A universal grid-forming VSC control for future power system

To decarbonise the electricity sector, power systems are facing a significant transition to converter-dominated systems with higher penetration of renewable energy generations to replace conventional power generations using synchronous generators (SGs), changing the characteristics of power grid. Unlike SGs, power electronic converters do not contain rotating mechanical components. Accordingly, the mechanical properties owned by SGs will not be exhibited in the future power system, which can result in various issues in term of power system stability and the ability of faults and disturbances ride through. As power electronic converters are used to interface renewable resources with the power grid, they rely on the control dynamics and algorithms to maintain the entire system power balance and stability. However, there are lots of different control requirements considering the various grid conditions, including weak and strong grid connection, islanding, symmetrical and asymmetrical AC faults, which brings a big challenge for the control design of the power electronic converters. This thesis proposes a universal grid-forming (GFM) VSC (Voltage source converter) control for future power system with consideration to the corresponding various grid conditions. In this thesis, the control of grid-following (GFL) and GFM converters are reviewed firstly. The GFL control usually contributes to the regulation of active and reactive power output by injecting current through a vector current controller at a given phase. The grid phase is tracked by using a phase-locked loop (PLL) at all times. Different outer controller can be applied for different control purposes such as active power and voltage control. The GFL converters are predominantly applied in present renewable power generations, due to the capability in handing transient current during large transient events, precise control of current and good control dynamics, etc. However, as the GFL converters cannot regulate the system voltage and frequency directly, which makes them lack the capability of islanded operation. In addition, another constraint comes along with the use of vector current controller that causes the risk of instability on a weak grid. Intrinsically different from the GFL converters, the GFM converters use voltage regulation as the inner loop combined with power droop controller as the outer loop, to actively control their voltage and frequency outputs for the aim of voltage support. Hence, the GFM converters have the ability to work stably on islanding network, as well as weak grid connection network. However, the most common issue for GFM converters is the absence of effective current control loop, which limits their overcurrent capability. To synthesise the advantages of both GFM and GFL converters, a universal GFM VSC control is proposed. A direct voltage control in the dq reference frame is combined with a frequency droop control to regulate the AC voltage and frequency. Hence, the VSC has the capability of handling islanded operation. To ensure a stable grid connected operation, an adaptive power droop control is added as the outer loop to regulate the power exchanged between the converter and grid. A universal current limit control is also developed to limit the overcurrent and share the active and reactive current on both grid connection and islanding networks. In order to enable the ability of asymmetrical faults ride-through, the GFM VSC control is built in double synchronous frames to enable independent control of positive- and negative sequence components. An enhanced AC fault current control that employs both positive and negative-sequence current control is proposed. An additional voltage balancing control is also developed to retain the AC voltage controller for fault current limiting. By applying this controller, the general fault current limiting, dq current distribution and negative sequence current control when required can be achieved on a weak grid connection. Finally, small signal analysis is carried out to compare the stability of the GFM and GFL VSCs on weak networks. The impedance-based method is adopted to derive the admittances of the VSCs and connected grid in the positive- and negative-sequence (pn) reference frame. Time-domain simulations are also performed to verify the accuracy of the small signal admittances. Stability improvement with the GFM VSC on a very weak grid is validated.To decarbonise the electricity sector, power systems are facing a significant transition to converter-dominated systems with higher penetration of renewable energy generations to replace conventional power generations using synchronous generators (SGs), changing the characteristics of power grid. Unlike SGs, power electronic converters do not contain rotating mechanical components. Accordingly, the mechanical properties owned by SGs will not be exhibited in the future power system, which can result in various issues in term of power system stability and the ability of faults and disturbances ride through. As power electronic converters are used to interface renewable resources with the power grid, they rely on the control dynamics and algorithms to maintain the entire system power balance and stability. However, there are lots of different control requirements considering the various grid conditions, including weak and strong grid connection, islanding, symmetrical and asymmetrical AC faults, which brings a big challenge for the control design of the power electronic converters. This thesis proposes a universal grid-forming (GFM) VSC (Voltage source converter) control for future power system with consideration to the corresponding various grid conditions. In this thesis, the control of grid-following (GFL) and GFM converters are reviewed firstly. The GFL control usually contributes to the regulation of active and reactive power output by injecting current through a vector current controller at a given phase. The grid phase is tracked by using a phase-locked loop (PLL) at all times. Different outer controller can be applied for different control purposes such as active power and voltage control. The GFL converters are predominantly applied in present renewable power generations, due to the capability in handing transient current during large transient events, precise control of current and good control dynamics, etc. However, as the GFL converters cannot regulate the system voltage and frequency directly, which makes them lack the capability of islanded operation. In addition, another constraint comes along with the use of vector current controller that causes the risk of instability on a weak grid. Intrinsically different from the GFL converters, the GFM converters use voltage regulation as the inner loop combined with power droop controller as the outer loop, to actively control their voltage and frequency outputs for the aim of voltage support. Hence, the GFM converters have the ability to work stably on islanding network, as well as weak grid connection network. However, the most common issue for GFM converters is the absence of effective current control loop, which limits their overcurrent capability. To synthesise the advantages of both GFM and GFL converters, a universal GFM VSC control is proposed. A direct voltage control in the dq reference frame is combined with a frequency droop control to regulate the AC voltage and frequency. Hence, the VSC has the capability of handling islanded operation. To ensure a stable grid connected operation, an adaptive power droop control is added as the outer loop to regulate the power exchanged between the converter and grid. A universal current limit control is also developed to limit the overcurrent and share the active and reactive current on both grid connection and islanding networks. In order to enable the ability of asymmetrical faults ride-through, the GFM VSC control is built in double synchronous frames to enable independent control of positive- and negative sequence components. An enhanced AC fault current control that employs both positive and negative-sequence current control is proposed. An additional voltage balancing control is also developed to retain the AC voltage controller for fault current limiting. By applying this controller, the general fault current limiting, dq current distribution and negative sequence current control when required can be achieved on a weak grid connection. Finally, small signal analysis is carried out to compare the stability of the GFM and GFL VSCs on weak networks. The impedance-based method is adopted to derive the admittances of the VSCs and connected grid in the positive- and negative-sequence (pn) reference frame. Time-domain simulations are also performed to verify the accuracy of the small signal admittances. Stability improvement with the GFM VSC on a very weak grid is validated

