Power Hardware-in-the-loop System Research Articles

Impact of Multi-Physics HiL Test Benches on Wind Turbine Certification

Recently developed nacelle test benches for wind turbines, equipped with multi-physics Hardware-in-the-Loop (HiL) systems, enable advanced testing and even certification of next-generation wind turbines according to IEC61400-21. On the basis of three experiments carried out with a commercial 3.2 MW wind turbine, this paper shows to which extent test bench hardware and HiL systems influence certification results. For the crucial Fault-Ride-Through tests, all deviations were found to be below 1% compared to field and simulation results. For this test, the power HiL system and the accuracy of its impedance emulation are found to be of most relevance. The results for the test items Frequency Control and Synthetic Inertia were found to be more sensitive to shortcomings of the mechanical HiL with its control system. Based on these findings, the paper mentions general procedures to ensure the quality of test benches with HiL systems and, with that, ensure the quality of certification.

A Framework for Sensitivity Analysis of Real-Time Power Hardware-in-the-Loop (PHIL) Systems

Power hardware-in-the-loop (PHIL) simulation leverages the advanced real-time emulation based technique to carry out in-depth investigations on novel real-world power components. Power amplifiers, sensors, and signal conversion units based power interfaces (PI) incorporate physical hardware systems and real-time simulation platforms into PHIL setups. However, the employment of any interfacing technique inevitably introduces disturbances such as sensor noise, switching harmonics, or quantization noise to PHIL systems. To facilitate quantitatively analyzing and assessing the impact of external disturbances on PHIL simulation systems, a framework for sensitivity analysis of PHIL setups has been developed in this paper. Detailed modelling principles related to the sensitivity analysis of PHIL systems and the inherent relationship between sensitivity transfer functions and stability criteria are elaborated along with theoretical and experimental validation. Based on this concept, accuracy assessment methods are employed in this framework to quantify generic sensitivity criteria. Moreover, physical passive load and converter-based PHIL setups are applied and experimental results are presented to characterize and demonstrate the applicability of the proposed framework.

Interface Algorithm Design for Power Hardware-in-the-loop Emulation of Modular Multilevel Converter Within High-Voltage Direct Current Systems

Power hardware-in-the-loop (PHIL) simulation combines the advantages of digital simulation and physical experiment, which provides great convenience for research and verification of a complex electric system, such as the high-voltage direct current (HVdc) system. The digital interface algorithm realizes interconnection of the digital and physical system, but it is also the main factor to affect stability and accuracy of the PHIL results. In this article, the damping impedance interface algorithm is adopted to test the performance of a modular multilevel converter (MMC) connected into an HVdc system. Instead of online acquiring the exact MMC dc impedance, a simple RLC damping impedance is proposed to ensure stability and particularly, low-frequency accuracy for the PHIL system. Moreover, as testing in the laboratory does not allow hundreds of MVA-rated physical MMC, suitable scaling factors are proposed to design a scaled-down MMC prototype which presents identical dynamic characteristic with a full-scale MMC. A PHIL setup including RT-LAB real-time simulator, power amplifier, and a scaled-down MMC prototype is built and the experiments confirm the effectiveness of the proposed interface algorithm.

Optimal Power Hardware-in-the-Loop Interfacing: Applying Modern Control for Design and Verification of High-Accuracy Interfaces

In this article, we develop a novel approach to design power hardware-in-the-loop (PHIL) simulation interfaces that maximize simulation bandwidth and accuracy by leveraging a modern control framework that explicitly considers objectives on accuracy and can automatically synthesize an optimal controller that meets these objectives while stabilizing the closed-loop system. The method developed improves upon common approaches to PHIL interface design that typically involve multiple design steps for manual compensation and stabilization that can result in interfaces that are stable but have suboptimal bandwidth and accuracy. The approach developed is general and can be applied to most PHIL system configurations. The modeling framework allows for the inclusion of scaling factors and hardware-under-test current injection models that are common to practical PHIL simulations and have significant impacts on stability. We also present practical methods and metrics for verifying the absolute accuracy of PHIL simulation results without relying on relative comparisons to potentially inaccurate models or previous simulation results. We demonstrate the accuracy evaluation method and show the improved performance achievable when using the optimal PHIL interface design approach in an experimental case study involving a 100-kVA battery inverter.

