Mitigating the erosion of transient stability margins in Great Britain through novel wind farm control techniques

The predominant North-to-South active power flow across the border between Scotland and England has historically been limited by system stability considerations. As the penetration of variable-speed wind power plants in Great Britain grows (reducing the generation share of traditional synchronous generation), it is imperative that stability limits, operational flexibility, efficiency and system security are not unduly eroded as a result. The studies reported in this thesis illustrate the impacts on critical fault clearing times and active power transfer limits through this North-South corridor, known as the B6 boundary, in the presence of increasing penetrations of wind power generation on the GB transmission system. By focussing on the transient behaviour of a representative reduced test system following a three-phase short-circuit fault occurring on one of the two double-circuits constituting the B6 boundary, the impacts on the transient stability margins are qualitatively identified. There is a pressing necessity for new wind farms to be able to mitigate, as much as possible, their own negative impacts on system stability margins. The transient stability improvement achieved by tailoring the low voltage ride-through reactive power control response of wind farms is first investigated, and a novel control technique is then presented which can significantly mitigate the erosion of the transient stability performance of power systems, in the presence of in-creasing amounts of wind power, by tailoring the immediate post-fault active power recovery ramp-rates of the wind power plants around the system. The impacts of these control techniques on critical fault clearing times and power transfer limits are investigated. In particular, it has been found that the use of slower active power recovery from wind farms located in exporting regions when a short circuit fault occurs on the export corridor will provide significant benefits for both of these metrics, while a faster active power recovery in importing regions will provide a similar transient stability benefit. However, it is also shown that there are potential detrimental effects for system frequency stability. In addition, important impacts of wind farm settings in respect of low voltage ride through are revealed whereby the LVRT controls can act to erode stability margins if careful consideration of their settings is not taken. Assuming a future power system with high levels of centralised observability and controllability (or decentralised co-operative control systems), it may be possible to continually “dispatch” the reactive power gains and active power recovery ramp rates discussed in this thesis to match the current system setpoint and to seek an optimal transient response to a range of credible contingencies.The predominant North-to-South active power flow across the border between Scotland and England has historically been limited by system stability considerations. As the penetration of variable-speed wind power plants in Great Britain grows (reducing the generation share of traditional synchronous generation), it is imperative that stability limits, operational flexibility, efficiency and system security are not unduly eroded as a result. The studies reported in this thesis illustrate the impacts on critical fault clearing times and active power transfer limits through this North-South corridor, known as the B6 boundary, in the presence of increasing penetrations of wind power generation on the GB transmission system. By focussing on the transient behaviour of a representative reduced test system following a three-phase short-circuit fault occurring on one of the two double-circuits constituting the B6 boundary, the impacts on the transient stability margins are qualitatively identified. There is a pressing necessity for new wind farms to be able to mitigate, as much as possible, their own negative impacts on system stability margins. The transient stability improvement achieved by tailoring the low voltage ride-through reactive power control response of wind farms is first investigated, and a novel control technique is then presented which can significantly mitigate the erosion of the transient stability performance of power systems, in the presence of in-creasing amounts of wind power, by tailoring the immediate post-fault active power recovery ramp-rates of the wind power plants around the system. The impacts of these control techniques on critical fault clearing times and power transfer limits are investigated. In particular, it has been found that the use of slower active power recovery from wind farms located in exporting regions when a short circuit fault occurs on the export corridor will provide significant benefits for both of these metrics, while a faster active power recovery in importing regions will provide a similar transient stability benefit. However, it is also shown that there are potential detrimental effects for system frequency stability. In addition, important impacts of wind farm settings in respect of low voltage ride through are revealed whereby the LVRT controls can act to erode stability margins if careful consideration of their settings is not taken. Assuming a future power system with high levels of centralised observability and controllability (or decentralised co-operative control systems), it may be possible to continually “dispatch” the reactive power gains and active power recovery ramp rates discussed in this thesis to match the current system setpoint and to seek an optimal transient response to a range of credible contingencies

