A Novel Algorithm to Determine Fault location in a Transmission Line Using PMU

Measurements

Mahesh Venugopal Department of Electrical Engineering

Indian Institute of Technology Kanpur, India

maheshv9249@gmail.com

Chandrakant Tiwari Department of Electrical Engineering

Rajeev Gandhi Institute of Petroleum Technology Raebareli, India

chandrakant2806@gmail.com

Abstract— In recent years a lot of research activities on determining fault location in a transmission line using PMU measurements have been done. Most of the methods use voltage and current measurement at one or both the ends of the transmission line. Also many methods consider source side and load side impedance to be known and constant. This paper presents a method for the determination of fault location in a transmission line using synchronized measurements considering the variation in source and load side impedance. The proposed algorithm is valid for symmetrical as well as unsymmetrical faults. This method is also less dependent on Current Transformer, so that error will be less. Algorithm has been tested in various systems –including IEEE 7-bus system- using Real Time Digital Simulation platform. Simulation result on a general two bus system is presented in the paper.

Index Terms— Equivalent bus side impedance, Fault location, Phasor Measurement Units, RTDS

I. INTRODUCTION

Determination of fault location in a transmission line is not a new topic in the field of power system and it has been matter of research for a long time. It is a process aimed at locating the occurred fault with the highest possible accuracy. A fault locator is mainly the supplementary protection equipment, which apply the fault-location algorithms for estimating the distance to fault. Automatic fault location can be classified into the following main categories: technique based on fundamental frequency currents and voltages-mainly on impedance measurement, technique based on traveling wave phenomenon, technique based on high frequency components of currents and voltages generated by faults, knowledge based approaches. We need fault locator using synchronized measurements for maintaining healthy transmission system, to detect and locate the fault as soon as possible and as accurately as possible.

Fault location techniques can be classified into those that use data from just one end of the transmission line and those that use data from both (all) ends of the line. Protective relays are basically single-ended fault locators. The accuracy of single-ended fault locators is affected by the assumptions that are made about the fault impedance, the source impedance and the in feed into the fault from the remote end source [11]. The distance relay is capable of finding the fault in pre-defined zone, sends the trip signal to corresponding circuit

breaker to isolate the fault. Distance relay have multiple protection zones to provide backup capabilities. The relay which detects the fault in the 1st zone is designed to trip it first. In this type of protection pair of distance relays is used to protect the two terminal transmission systems [9]. The operation of these distance relays are influenced by combine effect of load and fault resistance, which is known as ‘reactance effect’ [6] [11]. Due to this effect the relay may incorrectly operate or may not operate for the faults, which have high fault resistance. Generally ground faults, which are more frequent in the power system, have high fault resistance. Reference [11] proposed an algorithm to find the fault location by compensating the apparent reactance. Reactive power based fault locator is developed using the one-terminal voltage and current data of the transmission line [1], a microprocessor is used in this paper to locate the fault using only one terminal voltage and current data. References [3], [4] proposes a new Discrete Fourier transform (DFT) based algorithm i.e., Smart Discrete Fourier Transform (SDFT) to accurately extract the phasor quantities of fundamental frequency from the measured waveform and to eliminate system noise and measurement errors such that extremely accurate fundamental frequency components can be extracted for the calculation of fault location. In part II they proposed a new timing device rather than GPS clock which is named as global synchronism clock generator (GSCG) and this equipment GSCG is tested by the measurement results between Chung-jeng and chang-Te 161 KV substation of Taiwan Power Company. Reference [2] proposed a technique of fault location for two-and three-terminal lines using current and voltage phasor of faulty line, independent of fault resistance. This method can be used on-line following the operation of digital relays or off-line using data transferred to a central processor from a digital transient recording apparatus. Fault location by solving the line model equation in time domain is studied in reference [8]. In this the lossless line model is modified to account for the series losses in the line. This method is efficient of locating faults within or outside the mutually coupled section based on the current and voltage data from the various line terminals. Reference [12] proposed an algorithm to find the fault location using the travelling wave propagation. These types of methods consider the voltage and current waves, travelling at speed of light from the

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fault to the line terminals. These methods are considered to be accurate but are quite complex to implement for the fault location application. The knowledge based approaches are like Artificial Neural Network (ANN), Fuzzy Logic (FL) techniques have been used to estimate the fault location [13] [14].

