Grid Emulator Research Articles

A Comprehensive Study and Validation of a Power-HIL Testbed for Evaluating Grid-Connected EV Chargers

Integration of excessive electric vehicle (EV) chargers into the low-voltage (LV) network may introduce new challenges. Power hardware in the loop (PHIL) simulations can be used for evaluating such systems as it provides a flexible testing platform to study the overall system as well as individual devices. To facilitate a proper PHIL simulation, a precise mathematical model of the PHIL testbed is required. This article presents a comprehensive small-signal model capable of describing the dynamics of a PHIL testbed developed for evaluating grid-connected EV chargers. The PHIL testbed consists of a PHIL-based battery emulator (BE) and a grid emulator (GE) to mimic the dc side battery energy storage system (BESS) and the ac side LV grid behavior, respectively. A mathematical framework is developed to analyze the stability and predict the accuracy of both PHIL-based emulators. The BE in this article considers a switch-mode power amplifier (PA). Thus, design strategies for its linear controller are also discussed in the context of cascaded dc–dc configuration. An experimental PHIL platform based on a real-time simulator (RTS) has been used to validate theoretical predictions and confirm developed models. Finally, the validated PHIL test has been employed for analyzing the performance of a commercial EV charger and its interactions with a weak LV network simulated in RSCAD/EMTDC.

Generic Multi-Frequency Modelling of Converter-Connected Renewable Energy Generators Considering Frequency and Sequence Couplings

Frequency and sequence couplings can compromise the trustworthiness of multi-frequency models for converter-based systems. There have been effective attempts to address the couplings mainly by linearized averaged models. Only a few studies have been conducted on practical optimization of such models with enormous matrices and experimental results. This paper provides a generic theory for coupling patterns and proposes a multi-frequency modelling method to detect and address only the main couplings in the sequence domain for converter-connected renewable energy generators. The proposed generic model is based on empirical tests using small-signal perturbations and adopting Fourier transform on the switching converter response. The proposed theory and modelling methodology are verified using a 7MVA grid emulator for voltage perturbation tests on a 2MVA photo-voltaic converter. Accordingly, the couplings can exist in more generic forms, including multiples of perturbation and fundamental frequencies. To the best of our knowledge, the patterns with the multiples of the perturbation frequency have been overlooked in the literature. Furthermore, the mirror frequency concept is valid for all coupling patterns and is included in the proposed model. Besides, the proposed patterns and the environment noise levels have been practical criteria for selecting the main couplings.

Extension of the Generic Multi-Frequency Modelling Method for Type 3 Wind Turbines

Reflecting potential non-linearities of converter-based systems, especially frequency and sequence couplings, is an ongoing challenge for linearized multi-frequency models. Besides, design details are required to develop such models, which either are the intellectual property of manufacturers or require experimental tests. The generic multi-frequency modelling method has been proposed to fill this gap; however, it is only developed for converter-connected systems, e.g., Type 4 Wind Turbines (WT). This paper proposes to extend the application of the generic multi-frequency modelling method for Type 3 WTs. In this way, a theory for patterns of the couplings in Type 3 WTs is proposed. Accordingly, a group of emissions and couplings are Rotor-Speed-Dependent (RSD). The RSD emissions and couplings are particular characteristics of Type 3 WTs, which should be addressed in the generic multi-frequency models. The proposed theory is verified by unique-worldwide experimental perturbation tests on a 2 MVA Type 3 WT using a 7 MVA grid emulator. Accordingly, a limited number of RSD couplings and emissions are observed in the test results, mainly in low frequencies (below 1 kHz). Therefore, addressing the RSD couplings is practical and important to extend the generic multi-frequency modelling for Type 3 WTs.

A Controller Improving Photovoltaic Voltage Regulation in the Single-Stage Single-Phase Inverter

While substantial research covers current control and synchronization of grid-connected photovoltaic (PV) inverters, issues concerning control of the PV input voltage deserve more attention, as they equally affect the reliable and stable operation of the system. Hence, this article analyses the PV voltage regulation in the single-stage single-phase PV inverter. In contrast to previous work, the PV source influence on the input voltage dynamic is analytically formalized, exposing a potential instability when the PV source is operating in its constant current region. A traditional proportional-integral PV voltage controller fails to ensure a consistent and stable voltage regulation. On the other hand, this issue is resolved by the proposed feedback linearization based controller. The new controller is validated on a test setup comprising of a PV source emulating a 1.2 kW PV array, interfaced to a single-phase inverter connected to a grid emulator. Confirming the issues predicted by the theoretical analysis, the experiments prove two main advantages of the proposed controller. First, PV voltage regulation instability is eliminated when the PV array operates in its constant current region. Second, the PV voltage transient behavior is now independent of the operating point of the PV source.