Control and Stability of Residential Microgrid with Grid-Forming Prosumers

The rise of the prosumers (producers-consumers), residential customers equipped with behind-the-meter distributed energy resources (DER), such as battery storage and rooftop solar PV, offers an opportunity to use prosumer-owned DER innovatively. The thesis rests on the premise that prosumers equipped with grid-forming inverters can not only provide inertia to improve the frequency performance of the bulk grid but also support islanded operation of residential microgrids (low-voltage distribution feeder operated in an islanded mode), which can improve distribution grids’ resilience and reliability without purposely designing low-voltage (LV) distribution feeders as microgrids. Today, grid-following control is predominantly used to control prosumer DER, by which the prosumers behave as controlled current sources. These grid-following prosumers deliver active and reactive power by staying synchronized with the existing grid. However, they cannot operate if disconnected from the main grid due to the lack of voltage reference. This gives rise to the increasing interest in the use of grid-forming power converters, by which the prosumers behave as voltage sources. Grid-forming converters regulate their output voltage according to the reference of their own and exhibit load sharing with other prosumers even in islanded operation. Making use of grid-forming prosumers opens up opportunities to improve distribution grids’ resilience and enhance the genuine inertia of highly renewable-penetrated power systems. Firstly, electricity networks in many regional communities are prone to frequent power outages. Instead of purposely designing the community as a microgrid with dedicated grid-forming equipment, the LV feeder can be turned into a residential microgrid with multiple paralleled grid-forming prosumers. In this case, the LV feeder can operate in both grid-connected and islanded modes. Secondly, gridforming prosumers in the residential microgrid behave as voltage sources that respond naturally to the varying loads in the system. This is much like synchronous machines extracting kinetic energy from rotating masses. “Genuine” system inertia is thus enhanced, which is fundamentally different from the “emulated” inertia by fast frequency response (FFR) from grid-following converters. Against this backdrop, this thesis mainly focuses on two aspects. The first is the small-signal stability of such residential microgrids. In particular, the impact of the increasing number of grid-forming prosumers is studied based on the linearised model. The impact of the various dynamic response of primary sources is also investigated. The second is the control of the grid-forming prosumers aiming to provide sufficient inertia for the system. The control is focused on both the inverters and the DC-stage converters. Specifically, the thesis proposes an advanced controller for the DC-stage converters based on active disturbance rejection control (ADRC), which observes and rejects the “total disturbance” of the system, thereby enhancing the inertial response provided by prosumer DER. In addition, to make better use of the energy from prosumer-owned DER, an adaptive droop controller based on a piecewise power function is proposed, which ensures that residential ESS provide little power in the steady state while supplying sufficient power to cater for the demand variation during the transient state. Proposed strategies are verified by time-domain simulations

Development of Robust and Dynamic Control Solutions for Energy Storage Enabled Hybrid AC/DC Microgrids

Development of Robust and Dynamic Control Solutions for Energy Storage Enabled Hybrid AC/DC Microgrid

Advances in Modelling and Control of Wind and Hydrogenerators

Rapid deployment of wind and solar energy generation is going to result in a series of new problems with regards to the reliability of our electrical grid in terms of outages, cost, and life-time, forcing us to promptly deal with the challenging restructuring of our energy systems. Increased penetration of fluctuating renewable energy resources is a challenge for the electrical grid. Proposing solutions to deal with this problem also impacts the functionality of large generators. The power electronic generator interactions, multi-domain modelling, and reliable monitoring systems are examples of new challenges in this field. This book presents some new modelling methods and technologies for renewable energy generators including wind, ocean, and hydropower systems

Advances in Modelling and Control of Wind and Hydrogenerators

Rapid deployment of wind and solar energy generation is going to result in a series of new problems with regards to the reliability of our electrical grid in terms of outages, cost, and life-time, forcing us to promptly deal with the challenging restructuring of our energy systems. Increased penetration of fluctuating renewable energy resources is a challenge for the electrical grid. Proposing solutions to deal with this problem also impacts the functionality of large generators. The power electronic generator interactions, multi-domain modelling, and reliable monitoring systems are examples of new challenges in this field. This book presents some new modelling methods and technologies for renewable energy generators including wind, ocean, and hydropower systems

