Grid-forming Converters Research Articles

A fault-tolerant grid-forming converter applied to AC microgrids

A crucial converter for the islanded operation of AC microgrids is the grid-forming converter. This central converter supplies electrical loads, and assures the distributed generators operation, since the malfunctioning of grid-forming converter can jeopardize the whole microgrid. This paper proposes a bidirectional grid-forming converter with a fault-tolerant functionality applied to islanded AC microgrids using a centralized control architecture. The converter is based on a delta connection of three single-phase voltage source inverters (VSIs) using an H-bridge topology for three-phase three- or four-wire systems. For this latter case, a zigzag transformer is used to provide the neutral conductor for the system. In addition to establishing voltage and frequency references for the microgrid, the converter is capable of changing its configuration from delta to open-delta, and vice-versa, in a smooth mode and without interrupting the microgrid operation. Thus, in case of failure of one of the grid-forming VSIs, the system can keep supplying the microgrid. A control strategy is developed to allow a stable operation in all operating modes. Experimental results are presented with a converter prototype to validate the proposed topology, control and mode of connection.

Novel Control Approach for a Hybrid Grid-Forming HVDC Offshore Transmission System

This article describes a hybrid topology of high-voltage direct current (HVDC) for offshore wind farms using a series connection of a voltage source converter (VSC) and six-pulse diode rectifier (6P-DR). In this topology, the offshore side VSC (OF-VSC) acts as a grid-forming converter to maintain the PCC (point of common coupling) voltage of offshore wind farms (WF) and frequency. In addition, the OF-VSC functions as an active power filter to suppress the 5th, 7th, 11th, and 13th order harmonic current components produced by the 6P-DR, making it almost sinusoidal. Due to the 6P-DR being used in the hybrid converter, this new configuration reduces the total cost of the converters and losses, while preserving the power flow to the onshore gird. Compared to the fully-rated converter and hybrid converter based on a 12-pulse diode rectifier, the power loss and cost are reduced, and in addition, the proposed hybrid converter does not require a phase shift transformer nor a high number of diodes. A 200 MW in an HVDC transmission system using the hybrid configuration was simulated in PSCAD. The results show that the system operated correctly and the harmonic components were filtered.

Load-Dependent Active Thermal Control of Grid-Forming Converters

Grid-forming converters enable an asynchronous connection of grids, which opens new opportunities for the power system management. This includes the potential to control the amplitude and the frequency of the grid voltage to vary the power consumption of the loads and the generation in the grid. Depending on the load sensitivity to voltage and frequency, the power transfer of the grid-forming converter changes. Remarkably, this has an impact on the power semiconductors by means of thermal stress, which is limiting the lifetime of the devices. This article aims to investigate and mitigate the thermal stress in grid-forming converters by applying a load voltage-sensitivity control. Depending on the nature of the loads, their power consumption varies with the voltage. This response is dependent on the connected loads and can be characterized by constant power (independent from the voltage), constant current (linearly dependent on the voltage), and constant impedance behavior (squared-dependent on the voltage). The sensitivity to voltage is exploited to control the load power consumption, reducing the junction temperature fluctuations and, thus, the power semiconductors stress.

A research into the operation of a system of electric energy accumulation as part of a cyber-physical real time simulation facility

Introduction: the frequency and capacity control, as well as the maintenance of electric energy systems rely on the choice of the items of equipment that comprise a particular electric energy system. Generators are the core items of equipment comprising traditional electricity systems, while in off-grid electricity systems this function is assumed by the power-driven converter equipment coupled with energy accumulation systems. The main problem of these systems consists in a fast response generated by the power-driven converter equipment to the changing environment. Excessively fast responses, given by the controllers, make the whole off-grid electricity system unstable.Methods: the resolution of the problem of an unstable off-grid electricity system requires the use of algorithms for the control over inverters and frequency converters, designed according to the principle of a virtual synchronous machine that applies voltage and frequency droops. The model of an electricity system has been produced. It has six key elements: a basic balancing inverter, two generators, lithium-oil battery simulation, an interface converter and a real time digital simulator (RTDS). The model was used to perform an experiment to implement two-way data transmission from RTDS to converting facilities and to verify the performance capability of the algorithm and the electricity system as a whole.Results and discussion: as a result of this experiment, the contact was made between RTDS, Generator 1, Generator 2 and the basic balancing inverter through interface converters. This electricity system is resilient and failure-free.Conclusion: data communication was organized between the real time module of digital simulation and Generator 2. Control commands were delivered from the digital simulation module through interface converters, and their execution monitoring was used as a feedback. The operation of grid-forming and grid-filling converters of a self-contained electricity system was stand tested at the MIPT Centre for Engineering, and optimization algorithm performance results were obtained in respect of a battery used in the course of the application of virtual synchronous machines.