A fast finite sample count symmetric component extraction method for use in grid side converters

The considerable increase in the number of renewable energy sources in power systems in recent years has led to the grid side converters’ connection requirements being more and more strict. In order to comply with the newly arising grid codes, the synchronization unit has to provide a fast and robust response even under unbalanced and distorted line voltage. This paper proposes a new method for very fast extraction of symmetrical components. The method uses a finite number of grid voltage samples to solve a set of algebraic equations and obtain sine and cosine factors of the direct and inverse sequence components. The corresponding amplitudes and angles are then found by using √ and arctan functions. The proposed extraction algorithm requires a derivative estimation technique to estimate the frequency from the direct sequence angle. The performance of the proposed finite sample count (FSC) method is compared to the all-pass filter (APF) based synchronous reference frame phase-locked-loop (SRF PLL) in simulations and experimentally using a DSP based platform and grid emulator with analog outputs. Under the voltage dip, FSC responds within several hundreds of µs, whereas it takes around 3 ms for the APF SRF PLL to settle.

A passive communication based islanding detection technique for AC microgrid

This paper presents a secured communication based passive anti-islanding technique, using phase angle difference information of superimposed impedance computed at both distributed generation (DG) and point of common coupling (PCC) ends. In the approach, the PCC end signal information is transmitted every cycle to DG sites to analyze the event, hence more effective and reliable. The performance of the method is evaluated for different test conditions such as low power mismatch islanding events, different fault types and switching events. A standard distribution system model is considered and simulated in EMTDC/PSCAD. Additionally, islanding detection is also performed on experimental signals obtained on PV system integrated with grid emulator. The complete procedure is presented, and performances are evaluated by conducting selectivity, dependability and security tests.

Real Time Simulation of Power Electronics Medium Voltage DC-Grid Simulator

Power electronics medium-voltage (MV) systems must comply with the requirements defined in grid codes. These systems’ compatibility with the standards can be validated by specialized testing equipment: grid simulators. This paper presents a hardware in the loop (HiL) implementation and the simulation results of a MV multiphase DC/DC converter designed for MV DC grid emulation. By using ABB’s reliable, patented power converter hardware topology (US 10978948 B2) and by applying advanced control algorithms, the presented system can be used for special purposes, such as the emulation of fault events in a DC-grid used for the certification of other devices, or for other research goals. The presented concept of a power electronics DC-grid simulator (PEGS-DC) is characterized by high power capability and high voltage quality. In this paper, the general idea of a power electronics grid simulator applied for the testing of MV electrical systems is discussed. Then, details related to the PEGS-DC, such as its hardware topology and the applied modulation method are presented. Subsequently, the HiL setup is described. The main scope of this article focuses on model the description and presenting recorded HiL simulations.

Test methodology for validation of multi‐frequency models of renewable energy generators using small‐signal perturbations

Providing trustworthy and accurate multi-frequency (or harmonic) models for renewable energy generators (REG) is an ongoing challenge for harmonic studies. There have been effective attempts to propose and design a test device to validate the harmonic models, mainly based on shunt current perturbations. However, using additional devices for perturbations is costly for converter-based test sites. This paper provides the test specifications to extend the application of the grid emulators for voltage perturbations and appropriate harmonic model validation. Besides, the effects of the sequence couplings, initial emissions, and power set-points on the test results have been overlooked in the literature. Considering these effects, this paper proposes a generic test methodology to obtain more accurate models in the sequence domain. The experimental verification of the proposed methodology is demonstrated using a 7 MVA grid emulator for testing of a 2 MVA photo-voltaic converter and a 2 MVA Type 3 wind turbine. This way, the test challenges, specifications, and recommendations are presented using the MW-scale experiments on different REGs. Furthermore, the effects of sequence couplings and initial emissions on the calculation results are investigated and compared. The proposed methodology is applicable for harmonic model validation as well as empirical modelling.

