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Paper accepted for publication in JESTPE https://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=6245517 Virtual synchronous machines (VSMs) have become popular solutions for grid-tied inverters, suited to work on AC-grids with an increasing share of distributed energy sources and storage. However, just like real synchronous machines, VSMs may experience power oscillations. Recently, a new damping concept called virtual friction (VF) has been proposed that makes use of real time communication between several inverters. VF introduces an additional damping torque in the swing equation of the VSM, proportional to the deviation of the virtual rotor frequency from the center of inertia (COI)-frequency. VF can apply high damping without imposing a strong response in the output power of the VSM during frequency transients. However, VF requires the transmission of frequencies over a communication network, which may suffer from transmission delays. We give an experimental proof of concept of VF both in an isolated microgrid and in grid connected operation of a 45kVA setup consisting of three VSMs realized by three-phase two-level IGBT-based inverters1. We consider continuous constant and discontinuous varying time delays and show theoretically and experimentally under what conditions delays can impact the output powers of the VSMs. Further, we show how to correctly estimate the COI-frequency with transmission delays using timestamps to align individual signals.

Virtual synchronous machines (VSM) are inverters that behave towards the power grid like synchronous generators. Hence, they can be used as grid forming inverters and they can support the grid with inertia, droops, fault ride-through and more. Synchronverters are a much studied type of VSM. In this paper we present several improvements and innovations to the synchronverter control algorithm. We offer a complete design procedure for this algorithm. One change is meant to mitigate the fact that earlier designs are very sensitive to grid voltage measurement errors and processing delay, which may cause harmonic distortion and fluctuating amplitude of the grid-side currents. We propose to include a fast current controller as the internal control loop of the inverter. The design of this current controller involves delicate issues of compensating the delays and eliminating the DC components of the currents. The new design enables also a natural way for distortion-less current limitation. We present a smooth start?up procedure. Our simulations and experiments show that the current controller leads to a dramatic reduction of the sensitivity of the VSM to measurement errors, processing delay and grid voltage imbalance.

We prove that virtual friction control can stabilize a power grid containing several virtual synchronous machines (VSMs), connecting line impedances and loads. Virtual friction is a torque added to the swing equation of each VSM, proportional to the deviation of its frequency from the overall center of inertia (COI) frequency. Our analysis is based on the network reduced power system (NRPS) model. We support our results with simulations for a two-area network of four VSMs, looking at the transients induced by a change of tie-line impedance and an asymmetric load change. We compare the results for the NRPS model with the corresponding results using detailed models of synchronverters and line impedances. We find that virtual friction has a strong stabilizing effect both for the NRPS model and for the full grid model.

We prove that virtual friction can stabilize a power grid containing several virtual synchronous machines (VSMs), connecting line impedances and loads. Virtual friction is a torque added to the swing equation of each VSM, proportional to the deviation of its frequency from the overall center of inertia (COI) frequency. Our analysis is based on the network reduced power system (NRPS) model. We support our results with simulations for a two-area network of four VSMs, looking at the transients induced by a change of tie-line impedance and an asymmetric load change. We compare the results for the NRPS model with the corresponding results using detailed models of synchronverters and line impedances. We find that virtual friction has a strong stabilizing effect both for the NRPS model and for the detailed model. Furthermore we show simulation results on the influence of time delays in the communication between the inverters. This communication is used to compute the COI frequency. It turns out that communication delays of less than 100ms have practically no effect on the overall system.

Measurements of the lift coefficient history obtained from a NACA 0018 airfoil undergoing pitching maneuvers with blowing-type active flow control were used to obtain low-dimensional models for con...

Wind-tunnel experiments were conducted on two symmetric airfoils with different thickness ratios as a test of the ability of Goman–Khrabrov-type models to predict the lift coefficient history during pitching maneuvers. The primary difference between the two airfoils was that the thin airfoil had no hysteresis in its lift curve during quasi-steady maneuvers, but static hysteresis was observed with the thick airfoil over a range of 16

This article belongs to a Topical Collection of articles entitled Extreme Flow Workshop 2014. Guest editors: I. Marusic and B. J. McKeon.

Wind tunnel experiments were conducted on two symmetric airfoils with different thickness ratios as a test of the ability of Goman-Khrabrov-type models to predict the lift coefficient history during pitching maneuvers. The primary difference between the two airfoils was that the thin airfoil had no hysteresis in its lift curve during quasi-steady maneuvers, but static hysteresis was observed with the thick airfoil over a range of 16 < α < 22. Both airfoils exhibited dynamic hysteresis in the lift coefficient when the wing was pitching. The existence of static hysteresis had a strong effect on the lift response during periodic pitching, and must be accounted for in the model. A modified version of the Goman-Khrabrov model was introduced that captured the static hysteresis behavior and the large-scale features of the dynamic hysteresis for the thick airfoil. Large-scale variations in the lift trajectory at different pitching frequencies are reproduced by the modified model, even when the thick airfoil is in a deep stall condition. Some deviations between model and experiment are observed at the highest angles of attack, which are attributed to the dynamic stall vortex and trailing edge vortex shedding.

Dynamic hysteresis appearing in the lift force during pitching maneuvers is distinctly different from conventional static hysteresis. The size and shape of dynamic hysteresis loops are dependent on the degree of flow attachment, the dimensionless pitching frequency, and two time delays associated with the flow separation process. A linearized version of the Goman–Khrabrov model is derived and shown to capture the dynamic hysteresis characteristics when the pitching amplitude is small. Closed-loop control using a linearized version of the Goman–Khrabrov model is demonstrated, which incorporates a disturbance model into the feed-forward controller. The controller is shown to reduce the dynamic hysteresis during periodic pitching, step-up and step-down maneuvers, and quasi-random pitching maneuvers.

The synchronverter algorithm is a way to control a switched mode power converter that connects a DC energy source to the AC power grid. The main features of this algorithm are frequency and voltage droops as well as synthetic inertia, so that the inverter resembles a synchronous generator (SG). Many versions of this algorithm have been proposed and tested, but all share the same “basic control algorithm”, which is based on the equations of a SG. We analyze the sensitivity of the output currents of a synchronverter, with respect to the measurement errors. We show that some of the sensitivity functions exhibit high gains at the relevant frequencies, leading to distorted grid currents, which makes the use of this inverter control algorithm problematic. We then do a similar analysis assuming that we have controlled current sources available at the grid output of the converter, that we control using virtual currents generated in the algorithm. The virtual currents are flowing through virtual output inductors, that we can choose to be significantly larger than the actual output inductors. We show that using the current sources reduces the sensitivity considerably, thus indicating a better approach to synchronverter design.