Performance of Oman Transmission System with Distributed Generation

Transcription

1 Helwan University From the SelectedWorks of Omar H. Abdalla June 2, 2010 Performance of Oman Transmission System with Distributed eneration Omar H. Abdalla Hilal S. Al-Hadi Hisham A. Al-Riyami Available at:

2 1 Performance of Oman Transmission System with Distributed eneration Omar H. Abdalla* 1, Hilal S. Al-Hadi *, and Hisham A. Al-Riyami * Abstract The paper presents simulation studies of installing distributed generation (D) at a number of grid stations in the main transmission system of Oman. The diesel-engine driven generator units are required on a temporary basis to aid in meeting the peak demand. A digital model is developed to simulate the system including power plants, transmission system, loads and the proposed D. The simulation studies are performed by using the DIgSILENT software package. The objective of the studies is to demonstrate effects of the D in improving system performance in terms of voltage profile, line and transformer loadings, and transmission losses. The results include comparison of the contribution of individual generation at each site and the case with all proposed distributed generators installed. Keywords Distributed eneration, Transmission System Performance. 1. INTRODUCTION There has been a growing interest in installing small-scale distributed generations (Ds) in electric power systems in recent years [1]-[3]. Although most Ds are based on renewable energy sources such as wind, solar and fuel cells, conventional small gas turbines and diesel engines can be employed. Distributed generation can provide an alternative to electric utility investments in system capacity [1]. Optimal investment planning for employing D in a competitive electricity market is investigated in [4]. Introducing Ds in a main power system as Independent Power Producers (IPP) can help in meeting peak demand [5]. Determination of optimal location and size of D is a key factor of successful operation. Evaluating the impact of network investment deferral on distributed generation expansion is explored in [6]. Evaluation of technical benefits, impacts, and trade-offs of D are investigated in [7]-[9]. D units with their goodness factors can contribute to improved operation of electric networks [10]. Ds can provide ancillary services in power grids [11]. This paper concerns with evaluation of performance of the main electric power grid of Oman with Ds at five locations. These Ds are required to aid in meeting peak demand in summer Benefits of installing new Ds are evaluated. These include improvement in voltage profile, reducing transformer and line loadings, and reducing active and reactive power losses in the transmission grid. A digital model of the power system [12] is developed to study the system performance with the added Ds using DIgSILENT software. Section 2 describes the main transmission system in Oman and the associated generation and distribution systems. Section 3 briefly describes the basis of selecting the location and power of each D. The results are given in Section 4. Proposed key performance indicators are * Oman Electricity Transmission Company, P. O. Box 1224, Al- Hamriyah, P. C.: 131, Muscat, Sultanate of Oman. 1 Corresponding author; Tel: , Fax: ohabdalla@ieee.org. given in Section 5 to provide a basis for evaluating the improvements gained by employing Ds. Finally, conclusions are summarized in Section OMAN ELECTRICITY TRANSMISSION SYSTEM The transmission system extends across the whole of northern Oman and interconnects bulk consumers and electricity generators located in the overnorate of Muscat and in the regions of Batinah, Dhahirah, Dakhliyah and Sharquiya [13]. Fig. 1 shows a geoschematic diagram of the system in It has two operating high voltages, i.e. 220 kv and 132 kv. 2.1 Power Plants The main transmission system is supplied with electricity generated from gas-based power stations located at hubrah, Rusail, Wadi Al-Jizzi, Manah, Al-Kamil, Barka and Sohar. For updated generation information, see [14]. Table 1 summarizes the expected capacity of each power plant during the years Rusail, Wadi Al-Jizzi, Manah and Al-Kamel power plants have open-cycle gas turbines. The remaining plants are of combined-cycle type; gas and steam turbines. Import power generations from internal and external sources are also listed in the table. 115 MW and 250 MW are expected from Ds in 2010 and 2011 respectively. The transmission system may import power from direct customers as shown in Table Transmission System The OETC transmission system consists of: 665 circuit-km of 220 kv overhead transmission lines 2829 circuit-km of 132 kv overhead transmission lines 12 circuit-km of 220 kv underground cables 50 circuit-km of 132 kv underground cables 6630 MVA of 220/132 kv transformer capacity 7488 MVA of 132/33 kv transformer capacity 150 MVA of 132/11 kv transformer capacity Two 220 kv interconnection grid stations Two 220/132 kv grid stations Five 220/132/33 kv grid stations

