Limiting Short Circuit Currents in Oman Transmission System

Helwan University From the SelectedWorks of Omar H. Abdalla October 18, 2010 Limiting Short Circuit Currents in Oman Transmission System Omar H. Abdalla Hilal S. Al-Hadi Hisham A. Al-Riyami Available at: https://works.bepress.com/omar/49/

Proc. of the 8 th Regional Conference for National Committee of Cigre in the Arab Countries, CC-Cigre Power 2010, Doha, Qata, Paper No. B302, pp. 332-341, 18-19 October 2010 Limiting Short-Circuit Currents in Oman Transmission System Omar H. Abdalla*, Senior Member, IEEE, Hilal Al-Hadi, and Hisham Al-Riyami, Member, IEEE Oman Electricity Transmission Company (Sultanate of Oman) Summary: The paper describes the applications of fault current limiting techniques to Oman electricity transmission system to overcome high short-circuit currents in some parts of the grid. These include splitting busbars at selected grid stations, regrouping generators at power stations, opening transmission lines at critical points, and introducing fault current limiters at strategic places in the network. Computer simulation results, using DIgSILENT software package, are presented to show the effectiveness of these techniques in reducing the shortcircuit currents at critical busbars. The results have shown that the calculated short-circuit currents can be reduced to be within the fault level capacity of the existing switchgears. Splitting busbars and regrouping generators are considered as short-term temporary solutions with no cost. Practical implementations of this technique at Rusail and hubrah power plants are described. Employing fault current limiting reactors is considered as a long-term permanent solution. Keywords: Fault current limiter, Short-circuit currents, Splitting busbars, Transmission system. 1. INTRODUCTION Continuous growth of electricity demand leads to upgrading transmission systems to increase the capability of power transfer. This results in the need for higher fault current capability. Interconnected systems with more parallel paths exhibit reduced source impedances and increased number of sources contributing to fault currents. Fault currents increase also with the introduction of new generation. To avoid damages or malfunctioning system assets and to increase reliability, it is crucial to properly manage increased fault currents [1]. High fault currents produce mechanical forces and thermal effects [2] that can damage or destroy substation equipment, circuit breakers, earthing grids, transmission lines and transformers. Protection and control systems can be badly affected by high fault currents [3]-[8]. Various methods and technologies of fault current limitation are discussed in [1] and [9]. These methods include splitting grids at strategic points, splitting busbars, introduction of higher-voltage levels, use of transformers with increased short-circuit impedance, and installing fault current limiting reactors. Technologies include conventional solid-state, and superconductor techniques. This paper presents simulation studies and field applications of conventional techniques to the main transmission system of Oman in order to solve the high fault-current problem at some locations in the system. A short-term solution with no-cost is to split the HV busbars at grid stations directly connected to power plants. A long-term permanent solution is to install fault current limiting reactors at strategic locations in the network. A digital model of the system is developed based on the PowerFactory DIgSILENT software [10]. Simulation results are presented to show the reduction in fault levels achieved by using the proposed methods. Busbar splitting and generator re-grouping are successfully implemented at two power plants. Sections 2 and 3 review various methods and solutions to reduce fault currents in electric power systems. Sections 4 and 5 describe Oman transmission system configuration and modeling, respectively. Section 6 describes the proposed splitting options. Section 7 illustrates the results of the proposed short-term temporary options. Section 8 presents fault current limiting reactors as a long-term solution. Practical implementation and operational considerations are discussed in section 9. Conclusions are summarized in section 10. 2. LIMITIN FAULT CURRENTS Short circuits in power grids can result in high currents flow in connected equipment. These high fault currents * Email: ohabdalla@ieee.org

