Winding insulation resistance

[setiawanmarten]

Dear all,

We just test transfomer 3000 KVA , primer 20.000 V and secunder 90 V.
We test the winding insulation resistance :
P to ground (with secunder Short to Ground) result : 13.200 MegOhm
S to ground (with primer short to ground) result : 2000 MegOhm
Primer to secunder was 191 Meg Ohm,

The minimum insulation resistance according to IEEE C57.125 (EASA AR200 section 2 page 2) for primer to ground we use the primer voltage for the formula(E),
For secunder to ground we use secunder voltage to the formula (E),

My question if we already measure winding insullation resistance P to Ground (secunder short to ground) and winding insulation resistance S to ground ( primer short to ground) do we have to also measure the value of winding insulation resistance of Primer to secunder? And if we have to measure then what is formula for the minimum value of primer to ground ( i mean for the E value, which voltage do i have to put to the formula? Secunder or primer? )

Thank you

 

[7anoter4]

The transformer windings have 3 different insulations. Primary to magnetic core called Primary Winding to Ground
secondary to magnetic core called Secondary Winding to Ground and the insulation between primary and secondary
winding. The electric field of primary voltage produces stress in the primary insulation to ground and the primary insulation to secondary. So, you need to test all three insulations and to take for E the primary rated voltage for delta connected winding or Primary voltage/sqrt(3)= 20000/sqrt(3)= 11547 V for wye connected winding.
For instance: Rmin=C*E*(kVA)^(1/2) where C=30 for untanked coils and core Rmin=30*20000/sqrt(3000)=11000 Megohms for all the primary and secondary winding [all three phases together].
It seems that primary to secondary is less than permitted.