A Fuzzy Based Under-Frequency Load Shedding

International Review of Electrical Engineering (I.R.E.E.), Vol. 7, N. 4 ISSN 1827- 6660 July - August 2012

A Fuzzy Based Under-Frequency Load Shedding Scheme for Islanded Distribution Network Connected with DG H. Mokhlis1, 2, J. A. Laghari1, 3, A. H. A. Bakar2, M. Karimi1, 2 Abstract – The frequency of power system is very sensitive to load changing when operating in islanded mode. This may causes overloading or loss of generation cases. Under-Frequency Load Shedding (UFLS) Scheme is commonly applied to stabilize the frequency during these cases. Conventional UFLS scheme operates successfully in interconnected grid system and may not work well when applied to DG based system operating in islanded mode. This paper presents a new fuzzy logic based under- frequency load shedding scheme implemented on mini hydro type-DG operating in islanded mode. The proposed strategy is based on frequency, rate of change of frequency and load prioritization. In proposed UFLS scheme, a fuzzy logic load shedding controller (FLLSC) with Load Shed Controller Module (LSCM) is modelled. FLLSC measures amount of load to be shed and LSCM shed the respective load to stabilize frequency. The proposed scheme is validated on different event-based and response-based cases. Simulation results show that proposed scheme is effective in shedding optimal number of loads while stabilizing the frequency. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved.

Keywords: Distributed Generation (DG), Fuzzy Logic Load Shedding Controller (FLLSC), Load Shed Controller Module (LSCM), Islanded Distribution Network

Nomenclature fc f df/dt H P P0 Q Q0 P DG FLLSC RCB UFLS LSCM

Frequency of the center of inertia Frequency deviation Rate of change of frequency Generator inertia constant Active power at new voltage/frequency Active power at base voltage/frequency Reactive power at new voltage/ frequency Reactive power at base voltage/ frequency Imbalance power Distributed Generation Fuzzy logic load shedding controller Remote Circuit Breaker Under-frequency load shedding Load shedding controller module

I.

Introduction

Distributed generation (DG) refers to small type of electric power generation having capacity less than 10 MW [1]. DG units are located near load centers in order to avoid expansion of the existing network and supply new load areas. Due to market deregulation and environmental constraints, the use of DG resources has been widely employed in power industry[2]. The increasing trend of DG penetration in power system network has opened the new challenging issues in the field of power system. Manuscript received and revised July 2012, accepted August 2012

The use of DG has provided benefits to the end user, power utilities and DG’s owner in terms of reliability, efficiency of power and economics [3]-[5]. However, penetration of DG causes various problems to the existing network and power system need to be modified. One of the constraints is operation of DG during islanded mode, in which DG is electrically isolated from the main grid [6], [7]. This causes overloading or loss of generation cases and requires load shedding technique to stabilize the frequency within acceptable range. The conventional load shedding scheme employs frequency relay to stabilize frequency under abnormal conditions. In this scheme, the under frequency relay operates when system frequency falls below a certain threshold value. Conventional UFLS scheme shed a fixed amount of electrical power in fixed steps. This scheme is unreliable in shedding the optimal number of loads [6]-[9]. Efforts have been made to improve the existing conventional UFLS scheme by combining the generator tripping with UFLS scheme in order to stabilize power system during unstable faults[10]. Another effort for improving the conventional load shedding scheme is presented by simultaneously studying the transmission and distribution network [11]. Since, conventional load shedding scheme shed the load in fixed steps without estimating the amount of load to shed. Hence, it often shed more load than required. This problem is solved by employing power swing equation to estimate the amount of load to be shed. The technique is known as an adaptive UFLS scheme. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved

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H. Mokhlis, J. A. Laghari, A. H. A. Bakar, M. Karimi

