650 IEEE Transactions on Energy Conversion, Vol. 11, No. 3, September 1996 zy Efficiency of three wind energy generator systems Anders Grauers Department of Electric Power Engineering Chalmers University of Technology S-412 96 Goteborg, Sweden Abstract-This paper presents a method to calculate the average efficiency from the turbine shaft to the grid in wind energy converters. The average efficiency of three 500 kW systems are compared. The systems are: a conventional grid-connected four-pole induction generator equipped with a gear, a variable- speed synchronous generator equipped with a gear and a frequency converter, and a directly driven variable- speed generator equipped with a frequency converter. In this paper it is shown that a variable-speed generator system can be almost as efficient as one for constant speed, although it has much higher losses at rated load. The increased turbine efficiency that variable speed leads to has not been included in this paper. It is also found that a directly driven generator can be more efficient than a conventional four-pole generator equipped with a gear. KEYWORDS Losses, Efficiency, Wind energy, Constant speed, Variable speed, Directly driven generator, Gear, Generator, Frequency converter. I. INTRODUCTION A standard wind energy converter of today has a constant turbine speed of 30 to 50 rpm and uses a gearbox and a four- or six-pole induction generator, directly grid- connected. This concept is very simple and reliable and it can be made of standard components. Therefore, it is the most common system today. There are, however, some drawbacks. The gear is an expensive and heavy component and it causes losses. An other disadvantage of the directly grid-connected generator is that the constant 96 WM 047-1 EC zyxwvutsrqpo A paper recommended and approved by the IEEE Energy Development and Power Generation Committee of the IEEE Power Engineering Society for presentation at the 1996 IEEE/PES Winter Meeting, zyxwvut January 21-25, 1996, Baltimore, MD. Manuscript submitted July 27, 1995, made available for printing January zyxwvutsrqponmlk 4, 1996 speed only allows the turbine to operate at its maximum efficiency at one wind speed. These drawbacks can be eliminated by using variable speed and a directly driven generator. Variable speed is considered mainly because it can slightly increase the energy captured by the turbine and because it can reduce some mechanical loads. A directly driven generator leads to a system without the expensive gearbox and its losses. Such generators have been proposed by, for instance, [1,2] and today one wind energy converter manufacturer produces 200 kW and 500 kW directly driven wind energy generators. The drawbacks of directly driven generators are that they are large and often have high losses. In the design of a directly driven generator, variable speed is also an advantage for two further reasons. The first is that the nominal generator frequency can be chosen freely allowing a better generator design. The second one is that the frequency converter can control the generator power and, thereby, reduce the demands on efficient damping in the generator. (It is difficult to obtain sufficient electrical damping in low- speed generators with a small pole pitch.) Different wind energy converter systems are compared mainly regarding cost per kWh. Therefore, several things should be investigated: the average power captured by the turbine, the system cost, the efficiency, and the availability of the systems. In this paper one of these aspects, the efficiency, is investigated. The efficiency is important when comparing different systems because the losses reduce the average power produced by the wind energy converter and, thereby, they reduce the incomes. The average power production is, of course, determined by the average efficiency and not by the efficiency at rated load. Earlier, it has often been assumed that a variable-speed system must be less efficient than a constant-speed system simply because a frequency converter is added to the system and it has losses [3]. The purpose of this paper is to show, theoretically, that a variable-speed generator and converter system can be about as efficient as a directly grid-connected generator, although its efficiency at rated load is much lower. The paper also shows that a directly driven variable-speed system can be more efficient than a four-pole generator equipped with a gearbox. 0885-8969/96/$05.00 zyxwv 0 1996 IEEE