Thyristor Rectifier Surge Protection via NTC Resistor
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Solution Overview
Problem
Thyristor rectifiers in AC-DC conversion applications face limitations due to their limited di/dt capability at the start of conduction, and existing protection methods, such as stopping the triggering signal during overvoltage, fail to provide comprehensive protection, leaving a vulnerable time slot.
Innovation Solution
Incorporating a NTC resistor in series with a capacitor between the positive and negative rectifier outputs to limit current during thyristor turn-on and enhance surge protection, while maintaining motor cable fault capability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an impedance is added to limit current during thyristor turn-on, then surge protection is improved, but device complexity increases
Solution Approach 1:
An NTC resistor is introduced as an intermediary component in series with the capacitor to limit surge current during thyristor turn-on. The NTC resistor acts as a current-limiting mediator that protects the thyristor from excessive di/dt stress while allowing normal operating current to pass through once the capacitor is charged.
Solution Approach 2:
The NTC resistor exploits the temperature-dependent resistance parameter change of thermistors. At cold temperatures (initial state), the NTC resistor has high resistance to limit surge current. As current flows and the resistor heats up, its resistance automatically decreases to allow normal operating current levels, providing dynamic parameter adaptation without additional control circuitry.
2Reliability
If a capacitor is added across the rectifier to protect against voltage peaks, then voltage protection is improved, but current path problems arise that can damage thyristors during surge
Solution Approach 1:
The NTC resistor is placed in series with the capacitor to act as a protective intermediary. During surge conditions, the NTC resistor's high resistance limits the current that can flow through the capacitor, preventing the capacitor from creating a damaging current path for the thyristor. During normal operation, the NTC resistance decreases, allowing the capacitor to function as intended for voltage protection.
Solution Approach 2:
The NTC resistor provides beforehand cushioning by being pre-positioned in the circuit to limit current before surge damage can occur. The resistor is designed to present high impedance during the critical initial surge period, cushioning the thyristor from excessive current stress before the surge current can damage the device.
3Reliability
If MOVs are used to protect against high voltage peaks, then voltage protection is improved, but overheating occurs that limits long-term protection capability
Solution Approach 1:
The NTC resistor serves as a current-limiting intermediary that reduces the overall power dissipation in the MOVs. By limiting surge current through the NTC resistor before it reaches the MOVs, the thermal load on the MOVs is significantly reduced, allowing them to provide voltage protection without overheating and maintaining their protection capability over longer periods.
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 provides improved surge robustness and protection during normal operation and fault conditions, complementing existing protective measures without overheating, thus enhancing the robustness of the thyristor rectifier system.
Implementation Method 1
The thyristor rectifier comprises an impedance and a capacitor arranged in series to each other, wherein the impedance and the capacitor connect the positive rectifier output to the negative rectifier output and wherein the impedance is a NTC-resistor
Data Source
AI summary
The present invention relates to AC-DC conversion in power electronics. In particular, the present invention is directed at a thyristor rectifier, which may be employed in an electric motor drive. The rectifier includes three thyristors and three diodes, wherein the thyristors are connected to a positive rectifier output and the three diodes are connected to a negative rectifier output. The present invention also relates to an electric motor drive comprising a corresponding thyristor rectifier. In general, the present invention may be used in a frequency converter for an electric motor drive, but it can also be used in non-drive application.