Research on System Security Coordination Problemiin Multi-level Electricity Market Rui Bo, Yang Li, and Guoqing Tang

Abstract-With further development of restructuring in power industry, it's urgent and necessary to establish multi-level power market in large interconnected power systems in order to better configure energy resources in a larger scope and gain greater economic benefit of power exchange. This paper puts strong emphasis upon system security coordination problem in a multi-level market, which is one ofthe key problems encountered in such environment. After careful analysis and comparison of several proposed alternatives, a sequential security coordination scheme that is based on h+o.sets of coordination information is recommended. Under this scheme the upper-level and lower-level power markct can cooperate smoothly far the system security, and potential power exchanges among lower-level markets can be greatly boosted. The related topics and results of a case study are also introduced. It is concluded that this scheme is,excellent for its high economics, low complexity, fair flexibility and easy generalization. Inder Terms-multi-level power market, system security coordination scheme, coordination information, power exchange, pre-scheduling, transaction scheduling, ATC.

I. INTRODUCTION n the environment of restructuring in power industry, it's suitable for some large countries'and multinational markets to develop multi-level power market in large interconnected power systems. Because of the diversity of energy resources such as generation resource, fuel price, labor price and power consumption level, establishing multi-level power market can be helpful to make better resource configuration and greater economic benefit. In a two-level market, for instance, several lower-level markets constitute an upper-level market, which is designed to facilitate and boost potential power exchanges among the lower-level markets. In some market design, besides lowerlevel markets themselves, generators and bulk customers in lower-level markets can directly take part in the upper-level market. Very few researches have been done on this field up to now. Although there exist a few researches"4' on related topics such as approaches for optimal power exchange in decentralized power systems, the coordination problem between upper-level and lower-level markets hasn't been well

1.

Rui Bo is with the Depamnmt of Electricd Engineenng, Southeast University, Nanjing, 210096 Chinn(e-mail: [email protected]).

Yang Li is with the Department of Eleceical Engineering, Southeast Universir?.,Nanjing, 2 I0096 China(emai1 [email protected] I. Guoqing Tang IS with the Department of Electrical Engineenng, Souheart University, Nanjing, 2 10096 China(e-mail: [email protected]).

addressed. It should be notided that, in multi-level power market, the upper and lower-market are tightly related rather than entirely independent of each other. So when technical problems such as transaction scheduling in upper-level market are examined, their consequential influence on lower-level market must be investigated simultaneously. In other words, both the upperlevel and lower-level markets contribute to the system status, for instance, the system security. This paper therefore puts strong emphasis upon system security coordination problem in multi-level power market. The remainder of this paper is organized as follows: firstly, in the foll?wing section, market organization mode will be introduced and system security coordination problem will be examined more carefully. Afterward, three altematives for solving the problem are proposed and analyzed in section 111. By comparison, this paper will recommend a sequential security coordination scheme that is based on two sets of coordination information. The next section will review the results of a case study under the proposed scheme. The features of this scheme will be summarized in section V. Finally, conclusions will be drawn in section VI. 11. MARKET ORGANIZATION MODEAND SYSTEM SECURITY COORDINATION PROBLEM

In reference [5], we analyze and compare the dispatch principle of two organization modes and their relations: centralized mode and decentralized mode. And it is concluded by proof that, with fully power exchange between sub-systems in an interconnected power system, the decentralized dispatch mode can produce the same optimal result as that in centralized mode. Furthermore, by discussion from various aspects such as power market organization theory, the current situation of power industry, requirements for technical supporting system and economic benefits, the decentralized dispatch mode is recommended as the organization mode for Chinese multi-level power market (named Regional Electricity Market). In an interconnected power system whose market is organized in decentralized dispatch mode, the lower-level market retains control of its own grid and makes its own control decision. Besides, it can achieve greater economic benefits through power exchanges with other lower-markets by participating the upper-level market. Although the grid of a lower-level market is only controlled by the market itself (precisely, the I S 0 of the lower-level

