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Power Conversion for HTS Wind Generators

IP.com Disclosure Number: IPCOM000169600D
Publication Date: 2008-Apr-23
Document File: 6 page(s) / 337K

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Abstract

Direct-drive high temperature superconductor (HTS) wind generators require power conversion to connect to the grid. A standard 6-pulse rectification generates too much harmonic content and will result in heating in the cold rotor components. Two approaches are proposed to address this issue: an active rectification and a passive 9-phase rectification with multilevel conversion.

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Technical Publication - Power Conversion for HTS Wind Generators

Abstract

Direct-drive high temperature superconductor (HTS) wind generators require power conversion to connect to the grid.  A standard 6-pulse rectification generates too much harmonic content and will result in heating in the cold rotor components.   Two approaches are proposed to address this issue: an active rectification and a passive 9-phase rectification with multilevel conversion.

Technical Description

Direct-drive high temperature superconductor (HTS) wind generators require power conversion to connect to the grid.  A standard 6-pulse rectification generates too much harmonic content and will result in heating in the cold rotor components. This is an important issue because of potentially increased need for refrigeration requiring more space and weight and reducing efficiency.  In the nacelle of a wind turbine, space and weight for housing refrigeration equipment are particularly critical.  This technical note addresses approaches for reducing these losses.

Table 1 – Possible harmonic content from a passive rectification process

A first approach to lower the harmonic content back to the generator is active rectification.  The active control can be connected to a system controller that evaluates the current waveforms in the generator to actively mitigate rotor ac losses in the lower frequency domain to prevent penetration into the cold rotor space.   Converteam demonstrated how active rectification can be implemented in motor drives[1].  General explanations can be found from several sources.[2],[3]

AMSC has performed rectification analysis for several cases as shown below. Figure 1 shows the use of an active rectification as applied to direct drive wind generators with full conversion power electronics playing the role of the motor drive. Figure 2 shows a passive method with the phase groups stacked in series. Figure 3 shows a passive method where the 3-phase groups are connected in parallel or operated separately.  That parallel method is more effective in lowering harmonics in a generator than the series method as described in reference [2].

Figure 1– Single level active front end

Figure 2–  Multilevel active (or passive) front end in series

Figure 3 - Multilevel level front end in parallel or isolated

A second option is passive rectification with multilevel conversion methods, for example with a 9-phase stator.

Selection of the correct power converter will impact the electrical parameters, cost, size, and efficiency for the power conversion. Harmonic analysis from a passive rectification of a 9-phase system wired with isolated neutrals for each 3-phase group shows very little current harmonic as shown in the next section. The result of this analysis is very low power loss imposed onto the rotor.

Harmonic Analysis

Summa...