can produce high thermal and mechanical stresses that leading to badly damage or even destroy electrical equipment connected to the grid [1], [16] and [17]. The following main power system components can be affected by high fault currents. Transmission Lines Power Transformers as Insulated Substations (IS) Insulators Busbars Circuit Breakers rounding System Protection and Metering Systems Various technologies and methods are available in practice to limit fault currents in electric power systems [9] and [17]. These include: Conventional technologies such as Current Limiting Reactors () and high resistance earthing systems. New technologies such as Solid State Current Limiters (SSCL) and Superconducting Current Limiters (SCCL). Passive methods in which the circuit impedance is increased at both normal and fault conditions. Two main passive methods are available: Topological methods which include splitting of grids, splitting of busbars, or introducing high voltage systems. Apparatus methods using transformers with high impedance to short-circuit, or fault current limiting reactors. Active methods which provide small circuit impedance in normal operation and increased circuit impedance at fault. One method is to use a HV current limiting fuse. The other method is to employ a pyrotechnic device commercially known as Is-Limiter [18]. It is activated by a small explosive charge that opens a link to divert the fault current to a parallel current limiting fuse. An electronic module is used to trigger the device by sensing the rate of rise of the fault current. Novel active methods include: apparatus of positive temperature coefficient, liquid metal FCL, solid-state FCL, hybrid FCL, etc. 3. SOLUTION METHODS Various practical solutions to high fault current problems are briefly described here; more details concerning conventional and advanced solutions can be found in [1]. A. Construction of new substations Utilities may consider construction of new substations with new higher short-circuit rating switchgear to overcome immediate problems of fault current overduty of existing switchgear. This solution can overcome the problems and accommodate future load growth. However, this is relatively the most expensive solution. B. Introducing Higher Voltage Level At higher voltage, the current levels (nominal and short-circuit values) can be readily kept within the standard ratings of commercially available equipment and switchgears. This solution requires major investments and thus will not be a preferred option in many cases. C. Splitting Busbars Splitting busbars at substations can result in separation of power sources that could possibly feed a fault. This can be achieved by opening normally closed bus ties, or splitting existing busbars. This solution can effectively reduce the number of sources contributing to fault currents. In the meantime, the number of power sources that supply grid loads during normal or contingency operating conditions is reduced. Thus, special polices are required for successful operation and control of the power system. D. Splitting the System into Sub-rids In this method the power system is divided into smaller sub-grids at a certain voltage level. These sub-grids are connected to the next higher voltage level. This splitting configuration reduces the short-circuit currents in the sub-grids to the allowable switchgear rating. E. Upgrading Circuit Breakers An expected high fault current level in a transmission system will normally affect more than one circuit breaker. To overcome this problem, utilities might upgrade these breakers with higher fault level ones. However, this solution will not reduce fault currents in the system and requires high replacement cost and in some cases considerable implementation time. This expensive solution might only be justified in cases of aged switchgear that lost acceptable reliability level and requires high maintenance costs. F. Current Limiting Reactors Fault currents can be reduced to the switchgear rating by connecting series reactors in the circuit known as Current Limiting Reactors (). The voltage drop across the terminals of the reactor increases during the fault. Air core reactors can provide an economical choice. Disadvantages of these conventional s include unavoidable voltage drop across them under normal operating conditions and present a constant source of losses. They can interact with other system components and cause instability. Transient voltage can increase to a sever limit so that appropriate capacitors might be required to mitigate this problem. Figures 1 to 4 show possible locations of installing s in power systems. Installing a between bus sections as shown in Fig. 1 is effective in reducing the short-circuit level but not limit individual contributions of the incoming sources. In the configuration shown in

