Multi-converter Power Systems Research Articles

Small-Signal Synchronous Stability of a New-Generation Power System With 100% Renewable Energy

The traditional power system dominated by synchronous generators (SGs) is now evolving into a new-generation power system dominated by voltage source converter (VSC)-based renewable energy, causing substantial changes in the dynamical behavior. This paper investigates the small-signal synchronous stability of a new-generation power system composed solely of grid-following and/or grid-forming power electronics devices in the absence of an infinitely strong bus. The phase-lock-loop (PLL)-based VSC is taken as a representative of grid-following devices and treated as a controlled-current source whose phase is driven by the power factor angle. In sharp contrast, the virtual synchronous generator (VSG)-based VSC is taken as a representative of grid-forming devices and treated as a controlled-voltage source whose phase is driven by the active power. Small-signal synchronization stability models are established for multiconverter power systems within the framework of the classical Phillips–Heffron model for traditional power systems. Explicit expressions for equivalent inertia, damping, and synchronization coefficients are obtained. We find that both PLL-based and VSG-based VSCs show similar synchronization principles and contribute to the inertia and damping of the system. All these findings are well supported and verified by our modal analysis and time-domain simulations.

Stability Analysis of Multi-Converter Power Systems with Angle Feedback

This paper presents a stability analysis for inverter-based renewable resources in multi-converter power systems. Stability conditions are obtained based on the port-Hamiltonian structure of the droop control in grid-forming converters. We show that conventional grid-forming converters have a port-Hamiltonian structure in the multi-converter power system. A simple and direct estimation of the region of attraction is obtained. In addition, we proposed droop control with an angle feedback that ties two converters in order to improve transient response. This control maintains the port-Hamiltonian structure and hence the stability properties of the conventional droop. Numerical experiments show the accuracy of the model and the superior performance of the proposed control.

Small-Signal Stability Boundary of Heterogeneous Multi-Converter Power Systems Dominated By the Phase-Locked Loops’ Dynamics

While renewable resources are increasingly integrated into the electric power system, the small-signal instability risk may be induced by grid-following converters using phase-locked loops (PLLs) for grid synchronization, especially in low short-circuit strength systems. The stability analysis complexity is further increased in such a heterogeneous multi-converter system (HMCS), where multiple converters may have different PLLs’ control parameters provided by various vendors. To understand how the different PLLs’ dynamics collectively affect the small-signal stability of the HMCS, we transform the HMCS into an equivalent homogeneous multi-converter system for the small-signal stability analysis. Then, we analytically derives the small-signal stability boundary condition of the HMCS dominated by the PLLs’ dynamics (HMCS-DPLL). The derived stability boundary allows us to obtain analytical results about how the small-signal stability of the HMCS-DPLL is collectively affected by network structure, converter operating conditions, and different PLLs’ control parameters. Based on the stability boundary condition, a computationally efficient method is also proposed to identify the design rationality of PLL control parameters as well as the small-signal stability and stability margin of the HMCS-DPLL. The analytical results and proposed method are validated by modal analysis and electromagnetic transient simulation with the full-order models on a 9-converter heterogeneous system.

Frequency Synchronization of a High-Order Multiconverter System

We investigate the stability properties of a multi-converter power system model, defined on a high-order manifold. For this, we identify its symmetry (i.e., rotational invariance) generated by a static angle shift and rotation of AC signals. We characterize the steady state set, primarily determined by the steady state angles and DC power input. Based on eigenvalue conditions of its Jacobian matrix, we show asymptotic stability of the multi-converter system in a neighborhood of the synchronous steady state set by applying the center manifold theory. We guarantee the eigenvalue conditions via an explicit approach. Finally, we demonstrate our results based on a numerical example involving a network of identical DC/AC converter systems.

Stability Analysis and Location Optimization Method for Multiconverter Power Systems Based on Nodal Admittance Matrix

This article presents an impedance-based method for stability analysis of multiconverter power systems based on the generalized Nyquist stability criterion. The return-ratio matrix of the system is formulated on the basis of the nodal admittance matrix and output admittance of the converters, which can be applicable for large-scale systems with complicated structure. A modified IEEE 13-bus system is used as the testing system, which shows that the proposed method can be easily implemented for location optimization of the converters. And the participation factors (PFs) of the return-ratio matrix’s critical eigenvalues [which encircle the critical point (−1, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0 j$ </tex-math></inline-formula> )] at the instability frequencies are used to identify which converters have significant contributions to the instability, and then guidance for the design of system parameters can be provided. Simulations in PSCAD/EMTDC validate the effectiveness of the proposed stability analysis method.

