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

Wind Power Integration into Power Systems: Stability and Control Aspects

Power network operators are rapidly incorporating wind power generation into their power grids to meet the widely accepted carbon neutrality targets and facilitate the transition from conventional fossil-fuel energy sources to clean and low-carbon renewable energy sources. Complex stability issues, such as frequency, voltage, and oscillatory instability, are frequently reported in the power grids of many countries and regions (e.g., Germany, Denmark, Ireland, and South Australia) due to the substantially increased wind power generation. Control techniques, such as virtual/emulated inertia and damping controls, could be developed to address these stability issues, and additional devices, such as energy storage systems, can also be deployed to mitigate the adverse impact of high wind power generation on various system stability problems. Moreover, other wind power integration aspects, such as capacity planning and the short- and long-term forecasting of wind power generation, also require careful attention to ensure grid security and reliability. This book includes fourteen novel research articles published in this Energies Special Issue on Wind Power Integration into Power Systems: Stability and Control Aspects, with topics ranging from stability and control to system capacity planning and forecasting

Frequency regulation in wind integrated power system

This Thesis has broader implications in terms of improvement in wind generation modeling which is a current requirement for prospective operational planning tools for future grid. This thesis mainly deals with various modelling issues encountered in wind integrated power system for frequency regulation. Thesis provides development of grid code compatible, frequency responsive type 4 wind turbine generator system and analysis of the wind energy systems frequency regulation capability and their integration impact on interconnected power system.<br /

A novel approach to frequency support in a wind integrated power system

This paper discusses the impact of wind penetration on frequency control of a thermal dominated system considering Generation Rate Constraints (GRC) and dead band non-linearities. The hidden inertia emulation and coordinated operation of conventional power generation systems with wind energy can effectively alleviate the frequency excursions during sudden load disturbances. Conventional energy storage device like Flywheel Energy Storage (FES) system can be used in conjunction with wind integrated power system to overcome the intermittent nature of power generation. Thyristor Controlled Series Compensator (TCSC) is found to be effective in damping low frequency oscillations in weak tie-lines and supplement the frequency regulation. A stochastic population based evolutionary computation technique - Particle Swarm Optimization (PSO) is used to tune the controller gains. A strategy comprising inertia control, coordinated operation of conventional generation units with wind energy and TCSC-FES has been proposed to enhance the frequency regulation which is effective in controlling low frequency oscillations as established by the simulation results

Novel Control of PV Solar Farms as STATCOM (PV-STATCOM) for Frequency Control and Power Oscillation Damping

Frequency stability and low-frequency power oscillations are two major concerns in modern power systems. PV-STATCOM is a patented concept which enables PV inverters to provide STATCOM functions, during day and night, as well as real power modulation during daytime. This thesis aims to utilize PV-STATCOM capability effectively for enhancing frequency stability and power oscillations damping. A novel, simultaneous real power-based Fast Frequency Response (FFR) and reactive power-based Power Oscillation Damping (POD) control is proposed for PV-STATCOMs. This control not only significantly reduces system under- and over-frequency deviations, but also uses the unutilized capacity of PV inverters to enhance damping of critical modes. A novel night and day Reactive power-based Frequency Control (RFC) is proposed for PV-STATCOMs, that deploys the unutilized reactive power capacity of PV-STATCOM for frequency stability improvement. RFC modulates the system voltage, via PV-STATCOM voltage control loop, to control the power of voltage-sensitive loads and reduce the generation-demand imbalance. Sensitivity studies show that the load type and its composition, and location of RFC-equipped PV-STATCOM play a significant role in the efficacy of proposed controls. RFC not only provides a 24/7 complementary frequency support service but potentially obviates the impact of system inertia loss due to replacement of conventional synchronous generators by inverter-based generators. A new combined RFC and POD controller is also proposed for PV-STATCOM utilizing unused reactive power capacity of PV inverters. The studies show that depending on PV-STATCOM location, the proposed combined RFC+POD controller can effectively enhance system frequency stability and power oscillations damping. This thesis also proposes a fast power-frequency droop for PV generators and an enhanced synthetic inertial response for wind generators. These two controls operate in a harmonized manner to provide improved frequency support while reducing the stresses on wind generators. The proposed PV-STATCOM grid support functionalities can potentially open up new revenue streams for solar farms. The mechanisms of such financial compensations are expected to develop in near future with the unprecedented growth of solar farms globally. MATLAB based simulation studies are performed on two-area-four-machine and 12-bus study systems using modified WECC generic dynamic models for PV plants, wind plants and loads

Active power control response from large offshore wind farms

This thesis was submitted for the degree of Doctor of Engineering and awarded by Brunel University LondonThe GB power system will see huge growth in transmission connected wind farms over the next decade, driven by European clean energy targets. The majority of the UK’s wind development is likely to be offshore and many of these wind farms will be interfaced to the grid through power converters. This will lead to a loss of intrinsic inertia and an increasing challenge for the system operator to keep grid frequency stable. Given this challenge, there is increasing interest in understanding the capabilities of converter control systems to provide a synthesised response to grid transients. It is interesting to consider whether this response should be demanded of wind turbines, with a consequential reduction in their output, or if advanced energy storage can provide a viable solution. In order to investigate how large offshore wind farms could contribute to securing the power system, wind turbine and wind farm models have been developed. These have been used to design a patented method of protecting permanent magnet generator’s converters under grid faults. Furthermore, these models have enabled investigation of methods by which a wind turbine can provide inertial and frequency response. Conventionally inertial response relies on the derivative of a filtered measurement of system frequency; this introduces either noise, delay or both. This research proposes alternative methods, without these shortcomings, which are shown to have fast response. Overall, wind farms are shown to be technically capable of providing both high and low frequency response; however, holding reserves for low frequency response inevitably requires spilling wind. Wind’s intermittency and full output operation are in tension with the need of the power system for reliable frequency response reserves. This means that whilst wind farms can meet the technical requirements to hold reserves, they bid uncompetitive prices in the market. This research shows that frequency response market prices are likely to rise in future suggesting that the Vanadium Redox Flow Battery is one technology which could enter this market and also complement wind power. Novel control incorporating fuzzy logic to manage the battery is developed to allow a hybrid wind and storage system to aggregate the benefits of frequency response and daily price arbitrage. However, the research finds that the costs of smoothing wind power output are a burden on the store’s revenue, leading to a method of optimising the combined response from an energy store and generator that is the subject of a patent application. Furthermore, whilst positive present value may be derived from this application, the long payback periods do not represent attractive investments without a small storage subsidy.The Engineering and Physical Sciences Research Council (EPSRC) and GE Energ