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International Journal of Electrical and Computer Engineering (IJECE)
Vol. 10, No. 5, October 2020, pp. 4552~4561
ISSN: 2088-8708, DOI: 10.11591/ijece.v10i5.pp4552-4561  4552
Journal homepage: http://ijece.iaescore.com/index.php/IJECE
Islanded microgrid congestion control by load prioritization
and shedding using ABC algorithm
L. O. Mogaka1
, G. N. Nyakoe2
, M. J. Saulo3
1
Department of Electrical and Electrinics Engineering, Pan African University Institute for Basic Sciences,
Technology and Innovation (PAUSTI), Kenya
2
Department of Electrical Engineering, Jomo Kenyattan University of Agriculture and Technology, Kenya
3
Department of Electrical Engineering, Technical University of Mombasa, Kenya
Article Info ABSTRACT
Article history:
Received Feb 15, 2019
Revised Mar 23, 2020
Accepted Apr 3, 2020
The continued growth in load demand and the gradual change of generation
sources to smaller distributed plants utilizing renewable energy sources (RESs),
which supply power intermittently, is likely to strain existing power systems
and cause congestion. Congestion management still remains a challenging
issue in open access transmission and distribution systems. Conventionally,
this is achieved by load shedding and generator rescheduling. In this study,
the control of the system congestion on an islanded micro grid (MG)
supplied by RESs is analyzed using artificial bee colony (ABC) algorithm.
Different buses are assigned priority indices which forms the basis of
the determination of which loads and what amount of load to shed at any
particular time during islanding mode operation. This is to ensure as minimal
load as possible is shed during a contingency that leads to loss of mains and
ensure a congestion free microgrid operation. This is tested and verified on
a modified IEEE 30-bus distribution systems on MATLAB platform.
The results are compared with other algorithms to prove the applicability of
this approach.
Keywords:
ABC algorithm
Congestion control
Islanding
Load shedding
Power losses reduction
Renewable energy sources
Copyright © 2020 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
L. O. Mogaka,
Department of Electrical and Electronics Engineering,
Pan African University Institute for Basic Sciences, Technology and Innovation (PAUSTI),
P. O. Box 62000-00200, Nairobi, Kenya.
Email: Mogaka.Lucas@gmail.com
1. INTRODUCTION
The electrical power market deregulation and the gradual evolution of the grids towards smart grid
(SG) is currently being witnessed in an effort to supply power to the increasing population and flourishing
industries. The RESs in the SGs are expected to be more resilient with the ability to survive system
difficulties during contingencies [1, 2]. However, this change from conventional monopolistic system to
deregulated system may make the system to operate beyond its voltage and thermal limits hence transmission
and distribution system congestion [3]. Congestion can be defined as difference in the scheduled and
the actual power flow in a given line without violating the set limits [4]. It is a condition where more power is
scheduled to flow across transmission, distribution lines and transformers than what those lines can carry [5].
Operation in islanded mode is recommended in order to prevent total blackouts in the power system [6].
This ensures power is supplied to customers reliably in a safe way and maintains the system security.
The methods used for congestion management (CM) can be classified into two; Cost free and non-
cost-free methods [7]. The cost-free method includes use of FACTS devices, outaging congested lines, and
the operation of transformer taps. On the other hand, the non-cost-free methods include shedding of some
loads, generator rescheduling among others. The increase in competitive pressure in the electrical power
industry necessitates removal of overloads through load shedding bearing in mind the economic and the cost
Int J Elec & Comp Eng ISSN: 2088-8708 
Islanded microgrid congestion control by load prioritization… (L. O. Mogaka)
4553
of shedding the load [8]. In addition to the above conventional methods, evolutionary algorithms like ABC,
particle swarm optimization (PSO), ant colony optimization (ACO), cuckoo search algorithm (CS), harmony
search algorithm (HS), shuffled frog leaping algorithm (SFLP), simulated annealing algorithms (SA) among
others are also increasingly being used in CM [9-11].
A number of studies on CM have been done so far. For instance, reference [12] proposes two
measures that can be used to control congestion; generator rescheduling and loss reduction of real power.
In [13], the author proposes the use of FACTS devices in the management of congestion at transmission level.
In [14], rescheduling of sensitive generators and load shedding is proposed for CM using PSO algorithm.
In [15], the author used the sensitivities of overloaded lines together with the cost of power generation and
load shedding in CM. The load-shedding schedule and new power generation for affected buses was
calculated using line sensitivities and the cost.
It is challenging to control the power produced in an islanded MG by RESs because these sources
produce power intermittently. Load shedding is the main method of controlling system stability in this case
and avoid any danger within the MG [16]. Optimal shedding of connected loads assists in reducing
the difference between the power that DGs can supply and the connected loads [17]. In [18], ABC algorithm
is used for overload control by rescheduling generators. The author uses generator sensitivity factors in
selecting the generators that participate in real power rescheduling. In [19], ABC and PSO algorithms were
used in generator rescheduling based on bus and generator sensitivity factors for CM in a system with wind
energy resources only. In [20], the authors used PSO in generation rescheduling and load shedding in CM.
In [21], congestion alleviation in a deregulated power system is proposed using cuckoo search algorithm.
This was tested on IEEE 30-bus system. A modification of ABC algorithm is applied in the determination of
the DG power output and their location in reference [22]. This was tested on 33 bus system.
Through load shedding, the amount of power demand that can be curtailed so as to mitigate CM
problem is determined. From the literature, extensive research on CM on MGs has been done. However,
CM using load shedding algorithms still remains an open issue and needs further research. In this paper,
the CM problem in an islanded MG supplied with RESs is analyzed. This is formulated as an optimization
problem. ABC algorithm is used in the determination of the optimal amount of load to be shed and from
which buses based on their priority index. This rest of this paper is organized as follows; section 2 discusses
the proposed congestion management method, its mathematical formulation and how ABC algorithm will be
applied; section 3 analysis the simulation results and 4 summarizes and concludes this paper.
