Grid-Connected Inverter Seamless Switching Control
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Solution Overview
Problem
Existing grid-connected inverter systems face challenges in achieving seamless power switching due to long switching times when using relay-type low-speed breakers, which can lead to safety issues and inefficiencies, especially during system abnormalities like power failures.
Innovation Solution
A grid-connected inverter system with a controller that operates in current control mode until the breaker turns off, then transitions to voltage control mode, allowing for zero switching time and seamless power supply, even with low-speed breakers like relays or magnetic connectors, eliminating the need for high-speed semiconductor devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a relay-type low-speed breaker is used to interrupt current to the grid, then the device complexity is reduced and cost is lowered, but the switching time increases to about 4 ms or later, making seamless switching impossible
Solution Approach 1:
The inverter performs preliminary action by generating and storing power before the breaker interrupts current to the grid. When abnormality is detected, the inverter has already prepared power that can be immediately supplied to the load, eliminating the 4ms or later switching delay inherent in relay-type breakers.
Solution Approach 2:
The inverter acts as an intermediary between the power source and the load. When the breaker interrupts current to the grid, the inverter seamlessly transfers power supply to the load using its generated power, preventing any interruption in load power supply despite the breaker's slow switching speed.
2Loss of time
If a high-speed semiconductor device such as SCR is used to reduce switching time, then the switching time is reduced, but the device lacks self-turn-off ability and requires reverse bias, creating safety risks when current interruption fails
Solution Approach 1:
The breaker performs self-service by automatically detecting grid abnormalities and interrupting current to the grid without requiring complex control circuits. The inverter simultaneously performs self-service by automatically detecting the breaker's turn-off status and switching from grid-connected mode to independent operation, ensuring reliable current interruption without the safety risks of semiconductor devices.
Solution Approach 2:
The patent replaces the semiconductor device's electronic switching mechanism with a combination of mechanical breaker and inverter control. The inverter uses its switching elements to seamlessly transfer load power supply, eliminating the need for high-speed semiconductor breakers and their associated safety risks while achieving zero switching time.
3Reliability
If the inverter switches from grid-connected operation to independent operation during system abnormality, then seamless power supply to the load is achieved, but the control complexity increases due to mode switching requirements
Solution Approach 1:
The controller uses feedback by continuously monitoring the breaker's operation status and grid conditions. When abnormality is detected and the breaker interrupts current, the controller receives feedback and automatically switches the inverter from grid-connected operation to independent operation, ensuring seamless power supply while managing control complexity through intelligent feedback-based decision making.
Data Source
AI summary
A grid-connected inverter system having a seamless switching function. An inverter converts DC power into AC power. A breaker is connected between the inverter, a grid, and a load to switch between a grid-connected operation and an independent operation. A filter converts an output of the inverter into a sine wave. A controller operates the inverter in a current control mode or a voltage control mode. The controller operates the inverter in the current control mode for a period of time longer than a turn-off time of the breaker when an abnormality in the grid is detected, and operates the inverter in the voltage control mode when the grid is disconnected from the load due to turn-off of the breaker.