• Energy Research

The circuit topology of a submodule (SM) in an modular multilevel converter (MMC) defines many of the functionalities of the complete power electronics conversion system and the specific applications that a specific MMC configuration can support. Most prominent among all applications for the MMC is its use in high-voltage direct current (HVDC) transmission systems and multiterminal dc grids. The aim of the paper is to provide a comprehensive review and classification of the many different SM circuit topologies that have been proposed for the MMC up to date. Using an 800-MVA, point-to-point MMC-based HVDC transmission system as a benchmark, the presented analysis identifies the limitations and drawbacks of certain SM configurations that limit their broader adoption as MMC SMs. A hybrid model of an MMC arm and appropriate implementations of voltage-balancing algorithms are used for detailed loss comparison of all SMs and to quantify differences among multiple SMs. The review also provides a comprehensive benchmark among all SM configurations, broad recommendations for the benefits and limitations of different SM topologies which can be further expanded based on the requirements of a specific application, and identifies future opportunities.

This paper proposes a general methodology for the development of power system models suitable for distributed real-time simulations (D-RTS) based on topology, simulator interfaces and data exchange. D-RTS have risen as functional alternatives that can combine remote and multi-vendor resources for large-scale power system simulations via a virtual connection. However, previous work focused on combination of separate models and not the performance of D-RTS when splitting a single monolithic model, failing to study the behaviors of D-RTS, including, synchronization and accuracy. The proposed methodology is used to develop distributed models of two widely used testbed large power systems, the IEEE Australian Benchmark model and the IEEE 300-Bus system. These testbeds are selected as they can be simulated as both monolithic and distributed models in available simulators in order to validate both the methodology and resulting model performance. The obtained results and comparison between monolithic and distributed models support the proposed approach and demonstrate the performance of D-RTS under both steady-state and transient operations in multiple scenarios.

Energy balancing of the alternate arm converter (AAC) is a major challenge and overlap period-based circulating current control methods are commonly used. Component parameter variation occurs due to tolerances, component aging, and internal faults leading to stored energy imbalances of modular voltage source converter (VSC) topologies. If not controlled, such variations can severely impact the operation of the AAC. Hence, component parameter variation is a key consideration for energy balancing of all sorts of modular VSC topologies, but has not been addressed in the existing literature for the AAC. This paper investigates the energy balancing capability of the AAC in high-voltage direct current (HVDC) applications under component parameter variations and proposes a compensation method to further improve its energy balancing performance. The effectiveness of the proposed method is verified in a real-time model of an AAC-HVDC transmission system from widely accepted existing CIGRE benchmark models.

The continuous growth of Renewable Energy (RE) in the grid reduces system inertia and damping, impacting the overall stability of power systems. To alleviate this issue, “virtual” inertia can be provided by power electronics converters. Under this functionality they are controlled to emulate the behavior of rotating machines, operating as Virtual Synchronous Generators (VSGs). However, grid-connected inverters are susceptible to voltage and frequency disturbances which negatively affect their performance. Therefore, this paper introduces a Unified Power Flow Controller (UPFC) with VSG functionality that can compensate for voltage variations at the point of common coupling and provide a constant voltage reference for the connection of remote RE systems. The detailed controlled strategy together with the small-signal analysis are also developed in this work. The UPFC-VSG is compared with an equivalent Static Synchronous Compensator (STATCOM) with centralized energy storage also under VSG control to illustrate the major benefits of the UPFC-VSG. Validation of the analysis and the proposed control method is provided through simulation of a UPFC-VSG and a STATCOM-VSG supporting the grid-connection of a 100-MVA RE system.

Abstract Load imbalances in the supply of passive loads with modular multilevel converter (MMC)-based medium (MVDC) and high voltage (HVDC) dc-links are inevitable. In such applications, the maximum-allowable voltage ripples and peak arm currents define the operating limits of the MMCs. This paper demonstrates the impact of circulating current control in capacitor voltage ripples of sub-modules and peak arm currents under unbalanced load conditions when supplying passive networks, comparing the effects of three circulating current control methods on internal variables of the MMCs. The efficacy of the circulating current control is validated through real-time digital simulations of an 800 MVA benchmark MMC-based HVDC transmission system.

This paper demonstrates the interoperability of an emerging alternate arm converter (AAC) with the state-of-the-art modular multilevel converter (MMC) in high-voltage direct current (HVDC) systems based on a hybrid VSC-HVDC system. The paper also showcases the parameter derivation of the hybrid HVDC system and its detailed control structure. The study provides preliminary steps towards detailed analysis of AAC interoperability in complex hybrid dc grid configurations. A detailed set of results based on the 800 MVA hybrid voltage source converter (VSC)-HVDC system showcases the interoperability performance of the AAC under different operating scenarios and verifies its associated control functions.

Estimation-based indirect dc-voltage control in MMCs interacts with circulating current control methods. This paper proposes an estimation-based indirect dc-voltage control method for MMC-HVDC systems and analyzes its performance compared to alternative estimations. The interactions between estimation-based indirect dc-voltage control and circulating current control methods, active/reactive power regulation are also investigated. The proposed method delivers similar performance to measurement-based direct dc-voltage control, regardless of the circulating current control method. Steady-state and transient performance is demonstrated using a benchmark MMC-HVDC transmission system, implemented in a real-time digital simulator. The results verify the theoretical evaluations and illustrate the operation and performance of the proposed indirect dc-voltage control method.