Maximum current injection method for grid-forming inverters in an islanded microgrid subject to short circuits

In islanded microgrids, when a short circuit or a sudden overload occurs, it provokes an abrupt increment in the currents supplied by the generation nodes, which feed the load collaboratively. This is particularly challenging for inverter-based nodes, due to its reduced power capacity. This work takes advantage of the droop-method basic configuration to propose an additional closed-loop control, which ensures maximum current injection during any kind of short circuit maintaining the underlying droop control. Ensuring that any node injects its maximum rated current during the short circuit, it emulates the most common low-voltage ride-through protocols for grid-feeding sources oriented to support the grid and, in this way, the voltage unbalance is reduced. To develop the control proposal, a model of the faulted system is presented in order to evaluate the stability of the closed-loop system. A general modelling methodology is introduced in order to derive the control for any microgrid configuration. Finally, selected experimental results are reported in order to validate the effectiveness of the proposed control.Peer ReviewedPostprint (author's final draft

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

Advanced control methods on three-phase inverters in distributed energy resources

“This research is an endeavor to apply new and well-established control methodologies to improve transient response, stability and reliability of three-phase inverters in grid-connected and isolated mode of operation. In the course of studying the effect of these methodologies, model-based control is introduced and is extensively applied which is relatively a new approach. In addition, the application of this concept has been studied on developing “grid-forming” controls to allow wind and solar inverters to support voltage and frequency levels like traditional generators. This research encloses the details of three major works of this research and their possible contributions on improving the performance of three-phase inverters in gridconnected and isolated mode of operation. The first one employs the concept of adaptive control using multiple models and a hierarchical control approach to smoothly switch between isolated and grid-connected modes of operation. In the second work, the features of the first research work have been applied and more nourished to control a grid-forming unit. The interactions of this grid-supporting converter with a grid- forming unit is the main subject of discussion in this work. The last work applies the concept of internal-model control to introduce a new control methodology in power-synchronization method. This approach has tackled the non-minimum phase issue attributed to power-synchronization methodology and offers a robust solution. Furthermore, in this research, detailed stability analysis of all the proposed control structures have been presented. Along with all simulation verification, FPGA-Based Hardware-in-the-Loop (HIL) has been utilized to verify the performance of the discrete control structure. The details of plant modeling, controller design, HIL and experimental results are presented for all of the proposed schemes in each section”--Abstract, page iv

Microgrids/Nanogrids Implementation, Planning, and Operation

Today’s power system is facing the challenges of increasing global demand for electricity, high-reliability requirements, the need for clean energy and environmental protection, and planning restrictions. To move towards a green and smart electric power system, centralized generation facilities are being transformed into smaller and more distributed ones. As a result, the microgrid concept is emerging, where a microgrid can operate as a single controllable system and can be viewed as a group of distributed energy loads and resources, which can include many renewable energy sources and energy storage systems. The energy management of a large number of distributed energy resources is required for the reliable operation of the microgrid. Microgrids and nanogrids can allow for better integration of distributed energy storage capacity and renewable energy sources into the power grid, therefore increasing its efficiency and resilience to natural and technical disruptive events. Microgrid networking with optimal energy management will lead to a sort of smart grid with numerous benefits such as reduced cost and enhanced reliability and resiliency. They include small-scale renewable energy harvesters and fixed energy storage units typically installed in commercial and residential buildings. In this challenging context, the objective of this book is to address and disseminate state-of-the-art research and development results on the implementation, planning, and operation of microgrids/nanogrids, where energy management is one of the core issues