In mathematical terms, the SO-OPF problem and MO-OPF problems may be represented as:
Minimize f(s,a)
Subject to \(\left\{ {\begin{array}{*{20}{c}} {g(s,a)=0} \\ {h(s,a) \leqslant 0} \end{array}} \right.\) (1)
Minimize fi(s,a)
Subject to \(\left\{ {\begin{array}{*{20}{c}} {{g_i}(s,a)=0} \\ {{h_i}(s,a) \leqslant 0} \end{array}} \right.\) (2)
Where, f(s,a) represents the objective function. s represents dependent variables arranged in a vector, whereas a represents independent variables arranged in a vector. g(s,a) and h(s,a) represents the equality constraints and inequality constrains.
The variables of OPF objectives can be formulated mathematically as:
$$\begin{gathered} {a^T}=\left[ {{P_{Gen2}} \cdot \cdot \cdot {P_{GenNG}},{V_{Gen1}} \cdot \cdot \cdot {V_{GenNG}},{Q_{C1}} \cdot \cdot \cdot {Q_{CNC}},{T_1} \cdot \cdot \cdot {T_{NT}}} \right] \hfill \\ {s^T}=\left[ {{P_{Gen1}},{V_{D1}} \cdot \cdot \cdot {V_{DNPQ}},{Q_{Gen1}} \cdot \cdot \cdot {Q_{GenNG}},{S_{L1}} \cdot \cdot \cdot {S_{LNL}}} \right] \hfill \\ \end{gathered}$$
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Where NG, NC, NT, NL and NPQ represent the number of generator bus, VAR compensator, tap changing transformer, transmission line (TL) and PQ bus respectively. QC and QGen represents the VAR compensations and generator buses reactive power output respectively. SL represents the line flow through the TL.
2.1. Objective functions:
The traditional OPF problem basically concerned with three major Single objective such as:
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Case-1: Minimization of the active power loss in TL (PLoss)
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Case-2: Minimization of the Voltage deviation at load busses (TVD)
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Case-3: Enhance the Index of Voltage stability (L-Index)
The MO-OPF problems can be formalized by the combination of following objectives:
2.1.1. Minimization of the active power loss in TL: The formula for minimizing PLoss in TL is [21]:
Minimize F1 = Ploss = \(\sum\limits_{{k=1}}^{{NT}} {{g_k}} \left( {{V_a}^{2}+{V_b}^{2} - 2{V_a}{V_b}\cos \left( {{\delta _a} - {\delta _b}} \right)} \right)\) (4)
Where gk represents the kth TL conductance. Va and Vb represents the voltage magnitude of ath and bth buses respectively. δa and δb represents the voltage angle.
2.1.2. Voltage deviation minimization at load busses: Sometimes in power system operation the load voltage may violate its limit. A problem formulation has been done in the OPF to minimize the violations from all the load buses. Minimization of TVD might enhance the voltage profile and allow power systems to operate more safely. TVD minimization at load busses can be formulated as [21]:
Minimize F2 = VD =\(\sum\limits_{{a=1}}^{{NPQ}} {\left| {{V_a} - {V_{re{f_a}}}} \right|}\) (5)
where Vrefa represents the reference voltage magnitude in ath PQ bus (1pu).
2.1.3. Voltage stability enhancement: Currently, power systems are run near to their stability limitations for operational reasons that are both economical and ecologically benign. Therefore, Voltage stability is considered as an important issue. The ability of a power system to keep the voltage on any bus within a certain range is known as voltage stability. When there is a sudden disruption in the system, voltage instability occurs which may cause uncontrollably reducing in voltage. We can increase voltage stability by minimizing the maximum LIndex. The L-Index might have a range anywhere from 0 to 1, inclusive. The value of L-index near 0 would be considered the best solution, and a value near 1 would be considered the worst solution. L-index for jth bus can be formulated as [21]:
L-indexj = \(\left| {1 - \sum\limits_{{i=1}}^{{NPV}} {{F_i}_{j}\frac{{{V_i}}}{{{V_j}}}} } \right|\) (6)
\({F_{ij}}={\text{ }} - {\left[ {{Y_1}} \right]^{ - 1}}\left[ {{Y_2}} \right]\)
$$\left[ {\begin{array}{*{20}{c}} {{I_{PQ}}} \\ {{I_{PV}}} \end{array}} \right]=\left[ {\begin{array}{*{20}{c}} {{Y_p}}&{{Y_q}} \\ {{Y_r}}&{{Y_s}} \end{array}} \right]\left[ {\begin{array}{*{20}{c}} {{V_{PQ}}} \\ {{V_{PV}}} \end{array}} \right]$$
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where Yp and Yq represents the YBUS submatrices that were created by isolating the PQ bus and PV bus characteristics.