Fig. 2, individual contribution from the feeding sources can be reduced to acceptable level. However, this configuration suffers from high losses and poor regulation. These disadvantages can be overcome by using the configuration shown in the Fig. 3 where a is installed in each outgoing feeder. In Fig. 4, a is installed in each generator feeder, thus limiting the fault current of individual sources. breakers. With this sequential tripping scheme, interrupting excessive fault currents can be prevented. HV Busbar HV Busbar TX1 TX2 TX1 TX2 Figure 1: Connecting between MV bus sections. Figure 4: Connecting in generator feeders. HV Busbar TX1 TX2 Figure 2: Connecting in incoming feeders. HV Busbar The operation is explained as follows. When a fault is detected, the breaker upstream to the source of shortcircuit current is tripped first, CB1 in the configuration shown in Fig. 5. This arrangement reduces the fault current seen by the circuit breaker within the protection zone. This breaker (CB3) can then open safely. A disadvantage of this tripping scheme is that it delays final fault clearing by one tripping stage. Also, opening CB1 affects zones that were not originally impacted by the fault. The overall short-circuit level is not reduced. The reliability decreases and equipment overstress may result over a longer operation time. BB1 BB2 TX1 TX2 CB1 CB2 CB3 Figure 3: Connecting in outgoing feeders.. Sequential Breaker Tripping This method is shown in Fig. 5 and can be used to manage high fault currents without replacing the circuit New Source Existing Source Figure 5: Sequential tripping scheme. Fault

37km 800mm2 XLPE CABLE 2500mm2 XLPE CABLE 2.2km 2500mm2 XLPE CABLE ARCURIA x 2 3 km ARCURIA x 2 240 ZTACIR 4 km 240 ZTACIR 7 km 1 km 7 km YEW x 1 Al Batinah North UAE Auha 47km ARCURIA x 2 33 km ELM x1 Bureimi Shinas Liwa 20 km Mhadah (Alwasit) 33 km ELM x1 28 km Wadi Al Jizzi SRC SIA-1 Smelter 13km 24km 3 km Sohar 28 km SIS SPS 41 km 0.5km Aluminium 30 km 121 km Oman ulf Legend 220kv rid Station 220kv Double Circuit 220kv Double Circuit Cable 132kv rid Station 132kv Double Circuit 132kv Single Circuit wooden pole 132 kv Double Circuit Cable Power Station Saham 40km ARCURIA x 2 Bureimi Ad Dhahirah Al Dhahirah KSA 225 AAAC 43 km Dank 225 AAAC 52km 225 AAAC 54 km Ibri Alhayl Khaburah 54 km Al Batinah South Barka Muladah MIS 12 km 43km 25km 64 km ARCURIA x 2 Filaj 11km Seeb Main Rustaq 59 km houbrah Barka Mawalih Main 15km Rusail 10km 28 km Bawsher Mabailah Al-Dakhiliah Sumail Muscat 9km 35 km 8 km 240 ACSR 8 km 240 ZTACIR ARCURIA x 2 MSQ 46 km AlFalaj 8.2km 8 km Wadi Adai 43 km 3 km Wadi Kabir Jahloot 61km 120km Bahla 32km Nizwa 33 km Izki Al-Sharqiyah 67km ELM x1 20 km Manah 47km 63 km Mudaybi 60km Mudhirib OMIFCO 3km Sur Al Wusta Nahada PDO Adam 51km Alkamil 73km 55km JBB Ali Figure 6: Electricity Transmission System of Oman. H. Impedance Earthing Solidly earthed systems can be converted to impedance earthed systems to reduce ground faults such as single line to ground fault. The following configurations are applicable: (i) Low-resistance earthing, (ii) Inductance earthing, and (iii) High-resistance earthing 4. OMAN TRANSMISSION SYSTEM The existing transmission system extends across the whole of northern Oman and interconnects bulk consumers and power plants [11]. Fig. 6 shows a geoschematic diagram of the system. It has two operating high voltages, i.e. 220 kv and 132 kv. The present OETC transmission system consists of: 665 circuit-km of 220 kv OH transmission lines 2829 circuit-km of 132kV OH 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 Thirty one 132/33 kv grid stations One 132/11 kv grid station The transmission system is interconnected at 220 kv with the transmission system of the United Arab Emirates (UAE). This will provide increased security of supply and benefits to both countries in the form of cost savings from the sharing of spinning reserve capacity and energy resources. The Oman-UAE interconnector will be brought into service in 2011. Internally, the main transmission system is interconnected with other systems such as Sohar Aluminum Company and Petroleum Development of Oman (PDO) [12]. The main transmission system is supplied with electricity generated from eight gas-based power stations located at hubrah (482MW), Rusail (684MW), Wadi Al-Jizzi (290MW), Manah (279MW), Al-Kamil (282MW), Barka AES (434MW), Barka SMN (683MW) and Sohar (590MW) [13]. 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, namely, Muscat Electricity Distribution Company, Mazoon Electricity Company and Majan Electricity Company, in addition to directly-connected large private customers. In summer 2009 the system peak demand of 3546 MW occurred at 15:00 hours on 31 May, which was an increase of 13% compared to 2008 peak demand. 5. SYSTEM MODELIN A digital model of the system is developed [10] based on a commercially available power system simulation

