Scenario-Based Stochastic Framework for Optimal Planning of Distribution Systems Including Renewable-Based DG Units

: Renewable energy-based distributed generators are widely embedded into distribution systems for several economical, technical, and environmental tasks. The main concern related to the renewable-based distributed generators, especially photovoltaic and wind turbine generators, is the continuous variations in their output powers due to variations in solar irradiance and wind speed, which leads to uncertainties in the power system. Therefore, the uncertainties of these resources should be considered for feasible planning. The main innovation of this paper is that it proposes an efﬁcient stochastic framework for the optimal planning of distribution systems with optimal inclusion of renewable-based distributed generators, considering the uncertainties of load demands and the output powers of the distributed generators. The proposed stochastic framework depends upon the scenario-based method for modeling the uncertainties in distribution systems. In this framework, a multi-objective function is considered for optimal planning, including minimization of the expected total power loss, the total system voltage deviation, the total cost, and the total emissions, in addition to enhancing the expected total voltage stability. A novel efﬁcient technique known as the Equilibrium Optimizer (EO) is actualized to appoint the ratings and locations of renewable-based distributed generators. The effectiveness of the proposed strategy is applied on an IEEE 69-bus network and a 94-bus practical distribution system situated in Portugal. The simulations verify the feasibility of the framework for optimal power planning. Additionally, the results show that the optimal integration of the photovoltaic and wind turbine generators using the proposed method leads to a reduction in the expected power losses, voltage deviations, cost, and emission rate and enhances the voltage stability by 60.95%, 37.09%, 2.91%, 70.66%, and 48.73%, respectively, in the 69-bus system, while in the 94-bus system these values are enhanced to be 48.38%, 39.73%, 57.06%, 76.42%, and 11.99%, respectively.


Problem Statement
Uncertainty is essential in the optimal power planning problem of electrical systems, and it is a main consideration adding to its complexity, specifically the uncertainties of the renewable energy resources (RERs) and load demands. Many research efforts related to the optimal integration of Distributed Generators (DGs) are expressed as ideal optimization problems, and only few have considered uncertainty. The optimal allocation of the photovoltaic (PV) and wind turbine (WT) units in power systems and techniques existing in the literature considering the uncertainty of systems have an incredible impact on the planning of renewable DGs, and uncertainties effect the load demand and the output powers of solar and wind-based DGs in the distribution systems (DSs). The contribution and the research gap are referenced in detail. Electric power generation organizations will in general utilize (DGs) near the load to convey the electrical power to the consumers for technical, economic, and environmental reasons. As of late, integration of the RERs including wind and photovoltaic energies have become a favored solution for defeat increasing the load growth as they are sustainable and clean resources. Nonetheless, the inclusion of the RERs in the distribution grids face numerous issues due to their intermittency and the fluctuations of the output power, which increases the uncertainties in electrical systems. In this way, the uncertainties in power systems should be taken into consideration for correct planning and the secure operation of power systems.

