Impact of Distributed Generation on Voltage Profile in Deregulated Distribution System

Transcription

1 Impact of Distributed Generation on Voltage Profile in Deregulated Distribution System W. EL-KHATTAM M. M. A. SALAMA Electrical & Computer Engineering, Waterloo University, Ontario, Canada Abstract: Due to the deregulation trend in electric power system networks, many factors have to be taken into consideration such as lack of supplying electric power, system reliability, power quality, electric system losses and voltage disturbance and profile problems. The excessive growing needs for electricity force electrical researchers to implement new approaches through the electric system. Introducing Distributed Generation (DG) in the distribution network is considered to be a promising new approach to solve these problems. DG is capable of providing some, or all of the required power for the demand increase and at the same time improves system s performance. This paper focuses on introducing a new approach to generate power in the distribution network and in addition enhance the distribution system s voltage profile and reduce the electric system losses by installing DG in the distribution system. Results of computer simulations are presented to confirm the proposed ideas. Key Terms: Deregulation, Voltage Profile, Electric Power Losses, Distributed Generation (DG), Distribution System. I. INTRODUCTION Applying deregulation to the electric power system has divided the electric power system into three different categories; electric power generation, transmission and distribution sectors. Each one of these categories owned and operated by a separate company and has its individual identity. The main concerns of implementing deregulation are to reduce the electricity cost especially for retail prices and in the same time improve the power quality of the delivered power to customers. These aims create a worldwide competition in the electricity market to reduce the prices fluctuations, costs in the electric power prices and supply the best customer s service as possible. Transmission companies and distribution comp anies are now operating for profits, this lead to an increase in the retail electricity bills. Electric companies tried to maximize their profits and minimize their costs. This is done by reducing spending on the maintenance, which will lead to a lot of technical problems in the electric network []. The most expensive problems usually appear in the distribution system. Distribution system considered the most expensive part in the electric network []. As load demand densities increase the more complication and problems will occur in the distribution network. Voltage regulation is one of the main problems in the distribution systems especially at the much far-end load and in the rural areas. Voltage regulation and maintaining the voltage level are well known problems in the radial distribution network. Several techniques have been applied by implementing many devices in the distribution network to solve these problems. The most common devices and techniques used are transformer equipped by load tap changer (LTC), supplementary line regulators installed on distribution feeders, shunt capacitor switched on distribution feeders [] and shifting transformers towards the load center []. A substation s LTC transformer equipped with a Line Drop Compensator (LDC) operation for radial distribution voltage regulation is based on the amount of current passing through the transformer. Where, the main objective of LDC is to maintain a certain voltage level for predetermined load values. As the current passing through this transformer increases, the feeder voltage drop increases (and vice versa). So the output voltage of this transformer must increase to readjust the voltage level and to compensate the expected increase of the feeder s voltage drop [,]. Supplementary line regulators installed on distribution feeders are either induction type or step type. Feeder s voltage regulator is used to get a constant voltage at the utilization point which some times called Constant Voltage Point (CVP). The regulation is usually between ± % of the base voltage []. Shunt capacitor switched on distribution feeder is placed at nearly / of the radial feeder length from the substation according to the Two- Thirds Rule and its size will be around / of the total load reactive power in the distribution network []. The main concept of using shunt capacitor is that it regulates the voltage and the reactive power flow at the point of connection with the distribution feeder. By regulating the voltage at the shunt capacitor s connection point, the voltage profile of the far-end feeder point is improved depending on the shunt capacitor size (the amount of reactive power injected by the shunt capacitor). Switched shunt capacitor is considered to be the cheapest device used to improve the voltage regulation and usually place in the distribution feeder combined with voltage regulators in the same distribution network to maintain adequate voltage profile especially for long feeders []. This paper presents a new effective approach to solve the voltage regulation problem, improve the voltage profile along the distribution network and reduce the electric power losses as well. This is done by implementing DG