package called PowerFactory DIgSILENT software [14]. In the following, modeling of the main components is summarized. A. Synchronous enerators The system comprises 56 synchronous generators of a round-rotor type in the 8 power stations. The rating of these turbo-generators ranges from 13.4 MVA for the smallest old unit to 280 MVA for the largest unit in the system. Each generator is represented by an 8th order model [15] based on the two-reaction theory. The stator has three windings ABC, and the round rotor has the field winding in the d-axis. Also, it is assumed that the rotor has one damper winding in the d-axis and two damper windings in the q-axis. B. Transformers The generating units in the 8 power stations are equipped with step-up transformers connecting the generators to the corresponding 132 kv or the 220 kv transmission network. Auto transformers of 500 MVA and 315 MVA are used at the interconnection substations between the 220 kv and 132 kv transmission systems. At connection points with the distribution companies, 132/33 two-winding transformers are used in the substations. Most of these transformers are 125 MVA rating; in some smaller substations 63 MVA, 40 MVA or 15 MVA ratings are used. Three-phase power transformers are represented by equivalent circuit models with parameters in ohms or in p.u. The models include representation of the magnetization reactance and iron loss admittance in addition to the leakage reactances and winding resistances. On-load tap changers with their automatic control facilities; and off-load tap changers are simulated in the transformer model. The representations include various connection types and vector groups of transformers. Earthing transformers with associated earthing resistors are also simulated in the model. C. Transmission Lines The system comprises 53 double-circuit transmission lines; most of them are overhead lines and only a few are cables. The majority of these lines are within the short length range; only a few are in the medium length range. Lumped-parameters π-equivalent circuit models are used to simulate the lines. D. Loads An electric power system normally includes residential, industrial and commercial main load types. These may be represented as constant P & Q load, voltage dependant load or dynamical load models. Selection of load representation method depends on the objective of the study and availability of accurate data. In the studies presented here, the load at each substation is represented as constant P and Q model. Contribution of induction motor loads to fault currents is neglected. E. Shunt Capacitors A number of 132/33 kv substations are equipped with capacitor banks at the 33 kv load side to provide reactive power and voltage support. The capacitor banks are arranged in a number of groups called steps (1 to 4); each has a capacity of 5 MVAr. They are set to power factor control mode. 6. SHORT-TERM FAULT CURRENT LIMITATION METHODS Short-circuit studies have shown that, the fault currents levels at Rusail, hubrah, and MSQ grid station busbars are higher than the corresponding short-circuit ratings of the switchgear. The rating of the 132 kv switchgear at Rusail and MSQ grid stations is 31.5 ka. At hubrah, it is 31.5 ka for section A and 26.5 ka for section B. It is observed that single-phase-to ground fault currents are significantly higher than three-phase fault currents at these busbars. Four options are considered here to reduce fault currents. All these methods are available for direct applications by the system operator at no cost. They are considered as short-term temporary solutions and described as follows: Option 1: Disconnect Rusail- Mobella Line The double-circuit transmission line between Rusail power station and Mobella grid station is opened at both sides. This results in eliminating the fault current coming to Rusail from Barka Power Stations through Mobella when a short circuit fault occurs at the 132 kv of Rusail. Option 2: Splitting Busbars at Rusail Power Plant The 132 kv busbars at Rusail power plant are split into two groups: Rusail Busbar roup A: Three generating units: T-1, T-2, and T-6 Three transmission lines: Rusail-Boushar, Rusail-Sumail, and Rusail-Mobellah Four transformers: 75 MVA, 132/33 kv feeding Rusail distribution grid Rusail Busbar roup B: Five generating units: T-3, T-4, T-5, T- 7, and T-8 Two transmission lines: Rusail-Mawaleh, and Rusail-Wadi Adai Option 3: Splitting Busbars at both Rusail and hubrah Power Plants In addition to splitting the busbars at Rusail power plant as shown in option 2, the 132 kv busbars at hubrah power plant are split into two groups:

