Zero Sequence Impedance

[Humble2000]

Dear All,

If the cable has neutral conductor, is the zero sequence impedance calculated different?.I have difficulty figuring out how the consultant has come up with the Z(on) at the end.

PRIMARY SUPPLY SYSTEM

Z0 0.586 < 82.88 100 MVA
0.0726 +j 0.5812 100 MVA

SUPPLY TRANSFORMER

Primary Voltage 44 KV
Secondary Voltage .6 KV
ONAN Rating 2.5 MVA
Ratio X/R 9.846 [typical]
Impedance 5.65% on base

Include tolerance -7.50% [MIN]
5.226% < 84.20
Zmin=0.528%+j5.200%
0.2112+j2.080

600 V CABLES
750 MCM Cu
14.9352 m RUN LENGTH
49 ft RUN LENGTH
6 CONDUCTOR/PHASE
6 NEUTRAL CONDUCTOR

Z 0= 0.05689 +j 0.60363 ? per mi
= 0.00053 +j 0.00560 ? actual
= 0.14665 +j 1.55607 ? on
= 1.56297 < 84.62 ? p.u.

Z(on)=.38990+j3.89204
Max Line to ground =33200A

 

[cuky2000]

Yes the zero sequence impedance is different with or without ground wire since is direct related with the neutral and ground return.

 

[Humble2000]

Thanks Cucky2000. I checked your graph webpage,didn't find any info about the subject( great graphs!)

Zn0=.14665+.2112+.0726=.43045
compare to Zn0=.38990

So, where the difference is come from?
I really appreciate, if you know any literature source on the subject that I can read

 

[Humble2000]

I appreciate any further reply on the subject issue.

 

[alehman]

Zero sequence impedance of conductors in raceway is difficult to calculate accurately. Analytical calculations are not very helpful unless the geometry (number, spacing and lay of conductors) is consistent and type of raceway known.

Kerite has published some typical values for shielded cables.

http://www.kerite.com/catalog/catalogfiles/impedance_data_power.htm

 

[Humble2000]

In above calculation the cable zero sequence is given; however I don't get the same result when I add all zero sequence impedances together as the final result.
It seems having neutral conductor has some effects on conductor zero sequence.

Zn0=.14665+.2112+.0726=.43045
compare to Zn0=.38990

I appreciate if you could go through the calculations and see if you will get the same results or not.

 

[alehman]

I'm not following his math either.
It appears we're assuming the transf. is connected Dy?

 

[alehman]

Meant to say 'It appears we're assuming the transf. is connected YG-YG?'

 

[Humble2000]

No! Transformer is Delta-Wye.
Cables are inside PVC ducts

6X(4-750 MCM + 500 MCM GND)

 

[jghrist]

It's difficult to figure out what your calculations mean.

PRIMARY SUPPLY SYSTEM

Z0 0.586 < 82.88 100 MVA
0.0726 +j 0.5812 100 MVA

What are the two values?

Z 0= 0.05689 +j 0.60363 ? per mi
= 0.00053 +j 0.00560 ? actual
= 0.14665 +j 1.55607 ? on
= 1.56297 < 84.62 ? p.u.

What do these four lines mean?

 

[Humble2000]

Sorry for bad representation of equations.

Z0=.586<82.88 This value is provided by the utility.
Z0= .586*cos(82.88)+j*.586*sin(82.88)

The second line is for cable based on 49' run

Z(49')=.00053+j.00560
Per unit value for 600V,100MVA
Z0=.14665+j1.55607
Z0=sqrt((.14665^2)+(1.55607^2))<atan(1.55607/.14665)
Z0=1.5629<84.62

 

[alehman]

The primary Z0 should not be included if the transformer is D-YG.

I think I understand the impedance representations, but I agree it seems to not add up. Excluding the primary, I get:

0.358 + j3.636 pu

 

[jghrist]

Could the consultant be using a different MVA base? The answer would be close if the base used was 75 MVA. I can't think of a reason to do this. How does the consultant's fault current calculation compare with yours?

 

[DanDel]

Z0 = 0.586<82.88°, which equals 0.07263 + j0.58148

Z0(49') = (0.05689 + j0.60363)(5280/49) = 0.00052795 + j0.00560187, which equals 0.00562669<84.616°

Z0(49')pu = (0.00052795 + j0.00560187)/(600V^2/100000000) = 1.56297<84.616° which equals 0.14665 = j1.556075