Transformerless Power Converter Precharge for DC Grid Insulation Faults
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Solution Overview
Problem
When a DC grid connected to a grounded AC supply grid, uncontrolled ground currents can flow, potentially damaging components, especially when insulation faults are present, and existing insulation monitoring systems are inadequate for transformerless power converters.
Innovation Solution
A power converter with a galvanically isolating AC precharging circuit and an insulation monitor that measures insulation resistance before connecting the DC grid, allowing safe and controlled power transfer by precharging the DC link circuit and monitoring insulation resistance to prevent ground faults.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a transformerless power converter is used to connect a DC grid to a grounded AC supply grid, then cost is reduced and device complexity is lowered, but uncontrolled ground currents can flow causing component damage
Solution Approach 1:
The patent applies preliminary action by precharging the DC link circuit through a galvanically isolating AC precharging circuit before connecting the DC grid to the AC supply grid. This precharging process establishes controlled current paths and ensures proper potential distribution, preventing uncontrolled ground currents from flowing when the transformerless power converter connects the ungrounded DC grid to the grounded AC supply grid.
Solution Approach 2:
The patent introduces an intermediary element - the galvanically isolating AC precharging circuit with insulation monitor - that mediates between the ungrounded DC grid and the grounded AC supply grid. This intermediary provides galvanic isolation during the precharging phase and enables controlled connection, acting as a buffer that prevents direct harmful current paths while maintaining the transformerless cost-effective design.
2Reliability
If insulation monitoring is implemented in a transformerless power converter, then protection against ground faults is improved, but device complexity increases
Solution Approach 1:
The patent applies universality by integrating the insulation monitoring function into the existing AC precharging circuit components. The insulation monitor utilizes the same galvanic isolation transformer and control circuitry already present in the precharging circuit, allowing a single system to perform both power transfer and insulation monitoring functions, thereby improving ground fault protection without proportionally increasing device complexity.
Solution Approach 2:
The insulation monitoring system is designed to self-test and self-report insulation status during the precharging operation. The system automatically monitors insulation resistance between the DC link circuit and ground, and can independently detect and signal ground faults without requiring external monitoring equipment, thus providing reliable protection with minimal additional complexity.
3Reliability
If the DC link circuit is precharged from the AC supply grid, then safe power transfer is enabled, but additional circuitry is required
Solution Approach 1:
The galvanically isolating AC precharging circuit serves as an intermediary that enables safe power transfer by providing controlled charging paths and galvanic isolation. This intermediary circuit allows the ungrounded DC link circuit to be safely charged from the grounded AC supply grid while preventing direct ground current paths, and the same intermediary structure is later utilized for insulation monitoring, justifying the added complexity through multi-functionality.
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
Enables safe and cost-effective connection of an ungrounded DC grid to a grounded AC grid, reducing the risk of component damage and ensuring compliance with insulation monitoring standards, even in the absence of a DC source, while maintaining galvanic isolation.
Implementation Method 1
a galvanically isolating AC precharging circuit for precharging a DC link circuit of the power converter from the AC supply grid
Implementation Method 2
an insulation monitor that measures insulation resistance before connecting the DC grid
Data Source
AI summary
A power converter between an AC side and DC of the power converter is disclosed. The AC side is connected to an AC supply grid and the DC side is connected to a DC grid. The power converter includes a bridge circuit connected to the AC side of the power converter via AC switches and connected to the DC side of the power converter via circuit breakers, wherein a DC link circuit of the power converter is chargeable from the AC supply grid via an AC precharging circuit. The power converter has an insulation monitor to measure the insulation resistance of the DC side when the AC precharging circuit is connected to the DC link circuit. The disclosure also includes a related method.