The phasor measurement unit (PMU) hardware is now based on proven technology and is considered as the most accurate and advanced time-synchronized technology available to power engineers [8]. The accuracy of synchronization is better than one microsecond, and the set of measurements provide a real-time snapshot of the state of the power system [15]. The synchronized phasor estimates can be extremely accurate with phasor measurement units (PMUs) using a synchronizing pulse from a Global Positioning System (GPS). In this paper impedance model is developed based on voltage measurements using PMUs therefore they are much accurate and reliable for estimating the location of fault. This method is free from errors due to CT and can be applied to transposed and un-transposed transmission lines. The line was tested with RTDS for more realistic results.

II. PROPOSED ALGORITHM

In this section, algorithm for determining fault location using Phasor Measurement Units is developed. During faulted conditions, current transformer reading will not be accurate because of saturation of its core. So it is preferred to formulate algorithms, which are independent of current measurements.

Fig.1: Two bus system

Consider a two bus system as shown in figure 1. Here the

algorithm utilizes two end synchronized measurements. So both the buses should be observable by PMU. Phasor Measurement Units will measure Current, Voltage, Phase angle, frequency etc. It will send these measurements to the nearby data concentrator at a predefined speed. The frequency of system is considered as 50 Hz.

Figure 1 shows a transmission line that Zs1abc and Zs2abc are impedance matrices of three-phase sources and L denotes the length of transmission line and Zabc is three phase impedance of transmission line which is obtained through the following parameter matrices:

(1)

Zaa, Zbb and Zcc consist of series resistance and self-reactance part. Other elements in the matrix Zabc represent the mutual inductances and capacitances between lines. Since mutual coupling between lines are negligible compared to its resistance and self-reactance, which can be neglected. It means that matrix Zabc will be a diagonal matrix.

Consider a fault that has been occurred at a length L1 from bus 1. The faulted point is considered to be a third bus. From

the PMU data it is possible to extract pre fault and during fault voltages. The change in the bus voltages on all three buses due to the fault can be obtained by the following equation:

(2)

[0]-null matrix of dimension 3x1 IFabc-Three phase fault current at the faulted bus of dimension 3x1

- Change in three phase voltage at bus i due to fault in bus 3, and it has dimension of 3x1

- Impedance matrix when fault has been occurred

Fig.2: Three bus system considering fault location as third

bus

(3)

Ybusfault is admittance matrix when fault has been occurred and is having dimension of 9x9.The elements of Ybusfault is given as per the following:

(4)

; ; (5)

; ; (6)

; ; (7)

Where, (8) From (3),

(9)

(10)

ysrc1 and ysrc2 are the source admittance or equivalent bus side admittance of bus 1 and bus 2 respectively. By solving (9) & (10) it is possible to obtain expression for fault distance L1. The expression is:

In general,

(12)

(13)

(14)

But for the algorithm proposed, equivalent bus side impedance has to be found out in near real time. Equivalent bus side impedance is same as thevenin impedance. There are many methods to track thevenin equivalent circuit. In this work thevenin impedance is found out using a method which utilizes the local voltage and current measurements [16].

Fig.3: Thevenin equivalent of system [16]

Here Voltage and Current are measured using PMU’s. Zth

and Eth has to be calculated.

(15) (16)

(17) (18)

By applying Kirchhoff’s voltage law,

(19) (20) (21)

The above two equations can be written in matrix form:

(22)

Note that here the number of unknowns are more than the number of equations. So to solve these equations we need two different sets of voltage and current measurements. Calculate Thevenin impedance for three phases separately using corresponding phase voltage and current.

