Probability Distributions.- Linear Models for Regression.- Linear Models for Classification.- Neural Networks.- Kernel Methods.- Sparse Kernel Machines.- Graphical Models.- Mixture Models and EM.- Approximate Inference.- Sampling Methods.- Continuous Latent Variables.- Sequential Data.- Combining Models.

Pattern Recognition and Machine Learning

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

(1990). ENERGY FUNCTION ANALYSIS FOR POWER SYSTEM STABILITY. Electric Machines & Power Systems: Vol. 18, No. 2, pp. 209-210.

Energy function analysis for power system stability

Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.

Evolution of Topologies, Modeling, Control Schemes, and Applications of Modular Multilevel Converters

The modular multilevel converter (MMC) is distinguished by its modularity that is the use of standardized submodules (SMs). To enhance reliability and avoid unscheduled maintenance, it is desired that an MMC can remain operational without having to shut down despite some of its SMs are failed. Particularly, in this paper, complete fault diagnosis and tolerant control solution, including the fault detection, fault tolerance, fault localization, and fault reconfiguration, have been proposed to ride through the insulated gate bipolar transistor open-circuit failures. The fault detection method detects the fault by means of state observers and the knowledge of fault behaviors of MMC, without using any additional sensors. Then, the MMC is controlled in a newly proposed tolerant mode until the specific faulty SM is located by the fault localization method; thus, no overcurrent problems will happen during this time interval. After that, the located faulty SM will be bypassed while the remaining SMs are reconfigured to provide continuous operation. Throughout the fault periods, it allows the MMC to operate smoothly without obvious waveform distortion and power interruption. Finally, experimental results using a single-phase scaled-down MMC prototype with six SMs per arm show the validity and feasibility of the proposed methods.

Fault Diagnosis and Tolerant Control of Single IGBT Open-Circuit Failure in Modular Multilevel Converters

The modular multilevel converter (MMC) with half-bridge submodules (SMs) is the most promising technology for high-voltage direct current (HVDC) grids, but it lacks dc fault clearance capability. There are two main methods to handle the dc-side short-circuit fault. One is to employ the SMs that have dc fault clearance capability, but the power losses are high and the converter has to be blocked during the clearance. The other is to employ the hybrid HVDC breakers. The breaker is capable of interrupting fault current within 5 ms, but this technology is not cost effective, especially in meshed HVDC grids. In this paper, an assembly HVDC breaker and the corresponding control strategy are proposed to overcome these drawbacks. The assembly HVDC breaker consists of an active short-circuit breaker (ASCB), a main mechanical disconnector, a main breaker, and an accessory discharging switch (ADS). When a dc-side short-circuit fault occurs, the ASCB and the ADS close immediately to shunt the fault current. The main breaker opens after a short delay to isolate the faulted line from the system and then the mechanical disconnector opens. With the disconnector in open position, the ASCB opens and breaks the current. The proposed breaker can handle the dc-side fault with competitively low cost, and the operating speed is fast enough. A model of a four-terminal monopolar HVDC grid is developed in Power Systems Computer Aided Design / Electromagnetic Transients including DC, and the simulation result proves the validity and the feasibility of the proposed solution.

Assembly HVDC Breaker for HVDC Grids With Modular Multilevel Converters

This paper studies the techniques for modeling modular multilevel converter (MMC) based multi-terminal HVDC (MTDC) systems in the electromechanical transient mode. Firstly, the mathematical model of the MMC and its corresponding equivalent circuit are established, which are similar to those of the two level converters. Then, a power flow calculation method for AC/DC systems containing MMC-MTDC systems is developed. Two dynamic models for MMC-MTDC systems are developed in the paper. One is the detailed model, taking into account of the AC side circuit, the inner controllers, the modulation strategies, the outer controllers and the MTDC circuit. The other is the simplified model, which only reserves the outer controllers and partial dynamics of the MTDC circuit based on a quantitative analysis of the detailed model's dynamic processes, and it can be used in electromechanical transient simulation with a larger step size. Both the detailed and the simplified models are implemented on PSS/E and compared with the accurate electromagnetic transient models on PSCAD in a four terminal MMC-MTDC system; the result proves the validity of the developed models. Lastly, a stability study of a modified New England 39-bus system is executed, and the result shows that the AC fault can be isolated well in an MMC-MTDC asynchronously connected AC grid.

