Variable Choke Overvoltage Protection With Low Normal Impedance
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Solution Overview
Problem
Chokes in power grid protection devices cause voltage drops and unwanted reactive power during normal operation, and can lead to L-C series resonances in DC networks, making them unsuitable for signal transmission paths.
Innovation Solution
A protective device with two variable chokes, each having a soft magnetic core pre-magnetized by a source magnet, allowing the chokes to act like air chokes during normal operation, reducing impedance and preventing resonances, while increasing impedance at higher currents to provide protection against overvoltages and short circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a choke with ferromagnetic core and permanent magnets is used to provide impedance for protection, then protection coordination and current limiting are improved, but voltage drop and reactive power increase during normal operation
Solution Approach 1:
The choke uses a variable inductance design where the impedance changes dynamically based on current magnitude. During normal operation, the inductance is minimized to reduce voltage drop and reactive power. During fault conditions, the inductance increases automatically to provide protection coordination and current limiting.
Solution Approach 2:
The magnetic core utilizes saturation effects where the permeability changes with magnetic field strength. By designing the permanent magnet strength and air gap to create a specific operating point, the choke maintains low impedance at normal currents but automatically increases impedance when current exceeds a threshold, resolving the contradiction between normal operation efficiency and protection capability.
2Reliability
If a choke with permanent magnets is used to provide impedance for protection, then protection coordination is improved, but L-C series resonances occur in DC networks
Solution Approach 1:
The variable inductance design allows the choke to present a dynamic impedance characteristic that prevents resonance. During normal operation, the minimized inductance breaks the resonant circuit conditions. The impedance only increases when current exceeds the threshold, at which point protection action is already indicated, thus avoiding sustained resonant conditions.
3Reliability
If a choke is used to provide impedance for protection, then current limiting is improved, but transmission behavior is influenced in signal transmission lines
Solution Approach 1:
The choke's inductance parameter changes with current magnitude. At normal operating currents, the inductance is minimized to maintain proper transmission behavior in signal lines. When current exceeds the protection threshold, the inductance increases to provide current limiting, at which point transmission function is already compromised by the fault condition.
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 solution minimizes disruptions to power networks by maintaining low impedance during normal operation and increasing impedance only when necessary, effectively protecting against overvoltages and short circuits without affecting signal transmission.
Implementation Method 1
the source magnet is designed such that it pre-magnetizes the soft magnetic part in a pre-magnetization direction
Implementation Method 2
increasing impedance at higher currents to provide protection against overvoltages and short circuits
Data Source
Figure 1~2a
Figure 2b~2c
Figure 2d~3a
AI summary
A protective device (16) for an electrical grid (10) has a variable choke (22) which comprises a conductor (34) having at least one turn (36) and comprises a core (32) which is arranged at least partly within the at least one turn (36). The core (32) has a soft magnetic part (40) and at least one source magnet (42), the source magnet (42) being designed in such a way that the source magnet pre-magnetizes the soft magnetic part (40) in a pre-magnetization direction (MV). The protective device (16) is an overvoltage protection device (52), comprising a coarse protection means (54) and a fine protection means (56), the variable choke (22) being arranged between the coarse protection means (54) and the fine protection means (56).