From Thevenin equivalent impedance find and . Determine fault location using (11).

III. MODIFICATIONS IN PROPOSED ALGORITHM FOR DOUBLE

CIRCUIT LINES

In a typical power system, there will be many double circuit lines also. The proposed algorithm can be easily modified for double circuit lines. Only change in the algorithm will be equations (5)-(7)

(23)

(24) (25)

Location of fault,

(26)

Where,

(27)

IV. SIMULATION RESULTS

A 765 kV, 250 miles long single circuit line was chosen for simulation. The calculations to be done were programmed in MATLAB. The program first creates the three phase bus impedance matrix for buses 1 and 2. It then removes the line and introduces the third bus which is faulty bus as shown in fig. 2. And the length of bus 1 to bus 3 and bus 3 to bus 2 depends on the fault location point on the transmission line. Then by varying the fault resistance, different types of faults are created and find the changes in three phase voltage at bus 1 and 2. The values of voltage are taken to find the fault location using equations (11) and (22). The same line was tested with RTDS for more realistic results. Table 1 shows the results for all types of faults at two different fault locations for two different values of fault resistances. The percentage error in table 1 is calculated as,

Here, the estimated location is found out through the proposed algorithm. Since Real Time Digital Simulator (RTDS) gives the most realistic results, the two bus system has been simulated in it. The results obtained from RTDS are considered as actual measurements. Different types of faults at various locations were simulated in RTDS and the measurements were made using PMU. These measurements have been used by the algorithm to predict fault location. To show the significant effect of varying bus side impedance, the results are compared for two cases: Case 1: Proposed Algorithm -Considering the variation in bus side impedance. Here equivalent bus side impedance has to be found in real time using some algorithm. Case 2: Considering bus side impedance to be known and constant even during fault. Table 1 shows the results obtained. Results show that the proposed algorithm is valid for both symmetrical and unsymmetrical faults. ‘error1’ is the percentage error for case 1 and ‘error2’ is that for case 2. With varying bus side impedance, it gives much accurate results than by taking constant bus side impedance. For the validation of algorithm different fault resistances with different fault locations has been chosen. In Table 1 fault location from 10 miles to 200 miles are included in a random fashion.

LG refers to single line to ground fault LL refers to line to line fault LLG refers to double line to ground fault LLL refers all lines are shorted or three phase fault

TABLE 1 Performance of algorithms to find fault location on different

type of faults

V. CONCLUSION

The method proposed in this work is almost free from Current Transformer (CT) error. CT measurements are used only in evaluating equivalent bus side impedance. In general CT may cause 5-10% error (class 5P and class10P). Also during fault, probability of getting saturation is high. But the proposed method is free from these problems. The algorithm has been tested for various types of faults and with different fault resistances. In all the cases results were quite accurate. For the simulation purpose very long transmission line has been chosen. But in practice the line may be shorter. Since the method is giving accurate results for long transmission lines, the results will be better as the length of line is decreasing. In a typical power system, double circuit lines also will present. So a modified algorithm for double circuit lines is also presented in this work. Another advantage of proposed method is its less computational time. The calculations required for determining fault location are also very less.

ACKNOWLEDGMENT

The Authors would like to sincerely thank Indian Institute of Technology Kanpur for providing them with all the facility necessary for carrying the research and analysis. The Authors would also like to thank Dr. Saikat Chakrabarti, Assistant Professor, IIT Kanpur for the kind support.

REFERENCES

[1] K. Takagi, Y. Yomakoshi, M. Yamaura, R. Kondow, and T.

Matsushima, “Development of a new type fault locator using the one terminal voltage And current data,”IEEE Trans Power App. Syst., vol. PAS-101, pp.2892–2898, Aug. 1982.

[2] A. Girgis, D. Hart, and W. Peterson, “A new fault location technique for two- and three-terminal lines,”IEEE Trans. Power Delivery, vol. 7, pp.98–107, Jan. 1992.

