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In this paper, the cascaded H-bridge (CHB) multilevel inverter is employed as a grid connected inverter. This kind of inverter has interesting features compared to conventional inverters, including better efficiency, lower THD, lower common mode current injection, and better waveform quality. Using a series connection of H-bridge cells in this topology, a high modularity is achieved and a stepwise voltage waveform is synthesized at the ac side, leading to lower filter size and complexity. This topology processes the electric power in one stage and does not need any step-up voltage transformer for grid connection. The dynamic performance of control system is studied through simulations and is compared with the conventional approach. In addition, a new voltage estimation approach is proposed to reduce the number of voltage sensors compared to existing methods. The simulation results are presented for a 1.5 kW grid connected 7-level CHB inverter and then the hardware-in-the-loop implementation of the CHB inverter is presented to verify the performance of control system.

This study presents a novel time-domain protection technique for application to DC grids. The technique utilises the power developed by the forward and backward travelling waves produced by a fault to distinguish between internal and external faults. For an internal fault, the calculated travelling wave power must exceed a predetermined setting; otherwise the fault is external. The ratio between the forward travelling wave power and the backward travelling wave power provides a directional comparison. For a forward directional fault, this ratio is less than unity, whereas the ratio is greater than unity for reverse directional faults. To improve the sensitivity of the protection scheme for long-distance remote internal fault, a second element utilising the concavity of the forward travelling wave power is proposed. The proposed technique is time domain based and does not require complex mathematical burden; moreover, as such can be easily implemented since it will require fewer hardware resources. Simulations were carried out in power systems computer-aided design/electromagnetic transient simulations, and the results presented considering wider cases of fault scenarios including 500 Ω remote internal fault shows the suitability of the proposed scheme as all fault scenarios indicated were identified within 500 µs following the application of the fault.

This paper presents a transient based protection principle utilising the magnitude of the travelling wave power derivative at only local terminal to formulate a non-unit protection scheme, where the directional transient relay and boundary relay have been involved. In terms of boundary relay, for an internal fault in the forward direction with respect to a local relay, the traveling wave power derivative will exceed a pre-determined setting, otherwise the fault is external to the relay, due to the discontinuity of surge impedance at the boundary point. Directional detection is achieved by the comparison of the forward travelling wave power to the backward travelling wave power. The ratio of forward traveling wave to backward traveling wave is less than unity for all forward directional faults and much greater than unity for reverse directional faults. Simulation studies carried out in PSCAD and the results presented shows the suitability of the proposed protection principle as all fault scenario indicated were detected within 250μs following the application of the faults.

Modular multilevel converters (MMCs) will be extensively used in the high-voltage direct-current transmission networks because of its superior characteristics over line commutated converter. Increasing the reliability of the MMC is directly related to the balancing of the MMC submodule capacitors voltages, which guarantees the proper operation of the converter and lowers the stress on the submodules. This paper presents an adaptive voltage-balancing strategy based on the capacitor voltage estimation, utilizing a hybrid adaptive linear neuron recursive least squares scheme. The proposed strategy eliminates the need of measuring submodules capacitor voltages and associated communication link with the central controller. Furthermore, the estimated capacitor voltages are utilized to detect and localize different types of submodule faults. After isolating the faulty submodules, the proposed fault-tolerant control unit modifies the parameters of the voltage-balancing strategy to overcome the reduction of the active submodules. The dynamic performance of the proposed strategy is investigated, using PSCAD/EMTDC simulations and hardware-in-the-loop real-time simulations, under different normal and faulty operating conditions. The accuracy and the time response of the proposed fault detection and tolerant control units result in stabilizing the operation of the MMC under different types of faults. Consequently, the proposed integrated control strategy improves the reliability of the MMC.