3 2500mm2 XLPE 2500mm2 XLPE 240 ZTACIR 2.25 km 2 Thirty one 132/33 kv grid stations One 132/11 kv grid station 2.3 Distribution System The bulk of the power transmitted through the main grid, is fed, through 220/132/33 kv and 132/33 kv grid stations, to the three distribution licence holders, i.e. Muscat Electricity Distribution Company, Mazoon Electricity Company and Majan Electricity Company. In addition to the distribution companies a number of large private customers are directly connected to the main transmission system at 220 kv or 132 kv level. In 2009 the system gross peak demand of 3546 MW occurred at 15:00 hours on 31 May, which was an increase of about 13% from 2008 peak demand. Table 1 shows the peak demand during the years 2010 to Oman-UAE Interconnection The transmission system is interconnected at 220 kv from Al-Wasit in Mahadah with the transmission system of the United Arab Emirate. This should provide increased security of supply and benefits to both countries in the form of cost savings from the sharing of reserve capacity and energy resources. Table 1. eneration capacity and peak demand (. Expected Power Year eneration Power Stations hubrah PP Rusail IPP Wadi-Jizzi PP Manah IPP Al Kamil IPP Barka-I IWPP Sohar-I IWPP Barka-II IWPP Barka-III IPP Sohar-II IPP New hubrah IWPP Import Sohar Aluminum Company United Arab Emirates Sohar Refinery Company Oman Mining Company Distributed eneration Expected Peak Demand Peak Demand UAE Auha 47km Bureimi 33km ELM x1 Al Bureimi Shinas 20 km Al Batinah North Mhadah (Alwasit) 225 AAAC 37km 33 km Liwa ELM x1 28 km Wadi Al Jizzi Smelter SRC SIA-1 24km 800mm2 XLPE 2.2km 3km 13km Sohar 28km SPS 3km ARCURIA x 2 SIS 41 km 0.5km Aluminium 30 km Saham 121 km 40km Oman ulf Legend 220kv rid Station 220kv Double Circuit 220kv Double Circuit Cable 132kv rid Station 132kv Double Circuit 132kv Single Circuit wooden pole Power Station 132 kv Double Circuit Cable Wadi Sa a KSA Al Dhahirah Ad Dhahirah 43km 225 AAAC Dank 225 AAAC 54 km 52km 225 AAAC Ibri Alhayl Khaburah 54 km MIS Al Batinah South Rustaq Barka Muladah Barka Main 12 km 25km 43km 64km 11 km Filaj 6.3 km 28km Wave Seeb Main Mabailah Rusail Al-Dakhiliah Sumail Muscat 15km 35 km 8 km YEW x 1 10km Mawalih 240 ZTACIR 8 km 2000mm2 1c Cu XLPE 10km 28 km Bawsher 46 km 28km houbrah AlFalaj 2500mm2 XLPE 5km 2500mm2 XLPE 5 km 2500mm2 XLPE MSQ Wadi Adai 8.2km 29km 3 km YEW x 1 43 km Wadi Kabir 6 km Yitti Airport High 40 km 120km Bahla 32km Nizwa 33 km 61km Izki Al-Sharqiyah Al Wusta Nahada PDO 67km ELM x1 Adam 19.7 km Manah 47km 63 km Mudaybi 60km Mudhirib 51km Alkamil OMIFCO 55km 3km 73km Sur JBB Ali Fig. 1. Main electricity transmission system of Oman in DISTRIBUTED ENERATION Various sites for installing distributed generation have been surveyed to determine the most suitable places. The sizes and locations of Ds have been selected based on practical considerations; including availability of spacing, short-circuit ratings of existing switchgears, circuit capacity, feasibility of connection, noise, site access roads for fuel delivery, environmental effects, etc. Five locations have been selected to install Ds to assist meeting peak demand in summer The locations and powers of these Ds are: Sur (27, (22, Mudairib (18, Mudaibi (24, and JBB Ali (24 grid stations. The total power is 115 MW. The Ds are connected at the 33 kv busbars of the 132/33 kv grid stations. All generators are driven by diesel engines and connected to the system through step-up transformers.

