DC Bus Pre-Charge Control for AC Power Surge Prevention
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Solution Overview
Problem
Conventional transport refrigeration systems face issues with high surge currents when connecting to external AC power supplies, leading to energy dissipation and size/mass penalties, and the use of passive resistors does not address these issues.
Innovation Solution
The use of an electrical apparatus with a pre-charge circuit to provide a pre-charge circuit to the DC bus, which includes a controllable current source and a control system to monitor and manage the voltage, reducing the risk of surge currents and energy dissipation, and a control system to manage the connection to the external AC power supply.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large passive resistor is used to limit initial current drawn from external AC power supply, then surge current risk is reduced, but energy dissipation as heat increases and device size/mass increases
Solution Approach 1:
The pre-charge circuit is activated before the main power connection to gradually charge the DC bus capacitors to a predetermined voltage level. This preliminary action prevents surge current when the main contactor closes, eliminating the need for large passive resistors that would continuously dissipate energy as heat.
Solution Approach 2:
The system transitions from a static passive resistor approach to a dynamic controlled pre-charge circuit with switching elements (MOSFETs/IGBTs) and control logic. The pre-charge contactor and main contactor are sequentially operated based on real-time voltage monitoring, optimizing current limiting only when necessary during the pre-charge phase without continuous energy loss.
2Reliability
If a large passive resistor is used to limit initial current, then surge current risk is reduced, but device complexity and mass increase
Solution Approach 1:
The pre-charge circuit performs the current-limiting function in advance before main power connection, allowing the main power path to remain simple without requiring large passive resistors. The complexity is confined to the temporary pre-charge path with controlled switching elements rather than the main power architecture.
Solution Approach 2:
The pre-charge circuit acts as an intermediary between the external AC power supply and the main DC bus. It provides a controlled intermediate charging path that prepares the DC bus voltage before main power connection, isolating the main power system from surge current effects without requiring complex protection circuitry in the main path.
3Reliability
If pre-charge circuit is activated to provide pre-charging current to DC bus, then surge current is prevented, but system operation complexity increases
Solution Approach 1:
The control system continuously monitors the DC bus voltage and uses this feedback to control the pre-charge circuit operation. When the monitored voltage reaches the predetermined threshold, the control system automatically deactivates the pre-charge contactor and activates the main contactor, creating a self-regulating system that reduces operational complexity through automated decision-making.
Solution Approach 2:
The pre-charge circuit monitors its own operational status through voltage sensing and automatically manages its own deactivation when the DC bus is sufficiently charged. The system serves itself by using the DC bus voltage level as both the control signal and the termination criterion, eliminating the need for external timing circuits or complex control logic.
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
This approach reduces energy dissipation and the need for passive cooling, minimizing the size and mass of the electrical apparatus while ensuring safe and efficient power transfer.
Implementation Method 1
The pre-charging current provided to the DC bus is for simultaneously pre-charging the AC power module and the core power module
Data Source
AI summary
The present disclosure relates to an electrical apparatus comprising: an AC power module including a rectifier system; a DC bus configured to electrically couple the AC power module to a core power module including at least one DC electrical component; a pre-charge circuit; and a control system. The control system is configured to: responsive to a pre-charge initiation signal, perform a pre-charging procedure for pre-charging the DC bus, the pre-charging procedure including: activating the pre-charge circuit to provide a pre-charging current to the DC bus; monitoring a voltage on the DC bus; and responsive to a determination that the monitored voltage on the DC bus has reached a pre-charging threshold, perform a run procedure including: deactivating the pre-charge circuit; and electrically coupling the AC power module to an external AC power supply via an AC bus.