A linearized small-signal Thévenin-equivalent model of a voltage-controlled modular multilevel converter

This paper presents a linearized Thévenin-equivalent circuit for a grid-forming Modular Multilevel Converter (MMC) under double closed-loop control, comprehending an outer voltage control-loop together with an inner current control-loop. Both the AC-side Thévenin model and the equivalent admittance of the DC-side were derived in this work. The proposed MMC model takes into account the influence of passive elements and the control-loop effects on the impedances and gains. With this model it was possible to observe how the control system parameters changes resonant frequencies in both DC-side equivalent admittance and AC-side equivalent model. A comparison between the frequency response of the proposed analytical model and the frequency response obtained from the nonlinear time-domain model is presented to validate the proposed MMC model. The proposed model allows analyzing the dynamic response of the MMC as grid-forming converter in the frequency domain. Finally, as an application example, the stability of an ordinary power-electronic-based system (comprised of a grid-forming MMC supplying a current controlled MMC) was assessed and PSCAD/EMTDC simulations are presented to validate the results predicted by the proposed model.

Selective Harmonic Current Rejection for Virtual Oscillator Controlled Grid-Forming Voltage Source Converters

Virtual oscillator control (VOC) is a nonlinear grid-forming controller that simultaneously achieves the functionality of output voltage control and primary control layers in a power electronics interfaced distributed generation (DG) unit. Unlike conventional phasor-based primary control methods such as the droop control and virtual synchronous machine control, VOC is a time-domain controller that can guarantee almost global asymptotic synchronization. However, the high-frequency dynamics has largely been ignored in the analysis of VOC in prior art; as a result, VOC-based DGs fail to suppress harmonic current in the presence of harmonic distortion in the network-side voltage. In this work, we demonstrate that frequency-domain method can be used for oscillator-based converters in the high-frequency range, specifically, for harmonic mitigation in the converter output current. We propose a virtual impedance-based selective harmonic current suppression method, and demonstrate that it is better to use the network-side current feedback rather than that of the converter-side current for VOC implementation with virtual impedance control. Established harmonic rejection strategies for grid-following and conventional grid-forming converters are compared with the proposed method for VOC. Through impedance-based analysis, we demonstrate that the proposed method augments the passivity range of the converter terminal response with a much simpler implementation. The proposed harmonic suppression strategy is validated through hardware experiments using a single-phase inverter prototype.

Critical Clearing Time Determination and Enhancement of Grid-Forming Converters Embedding Virtual Impedance as Current Limitation Algorithm

This article deals with the postfault synchronization of a voltage source converter based on the droop control. In the case of large disturbances on the grid, the current is limited via current limitation algorithms such as the virtual impedance. During the fault, the power converter internal frequency deviates, resulting in a converter angle divergence. Thereby, the system may lose the synchronism after fault clearing and which may lead to instability. Hence, this article proposes a theoretical approach to explain the dynamic behavior of the grid-forming converter subject to a three-phase bolted fault. A literal expression of the critical clearing time is defined. Due to the precise analysis of the phenomenon, a simple algorithm can be derived to enhance the transient stability. It is based on adaptive gain included in the droop control. These objectives have been achieved with no external information and without switching from one control to the other. To prove the effectiveness of the developed control, experimental test cases have been performed in different faulted conditions.

