UPS Inverter Pre-Charge for Relay Inrush-Free Startup
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Solution Overview
Problem
The existing uninterruptible power supplies (UPS) experience a high failure rate due to arcing of relay contacts caused by a large inrush current during the power-on process, which is a result of the voltage difference between the mains power and the bus capacitor, leading to reduced reliability.
Innovation Solution
The UPS is designed with an inverter circuit that performs energy storage and freewheeling operations to charge the bus capacitor, ensuring its voltage exceeds the mains power peak, and the mains relay is powered on when the rectification circuit limits current direction, preventing inrush currents and arcing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the pre-charge circuit charges the bus capacitor through uncontrolled rectification with current-limiting resistor, then the bus capacitor can be pre-charged, but a large inrush current occurs when the mains relay is closed causing arcing of relay contacts and increasing failure rate
Solution Approach 1:
The inverter circuit performs energy storage operation before the mains relay is closed, pre-charging the bus capacitor to a voltage higher than the peak mains voltage. This preliminary action ensures that when the mains relay closes, no inrush current occurs because the capacitor voltage is already sufficient, eliminating the need for current-limiting resistors and preventing relay contact arcing.
Solution Approach 2:
The patent replaces the traditional mechanical pre-charge circuit with current-limiting resistors and relays with an inverter circuit performing energy storage operation. This substitution uses electronic control to achieve pre-charging, eliminating the mechanical wear and arcing issues associated with relay contacts while providing more precise voltage control.
2Device complexity
If the pre-charge circuit is connected directly to the direct-current bus, then the charging process is simple, but the voltage difference between mains power and bus capacitor causes large inrush current
Solution Approach 1:
The inverter circuit is designed to perform multiple functions: it can operate as a pre-charge circuit through energy storage operation, function as the main rectification circuit, and provide freewheeling capability. By connecting the pre-charge circuit to the inverter circuit output, the system achieves multi-functionality without increasing overall complexity, as the same inverter hardware serves multiple purposes at different stages.
3Productivity
If the bus capacitor voltage is lower than mains peak voltage at power-on, then the capacitor needs to be charged, but this creates voltage difference leading to inrush current when relay closes
Solution Approach 1:
The system performs preliminary energy storage operation through the inverter circuit before closing the mains relay, ensuring the bus capacitor is pre-charged to a voltage higher than the peak mains voltage. This preliminary action eliminates the voltage difference that would otherwise cause inrush current, while the inverter's controlled charging provides efficient productivity without harmful effects.
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 arcing of relay contacts and enhances the stability of the UPS during power-on processes by eliminating inrush currents, thereby improving reliability.
Implementation Method 1
the inverter circuit performs energy storage and freewheeling operations to charge the bus capacitor
Implementation Method 2
the inverter circuit performs energy storage and freewheeling operations to charge the bus capacitor
Implementation Method 3
the mains relay is powered on when the rectification circuit limits current direction, preventing inrush currents and arcing
Data Source
AI summary
Provided are an uninterruptible power supply (UPS), a method and an apparatus for soft starting the UPS, and a controller. In the UPS, an inverter circuit is provided with three-phase output terminals, and two output terminals of a pre-charge circuit are electrically connected to output terminals of any two phases, respectively. The UPS charges a bus capacitor through an energy storage operation and a freewheeling operation, 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 limitation of a conduction direction in a rectification circuit, no inrush current occurs, thereby reducing arcing of relay contacts and improving stability of the UPS during a power-on process.