M. J. E. Alam

Mitigation of Rooftop Solar PV Impacts and Evening Peak Support by Managing Available Capacity of Distributed Energy Storage Systems

A high penetration of rooftop solar photovoltaic (PV) resources into low-voltage (LV) distribution networks creates reverse power-flow and voltage-rise problems. This generally occurs when the generation from PV resources substantially exceeds the load demand during high insolation period. This paper has investigated the solar PV impacts and developed a mitigation strategy by an effective use of distributed energy storage systems integrated with solar PV units in LV networks. The storage is used to consume surplus solar PV power locally during PV peak, and the stored energy is utilized in the evening for the peak-load support. A charging/discharging control strategy is developed taking into account the current state of charge (SoC) of the storage and the intended length of charging/discharging period to effectively utilize the available capacity of the storage. The proposed strategy can also mitigate the impact of sudden changes in PV output, due to unstable weather conditions, by putting the storage into a short-term discharge mode. The charging rate is adjusted dynamically to recover the charge drained during the short-term discharge to ensure that the level of SoC is as close to the desired SoC as possible. A comprehensive battery model is used to capture the realistic behavior of the distributed energy storage units in a distribution feeder. The proposed PV impact mitigation strategy is tested on a practical distribution network in Australia and validated through simulations.

A Novel Approach for Ramp-Rate Control of Solar PV Using Energy Storage to Mitigate Output Fluctuations Caused by Cloud Passing

The variability of solar irradiance with a high ramp-rate, caused by cloud passing, can create fluctuation in the PV output. In a weak distribution grid with a high PV penetration, this can create significant voltage fluctuations. Energy storage devices are used to smooth out the fluctuation using traditional moving average control. However, moving average does not control the ramp-rate directly; rather the ramp-rate depends on previous values of PV output. This paper proposes a strategy where the ramp-rate of PV panel output is used to control the PV inverter ramp-rate to a desired level by deploying energy storage (which can be available for other purposes, such as storing surplus power, countering voltage rise, etc.). During the ramping event, the desired ramp-rate is governed by controlling the energy storage based on an inverse relationship with the PV panel output ramp-rate to improve the fluctuation mitigation performance. In contrast to the moving average method, the proposed strategy is able to control the desired ramp-rate independent of the past history of the PV panel output. A dynamic model of the PV-storage integrated system is developed to verify the proposed strategy in the presence of physical device time lags. The proposed strategy is verified using simulation results based on an Australian distribution system. A laboratory experiment is also conducted to validate the concept of the proposed control strategy.

A Three-Phase Power Flow Approach for Integrated 3-Wire MV and 4-Wire Multigrounded LV Networks With Rooftop Solar PV

With increasing level of rooftop solar photovoltaic (PV) penetration into low voltage (LV) distribution networks, analysis with realistic network models is necessary for adequate capturing of network behavior. Traditional three-phase 3-wire power flow approach lacks the capability of exact analysis of 4-wire multigrounded LV networks due to the approximation of merging the neutral wire admittance into the phase wire admittances. Such an approximation may not be desirable when neutral wire and grounding effects need to be assessed, especially in the presence of single-phase solar power injection that may cause a significant level of network unbalance. This paper proposes a three-phase power flow approach for distribution networks while preserving the original 3-wire and 4-wire configurations for more accurate estimation of rooftop PV impacts on different phases and neutrals. A three-phase transformer model is developed to interface between the 3-wire medium voltage (MV) and the 4-wire LV networks. Also an integrated network model is developed for an explicit representation of different phases, neutral wires and groundings of a distribution system. A series of power flow calculations have been performed using the proposed approach to investigate the impacts of single-phase variable PV generation on an Australian distribution system and results are presented.

Distributed energy storage for mitigation of voltage-rise impact caused by rooftop solar PV

A high penetration of solar photovoltaic (PV) resources into distribution networks may create voltage rise problem when the generation from PV resources substantially exceeds the load demand. To reduce the voltage rise, the excess amount of power from the solar PV units needs to be reduced. In this paper, distributed storage systems are proposed for the mitigation of voltage rise problem. The surplus energy from the solar PV is used to charge the distributed storage units during midday, when the power from the solar PV would be typically higher than the load level. This stored energy is then used to reduce the peak load in the evening. An intelligent strategy for charging and discharging control to make effective use of the storage capacity is discussed. The proposed voltage rise mitigation strategy is verified on a practical low voltage distribution feeder in Australia.

An Approach for Online Assessment of Rooftop Solar PV Impacts on Low-Voltage Distribution Networks

Assumption-based offline analysis tools may not be able to provide sufficient and accurate information for the corrective decision making to mitigate solar photovoltaic (PV) impacts in the future distribution grids. This is mainly due to the increasing penetration level of intermittent power generation resources and also the fluctuating behavior of consumer demand. Online assessment tools can assist to manage PV impacts and aid to mitigate those on a real-time basis. This paper proposes an approach for online assessment of rooftop PV impacts on low-voltage (LV) networks using real-time network data. A variable-width sliding window will be used to provide the analysis an outcome-based online data. The width of the sliding window can be varied according to user input so that the changes in network behavior caused by PV integration can be investigated conveniently. Several numerical indices are proposed in this paper to assess solar PV impacts on the LV networks. This approach also uses the online data to develop real-time distribution network models for a dynamic “what-if” analysis. The usefulness of the proposed online assessment approach is verified using an Australian LV distribution feeder.

