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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 60 61 Abstract Based on different grid development periods this paper proposes a new method on how to select main electrical connection in different development periods The annual investment cost function and minimal cut set reliability model is combined in the proposed method In different 110kV and 220kV grid development periods the main electrical connection model based on much survey is helpful for grid planners to find the fragile part and enhance reliability Thus the research could provide some valuable help in decision making process for planning in power system Index Terms different power grid development periods main electrical connection minimal cut set method reliability assessment economic assessment annual investment cost function I INTRODUCTION N different grid investment and construction periods the DEMAND of grid reliability and economic is different As the main electrical connection in substation is the most important energy transfer point in grid generation transmission and distribution its reliability assessment and economic assessment are very important Firstly this paper gives a definition for different grid development periods Then contrast various reliability assessment methods and composite the advantages of the state space method and the network method the reliability assessment is carried out by analyzing the impact on components state or state combination to the system minimal road Finally the introduction of annual investment cost function is running to select three kinds of main electrical connection schemes After carrying out its economic This work was financially supported by the scientific research project Research on N 1 principle and reliability and economic for 35 220kV power network This project is supported by the State Grid Corporation of China SGCC Hui Hui is with Distribution System Planning Research Centre China Electric Power Research Institute CEPRI Beijing China e mail huihui Sige Liu is with Distribution System Planning Research Centre CEPRI Beijing China e mail sgliu Mingxin Zhao is with Distribution System Planning Research Centre CEPRI Beijing China e mail zhaomingxin Hai Chen is with Distribution System Planning Research Centre CEPRI Beijing China e mail chenhai assessment the final main electrical connection scheme could be confirmed It concludes that the method of selected main electrical connection proves practical II DEFINITION OF DIFFERENT STAGES OF GRID DEVELOPMENT PERIOD Mainly defined by the load and the demand level of power grid the grid construction period could be divided into three stages early stage transition stage and perfecting stage Under normal circumstances when the load level of the transformer or line load level defined as the maximum load rate is below 25 or less this grid construction period could be considered as the early stage when below the level of 25 50 it could be the transition stage and when below the level of 50 or more it could be the perfect stage In addition as the important users have special requirements on the power supply reliability compared to general users the corresponding grid construction should also be ahead of the general users The impact could be represented by the adjustment coefficient k1 On the other hand the electrical equipment reliability has a great impact on the entire grid reliability The overload ability may be appropriate to delay the grid construction period of the corresponding transformer load level The impact could be represented by the adjustment coefficient k2 In summary the division of the grid construction periods could be used the following formula to determine The function is as follows f k1k2T 1 Where T is the maximum load of the grid equipment Then the gird construction period could be defined as follows 25 50 earlyf Grid construction periodtransitionf perfectf 2 The Reliability and Economic Assessment Methods for Main Electrical Connection in Substation Considering Adapting to Power Grid Development Hui Hui Sige Liu Mingxin Zhao and Hai Chen I IEEE PES ISGT ASIA 2012 1569537699 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 60 61 III RELIABILITY ASSESSMENT OF MAIN ELECTRICAL CONNECTION A Models by the minimal cut set algorithm based on components state space method Main electrical connection reliability evaluation uses the improved wiring components model of four states The model is divided into normal status N abnormal status A fault status R scheduled maintenance state M probability of components from the state N to A A probability of components from the state A to M M components maintenance rate M components failure state R probability of components from the state A to R A components repair state R and maintenance repair rate M shown in Figure 1 R R A A M M M