2 as a source of active power in the distribution network. DG is a new generation technologies such as renewable resources; photovoltaic, wind turbines and hydro, storage energy devices; batteries and flywheel, fuel cells [,]; combined heat and power modules (CHP) and microturbines. Their technologies have become a very important issue. The main purpose of using DG is to supply safe, clean, reliable, low price, reduced losses and more efficient electricity than the traditional centralized power generation electricity. A couple of years ago, as a result of deregulation, some customers are going to install small generation units (DG) at their load locations in parallel with the existing large generation stations. As a result of implementation of DG into an electric distribution system, the system performance and operation will be strongly affected. In the past, distributed generations were available in small size and designed to serve a single end-user s site. But nowadays, distributed generations can be included as large co-generation providing up to hundreds of MWatts to customers or back to the grid. If distributed generation is properly sized, installed and operated [] it can have significant effect in lowering the costs [9], improving the system reliability and power quality and minimizing the substation, transmission and distribution system exp ansion [,] as it will be later discussed in this paper. Distributed generation can be the solution for the future electricity generation by spreading distributed generation units along the distribution network for on-site generation. So the proposed distributed generation change the distribution network from passive network to an active one. Therefore, the new electric power system is no more vertically operated. Recently, due to need for more electricity, deregulation policies, restructuring electricity markets investments and the development of DG technologies, DG can be implemented and operated by utilities and/or customers, which change the electric power infrastructure industry []. This paper consists of three sections. First the system configuration is simulated on computer software in section II. The results are illustrated and explained in section III. Finally, section IV concludes the paper. II. SYSTEM CONFIGURATION A. The System Model The system under study as shown in Figure () is a small portion of a distribution network. It consists of a main feeder and three laterals. Three industrial loads are connected directly to the main feeder at different points while another seven loads are pointed out from the three laterals. The cross section areas of the laterals are different from that of the main feeder. The loads are represented as constant impedance loads. The distribution main feeder and laterals are connected to the rest of the electric system network through a distribution substation s bus (DS) with the voltage level of. kv three phase line voltage. As shown in figure () the main feeder has six feeder-segments, which contain the far-end load point; L, and the much far-end load point; L, in the network under study exists in the four-segment lateral;. A small size controllable [] DG with an output active power of KW [] is used to study its effect on the distribution network s voltage profile, electric power losses and the substation size and loading. The computer simulations are carried out using the PSCAD/EMTDC software. DS Lat L L9 L L L L Fig. (): The Distribution System Under Study B. The Simulation Process First, the simulation is carried out without inserting the DG into the network. The voltage at each load points and laterals connections with the main feeder, the current flows in the main feeder s and laterals sections and the active & the reactive power feed by the distribution substation are measured. Second, the simulation is run with implementing the DG into the network. Again, the voltage at each load points and laterals connections with the main feeder, the current flows in the main feeder s and laterals sections and the active & the reactive powers feed by the distribution substation and the installed DG are measured. Third, repeat the second step six times and at each time vary the location of the DG at several load node points; L, L, L, L, L, L. The output power of the DG is kept constant at all cases. Fourth, place a shunt capacitor switched at load point L. Vary the value of the shunt capacitor until we obtain the best adequate voltage regulation value at the much far-end load; L. Then at that shunt capacitor value we calculate the voltage at each load point, the current flows in the main feeder s and laterals sections, the complex power feed by the distribution substation and the reactive power injected by the shunt switched capacitor to the distribution network. III. THE SIMULATION RESULTS L First, the voltage profile across the whole system is shown in figure (). The voltage values are represented by the voltage along the main feeder () and the three laterals (Lat,, ) in the same graph as a y-axis while the x-axis represent the main feeder s and the laterals L L L