A Multi-Mode Control Strategy for VAr Support by Solar PV Inverters in Distribution Networks

This paper proposes a multi-purpose VAr control strategy for solar PV inverters for voltage support in distribution networks. The proposed strategy can be applied under various PV power generation conditions. The inverters will normally operate in a dynamic VAr compensation mode for voltage support (including low PV and no PV periods). During mid-day when PV has surplus power, the proposed strategy will control the PV inverters to absorb VAr for voltage rise mitigation using a droop characteristic approach. During passing clouds, the strategy will mitigate voltage fluctuations by ramp-rate control of inverter VAr output. A dynamic model of the proposed PV inverter control has been developed to analyze its performance in terms of fast VAr control and voltage support under various PV generation conditions. The results of the analysis performed on an Australian distribution system show that the proposed VAr control strategy can mitigate voltage rise, and improve the voltage profile despite potential vast changes in the sun irradiation during passing cloud and also in the absence of PV output during the evening.

A Controllable Local Peak-Shaving Strategy for Effective Utilization of PEV Battery Capacity for Distribution Network Support

Plug-in electric vehicles (PEVs) have a potential amount of battery energy storage capacity, which is not fully utilized in regular day-to-day travels. The utilization of spare PEV battery capacity for grid support applications using vehicle-to-grid concept is becoming popular. Depending on the stress on the grid during peak load periods, a small amount of peak-shaving support from the PEVs in a feeder can be useful in terms of grid support. However, as the PEV batteries have limited capacity and the capacity usage is also constrained by travel requirements, a strategy is proposed in this paper for an effective utilization of the available PEV battery capacity for peak shaving. A controllable discharging pattern is developed to most utilize the limited PEV battery capacity when peak shaving is most valuable based on the demand pattern. To ensure an effective use of the available PEV battery capacity for travel, which is the main usage of the PEVs, and for grid support application, dynamic adjustments in PEV discharging rates are made. The effectiveness of the proposed strategy is tested using a real distribution system in Australia and based on practical PEV data.

Effective Utilization of Available PEV Battery Capacity for Mitigation of Solar PV Impact and Grid Support With Integrated V2G Functionality

Utilizing battery storage devices in plug-in electric vehicles (PEVs) for grid support using vehicle-to-grid (V2G) concept is gaining popularity. With appropriate control strategies, the PEV batteries and associated power electronics can be exploited for solar photovoltaic (PV) impact mitigation and grid support. However, as the PEV batteries have limited capacity and the capacity usage is also constrained by transportation requirements, an intelligent strategy is necessary for an effective utilization of the available capacity for V2G applications. In this paper, a strategy for an effective utilization of PEV battery capacity for solar PV impact mitigation and grid support is proposed. A controllable charging/discharging pattern is developed to optimize the use of the limited PEV battery capacity to mitigate PV impacts, such as voltage rise during midday or to support the evening load peak. To ensure an effective utilization of the available PEV battery capacity when used for travel (which is the main usage of the PEVs) or when interventions in the charging operation is caused by passing clouds, a strategy for dynamic adjustments in PEV charging/discharging rates is proposed. The effectiveness of the proposed strategy is tested using a real distribution system in Australia based on practical PV and PEV data.

A SAX-Based Advanced Computational Tool for Assessment of Clustered Rooftop Solar PV Impacts on LV and MV Networks in Smart Grid

Future distribution networks with increasing level of solar PV penetration will be managed using smart grid technologies capable of producing appropriate and timely response during normal and abnormal operational events. Distribution feeder loads vary throughout the day according to the trend of consumption of the customers. Solar PV outputs fluctuate in proportion to irradiance level of sun. Simultaneous occurrence of both of these variations would result in various operating conditions that may lead to unexpected events, and would require a large amount of network data to be processed and analyzed for decision making. It is envisaged that such data will be available in the future grids with the availability of smart technologies and advanced communication in residential dwellings, commercial buildings and industrial complexes. In this paper, an advanced intelligent computational tool is developed to characterize and analyze the large amount of data associated with wide variations in network behavior using SAX (Symbolic Aggregate Approximation) and pattern recognition. The proposed tool is capable of dealing with network asymmetry, load unbalance, single-phase solar PV integration and their impacts on upstream networks and will assist in making right and timely decision to mitigate adverse impacts of solar PV. The proposed tool has been tested with a practical three-phase distribution system in Australia and can provide an extensive assessment with less computational efforts and time.

Effectiveness of traditional mitigation strategies for neutral current and voltage problems under high penetration of rooftop PV

A high penetration of single phase rooftop photovoltaic (PV) units have the potential to exacerbate the existing neutral current and voltage problems in low voltage 4-wire distribution networks. This paper investigates the effectiveness of traditional strategies for mitigation of neutral current and voltage problems in the presence of a high penetration of rooftop PV. This paper analyses the limitations of traditional methods to mitigate neutral current and voltage problems caused by rooftop PV. The analysis shows that in the presence of unbalanced rooftop PV allocation, new mitigation strategies are required. The application of energy storage is explored in this paper as a potential mitigation strategy to reduce the neutral current and voltage problems. Results show that the reduction in neutral voltage by the application of energy storage can be achieved, within the acceptable limit, under daily variations of load demand and PV output.