132

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market), power flow on the grid can be envisaged as two components: one i s caused by transactions inside the lowerlevel market: the other by transactions among lower-level markets (including wheeling power). These two types of transactions are scheduled respectively in lower-level market and upper-level market. On the other hand, the two types of transactions are interdependent. For example, when the upperlevel market schedules transactions among lower-level markets, it needs to know the transactions inside the lowerlevel market so as to consider system security of the whole grid (including both the grids o f lower-level markets and the tie-lines connecting lower-level markets). However, when lower-level market perform scheduling for inner transactions, it also needs to know the exchange power scheduled in upperlevel market. So, the maintenance of system security requires the cooperation between upper-level and lower-level markets. It i s a new problem, that is, how to design a coordination mechanism under which the upper-level and lower-level power markets can cooperate smoothly for the system security, and at the same time potential power exchanges among lowerlevel markets could be greatly boosted. In fact, this problem i s one of the key problems encountered in multi-level power market in the restructuring environment.

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111. DISCUSSION OF ALTERNATIVES TO THE PROBLEM AND FINAL RECOMMENDATION

B. A TL' b a . d s q i r d n i r d roorrlinutron schtnrr

T h r e e possible solutions to the system coordination problem

are proposed and analyzed in the case of transaction scheduling. A. 1feralive coordination scheme

In order to assure the security o f the whole grid, one alternative for the upper-level and lower-level dispatching systems i s to cooperate in a n iterative way as shown in Figure 1. Under this scheme, the upper-level market negotiates with the lower-level markets about the secure power exchange in an iterative way. However, this scheme requires tight coupling o f the upperlevel and lower-level markets and frequent, iterative data communication between them. Thus it burdens much with the supporting systems. Moreover, the convergence speed and characteristics of corresponding solution algorithm is uncertain because of the iterative mode. So this scheme is unpractical in application.

To o\ercomc the disadvantages o f the iterative coordination scheme, this paper proposes 3 sequential scheme instead. Undcr this scheme, luwcr-level markcts submit coordination information to the upper-lcvel market. and hy these infurmarim the upper-level market ;an csiimate thr. intluencc o f power exchange m w n g lower-level niarkels on the securit) of the grids o f loner-level markets. In other words, the coordination iniurmation takes pl3cr o f iterative ncgotiatim in the prcvious scheme in pan A and makes the sequential pruccss possible as s h w n in Figure ?. hlxker Open . .. -.

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The scheme is classified into two types by coordination information: one is based on ATC and the other is based on two sets of factors (i.e., LCPP and BPPF) proposed later. The former will be discussed in this part and the other in part C. Since ATC represents available transmission capacity, it 'can be used for validating scheduled transactions from the system security perspective. For ATC based sequential coordination scheme, we design three types of ATC information for power grids of lower-level markets: Power purchase ATC (denoted by ATCIA),Power sale ATC (AT'&) and Wheeling ATC (ATCw), respectively representing the maximum purchasing capacity, selling capacity and wheeling capacity of the grid subjected to the network security constraints. Power purchase ATC of the grid of a lower-level market can be calculated by standard ATC computation algorithms with considering the load buses in the grid as power consumption area and 'considering the-outside grids as power generation area. Power sale ATC can be calculated with reverse roles considered. Wheeling ATC can be worked out in a similar way. For instance, as shown in Figure 3, the Wheeling ATC of the grid of lower-level market A can be calculated by standard ATC algorithms with considering boundary bus 1 as the generation bus and bus 2 as the power consumption bus. The results, denoted by 21TCiJ2, represents the maximum wheeling capacity from bus 1 to bus 2 undertaken by the grid of lowerlevel market A. c '\Grid of lower-level market B

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Fig. 3. ATC based coordination information