3 segments starting from the distribution substation to the far-end loads in the distribution network under study Lat. Fig. (): The Distribution s Network Voltage Profile Without DG Second, the electric power losses along the main feeder s and the laterals segments are calculated at each segment and added sequentially from the substation till the end of the main feeder and the laterals as shown in figure (). Lat Fig. (): The Electric Network Power Losses Without DG Implementation in the Distribution Network Third, introducing the DG in the distribution network at several points are carried out. Figure () shows the voltage profile through the whole system s nodes under study with connecting the DG at the much far-end load point L Lat. Fig. (): The Distribution s Network Voltage Profile With DG at L The electric power losses are calculated at each segment and added sequentially from the substation till the end of the main feeder and the laterals after introducing the DG at node L as shown in figure (). Lat Fig. (): The Electric Network Power Losses With DG Implementation in the Distribution Network at L It is very clear from figure () that without implementing the DG in the network, the phase voltage at node L (end of segment in the graph), is. KV instead of. KV due to the network loading. Node L has the maximum voltage regulation percentage (.% at load point L) through the entire network. While after connecting the DG in the network at load point L, as shown in figure (), the voltage of the point L is regulated to get the required voltage value and the maximum voltage regulation percentage in the whole distribution network found to be ( %), which occurs at point L. Also, figures (,) show that adding the DG into the distribution network reduces the electric power losses significantly along the main feeder and the laterals in the distribution network. For example, the electric power losses from the distribution substation to the end of that has the far-end load; L, is reduced from approximately. KW to. KW (more than % reduction) by inserting the DG at L. The previous results emphasize the voltage profile improving and the electric power losses reduction all over the distribution network under study by placing the DG into the distribution network. This is because DG injecting active power (current) directly beside the load to satisfy its demand, which in turn reduces the power taken from the distribution substation. As a result of reducing the power taken from the distribution substation, the value of the total current flows from the distribution substation to the loads through the main feeder s and the laterals segments is reduced. The substation current reduction means that the voltage drop across the main feeder s segments is less and the electric power losses in the main feeder s and the laterals segments are greatly decreased by a square proportion. The current injected by the DG will not increase the electric power losses as a great part of it goes directly to the load on which the DG is connected to supply its demand.

4 Fourth, figures (&) give the network voltage profile along and respectively due to the DG connected to different load points; L, L, L, L and L, L Without DG L L. L L L L. Fig. (): The Distribution s Network Voltage Profile Along Points With DG at Different Load Points. Without DG L L L L L L Fig. (9): The Electric Network Power Losses along With DG Implementation in the Distribution Network at Different Load Points Fifth, introducing a shunt capacitor switched at node L and by varying the shunt capacitor s value, the best adequate voltage profile is shown in figure ()..... Without DG L L. L L L L. Fig. (): The Distribution s Network Voltage Profile Along Points With DG at Different Load Points Also, the electric power losses calculated through the longest two paths from the distribution substation to the end of and, as DG connected at different load points are shown in figures (&9) respectively. Without DG L L L L L L Fig. (): The Electric Network Power Losses along With DG Implementation in the Distribution Network at Different Load Points Lat. Fig. (): The Distribution s Network Voltage Profile With installing a shunt capacitor Also the electric system power losses can be calculated as shown in figure (). The value of the shunt capacitor chosen is at C= µf and it injects approximately KVAR, as the value of C increases the injected reactive power increases and may lead to system over compensation. It is clear that the network s voltage profile and the system electric power losses are slightly improved which is due to the low inductive load of the distribution network under study. By increasing the capacitor injected reactive power beyond KVAR, the system voltage profile improves but it was found that the distribution substation s apparent power and the total electric power losses increase. That describe why this capacitor s value has been chosen.

5 Table (): Distribution System s Operating Cases Lat Fig. (): The Electric Network Power Losses With installing a shunt capacitor Different values for the maximum voltage regulation in the entire distribution network under study are shown in figure (). The electric power losses percentage with respect to the total power generated and delivered to the distribution network as shown in figure () due to different distribution system s operating cases as shown in table (). Case () Case () Case () Case () Case () Case () Case () Case () The distribution system operates alone a shunt capacitor connected at L DG connected at L DG connected at L DG connected at L DG connected at L connected at L connected at L VR% Maximum Voltage Regulation % (VR%) Distribution System's Operating Cases Fig. (): The Distribution s Network Maximum Voltage Regulation at Different Operating Cases PL %..... Maxinum Power Losses % (PL%) Distribution System's Operating Cases Fig. (): The Electric Network Powe r Losses Percentage at Different Operating Cases Figure () gives the maximum system s voltage regulation percentage points under the above operating cases. We found that the maximum voltage regulation occurred at point L in cases (,,,,), while happened at point L in case (,,). From figures (,), It is obvious that the distribution system operating cases with DG have the privileges over the case of operating the distribution system alone or with a shunt switched capacitor. DG s locations strongly affect the system performance. The DG s point of connection with the distribution network that gives the minimum voltage regulation percentage is not the same connecting point used to get the lowest total electric power losses percentage. These points are expected to differ according to the distribution network s configuration, loading and the output power of the used DG. This means that to find the optimum DG s location to get the best system performance is not an easy task and need more further investigation and study. Implementing the DG approach in the distribution system achieves great advantages on the distribution system s performance. But the hidden benefit of applying DG approach is to inject active power to the distribution system. So the amount of power taken from the distribution substation will be reduced as shown in figure (), which means that the distribution substation s capacity can be reduced according to different distribution system operating cases as shown in table ().

