Coordinated transmission substations and sub-transmission networks expansion planning incorporating distributed generation

Highlights

• A new formulation is developed for the coordinated network expansion planning.

• Transmission, sub-transmission, and distribution networks are considered.

• Distributed generation (DG) units are incorporated in the problem.

• An efficient genetic algorithm is applied for the problem optimization.

• The planning approach is implemented on a real-case network.

Abstract

Transmission and distribution systems are linked to each other through the sub-transmission network, where it receives the electric energy from the transmission network at extra high-voltage levels, and delivers it to the distribution network at low-voltage level. Hence, if the sub-transmission network is designed in an optimal manner, it will supply the loads of distribution system adequately on one hand, and this leads to an efficient design of transmission network on the other hand. As the design optimality of transmission, sub-transmission, and distribution systems are dependent on each other, the best result for the network expansion is achieved when these three networks are considered coordinately. Since implementing the coordinated design of transmission, sub-transmission, and distribution systems is very complicated, traditionally, they are designed separately. In this paper, a new formulation has been developed for the coordinated expansion planning of transmission substations, sub-transmission network, and distribution system. In addition, to obtain more efficient results for the network expansion, distributed generation (DG) units are considered in the problem. The proposed approach is implemented on a real network of Zanjan Regional Electrical Company, and the results are discussed. The obtained results demonstrate the effectiveness of the conducted planning method.

Introduction

Power system expansion planning, as a major part of the power system studies, determines the characteristics of network equipment to be installed or upgraded in order to reliably supply the loads in an economical manner. The expansion planning study is performed in three levels of generation, transmission, and distribution systems. Sub-transmission system is an intermediate system between the transmission and distribution networks that receives the electric energy from the transmission system at extra high-voltage (EHV) levels, and delivers it to the distribution network at low-voltage (LV) levels [1]. In this regard, the adequate design and operation of sub-transmission system will lead to an efficient design of transmission network on one hand and the adequacy of power delivery to the distribution loads on the other hand [1], [2]. Different models have been presented in the literature for the optimal design of HV/MV sub-transmission substations [3], [4], [5], [6], [7]. In Ref. [4], a constructive heuristic algorithm (CHA) has been used to solve the power distribution system expansion planning problem. By employing a local improvement phase and a branching technique in CHA, the algorithm seeks for the optimal location and capacity of MV feeders and HV/MV substations. The objective function to be minimized includes the system operating costs and cost of constructing feeders and substations by taking into account the power flow, voltage drop, and radial configuration as the problem constraints. Authors in Ref. [5] proposed a model for solving the multistage planning problem of a distribution system. The objective function is the net present value of the investment cost to add, reinforce, or replace the feeders and substations, and also, the losses cost, and operation and maintenance costs. The nonlinear objective function of the problem is approximated by a piecewise linear function, resulting in a mixed integer linear model that is solved using standard mathematical programming. Moreover, the reliability indices and the related costs are computed for each solution based on the regulation model of Brazil. An approach is proposed in Ref. [6] for the planning of low-voltage distribution systems where the location, number, and capacity of distribution transformers are optimally determined for the sake of improving the system reliability and minimizing the power and energy losses under energy demand growth. The considered objective function is composed of investment, maintenance, losses, and reliability cost. It was shown that the appropriate placement of distribution substations leads to considerable reduction of interruption cost. Ref. [7] has proposed a multi-objective mixed-integer non-linear programming (MINLP) model for the planning problem of primary distribution networks in order to minimize the expansion and operation costs of the network as well as the system's reliability cost in contingency events. This study has considered the expansion and operation costs of transformers, lines, and sectionalizing switches, as well as reliability cost as the objective functions to be optimized by a Multi-objective Reactive Tabu Search (MORTS) algorithm. A novel mixed-integer second-order cone programming model is developed in Ref. [8] for the robust multi-stage expansion planning of sub-transmission substations considering the stochastic nature of demand. To take into account the total expected expansion planning cost and the robustness probability, a multi-objective formulation has been proposed, and the classical optimization techniques have been used to optimally solve the problem. The solution gives the construction of new substations, reinforcement of existing substations, and service area of each substation. In addition, the Monte Carlo (MC) simulation is carried out in order to evaluate the extent of the problem constraints satisfaction.