2. PROPOSED CM SCHEME IN AN ISLAND WITH RESs
This simulation assumes that MG is suddenly switched to island operation mode due to
a contingency that leads to loss of mains. After a utility grid disturbance that lead to islanding mode,
the power in the MG is allocated to the loads as per their priority index. Figure 1 below is a sketch of a MG
with RESs that was used for CM analysis in this study. The loads and buses within the islanded MG were
given priority indices, and then ABC algorithm was modified and applied in shedding loads in order to
ensure the system is not stretched beyond its limits. Figure 2 is the proposed load shedding power flow for
this study.
Figure 1. Micro grid with RESs
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Figure 2. Load shedding power flow for CM
2.1. Mathematical problem formulation
The main objective of this paper is to alleviate elements of system congestion in an islanded
MG through minimizing overloads by optimal load shedding using ABC algorithm. This is expressed
the expressions 1-3 below;
𝑀𝑖𝑛𝑖𝑚𝑖𝑧𝑒 𝐹1 = ∑(𝑆𝑖 − 𝑆𝑖
𝑚𝑎𝑥
)2
𝑂𝐿
𝑖=1
(1)
𝑀𝑖𝑛𝑖𝑚𝑖𝑧𝑒 𝐹2 = ∑(𝑃𝑖 + 𝑄𝑖 𝑃𝑔𝑖 + 𝑅𝑖 𝑃𝑔𝑖
2
) + |𝑒𝑖 ∗ sin(𝑓𝑖 ∗ (𝑃𝑔𝑖 − 𝑃 𝑚𝑖𝑛,𝑖))|
𝑁𝐺
𝑖=1
+ ∑(𝑃𝑘
′
+ 𝑄 𝑘
′
𝐿 𝑠ℎ𝑑,𝑘)
𝑃𝐿
𝑘=1
(2)
𝑀𝑖𝑛𝑖𝑚𝑖𝑧𝑒 𝑃𝑙 = 𝐹(𝑃𝑔,𝑠𝑙𝑎𝑐𝑘) (3)
Start
Initialize food positions and algorithm parameters
Evaluate connected loads ( 𝑃𝐿𝑑) and
system generation (𝑃𝑔𝑒𝑛) capacity
Assign important loads using
priority index
Is 𝑃𝑔𝑒𝑛 < 𝑃𝐿𝑑?
Is Pgen ≥
Important load′
s demand??
Power all
loads
No
Yes
Power important loads
Yes
Shed loads
equivalent to
the difference
No
Operational
constraints violated?
Yes
No
End
Int J Elec & Comp Eng ISSN: 2088-8708 
Islanded microgrid congestion control by load prioritization… (L. O. Mogaka)
4555
where 𝑃𝑙 is the system real power loss, 𝑆𝑖
𝑚𝑎𝑥
is the maximum capacity of line i, 𝑂𝐿 is the number of the lines
that are overloaded ,𝐿 𝑠ℎ𝑑,𝑘is the amount of load to be shedded at bus k, 𝑃𝑔𝑖 is the power generated by
generator i, 𝑆𝑖 is the power flow on line i, 𝑁𝐺 is the number of participating generators, 𝑃 𝑚𝑖𝑛,𝑖 is
the minimum power generated by generator i, 𝑓𝑖 and 𝑒𝑖are the generator coefficients for generator.
Subject to the equality and inequality constraints (4-10) below:
Equality constraints,
𝑃𝑔𝑖 − 𝑃𝑑𝑖 = ∑|𝑉𝑖||𝑉𝑗||𝑌𝑖𝑗| cos(𝛿𝑖 − 𝛿𝑗 − 𝜃𝑖𝑗)
𝑁𝐵
𝑗=1
(4)
𝑄 𝑔𝑖 − 𝑄 𝑑𝑖 = ∑|𝑉𝑖||𝑉𝑗||𝑌𝑖𝑗| sin(𝛿𝑖 − 𝛿𝑗 − 𝜃𝑖𝑗)
𝑁𝐵
𝑗=1
(5)
where |𝑉𝑖| 𝑎𝑛𝑑 |𝑉𝑗| are voltage magnitudes.
The distributed generation RESs constraints
𝑃𝑔𝑖
𝑚𝑖𝑛
≤ 𝑃𝑔𝑖 ≤ 𝑃𝑔𝑖
𝑚𝑎𝑥
(6)
𝑄 𝑔𝑖
𝑚𝑖𝑛
≤ 𝑄 𝑔𝑖 ≤ 𝑄 𝑔𝑖
𝑚𝑎𝑥
(7)
where 𝑃𝑔𝑖
𝑚𝑖𝑛
𝑎𝑛𝑑 𝑃𝑔𝑖
𝑚𝑎𝑥
are Real power generation limits of the RES i and 𝑄 𝑔𝑖 is the VAR generated by RES i
Voltage constraint
𝑉𝑔𝑖
𝑚𝑖𝑛
≤ 𝑉𝑔𝑖 ≤ 𝑉𝑔𝑖
𝑚𝑎𝑥
(8)
𝑉𝑑𝑖
𝑚𝑖𝑛
≤ 𝑉𝑑𝑖 ≤ 𝑉𝑑𝑖
𝑚𝑎𝑥
(9)
where 𝑉𝑔𝑖 and 𝑉𝑑𝑖is voltage magnitude at generator and load bus i respectively.
∑ 𝑃𝑔𝑖
𝑁𝐺
𝑖=1
= ∑ 𝑃𝑑𝑘 + 𝑃𝑙
𝑁𝐷
𝑘=1
(10)
where 𝑃𝑑𝑘 is the real power load at bus k, 𝑁𝐺 and 𝑁𝐷 are the sets of generator and load buses respectively.
2.2. ABC algorithm
Swarm intelligence algorithms are the currently preferred approaches in solving dynamic
optimizationNP-hard problems in engineering [23]. ABC algorithm is a meta-heuristic optimization
algorithm that was proposed by Karaboga from Erciyes University of Turkey [24]. This algorithm mimics
the foraging movement of a swarm of bees in search of nectar around the hive as shown in Figure 3 [22].
In this algorithm, the location of food sources represents a possible solution to an optimization problem being
solved. The quantity and quality of nectar indicates the fitness of that particular solution.
