MV Soft Starter Harmonics Limits and Captive Transformer

We will install solid state soft starter to drive 2MW 6KV motors. The soft starter will fed by 13.8kV switchgear via 13.8/6kV captive transformers. The solid state reduced voltage starter switches on-off during starting and causes harmonic component in the network. I know VFD generally needs input/output filter to limit THD. However, I do not find any filter equipped in the catalogue of soft starter manufacturer.

How can we control input/output THD of the soft starter? If we do not limit THD, shall the captive transformer be designed specially to withstand harmonics?


A soft-starter will produce harmonics for the 10-30 seconds that the motor is accelerating. The filter would be very large and expensive to filter these harmonics.

The transformer will not require any special considerations because of the harmonics. The duration of the start is short enough that the harmonics won’t have much of a heating effect. You need to be more concerned about the overload MVA draw during the start and excessive voltage drop.

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