Grain Philip Adam

New Breed of Network Fault-Tolerant Voltage-Source-Converter HVDC Transmission System

This paper proposes a new breed of high-voltage dc (HVDC) transmission systems based on a hybrid multilevel voltage source converter (VSC) with ac-side cascaded H-bridge cells. The proposed HVDC system offers the operational flexibility of VSC-based systems in terms of active and reactive power control, black-start capability, in addition to improved ac fault ride-through capability and the unique feature of current-limiting capability during dc side faults. Additionally, it offers features such as smaller footprint and a larger active and reactive power capability curve than existing VSC-based HVDC systems, including those using modular multilevel converters. To illustrate the feasibility of the proposed HVDC system, this paper assesses its dynamic performance during steady-state and network alterations, including its response to ac and dc side faults.

Quasi Two-Level Operation of Modular Multilevel Converter for Use in a High-Power DC Transformer With DC Fault Isolation Capability

DC fault protection is one challenge impeding the development of multiterminal dc grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and dc fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode dc component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a dc to dc transformer, where two converters operating in the proposed mode are coupled by an ac transformer for voltage matching and galvanic isolation. The proposed dc transformer is shown to be suitable for high-voltage high-power applications due to the low-switching frequency, high efficiency, modularity, and reliability. The dc transformer facilitates dc voltage regulation and near instant isolation of dc faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.

Inertia Emulation Control Strategy for VSC-HVDC Transmission Systems

There is concern that the levels of inertia in power systems may decrease in the future, due to increased levels of energy being provided from renewable sources, which typically have little or no inertia. Voltage source converters (VSC) used in high voltage direct current (HVDC) transmission applications are often deliberately controlled in order to de-couple transients to prevent propagation of instability between interconnected systems. However, this can deny much needed support during transients that would otherwise be available from system inertia provided by rotating plant.

Small-Signal Stability Analysis of Multi-Terminal VSC-Based DC Transmission Systems

A model suitable for small-signal stability analysis and control design of multi-terminal dc networks is presented. A generic test network that combines conventional synchronous and offshore wind generation connected to shore via a dc network is used to illustrate the design of enhanced voltage source converter (VSC) controllers. The impact of VSC control parameters on network stability is discussed and the overall network dynamic performance assessed in the event of small and large perturbations. Time-domain simulations conducted in Matlab/Simulink are used to validate the operational limits of the VSC controllers obtained from the small-signal stability analysis.

Capacitor Balance Issues of the Diode-Clamped Multilevel Inverter Operated in a Quasi Two-State Mode

A new operational mode for diode-clamped multilevel inverters termed quasi two-level operation is proposed. Such operation aims to avoid the imbalance problem of the dc-link capacitors for multilevel inverters with more than three levels and reduces the dc-link capacitance without introducing any significant voltage ripple at the dc-link nodes. The proposed operation can be generalized for any number of levels. The validity of the proposed multilevel inverter operational mode is confirmed by simulations and experiments on a prototype five-level diode-clamped inverter.

Single-Phase Single-Stage Transformer less Grid-Connected PV System

In this paper, a single-phase, single-stage current source inverter-based photovoltaic system for grid connection is proposed. The system utilizes transformer-less single-stage conversion for tracking the maximum power point and interfacing the photovoltaic arrays to the grid. The maximum power point is maintained with a fuzzy logic controller. A proportional-resonant controller is used to control the current injected into the grid. To improve the power quality and system efficiency, a double-tuned parallel resonant circuit is proposed to attenuate the second- and fourth- order harmonics at the inverter dc side. A modified carrier-based modulation technique for the current source inverter is proposed to magnetize the dc-link inductor by shorting one of the bridge converter legs after every active switching cycle. Simulation and practical results validate and confirm the dynamic performance and power quality of the proposed system.

Half- and Full-Bridge Modular Multilevel Converter Models for Simulations of Full-Scale HVDC Links and Multiterminal DC Grids

This paper presents an improved electromagnetic transient (EMT) simulation model for the half- and full-bridge modular multilevel converters (MMCs) that can be used for full-scale simulation of multilevel high-voltage dc (HVDC) transmission systems, with hundreds of cells per arm. The presented models employ minimum software overhead within their EMT parts to correctly represent MMCs behavior during dc network faults when converter switching devices are blocked. The validity and scalabilities of the presented models are demonstrated using open-loop simulations of the half- and full-bridge MMCs, and closed-loop simulation of a full-scale HVDC link, with 201 cells per arm that equipped with basic HVDC controllers, including that for suppression of the second harmonic currents in the converter arms. The results obtained from both demonstrations have shown that the presented models are able to accurately simulate the typical behavior of the MMC during normal, and ac and dc network faults.

Hybrid Multilevel Converter With Cascaded H-bridge Cells for HVDC Applications: Operating Principle and Scalability

Hybrid multilevel converters are contemplated in an attempt to optimize the performance of voltage source converters in terms of magnitude of semiconductor losses and converter footprint, and to achieve additional features such as dc short circuit proof, which is essential for a high integrity multiterminal HVDC grid. Therefore, this paper considers an emerging hybrid cascaded converter that offers the dc side short circuit proof feature at reduced loss and footprint compared to the existing multilevel and other hybrid converters. Its operating principle, modulation, and capacitor voltage balancing strategies are described in detail. Furthermore, hybrid converter scalability to high voltage applications is investigated. The validity of the modulation and capacitor voltage strategy presented are confirmed using simulation and experimentation. The hybrid cascaded converter is extendable to a large number of cells, making it applicable to high voltage applications, and operation is independent of modulation index and power factor. On these ground, the converter is expected to be applicable for both real and reactive power applications.

Analysis and Design of a Modular Multilevel Converter With Trapezoidal Modulation for Medium and High Voltage DC-DC Transformers

Conventional dual-active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual-active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique, where staircase approximated trapezoidal voltage waveforms are produced; thus, alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. This way the analyzed dc-dc transformer employs modular multilevel converters operated in a new mode with minimal common-mode arm currents, as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft-switching characteristics will be shown to be comparable to those of the two-level dual-active bridge. Experimental results from a scaled test rig validate the presented concept.

Hybrid Multilevel Converter: Capacitor Voltage Balancing Limits and its Extension

This paper presents theoretical analysis of a network fault tolerant hybrid cascaded multilevel converter when operated as one unit using multilevel pulse width modulation. The analysis establishes the modulation index range where the voltage balance of the H-bridge floating capacitors of the hybrid converter is attainable independent of load power factors. To realize the established modulation range, a new modulation strategy is proposed that exploits third harmonic subtraction modification of the reference voltage in order to extend the regions around zero voltage crossing where the cells capacitor voltage balancing can be achieved, with a minimum number of cells; independent of load power factor. The significance of the proposed modulation strategy is that it permits increased utilization of dc link voltage independent of operating condition over an extended modulation linear range. Therefore this hybrid converter is applicable to real and reactive power applications, with a higher power density than existing multilevel converters. The validity of the theoretical analysis and proposed extended modulation index linear range are confirmed using simulations and experimentations. The presented analysis and proposed modulation linear range extension can be extended to a hybrid converter with a large number of series H-bridge cells.