Distribution transformer protection settings

[ScottyUK]

At a hypothetical plant with an extensive HV and LV private distribution system it has been noted that the overload protection for the distribution transformers are set 'a little on the high side'. At this plant a hypothetical electrical engineer is mildly concerned that the overload protection is not properly protecting the assets on his system, principally the transformers and the switchgear. The engineer, who has inherited this hypothetical mess from his predecessors, faces opposition to changing relay settings "because that's how they've always been and there's never been a problem". The engineer privately wonders which substation will be the first to burn down.

For those who work in the distribution industry or who operate a private system - ideally a British DNO, but I'm really not all that bothered about location - roughly how far above continuous maximum rating do you set the pickup of your transformer secondary overcurrent protection? Transformer sizes are in the 1MVA - 10 MVA range, secondary voltages being 3,300V and 400V, principal load is motors. I'll not initially share some of the settings from this hypothetical plant because I want to test opinion rather than inject my own.

Thanks!

 

[slavag]

Im usually ( 90% of cases ) put 1.2 transformer rated current

 

[waross]

In our code (Canada) there is a distinction between overload (long time current) and overcurrent (short circuit protection).
Primary overcurrent settings are allowed to be quite high to allow for inrush currents. Our overcurrent protection is designed to protect the system rather than to protect the equipment.
Overload protection is intended to protect the equipment. 125% is generally the maximum allowable overload protection setting.
Your proposed 1.2 setting sounds like a safe and mildly conservative setting under our code.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[slavag]

Bill.
Im use two or three stages overcurrent protection
1 stage 1.2In with IDMT ( very inverse or extremely inverse )
2 stage depend on the short circuit current on the low voltage side of transformer with time delay 0.4-0.6sec
3 stage depend on the in-rush current with time delay 0.

stage 1 and 2 are used as back-up protection for LV side of transformer and selectivity with LV side transformer and outgoings protection.

Overload is installed on the LV side, or used PT100.