Quality power from induction generator feeding variable speed motors

Abstract

A prime mover (28) in a heat recovery system drives an induction generator (8) which feeds power to a utility grid (9) through a breaker (10) but also powers a load including auxiliary induction motors (11, 12). To provide acceptable waveform and power factor, the auxiliary equipment is driven through IGBT switched bridge converters (13) by DC voltage (15) generated by an IGBT switched bridge converter (13a), instead of three-phase diode rectifiers (16). The switched bridge converter controller (14a) is responsive to a system process controller (23a) which causes the switched bridge controller (13a) to be driven in response to the voltage (26) and current (27) on the generator bus (17). This eliminates the need for harmonic filters (18) and power factor capacitors (20) while improving the quality of the power generated. The controller (23a) trips the breaker if the voltage, frequency or power factor is out of limits.

Description

TECHNICAL FIELD [0001] This invention relates to providing power to a utility power grid from an induction generator which also provides power to variable speed motor drives associated therewith, such as in combined heat and power systems; the power factor and harmonics content (shape) of the power at the grid interface is controlled to be well within requirements for power provided to the utility, by actively controlling power in a manner to complement and compensate for undesirable harmonics and power factor. BACKGROUND ART [0002] It has become common for entities other than power utilities to generate electric power and to provide it to the utility grid, thereby to derive revenue or offset customer electric utility bills. Recapture and utilization of energy (such as heat) which would otherwise be wasted, as a byproduct of some useful function, is becoming commonplace. [0003] In some electric power generating systems, the manner of harnessing the energy that will operate the gener...

AI Technical Summary

Benefits of technology

The invention aims to produce high-quality power with a generator and use it in a system with auxiliary variable speed motors without degrading the quality of the generated power. It also aims to improve power factor correction, reduce harmonic distortion, and improve heat recovery. Additionally, it aims to generate electrical power with low harmonic distortion, high power factor, and suitable anti-islanding in an organic Rankin cycle heat recovery system. The invention uses an insulated gate bi-polar transistor (IGBT) switched bridge converter (SBC) to provide DC power input to the variable speed motor. Overall, the invention aims to improve the efficiency and reliability of power generation and utilization.

Problems solved by technology

Synchronous generators are expensive and require additional controls compared with other, cheaper generators such as induction generators, in which the rotor consists of simple conductive bars, short-circuited at the ends, which is much less expensive than synchronous generators.

However, induction generators have an inherently lower power factor than what is acceptable to utility grids.

This amounts to about 8% distortion of the 200 kW being generated at the utility interface.

The filter traps may result in the power factor to be more lagging.

Harmonic filter traps also increase the cost and space requirements of the installation where used.

This may be corrected by large power factor correcting capacitors 20, which increase the cost of the system, consume space, and are possible sources for self-excitation, all of which can be inappropriate in many installations.

These capacitors may require additional bulky series inductors for limiting harmonic current to the capacitors.

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