 Open Access
 Total Downloads : 252
 Authors : Trong Nghia Le, Ngoc Au Nguyen, Huy Anh Quyen
 Paper ID : IJERTV4IS100218
 Volume & Issue : Volume 04, Issue 10 (October 2015)
 Published (First Online): 13102015
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Under Frequency Load Shedding with Application of Heuristic Algorithm
Trong Nghia Le , Ngoc Au Nguyen , Huy Anh Quyen
Department of Electrical and Electronics Engineering HCMC University of Technology and Education,
Ho Chi Minh City, Vietnam
Abstract Load shedding is considered the method to be applied in emergency situations to prevent blackout and to maintain the stability of the power system. The traditional load shedding methods are complex and cannot be expanded. This paper proposes a new load shedding method based on heuristic algorithms to overcome the above limitations. One of the important characteristics of this method is that amount of loads shed and load placement shed at each bus are determined and implemented according to plans corresponding to the cases when the problems occur. This contributes to reduce the operation and recovery time of the system when the problem occurs. Case studies proposed in this paper include: generator failure, the bus failure when the system operates at 80%, 90% and 100% of maximum capacity. The method proposed is tested on the IEEE 37 bus system, using PowerWorld software and the simulation results proved the effectiveness of the method.
Keywords Load Shedding; Heuristic Algorithm; The Power System Stability; Under Frequency; Recovery Time; Generator Failure

INTRODUCTION
Voltage and frequency are the two most important parameters affecting the stability of the power system. When there is instability the frequency or voltage will cause disturbance and if it extends disturbance that lead to the collapse of the system. Therefore, the need to quickly put the parameters on which the original value or reestablish a new stability point is very important in order to minimize the system collapse. When all the available controls cannot maintain stable power system frequency, load shedding method is applied in emergency situations to maintain the power system stability or restore steady state the fastest.
The traditional load shedding methods based on under frequency or voltage relay [1], [2] the results are often less accurate, the mount of load is shed sometimes so large in a step, the plans do not have the flexibility to increase the number of load shedding steps or take a long time [4], [5]. To increase the efficiency of load shedding, some load shedding methods based on frequency or the rate of change of frequency (df/dt) [6], voltage, sensitivities QV at each load bus. However, this case shows the processing speed algorithm is too slow [7].
The load shedding algorithm proposes below aims to load shedding based on based on heuristic algorithm to resolve the limitations of previous studies. One of the important characteristics of this method is that amount of loads shed and load placement shed at each bus are determined and implemented according to plans corresponding to the cases when the problems occur. This contributes to reduce the operation and recovery time of the system when the problem occurs.

APPROACH METHOD

Heuristic Algorithm
Heuristic algorithms often show quite natural, close to the way of thinking and acting of human beings. The function of heuristic is search from a available target, it provides information from neighboring nodes to serve the search results for targeted needs identified. Heuristic algorithms based on the basic principles:

The principle of exhaustion intelligent: A criterion of this principle is when the search area is too large, it must seek to limit the maximum search area to quickly identify problems and produce results soon.

Principle greedy: release criteria are obtained optimal standards of big problems to select for each solve step within the limits of each region.

Principle of order: the criteria set out implementation actions are based on search structure predefined order.
To problems are solved the fastest and shortest path, Greedy principle is often used. Specifically as follows:

From the start node, list all distance traveled from the starting point to the node n two level of the system, and then choose the shortest path.

When go to a node level 2 is also in accordance with the above principle, list all the way from the current location node next to the node and also according to the selection criteria shortest path. Programming will be done until there is no node to go.
Fig. 1. Describe heuristic algorithm



The application of heuristic algorithm for load shedding
The proposed method uses the "decision tree" to implement load reduction. The goal of the "decision tree" is to find the shortest way to recover the system with the fastest time from a predefined node. The idea of the method is implement load shedding through from the search node cause problems in areas that have been identified before. The implementation of this model is effective because it avoids having to calculate the optimal the whole system. After identifying the node causes the problem, the level of disturbance is estimated using the rate of change of frequency. Load location is shed at each bus has been determined based on the calculated voltage change at each load position when problems occur.
Step 1: Search node /generator is faulted.
Step 2: Determine the unbalanced load based on equations of motion of the rotor:


