Steady-State and Dynamic Performance of Oman Transmission System with Diesel-Engine Driven Distributed Generation Omar H

Helwan University

From the SelectedWorks of Omar H. Abdalla

September 5, 2011

Steady-State and Dynamic Performance of Oman Transmission System with Diesel-Engine Driven Distributed Generation Omar H. Abdalla Rashid Al-Badwawi Hilal Al-Hadi Hisham Al-Riyami Ahmed Al-Nadabi

Available at: https://works.bepress.com/omar/12/

Steady-State and Dynamic Performance of Oman Transmission System with Diesel-Engine Driven Distributed Generation

Omar H. Abdalla1, Rashid Al-Badwawi2, Hilal S. Al-Hadi3, Hisham A. Al-Riyami4, and Ahmed Al-Nadabi5 Oman Electricity Transmission Company, P. O. Box 1224, P. C. 131 Al-Hamriyah, Muscat, Sultanate of Oman 1 [email protected] 2 [email protected] 3 [email protected] 4 [email protected] 5 [email protected]

Abstract— The paper presents simulation studies of installing can contribute to improved operation of electric networks [10]. a number of diesel-engine driven generating units at selected DGs can provide ancillary services in power grids [11]. locations in the main transmission system of Oman. A total of This paper concerns with evaluation of the performance of 300MW generator units are proposed on a temporary basis to aid the main electric power grid of Oman with DGs at six in meeting the peak demand in summer 2011. locations. These DGs are required to aid in meeting peak A digital model is developed to simulate the system including central power plants, transmission system, loads and the demand in summer 2011. Benefits of installing new DGs are proposed Distributed Generation (DG). The model includes evaluated. These include improvement in voltage profile, representation of governor and excitation systems. The reducing transformer and line loadings, and reducing active simulation studies are performed by using the DIgSILENT and reactive power losses in the transmission grid. A digital PowerFacrory software package. The objective of the studies is to model of the power system [12] is developed to study the demonstrate effects of the DGs in improving system performance system performance with the added DGs using DIgSILENT in terms of voltage profile, line and transformer loadings, and PowerFacrory software. Section II describes the main transmission losses. transmission system in Oman and the associated generation The sizes and locations of DGs have been selected based on and distribution systems. Section III briefly describes the basis practical considerations; including availability of spacing, short- circuit ratings of existing switchgears, circuit capacity, feasibility of selecting the location and power of each DG. The results of connection, operation noise, site access roads for fuel delivery, are given in Section IV. Proposed key performance indicators environmental effects, etc. A number of performance indicators are given in Section V to provide a basis for evaluating the are defined to assess the contribution of the selected DGs. These improvements gained by employing DGs. Section VI include transformer loading index, line loading index, and describes the practical implementation and operation of the voltage deviation index. In addition, active and reactive DGs in summer 2011. Finally, conclusions are summarized in transmission losses are calculated and used for assessing the Section VII. benefits of the DGs. The results include comparison of the contribution of II. OMAN ELECTRICITY TRANSMISSION SYSTEM individual distributed generation at each site and the case with all proposed DGs. In addition the system dynamic performance is The transmission system extends across the whole of assessed by simulating various DG outages at peak demand. northern Oman and interconnects bulk consumers and electricity generators located in the Governorate of Muscat Index Terms — Distributed Generation, Electricity and in the regions of Batinah, Dhahirah, Dakhliyah and Transmission System, Performance Indicators. Sharquiya [13]. Fig. 1 shows a geo-schematic diagram of the system in 2011. It has two operating high voltages, i.e. 220kV I. INTRODUCTION and 132kV. There has been a growing interest in installing small-scale The main transmission system is supplied with electricity distributed generations (DGs) in electric power systems in generated from eight gas-based power stations located at recent years [1]-[3]. Although most DGs are based on Ghubrah (469MW), Rusail (687MW), Wadi Al-Jizzi renewable energy sources such as wind, solar and fuel cells, (336MW), Manah (279MW), Al-Kamil (297MW), Barka AES conventional small gas turbines and diesel engines can be (434MW), Barka SMN (681MW) and Sohar (605MW). For employed. Distributed generation can provide an alternative to updated generation information, see [14]. Rusail, Wadi Al- electric utility investments in system capacity [1]. Optimal Jizzi, Manah and Al-Kamel power plants have open-cycle gas investment planning for employing DG in a competitive turbines. The remaining plants are of combined-cycle type; electricity market is investigated in [4]. Introducing DGs in a gas and steam turbines. In addition, a number of direct main power system as Independent Power Producers (IPP) can customers are connected directly to the transmission system. help in meeting peak demand [5]. Determination of optimal The transmission system may be supplied from these direct location and size of DG is a key factor of successful operation. connected customers such as Sohar Aluminum Company, Evaluating the impact of network investment deferral on Oman Mining Company, Sohar Refinery Company and distributed generation expansion is explored in [6]. Evaluation Petroleum Development of Oman (PDO). Also, up to 300MW of technical benefits, impacts, and trade-offs of DG are can be supplied from DGs in 2011. investigated in [7]-[9]. DG units with their goodness factors