The increasing demand for high voltage DC grids resulting from the continuous installation of offshore wind farms in the North Sea has led to the concept of multi-terminal direct current (MTDC) grids, which face some challenges. Power (current) flow control is a challenge that must be addressed to realize a reliable operation of MTDC grids. This paper presents a reduced switch count topology of a current flow controller (CFC) for power flow and current limiting applications in MTDC grids. A simple control system based on hysteresis band current control is proposed for the CFC. The theory of operation and control of the CFC are demonstrated. The key features of the proposed controller, including cable current balancing, cable current limiting, and current nulling, are illustrated. An MTDC grid is simulated using MATLAB/SIMULINK software to evaluate the steady state and dynamic performance of the proposed CFC topology. Furthermore, a low power prototype is built for a CFC to experimentally validate its performance using rapid control prototyping. Simulation and experimental studies indicate the fast dynamic response and precise results of the proposed topology. Furthermore, the proposed controller offers a real solution for power flow challenges in MTDC grids.

One of the main problems that need to be solved to allow the realisation of multi-terminal high-voltage direct current (HVDC) transmission systems is the absence of a practical power flow control method. Industry leaders and researchers have proposed a few methods of power flow control based on either the control of converter station or the connection of new power electronic equipment to the grid. This study presents the operation and control of a three-port insulated gate bipolar transistor-based current flow control (CFC) device suitable for multi-terminal HVDC systems. Key features and functionalities of the proposed controller including the balancing of cable currents, limiting the magnitude of cable current and current nulling are demonstrated. The three-port CFC was simulated using power system computer aided design (PSCAD)/electromagnetic transient and direct current (EMTDC), network simulation software to evaluate its steady state and dynamic performance. Furthermore, low-power prototypes are implemented for a two and three-ports CFC to experimentally validate their different functionalities. Simulation and experimental studies explore the fast dynamic response and the results show that the CFC studied may have a significant role to play in the control of power flows in multi-terminal high-voltage DC systems.

A high voltage DC-DC converter is a key component of future HVDC grids. This paper presents a comparison of a single-stage and multi-stage converter. Both topologies are based on the Marx principle where charged capacitors are charged in parallel and discharged in series to achieve the voltage transformation. Detailed models of both converter topologies at 50 MW, 6kV/72kV are designed and simulated using Matlab/Simulink package software.

Wind power is the most dominant renewable source and the global wind power capacity, both on shore and off shore are rapidly increasing. The United Kingdom is leading the use of off shore wind energy and has the largest off shore wind farm in the world. Power electronics is essential for the harnessing of wind energy for use by the consumers. Power electronic converters required are line commutated thyristor converters, two level and multilevel voltage source converters including energy diverting converters for facilitating fault ride through and reactive power compensators. With the advent of DC grids DC-DC converters are required for interfacing different voltage levels and novel designs are required. All these converters require either series connection or modularization of Thyristors and IGBTS with associated voltage equalization networks.

This paper presents an improved cascaded DC-DC resonant converter for offshore windfarms. The improvements are reduced losses and the number of components. The topology is based on the Marx principle where charged capacitors are charged in parallel and discharged in series to achieve the voltage transformation. The four inductors of the converter are designed to resonate with the capacitors to create resonance forcing current zeros to enable zero current switching thereby reducing switching losses. The operating principles and design considerations of the proposed converter are discussed and the design equations are presented. In order to evaluate the operation of 50 MW converter aimed at connecting a 30 kV DC Busbar in a wind power collection system to a 360 kV high voltage DC bus for transmission to the onshore grid was simulated and the results are presented.

A decoupled active-reactive power control technique is proposed for an oscillating water column (OWC) based wave energy conversion system adopting grid connected doubly fed induction generator (DFIG). The effectiveness of the technique is investigated under various rotor speed and reactive power reference commands. The results show enhanced performance, better tracking, and improved transient response.