Literature Survey
The essential purpose of efficient planning in distribution systems is to provide excellent solutions that guarantee the security, quality, and reliability of power supply to clients at the least cost [1]. The cost of power generation from conventional generators is expanding quickly because of the increase in fuel costs, although lately the generation cost of RERs has diminished. Alongside financial contemplations, another advantage of RERs is the eco-friendly power generation from these sources [2]. A stochastic scenario modeling of a multistage joint for the distribution systems planning has been utilized to decrease the operational and investment costs [3]. Sensible application of DGs can bring numerous points of interest, for example, voltage profile improvement, reducing emissions and energy cost [4][5][6].
Nonetheless, improper placement of DGs may lead to the fluctuations of voltage and also system instability because of the uncertain nature of RERs [7,8]. The issue of optimal integration of DGs has been explored in the several papers from different points of view. The authors in [9] suggested an improved adaptive genetic algorithm for resolving the optimal DG allocation problem. In [10], an efficient framework has been suggested for the optimal DG allocation problem to reduce the system costs. The authors in [11] offered a genetic algorithm along with the Monte Carlo method for solving the optimal DG integration problem under the uncertainties of RER generation. The cost of energy losses and DGs have been considered in the model. In [12], an efficient method has been presented for the optimal planning of accommodating the integration of PEV along with renewable DGs under uncertainties of the system. In [13], the optimal power planning problem in the active distribution system is solved to reduce total cost and emissions using a cuckoo search (CS) with optimal integration of WTs and demand response, considering the uncertainties of the system by the scenario synthesis method. The authors in [14] applied the Crisscross Optimization Algorithm and Monte Carlo Simulation for assigning the rating and location of DGs in the distribution system for reducing the power losses and the cost. Esmaeili et al. presented a multi-objective framework for optimizing the DG allocation and reconfiguration of the distribution network using the Big Bang-Big Crunch algorithm [15]. In [16], a probabilistic planning method was suggested based on mixed integer nonlinear programming (MINLP) and has been implemented to assign energy loss reduction with optimal integration of RERs in a rural distribution system. The author in [17] proposed a stochastic model for optimizing the investment of the DGs under uncertain conditions in distribution networks. Ref. [18] proposed a planning strategy for a hybrid solar-wind generation MG system with hydrogen energy storage using a novel multi-objective optimization algorithm to minimize the following three objective functions: loss of load expected, annualized cost of the system, and loss of energy expected. Ref. [19] proposed an algorithm for DG allocation planning based on using the probabilistic uncertainty modeling method. Several optimization algorithms have been used to determine the best size and location of DGs in a radial distribution network (RDN) considering the uncertainties of systems such as Particle Swarm Optimization (PSO) [20], modified sine cosine algorithm [21], Cuckoo Search Algorithm (CA) [22], water

Contribution of Paper
From the previous survey, the main concern or the problem statement related to the optimal planning of distribution systems with the inclusion of optimal RERs is the uncertainties of load demand and the output power of the RERs. Therefore, the planning problem became more complex and needs an efficient method to be solved.
This paper contributes to the existing body of knowledge as it solves the optimal planning problem in a distribution system for optimal incorporation of DGs using a scenario-based stochastic framework considering the uncertainties of load demands and the output powers of RERs. The innovation and contributions of this paper are summarized as follows:

•
Proposing an efficient framework for the optimal planning of distribution systems considering the uncertainties of load and the output powers of renewable based DGs.

•
The application of scenario-based methods for modeling the uncertainties in the electrical systems.

•
The application of an efficient algorithm, called the EO, for solving the planning problem.

•
The developed algorithms are applied for optimal integration of the renewable-based DGs for loss reduction, voltage improvements, system voltage deviation, the total cost, and the total emissions of the IEEE 69-bus and 94-bus distribution networks. • A comparison is presented between the EO and other well know techniques for solving the planning problem.

Paper Layout
The paper is arranged as follows: Section 2 displays the problem formulation including the objective function. Section 3 illustrates the uncertainty modeling methods. Section 4 introduces an overview of the EO technique. Section 5 shows the obtained outcomes, while Section 6 lists the paper's conclusions.

Problem Formulation
In this study, five objective functions are considered in a multi-objective function. It is worth mentioning that in case of modeling or considering the uncertainties in power systems, a set of scenarios will be generated. Thus, these scenarios should be considered for the efficient solving of planning problems, and each scenario has its expected values as depicted in the following sections. In this work, the considered objective function is The expected power losses of the radial distribution network are determined as follows: where

Minimization of the Expected Voltage Deviations (ETVD)
The expected summation of the voltage deviations of the radial distribution network are given as follows:

Enhancement of the Expected Voltage Stability (ETVSI)
The expected summation of the voltage stability indices can be expressed as follows: where VSI n = |V n | 4 − 4(P n X nm − Q n R nm ) 2 − 4(P n X nm + Q n R nm )|V n | 2 (7)

Minimization of the Expected Total Cost (ETCost)
The expected total annual cost (ETCost) is considered, which consists of the expected annual energy loss cost (ECost loss ), the expected cost of the electric energy savings from the main substation (ECost Grid ), the expected PV units cost (ECost PV ), and the expected WT cost (ECost WT ). It can be represented as follows: The items detailed in Equation (8) are defined as follows: Cost Loss = 8760 × K Loss × P Total_loss (11) where a 1 is the annual installment of the wind turbine, and b 1 is the annual operation and maintenance cost of the wind turbine.
where a 2 is the annual installment of the PV unit, and b 2 is the annual operation and maintenance cost of the PV unit. In this paper the cost coefficients of PV are selected to be CSDG PV = 770 USD/kW, Cost_PV O&M = 0.01 USD/kWh, Cost_PV Fuel = 0 USD/kWh, and the cost coefficients of the wind are selected to be CSDG WT = 4000 USD/kW, Cost_WT O&M = 0.01 USD/kWh, Cost_WT Fuel = 0 USD/kWh [36].

