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A PPLICATION E XAMPLES

dynamics.

A schematic diagram showing the network integration of voltage regulator dynamics and PV inverter dynamics is shown in Fig. 5.7 which will be used for investigating the dynamics of the distribution network with the effect of the proposed reactive power control strategy including the impact on the voltage regulator tap operation.

Fig. 5.8. An Australian distribution system. (a) The MV system. (b). An LV feeder in the MV system.

A realistic PV output profile with a 1-second resolution, collected from the Oahu Island, Hawaii on the 18th of March 2010 by NREL [16] is used for testing the proposed strategy. The load data is captured from a real LV residential feeder in New South Wales.

Fig. 5.9. Reactive power support profile obtained from the dynamic model. (a) During reduction of PV output power. (b) During increase of PV output power.

Fig. 5.10. Dynamic behaviour of the PCC voltage at the household HH16.

Using the differential-algebraic model of the integrated network, the dynamic changes of PV inverter reactive power in response to PV inverter active power change is investigated. Fig. 5.9(a) shows how the reactive power is increased in response to a sudden decrease in active power caused by the reduction in the irradiance level. Initially the controller was in Mode 2 for controlling the consumption of reactive power for voltage rise mitigation caused by reverse power flow. With the sudden reduction of PV output power, the controller switches to Mode 3 operation and starts to generate reactive power. At the end of the negative ramping event, the controller continues the Q-support over the low PV period until a positive ramping event is detected. Fig. 5.9(b) shows that the controller was initially in Mode 3 for providing reactive support during the low PV

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period between the negative and positive ramping events. As the PV output starts to increase at the end of cloud passing, a positive ramping event is detected. Then the controller starts to reduce the Q-injection, and eventually starts to consume Q, which is then followed by a transition to Mode 2 for voltage rise mitigation. The dynamic behaviour of the PCC voltage at household HH16 as a result of the reactive power control is shown in Fig. 5.10 that shows, without reactive power support, the decrease in PCC voltage is higher.

To understand the effectiveness of the proposed strategy in providing network support under different weather conditions, a longer term analysis is performed using quasi-steady-state simulation by solving the load-flow model of the network at each instant of time. A 570-second window from the real PV output profile used in the dynamic simulation is used for the long term quasi-steady-state analysis.

As shown in Fig. 5.11, the PV output profile consists of events such as, a high PV output producing reverse power flow, a sharp decrease in the PV output due to the starting of the passing cloud period, a low PV output during cloudy sky, and a sharp increase in PV output at the end of the passing cloud period. When the PV output is high and produces reverse power flow, reactive power is consumed according to Mode 2 to mitigate voltage rise. While this approach consumes VAr from the system, at the same time it injects active power to the system which relieves the system from active power supply burden. Therefore the system capacity will be relieved. As the present version of this work mainly concerns PV inverter installations at the LV distribution level which would be connected to the high voltage transmission system through large sub-transmission networks with numerous VAr support devices (capacitor banks, SVC, STATCOM etc.), the impact of the VAr demand created by the proposed strategy on the transmission system operation would be insignificant.

When the PV output decreases sharply at a high ramp-rate at the starting of the cloud passing period, the reactive power generation also increases sharply because of triggering the control Mode 3, to mitigate the sudden decrease in voltage. Following the sudden decrease in PV output, when the PV active power generation remains low for a certain period of time, the reactive power support is continued over that period according to (7). When the PV output again sharply increases at the end of the cloud passing, reactive power is consumed according to Mode 3 control for mitigating sharp

increase in voltage. Once the PV output ramping event is over, the reactive power is controlled according to Mode 2 to consume reactive power to mitigate voltage rise.

Fig. 5.11. Control of reactive power according to the proposed strategy under different conditions of PV generation.

Fig. 5.12. PCC voltage at HH16 without the regulator REG3 operation.

Fig. 5.13. PCC voltage at HH16 with the regulator REG3 operation.