It has a superior performance in solving engineering problems when compared to other algorithms.
For instance, it can handle both continuous and discrete variables. It is mostly applied to optimization
problems in power systems that are not smooth. It is divided into three groups as observed from the behavior
of a swarm of bees where each group performs a particular function as highlighted below [25];
 The employed bees. They are equal to the possible solutions of the problem. This group continuously
updates the rest of the bees in the hive about the quantity, quality and the direction of the food source
through the performance of a waggle dance. The duration of the waggle dance depends on the quality and
quantity of food source.
 The onlooker group of bees. These bees pick on a food source to exploit based on the information
provided by the employed bees waggle dance. More onlookers move to food sources with high fitness and
few to food sources with lesser fitness values.
 The scout bees. These bees work is to look for new sources of food for exploitation. They randomly
choose food sources around the hive.
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Figure 3. ABC search behavior
2.3. ABC algorithm steps
The initial phase where random population is generated. The solution size is taken to be equal to
the number of employed bees. Each of the above solution is a vector of dimension D. This dimensional
vector D corresponds with the number of parameters being optimized. The fitness (quality) of a food source
can be expressed by (11):
𝑓𝑖𝑡𝑛𝑒𝑠𝑠𝑖 =
1
1 + 𝑓𝑖
(11)
where objective function 𝑓𝑖 is the problem formulation target.
The above generated initial population of possible solutions is then sequentially subjected through
the three categories of bees in cycles till the specified maximum cycle number (MCN) is reached. Employed
bees keeps on modifying the memories of the food locations (solutions) based on the visual observations and
nectar quality. The employed bees share the food source information with the rest of the bees in the hive
through waggle dance.
The onlooker bee will choose a preferred food source based on the amount of nectar available and as
per the probability guided by the equation below [26];
𝑝𝑖 =
𝑓𝑖𝑡𝑛𝑒𝑠𝑠𝑖
∑ 𝐹𝑖𝑡𝑛𝑒𝑠𝑠𝑗
𝑡𝑠
𝑗=1
(12)
where 𝑡𝑠 is the number of employed bees (= number of the sources of food) and 𝑓𝑖𝑡𝑛𝑒𝑠𝑠𝑖 is the fitness value
of the 𝑖 𝑡ℎ
solution.
The onlooker bee then compares the food sources from the information given by employed
bees and then chooses a neighbor food source if it is better than the current one using the following
equation;
𝑋𝑖𝑗
𝑛𝑒𝑤
= 𝑋𝑖𝑗
𝑜𝑙𝑑
+ 𝑄𝑖𝑗(𝑋𝑖𝑗
𝑜𝑙𝑑
− 𝑋 𝑘𝑗) (13)
where 𝑄𝑖,𝑗 is a random number that lies between -1 and 1, 𝑋 𝑘,𝑗 is the neighbouring food source that can
be randomly selected and 𝑋𝑖𝑗
𝑜𝑙𝑑
and 𝑋𝑖𝑗
𝑛𝑒𝑤
are old and new food sources respectively.
If the food source does not improve after a number of trials, it is abandoned and the associated bee
becomes a scout. These steps are continuously repeated until the maximum cycle or stopping criteria is met.
Then, the food with the highest fitness value is selected and printed.
3. RESULTS AND DISCUSSIONS
IEEE 30-bus is used to test applicability of this approach in congestion management. This system
has a total of six generators, 41 lines, 24 load buses and 21 loads. It has a total of 283.400MW active and
126.200 MVAR reactive loads connected to the system. This is shown in Figure 4. The simulation was
carried out on MATLAB/SIMULINK platform on an AMD 4C+6G 2.10 GHz processor with 4GB RAM.
The control parameters for ABC algorithm were set as shown in the Table 1.
Int J Elec & Comp Eng ISSN: 2088-8708 
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Figure 4. IEEE standard 30 bus system
Table 1. ABC parameter settings
S/N Parameter Value
1 Colony size 40
2 Employed bees 20
3 Limit value 100
4 Number of onlooker bees 20
5 MCN 100
Two test cases were simulated and Newton Raphson power flow run to test the congestion status in
the island. The test cases done are shown in Table 2. In this case, congestion is simulated by creating an
outage on lines 1-2 and 1-7 and overloading the lines.
Table 2. Test cases description
Test case Contingency Description
1 Outage of line 1-2
2 Outage of line 1-7 with increase in load at all buses by 50%
3.1. Case 1
The outage of line 1-2 brings congestion between lines 1-7 and 7-8 of the modified system. The total
system loss increases to 16.069 MW from 7.376 MW. This is compared with the CM using firefly algorithm
as shown in the Table 3.
Table 3. Total system loss for case 1
MW MVar
Normal condition 7.376 -15.043
Outage of line 1-2 contingency 16.069 21.303
After load shedding (20.552MW) using ABC algorithm 11.649 4.305
Using Firefly algorithm 13.10 Not given
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From the load flow, the power flowing between the two congested lines becomes 147.509 and
136.348 MW respectively against the set limit of 130MW. This condition is alleviated by shedding 20.552
MW load from the buses with least priority indices using ABC algorithm. After load shedding, the power
flow between the two congested lines comes down to 122.537 MW and 114.093 MW which is well within
the limit. This is as shown in Table 4. The Figure 5 shows the voltage profiles at various buses for normal,
during the fault and after load shedding using ABC algorithm. There is a great improvement in the voltage
profiles on the system buses after load shedding.
Table 4. Power flow for case 1
Congested line Actual power flow (MW) Power flow after load shedding using ABC Line limit (MW)
1-7 147.348 122.537 130
7-8 136.348 114.093 130
Figure 5. Voltage profile for test case 1
3.2. Case 2
In this section, the system loads were increased by 50% and then line 1-7 outaged. This caused
congestion on lines 1-2, 2-8 and 2-9 of 313.914MW, 98.411MW and 104.769 MW respectively against
the set limits of 130MW for line 1-2 and 65MW for lines 2-8 and 2-9. In this study, the total power Flow
violation on the lines due to congestion is 257.088MW while it is 251.794MW in the study by reference [4].
This is shown in the Table 5.