Minimization of the Expected Total Emissions (ETEmission)
The expected total annual emissions in kilotons (Kt) can be expressed as follows: According to ER Grid , the emission rate of grid values of NOx, CO 2, and SO 2 are 2.2952 kg/MWh, 921.25 kg/MWh and 3.5834 kg/MWh, respectively [15].

The Multi-Objective Function
In this work, the previous objective functions are considered simultaneously. To consider these objective functions concurrently, the weight approach method is utilized. In addition, the objectives should be normalized as follows via division by its base value (without PV or WT), which makes the objective function dimensionless and also prevents any scaling problems. The augmented objective function can be described as follows: where ∝ 1 , ∝ 2 , ∝ 3 , ∝ 4 , and ∝ 5 are weighting factors. The summation of weight factors should equal 1 as follows: The normalized objective functions can be formulated as follows:

Uncertainty Modeling
In this work, the uncertainties that existed in the power system are considered by solving the problem of optimal power planning. The proposed stochastic framework considered three uncertain parameters including load demand, solar irradiance, and wind speed. The continuous probability density functions (PDFs) of wind speed, solar irradiance, and loads are used for representing the uncertainties of these parameters; then, the scenariobased method is utilized for generating a set of scenarios from combinations of these parameters. The proposed stochastic framework can be depicted as follows:

Modeling of Load Demand
The normal PDF ( f d (P d )) is used for uncertainty representation of the load demand, which can be described using the following equations [37]: The generated load scenarios and their probabilities obtained from (33) can be obtained as follows [38]: In this work, three load scenarios are presented. The load scenarios are obtained by dividing the normal PDF into three intervals. Table 1 provides the load scenarios and their probabilities when µ d and σ d are 70 and 10 [39].

Modeling of Wind Speed
The Weibull PDFs ( f v (v)) are used to describe the uncertainties of wind speed which can be expressed as follows [38]: The wind turbine output power can be specified as follows [40,41]: Additionally, a set of scenarios can be obtained from (36) by dividing the f v (v) into of a set of wind speed intervals. The generated wind speeds and their probabilities can be obtained as follows [42]: In this paper, three scenarios of wind speed are generated from the previous equations. The wind speed scenarios and their probabilities are listed in Table 1 in the case of selecting c and k to be 10.0434 and 2.5034, respectively, as given in [38].

Modeling of Solar Irradiance
The Beta PDF is used to specify the uncertainty of the solar irradiance, which can be given as follows [43]: The yield power from the PV system can be calculated as follows [44,45]: In the previous equation, G std is set to be 1000 W/m 2 , and X c is a certain irradiance point is set to be 120 W/m 2 [41]. Three scenarios can be obtained from the previous equations by dividing the PDF into three intervals. The portability of solar irradiance and its corresponding solar irradiance for each scenario are given as follows [42]: The generated scenarios of the solar irradiance and their probabilities are listed in Table 1 in the case of selecting α and β to be 6.38 and 3.43, respectively, as given in [38].

The Combined Load-Generation Model
To consider the uncertainties of the load demand, wind speed, and irradiance simultaneously, the probabilities of these parameters depicted in (34), (38), and (44) are multiplied together according to (46) as follows: A total of 27 scenarios can be obtained from (46). Table 2 shows the obtained scenarios and the value of the uncertain parameters and their probabilities.