The literature on adaptive UFLS scheme has reported that the most schemes employ voltage variation to identify and shed the sensitive load buses [12], a combination of frequency, df/dt and voltage changes [13] and initial slope of df/dt for setting the under-frequency relays [14]. These schemes can operate successfully, provided that grid is facilitated with high speed communication technology. This leads to intelligent UFLS schemes. The intelligent UFLS schemes employ two-way high speed communication system, power system automation. The intelligent UFLS schemes require fast and accurate measurements units to acquire knowledge and information regarding the network for optimally shedding the load [15]-[17]. These UFLS scheme may not ensure system security when applied in islanded distribution network connected with DG. This is due to the fact that system frequency severely disturbed during islanding mode. Also, DG system has smaller inertia which causes to drop frequency quickly. Hence, a DG system operating in islanded mode requires an appropriate adaptive load shedding scheme. The research on UFLS schemes based on islanded system has shown that not much work has been conducted in this area. The developed load shedding schemes by various authors are based on frequency and df/dt information, customers willingness to pay and load histories [18] and best time to shed the loads [19]. This paper proposes a new fuzzy based underfrequency load shedding scheme for islanded distribution network connected with DG. The proposed strategy uses frequency, rate of change of frequency and load prioritization to shed the optimal number of loads. The proposed UFLS scheme is tested at event-based and response-based load disturbance as proposed in [20], [21]. It consists of fuzzy logic load shedding controller (FLLSC) with load shedding controller module (LSCM). FLLSC estimates the power imbalance and LSCM shed the required load to stabilize frequency.

II. II.1.

monitors system status at every instant of time, and determines frequency of the equivalent inertial center fc as below [22]: N

H i fi fc

i 1 N

(1) Hi

i 1

where, Hi is inertia constant of ith generator in s and fi is frequency of ith generator in Hz and N shows number of generators. Fig. 1 illustrates layout of load shedding scheme. Standard frequency pick value to begin load shedding scheme is set to 49.5 Hz as practised in TNB, Malaysia [23]. When DG system encounter load disturbance, FLLSC check frequency limit of 49.5 Hz. If frequency goes below 49.5Hz, FLLSC investigate about the type of disturbance (Event-based or Response-based) occur on DG and estimates amount of load to be shed.

Methodology Fig. 1. Proposed fuzzy based UFLS scheme layout

Proposed Load Shedding Scheme Description

The scheme is proposed to operate and monitor distribution network after the occurrence of grid disconnected. The proposed load shedding scheme uses fuzzy logic control approach to stabilize frequency by shedding some amount of load. The scheme has two parts. Fuzzy logic load shedding controller (FLLSC) and load shed controller module (LSCM). The FLLSC uses frequency and rate of change of frequency as input, and determines type of load disturbance whether Event or Response-based and estimates the power imbalance. FLLSC sends this value to LSCM, which shed the required load according to load priority. The loads are prioritized into three categories; vital, semi-vital and nonvital. Non-vital loads have the lowest priority and will be shed first followed by semi-vital and vital loads. FLLSC Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved

In order to avoid unnecessary shedding of load due to smaller disturbances, the absolute value of Pmin is fixed to 50 kW as this is minimum load value in the distribution network. If estimated value is greater than Pmin, FLLSC sends estimated P to LSCM for shedding respective loads. FLLSC sends estimated value to LSCM via communication link. The delay time which includes calculation time, communication time and circuit breaker operation time is assumed as 100 ms, which is according to practical considerations [6], [12]. The co-ordination of under-frequency protection of generator with under-frequency load shedding scheme is very important. If system frequency goes below certain threshold value, under frequency protection relay of International Review of Electrical Engineering, Vol. 7, N. 4

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generator will operate and system will collapse unnecessarily. Hence, under-frequency load shedding technique should be applied in such a way that frequency recovers without going below threshold value. The minimum allowed operating frequency usually specified by the manufacturer according to the type of turbine is 47.5 Hz [24]. In this paper, distribution network is assumed to have reliable monitoring devices and fast communication system for transmitting data. Real-time measurement and Remote Circuit Breaker (RCB) are facilitated at each of the load feeder. The system state variable measurement (i.e. active power, frequency and voltage) are monitored by FLLSC whereas breakers status of load feeders is monitored by LSCM. The flow chart of proposed scheme is shown in Fig. 2. In FLLSC, there are two strategies; (1) Event-based and (2) Response-based scheme. FLLSC decides right strategies based on frequency, df/dt and breaker status at the DG units. The description of these strategies is as follows:

II.2.