The system losses were also monitored during and after load shedding using ABC algorithm.
Applying ABC algorithm, 41.268 MW was shed to mitigate congestion levels. The total losses were recorded
as shown in Table 6. The total losses decreased from 38.164 during the contingency to 17.485 MW after load
shedding. This is compared by the congestion management approach using firefly algorithm in reference [4].
As can be observed from the figure, the total power losses for the two approaches is almost the same. This is
elaborated in Figure 6.
Table 5. Congested lines for case 2
Congested line Actual power flow (MW) Line limit (MW)
1-2 313.914 130
2-8 98.411 65
2-9 104.769 65
Table 6. Total losses for case 2
MW MVar
Normal condition 7.376 -15.043
Outage of line 1-7 and 50% load increase 38.164 97.773
After load shedding (41.268MW) using ABC algorithm 26.770 54.706
0,96
0,98
1
1,02
1,04
1,06
1,08
1,1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
VOLTAGEMAGNITUDE
BUS NUMBER
Normal operating condition
Overloaded Condition
After loadshedding
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Figure 6. System losses comparison for case 2
The CM by load shedding using ABC algorithm was compared with CM by generator rescheduling
using FFA, PSO, RSM and SA as reported in [4]. The power flows comparison is as shown in Table 7.
The voltage profiles before, during the contingency and after CM by load shedding using ABC algorithm
were monitored. The CM by load shedding using ABC algorithm improves the voltage profiles within
the islanded MG. This is shown in Figure 7.
Table 7. Comparison of various algorithms in CM
Power flow (MW) FFA PSO RSM SA ABC
Line 1-2 130 129.7 129.91 129.78 204.225
Line 2-8 62.713 61.1 52.36 51.47 59.509
Line 2-9 64.979 64.67 55.43 54.04 63.396
Figure 7. Voltage profiles for case 2
Convergence characteristics for this case was also monitored before the contingency, during
the contingency and after load shedding. The iteration converged 14 iterations before the contingency,
after 35 iterations during the contingency and converged after 29 iterations after load shedding. This is shown
in the Figure 8.
0
5
10
15
20
25
30
35
40
45
Using Firefly algorithm Using ABC algorithm
Losses(MW)
Total losses during contingency
Losses after CM
0,94
0,96
0,98
1
1,02
1,04
1,06
1,08
1,1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
VoltageMagnitude
Bus Number
Normal Voltage Mag
Line 1-7 Outage and 50% load increse Voltage Mag
Voltage Mag after shedding 41.268MW
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Figure 8. Convergence characteristics
4. CONCLUSION
This paper has presented an approach for congestion management on an islanded MG with RESs.
Here, load shedding using ABC algorithm has been successifully employed for congestion management in an
islanded microgrid. Various loads and buses were chosen for shedding based on their priority index to help
mitigate the congestion problem. This was tested and validated on a modified IEEE 30 bus system with
contingencies and sudden load increase in the micrgrid introduced. The results were compared with those
from other algorithms like firefly algorithm and PSO as reported in the literature. The results show a great
superiority of this approach over these algorithms. As it can be observed, this proposed method can greatly
improve the system stability of the islanded MG through the minimization of the load shed and maintain
the voltage profile within the required limits. This is due to superior convergence characteristics of ABC
algorithm. This approach can therefore berecommeded in the solution of optimization problems in
engineering and other fields as well.
ACKNOWLEDGMENT
This research was supported by the Pan African University Institute for Basic Sciences, Technology
and Innovation in the form of a postgraduate student research funding.
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cost-efficient generation rescheduling and load shedding for congestion management," International Journal
Electrical Power Energy System, vol. 27, no. 5, p. 379-388, 2005.
[16] C. Yeongho, L. Yujin and K. Hak-Man, "Optimal load shedding for maximizing satisfaction in an islanded
microgrid," Energies, vol. 10, no. 45, pp. 1-13, 2017.
[17] S. Fatma and K. Salah, "Optimal load shedding for voltage collapse prevention using improved harmony search
algorithm," Nineteenth International Middle East Power Systems Conference, 2017.
[18] R. Saranya, K. Balamurugan and M. Karuppasamypandiyan, "Artificial bee colony algorithm based congestion
management in restructured power system," Indian Journal of Science and Technology, vol. 8, no. 7, pp. 171-178, 2015.
[19] R. Mageshvaran and T. Jayabarathi, "Steady state load shedding to prevent blackout in the power system using
artificial bee colony algorithm," Jurnal Teknologi (Sciences & Engineering), vol. 74, no. 1, pp. 113-124, 2015.
[20] T. Bhattacharjee and A. K. Chakraborty, "Congestion management in a deregulated power system by rescheduling
of sensitive generators and load curtailment using PSO," IJETAE, vol. 2, no. 3, pp. 284-289, 2012.
[21] N. Chidambararaj and K. Chitra, "Congestion management based on active power rescheduling of generator units
using cuckoo search algorithm," International Journal of Scientific & Engineering Research, vol. 5, no. 4,
pp. 33-38, 2014.
[22] N. B. M. Mohd and F. Goro, "Distribution network power loss by using artificial bee colony," IEEE, pp. 1-5, 2013.
[23] V. Z. Manusov, P. V. Matrenin and N. Khasanzoda, "Swarm algorithms in dynamic optimization problem of
reactive power compensation units control," International Journal of Electrical and Computer Engineering
(IJECE), vol. 9, no. 5, pp. 3967-3974, 2019.
[24] D. Karaboga, B. Gorkemli, C. Ozturk and and N. Karaboga, "A comprehensive survey: Artificial bee colony (ABC)
algorithm and applications," Artificial Intelligence Reviews, vol. 42, pp. 21-57, 2012.
[25] K. M. Sagayam and D. J. Hemanth, "ABC algorithm based optimization of 1-D hidden Markov model for hand
gesture recognition applications," Computers in Industry, vol. 99, pp. 313-323, 2018.
[26] B. Hamid, B. Bachir and R. Abdelhadi, "Analysis of the impact of metal thickness and geometric arameters on
the quality factor-Q in integrated spiral inductors by means of artificial bee colony technique," International
Journal of Electrical and Computer Engineering (IJECE), vol. 9, no. 4, pp. 2918-2931, 2019.