Use of voltage limits for current limitation in grid-forming converters

Renewable generation interfaced through grid-forming converters are proposed as a replacement for synchronous generators in power systems. However, compared to the synchronous generator, the power electronics converter has a strict limit on the current to avoid overcurrent damage. The grid-forming converter acts like a voltage source, directily controlling the voltage. This conflicts with the operation of the conventional current limit control, which is applied to a current source. The switch between the voltage control and current control aimed to impose the current limit leads to synchronization instability. This paper proposes a novel control scheme which can be applied to the grid forming voltage control in order to enforce current limits. The proposed method has been verified through simulation and hardware tests in both symmetrical and asymmetrical faults to perform current suppression while maintaining synchronization stability in the voltage control mode.

Sequence Impedance-Based Stability Analysis of AC Microgrids Controlled by Virtual Synchronous Generator Control Methods

In this paper, sequence impedance-based modelling is applied to two different grid-forming converters which are based on virtual synchronous generator (VSG) concepts including a dual loop voltage control. The considered controls only differ in the feedback design (PLL-driven or not) of the power-related control loop. In general, impedance modelling is a suitable method to analyse stability issues related to converter controls for use in larger power networks. In this work, the analytical model of a voltage-controlled converter is illustrated first. Sequence impedance models are then proposed, which do not only predict the effect of two different VSG controls on the systems stability, but also reveal its frequency coupling effect and analogy to the classical droop control. In addition, a small power system consisting of VSG-controlled converters is analysed by their equivalent output impedances. These models and the stability of the converter cluster are validated by time-domain simulations and laboratory experiments. The close correlation between sequence impedance model, time-domain simulation and experimental results confirms the effectiveness of the derived models.

Grid-Forming Power Converters Tuned Through Artificial Intelligence to Damp Subsynchronous Interactions in Electrical Grids

The integration of non-synchronous generation units and energy storage through power electronics is introducing new challenges in power system dynamics. Specifically, the rotor angle stability has been identified as one of the major obstacle with regards to power electronics dominated power systems. To date, conventional power system stabilizer (PSS) devices are used for damping electromechanical oscillations, which are only tuned sporadically leading to significant deterioration in performance against the ever-changing operating conditions. In this paper, an intelligent power oscillation damper (iPOD) is proposed for grid-forming converters to attenuate electromechanical inter-area power oscillation. In particular, the iPOD is applied to a synchronous power controller (SPC) based grid-forming power converter to increases gain of the active power control loop at the oscillatory frequency. Predictions regarding the mode frequency, corresponding to the current operating points, are given by an artificial intelligence ensemble model called Random Forests. The performance of the proposed controller is verified using the two area system considering symmetrical fault for random operating points. In addition, a comparison with PSS installed in each generator reveals the individual contribution with respect to the inter-area mode damping.

Passivity-Based Analysis and Design of Linear Voltage Controllers For Voltage-Source Converters

This article presents a systematic evaluation on the impedance passivity of voltage-controlled voltage-source converters. The commonly used single- and dual-loop control structures with different linear controllers are compared extensively, considering the effect of the time delay involved in the control loop. A virtual impedance control, co-designed with different voltage control schemes, is then proposed to eliminate the negative output resistance till half of the sampling frequency, which improves the system stability for grid-forming converters in grid-connected applications. Both frequency-domain analysis and experimental results validate the theoretical findings.

Coordinated Control of HVDC and HVAC Power Transmission Systems Integrating a Large Offshore Wind Farm

