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Elektroenergetyczne sieci rozdzielcze – SIECI 2004 V Konferencja Naukowo-Techniczna Politechnika Wrocławska Instytut Energoelektryki Emil DVORSKÝ Pavla HEJTMÁNKOVÁ Miloslava TESAŘOVÁ University of West Bohemia in Pilsen, Faculty of Electrical Engineering Department of Electrical Power Engineering and Ecology, Univerzitní 8, 304 16 Plzeň, Czech Republic e-mail: [email protected], [email protected], [email protected] UTILIZATION OF MULTIPLE PHOTOVOLTAIC POWER GENERATION SYSTEM AT WBU IN PILSEN Photovoltaic cells (PV) are one of the perspective technologies of distributed energy sources (DER). PV grid interconnection must satisfied the technical requirements from both the utility power system grid side and the PV system side in order to ensure the safety of the PV installer and the reliability of the utility grid. Clarifying the technical requirements for grid interconnection and solving the problems are therefore very important issues for widespread application of PV systems. PV grid connection is determinate by many questions. Test results shows that grid interconnection of multiple photovoltaic power generation systems has little effect on distribution line short circuit, AC-DC mixing fault, and output variation, but does affect harmonics and islanding. Further consideration of these effects, especially the islanding conditions, is required. The PV system of the University of West Bohemia in Pilsen has possibility to investigate impact on PV penetration in distribution network and other aspects. 1. INTRODUCTION Small, distributed generation (DG) technologies such as micro-turbines, photovoltaic and fuel cells are gaining wide interest because of rapid advances in technologies. The deployment of these generation units on distribution networks could potentially lower the cost of power delivery by placing energy sources nearer to the demand centres. The capacity of the devices ranges from 1 kW to 2 MW in power level. The instantaneous power production from PV in the near future will often exceed the instantaneous power consumption in residential areas with a high concentration of PV systems. The power flow backwards through the MV/LV transformers, i.e. the power flows from the LV network to the medium voltage (MV) network will increase. There is need to know the impact of the backward power flow to the Electric power system (EPS) and to set the upper limits to of PV amount that can be fed into a power system without causing problems to the power systems and find the possibilities of stretching the limits. The PV system of the University of West Bohemia in Pilsen has possibility to investigate impact on PV penetration in distribution network and other aspects regarding the future PV cells utilisation and interconnection to EPS. This system is one of the greatest systems of Central Europe. Further, the other DER are concentrate in new campus of Faculty of power engineering, such as the wind power plant, heat pumps, combined heat and power unit (CHP). These sources together with storage possibilities and interconnection to EPS and central systems system of municipal power and heat station allow to investigate cooperation integrate power systems. The operation and regulation of the micro grid will be investigated. 128 First the distributed power systems will be step by step equipped by the measure and regulation equipments in dependence on the operation results in order to the backward impact on the interconnected power systems should be minimised and maximal cooperation efficiency achieved. 2. PV LOAD PLACEING Regarding to the load place and the interconnection to the power supply system, the load or DG systems can be divided to: • Stand alone (SAPV systems) – grid off • Grid on (IPV) – interconnected to EPS In SAPV systems (Fig. 1), special attention must be paid to the used appliances and loads. In appropriate loads is very often the origin of PV system malfunction or failure. Start-up power peaks, or reactive power and harmonic distortion can cause system signal instability and protective devices will close the system down. Available solar energy ηP PV modules Regulator Load Loadfactor Variation in insolation Battery Load profile Back-up energy system? Figure 1. SAPV system A well-matched load together with a carefully selected choice of appliances can lead to significant savings in terms of reduced need for PV and electricity storage capacity. Conversely, inefficient appliances and processes, standby loads and inappropriate loads will increase the requirement for expensive PV and storage capacity. In IPV systems, the power flow from PV cells can partly covers the power consumption and the rest is covered by the EPS (Fig. 2). If the PV output is higher then consumption, a part of backward power flow to the EPS. In any case PV cells have influences on the electric power parameters changes. The major issue related to interconnection of distributed resources onto the power grid is the potential impacts on the quality of power provided to other customers connected to the grid. Attributes which define power quality include: • Voltage regulation - The maintenance of the voltage at the point of delivery to each customer within an acceptable range. • Flicker - The repetitive and rapid changes of voltage, which has the effect of causing unacceptable variations in light output and other effects on power consumers and their equipment. • Voltage imbalance - The grid voltage does not have identical voltage magnitude on each phase, and a 120° phase separation between each pair of phases. 