Towards a Versatile Cyber Physical Power System Testbed: Design and Operation Experience

The present trends in the area of smartgrids indicate that future transmission and distribution systems will heavily rely on digital and on communication technologies to operate. Indeed, the power systems are evolving progressively towards what is denoted as a cyber-physical system. This transition challenges the classical approaches for experimental testing and requires the development of testing platforms for cyber-physical systems able to capture the interactions between physical components, control and monitoring software and the communication infrastructure. This paper presents general considerations and requirements for a cyber-physical testing platform for power systems. The paper provides also examples of a testing platform specifying the characteristics of the major components and a summary of the experience matured in its setup and configuration. Finally, an example of an experiment on a notional smartgrid and the related results are reported.acceptedVersio

Impedance-compensated grid synchronisation for extending the stability range of weak grids with voltage source converters

This paper demonstrates how the range of stable power transfer in weak grids with voltage source converters (VSCs) can be extended by modifying the grid synchronisation mechanism of a conventional synchronous reference frame phase locked loop (PLL). By introducing an impedance-conditioning term in the PLL, the VSC control system can be virtually synchronised to a stronger point in the grid to counteract the instability effects caused by high grid impedance. To verify the effectiveness of the proposed approach, the maximum static power transfer capability and the small-signal stability range of a system with a VSC HVDC terminal connected to a weak grid are calculated from an analytical model with different levels of impedance-conditioning in the PLL. Such calculations are presented for two different configurations of the VSC control system, showing how both the static power transfer capability and the small-signal stability range can be significantly improved. The validity of the stability assessment is verified by time-domain simulations in the Matlab/Simulink environment.Peer ReviewedPostprint (published version

Phase Angle Feed-Forward Control for Improving the Power Reference Tracking of Virtual Synchronous Machines

A method for improving the power reference tracking capability of Virtual Synchronous Machines (VSM) by phase angle feed-forward (PAFF) control is proposed in this paper. The presented implementation includes compensation for the ac power flow dynamics and can be applied to any VSM-based control relying on a virtual swing equation. Fast and accurate power reference tracking is achieved independently from the emulated inertia time constant, and without influencing the grid synchronization mechanism of the VSM. Therefore, the grid forming capability of the VSM and the inertial response to grid frequency variations are not influenced by the proposed PAFF control. The effectiveness of the derived feed-forward terms is first validated with a simplified VSM scheme. The performance is also thoroughly evaluated for a detailed VSM implementation including closed loop current control and a virtual impedance. This includes operation with different grid impedance values and studies of the sensitivity of the PAFF function with respect to parameter variations in the VSM control or deviations from the assumed grid impedance. Experimental results with a 50 kVA converter controlled as a VSM are presented as verification of the improvement in power reference tracking capability.Phase Angle Feed-Forward Control for Improving the Power Reference Tracking of Virtual Synchronous MachinesacceptedVersio

Generalized Voltage-based State-Space Modelling of Modular Multilevel Converters with Constant Equilibrium in Steady-State

This paper demonstrates that the sum and difference of the upper and lower arm voltages are suitable variables for deriving a generalized state-space model of an MMC which settles at a constant equilibrium in steady-state operation, while including the internal voltage and current dynamics. The presented modelling approach allows for separating the multiple frequency components appearing within the MMC as a first step of the model derivation, to avoid variables containing multiple frequency components in steady-state. On this basis, it is shown that Park transformations at three different frequencies ($+\omega$, $-2\omega$ and $+3\omega$) can be applied for deriving a model formulation where all state-variables will settle at constant values in steady-state, corresponding to an equilibrium point of the model. The resulting model is accurately capturing the internal current and voltage dynamics of a three-phase MMC, independently from how the control system is implemented. The main advantage of this model formulation is that it can be linearised, allowing for eigenvalue-based analysis of the MMC dynamics. Furthermore, the model can be utilized for control system design by multi-variable methods requiring any stable equilibrium to be defined by a fixed operating point. Time-domain simulations in comparison to an established average model of the MMC, as well as results from a detailed simulation model of an MMC with 400 sub-modules per arm, are presented as verification of the validity and accuracy of the developed model

Accuracy Analysis of a Dynamic State Estimator with a Hardware in the Loop Approach