Stability Analysis of Power Hardware-in-the-Loop Architecture With Solar Inverter

Power hardware-in-the-loop (PHIL) simulations have been rapidly growing in recent times due to the flexibility it offers in conducting various system-level studies as well as individual device evaluation. Evaluating a power converter has been one of the major applications of PHIL at recent times in the industry. Following this trend, this article proposes a way to evaluate a photovoltaic (PV) microinverter in PHIL arrangement. The mathematical background to quantify the stability criteria for a PHIL network is presented along with theoretical and experimental verifications. The methodology in this article is based on the model of interface devices and accurate delay model to analyze the stability of a PHIL system employing Routh–Hurwitz's formulation. An extensive analysis on stability along with the compensator design to enhance the stability limit of a PHIL system is presented. The workflow developed is applied to evaluate a 250-W PV microinverter, which showed a stable performance with more than 97% efficiency during steady state and transients.

A Review of PHIL Testing for Smart Grids—Selection Guide, Classification and Online Database Analysis

The Smart Grid is one of the most important solutions to boost electricity sharing from renewable energy sources. Its implementation adds new functionalities to power systems, which increases the electric grid complexity. To ensure grid stability and security, systems need flexible methods in order to be tested in a safe and economical way. A promising test technique is Power Hardware-In-the-Loop (PHIL), which combines the flexibility of Hardware-In-the-Loop (HIL) technique with power exchange. However, the acquisition of PHIL components usually represents a great expense for laboratories and, therefore, the setting up of the experiment involves making hard decisions. This paper provides a complete guideline and useful new tools for laboratories in order to set PHIL facilities up efficiently. First, a PHIL system selection guide is presented, which describes the selection process steps and the main system characteristics needed to perform a PHIL test. Furthermore, a classification proposal containing the desirable information to be obtained from a PHIL test paper for reproducibility purposes is given. Finally, this classification was used to develop a PHIL test online database, which was analysed, and the main gathered information with some use cases and conclusions are shown.

Improving the Performance of a Power Hardware-In-the-Loop System by a New Interface Algorithm

Power Hardware-In-the-Loop (PHIL) is an efficient way to analysis and test electrical equipment which is introduced into power system. To improve the performance of a PHIL system, the feasibility of several interface algorithms for PHIL system are analyzed. And a new interface algorithm, which is based on the virtual impedance method (VIM), is proposed. Therefore, the proposed interface algorithm is compared and assessed with the current interface algorithms. Through the comparing and testing, the proposed interface algorithm is verified that it obtains a better overall performance than another.

Multirate Partitioning Interface for Enhanced Stability of Power Hardware-in-the-Loop Real-Time Simulation

Power hardware-in-the-loop (PHIL) technology allows for the testing of physical equipment including power electronic converters in a simulation environment that closely mirrors the reality of the electric power grid. A challenge involved in PHIL systems is the design of the interface between the digital network model and physical equipment as this has a significant influence on stability of the real-time simulation. While a small time step supports stability and accuracy, its lower limit is given by the real-time constraint. The proposed multirate partitioning (MRP) interface addresses this issue by using a rather small time step at the hardware–software interconnection and then employing a staged adaptation of the time step within the digital model in accordance with the real-time constraint. Thus, multiple rates are used. The Nyquist stability criterion confirms enhanced stability and bandwidth by the MRP compared with single-rate (SR) counterparts. Moreover, a PHIL test of two parallel photovoltaic converters feeding a low-voltage network reveals different behaviors depending on the interface. Using the MRP, the waveforms more closely track the real-world curves of active and reactive power with the relative accuracy increasing with the speed of the transients. Informed decision making regarding the integration of renewables in the grid is so supported.

Stability of a Switched Mode Power Amplifier Interface for Power Hardware-in-the-Loop

Power hardware-in-the-loop (PHIL) is an attractive, real-time system testing and validation technique. Virtual models can be replaced with physical equipment to emulate system integration testing, and perform model validation without the cost and risk of a full scale power system. Power amplifiers are the principal components that facilitate the power interface between the physical and the virtual systems. This necessary interfacing introduces dynamics that do not exist in the real system and is the source of stability and accuracy issues affecting PHIL applications. This paper presents a stability analysis of a practical PHIL system based on the ideal transformer model interface algorithm that utilizes a voltage source converter for power amplification. A systematic approach to evaluate the expected stability characteristics of PHIL experiments is developed. The analysis in this paper shows that in a practical system, the power amplifier's filter, sampling, and measurement filtering are all shown to influence the stability in a counterintuitive way, leading to a nonmonotonic stability characteristic. An experimental PHIL platform based on a real-time digital simulator real-time computer has been used to validate the analysis and confirm the effects of the practical interface. Switching dead-time is also shown to have a significant stabilizing effect.