4 3 4. RESULTS Table 2 shows the loadings (%) of power transformer at the concerned grid stations. Significant reductions in transformer loadings are resulted by introducing the Ds. To comply with the Transmission Security Standard, the loading on each transformer should not exceed 50%, thus satisfying the (N-1) security criterion. The non-firm loading conditions on the transformers at Mudairib (54.4%), Mudaibi (73.3%), and JBB Ali (53.5%) are completely removed with the Ds as indicated in the last column. The loadings become <50%. The contribution of each D is shown in the corresponding column. Table 3 shows the percentage loading of the concerned transmission lines. enerally, significant reductions in line loadings can be achieved by installing Ds at the proposed locations shown in the table. In particular, over-firm loadings on three transmission lines can be relived. These lines are: 1. Ibri Dank 2. Mobela Barka 3. Wadi Jizzi - Al Wasit The loading on these lines is reduced to be less than 50%, thus satisfying (N-1) security criterion. Table 4 shows the voltage improvement obtained by adding the Ds to the system. The voltages at all busbars are improved. Voltages at some concerned 132 kv busbars are only listed in the table. The 132 kv busbars are connected to the 33 kv busbars through 132/33 kv transformers at grid stations. The Ds are connected at the 33 kv load busbars. It should be noted that the rid Code [15], determines the allowable 132kV voltage range to be within ± 10% from its nominal value. Without the Ds, the voltage at Al Hail grid station, which is 115 kv = 0.87 p.u., breaches the allowable range. Installation of the Ds, although far from Al Hail, brings the voltage at this busbar just within the range. Table 5 shows the reduction in transmission losses due to introducing the Ds. Significant reductions in both active and reactive power losses can be achieved. A reduction of MW is resulted by installing the Ds at the selected locations. This represents a reduction of active power losses of 17.44%. Also, a reduction of 194 MVAr (18.88%) is achieved with the Ds. In addition to energy and cost saving, the reduction in losses can contribute in allowing more flow of useful power through the grid. Table 2. Transformer loadings (%). rid Station Number of Transformers and Rated Capacity (MVA) Without D Sur (27 (22 Loading (%) Mudairib (18 Mudaibi (24 JBB Ali (24 All Ds (115 Sur 2x x Mudairib 2x Mudaibi 2x JBB Ali 2x Table 3. Transmission line loadings (%) Lines Name Rated Capacity (MVA) Number Of Circuits Without D Sur (27 (22 Loading (%) Mudairib (18 Mudaibi (24 JBB Ali (24 All D (115 AlKamil - JBB Ali AlKamil - Sur Barka - Filaj Ibri - Dank Izki - Mudaibi Izki - Nizwa Mobela - Barka Mudairib - Mudaibi MSQ Rusail - Sumail Sumail - Izki Wadi Jizzi - Al Wasit

5 4 Table 4. Busbar voltages (kv). Voltage (kv) Busbars Name Nominal Voltage (kv) Without D Sur (27 (22 Mudairib (18 Mudaibi (24 JBB Ali (24 All D (115 Al Kamil Al Hail JBB Ali Mudaibi Mudairib Izki Yitti Sur Table 5. Active and reactive power losses in the transmission grid. Losses Without D Sur JBB Mudairib Mudaibi Ali All D P ( Q (MVAr) PROPOSED PERFORMANCE INDICATORS The benefits of the Ds can be better evaluated in terms of performance indicators defined as follows: 5.1 Transformer Loading Index (TLI) The transformer loading index is calculated as given in the following equation. 5.3 Transmission Losses Active and reactive transmission losses are calculated in the load flow program. Absolute values of losses in MW and MVAr are listed in Table 5. Using all Ds at the selected locations results the lowest losses of MW and MVAr. TLI = (All Transformer Loadings) N (1) N = Number of transformers in the grid = 99. Fig. 2 shows a comparison of the transformer loading index (TLI) as the Ds installed at the selected locations. It should be noted that although the TLI is used to compare loadings, it does not mean that the loading of each transformer in the grid is below 50%. The TLI is used only for comparing the contribution of Ds. 5.2 Line Loading Index (LLI) The loading index of the transmission lines can be calculated from the following equation: Fig. 2. Comparison of transformer loading index. LLI = (All Transmission Line Loadings) L (2) L = Total number of lines = 105. Fig. 3 shows the effect of the Ds on the line loading index. A reduction of about 4% is achieved by using the selected Ds. Again, the line loading index is used only for comparing the contribution of Ds in reducing line loadings. Fig. 3. Comparison of line loading index.

