UPS Soft-Start Inverter Pre-Charge to Prevent Relay Arcing
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Solution Overview
Problem
The reliability of uninterruptible power supplies (UPS) is compromised due to large inrush currents during the power-on process, causing arcing of relay contacts and increasing failure rates, primarily due to the voltage division by current-limiting resistors in the pre-charge circuit.
Innovation Solution
The UPS design includes an inverter circuit with three-phase output terminals, where the pre-charge circuit terminals are connected to any two phases of the inverter circuit, allowing the bus capacitor to be charged through energy storage and freewheeling operations, ensuring the bus capacitor voltage exceeds the mains power peak, thus preventing inrush currents and reducing arcing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the pre-charge circuit uses current-limiting resistors to charge the bus capacitor, then the capacitor can be charged, but large inrush current occurs when the mains relay is closed causing relay contact arcing and increased failure rate
Solution Approach 1:
The inverter circuit performs preliminary energy storage operation before the mains relay is closed, charging the bus capacitor to a voltage higher than the peak mains voltage. This preliminary action ensures that when the relay closes, no inrush current can flow from the mains to the bus, eliminating relay contact arcing and improving reliability
Solution Approach 2:
The invention changes the voltage parameter of the bus capacitor from a traditionally lower value to a value higher than the peak mains voltage. By controlling the inverter circuit to charge the bus capacitor to this elevated voltage level before relay closure, the voltage differential that would normally cause inrush current is reversed, preventing the harmful effect
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 enhances the stability of the UPS during power-on by eliminating inrush currents, thereby reducing relay contact arcing and improving reliability.
Implementation Method 1
the bus capacitor can be charged through the energy storage operation and the freewheeling operation of the inverter circuit
Implementation Method 2
the bus capacitor can be charged through the energy storage operation and the freewheeling operation of the inverter circuit
Data Source
Figure 1
Figure 2
Figure 3A
AI summary
Provided in the present disclosure are an uninterruptible power supply, a method and an apparatus for soft starting the uninterruptible power supply, and a controller. In the uninterruptible power supply, an inverter circuit is provided with three-phase output terminals, and two output terminals of the pre-charge circuit are electrically connected to output terminals of any two phases of the inverter circuit, respectively, rather than directly connected to a direct-current bus. When the uninterruptible power supply charges a bus capacitor with the pre-charge circuit, the bus capacitor can be charged through an energy storage operation and a freewheeling operation of the inverter circuit, so that a voltage of the bus capacitor is greater than a peak value of a mains power. When the mains relay is powered on, a voltage of an end of the mains relay that is electrically connected to the mains power is lower than the voltage of the bus capacitor. Due to the limitation of a conduction direction of a rectification device in a rectification circuit, no inrush current occurs. Hence, arcing of relay contacts caused by the inrush current is reduced, and thereby the stability of the uninterruptible power supply during a power-on process is improved.