Legend

UAE Existing OETC Transmission System (2011) Power Station Al Batinah North G

SRC

G A lu 220kV Grid Station m in iu m G S o h SIP-A a r Shinas I 132kV Grid Station G IP Al Bureimi SPS 220kV Double Circuit Liwa 220kV Double Circuit Cable

132kV Double Circuit

Auha 132kV Single Circuit Wooden Pole

132 kV Double Circuit Cable Oman Sea Sohar Private Customer

Mhadah G SIS G (Alwasit)

A Wadi Al Jizzi BA ES RK A SM Saham Al Batinah South N G G Bureimi Muscat Ghoubrah G Khaburah Seeb Main

Madinat Barka

Wadi Sa’a Wave 2 4 Muladah

Qurum AlFalaj MIS

Bawsher Muttrah

Filaj Mabailah MSQ Wadi Kabir Rustaq

Dank Wadi Adai

Alhayl Mawalih Barka Main

Nakhal Airport High

Ibri KSA Al-Dakhiliah Rusail GG Misfah Yitti

Sumail Bahla Jahloot

Madiant Nizwa Ad Dhahirah Nizwa Izki

Al-Sharqiyah

Al Wusta Manah

G G OMIFCO

Sur

Nahada PDO Adam Mudhirib G Mudaybi

Alkamil

JBB Ali

Prepare By: Strategic Planning & Studies Section

Fig. 1. Main Electricity Transmission System of Oman.

The OETC transmission system consists of: In 2010 the system gross peak demand of 3613MW  835 circuit-km of 220 kV overhead transmission line occurred at 15:00 hours on 1 June, which was an increase of  2969.86 circuit-km of 132 kV overhead transmission line about 1.9 % from 2009 peak demand. The transmission system is interconnected at 220kV from  12 circuit-km of 220 kV underground cable Al-Wasit in Mahadah with the transmission system of the  63.798 circuit-km of 132 kV underground cable United Arab Emirate. This should provide increased security  6630 MVA of 220/132 kV transformer capacity of supply and benefits to both countries in the form of cost  9239 MVA of 132/33 kV transformer capacity savings from the sharing of reserve capacity and energy resources.  150 MVA of 132/11 kV transformer capacity

 Two 220 kV interconnection grid stations III. DISTRIBUTED GENERATION  Two 220/132 kV grid stations Various sites for installing distributed generation have been  Five 220/132/33 kV grid stations surveyed to determine the most suitable places. The sizes and  Thirty seven 132/33 kV grid stations locations of DGs have been selected based on practical  One 132/11 kV grid station considerations; including availability of space, short-circuit ratings of existing switchgears, circuit capacity, feasibility of connection, operation noise, site access roads for fuel delivery, The bulk of the power transmitted through the main grid, is environmental effects, etc. Six locations have been selected to fed, through 220/132/33kV, 132/33kV and 132/11kV grid install DGs to assist meeting peak demand in summer 2011. stations, to the three distribution licence holders, i.e. Muscat The locations and powers of these DGs are: Sur (80MW), Electricity Distribution Company, Mazoon Electricity Mudaibi (78MW), JBB Ali (24MW), MIS (19MW), Khaborah Company and Majan Electricity Company. In addition to the (59 MW) and Liwa (40MW) grid stations. The total power is distribution companies a number of large private customers 300MW. The DGs are connected at the 33kV busbars of the are directly connected to the main transmission system at 132/33kV grid stations. All generators are driven by diesel 220kV or 132kV level. engines and connected to the system through step-up transformers.

IV. RESULTS only listed in the table. The 132kV busbars are connected to the 33kV busbars through 132/33kV transformers at grid Table I shows the percentage loading of power transformers stations. The DGs are connected at the 33kV load busbars. It at the concerned grid stations. Significant reductions in should be noted that the Grid Code [15], determines the transformer loadings are resulted by introducing the DGs. To allowable 132kV voltage range to be within ± 10% from its comply with the Transmission Security System Standard, the nominal value. loading on each transformer should not exceed 50%, thus Table IV shows the three-phase and single-phase to ground satisfying the N-1 security criterion. The non-firm loading faults when the DGs are connected to the 33 kV busbars. The conditions on the transformers at Mudaibi (66.41%), MIS fault current at all busbars increase significantly but these (51.18%) and Khaborah (50.55%) are completely removed values are still remaining within the allowable limit of the with the DGs as indicated in the last column. The loadings busbar (25 kA). become <50%. The contribution of each DG is shown in the Table V shows the reduction in transmission losses due to corresponding column. introducing the DGs. Significant reductions in both active and Table II shows the percentage loading of the concerned reactive power losses can be achieved. A reduction of 11.04 transmission lines. Generally, significant reductions in line MW (15.55%) is resulted by installing the DGs at the selected loadings can be achieved by installing DGs at the proposed locations. Also, a reduction of 120.11 MVAr (15.39%) is locations shown in the table. achieved with the DGs. In addition to energy and cost saving, Table III shows the voltage improvement obtained by the reduction in losses can contribute in allowing more flow of adding the DGs to the system. The voltages at all busbars are useful power through the grid. improved. Voltages at some concerned 132kV busbars are