FLLSC for Event-Based Case

Event-based case may occur when one of DG unit is tripped during islanded mode. This tripping incident may be initiated by the failure operation or malfunction of generator differential protection. It may also happen due to transmission line failure in power system. FLLSC will estimate the amount of load to be shed thus, preventing system from blackouts. When Event-based occurs, FLLSC will estimate total power imbalance as given in Equation (2): P

PDG

PLoad

(2)

where, P is power imbalance, PDG is DG dispatching power, PLoad is total load demand. FLLSC sends estimated value of P to LSCM which sheds the load according to load priority defined in load look-up table. II.3.

FLLSC for Response-Based Case

Response-based case occurs due to sudden increment of load in an islanded system. In this case, number of load to be shed depends on the disturbance magnitude. FLLSC estimates the power imbalance by using frequency and df/dt as input signals. After estimating power imbalance, FLLSC sends estimated value to LSCM for shedding the required load. II.4.

Fuzzy logic Load Shedding Controller (FLLSC) Modelling

Fuzzy logic load shedding controller is modelled in PSCAD software. Since, PSCAD does not provide fuzzy logic tool box, FLLSC is modelled by writing C-coding. Fuzzy logic load shedding controller for UFLS scheme consists of two inputs and one output. The inputs of fuzzy logic load shedding controller are frequency (f) and rate of change of frequency (df/dt). The output is amount of load shed (Lshed). Depending upon the input values, fuzzy logic will estimates the amount of load required to be shed. The FLLSC comprises of fuzzification, rule base, inference mechanism and defuzzification steps as shown in Fig. 3.

Fig. 3. Fuzzy logic load shedding controller block diagram

The respective membership function of frequency and rate of change of frequency (df/dt) and Lshed are shown in Figs. 4-6. The linguistic variables of input frequency are Low (Low), Vlow (Very Low), Extlow (Extremely Low), Vextlow (Very Extremely Low) and input rate of change of frequency (df/dt) membership functions are HN (High Negative), LN (Low Negative), LP (Low Positive), HP (High Positive).

Fig. 2. Flow chart of proposed FLLSC controller and LSCM module

Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved

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The linguistic variables of output Lshed are Vsshed (Very Small Shed), Sshed (Small Shed), Bshed (Big Shed), Vbshed (Very Big Shed).

defuzzification is carried out through weighted average to convert the fuzzy linguistic variable into real crisp values. TABLE I RULE TABLE FOR FUZZY LOGIC LOAD SHEDDING CONTROLLER (FLLSC) Frequency Low Vlow Extlow Vextlow HN Sshed Bshed Bshed Vbshed LN Sshed Sshed Bshed Vbshed LP Vsshed Vsshed Ssshed Sshed HP Vsshed Vsshed Vsshed Vsshed

III. Test System for Under-Frequency Load Shedding Scheme (UFLS) Fig. 4. Frequency membership functions

The test system for analyzing the proposed UFLS scheme is shown in Fig. 7. The DG test system consists of two mini-hydro power plants units. Each DG unit has 2 MVA (maximum power dispatch is 1.83MW) capacity and is modelled in PSCAD/EMTDC software. The system consists of 27 buses and 20 lumped loads. The DG units are operated at 3.3 kV voltage level and are connected with transformer to step-up the voltage level to 11 kV. Each node is connected with remote circuit breaker (RCB) that can be remotely controlled for load shedding purposes.

Fig. 5. Rate of change of frequency (df/dt) membership functions

Grid

Load 5 1047

1079 Disconnected from Grid 1058

1046 1056 Load 12

1075

1057

1039

1018

Load 6 Vital Load Load 7

Load 9

1154

1010

1019

1013

Load 13

Load 1 Vital Load

1050

1020

Load 8

1012 Load 3

1004

1141

1151

Load 4

1064

1029

Vital Load

Load 14

Load 11 1000

DG 1

Load 10

Load 2

Load 15

DG 2

Fig. 6. Lshed (p.u.) membership functions

The fuzzy logic load shedding controller input and output membership functions are formed in C-coding by using one dimensional array concept. The triangular membership functions are divided into two slope equations for fuzzification. In fuzzification, the real input values are converted into fuzzy set values which assign degree to which these inputs belong to each of the appropriate fuzzy sets. Fuzzy rule base is used in IF-THEN rule form to assign the input and output control such as: IF frequency is low and df/dt is HN THEN Lshed is Sshed. IF frequency is Vextlow and df/dt is HN THEN Lshed is Vbshed. The other rules of fuzzy logic controller are summarized in Table. I. The inference mechanism evaluates active signals for taking control actions from the fuzzy rules. Finally, Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved

Fig. 7. Mini hydro power plant with distribution network disconnected from Grid

The distribution network having two DG units is assumed to be disconnected from grid. The standard model for exciter and governor and hydraulic turbine provided in PSCAD/EMTDC library are used in this study. The IEEE type AC1A excitation standard model is chosen in this work and its model parameters are shown in Table II. The governor consists of PID controller including pilot and servo dynamics models and its parameters are shown in Table III. The hydraulic turbine for this study is considered as Non-elastic water column without surge tank and its parameters are shown in Table IV. The distribution network has load profile consisting of base load and peak load capacity. International Review of Electrical Engineering, Vol. 7, N. 4

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TABLE II EXCITATION PARAMETERS Value Parameter 0 KF 0 TF 400 TE 0.02 KE 14.5 KC -14.5 KD 6.03 VRMIN 0.1 SE(VE2) 4.18 VE2

Parameter TC TB KA TA VAMAX VAMIN VRMAX SE(VE1) VE1

TABLE III GOVERNOR PARAMETERS Value Parameter 2 TC 0.35 TD 0.9 Max gate opening 0.05 Max gate closing 0.04 Dead band value 1.0 Min gate position

Parameter KP KI KD TA RP Max gate position

Parameter TW fP D

TABLE IV HYDRAULIC TURBINE PARAMETERS Value Parameter 2 Initial output power Initial operating 0.02 head 0.5 Rated output power

where P is active power at new voltage and frequency, and P0 is active power at base voltage and frequency, Q is reactive power at new voltage and frequency, and Q0 is reactive power at base voltage and frequency. Kpf and Kqf are the co-efficient of active and reactive load dependency on frequency. The parameter of this model are the exponents a and b. With these exponents equal to 0, 1 or 2, the model represents constant power, constant current, or constant impedance characteristics, respectively [25]. f is frequency deviation (f - f0). In this study, the exponent value for a and b is set to 1.0 and 2.0 respectively. Meanwhile, Kpf and Kqf is set to 1.0 and -1.0 respectively. Thus, by choosing these values, load is voltage and frequency dependent type. The power consumption of each load and its rank type is presented in Table V. The loads are ranked based on their load priority and look-up table is created according to prioritization. This load priority is created on active power value of each load. Both event-based and response-based strategies of UFLS scheme are considered. Depending upon the system load capacity, generating system can be operated at base load and peak load capacity. respectively [25].

Value 0.03 1 0.8 1 0.2 0.38 -5.43 0.03 3.14

Value 0.2 0.2 0.16 0.16 0 0

Value 0.7

IV.

1.0 1.0

In a real system, the loads are always frequency and voltage dependent. This dependency of load characteristics is usually modelled as:

Load Ranked 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

P

P0

V V0

Q

Q0

V V0

Under-frequency load shedding scheme is tested on Event-based and Response-based case studies. All case studies of load shedding scheme are tested on islanded distribution network connected with DG. The descriptions of case studies are summarized in Table VI.

V.

a

1 K pf f

(3)

1 K pf f

(4)

b

Bus Number 1013 1141 1012 1050 1047-1079 1057 1058 1010-1039 1064 1018 1154 1004 1046 1020 1029 1019 1151 1056

P(MW) 0.0684 0.0795 0.0795 0.1095 0.1794 0.189 0.198 0.234 0.1488 0.1743 0.2097 0.2121 0.2535 0.2745 0.3468 0.1902 0.2208 0.345

V.1.

Result and Discussions

Case I: Event-Based Case without Load Shedding Scheme

To simulate Event-based case without applying load shedding scheme, one of the DG unit is tripped-off from islanded distribution system.

