Salvatore D'Arco

Comparing the Dynamic Performances of Power Hardware-in-the-Loop Interfaces

This paper presents an evaluation of two interface design solutions for power hardware-in-the-loop experiments. The evaluation is performed according to four figures of merit that focus on both the dynamic and steady-state performances of the interface systems. An analytical process is then used to both compare the different interfaces and, more importantly, to define a design procedure based on an optimization problem. The primary advantage of the proposed approach is that the figures of merits can be calculated by accounting not only for the topology of the interface but also for the structure and characteristics of the controller adopted for the interface itself.

Equivalence of Virtual Synchronous Machines and Frequency-Droops for Converter-Based MicroGrids

Over the last decade, frequency-droop-based control schemes have become the preferred solution in microgrids dominated by power electronic converters. More recently, the concept of virtual synchronous machines (VSMs) has emerged as an effective method for adding virtual inertia to the power system through the control of power electronic converters. These two approaches have been developed in two separate contexts, but present strong similarities. In fact, they are equivalent under certain conditions, as demonstrated in this letter. Analysis of this equivalence provides additional physics-based insight into the tuning and operation of both types of controllers.

Virtual synchronous machines — Classification of implementations and analysis of equivalence to droop controllers for microgrids

The concept of Virtual Synchronous Machines (VSM) is emerging as an alternative approach for control of power electronic converters operating in the power system. One main motivation for applying VSM-based control is to achieve a simple approach for emulating the inertia effect of traditional synchronous machines. This paper provides a comprehensive literature review on VSM and a possible classification of the different schemes. In addition, the small-signal response of the inertia emulation characteristics of VSM-based control is proved to be equivalent to conventional droop-based control for standalone and microgrid operation of converters. Thus, the droop gain and the filter time constant of the power feedback in a droop controller can be directly related to the damping factor and the inertia constant of a Virtual Synchronous Machine. The derived results are providing additional physics-based insight into the operation and tuning of both types of controllers.

Identification and Small-Signal Analysis of Interaction Modes in VSC MTDC Systems

In this paper, a methodology is presented to identify and analyse interaction modes between converters in Voltage Source Converter Multi-Terminal High Voltage Direct Current (VSC MTDC) systems. The absence of a substantial level of energy stored in such power electronics based systems results in fast system dynamics, governed by electromagnetic phenomena. Moreover, interactions between converters are largely influenced by the control parameters and in general, an a-priori identification of interaction modes based on associated time constants is less straight-forward than in AC systems. Furthermore, the extent to which converters interact not only depends on the controller parameters, but is also influenced by the physical characteristics of the HVDC system. The methodology introduced in this paper is based on aggregated participation factors to distinguish between local modes, primarily associated with one terminal, and interaction modes involving multiple terminals. To illustrate the proposed methodology, the influence of droop control parameters, as well as DC breaker inductors, on the system dynamics and the participation of the terminals in system interactions are investigated for a three-terminal MTDC system.

Control system tuning and stability analysis of Virtual Synchronous Machines

Virtual Synchronous Machines (VSMs) have been introduced as a control concept for emulating the behavior of traditional synchronous machines with power electronic converters. This paper analyses a VSM control scheme where an outer loop for inertia emulation provides references for two cascaded voltage and current controllers. This scheme offers several advantages compared to alternative VSM implementations. However, the interactions between the cascaded control loops and the complex functional dependency of the system dynamics with respect to the controller parameters prevents the effective application of classical tuning methods to this scheme at low switching frequencies. Therefore, a tuning approach for the VSM guided by the eigenvalue sensitivity matrix of a linearized system model is proposed in the paper. The method is implemented in the form of an iterative optimization procedure enforcing stability of the system and ensuring that the system eigenvalues are moved away from critical locations. Numerical simulations in the time domain are presented to verify the improvement in dynamic performance of the system when tuned with the presented algorithm compared to the results achieved by a more conventional tuning method.

