Grid Emulator Research Articles

Adaptive Anti-Saturation Control Design of Transformers in Converter-Based Grid Emulators

Adaptive Anti-Saturation Control Design of Transformers in Converter-Based Grid Emulators

Medium-Voltage Megawatt Power-Electronic-Based Grid Emulators: Testing Capability Requirements and Dynamics Challenges—A Review

Power-electronic-based grid emulators are emerging as a promising way to test the grid-code compliance of renewable energy resources. Yet, the ever-increasing power and voltage levels of device under test pose new requirements and challenges to power-electronic-based grid emulators. This paper first gives an overview of testing capability requirements based on the recent grid codes and standards, then discusses the resulted dynamic interactions and solutions for power-electronic-based grid emulators. Perspectives on the open issues and emerging trends of grid emulators are finally shared.

Dc-Link Ripple Suppression of Cascaded H-Bridge Based MV Grid Emulator Apparatus for Analysis and Testing of Grid-Tied Converters in Distribution System

Dc-Link Ripple Suppression of Cascaded H-Bridge Based MV Grid Emulator Apparatus for Analysis and Testing of Grid-Tied Converters in Distribution System

Electrical modelling, design, and implementation of a hardware PEM electrolyzer emulator for smart grid testing

Abstract The high cost and complexity of Proton Exchange Membrane (PEM) electrolyzers pose substantial challenges for their integration and testing within smart grid emulators. Addressing this, our research offers two pivotal contributions. First, we introduce an innovative Equivalent Electrical Circuit (EEC) for PEM electrolyzers. This electrical model serves as an essential tool for evaluating the performance of PEM electrolyzers, especially within the framework of renewable energy systems in smart grids. Our second major contribution is the design and implementation of a hardware PEM electrolyzer emulator based on a DC/DC boost converter. Unlike conventional approaches, our emulator accurately reproduces the nonlinear polarization curve and dynamic behavior of PEM electrolyzers under a large range of operating conditions. By seamlessly integrating into a smart grid emulator, it provides an environment for efficient and cost-effective testing of PEM electrolyzers. Experimental results offer strong validation of the emulator’s ability to replicate the characteristics of the PEM electrolyzer, establishing it as a practical alternative for evaluating and improving smart grid emulators and their management strategies.

Analysis and Mitigation of Transient Overvoltage and Voltage Imbalance in Submodule Capacitors for MMC-Based Grid Emulators

Analysis and Mitigation of Transient Overvoltage and Voltage Imbalance in Submodule Capacitors for MMC-Based Grid Emulators

Recursive Secondary Controller for Voltage Profile Improvement Based on Primary Virtual Admittance Control

This paper proposes a recursive, virtual admittance based, secondary controller for DG units that improves the voltage profile in distribution networks. First, the adaptation of the virtual admittance concept for the goal of voltage regulation is explained. Then, a recursive secondary controller is developed to periodically update the virtual admittance gains. The controller is formulated as an optimization problem with current and stability limitations as constraints. Measurements across the grid, transmitted through low-bandwidth communications, are used to simplify the calculations, resulting in a recursive algorithm. Weight vectors are included in the objective function to allow participation flexibility of each converter. Results show that the primary virtual admittance controller is able to mitigate over- and under-voltages in steady state and under transient conditions. Subsequently, the secondary controller is shown to further improve the voltage profiles across the grid. Experimental results obtained from a laboratory environment, comprising three DG units and a grid emulator, validate the functionality of the complete control structure.

Multivariable-Controlled Shunt Compensator for Damping Subsynchronous Interactions in Electrical Distribution Networks

In recent years, the massive integration of converter-interfaced devices to electrical distribution networks has lead to appearance of poorly-damped subsynchronous interactions in the grid. These oscillation modes amplify electrical disturbances originated from the grid and may cause an interaction between grid elements threatening the system stability. Shunt compensators at the distribution level have already been used to improve the damping of these networks. However, their control system strongly depends on the distribution network dynamics, that is difficult to know at all times. In this paper, a multivariable controller for a shunt compensator is proposed. First, the dynamics of a distribution network with high penetration of electronic interfaces is explored in order to understand the interactions. Then, the controller of the compensator is designed based on an estimated model of the plant so that no previous information of the network is required. This controller is based on a state-feedback controller and a state observer, both designed by using the linear quadratic regulator (LQR) theory. Analytical results and computer simulations are used to verify the effectiveness of the controller. The main findings are validated in a laboratory environment comprising two 15 kW distributed generators, a 15 kW compensator and a 75 kW grid emulator.