TABLE V LOAD RANKING TABLE Peak Load Q(MVAr) 0.0423 0.0495 0.0495 0.0576 0.0792 0.1152 0.123 0.1101 0.0867 0.108 0.1275 0.1314 0.1578 0.1716 0.2148 0.099 0.0996 0.3282


Case Studies

Base Load P(MW) 0.0456 0.0531 0.0531 0.063 0.11721 0.126 0.132 0.15009 0.093201 0.11619 0.1401 0.14151 0.1701 0.1845 0.2313 0.10671 0.107199 0.35259

Q(MVAr) 0.0282 0.033 0.033 0.0384 0.07281 0.0768 0.0819 0.0933 0.057801 0.072 0.0849 0.0876 0.1053 0.11439 0.1431 0.06609 0.06639 0.2187

Load Category Non-vital Non-vital Non-vital Non-vital Non-vital Non-vital Non-vital Non-vital Semi-vital Semi-vital Semi-vital Semi-vital Semi-vital Semi-vital Semi-vital Vital Vital Vital


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Case Studies Case I Case II

Case III

one DG Trip

TABLE VI CASE STUDIES Description Event-Based case without load shedding scheme Event-Based case with Load Shedding Scheme (1) Peak Load Scenario (2) Base Load Scenario Response-Based case with Load Shedding Scheme (1) Peak Load Scenario (2) Base Load Scenario

50.5 50 49.5 49 48.5 48 0

Since, loads in islanded system are supplied by two DGs, loss of one DG will give a great impact to islanded system. As a result, all load is shifted to the remaining operating DG unit. The frequency response of DG without applying load shedding scheme is shown in Fig. 8. From Fig. 8, it can be noticed that when event-based case occurred and load shedding scheme is not applied, frequency of operating DG unit goes below the safe frequency limit of 47.5 Hz, stables at 28.67 Hz and never recovered to normal value. This will cause tripping of operating DG unit due to operation of protection relay which will leads to power blackouts. Hence, load shedding scheme is necessary in order to able DG unit operating continuously, and prevent power system from power collapse.

10

20

30

40

50

Time (sec)

Fig. 9. Frequency response for Event-based case at Peak Load

Fig. 9 shows that by applying proposed load shedding scheme, DG frequency has undershoot of 48.56 Hz and recovered to 50Hz after some time. The proposed scheme shed load up to 11th load ranked for this case. It is noticed that DG frequency while stabilizing has an overshoot up to 50.3 Hz. This frequency overshoot explains that amount of load shed is not optimal. To investigate this issue, load ranked 11th is not shed to confirm whether amount of load shed is optimal or not and response is shown in Fig. 10.

one DG Trip

55 50 45 40 35 30 25 0

10

20

30

40

50

Time (s)

Fig. 10. Frequency response at peak load when load ranked 11th is not disconnected Fig. 8. Frequency response at Event-based case without load shedding scheme

V.2.

Fig. 10 shows that proposed fuzzy based UFLS scheme sheds the optimal number of loads as the frequency does not rise to normal value and stables at 46 Hz when one less load is shed. DG unit enable to supply 1.83 MW load after one DG being tripped and its graph is shown in Fig. 11.

Case II: Event-Based Case with Load Shedding Scheme V.2.1. Peak Load Scenario

To simulate Event-based case with load shedding scheme at Peak load (3.66 MW), one of the DG unit is tripped-off from islanded distribution system at t=10 s. The FLLSC checks first frequency limit of 49.5 Hz. After checking this, FLLSC check about type of load disturbance applied on islanded distribution system. FLLSC by monitoring RCB status of DG units determines that system encountered Event-based load disturbance. FLLSC estimates the amount of load to be shed and sends signal to LSCM, which immediately trip significant number of load feeders to stabilize frequency. The frequency response of DG for this case is shown in Fig. 9.


Fig. 11. Power graph for Event-based case at Peak Load


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V.2.2. Base Load Scenario To simulate Event-based case with load shedding scheme at Base load (2.5 MW), one of the DG unit is tripped-off from islanded distribution system at t=10 s. The frequency response of DG for this case is shown in Fig. 12. It can be noticed from Fig. 12 that by applying proposed load shedding scheme, DG frequency has undershoot of 47.62 Hz and recovered to 50 Hz after some time. The proposed scheme shed load up to 10th load ranked. Fig. 13 shows DG frequency when one less load is shed. It is observed that when ranked 10th is not shed, DG frequency goes to 46.141 Hz. Since, DG frequency goes below the safe limit of 47.5 Hz and will cause tripping of generator due to operation of protection relay. Hence, proposed fuzzy based UFLS scheme sheds the optimal number of loads.

