Paper accepted for presentation at 2003 IEEE Bologna Power Tech Conference, June 23th-26th, Bologna, Italy
Fault Location in Distribution Systems Based on Traveling Waves David W. P. Thomas, Member, IEEE, Ricardo J. O. Carvalho and Elisete T. Pereira
Abstract—A single end fault location method for distribution systems, based on the traveling waves theory, is presented. The proposed scheme operates on the incident transient currents traveling from the fault point and measured at the substation. A cross-correlation function is used to identify the transient traveling waves. The voltages are estimates by using the transmission line modeling method while the distance to the fault is computed using the cross-correlation function. Although the complexity introduced by the many discontinuities created by the sub-feeders in the system is considered, the results of several simulations and a field trial on a 23.8 kV system demonstrate the viability of this technique for distribution systems.
Index of Terms-- Fault location, transient traveling-waves, distribution systems, digital signal processing. .
I. INTRODUCTION
F
AULT location using fault transients and based on traveling waves theory has been successfully applied as a unit or double ended scheme on extra high-voltage (ehv) transmission lines [1]. A single end fault location scheme is also possible when the current and voltage transients are available at the relaying point [2]. A single ended fault location scheme has its origin in Type-A offline travelingwave fault locators, which, inject a signal and fault locate from the time it takes the signal to reflect back from the fault location. It has also been shown [3] that a single ended fault location scheme is possible using just the transient fault currents on an ehv system and it is this scheme which is demonstrated here on a distribution system. Fault transient signals are high frequency signals superimposed on the steady state voltage and currents. The transient signals can then be extracted by applying a suitable high pass filter. In this work the transient currents are obtained via a current probe with a high pass transfer characteristic which is attached to the secondary circuit of the relay current transformers as given in Fig. 1.
David W P Thomas, is with the School of Electrical and Electronic Engineering, the University of Nottingham, Nottingham, UK, NG7 2RD (phone: +44 115 9515594), e-mail:
[email protected]) Ricardo J. O. Carvalho and Elisete T. Pereita, are with Electrical Engineering Department. The University of Blumenau, Blumenau-SC, Brazil, CEP.89010-971 (e-mail:
[email protected] and
[email protected] ).
0-7803-7967-5/03/$17.00 ©2003 IEEE
Current Transformer 23.8 kV line
1 kΩ
A/D
Transient Recorder
10-100m
relay
Figure 1 Fault recording instrumentation arrangement The data is sampled at 1.25 MHz sampling rate with 8 bit resolution. The voltage transients are deduced from the current transients as described in the theory section.
II. Theory Transient on three phase transmission lines can be considered to propagate as three independent voltage and current modes [4]. The relationship between the phase voltages V ph and currents I ph and the modal voltages Vm and current I m is given by
[Vm ] = [S ]−1 [V ph ] [I m ] = [Q]−1 [I ph ] [S ] and [Q]
(1) (2)
where are the voltage and current transformation matricies. Assuming the transmission lines can be approximated as fully transposed the [S] and [Q] transform matricies have the form 1 1 1 [S ] = [Q] = 1 0 − 2 (3) 1 − 1 1 Each modal transient voltage is related to the modal current by the surge impedance of that mode Z m such that Vm = ± Z m I m (4) where the sign depends on the direction of propagation of the transients with respect to the defined direction of positive current. If a travelling wave transient on a transmission line, due to a fault or switching event, is incident at a substation busbar as shown in Fig. 2 then there will be transmitted and reflected transients created as shown. Initially on the line from which the transients originated there will be both an incident and reflected transient, however, on all the other transmission lines
there is only the transmitted transient present which is propagating away from the substation busbar. On the transmission lines with just transmitted waves the transient modal voltage Vt can therefore be found from the transient modal current I t using (4) to give Vt = Z m I t (5) v 1i
vt
B u sb a r
L in e 1 v 1r
Single ended In the single ended method only one receiver is used and for substation A the fault location is found from identifying the time of arrival of the incident waves from the fault location t2 and t5. This is usually achieved by crosscorrelating the reflected wave amplitude with the incident wave amplitudes [2].
vt vt
F a u lt
Fig. 2 The direction of propagation of the incident, reflected and transmitted waves at a busbar due to a fault on line 1.
Thus from just measurements of the transient current it should be possible to deduce the transient voltages on the lines with just one propagating wave [4]. For voltage continuity the voltages on the transmission lines should all be the same, therefore, it should be possible to identify the lines with just one propagating wave transient as they should all indicate the same voltage amplitude. The fault transient waves propagate around the system as shown by the Bewley lattice diagram in Fig. 3. A
Fault
Double ended In the double ended method two fault recorders are used. For the system given in Fig. 3 there would be one recorder at substation A and one at substation B. The fault location is given for faults between substations A and B by comparing the time of arrival of the initial transients at each substation (t1 and t2). Accurate timing is provided from GPS receivers at each substation.
