Power Hardware-in-the-loop Research Articles

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.

Accurate and Stable Hardware-in-the-Loop (HIL) Real-Time Simulation of Integrated Power Electronics and Power Systems

Power hardware-in-the-loop (PHIL) technology allows for the testing of physical equipment in a real-time simulation environment. An important role is attributed to the power interface (PI). This PI connects a power system model, which is implemented on a digital computer, to physical hardware under test, such as a power electronic converter. Several hardware-in-the-loop (HIL) test setups with distinct PIs are proposed and compared. Based on detailed modeling of the different interfaces, system analysis is performed for each HIL test setup with respect to overall stability and accuracy. To verify stability for PHIL simulation systems, transfer function representations of the entire PHIL simulation processes are developed, and all involved time delays are quantified. The Nyquist stability criterion is applied to analyze all considered interfacing methods to enhance PHIL simulation stability, and the accuracy is evaluated. Moreover, experimental test results are given to demonstrate both the applicability and the functioning of the proposed interfacing methods. A particular focus is laid on the interfacing of physical power electronic inverters tested as part of a network in a PHIL real-time simulation.

Practical Estimation of Accuracy in Power Hardware-in-the-Loop Simulation Using Impedance Measurements

Power hardware-in-the-loop (PHIL) simulation is a technique whereby actual power hardware is interfaced to a virtual surrounding system, simulated in real-time, through PHIL interfaces making use of power amplifiers and/or actuators. While PHIL simulation affords many advantages for flexible testing of power hardware, the accuracy of the experiment can be degraded through the non-ideal aspects of the PHIL interface, including distortion in the frequency domain, delays, and disturbances. This paper presents a practical method to estimate a subset of the closed-loop transfer function accuracy metrics of a PHIL experiment based on impedance measurements from the experiment. The method of assessing the impacts of the PHIL interfaces is demonstrated through application to an experiment in which two simulated systems are coupled through two PHIL interfaces with high bandwidth power amplifiers, enabling comparison of results with those from the ideal system.

Reduced-Scale Models of Variable Speed Hydro-Electric Plants for Power Hardware-in-the-Loop Real-Time Simulations

Variable Speed Hydro-Electric Plant (VS-HEP) equipped with power electronics has been increasingly introduced into the hydraulic context. This paper is targeting a VS-HEP Power Hardware-In-the-Loop (PHIL) real-time simulation system, which is dedicated to different hydraulic operation schemes tests and control laws validation. Then, a proper hydraulic model will be the key factor for building an efficient PHIL real-time simulation system. This work introduces a practical and generalised modelling hydraulic modelling approach, which is based on ‘Hill Charts’ measurements provided by industrial manufacturers. The hydraulic static model is analytically obtained by using mathematical optimization routines. In addition, the nonlinear dynamic model of the guide vane actuator is introduced in order to evaluate the effects of the induced dynamics on the electric control performances. Moreover, the reduced-scale models adapted to different laboratory conditions can be established by applying scaling laws. The suggested modelling approach enables the features of decent accuracy, light computational complexity, high flexibility and wide applications for their implementations on PHIL real-time simulations. Finally, a grid-connected energy conversion chain of bulb hydraulic turbine associated with a permanent magnet synchronous generator is chosen as an example for PHIL design and performance assessment.

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.

Adaptive inertia emulation control for high‐speed flywheel energy storage systems

Low-inertia power systems suffer from a high rate of change of frequency (ROCOF) during a sudden imbalance in supply and demand. Inertia emulation techniques using storage systems, such as flywheel energy storage systems (FESSs), can help to reduce the ROCOF by rapidly providing the needed power to balance the grid. In this work, a new adaptive controller for inertia emulation using high-speed FESS is proposed. The controller inertia and damping coefficients vary using a combination of bang–bang control approaches and self-adaptive ones, to simultaneously improve both the ROCOF and the frequency nadir. The performance of the proposed adaptive controller has been initially validated and compared with several existing adaptive controllers by means of offline simulations, and then validated with experimental results. The proposed controller has been implemented on a real 60 kW high-speed FESS, and its performance has been evaluated by means of power hardware-in-the-loop (PHIL) testing of the FESS in realistic grid conditions. Both simulations and PHIL testing results confirm that the proposed inertia emulation control for the FESS outperforms several previously reported controllers, in terms of reducing the maximum ROCOF and improving the frequency nadir during large disturbances.