Analysis of small signal model of DC/DC converter without source and load effects

The small signal model of DC/DC converter without source and load effects is established and analyzed in detail. Characteristics of input and output impedance of converter in frequency domain in the model are applicable to the stability analysis and design in DC multi-converter power system. Investigation shows that the resonant characteristics of input and output impedance under closed-loop control are closely related to the crossover frequency and phase margin of control loop. The input impedance resonant valley and output impedance resonant peak locate at the same frequency as the loop crossover frequency. Furthermore, when the phase margin decreases, the resonance valley of the input impedance becomes lower and the resonance peak of the output impedance becomes higher. Finally, the accuracy of the model is verified by MATLAB Simulink time domain simulation.

Small-Signal Stability of Multi-Converter Infeed Power Grids with Symmetry

Traditional power systems have been gradually shifting to power-electronic-based ones, with more power electronic devices (including converters) incorporated recently. Faced with much more complicated dynamics, it is a great challenge to uncover its physical mechanisms for system stability and/or instability (oscillation). In this paper, we first establish a nonlinear model of a multi-converter power system within the DC-link voltage timescale, from the first principle. Then, we obtain a linearized model with the associated characteristic matrix, whose eigenvalues determine the system stability, and finally get independent subsystems by using symmetry approximation conditions under the assumptions that all converters’ parameters and their susceptance to the infinite bus (Bg) are identical. Based on these mathematical analyses, we find that the whole system can be decomposed into several equivalent single-converter systems and its small-signal stability is solely determined by a simple converter system connected to an infinite bus under the same susceptance Bg. These results of large-scale multi-converter analysis help to understand the power-electronic-based power system dynamics, such as renewable energy integration. As well, they are expected to stimulate broad interests among researchers in the fields of network dynamics theory and applications.

Electromagnetic compatibility assessment in multiconverter power systems – Conducted interference issues

Abstract An assessment of electromagnetic compatibility is important from a technical as well as legal point of view. The harmonized standards should provide detailed guidelines enabling assessment of electromagnetic compatibility for the typical application of devices in systems. This paper presents the specific issues accompanying measurements of conducted electromagnetic interference in multiconverter systems. The theoretical considerations have been confirmed by experimental results obtained in a laboratory and in a real 1 MW photovoltaic power plant. The presented theoretical and experimental results might constitute the recommendations for practitioners dealing with conducted interference measurements, while the proposed approach can be used as a basis for the elaboration of reliable standards for electromagnetic compatibility assessment in multiconverter systems within the scope of the studied interference frequency range.

Transient Stability Assessment of Multi-Machine Multi-Converter Power Systems

Transmission faults caused by recent wildfires in California have induced the disconnection of utility-scale converters in photovoltaic (PV) power plants. Postmortem investigations reported that tripping commands were caused by phase-locked loops (PLLs) and dc-side dynamics, which are typically unmodeled in classical transient stability studies. Since existing simulation packages rely on simplified models that neglect these dynamics, they have a limited capability to predict converter behavior during faults. To address this shortcoming, we set forth a positive-sequence model for PV power plants that is derived from physics and controls first principles. As seen on utility-scale three-phase converters, the model includes PLLs, dc-side dynamics, and closed-loop controllers. Instances of the developed model are integrated into illustrative power systems containing conventional generators. Numerical simulations of the obtained multi-machine multi-converter power systems are assessed via a suitable set of stability and performance metrics.

Parallel processing and genetic algorithm‐based efficiency optimization for multiconverter power system

An efficient optimization approach based on parallel processing and genetic algorithm for a multiconverter power system is presented in this paper. The system consists of several clients and one server. The genetic algorithm helps the system to reach maximum efficiency over the entire load range. Meanwhile, WiFi is employed to achieve high‐speed data transfer. Also, a benefit from the parallel processing and structure is that the optimization time has been exponentially reduced. The experimental results show that the prototype can achieve optimal efficiency over the entire load range, and the optimizing process is accelerated five times in average because of the parallel processing © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

Robust sliding‐mode control of dc/dc boost converter feeding a constant power load

Tightly regulated power electronic converters show negative impedance characteristics and behave as a constant power load (CPL) which sink constant power from their input bus. This incremental negative impedance characteristics of tightly regulated point-of-load converters in multi-converter power systems have a destabilising effect on source converters and may destabilise the whole system. Similar phenomena also occur in many situations like dc microgrid, vehicular power system. Here, the authors present a robust pulse-width modulation-based sliding-mode controller for a dc/dc boost converter feeding the CPL in a typical dc microgrid scenario. A non-linear surface is proposed which ensures constant power to be delivered to the load. The existence of sliding mode and stability of the sliding surface are proved. The proposed controller is implemented using OPAL-RT real-time digital simulator on a laboratory prototype of dc/dc boost converter system. The effectiveness of the proposed sliding-mode controller is validated through simulation and experimental results under different operating conditions.