CALCULATION, EXPERIMENTS, THE SIMULATION ON THE TEST SYSTEM
Experiments on typical systems IEEE with f= 60Hz include 37 buses 9 generators shown in Fig. 3.
Fig. 2. 37 buses 9 generators test system
Case studies proposed in this paper include: generator failure, the bus failure when the system operates at 80%, 90% and 100% of maximum capacity. Corresponding to each case will build "decision tree" in load shedding to restore the parameters return to the original steady state.
Consider the case loss a generator at bus No. 4 when the system is operating 80% of maximum capacity, use PowerWorld software to simulate and observer getting results when apply traditional load shedding programs and the load shedding program proposed. Result frequency graph when the power system is not load shedding is shown in Fig. 4. Frequency graphs after applying the previous load shedding methods are shown in Fig. 5, Fig. 6.
The obtained results, the frequency before using the load shedding program is 59.6 Hz, after application proposed load shedding program, the frequency has improved to a stable
2Heq
fo
dfn
dt
Pdiff
Pmn Pen
(1)
value near 60 Hz (59.99 Hz) in 24s. Compare with the case load shedding common: load shedding based on frequency and voltage sensitivity, loads are shed according to ascending order [7], follow the steps based on the change of frequency [3],
Where: Heq is the equivalent inertia constant of the
generators; f0 is the nominal frequency of the system; Pdiff is the difference in the generated power and the load power; Pmn is the mechanical shaft power for n machines, Pe is the electrical power for n machines.
Step 3: calculate dV/dt to determine priority load. Bus has dV/dt magnitude greater is shed before.
Step 4: Load shedding until recovery frequency close to the value of the original steady state.
Flowchart algorithm load shedding based on Heuristic is presented in Fig. 2.
comparing the results presented in Table 1.
Begin
Measure the frequency and its rate
f<59.7 Hz
Search node /generator is faulted
True/p>
False
60
59.95
No load shedding
59.9
Hz
59.85
59.8
59.75
59.7
59.65
The initial imbalance load Pdiff is calculated using the swing equation
59.6
diff
f dt
df
eq
0
2H
. n P
5 10
15 20
25 30 35
Sec
40 45 50
0
Fig. 5. Load shedding based on frequency and voltage sensitivity
Calculate dV/dt at f <59.7 Hz to decide load shedding
60
59.95
59.9
Load shedding implement according the plan
59.85
Hez
59.8
Fig. 3. Flowchart algorithm load shedding based on Heuristic algorithm
60
59.9
59.8
59.7
Hez
59.6
59.75
59.7
59.65
59.6
59.55
0
5 10
15 20
25 30 35
Sec
40 45 50
59.5
59.4
59.3
59.2
0
5 10
15 20
25 30 35
Sec
40 45 50
Fig. 6. Load shedding based on these steps based on the change of frequency
60
59.98
59.96
59.94
59.92
59.9
59.88
59.86
Fig. 4. Frequency of system in case of failure at the generator bus 4
59.84
Hz
59.82
59.8
59.78
59.76
59.74
59.72
59.7
59.68
0
5 10
15 20
25 30 35
Sec
40 45 50
Fig. 7. Frequency of system after using load shedding based on Heuristic algorithm
Consider the case bus No.17 is failed to shortcircuit problem when the system is operating 80% of maximum capacity, use Powerworld software to simulate and observer getting results from the load shedding program proposed.
Result frequency graph when the power system is not load shedding is shown in Fig. 8.
60
59.5
59
58.5
Hez
58
57.5
57
56.5
60
59.95
59.9
59.85
59.8
59.75
Hez
59.7
59.65
59.6
59.55
59.5
59.45
59.4
59.35
59.3
0
5 10 15
20 25 30
Sec
35 40
45 50
56
55.5
55
0
0.5
1 1.5
2
Sec
2.5
3 3.5
Fig. 9. System frequency after applying the load shedding program the order ascending load
Frequency graph after applying the proposed load shedding
4 program is presented in Fig. 10.
60
Fig. 8. Frequency of system in case of problems at the bus
TABLE I. RESULTS OF THE COMPARISON BETWEEN THE METHODS LOAD SHEDDING IN CASE OF GENERATOR FAILURE
59.95
59.9
59.85
59.8
Hez
59.75
59.7
59.65
59.6
59.55
59.5
59.45
0
5 10
15 20
25 30 35 40
Sec
45 50
Methods load shedding
Recovery frequency (Hz)
Load Shedding capacity
(MW)
Recovery time (s)
Load shedding based on frequency and
voltage sensitivity
59,99
149,1
30
Load shedding based on these steps based on the change of
frequency
59,94
127.14
13
Load shedding based on ascending order
59,98
140
15
Load shedding based
on Heuristic algorithm
59,9
119.83
10
Frequency graph after applying the traditional load shedding program: loads are shed according to ascending order is shown in Fig. 9.
Fig. 10. Frequency system after applying the load shedding program according to Heuristic algorithm
The obtained results, if the load shedding is not implementing, the power system will black out. After applying the load shedding program proposed, the frequency has improved to a stable value close to 59.97 Hz within 12 seconds faster than the case load shedding in order to the smallest value being shed before. Decision tree diagram is shown in Fig. 11.
Fig. 11. Decision tree diagram when there is a problem on power system
Fig. 12. Decision tree diagram when a generator is faulted and load reaches 70% of maximum capacity
Fig. 13. Decision tree diagram when a generator is faulted and load reaches 80% of maximum capacity
Fig. 14. Decision tree diagram when a generator is faulted and load reaches 90% of maximum capacity
Fig. 15. Decision tree diagram when a generator is faulted and load reaches 100% of maximum capacity
Fig. 16. Decision tree diagram when a bus is faulted

