PV Inverter Backfeed Soft-Start Circuit for Inrush Current Suppression
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Solution Overview
Problem
Conventional string photovoltaic inverters face issues with inrush currents due to unidirectional DC/DC converters, leading to potential damage and operational challenges during photovoltaic module testing, especially when a direct current backfeed current is transmitted, and require multiple MPPTs and DC/DC converters, increasing device size and security risks.
Innovation Solution
A backfeed soft-start circuit with branch switches and a primary switching switch, connected between the DC/AC converter and the photovoltaic array, controls the turning-on and turning-off of branch switches to selectively configure module testing, and includes current-limiting components to suppress inrush currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional start-up circuit is used, then the inverter can start up, but the IGBTs are vulnerable to overcurrent damage during start-up and backward flowing conditions
Solution Approach 1:
The patent extracts the overcurrent protection function from a complex protection circuit and implements it through a simple current mirror mechanism. The current mirror circuit (Q1-Q2 transistors) independently replicates the emitter current to control the base current, providing automatic overcurrent protection without requiring additional complex control logic or sensing circuits.
Solution Approach 2:
The start-up circuit uses the IGBT's own emitter current to control its base current through the current mirror mechanism. The circuit automatically regulates the base current based on the emitter current without external intervention, enabling the IGBT to protect itself during start-up and backward flowing conditions.
2Productivity
If the inverter starts up quickly, then productivity is improved, but the IGBTs may be damaged by excessive current
Solution Approach 1:
The start-up circuit is activated before the main inverter operation to pre-charge the DC link capacitor and gradually build up voltage. The circuit enables controlled current flow during the start-up phase, allowing the system to reach operational voltage without subjecting the IGBTs to damaging inrush currents.
Solution Approach 2:
The current mirror mechanism provides automatic feedback control where the emitter current directly influences the base current. When the emitter current exceeds safe levels during start-up, the feedback mechanism automatically reduces the base current, thereby limiting the collector current and protecting the IGBT from overcurrent damage.
3Loss of energy
If the inverter handles backward flowing, then energy recovery is improved, but the start-up circuit may be damaged by reverse current
Solution Approach 1:
The start-up circuit incorporates protective mechanisms that act in advance to prevent reverse current damage. The circuit design includes current direction control and protection elements that are activated before backward flowing occurs, preventing the start-up circuit from being exposed to damaging reverse currents while still allowing energy recovery functionality.
4Reliability
If external resistance is used for start-up, then IGBT protection is improved, but energy loss increases
Solution Approach 1:
The patent replaces the mechanical/electrical resistance-based current limiting method with an active electronic control mechanism using the current mirror circuit. Instead of dissipating excess energy as heat through resistance, the circuit actively regulates base current to limit collector current, enabling IGBT protection during start-up without the continuous energy losses associated with resistive damping.
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 suppresses inrush currents, protects the inverter from damage, and allows flexible configuration of photovoltaic modules for testing, ensuring safe operation and efficient energy utilization.
Implementation Method 1
the start-up circuit comprising a current mirror circuit configured to mirror a current flowing through an emitter of the IGBT to a base of the IGBT
Data Source
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AI summary
A backfeed soft-start circuit (153) for a photovoltaic inverter is provided. The backfeed soft-start circuit (153) includes a plurality of branch switches (K1, K2, ..., and KN). A solar photovoltaic array includes a plurality of photovoltaic modules (100, 101, ..., and 102), where the plurality of branch switches (K1, K2, ..., and KN) are in a one-to-one correspondence with the plurality of photovoltaic modules (100, 101, ..., and 102), and when the plurality of branch switches (K1, K2, ..., and KN) are turned on, the corresponding photovoltaic modules (100, 101, ..., and 102) are connected to the backfeed soft-start circuit; and a primary switching switch (Kc). One end of each of the branch switches (K1, K2, ..., and KN) is connected to a corresponding photovoltaic module (100, 101, ..., or 102), and the other end is connected to the primary switching switch (Kc). The primary switching switch (Kc) is turned on after the branch switch (K1, K2, ..., or KN) is turned on.