Real-time optimization-based reference calculation integrated control for MMCs considering converter limitations

© 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksThe paper addresses a real-time optimization-based reference calculation integrated with a control structure for Modular Multilevel Converters (MMC) operating under normal and constrained situations (where it has reached current and/or voltage limitations, e.g. during system faults). The algorithm prioritizes to satisfy the Transmission System Operators (TSO) AC grid current demanded set-points. The constrained optimization problem is formulated based on the steady-state model of the MMC, whereby the prioritization is achieved through distinct weights defined in the Objective Function’s (OF) terms. The resultant optimization problem, however, is highly nonlinear requiring high computation burden to be solved in real-time. To overcome this issue, this paper applies a Linear Time-Varying (LTV) approximation, where the nonlinear dynamics of the system are represented as constant parameters, while a Linear Time-Invariant (LTI) system is used to formulate the optimization constraints. The converter's current references are determined in real-time by solving a constrained linearized optimization problem at each control time step, considering the TSO's demands, the current MMC operating point and its physical limitations. Finally, the linearized-optimization problem is integrated with the MMC controllers and evaluated under different network conditions, where the results indicated that method can be potentially employed to obtain the MMCs current references.Peer ReviewedPostprint (author's final draft

A fault detection and location algorithm for the LVDC interconnection network in rural area

Low voltage DC (LVDC) microgrids (MGs) can be linked together through an interconnection network to enhance the utilization of their energy resources in remote locations, particularly in rural low-income areas. However, the identification of the fault is challenging due to the fast fault transients and equipment limitations, where there are no sensors and DC circuit breakers (DCCBs) in the lines. To solve this problem, this article proposes a fault detection and location algorithm without requiring extra sensors and DCCBs in lines. The proposed algorithm uses the sensors of the interface converters to detect the fault. Following this, a coordinated current injection method is used to identify the faulty element by coordinating converters with disconnectors. This process employs two strategies “weight check” and “scope check” to minimize the time and the number of actions. The algorithm is robust to various fault impedance, fault types and network topology modifications. The effectiveness of the algorithm is validated through a series of simulation case studies

Implementation of model predictive control strategies in grid-tied power electronic voltage source converters

This thesis explores a different approach in the use of MPC applied to grid-tied converters, which aims to improve their performance, either during abnormal operating conditions or by exploiting the full potential of the converter in steady-state. The design of the MPC is firstly introduced in a single-phase converter. Although this device is mostly used in low voltage and low power applications, this analysis brings insight into the design of more complex converter topologies. Besides, the study of the single-phase converter presents interesting challenges due to the pulsating power of the single-phase system. It is found that the nonlinear behaviour of the converter, even at a high level of abstraction, leads to a formulation of the MPC that requires the use of nonlinear solvers. Consequently, the time-to-resolution of the optimisation problem becomes cumbersome, and a strategy to reduce the overall computation burden is sought. Given that the steady-state operation of grid-tied converters is based on sinusoidal trajectories, a simplification approach where the converter is linearised over LTV trajectories is introduced. This approach is not common in the control of power converters but it has been successfully implemented in other fields such as in the automotive industry. The results obtained prove the good performance of this approach and the controller is validated in a laboratory test-bench. The benefits of the MPC strategy applied to a MMC-HVDC converter are also discussed. The algorithm is applied to a single-phase structure to facilitate its analysis. The results considering HVDC-scaled parameters suggest potential benefits in terms of hardware optimisation. It is shown that the predictive controller can enhance the operating area of the converter by naturally combining different known harmonic injection techniques. Finally, this document also presents a preliminary investigation of the coordination of the different curtailment mechanisms integrated into a RE system using an MPC algorithm.Open Acces

Fault Detecting and Isolating Schemes in a Low-Voltage DC Microgrid Network from a Remote Village

Fault detection and isolation are important tasks to improve the protection system of low voltage direct current (LVDC) networks. Nowadays, there are challenges related to the protection strategies in the LVDC systems. In this paper, two proposed methods for fault detection and isolation of the faulty segment through the line and bus voltage measurement were discussed. The impacts of grid fault current and the characteristics of protective devices under pre-fault normal, under-fault, and post-fault conditions were also discussed. It was found that within a short time after fault occurrence in the network, this fault was quickly detected and the faulty line segment was efficiently isolated from the grid, where this grid was restored to its normal operating conditions. For analysing the fault occurrence and its isolation, two algorithms with their corresponding MATLAB/SIMULINK platforms were developed. The findings of this paper showed that the proposed methods would be used for microgrid protection by successfully resolving the fault detection and grid restoration problems in the LVDC microgrids, especially in rural villages

Experimental validation of a single phase Intelligent Power Router

Current power networks are facing numerous challenges in transmission and distribution in order to accommodate distributed generation from renewables. As a result, the current grid needs to evolve towards a system with more control and resiliency. The Intelligent Power Router device, located at strategic nodes, permits to add novel functionalities. This device allows to fully control the power flow by the means of Voltage Source Converters. In this article, the operation modes of the Intelligent Power Router are proposed and discussed. Moreover, this paper presents the design of a single phase Intelligent Power Router, simulations that prove its capability to control the power flow; and finally, an experimental validation of the device operation is also offered.Peer ReviewedPostprint (published version

Experimental validation of a single phase Intelligent Power Router

Current power networks are facing numerous challenges in transmission and distribution in order to accommodate distributed generation from renewables. As a result, the current grid needs to evolve towards a system with more control and resiliency. The Intelligent Power Router device, located at strategic nodes, permits to add novel functionalities. This device allows to fully control the power flow by the means of Voltage Source Converters. In this article, the operation modes of the Intelligent Power Router are proposed and discussed. Moreover, this paper presents the design of a single phase Intelligent Power Router, simulations that prove its capability to control the power flow; and finally, an experimental validation of the device operation is also offered.Peer Reviewe