CONCLUSIONS
Load shedding based on FuzzyAHP algorithm is applied in the emergency situations to maintain stability of the power system. The number of loads nodes and load shedding position at each node were predetermined. The implementation of strategic control with this method contributed to simplifying the operation, reducing the total power load shedding and decreasing the economic damage in emergency.
Simulation results showed that although the recovery time was a little longer but FuzzyAHP algorithm allowed capacity lower load shedding than the AHP algorithm demonstrated the effectiveness of the proposed method.
Load shedding method based on Heuristic algorithm is applied in emergency situations to maintain the stability of the power system. The important characteristics of this method is that amount of loads shed and load placement shed at each bus are determined and implemented according to plans corresponding to the cases when the problems occur. This contributes to reduce the operation and recovery time of the system when the problem occurs.
The simulation results show that the amount of capacity load shedding less and faster recovery time than previous methods demonstrate the effectiveness of the proposed method.
ACKNOWLEDGMENT
This research was supported by Ho Chi Minh City University of Technology and Education under a research at the Power System and Renewable Lab.]
REFERENCES

Behzad Farhangi Rad, Mehrdad Abedi, An optimal loadshedding scheme during contingency situations using metaheuristics algorithms with application of AHP method, Optimization of Electrical and Electronic Equipment, 11th International Conference on, pp. 167173, 2008.

M. Moazzami, A. Khodabakhshian, A new Optimal adaptive Under Frequency Load Shedding Using Artificial Neural Networks, Proceeding of Electrical Engineering (ICEE), 2010 18th Iranian Conference on, pp. 824829, 2010.

Florida Reliability Coordinating Council Inc, FRCC standards handbook, 2011.

P. Mahat, et al., "Under frequency Load Shedding for an Islanded Distribution System With Distributed Generators", IEEE Transactions on Power Delivery, vol. 25, pp. 911918, 2010.

The Northern Regional Power Committee (NRPC), Under frequency load shedding scheme in Northern Region (NR), 2010.

R. Hooshmand, "Optimal design of adaptive under frequency load shedding using artificial neural networks in isolated power system" Electrical Power and Energy Systems 42 (2012), pp. 220228.

Bui Quang Anh, Optimal Load Shedding, Masters Thesis at HCMUTE, 2011.