Equilibrium Optimizer
The EO is a modern optimizer which simulates models of the control volume mass balance to describe the dynamic and equilibrium states. In the EO, the concentrations denote the positions or the locations, while the particles represent the search agents of the optimizer. The particles update their location randomly around a vector known as equilibrium candidates. In addition, the generation rate is utilized for boosting the exploration and exploitation of the optimizer [31]. The mass balanced equation is described according to Equation (47) as follows: where V dX dt describes the rate of mass changing in a volume. X refers to the concentration, and V represents the control volume. Q denotes the flow rate. G denotes the mass generation rate. By integration and manipulation of Equation (47), it is formulated as follows: where λ = D V . X 0 refers to the initial concentration, and t 0 is the initial start time.

The Steps of EO
Step 1: Initialization The initial concentrations are generated randomly according to (49).
where X max is the upper boundary of the control variable, while X min is its lower limit. rand is a random value in the range [0,1]. Then, the objective function is evaluated for each obtained concentration.
Step 2: Assignment of the Equilibrium Candidates The concentrations will be sorted according to their objective functions. The best four concentrations and their average vector represent the equilibrium candidates or the pool vector (X pool ), which can be expressed using (50) the following: where Step 3: Updating of the concentrations Two vectors (r, λ) are created randomly, and they are used to control the exponential factor (F) to update the concentrations according to the following equations: where L 1 and L 2 are constant values, which equal 2 and 1, respectively. These values are employed to adjust the exponential factor. T Max is the maximum number of iterations, T refers to the T-th iteration. It should be indicated here that a 1 is employed to control the exploration process, while a 2 is employed to control the exploitation phase o. Sign (r − 0.5) can also control the exploration direction.
Step 4: Applying the generation rate It worth mentioning here that the generation rate is a robust approach for exploitation enhancement, and it can be defined as follows: where where r 1 and r 2 refer to a random value in the range of [0,1]. GP is the probability of generation, which is utilized to control the participation probability of concentration where it is updated by the generation rate. When GP = 1, the generation rate will not participate in the optimization process, while when GP = 0, the generation rate will greatly participate in the process. If GP = 0, the generation rate offers an admirable balancing between the exploration and exploitation procedures. According to the mentioned steps, the updated equation can be described using Equation (57): Step 5: Adding memory saving. The obtained solutions or concentration will be compared with the previous solution. It is worth mentioning here that the EO is proposed to solve the presented optimal planning problem, where the main advantages of the Equilibrium Optimizer lie in its ability to assign optimal solutions with higher efficiency (i.e., less computational time or fewer number of iterations) when compared with other optimization techniques, as well as its high simplicity in updating the algorithm structure and its controllability between the exploitation and exploration phases. Its related disadvantage is that it is very sensitive to its selected parameters. Figure 1 describes application of the EO for the solution of the optimal power planning problem.

Results and Discussion
The optimal power planning problem has been solved by the suggested algorithm (EO), and the optimal ratings and placement of wind turbines and solar PV units are assigned under the uncertainties of renewable energy and load demand. The objective function is a multi-objective function which comprises of (1) the expected power loss, (2)

Results and Discussion
The optimal power planning problem has been solved by the suggested algorithm (EO), and the optimal ratings and placement of wind turbines and solar PV units are assigned under the uncertainties of renewable energy and load demand. The objective function is a multi-objective function which comprises of (1) the expected power loss, (2) the expected summation of voltage deviations, (3) the expected voltage stability index, (4) the expected cost, and (5) the expected emissions. It should be highlighted here that the value of each weight factors in (21) is selected to be 0.2 for all studied cases. The EO algorithm is implemented for IEEE 69 and 94-bus systems, and the outcomes are compared with those obtained by the Sine Cosine Algorithm (SCA) [46], Particle Swarm Optimizer (PSO) [47], and the Anti Lion Algorithm (ALO) [48]. The single line diagram of the IEEE 69 and 94-bus systems are illustrated in Figures 2 and 3, respectively. The systems data of the 69-bus and 94-bus systems are given in [49,50], respectively. The system data and the initial load flow are provided in Table 3, while the constraints of the system are given in Table 4. The used parameters of the applied optimization techniques are tabulated in Table 5. It should be pointed out that the maximum number of search agents' and iterations or populations of the applied algorithms are selected to be the same for a fair comparison. The proposed EO technique as well as the other algorithms have been conducted on a I7-8700 CPU 3.2GHz and 24 GB RAM PC using MATLAB 2014a. The studied cases are presented below.