The development of efficient and reliable offshore electrical transmission infrastructure is a key factor in the proliferation of offshore wind farms (OWFs). Traditionally, high-voltage AC (HVAC) transmission has been used for OWFs. Recently, voltage-source-converter-based (VSC-based) high-voltage DC (VSC-HVDC) transmission technologies have also been considered due to their grid-forming capabilities. Diode-rectifier-based (DR-based) HVDC (DR-HVDC) transmission is also getting attention due to its increased reliability and reduced offshore platform footprint. Parallel operation of transmission systems using such technologies can be expected in the near future as new OWFs are planned in the vicinity of existing ones, with connections to more than one onshore AC system. This work addresses the control and parallel operation of three transmission links: VSC-HVDC, DR-HVDC, and HVAC, connecting a large OWF (cluster) to three different onshore AC systems. The HVAC link forms the offshore AC grid, while the diode rectifier and the wind farm are synchronized to this grid voltage. The offshore HVDC converter can operate in grid-following or grid-forming mode, depending on the requirement. The contributions of this paper are threefold. (1) Novel DR- and VSC-HVDC control methods are proposed for the parallel operation of the three transmission systems. (2) An effective control method for the offshore converter of VSC-HVDC is proposed such that it can effectively operate as either a grid-following or a grid-forming converter. (3) A novel phase-locked loop (PLL) control for VSC-HVDC is proposed for the easy transition from the grid-following to the grid-forming converter in case the HVAC link trips. Dynamic simulations in PSCAD validate the ability of the proposed controllers to ride through faults and transition between grid-following and grid-forming operation.

Current Limiting Control With Enhanced Dynamics of Grid-Forming Converters During Fault Conditions

With an increasing capacity in the converter-based generation to the modern power system, a growing demand for such systems to be more grid-friendly has emerged. Consequently, grid-forming converters have been proposed as a promising solution as they are compatible with the conventional synchronous-machine-based power system. However, most research focuses on the grid-forming control during normal operating conditions without considering the fundamental distinction between a grid-forming converter and a synchronous machine when considering its short-circuit capability. The current limitation of grid-forming converters during fault conditions is not well described in the available literature and present solutions often aim to switch the control structure to a grid-following structure during the fault. Yet, for a future converter-based power system with no or little integration of synchronous machines, the converters need to preserve their voltage-mode characteristics and be robust toward weak-grid conditions. To address this issue, this article discusses the fundamental issue of grid-forming converter control during grid fault conditions and proposes a fault-mode controller which keeps the voltage-mode characteristics of the grid-forming structure while simultaneously limiting the converter currents to an admissible value. The proposed method is evaluated in a detailed simulation model and verified through an experimental test setup.

Voltage Source Operation of the Energy-Router Based on Model Predictive Control

The energy router (ER) is regarded as a key component of microgrids. It is a converter that interfaces the microgrid(s) with the utility grid. The energy router has a multiport structure and bidirectional energy flow control. The energy router concept can be implemented in nearly zero energy buildings (NZEB) to provide flexible energy management. We propose a concept where ER is working as a single grid-forming converter with a predefined voltage reference. The biggest challenge is to maintain regulated voltage and frequency inside the NZEB in the idle operation mode, where traditional regulators, e.g., proportional-resonant (PR), proportional-integral-derivative (PID), will not meet the control design requirements and could have unstable behavior. To gain the stability of the system, we propose model predictive control (MPC). The design of the MPC algorithm is explained. A simulation software for power electronics (PLECS) is used to simulate the proposed algorithm. Finally, the simulation results are verified on an experimental prototype.

An adaptive time-graded control method for VSC-MTDC networks

Power flow and voltage control, beside other challenges such as DC faults, are the main concerns in the operation of multi-terminal HVDC (MTDC) networks. This paper proposes a novel DC voltage control method, which is inspired from the principles of power system protection, for VSC-based MTDC networks. This time-graded control method periodically ranks all converters; the highest priority one is dedicated to regulate the network voltage while the others operate in the power control mode. In abnormal conditions in which the grid-forming converter cannot solely control the network voltage, a sufficient number of converters are sequentially turned to the voltage control mode according to their priority. Upon recovering from the abnormal condition, the proposed controller with its conditional reclosing ability returns these mode-changed converters to the power control mode. Since the proposed method regulates the voltage of MTDC network by the minimum number of converters, the maximum ones can be employed in power flow control. Proposing an adaptive procedure to calculate appropriate mode change delays is another contribution of this communication-less control method. Simulation of a test network in MATLAB/Simulink under various conditions with both proposed and conventional droop controllers verifies the outstanding abilities of the proposed method.

