Presented by
V.Kalyani
B.Padmini

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VOLTAGE SOURCE CONVERTER TOPOLOGY FOR HVDC GRID CONNECTION OF OFFSHORE WIND FARMS
ABSTRACT
Large offshore wind farms are recently emerging as promising alternative power Sources. Long distances between offshore generation and onshore distribution grid demand new solutions for their connection to the AC network. HVDC systems based on voltage source converters (VSC) are a promising alternative to conventional AC transmission above a certain cable length. This paper presents a new VSC transmission topology for HVDC grid connection of offshore wind farms.
I. INTRODUCTION
The recent emergence of larger and more efficient wind turbines establishes bright prospects in wind power generation. The first 5 MW wind turbine was erected in Brunsbüttel near Hamburg in summer 2004 by REpower Systems AG. These latest developments extend the potential of large-scale offshore wind generation, which becomes a rapidly growing worldwide alternative power source.
Today, especially large offshore wind farms in the power range of several hundred megawatts are getting into focus. Limited availability of onshore sites and better offshore wind conditions are driving the wind turbines offshore. Environmental requirements regarding noise pollution and the visible impact as well as colliding interests in the nearshore areas (recreation, military, coastal shipping, fishing etc.) lead to increasing distances between offshore wind farms and onshore distribution grids. Remote locations, however, often imply deep water depths, complicating the foundation of the wind turbines in the seabed. Recent improvements in submarine foundations (i.e. tripod, quadropod or lattice structures) allow deeper water depths, whereas the current economic limit of such installations lies in the range of 30 to 35 m. Another important factor that causes prolonged transmission distances is the necessity of a strong grid connection point with a significant short-circuits capacity. Reaching a suitable AC network connection point requires often a long onshore transmission line. In exchange, expensive grid enforcement measures can be avoided. As a consequence of the ongoing trend, the generated power from the wind farms has to be transported over longer distances in order to make a connection to the AC network for onward transmission and distribution. For longer transmission distances, HVDC transmission is a feasible solution compared to traditional AC transmission. AC cables inherently generate reactive power that limits the maximum permissible AC cable length. This is known as the critical AC cable length.
The critical cable length for AC transmission cannot be determined generally. It varies for every individual project and is given by economical and technical constraints. Nowadays, it lies in the range of approximately 100 km. As mentioned before, the AC cable length is limited due to capacitive charging currents. Above a certain cable length, a compensation unit is required. This is particularly costly and troublesome for submarine cables.
HVDC transmission systems based on VSCs, also called VSC transmission, have lately emerged as a competitive alternative to conventional AC transmission. Their operation principle is shown in Fig. 1. VSC transmission provides an efficient and reliable solution overcoming the major technical difficulties facing traditional AC solutions. Mainly, DC cables are not affected by cable charging currents and thus can have any length required. The DC cable capacitance offers instead the advantage of energy storage. Only the cost of the complex onshore and offshore converter stations and the losses of the semiconductor devices limit an augmented application of VSC transmission. Especially the high frequency PWM switching leads to excessive switching losses. Thus, an improvement in efficiency and lower initial converter costs are highly attractive and help to decrease the minimum length at which VSC transmission can compete with traditional AC transmission.
VSC transmission offers a couple of additional advantages compared to AC transmission. If there are environmental constraints on the quantity of submarine cables, DC cables with a higher capacity are the preferable choice. Furthermore, VSC transmission allows frequency control of the local wind farm grid, as the frequencies on both ends of the DC page link are independent. Both the active power flow and reactive power at both ends of the DC page link are fully defined and controllable. This allows the control of voltage fluctuations and increases the exchange limits of reactive power, thus stabilizing the AC grid at the connection point. The electrical impact of the VSC transmission on the existing AC network, as i.e. the harmonic distortion must be studied.
In this paper, a new soft-switched VSC topology is proposed that promises both lower initial costs and a higher efficiency. As a consequence, the minimum cable length at which VSC transmission gets interesting can be further decreased and the HVDC grid connection of large offshore wind farms gets far more attractive. The operation principle of this new VSC topology is presented with respect to the commutation of the semiconductor switches. The advantages and challenges of the proposed topology are also outlined. Furthermore, a comparison with a state-of-the-art variable-speed wind turbine reveals the potential of this new technology. Thereby, this paper concentrates on the losses and ratings of the converters.
II. DESCRIPTION OF PROPOSED TOPOLOGY
The topology of the proposed AC/DC converter is shown in Fig. 2. It incorporates a VSC and cycloconverters (direct converters) connected via a medium frequency (MF) AC bus.