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Islanded microgrid congestion control by load prioritization and shedding using ABC algorithm

  • 1. International Journal of Electrical and Computer Engineering (IJECE) Vol. 10, No. 5, October 2020, pp. 4552~4561 ISSN: 2088-8708, DOI: 10.11591/ijece.v10i5.pp4552-4561  4552 Journal homepage: http://ijece.iaescore.com/index.php/IJECE Islanded microgrid congestion control by load prioritization and shedding using ABC algorithm L. O. Mogaka1 , G. N. Nyakoe2 , M. J. Saulo3 1 Department of Electrical and Electrinics Engineering, Pan African University Institute for Basic Sciences, Technology and Innovation (PAUSTI), Kenya 2 Department of Electrical Engineering, Jomo Kenyattan University of Agriculture and Technology, Kenya 3 Department of Electrical Engineering, Technical University of Mombasa, Kenya Article Info ABSTRACT Article history: Received Feb 15, 2019 Revised Mar 23, 2020 Accepted Apr 3, 2020 The continued growth in load demand and the gradual change of generation sources to smaller distributed plants utilizing renewable energy sources (RESs), which supply power intermittently, is likely to strain existing power systems and cause congestion. Congestion management still remains a challenging issue in open access transmission and distribution systems. Conventionally, this is achieved by load shedding and generator rescheduling. In this study, the control of the system congestion on an islanded micro grid (MG) supplied by RESs is analyzed using artificial bee colony (ABC) algorithm. Different buses are assigned priority indices which forms the basis of the determination of which loads and what amount of load to shed at any particular time during islanding mode operation. This is to ensure as minimal load as possible is shed during a contingency that leads to loss of mains and ensure a congestion free microgrid operation. This is tested and verified on a modified IEEE 30-bus distribution systems on MATLAB platform. The results are compared with other algorithms to prove the applicability of this approach. Keywords: ABC algorithm Congestion control Islanding Load shedding Power losses reduction Renewable energy sources Copyright © 2020 Institute of Advanced Engineering and Science. All rights reserved. Corresponding Author: L. O. Mogaka, Department of Electrical and Electronics Engineering, Pan African University Institute for Basic Sciences, Technology and Innovation (PAUSTI), P. O. Box 62000-00200, Nairobi, Kenya. Email: Mogaka.Lucas@gmail.com 1. INTRODUCTION The electrical power market deregulation and the gradual evolution of the grids towards smart grid (SG) is currently being witnessed in an effort to supply power to the increasing population and flourishing industries. The RESs in the SGs are expected to be more resilient with the ability to survive system difficulties during contingencies [1, 2]. However, this change from conventional monopolistic system to deregulated system may make the system to operate beyond its voltage and thermal limits hence transmission and distribution system congestion [3]. Congestion can be defined as difference in the scheduled and the actual power flow in a given line without violating the set limits [4]. It is a condition where more power is scheduled to flow across transmission, distribution lines and transformers than what those lines can carry [5]. Operation in islanded mode is recommended in order to prevent total blackouts in the power system [6]. This ensures power is supplied to customers reliably in a safe way and maintains the system security. The methods used for congestion management (CM) can be classified into two; Cost free and non- cost-free methods [7]. The cost-free method includes use of FACTS devices, outaging congested lines, and the operation of transformer taps. On the other hand, the non-cost-free methods include shedding of some loads, generator rescheduling among others. The increase in competitive pressure in the electrical power industry necessitates removal of overloads through load shedding bearing in mind the economic and the cost
  • 2. Int J Elec & Comp Eng ISSN: 2088-8708  Islanded microgrid congestion control by load prioritization… (L. O. Mogaka) 4553 of shedding the load [8]. In addition to the above conventional methods, evolutionary algorithms like ABC, particle swarm optimization (PSO), ant colony optimization (ACO), cuckoo search algorithm (CS), harmony search algorithm (HS), shuffled frog leaping algorithm (SFLP), simulated annealing algorithms (SA) among others are also increasingly being used in CM [9-11]. A number of studies on CM have been done so far. For instance, reference [12] proposes two measures that can be used to control congestion; generator rescheduling and loss reduction of real power. In [13], the author proposes the use of FACTS devices in the management of congestion at transmission level. In [14], rescheduling of sensitive generators and load shedding is proposed for CM using PSO algorithm. In [15], the author used the sensitivities of overloaded lines together with the cost of power generation and load shedding in CM. The load-shedding schedule and new power generation for affected buses was calculated using line sensitivities and the cost. It is challenging to control the power produced in an islanded MG by RESs because these sources produce power intermittently. Load shedding is the main method of controlling system stability in this case and avoid any danger within the MG [16]. Optimal shedding of connected loads assists in reducing the difference between the power that DGs can supply and the connected loads [17]. In [18], ABC algorithm is used for overload control by rescheduling generators. The author uses generator sensitivity factors in selecting the generators that participate in real power rescheduling. In [19], ABC and PSO algorithms were used in generator rescheduling based on bus and generator