This paper investigates how black-box representation of sub-systems in a larger small-signal state-space model influences the results from participation factor analysis. The results show that the value of individual participation factors associated with the states of a black-box sub-system on diagonalized form can be misleading. To avoid this issue, participation factor analysis involving a black-boxed sub-system should be based on aggregation by the complex sum of participation factors associated with the subsystem. The reason is that individual participation factors of the diagonalized sub-system can assume high values that do not reflect a physical interaction with other sub-systems. However, the individual participation factors of any transparent part of the system model can still be studied according to established conventional methods. These findings are obtained from, and demonstrated by, analysis of the small-signal dynamics in an offshore islanded AC grid with two HVDC converter terminals applying grid forming control and two equivalent grid-following converters representing separate wind farm clusters.

This paper presents a continuous control set model predictive control (CCS-MPC) method for reducing oscillations in an inductive power transfer (IPT) charging system with constant voltage load (CVL). The proposed power control method inherently reduced the output fluctuations by minimizing the error between the actual power and the reference power, which is expressed as a cost function. Then, by solving this optimization problem, the optimal pulse density can be obtained for pulse density modulation (PDM) techniques in order to adjust the average sending voltage. This optimal pulse density is applied to enhanced/pulse density modulation (E/PDM) based on delta-sigma modulator (DSM) approach. The proposed CCS-MPC based on EPDM regulates power transfer control to ensure high efficiency by maintaining the low current/power ripple, fast dynamic response and achieving zero voltage switching (ZVS) in a full operation range. To verify the performance and effectiveness of the proposed control method, a comparison between the proposed CCS-MPC and PI control methods based on different modulators has been tested in MATLAB/Simulink.

This paper presents a finite-control-set model predictive control (FCS-MPC) strategy for pulse density modulation (PDM) in inductive power transfer (IPT) systems. The main purpose of this method is to suppress current/power ripples while ensuring high efficiency and fast dynamic response. The proposed control method is based on a nonlinear model of the IPT system when charging a battery appearing as a constant voltage load (CVL) and considers all the possible switching states for each period within the control horizon. The switching states are obtained based on optimal decision variables at each sampling time which provide the minimum value of the cost function. This results in the skipping of voltage pulses, which ensures that zero voltage switching (ZVS) occurs over the full range of operation. Moreover, by predicting the future behaviour of the studied system, the proposed control method will inherently suppress oscillations that can be excited by the skipped voltage. As a result, the system's performance is improved with respect to high efficiency and fast dynamic response. The effectiveness and performance of the proposed control method are verified by the simulation results.

The high switching frequency of wireless power transfer systems makes soft switching important due to switching losses. In the current literature, numerous solutions have been proposed for maintaining soft switching within the entire operating range. However, this study shows that variable frequency control can cause loss of zero voltage switching (ZVS) during system transients, even if soft switching during steady-state operation is ensured. The causes of this phenomena are analyzed thoroughly and two approaches to address such issue are discussed. Firstly, a limitation on the changing rate of the frequency can alleviate the issue. Secondly, phase advance control could also be possible, although this approach requires further studies. Results from both simulations and laboratory experiments are presented to verify the analysis and the control method proposed to avoid losing ZVS during transient conditions.

The paper presents a hall-effect sensor-assisted non-linear observer-based solution for position locking of a surface-mounted permanent magnet synchronous motor (SMPMSM) propeller drive in drone applications. The purpose of the position locking is to ensure a fixed motor position at the landing instant to avoid mechanical damage to the propeller. To evaluate the proposed solution, the position locking sequence of the motor drive is studied for two cases, implemented with two different state machines. The first case is relying on an encoder to provide the position feedback signal and serves as a reference for assessing the performance of the proposed solution based on the position estimate from the hall-effect sensor-assisted nonlinear observer. Experimental results show how the proposed solution can provide sufficient performance of position locking without the encoder.