Power Hardware-in-the-Loop-Based Performance Analysis of Different Converter Controllers for Fast Active Power Regulation in Low-Inertia Power Systems

Future electrical power systems will be dominated by power electronic converters, which are deployed for the integration of renewable power plants, responsive demand, and different types of storage systems. The stability of such systems will strongly depend on the control strategies attached to the converters. In this context, laboratory-scale setups are becoming the key tools for prototyping and evaluating the performance and robustness of different converter technologies and control strategies. The performance evaluation of control strategies for dynamic frequency support using fast active power regulation (FAPR) requires the urgent development of a suitable power hardware-in-the-loop (PHIL) setup. In this paper, the most prominent emerging types of FAPR are selected and studied: droop-based FAPR, droop derivative-based FAPR, and virtual synchronous power (VSP)-based FAPR. A novel setup for PHIL-based performance evaluation of these strategies is proposed. The setup combines the advanced modeling and simulation functions of a real-time digital simulation platform (RTDS), an external programmable unit to implement the studied FAPR control strategies as digital controllers, and actual hardware. The hardware setup consists of a grid emulator to recreate the dynamic response as seen from the interface bus of the grid side converter of a power electronic-interfaced device (e.g., type-IV wind turbines), and a mockup voltage source converter (VSC, i.e., a device under test (DUT)). The DUT is virtually interfaced to one high-voltage bus of the electromagnetic transient (EMT) representation of a variant of the IEEE 9 bus test system, which has been modified to consider an operating condition with 52% of the total supply provided by wind power generation. The selected and programmed FAPR strategies are applied to the DUT, with the ultimate goal of ascertaining its feasibility and effectiveness with respect to the pure software-based EMT representation performed in real time. Particularly, the time-varying response of the active power injection by each FAPR control strategy and the impact on the instantaneous frequency excursions occurring in the frequency containment periods are analyzed. The performed tests show the degree of improvements on both the rate-of-change-of-frequency (RoCoF) and the maximum frequency excursion (e.g., nadir).

A new real time hydraulic test platform dedicated to Ancillary Services

Characterization of Hydraulic turbines, storage pumps and pump-turbines is a well-known process formalized in the IEC60193 standard. Test platforms ensure the acceptance of hydraulic design with respect to hydraulic transpositions such as efficiency, range of operation, cavitation, stability etc. These test platforms only focuses on the static operation of a turbine. The HydroPHIL platform - which stands for Power Hardware In the Loop for Hydro-electrical conversion - focuses on studying transients operations of Pump Storage Plants. It includes a fully operational scale model of a hydraulic units from water to wire. The scaled model interacts with a real time grid emulator and a real time penstock emulator. The test platform allows the simulation of technologies in relevant environment related to three main topics of investigation: a) system behavior study, whether it is solicited by the grid (frequency or voltage dips, etc.) or the piping system (water hammer, etc.) b) new conversion technologies with real electrical components (such as variable speed PSP), and c) development of new control strategies to improve provision of ancillary services and other flexibility services. This paper presents the platform architecture and features.

Optimal control of grid-connected microgrid PV-based source under partially shaded conditions

Microgrids are gaining increasing attention globally and becoming increasingly powered by photovoltaic (PV) systems, thereby requiring high-efficiency control to function as a microgrid distributed generation unit. Accordingly, this study presents an optimal control of a grid-connected Microgrid PV Source (MPVS) under partially shaded conditions. The objective is to ensure the MPVSs ability to rapidly and precisely deliver the amount of power assigned by the supervisory controller. Thus, MPVS must shift rapidly and smoothly between the maximum and intermediate power point modes. The proposed system is composed of PV array, grid emulators, and two converters coupled to a common DC bus. The control strategy of the boost converter is based on the combination of two algorithms: particle swarm optimization algorithm and the proposed intermediate power point tracker algorithm. The voltage source inverter is controlled to keep the DC bus voltage constant and inject the power to the grid, in which the voltage-oriented control technique is applied and combined with the phase-locked loop algorithm for voltage synchronization. Lastly, all control algorithms are implemented in a DSpace 1104 environment and largely tested under various partially shaded patterns.

