Delta-Connected Multilevel Converter Control for Circulating Current
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Solution Overview
Problem
Delta-connected cascaded multilevel converters face inefficiencies due to circulating currents, which limit their power capacity and efficiency, especially in voltage drive applications, where conventional solutions for STATCOM do not directly apply and fail to extend power capacity effectively.
Innovation Solution
A controller system that determines and controls the amplitude of circulating currents in delta-connected cascaded multilevel converters by generating harmonic voltage signals based on current and reference signals, allowing for the injection of desired circulating currents into phase arms, thereby extending the rated power range with minimal hardware modifications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a delta-connected cascaded multilevel converter is used, then the rated current of each converter cell is reduced compared to Y-connected topology, but circulating current flows through the three phase arms causing decreased converter efficiency
Solution Approach 1:
The patent converts the harmful circulating current into a beneficial tool by intentionally injecting a controlled third-order harmonic circulating current into the phase arms. This injected circulating current is used to extend the rated power range of the converter, transforming the previously harmful effect into a useful function for power capacity extension.
Solution Approach 2:
The patent changes the parameter of circulating current from an uncontrolled harmful phenomenon to a controlled parameter with specific amplitude (0.167 times the rated current). By precisely controlling the amplitude and harmonic order of the circulating current, the system achieves both efficiency improvement and power capacity extension.
2Loss of energy
If conventional STATCOM control methods are applied to delta-connected cascaded multilevel converter, then circulating current limitation is achieved, but power capacity extension is not realized and the solution cannot be directly applied for voltage drive
Solution Approach 1:
The patent transitions from static circulating current limitation to dynamic circulating current injection. The control system dynamically adjusts the third-order harmonic circulating current amplitude and phase to achieve both limitation of harmful circulating currents and extension of power capacity, making the system adaptable to different operating conditions.
Solution Approach 2:
The patent creates a control method that serves multiple functions: it limits harmful circulating currents, extends the rated power range, and enables voltage drive applications. The same control apparatus handles both STATCOM and voltage drive modes, making the solution universally applicable to delta-connected cascaded multilevel converters.
3Loss of energy
If Y-connected cascaded multilevel converter topology is used, then circulating current is naturally limited, but the rated current of each converter cell is much greater requiring parallel operation of semiconductor devices
Solution Approach 1:
The patent adopts the delta-connected topology and converts its harmful circulating current characteristic into a beneficial feature by intentionally injecting controlled third-order harmonic circulating current. This approach maintains the advantage of lower rated current per converter cell while eliminating the harmful effect of uncontrolled circulating current.
Data Source
AI summary
Embodiments of present disclosure relate to an apparatus and a method for controlling a delta-connected cascaded multilevel converter. The apparatus (100) for controlling a delta-connected cascaded multilevel converter (110) comprises: a converter controller (102) configured to: receive current signals indicating phase currents flowing through respective phase arms of the converter (110); determine a harmonic current signal indicating a circulating current of the converter (110) from the current signals; and generate, based on the determined harmonic current signal and a reference current signal, a harmonic voltage signal to cause an amplitude of the circulating current flowing through the phase arms of the converter (110) to be a predetermined amplitude.