Hybridized Symbiotic Organism Search Algorithm for the Optimal Operation of Directional Overcurrent Relays

This paper presents the solution of directional overcurrent relay (DOCR) problems using Simulated Annealing based Symbiotic OrganismSearch (SASOS).Theobjective function of the problem is tominimize the sumof the operating times of all primary relays. The DOCR problem is nonlinear and highly constrained with two types of decision variables, namely, the time dial settings (TDS) and plug setting (PS). In this paper, three models of the problem are considered, the IEEE 3-bus, 4-bus, and 6-bus, respectively. We have applied SASOS to solve the problem and the obtained results are compared with other algorithms available in the literature.


Introduction
Due to rapidly growing power systems, the stability and security issues are highly important for the power system researchers [1][2][3].The protection systems are mainly used to detect and clear faults as fast and selective as possible [4][5][6].The protection relays are used for detecting the faults in the system and to detach the faulty parts from the system in real time.Proper coordination of relays is essential to maintain the appropriate operation of the overall protection system.There are various types of relays with different operating principles.An example of these relays, which are used as a good technical tool for the protection of power systems, is directional overcurrent relay [7][8][9].Such a relay is divided into two units, that is, the instantaneous unit and the time overcurrent unit.The parameters to be defined in the overcurrent unit are the time dial settings (TDS) and the plug settings (PS).Computers have made the huge calculations in DOCR problems in power systems easy [10,11].Different optimization algorithms are used to solve the relays coordination problems in which the objective function is to minimize activity time of all main relays.The constraints of this optimization problem are considered in the second layer of relay, which should respond, if the main layer of relay fails to operate on nearby fault.This depends on the variables, TDS, PS, and the minimized working time of relay.There is a nonlinear relationship between the operating time of overcurrent relays, TDS, and PS.
In electrical engineering, the power system engineering has the longest history among all other areas.Ever since, different numerical optimization techniques have been applied to power systems engineering and played an important role [12].Optimization problems are usually nonlinear, which have nonlinear objective functions and constraints [13,14].

Problem Formulation
There are two important settings in each overcurrent relay for its satisfactory operations.The time dial settings (TDS) represent the activation time of each relay and the relay operation is decided by the plug settings (PS).The plug settings (PS) depend on the maximum load current and fault current due to short circuit.The main factors which control the total operating time of the relay are TDS and PS, and the fault current is represented by   [15,22], where where   denotes the fault current at the current transformer (CT) initial terminal and CT pri-rating is the primary rating of CT.Constants , , and  are assigned values 0.14, 0.02, and 1.0, respectively, and that is according to IEEE standards [26].The current, seen by the relay, denoted by  relay , is equal to the ratio between   and CT pri-rating , which is a nonlinear equation: 2.1.Objective Function.In coordination studies [22], the main objective is to minimize the total time taken in operation of primary relays for clearing a fault.The objective function takes the following form: where where  pri-cl-in is the relay operation time to clear a near-end fault while  pri-far-bus is its operation time in case of a far end fault. cl and  far represent the relays fixed at the ends of the front line.

Constraints.
The objective function is bound to the three constraints related to TDS, PS, and   .Equation ( 5) represents the bound constraints on TDS: TDS min

𝑖
and TDS max  are the lower and upper bounds for TDS, whose values are given by 0.05 and 1.1, respectively, while  varies from 1 to  cl .
The second constraint is PS of the relay that takes the following form: PS min  and PS max  are the lower and upper bounds of PS, whose values are given by 1.25 and 1.50, respectively, while  varies from 1 to  far .
The third constraint is related to the fault current and pickup current.The operating time of relay depends on the type of relay.According to [24,33], the operating time of relay is defined by min  and  max  are the lower and upper bounds for relay functioning time, whose values are adopted as 0.05 and 1, respectively.
During the optimization procedure, the time taken by primary relays to coordinate with the backup relays is constrained as in where CTI represents the specified coordination time.
backup and  primary represent the working time of primary and backup relays, which can be obtained using the following equations: DOCR relay (3) (5) Figure 1: The 3-bus model.