This paper proposes a hysteresis ON-OFF control method for inductive power transfer (IPT) systems with low output ripples and fast transient responses. System power is regulated by skipping pulses, thus achieving zero voltage switching over the full operating range. Moreover, each switching state is determined by a hysteresis comparator operating on the measured power. This prevents the skipped pulses from exciting poorly damped oscillations. The dynamic behavior of IPT systems with the hysteresis ON-OFF control method is excellent because the controlled state of the system is directly adjusted in every half cycle. The effectiveness and feasibility of the proposed method are validated by simulations and experimental results from a small-scale laboratory prototype.

In this paper, backstepping is applied as a first step of modulation control in the abc reference frame for modular multilevel converters (MMCs). In the second step, reduced indirect FCS-MPC is applied where the number of inserted modules are allowed to change by maximum one from the rounded result of the continuous outcome from backstepping. The backstepping method uses the ac-side current, differential current and summation of capacitor voltages in one arm as the state variables to form the Lyapunov functions. An established bilinear model of MMCs is used in the proposed design. The proposed approach offers similar dynamic performance as the full indirect FCS-MPC, at a much lower computational burden. The performance of the proposed method is validated by simulation.

This paper presents an experimental performance evaluation of different control strategies for providing virtual inertia from power electronic converters. The evaluation is based on a laboratory-scale prototype of a point-to-point HVDC transmission system with Modular Multilevel Converters (MMCs). Operation with a grid emulator connected to a realtime simulator reproduces the behavior of a power system and allows Power-Hardware-in-the-Loop (P-HiL) experiments. Control of the inverter terminal for providing virtual inertia to the simulated power system is evaluated with four different control system implementations. The four cases include a conventional control system enhanced with df/dt-based inertia emulation functionality and three different Virtual Synchronous Machine (VSM) implementations based on emulation of a synchronous machine swing equation. The dynamic response in the power flow and the frequency transients are evaluated for all cases, and the inertial energy exchanged with the isolated power system is assessed in comparison to a conventional power controller without inertia emulation. The results demonstrate that all the evaluated implementations can provide similar inertial response when operated in a small isolated grid, while clear differences in dynamic response and stability properties are revealed during operation under strong grid conditions.

Safety-critical power conversion systems play a major role in the paradigm shift towards more electric aircraft (MEA) architectures. This paper reviews the electrical machines and their power electronic systems that are currently competing in the application of integrated starter-generators (S/Gs) in MEA power systems. Motivated by the strict requirements of sufficient electrical starting capability, super-high power density and ultra-high reliability, additional considerations on the overall system design are necessary, including the power electronic converters (PECs) and integrated thermal designs. These aspects are discussed not only in the light of their many benefits but also of the challenges introduced by the continuous advancements and emerging innovations in the power conversion technology. In achieving the MEA goals and capitalize on all potential benefits, optimization-based design approaches will be necessary, where the aggregation of electric machines, PECs and the aircraft grid is considered as an integrated system to be optimized. This review highlights the importance of these aspects and offers a view on future perspectives and open issues.

This paper presents a comparative eigenvalue analysis of the stability characteristics and small signal dynamics of four different control strategies for Synchronous Machine Emulation (SME) by power electronic converters, considering a Synchronous Machine (SM) as the benchmark system. The four SME techniques are selected to represent the most established general approaches for emulating the inertial characteristics of SMs in the control of power electronic converters. The small-signal stability assessment is based on the analysis of system eigenvalues, including evaluation of participation factors and parametric sensitivities. All the investigated techniques can be tuned to obtain similar inertial dynamics under grid frequency variations, but exhibit differences in other small-signal characteristics due to the distinct control system implementations. Among the analyzed cases, the current-controlled virtual synchronous machine has the highest damping of the most oscillatory mode. However, the study shows that the most oscillatory modes of the other techniques are associated with the LCL impedance, and could be further attenuated by active damping techniques.

