Equalizing networks for series-connected thyristors For static (steady-state) voltage sharing across a string of series-connected thyristors, which equalization network is appropriate?

Introduction / Context:When connecting thyristors in series to withstand higher DC voltages, device parameter tolerances cause unequal voltage sharing. Proper equalizing networks are required to prevent over-stressing an individual device.

Given Data / Assumptions:

• Concern is steady-state (static) off-state voltage distribution.
• Dynamic equalization during transients is considered separately.

Concept / Approach:Static inequality arises from variations in leakage currents. Placing a relatively large-value resistor in parallel with each device provides a defined sharing current that dominates leakage variations, equalizing the DC voltage division.

Step-by-Step Solution:For static sharing: use parallel resistors (R) across each device.Choose R so that balancing current >> worst-case leakage difference.For dynamic sharing (not asked), add capacitors (RC) to address dv/dt and transient splits.

Verification / Alternative check:Application notes show “R-only” networks for static sharing and “R//C” networks for both static and dynamic sharing in high dv/dt environments.

Why Other Options Are Wrong:

• Series resistors do not correct off-state voltage sharing.
• RC across each device is a dynamic (and static) solution but is more than needed if only static sharing is asked.
• A single shunt across the whole string does not enforce per-device sharing.

Common Pitfalls:

• Selecting R too large, giving negligible balancing current and poor sharing.
• Omitting RC when high dv/dt transients are present.

Final Answer:One resistor in parallel with each thyristor