Blake Lundstrom

A Power Hardware-in-the-Loop Platform With Remote Distribution Circuit Cosimulation

This paper demonstrates a novel cosimulation architecture that integrates hardware testing using power hardware-in-the-loop (PHIL) techniques with larger-scale electric grid models using off-the-shelf non-PHIL software tools. This test bed for distributed integration enables utilities to study the impacts of emerging energy technologies on their system and manufacturers to explore the interactions of new devices with existing and emerging devices on the power system, both without the need to convert existing grid models to a new platform or to conduct in-field trials. This paper describes an implementation of this architecture for testing two residential-scale advanced solar inverters at separate points of common coupling (PCCs). The same hardware setup is tested with two different distribution feeders (IEEE 123 and 8500 node test systems) modeled using GridLAB-D. In addition to simplifying testing with multiple feeders, the architecture demonstrates additional flexibility with hardware testing in one location linked via the Internet to software modeling in a remote location. In testing, the inverter current, real and reactive power, and PCC voltage are well captured by the cosimulation platform. Testing of the inverter advanced control features is currently somewhat limited by the software model time step (1 s) and tested communication latency (24 ms). These limitations could be overcome using faster modeling and communication within the same cosimulation architecture.

An overview of real time hardware-in-the-loop capabilities in digital simulation for electric microgrids

This paper explores and presents the capabilities of real time hardware-in-the-loop (RT-HIL) digital simulation for electric microgrids. RT-HIL is used by academia and industry for the simulation and testing of power systems at both the component and system levels. A literature survey is presented to identify existing real time simulation systems and some relevant applications. Simulations using RT-HIL involve modeling the interface between the components and systems in greater detail for increased understanding of interface issues. This is aimed at de-risking and hastening the penetration of new technologies such as microgrids.

Implementation and validation of advanced unintentional islanding testing using power hardware-in-the-loop (PHIL) simulation

Unprecedented investment in new renewable power (especially solar photovoltaic) capacity is occurring. As this new generation capacity is interconnected with the electric power system (EPS), it is critical that their grid interconnection systems have proper controls in place so that they react appropriately in case of an unintentional islanding event. Advanced controls and methods for unintentional islanding protection that go beyond existing standards, such as UL 1741 and IEEE Std 1547, are often required as more complex high penetration photovoltaic installations occur. This paper describes the implementation, experimental results, and validation of a power hardware-in-the-loop (PHIL)-based platform that allows for the rapid evaluation of advanced anti-islanding and other controls in complex scenarios. The PHIL-based approach presented allows for accurate, real-time simulation of complex scenarios by connecting a device under test to a software-based model of a local EPS. This approach was validated by conducting an unintentional islanding test of a photovoltaic inverter, as described in IEEE 1547.1, using both PHIL and discrete hardware-based test configurations. The comparison of the results of these two experiments demonstrates that this novel PHIL-based test platform accurately emulates traditional unintentional islanding tests. The advantage of PHIL-based testing over discrete hardware-only testing is demonstrated by completing an IEEE 1547.1 unintentional islanding test using a very precisely tuned resonant circuit that is difficult to realize with discrete hardware using PHIL.

Evaluation of system-integrated smart grid devices using software- and hardware-in-the-loop

This paper presents a concise description of state-of-the-art real-time simulation-based testing methods and demonstrates how they can be used independently and/or in combination as an integrated development and validation approach for smart grid DERs and systems. A three-part case study demonstrating the application of this integrated approach at the different stages of development and validation of a system-integrated smart photovoltaic (PV) inverter is also presented. Laboratory testing results and perspectives from two international research laboratories are included in the case study.

Viability and analysis of implementing only voltage-power droop for parallel inverter systems

In microgrids that are predominantly resistive, real and reactive power can be controlled by implementation of voltage and frequency droop laws respectively. However, the variable frequency displayed by such a system complicates analysis such that design approaches rely on approximations and linearized models. In this work, we present a modified form of droop control where only the voltage versus real power relationship is upheld and the frequency is held constant. Since the frequency is not explicitly controlled and the reactive power is not measured, the controller can be simplified. In such a setting, the only assumption we make is that all inverters have access to a common time-reference. Because fixed frequency operation is enforced by design, a variety of analytical tools can be leveraged to formulate a comprehensive analytical framework which facilitates a precise design methodology. In particular, closed-form expressions on the output current phase differences are obtained which yield practical selection guidelines on the voltage-power droop gains such that reactive flows between inverters are kept small. As a corollary, it is demonstrated that there are no reactive power flows in the presence of purely resistive loads. For the particular case of a single inverter, an almost exact solution describing the nonlinear dynamics of the inverter output voltage, current, and power are derived. Accompanying simulation results validate the analytical results and demonstrate the feasibility of the proposed control approach.

