Uninterruptible Power Supply Capacitor Voltage Balancing
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Solution Overview
Problem
In three-phase four-wire uninterruptible power supply systems, terminal-to-terminal voltage imbalances between capacitors can occur during power failures, especially when the load current is small, leading to unbalanced capacitor voltages.
Innovation Solution
The system incorporates a neutral line connected to both the AC power supply and load, along with switches and an AC input filter, utilizing multi-level circuits in converters and inverters to manage voltage differences between capacitors by controlling the switching elements to discharge and charge capacitors as needed, ensuring balanced voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the system operates as a three-phase four-wire system with neutral line current flow, then the system can handle asymmetric loads, but terminal-to-terminal voltages of capacitors become unbalanced
Solution Approach 1:
The control unit continuously monitors the terminal-to-terminal voltages of the first and second capacitors and adjusts the switching states of the converter based on the detected voltage difference. This feedback mechanism dynamically compensates for voltage unbalance caused by neutral line current, maintaining capacitor voltage balance while operating in four-wire mode
Solution Approach 2:
The system changes the operating parameters of the converter by adjusting the switching duty ratios of the semiconductor switches based on the detected voltage unbalance. This parameter adjustment allows the system to maintain capacitor voltage balance while adapting to asymmetric load conditions in four-wire operation
2Loss of energy
If the load current is small during power failure, then energy consumption is reduced, but voltage unbalance between capacitors increases
Solution Approach 1:
The control unit detects the voltage difference between the first and second capacitors and adjusts the converter operation accordingly. Even when load current is small, the feedback control actively compensates for voltage unbalance by modulating the switching states, preventing excessive voltage divergence while maintaining low energy consumption
Solution Approach 2:
The converter performs dual functions: it provides power conversion during normal operation and simultaneously acts as a voltage balancing mechanism during power failure. By utilizing the converter's switching capability, the system self-regulates capacitor voltage balance without requiring additional active components or significant energy input
3Stability of the object's composition
If the converter operates at full capacity during power failure, then capacitor voltage unbalance is reduced, but energy consumption increases
Solution Approach 1:
The converter operates at partial capacity specifically for voltage balancing purposes rather than full power conversion. The control unit applies just enough switching action to correct the voltage unbalance, avoiding excessive energy consumption while still achieving the desired voltage balance. This partial action is sufficient to maintain system stability without wasting energy
Data Source
AI summary
In the present uninterruptible power supply apparatus (U1), in a power failure of a commercial AC power supply (41), a switch (1) is turned off to electrically cut off the commercial AC power supply (41) from an AC input filter (2), and when DC voltage (ΔE=Ep−En) that is the difference between terminal-to-terminal voltages (Ep, En) of first and second capacitors (C1, C2) exceeds a threshold voltage (ETH), first and second IGBT devices (Q1, Q2) or third and fourth IGBT devices (Q3, Q4) included in the converter (3) are turned on and off to reduce DC voltage (ΔE).