The IEEE 69-Bus System
The proposed algorithm is utilized to solve the optimal planning of the 69-bus system with optimal integration of RERs considering the uncertainties of the system. Initially, without integration of RERs, the total of the expected values of the power losses ( ), the total of the expected values of the voltage deviations ( ), the total of the expected values of the voltage stability index ( ), the expected values of the total cost ( ),

The IEEE 69-Bus System
The proposed algorithm is utilized to solve the optimal planning of the 69-bus system with optimal integration of RERs considering the uncertainties of the system. Initially, without integration of RERs, the total of the expected values of the power losses (ETP Loss ), the total of the expected values of the voltage deviations (ETVD), the total of the expected values of the voltage stability index (ETVSI), the expected values of the total cost (ETCost), and the expected values of the emissions (ETEmission) are 144.0507 kW, 1.4014 p.u, 62.7261 p.u, 2,434,700 USD, and 15.947 × 10 3 kg/MWh, respectively. As mentioned, in Section 3, by combining the load demand, wind speed, and solar irradiance uncertainties, 27 scenarios have been generated to model the uncertainties of the system as depicted in Table 2. By application of the EO, the optimal sites for PV and wind turbine-based DGs are at buses number 26 and 62, respectively, while the optimal rating of the PV and wind turbine-based DGs are 177.5 kW and 1151 kW, respectively. Table 6 and Figure 4 provide the output power of the PV and wind turbine-based DGs for each scenario, as well as the corresponding P Loss (MW), VD (pu), VSI (pu), Cost (USD), and Emision (kg/MWh). As solar irradiance, wind speed, and load demand have different values in each scenario, the yielded result will also be different. At a high probability value which occurred in scenario 13, according to Table 2 the output power of the wind turbine and PV systems is 394.2 kW and 108.4228 kW, respectively. Table 7 provides the expected values for each scenario with optimal integration of RERs. According to Table 7, the summation of the expected values including ETP Loss , ETVD, ETVSI, ETCost and ETEmission are enhanced to be 56.2394 kW, 0.8816 p.u, 64.6087 p.u, 1,248,050 USD, and 4,677.936 × 10 3 kg/MWh, respectively.    In other words, the enhancement in the summation of the expected values with optimal integration of the RERs including ETP Loss , ETVD, ETVSI, ETCost, and ETEmission are 60.95%, 37.09%, 2.91%, 48.73%, and 70.66%, respectively. Figure 5 shows the voltage profile for the obtained scenarios. From this figure, it is obvious that the voltage magnitudes of all scenarios are within the allowable limits, and there is no violation which occurred. Table 8 shows a comparison of the obtained results by the application of other algorithms for the IEEE 69-bus system. Judging from Table 8, the minimum objective function has been obtained by the application of the EO compared with SCA, ALO, and PSO. In other words, the enhancement in the summation of the expected values with optimal integration of the RERs including ETP , ETVD, ETVSI, ETCost, and ETEmission are 60.95%, 37.09%, 2.91%, 48.73%, and 70.66%, respectively. Figure 5 shows the voltage profile for the obtained scenarios. From this figure, it is obvious that the voltage magnitudes of all scenarios are within the allowable limits, and there is no violation which occurred. Table 8 shows a comparison of the obtained results by the application of other algorithms for the IEEE 69-bus system. Judging from Table 8, the minimum objective function has been obtained by the application of the EO compared with SCA, ALO, and PSO.

The IEEE 94-Bus System
The optimal planning of the 94-bus system with the optimal integration of RERs considering the uncertainties of the system has been solved by the proposed algorithm. Initially, without the integration of RERs, the total of the expected values of the power losses (ETP ), the total of the expected values of the voltage deviations (ETVD), the total of the

