on the voltage conditions in the network, transformer s ta changer can be adjusted manually under the off-load conditions. The ta changer should be set so that the voltage along the entire length of the distribution line is within the rescribed limits (6/ 10% for the low voltage in Slovenia) [5, 6]. Fig. 2b shows voltage rofile along the feeder for an examle without DG (dotted line). As it can be seen, the voltage is raised at both transformers to ensure the aroriate voltage level. Thus, it can haen that the voltage at the end of the feeder exceeds the maximum allowed value (full line). In rincile, this can be revented by lowering the voltage of the HV/MV transformer and thus bring the voltage at the end of the feeder below the uer limit. However, one should be aware that there are usually several feeders connected to the HV/MV and MV/LV transformers. If there are no DG units connected to these feeders, it is very likely that their voltage will be unaccetably low if the voltage at the HV/MV transformer is lowered [7]. Voltage regulation facilities In networks with distributed generation the following methods of maintaining roer voltage levels are ossible: - reinforcement of the network, - DG reactive ower control, - DG active ower control, - installation of voltage regulators, - use of comensators. Aart from the reinforcement of the network all other methods reresent an active aroach of voltage regulation. For the last two methods from the list installation of some additional equiment is required, while regulating the voltage with active and reactive ower control usually does not require any additional equiment. Distributed generation ower control The voltage dro value along the LV line is given by (see Fig. 2): 2 R P X X P R j L L L L LV1, (1) where LV2 = LV2 0 is set as a reference voltage and P and are, consecutively, the real and imaginary arts of the aarent ower at DG oint of common couling (PCC). Fig. 3 shows the voltage hasor diagram described by (1). From the Fig. 3 it can be concluded that the difference in voltage amlitudes LV1 and LV2 is mainly due to the comonent Δ, which is in hase with the hasor LV1. The difference in hase angle between LV1 and LV2 is mainly due to the imaginary comonent δ. Voltage amlitude at the generator PCC can therefore be aroximated as: X X L G L G LV1, (2) LV 2 where the relations P = P L P G and = L ± G were considered. Assuming the extreme case (P L = 0 and L = 0), the voltage at DG connection oint can be exressed as: LV2 Fig. 3. Voltage dro hasor diagram. X G G LV1. (3) From (3) it can be seen, that the voltage LV2 at the DG PCC may be higher than the voltage LV1 at the MV-side of the transformer and is in direct relationshi with the ower roduced by the DG. Therefore, controlling the DG ower roduction offers a very straightforward method of voltage control. This can be done either by reducing the generated active ower P G or by absorbing the reactive ower G. Maintaining an adequate voltage rofile by regulating the reactive ower is more common in the transmission system, where ower lines are longer and the R/X ratio is lower, which means that the imact of reactive ower on voltage is higher. Desite the fact that the distribution network s R/X ratio is larger (tyically around 1) and the imact on voltage by reactive ower is lower, the reactive ower control can still contribute to maintaining voltage within the limits set by the standards [8-9]. CIRED /5

3 VOLTAGE REGLATION BY DISTRIBTED GENERATION REACTIVE POWER CONTROL In this section a decouled current control method alied to the roduction of reactive ower of a PV-VRB system is resented. Beside the basic function, i.e. delivering the active ower to the grid, the algorithm also enables the reactive ower control. Derivation of the voltage control algorithm The voltage control algorithm is based on the VSC d-q mathematical model [4]. At first, the d- and q-axis current comonents have to be decouled. This is done by removing all the adjustable arameters and elements containing ω and introducing three new variables (v d, v and v dc). The modified VSC mathematical model is given by equations 4, 5 and 6. The model is simlified to three first order functions. R B 0 0 d d v d d R B 0 0 v dt u DC u DC v dc BC 0 0 R C A 1 (4) B u id v d i d B v A 1 u iq v DC u DC C Bi DC, sol k BC 3 k BC Sd S 2 2 q k B Sd k B S q 0 Figures 4 and 5 show the rincile of reactive ower control. A linear droo characteristic is used to calculate the reactive ower set oint ref, using voltage deviation at the PCC of DG as the control inut. Droo control is widely used in multile generator systems as a loadsharing scheme. The droo characteristic describes how a (5) (6) * max max-l reactive comensating unit resonds to voltage changes. Each sloe characteristic is determined by a nominal voltage set oint and two droos (7) and (8). s s c l max c min max max l uantities used in (7) and (8) are as follows: ial reactive ower injection/consumtion, ial (nominal) voltage and max-l, max-c inductive and caacitive reactive ower boundary values. For examle, a 0.1 reactive ower sloe means that a 10 % voltage deviation causes 100% change in reactive ower outut. In general, the sloes (7) and (8) may be different (as shown in Fig. 4) and are determined with the ermitted voltage deviation and with reactive ower boundary values [10]. The DG reactive ower reference values are calculated according to the equations (9) and (10): ref c max c ref l max l c (7) (8), (9) s. (10) s Maximum reactive ower values max-c and max-l can be determined in a fixed manner or deending on the roduction of the active ower [11]. In this aer, reactive ower boundary values are fixed according to the rated aarent ower of the converter. Fig. 6 shows a simlified control algorithm scheme. The inuts to the algorithm are formed by the difference between the reference and actual values of currents (i d l max-c min Fig.4. Voltage control droo characteristic. - max-c max-l Equations ref-c (12), (13) ref-l Fig.5. Reactive ower control block diagram. CIRED /5