Once the voltage and current transient amplitudes are known the incident S1 and reflected S2 waves on the faulted line (line 1) can be found from S1 (t ) = Z m I m (t ) − V m (t ) S 2 (t ) = Z m I m (t ) + V m (t )
(6) (7)
The discrete normalized cross-correlation function of the S1 and S2 signals is defined as: φ(τ )S1− S 1, S 2 − S 2 =
B
1 NA
N
∑ [S (k∆t + τ) − S ][S (k∆t ) − S ] 1
1
2
2
(8)
k =1
where distance A=
t1 t2 t3 t4 t5
time Figure 3 Bewely lattice diagram showing propagation of fault transients between system busbars A and B
The fault location can be found from one of two methods.
1 N
N
∑ [S
2
(k∆t ) − S 2 ][S 2 (k∆t ) − S 2 ]
(9)
k =1
The reflected transient is then cross-correlated with the incident transient and the peaks in the cross-correlation should give the arrival time of the first and second incident waves (t2 and t5) from which the fault location can be deduced. While the traveling wave phenomena is well behaved on ehv transmission lines [3], a much more complex situation appears in a distribution system due to the discontinuities introduced by the numerous sub-feeders that are characteristic of the system and the highly interconnected nature of a distribution system. The discontinuities may occur between the end of the line and the fault point, adding several reflections to the already transient waves arising from the fault. There may also be more than one path for transients between a fault and a substation. A typical system is shown in Fig. 4. It has been found, however, that the double ended method should provide accurate fault location if the fault is located directly on the line of interest or it should indicate the location of the feeder circuit if the fault is on a feeder. For the single
ended case a long correlation window has to be used but fault location may be possible even for faults on a feeder circuit.
2 T2
T1
1.5
Cross-correlation
1
0.5
0
-0.5
-1
-1.5
0
50
100
150
200
250
300
350
Samples
Figure-6: Cross-correlation considering current and voltage measures Figure 4: Single-line diagram with sub-feeder 1.2
III. EXAMPLE SIMULATION
T1
T2
1
0.04 0.03
0.8
Cross-correlation
A three-phase fault occurring in feeder 2 in Fig. 4, at 18 Km from the substation, was simulated. Feeder 2 is 36 Km long. The results of the transient current, measured at the substation, are shown in Figure 5.
0.6 0.4 0.2 0 -0.2
0.02 -0.4
Current (P U)
0.01
-0.6
0
-0.8
50
100
150
200
250
300
350
Figure-7: Cross-correlation considering the estimated voltage
-0.02
The results point to a nearly exact fault distance, calculated by difference (t2 – t1), which was equal to 18.15 Km.
-0.03 -0.04 -0.05 1.7
0
Samples
-0.01
1.75
1.8
1.85
1.9 1.95 Tim e (s ec .)
2
2.05
2.1 x 10
IV. FIELD TRIAL RESULTS
-3
Figure 5: Transient Current after filtering.
As may be observed, the presence of sub-feeders results in additional reflections, increasing the complexity of both the signal and the corresponding fault location algorithm. The case study required a cross-correlation function for a fault in feeder 2, at 18 Km from the substation bus-bar. Figure-6 shows the result of the cross-correlation function obtained using transient current and voltage signal measurements. In this case, all the sub-feeders are considered. In figure-7, the cross-correlation result was obtained by estimating voltage using busbar current and the TLM solution of the busbar impedance and disregarding the discontinuities introduced by the sub-feeders.
A 23.8 kV distribution system in Brazil, which, is part of the CELSC system of Santa Catarena state was chosen to demonstrate the viability of this method. The circuit diagram of the selected circuit is given in Fig. 8. Two sets of traveling wave fault recorders were installed on a common circuit to provide both single ended and double ended fault location. One fault recorder was installed at the substation (Blumenau) and the other 14.6 km along the line at Pomerode. To test the accuracy of both methods a capacitor bank was switched twice. The location of the capacitor bank was 1.1km down a feeder that is 9.9km from Blumenau and 4.7km from Pomerode as shown in Fig. 8. On both occasions the time of arrival of the initial transients at Pomerode and Blumenau gave the correct location of the feeder terminal (i.e. 9.9km from Blumenau) to within 0.1 km (double ended fault
location).
recorded at Blumenau substation.