The impact of the proposed strategy on the PCC voltage of household HH16 in the LV feeder is shown in Fig. 5.12 (without the operation of the regulator) and in Fig. 5.13 (with the operation of the regulator). The mitigation of voltage rise by consuming reactive power, mitigation of voltage fluctuation by ramp-rate control of reactive power

Voltage [V]

(generation /consumption) and voltage support during intermittent low PV generation are identified in Fig. 5.12. The voltage profile for the same PCC in household HH16 is shown in Fig. 5.13, where the impact of the upstream step voltage regulator is considered. Due to the voltage regulator operation, the LV feeder voltage without VAr support is improved during the cloudy period. Fig. 5.12 and 5.13 also show that with appropriate reactive power support, the voltage profile does not exceed the operational limits.

The proposed versatile VAr control strategy not only features the capabilities to provide dynamic VAr compensation for voltage support (Mode 1), voltage rise mitigation (Mode 2) and voltage fluctuation mitigation (Mode 3), but also ensures smooth transitions among the control modes so that the mode transitions do not create any fluctuation or abrupt change in voltage. The performance of the proposed strategy is compared in Fig. 5.14 with that using the simple reactive power to PV active power (Q-P) droop and the reactive power to voltage (Q-V) droop.

Fig. 5.14. Comparison of the proposed strategy with Q-P and Q-V regulation modes.

In the Q-P regulation mode, the inverter will consume VAr in proportion to the active power generation of the inverter to mitigate voltage rise. However, when the inverter experiences fluctuations in the active power generation due to unstable weather conditions, the amount of reactive power (Q) will also experience fluctuation at the same proportion and this will also be reflected in the voltage, as shown in Fig. 5.14.

In a Q-V regulation mode, the inverter will consume or generate VAr according to a Q-V droop characteristic. However, the Q-V droop itself will not cater for the fluctuations created by the shifting of operating points from a high voltage

(Q-consumption) to a low voltage (Q-generation) region. As the high voltage to low voltage transition can appear quite often and at a high ramp-rate in a practical PV generation scenario, the problem of the voltage fluctuation will still remain unaddressed, as shown in Fig. 5.14. In contrast, the proposed strategy carefully controls the mode transitions and in-mode operations using strategic applications of a ramp-rate limiting function. Therefore, an overall satisfactory performance in voltage support, voltage rise mitigation and fluctuation reduction can be achieved using the proposed method.

The benefit of the reactive power support on the voltage regulator operation is shown in Fig. 5.15 and 5.16. The total reactive power support from the all the distributed inverters in the downstream LV feeder, shown in Fig. 5.15(a), aids to keep the load centre [MV Bus 27] voltage profile of the regulator REG3 within the regulator control limits, shown in Fig. 5.15(b). Therefore, the tap operation is reduced as shown in Fig.

5.16. The cumulative tapping profile showing the total number of tap operations performed at the end of the period under consideration indicates a significant reduction in tap operation with the proposed strategy which will eventually result in less maintenance of voltage regulators.

Fig. 5.15. Impact of the proposed reactive power control strategy on REG3 load centre voltage. (a) Total reactive power generation and consumption in the feeder. (b) Load centre [Bus 27] voltage profile.

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Fig. 5.16. REG3 tap operation profile.

The application of the proposed control strategy for VAr support during a low voltage period using Mode 1 control is investigated and the results are presented in Fig. 5.17. A one hour period is selected from 20:00 hour to 21:00 hour that contains the peak load demand of the demand curve used in this thesis.

 

Fig. 5.17. Effects of reactive power support from PV inverters during evening load period. (a) Reactive power profile. (b) Voltage improvement at the PV inverter connection point at the LV customer location.

(c) REG3 load centre voltage profile. (d) REG3 tap operation.

The power drawn from the grid and VAr support profile from the inverter at the household HH16 is shown in Fig. 5.17(a), and the voltage improvement achieved by the VAr support during this period shown in Fig. 5.17(b). Without the VAr support, the voltage profile is at the edge of the lower voltage limit, whereas with the VAr support

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the voltage profile does not exceed the limit. Due to this VAr support, the load centre voltage also improves in such a way that the tap operation is reduced, as presented in Fig. 5.17(c) and (d).

Although VAr support from customer installed distributed PV inverters is a highly discussed topic, most distribution utilities are yet to allow this to happen in their networks. In autonomous microgrids, however, this concept could be more realistic.