Review of wind farm power collection schemes

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

Bidirectional Marx DC-DC Converter for Offshore Wind Farm Application

The bidirectional DC-DC converter explained in this paper is based on the Marx principle, and is capable of achieving step-up and step-down voltage transformations at kV level and is able to handle MW level power transfers in both directions. The main features of this topology is the absence of a high frequency transformer, reduced weight, volume, and soft switching to reduce the switching losses. In the boost mode, five capacitors are charged in parallel and discharged in series to achieve the step-up action, and in the buck mode the converse action takes place. The operating principle is explained, and the steady-state analysis of the converter is given. Matlab/Simulink simulation of a 50MW converter, interfacing 6kV, and 30kV systems supports and validates the theoretical analysis, and enables positive supporting conclusions to be made

A Compact DC-DC Converter for Offshore Wind Farm Application

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 simulatio

Investigation of a Cascaded H-bridge Photovoltaic Inverter under non-Uniform Insolation Conditions by Hardware-in-the-Loop Test

Abstract- 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. Keywords- Cascaded H-bridge Inverter, Grid Connection, Hardware-in-the-loop implementation, Photovoltaics, and Real-time simulation

Capacitor voltage balancing strategy based on sub-module capacitor voltage estimation for modular multilevel converters

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

Flicker Transfer in Radial Power Systems

Loads which exhibit continuous and rapid variations in their current can cause voltage fluctuations that are often referred to as flicker. One good example for such loads is arc furnaces which are usually fed by dedicated feeders from the high voltage busbars in transmission systems. The flicker generated from such loads will propagate to the upstream HV point of common coupling (PCC), and from there to the downstream through the transmission and sub transmission systems. This paper demonstrates how the generated flicker is propagated from the HV PCC to the downstream in radial networks exhibiting different levels of attenuation depending upon the load composition of the downstream. Theoretical investigations on flicker transfer have been carried out using simple and more advanced modelling of loads and simulations of radial transmission and sub transmission networks having different load types. The behaviour predicted by the theoretical work is supported through field measurements that have been carried out in an actual network

Minimum DC link Voltages for the Generator Bridge Converter of a SCIG Based Variable Speed Wind Turbine with Fully Rated Converters

Squirrel Cage Induction Generator (SCIG) based variable speed wind turbine with Fully Rated Converters (FRC) is a popular choice in the industry for the modern multi mega-watt wind turbines. Typical FRC system uses a fixed DC link voltage that allows operation in all steady state and dynamic operating conditions while allowing the modulation index of the PWM scheme to vary. However, the analysis made in this paper shows that at steady state, in the maximum power point tracking region where the turbine is operated at variable speeds with generator controlled using Rotor Flux Oriented Control (R-FOC), it is possible to operate the Generator Bridge (GB) converter with significantly lower DC link voltages than the fixed value used, by maintaining maximum modulation index in the PWM scheme. This paper presents a methodology of determining the minimum DC link voltages for such a system supported by simulation results showing the successful operation of a GB converter with minimum DC link voltages in the maximum power point tracking region

Comparison of Single-stage and Multi-stage Marx DC-DC converters for HVDC application

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

High voltage cascaded step-up DC-DC Marx converter for offshore wind energy systems

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 presente