A Power Hardware-in-the-Loop Based Method for FAPR Compliance Testing of the Wind Turbine Converters Control

A task for new power generation technologies, interfaced to the electrical grid by power electronic converters, is to stiffen the rate of change of frequency (RoCoF) at the initial few milliseconds (ms) after any variation of active power balance. This task is defined in this article as fast active power regulation (FAPR), a generic definition of the FAPR is also proposed in this study. Converters equipped with FAPR controls should be tested in laboratory conditions before employment in the actual power system. This paper presents a power hardware-in-the-loop (PHIL) based method for FAPR compliance testing of the wind turbine converter controls. The presented PHIL setup is a generic test setup for the testing of all kinds of control strategies of the grid-connected power electronic converters. Firstly, a generic PHIL testing methodology is presented. Later on, a combined droop- anFd derivative-based FAPR control has been implemented and tested on the proposed PHIL setup for FAPR compliance criteria of the wind turbine converters. The compliance criteria for the FAPR of the wind turbine converter controls have been framed based on the literature survey. Improvement in the RoCoF and and maximum underfrequency deviation (NADIR) has been observed if the wind turbine converter controls abide by the FAPR compliance criteria.

Wide Frequency Band Single-Phase Amplitude and Phase Angle Detection Based on Integral and Derivative Actions

Numerous applications, such as the synchronization of distributed energy resources to an existing AC grid, the operation of active power filters or the amplification of signals for Power-Hardware-In-The-Loop (PHIL) systems require a few tasks in common. Amplitude, phase angle and frequency detection are crucial for all these applications and many more. Various techniques are presented for three-phase and single-phase applications but only a few of them are able to identify the signals’ attributes for a wide range of frequencies and amplitudes. Single-phase systems are typically burdensome, considering the challenge to create an internal signal, orthogonal with the input, in order to perform the phase angle detection. This matter is even more critical when the amplitude and frequency of the input signal varies in a wide range. This paper presents an Orthogonal Signal Generator (OSG) based on integral and derivative actions. It includes a detailed design procedure and a design example. The performance of a single-phase wide range amplitude and frequency detector based on the discussed OSG is experimentally validated under steady state and dynamic conditions.

Integration of SCADA Services and Power-Hardware-in-the-Loop Technique in Cross-Infrastructure Holistic Tests of Cyber-Physical Energy Systems

Cyber-physical energy system (CPES)—complex juxtaposition of multiple energy domains and communication and automation technologies—requires a sophisticated testing framework to achieve holistic assessment and validation, especially at large scale. In this article, the real supervision, control, and data acquisition (SCADA) system of CPES is integrated with the advanced techniques—real-time simulation (RTS) and power-hardware-in-the-loop (PHIL), in a cross-infrastructure manner to create a realistic validation environment for CPES. On the one hand, the method can be applied to extend the capacity of the infrastructures as well as creating a common resource and expertise pool. On the other hand, this approach combines the realistic data and advanced SCADA services with the flexibility of the RTS platform, which provides the possibility to emulate extreme and faulty scenarios with virtual equipment and topology. The proposed approach is demonstrated via a case study of analyzing the impact of communication on advanced voltage and frequency restoration in an isolated microgrid. The case study is implemented on two remote platforms PREDIS-PRISMES (70 km apart). The validation framework comprises the RTS and PHIL platform coupled with the OPC UA SCADA system in PRISMES, the control algorithm and the communication network simulator located in PREDIS platform.

Impedance Analysis and PHIL Demonstration of Reactive Power Oscillations in a Wind Power Plant Using a 4-MW Wind Turbine

This paper presents impedance-based analysis, mitigation, and power-hardware-in-the-loop (PHIL) demonstration of reactive power oscillations in a wind power plant using a 4-MW Type III wind turbine. Because such oscillations result from interactions among slower control loops of wind turbines regulating the phasor quantities—such as active power, reactive power, and the magnitude of voltages at the point of interconnection (POI)—we propose a new type of admittance called power-domain admittance for the analysis. The power-domain admittance of a wind turbine gives the transfer function from the frequency and magnitude of the voltages at the POI to the active and reactive power outputs of the turbine. The power-domain admittance responses of the 4-MW wind turbine are measured using a 13.8-kV/7-MVA grid simulator to identify the source of the reactive power oscillations. The power-domain impedance analysis and PHIL experiments are performed to explain how a resonant mode inside the wind turbine manifests as turbine-to-turbine and plant-to-grid reactive power oscillations. It is discovered that higher grid impedance during weak grid conditions improves damping of reactive power oscillations between a wind power plant and the grid; however, a higher grid impedance also increases coupling among wind turbines and the risk of turbine-to-turbine reactive power oscillations. The efficacy of a simple droop-based solution in mitigating reactive power oscillations is demonstrated using power-domain impedance measurements and PHIL experiments. The paper presents power-domain impedance measurement as a powerful tool for the analysis and mitigation of reactive power oscillations in wind power plants.