Recently, the integration of distributed generation (DG) resources into distribution networks has affected the planning and operation problems of the power system [9]. These small-scale power generating units bring various benefits to power system such as peak cutting, losses reduction, voltage profile improvement, reliability enhancement, and deferring the capacity reinforcement of network equipment [10], [11], [12], [13]. Due to potential advantages of the DG units, several researches have incorporated these resources as an alternative in the expansion planning problems [14], [15], [16], [17], [18], [19]. Ref. [14] has integrated the distributed generation in distribution network expansion planning, and employed a hybrid simulated annealing (SA) and MILP technique for the optimization planning problem. The genetic algorithm (GA) and interior-point methods are employed in Ref. [16] for the optimal placement of DGs and smart metering devices along with the reinforcement of sub-transmission substations and MV feeders. Bagheri et al. [17] incorporated renewable power generation in an integrated distribution system expansion planning to obtain a low-cost, reliable, and non-polluting network. The sub-transmission substations, MV feeders, and DG units are optimally located and sized using an efficient GA-OPF method. A new framework has been developed in Ref. [19] for multi-year expansion planning of distribution networks. This model determines the optimal expansion of primary distribution system including the reinforcement of MV feeders as well as location and size of DGs for a multi-year planning period. Also, for the sake of network reliability optimization, an analytical approach based on minimum load shedding is introduced to restore the loads by means of DGs' restoration capability. In order to solve the objective function, which consists of the total investment and operation costs, a new method from the family of evolutionary algorithms called BCSSO (Binary Chaotic Shark Smell Optimization) is proposed. The superiority of the proposed optimization method is indicated by comparing its results with those of other solution methods.

Sub-transmission substations are fed from the transmission substations via sub-transmission lines. Few researchers have focused on the sub-transmission lines and substations expansion planning. Shayeghi et al. [20] employed a hybrid LP and GA technique to dynamically expand the sub-transmission lines and substations in the presence of distributed generation. They utilized a three-level load model for optimal siting and sizing of DG units concurrent with upgrading of the sub-transmission lines and substations. They regarded the distribution system as the lumped load of sub-transmission substations, and disregarded the MV feeders' layout. In Ref. [21], the sub-transmission lines and substations have been optimally designed simultaneous with the MV Feeders' layout; while the configuration of sub-transmission and transmission networks are dependent on each other, this study has considered some pre-determined locations for the transmission substations. A new transmission substation expansion planning (SEP) technique is presented in Ref. [22]. This technique, at first, employs a mathematical clustering algorithm to determine an initial list of feasible candidates considering the voltage drop and capacity limitations of feeders and substations; then, the GA is used to optimally decide the allocations and capacities of new substations along with the expansion requirements for the existing ones. This work has neglected the distribution network details in the SEP problem; In addition, the DG units have not been regarded.

The above literature survey reveals that no significant research has been implemented for the simultaneous expansion planning of transmission substations, sub-transmission system, and distribution network. In this regard, the current paper develops a model for coordinated expansion planning of transmission substations, sub-transmission systems, and distribution networks. This study will regard the dependency of the three networks' configuration on each in order to find the best result for the network expansion. The distribution network is modeled as MV/LV load points which must be optimally supplied from the sub-transmission substations through the MV feeders. The HV/MV sub-transmission substations are linked to transmission substations via appropriate connections of HV sub-transmission lines. The EHV/HV transmission substations in turn, are fed through the EHV transmission lines. In addition, the DG units are considered in the problem as another alternative for the network expansion. The proposed coordinated planning problem along with its constraints is formulated as an optimization problem where an efficient genetic algorithm is employed to optimize this integrated planning problem. The developed planning framework is applied to the real network of Zanjan Regional Electrical Company (ZREC) to validate its effectiveness in different experiments. The main contributions of this paper can be outlined as the following items:

• Proposing an integrated formulation for the coordinated design of transmission substations, sub-transmission networks, and distribution systems expansion planning;

• Considering the distributed generation as an alternative for the planning problem;

• Using an efficient genetic algorithm as the optimization method for the integrated planning problem.

The remainder of this paper has been organized as follows:

Part 2 describes the proposed formulation for the problem. The solution method has been detailed in part 3. The simulation results are given in part 4; and finally, the last part concludes the paper.