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Cheng-Ting Hsu Chao-Shun Chen

Islanding Operations for the Distribution Islanding Operations for the Distribution Systems with Dispersed Generation SystemsSystems with Dispersed Generation Systems

Department of Electrical Engineering

Southern Taiwan University of Technology

Tainan, Taiwan

Department of Electrical Engineering

National Sun-Yat Sen University

Kaohsiung, Taiwan

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OutlineOutline

Description of Study System and DGSDescription of Study System and DGS

Load ModelsLoad Models

Load Shedding SchemesLoad Shedding Schemes

ConclusionConclusion

IntroductionIntroduction

Transient Stability Analysis of Islanding SystemTransient Stability Analysis of Islanding System

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IntroductionIntroduction

DGS are growing quickly due to the environmental issueDGS are growing quickly due to the environmental issue and and most of DGS have smaller installation capacity so that they most of DGS have smaller installation capacity so that they will be connected to the distribution system. will be connected to the distribution system.

It is possible to have the islanding operation, although it is It is possible to have the islanding operation, although it is

prohibited by utility since it may endanger the safety of the prohibited by utility since it may endanger the safety of the equipment and utility staff. However, it is also a feasible equipment and utility staff. However, it is also a feasible condition because the probability of power failure can be condition because the probability of power failure can be reduced if the utility can solve the problems. reduced if the utility can solve the problems.

This paper investigates the operation feasibility for the This paper investigates the operation feasibility for the islanding system with different types and control schemes of islanding system with different types and control schemes of DGS by executing transient stability. Also, different load DGS by executing transient stability. Also, different load models and load shedding schemes are applied to know their models and load shedding schemes are applied to know their impact on the islanding system. impact on the islanding system.

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Description of Study System and DGSDescription of Study System and DGS

55

/),(CpUR5.0T 23W

66

-+

PID control scheme

f

reff

sT

sT

sTK

dp

dp

ipp 1

11

f 0

min

max

PC MP

77

Load Models

CP, RCI and RCIM load models are applied in this paperCP, RCI and RCIM load models are applied in this paper

1. CP: constant power1. CP: constant power

2. RCI: RCI load model is the combination of the residential, 2. RCI: RCI load model is the combination of the residential, commercial and industrial type customers. The load commercial and industrial type customers. The load composition at each bus of the feeder can be applied to the composition at each bus of the feeder can be applied to the static RCI load model derived by the EPRI to know the static RCI load model derived by the EPRI to know the variation of the load on the voltage and frequency deviations. variation of the load on the voltage and frequency deviations.

3. RCIM: The RCIM load model is composed of the typical 3. RCIM: The RCIM load model is composed of the typical dynamic model of induction motor and the static RCI load dynamic model of induction motor and the static RCI load model. model.

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Load Models

The percentages of the load composition at different busesThe percentages of the load composition at different buses

LoadP

(MW) Q

(MVAR)

Residential (24.1%) Commercial (11.5%) Industrial (64.4%)

A/C R IL A/C FIL RFG IM IM A/C FIL R

L1 1.7 0.42 72 18 10 46 39 9 6 56 20 21 3

L2 1.94 0.46 67 19 14 65 22 9 4 80 10 7 3

L3 1.31 0.2 58 18 24 40 45 10 5 45 25 23 7

LoadP

(MW) Q

(MVAR)

Residential (42.1%) Commercial (53%) Industrial (4.9%)

A/C R IL A/C FIL RFG IM IM A/C FIL R

L4 1.5 0.41 66 17 17 45 39 10 6 40 25 30 5

L5 1.5 0.37 56 24 20 50 32 13 5 60 20 17 3

L6 1.8 0.42 56 24 20 40 42 10 8 70 15 11 4

LoadP

(MW) Q

(MVAR)

Residential (29.4%) Commercial (19.1%) Industrial (51.5%)

A/C R IL A/C FIL RFG IM IM A/C FIL R

L7 2.7 0.71 71 17 12 60 20 13 7 85 10 4 1

L8 1.31 0.2 71 22 7 45 33 15 7 85 10 4 1

L9 2.33 0.6 66 12 22 65 18 12 5 50 20 24 6A/C : air conditioner load IL : incandescent lighting RFG : refrigerator loadIM : induction motors R : resistive load FIL : fluorescent and incandescent lighting

