title: Evidence and Techniques to Derive Benefits from Multifunctional Energy Storage Systems at Low Voltage Distribution Level Including Renewable Generation
Creators: name: Katsanevakis, Markos
Publisher: name: Griffith University
Year: 2022
Subjects: subject: renewable energy
subject: energy storage systems
subject: High Voltage Engineering

description: Reducing the combustion of fossil fuels for electricity generation and replacing them with renewable energy results in two major benefits for people. Public health is promoted with the breathing air being cleaner with reduced particulate concentration. Higher energy production from renewable energy sources (RESs) results in larger portion of the produced electricity being decarbonised thereby decelerating the climate change. Complementary to the benefits that are brought by the RESs to the public health and to the battle with the climate change, are the technological advances of components that are used in RESs installations. For photovoltaic (PV) panels and dc/ac converters, the purchase costs have decreased, and the efficiency and reliability have increased. Considering the significant benefits that RESs bring to people’s lives global scale efforts are in progress to increase the share of renewable energy in the energy mix. The central aim of this research work was to allow higher PV penetration levels to be achieved while using energy storage systems (ESSs) for grid support purposes. ESSs powered by RESs add the missing flexibility to the renewable energy thereby the grid integration of RESs can be increased. The renewable energy that was charged by the ESSs to facilitate the integration of RESs, is dispatched to another period of the day where the grid needs support, thereby complementary applications are performed by the ESSs. For the research a real-life low voltage (LV) distribution network was modelled and it was used in the simulations. The real-life LV network maintains a complex loop arrangement and it is predominantly resistive. The practical LV network is situated at the city of Brisbane in Queensland, Australia. Load profiles measured at the LV network, were proliferated by using Normal distributions, and they were used in the simulations. Environmental data (i.e., solar irradiation and ambient temperature) that were measured at the city of Brisbane, PV panel datasheet values, the one-diode model, as well as the efficiency of the inverter that is representative of a wide range of operation were employed for the calculation of the power generation profiles of the rooftop PV installations that existed at the LV network. Depending on the operational phase of the ESS (i.e., charging, discharging or idling) another equation was used to estimate the energy stored in each storage unit each operating state. The association of power sign and power direction that is used for generators was also followed by the ESSs for power-flow purposes. The comprehensive Newton-Raphson method determined the power-flow and the loading of grid components. The total of the operational grid constraints were included in the simulations namely transmission lines loading, terminal voltage variation, and substation transformer loading. The droop control introduced in this work generated dispatch commands for the ESSs, and maintained a sound grid operation during periods of excessive PV generation. Voltage stability analysis was performed, and to the voltage stability curves was applied for the first time in the literature the Pythagorean theorem, that jointly captures the effect of ESSs on both voltage stability and voltage quality. Key research findings: - Low PV penetration (i.e., 9.12%) is beneficial for the grid because it decreases the loading levels of grid components and it improves voltage profiles. - Grid operation with no support is unable to accommodate high PV penetration levels such as 70%, because the reverse power-flow challenges both the thermal limits of grid components and the statutory voltage limits. - The lowest cost solution was identified for placement and sizing of the community level (CL) ESS. - When the operation of the electrical network is supported by the CL ESS, that absorbs excessive power surpluses according to the dispatch commands it receives from the herein developed droop control, 70% PV penetration becomes feasible, while a sound grid operation is maintained at all times. - For 70% PV penetration, the CL ESS was first dispatched by using fixed droop (FD) control, and then, the dispatch commands of the CL ESS were generated by employing variable droop (VD) control. For 70% PV penetration, both FD control and VD control were able to generate dispatch commands that maintain the total of the grid parameters within the allowable range. - Two ESSs deployment options have been explored in this work. It was found that the CL ESS has higher sizing requirements and cost as compared with the sizing requirements and cost of three ESSs distributed (DIST) at selected terminals of the LV network. The three ESSs administer less power to achieve the control aim, iii therefore their sizing requirement and in turn their cost is lower as compared with the higher sizing requirement and higher cost of the CL ESS. - The cost oriented sensitivity analysis that was performed indicated that the longer the distance of the three ESSs from the critical grid parameters the higher their sizing requirements and therefore their cost. The lowest cost solution was found when the three ESSs were placed directly at the critical terminals of the LV network. When the ESSs were placed directly at the critical terminals of the LV network, they had to manage the minimum amount of power and energy to achieve the control aim. This reduced the sizing requirements of the ESSs and led to the lowest cost solution for ESSs placement. - Steady-state voltage stability analysis was performed. When only one of the three ESSs supported the grid operation at night the voltage stability improved. The voltage stability kept improving as the second and the third ESSs also supported the grid operation. When the CL ESS injected power into the grid at night, the voltage stability benefit was greater as compared with the voltage stability benefit that was recorded when all three distributed ESSs supported the grid operation. The higher amount of energy that was discharged into the grid by the community level ESS, was available at the expense of greater sizing requirement and higher cost compared with the distributed ESSs case. - Initially, the effect of ESSs on voltage stability and on voltage quality was identified independently. In turn, voltage stability curves and the Pythagorean theorem were combined to produce the novel voltage stability and quality index (VSQI ). The comparison of the novel VSQI value in the absence of ESSs and as more ESSs were included in the dispatch, was indicative of the beneficial role of ESSs on both voltage stability and voltage quality. - The impeccable grid support capabilities of ESSs have been demonstrated in this work, by the aggregated applications that have been performed here by the same ESSs. The present PhD research work has made the following contributions: - A novel droop control was introduced in this work that generated dispatch commands for the ESSs and maintained the total of the grid parameters within iv their statutory limits. The novel droop control generated dispatch commands in response to the expected power surplus at the substation transformer. - The herein developed droop control outperforms other droop control methods that exist in the literature that only cater for one of the grid parameters. Further, the droop control introduced in this work is applicable to any grid arrangement because the slope of the droop characteristic is identified iteratively. - With the application of technically sound methods the impeccable grid support capabilities of ESSs have been demonstrated. Specifically, the storage units shifted renewable energy from morning to night, shaved the peak load, enhanced voltage stability, improved voltage quality, and reduced the loading levels of transmission lines and of the substation transformer. Solely the present research work has applied sound technical methods to demonstrate that the abovementioned applications have been performed by the same ESSs. - The present research work has introduced the novel VSQI that jointly captures the beneficial effect of ESSs on voltage stability and on voltage quality. For each scenario, the voltage stability curve was obtained, therefrom the coordinates that correspond to voltage stability and voltage quality were identified, while the Pythagorean theorem was used to calculate the VSQI of the scenario. The increase of the VSQI value as more ESSs were included in the dispatch, represents the benefit of ESSs on both voltage stability and voltage quality. - Comparison of FD control and VD control was performed in terms of compensation accuracy and CL ESS cost. The repetitive overcompensation of FD control increased the rated energy of the CL ESS that in turn elevated the cost of the storage unit. The dispatch commands of higher accuracy generated by VD control, additively diminished the amount of energy the CL ESS had to handle, therefore the rated energy of the storage unit was reduced. Eventually, the reduced rated energy when the CL ESS was dispatched by VD control improved the cost of the storage unit. - The present research work has demonstrated that the same ESSs are able to provide multiple support to the grid. During the planning stage, the present combined grid support should be compared with the grid support reported in other works, for the grid planner to select the appropriate grid support device.