Validation of Wind Turbine Models Based on Test Bench Measurements: A System for Theoretical Representation of the Grid Replica

In recent years, nacelle test benches for wind turbines have been developed internationally. New standards are currently being developed that explicitly refer to the measurement of the electrical properties of wind turbines on these test benches. Thus, they are suitable for measuring the electrical properties required for certification. Another part of the certification is the creation and validation of suitable models of the wind turbine, which are used for stability analyses of the utility grid. Validation requires a suitable model of grid replication on the test benches, which is not yet covered by any applicable standard. Such models should be as simplified a representation of the artificial grid replication as possible to ensure that they are accessible to certification bodies. A model of the grid emulator installed at the CWD of RWTH Aachen University, which was validated with real measurement data, serves as a reference. A step-by-step reduction of the model’s depth up to the system’s technical representation is followed by a model evaluation with respect to the level of detail and an analysis of time and frequency. The evaluation shows that even a highly simplified model consisting of a reference voltage and an impedance replica meets the requirements for the validation of wind turbine models according to IEC 61400-27-2.

Hidden Modes of Instability for Inverters in Weak Grids

This article introduces a hidden instability mode of grid-following inverters at low power levels. It is known that grid-following inverters become unstable due to weak grid conditions and improper control parameters. However, this article elaborates on a hidden instability mode, which can occur at a power level below the inverter's nominal value. The nonlinearity of PQ control loops explains how the inverter becomes unstable for a narrow range of active power setpoints below the inverter's nominal value. This phenomenon is explained using a full-order dynamic model of PQ-controlled inverters. This work also presents a stability criterion, identifying a margin for this hidden instability mode. This phenomenon and the stability criterion are verified by a set of experiments using a 3kVA inverter feeding a programmable 208V, 12kW power grid emulator.

A discussion on the characterization of grid emulators by apparent power rating and short-circuit power

Wind turbines are a driving force in the change towards renewable energies today. Offshore wind turbines have reached power ratings around 15 MW, and the growth continues. Grid emulators have become inevitable for the test and validation of wind turbines and subsystems at full scale. Yet, large investments for new installations present major challenges.Therefore, the following discussion outlines basic design aspects for the grid emulator main components: converter, transformer, and filter. The aim is to clarify the implications of requirements on the design to exhaust ratings for minimum investments, and to satisfy testing against the most recent standards and grid codes. Appropriate ratings are derived based on steady-state and transient test needs. It is shown how much overrating is specifically needed and why the converter is usually rated a multiple compared to the device under test. Further dimensioning criteria such as power quality, switching frequency, and filter size are discussed in this context.Finally, the capacity of grid emulators is often characterized by the short-circuit power. Two essentially different variants of short-circuit power are introduced, either being dependent on the converter peak current or the passive components impedance. While the first one reveals the overload capability, the latter represents a proper measure for grid strength. However, the emulated grid strength can be adjusted in a limited bandwidth using impedance control. The concept might be confusing at first glance, but it effectively decouples the virtual short-circuit power from the ratings of the power hardware.

Power oscillation damping method suitable for network reconfigurations based on converter interfaced generation and combined use of active and reactive powers

Power system stabilisers (PSSs) are commonly used in synchronous generators for damping low-frequency oscillations. However, many of these machines have been replaced by converter-interfaced generators (CIGs), such as those used in solar power plants. This means that CIG-based power plants should also contribute to oscillation damping. Moreover, CIGs may use both active and reactive power for power oscillation damping (POD). However, their combined use has been seldom studied in the literature. Moreover, only a few articles have addressed the adaption of POD controllers when the power system changes (e.g., after a fault). In this paper, a POD controller for CIG-based power plants is proposed that is suitable for operation in power systems exposed to reconfiguration. This controller takes advantage of both active and reactive power injection to maximise the damping of the power system. This controller is based only on local measurements so communication systems are not required. Theoretical developments were validated in a laboratory using real power converters including a 75 kVA grid emulator where the two-area benchmark model is emulated and four 15 kVA CIGs operating in parallel connection. The applicability of the proposed controller was also explored for the IEEE 39-bus system.