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Load Shedding Schemes

1. Low frequency relay load shedding

Step Frequency (Hz)Shedding

Amount (MW)

1 59 2

2 58.8 1

3 58.6 1

4 58.4 1

5 58.2 1

6 58 0.5

2. Frequency decay-rate load shedding

060

2

60

2m

H

dt

fdHP syssysstep

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Transient Stability Transient Stability Analysis of Islanding SystemAnalysis of Islanding System

The utility network is disconnected from the distribution substatThe utility network is disconnected from the distribution substation at 16 cycles. To investigate the effects of the DGS on the islion at 16 cycles. To investigate the effects of the DGS on the islanding distribution network, three operation scenarios are selectanding distribution network, three operation scenarios are selected for transient stability analysis. Besides, different load modeled for transient stability analysis. Besides, different load models and load shedding schemes as described above are applied in ts and load shedding schemes as described above are applied in the computer simulation. he computer simulation.

In this case study, the WG is out of service and the GTG is opeIn this case study, the WG is out of service and the GTG is operated alone. The initial active and reactive power outputs of Grated alone. The initial active and reactive power outputs of GTG are 10MW and -0.3Mvar. Also, the total load demands of tTG are 10MW and -0.3Mvar. Also, the total load demands of the distribution feeders are 16.2MW and 3.8Mvar.he distribution feeders are 16.2MW and 3.8Mvar.

Case A :Islanding system with GTG aloneCase A :Islanding system with GTG alone

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This figure shows the voltage responses of the islanding system without considering the load shedding. It is found that the voltage responses of the islanding system are almost recovered to the nominal value finally. However, the voltages have ever dropped to the values of 0.86, 0.87 and 0.91pu for the CP, RCI and RCIM load models respectively. The RCIM load model gives the better dynamic responses than the CP and RCI load models.

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This figure gives the frequency responses of the islanding system without considering the load shedding. Without executing the load shedding, the frequencies of the islanding system decline very quickly and reach an unacceptable value for any kind of the load model even the GTG has increased its mechanical input power to the maximal value. For the CP load model, it produces the largest frequency decay rate because the constant load demand is assumed during the transient period.

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This figure shows the frequency responses of the islanding system with considering the under-frequency load shedding. Two shedding steps with a total amount of 3MW load are executed to recover the islanding system frequency to 59.5Hz for the RCI and RCIM models. For the CP load model, three shedding steps with a total amount of 4MW load are necessary to restore the frequency.

CP: 3steps4MWRCI: 2steps3MWRCIM: 2 steps3MW

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CP: 5.46MWRCI: 4.72MWRCIM: 3.61MW

This figure shows the frequency responses of the islanding system with considering the frequency decay rate load shedding. After the tripping of the utility, the frequency decay rates are 5.9, 5.1 and 3.9 Hz/sec for CP, RCI and RCIM load models. The total shedding loads are therefore calculated as 5.5, 4.7 and 3.6MW for CP, RCI and RCIM models. The frequencies recover quickly after the load shedding have been executed.

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Case B: Islanding system with WG aloneCase B: Islanding system with WG alone

For study case B, the WG is operated to generate active power For study case B, the WG is operated to generate active power while the GTG is considered as a synchronous condenser to rewhile the GTG is considered as a synchronous condenser to regulate the voltage by its excitation system. Also, a capacitor bagulate the voltage by its excitation system. Also, a capacitor bank with rated capacity of 3.5Mvar is installed to provide the renk with rated capacity of 3.5Mvar is installed to provide the reactive power absorbed by the WG. The initial active power outactive power absorbed by the WG. The initial active power outputs for the WG is 10MW.puts for the WG is 10MW.

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This figure gives the frequency responses of the islanding system without considering the load shedding schemes. It is found that the frequency responses are worse than the case A because the WG has provided the constant power output of 10MW. It is also observed that the islanding system collapsed very quickly for all kinds of load models.