129 • Harmonic distortion - The injection of currents having frequency components which are multiples of the fundamental frequency. • Direct current injection - A situation which can cause saturation and heating of transformers and motors, and can also cause these passive devices to produce unacceptable harmonic. PV array System User load Invertor Parray Uarray Grid Pouver Pout,inv P = Pused Pback-up N ~ ~ load Figure 2. IPV system 3. PHOTOVOLTAIC POWER GENERATION SYSTEM AT WBU IN PILSEN The PV system of WBU is the part of the new renewable mini-park sources, which was created in the new campus of Faculty of electrical engineering. The park includes PV systems (IPV, SAPV), solar systems, heat pumps and wind power station. That means, that together with other power means as batteries, heat pumps, thermal load, it can be possible to simulate the DER work and their influences on power quality in the interconnection place. The interconnections of the PV array to EPS and measurements possibilities are shown in Fig. 3. The PV trecker is used for the research activity connected with influence of air pollution on PV cells. In the near future, the system will be equipped with others measurement and regulation means for the purpose of the micro-grid utilization and control. 4. PRESENT INVESTIGATION RESULT Voltage in electric power systems with DER sources One of the problems which are solved at present is setting which PV operation state and interconnection to EPS is suitable and maintenance of the voltage in interconnection place in definite limits. It is the main reason which limits penetration of PV systems in electric power networks. The results of the IEA Photovoltaic Power Systems Programme (PVPS) Task V was applied one the WBU PV system interconnection conditions. The fundamental condition for the computation is that the supply voltage at the delivery point (I J, Fig. 4) must lie within ±10% of the nominal voltage for European LV networks according to EN 50160. Stricter limits may apply nationally, both in Europe and elsewhere. Accepted voltages typically lie between 90% and 106% of the nominal voltage. Stricter limits 130 may apply nationally, both in Europe and elsewhere. Accepted voltages typically lie between 90% and 106% of the nominal voltage. Figure 3. WBU PV system VHV/VHV A B C VHV/HV D E HV/HV F HV/LV G 110 UPPER LIMIT 105 100 95 90 LOWER LIMIT 85 Figure 4. Voltage drops in the central supply EPS H I J 131 The PV penetration can be simplified and limited to the MV and LV due to automatic voltage regulation and therefore approximately 100 % voltage values in point A (Fig. 5). The results are in Fig. 6. As the computation data, the typical values for the EPS were assumed - five LV lines per MV/LV transformer, 10 MV/LV transformers per ring, average values for line impedances, line lengths and transformer sizes. The minimum load is set to 25% of the maximum load. HV EQVIVALENT C2 B2 HV RING D2 HV/LV A 0,4 kV B1 C1 D1 Figure 5. Schema for PV penetration computation Three cases of high PV penetrations are investigated: • high PV penetration from a single LV line (in the order of 30-80 kWp) • high PV penetration from all the LV lines connected to a single MV/LV transformer (in the order of 200-400 kWp) • high PV penetration from all the MV/LV transformers connected to a 10 kV ring (in the order of 1-2 MWp) MAXIMUM PV PENETRATION (% of MAX. LOAD) MINIMUM LOAD 160 140 120 A 100 B 80 C 60 40 20 0 0 10 20 30 40 50 60 70 80 90 LOAD (% OF MAXIMUM LOAD) Figure 6. Overall limits of PV penetration 100 132 In principle, no PV penetration is acceptable at minimum load. However, the excess voltages are rather limited for PV penetrations up to the minimum load. In addition, only a small increase in the load from the minimum load opens up for a considerable amount of PV, especially if PV power only penetrates from a single LV line. Economy of SAPV application 4 3,5 Power needed (kWp) 3 Decreasing PV module cost Grid connection is cheaper 2,5 2 1,5 1 PV is cheaper 0,5 0 0 0,5 1 1,5 2 2,5 Distance from existing grid (km) The SAPV are usually applied in the place with the higher distance from EPS. For the investors and operators, in this case, the economy is important. They are trying to find the best economy solution for the load requests. It mainly depends on costs. The computation can be solved by the objective function creation and the minimum function value finding. It is clear that this function depends on the load place distance from the EPS and the PV cells installation cost (Fig. 7). The present situation will be change by the next PV cells costs decreasing. Figure 7. SAPV economy 5. CONCLUSION In the longer term, it is expected that more flexible and accommodated consumption will remove the barriers of limits to PV penetration. It is essential that PV, together with other elements of distributed generation must be considered in the future network planning. For the fulfillment of the renewable power source increasing in Europe, as was announced by European committee, it is necessary to find the more suitable technology and economy solutions in DER enlargement. Therefore, it is need to do new investigation in the DER utilization and their interconnection to the present power systems. The PV system of WBU in Pilsen has the possibilities to improve PV utilization efficiency in West Bohemia region and make the research and education work on the field cooperation with other European institutions. This paper was written as a part of the GAČR project (GAČR 102/02/0949). REFERENCES [1] [2] [3] [4] Impacts of Power Penetration from Photovoltaic Power Systems in Distribution Networks Report IEA-PVPS T5-10: 2002. Grid -connected photovoltaic power systems, Report IEA PVPS T5-03: 1999. Dvorský E., Hejtmánková P., Mulhbacher J.: Influence of photovoltaic cells interconection to distribution power networks. 13 th International expert meeting “Power Engineering”, Maribor, Slovenia, 2004, ISBN 86-435-0617-6 Noháčová, L. Mühlbacher, J. Noháč, K.: The Problematic of Distributed Power Systems, Second International Conference of Central European Energy, Efficiency and Renewable Energy Sources, Praha, Czech Republic, 2003, ISBN 80-239-1142-2