Emulation of grid-forming inverters using real-time PC and 4-quadrant voltage amplifier

A test bed for the evaluation of novel control methods of inverters for renewable power generation is presented. The behavior of grid-following and grid-forming control in a test scenario is studied and compared.Using a real-time capable control platform with a cycle time of 50 µs, control methods developed with Matlab/Simulink can be implemented. For simplicity, a three-phase 4‑quadrant voltage amplifier is used instead of an inverter. Thus, the use of modulation and switched power semiconductors can be avoided. In order to show a realistic behavior of a grid-side filter, passive components can be automatically connected as L‑, LC- or LCL-filter. The test bed has a nominal active power of 43.6 kW and a nominal voltage of 400 V.As state-of-the-art grid-following control method, a current control in the d/q-system is implemented in the test bed. A virtual synchronous machine, the Synchronverter, is used as grid-forming control method. In combination with a frequency-variable grid emulation, the behavior of both control methods is studied in the event of a load connection in an island grid environment.

First comparison of the electrical properties of two grid emulators for UVRT test against field measurement

The technical rules for connecting turbines to the medium, high or extra-high voltage grid in Germany require the certification of the UVRT characteristics of wind turbines. The state-of-art voltage divider-based test equipment, also named UVRT-Container, is well equipped for executing UVRT tests in field. To conduct the UVRT in field the full wind turbine should be already installed. A second option to perform UVRT tests are system level test benches. They enable the testing of the nacelle. The components that are not actually present, such as the turbine tower or the blades, are emulated via a mechanical hardware in the loop (HiL) system. In this work, for the first time, the performance of two different grid simulators installed at the DyNaLab at Fraunhofer IWES and at the CWD at RWTH Aachen University is compared with a field measurement of the same type of wind turbine. Thus, not only a system test bench measurement is compared to a field measurement. Rather, two system test benches with individual technical approaches are additionally compared with each other. The focus of this work is to investigate the characteristics of the grid simulators within the steady-state range of the UVRT tests to replicate identical fault shapes on the test benches and in the field.

CertBench: conclusions from the comparison of certification results derived on system test benches and in the field

This paper presents measurement results of the world wide first successful certification the electrical properties of a wind turbine, solely based upon measurements obtained at a system test bench with HiL-System and grid emulator. For all certification relevant tests the results are compared to field measurements. The impact of the real-time models in the HiL-System as well as the converter-based grid emulator are discussed in this paper. For full converter wind turbine, different requirements for the model depth could be determined depending on the tests. Nevertheless, higher-quality models that reflect the plant behaviour better are recommended to reduce uncertainties within the certification process. This paper also shows that especially for grid failure events grid emulators require real-time impedance control, in order to emulate grid failures properly. Based on these findings, recommendations for the requirements on test bench components are formulated in this paper, in order to contribute to new certification guidelines. Overall, we conclude that based on the experiences made at two different system test benches, the vast majority of certification measurements can be carried out without limitation at such system test benches.

Primary Frequency Control Using Motor Drives for Short Term Grid Disturbances

Industrial motor systems make up a quarter of all electric sales in the United States. Variable speed drives (VSDs) can provide energy efficiency savings to the customer by regulating motor speed based on specific and varying needs. In addition to the benefits provided to the customer, VSDs can provide support to the grid through ancillary services. The Center for Ultra-Wide-Area Resilient Electric Energy Transmission Networks (CURENT) developed a power electronics converter-based grid emulator to allow testing of various power system architectures and demonstration of key technologies in monitoring, control, actuation, and visualization. This paper proposes using an active front-end VSD's connected motor load to provide frequency regulation to a large scale power grid. Each part of the emulator is described including motor and power electronics model and control. The proposed frequency regulation is implemented in VSDs and modeled in both a transmission system in EMTDC/PSCAD and verified on CURENT's hardware testbed.

AC Grid Emulations for Advanced Testing of Grid-Connected Converters—An Overview

High penetration of distributed generations and active loads have enabled the power electronics converter to become a vital component in the modern power grid system, and the broad employment of grid-connected converters at various power levels is making the grid impedance and characteristics complicated. Consequently, in order to validate more advanced features such as the reliability and stability performances of the grid-connected converters, it is becoming an emerging need to emulate the grid behaviors from more aspects. This article serves to foster and investigate the state-of-the-art techniques in the field of ac grid emulation from the perspective of multiple spatial-scales and multiple time-scales. Four major concepts used for grid emulation with featured principles, including concept I (analog simulation with under-scaled components), concept II (grid characteristics in the real-time simulator), concept III (grid characteristics in the converters structure), and concept IV (grid characteristics in the converters controller), are summarized, respectively, in this article. The practical implementation regarding to the circuit topology and the power supply for the grid emulation system are also discussed. Finally, the future trends and conclusions in the field of ac grid emulation are provided.