TABLE I TRANSFORMER LOADING (%) Number of Transformers and Loading (%) Grid Station Rated Capacity Without DG JBB Ali Liwa MIS Mudaibi Sur Khabourah All DG (MVA) Sur 2x125 41.23 41.08 41.24 41.23 41.02 15.94 41.24 23.33 Mudaibi 2x63 66.41 66.24 66.44 66.41 23.36 65.99 66.43 33.51 JBB Ali 2x125 47.25 38.27 47.26 47.25 47.02 46.84 47.26 41.45 MIS 2x125 51.18 51.15 51.18 44.60 51.10 51.11 51.02 47.10 Khaborah 2x125 50.55 50.51 50.53 50.47 50.46 50.47 29.09 36.53 Liwa 2x125 40.95 40.93 27.71 40.94 40.92 40.93 40.90 32.57

TABLE II LINE LOADING (%) Loading (%) Number of Circuits and Rated 132 kV Lines Name Without JBB All Capacity (MVA) Liwa MIS Mudaibi Sur Khabourah DG Ali DG Filaj-Barka 2x261 69.78 67.16 70.49 70.50 62.05 62.02 71.29 59.18 Ibri-Dank 2x89 36.30 33.30 38.91 36.74 26.79 26.60 37.64 22.77 Izki-Mudhabi 2x261 20.23 16.00 20.57 20.48 7.69 7.67 20.57 1.44 Khaborah-Saham 2x261 21.41 21.34 22.44 20.85 20.39 20.36 16.64 15.99 Mobalah-Barka 2x261 39.37 36.71 40.11 40.09 31.43 31.30 40.86 28.28 Nizwa-Izki 2x261 20.97 19.76 21.02 20.87 16.58 17.06 20.41 13.56 Rusail-Mobalah 2x261 23.61 21.05 24.32 24.27 16.14 15.92 24.99 13.02 Rusail-Sumail 2x261 30.17 27.09 30.42 30.56 21.83 21.77 31.23 18.83 SIS-Saham 2x261 41.57 41.49 42.58 40.99 40.52 40.50 36.69 36.02 Wadi Sa'a-Dank 2x89 75.03 72.01 77.62 75.47 65.48 65.43 76.38 61.69

TABLE III BUSBAR VOLTAGES (KV) Nominal Voltage Voltage (kV) Busbars Name (kV) Without DG JBB Ali Liwa MIS Mudaibi Sur Khabourah All DG Al Hail 132 122.77 123.14 122.61 122.77 123.72 123.56 122.76 123.97 Dank 132 128.07 128.41 127.93 128.08 129.01 128.86 128.07 129.23 Ibri 132 127.49 127.86 127.32 127.49 128.55 128.36 127.44 128.74 Izki 132 130.91 131.22 130.86 130.92 131.94 131.54 130.88 132.00 JBB Ali 132 131.96 132.94 131.94 131.96 132.66 133.13 131.95 133.85 Mudaibi 132 129.86 130.29 129.81 129.85 131.63 130.74 129.82 131.79 Sumail 132 129.27 129.49 129.24 129.29 129.93 129.72 129.28 130.05 Sur 132 132.28 132.80 132.26 132.28 132.98 134.93 132.27 134.96 khaborah 132 129.38 129.45 129.41 129.54 129.56 129.53 130.34 130.33

TABLE IV THREE AND SINGLE PHASE SHORT CIRCUIT CURRENTS AT CONNECTED BUSBARS Short Circuit at 33 kV Busbar JBB Ali Liwa MIS Mudaibi Sur Khabourah Switchgear Rated (kA) 25 25 25 25 25 25 Three Phase Ik'' (kA) 14.475 20.702 22.798 18.716 18.976 23.679 Single Phase Ik'' (kA) 6.128 9.388 5.803 13.278 13.537 12.471