Figure1 Diagram for improved wiring components model with four states By using the state space method assume the probability of the component normal status N abnormal status A fault status R and scheduled maintenance state M respectively as PN PM PA and PR As components in any one state are inter exclusion there is PN PM PA PR 1 3 In the steady state the Markov state equation of the components is 0 0 0 0 RAMNMMRR RRRNAA ANAMA MMMAMN PPP PPP PP PPP 4 The probability results of any components state to simultaneous solution of the two equations 3 and 4 are as follows 1 1 RMRMAMAARAM N RMRMAM RARMAA RN RAM AMMMAM MN MAM A AN AM P PP PP PP 5 Isolation switch CT PT and other components are generally unplanned maintenance The consequences of these components faults are same to their connect component faults To simplify the model and improve the computational efficiency these components could be merged in their ends of the bus or circuit breaker accordance with the reliability logic Then the equivalent failure rate eqR and equivalent repair rate eqR of this component are 1 n eqRjR j R R eqReqRjRjR j 6 Where jR fault probability jR j 1 2 n repair rate Through the analysis of the main electrical connection status the changes of the system status and events occurrence are caused by main electrical connection components Thus the reliability calculation must be based on the reliability of the main electrical connection components and the grid topology The system failure probability and frequency could be calculated by using minimal cut set method The expression is as follows Fi i F Fii i F PP C fP C M 7 Where PF fault probability of the system Ci minimal cut set status fF fault frequency of the system Mi metastasis rate of the system from the status of minimal cut set to normal B The reliability assessment process The reliability assessment process is shown in Figure 2 It broadly includes the following establish a network structure use the minimal cut set method to search system failure event then calculate the fault probability frequency and loss of power for each event Finally cumulate calculation results and output them Figure 2 Reliability Assessment Flowchart The specific results are shown in Table I 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 60 61 TABLE I THE TYPICAL MAIN ELECTRICAL CONNECTION RELIABILITY INDEX Wiring Type System Reliable rate System Failure Rate System Average Time between Failures A single bus of one source connection and isolating switch by the power 0 995601 2 860006 3062 93 A single bus connection of one work source one contact power and breaker by the power 0 996735 1 363459 6424 83 A single bus connection with a tie of two work sources and breaker by the power 0 996739 1 383458 6331 96 Two bus connection with two sources breaker by the power and breaker contact 0 998764 0 505928 17314 73 IV ECONOMIC ASSESSMENT OF MAIN ELECTRICAL CONNECTION A Economic assessment of the objective function The total load is different for different main electrical connections in substations To make differences in different main electrical connections unit load in costs is used as the economic assessment target Substation construction and operation costs in the objective function could be expressed as 1 1 1 n IOR n rr CCCC r 8 Where CI investment running costs CO Operating costs CR Risk loss costs n economic useful life r payback Investment running costs of the substation CI include the basic operation and maintenance costs and transformer loss The transformer loss is related to the electricity loss of the substation CI could be calculated as follows Pr 8760 OOMice CCPk 9 Where COM basic operation and maintenance costs transformer loss coefficient P the average load of the substation kW kprice tariffs kwh The expected to lack expectations of energy not supplied EENS multiplied by the unit outage cost are risk loss costs Considering the social benefits EENS could be calculated in accordance with the GDP generated by the unit of electricity Risk loss cost could be calculated as follows Pr 8760 1 RSiceGDP CPPkk 10 Where Ps reliability probability kGDP economic benefits per unit of electricity B Economic assessment program Considering the future demand of the load development this paper aims at different main electrical connections selection in 110kV and 220kV substations below different load levels 10 80 We could choose the main electrical connection scheme in different substations below specific load level In this study the following three kinds of programs are as examples 1 In 110kV substation below the load level of 10 25 respectively compare the single bus connection and single bus connection with a tie the load levels of 15 20 and 25 and transformers capacity with 31 5 MVA 40 MVA and 50MVA were selected For example for a single bus connection with a tie the transformers capacity with 