Demand response for frequency regulation with neural network load controller under high intermittency photovoltaic systems

The rate of change of frequency (ROCOF) has become a key parameter to be monitored under high penetration of renewable energy. Any significant ROCOF should be mitigated immediately to avoid any shutdown of power plants and hence power interruptions to the customers. However, ROCOF is found to be significant and often in a region near to equatorial line because the power output of PV is highly intermittent due to a large number of passing clouds. Demand response (DR) is one of the most cost-effective means for mitigating any high ROCOF through controlling heating elements, refrigerators, and air-conditioners. However, most of the DR controllers use pre-determined conditions, also known as the condition-based approach, to switch the loads for reducing frequency changes. This approach may not be effective enough to respond to a new load condition. Furthermore, many DR controllers, including those accompanied by machine learning, are mainly developed to mitigate frequency changes with little focus on the mitigation of high ROCOF. Hence, an artificial neural network (ANN)-based controller is proposed in this article for frequency regulation with auto corrective efforts on high ROCOF under high intermittency of PV systems. The proposed controller is designed to manage several controllable loads on a grid emulator with PV systems at Universiti Tunku Abdul Rahman, Malaysia. It is shown that the proposed controller can reduce the frequency deviation by 23% and improve ROCOF by 19.7% as compared to that without any controller.

Inertia and Grid Impedance Emulation of Power Grid for Stability Test of Grid-Forming Converter

The virtual synchronous generator (VSG) control is a promising solution for the grid-forming converters to enhance the inertia of the converter-based power grid. The virtual inertia and output impedance of the VSG are the important behaviors to be focused. However, the virtual inertia of grid-forming converters may cause active power oscillation due to the interaction with the power inertia of grid through the low-frequency band grid impedance. Besides, the resonance can be also triggered by the interaction between middle-frequency impedance of power grid and output impedance of converter. In order to test the stability of grid-forming converter, accurate emulation of grid behaviors from more aspects, including inertia and impedance under wider frequency ranges, is becoming critical. This article proposes a grid emulation method to mimic the inertia and impedance characteristics of power grid, targeting for the stability test of grid-forming converter under multifrequency bands. A high-switching-frequency converter and a low-switching-frequency converter are adopted in the proposed emulator structure to achieve both high capability in respect to control bandwidth and power level. Moreover, the virtual impedance control is integrated with the VSG control to emulate the line impedance and inertia within relatively wider frequency ranges. The realization of virtual impedance is achieved without derivation terms nor low pass filter. Finally, the performance of the proposed emulator is verified by various simulations and experimental measurements.

Three-Phase Four-Wire Inverter for Grid Emulator Under Wide Inductance Variation to Evaluate the Performance of Distributed Generator

This paper presents a three-phase four-wire inverter to generate voltage sources under wide filter inductance variation. The voltage sources with distortion and unbalance are to emulate grid voltages. The distortion includes voltage sags, voltage swells, and harmonic components, and the unbalance includes voltage and phase-angle differences. With a conventional PI control and direct digital control (DDC) only, the voltage distortion cannot fulfill precisely because of the improper controls. While with the proposed modified direct digital control (MDDC) and harmonic-angle adjustment algorithm, the soft core saturation can be taken into account and the harmonic voltage distortion can be accurately emulated, respectively. Moreover, the proposed control scheme can achieve fast transient response. The control law is derived and the inverter with the control to generate grid voltage is described in detail. Simulated and experimental results from 10 kW prototype have verified the analyses and discussions

Dual-Frequency Bands Grid Impedance Emulator for Stability Test of Grid-Connected Converters

The grid-connected converters are becoming a fundamental building block in the modern power grid, and the stability of grid-connected converters, especially when they are connected to weak grid, has drawn increasing attention. In order to test the interaction with various grid impedances and validate the stability behaviors, more accurate and advanced emulation of grid impedance is becoming an emerging need. However, the existing approaches are still difficult to flexibly shape the grid impedance under medium-to-high frequency range. In this article, a novel configuration for programable impedance emulation of power grid is proposed. The proposed grid emulator is composed of two power electronics converters with special control functions, i.e., the impedance forming converter (IFC) and power supporting converter (PSC). The IFC is designed with a higher switching frequency to virtually emulate different grid impedances under medium-to-high frequency band. On the other hand, the PSC is designed with a lower switching frequency to generate/absorb the power of the converter under test. Besides, a Norton-circuit-based virtual impedance control strategy is introduced to eliminate the derivation term in the control structure of virtual impedance. Finally, the performance of the proposed emulator is verified by various simulations and experimental prototype.