Life prediction model for grid-connected Li-ion battery energy storage system

Lithium-ion (Li-ion) batteries are being deployed on the electrical grid for a variety of purposes, such as to smooth fluctuations in solar renewable power generation. The lifetime of these batteries will vary depending on their thermal environment and how they are charged and discharged. To optimal utilization of a battery over its lifetime requires characterization of its performance degradation under different storage and cycling conditions. Aging tests were conducted on commercial graphite/nickel-manganese-cobalt (NMC) Li-ion cells. A general lifetime prognostic model framework is applied to model changes in capacity and resistance as the battery degrades. Across 9 aging test conditions from 0°C to 55°C, the model predicts capacity fade with 1.4% RMS error and resistance growth with 15% RMS error. The model, recast in state variable form with 8 states representing separate fade mechanisms, is used to extrapolate lifetime for example applications of the energy storage system integrated with renewable photovoltaic (PV) power generation. Uncertainty quantification and further validation are needed.

Examining system-wide impacts of solar PV control systems with a power hardware-in-the-loop platform

High penetration levels of distributed solar PV power generation may lead to adverse power quality impacts. Advanced inverter control schemes have the potential to mitigate many power quality concerns. However, interactions between local closed-loop controls may lead to unintended behavior in deployed systems. To study the performance of advanced control schemes in a detailed distribution system environment, a test platform has been developed that integrates Power Hardware-in-the-Loop (PHIL) with concurrent time-series electric distribution system simulation. In the test platform, GridLAB-D, a distribution system simulation tool, runs a detailed simulation of a distribution feeder in real-time mode at the Pacific Northwest National Laboratory (PNNL) and supplies power system parameters at a point of common coupling. At the National Renewable Energy Laboratory (NREL), a hardware inverter interacts with grid and PV simulators emulating an operational distribution system. The platform is described and initial test cases are presented.

Learning Exact Topology of a Loopy Power Grid from Ambient Dynamics

Estimation of the operational topology of the power grid is necessary for optimal market settlement and reliable dynamic operation of the grid. This paper presents a novel framework for topology estimation for general power grids (loopy or radial) using time-series measurements of nodal voltage phase angles that arise from the swing dynamics. Our learning framework utilizes multivariate Wiener filtering to unravel the interaction between fluctuations in voltage angles at different nodes and identifies operational edges by considering the phase response of the elements of the multivariate Wiener filter. The performance of our learning framework is demonstrated through simulations on standard IEEE test cases.

A fuzzy-logic subsumption controller for Home Energy Management Systems

Home Energy Management Systems (HEMS) are controllers that manage and coordinate the generation, storage, and loads in a home. These controllers are increasingly necessary to ensure that increasing penetrations of distributed energy resources are used effectively and do not disrupt the operation of the grid. In this paper, we propose a novel approach to HEMS design based on behavioral control methods, which do not require accurate models or predictions and are very responsive to changing conditions. We develop a proof-of-concept behavioral HEMS controller and show by simulation on an example home energy system that it capable of making context-dependent tradeoffs between goals under challenging conditions.

A high-speed, real-time visualization and state estimation platform for monitoring and control of electric distribution systems: Implementation and field results

Continued deployment of renewable and distributed energy resources is fundamentally changing the way that electric distribution systems are controlled and operated; more sophisticated active system control and greater situational awareness are needed. Real-time measurements and distribution system state estimation (DSSE) techniques enable more sophisticated system control and, when combined with visualization applications, greater situational awareness. This paper presents a novel demonstration of a high-speed, real-time DSSE platform and related control and visualization functionalities, implemented using existing open-source software and distribution system monitoring hardware. Live scrolling strip charts of meter data and intuitive annotated map visualizations of the entire state (obtained via DSSE) of a real-world distribution circuit are shown. The DSSE implementation is validated to demonstrate provision of accurate voltage data. This platform allows for enhanced control and situational awareness using only a minimum quantity of distribution system measurement units and modest data and software infrastructure.