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A DC-DC converter suitable for the grid integration of windfarms through a DC grid is presented. The operation is based on the Marx principle where charged capacitors are connected in series and parallel in turn to achieve the voltage transformation. The two inductors at the two ends 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 design of a 50 MW, 6kV/30kV DC-DC converter was carried out by analysis and simulation

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.

This paper presents a voltage regulated step up Marx DC-DC converter suitable for HVDC offshore wind farm applications. This configuration has the capability to generate a variable high voltage adding controllability to the standard Marx converter with a fixed gain. For voltage control, an auxiliary controlled buck-boost converter was combined with the Marx converter to regulate the output voltage. The studies were carried out by simulating the converter with the design specifications of 6kV to 30kV voltage on LV and HV sides and 5MW rated power. The simulation results showed good voltage stability on the output DC line when the converter input voltage varies.

Experimental results on atmospheric-pressure arc plasma convolutes in air around a polytetrafluoroethylene cylindrical shroud containing a magnetic field ( B-field) producing coil are presented. In this paper, the B-field coil is energized by a current separate from that flowing through the arc, and a separate RLC circuit was connected across the arc gap. Thus the magnitude and time duration of the B-field are independent of the arc current and the high-frequency current oscillations produced by the parallel RLC circuit. Experimental results for the time variation of the current through and voltage across the arc plasma for these different conditions are presented, along with high-speed photographs of the oscillating current arc. The effects of varying the B-field upon plasma pulsations formed by the independent B-field and RLC current oscillations are discussed.

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.

The absence of a reliable current flow control method is one of the main problems that need to be solved to allow the realization of multi terminal high-voltage direct current (HVDC) transmission. Industry leaders and researchers have introduced 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 paper presents the operation and control of an extended three-port IGBT based current flow control (CFC) device suitable for multi terminal HVDC systems. 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 is simulated using PSCAD/EMTDC, network simulation software to evaluate its steady state and dynamic performance. Simulation studies explore the 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.

High-voltage switches required in present power electronics applications are realized by connecting existing devices in series. Unequal sharing of voltage across series-connected devices can be minimized by using active gate control techniques, snubber circuits, and active clamping circuits. The primary objectives of this paper are to discuss existing voltage-balancing techniques, to present a novel hybrid voltage-balancing technique, and to optimize the number of insulated gate bipolar transistors (IGBTs) in a series string in terms of power losses. The novel voltage-balancing technique can achieve good voltage balancing with a minimum number of components and minimum total losses (i.e., IGBT losses and balancing circuit losses). This technique was validated by both simulation and experimental work. The power loss of a high-voltage switch depends on the voltage-balancing circuit and the number of IGBTs in series and switching frequency. For a given application, the optimum number of IGBTs, in terms of power losses, depends on device characteristics and switching frequency.

The main development trend of wind power generation systems is large offshore wind farms (OWFs) with grid connection. However offshore wind farms have grown rapidly due to much better wind conditions. Hence, several large scale offshore wind farms are planned to be built and installed at distances greater than 100 km from the coast. Traditionally, an AC collector scheme collects energy from the wind farm and step up the voltages by power transformers and transmit power via AC submarine cables to the onshore substation. However, this is suitable for shorter distances about 50 km. When the distances are greater the AC transmission of bulk power from the wind farm to the onshore grid via undersea cables is not viable due the reactive power issues. Therefore HVDC transmission is now being considered for the grid connection of wind farms. However as wind farms constitute weak systems Line commutated converter (LCC) based HVDC is not viable and newer Modular Multilevel Converter (MMC) based Voltage Source Converters(VSC) are needed for AC-DC conversion. Opting for dc systems for both power collection and transmission pose a number of technical challenges in terms of developing HVDC breakers and DC -DC converters.

The modular multilevel converters (MMC) form the backbone of the modern voltage source converter (VSC) based HVDC system due to their unique features. Faults within the MMC affect the performance of the HVDC system. In this paper the fault types that can occur in the MMC are investigated. The reported work focuses on sub-module faults. A nine level MMC was simulated using PSCAD/EMTDC in this investigation.

Some significant problems prevent the formation of multi terminal high voltage DC grids until now. Absence of power flow control is considered one of the main challenges that have to be solved in the near future to allow building of an efficient DC grid. This paper investigates different power flow control methods. Simulation of series insertion of AC/DC converters is carried out using MATLAB/SIMULINK software package and results are discussed.

Grid codes imposed by utilities regulate the operation of Voltage Source Converter - High Voltage Direct Current (VSC-HVDC) interconnected offshore wind farms. Fault ride-through (FRT) specifications require the adoption of specific measures to avoid over-voltages of the HVDC link during faults in order to protect the HVDC equipment. Implementing Energy Diverting Converters (EDC), for instance Dynamic Braking Resistor (DBR) circuits, at the DC link is an established method to comply with the grid codes, where the excess energy of the wind farm is diverted into the parallel circuit during the fault. In this paper an evaluation of three different state-of-the-art DBR circuits is performed in order to establish the advantages and disadvantages of each circuit. The evaluation has shown that although the three solutions meet the FRT requirements, the modular topologies generate reduced slope current and voltage step changes during their operation, while being larger in size and requiring a higher number of semiconductors as compared to the traditional DC chopper circuit employing hard switched series connected semiconductor arrangements.

This paper describes a novel technique for interrupting direct currents (D.C.) with an electromagnetically convoluted arc in air at atmospheric pressure. Investigations are reported on the effects of using a separate current for producing the arc convolving electromagnetic field (B-field) to that being interrupted and using a separate R, L, C circuit in parallel with the arc contacts. Experimental results are presented for the time variation of currents flowing through the arc gap, the B-field coil and the parallel R, L, C circuit, along with the voltage across the arc gap.

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.

A Multi-Terminal High Voltage Direct Current (MT-HVDC) network is being considered for utilising the full potential of offshore wind power whereas its realisation is currently being hampered by protection issues. In this paper, a protection strategy for future DC grids based on Modular Multilevel Converter (MMC) based HVDC system is presented. Firstly, a fault detection technique based on initial di/dt measurement is presented and thereafter protection strategies for future DC grids are presented. The fault detection technique presented is based on estimating the initial rate of rise of the current, IRRC (di/dt) at fault inception using measured data and thereafter calculating the line inductance. The calculated line inductance is compared with a setting value to determine whether or not a fault has occurred, thus paving the way for a distance protection strategy. Simulations were carried out using Matlab/SIMULINK for varying fault distances. The results obtained show the validity of the technique in detecting and locating DC side short circuits. An advantage of this technique is that it relies only on information from the local end terminal and as such, no communication channel is required, hence satisfying the protection requirement of fast fault detection and location technique for MT-HVDC systems.

Protection issues are major challenges in realising multiterminal (MT) HVDC systems. In this paper, the findings of an investigation carried out on a di/dt based protection technique with the view to ascertaining its suitability for the protection of DC lines are presented. Firstly the main issues regarding the protection of MT-HVDC system are explored and thereafter the di/dt protection technique is evaluated. Some of the limitations of di/dt protection technique were due to the effect of travelling waves and oscillations. These lead to errors in estimating the actual di/dt since the resulting fault current profile may attain its local maximum or minimum before or during the time window set for the measurement. This is particularly the case during directional comparison. Simulations have been carried out in PSCAD and the results presented show that the di/dt protection technique will not be a reliable method for the protection of DC lines, in particular for the protection of DC grid.