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Modelling of power electronic components for evaluation of efficiency, power density and power-to-mass ratio of offshore wind power converters
Abstract
In an offshore environment, the efficiency (η), power density (ρ) and the power-to-mass ratio (γ) are performance indices of paramount importance in the design of the wind energy conversion systems in order to reduce investment costs, especially when most of the electrical conversion components are going to be located in the nacelle or tower of the wind turbine. This chapter describes a simple procedure to calculate the η, the ρ and the γ of power converters for offshore wind turbines via the calculation of power losses, volume and mass of the main components of the wind energy conversion systems (WECS): the power electronics valves, the magnetics components and the capacitors. A base topology known as the two-level voltage source converter (2L-VSC) has been considered in order to illustrate the evaluation procedure.
Keywords
Design methodology; Efficiency; Mass; Power density; Power electronics converter; Power losses; Power-to-mass ratio; Volume; Wind power generation9.1. Introduction
In an offshore environment, the design of the wind energy conversion systems (WECS) requires taking into account not only efficiency and reliability but also size and weight, as expensive platforms must be placed to support each component. A contribution of around 15% in active volume and around 10% in active weight is normally reported for power electronics converters in wind turbine applications (Blaabjerg et al., 2006). Therefore power density is also a performance index of paramount importance, especially when most of the electrical power conversion components are going to be located in the nacelle or tower of the wind turbine (WT).
The efficiency (η) of an electrical system is the ratio of power output and power input (Pin) (Eq. [9.1]), and the power output is the difference between power input and the total losses from the input to output stages of the WECS, including power semiconductors and passive elements like inductors and capacitors.
[9.1]
On the other hand, the power density (ρ) defined by Eq. [9.2] characterizes the degree of compactness of a WECS. ρ depends on the total converter volume and power losses of the system. The converter volume (VolTotal) is the summation of the individual volumes Vol(i) of the components, and the utilization of the VolTotal by active parts is characterized by the volume utilization factor CPV, which has typical values between 0.5 and 0.7 (Kolar et al., 2010).
[9.2]
The WECS should be located in the nacelle, tower or pillar of the WT, so the weight of the converter is also relevant in order to compare different designs. The power-to-mass ratio (γ), defined by Eq. [9.3], indicates the level of heaviness of the WECS. The total converter active mass (MassTotal) is calculated via summation of the individual masses (Mass(i)) of the components.
[9.3]
This chapter describes a simple procedure to calculate η, ρ and γ of WECS for offshore WTs via the calculation of power losses, volume and mass of the main components: the power electronics valves, the magnetic components (AC and DC filter inductors) and the capacitors (DC link capacitors and AC filter capacitors).
A base topology known as the two-level voltage source converter (2L-VSC) has been considered in order to illustrate the evaluation procedure. The nominal η, ρ and γ are obtained for a set of design parameters and constraints. Even more, the η–ρ Pareto-front and the ρ–γ Pareto-front are considered in order to compare different parameters of design.
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