'High-efficiency classic' takes on PMG

WORLDWIDE: Emerson's European subsidiary Leroy Somer (LS) recently introduced a new generator design to compete with the permanent-magnet generator (PMG) - a copper-only classic synchronous generator with a dedicated optimised power-electronic converter.

LS power expert Pierre Rigaud claims the new solution matches the PMG in terms of efficiency but avoids its inherent disadvantages - dependence on rare earth metals, servicing challenges and issues related to high speed.

The PMG is increasingly popular for wind turbines with a high-speed drive train. This generator is traditionally seen to have superior partial-load efficiency, greater compactness and reduced mass when compared with conventional synchronous generators. The PMG is also a claimed improvement on the doubly fed induction generator (DFIG) in terms of meeting future grid-energy demands. The decoupling between PMG and grid is said to offer potentially enhanced gearbox life due to reduced drive-train peak loads.

In order to maximise power efficiency, the new LS design has treated the generator and converter as one integrated system as opposed to two separate components.

Rigaud explained the logic: "During the operation of a classic brushless electrically excited synchronous generator (EESG), rotor copper (power) losses - not present in PMGs - are balanced by reduced stator iron and copper losses."

Air-gap magnetic flux can also be varied in order to find the optimal efficiency point.

By contrast, adjusting the flux strength in a PMG to control the voltage increases reactive current, creating generator and converter power losses. Rotor-efficiency losses also occur through eddy currents in PMG magnets or pole caps, he adds.

"One of the system-optimising measures we accomplished (with the new design) is to continuously adapt field-current levels to maximise total efficiency of both generator and converter during partial load," he said. "This control strategy means finding voltage and power factor points in order to reach an optimal balance between system efficiency and losses."

LS's power-electronic converter choice also has substantial efficiency gain potential. With regard to different generator topographies in relation to required power-electronics technology, Rigaud determined two main generator groups or families - induction (asynchronous and DFIG) and synchronous (EESG and PMG).

EESGs can operate with either 4-Quadrant (4-Q) or 2-Quadrant (2-Q) converters, whereas a PMG requires a 4-Q.

Rigaud said: "4-Q converters contain complex IGBT-type switches versus cheaper diode-type switches used in 2-Q equivalents. Internal losses in 4-Q converters add up to around 3% against about 2% efficiency loss in 2-Q. The difference adds 1% efficiency gain for our solution."

He is convinced that with their new design LS can offer the market a proven solution for both high-speed and medium-speed turbines. High-speed system, size and mass will thereby be comparable to the PMG, while a slight 5-10% mass penalty for medium-speed systems has to be expected.

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