Bipolar Thyristor - Asymmetric Bypass Thyristors

In High Voltage Direct Current (HVDC) transmission systems, Silicon Controlled Rectifiers (SCRs) are commonly used in rectification for converting AC to DC,. In some scenarios, bypass SCRs might be necessary for reasons such as improving system efficiency, enhancing performance, or during maintenance.

Purpose of Bypass SCRs in HVDC Applications

During a fault in the HVDC converter station (like overcurrent, overvoltage, or converter malfunctions), bypassing the SCR helps protect sensitive components by allowing the current to bypass the faulty thyristor valve.

SCR valves can be bypassed for maintenance, ensuring that the system continues operating even when certain components need repair or replacement. Bypass circuits provide operational flexibility and redundancy.

Silicon Controlled Rectifiers are designed to switch large currents, but they also have inherent losses due to forward voltage drops and switching losses. Bypassing the SCRs when they are not needed reduces energy losses, improving overall efficiency.

In low-load conditions, SCRs may not be fully utilized, so bypassing them can help maintain efficient operation without relying on high-voltage components unnecessarily.

HVDC technology enables efficient transmission of large power amounts over long distances using direct current.

There are two main HVDC technologies: line-commutated converters (LCC) with thyristors and self-commutated voltage source converters (VSC) with insulated gate bipolar transistors (IGBTs):

LCC-based HVDC systems consume reactive power and require AC filters for compensation. Power reversal involves voltage polarity reversal, which can be problematic for HVDC grids.

VSC-HVDC systems produce high-frequency harmonics but require smaller AC filters. For power reversal, voltage polarity remains unchanged, but they have higher losses than LCC technology.

Bypass thyristors enhance HVDC system reliability by reducing fault-related downtime.

During faults, bypass thyristors divert fault current away from the faulty section, allowing uninterrupted operation.

This reduction in fault-related downtime increases system reliability and minimizes disruptions.

In summary, bypass thyristors contribute significantly to HVDC system reliability by swiftly handling faults and ensuring continuous power transmission.

Dynex Asymmetric Thyristors are designed for the protection of IGBT modules in VSC (voltage source converter) multi-level applications, where a reduced forward blocking voltage is required.

The design of the thyristor provides greater flexibility to the system designer, allowing compactness, better reliability and facilitating a push towards higher scheme voltages.

They are resistant to fast voltage transients, which can be exposed due to the switching of the IGBT diode. The device structures have greater enhanced hardness to cosmic ray induced failures, which become significant at high DC voltage duty cycles.