sensitivity factors for CM in a system with wind energy resources only. In [20], the authors used PSO in generation rescheduling and load shedding in CM. In [21], congestion alleviation in a deregulated power system is proposed using cuckoo search algorithm. This was tested on IEEE 30-bus system. A modification of ABC algorithm is applied in the determination of the DG power output and their location in reference [22]. This was tested on 33 bus system. Through load shedding, the amount of power demand that can be curtailed so as to mitigate CM problem is determined. From the literature, extensive research on CM on MGs has been done. However, CM using load shedding algorithms still remains an open issue and needs further research. In this paper, the CM problem in an islanded MG supplied with RESs is analyzed. This is formulated as an optimization problem. ABC algorithm is used in the determination of the optimal amount of load to be shed and from which buses based on their priority index. This rest of this paper is organized as follows; section 2 discusses the proposed congestion management method, its mathematical formulation and how ABC algorithm will be applied; section 3 analysis the simulation results and 4 summarizes and concludes this paper. 2. PROPOSED CM SCHEME IN AN ISLAND WITH RESs This simulation assumes that MG is suddenly switched to island operation mode due to a contingency that leads to loss of mains. After a utility grid disturbance that lead to islanding mode, the power in the MG is allocated to the loads as per their priority index. Figure 1 below is a sketch of a MG with RESs that was used for CM analysis in this study. The loads and buses within the islanded MG were given priority indices, and then ABC algorithm was modified and applied in shedding loads in order to ensure the system is not stretched beyond its limits. Figure 2 is the proposed load shedding power flow for this study. Figure 1. Micro grid with RESs
  • 3.  ISSN: 2088-8708 Int J Elec & Comp Eng, Vol. 10, No. 5, October 2020 : 4552 - 4561 4554 Figure 2. Load shedding power flow for CM 2.1. Mathematical problem formulation The main objective of this paper is to alleviate elements of system congestion in an islanded MG through minimizing overloads by optimal load shedding using ABC algorithm. This is expressed the expressions 1-3 below; 𝑀𝑖𝑛𝑖𝑚𝑖𝑧𝑒 𝐹1 = ∑(𝑆𝑖 − 𝑆𝑖 𝑚𝑎𝑥 )2 𝑂𝐿 𝑖=1 (1) 𝑀𝑖𝑛𝑖𝑚𝑖𝑧𝑒 𝐹2 = ∑(𝑃𝑖 + 𝑄𝑖 𝑃𝑔𝑖 + 𝑅𝑖 𝑃𝑔𝑖 2 ) + |𝑒𝑖 ∗ sin(𝑓𝑖 ∗ (𝑃𝑔𝑖 − 𝑃 𝑚𝑖𝑛,𝑖))| 𝑁𝐺 𝑖=1 + ∑(𝑃𝑘 ′ + 𝑄 𝑘 ′ 𝐿 𝑠ℎ𝑑,𝑘) 𝑃𝐿 𝑘=1 (2) 𝑀𝑖𝑛𝑖𝑚𝑖𝑧𝑒 𝑃𝑙 = 𝐹(𝑃𝑔,𝑠𝑙𝑎𝑐𝑘) (3) Start Initialize food positions and algorithm parameters Evaluate connected loads ( 𝑃𝐿𝑑) and system generation (𝑃𝑔𝑒𝑛) capacity Assign important loads using priority index Is 𝑃𝑔𝑒𝑛 < 𝑃𝐿𝑑? Is Pgen ≥ Important load′ s demand?? Power all loads No Yes Power important loads Yes Shed loads equivalent to the difference No Operational constraints violated? Yes No End
  • 4. Int J Elec & Comp Eng ISSN: 2088-8708  Islanded microgrid congestion control by load prioritization… (L. O. Mogaka) 4555 where 𝑃𝑙 is the system real power loss, 𝑆𝑖 𝑚𝑎𝑥 is the maximum capacity of line i, 𝑂𝐿 is the number of the lines that are overloaded ,𝐿 𝑠ℎ𝑑,𝑘is the amount of load to be shedded at bus k, 𝑃𝑔𝑖 is the power generated by generator i, 𝑆𝑖 is the power flow on line i, 𝑁𝐺 is the number of participating generators, 𝑃 𝑚𝑖𝑛,𝑖 is the minimum power generated by generator i, 𝑓𝑖 and 𝑒𝑖are the generator coefficients for generator. Subject to the equality and inequality constraints (4-10) below: Equality constraints, 𝑃𝑔𝑖 − 𝑃𝑑𝑖 = ∑|𝑉𝑖||𝑉𝑗||𝑌𝑖𝑗| cos(𝛿𝑖 − 𝛿𝑗 − 𝜃𝑖𝑗) 𝑁𝐵 𝑗=1 (4) 𝑄 𝑔𝑖 − 𝑄 𝑑𝑖 = ∑|𝑉𝑖||𝑉𝑗||𝑌𝑖𝑗| sin(𝛿𝑖 − 𝛿𝑗 − 𝜃𝑖𝑗) 𝑁𝐵 𝑗=1 (5) where |𝑉𝑖| 𝑎𝑛𝑑 |𝑉𝑗| are voltage magnitudes. The distributed generation RESs constraints 𝑃𝑔𝑖 𝑚𝑖𝑛 ≤ 𝑃𝑔𝑖 ≤ 𝑃𝑔𝑖 𝑚𝑎𝑥 (6) 𝑄 𝑔𝑖 𝑚𝑖𝑛 ≤ 𝑄 𝑔𝑖 ≤ 𝑄 𝑔𝑖 𝑚𝑎𝑥 (7) where 𝑃𝑔𝑖 𝑚𝑖𝑛 𝑎𝑛𝑑 𝑃𝑔𝑖 𝑚𝑎𝑥 are Real power generation limits of the RES i and 𝑄 𝑔𝑖 is the VAR generated by RES i Voltage constraint 𝑉𝑔𝑖 𝑚𝑖𝑛 ≤ 𝑉𝑔𝑖 ≤ 𝑉𝑔𝑖 𝑚𝑎𝑥 (8) 𝑉𝑑𝑖 𝑚𝑖𝑛 ≤ 𝑉𝑑𝑖 ≤ 𝑉𝑑𝑖 𝑚𝑎𝑥 (9) where 𝑉𝑔𝑖 and 𝑉𝑑𝑖is voltage magnitude at generator and load bus i respectively. ∑ 𝑃𝑔𝑖 𝑁𝐺 𝑖=1 = ∑ 𝑃𝑑𝑘 + 𝑃𝑙 𝑁𝐷 𝑘=1 (10) where 𝑃𝑑𝑘 is the real power load at bus k, 𝑁𝐺 and 𝑁𝐷 are the sets of generator and load buses respectively. 2.2. ABC algorithm Swarm intelligence algorithms are the currently preferred approaches in solving dynamic optimizationNP-hard problems in engineering [23]. ABC algorithm is a meta-heuristic optimization algorithm that was proposed by Karaboga from Erciyes University of Turkey [24]. This algorithm mimics the foraging movement of a swarm of bees in search of nectar around the hive as shown in Figure 3 [22]. In this algorithm, the location of food sources represents a possible solution to an optimization problem being solved. The quantity and quality of nectar indicates the fitness of that particular solution. It has a superior performance in solving engineering problems when compared to other algorithms. For instance, it can handle both continuous and discrete variables. It is mostly applied to optimization problems in power systems that are not smooth. It is divided into three groups as observed from the behavior of a swarm of bees where each group performs a particular function as highlighted below [25];  The employed bees. They are equal to the possible solutions of the problem. This group continuously updates the rest of the bees in the hive about the quantity, quality and the direction of the food source through the performance of a waggle dance. The duration of the waggle dance depends on the quality and quantity of food source.  The onlooker group of bees. These bees pick on a food source to exploit based on the information provided by the employed bees waggle dance. More onlookers move to food sources with high fitness and few to food sources with lesser fitness values.  The scout bees. These bees work is to look for new sources of food for exploitation. They randomly choose food sources around the hive.
