PWM Phase Shift Harmonic Cancellation in Parallel Power Converters
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Solution Overview
Problem
The connection of variable voltage and frequency generators to a nominally fixed voltage and frequency power grid or supply network results in harmonic distortion, exceeding the limits defined by standards and grid codes, especially when multiple power converters are connected in parallel, leading to increased harmonic voltage distortion in the supply network.
Innovation Solution
A method that involves generating a common time signal and providing it to each power converter, allowing each network bridge to operate with a different time offset relative to this signal, effectively cancelling unwanted harmonics by synchronizing the switching periods of the PWM strategy across multiple power converters, thereby reducing harmonic distortion in the supply network.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If multiple power converters are connected in parallel to interface variable voltage and frequency generators to the supply network, then the power export capability is improved, but the harmonic voltage distortion increases and exceeds the limits defined by standards and grid codes
Solution Approach 1:
The patent combines multiple power converters operating in parallel and synchronizes their PWM switching strategies by providing a common time signal to all converters. This merging approach allows the converters to work together as a coordinated system, where their combined power export capability is enhanced while their harmonic distortions cancel each other out through phase shifting, thus resolving the contradiction between increased power capability and reduced harmonic distortion.
Solution Approach 2:
The patent converts the harmful effect of harmonic distortion into a beneficial cancellation effect. By intentionally introducing phase shifts between the PWM strategies of parallel converters and providing a common time signal, the harmonic distortions generated by each converter are transformed into counteracting waveforms that cancel each other out, turning the harmful harmonics into a benefit that reduces overall distortion and meets grid code requirements.
2Object-generated harmful factors
If the PWM switching frequency is increased to reduce harmonic distortion, then the power quality is improved, but the switching losses and device stress increase
Solution Approach 1:
The patent segments the PWM switching strategy of each power converter by introducing different phase shifts to their switching signals while providing a common time signal. This segmentation allows each converter to operate at a moderate switching frequency with acceptable switching losses, while the combined effect of phase-shifted converters reduces overall harmonic distortion, thus resolving the contradiction between power quality and energy losses.
Data Source
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AI summary
The present invention provides a method of controlling a plurality of power converters (1a... 1c) that can be used to interface to a supply network, ac busbar etc. Each power converter includes a network bridge (14) operating in accordance with a pulse width modulation (PWM) strategy having the same switching period and which causes at least one unwanted harmonic in the supply network voltage. The method includes the step of providing the switching period of the PWM strategy of each network bridge with a different time offset relative to a time datum such that the at least one unwanted harmonic in the supply network voltage is at least partially cancelled. Two alternative ways of providing the time offset are described.