TABLE VII OPTIMAL LOAD SHED DURING EVENT-BASED CASE Event-Based Power Undershoot Load Shed Case Imbalance Peak Load 48.56Hz 1.83MW 1.83MW Base Load 47.62Hz 1.25MW 1.03MW One DG Trip

2.6 2.4 2.2 2 1.8 1.6 1.4 0

10

20

30

40

50

Time (s)

one DG Trip

Fig. 14. Power graph for Event-based case at Base capacity

50.5 50

By adapting proposed UFLS scheme, FLLSC checks for frequency limit of 49.5 Hz. After checking this, FLLSC check about type of load disturbance applied on system. FLLSC by monitoring RCB status of DG units determines that system encountered Response-based load disturbance. The FLLSC by measuring frequency and df/dt, estimates the amount of load to be shed for this case. If estimated amount is greater than Pmin, FLLSC sends signal to LSCM, which immediately trip significant number of load feeders to stabilize the frequency. However, if estimated amount is less than Pmin, FLLSC does not sends signal to LSCM, DG unit’s remains operating without requiring any load to be shed. The frequency response of DG for this case is shown in Fig. 15.

49.5 49 48.5 48 47.5 0

10

20

30

40

50

Time (s)

Fig. 12. Frequency response during Event-based case at Base load One DG Trip

50.5 49.5 48.5

Load Increment

47.5 50.2

46.5

50

45.5 0

10

20

30

40

49.8

50

49.6

Time (s)

49.4

Fig. 13. Frequency response without shedding load ranked 10th

49.2

The power supplied by operating DG unit is 1.47 MW and its graph is shown in Fig. 14. The optimal amount of load shed during peak load and based load are shown in Table VII. V.3.

Case III: Response-Based Case with Load Shedding Scheme V.3.1. Peak Load Scenario

To simulate Response-based condition at peak load capacity (3.66 MW); a new load feeder rated 0.54 MW is suddenly connected to bus number 1056 in islanded distribution network. Upon addition of this amount of load, total load becomes 3.66 MW + 0.54 MW = 4.2 MW. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved

49 0

10

20

Time (s)

30

40

50

Fig. 15. Frequency response for Response-based case at Peak Load

Fig. 15 shows that DG frequency has undershoot of 49.078 Hz and recovers to normal value after some time. The proposed methodology shed load up to 5th load ranked. It can be observed that DG frequency while stabilizing has an overshoot up to 50.15 Hz. This frequency overshoot explains that amount of load shed is not optimal. To investigate this issue, load ranked 5th is not shed to confirm whether amount of load shed is optimal or not and response is shown in Fig. 16. It can be noticed that the proposed fuzzy based UFLS scheme sheds optimal number of loads as frequency does not rise International Review of Electrical Engineering, Vol. 7, N. 4

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to normal value and stables at 47.8 Hz when one less load is shed. The power supplied by DG is 3.68237 MW and is shown in Fig. 17.

The power supplied by DG is 2.9413 MW as shown in Fig. 19. The optimal load shedding values for responsebased case are shown in Table VIII. Load Increment

Load Increment

3.1

50.5 50

2.9

49.5

2.7

49 48.5

2.5

48

2.3

47.5 0

10

20

30

40

0

50

10

20

Time (s)

30

40

50

Time (s)

Fig. 16. Frequency response at peak load when load ranked 5th is not disconnected

Fig. 19. Power graph for Response-based case at Base load TABLE VIII OPTIMAL LOAD SHED DURING RESPONSE-BASED CASE Event-Based Power Undershoot Load Shed Case Imbalance Peak Load 49.078 Hz 0.54 MW 0.5163 MW Base Load 47.92 Hz 0.54 MW 0.0987 MW

Load Increment

4.2 4.1 4 3.9 3.8

VI.

3.7 3.6 0

10

20

30

40

50

Time (s)

Fig. 17. Power graph for Response-based case at Peak load

V.3.2. Base Load Scenario To simulate Response-based load shedding case at Base load capacity (2.5 MW); a new load feeder rated 0.54 MW is suddenly connected to bus number 1056 in islanded distribution network. Upon addition of this amount of load, total load becomes 2.5 MW + 0.54 MW = 3.04 MW. Response-based load shedding is applied at t=10s. The frequency response of DG for this case is shown in Fig. 18.