  • 5.  ISSN: 2088-8708 Int J Elec & Comp Eng, Vol. 10, No. 5, October 2020 : 4552 - 4561 4556 Figure 3. ABC search behavior 2.3. ABC algorithm steps The initial phase where random population is generated. The solution size is taken to be equal to the number of employed bees. Each of the above solution is a vector of dimension D. This dimensional vector D corresponds with the number of parameters being optimized. The fitness (quality) of a food source can be expressed by (11): 𝑓𝑖𝑡𝑛𝑒𝑠𝑠𝑖 = 1 1 + 𝑓𝑖 (11) where objective function 𝑓𝑖 is the problem formulation target. The above generated initial population of possible solutions is then sequentially subjected through the three categories of bees in cycles till the specified maximum cycle number (MCN) is reached. Employed bees keeps on modifying the memories of the food locations (solutions) based on the visual observations and nectar quality. The employed bees share the food source information with the rest of the bees in the hive through waggle dance. The onlooker bee will choose a preferred food source based on the amount of nectar available and as per the probability guided by the equation below [26]; 𝑝𝑖 = 𝑓𝑖𝑡𝑛𝑒𝑠𝑠𝑖 ∑ 𝐹𝑖𝑡𝑛𝑒𝑠𝑠𝑗 𝑡𝑠 𝑗=1 (12) where 𝑡𝑠 is the number of employed bees (= number of the sources of food) and 𝑓𝑖𝑡𝑛𝑒𝑠𝑠𝑖 is the fitness value of the 𝑖 𝑡ℎ solution. The onlooker bee then compares the food sources from the information given by employed bees and then chooses a neighbor food source if it is better than the current one using the following equation; 𝑋𝑖𝑗 𝑛𝑒𝑤 = 𝑋𝑖𝑗 𝑜𝑙𝑑 + 𝑄𝑖𝑗(𝑋𝑖𝑗 𝑜𝑙𝑑 − 𝑋 𝑘𝑗) (13) where 𝑄𝑖,𝑗 is a random number that lies between -1 and 1, 𝑋 𝑘,𝑗 is the neighbouring food source that can be randomly selected and 𝑋𝑖𝑗 𝑜𝑙𝑑 and 𝑋𝑖𝑗 𝑛𝑒𝑤 are old and new food sources respectively. If the food source does not improve after a number of trials, it is abandoned and the associated bee becomes a scout. These steps are continuously repeated until the maximum cycle or stopping criteria is met. Then, the food with the highest fitness value is selected and printed. 3. RESULTS AND DISCUSSIONS IEEE 30-bus is used to test applicability of this approach in congestion management. This system has a total of six generators, 41 lines, 24 load buses and 21 loads. It has a total of 283.400MW active and 126.200 MVAR reactive loads connected to the system. This is shown in Figure 4. The simulation was carried out on MATLAB/SIMULINK platform on an AMD 4C+6G 2.10 GHz processor with 4GB RAM. The control parameters for ABC algorithm were set as shown in the Table 1.
  • 6. Int J Elec & Comp Eng ISSN: 2088-8708  Islanded microgrid congestion control by load prioritization… (L. O. Mogaka) 4557 Figure 4. IEEE standard 30 bus system Table 1. ABC parameter settings S/N Parameter Value 1 Colony size 40 2 Employed bees 20 3 Limit value 100 4 Number of onlooker bees 20 5 MCN 100 Two test cases were simulated and Newton Raphson power flow run to test the congestion status in the island. The test cases done are shown in Table 2. In this case, congestion is simulated by creating an outage on lines 1-2 and 1-7 and overloading the lines. Table 2. Test cases description Test case Contingency Description 1 Outage of line 1-2 2 Outage of line 1-7 with increase in load at all buses by 50% 3.1. Case 1 The outage of line 1-2 brings congestion between lines 1-7 and 7-8 of the modified system. The total system loss increases to 16.069 MW from 7.376 MW. This is compared with the CM using firefly algorithm as shown in the Table 3. Table 3. Total system loss for case 1 MW MVar Normal condition 7.376 -15.043 Outage of line 1-2 contingency 16.069 21.303 After load shedding (20.552MW) using ABC algorithm 11.649 4.305 Using Firefly algorithm 13.10 Not given
  • 7.  ISSN: 2088-8708 Int J Elec & Comp Eng, Vol. 10, No. 5, October 2020 : 4552 - 4561 4558 From the load flow, the power flowing between the two congested lines becomes 147.509 and 136.348 MW respectively against the set limit of 130MW. This condition is alleviated by shedding 20.552 MW load from the buses with least priority indices using ABC algorithm. After load shedding, the power flow between the two congested lines comes down to 122.537 MW and 114.093 MW which is well within the limit. This is as shown in Table 4. The Figure 5 shows the voltage profiles at various buses for normal, during the fault and after load shedding using ABC algorithm. There is a great improvement in the voltage profiles on the system buses after load shedding. Table 4. Power flow for case 1 Congested line Actual power flow (MW) Power flow after load shedding using ABC Line limit (MW) 1-7 147.348 122.537 130 7-8 136.348 114.093 130 Figure 5. Voltage profile for test case 1 3.2. Case 2 In this section, the system loads were increased by 50% and then line 1-7 outaged. This caused congestion on lines 1-2, 2-8 and 2-9 of 313.914MW, 98.411MW and 104.769 MW respectively against the set limits of 130MW for line 1-2 and 65MW for lines 2-8 and 2-9. In this study, the total power Flow violation on the lines due to congestion is 257.088MW while it is 251.794MW in the study by reference [4]. This is shown in the Table 5. The system losses were also monitored during and after load shedding using ABC algorithm. Applying ABC algorithm, 41.268 MW was shed to mitigate congestion levels. The total losses were recorded as shown in Table 6. The total losses decreased from 38.164 during the contingency to 17.485 MW after load shedding. This is compared by the congestion management approach using firefly algorithm in reference [4]. As can be observed from the figure, the total power losses for the two approaches is almost the same. This is elaborated in Figure 6. Table 5. Congested lines for case 2 Congested line Actual power flow (MW) Line limit (MW) 1-2 313.914 130 2-8 98.411 65 2-9 104.769 65 Table 6. Total losses for case 2 MW MVar Normal condition 7.376 -15.043 Outage of line 1-7 and 50% load increase 38.164 97.773 After load shedding (41.268MW) using ABC algorithm 26.770 54.706 0,96 0,98 1 1,02 1,04 1,06 1,08 1,1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 VOLTAGEMAGNITUDE BUS NUMBER Normal operating condition Overloaded Condition After loadshedding
