Grid Connection Inverter Voltage Stepwise Adjustment
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Solution Overview
Problem
Existing grid connection power conversion devices face challenges in reducing inrush currents to capacitors during start-up, which can lead to relay welding and increased manufacturing costs due to the need for additional circuitry, and are unable to effectively manage inrush currents to LC filters between the inverter and commercial power systems.
Innovation Solution
A grid connection power conversion device with an isolated operation function that includes an inverter, a capacitor, a commercial voltage amplitude detection circuitry, and a start-up control circuitry to adjust the output voltage stepwise to match the commercial system voltage, thereby reducing inrush currents without using an inrush current prevention circuitry, and further includes phase angle adjustment to synchronize with the commercial power system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the inverter is connected to the commercial power system at start-up, then grid-connected operation can begin immediately, but an inrush current flows to the capacitor causing relay welding and potential circuit damage
Solution Approach 1:
The control unit performs preliminary voltage application to the capacitor through the inverter before closing the grid connection relay. This preliminary action charges the capacitor gradually, preventing inrush current when the relay closes. The control unit applies voltage in a controlled manner during a predetermined period before relay closure, ensuring the capacitor is pre-charged to avoid sudden current surges that would weld the relay contacts.
2Object-affected harmful factors
If an inrush current prevention circuitry with parallel switch and resistor is added, then inrush current to the capacitor is reduced, but manufacturing cost increases
Solution Approach 1:
The patent replaces the mechanical/electrical inrush current prevention circuitry (parallel switch and resistor) with a control-based solution. The control unit uses software control to manage the inverter's voltage output, applying voltage gradually to the capacitor before relay closure. This substitution eliminates the need for additional physical components while achieving the same inrush current reduction effect.
3Speed
If voltage is applied abruptly to the capacitor at start-up, then grid connection can be established immediately, but inrush current causes relay welding and circuit damage
Solution Approach 1:
The control unit applies voltage to the capacitor in a periodic, controlled manner during a predetermined period before relay closure. Instead of abrupt voltage application, the system uses periodic voltage steps or gradual increase, ensuring the capacitor charges smoothly. This periodic action pattern prevents inrush current while still achieving quick start-up within the predetermined time frame.
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
The solution effectively reduces inrush currents to capacitors during start-up, prevents relay welding, and eliminates the need for additional circuitry, while enabling reliable detection of disconnection and welding of grid connection relays, thus enhancing the efficiency and cost-effectiveness of the power conversion process.
Implementation Method 1
The inverter comprises switching elements such as IGBTs (Insulated Gate Bipolar Transistors) which are switched by a PWM (Pulse Width Modulation) signal sent from a control unit
Implementation Method 2
an LC (inductor-capacitor) filter for removing high frequency components, which is provided between the inverter and the commercial power system
Implementation Method 3
the voltage applied to the capacitor of the LC filter abruptly increases from 0V to the commercial system voltage. Therefore, an inrush current may flow in the capacitor
Data Source
AI summary
A grid connection power conversion device having isolated operation function for connecting a distributed power supply to a commercial power system is provided which comprises an inverter for converting DC power to AC power, a capacitor connected between the inverter and the commercial power system, a commercial voltage amplitude detection circuitry, an amplitude adjustment circuitry to increase an amplitude of an output voltage from the inverter stepwise from a given value to match the amplitude of the commercial system voltage, and a start-up control circuitry to control the grid connection power conversion device so that after the amplitude adjustment circuitry adjusts the amplitude of the output voltage from the inverter, after the start-up, to match the amplitude of the commercial system voltage detected by the commercial voltage amplitude detection circuitry, the start-up control circuitry connects the inverter to the commercial power system to start grid-connected operation.