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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.

The virtual infinite capacitor (VIC) is a nonlinear active capacitor for dc voltage filtering (ripple elimination). It can replace large and unreliable electrolytic capacitors, and it is intended for dc systems where random fluctuations of the dc bus voltage may occur. We propose a plug-and-play (PnP) realization of the VIC, which can be connected directly to the dc bus like a passive capacitor without any extra connections. The proposed control algorithm automatically adjusts the reference voltage of the VIC so that it will be equal to the equilibrium voltage of the dc bus. For the stability of complex interconnected systems, it is recommended that the impedance of the components should be strictly positive real. By studying the output impedance of PnP VIC, we discuss how to achieve the strictly positive-real property of the overall dc bus by a careful choice of controllers and filters. The results were tested by simulations and experiments on a circuit comprising a commercial power factor compensator with an average of 390 V output and a variable resistive load (up to 345 W).

Synchronverters are inverters that mimic the behavior of synchronous generators. In this paper, we propose five modifications to the synchronverter algorithm from the paper “Synchronverters: Inverters that mimic synchronous generators,” ( IEEE Trans. Ind. Electron. , vol. 58, no. 4, pp. 1259–1267, Apr. 2011), to improve its stability and performance. These modifications are implemented in software and do not require any changes in the inverter hardware. The first two modifications concern the control of the virtual field current in the synchronverter so that it is more robust to faults. We prove the stability of the grid-connected synchronverter with this improved field current controller. The third modification is to increase the effective size of the filter inductors virtually. This is motivated using results from the stability analysis of a constant field current synchronous generator connected to an ac power grid and also by practical considerations. Simulations and experiments show that this leads to a much better response to changes in grid frequency, voltage, or to imbalance in the grid. The fourth modification is to change the formula for the (virtual) nominal active mechanical torque to take into account the (virtual) losses in the output impedance of the converter. This makes the tracking of the desired active power much more accurate. The fifth modification is to introduce virtual capacitors in series with the inverter outputs to filter spurious dc components from the current supplied to the grid.

We study the optimal control for the maximization of profit in a grid-connected energy storage system. The changing price of electricity is assumed to be known in advance over the optimization horizon. The system has a storage device (such as a battery) which we model as a capacitor-type device, with natural constraints on its voltage and current. We prove the existence of an optimal control and the fact that a bang-off-bang type policy is optimal. The proof uses Filippov's theorem and the Pontryagin minimum principle.

This paper is about the state feedback regulator problem for infinite-dimensional linear systems. The plant, assumed to be an exponentially stable regular linear system, is driven by a linear (possibly infinite-dimensional) exosystem via a disturbance signal. The exosystem has its spectrum in the closed right half-plane and also generates the reference signal for the plant output. The regulator problem is to design a controller that, while guaranteeing the stability of the closed-loop system without the exosystem, drives the tracking error to zero. A particular version of this problem is the state feedback regulator problem in which the states of the exosystem and the plant are known to the controller. Under suitable assumptions, we show that the latter problem is solvable if and only if a pair of algebraic equations, called the regulator equations, is solvable. We derive conditions, in terms of the transfer function of the plant and eigenvalues of the exosystem, for the solvability of the regulator equations. Three examples illustrating the theory are presented.

In this paper, the idea of operating an inverter to mimic a synchronous generator (SG) is motivated and developed. We call the inverters that are operated in this way synchronverters. Using synchronverters, the well-established theory/algorithms used to control SGs can still be used in power systems where a significant proportion of the generating capacity is inverter-based. We describe the dynamics, implementation, and operation of synchronverters. The real and reactive power delivered by synchronverters connected in parallel and operated as generators can be automatically shared using the well-known frequency- and voltage-drooping mechanisms. Synchronverters can be easily operated also in island mode, and hence, they provide an ideal solution for microgrids or smart grids. Both simulation and experimental results are given to verify the idea.

