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

Variable speed pumped storage hydropower for integration of wind energy in isolated grids : case description and control strategies

This paper presents the use of variable speed pumped storage hydropower plants for balancing power fluctuations from wind power in an isolated grid. A topology based on a synchronous machine and a full scale back-to-back voltage source converter is suggested for obtaining variable speed operation of a pump-turbine unit. This topology has not been previously investigated for variable speed pumped storage power plants, but can now be considered relevant for small and medium sized pumped storage units because of the development of voltage source converter drives for higher voltage levels. A possible case for implementation of such a system is described based on the situation on the Faroe Islands, where controllable energy storage can help to allow for a higher share of renewable energy in the power system and by that to reduce the dependency on fossil fuels. Power control of the pumped storage unit by load following for direct compensation of the fluctuations in power output from a wind farm will limit the influence on the operation of the rest of the grid. By utilizing the pumped storage to take part in the primary frequency control of the power system, the frequency response to other changes in production or load will also be improved.reviewe

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

Bisection Algorithm based Indirect Finite Control Set Model Predictive Control for Modular Multilevel Converters

In this work, an idea based on the bisection algorithm is used to reduce the computational burden of indirect finite control set model predictive control (FCS-MPC) for modular multilevel converters (MMCs). The proposed method greatly reduces the search space for reaching the optimal insertion index (number of submodules to be inserted). Therefore, the strategy proposed offers similar steady-state and dynamic performance compared to full indirect FCS-MPC at a much lower computational burden. A new cost function is also proposed for indirect FCS-MPC which eliminates the need for an outer loop or additional control of differential current to regulate the summation voltages in each arm. The results of the proposed strategy are validated through simulations in MATLAB/Simulink.acceptedVersio

Operation-based Reliability Assessment of Shore-to-Ship Charging Systems Including On-Shore Batteries

In this paper, an operation-based reliability assessment framework is proposed for Shore-to-Ship Charging (S2SC) systems including On-Shore Batteries (OSB). The OSB is considered to support the grid under fast charging loads. By the proposed approach, the impact of operational planning on reliability is identified. The main operational parameters considered in the reliability analysis include the charging load power and the charging- and discharging scheduling of the OSB. A hierarchical reliability framework is established where the failure rates of the components are estimated based on the FIDES methodology for physics-of-failure-based reliability prediction. Then, a dynamic failure threshold is introduced to translate the component failure consequences to the system performance into three states – failed, normal, and de-rated operation. Hence, the failure threshold is obtained for a specific set of operational and system design parameters. Additionally, to benchmark the characteristics of the SoC profiles of the OSB, an operation-based battery lifetime analysis is conducted. The evaluation of system-level reliability and on-shore battery lifetime is carried out for a 4MW dc S2SC system with a specified range of operation parameters. The results show that batteries and the IGBTs in the power electronics converters are the most reliability-critical elements. Moreover, it is apparent from the results that adjustments to the OSB power profile planning can potentially improve the reliability of the system for specific system sizing. It is also found that the OSB lifetime can be extended up to 2.5 times by increasing the capacity by 50 % and keeping the SoC close to around 50%.Operation-based Reliability Assessment of Shore-to-Ship Charging Systems Including On-Shore BatteriesacceptedVersio

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

A multi‐layer framework for energy efficiency assessment of shore‐to‐ship fast charging systems including onshore batteries

This paper proposes a three-layer framework for energy efficiency evaluation of Shore-to-Ship Charging (S2SC) systems using load-dependent loss models of the components. The considered S2SC system is supplied by the grid but is also supported by On-Shore Batteries (OSB). The presented approach is then used to investigate the impact of the specific design and operational parameters on energy efficiency. Power system architectures for three general S2SC solutions for ac, dc, and inductive charging are defined and compared in terms of energy efficiency. Operational parameters are also considered in the analysis, namely, the grid power ratio, determining the load sharing between the grid and the OSB, as well as the OSB charging profile. A case study is performed with peak charging power of 1 MW, and the most efficient S2SC solutions are identified for both ac- and dc-based onboard power systems. Moreover, it is shown that charging OSB with the highest available power from the grid between the charging breaks would often lead to higher energy efficiency than the maximum utilization of the available charging time. Field data from a real S2SC system is used to verify the estimated energy efficiency by the proposed framework. The analysis of the real case S2SC is then extended to include and verify a projected OSB. © 2022 The Authors. IET Electrical Systems in Transportation published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology.A multi‐layer framework for energy efficiency assessment of shore‐to‐ship fast charging systems including onshore batteriespublishedVersio

Position Locking for Permanent Magnet Synchronous Machine Propeller Drives in Drones by Hall-Effect Sensor-Assisted Nonlinear Observer

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.Position Locking for Permanent Magnet Synchronous Machine Propeller Drives in Drones by Hall-Effect Sensor-Assisted Nonlinear ObserveracceptedVersio

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