With Power purchase ATC and Power sale ATC submitted by one lower-level market, the upper-level market can validate the purchase and sale amount of power exchange between the lower-level market and others. Likewise, the upper-level market can validate the wheeling power undertaken by lowerlevel markets with the Wheeling ATC they submit. As a matter of fact, with Power purchase ATC and Power sale ATC, the grid of a lower-level market can be viewed as a virtual bus whose maximum generation and consumption capacity are represented by ATCTBand ATCls respectively; with Wheeling ATC, the grid of a lower-level market can be viewed as a virtual transmission channel (in Fieure 3 for

and thus need revising. 2) For lower-level markets with several tie-lines or more than two boundary buses such as market B in Figure 3, no uniform Wheeling ATC can be defined because Wheeling ATC values vary in different wheeling paths. If more than one Wheeling ATC are defined, it will be difficult to use them; ifthe minimum Wheeling ATC is selected for use, the results may be too conservative. 3) The value of Power purchase ATC, Power sale ATC and Wheeling ATC are not usable simultaneously since they are usually calculated in the same base case. C. LCPP and BPPF basedseqirenrial coordination scheme

Under this scheme, each lower-level market submits two types of coordination information. With this information, the upper-level market can evaluate the network security of the whole grid when performing transaction scheduling. The first type of coordination information is called Line Capacity Participation Percent (LCPP), which is defined as the percent of transmission lines capacity of the grid of lowerlevel market available for carrying power exchange transactions among lower-level markets. The other type of coordination information is called Bus Power Participation Factor (BPPF) which represents the proportion of contributed power of each bus in undertaking the power exchange transactions. BPPF can further be classified into two kinds: one kind related with sold power is called Generation Contribution Factor (GCF); the other kind of BPPF which is related with purchased power is called Load Extraction Factor (LEF). It should be noted here that it's proper to define both GCF and LEF at generation buses because whether the lowerlevel market sells or buys power, the influenced buses are always generation buses. LCPP reflects the remaining available capacity of transmission lines in the grid of lower-level market afler prescheduling of the market with zero power exchange transactions. BPPF reflects how the pre-scheduling results of the lower-level market will change in terms of different level of power exchange transactions. So, with these two types of coordination information, the upper-level market is able to know how power flow in the grid of a lower-level market will change with power exchange transactions and furthermore estimate the network security of the grid. It should be pointed out that this paper mainly discusses transmission constraints of network security. Here give a simple example to illustrate the principle of this scheme. Suppose the two-bus system in Figure 4 represents the grid of a lower-level market A. The network and load parameters are labeled in the figure.

Grids of other lower-level markets

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with zero power exchange transactions, it’s found that line 1-2 still have 40 percent available capacity. Also, it’s obvious that bus 1 will undertake all the possible power exchange transactions since there is no generator at bus 2. Based on these facts, lower-level market A will submit the following data to upper-level market: LCPP of line 1-2 is LCPPlp=40%; BPPF at bus I is a,=I,BPPF at bus 2 is UFO. With these coordination information instead of the detailed pre-scheduling result of each lower-level market, the upperlevel market can deduce as follows the maximum power sale amount that the network of lower-level market A can undertake: firstly, a,=l and a,=O represent that the power exchange transactions will be totally provided at bus 1 and the power will flow through line 1-2 as can be seen from Figure 4; then, LCPPl,=40% indicates that line 1-2 have 40 percent capacity available. So, it can be concluded that the maximum sale amount of lower-level market A must be no more than

* 40% = 40MW ;otherwise, transmission constraints 1 will be violated. This conclusion is identical to the truth. From the above example, we can see that the proposed security coordination scheme can achieve accurate results and be used for transaction scheduling as long as the submitted coordination information is accurate enough. looMW

D. Recommendation of coordination scheme Grounded on the analysis of three alternatives discussed above, we recommend the sequential coordination scheme that is based on LCPP and BPPF. The result of a case study under this scheme will be briefly reviewed in the next section and characteristics of the scheme will be discussed in section V. Elaborate description, definition and calculation method of LCPP and BPPF can be referenced in [SI. The mathematical model of transaction scheduling for the upper-level market is also introduced in [ 5 ] . IV. REVIEWOF A CASE SlIJDY UNDER RECOMMENDED SCHEME

The sequential coordination scheme that is based on LCPP and BPPF is simulated on IEEE 30 bus system in [6].Here we review some important conclusions ofthe case study. The results show that the proposed scheme and related algorithms are feasible to maintain the system security of the whole grid, and that it can achieve great economic benefit that is near to the ideal optimal results obtained under centralized dispatch mode. It indicates that under the proposed scheme, power exchanges are fully boosted, which rightly meets one of the original intentions of the establishment of multi-level power market.

v.

