Coupling Neighboring Microgrids for Overload Management Based on Dynamic Multicriteria Decision-Making

Abstract:

A microgrid (MG) is expected to supply its local loads independently; however, due to intermittency of wind and solar-based energy resources as well as the load uncertainty, it is probable that theMG experiences power deficiency (overloading). This problem can be mitigated by coupling the overloaded MG to another neighboring MG that has surplus power. Considering a distribution network composed of several islanded MGs, defining the suitable MGs (alternative) to be coupled with the overloaded MG is a challenge. An MG overload management technique is developed in this paper, which first identifies the overloaded MG(s) and then selects the most suitable alternative. The alternative selection is based on different criteria, such as available surplus power, reliability, supply security, power loss, electricity cost, and CO2 emissions in the alternative MGs. Moreover, the frequency and voltage deviation in the system of coupled MGs are considered in the selection. A dynamic multicriteria decision-making algorithm is developed for this purpose. To contemplate the uncertainties in the considered distribution network, a cloud theory-based probabilistic analysis is deployed as the research framework and the performance of the developed technique is evaluated in MATLAB

EXISITNG SYSTEM:

 The intermittency of nondispatchable (e.g., solar and wind-based) DERs in addition to load uncertainties can lead to imbalance between the instantaneous power generation and demand in an MG [8]. Any generation deficiency (overloading) will lead to voltage/frequency drop. To address power imbalance problems in MGs, several solutions can be considered as follows: 1) under frequency/voltage load-shedding [9]; 2) utilization and control of battery energy storages [10]; 3) optimal capacity design of dispatchable DERs (e.g., diesel generators) [11], [12]; 4) interconnection of the MG to utility [13]; 5) coupling of one MG to one/more neighboring MG(s) [14]. MGs coupling is introduced in [15] as a solution to proliferate the number of DERs in distribution networks. Each MG in Fig. 1 may be supported by one/more of its neighboring MG(s) during power deficiency. This can be achieved by closing the normally open ISS which is located between every two adjunct MGs. It is to be noted that the structure and control mechanism of the ISS is beyond the scope of this paper. Wang and Wang [16] and Wang et al. [17] proposed a transformative architecture for coupling the neighboring MGs as a technique for improving the self-healing of the distribution system in case of short-circuit faults in the network. The trade of power a MGs in the system of CMGs is addressed in [18]. Optimal control of a distribution network composed of utility-connected MGs forming a CMG is also studied in [19]. Dynamic operation of DERs within CMGs is investigated in [20] and the dynamic security of the CMGs is examined in [21]. The conditions under which two MGs are interconnected are addressed in [22]. The stability analysis of a CMG prior to the interconnection of the MGs is discussed in [23], as a preliminary step to prevent any interconnection that may lead to system instability. Selection of the suitable MG(s) a the available neighboring MGs when interconnecting them during overloading has not been addressed in the previous literature and is the main focus of this paper.

PROPOSED SYSTEM:

This paper proposes an OMT, based on coupling the neighboring islanded MGs, and utilizes a dynamic multicriteria DMA. The proposed OMT assumes a data communication system is available to receive the power generation of all DERs and consumption of essential/nonessential loads in all MGs. The communication system also transmits the command (output) of the OMT to the relevant ISS(es) to couple the MGs. Under such a case, the power flow control in the considered system is based on the proper operation of the ISS, i.e., if the OMT decides that some MGs should be interconnected, the ISS of each of those MGs will be closed and thereby, the power flow will occur between the interconnected MGs automatically based on the dynamic operation of the DERs in each MG and no further power control is required. The main contributions of this paper are as follows. 1) Develop an OMT to reduce load-shedding rate in MGs during overloading conditions. 2) Develop a dynamic multicriteria DMA to select the suitable MGs.  3) Define the different criteria required for DMA. 4) Qualify the selected MG(s) based on the deviations in voltage and frequency after coupling the MGs. 5) Define the portion of the nonessential loads to be shed from each MG based on the proposed DMA such that all essential loads of all MGs are always supplied. 6) Develop a suitable PFA technique, applicable for MGs.

 CONCLUSION:

An OMT is developed in this paper to reduce the loadshedding rate of a remote area MG, during overloading conditions, by interconnecting it with suitable neighboring MG(s). A dynamic multicriteria DMA is presented to formulate the possible alternatives, qualify them based on four proposed criteria and then assess and select the most suitable one to achieve the highest satisfaction and minimum risk considering another six criteria. All criteria have different weightings. In case, surplus power is not available in the other MGs, the developed OMT proceeds to define the required ALS from each MG. If this level is less than the nonessential part of the loads of an MG, the developed technique defines which neighboring MG(s) have extra nonessential loads. It then proceeds to identify those MGs, defines the portion of nonessential load to be disconnected from them, and then interconnects those MGs. The successful performance of the developed algorithms is validated in a stochastic frame in MATLAB for a small network composed of three MGs as well as a large network, composed of six MGs. It is worth mentioning that the proposed method of coupling the MGs can be expanded to be used when extra surplus power is available in the DERs of an MG. Thus, the extra available power can be exported to other MGs with a lower electricity price, to be stored in the energy storage units or to be consumed by the controllable loads of the other MGs, under demand dispatch concept. It is also noteworthy that analyzing the effect of different weightings for each criterion and thereby the outcome of the DMA can be another future research topic.  It is to be noted that in this paper, it was assumed that the MGs are interconnected through one bus only. However, in general, it is possible to consider the interconnection of one MG through different buses to different MGs. This can be a future research topic and the power flow control under such conditions needs to be investigated.

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