Robust High-Rate Secondary Control of Microgrids With Mitigation of Communication Impairments

The dynamic response speed of the secondary control (SC) in microgrids (MGs) is limited both by the communication network time delays and the bandwidth (BW) of the primary control (PC). By increasing the PC BW, which depends on how to design and tune the PC, the time delay issues will be multiplied due to the speed range of the communication modules and technologies. This article proposes a communication compensation block (CCB) to enhance the robustness of distributed SC against communication impairments in the MGs operated at the higher BW. The proposed method mitigates malicious time delays and communication non-ideality in distributed networked controls employed in the secondary layer of the MG by prediction, estimation and finally decision on transmitted data. A comprehensive mathematical model of the employed communication network is presented in details. Then, a robust data prediction algorithm based on the temporal and spatial correlation is applied into the SC to compensate for time delays and data packet loss. Furthermore, the effect of the number of stored packets and burst packet loss on the average success rate of the communication block, and the small signal stability analysis of the system in the presence of the CCB are investigated. Power hardware-in-the-loop (PHiL) experimental tests show the merits and applicability of the proposed method.

Distributed Power Hardware-in-the-Loop Testing Using a Grid-Forming Converter as Power Interface

This paper presents an approach to extend the capabilities of smart grid laboratories through the concept of Power Hardware-in-the-Loop (PHiL) testing by re-purposing existing grid-forming converters. A simple and cost-effective power interface, paired with a remotely located Digital Real-time Simulator (DRTS), facilitates Geographically Distributed Power Hardware Loop (GD-PHiL) in a quasi-static operating regime. In this study, a DRTS simulator was interfaced via the public internet with a grid-forming ship-to-shore converter located in a smart-grid testing laboratory, approximately 40 km away from the simulator. A case study based on the IEEE 13-bus distribution network, an on-load-tap-changer (OLTC) controller and a controllable load in the laboratory demonstrated the feasibility of such a setup. A simple compensation method applicable to this multi-rate setup is proposed and evaluated. Experimental results indicate that this compensation method significantly enhances the voltage response, whereas the conservation of energy at the coupling point still poses a challenge. Findings also show that, due to inherent limitations of the converter’s Modbus interface, a separate measurement setup is preferable. This can help achieve higher measurement fidelity, while simultaneously increasing the loop rate of the PHiL setup.

A Comparison of DER Voltage Regulation Technologies Using Real-Time Simulations

Grid operators are now considering using distributed energy resources (DERs) to provide distribution voltage regulation rather than installing costly voltage regulation hardware. DER devices include multiple adjustable reactive power control functions, so grid operators have the difficult decision of selecting the best operating mode and settings for the DER. In this work, we develop a novel state estimation-based particle swarm optimization (PSO) for distribution voltage regulation using DER-reactive power setpoints and establish a methodology to validate and compare it against alternative DER control technologies (volt–VAR (VV), extremum seeking control (ESC)) in increasingly higher fidelity environments. Distribution system real-time simulations with virtualized and power hardware-in-the-loop (PHIL)-interfaced DER equipment were run to evaluate the implementations and select the best voltage regulation technique. Each method improved the distribution system voltage profile; VV did not reach the global optimum but the PSO and ESC methods optimized the reactive power contributions of multiple DER devices to approach the optimal solution.

An Investigation of Power-Hardware-in-the-Loop- Based Electric Machine Emulation for Driving Inverter Open-Circuit Faults

Power-hardware-in-the-loop (PHIL) simulations, in the form of machine emulation, are gaining popularity for electric drive system testing in transportation applications. Since there is a reduced chance of equipment damage with PHIL simulations, they are particularly suited for the testing of electric drive systems, for drive inverter faults. This article investigates PHIL simulations to emulate machine behavior in the event of drive inverter faults resulting due to a gate-drive failure of one or more switches. Modifications are proposed to the closed-loop current control of the machine emulator system. These modifications ensure that various lower order harmonics in the emulated machine currents, resulting due to the drive inverter faults, are accurately emulated. Drive inverter damages resulting due to machine behavior emulation in fault conditions are avoided by proposing appropriate emulator control structure modifications. Experimental results are obtained for the emulation of machine behavior for various drive inverter gate-drive faults. These experimental results are compared with that obtained from a faulty drive inverter feeding a prototype machine. A close match between the two results is shown to prove the sufficiency and utility of the proposed controller modifications in the developed PHIL-based machine emulation system.

