Three-Phase Power Conversion Startup With Sequential Relay Closure
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Solution Overview
Problem
Solid-state transformers (SSTs) experience high contact power and reliability risks due to large currents during startup, leading to increased component volume and cost.
Innovation Solution
A power conversion circuit design where relays on each phase line are sequentially closed, with resistors connected in parallel to limit current increments and reduce contact power, and a method to control the sequential closure and opening of relays to manage startup and shutdown, ensuring balanced phase line settings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If relays are closed simultaneously during startup, then the power conversion circuit starts quickly, but large impact currents are generated causing high contact power and reliability risks
Solution Approach 1:
The startup process is segmented into multiple stages by dividing the relay closure into sequential steps. Instead of closing all relays simultaneously, the controller closes relays on different phase lines in sequence (e.g., first phase A, then phase B, then phase C), with each relay closure being a separate stage. This segmentation reduces the peak current impact at each stage while maintaining overall startup efficiency.
Solution Approach 2:
The controller performs preliminary action by pre-planning and executing relay closures in a predetermined sequence before full power conversion begins. The control device prepares the startup sequence in advance, closing relays gradually on each phase line before the bus capacitor is fully charged, thereby preventing sudden large currents while ensuring systematic startup.
2Reliability
If resistors are used to limit current during relay closure, then contact power is reduced, but component volume and cost increase
Solution Approach 1:
The external current-limiting resistors are extracted and replaced by the inherent impedance characteristics of the power conversion circuit itself. The bus capacitor's charging process naturally limits the current without requiring separate resistor components. The controller utilizes the circuit's own parameters (capacitor charging curves, phase line impedance) to achieve current limitation, eliminating the need for additional protective resistors.
Solution Approach 2:
The controller acts as an intermediary that manages the relay closure timing based on the charging state of the bus capacitor. Instead of using passive resistors to mediate current flow, the active controller monitors capacitor voltage and triggers relay closures at optimal moments, using control signals as the intermediary mechanism to coordinate between the power source and the relay switches.
3Power
If sequential relay closure is implemented, then contact power is reduced, but control complexity increases
Solution Approach 1:
The control device implements feedback by monitoring the bus capacitor voltage and using this information to determine the optimal timing for each relay closure. The controller continuously detects the charging state and adjusts the relay closure sequence accordingly, creating a closed-loop control system that automatically optimizes the startup process without requiring complex external control circuits.
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 design reduces relay contact power and volume, improves system reliability by gradually increasing current, and minimizes large current impacts during startup, resulting in more efficient and reliable power conversion.
Implementation Method 1
each of the plurality of first start circuits includes a first relay and a first resistor, the first relay and the first resistor are connected in parallel
Implementation Method 2
a solid-state transformer (SST) is a still electrical device that combines a power electronics conversion technology and an electromagnetic induction principle-based high-frequency electric energy conversion technology
Implementation Method 3
a capacitance of a bus capacitor in the power converter is 0, and mains electricity charges the bus capacitor
Data Source
AI summary
Embodiments of this application provide a power conversion circuit. The power conversion circuit includes at least one first power conversion unit connected in series to a first phase line, at least one second power conversion unit connected in series to a second phase line, at least one third power conversion unit connected in series to a third phase line, a plurality of first start circuits connected in series to the first phase line, and a plurality of second start circuits connected in series to the second phase line. Each first start circuit includes a first relay and a first resistor that are connected in parallel, and first relays in all the first start circuits are sequentially closed after the power conversion circuit is powered on, to start the power conversion circuit. Each second start circuit includes a second relay and a second resistor that are connected in parallel.