This paper presents a transient and non-unit-based protection scheme for consideration in DC grids of the future. The technique utilises the rate-of-change (R-o-C) of the associated travelling wave components following the occurrence of a fault to determine whether the fault is internal or external. For an internal fault, the product of the magnitude of the R-o-C of the fault induced voltage and current travelling wave following fault inception must exceed a predetermined setting, otherwise the fault is external. The DC inductor located at the cable ends provides attenuation for the high frequency contents of the fault generated components resulting from an external fault. The ratio between the forward voltage travelling wave and the backward voltage travelling wave provides directional discrimination. This ratio is less than unity for a forward directional fault and greater than unity for reverse directional faults. The protection algorithm has been validated using PSCAD/EMTDC simulations based on full scale modular multilevel converter (MMC)-based HVDC grid. The simulation results presented, including the performances indices compared to existing and proposed methods available in literature utilising the derivative of the fault induced components show the suitability and reliability of the proposed technique in distinguishing between internal and external faults. Key advantages of the proposed technique is that it simple, easily implemented, and does not rely on complex signal processing technique; and therefore it can easily be implemented to provide autonomous tripping for all relays located on the DC grid.

Interconnecting several offshore wind farms using High Voltage dc (HVDC) connections and forming multi terminal HVDC grid are some of the targets set by industry leaders to be achieved in the near future. Researchers have introduced a few methods of power flow control based on either the control of AC/DC converter stations or the connection of new power electronic equipment to the grid. In this paper, the operation and control of an extended three-port IGBT based current flow control (CFC) device suitable for multi terminal HVDC systems is presented. Features and functionalities of the proposed controller including the balancing of cable currents, limiting the magnitude of cable current and current nulling are demonstrated. The proposed three-port CFC is real time simulated using OP4510 real time simulator. Real time simulation studies explore fast dynamic response and the results show that the CFC studied may have a significant role to play in the control a of power flows in multi terminal high voltage DC systems.

This paper presents a new fault detection technique for the diagnosis and localization of sub-module faults that are developed due to failures in switching devices inside Modular Multilevel Converters (MMC). Unlike other fault detection strategies that have been proposed in literature, the proposed fault detection technique does not need extra sensors or special power circuits as it depends on an ADAptive LINEar Neuron (ADALINE) capacitor voltage estimation algorithm. The proposed fault detection technique is validated by Hardware In the Loop (HIL) real time simulations through which different case studies are simulated to test the algorithm under different operating conditions.

The modular multilevel converter (MMC) is expected to be used extensively in high-voltage direct current (HVDC) transmission networks because of its superior characteristics over the line-commutated converter (LCC). A key issue of concern is balancing sub-module capacitor voltages in the MMCs, which is critical for the correct operation of these converters. The majority of voltage balancing techniques proposed thus far require that the measurement of the capacitor voltages use a reliable measuring system. This can increase the capital cost of the converters. This paper presents a voltage balancing strategy based on capacitor voltage estimation using the adaptive linear neuron (ADALINE) algorithm. The proposed estimation unit requires only three voltage sensors per phase for the arm reactors and the output phase voltages. Measurements of sub-module capacitor voltages and associated communication links with the central controller are not needed. The proposed strategy can be applied to MMC systems that contain a large number of sub-modules. The method uses PSCAD/EMTDC, with particular focus on dynamic performance under a variety of operating conditions.

Fault vulnerability and protection issues are major challenge in realising multi-terminal HVDC transmission system, also termed HVDC grids. This paper presents a novel time domain and transient based protection technique for application to HVDC grids. The technique utilises the energy of the forward and backward travelling waves produced by a fault to distinguish between internal and external faults. For an internal fault, the calculated forward or backward travelling wave energy for a pre-set time duration following the occurrence of a fault must exceed a predetermined setting otherwise the fault is external. This characteristic is largely due to the DC inductor located at the cable ends, as per HVDC breakers or fault current limiters, which provides attenuation for the high frequency transients resulting from an external fault. The ratio between the forward travelling wave energy and the backward travelling wave energy provides directional comparison. For a forward directional fault with respect to a local relay, this ratio must be less than unity whereas the ratio is greater than unity for a reverse directional fault. The simulation results presented based on full scale Modular Multilevel Converter Based HVDC grid shows the suitability of the proposed technique. An advantage of this technique is that it is non-unit based and as such no communication delay is incurred. Furthermore, it is simple as it does not require complex mathematical/DSP technique; and as such can be easily implemented at each independent relay since it will require minimal hardware resources hence reduces cost.