Multiconverter System Design for Fuel Cell Buffering and Diagnostics Under UAV Load Profiles

This paper presents a multisource, multiconverter power system for electrically propelled unmanned aerial vehicles (UAVs) with a focus on promoting fuel cell health. Linearized multiconverter system analysis and the two extra element theorem (2EET) inform a system design for fuel cell current buffering and integral diagnostics. Integral diagnostics is in situ impedance spectroscopy achieved by controlling the power system to superpose a frequency-swept excitation current at the fuel cell terminals. An experimental system demonstrates hybridization of a solid-oxide fuel cell (SOFC) with a lead-acid battery having suitable current buffering and integral diagnostics performance under UAV load profiles. Experimental behavior is demonstrated with an electrically emulated SOFC stack or “reference simulator.” Impedance spectroscopy data measured during run-time clearly indicate both degradation and recovery phenomena in the SOFC.

Start-Up Process and Step Response of a DC–DC Converter Loaded by Constant Power Loads

Constant power loads (CPLs) are very common in multiconverter power system applications. Buck converters loaded by CPLs have different start-up and step-response processes from the resistive load. This paper analyzes the start-up process and step response of an average current-controlled buck converter with CPLs based on the phase portrait method. Design criteria rules have been proposed to achieve system stability during start-up and load transient processes. The proper current-limiting value is proposed accordingly to avoid the instability during the start-up and load change dynamics. Simulation and experimental results verify the analysis and the proposed proper current-limit value.

An Analytical Investigation of DC/DC Power Electronic Converters With Constant Power Loads in Vehicular Power Systems

Stability of multi-converter power systems, which exist in advanced more electric vehicles, is of great importance. The stability issue is investigated in this paper, and design considerations and limitations of the methods that stabilize the open-loop converters are presented. By stabilizing the open-loop converter, it is shown that the feedback loop design is usually translated into a conventional feedback design task. The behavior of the unstable converter is also discussed, and a method for decreasing the amplitude of the output voltage oscillations is proposed. The model of a tightly regulated practical converter is presented. This model is used to decide how much damping should be added to make the feeder converter stable. Furthermore, because we have this information about the load converter, the feeder converter can be redesigned so that it does not see the load converter as a constant power load.

Realization of Parasitics in Stability of DC–DC Converters Loaded by Constant Power Loads in Advanced Multiconverter Automotive Systems

Distributed DC power systems are becoming increasingly common in advanced automotive systems. Constant power load (CPL) behavior of tightly regulated DC-DC converters in multiconverter power systems is equivalent to the dynamic negative impedance, destabilizing the DC bus and, consequently, the system. Various techniques have been developed to control ideal/lossless converters loaded by CPLs. Loss components significantly enhance the stability of the cascaded converters. In this paper, the effects of parasitics in the behavior of DC-DC converters loaded by CPLs are investigated. Furthermore, the stability of power converters loaded by CPLs in parallel to conventional constant voltage loads in the presence of loss components is analyzed. Design criteria are presented for converter operation in continuous and discontinuous conduction modes, which gives recommendations on the design of multiconverter DC power systems to avoid negative impedance instability. The proposed stable operation criteria are validated with hardware prototypes and simulation analyses.

Multiconverter Power Systems and Their Applications

As the power systems evolve to contain more and more solid-state devices, a new type of power system emerges. We call this solid-state, silicon rich, silicon intensive, power electronics based, or, as is named in this paper, multiconverter power systems. In these systems, solid-state switching power converters are extensively used for generating, distributing, and utilizing of electrical energy throughout the system. Different converters, such as DC/DC choppers, DC/AC inverters, AC/AC cycloconverters, and AC/DC rectifiers, are used in source, load, and distribution subsystems to provide power at different voltage levels and both DC and AC forms. Most of the loads are also in the form of power electronic converters and motor drives. These systems have a broad variety of applications from small multiconverter power systems, such as automotive power systems, Electric and Hybrid Electric Vehicles (EV & HEV), advanced industrial electrical systems, telecommunications, and terrestrial computer systems with a few converters, to large systems, such as International Space Station (ISS), spacecraft, modern aircraft, submarine, and More Electric Ship (MES) power systems with many converters. Furthermore, these unconventional power systems have unique characteristics, dynamics, and stability problems that are just beginning to be appreciated. In this paper, we take a closer look at multiconverter power systems and address the fundamental problems faced in these systems. We also propose the multiconverter power systems as a new area of specialized research, which will have benefits in many applications.