Figure 8. Section of CELESC 23.8 kV system studied
2 Cross-correlation
Typical current transient waveforms at the substation are given in Fig. 9. and the cross-correlation function for the mode 1 wave are given in Fig 10. From Fig. 10 it appears that only weak cross-correlation peak is observed at approximately the correct time for the location of the capacitor bank switching event. A capacitor bank switching event, however, is not a true fault and it is hoped that better results will be possible for a true fault.
1
0
1
Current
100
5
10 15 Distance (km)
20
25
Cross-correlation
Figure 10. Cross-correlation of capacitor bank switching transients recorded at Blumenau substation
0 100 200
0
0.0016 0.0018 Time (seconds)
phase-a phase-b phase-c
Figure 9 Transient currents due to a capacitor bank switching
To Assess if fault location is possible the CELESC system was simulated with a three phase fault at the same location as the capacitor bank. A MATLAB Power System Blockset [6] simulation was used which included all the line sections and loads indicated in figure 8. The cross-correlation function for the simulated transients is shown in Fig. 11. In Fig. 11 there is a clear correlation peak of the correct amplitude for a three phase fault (unity) at about 11.5 km which corresponds to the true distance of the fault from the substation. Simulation results therefore seem to indicate that single ended fault location should be possible on a distribution system.
BIOGRAPHIES
Cross-correlation
2 1 0 1 2
0
5
10 15 Distance (km)
20
Cross-correlation
Figure 11. Cross-correlation of simulated fault transients at Blumenau substation for a fault at the location of the capacitor bank switching event in Fig. 8.
V. CONCLUSIONS Methods for accurately locating faults on distribution systems using traveling wave transients have been investigated. Despite the complexity of distribution systems it has been found that it may be possible to accurately fault locate using standard techniques developed for ehv transmission systems. Initial results from simulated transients and recorded transients from a 28.8 kV distribution system are given which demonstrate the accuracy of the methods using a 1.25 MHz sampling rate and 8 bit resolution. Further work is needed to confirm the reliability of the methods proposed. V ACKNOWLEDGEMENTS The authors would like to acknowledge the efforts and funding received from CELESC, the distribution electrical power company of the State of Santa Catarina, Brazil. VII. [1] [2] [3] [4]
[5] [6]
REFERENCES
Roger Jensen and Philip Gale, At last, Locate faults by recording traveling waves, ELECTRICAL WORLD, February 1996. P.A. Crossley and P.G. McLaren, “Distance protection based on traveling waves”, IEEE, PAS-102, no. 9, September 1983. David W. P. Thomas, C Christiopoulos, Y Tang and P Gale “A single ended fault location scheme” Developments in Power system protection, IEE 2001, Amsterdam, Holland, pp 414-417. David W. P. Thomas, Richard E. Batty, Christos Christopoulos and Anding Wang, “A novel transmission line voltage measuring method”, IEEE, Trans. on Instrumentation and Measurement, Vol. 47, no. 5, October 1998. Christos Christopoulos, The Transmission-Line Modeling Method TLM, University of Nottingham, IEEE Pres, 1995 Matlab – Power System Blockset User’s Guide – For use with Simulink, TEQSIM International, Hidro-Québec, Math Works, 1998.
David W. P. Thomas MIEE MIEEE CEng. was born in Padstow, UK, on May 5, 1959. He received the B.Sc. degree in Physics from Imperial College of Science and Technology, the M.Phil. degree in Space Physics from Sheffield University, and the Ph.D. degree in Electrical Engineering from Nottingham University, in 1981, 1987 and 1990, respectively. In 1990 he joined the Department of Electrical and Electronic Engineering at the University of Nottingham as a Lecturer where he is now a Senior Lecturer. His research interests are in Electromagnetic Compatibility, Electrostatic Precipitation, and the Protection and Simulation of Power Networks. He has over 100 research papers and 2 patents. Ricardo J. O. Carvalho was born in João Pessoa, Brazil, in 1961, He received the degree in Electrical Engineering from Federal University of Amazonas in 1983. He M.Sc. in 1987 from Federal University of Brasília and his Ph.D. in 1995 from Santa Catarina University, Brazil. He is with Electrical Engineering Department at Blumenau University (Brazil). His research include power systems optimization and simulation. Elisete T. Pereira was born in the south of Brazil, on January 23rd, 1956. She first graduated from UFSC, the federal university of Florianópolis, Brazil, receiving a degree in Electrical Engineering, in 1979. In 1983, she obtained the Master of Applied Science degree from the University of Waterloo, Canada and, in 1993, the Doctor of Philosophy degree from the University of Nottingham, English, UK. - both degrees in the electrical engineering area. She is now a Professor of Electrical Engineering at FURB, the regional university of Blumenau, Brazil.
