open delta transformer

[hasanhadi]

hi
we have vt transformer Y/y/d 33kv/110/110
now d is connected open to detection earth fault true?
the end for open delta is connected to damping resistance
now
1- Why take advantage for using damping resistance?
2- how calculated value damping resistance?
3-when not using damping resistance i mean the end for open delta isn't connected to damping resistance this effect to transformer primary ?

 

[7anoter4]

See:
thread238-87156

 

[hasanhadi]

thanks sir
but can you help me for calculation damping resistance?

 

[7anoter4]

Typical resistance values across the secondary winding,
derived from experience, are shown in Table 7.1.
from "Protective Relaying Principles and Applications" by J. Lewis Blackburn and Thomas J. Domin
But maximum primary voltage in this table is only 13.8 [not 33 KV as you need].I think nevertheless 85 ohm 510 w will be ok even for 33 KV/120 V transformer.
The potential transformer manufacturer has to know more.
The required rated power will be P=Vo^2/R
Vo=3*120/sqrt(3)[if 120 V is VT secondary rated voltage.]

 

[7anoter4]

See also:

http://www.basler.com/downloads/pc-59n01.pdf

 

[burnt2x]

The phase winding turns ratio of each phase of the transformer is (33,000/1.732)/110 = 19.052kV/110V = 173.2

The normal secondary voltage during unfaulted conditions on each leg of the delta will be = 110 Volts. During a line to ground fault, secondary voltage on the two unfaulted phases rises to 190.52V (=33,000/173.2)

The maximum broken delta voltage will be:
V[sub]broken delta[/sub] = 3 x Vpri L-N /turns ratio = 3(19,052)/173.2 = 330 volts.

Assuming the VT tertiary VA is 600VA, the VT secondary current if delta is closed will be:

                  VA[sub]1-PHASE[/SUB]   
I sec(rating) = -------------
                  V[SUB]SEC[/SUB]
                  200VA
I sec(rating) =  -------  = 1.82 amps
                  110

The loading resistor will be:

      V[sub]broken delta[/sub]         330
R  = ---------------   = -------   =  181 ohms
       I                   1.82 

P[sub]R[/sub] = I x V = 1.82 x 330V = 600 W

 

[hasanhadi]

thank yor dear
sorry dear ratio for transformer (33kv/1.73)/110/1.73

 

[Fuse4]

The resistance depends of the transformers brand. In same time ago I saw a unit with a variable resistor. Here an example:

Cahier technique n° 190 Ferroresonance schneider-electric.com

http://www2.schneider-electric.com/...ntal-constraints/general-knowledge/ect190.pdf

VT Guard from ABB is a electronic device

http://www05.abb.com/global/scot/sc...a5e2113c02c1257346003fdc09/$file/VT Guard.pdf

 

[burnt2x]

thank yor dear
sorry dear ratio for transformer (33kv/1.73)/110/1.73

See how people get things wrong with wrong data! Your secondary voltage unfaulted is 110/1.732 = 63.5V. This rises to 110V when one line gets grounded. V[sub]broken-delta[/sub] = 63.5 x 3 = 190.52V
With a thermal rating of 600VA, your loading resistor will be 105 ohms
P[sub]R[/sub] = (200/110)[sup]2[/sup] x 105 = 347W, say 400W