We consider coupled systems consisting of a well-posed and strictly proper (hence regular) subsystem and a finite-dimensional subsystem connected in feedback. The external world interacts with the coupled system via the finite-dimensional part, which receives the external input and sends out the output. Under several assumptions, we derive well-posedness, regularity, exact (or approximate) controllability and exact (or approximate) observability results for such coupled systems.

D-class amplifiers use pulsewidth modulation to transform an input signal u into a rectangular signal p whose duty cycle linearly depends on the input signal u. After low-pass-filtering p to suppress the switching frequency and its harmonics, we obtain the output signal y that is a good approximation of u. This kind of an amplifier is attractive (as compared with the classical A-class and B-class amplifiers) due to its high efficiency. A buck power supply may be regarded as a particular instance of a D-class amplifier. While the operating principle of the D-class amplifier is intuitively clear, a rigorous mathematical analysis of this system is not trivial, as the amplifier is time-varying and nonlinear. We derive a rigorous proof of the fact that the signal p is a good approximation of u in the low-frequency range, and we provide precise error bounds for this approximation.

We show that the state of a strictly output passive system with an L(2) input converges to zero. The result is applied to the disturbance rejection problem (with reference signal zero), where the disturbance can be decomposed into a finite superposition of sine waves of arbitrary but known frequencies and an L(2) signal. Using an LTI controller, constructed based on the internal model principle, the state trajectories of the plant (and hence also the error signal) converge to zero.

Three-phase four-wire inverters, with either three-leg or four-leg topology, are useful for interfacing distributed generation to networks of unbalanced loads, but neither of the available circuit topologies is ideal. The split-link three-leg topology (with six switches) suffers from poor DC voltage utilization compared with the four-leg topology (with eight switches). The four-leg topology has an electromagnetic compatibility (EMC) difficulty because it imposes large-amplitude high-frequency voltages between the DC-link busbars and ground. To obtain both good dc voltage utilization and good EMC performance, it is proposed to use a split-link inverter with an active balancing circuit (also eight switches). The balancing circuit is used to modulate the DC busbar offset voltage to make better use of the available DC-link voltage. The optimum voltage term is established to be a third harmonic term, and the DC voltage utilization is improved. A deadbeat controller supplemented with a repetitive controller is designed to give good tracking and good disturbance rejection for the busbar offset voltage. System performance is studied through an experimental test rig.

This paper investigates the integral input-to-state stability (iISS) property for passive nonlinear systems. We show that under mild technical assumptions, a passive nonlinear system which is globally asymptotically stable is also iISS. Moreover, the integral term from the definition of the iISS property has a very simple form (like an L 1 norm). We illustrate the result by proving that the drive-train of a wind turbine with quadratic torque control is iISS.

This paper proposes a voltage controller design method for DC-AC converters supplying power to a microgrid, which is also connected to the power grid. This converter is meant to operate in conjunction with a small power generating unit. The design of the output voltage controller is based on H/sup /spl infin// and repetitive control techniques. This leads to a very low harmonic distortion of the output voltage, even in the presence of nonlinear loads and/or grid distortions. The output voltage controller contains an infinite-dimensional internal model, which enables it to reject all periodic disturbances which have the same period as the grid voltage, and whose highest frequency components are up to approximately 1.5 kHz.

The controller in a pulse-width-modulation (PWM) power converter has to stabilize the system and guarantee an almost constant output voltage in spite of the perturbations in the input voltage and output load over as large a bandwidth as possible. Boost and flyback power converters have a right-half-plane zero (RHPZ) in their transfer function from the duty cycle to the output voltage, which makes it difficult to achieve the aforementioned goals. Here, the authors propose to design a controller using H/sup /spl infin// control theory, via the solution of two algebraic Riccati equations. The almost optimal H/sup /spl infin// controller is of the same order as the converter and has a relatively low DC gain. The closed-loop characteristics of a typical low-power boost power converter with four different control schemes were compared by computer simulation. The H/sup /spl infin// control was found to be superior in a wide frequency range, while being outperformed by the others at extremely low frequencies. Good agreement was found between simulation results and experimental measurements.