The IEEE 94-Bus System
The optimal planning of the 94-bus system with the optimal integration of RERs considering the uncertainties of the system has been solved by the proposed algorithm. Initially, without the integration of RERs, the total of the expected values of the power losses (ETP Loss ), the total of the expected values of the voltage deviations (ETVD), the total of the expected values of the voltage stability index (ETVSI), the expected values of the total cost (ETCost), and the expected values of the emissions (ETEmission) are 204.6913 kW, 7.2162 p.u, 67.8052 p.u, 3,103,600 USD, and 20,254 × 103 kg/MWh, respectively. As referenced in Section 3, by combining load demand, wind speed, and solar irradiance uncertainties, 27 scenarios have been generated to model the uncertainties of the system as shown in Table 2. By utilization of the EO, the optimal sites for PV and wind turbine-based DGs are at buses number 91 and 23, respectively, while the optimal rating of the PV and wind turbine-based DGs are 107.5 kW and 1261.6 kW, respectively. Table 9 and Figure 6 provide the output power of the PV and wind turbine-based DGs for each scenario, as well as the corresponding P Loss (MW), VD (pu), VSI (pu), Cost (USD), and Emision (kg/MWh). As solar irradiance, wind speed, and load demand have different values in each scenario, the yielded result will also be different. At a high probability value which occurred in scenario 13 according to Table 2, the output power of the wind turbine and PV systems are 432.2 kW and 65.1755 kW, respectively.  Table 10 provides the expected values for each scenario with the optimal integration of RERs. According to Table 10, the summation of the expected values including ETP Loss , ETVD, ETVSI, ETCost, and ETEmission are enhanced to be 105.6493 kW, 4.3489 p.u, 77.0479 p.u, 1,332,680 USD, and 4774.689 × 10 3 kg/MWh, respectively. In other words, the enhancement in the summation of the expected values with optimal integration of the RERs including EP Loss , EVD, ETVSI, ETCost, and EEmission are 48.38, 39.73%, 11.99%, 57.06%, and 76.42%, respectively. Figure 7 shows the voltage profile for the obtained scenarios.      From this figure, it is obvious that the voltage magnitudes of all scenarios are within the allowable limits, and no violation occurred. Table 11 shows a comparison of the obtained results by the application of other algorithms for the 94-bus system. Judging from Table 11, the minimum objective function has been obtained by the application of the EO compared with SCA, ALO, and PSO.

Conclusions
In this paper, the optimal planning for distribution systems has been solved using an efficient stochastic framework by assigning the optimal sites and sizes of solar PV and wind turbine-based DGs under uncertainties of load demands, wind speeds, and solar radiation. The proposed framework is based on application of the Equilibrium Optimizer (EO) and the scenario-based method for reducing the expected power loss, the expected system voltage deviations, the expected total cost, the expected total emissions, and maximizing the expected voltage stability. The EO has been applied for solving the allocation problem of solar PV and wind turbine-based DGs, while the scenario-based method was utilized to represent the combination the uncertainties of load demands, wind speeds, and solar radiation. The proposed technique has been implemented on an IEEE 69-bus and 94-bus practical distribution system located in Portugal, and the obtained results were compared with those obtained by SCA, PSO, and ALO. The obtain results verified the following: - The effectiveness of the proposed framework for solving the optimal planning problem for distribution systems. - The superiority of the EO for assigning the optimal placement and sizes of the The resistance of the line between buses k and k + 1, P k Real power Q k Reactive powers P L The active load demand The expected total annual cost ECost loss The expected annual energy loss cost Cost Grid Cost of the power injection at substation ECost Grid The expected cost of the power injection at substation ECost PV The expected PV units cost P Grid Power of the grid K Grid Cost of electricity in USD/kW h Cost Loss Cost of the losses K Loss The energy loss cost P Total_loss The total power losses ECost Loss The expected loss cost π S The Combined probabilities π d,i The portability of load demand of i-th interval π wind,z The probability of the wind speed of z-th interval π Solar,m The probability of the solar irradiance of m-th interval ECost wind The expected WT cost CF The capital recovery factor P wr Rated output power of the WT ECost PV The expected PV cost CSDG PV Installation cost of the PV P sr The rated power of PV unit P s The output power of PV unit P wr The rated power of WT P w The output power of WT The maximum limit of the selected interval i P min d,i Multi-objective function f 1 The objective function representing the normalized active total power losses f 2 The objective function representing the normalized total voltage deviations f 3 The objective function representing the normalized voltage stability index f 4 The objective function representing the normalized total cost f 5 The objective function representing the normalized total emission