Investigation of Fast-Front Transients in HVDC Switchgear for CMS Multiterminal HVDC System

These fast transients caused unexpected voltage and current readings from the Direct Voltage

Dividers (DVDs) and Direct Current Optical Current Transformers (DCOCTs) as used for operational measurement and protection, leading to unintended Pre-Insertion Resistor (PIR) overload protection trips and requiring an extension of the commissioning programme.

A comprehensive root-cause investigation considered a range of potential contributing factors, for example, earthing arrangements, electromagnetic interference, and stray capacitance/inductance. Ultimately, site measurements confirmed that the disturbances were fast-front transients generated by stray capacitances and inductances energised during highspeed switch (HSS) operations, a phenomenon well known in Gas Insulated Switchgear (GIS) environments. The findings demonstrated how commonly used DC measurement devices can be affected by such transients and highlighted lessons applicable to HVDC protection and measurement design.

This paper describes:

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5. The operating scenario that produced the transient behaviour;

The investigation methodology;

Equipment and design factors contributing to the issue;

The protection system response;

General conclusion and recommendations.