Robust dynamic grid emulator control

This paper proposes an enhanced dynamic control strategy for a grid emulator. The proposed control switches between resonant and predictive controls in order to benefit from the advantages of both controls and to cope with their drawbacks. It ensures good dynamic performance in transient state and very low error in case of parametric or load variation during steady state. The most used control for grid emulator based on power converters is the one based on resonant controllers. Although this control is characterized by its robustness against parameters and load variation, it suffers from low performance in transient operation. The predictive control, which is characterized by its fast dynamic performance in transient state, can be used to overcome the resonant controllers limit. An exhaustive theoretical study reinforced by simulation and experimental results validate the capability, robustness and high performances of the proposed dynamic control strategy.

Generic characterization of electrical test benches for AC- and HVDC-connected wind power plants

Abstract. The electrical test and assessment of wind turbines go hand in hand with standards and network connection requirements. In this paper, the generic structure of advanced electrical test benches, including grid emulator or controllable grid interface, wind torque emulator, and device under test, is proposed to harmonize state-of-the-art test sites. On the other hand, modern wind turbines are under development towards new features, concerning grid-forming, black-start, and frequency support capabilities as well as harmonic stability and control interaction considerations, to secure the robustness and stability of renewable-energy-based power systems. Therefore, it is necessary to develop new and revised test standards and methodologies to address the new features of wind turbines. This paper proposes a generic test structure within two main groups, including open-loop and closed-loop tests. The open-loop tests include the IEC 61400-21-1 standard tests as well as the additional proposed test options for the new capabilities of wind turbines, which replicate grid connection compliance tests using open-loop references for the grid emulator. In addition, the closed-loop tests evaluate the device under test as part of a virtual wind power plant and perform real-time simulations considering the grid dynamics. The closed-loop tests concern grid connection topologies consisting of AC and HVDC, as well as different electrical characteristics, including impedance, short-circuit ratio, inertia, and background harmonics. The proposed tests can be implemented using available advanced test benches by adjusting their control systems. The characteristics of a real power system can be emulated by a grid emulator coupled with real-time digital simulator systems through a high-bandwidth power-hardware-in-the-loop interface.

Robust and Cost-Effective Synchronization Scheme for a Multicell Grid Emulator

Steadily increasing requirements for grid-connected converters, their growing unit power, and output voltage call for flexible high-power medium-voltage grid emulators. Four-quadrant cascaded H-bridge topology features high output voltage resolution and high effective switching frequency, which enable high-dynamic high-fidelity grid emulation. This article shows that despite nonidealities (turn ratios, phase-shift angles, and different stray inductances) in a typical phase-shifting multiwinding transformer, active front-ends of the cells can be effectively synchronized to their respective supply voltages using transformer primary-side voltage measurements and nameplate values. Consequently, cell input filters can be omitted, making the grid emulator more compact, cheaper, faster, and exhibiting more robust grid synchronization. Theoretical developments and designs are verified by means of high-fidelity time-domain simulations.

On the Dynamic Performance of Variable-Frequency AC–DC Converters

Variable-frequency schemes have increased their presence in aerospace applications while maintaining their popularity in hybrid electric vehicles. In this article, the dynamic performance under parameter mismatch of a variable-frequency ac–dc converter is analyzed. A Lyapunov-based adaptive parameter estimation algorithm is developed, and included in the control scheme to address the performance degradation that occurs from the parameter mismatch between the hardware and the controller due to the variable-frequency and ambient operating conditions. Because of this adaptive parameter estimation technique, the proposed controller can effectively regulate the output dc-bus voltage and maintain a unity power factor. The validity and performance effectiveness of the proposed controller are verified experimentally through a laboratory-scaled three-phase 1.5 kW 270 V SiC-based two-level voltage-source-converter using a variable-frequency programmable grid emulator. The results demonstrate that by including the developed adaptive parameter estimation algorithm in the control scheme, the performance of the ac–dc converter can be improved under rapid input frequency changes and filter parameter variations.