FEATURES OF THE RECOh4MENDED SCHEME

The proposed sequential coordination scheme that is based on LCPP and BPPF has the following merits: I) Having the characteristics of both the centralized and decentralized dispatch mode On one hand, under the proposed scheme, the multi-level power market is organized in decentralized mode, that is, each

lower-level market retains control of its own grid. On the other hand, with the help of coordination information submitted by lower-level markets, the upper-level market can take the security of the whole grid into account and seeks for benefit optimization for the whole system, as if organized in centralized dispatch mode. Moreover, the case study reviewed in section IV shows that the scheme can reach the same optimal result as that in centralized mode under certain condition. ’ 2) Low complexity Under this scheme, each lower-level market should only submit a little amount of information to the upper-level market. The complex dispatch characteristics of each lower-level market are replaced by this simple information and thus greatly reduce data communication between the two level markets, and also reduce the complexity of the related algorithms (such as security validation, transaction scheduling and congestion management) in upper-level market. So the scheme is suitable for multi-level power market especially in large interconnected power systems and even the future National Power Market. 3) Fair flexibility and easy generalization The scheme is flexible to changes. For example, when generators and bulk customers in lower-level markets are permitted to participate in the upper-level market directly, they can be easily incorporated into the scheme with the only requirements to submit coordination information. Generally speaking, this scheme is suitable for the mixed structure of coexistence of market members based on bus and those based on grid. VI. CONCLUSION

Multi-level power market is now gradually becoming important in power industry, especially in large countries such as China. Members in interconnected power systems can achieve great economic benefit by power exchange with neighbors through multi-level power market. This paper firstly introduces market organization mode in multi-level market and puts forward the problem of system security coordination. Then three alternative solutions are proposed an iterative coordination scheme, a sequential coordination scheme that is based on ATC and a sequential coordination scheme that is based on LCPP and BPPF. By careful analysis and comparison, the sequential coordination scheme that is based on LCPP and BPPF is recommended. With this information, the upper-level market can estimate the influence of power exchange among lowerlevel markets on the security of the grids. A review of a case study under this scheme shows the scheme is feasible to maintain the system security of the whole grid, and it can greatly boost power exchanges. At the end of this paper, it is concluded that the recommended scheme is advantageous for high economics, low complexity, fair flexibility and easy generalization.

135

VII.

REFERENCES

[I] K. W.Dory, P. L. McEntire, " An analysis of electnc power brokerage system," IEEE Trans Power Apparatus ondSysrems, vol. PAS-IOI, no. 2, pp. 389-396, Feb. 1982. [2] Mark I. Huggins, Michael S. Mink, I' Optimal energy transactions in interconnected electric systems," IEEE Trons. Power Apporolus and Systems, vol. PAS-104, no. II,pp. 2994-3003, Nov. 1985. [3] George Fahd, Gerald B.'Shebls, " Optimal power flow emulation of interchange brokerage systems using linear programming," IEEE Tram PowerSyslems, YOI. 7, no. 2, pp. 497-504, May. 1992. [4] Fuyuhiko Nishimura, Richard D. Tabors, Man@ D. Ilic, Jaaquin R. Lacalle-Melem, Benefit optimization of centnlized and decentralized power systems in a multi-utility environment," IEEE Trans. Power Syslems, vol. 8, no. 3, pp. 1180-1 186, Aug. 1993. [5] 'Bo Rut, " Research on transaction scheduling and congestion management in regional electricity market," M.S. dissertation, Dept. E l s . Eng ,Southeast Universily, China, ZOO?. [6] Rui Bo, Yang Li, Fubin Liu, " A Novel Coordination Scheme of Transaction Scheduling in Multi-level Power Market," to be presented at the ?W3 IEEEmES Transmission and Distribution Conference and Exposition, Dallas, Texu, USA, 2003. 'I

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Research on System Security Coordination Problemiin ...

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