Enhanced Computation Performance of Photovoltaic Models for Power Hardware-in-the-Loop Simulation

For power hardware-in-the-loop (PHIL) simulation, a real-time simulator has to complete the target model calculations in a real-time manner without overrun errors. However, a photovoltaic (PV) simulation model contains a nonlinear equation that demands a numerical method with long computation time. Therefore, the complexity and operating condition of the target PV model are limited to reduce its computation burden. Besides, it sacrifices the model accuracy and limits the simulation scenario of the PHIL simulation. In this article, the PV simulation model employing an effective initial value selection method is proposed to enhance the real-time simulation performance for the PV PHIL simulation. The proposed initial value selection method can reduce the number of iterations of the numerical method so that it can drastically reduce the computation time of the real-time simulation. Therefore, the PHIL simulation model can increase it complexity with a fixed time step. Moreover, the PV model can be scaled up with various operating conditions, which can increase the PHIL simulation accuracy. The accuracy and the performance of the proposed PV model are evaluated by the PHIL simulation results. The maximum number of the PV models for the target PHIL simulation is also discussed.

Reduced Scale PHIL Emulation Concepts Applied to Power Conversion Systems With Battery Storage

This article deals power hardware in the loop (PHIL) real-time simulation (also called emulation) of electric power conversion systems, especially the ones involving battery storage. A case study related to a wind turbine power conversion system hybridized with a lithium-ion battery illustrates the main concepts. A similitude reduction process is presented to scale the model parameters in order to fulfill the sizing of a reduced scale test bench allowing to achieve the PHIL real-time emulation. The original “time compression” concept is also reminded. However, the main contribution of this article is the comparison between two concepts of real-time emulated devices (RTED) depending on whether these concepts are based on a model (“model based” RTED) or on a physical image of the actual device (“image based” RTED). Advantages and drawbacks of both concepts are discussed based on experimental results.

Microgrid Controller Testing Using Power Hardware-in-the-Loop

Required functions of a microgrid become divers because there are many possible configurations that depend on the location. In order to effectively implement the microgrid system, which consists of a microgrid controller and components with distributed energy resources (DERs), thorough tests should be run to validate controller operation for possible operating conditions. Power-hardware-in-the-loop (PHIL) simulation is a validation method that allows different configurations and yields reliable results. However, PHIL configuration for testing the microgrid controller that can evaluate the communication between a microgrid controller and components as well as the power interaction among microgrid components has not been discussed. Additionally, the difference of the power rating of microgrid components between the deployment site and the test lab needs to be adjusted. In this paper, we configured the PHIL environment, which integrates various equipment in the laboratory with a digital real-time simulation (DRTS), to address these two issues of microgrid controller testing. The test in the configured PHIL environment validated two main functions of the microgrid controller, which supports the diesel generator set operations by controlling the DER, regarding single function and simultaneously activated multiple functions.

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.

Real-time test-bed system development using power hardware-in-the-loop (PHIL) simulation technique for reliability test of DC nano grid

Since various power sources such as renewable energy and energy storage systems (ESSs) are connected to the DC grid, the reliability of the grid system is significant. However, the configuration of an actual DC grids for testing the reliability of the grid system is inconvenient, expensive and dangerous. In this paper, a test-bed system made up of a 20-kW DC nano grid and a control algorithm considering an external grid based on power hardware-in-the-loop (PHIL) simulation are proposed to demonstrate the reliability of the DC grid. Using the PHIL simulation technique, target grids can be safely implemented with laboratory-level instruments and simulated by real-time simulators, which emulates grid operations that are similar to the actual grid. In addition, using the proposed control algorithm, the operations of grid-connected converters are demonstrated according to the grid-connected or islanding modes. Finally, the reliability of the simulated DC nano grid and the effectiveness of the grid-connected converter are verified using the PHIL simulation system with 3-kW prototype converters.

Leveraging National Laboratory Assets to Address Stability Challenges due to Declining Grid Inertia Using Geographically Distributed Electrical–Thermal Co-Emulation

Abstract Due to increased penetration of low-inertia resources into the electric grid, challenges are increasing for maintaining wide-area system stability. Grid stability assessment requires a faithful representation of the multiple-physics interaction at the system level, and timescales of interaction varying in orders of magnitude, from microseconds to seconds to several minutes. Along with the simulation-based techniques, hardware-in-the-loop (HIL), controller HIL, and power HIL techniques have been developed to better understand the emergent behavior of the system with emerging technologies. US National Laboratories have played a vital role in research and development to understand the behavior of individual technologies and devices integrated to the electric grid. Each national laboratory forwards a technological and strategic initiative tied core and enabling capabilities. Due to strategic, efficiency, and economic reasons, not all the labs have assets to conduct research on all technologies concomitantly, so it becomes crucial to integrate the labs across geographies to understand the interplay of different technologies together at the system level. This approach avoids duplication of the assets at different lab facilities and helps understand the integrated system behavior of various technologies representative of actual grid conditions by connecting multiple national labs. This paper talks about techniques of connecting three national laboratories to enable co-emulation of electrical–mechanical–thermal characteristics of devices and systems. Such an approach can be used to understand the dynamic and transient interaction of multi-physics in a system level, at-scale emulation using real-time simulation tools and techniques.