Three-Phase Inverter Protection Circuit for STG Relay Isolation
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Solution Overview
Problem
Photovoltaic system cables are prone to short-to-ground faults, leading to kiloampere-level short-circuit currents that risk damaging circuit components.
Innovation Solution
A protection circuit for a three-phase inverter system that includes a controllable power device, fault detection module, and a controller to safely disconnect relays and shunt short-circuit currents, using a controllable power device to prevent damage by shunting currents through flyback diodes and disconnecting relays at zero-crossing conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If relay assembly is disconnected during short-circuit current flow, then circuit protection is achieved, but relay damage occurs due to high current
Solution Approach 1:
The controller detects short-circuit current in advance and prepares for relay disconnection by first activating the controllable power device to establish an alternative current path, ensuring the relay is only disconnected when current is safely diverted, thus preventing relay damage while achieving circuit protection
Solution Approach 2:
The controllable power device serves as an intermediary element that provides an alternative current path during short-circuit conditions. When activated, it diverts the short-circuit current away from the relay assembly, allowing the relay to be disconnected safely without承受ing the full current stress
2Strength
If controllable power device is activated to shunt short-circuit current, then relay is protected from damage, but circuit complexity increases
Solution Approach 1:
The controllable power device is integrated into the existing inverter circuit architecture, serving dual functions: normal power conversion operations and short-circuit current diversion. This multi-functionality approach protects the relay without requiring entirely separate protection circuitry, thereby limiting the increase in overall circuit complexity
3Strength
If relay disconnects at zero-crossing point, then relay is safely disconnected, but response time is delayed
Solution Approach 1:
The controller prioritizes relay protection by waiting for the nearest zero-crossing point of the AC current waveform before executing disconnection. This approach safely skips through the brief period where current is non-zero but minimal, achieving safe relay disconnection while minimizing the time delay to the next available zero-crossing instant
Data Source
AI summary
A protection circuit includes: a controllable power device in parallel connection with a bridge arm in a three-phase inverter circuit, wherein a DC side of the three-phase inverter circuit is configured to connect a power supply assembly, an output terminal of each phase in the three-phase inverter circuit is provided with a relay assembly; a fault detection module electrically connected to the three-phase inverter circuit, and configured to generate a fault signal when detecting a short-to-ground (STG) fault; a current detection circuit configured to detect an output current of each phase in the three-phase inverter circuit; and a controller electrically connected to the fault detection module, the current detection circuit and the relay, and configured to control the controllable power device to be on after receiving the fault signal, and disconnect the relay assembly of each phase when the output current of each phase meets a zero-crossing condition.