Operational Range of Several Interface Algorithms for Different Power Hardware-In-The-Loop Setups

The importance of Power Hardware-in-the-Loop (PHIL) experiments is rising more and more over the last decade in the field of power system and components testing. Due to the bidirectional exchange between virtual and physical systems, a true-to-reality interface is essential; however, linking several dynamic systems, stability issues can challenge the experiments, the components under test, and the individuals performing the experiments. Over the time, several interface algorithms (IA) have been developed and analyzed, each having different advantages and disadvantages in view of combining virtual simulations with physical power systems. Finally, IA are very specific to the kind of PHIL experiment. This paper investigates the operational range of several IA for specific PHIL setups by calculations, simulations, and measurements. Therefore, a selection of the mainly used respectively optimized IA is mathematically described. The operational range is verified in a PHIL system testing environment. Furthermore, in order to study the influence of different PHIL setups, according to software and hardware impedance, different tests using linear and switching amplifiers are performed.

A Stable and Fast-Transient Performance Switched-Mode Power Amplifier for a Power Hardware in the Loop (PHIL) System

Power Hardware in the Loop (PHIL) systems are used to test a power system with the help of combined software and hardware. Generally, to construct a PHIL system, a switched-mode power amplifier that has a stable performance is used, because of their large, linear signal control-to-output characteristics. However, the fundamental limitations of a switch-mode power amplifier (PA) are the dynamic performance and output bandwidth. In this paper, a compound controller has been used for the rectifier part of a PA, which can ensure the stability of a PA under transient or fault operating conditions. Moreover, a compound controller, which involves a feed-forward controller, a proportional controller and a repetitive controller, is proposed in the inverter part of a PA, and it can be used for PHIL applications. Experimental results are obtained under various operating conditions, such as transient responses under load step change, and output voltage bandwidth testing for a PHIL system, it is concluded that a proposed switched-mode power amplifier is useful for the PHIL system.

Contribution to stability analysis of power hardware‐in‐the‐loop simulators

This study establishes a new basis for understanding the stability of power hardware-in-the-loop (PHIL) systems considering their hybrid (analogue/digital) nature. Such systems are known to have closed-loop stability issues due to delays between the simulator and the power amplifier (PA). This work demonstrates that the conventional method for determining the stability criterion, which considers the system as a continuous model, is not appropriate. A new method of assessing the stability of a PHIL system based on discrete-time impedance frequency responses is thus presented. Hydro-Quebec's Research Institute will use this innovative approach for the development of its own PHIL system, which will involve connecting the institute's real-life experimental distribution test line to its large-scale real-time digital simulator through a 25-kV, 10-MVA PA. The validity of the new method is demonstrated for simulation models of a well-known inductive system and of a distribution feeder connected to a large-scale power system.

Stability and Accuracy Considerations of Power Hardware- in-the-Loop Test Benches for Wind Turbines

In Power-Hardware-In-the-Loop (PHIL) setups, the device under test (DUT) is connected to the rest of the system (ROS) running in a real-time simulator through an Interface Algorithm (IA) and power amplifier, which strongly influences stability and accuracy of the test setup. This paper presents a comprehensive theoretical framework for the analysis of the two aspects of a PHIL system with different IAs, and demonstrates its applicability to the design of PHIL test benches for wind turbines. For the theoretical modelling derivation, the interaction of two lumped active systems is considered.

Improving the Stability and Accuracy of Power Hardware-in-the-Loop Simulation Using Virtual Impedance Method

Power hardware-in-the-loop (PHIL) systems are advanced, real-time platforms for combined software and hardware testing. Two paramount issues in PHIL simulations are the closed-loop stability and simulation accuracy. This paper presents a virtual impedance (VI) method for PHIL simulations that improves the simulation’s stability and accuracy. Through the establishment of an impedance model for a PHIL simulation circuit, which is composed of a voltage-source converter and a simple network, the stability and accuracy of the PHIL system are analyzed. Then, the proposed VI method is implemented in a digital real-time simulator and used to correct the combined impedance in the impedance model, achieving higher stability and accuracy of the results. The validity of the VI method is verified through the PHIL simulation of two typical PHIL examples.

