|Advanced control strategies of renewables supporting power system operation
|Xu, Zhao (EE)
|Hong Kong Polytechnic University -- Dissertations
Electric power systems -- Control
Renewable energy sources
Interconnected electric utility systems
|Department of Electrical Engineering
|xiii,122, B1-B10 pages : color illustrations
|Renewable energy has gained significant attentions in recent years due to energy crisis and environmental awareness. Specifically, rapid development of wind and solar power generation has been witnessed around the world. However, the high penetration of renewable generation imposes severe challenges towards power grids operation. One negative influence is that system inertia becomes less and less as synchronous generators (SGs) are replaced by these electronic interfaced renewable generators. In addition, the power reserve requirement is accordingly increased due to the fluctuating and un-dispatchable characteristic of renewable generation. To cope with these challenges, this thesis is mainly focused on advanced active power control strategies of solar and wind power generation systems for providing system ancillary service. In this thesis, a novel frequency control strategy is proposed for the two-stage three phase photovoltaic (PV) generation system, which involves simultaneously utilizing DC-link voltage control and the deloading control to support system frequency. When frequency variation is detected, the DC-link capacitor is utilized to provide inertia or frequency response. In addition, the deloading control is applied to reserve some PV generation beforehand. Therefore, the PV generation output can be up/down-regulated accordingly to also provide primary frequency support based on a droop control logic. Considering time varying characteristics of the irradiance and ambient temperature, the droop coefficient is adaptively adjusted in the proposed control strategy to improve the efficiency of the primary frequency regulation under changing working conditions. The effectiveness of the proposed control strategy is verified through extensive case studies.
The maximum power point tracking (MPPT) control is usually adopted by wind turbines to maximize wind power harvesting. Besides the MPPT operation mode, the wind turbine can also regulate its output power according to system operator's commands. To achieve this, two power regulation control schemes are proposed for permanent magnet synchronous generator (PMSG) based wind turbine (WT). The first strategy aims to regulate power output through simultaneously utilizing the DC-link voltage control, rotor speed control and pitch angle control, while the second one coordinates these three controls in a hierarchical manner to reduce the overall impacts wind turbine operation, in which the power regulation tasks are allocated to individual control modules or their combinations dynamically in line with WT's operation states. Both control strategies implement active power regulation successfully, while the second control strategy outperforms the first one in the following aspects 1) requiring less activations of pitch angle control, and 2) imposing less impacts on wind energy harvesting. Case studies of the proposed control strategies are conducted to compare performance of the control strategies in active power regulation. Owing to the increasing penetration of wind power in modern power systems, wind farms are expected to operate in a dispatchable manner to a certain extent by actively fulfilling dispatch orders sent from system operators. To coordinate mutually influenced wind turbines (WTs) within a wind farm due to non-negligible wake effects, a novel wind farm control strategy is proposed by 1) using the existing resources (i.e. kinetic energy (KE) and adjusting blade pitch angle) of doubly fed induction generator (DFIG) based wind turbine to satisfy the dispatch order efficiently and reliably; 2) dynamically allocating power regulation tasks to individual wind turbine so that the loss of energy harvest can be reduced and wear outs caused by pitch control can be somehow mitigated. Extensive case studies are conducted out to verify the effectiveness of the proposed control strategy. Simulation results exhibit that the proposed strategy has better control performance in terms of total energy harvesting and pitching manipulation. The ever-increasing penetration of wind energy in today's power system exposes the necessity of smoothing out power fluctuations in an effective and conducive way. Considering that adjusting wind power output using hard-coded filtering algorithms that can result in visually smoothed power output with unmeasurable impacts on system generation-demand balance. Therefore, a novel wind power smoothing control paradigm in context of performance-based regulation service is also proposed. Distinguished from conventional methods, the newly proposed control method smooths wind power output from a power system perspective by using the regulation mileage as a key performance indicator. To simultaneously address the system needs and maximize wind energy harvesting, a mileage-responsive framework is developed to enable wind farms to optimally generate smoothing power. The effectiveness of the proposed method is well demonstrated through case studies, of which the simulation results show a great potential for practical applications.
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