6 Complex Power (KVA) Distribution System's Equipments Capacities Distribution System's Operating Cases S_Substation S_DG S_Capacitor Fig. (): The Distribution System Equipments Complex Powers at Different Operating Cases Case () Case () Case () Case () Case () Case () Table (): Distribution System s Operating Cases The distribution system operates alone a shunt capacitor connected at L connected at L connected at L connected at L connected at L IV. CONCLUSIONS This paper proves that the DG implementation as a source of active power in the distribution network will change the electric distribution system operation s map. DG mainly provides part of the required demand in the distribution network. In addition to its main purpose, DG has a great positive impact on improving the voltage profile and reducing the total electric power losses through the entire distribution network over the traditional methods. Also, DG reduces the distribution substation required capacities all over the distribution system. [] Gõnen, T.; Electric Power Distribution System Engineering, McGraw-Hill, New York, 9 [] Barker, P.P.; De Mello, R.W., Determining the impact of distributed generation on power systems. I. Radial distribution systems, Power Engineering Society Summer Meeting,. IEEE, Volume:,, Page(s): - vol. [] Ijumba, N.M.; Jimoh, A.A.; Nkabinde, M., Influence of distribution generation on distribution network performance, Africon, 999 IEEE, Volume:, 999, Page(s): 9-9 vol. [] Joon-Ho Choi; Jae-Chul Kim, Advanced voltage regulation method of power distribution systems interconnected with dispersed storage and genera tion systems, Power Delivery, IEEE Transactions on, Volume: Issue:, April, Page(s): 9 [] Del Monaco, J.L., The role of distributed generation in the critical electric power infrastructure, Power Engineering Society Winter Meeting, IEEE, Volume:,, Page(s): - vol. [] Lasseter, B., Microgrids [distributed power generation], Power Engineering Society Winter Meeting, IEEE, Volume:,, Page(s): -9 vol. [] Hadjsaid, N.; Canard, J. -F.; Dumas, F., Dispersed generation impact on distribution networks, IEEE Computer Applications in Power, Volume: Issue:, April 999 [9] Coles, L.; Beck, R.W., Distributed generation can provide an appropriate customer price response to help fix wholesale price volatility, Power Engineering Society Winter Meeting, IEEE, Volume:,, Page(s): - vol. [] Barker, P.P.; De Mello, R.W., Determining the impact of distributed generation on power systems. I. Radial distribution systems, Power Engineering Society Summer Meeting,. IEEE, Volume:,, Page(s): - vol. [] Marnay, C.; Robio, F.J.; Siddiqui, A.S., Shape of the Microgrid, Power Engineering Society Winter Meeting, IEEE, Volume:,, Page(s):, vol. [] Kirkham, H.; Klein J., Dispersed Storage and Generation Impacts on Energy Management Systems, IEEE Transactions Power Apparatus and Systems, Vol. PAS-, No., February 9, Page(s): 9- [] Ackermann, Thomas; Andersson, Göran; Söder, Lennart, Distributed generation: a definition, Electric Power Systems Research, Vol:, Issue:, pp. 9 -, April, VI. REFERENCES [] Ding Xu; Girgis, A.A., Optimal load shedding strategy in power systems with distributed generation, Power Engineering Society Winter Meeting, IEEE, Volume:,, Page(s): -9 vol.