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Induction Generator

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ADVANCED ELECTRIC GENERATOR & CONTROL
FOR
HIGH SPEED MICRO/MINI TURBINE BASED POWER SYSTEMS

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INTRODUCTION

High-speed micro-turbines and mini-turbines play a significant role in the Distributed Power Systems that provide dependable electric power close to the user. Several high-speed turbo-generators manufactured by various corporations are now available in the 30 kW to 90 kW range. These systems operate at speeds from 50000 RPM to 120000 RPM. The generator is directly coupled to the turbine shaft. This obviates the need for a gearbox, helps reduce the size of the generator, and lowers the cost of the overall system. The output power is electronically processed and conditioned to provide constant voltage dc or multi-phase ac power at constant frequency.

Technology of micro-turbines is moving forward to address ratings above 100 kW due to the growing demand for larger units. There is a tendency to use multiple units of the existing 30 to 90 kW packages to satisfy this demand for higher power capacity. However, use of turbo-generators of higher ratings is likely to be beneficial to the user for the following reasons:

a) lower cost of investment per kW for purchase and installation

b) lower cost of maintenance because of reduced parts count

c) higher efficiency

d) safer operation.

At the present time most generators used with micro-turbines are based on permanent magnet technology. It is the objective of this paper to compare alternatives to the PM generator technology, and introduce induction generator technology as a more viable alternative in the power range exceeding 100 kW. The approach in this paper is to present the concept in all its dimensions including the issues of generator and controller design. The authors are currently engaged in the development of the high-speed induction generator systems. Their experience in the field of the technology forms the basis supporting the discussions in this paper.



SYSTEM DEFINITION AND CONSTRAINTS

It is realized that one specific technology does not necessarily provide the best answer under all situations. We must therefore limit our discussions to applications within certain constraints. At this time the following broad limits are applicable for the technology under consideration:

i) The micro or mini turbine systems considered here are in the 100 kW to 500 kW power

range. The system comprises mainly of high-speed turbine, generator, controller, protection, and instrumentation.

The generator and the turbine are directly coupled. Figure 1 shows the components in a block diagram.

ii) The prime mover operates at speeds between 30000 to 80000 RPM depending upon the

rated output. Typically, the operating speed of the prime mover varies inversely with the rated output.

iii) Constant speed of operation is considered. However, certain narrowly defined operating

speed range may be required in specific applications.

iv) The generator must be designed for a cooling system that is compatible with the system

requirements. Typically either air, or lubricant oil, or water glycol mixture is used.

v) The integrated power system is located close to the user such as in a factory building,

hospital, department store, and office complex. Alternatively, vehicle mounted applications in airborne, land based or marine situations are also considered. These mobile applications are valuable particularly for military requirements.

vi) The electrical power output is typically 3-phase ac with single or multiple voltages.

Alternatively, DC output may be required. In case of AC power systems, 50/60 Hz.

frequency is common for commercial applications, and 400 Hz. frequency is used in military / aerospace applications.

vii) Compatibility with utility power systems may or may not be required. In most situations

stand-alone capability in isolation from a utility system is required. In some other situations, power transfer from utility to the turbo-generator and vice versa may be necessary.

viii) The generator must also provide electric start capability during the initial start up of the turbine.

ix) The system must provide protection against hazards. Safety of operation is an important

consideration.

In approaching various issues, we have considered the following issues to define relative merits:

i) Cost: Investment and Operational

ii) Reliability and safety

iii) Size, Power Density.

The issues listed above are not necessarily listed in the order of their importance. GENERATOR TECHNOLOGIES

We plan to review three different generator technologies for comparison: permanent magnet (PM), induction, and switched reluctance (SR). All these three are suitable for high-speed operation in the speed range considered here. There are other technologies such as synchronous reluctance and homopolar that are suitable for high-speed operation but are not considered in this paper. We also limit our discussion to radial geometric configurations for the three technologies. Axial gap geometric configurations are not considered.


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