hubrah Busbar roup A: Seven generating units: T-4, T-5, T-6, T-10, T-11, T-12, ST-4 One transmission line: hubrah-bousher hubrah Busbar roup B: Ten generating units: T-1, T-2, T-3, T-7, T-8, T-9, T-13, ST-3, ST-5, and ST-6 One transmission line: hubrah-msq Figures 7 and 8 show the connection diagrams of this part of the network before and after splitting both Rusail and hubrah busbars. Option 4: Splitting Busbars at both Rusail Power Plant and Madienet as-sultan Qaboos (MSQ) grid station In addition to splitting the busbars at Rusail power plant as shown in option 2, the 132 kv busbars at MSQ are split into two groups as shown in Fig. 9. Figure 7: Before splitting. 7. SHORT-TERM FAULT CURRENT LIMITATION RESULTS A. Short-Circuit Results Table 1 and Table 2 show the results of three-phase and single-phase to ground faults, respectively. If no action is taken, the fault current levels are well higher than the short-circuit rating at Rusail, hubrah and MSQ grid stations. As shown in Table 1, the three-phase fault currents at these busbars are 40.8, 33.0, and 31.8 ka, respectively. Table 2 shows that the single-phase to ground fault currents at the same busbars are much higher. The IEC fault current calculation method is employed. Table 1: Three-phase short-circuit levels I K (ka). rid Stations Max 3-Phase Short Circuit Current I K (ka) No Action 132 kv Busbars: 31.5kA Fault Rated Option 1 2 3 4 Rusail 40.8 34.1 NA NA NA Rusail-A NA NA 26.2 23.9 25.5 Rusail-B NA NA 18.8 17.6 16.2 hubrah 33.0 32.7 31.5 NA 21.1 hubrah-a NA NA NA 14.6 NA hubrah-b* NA NA NA 19.8 NA MSQ 31.8 31.6 30.5 20.8 NA MSQ-A NA NA NA NA 18.5 MSQ-B NA NA NA NA 14.5 Bousher 28.1 27.6 27.0 14.7 20.0 Wadi Adai 25.9 25.4 24.4 18.5 14.0 Wadi Kabir 17.0 16.9 16.4 13.5 10.7 NA: Not Applicable * Fault Rated = 26.5 ka Figure 8: After splitting Rusail and hubrah Figure 9: Splitting both Rusail & MSQ.

Table 2: Single-phase short-circuit levels I K (ka). rid Stations NA: Not Applicable * Fault Rated = 26.5 ka Max 1-Phase Short Circuit Current I k (ka) No Action 132 kv Busbar, 31.5kA Fault Rated Option 1 2 3 4 Rusail 48.3 40.9 NA NA NA Rusail -A NA NA 29.9 27.9 29.4 Rusail -B NA NA 22.2 21.1 19.6 hubrah 40.4 39.9 38.2 NA 27.1 hubrah-a NA NA NA 18.2 NA hubrah -B* NA NA NA 23.4 NA MSQ 36.2 35.9 34.8 24.8 NA MSQ -A NA NA NA NA 21.5 MSQ -B NA NA NA NA 15.4 Bousher 29.6 29.2 28.5 17.4 22.6 Wadi Adai 28.7 28.3 27.4 22.0 16.7 Wadi Kabir 17.5 17.4 17.0 14.8 12.1 Option 1 leads to significant reduction in fault current levels for both 3-phase and 1-phase to ground cases, but fault currents are still higher than the ratings of circuitbreakers, thus this option is not acceptable. Option 2, i.e. splitting the busbars at Rusail, provides better results than option 1. It makes the 3-phase fault current levels below short-circuit ratings except at hubrah bus section rated at 26.5 ka. However, with option 2, the 1- phase fault current level although significantly reduced at Rusail but it is still high at hubrah and MSQ. With option 3, i.e. splitting the busbars at both Rusail and hubrah, the fault level currents are below the short-circuit ratings of the concerned switchgear. This is true for either three-phase or single-phase to ground faults. From the short-circuit point of view, option 3 is the best short-term temporary solution. Option 4 may provide an alternative solution. B. Load Flow Results Load flow studies are performed to determine the impacts of applying the busbar splitting method to Oman main power grid. The DIgSILENT software is employed. Table 3 shows the voltage profile at concerned busbars. In general, the voltages at most busbars are improved with option 3. For example, improvements of 1% to 3% are achieved in Rusail Busbar roup A, hubrah A and B, MSQ, Wadi Adi, and Wadi Kabir grid stations. A reduction of 1% is observed at Rusail Busbar roup A, but the voltage level (0.95 p.u.) is still acceptable. Table 4 shows that there will no significant changes in the transformer loadings due to the application of busbar splitting techniques. Table 5 shows that the percentage line loadings are not significantly changed. (1) (2) (3) 132 kv Busbars Table 3: Voltage profile (pu). No Action Voltage Profile (p.u.) Option 1 2 3 4 Rusail 0.96 0.98 NA NA NA Rusail -A NA NA 0.94 0.95 0.98 Rusail -B NA NA 0.97 0.99 1.01 hubrah 0.94 0.95 0.92 NA 0.98 hubrah-a NA NA NA 0.95 NA hubrah -B NA NA NA 0.97 NA MSQ 0.93 0.94 0.92 0.96 NA MSQ-A NA NA NA NA 0.97 MSQ-B NA NA NA NA 1.00 Bousher 0.94 0.95 0.91 0.94 0.97 Wadi Adai 0.93 0.94 0.91 0.96 0.99 Wadi Kabir 0.92 0.93 0.90 0.95 0.98 Table 4: Sample of transformer loadings (%). 