2.1.4. Minimization of simple fuel cost and Total Emission: Fcostsimple and Femissionsimple for nth thermal generating units can be mathematically formulated as [24]:
$$F_{{TFC}}^{{simple}}=\sum\limits_{{n=1}}^{{NG}} {\left[ {{a_n}+{b_n}{P_{gn}}+{c_n}P_{{gn}}^{2}} \right]} \$ /h$$
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$$F_{{TE}}^{{simple}}=\sum\limits_{{n=1}}^{{NG}} {\left[ {{s_n}+{t_n}{P_{gn}}+{u_n}P_{{gn}}^{2}+{v_n}\exp \left( {{w_n}{P_{gn}}} \right)} \right]} ton/h$$
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Where an, bn and cn represents the cost coefficient of nth generating unit. Sn, tn, un, vn and wn represents the emission coefficient of nth generating unit. Pgn represents the real power generated in nth generating units.
2.2. Multi-Objective Optimal power flow
In typical MO-OPF problem, multiple objectives are simultaneously optimized. The technique that solves the MO problem gets the Pareto-optimal front (POF), which is an array of solutions that are less significant than the others. Consider that the system has two alternative solutions, Xp and Xq. Xp is considered the non-dominant solution if the subsequent conditions are met [25].
$$\begin{gathered} \begin{array}{*{20}{c}} {\forall a \in \left\{ {1,2, \cdot \cdot \cdot ,{N_{obj}}} \right\}}&{{F_a}({X_p}) \leqslant {F_a}({X_q})} \end{array} \hfill \\ \begin{array}{*{20}{c}} {\exists b \in \left\{ {1,2, \cdot \cdot \cdot ,{N_{obj}}} \right\}}&{{F_b}({X_p})<{F_b}({X_q})} \end{array} \hfill \\ \end{gathered}$$
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Where Nobj represents number of total objective functions used for formation of the multi objective optimization problem.
The objectives of a multi-objective problem often contradict with one another. Since there is not a single best option, the Pareto method instead seeks to provide an array of solutions. The best possible solution among them is named as the Pareto-optimal Solution (POS). After storing all of the POS, a fuzzy membership function (𝝻a) is utilised to report the most preferred solution. The ath objective function fa may be stated in terms of a fuzzy membership function 𝝻a as follows [25].
$${\mu _a}=\left\{ {\begin{array}{*{20}{c}} 1&{{f_a} \leqslant f_{a}^{{\hbox{max} }}} \\ {\frac{{f_{a}^{{\hbox{max} }} - {f_a}}}{{f_{a}^{{\hbox{max} }} - f_{a}^{{\hbox{min} }}}}}&{f_{a}^{{\hbox{min} }}<{f_a}<f_{a}^{{\hbox{max} }}} \\ 0&{{f_a} \geqslant f_{a}^{{\hbox{max} }}} \end{array}} \right.$$
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Where, famax and famin represents the maximum and minimum value of ath objective function.
The membership function 𝝻k can be estimated for each Pareto front k can be mathematically formulated as,
$${\mu ^k}=\frac{{\sum\limits_{{a=1}}^{{Nobj}} {{W_a} \times \mu _{a}^{k}} }}{{\sum\limits_{{k=1}}^{M} {\sum\limits_{{a=1}}^{{Nobj}} {{W_a} \times \mu _{a}^{k}} } }}$$
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Where, 𝝻k represents the best optimized solution. Wa represents the weight coefficient. The significance of the objective function is used to establish the weight coefficient. M represents the number of non-dominated solution. Pareto analysis might be used to determine the nondominated solutions [19].
The current study takes a multi-objective optimisation approach by simultaneously minimizing the combination (PLoss – TFC), and (TFC - TE). The formulation for PLoss, TFC and TE and are given in equations (5, 8 and 9) respectively.