A Sliding Mode Control Strategy for a Grid-Supporting and Grid-Forming Power Converter in Autonomous AC Microgrids

A Sliding Mode Control Strategy for a Grid-Supporting and Grid-Forming Power Converter in Autonomous AC Microgrids

Full Discrete Modeling, Controller Design, and Sensitivity Analysis for High-Performance Grid-Forming Converters in Islanded Microgrids

Recent works have shown that the state-feedback decoupling of capacitor voltage allows for drastic bandwidth enlarging of current controllers for grid-forming converters in islanded microgrids. Furthermore, Smith predictor and lead compensation have been also proved as very effective implementations for compensating the controller delays. These features are key to fulfill demanding requirements in terms of voltage regulation in islanded applications. This work deepens in the discrete-time domain modeling and implementation issues of the above-mentioned techniques. A full discrete-time and sensitivity analyses reveal phenomena not properly modeled in previous works, which limits the performance: the presence of high-frequency oscillations due to discrete poles with negative real part. Subsequently, proper design countermeasures (i.e., limit bandwidth) are proposed. Discrete implementation of the voltage controller is also addressed, and design guidelines are provided. Experimental tests in accordance with the high demanding standards for uninterruptible power supply systems verify the theoretical analysis.

Operation paradigm of an all converter interfaced generation bulk power system

This study investigates the viability and the associated operational implications of an all converter interfaced generation system operating at constant frequency using grid forming converters. Grid-forming converters set the frequency and voltage of the grid and enable its operation under constant frequency, thus creating a new paradigm for the frequency response of a power system. Power sharing between converters is addressed while assessing the implications of operating the bulk power system at a constant frequency. Case studies performed on a small 9-bus system as well as a large 2000-bus system, using a developed positive sequence as well as a detailed three-phase point on wave models of a grid forming converter are discussed.

A new multifunctional converter based on a series compensator applied to AC microgrids

This paper proposes a new multifunctional converter based on a series voltage compensator applied to centralized AC microgrids. Besides the basic functionalities required for a centralized converter in microgrids, such as grid-supporting and grid-forming voltage support and smooth transition from operating modes, the proposed converter can change its connection topology from series to parallel and vice-versa, depending on the system’s condition. Thus, it assumes all the basic defining features of series and shunt power converters. The devised control algorithm allows on-line change of converter operating modes either as voltage- or current-controlled sources. These flexible functionalities allow the converter to operate as: (i) a grid-forming converter providing voltage and frequency references for the entire microgrid, keeping the voltages at the point of common coupling (PCC) synchronized with the utility voltage; (ii) grid-feeding active power into the microgrid; and (iii) grid-supporting, performing ancillary services such as voltage regulation, voltage sag and swell compensation, along with active power filtering, reactive power, unbalance and harmonic compensation. The converter can be applied to both single-phase and three-phase three- or four-wire networks. The multifunctional control is implemented using a Texas Instruments TMS320F28335 digital signal processor, and then validated through a hardware-in-the-loop simulation developed in the Typhoon HIL 600 platform.

Improving Transient Response of Power Converter in a Stand-Alone Microgrid Using Virtual Synchronous Generator

Multiple power converters based on the droop controllers have been used widely in the microgrid (MG) system. However, owing to the different response time among several types of power converters such as grid-feeding and grid-forming converters, low frequency oscillation occurs with high overshoot in the transient state. This paper proposes a novel control strategy based on the virtual synchronous generator (VSG) for improving transient response of parallel power converters during large disturbance in the stand-alone microgrid. The proposed VSG control, which inherits the transient state characteristic of the synchronous generator, can provide inertia virtually to the system. The transient response of voltage and frequency is improved, while the total system inertia response is compensated. Thus, the system stability can be enhanced by using the proposed VSG control. Additionally, the small signal analysis of the conventional VSG controller and the proposed VSG controller are carried out to show the effectiveness of the proposed VSG controller. The derivation of frequency, which is used to evaluate the inertia support of the VSG controller to the MG system, is discussed. The simulation result demonstrates that the overshoot of the transient response can be reduced, and the system stability is improved when the proposed VSG controller is applied. The MG system based on the real-time simulator OP5600 (OPAL-RT Technologies, Montreal, QC, Canada) is carried out to verify the feasibility of the proposed VSG controller.