Small-signal state-space modeling of modular multilevel converters for system stability analysis

Small-signal state-space analysis of power system stability relies on linearized models where steady state operation corresponds to constant values of all state variables. Such models are available and commonly used for analysis of configurations with three-phase 2- or 3-level Voltage Source Converters (VSC). However, the Modular Multilevel Converter (MMC), which is emerging as a preferred topology for VSC-based HVDC transmission, is based on single-phase modules with internal capacitors experiencing double frequency voltage oscillations in steady state. Thus, well-established VSC models applied for small-signal stability studies of MMC-based HVDC systems will ignore the internal energy dynamics of the MMC. This paper presents a simplified model of an MMC HVDC terminal, suitable for small-signal linearization while including the aggregated effect of the internal energy dynamics, the internal circulating currents and the corresponding control loops. This model can be combined with models of ac and dc systems and is intended for including the average energy dynamics of MMC-based HVDC terminals in power system stability studies.

Virtual synchronous machine-based control of a single-phase bi-directional battery charger for providing vehicle-to-grid services

This paper presents a single-phase Virtual Synchronous Machine (VSM) and its possible application for providing Vehicle-to-Grid (V2G) services from domestic battery chargers of Electric Vehicles (EVs). In a VSM, the power converter is controlled to emulate the inertia and the damping effect of a synchronous machine. Thus, a VSM-based EV charger can contribute to the spinning reserve and frequency regulation of the power system. In case of grid outages, the VSM can seamlessly establish an islanded grid and supply local loads from the battery onboard the EV. In order to avoid the influence on the virtual inertia from power oscillations associated with a single phase circuit, the proposed control scheme relies on a virtual two-phase system for calculating active and reactive powers. The proposed VSM implementation is described in detail and its dynamic performances in grid-connected as well as standalone operation are demonstrated by numerical simulations and by laboratory experiments.

Automatic Tuning of Cascaded Controllers for Power Converters Using Eigenvalue Parametric Sensitivities

Control structures containing cascaded loops are used in several applications for the stand-alone and parallel operation of three-phase power electronic converters. Potential interactions between these cascaded loops and the complex functional dependence between the controller parameters and the system dynamics prevent the effective application of classical tuning methods in the case of converters operating with a low switching frequency. A tuning approach guided by the eigenvalue parametric sensitivities calculated from a linearized model of the converter and its control system is proposed in this paper. The method is implemented in the form of an iterative procedure enforcing the stability of the system and ensuring that the system eigenvalues are moved away from critical locations. Numerical simulations in the time domain are presented to verify the improvement in the dynamic performance of the system when tuned with the presented algorithm compared with a conventional rule-based tuning method.

A new architecture for low cost Power Hardware in the Loop testing of power electronics equipments

The paper presents a critical analysis of the current status in the field of power hardware in the loop testing. The authors first summarize their experience in the development of a platform for an industrial application. Starting from the results obtained in this previous project, the authors then illustrate the path towards a more standardized and user-friendly platform. The new platform architecture presented in this paper is supposed to achieve significant improvement both in the bandwidth of the hardware/software interface and in the quality of the user interface.

Implementation and analysis of a control scheme for damping of oscillations in VSC-based HVDC grids

This paper proposes a simple method for damping of oscillations in dc grids based on Voltage Source Converters (VSCs). The damping effect is achieved by a control loop for counteracting measured dc voltage oscillations by acting on the active current reference of the ac-side VSC controller. The design of the damping method is supported by participation factor analysis and parametric sensitivities of a small-signal model representing an investigated test case. This investigated system is consisting of a single HVDC converter station connected to a dc cable equivalent and an ac grid. The validity of the developed small-signal model is verified by comparison to a simulation model including nonlinear effects of the investigated converter configuration. The small-signal model is then used to analyze the stability and dynamic characteristics of the system with and without the proposed active damping and to identify a suitable tuning of the damping controller.