Equalizing networks for series-connected thyristors Why is an equalizing network required when thyristors are connected in series for high-voltage applications?

Introduction / Context:
To withstand high voltages, multiple thyristors are often connected in series. However, device parameter tolerances cause unequal blocking voltage distribution, risking over-voltage on a single device. Equalizing networks mitigate this risk.

Given Data / Assumptions:

• Series string of SCRs used for high DC or AC blocking.
• Devices exhibit tolerances in leakage current, junction capacitances, and recovery behavior.
• We consider both steady-state and transient conditions.

Concept / Approach:
Static equalizing resistors help share DC (steady-state) blocking voltage by equalizing leakage currents. Dynamic equalizing capacitors (often with resistors) help share transient voltages during switching, countering dv/dt differences and recovery disparities.

Step-by-Step Solution:
Problem: Without equalization, the device with lowest leakage or different capacitance may block a disproportionate voltage.Remedy: Add parallel resistors across each SCR for static sharing and RC snubbers for dynamic sharing.Outcome: Each device sees approximately its designed portion of the total voltage, improving reliability.

Verification / Alternative check:
Waveform measurements across devices with and without equalizing networks show reduced voltage imbalance and lower over-voltage spikes when networks are used.

Why Other Options Are Wrong:

• Proper firing is controlled by gate drive timing and magnitude, not by equalizing networks.
• Current division pertains to parallel connections; series strings need voltage, not current equalization.
• “None” ignores the well-established engineering practice.

Common Pitfalls:

• Using only resistors and neglecting dynamic sharing; both static and dynamic effects matter.
• Assuming perfectly matched devices; real components vary.

Final Answer:
ensure proper voltage division