Frequency support using multi-terminal HVDC systems based on DC voltage manipulation

This paper investigates the use of multi-terminal HVDC systems to provide primary frequency support to connected AC networks via coordinated DC voltage manipulation. Control schemes for multi-terminal HVDC systems to allow redistribution of active power, based on the idea of “power priority” are proposed. Inertia response from DC connected large offshore wind farms can also be incorporated based on the detection of DC voltage derivation at the offshore converter terminal without the need for telecommunication between the DC terminals. Simulation studies based on a three - terminal HVDC system connecting one large wind farm and two separate AC networks validate the operation of the system during frequency events

Multi-port converter for medium and high voltage applications

This work presents a multi-port converter (MPC) that is well-suited for use as a hybrid hub in complex multi-terminal high-voltage direct current (MTDC) networks. The proposed MPC generates several and controllable DC voltages from a constant or variable input DC voltage or AC grid. Its operating principle is explained and corroborated using simulations and experimentations

A new hybrid multilevel thyristor-based DC-DC converter

The rapid growth in HVDC grids is becoming inevitable for long-distance power transmission. Therefore, the idea of interconnection between the point-to-point links becomes essential. However, these point-to-point connections face several challenges such as the requirement of DC fault blocking capability, interfacing of different grounding schemes, offering multi-vendor interoperability, and difficulty to achieve high DC voltage stepping. DC-DC converters are considered the optimum solution to tackle these challenges in DC grids interconnection. In this paper, a new hybrid modular DC-DC converter is proposed that achieves a low number of semiconductors, low losses, and cost in comparison to other DC-DC converters due to the utilization of thyristors. The new DC-DC converter consists of two hybrid MMC bridges connected through an isolating transformer. Each MMC bridge is comprised of half bridge submodules and bidirectional thyristors. Detailed mathematical analysis, design, and control are illustrated. A comparison is carried out between different topologies in terms of semiconductor count, power loss, and cost. Also, both simulation model and experimental test rig are built to validate the proposed hybrid modular DC-DC converter under different scenarios. Finally, another variant of the hybrid-thyristor based converter (version two) is proposed for multiport DC-Hub application to achieve DC fault blocking without turning off all connected bridges

Monolithic modular thyristor-based DC-Hub with zero reactive power circulation

The promising features of HVDC technology have led to the possibility of numerous renewable resources integration and enormous DC grids interconnection. In spite of the obstacles, these interconnections encounter such as the necessity to block DC faults, achieving isolation between different schemes, the ability to maintain power flow throughout different power flow profiles, and the interfacing with various infrastructures, the DC-Hub arises to overcome these interconnection obstacles being the excellent approach to enhance the DC grid capabilities. This paper proposes a new monolithic modular thyristor-based multilevel converter, which serves as the fundamental building block of the DC-Hub, offering advantages such as lower switch count, bidirectional power flow, and DC fault blocking capability. Moreover, a control algorithm, for zero reactive power circulation in the DC-Hub, is introduced. The proposed algorithm successfully mitigates the circulation of reactive power throughout the entire range of power flow. A comprehensive mathematical analysis, optimum design of converter parameters, and the proposed control technique, which suppress the circulating reactive power at full range of power flow, are illustrated. Finally, simulation modelling and hardware test rig are established to validate the claims of the DC-Hub at different normal and faulty scenarios

New analysis of VSC-based modular multilevel DC-DC converter with low interfacing inductor for hybrid LCC/VSC HVDC network interconnections

The integration of multiterminal hybrid HVDC grids connecting LCC- and VSC-based networks faces several technical challenges such as DC fault isolation, ensuring multi-vendor interoperability, managing high DC voltage levels, and facilitating high-speed power reversal without interruptions. The two-stage DC-DC converter emerges as a key solution to address these challenges. By implementing the modular multilevel converter (MMC) structure, the converter's basic topology includes half-bridge sub-modules on the VSC side and full-bridge sub-modules on the LCC side. However, while this topology has been discussed in the literature, its connection to an LCC-based network with controlled current magnitude lacks detailed analysis regarding operational challenges, control strategies under various scenarios, and design considerations. This paper fills this gap by providing comprehensive mathematical analysis, design insights, and control strategies for the modular DC-DC converter to regulate DC voltage on the LCC-HVDC side. Additionally, the proposed control scheme minimizes the interfacing inductor between the two bridges, ensuring uninterrupted power flow during reversal and effective handling of DC faults. Validation through Control-Hardware-in-the-Loop testing across diverse operational and fault scenarios, along with a comparative analysis of different converters, further strengthens the findings

Limitations of voltage source converter in weak ac networks from voltage stability point of view

