X/R Ratios and Earth Faults 1

[Dannielle]

I have been asked to quote X/R ratios for faults. Historically we have quoted the Z1 X/R but now I am wondering if the Z0 X/R should be quoted as well.
I have also been asked for the X/R for a 2 phase to earth fault.
I am concerned that Z0 X/R could be confused with the single phase to earth fault X/R, am I going down the right track ???
Also could anyone provide me with references that calculate these values so I can "sanity check" the values I am getting.
Kind Regards
Dannielle

 

[Kiribanda]

Hi Dannielle,

Normally the value of X/R ratio is required to estimate

1)Peak short circuit which the circuit breaker should handle during closing and for bus bar sizing for magnetic forces etc.

2)The value of the percentage D.C. component that the circuit breaker should handle while the asymmetrical short circuit current is being interrupted.

3)To check the non-availability of non-current zero crossings in the decaying short circuit current.

The highest possible short circuit current at a fault location is usually due to a 3-phase short circuit (There are exceptional cases in the field). That means it is obvious that we have to define the X/R ratio at a fault location with Z1 (Positive Seq.) ONLY. Further, the value of X/R becomes low for earth faults due to high resistive component of the zero seq. impedance.

Therefore we have to define the X/R ratio by calculating Z1 (positive seq.) ONLY, at the fault location to cater for the worst possible condition.

Regards!
Kiribanda

 

[Dannielle]

Thankyou Kiribanda,
In our system (500kV, 330kV,132kV,33kV,66kV,11kV) we have, I would estimate, over 40% of sites with 1ph-e or 2ph-e (either Gnd or phase) having the highest fault level. I realize the ground component of the 2ph-e is not relevent to CBs but for our working earths and the design of the earth grid it has relevance.
eg one of our 132kV sites has 25kA 3ph, 31kA 2ph-e phase component,and 41kA 2ph-e ground component.
We are now looking very closely at all fault modes.
I am really just trying to get some sense of magnitudes (if there is any). Maybe someone out there may know a valid reason why we have been ignoring these values up till now, the only response I have been able to get in my camp is that they were considered to be less onerous than 3ph, which is clearly (now) not the case.
Regards
Dannielle

 

[stevenal]

The effective X/R for a line to ground fault can be found from the equivalent circuit. The three sequence impedances are in series, so they can simply be added. If we assume Z1 = Z2, than the effective X/R=(2X1+X0)/(2R1+R0).

Note that IEEE 367 uses the words "effective X/R" and IEEE 80 states that X/R should be determined for the particular fault type.

 

[Kiribanda]

Hi Dannielle,

Depending on the values of ( Z2/Z1) and (Z2/Z0) at the fault location the phase to earth fault currents can be greater than 3-phase fault currents. For example, at a fault location if Z2/Z1 = 0.5 and Z2/Z0 = 0.65 then it can be proved that the phase to earth fault current will be greater than 3-phase fault currents.

As such, by doing the fault calculation and obtaining the correct impedance values at each location it is possible to verify why the P-E fault current is higher than 3P faults. Then it is obvious that when specifying the circuit breaker ratings for each location we have to specify the highest X/R ratio irrespective of the fault type.

Regards!
Kiribanda

 

[alehman]

Danielle, can you describe a typical situation where this occurs? For a fault involving earth to be greater than for 3-phase, Z0 obviously must be less than Z1 at the point of the fault. I don't understand how this situation could occur in a transmission system. Z0 for lines is typically 2 to 3 times Z1 and for transformers Z0 is usually equal to Z1.

The most common situation for phase-ground faults > 3-phase is at the output of a generator with a grounded neutral. Z0 for most generators is less than Z1.

 

[jbartos]

Suggestion: Reference:
IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power System Analysis, indicates on page 363 Section: Typical Circuit Parameters for Transient Studies
11.4.1 Introduction
Tables 11-4 through 11-11 and Figures 11-34 through 11-39 depict typical parameters (some of them are Ro and Xo, as applicable and as required) used in switching transient analysis. Compared to conventional power system studies, switching transient analysis data requirements are often more detailed and specific. These requirements remain basically unchanged regardless of the basic analysis tools and aids that are employed, whether they are digital computer or transient network analyzer.

 

[stevenal]

alehman,

Close in line to ground fault on the secondary of a delta / wye transformer can exceed the three phase value. This is beside the point, though. For grounding studies, GPR etc. you need to simulate a fault that involves the ground. Three phase faults are useful for breaker duty studies, but not for grounding.

 

[alehman]

stevenal - According to Blackburn, 3-ph core type transformers can have X0 as low as 0.85 to 0.9 of X1. I don't recall seeing any cases where X0 < X1, but I haven't seen a lot of test reports on large transformers. Can you relate your experience?

//For grounding studies, GPR etc. you need to simulate a fault that involves the ground. Three phase faults are useful for breaker duty studies, but not for grounding. //
I agree with this.

 

[stevenal]

alehman

Positive sequence impedances add up from the source to the fault. But on the secondary of a delta wye, the only zero sequence impedance that counts is that between the fault and the transformer. Positive sequence source impedance can be high while zero sequence is low. This combined with the fact that sequence currents are in phase can cause the phase quantities for a line to ground fault to exceed that for a three phase fault.

 

[alehman]

Thanks. For some reason I wasn't thinking about the delta-wye case.

 

[jbartos]

Suggestion: The industry standards, e.g. ANSI/IEE C37.010-1979 &quot;Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis,&quot; page 30, indicates the 15% increase of the single line fault current over other types of fault currents.
A phase-to-ground fault may produce a higher fault current than a three-phase fault. This condition exists where the zero-sequence reactance at the point of fault is less than the positive-sequence reactance (Xo < X1). This will not necessarily result in a higher current through the breaker as positive and zero-sequence currents may flow in opposite directions.

 

[Dannielle]

Thankyou everyone,

Alehman, I have been looking through our system impedance data, the great majority of feeders with X0< X1 have oil filled cables, some are older style XLPE cables, and these are all in a geographically similar area. So this may explain why we are seeing this as a problem.

Here is our worst case problem, 132kV Subtransmission switching station and a Zone 132/11kV Sub fed from the switching station

Type Z1 (%) Z1 Angle Z0 (%) Z0 Angle &quot;X/R 3PH&quot; &quot;X/R1P-E&quot; &quot;X/R2P-E PH1&quot; &quot;X/R2P-EPH2&quot; &quot;X/R
2P-EEarth&quot; 3ph I Ph-Ph &quot;2ph/Ephase 2&quot; &quot;2ph/Ephase 1&quot; &quot;1ph/EGround&quot; &quot;2ph/EGround&quot;
STSS 1.704 86.1 0.755 71 14.67 8.61 1.40 -0.84 5.15 25668 22229 32111 28374 31682 40826
Zone 1.84 84.6 0.791 69.2 10.58 7.02 1.46 -0.78 4.50 23771 20586 29945 26446 29503 38345

These figures are from an excel spreadsheet, I hope you can make sense of them.
I will be trying to get copies of the IEEE documents, are they available on-line ??
Thankyou again
Dannielle
Australia

 

[busbar]

The three IEEE pubs are avaialbe as PDFs, but no for free.

399-1997

http://shop.ieee.org/store/product....5D&qu=%28%24all+399%2D1997%29&mh=10&sh=0&ae=1

80-2000

http://shop.ieee.org/store/product....%5D&qu=%28%24all+80%2D2000%29&mh=10&sh=0&ae=1

367-1996

http://shop.ieee.org/store/product....d%5D&qu=%28%24all+367%2D1996%5C%29&mh=10&sh=0