Cascaded H-Bridge Converter Control for DC-Link Voltage Stability
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Solution Overview
Problem
Cascaded H-bridge multilevel converters require large DC-link capacitance to smooth out DC-link voltage fluctuations due to 2nd order load current frequency oscillations, leading to bulky and heavy converter cells, and existing control methods are complex and prone to system failures.
Innovation Solution
Implementing a control subsystem with feed-forward load current compensation to minimize the difference between rectifier and inverter currents, allowing for a smaller capacitor size by using active front end rectifiers and pulse width modulation to control the ON/OFF durations of switching devices in each H-bridge converter cell.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If large DC-link capacitance is used to smooth out DC-link voltage fluctuations, then voltage stability is improved, but converter cell size and weight increase
Solution Approach 1:
The control subsystem performs preliminary action by predicting the load current using a feed-forward controller and adjusting the rectifier current in advance to compensate for power oscillations. This proactive control prevents voltage fluctuations before they occur, eliminating the need for large capacitance to reactively smooth out variations.
Solution Approach 2:
The control system implements feedback control where the actual load current is measured and compared with the reference current, and the error signal is used to adjust the rectifier current through PWM control. This closed-loop feedback ensures voltage stability while allowing for reduced capacitor size.
2Stability of the object's composition
If large DC-link capacitance is used to smooth out DC-link voltage fluctuations, then voltage stability is improved, but device complexity increases
Solution Approach 1:
The control system implements feedback control where the actual load current is measured and compared with the reference current, and the error signal is used to adjust the rectifier current through PWM control. This closed-loop feedback ensures voltage stability while allowing for reduced capacitor size.
Solution Approach 2:
The patent replaces the passive mechanical approach of using large capacitance to physically smooth voltage fluctuations with an active control system that uses electronic feedback and PWM switching to dynamically regulate the rectifier current and maintain voltage stability.
3Quantity of substance
If feed-forward load current compensation is implemented, then capacitor size is reduced, but control complexity increases
Solution Approach 1:
The control subsystem performs preliminary action by predicting the load current using a feed-forward controller and adjusting the rectifier current in advance to compensate for power oscillations. This proactive control prevents voltage fluctuations before they occur, eliminating the need for large capacitance to reactively smooth out variations.
Solution Approach 2:
The control system implements feedback control where the actual load current is measured and compared with the reference current, and the error signal is used to adjust the rectifier current through PWM control. This closed-loop feedback ensures voltage stability while allowing for reduced capacitor size.
4Quantity of substance
If active front end rectifiers with PWM control are used, then capacitor size is reduced, but device complexity increases
Solution Approach 1:
The patent employs dynamic PWM control where the switching duty cycles of the rectifier are continuously adjusted based on the instantaneous power oscillations. This dynamic control allows the system to actively compensate for voltage fluctuations in real-time, enabling the use of smaller capacitance compared to static passive filtering approaches.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces the size and weight of each H-bridge converter cell by minimizing the capacitor capacitance, maintaining stable DC-link voltage with smaller capacitors and simplifying control complexity, while ensuring sinusoidal primary side currents and non-sinusoidal secondary side currents.
Implementation Method 1
a capacitor suitable to receive a capacitor current ic, the capacitor smoothing the DC supply
Implementation Method 2
an active front end rectifier for receiving the three phase AC input supply and transforming it into a DC supply providing a rectifier current ii
Implementation Method 3
an inverter suitable to receive an inverter current io, wherein io = ii - ic, the inverter transforming the received inverter current io into a single phase AC supply
Data Source
Figure 1
Figure 2A~2B
Figure 3
AI summary
An electrical system including a three phase AC input supply and three or more H-bridge converter cells. Each H-bridge converter cell has: an active front end rectifier for receiving the three phase AC input supply and transforming it into a DC supply, thereby providing a rectifier current ii; a capacitor suitable to receive a capacitor current ic, the capacitor smoothing the DC supply; and an inverter suitable to receive an inverter current io, wherein io = ii - ic, said inverter transforming the received inverter current io into a single phase AC supply. The system also including a control subsystem, which provides a signal to each active front end rectifier to vary its respective rectifier current ii such that the difference between the rectifier current ii, provided by the active front end rectifier, and the inverter current io, received by the inverter, is substantially zero.