Experimental Analysis of Root Causes for Rotor-Speed-Dependent Emissions From Type 3 Wind Turbines

Multi-frequency (or harmonic) emissions and interactions are ongoing challenges for converter-dominated power systems. Consequently, multi-frequency models are introduced for power quality and harmonic stability studies. Rotor-Speed-Dependent (RSD) emissions from Type 3 Wind Turbines (WT) can lead to unexpected interactions. However, there are only a few specific studies on Type 3 WTs’ emissions. Besides, the provided analyses have been only based on field measurements. In this paper, the uncertainties of the field measurements are demonstrated experimentally. This paper provides an experimental analysis of emissions in different parts of a Type 3 WT, including stator, rotor, grid-side converter, and DC-link. In this way, theoretical emission patterns are formulated in sequence domain and verified by experimental measurements from a 2.2 MVA Type 3 WT tested by a 15 MVA grid emulator. The tests include emission measurements in different rotor speeds with identical output power set-point. To the best of our knowledge, the provided experiments are unique presentations from different parts of a Type 3 WT using MVA-scale torque and grid emulators. The test results provide a good impression of the existence, patterns, and amplitudes of the RSD emissions. Addressing such emissions in multi-frequency models can be necessary for the improvement of harmonic studies.

Self-Protective Inverters Against Malicious Setpoints Using Analytical Reference Models

This article presents the concept of self-protective inverters using steady-state and dynamic reference models. The self-protection strategy inspects incoming setpoints from the utility operator or third-party aggregators using analytical reference models before engaging the setpoints to the inverter's local controller. When a malicious setpoint passes the existing security layers, a smart inverter can examine the integrity of an incoming setpoint in real-time. The efficacy of the self-protection strategy has been tested using a laboratory setup, including a three-phase 3 kVA inverter and a 12 kW regenerative grid emulator. The results verify that the analytical steady-state and dynamic models can provide device-level protection for grid-interactive inverters by preventing harmful setpoints from getting engaged to the local controller.

Bifurcation Analysis of Active Electrical Distribution Networks Considering Load Tap Changers and Power Converter Capacity Limits

The aim of this article is to analyze the parameter stability and power transfer limits of active distribution networks with power converters and load tap changers by using bifurcation theory.First, the nonlinear model of a representative distribution grid with mixed loads and power converters is derived. Frequently neglected effects of converter capacity limits and transformers with load tap changers are considered, and for this purpose, static and dynamic saturations are modeled by using approximations based on smooth functions. In addition, a solution based on an antiwindup scheme is provided to model load tap changers without numerical convergence problems. These methods allow their integration to the model in a comprehensive way. Results show the impact of converter parameters and its limits on system stability as well as the impact of weak grid conditions. Moreover, they show that these effects can destabilize the system under, seemingly, safe operating conditions. The results indicate that reactive power support from converters can improve stability margins and the performed analysis can help in the design of the converter control parameters. Experimental results obtained from the laboratory environment comprising two distributed generators and a grid emulator are used for the validation of the contributions of this article.

Design and Analysis of Parallel-Connected Grid-Forming Virtual Synchronous Machines for Island and Grid-Connected Applications

Transmission system operators (TSOs) are increasingly requesting frequency support, reactive power compensation, and grid-forming capability to power plants interfaced by power converters. In these plants, a large number of converters is operated in parallel and all of them should be adequately coordinated to provide these services. For that purpose, the virtual synchronous machine (VSM) control technique is commonly adopted. However, the dynamic characteristics of the whole plant depend on the number of converters connected, their size, control parameters, and electrical interconnections. This makes it difficult to know the aggregated dynamic characteristic of the plant and how each VSM contributes to the plant response. In this article, the operation of VSMs in parallel configuration for island and grid-connected applications is studied. The developments make it possible to adapt the VSMs parameters so that the grid sees the power plant as an equivalent generator whose low-frequency characteristics do not vary with the number of converters in operation. The developments were validated in a laboratory consisting of four 15 kVA converters operating as VSMs, and a grid emulator.

Influence of the Phase-Locked Loop on the Design of Microgrids Formed by Diesel Generators and Grid-Forming Converters

In recent years, microgrids (MGs) with renewable energy sources, diesel gen-sets, and droop-controlled converters have been increasingly used to guarantee the continuity of power supply in remote areas. Renewable energy sources have been typically connected to MGs by using an electronic converter that features the two controllers: a current-control loop and a phase-locked loop (PLL). Stability issues related to the PLLs application in electrical grids have already been addressed in the literature; however, dynamic interactions in MGs caused by PLLs have not been sufficiently explored. In this article, an MG that includes a grid-feeding voltage source converter and a grid-forming device (diesel gen-set or converter) is studied. All network elements are modeled analytically and the eigenvalue and participation-factor analyses are used to analyze the interactions between the devices. It is demonstrated that MGs formed by diesel gen-sets have reduced stability limits. Also, it is shown that stability margins of MGs formed by droop-controlled converters can be improved by changing the control parameters (e.g., PLL and internal controllers bandwidths). The main findings and conclusions are summarized and presented as a practical MG design guide. Theoretical results are validated in a lab environment comprising two 15 kW converters and one 75 kW grid emulator.