Design and Control of Virtual Synchronous Machine Based Energy Systems

Conventionally, the operation and stability of power systems have been governed by the dynamics of large synchronous generators (SGs) which provide the inertial support required to maintain the resilience and stability of the power system. How-ever, the commitment of the UK to drive a zero-carbon economy is accelerating the integration of renewable energy sources (RESs) into the power system. Since the dynamics and operation of RESs differs from SGs, the large-scale integration of RESs will significantly impact the control and stability of the power system.This thesis focuses on the design of grid-friendly control algorithms termed virtual synchronous machines (VSMs), which mimic the desirable characteristics of SGs. Although several VSM topologies have been proposed in literature, most of them require further modifications before they can be integrated into the grid. Hence, a novel VSM algorithm for permanent magnet synchronous generator based wind turbines has been proposed in this thesis.The proposed VSM performs seamlessly in all operating modes and enables maxi-mum power point tracking in grid-connected operation (assuming strong grid), load following power generation in islanded mode and fault ride-through during faults. To ensure optimal performance of the VSM in all operating modes, a comprehensive stability analysis of the VSM was performed in the event of small and large per-turbations. The result of the analysis was used to establish design guidelines and operational limits of the VSM.This thesis further evaluates the impact of VSMs on the power systems low-frequency oscillations (LFOs). A detailed two-machine test-bed was developed to analyze the LFOs which exists when VSMs replace SGs. The characteristics of the LFO modes and the dominant states was comprehensively analyzed. The LFO modes which exists in an all-VSM grid was also analyzed. Further, the role of the power system stabilizers in an all-VSM grid was comprehensively evaluated. An IEEE benchmark two-area four-machine system was employed to validate the results of the small-signal analysis.The analysis and time-domain simulations in this thesis were performed in the MAT-LAB/SIMULINK environment

The load performance of multi-level alternating voltage provided by upgrade effect

In this paper, the applications of the multi-level inverter with a partial inductor are investigated on different loads. The operation of the inverter is given according to different conditions of the circuit at the six-part duty times. Mathematical equations for the inverter are forming according to the load which is resistive (R), inductive (L), and Capacitive (C) in serial connection. After describing the circuit structure, Matlab SIMULİNK also simulates the circuit for loads that are RLC, and a single-phase asynchronous motor. While the voltage and current measurements for the RLC load are performed; measurements of the main winding current, auxiliary winding current, electromagnetic torque, speed are made on the asynchronous motor. RLC loads are operated at high current values such as 297A in order to examine the suitability of the proposed system for systems that may require high current. When the current harmonic distortions of RLC loads are observed, the distortion values are acceptable values that are lower than % 5 that is international standards of the IEEE. In another hand, despite the voltage is insufficient for supplying the motor at the motor application, the voltage is upgraded with the boost effect of the inverter. So, the voltage is both a multilevel and sufficient. According to the obtained results, it is shown that multi-level inverter design has been successfully performed by using partial inductor source

Control strategy of grid connected power converter based on virtual flux approach

A la portada consta el nom del programa interuniversitari: Joint Doctoral Programme in Electric Energy Systems [by the] Universidad de Málaga, Universidad de Sevilla, Universidad del País Vasco/Euskal Erriko Unibertsitatea i Universitat Politècnica de CatalunyaDistributed Generation (DG) provides an alternative to the Centralized Generation (CG) by means of generating electricity near to the end user of power with the employment of small-scale technologies to produce electricity, mainly using Renewable Energy Sources (RES). The prospects of renewable energy integration during the next years are still very optimistic. This PhD dissertation is made to provide an alternative control framework for the grid connected power converter by adopting the virtual flux concept in the control layer. This dissertation can be divided into three main topics. The 1st topic presents the voltage sensorless control system for the grid-connected power converter. The control system presented is done without depending on AC-voltage measurement where the grid synchronization is based on the Virtual Flux (VF) estimation. In this regard, the Frequency Locked Loop (FLL) is used in conjunction with the estimation scheme to make the system fully adaptive to the frequency changes. This voltage sensorless application is useful for reducing cost and complexity of the control hardware. It is also can be utilized in case of limited reliability or availability of voltage measurements at the intended point of synchronization to the grid. Considering that most previous studies are based on the VF estimation for the case of power converter connected to the grid through the L-filter or LC-filter, this dissertation is focused on the power converter connected to the grid through the LCL filter. The Proportional Resonant (PR) current controller is adopted in the inner loop control of the power electronics-based converter to test the performance of such system. Another control method based on VF synchronization that permits to control the active and reactive power delivery in a remote point of the grid is also presented in this dissertation. This is due to the fact that the VF is implemented that the voltage in a remote point of the line can be estimated. As it will be shown in simulations and experiments, the proposed control scheme provides a good tracking and dynamic performance under step changes in the reference power. The fast synchronization and the smooth reference tracking achieved in transient conditions have demonstrated the effectiveness of the Dual Second Order Generalized Integrator controlled as Quadrature Signal Generator (DSOGI-QSG) and also the current controller used in the proposed system. In addition to the power control itself, this study could also benefit the frequency and the voltage regulation methods in distributed generation applications as for instance in microgrid. Considering the fact that the grid connected power converter can be controlled as a virtual synchronous generator where the flux is a variable to be used for controlling its operation, this dissertation also presents a Virtual Synchronous Flux Controller (VSFC) as a new control framework of the grid connected power converter. In this regard, a new control strategy in the inner loop control of the power converter will be proposed. The main components of the outer loop control of VSFC are based on the active and reactive power control. The results presented show that the VSFC works well to control the active and reactive power without considering any synchronization system. The inner loop control is able to work as it is required, and the measurement flux is able to track the reference flux without any significant delays. All the work presented in this dissertation are supported by mathematical and simulation analysis. In order to endorse the conclusions achieved, a complete experimental validations have been conducted before wrapping this dissertation with a conclusion and recommendation for future enhancement of the control strategies that have been presented.Postprint (published version