  • 8. Int J Elec & Comp Eng ISSN: 2088-8708  Islanded microgrid congestion control by load prioritization… (L. O. Mogaka) 4559 Figure 6. System losses comparison for case 2 The CM by load shedding using ABC algorithm was compared with CM by generator rescheduling using FFA, PSO, RSM and SA as reported in [4]. The power flows comparison is as shown in Table 7. The voltage profiles before, during the contingency and after CM by load shedding using ABC algorithm were monitored. The CM by load shedding using ABC algorithm improves the voltage profiles within the islanded MG. This is shown in Figure 7. Table 7. Comparison of various algorithms in CM Power flow (MW) FFA PSO RSM SA ABC Line 1-2 130 129.7 129.91 129.78 204.225 Line 2-8 62.713 61.1 52.36 51.47 59.509 Line 2-9 64.979 64.67 55.43 54.04 63.396 Figure 7. Voltage profiles for case 2 Convergence characteristics for this case was also monitored before the contingency, during the contingency and after load shedding. The iteration converged 14 iterations before the contingency, after 35 iterations during the contingency and converged after 29 iterations after load shedding. This is shown in the Figure 8. 0 5 10 15 20 25 30 35 40 45 Using Firefly algorithm Using ABC algorithm Losses(MW) Total losses during contingency Losses after CM 0,94 0,96 0,98 1 1,02 1,04 1,06 1,08 1,1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 VoltageMagnitude Bus Number Normal Voltage Mag Line 1-7 Outage and 50% load increse Voltage Mag Voltage Mag after shedding 41.268MW
  • 9.  ISSN: 2088-8708 Int J Elec & Comp Eng, Vol. 10, No. 5, October 2020 : 4552 - 4561 4560 Figure 8. Convergence characteristics 4. CONCLUSION This paper has presented an approach for congestion management on an islanded MG with RESs. Here, load shedding using ABC algorithm has been successifully employed for congestion management in an islanded microgrid. Various loads and buses were chosen for shedding based on their priority index to help mitigate the congestion problem. This was tested and validated on a modified IEEE 30 bus system with contingencies and sudden load increase in the micrgrid introduced. The results were compared with those from other algorithms like firefly algorithm and PSO as reported in the literature. The results show a great superiority of this approach over these algorithms. As it can be observed, this proposed method can greatly improve the system stability of the islanded MG through the minimization of the load shed and maintain the voltage profile within the required limits. This is due to superior convergence characteristics of ABC algorithm. This approach can therefore berecommeded in the solution of optimization problems in engineering and other fields as well. ACKNOWLEDGMENT This research was supported by the Pan African University Institute for Basic Sciences, Technology and Innovation in the form of a postgraduate student research funding. REFERENCES [1] K. Takumi, T. Hideyuki, S. Kazuto, K. Gen, K. Hak-Man and K. Tetsuo, "Priority-based hierarchical operational management for multiagent-based microgrids," Energies, vol. 7, pp. 2051-2078, 2014. [2] D. Mohammad, M. Hazlie and M. Saad, "Application of Meta-heuristic techniques for optimal load shedding in islanded distribution network with high penetration of solar PV generation," Energies, vol. 10, no. 150, pp. 1-24, 2017. [3] P. Oana, B. Constantin and K. Stefan, "Contingency-Constrained congestion management and transmission cost allocation," Acta Polytechnica Hungarica, vol. 11, no. 7, pp. 83-102, 2014. [4] V. Sumit and V. Mukherjee, "Firefly algorithm for congestion management in deregulated environment," Engineering Science and Technology, an International Journal, vol. 19, pp. 1254-1265, 2016. [5] K. Elango, S. R. Paranjothi and C. Sharmeela, "Transmission congestion management in restructured power systems by generation rescheduling and load shedding using rule based OPF," European Journal of Scientific Research, vol. 57, no. 2, pp. 380-390, 2011. [6] M. M. K. Seyedeh and K. Amangaldi, "Local power controller based load shedding scheme in islanded microgrids," International Journal of Renewable Energy Research, vol. 9, no. 2, pp. 1108-1115, 2019 [7] S. Thangalakshmi and P. Valsalal, "Congestion management using hybrid fish bee optimization," Journal of Theoretical and Applied Information Technology, vol. 58, no. 2, pp. 1405-1412, 2013. [8] J. Hazra, A. K. Sinha and Y. Phulpin, "Congestion management using generation rescheduling and/or load shedding of sensitive buses," Third International Conference on Power Systems, 2009. 25000 27000 29000 31000 33000 35000 37000 39000 41000 43000 45000 1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627282930313233343536373839405060 Iteration Normal Condition line 1-7 outage and 50% load increase After Shedding 41.268 MW
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