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CP: 6 steps (6.5MW)RCI: 5 steps (6MW)RCIM: 5steps (6MW)

This figure shows the frequency responses with considering the under-frequency load shedding scheme. Six shedding steps with a total amount of 6.5MW are executed for CP load model. However, the frequency kept rising to an unacceptable level due to over load shedding and constant active power output of WG. On the other hand, five shedding steps with a total amount of 6MW load are executed to recover the islanding system frequency to 59.5Hz for the RCI and RCIM models.

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This figure gives the blade angle responses of the wind turbines with considering the under-frequency load shedding and pitch controller. The initial blade angles are operated at 5 degree to produce 10MW mechanical power output. Due to the action of pitch controller, the blade angle reduces to 0° to result in the variation of the mechanical power from 10MW to 12.85MW. Finally, the blade angles keep at 1.05°, 1.21°and 1.32° for the CP, RCI and RCIM load models.

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CP: 3 steps (4MW)RCI: 3 steps (4MW)RCIM: 3 steps (4MW)

This figure gives the frequency responses of the islanding system with considering the under-frequency load shedding and pitch controller. The frequency has ever declined to a minimal value of 58.4Hz. Three shedding steps have been executed for all load models to recover the system frequency. After the load has been tripped, the frequencies of the islanding system reach the maximum value of 60.8, 60.2 and 60.4Hz for the CP, RCI and RCIM load models. With the proposed pitch controller to regulate the blade angle, the frequency can be recovered very well.

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Case C: Islanding system with Case C: Islanding system with WG and GTGWG and GTG

In this case study, the WG and GTG are operated at thIn this case study, the WG and GTG are operated at the same time. The initial active power outputs for the e same time. The initial active power outputs for the GTG and WG are 6MW and 4MW respectively. The GTG and WG are 6MW and 4MW respectively. The utility has provided the 6.2 MW active power and 3.utility has provided the 6.2 MW active power and 3.3 Mvar reactive power to the distribution feeders. 3 Mvar reactive power to the distribution feeders.

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This figure gives the frequency responses of the islanding system without considering the load shedding schemes. After the disconnection of utility, the frequencies of the islanding system decline very quickly and reach the minimal values of 58.4, 58.6and 58.8Hz for CP, RCI and RCIM models respectively. Due to the governor action of the GTG, the frequencies begin to rise and maintain at 58.8Hz even without executing load shedding.

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In this case study, the frequencies have reached the setting of load shedding schemes. This figure gives the frequency responses of the islanding system with considering the under-frequency load shedding scheme. For the CP load model, two shedding steps with a total amount of 3MW load are tripped and the frequency is restored to 59.4Hz. On the other hand, step one load shedding is executed only to recover the frequency to 59.3Hz for the RCI and RCIM models.

CP: 2steps (3MW)RCI: 1steps (2MW)RCIM: 1steps (2MW)

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CP: 5.46MWRCI: 5.42MWRCIM: 4.02MW

The above figure gives the frequency of the islanding system with considering the frequency decay rate load shedding scheme. The frequency decay rates after the tripping of utility are 5.7, 5.4 and 4.0 Hz/sec and the shedding loads are therefore calculated as 5.5, 5.4 and 4MW for CP, RCI and RCIM models respectively. It can be found that the frequencies of the islanding system can be maintained well for all the load models after the load shedding has been executed.

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ConclusionConclusion The load models have a great impact on the dynamic response

of the islanding system and the amount of load shedding. The CP load model has resulted in greater discrepancy than the RCI and RCIM models. With considering the static and dynamic characteristics of load, the RCIM load model should present the most accurate simulation results.

The islanding operation is difficult for the WG to be operated The islanding operation is difficult for the WG to be operated alone even different load shedding schemes have been alone even different load shedding schemes have been considered. However, it is feasible for the WG with the considered. However, it is feasible for the WG with the proposed frequency-based pitch controller. By executing the proposed frequency-based pitch controller. By executing the suitable load shedding, the islanding systems with a GTG alone suitable load shedding, the islanding systems with a GTG alone or the combination of WG and GTG can also be operated safely.or the combination of WG and GTG can also be operated safely.

It is concluded that the power islanding operation is feasible if It is concluded that the power islanding operation is feasible if the suitable load shedding schemes and proper DGS control the suitable load shedding schemes and proper DGS control schemes are applied. schemes are applied.