Current-limiting droop controller with fault-ride-through capability for grid-tied inverters

In this paper, the recently proposed current-limiting droop (CLD) controller for grid-connected inverters is enhanced in order to comply with the Fault-Ride-Through (FRT) requirements set by the Grid Code under grid voltage sags. The proposed version of the CLD extends the operation of the original CLD by fully utilizing the power capacity of the inverter under grid faults. It is analytically proven that during a grid fault, the inverter current increases but never violates a given maximum value. Based on this property, an FRT algorithm is proposed and embedded into the proposed control design to support the voltage of the grid. In contrast to the existing FRT algorithms that change the desired values of both the real and reactive power, the proposed method maximizes only the reactive power to support the grid voltage and the real power automatically drops due to the inherent current-limiting property. Extensive simulations are presented to compare the proposed control approach with the original CLD under a faulty grid

Current-Limiting Droop Control of Grid-connected Inverters

A current-limiting droop controller is proposed for single-phase grid-connected inverters with an LCL filter that can operate under both normal and faulty grid conditions. The controller introduces bounded nonlinear dynamics and, by using nonlinear input-to-state stability theory, the current-limiting property of the inverter is analytically proven. The proposed controller can be operated in the set mode to accurately send the desired power to the grid or in the droop mode to take part in the grid regulation, while maintaining the inverter current below a given value at all times. Opposed to the existing current-limiting approaches, the current limitation is achieved without external limiters, additional switches or monitoring devices and the controller remains a continuous-time system guaranteeing system stability. Furthermore, this is achieved independently from grid voltage and frequency variations, maintaining the desired control performance under grid faults as well. Extensive experimental results are presented to verify the droop function of the proposed controller and its current-limiting capability under normal and faulty grid conditions

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

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

Study of a current limitation strategy for grid-forming inverters in case of short-circuit faults

The use of renewable energies and their participation in the electricity market has been increasing in recent years with the aim of achieving a future where 100% of the generation comes from renewable sources. In this new scenario, in which most of the synchronous generators with high inertias will no longer participate, the main reference of the network will have to be formed in an alternative and robust way. Given that renewable energy generation and storage systems require power converters to adapt to an AC grid, they already now shall have power distribution control algorithms based on the network voltage and frequency. On the other hand, a power converter can behave and be modeled as a controlled voltage source or controlled current source according to the variable that is been regulated. In the absence of synchronous generators in the future, the formation and responsibility of a stable grid must be distributed among different converters working as voltage controlled sources with power distribution algorithms. When a converter works as a controlled voltage source, it presents a new challenge in the field of the corresponding control algorithm when facing a fault scenario at the grid side. In the event of a short circuit, the output current of the inverter must be saturated to prevent possible damage of the inverter. This limitation can have consequences for the upper control loops if they are not consistently adapted. This master thesis presents a new control strategy for dealing with short-circuit scenarios by saturating the amplitude of the reference voltage from the droop power distribution loop. In this way, possible windup effects are avoided, which would cause the instability of the system.El uso de las energias renovables y su participación en el mercado eléctrico viene en aumento durante los últimos años con el objetivo de alcanzar un futuro donde el 100% de la generación provenga de fuentes renovables. En este nuevo escenario, en el cual los generadores síncronos con grandes inercias dejarán de participar en gran medida, la red principal de referencia deberá formarse de una manera alternativa y rebusta. Teniendo en cuenta que los sistemas de generación renovables y almacenamiento de enegia eléctrica requieren de convertidores de potencia para adaptarse a una red AC, estos en la actualidad ya deben contar con algoritmos de control de distribución de potencia en función de la tensión y frecuencia de la red. Por otro lado, un convertidor de potencia puede comportarse y modelarse como una fuente de tensión controlada o fuente de corriente controlada según la variable a monitorizar. En un futuro ausente de generadores síncronos, la formación y responsabilidad de una red estable debe estar distribuida entre los distintos convertidores trabajando como fuente de tensión controlada con algoritmos de distribución de potencia. Cuando un convertidor trabaja como fuente de tensión controlada presenta un nuevo reto en el ámbito del correspondiente algoritmo de control frente a un escenario de falta en el lado de la red. En caso de cortocircuito, la corriente de salida del convertidor debe saturarse para evitar posibles daños de este. Esta limitación, puede tener consecuencias en los lazos de control superiores si estos no son adaptados de forma coherente. Este trabajo presenta una nueva estrategia de control para abordar escenarios de cortocircito mediante la saturación de la amplitud de la tensión de referencia proviniente del lazo de repartición de potencias "droop". De esta forma, se evitan posibles efectos de "windup", los cuales causarian la inestabilidad del sistema.L'ús de les energies renovables i la seva participació en el mercat elèctric ve en augment durant els darrers anys amb l'objectiu d'assolir un futur on el 100% de la generació provingui de fonts renovables. En aquest nou escenari, en el qual els generadors síncrons amb grans inèrcies deixaran de participar-hi en gran mesura, la xarxa principal de referència haurà de formar-se d'una manera alternativa i robusta. Tenint en compte que els sistemes de generació renovables i emmagatzemament d'energia elèctrica requereixen de convertidors de potència per a adaptar-se a una xarxa AC, aquests en la actualitat ja han de comptar amb algoritmes de control de distribució de potència en funció de la tensió i freqüència de la xarxa. Per altra banda, un convertidor de potència pot comportarse i modelarse segons una font de tensió controlada o font de corrent controlada segons la variable a monitoritzar. En un futur absent de generadors síncrons, la formació i responsabilitat d'una xarxa estable ha d'estar distribuida entre diferents convertidors treballant com a font de tensió controlada amb algoritmes de distribució de potència. Quan un convertidor treballa com a font de tensió controlada presenta un nou repte en l'àmbit del corresponent algoritme de control davant un escenari de falta al costat de la xarxa. En cas de curtcircuit, la corrent de sortida del convertidor s'ha de saturar per a evitar possibles danys d'aquest. Aquesta limitació, pot tenir conseqüènices ens els llaços de control superiors si aquests no són adaptats coherentment. Aquest treball presenta una nova estrategia de control per a afrontar escenaris de curtcircuit mitjançant la saturació de l'amplitud de la tensió de referència provinent del llaç de repartició de potència "droop". D'aquesta forma, s'eviten possibles efectes de "windup", els quals causarien la inestabilitat del sistema

Enhanced current-limiting droop controller for grid-connected inverters to guarantee stability and maximize power injection under grid faults

Droop controlled inverters are widely used to integrate distributed energy resources (DERs) to the smart grid and provide ancillary services (frequency and voltage support). However, during grid variations or faults, the droop control scheme should inherit a current-limiting property to protect both the inverter and the DER unit. In this brief, a novel structure of the recently developed current-limiting droop (CLD) controller is proposed to accomplish two main tasks: i) guarantee current limitation with maximum power injection during grid faults and ii) rigorously guarantee asymptotic stability of any equilibrium point in a given bounded operating range of the closed-loop system for a grid-connected inverter. Since the maximum power of the DER unit can be utilized under grid faults with the proposed enhanced CLD, then inspired by the latest fault-ride-through requirements, it is further extended to provide voltage support to a faulty grid via the maximum injection of reactive power. This is achieved by simply adjusting the reactive power reference opposed to existing control schemes which require adjustment of both the real and the reactive power. Hence, a unified current-limiting control scheme for grid-connected inverters under both normal and faulty grids with a simplified voltage support mechanism is developed and experimentally verified in this brief