Medium Voltage Gear

[kody916]

Hi Everyone

Had some questions in regards to the medium voltage gear.

1. Why do we use latching contactor for a transformer but not non-latching one?

2. I have a 4160V, FLA=31A, 250HP motor. How to select KVA rating of phase CT for this motor?

when it comes to power cable sizing, we typically use FLA*1.25. When it comes cable sizing for phase CT's inside the 4160V cubicle, how is that calculated?

Thank You

 

[jim dungar]

Latching vs non-latching: do you want to have to 'restart' the transformer after every power outage?

Phase CT sizing: current transformers are sized based on their primary amp rating not by KVA. The current transformers must be sized based on the settings of the protective device it is connected to as well as the amount of current it must be able to accurately transform.

CT secondary cabling: current transformer rated outputs are usually relatively low, however the conductor size is often relatively large in order to provide a reasonable maximum voltage drop when considering all of the impedance on the secondary of the CT (e.g. resistance of CT winding + secondary leads + protective device) and the expected maximum output (e.g. 12x motor inrush).

 

[Jraef]

Plus...
A latching contactor prevents the transformer from being energized, de-energized and energized again over and over if there is a "recloser" situation with the HV lines ahead of it. If allowed to do that, the likelihood of protective devices (fuses or breakers) on the primary feed of the transformer nuisance tripping is very high, in which case your outage become longer term. I've even seen it done on very large MV motors for basically the same reason.