Enhanced control of DFIG-based wind power plants to comply with the international grid codes

A review of the latest international grid codes shows that large wind power plants are stipulated to not only ride-through various fault conditions, but also exhibit adequate active and reactive power responses during the fault period in order to support the network stability. In particular, modern grid codes require wind power plants to: (1) ride-through various voltage sag and swell conditions, (2) inject reactive current into the grid during the fault period, and (3) attain swift active power restoration after the fault clearance. This thesis proposes a transient control scheme for DFIG-based wind power plants to comply with these requirements.In the first part of this thesis, the latest regulations enforced on large wind power plants are studied and compared. This study identifies the most stringent regulations defined by the international grid codes, to be further investigated in the following chapters. In the second part of this thesis, extensive simulation studies are carried out to examine the transient response of DFIG-based wind turbines under various symmetrical and asymmetrical fault conditions. Supplementary theoretical analyses are also presented to justify the observations made in the time-domain simulations results. For the first time, the impacts of phase-angle jump, voltage recovery process and sag parameters on the DFIG response are explored. The results of this study can assist researcher to identify the difficulties that hinder successful fault ride-through response of DFIG-based wind turbines, as requested by the international grid codes.In the third part of the thesis, an enhanced hysteresis-based current regulator (referred to as VBHCR) is proposed to be implemented in the rotor-side and grid-side converters of DFIG-based wind turbines. The main advantages of this current regulator are very fast transient response, simple control structure and insensitivity to the machine parameters variations. Simulation results show that on one hand the VBHCR has very good steady-state performance and on the other hand, it presents very fast/robust tracking response. Therefore, the DFIG equipped the proposed current regulator can fulfill the most stringent low-voltage ride-through requirements imposed by the international grid codes, i.e., those stipulated by the Australian grid code. In the fourth part of the thesis, a new hybrid current control scheme is introduced to enhance both low and high voltage ride-through capabilities of DFIG-based wind turbines. The proposed control scheme uses the standard PI current regulators under steady-state conditions but upon a voltage sag or swell occurrence, the supervisory control unit transfers the switching strategy of the rotor-side and grid-side converters to the hysteresis-based method. The VBHCR remains in action until the oscillation in the rotor current and dc-link voltage of DFIG suppress below the safety limit and then, the PI current regulator are activated through a re-initialization process.Finally, the conventional vector control scheme of DFIG-based wind power plants is modified to fulfill the regulations imposed on the active and reactive power responses of wind farms subject to various faults. New design strategies are suggested and their corresponding P-Q capability curves are thoroughly studied. Simulations results show that the proposed control scheme can meet the Australian regulations as the most demanding grid code. The best design strategy, with enhanced active and reactive power responses, permits the rotor-side and grid-side converters of DFIG to be temporarily overloaded during the fault period and also exploits the free capacity of the GSC to inject further reactive power to the grid. As a result, the active power generation of DFIG-based wind power plant can be retained during the fault period while its reactive power injection capacity of DFIG is also increased to further support the grid

Power Converter of Electric Machines, Renewable Energy Systems, and Transportation

Power converters and electric machines represent essential components in all fields of electrical engineering. In fact, we are heading towards a future where energy will be more and more electrical: electrical vehicles, electrical motors, renewables, storage systems are now widespread. The ongoing energy transition poses new challenges for interfacing and integrating different power systems. The constraints of space, weight, reliability, performance, and autonomy for the electric system have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this context, power converters and electric machines assume a key role in enabling higher performance of electrical power conversion. Consequently, the design and control of power converters and electric machines shall be developed accordingly to the requirements of the specific application, thus leading to more specialized solutions, with the aim of enhancing the reliability, fault tolerance, and flexibility of the next generation power systems

Identification and development of microgrids emergency control procedures

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200