Cost Effective Grid-Connected Inverter for a Micro Combined Heat and Power System

Abstract

This paper proposes a novel, cost effective grid-connected inverter for a domestic micro combined heat and power (micro-CHP) system. A high frequency (~12,000 rpm), high voltage (~425 Vdc) brushless DC (BLDC) machine is used as the generator. The output of the generator is injected into the grid through a novel grid-connected inverter, utilizing a digital algorithm for the implementation of a constant frequency hysteresis current control. This allows a simplified output filter with smaller components to be employed. A grid synchronization method has also been designed, based on a low-pass filter and a micro-controller to optimize/tune the output power factor.

Existing System

Simple Feedforward Bandwidth Prediction.

Proposed System

This paper proposes a reliable, low cost and long life-time mains power generation system which can be implemented within domestic boilers to construct a (micro-CHP) system. This micro-CHP system is expected to produce domestic heating output and electrical power is fed into the grid. The total one-off price for the electrical components in this power generation section, which would fall considerably if mass produced in the Far East for example. When implemented in a domestic boiler with a low cost, single-stage steam cycle turbine, this system has the potential to reduce the cost of a commercial micro-CHP. In terms of the power electronics design, the basic HCC method has been modified to include two additional digital potentiometers, varied by a micro-controller, to restrain the switching frequency of the inverter over a narrow bandwidth, and grid synchronization is based on a low-pass filter and a variable micro-controller delay to achieve reliable phase alignment and power factor control, even in the presence of grid harmonic distortion. These two features have been demonstrated to operate effectively in the grid-connected inverter, which has the advantages of simplicity and low costs compared with the conventional methods; typically, a feed-forward method with a phase-locked loop. Also, the output of the grid-connected inverter has been demonstrated to comply with the operating standards IEEE 1547 and EN 50160.

Architecture

Structure of the system