Wide-Area Synchrophasor Measurement Applications and Power System Dynamic Modeling

The use of synchrophasor measurements system-wide has been providing significant assistance for grid dynamic monitoring, situation awareness and reliability improvement. Frequency Monitoring Network (FNET), as an academia-run synchrophasor measurement system, utilizes a large number of Internet-connected low-cost Frequency Disturbance Recorders (FDRs) installed at the distribution level to measure power system dynamics and provide both online and off-line applications, such as event detection, oscillation modes estimation, event replay, etc. This work aims to further explore applications of the FNET measurements and utilize measurement-based method in dynamic modeling. Measurement-based dynamic reduction is an important application of synchrophasor measurement, especially considering the fact that when the system model is large, measurements provide a precise insight of system dynamics in order to determine equivalent regions. Another important application is to investigate Super Bowl games as an example to evaluate the influence of synchronized human activities on the power system. Featured characteristics drawn from the frequency data detected during the Super Bowl games are discussed. Increased penetration levels of wind generation and retirements of conventional plants have caused concerns about a decline of system inertia and primary frequency response. This work evaluates the impact of wind power on the system inertial response, simulation scenarios with different wind penetration levels are developed based on the U.S. Northeast Power Coordinating Council (NPCC) system. A user-defined electrical control model is also introduced to provide inertia and governor control to wind generations. Except for wind generation, frequency regulation can also be achieved by supplementary control of High Voltage Direct Current (HVDC) transmission line. A multi-terminal Voltage Source Converter (VSC) HVDC model is constructed to prove the effective control. In order to transmit large amount of intermittent and remote renewable energy over long distance to load centers, a potential solution is to upgrade the transmission system at a higher voltage by constructing an overlay HVDC grid on top of the original transmission system. The VSC HVDC model is utilized to build the HVDC overlay grid, and the overlay grid is tested with interconnection models. Conclusions and possible future research topics are given in the end

Electromechanical Dynamics of High Photovoltaic Power Grids

This dissertation study focuses on the impact of high PV penetration on power grid electromechanical dynamics. Several major aspects of power grid electromechanical dynamics are studied under high PV penetration, including frequency response and control, inter-area oscillations, transient rotor angle stability and electromechanical wave propagation.To obtain dynamic models that can reasonably represent future power systems, Chapter One studies the co-optimization of generation and transmission with large-scale wind and solar. The stochastic nature of renewables is considered in the formulation of mixed-integer programming model. Chapter Two presents the development procedures of high PV model and investigates the impact of high PV penetration on frequency responses. Chapter Three studies the impact of PV penetration on inter-area oscillations of the U.S. Eastern Interconnection system. Chapter Four presents the impacts of high PV on other electromechanical dynamic issues, including transient rotor angle stability and electromechanical wave propagation. Chapter Five investigates the frequency response enhancement by conventional resources. Chapter Six explores system frequency response improvement through real power control of wind and PV. For improving situation awareness and frequency control, Chapter Seven studies disturbance location determination based on electromechanical wave propagation. In addition, a new method is developed to generate the electromechanical wave propagation speed map, which is useful to detect system inertia distribution change. Chapter Eight provides a review on power grid data architectures for monitoring and controlling power grids. Challenges and essential elements of data architecture are analyzed to identify various requirements for operating high-renewable power grids and a conceptual data architecture is proposed. Conclusions of this dissertation study are given in Chapter Nine

Advances in Modelling and Control of Wind and Hydrogenerators

Rapid deployment of wind and solar energy generation is going to result in a series of new problems with regards to the reliability of our electrical grid in terms of outages, cost, and life-time, forcing us to promptly deal with the challenging restructuring of our energy systems. Increased penetration of fluctuating renewable energy resources is a challenge for the electrical grid. Proposing solutions to deal with this problem also impacts the functionality of large generators. The power electronic generator interactions, multi-domain modelling, and reliable monitoring systems are examples of new challenges in this field. This book presents some new modelling methods and technologies for renewable energy generators including wind, ocean, and hydropower systems