6 5 5.4 Voltage Deviation Index (VDI) The voltage deviation index is calculated by using the following formula: VDI = [ (V n V) 2 B] (3) V n = Nominal voltage at the busbar V = Actual voltage at the same busbar B = Total number of busbars = 46. Fig. 4 shows the improvement in busbar voltages expressed by the VDI. Fig. 4. Comparison of voltage deviation index. 6. CONCLUSIONS The paper has described the evaluation of a transmission performance by introducing distributed generations at some selected locations in the system. Significant improvements in system performance can be achieved by employing Ds. Transformer and line loadings have been reduced, and voltage profile is improved. Significant reduction of active and reactive losses can be obtained, thus resulting in energy and cost savings and relief the transformer and line loadings for possible useful power flows. A number of performance indicators are defined and calculated to provide a useful basis of comparison of contribution of individual and all Ds. These include the transformer loading index, line loading index and voltage deviation index. TLI N LLI L NOMENCLATURE Transformer loading index Number of transformers Line loading index Number of transmission lines VDI Voltage devistion index B Vn V Number of busbars Nominal voltage Actual voltage REFERENCES [1] Casten, S Assessment of distributed resource technologies. Electric Power Research Institute Report. EPRIEN, Palo Alto, CA: TR [2] Chowdhury, A.A., Agarwal, S.K., and Koval, D.O., Reliability modeling of distributed generation in conventional distribution systems planning and analysis. IEEE Transactions on Industry Applications 39 (5): [3] il, H. A. and Joos, On the quantification of the network capacity deferral value of distributed generation. IEEE Transactions on Power Systems 21 (4): [4] El-Khattam, W., Bhattacharya, K., Hegazy, Y.., and Salama, M.M.A Optimal investment planning for distributed generation in a competitive electricity market. IEEE Transactions on Power Systems 19 (3): [5] Lee, S.-H., and Park, J.-W Selection of optimal location and size of multiple distributed generations by using Kalman filter algoritm, IEEE Transactions on Power Systems 24 (3): [6] Piccolo, A., and P. Siano Evaluating the impact of network investment deferral on distributed generation expansion. IEEE Transactions on Power Systems 24 (3): [7] Chiradeja. P., and Ramakumar, R An approach to quantify the technical benefits of distributed generation, IEEE Transactions on Power Systems 21 (4): [8] Ochoa, L.F., Padiha-Felrin, A., and Harrison,.P Evaluating distribution generation impacts with multiobjective index. IEEE Transactions on Power Delivery 21 (3): [9] Harrison,.P., Siano, P., Piccolo, A., and Wallace, A.R Exploring the trade-offs between incentives for distributed generation developers and DNOs. IEEE Transactions on Power Systems 22 (2): [10] Fu, Y., Li, Z., Shahidehpour, M., Zheng, T., and Litvinove, E., Disco operation considering D units and their goodness factors IEEE Transactions on Power Systems 24 (4): Nov [11] Triggianese, M., Liccardo, F., and Marino, P Ancillary services performed by distributed generation in grid integration, In Proceedings of the International Conference on Clean Electrical Power Capri, Italy, May. Available online: IEEE Xplore. [12] Abdalla, O.H., Al-Hadi, H.S., and Al-Riyami, H.A Development of a digital model for Oman electrical transmission main grid. In Proceedings of the International Conference on Advanced Computations and Tools in Engineering Applications: Notre Dame University, Louaize, Lebanon, July. Available online: IEEE Xplore. [13] Five-Year Annual Transmission Capability Statement ( ) Oman Electricity Transmission Company. Available online: [14] Seven-Year Annual Capability Statement. Oman Power and Water Procurement Company. Available online: [15] rid Code. Oman Electricity Transmission Company Available online: