Active Protection for DC Distribution via Harmonic Injection
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Solution Overview
Problem
Existing protection systems for multi-terminal and distributed DC power systems face challenges in accurate fault detection, location, and fast isolation due to lack of standards, high non-linearity, and reliance on transient fault currents that disappear quickly, making it difficult to identify faulted areas and restore power efficiently.
Innovation Solution
A fault detection and location-based active protection system that uses a controller to monitor current values, apply harmonic analysis, and adjust switching frequencies of DC/DC converters to determine fault points, allowing for precise fault location and rapid system restoration without additional equipment or high data sampling rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If transient fault current methods are used for fault detection, then fault detection capability is provided, but accuracy deteriorates because transient fault currents disappear quickly
Solution Approach 1:
The system performs preliminary action by injecting a test signal into the DC cable before actual fault occurs. This test signal travels through the cable and reflects off the fault point, allowing the system to detect and locate faults based on the reflected signal characteristics rather than relying on transient fault currents that disappear quickly.
Solution Approach 2:
The patent uses an intermediary approach by introducing a test signal as a mediator between the detection system and the fault point. This test signal serves as an intermediary carrier that travels through the DC cable, interacts with the fault point, and returns reflected information to the detection system, enabling accurate fault location without directly measuring transient fault currents.
2Measurement precision
If additional equipment is added to improve fault detection accuracy, then measurement precision improves, but device complexity increases
Solution Approach 1:
The controller in the patent performs multiple functions: it generates the test signal, transmits it through the DC cable, receives the reflected signal, processes the signal to determine fault location, and triggers protection actions. By making the controller multi-functional, the system achieves accurate fault detection without adding separate dedicated equipment for each function.
Solution Approach 2:
The DC cable itself serves as the transmission medium for the test signal, and the fault point automatically generates the reflected signal that carries location information. The system uses the existing cable infrastructure and fault characteristics rather than requiring additional specialized equipment, enabling the system to detect and locate faults using its own components.
3Measurement precision
If high data sampling rates are used to capture fault information, then measurement precision improves, but use of energy increases
Solution Approach 1:
Instead of continuous high-rate sampling, the system uses periodic action by injecting a test signal at specific intervals and measuring the reflected signal at predetermined time points. This approach captures the essential fault information at critical moments without requiring continuous high-speed sampling, thereby reducing energy consumption while maintaining measurement precision.
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 system significantly reduces fault restoration time, improves renewable power output efficiency, and maintains load power supply with high selectivity and reliability, outperforming existing methods in speed and accuracy.
Implementation Method 1
apply the fast Fourier transform to the current waveform to extract a maximum frequency from the current waveform
Implementation Method 2
change a switching frequency of the first DC/DC converter to cause the first DC/DC converter to produce a first harmonic output current waveform comprising a first characteristic frequency corresponding to the switching frequency of the first DC/DC converter
Data Source
AI summary
A detection and location-based active protection method for flexible DC distribution systems with multi-terminal distributed photovoltaic sources is disclosed. The disclosed protection method actively utilizes the coordinated control between local protection and the converters in the DC distribution system. The converter can then be modified to become an injection source with characteristic signal, providing a known fault signal to build a clear protection boundary. The disclosed protection method can distinguish the correct faulted area by calculating the harmonic impedance of the characteristic signals. Compared with existing DC protection techniques, this disclosed method does not require additional injection equipment and modification of the DC distribution system configuration, nor does it need high data sampling frequency. The disclosed technique is also unaffected by measuring noise and cable-distributed capacitance. The disclosed protection method is therefore feasible for industry application in a large and complex DC network.