2.3. Constraints:
The solution to an optimization problem cannot be treated as a feasible solution if it doesn’t involve any sort of constraint violation. Several constraint management strategies, including Feasibility Rules (FR), ε -Constrained method, and the Penalty Factor (PF) method, have been created to handle impractical solutions and confine them to their minimum and maximum bounds [26]. The essential Equality Constraints (ECs) and Inequality Constraints (ICs) related to OPF problems and its handling process are discussed below.
2.3.1. Equality Constraints:
Power Flow Equation:
$$\begin{gathered} {P_{Ga}} - {P_{Lb}}=\sum\limits_{{b=1}}^{{NB}} {{V_a}{V_b}{G_{ab}}\cos \left( {{\delta _a} - {\delta _b}} \right)+} {V_a}{V_b}{B_{ab}}\sin \left( {{\delta _a} - {\delta _b}} \right)] \hfill \\ {Q_{Ga}} - {Q_{Lb}}=\sum\limits_{{b=1}}^{{NB}} {{V_a}{V_b}{G_{ab}}\cos \left( {{\delta _a} - {\delta _b}} \right) - } {V_a}{V_b}{B_{ab}}\sin \left( {{\delta _a} - {\delta _b}} \right)] \hfill \\ \end{gathered}$$
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Where PGa-PLa and QGa-QLa represent total real and power injection in the system. NB represents the total bus number. Gab and Bab represent the conductance and susceptance between ath and bth bus respectively.
Active power balance constraint
The sum of the load demand (PD) and PLoss must be equal to the amount of power produced, which can be mathematically formulated as
$$\sum\limits_{{n=1}}^{{NG}} {{P_{gn}} - {P_D} - {P_{Loss}}} =0$$
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Where PD represents the total load demand.
2.3.2. Inequality Constraints (ICs)
ICs on Independent Variables:
$$\left\{ {\begin{array}{*{20}{c}} {V_{{ge{n_a}}}^{{\hbox{min} }} \leqslant {V_{Ge{n_a}}} \leqslant V_{{ge{n_a}}}^{{\hbox{max} }}}&{a=1,2 \cdot \cdot \cdot NG} \\ {T_{a}^{{\hbox{min} }} \leqslant {T_a} \leqslant T_{a}^{{\hbox{max} }}}&{a=1,2 \cdot \cdot \cdot NT} \\ {Q_{{ca}}^{{\hbox{min} }} \leqslant {Q_{ca}} \leqslant Q_{{ca}}^{{\hbox{max} }}}&{a=1,2 \cdot \cdot \cdot NC} \end{array}} \right.$$
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Where, VGenamin and VGenamax represents the boundary limit of VGen. Tamin and Tamax represents the boundary limit of of transformer taps. QCamin and QCamax represents boundary limit of VAR compensation.
ICs on dependent Variables:
Reactive power has a significant impact on load voltage. Small changes in control variables may cause large deviation in load voltage and line power flow. Therefore, load bus voltage should be bounded within their limits and line power flow through each TL should be less than their upper limits.
$$\left\{ {\begin{array}{*{20}{c}} {Q_{{Ge{n_a}}}^{{\hbox{min} }} \leqslant {Q_{Ge{n_a}}} \leqslant Q_{{Ge{n_a}}}^{{\hbox{max} }}}&{a=1,2 \cdot \cdot \cdot NG} \\ {V_{{La}}^{{\hbox{min} }} \leqslant {V_{La}} \leqslant V_{{La}}^{{\hbox{max} }}}&{a=1,2 \cdot \cdot \cdot NL} \\ {{S_{la}} \leqslant S_{{la}}^{{\hbox{max} }}}&{a=1,2 \cdot \cdot \cdot TL} \end{array}} \right.$$
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Where QGenamin and QGenamax represents boundary limit of the reactive power of generator bus. VLamin and VLamax represents the boundary limit of PQ bus voltage.
2.4. Constraints Handling
Real-world optimization problems often include a number of constraints. Any optimization problem's solution can be considered practical if all of the restrictions are addressed. To keep the restrictions firmly within their boundaries, an effective approach to addressing constraints is necessary. As previously discussed, OPF is a real world highly nonlinear and mixed integer optimization problem that also requires an effective and reliable constrained handling approach for managing its ECs and ICs. Sections 2.3.1 and 2.3.2 of the current study define the methods used to deal with the constraints.