State estimation is critical for the monitoring and control of power systems. The conventional approach for state estimation is based on quasi static models for the power systems and as such neglects relevant dynamic effects. These dynamic characteristics are accounted in what are referred as dynamic state estimators (DSE). Several implementations for DSEs have been proposed in literature but mostly focusing on the modelling and numerical aspects. In this paper a laboratory assessment of a DSE for estimating the internal variables of a generation plant is presented. The assessment is conducted with a Hardware in the Loop approach in a laboratory configuration that includes a PDC, real phasor measurement units (PMUs) operating with IEC 61850 sampled values and a real time model of the nordic power system. Experimental results show the behaviour and root mean square error of the estimator for a few different hardware implementations. Thus, it is assessed if the utilization of the IEC 61850 standard may have an impact on performance of the dynamic state estimation and it is quantified the effect on the accuracy.Accuracy Analysis of a Dynamic State Estimator with a Hardware in the Loop ApproachacceptedVersio

Intra-arm Balancing Control for Modular Multilevel Converter with Cell Loading Operated under High Power Imbalances

Multiport converters with a large number of ports have been explored for integrated generation units or batteries. An attractive candidate multiport converter has been developed based on the modular multilevel converter (MMC) with cell loading. Accordingly, this study proposes a control strategy for a multiport converter based on the MMC operated under high power imbalances among cells. Owing to this power imbalance, the multiport converter requires an additional circulating current (intra-arm balancing current) to provide balanced three-phase grid currents and balanced capacitor voltages in all cells. In a previous study, the minimum required intra-arm balancing current was calculated offline and applied according to the loading condition. This method may require a high-capacity memory for the controller. In contract, this study proposes an active control method for minimizing the intra-arm balancing current online. Experimental results reveal that the multiport converter achieves balanced three-phase currents with a total harmonic distortion of 3.13% and balanced capacitor voltages with an error of 0.1% under maximum power imbalance among cells. © 2024 The Institute of Electrical Engineers of Japan.Intra-arm Balancing Control for Modular Multilevel Converter with Cell Loading Operated under High Power ImbalancesacceptedVersio

Emlékállítás és felejtés. A „málenkij robot” emlékezete a szatmári sváboknál

Commemorative practices and oblivion mechanism. The memory of the deportations of the Satu Mare Swabians) In 1945, a series of violent events took place, bringing the greatest individual and collective trauma in the history of the Satu Mare Swabians. The paper presents the various communication situations in which deportation narratives were drafted and transmitted. I wanted to reveal all the channels in which the commemoration of the deportations (both private and public) took/take place, paying special attention to changes in commemoration forms, as historical phenomena, and also making a comparison between the approaches in the Swabian villages from Romania and Hungary. In commemorating practicesof the deportations I distinguished four levels: individual memory, memory of generations, communicative memory and collective memory, all four having different functions and contexts

P-HiL Evaluation of Virtual Inertia Support to the Nordic Power System by an HVDC Terminal

This paper provides an assessment of the effect from virtual inertia provided by an HVDC converter terminal on the Nordic power system. The analysis is based on results from Power-Hardware-in-the-Loop (P-HiL) tests with a laboratory-scale Modular Multilevel Converter (MMC) representing an HVDC terminal interfaced with a real-time phasor simulation of the Nordic grid. The applied control method for providing virtual inertia is utilizing the derivative of the locally measured grid frequency to adapt the power reference for the studied converter terminal. The power injection provided by the converter and the resulting impact on the frequency dynamics of the power system are investigated as a function of the emulated inertia constant and the frequency droop gain. The results demonstrate how the HVDC converter can effectively support the dynamic response of the power system when exposed to large load transients by improving the frequency nadir and reducing the Rate-of-Change-of-Frequency (ROCOF). Keywords: HVDC Transmission , Power-Hardware-in-the-Loop , Real-time Simulation , Virtual InertiaacceptedVersio

Windowed PWM: a Configurable Modulation Scheme for Modular Multilevel Converter Based Traction Drives

This article introduces a modulation technique for modular multilevel converter (MMC) in variable speed traction drives for electrical transportation referred as windowed pulsewidth modulation (W-PWM). The windowed PWM (W-PWM) is derived by blending the principles of operation of conventional modulation schemes for MMC based on the nearest level control (NLC) and on PWM with the aim of combining their inherent strengths and offering a higher degree of flexibility. This can reduce switching losses compared to classical PWM schemes and lower the current harmonic distortion compared to NLC schemes. The window in which the PWM is applied can be seen as an additional degree of freedom that allows a dynamic optimization of the performance of the traction drive depending on its operating characteristics. The performance of the W-PWM technique is assessed in this article for several operating conditions and compared with conventional schemes based on NLC and on the phase opposition disposition PWM with both numerical simulation and experimental verification on a small-scale prototype. Results demonstrate the flexibility of the W-PWM and its potential for applications in electrical traction drives. © 1986-2012 IEEE.acceptedVersio