Thermal rating of Voltage Transformers - Secondary Fuse/MCB rating 1

[Tahir28]

thread238-209138

The thread is old but the problems are still the same. One such problem I had encountered and found useful information related to the old thread.

One thing that mikeangel was looking for was a calculation and justification to the selected MCB rating in the secondary of the Voltage Transformer.

The answer is simple and is hidden in the VT IEC standards!!

The rating of any VT is the per phase rating as per IEC definition.

Hence, to calculate the rating of the current interrupting device (MCB or fuse) the thermal rating has to be divided by the rated phase voltages (different than the line voltages in Wye connection).

Hence the MCB rating should be slightly above Thermal rating divided by the phase voltage.
MCB rating is to be calculated as:
1500/(100x√3) = 1500/57.7
= 26A.

 

[scottf]

Tahir28-

The fusing of a VT secondary is irrespective of if the VT's secondary windings are connected in a wye or delta.

If the rated secondary voltage of the VT is 100V, then the calculation for the current in the VT winding is 1500VA / 100V = 12A. In other words, the 1500VA thermal burden is based on 100V secondary voltage.

 

[Tahir28]

Appreciate your response Scottf.

Consider the VT rated 150kV/√3:100V/√3 and has a thermal burden rating of 1500VA (per phase as per IEC). What would be the secondary MCB or fuse rating in this case?

I believe your interpretation would be valid for a V-connection (Ratios would be 150kV:100V connected phase to phase in the primary). Applying as per your formula for the 3 VTs connected in star-star, we would have an undersized MCB/fuse rating which might work, though not required.

mikeangel mentioned three PTs were installed hence not a V-connection, understand that it would have been connected in star externally in a junction box probably.

 

[scottf]

The current through the VT secondary relative to the thermal burden rating doesn't depend on how the secondary windings of the phases are connected (wye or delta).

150kV/√3 : 100/√3 = 86.7kV : 57.8V, which means the current through the VT secondary at 1500VA would be 25.95A.