A weak ac grid could be envisioned as a voltage source behind a large synchronous impedance; thus the phase and magnitude of its terminal voltage vary significantly with the magnitudes of active and reactive powers it sinks or sources. Such characteristics present formidable challenges to control and stability of voltage source converters in weak ac grids. Therefore, this paper presents a theoretical analysis which establishes the maximum active power transfer that the voltage source converter (VSC) can exchange with a weak ac grid. The presented analysis reveals that the maximum active power that the VSC can exchange with a weak ac grid is determined by voltage stability limit. The validity of the presented theoretical analysis is confirmed by simplified simulation, and further corroborated by detailed simulations from a VSC model, developed in MATLAB-SIMULINK that incorporates all necessary control systems

A Novel Time-of-Use Pricing Based Energy Management System for Smart Home Appliances: Cost-Effective Method

Smart grids (SG) allow users to plan and control device usage patterns optimally, thereby minimizing power costs, peak-to-average ratios (PAR), and peak load demands. The present study develops a typical framework of a home energy management system (HEMS) for SG scenarios using newly limited and multi-limited planning approaches for domestic users. Time-of-use pricing (TOUP) is used to develop, handle, and manage the optimization problem properly. As a capable method for optimizing the proposed problem, this paper uses a robust meta-heuristic algorithm named wind-driven optimization algorithm (WDOA) and compares it to the other optimization algorithms in order to demonstrate its efficiency. In addition, it integrates a rooftop photovoltaic (PV) system with the system in order to show that all devices are cost-effective if managed properly. Eight diverse case studies are analyzed using a variety of time planning algorithms. The simulation results advocate for the quality and high performance of the proposed model by minimizing the total cost and managing energy consumption economically

Customized converter for cost-effective and DC-fault resilient HVDC Grids

This paper presents a comprehensive study that aims to establish the meaningful range for the ratios of full- bridge cells to the total number of cells per arm, in which the performance of the mixed cells modular multi-level converter (MC-MMC) can be customized by trading the number of FB cells for high value features such as: resiliency to DC faults, reduced capital costs of DC circuit breakers (DCCBs) and running costs of the semi-conductor, and continued operation. The MC-MMC design for particular ratio of FB cells to total number of cells per arm to achieve tailored features at system level is termed as an customized modular multilevel converter (CMMC). The primary motivation for the CMMC is to facilitate the use of low-cost and relative slow mechanical DC circuit breakers, with fault isolation times in the order of 8 ms to 12.5 ms. With these objectives to be achieved, interruption of power flows across the DC grid and surrounding AC grids must be minimized. It has been found that after certain ratios of FB cells to total cells per arm, the CMMC starts to exhibit current limiting mode, which helps to reduce DCCBs current breaking capacities and extend critical fault clearance times for remote converters from the fault point. Results of the pole-to-ground and pole-to-pole DC faults, semiconductor loss studies, and extended control range indicate that the presented CMMC is promising for future realization of DC grids

Improved damping control method for grid-forming converters using LQR and optimally weighted feedback control loops

Power grid pattern is expected to evolve from generator-based power systems towards converter-based systems in the forthcoming decades. Therefore, grid-forming converters will be pertinent to interconnected power grids in pursuance of enhancement their resilience against disturbances. This paper introduces a new efficient damping control method for grid-forming converters that provides a smooth power modulation and an efficient damping response against frequency and voltage deviations. First, an averaged state-space representation for a grid forming application in dq synchronization frame is derived. Based on this model, a new hybrid damping controller, including the concept of state feedback control and PI control, is proposed to address the main issues in existing controllers. The state feedback controller is optimally designed using a linear-quadratic regulator (LQR) approach to optimize the system performance. Moreover, the PI controller is optimally designed using the pattern search algorithm. The proposed damping control method integrates optimally between the control loops through a mapping matrix to rapidly synchronize with the grid and efficiently damp the oscillations. Simulations are carried out to prove the proposed method robustness. Finally, a comparative study using controller hardware-in-the-loop (CHiL) is employed against conventional system to validate the proposed damping method

Robust damping and decoupling controller for interconnected power network based on distributed energy resources

Converter based distributed energy resource (DER) units are intended to be integrated into the power system using grid-forming converters (GFCs). It proposes to emulate the synchronous machines dynamics and forms the grid voltage/frequency by instantaneously balance the load changes without peer-to-peer coordination. However, the damping and coupling characteristics of GFCs in multi-vendor interoperability-based network have not yet been fully explored. The paper proposes a step-by-step analytic method based on participation factor analysis to efficiently analyze the oscillatory modes and the coupling characteristics in the GFCs-based network, especially where the GFCs are controlled by different control techniques. Moreover, to efficiently damp these oscillatory modes and such control coupling, a hybrid damping method based on an oscillation damper and a decoupling controller is proposed. A mathematical modelling is derived to confirm the role of the proposed decoupling controller especially during transient periods. Design guidelines using pattern search algorithm are presented to identify the optimum parameters of the proposed hybrid damping method. Finally, the experimental results using Controller-hardware-in-the-loop (CHiL) verify the theoretical analysis and the efficient damping against large disturbances