132/33 kv Transformer No Action Transformer loading (%) Option 1 2 3 4 Rusail (1) 49.3 48.5 50.1 49.5 48.2 hubrah (2) 38.5 39.6 37.6 42.1 44.2 MSQ (3) 85.1 84.3 86.8 82.5 81.8 Wadi Adai (3) 52.7 52.4 53.7 51.2 49.5 Wadi Kabir (3) 57.7 57.2 58.6 55.8 53.9 Mawallah (3) 79.3 77.7 78.5 76.4 75.3 4 x 75 MVA Transformers 2 x 42 MVA Transformers 2 x 125 MVA Transformers 8. LON-TERM FAULT CURRENT LIMITATION As a long-term permanent solution for the short-circuit issue, fault current limiting reactors can be used. Various technologies are discussed in [16] and [17]. In fact, it has been already planned to introduce major developments in the main power system in Oman during the years 2010-2013 [11] and [13]. Three new power plants are planned to be in operation by 2013. These are Sohar-II IPP, Barka-II IPP, and New hubrah IWPP. Recently, a new large IWPP has been

proposed at Sur. New 220 kv and 132 kv overhead transmission lines will be constructed, in addition to about 20 new grid stations. A number of existing grid stations will be upgraded by replacing old transformers with larger capacity units. Details can be found in [11]. Line Table 5: Sample of line loadings (%). No Action 132 kv Lines, 261 MVA Circuit Rating Lines loading (%) Option 1 2 3 4 withstand rating. To avoid high fault currents coming from the new hubrah IWPP to old hubrah, Bousher and MSQ grid stations, fault current limiters can be employed. Fig. 10 shows this arrangement. A group of fault current limiting reactors will be installed between the new and old hubrah busbars. Another group of reactors will be inserted in the 132 kv underground cable lines between new hubrah and MSQ grid stations. The fault current levels at all busbars in the grid will be within the corresponding equipment shortcircuits rating, thus removing the high fault current problem completely. Rusail-Bousher 13.1 18.0 NA NA NA Rusail A-Bousher NA NA 14.5 6.2 7.3 Rusail-Sumail 44.8 41.6 NA NA NA Rusail A-Sumail NA NA 37.1 37.1 35.6 Rusail-Mobellah 33.1 NA NA NA NA Rusail A- Mobellah Rusail-Wadi Adai Rusail B-Wadi Adai NA: Not Applicable NA NA 29.2 28.0 15.1 23.7 21.4 NA NA NA NA NA 36.3 29.5 25.4 Rusail-Mowalleh 68.7 67.1 NA NA NA Rusail B- Mowalleh Barka Main- Mobellah NA NA 67.7 66.0 65.0 42.3 17.8 40.9 37.0 29.0 hubrah-bousher 39.8 41.6 33.8 NA 43.2 hubrah A- Bousher NA NA NA 40.5 NA hubrah-msq A 29.2 28.7 21.1 NA 19.6 hubrah B-MSQ NA NA NA 19.5 NA MSQ A-Wadi Adai 69.0 79.0 61.2 59.5 60.4 These developments will lead to major changes in the network configuration. Rusail-Mobellah transmission line will be disconnected; Rusail-Bousher and Rusail- Wadi Adai lines will no longer exist in 2013. Accordingly, the short-circuit problem at Rusail will be completely removed and the 132 kv busbars at this location will be brought back to operate together as normal. At existing hubrah power plant a number of old inefficient generating units will be retired by 2013; thus reducing the number of sources contributing to fault currents. In this case fault currents will be within the short-circuit rating of the existing busbars at hubrah and splitting will no longer be needed. The remaining generating units will be combined at the same busbar and operation will back to normal. For the new hubrah power plant, a new grid station will be constructed with appropriate short-circuit Figure 10: Fault current limiting reactors in 2013. 9. PRACTICAL IMPLEMENTATION AND OPERATION During summer 2009, splitting the busbars at Rusail was fully implemented with successful operation. However, splitting busbars at hubrah was not implemented due to some practical difficulties and to keep security of supply. The splitting scheme at both Rusial and hubrah is currently fully operational (summer 2010). During winter and also at off-peak times, the load demand is low, and therefore splitting the busbars may not be required because only a small number of generating units are operated. The proposed four short-term options give the operator the flexibility to select the most appropriate arrangement for system operation according to the actual situation and status of the network components and load requirements. 10. CONCLUSIONS The paper has described a number of methods to limit fault currents in some parts of the main transmission system in Oman. These include busbar splitting, opening of transmission lines, and the use of fault current reactors.