The Standard IEEE System of 3-Bus.
There are six overcurrent relays in this model.According to the number of relays, the value assigned to each of  cl and  far is 6 and the number of decision variables is 12, and this means TDS 1 to TDS 6 and the variables PS 1 to PS 6 .Figure 1 shows the 3-bus model.
Mathematically the objective function can be written as where The values of constants    , CT  pri-rating ,    , and CT  pri-rating are shown in Table 6.
Constraints imposed on the 3-bus system are as follows.
Limits on Variables TDS  .TDS min  ≤ TDS  ≤ TDS max
Another constraint limits each term of objective between 0.05 and 1.
Selectivity constraints on working time of backup relay and the primary relay are given in the following relation: where CTI takes the value 0.3.Here, Figure 2: The 4-bus model.
( The values of the constants for  backup and  primary of 3-bus model are given in Table 7.

The Standard IEEE System of 4-Bus.
In the IEEE 4-bus model, there are 8 overcurrent relays.Accordingly, the value of  cl and  far is 8, which is twice the number of transmission lines and the number of decision variables is 16.The 4-bus model is shown in Figure 2. CTI takes the value for this model as 0.3.Total selectivity constraints are 9.
Moreover, the values of constants for the 4-bus model are given in Tables 8 and 9.

The Standard IEEE System of 6-Bus.
In this section, the IEEE 6-bus model is elaborated.Here, value of  cl and  far is 14.Hence, there are 28 variables in this problem, namely, TDS 1 to TDS 14 and PS 1 to PS 14 .The 6-bus model is shown in Figure 3. CTI takes the value 0.2 for this model.There are 48 selectivity constraints in this problem.Based on the observation of [48], we have relaxed 10 constraints.The values of constants    , CT  pri-rating ,    , and CT  pri-rating for Model 3 are given in Tables 10 and 11.A summary of all components involved in models of IEEE 3-bus, 4-bus, and 6bus is presented in Table 5.For  = 1 to ecosize (4) Mutualism Phase (5) Simulated annealing on   (6) Simulated annealing on   (7) Transform   and   (8) Commensalism Phase (9) Simulated annealing on   (10) Apply Eq (15) to reduce the temperature (11) Transform   (12) Parasitism Phase (13) Create parasite vector (14) Update   (15) Transform   (16) Identify the best organism  best (17) End For (18) End While Algorithm 1: The pseudo code for SASOS [47].

Hybridization of Simulated Annealing (SA) and Symbiotic Organism Search (SOS)
The hybrid algorithm is named as SASOS [47].In SASOS algorithm, the new solutions are created using the three phases of SOS algorithm by shifting the earlier solutions towards randomly picked solutions from the system [49].The convergence rate of SOS algorithm is slow due to its strategy for locating the global optima.If the updated solution is better than previous solution, the convergence will be faster.Therefore, to further improve the convergence rate, Simulated Annealing technique is employed in the mutualism and commensalism phase of the SOS algorithm.The parasitism stage is left unchanged [47].A pseudo code for SASOS algorithm is given in Algorithm 1.The SA technique accepts poor neighbour solutions along with the current best solutions by implementing certain probability.In each generation, probability of selecting the bad solutions is higher at the initial stages but decreases at the later stages of the search.This probability of either selecting poor solutions or good ones is calculated as in where ( *  ) and (  ) are the fitness values of th solution and current best solutions, respectively, and here  is the control parameter.Equation ( 15) is used to reduce the temperature in Simulated Annealing:

Experimental Settings and Discussion
4.1.Experimental Settings.The parameters involved in SASOS are ecosize, initial temperature  init , final temperature  final , and cooling rate .The ecosize is taken as 50 for all the cases.The value of initial temperature  init is taken as 1, while the final temperature is 10 −10 and cooling rate  is taken as 1.
For a fair comparison, a total of 30 simulations were performed for each of the three cases and the current best solution through the simulation was recorded as the current best solution.
4.2.Discussion.Performance of the SASOS algorithm is analyzed based on the best objective values (see Figures 5, 7,  and 9) and total function evaluations taken by algorithms (see Figures 4, 6, and 8).The outcomes of SASOS are highlighted and compared with the values obtained by different versions of Differential Evolution and other algorithms available in the literature [17].The results for 3-bus, 4-bus, and 6-bus systems are given in Tables 1, 2, and 3, respectively.
The best solutions obtained by SASOS and other versions of DE for IEEE 3-bus model in terms of best decision features, minimum objective function value, and number of function evaluations taken to complete each simulation are given in Table 1 and Figures 4 and 5. Here, it is evident that, in terms of objective function value, DE gave the worse objective value as 4.8422 and all the other versions of DE algorithm gave almost similar values.On the other hand, SASOS gave us an objective value 4.7201 which is much better than other algorithms.However, if we compare the number of function evaluations (NFE), then the performances of DE and its variants are worse as compared to SASOS algorithm as it took only 31500 NFE to   Furthermore, the results for IEEE 6-bus model as in Table 3 and Figures 8 and 9, in terms of best objective function value, are again slightly different to each other with MDE-4 and MDE-5 giving slightly improved solutions than other variant algorithms of DE.On the other hand, SASOS gave us an objective value 9.8948 which is much better than other results in the table.However, in terms of NFE, the worst convergence rate was shown by DE while SASOS took 35000 NFE to get the best solution presented in the table.
Summarily, it can be observed from Tables 1, 2, and 3 that SASOS provides better solutions to the given problems compared to DE.Also, the number of function evaluations    values obtained by other algorithms such as DE, RST2, SOMA, and SOMGA for all the three models, as reported in [50], LX-POL and LX-PM [51], and MDE algorithms [17].It can be observed from Table 4 that SASOS provided better results for all the three systems among all results quoted in [17], in terms of best objective values.

Conclusion
We present a conclusion of this research as follows: (i) The problems of optimal coordination of directional overcurrent relays are highly nonlinear, NP-hard, and highly constrained in nature.
(ii) Dealing with the DOCR problems, use of efficient metaheuristic is needed.
(iii) SASOS is implemented to solve the problem of DOCRs for standard IEEE 3-, 4-, and 6-bus systems.(iv) The outcome of our simulations shows that SASOS can efficiently minimize all the three models of the problem.
(v) The efficiency of SASOS can be observed from the minimum function evaluations required by the algorithm to reach the optimum as compared to other standard algorithms.
(vi) In future, one can extend this work to solve problems of higher buses and complex power systems.Moreover, extensive statistical analysis and parameters tuning can further highlight and improve the efficiency of SASOS.

Nomenclature
, , : Constants according to IEEE standard : Cooling rate in Simulated Annealing CT: Current transformer CT pri-rating : Primary rating of current transformer CTI: Coordination time interval DE: Differential Evolution DOCR: Directional overcurrent relays 14 × TDS  (   / (PS  × CT  pri-rating ))

Figure 6 :
Figure 6: Comparison of number of function evaluations done, for obtaining the best results, by SASOS algorithm with DE and its modified versions for 4-bus model.

Figure 7 :
Figure 7: Comparison of best objective values obtained by SASOS algorithm with DE and its modified versions for 4-bus model.

Figure 9 :
Figure 9: Comparison of best objective values obtained by SASOS algorithm with DE and its modified versions for 6-bus model.

Table 1 :
Optimal values of design variables, objective values, and function evaluations obtained by SASOS, DE, and modified DE: IEEE 3-bus system.TDS 1 TDS 2 TDS 3 TDS 4

Table 2 :
Optimal values of design variables, objective values, and function evaluations obtained by SASOS, DE, and modified DE: IEEE 4-bus system.

Table 3 :
Optimal values of design variables, objective values, and function evaluations obtained by SASOS, DE, and modified DE: IEEE 6-bus system.NFE) taken by SASOS algorithm is considerably reduced in comparison to DE algorithms.In Table 4, the objective function values obtained by SASOS are compared with the (

Table 4 :
Comparison of results obtained by SASOS and other algorithms for IEEE 3-, 4-, and 6-bus systems.

Table 5 :
The summary table of different components involved in the problem of DOCR.

Table 6 :
Values of constants for objective function of 3-bus model.

Table 7 :
Values of constants for  backup and  primary of 3-bus model.

Table 8 :
Values of constants for objective function of 4-bus model.

Table 9 :
Values of constants for  backup and  primary of 4-bus model.

Table 10 :
Values of constants for objective function of 6-bus model.

Table 11 :
Values of constants for  backup and  primary of 6-bus model.