4 i d i u DC Δi d Δi Δu DC v d v v DC Decouling v dr v r Calc. of I* d i* d dq/3f Δu L1 Δu L2 Δu L3 u il1 u il2 u il3 Fig. 6. Simlified control system scheme. u L1 u L2 u L3 PWM u DC S VSC and i ) and the difference between the reference and actual value of dc voltage (u dc). The reference value of the current i is defined by ref see Fig. 5. From controllers the necessary voltage dros on the couling inductance and the required dc current to maintain a constant DC voltage are obtained. In the next ste v d and v are decouled and converted to the three-hase system. The reference current i d* is also calculated. After subtracting the actual voltages of all three hases (u il1, u il2 and u il3), we obtain the required voltages to be generated at the PV-VRB VSC outut (u L1, u L2 and u L3). Z 2 S SC TR 1 Distribution network Z SC Bus 2 20 kv 0.4 kv Z 1 Bus 1 SIMLATION RESLTS To illustrate some ractical imlications of the roosed reactive ower outut control and voltage regulation in distribution networks a simulation case is resented. Simulations were carried out in the PSCAD software. The simulated distribution network with integrated PV and VRB system is shown in Fig. 7. The network consists of a ower transformer TR 1 (S TR1 = 630 kva, u SC = 4 %), a PV-VRB system and two radial distribution lines (r = 0.86 Ω/km, x = 0.81 Ω/km, l 1 = 100 m, l 2 = 200 m), modelled with imedances Z 1 and Z 2. The ower consumtion at the end of each line is illustrated by the imedances Z L1 and Z L2 (R L1 = 1.44 Ω, L L1 = H, R L2 = 2.88 Ω, L L2 = H). The remaining distribution network was simulated as a stiff voltage source with a short-circuit imedance connected in series (S SC = 190 MVA). Droo characteristic data were as follows: the ial reactive ower set oint = 0.u. ( = 1.u.), the caacitive droo s C = 0.1 and the inductive droo s L = Simulation results are shown in Tab. 2. The results are given as er-unit values of voltages at bus 1 and bus 2 for four different cases: without DG (case 1), no voltage regulation (case 2), reactive ower voltage regulation (case 3) and voltage regulation by limiting the roduction of active ower (case 4). Due to the low voltage at bus 2, the voltage at the suly transformer is raised to 1.04.u. From the table it can be seen that the voltage at bus 1 exceeds the maximum allowed value of 1.06.u. in the case without voltage regulation (case 2). In case 3, the voltage is lowered using resented reactive ower voltage control. Reactive ower consumtion of 0.38.u. effectively reduces the voltage to 1.03.u. well below the maximum Z L2 limit. The same result can also be achieved by active ower curtailment. The active ower generation has to be limited to 0.22.u. (i.e. 31 % of the rated ower). CONCLSION TR 2 VSC Z L1 In this aer a decouled current control strategy for DG reactive ower voltage control was resented. Simulation results showed that PV-VRB DG with the roosed reactive ower control is caable of effectively comensating the voltage rise, while maintaining constant active ower injection (see Tab. 1). C i DCsol Fig.7. A simlified scheme of the modelled distribution network with shunt-connected VSC with PV-VRB generation system. Tab. 2: Simulation results voltages at buses 1 and 2, and ower injection for four cases in er-unit. Case 1 Case 2 Case 3 Case 4 Bus 1 voltage Bus 2 voltage P Note: 0.4 kv, 50 Hz, 150 kva, base. CIRED /5

5 REFERENCES [1] IEA, Distributed Generation in Liberalised Electricity Markets, Paris, [2] CIRED,: Disersed generation, Preliminary reort of CIRED working grou WG04,. 9, [3] T. Ackermann, G. Andersson, L. Soder, Distributed generation: a defion, Electric Power Systems Research, , [4] B. Blazic, I. Paic, Imroved d-statcom control for oeration with unbalanced currents and voltages, IEEE Transactions on Power Delivery, Vol. 21, No. 1, , [5] HD 472 S1 Nominal, Voltages for Low Voltage Public Electricity Suly Systems, Euroean Committee for Electrotechnical Standardization (CENELEC), [6] EN 50160, Voltage characteristics of electricity sulied by ublic distribution networks, Euroean Committee for Electrotechnical Standardization (CENELEC), [7] C. L. Masters, Voltage rise: the big issue when connecting embedded generation to long 11 kv overhead lines, Power Engineering Journal, Volume 16, Issue 1,.5 12, [8] L. Kojovic, "Imact of DG on voltage regulation," Proc.2002 IEEE/PES Summer Meeting, Chicago, IL, July 21-25, [9] S. Wijnbergen and S. W. H. de Haan, "Power electronic interface with indeendent active and reactive ower control for disersed generators to suort grid voltage and frequency stability," in EPE 2003,. 1-8., [10] S.D. Henry,; D.T. Rizy,; T.L. Baldwin,; J.D. Kueck, Li Fangxing, The alication of droo-control in distributed energy resources to extend the voltage collase margin, ICPS 2008, IEEE/IAS Volume, 1-8, 2008 [11] E.F. Mogos, X. Guillaud, A voltage regulation system for distributed generation, IEEE PES, vol.2, , CIRED /5