A model-based interfacing concept for accurate power hardware-in-the-loop systems

This article introduces a novel concept for the interface of power hardware-in-the-loop (PHiL) test systems. In a PHiL system, a real device under test is connected to dynamic simulation models that emulate high power flows running to and from the device under test. The states of these dynamic models are thus used to physically actuate the real hardware component and likewise the states of the latter serve as an external excitation of the dynamic model. Thus, a PHiL system constitutes a complex interaction of dynamic mathematical models and real hardware components. The purpose of PHiL systems is to operate and test the hardware component as closely to its future designated environment as possible. A crucial factor for a highly accurate PHiL system lies in the interface between virtual dynamic simulation and the physical hardware. Accuracy and performance is often impaired by signal communication delays, sensor noise and insufficient performance of test bed control systems. The approach presented in this article reduces unwanted oscillations based on an online receding optimization of specific state variables in the dynamic simulation environment. In addition, the adherence to physical conservation laws is assured. The interface algorithm makes use of an online model which incorporates knowledge about the dynamics in the underlying simulation. The performance of the proposed methodology is demonstrated and discussed by means of a highly dynamic automotive powertrain PHiL test bed in combination with a complex multibody real-time vehicle simulation.

Power Hardware-in-the-Loop Testing of an Air Coil Superconducting Fault Current Limiter Demonstrator

This paper discusses power hardware-in-the-loop (PHIL) tests performed at the Center for Advanced Power Systems (CAPS) with a 60-kVA, 400-V, z= 6$% air coil (AC) superconducting fault current limiter (AC-SFCL) demonstrator. The demonstrator was designed, built, and successfully tested for fault current limitation at Karlsruhe Institute of Technology (KIT) . Initial PHIL testing at CAPS focused on a single-phase setup to verify the results obtained at KIT. Subsequent PHIL testing in a three-phase system was accomplished by interfacing the physical AC-SFCL to the simulated system in phase A and simulating the effect of AC-SFCL devices in the other two phases. This was done by estimating the resistance and inductance of the AC-SFCL through instantaneous voltage and current measurements and inserting this impedance in phases B and C for cases in which these phases were part of the short-circuit path. For these PHIL experiments, the modified damping impedance (DIM) interface algorithm was employed. It was possible to reproduce earlier reference measurements using the modified DIM PHIL interface algorithm. The PHIL system was then used to create a three-phase power system consisting of a generator, a load, and a short-circuit path. Symmetrical and unsymmetrical short circuits have been performed with and without ground connection. The PHIL system maintained stability throughout the experiments, and the results show suitable current limitation for all performed types of short circuits.

Study of Bidirectional DC-DC Converter Interfacing Energy Storage for Vehicle Power Management Using Real Time Digital Simulator (RTDS)

The bidirectional dc-dc converter, being the interface between Energy Storage Element (ESE) and DC bus, is an essential component of the power management system for vehicle applications including electric vehicle (EV), hybrid electric vehicle (HEV), and fuel cell vehicle (FCV). In this paper, a novel multiphase bidirectional dc-dc converter interfacing with battery to supply and absorb the electric energy in the FCV system was studied with the help of real time digital simulator (RTDS). The mathematical models of fuel cell, battery and dc-dc converter were derived. A power management strategy was developed and first simulated in RTDS. A Power Hardware-In-the-Loop (PHIL) simulation using RTDS is then presented. The main challenge of this PHIL is the requirement for a highly dynamic bidirectional Simulation-Stimulation (Sim-Stim) interface. This paper describes three different interface algorithms. The closed-loop stability of the resulting PHIL system is analyzed in terms of time delay and sampling rate. A prototype bidirectional Sim-Stim interface is designed to implement the PHIL simulation.

An Effective Method for Evaluating the Accuracy of Power Hardware-in-the-Loop Simulations

Power hardware-in-the-loop (PHIL) simulations need to be accurate to truly reflect the behavior of the systems under test. However, a PHIL simulation may result in errors or even instability due to imperfections (e.g., time delay, noise injection, phase lag, and limited bandwidth) in the power interface, particularly in high-power applications. Additionally, it is usually difficult to determine the accuracy of a simulation because there is no reference available for people to know the ldquoshould-berdquo system responses in advance. Therefore, a method is demanded to predict the accuracy of PHIL simulations. In this paper, an effective method for evaluating the PHIL accuracy is proposed. This method provides a means to justify the result of a PHIL simulation analytically and quantitatively instead of empirically. While the method is based on linear system analysis, it is shown to be also applicable for nonlinear PHIL systems.