Varistor Overvoltage Protection with Thermal Segmentation
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Solution Overview
Problem
Existing overvoltage protective devices for lightning protection, such as those using varistors and PTC thermistors, face issues with short-circuit failures due to rapid degradation and reduced surge-withstand capability, leading to potential fires and equipment damage, as they fail to effectively transition from short-circuit to open-circuit mode quickly enough to prevent damage.
Innovation Solution
A novel overvoltage protective device comprising a first varistor with higher surge-withstand capability connected in parallel with a second varistor and a PTC thermistor in series, with a thermally-conductive terminal for heat management, allowing the first varistor to remain intact while the second varistor degrades, and utilizing a thermal disconnector to safely remove the device from the power grid when necessary.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thermal disconnector with low-melting alloy is used to protect the varistor, then the failure mode transitions from short-circuit to open-circuit, but the response time is too slow (several seconds) to prevent short-circuit breakdown when degradation is rapid
Solution Approach 1:
The patent divides the protective function into two independent components: a PTC thermistor that responds rapidly to temperature changes by changing resistance, and a thermal disconnector that provides ultimate protection by melting. This segmentation allows each component to optimize its response characteristics - the PTC for fast response and the thermal disconnector for reliable failure mode transition.
Solution Approach 2:
The PTC thermistor acts as an intermediary between the varistor and the thermal disconnector. It first responds to the heat from varistor degradation by increasing its resistance, which limits the current and buys time. This intermediary action allows the slower thermal disconnector to eventually activate without experiencing the full force of rapid short-circuit breakdown.
2Duration of action of moving object
If a PTC thermistor is used to limit leakage current and prolong degradation time, then the varistor can maintain thermal stability, but the follow-on current cannot be disconnected when varistor voltage drops, causing the PTC to short and the varistor to breakdown
Solution Approach 1:
The thermal disconnector serves as a mediator that ultimately disconnects the circuit when the varistor voltage drops too low. While the PTC thermistor limits current and prolongs degradation, the thermal disconnector provides the final safety mechanism to prevent the harmful scenario where follow-on current causes PTC shorting and varistor breakdown.
Solution Approach 2:
The thermal disconnector is designed to melt at a specific temperature threshold before the varistor can undergo complete breakdown. This beforehand cushioning ensures that even if the PTC thermistor fails to disconnect the follow-on current, the thermal disconnector will have already activated to prevent catastrophic failure.
3Reliability
If the varistor degrades to the accelerated degradation stage with milliampere-level leakage current, then thermal protection is needed, but the heating speed is too fast for the thermal disconnector to respond before short-circuit occurs
Solution Approach 1:
The protective system is segmented into two response mechanisms: the PTC thermistor that responds immediately to temperature increases by changing its electrical resistance, and the thermal disconnector that responds to sustained thermal conditions by melting. This segmentation allows the system to handle both fast heating scenarios (via PTC) and provide ultimate protection (via thermal disconnector).
Solution Approach 2:
The PTC thermistor provides immediate feedback to temperature changes by increasing its resistance when heated, which in turn reduces the leakage current and limits further heating. This feedback mechanism creates a self-regulating system that slows down the heating process, giving the thermal disconnector time to activate.
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 configuration ensures the first varistor remains functional while the second varistor degrades, providing sufficient time for thermal protection to activate, preventing short-circuit failures and reducing equipment failure rates by ensuring safe disconnection and easy maintenance, thereby enhancing safety and reliability.
Implementation Method 1
A PTC thermistor (positive temperature coefficient thermistor) B12 is connected in parallel with a GDT (gas discharge tube) 13 and then connected with a varistor B11 in series. When the varistor B11 degrades to the accelerated degradation stage, the temperature of PTC thermistor B12 elevates because of the heat conducted from the varistor B11. The elevated temperature results in the increase of its resistance value
Implementation Method 2
An elastic metal sheet A4 is soldered on one of the lead-out terminals A3 with a low-melting alloy A5 to form a thermal disconnector. When the heat reaches the melting point of the low-melting alloy A5, the thermal disconnector cuts off the power supply, so that the varistor A1 exits the power grid before the short-circuit occurs
Implementation Method 3
The thin copper sheet electrode A2 functions as a conductive electrode and also forms a thermal coupling with the varistor A1 to conduct the heat generated by the varistor A1 to the lead-out terminal A3
Data Source
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AI summary
Disclosed in the present invention is a novel overvoltage protective device for lightning protection, comprising a first varistor, a second varistor, a PTC Thermistor, and lead-out terminals. The first varistor and the PTC Thermistor are connected in parallel, and then further connected in series with the second varistor to form a single port combined circuit. The surge-withstand capability of the first varistor is higher than the surge-withstand capability of the second varistor. At least one of the two lead-out terminals of the single port combined circuit is a thermally-conductive end with low thermal resistance. The second varistor is thermally coupled to the PTC Thermistor. The thermally-conductive end with low thermal resistance is thermally coupled to one or both of the second varistor and the PTC Thermistor.