2 31 5 MVA 2 40 MVA and 2 50MVA were selected 2 In 110kV substation below the load level of 25 75 respectively compare the single bus connection with a tie and double bus connection with a tie the load levels of 25 50 and 75 and transformers capacity with 31 5 MVA 40 MVA and 50MVA were selected 3 In 220kV substation below the load level of 25 75 respectively compare the single bus connection with a tie and double bus connection with a tie the load levels of 25 50 and 75 and transformers capacity with 2 120MVA 2 180MVA and 2 240MVA were selected Based on the above calculation method the economic assessment of the three investment programs results are as follows TABLE II THE INVESTMENT RETURNS COMPARISON OF THE TWO MAIN WIRING IN 25 LOAD LEVEL OF 110KV SUBSTATION Load Level Wiring Substation capacity MVA Investment costs 104 Operation costs 104 Risk loss 104 Total costs 104 15 I 31 5 1039 5 75 87 95 59 1210 96 II 2 31 52142 151 74 141 72 2435 46 I 40 1320 82 85 121 39 1524 24 II 2 40 2720 165 70 179 97 3065 67 I 50 1650 91 06 151 73 1892 80 II 2 50 3400 182 13 224 96 3807 09 20 I 31 5 1039 5 84 49 127 46 1251 45 II 2 31 52142 168 99 188 97 2499 95 I 40 1320 93 80 161 85 1575 65 II 2 40 2720 187 60 239 96 3147 56 I 50 1650 104 75 202 31 1957 06 II 2 50 3400 209 50 299 95 3909 45 25 I 31 5 1039 5 93 12 159 32 1291 93 II 2 31 52142 186 23 236 21 2564 44 I 40 1320 104 75 202 31 1627 06 II 2 40 2720 209 50 299 95 3229 45 I 50 1650 118 44 252 89 2021 33 II 2 50 3400 236 88 374 93 4011 81 NOTE I UNIT CONNECTION II SINGLE BUS WITH A TIE TABLE III THE INVESTMENT RETURNS COMPARISON OF THE TWO MAIN WIRING IN 25 75 LOAD LEVEL OF 110KV SUBSTATION Load Level Wiring Substation capacity MVA Investment costs 104 Operation costs 104 Risk loss 104 Total costs 104 25 II 2 31 52142 186 23 236 21 2564 44 III 2 31 52583 186 23 89 53 2858 76 II 2 40 2720 209 50 299 95 3229 45 III 2 40 3280 209 50 113 69 3603 19 II 2 50 3400 236 88 374 93 4011 81 III 2 50 4100 236 88 142 11 4478 98 50 II 2 31 52142 272 46 472 42 2886 88 III 2 31 52583 272 46 179 06 3034 52 II 2 40 2720 319 00 599 89 3638 89 III 2 40 3280 319 00 227 37 3826 37 II 2 50 3400 373 75 749 87 4523 62 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 60 61 III 2 50 4100 373 75 284 22 4757 97 75 II 2 31 5 2142 358 69 708 62 3209 32 III 2 31 5 2583 358 69 268 59 3210 28 II 2 40 2640 428 50 899 84 3968 34 III 2 40 3280 428 50 341 06 4049 56 II 2 50 3300 510 63 1124 80 4935 43 III 2 50 4100 510 63 426 33 5036 95 NOTE II SINGLE BUS WITH A TIE III DOUBLE BUS WITH TWO TIES TABLE IV THE INVESTMENT RETURNS COMPARISON OF THE TWO MAIN WIRING IN 25 75 LOAD LEVEL OF 220KV SUBSTATION Load Level Wiring Substation capacity MVA Investment costs 104 Operation costs 104 Risk loss 104 Total costs 104 25 II 2 120 7920 528 50 899 84 9348 34 III 2 120 8400 528 50 341 06 9269 56 II 2 180 11880 692 75 1349 76 13922 51 III 2 180 12600 692 75 511 59 13804 34 II 2 240 15840 857 00 1799 68 18496 68 III 2 240 16800 857 00 682 12 18339 12 50 II 2 120 7920 857 00 1799 68 10576 68 III 2 120 8400 857 00 682 12 9939 12 II 2 180 11880 1185 50 2699 52 15765 02 III 2 180 12600 1185 50 1023 19 14808 69 II 2 240 15840 1514 00 3599 36 20953 36 III 2 240 16800 1514 00 1364 25 19678 25 75 II 2 120 7920 1185 50 2699 52 11805 02 III 2 120 8400 1185 50 1023 19 10608 69 II 2 180 11880 1678 25 4049 28 17607 53 III 2 180 12600 1678 25 1534 78 15813 03 II 2 240 15840 2171 00 5399 04 23410 04 III 2 240 16800 2171 00 2046 37 21017 37 NOTE II SINGLE BUS WITH A TIE III DOUBLE BUS WITH TWO TIES C Economic assessment conclusion 1 In 35kV 66kV 110kV substations below the load level of 25 or less the section should at least select the single bus connection 2 In 35kV 66kV 110kV substations below the load level of 25 75 the section should at least select the single bus connection with a tie 3 In 220kV substation below the load level of 25 75 the section should at least select double bus connection Through on site practical research the results prove that the above analysis is consistent with the actual demand V CONCLUSION This paper proposes a new method on how to select the main electrical connection in different power grid development periods In three actual planning schemes the advantages and disadvantages of the typical main electrical connection in substation below a variety of voltage levels is analyzed Application results shows that the proposed method is useful for main electrical connection selection in substation VI ACKNOWLEDGMENT The authors gratefully acknowledge the contributions of Zuping Zhang Jian Su and Renmu He for their work on the original version of this document VII REFERENCES 1 J Endrenyi Three State Models in Power System Reliability Assessments IEEE Trans on Power Apparatus and Systems vol 90 pp 1909 1916 1971 2 R Billinton J Oteng Adjei R 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