Derecho Comparado

The algorithm is tested using OMICRON's Test Universe software at a v oltage of 400kV. All parameters are entered using the TransPlay application. This section will

focusonlyonresultsobtainedfor the fault distance,arcresistance,totalfaultresistance

and tower footing resistance. Appendix D provides detailed analysis of simulated R-X plots, relay reaches and harmonics.

6.2.1 Short Line Tests (0-100km)

Figures 6.16 and 6.17 illustrates the line voltages and currents at both the sending

and receiving terminals of the ATL for a single phase-to-ground arcing fault with a

sample frequency of fs=3.2kHz (64 samples per 20ms cycle). VSA, VSB, VSC and ISA,

ISB, ISC are the three phase voltage and current signals at the sending end. VRA, VRB,

VRC and IRA, IRB, IRC are the three phase voltage and current signals at the receiving

end. Assuming the fault is at 10km, the fault current at the sending end can be seen to be slightly larger than the fault current at the receiving end. This is due to the fault being picked up closer to the sending end.

Figure 6.17: Line currents at the sending and receiving terminals

The arc voltage is assumed to have a distorted rectangular waveshape with an

amplitude of Varc=4kV as shown in Figure 6.18. The value of 4kV is obtained under

the assumption that the arc length or distance between arc electrodes is 3.25m and the arc voltage gradient is 12.5V/cm.

Figure 6.18: Fault arc voltage and current

Thearcresistance(Rarc)isdetermined as the ratioofthefundamentalarcvoltageand

fundamental arc current harmonic amplitudes:

1 arc 1 arc arc _I V = R (6.8) Figure 6.19 illustrates the arc resistance for four different cases in which the tower

footing resistance is modified from case to case. RF is set to 10, 20, 50 and 80Ω. In

practice,suchhighRFvaluesare caused by extremeweatherconditionswhereheavy

tree branches fall on overhead transmission lines. The diagram on the left shows the

theoretical results derived for Rarc, whereas the diagram on the right represents the

practical results obtained using the ATL. The theoretical results are calculated using equation (5.66) resulting in 0.48, 0.58, 1 and 1.57Ω. In both case, the arc voltage is the same, but the arc current fluctuates leading to different arc resistances.

Figure 6.19: Theoretical and practical arc resistance simulations (Rarc)

The results achieved in Figure 6.19 suggest good accuracy. Although there is a slight discrepancy between the simulations, the arc model used to derive the proposed algorithm is thought to be purely rectangular, whereas the arc simulated practically on the ATL is in effect a distorted rectangular wave shape as shown in Figure 6.18.

Figure 6.20 demonstrates the fault distances recorded at 10, 20,50 and 80km. The

towerfootingresistance is set to RF=8Ω.

Figure 6.20: Fault distance (l = 10, 20, 50 and 80km)

Figure 6.21 depicts the total fault resistance with varying tower footing resistances. The total fault resistance is the sum of the fault resistance and the arc resistance.

Figure 6.22 illustrates the tower footing resistance.

Figure 6.22: Tower footing resistance (RF)

In all cases, the algorithm accurately determined the fault distance, arc resistance,

totalfaultresistanceandtowerfooting resistance. Table 6.1summarisesthefindings.

Even though the majority of faults on ov erhead transmission lines are transient arcing faults, some faults are permanent metallic faults that do not produce an arc. If the proposed algorithm is to be used for other applications apart from fault location, it is important that it is able to distinguish between transient and permanent faults. A single phase-to-ground permanent metallic non-arcing fault with the fault resistance

set at RF=20Ω is simulated at 10km via the sending end. Figure 6.23 represents the

algorithmoutputwhereallunknownparameters are determined. Thealgorithmsuitably

establishes that the arc resistance is Rarc=0Ω.

Figure 6.23: Algorithm output for a non-arcing fault (l=10km, RF=20Ω, Rarc=0)

6.2.2 Long Line Tests (100-300km)

To assess the performance of the algorithm on long lines where shunt capacitances

areincluded, thevoltageand current data sampled ateach endoftheline isinjected

intotheATLusingtheCMC356 test set.

thefaultdistance()derivedusing Equation (5.37). Theinaccuraciesinthecalculation

of  in the first stage are caused by the shunt capacitance of the line and the

inaccuracies for ' in the second stage is due to  being inexact, therefore causing

minor inaccuracies in the shunt admittance.

Table 6.1: Algorithm outputs for both short and long line (D=300km)

Actual Fault

Distance (km) _(km) '_(km) Algorithm Outputs RF (Ω) Rarc (Ω) RT (Ω)

10 8.757 10.19 7.92 0.535 8.46 20 18.08 20.18 7.93 0.550 8.48 50 47.14 50.15 7.93 0.547 8.48 100 98.62 100.0 7.94 0.491 8.43 150 150.6 149.7 7.92 0.547 8.48 200 203.1 199.3 7.93 0.547 8.48 250 255.8 248.9 7.93 0.491 8.43 280 285.6 278.9 7.95 0.430 8.38

In the first stage, tests proved that the algorithm underestimated the fault distance

when the faults were nearer to the sending end o f the line (<150km) and

overestimated the fault distance when the faults were closer to the receiving end (

≥150km). In the second stage, the algorithm overestimated the fault distance when

the faults were closer to the sending end (<150km) and underestimated the fault

distance when the faults were closer to the receiving end ( ≥150km). This could

arise from a number of reasons, mainly component errors in the ATL. To improve the accuracy even further a third stage can be derived and tested. However, given the almost precise accuracy of the results from the second stage, the necessity of a third stage is questionable.

6.4 Conclusion

This chapter validated that the proposed algorithm is viable for the application of an single phase-to-ground arcing fault by testing it on a 300km Artificial Transmission

Line. In each case, the algorithm was slightly inaccurate in its calculation of RF,

although the error in the calculation was reduced as the fault distance was increased.

Thecalculationofthetotalfault resistance remained constant,onlydippingsomewhat

forthefinalcasefora fault at 280km.ThevaluesofRarcremainedreasonablyconstant

apart from in the last two cases where a combinationofanincreasedvalueof RF and

decreasedvalueofRTledtoareduction in the value of Rarc. The fault arc waveforms