This paper presents a control system implementation in dqz-coordinates for equalizing the average energies stored in each arm of a Modular Multilevel Converter (MMC); a control objective that is typically referred to as horizontal and vertical energy balancing. The proposed control scheme is obtained from analysis and simplification of a detailed time-invariant dqz-frame state-space representation of the MMC. The state variables of the model are the equivalent arm capacitor energies and the current components, and it will be shown that this representation is very suitable for designing outer-loop energy controllers in dqz coordinates that rely on linear inner current control loops. Moreover, a series of justified assumptions on the energy dynamics will be presented, providing significant insight that simplifies the control design. Finally, by proving that the unbalances of the average values of the converter equivalent arm capacitor energies in abc coordinates appear as undesired oscillations in dqz coordinates, active filtering is proposed as a mean to dissipate them and, therefore, achieve the desired balanced operation. Operation of the proposed control strategy is demonstrated by time-domain simulation of a 1 GW MMC-based HVDC converter terminal.

Two different methods for implementing inertial damping on a Virtual Synchronous Machine (VSM) and their potential for attenuating power system oscillations when utilized in a VSC HVDC terminal are investigated in this paper. As a reference case, the VSM is considered with only a frequency droop providing damping in the virtual swing equation. Then, the effect of damping based on high-pass filtering of the virtual speed is compared to damping based on high-pass filtering of the measured grid frequency. A simplified model of a power system with two equivalent generators and a VSC HVDC terminal is introduced as a case study. Analysis of the small-signal dynamics indicates that damping based on the VSM speed has limited influence on the power system oscillations, while improved attenuation can be obtained by introducing damping based on the locally measured grid frequency. The presented analysis and the operation of the proposed VSM-based damping strategy is validated by numerical simulation of a 150 MVA VSM controlled VSC HVDC terminal connected to a dynamic model of the assumed grid configuration.

This paper discusses the implementation of proportional resonant (PR) current controllers for a Voltage Source Converter (VSC) with LCL filter which is synchronized to the grid by virtual flux (VF) estimation with inherent sequence separation. Even though there is an extensive amount of literature and studies on the PR current controller for tracking the current reference of a VSC in the stationary reference frame, there is no discussion taking into account voltage sensor-less operation based on virtual flux estimation with an LCL-filter. Separate estimation of the positive and negative sequence virtual flux components at the grid-side of the LCL-filter, as well as current sequence separation, using the Second Order Generalized Integrator-Frequency Locked Loop (SOGI-FLL) is presented as part of a proposed method. The LCL filter is characterized in order to reduce the parameter deviation that might affect the virtual flux estimation. The stability of the proposed method is analyzed in the frequency domain while the operation and performance of the proposed system is verified by simulation studies.

A generalized and versatile approach for control of the circulating currents of Modular Multilevel Converters (MMC) has been recently proposed by using Lagrange Multipliers in the ABC frame. This approach is capable of analytically obtaining the differential current references associated with the desired operating conditions imposed to the optimization procedure. Previous implementations of this control approach have been sensitive to harmonic distortion of the control signals when applied to systems with a low number of levels, and have depended on average value calculations of sinusoidal variables, slowing down the dynamics of the control and making the tuning of the control system difficult under certain conditions. This work demonstrates that the performance of the versatile Lagrange-based control can be improved by introducing adaptive filtering based on Second Order Generalized Integrators (SOGIs) for processing the system variables needed for calculating the circulating current references. In addition to improved control dynamics and easier tuning of system controllers, the use of the adaptive filters provides inherent capability for operation under frequency variations, and is therefore suitable for control of MMCs operating in weak grids as well as for high- or medium-voltage motor drives.

This work presents a simple approach to eliminate DC power ripple in a three-phase Modular Multilevel Converter (MMC) while operating with balanced (load) currents during unbalanced grid faults. In this study, the MMC is connected to an AC grid on one of its terminals, and to a constant ideal DC source on the other, while a single phase fault takes place. The presented “first” approach is based on the combined application of a Notch filter and a Resonant PI controller. The main idea is to eliminate the zero-sequence oscillations that occur in the MMC differential currents when the unbalanced fault takes place. This strategy may be applied jointly with a constant circulating current controller on the one hand or with a constant energy per phase (implying an injection of a second harmonic to the differential current), on the other. Moreover, the load currents are kept balanced even during the fault in order to demonstrate the potential application of the MMC as a power oscillation firewall of some sort; i.e., to simultaneously keep constant DC power and balanced load currents at all time.