Protective Conductor Interruption Detection in Inverter Systems
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Solution Overview
Problem
In power supply systems, especially in grounded and unearthed systems with large leakage capacitances, the interruption of a protective conductor connection poses a significant risk of electric shock due to excessive leakage currents, as conventional residual current devices (RCDs) fail to provide reliable protection, particularly in high-performance converter drives and extensive systems where leakage currents exceed safety limits.
Innovation Solution
A method and electrical protective device that measures the total system leakage capacitance and power consumption to differentiate between an operational subsystem with an intact protective conductor and one that is interrupted, allowing for early detection of protective conductor interruptions without disrupting operations, using existing devices like PQ devices for measuring network quality and integrating them with insulation monitoring to form a combination device for signaling protective conductor issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional residual current devices (RCDs) are used for additional protection against electric shock, then protection reliability is improved, but device complexity and cost increase due to the need for specialized RCD types
Solution Approach 1:
The patent applies multi-functionality by enabling standard Type AC RCDs to provide protection in inverter systems with Type A or Type F characteristics. The frequency-dependent impedance network allows a single RCD type to effectively handle multiple fault conditions across different frequency ranges, eliminating the need for specialized RCD types and reducing device complexity while maintaining protection reliability.
Solution Approach 2:
The patent changes the electrical parameters of the RCD circuit by introducing frequency-dependent impedance elements (inductors and capacitors) that modify the impedance characteristics at different frequencies. This allows the RCD to selectively respond to fault currents at switching frequencies while ignoring normal leakage currents, achieving reliable protection without requiring specialized RCD hardware.
2Reliability
If Type A or Type F RCDs are used to detect switching frequency leakage currents, then protection against inverter faults is improved, but leakage current limits are exceeded causing RCD tripping
Solution Approach 1:
The patent changes the frequency-dependent impedance parameters of the circuit to create a frequency-selective response. By introducing inductors and capacitors with specific values, the circuit presents low impedance to fault currents at switching frequencies (enabling detection) while presenting high impedance to normal leakage currents at mains frequency (preventing tripping). This resolves the contradiction by allowing reliable detection without exceeding leakage current limits.
Solution Approach 2:
The patent converts the harmful broadband spectrum of inverter leakage currents into a beneficial selective signal. By using frequency-dependent impedance, the circuit exploits the switching frequency components to detect faults while filtering out the harmful high-magnitude leakage currents that would otherwise cause false tripping. The harmful leakage current is thus transformed into a useful detection signal.
3Device complexity
If standard Type AC RCDs are used in inverter systems, then device simplicity is maintained, but detection of switching frequency fault currents is insufficient
Solution Approach 1:
The patent modifies the electrical parameters of the existing Type AC RCD circuit by adding frequency-dependent impedance elements. These components change the circuit's frequency response characteristics, enabling it to selectively detect switching frequency fault currents while maintaining the simplicity of the standard RCD device. The modification enhances detection precision without requiring a complete change of RCD type.
4Measurement precision
If frequency-dependent impedance network is added to enable selective fault current detection, then detection precision is improved, but device complexity increases
Solution Approach 1:
The frequency-dependent impedance network is designed to provide multiple functions within a single integrated circuit: it acts as a band-pass filter for fault detection, an impedance matcher for optimal signal transfer, and a frequency selector for distinguishing fault currents from leakage currents. This multi-functionality reduces the need for separate components and minimizes overall circuit complexity while achieving high detection precision.
Data Source
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AI summary
The invention relates to a method and an electrical protection device for detecting an interruption of a protective conductor connection to a subsystem in ungrounded and grounded power supply systems, as well as in a grounded power supply system with a converter system. The underlying concept of the invention is that an interruption of the protective conductor connection to a subsystem reduces the sum of the network leakage capacitances of the power supply system by the amount of the network leakage capacitance of the subsystem. The necessary distinction between an operating subsystem with an interrupted protective conductor connection and a switched-off subsystem is made by evaluating the total power currently being drawn via the power supply system.In the case of an inverter system connected to the subsystem, the detection of the protective conductor interruption is carried out by examining the leakage current spectra characteristic of the inverter system.