Electromechanical Transient Modeling of Modular Multilevel Converter Based Multi-Terminal HVDC Systems

This paper analyzes the dc line fault transient stability characteristics of AC/DC power systems with three different modular multilevel converter based high voltage direct current (MMC-HVDC) configurations. The first configuration is half bridge sub-module based MMC (H-MMC) HVDC configuration, which clears dc line faults by tripping the ac circuit breakers. The clamp double sub-module (CDSM) based MMC (C-MMC) HVDC configuration with dc line fault clearance ability constitutes the second configuration. A line-commutated converter (LCC) and MMC hybrid HVDC configuration with dc line fault clearance ability, named LCC-diode-MMC (LCC-D-MMC), is the third configuration. The detailed processes of clearing dc line faults in three MMC-HVDC configurations are analyzed. The equal area criterion is utilized to analyze the dc line fault transient processes. Besides, to evaluate the power system transient stability characteristics in three MMC-HVDC configurations, an index, called critical ac transmitted power (P
 ac_critical
), is proposed. Under the same dc line to ground fault, transient stability characteristics of three test systems based on the three different MMC-HVDC configurations are compared. Excellent performance of the LCC-D-MMC HVDC configuration under the dc line fault is demonstrated through comparison of P
 ac_critical
 in the three test systems. Finally, the study is extended to a modified New England 39-bus system, and the simulation results also correspond with the theoretical analysis.

Impacts of Three MMC-HVDC Configurations on AC System Stability Under DC Line Faults

The newly developed modular multilevel converter based on a clamp double submodule (CDSM), namely, C-MMC, has been identified as a good solution for high-voltage direct-current (HVDC) applications, due to its inherent unique dc fault blocking capability. This paper presents a self-start control strategy for the C-MMC-based HVDC link. The equivalent charging circuit for the uncontrolled precharging stage is derived and the behaviors of the CDSM are investigated. In addition, the selection of the current-limiting resistor is described. To fully charge the capacitors in submodules, a simple grouping sequentially controlled charge (GSCC) method is proposed. This method makes the capacitors be charged group by group so that each capacitor can share sufficient stored energy. In particular, detailed startup procedures are designed for the HVDC link under two conditions, that is, connecting to active grids and supplying the passive network. The feasibility of the GSCC scheme under cell failure conditions is discussed. The validity of the proposed strategies is verified by PSCAD/EMTDC simulations.

Self-Start Control With Grouping Sequentially Precharge for the C-MMC-Based HVDC System

Considering the synthetical effects of the ac system and the dc system, this paper proposes two novel indexes, the dynamic redundancy and the utilization ratio of the submodules. On this basis, the nearest level control is applied as the modulation method and an optimized control strategy based on the dynamic redundancy for the modular multilevel converter (MMC) is proposed. One of the main innovations is that the reference value of the capacitor voltage is derived according to the maximum output voltage of each converter arm and the safety margin which can be adjusted artificially. Unlike previous strategies, the redundancy can be adjusted dynamically, and the utilization ratio of the submodules can be effectively improved. In addition, the capacitor voltage and the inner stress are reduced, and the fault ride-through capability of the system can be enhanced. In particular, the strategy under abnormal operating conditions is also detailed in this paper. A model of two-terminal MMC-high-voltage direct current system is built in PSCAD/EMTDC, and the simulation result proves the validity and the feasibility of the proposed strategy.

Geng Tang

Papers

Assembly HVDC Breaker for HVDC Grids With Modular Multilevel Converters

Electromechanical Transient Modeling of Modular Multilevel Converter Based Multi-Terminal HVDC Systems

Impacts of Three MMC-HVDC Configurations on AC System Stability Under DC Line Faults

Self-Start Control With Grouping Sequentially Precharge for the C-MMC-Based HVDC System

Optimized Control Strategy Based on Dynamic Redundancy for the Modular Multilevel Converter