1. 1 Cheng-Ting Hsu Chao-Shun Chen Islanding Operations for the Distribution Systems 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

2. 2 Outline Description of Study System and DGS Description of Study System and DGS Load Models Load Models Load Shedding Schemes Load Shedding Schemes Conclusion Conclusion Introduction Introduction Transient Stability Analysis of Islanding System

3. 3 Introduction DGS are growing quickly due to the environmental issue and most of DGS have smaller installation capacity so that they will be connected to the distribution system. DGS are growing quickly due to the environmental issue and most of DGS have smaller installation capacity so that they will be connected to the distribution system. It is possible to have the islanding operation, although it is prohibited by utility since it may endanger the safety of the equipment and utility staff. However, it is also a feasible condition because the probability of power failure can be reduced if the utility can solve the problems. It is possible to have the islanding operation, although it is prohibited by utility since it may endanger the safety of the equipment and utility staff. However, it is also a feasible condition because the probability of power failure can be reduced if the utility can solve the problems. This paper investigates the operation feasibility for the islanding system with different types and control schemes of DGS by executing transient stability. Also, different load models and load shedding schemes are applied to know their impact on the islanding system. This paper investigates the operation feasibility for the islanding system with different types and control schemes of DGS by executing transient stability. Also, different load models and load shedding schemes are applied to know their impact on the islanding system.

4. 4 Description of Study System and DGS

5. 5

6. 6 - + PID control scheme

7. 7 Load Models CP, RCI and RCIM load models are applied in this paper 1. CP: constant power 2. RCI: RCI load model is the combination of the residential, commercial and industrial type customers. The load composition at each bus of the feeder can be applied to the static RCI load model derived by the EPRI to know the variation of the load on the voltage and frequency deviations. 3. RCIM: The RCIM load model is composed of the typical dynamic model of induction motor and the static RCI load model.

8. 8 Load Models The percentages of the load composition at different buses Load P (MW) Q (MVAR) Residential (24.1%)Commercial (11.5%)Industrial (64.4%) A/CRILA/CFILRFGIM A/CFILR L11.70.427218104639965620213 L21.940.46671914652294801073 L31.310.258182440451054525237 Load P (MW) Q (MVAR) Residential (42.1%)Commercial (53%)Industrial (4.9%) A/CRILA/CFILRFGIM A/CFILR L41.50.416617 45391064025305 L51.50.3756242050321356020173 L61.80.4256242040421087015114 Load P (MW) Q (MVAR) Residential (29.4%)Commercial (19.1%)Industrial (51.5%) A/CRILA/CFILRFGIM A/CFILR L72.70.717117126020137851041 L81.310.2712274533157851041 L92.330.666122265181255020246 A/C : air conditioner load IL : incandescent lighting RFG : refrigerator load IM : induction motors R : resistive load FIL : fluorescent and incandescent lighting

9. 9 Load Shedding Schemes 1.Low frequency relay load shedding StepFrequency (Hz) Shedding Amount (MW) 1592 258.81 358.61 458.41 558.21 6580.5 2.Frequency decay-rate load shedding

10. 10 Transient Stability Analysis of Islanding System The utility network is disconnected from the distribution substation at 16 cycles. To investigate the effects of the DGS on the islanding distribution network, three operation scenarios are selected for transient stability analysis. Besides, different load models and load shedding schemes as described above are applied in the computer simulation. The utility network is disconnected from the distribution substation at 16 cycles. To investigate the effects of the DGS on the islanding distribution network, three operation scenarios are selected for transient stability analysis. Besides, different load models and load shedding schemes as described above are applied in the computer simulation. In this case study, the WG is out of service and the GTG is operated alone. The initial active and reactive power outputs of GTG are 10MW and -0.3Mvar. Also, the total load demands of the distribution feeders are 16.2MW and 3.8Mvar. In this case study, the WG is out of service and the GTG is operated alone. The initial active and reactive power outputs of GTG are 10MW and -0.3Mvar. Also, the total load demands of the distribution feeders are 16.2MW and 3.8Mvar. Case A :Islanding system with GTG alone

11. 11 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.

12. 12 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.

13. 13 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: 3steps  4MW RCI: 2steps  3MW RCIM: 2 steps  3MW

14. 14 CP: 5.46MW RCI: 4.72MW RCIM: 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.

15. 15 Case B: Islanding system with WG alone For study case B, the WG is operated to generate active power while the GTG is considered as a synchronous condenser to regulate the voltage by its excitation system. Also, a capacitor bank with rated capacity of 3.5Mvar is installed to provide the reactive power absorbed by the WG. The initial active power outputs for the WG is 10MW. For study case B, the WG is operated to generate active power while the GTG is considered as a synchronous condenser to regulate the voltage by its excitation system. Also, a capacitor bank with rated capacity of 3.5Mvar is installed to provide the reactive power absorbed by the WG. The initial active power outputs for the WG is 10MW.

16. 16 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.

17. 17 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.

18. 18 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.

19. 19 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.

20. 20 Case C: Islanding system with WG and GTG In this case study, the WG and GTG are operated at the same time. The initial active power outputs for the GTG and WG are 6MW and 4MW respectively. The utility has provided the 6.2 MW active power and 3.3 Mvar reactive power to the distribution feeders. In this case study, the WG and GTG are operated at the same time. The initial active power outputs for the GTG and WG are 6MW and 4MW respectively. The utility has provided the 6.2 MW active power and 3.3 Mvar reactive power to the distribution feeders.

21. 21 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.

22. 22 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)

23. 23 CP: 5.46MW RCI: 5.42MW RCIM: 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.

24. 24Conclusion 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 alone even different load shedding schemes have been considered. However, it is feasible for the WG with the proposed frequency-based pitch controller. By executing the suitable load shedding, the islanding systems with a GTG alone or the combination of WG and GTG can also be operated safely. The islanding operation is difficult for the WG to be operated alone even different load shedding schemes have been considered. However, it is feasible for the WG with the proposed frequency-based pitch controller. By executing the suitable load shedding, the islanding systems with a GTG alone or the combination of WG and GTG can also be operated safely. It is concluded that the power islanding operation is feasible if the suitable load shedding schemes and proper DGS control schemes are applied. It is concluded that the power islanding operation is feasible if the suitable load shedding schemes and proper DGS control schemes are applied.