2.4.1 Handling Equality Constraints (ECs)
In an optimization problem, equality constraints are thought to be the hardest or most rigid constraints because they make the possible search space smaller. This slows down the optimization process in an actual way. So, the most common way to deal with equality constraints is to turn them into inequality constraints by using a small threshold value. Several additional methods for overcoming EC problems have also been presented in [26]. In the current OPF problem, the number of ECs increases as the bus size rises. For each bus, two ECs are associated, one for active power balance and the other for reactive power balance (described in equations (13). If there are "N" total vehicles in the system, "2N" ECs must be satisfied. In power system, these ECs are being addressed by load flow solution techniques. In the present work, the power flow problem is addressed by using the Newton-Raphson Load Flow (NRLF) solution technique. It should be emphasized that the ECs stated in equations (13 and 14) must be fully satisfied for NRLF to converge.
2.4.2 Handling inequality constraints (ICs)
Inequality limitations are lenient because they are simple to satisfy. Both the independent and dependent sides of the OPF problem include ICs, as shown by equations (15 and 16). To meet ICs on the side of the independent variable, a randomly generated value within that range is sufficient. The independent variable's upper or lower limit might be used to correct any violations that were spotted when updating the variable's value. However, ICs in the dependent variable may be addressed by punishing the solution that includes the violation [26]. The overall objective function taking into consideration power system operation and security limits is mathematically formulated using the penalty approach and is discussed in Eq. (17).
$$F=Fn+{\lambda _{QGena}}{\left( {{Q_{Gena}} - Q_{{Gena}}^{{\lim it}}} \right)^2}+{\lambda _{Ln}}{\left( {{V_{La}} - V_{{La}}^{{\lim it}}} \right)^2}+{\lambda _{TL}}{\left( {{S_{la}} - S_{{la}}^{{\lim it}}} \right)^2}$$
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Where Fn is the minimization objective function. (QGn-QGnlimit), (VLn-VLnlimit) and (Sl-Sllimit) represents the limit violation related to dependent variables. 𝜆Gn, 𝜆Ln and 𝜆𝑇𝐿 are the penalty factors which can be chosen by trial and error method. QGnlimit, VLnlimit and Sllimit are the limiting values which can be mathematically formulated as:
$$Q_{{Gn}}^{{\lim it}}=\left\{ {\begin{array}{*{20}{l}} {Q_{{Gn}}^{{\hbox{min} }}}&{if,{Q_{Gn}} \leqslant Q_{{Gn}}^{{\hbox{min} }}} \\ {Q_{{Gn}}^{{\hbox{max} }}}&{if,{Q_{Gn}} \geqslant Q_{{Gn}}^{{\hbox{max} }}} \\ {{Q_{Gn}}}&{if,Q_{{Gn}}^{{\hbox{min} }} \leqslant {Q_{Gn}} \leqslant Q_{{Gn}}^{{\hbox{max} }}} \end{array}} \right.$$
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$$V_{{Ln}}^{{\lim it}}=\left\{ {\begin{array}{*{20}{l}} {V_{{Ln}}^{{\hbox{min} }}}&{if,{V_{Ln}} \leqslant V_{{Ln}}^{{\hbox{min} }}} \\ {V_{{Ln}}^{{\hbox{max} }}}&{if,{V_{Ln}} \geqslant V_{{Ln}}^{{\hbox{max} }}} \\ {{V_{Ln}}}&{if,V_{{Ln}}^{{\hbox{min} }} \leqslant {V_{Ln}} \leqslant V_{{Ln}}^{{\hbox{max} }}} \end{array}} \right.$$
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$$S_{l}^{{\lim it}}=\left\{ {\begin{array}{*{20}{c}} {{S_l}}&{if,{S_l} \leqslant S_{{_{l}}}^{{\hbox{max} }}} \\ {S_{l}^{{\hbox{max} }}}&{if,{S_l}>S_{{_{l}}}^{{\hbox{max} }}} \end{array}} \right.$$
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Where QGn, VLn, and Sl represent the reactive power at PV bus, voltage at load bus, and limit of line power flow respectively.