Fig. 2 and Fig. 3 show the rotor angle and the frequency The TLI is used only for comparing the contribution of responses respectively when the highest distributed generation DGs. at Sur grid station (80MW) tripped. The simulations show that the transmission system is stable and can withstand this type B. Line Loading Index (LLI) of disturbance. The loading index of the transmission lines can be calculated V. PROPOSED PERFORMANCE INDICATORS from the following equation: The benefits of the DGs can be better evaluated in terms of LLI = ∑ (All Line Loadings) ÷ L (2) performance indicators defined as follows: L = Total number of lines = 126. A. Transformer Loading Index (TLI) The transformer loading index is calculated as given in the Fig. 5 shows the effect of the DGs on the line loading index. following equation. A reduction is achieved by using the selected DGs. Again, the line loading index is used only for comparing the contribution TLI = ∑ (All Transformer Loadings) ÷ N (1) of DGs in reducing line loadings. N = Number of transformers in the grid = 110. C. Transmission Losses Active and reactive transmission losses are calculated in the Fig. 4 shows a comparison of the transformer loading index load flow program. Absolute values of losses in MW and (TLI) as the DGs installed at the selected locations. It should MVAr are listed in Table V. Using all DGs at the selected be noted that although the TLI is used to compare loadings, it locations results the lowest losses of 59.95MW and does not mean that the loading of each transformer in the grid 659.88MVAr. is below 50%.

TABLE V ACTIVE AND REACTIVE POWER LOSSES IN THE TRANSMISSION GRDID System Losses Without DG JBB Ali Liwa MIS Mudaibi Sur Khabourah All DG P (MW) 70.99 68.82 71.33 70.91 64.74 64.82 69.94 59.95 Q (MVAr) 779.99 756.42 768.87 771.28 720.89 722.26 754.23 659.88

20.00 DIgSILENT

10.00

0.00

-10.00

-20.00

-30.00 0.00 5.00 10.00 15.00 [s] 20.00 alk GT1-3: Rotor angle with reference to reference machine angle in deg brk GT1-2: Rotor angle with reference to reference machine angle in deg gbr_ST4: Rotor angle with reference to reference machine angle in deg mnh GT4-5: Rotor angle with reference to reference machine angle in deg rsl_GT1-2: Rotor angle with reference to reference machine angle in deg sps GT1-3: Rotor angle with reference to reference machine angle in deg wdj GT3: Rotor angle with reference to reference machine angle in deg Fig. 2. Rotor Angle Response after Disturbance.

50.01 DIgSILENT

50.00

49.99

49.98

49.97

49.96 0.00 5.00 10.00 15.00 [s] 20.00 MSQ 132kV: Electrical Frequency in Hz Nizwa 132kV: Electrical Frequency in Hz Sur 132kV: Electrical Frequency in Hz Al Wasit132kV: Electrical Frequency in Hz Fig. 3. Frequency Response after Disturbance.

43.5 27.5 43 27 26.5 42.5 26 42 25.5 41.5 25 24.5 41 24 40.5 23.5 40 23

Fig. 4. Comparison of Transformer Loading Index. Fig. 5. Comparison of Line Loading Index.

D. Voltage Deviation Index (VDI) The voltage deviation index is calculated by using the VI. PRACTICAL IMPLEMENTATION AND OPERATION following formula: Before summer 2011, the distributed generations at all 2 VDI = √[∑(Vn – V) ÷ B] (3) locations were installed. For example, Table VI shows the

Vn = Nominal voltage at the busbar actual generation, peak load demand and the distributed generation during the period from 16th May 2011 until the 22nd V = Actual voltage at the same busbar May 2011. The DGs contribute supplying the peak load B = Total number of busbars = 53. demand.

Fig. 6 shows the improvement in busbar voltages expressed by the VDI.

VII. 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 4.4 system performance can be achieved by employing DGs. 4.35 Transformer and line loadings have been reduced, and voltage 4.3 4.25 profile is improved. Significant reduction of active and 4.2 reactive losses can be obtained, thus resulting in energy and 4.15 cost savings and relief the transformer and line loadings for 4.1 possible useful power flows. In addition the system dynamic 4.05 performance is assessed by simulating various DG outages at 4 peak demand. A number of performance indicators are defined and calculated to provide a useful basis of comparison of contribution of individual and all DGs. These include the transformer loading index, line loading index and voltage deviation index. Fig. 6. Comparison of voltage deviation index.

TABLE VI ACTUAL GENERATION, MAXIMUM LOAD AND DISTRIBUTED GENERATION DURING ONE WEEK IN MAY 2011 Actual Power (MW) in 2011 Load Demand Actual Generations & Loads Demand 16-May 17-May 18-May 19-May 20-May 21-May 22-May Contracted Generation 3353 3123 3247 3288 3357 3429 3402 Noncontracted Generation 217 338 258 229 239 311 340 Distributed Generation 151 269 238 197 102 238 218 Load Demand 3721 3730 3743 3714 3698 3978 3960

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