From Motor Feeder to Transformer Feeder 1

[Arevaengineer]

A 3.3kV,3 pole,107A vacuum contactor feeding a motor is to be converted to a transformer feeder feeding 1000kVA 3.3kV/0.415kV Transformer.
In addition to some wiring modification and replacement of protection relay from motor protection to O/C and E/F relay as recommended by the manufacturer.Is there a need to remove the three 250A fuses installed on the contactor.If yes, why?
Thanks to answer my curiosity.

 

[dpc]

Before doing away with the fuses, you need to verify the interrupting rating of the contactor. Typically, motor starter contactors (medium-voltage) are not rated to interrupt short-circuit currents - this is the job of the fuse.

In a motor starter, you have to be careful about using an instantaneous or high-speed trip function in the protective relaying, because of the inability of the contactor to interrupt high levels of current. A time-delay generally is required to give the fuse time to clear the higher level short circuits.

So, I would not remove the fuses until you verify the short-circuit interrupting capability of the contactor is adequate for the maximum fault current it could ever see. You should also set your protective relays appropriately.

 

[sykimk]

As already mentioned by dpc, power fuses are used for protection against short circuit currents.
In addition, contactor's interrupting capability is much lower than those of system due to the function of power fuse (i.e. short circuit current interrupting), and rating of contactor is carefully selected by coordination with PF characteristic curve.

If you want to convert motor starter to transformer feeder, you need to consult manufacturer whether the ratings of existing PFs and contactors are suitable for transformer opeartion, and then if required, you may replace those componenet with new ones which are suitable for transformer.
In addition, I never recommend you to remove PFs in any case.

 

[Arevaengineer]

Thanks dpc and sykimk for your valuable contribution.

 

[MVRockwell]

Arevaengineer

When converting existing motor controller to a feeder controller there are at least four main areas to look at.
1. Control circuit and contactor configuration
2. Current Transformer selection
3. Protection
4. Power Fuse selection

The first point is to address the use of the existing control and to determine if it's suitable for a transformer load. What I mean by this, is the existing contactor control (typically electrically held for motor loads) suitable for a transformer load (typically mechanically held similar to a circuit breaker). This could require changing the control circuit and contactor.

The second point is to address the use of the existing current transformers to determine if the existing ratio is suitable for the new nominal current load and the over-capacity (125% or 133% for transformers as compared to a typical 1.15SF for motors)

The third point is to address the use of the existing motor protection relay (i.e. Rockwell Bul. 825, GE 269Plus, etc.) This point get complicated as it's based on what type of protection you're looking for and prefer. However typically it's not a good idea to protect a transformer with a motor protection relay.

The fourth point is to address the use of the existing motor controller power fuses. Typically MV motor controllers use an R-type fuse and a transformer feeders use an E-type. The major difference is in the curve profile (compare a couple of curves and you'll quickly notice). The issue with using an R-type fuse on a transformer load is that the fuse may not allow the high inrush transformers current (12 to 25x for 0.01 to 0.1s) without damage as it's only design for motor loads with a typical 6xFLC for 20s starts. As for the contactor, the above statements that contactors are not rated to open short-circuit is correct. However most contactor vendors have designed contactor to have inherent delays in the contactor to ensure that power fuses open faults. Also with the use of current limiting power fuse this is fairly easy.

I hope this helps,