An MMC based HVDC system with optimized AC fault ride-through capability and enhanced circulating current suppression control

Modular multilevel converter (MMC) is a proven technology for HVDC applications due to its salient features such as modularity and excellent power quality. To ensure best possible grid support, recent grid codes require incorporating fault ride-through (FRT) strategies so that HVDC converter stations remain connected and maintain reliable operation under various symmetrical and asymmetrical AC faults. In this paper, a communication-free enhanced fault ride-through technique without the need of DC chopper has been proposed. The proposed FRT strategy ensures quick post fault recovery operation and can effectively manage DC link and capacitor voltages within safe limits. Along with proposed FRT strategy, in order to avoid high circulating current (CC) inside an MMC, this paper has proposed an optimal circulating current control approach based on proportional resonant and PI controllers in an abc reference frame. The suggested technique lowers the ripple in capacitor voltages while reducing the magnitude of the CC. Under both balanced and unbalanced ac grid conditions, the ripple in the dc link voltage is also reduced without the use of dual synchronous reference frame or any additional controllers. Simulation results confirm the effectiveness of the proposed FRT and CC suppression techniques for a 580-kV, 850-MW MMC-based HVDC system

Design, Control and Protection of Modular Multilevel Converter (MMC)-Based Multi-Terminal HVDC System

Even though today’s transmission grids are predominantly based on the high voltage alternating current (HVAC) scheme, interests on high voltage direct current (HVDC) are growing rapidly during the past decade, due to the increased penetration of remote renewable energy. Voltage source converter (VSC) type is preferred over the traditional line-commutated converter (LCC) for this application, due to the advantages like smaller station footprint and no need for strong interfacing ac grid. As the state-of-the-art VSC topology, modular multilevel converter (MMC) is mostly considered. Most renewable energy sources, such as wind and solar, is usually sparsely located. Multi-terminal HVDC (MTDC) provides better use of transmission infrastructure, higher transmission flexibility and reliability, than building multiple point-to-point HVDCs. This dissertation studies the MMC-based MTDC system, including design, control and protection. Passive components design methodology in MMC is developed, with practical consideration. The developed arm inductance selection criterion considers the implementation of circulating current suppression control. And the unbalanced voltage among submodule capacitor is taken into account for submodule capacitance design. Circulating current suppression control is found to impact the MMC operating range. The maximum modulation index reduction is calculated utilizing a decoupled MMC model. A four-terminal HVDC testbed is developed, with similar control and communication architectures of the practical projects implemented. Several most typical operation scenarios and controls are demonstrated or proposed. In order to allow HVDC disconnects to online trip a line, dc line current control is proposed through station control. Utilizing the dc line current control, an automatic dc line current limiting control is proposed. Both controls have been verified in the developed testbed. A systematic dc fault protection strategy of MTDC utilizing hybrid dc circuit breaker is developed, including a new fast and selective fault detection method taking advantage of the hybrid dc circuit breaker special operation mechanism. Detailed criteria and control methods to assist system recovery are presented. A novel fault tolerant MMC topology is proposed with a hybrid submodule by adding an ultra-fast mechanical switch. The converter power loss can be almost the same as the half-bridge MMC, and 1/3 reduction compared to the similar clamp-double topology

Control design of Modular Multilevel Converters in normal and AC fault conditions for HVDC grids

This paper describes a control design strategy of Modular Multilevel Converters (MMC) for High Voltage Direct Current (HVDC) applications to operate during normal and AC fault conditions. First, a steady state analysis of the converter is performed to identify the uses of the current components within the control strategy. Based on the initial stationary study, a complete converter control structure is proposed, which enables full control of the MMC internal energy during normal and AC fault conditions. A detailed design procedure is included for the current and energy regulators, in order to be able to ensure a dynamic response under any grid condition. Finally, theoretical developments are validated through simulation results by means of a detailed model in normal operation and during an AC voltage sag

Strategies for decoupling internal and external dynamics resulting from inter-arm passive component tolerances in HVDC-MMC

Modular Multilevel Converter (MMC) performance may be adversely affected by passive component tolerances, such as submodule capacitance and arm inductance variations. Depending on control strategies, the differences in equivalent capacitances and/or inductances of the upper and lower arms of one phase-leg can cause unequal power distribution between upper and lower arms. Assuming passive component tolerances ranging between ±10%, this paper presents a comprehensive assessment of the internal/external coupling effects due to the passive component tolerances within one phase-leg, under the control of common MMC balancing methods. A novel control strategy is proposed to suppress the fundamental component that arises in the dc-link current due to such tolerances, and its effectiveness is demonstrated via simulation and experimentation. The investigation shows that voltage-based common and differential mode balancing control provides effective ac offset suppression while the proposed method offers superior performance in terms of dc-link fundamental current ripple suppression

A New MMC Topology Which Decreases the Sub Module Voltage Fluctuations at Lower Switching Frequencies and Improves Converter Efficiency

Modular Multi-level inverters (MMCs) are becoming more common because of their suitability for applications in smart grids and multi-terminal HVDC transmission networks. The comparative study between the two classic topologies of MMC (AC side cascaded and DC side cascaded topologies) indicates some disadvantages which can affect their performance. The sub module voltage ripple and switching losses are one of the main issues and the reason for the appearance of the circulating current is sub module capacitor voltage ripple. Hence, the sub module capacitor needs to be large enough to constrain the voltage ripple when operating at lower switching frequencies. However, this is prohibitively uneconomical for the high voltage applications. There is always a trade off in MMC design between the switching frequency and sub module voltage ripple

Practical Submodule Capacitor Sizing for Modular Multilevel Converter Considering Grid Faults

Submodule (SM) capacitors are key elements in the modular multilevel converter (MMC), the design of which influences the entire system performance. In practical cases, SM capacitor sizing must consider the abnormal system operation (e.g., grid faults). In order to find a clear design boundary for SM capacitors, a practical capacitor sizing method is presented for the first time in this paper, considering the grid-fault-ride-through operation of the MMC, impact of MMC control system, and aging mechanism of capacitors. The SM capacitor rated voltage, capacitance, ESR, thermal resistance, and lifetime can be decided to ensure reliable operations of the MMC during grid faults. The effectiveness of the proposed method has been verified through experimental tests on a down-scale MMC system.Published versio

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