Simulations studies have shown that splitting the busbars is an effective method to reduce fault currents to be within the equipment short-circuit rating. Splitting the busbars at Rusail and hubrah power plant grid stations has been successfully implemented in the field at no cost. The various short-term options described in the paper provide the system operator with flexible tools to select the most appropriate operational arrangement to avoid the problems of high fault currents. The long-term option using fault current limiting reactors can provide a permanent practical solution to the high fault current problem in transmission networks. 11. REFERENCES [1] Fault Current Management uidebook Update, EPRI, Palo Alto, CA, 1012419, 2006. [2] New implications of power system fault current limits, PSERC Publication 05-62, Oct. 2005. [3] The mechanical effects of short-circuit currents in open-air substations (rigid and flexible busbars), Cigre Brochure no. 105, vol. 1 and 2, April 1996. [4] W. Ruger, Mechanical short-circuit effects of single-core cables, IEEE Trans. Power Delivery, vol. 4, no. 1, pp. 68-74, 1989. [5] N.. Trinh, Risk of burn-through A quantitative assessment of gas-insulated switchgear to withstand internal arcs, IEEE Trans. Power Delivery, vol. 7, no. 1, pp. 225-236, 1992. [6] L. M. Popovi, Efficient reduction of fault current through the grounding grid of substation supplied by cable line, IEEE Trans. Power Delivery, vol. 15, no. 2, pp. 556-561, 2000. [7] Upgrading the EPRI transmission line reference book: Wind induced conductor motion (The Orange Book ), EPRI, Palo Alto, CA, 1010223, 2005. [8] W. J. McNutt, C. J. McMillen, P. Q. Nelson, and J. E. Dind, Transformer short-circuit strength and standards, IEEE Trans. Power Apparatus and Systems, vol. PAS-94, no. 2, pp. 432-443, March/April 1975. [9] P.. Slade, R. E. Voshall, J. L. Wu, E. J. Stubler, and J. Talvacchio, Study of fault-current-limiting techniques, EPRI Report EL-6903, 1990. [10] O. H. Abdalla, Hilal Al-Hadi, and Hisham Al-Riyami: Development of a Digital Model for Oman Electrical Transmission Main rid, Proc. of the 2009 International Conference on Advanced Computations and Tools in Engineering Applications, ACTEA, pp. 451-456, Notre Dame University, Louaize, Lebanon, 15-18 July, 2009. (Available online) IEEE Explore. [11] Five-Year Annual Transmission Capability statement (2009-2013), OETC, (Available online): http://www.omangrid.com. [12] A. Al-Busaidi, and I. French, Modeling of petroleum development Oman (PDO) and Oman electricity transmission company (OETC) power systems for automatic generation control studies, Proc. Int. Conf. on Communication, Computer, and Power, ICCCP 09, Sultan Qaboos University, Muscat, Oman, 15-18 Feb., 2009. [13] OPWP s 7-Year Statement 2009-2015, Oman Power and Water Procurement Company, (Available online): http://www.omanpwp.co.om. [14] PowerFactory DIgSILENT User Manual, http://www.digsilent.de. [15] P. Kundur, Power System Stability and Control, Mcraw-Hill, Inc.1994. [16] Fault current limiters Utility needs and perspectives, EPRI, Palo Alto, CA, 1008696, 1008694, 2004. [17] Survey of fault current limiters (FCL) Technologies Update, EPRI, Palo Alto, CA, 1016389, 2008. [18] H. remmel, and. Kopatsch, Switchgear Manual, 11 th edition, ABB A ermany, 2006.