This paper has presented a new fuzzy based adaptive under-frequency load shedding scheme, suitable for islanded distribution network connected with DG. The proposed scheme used frequency, rate of change of frequency, load priority to develop the load shedding strategy. The proposed UFLS scheme is suitable for both Event-based and Response-based cases. The effectiveness and robustness of this scheme has been investigated on different cases from base load to peak load. Form the simulation results it can be concluded that proposed load shedding scheme successfully stabilizes the frequency by shedding optimal number of loads.

Acknowledgements This work was supported by University of Malaya Kuala Lumpur under UMRG research grant (grant code: D000004-16001) and Quaid-e-Awam University of Engineering Science & Technology (QUEST), Nawabshah, Sindh, Pakistan.

Load Increment

50.5

Conclusion

50 49.5 49 48.5

References

48 47.5 0

10

20

30

40

[1]

50

Time (s)

Fig. 18. Frequency response for response-based case at Base load

[2]

In Fig. 18, The DG frequency has undershoot of 47.92 Hz and frequency recovers to normal value after some time. The proposed methodology shed load up to 2nd load ranked.


[3]

P. P. Barker and R. W. De Mello, "Determining the impact of distributed generation on power systems. I. Radial distribution systems," in IEEE Power Engineering Society Summer Meeting, , 2000, pp. 1645-1656 vol. 3. Akash Davda, M. D. Desai and B. R. Parekh, "Assessment of Reduction in Losses by Distributed Generation Penetration in a Radial Network: a Case Study," International Review on Modelling and Simulations (IREMOS), Vol. 4 N. 1, pp. 120-124, February 2011 (Part A) 2011. H. Mohamad, H. Mokhlis, A. H. A. Bakar and H. W. Ping, "A review on islanding operation and control for distribution network


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A Authors’ infformation 1

Deppartment of Eleectrical Engineeering, Faculty of Engineering,, Univ versity of Malaya, 50603 Kuala Luumpur, Malaysia. 2

UM M Power Energy Dedicated D Advannced Centre (UM MPEDAC), Levell 4, Wisma W R&D UM M, University off Malaya, 59990 Kuala Lumpur,, Malaaysia. 3

Deppartment of Electtrical Engineerinng, Quaid-e-Awaam University off Engineering Science & Technologyy (QUEST), 674450, Nawabshah,, Sindhh, Pakistan. Hazlie Mokh hlis received his h B. Eng inn Electrical Engiineering in 1999 and M. Eng. Scc in 2002 from University of Malaya, M Malaysia.. His obtained P PhD degree from the University off Manchester, U UK in 2009. Cuurrently he is a Senior Lectureer in the Departm ment of Electricall Engineering, University U of Malaya. M His mainn research intereest is in distribuution automationn area and power system m protection. J. A. Laghari received r his B. Engg E in Electricall engineering inn 2007 from BUET B Khuzdar,, Pakistan and joined as lectu urer in Quaid-e-Awam Univeersity of Engg g Science andd Technology, QUEST, Naw wabshah, Sindh,, Pakistan in 20008. He obtained his h M. Eng. from m University of Malaya, Malayssia in Electricall Engineering fieeld in 2012. He is currently PhD D Student in Universityy of Malaya, Malaysia. His main research r interestss are intelligent poweer system controol, power systeem optimization,, islan nding operation inn distributed geneeration and smart grid. Ab Halim Ab bu Bakar receivved his B.Sc. inn Electrical E Engineering in 1976 from m Southampton U University UK and a M. Eng andd PhD from University Technology Malaysia inn 1996 and 20003. He has 30 years of utilityy experience iin Malaysia before joiningg academia. Currrently he is a Lecturer in thee Department of Electricall Engineering,, Univ versity of Malaya, Malaysia. M. Karimi reeceived his B. Eng E from Islamicc Azad Universsity, Iran and M. Eng. from m University of Malaya, Malaysia in Electricall Engineering field in 20002 and 2011,, respectively. He H is currently PhD Student inn University off Malaya, Malaaysia. His mainn research intereest is in Distribbuted Generationn and Electric Poower System Stabbility.

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