[3] J.-A. Jiang, J.-Z.Yang, Y.-H.Lin, C.-W.Liu,and J.-C. Ma, “An adaptive PMU based fault detection/location technique for transmission lines part I:Theory and algorithms,”IEEE Trans. Power Delivery, vol. 15, pp. 486–493, Apr. 2000.

[4] J.-A. Jiang, Y.-H.Lin, J.-Z.Yang, T.-M.Too, and C.-W.Liu, “An adap-tive PMU based fault detection/location technique for transmission lines part II:PMU implementation and performance evaluation,”IEEE Trans.Power Delivery, vol. 15, pp. 1136–1146, Oct. 2000.

[5] E. Makram, A. Bou-Rabee, and A. Girgis, “Three phase modeling of un-balanced distribution systems during open conductor and/or shunt fault conditions using bus impedance matrix,”Elect. Power Syst. Res., vol. 13, pp.173–183, 1987.

[6] Murari Mohan Saha ,Jan Izykowski, Eugeniusz Rosolowski, “FaultLocation On Power Networks”,© Springer-Verlag London Limited A. Gopalakrishnan, D. Hamai, M. Kezunovic, and S.McKenna,“Fault location using the distributed parameter transmission line model,”IEEE Trans Power Delivery, vol. 15, pp. 1169–1174, Oct. 2000.

[7] D. Novosel, V. Madani, B. Bhargava, K. Vu, and J. Cole, “Dawn of the grid synchronization: Benefits, practical applications, and deploymentstrategies for wide area monitoring, protection, and control,” IEEE Power Energy Mag., vol. 6, no. 1, pp. 49–60, Jan. 2008.

[8] Ming Zhou, Student Member, IEEE, Virgilio A. Centeno, Senior Fellow, IEEE, “An Alternative for Including Phasor Measurements in State Estimators”, IEEE Transactions on power systems, vol.21, no. 4, November 2006.

[9] V.Cook and C.Eng, “Fundamental Aspects of Fault Location Algorithms used in Distance Protection”, IEE proceedings, partC, vol.133, No.6, September 2009.

[10] Leif Eriksson and Murari Mohan Saha, and G. D. Rockefeller, “An Accurate Fault Locator with Compensation for Apparent Reactance in the Fault Resistance Resulting from Remote-End Infeed”, IEEE Transaction on Power Apparatus and Systems, vol, PAS-104, No.2, February 1985.

[11] Aurangzeb, M. ; Crossley, P.A. ; Gale, P, “Fault location measured at a single line end”, IEEE Power Engineering Society Winter Meeting, Vol N0-4, 2000.

[12] P.F.Gale, P.V.Taylor, P.Naidoo, C.Hithin, D.Clowes, “Travelling Wave Fault Locator Experience on Eskom’s Transmission Network”, IEE Seventh International Conference on Development in Power System Protection, 2001,PP-327-330.

[13] Thammart, C. ;Nawikavatan, A. ; Niyomsat, T. ; Bunjongjit, S, “ANN-based technique for fault-location On transmission lines with ATP/EMTP program”, IEEE Trans on Advances in Power System Control, Operation and Management (APSCOM 2009), 2009 , PP, 1- 6.

[14] HongchunShu, Qianjin Wu, Xu Wang, XincuiTian, “Fault phase selection and distance location based on ANN and S-transform for transmission line in triangle network”, IEEE Transactions on Power Delivery, 2010.

[15] S.M Bhrama and A. Girgis,“Fault location on a transmission line using synchronized voltage measurements,”IEEE Trans. Power Delivery, vol. 19,N0-4 pp.1619–1622, Oct. 2004.

[16] Khoi Vu, Miroslav Begovic, Damir Novosel & Murari Mohan Saha,”Use of Local Measurements to Estimate Voltage-Stability Margin”, IEEE Transactions on PowerSystems, Vol. 14, No. 3, August 1999.

Fault

Distance (miles)

Fault Resistance

( ) %Error1

%Error2

LG

100

10 0.30 0.36 50 0.35 0.35

150

10 0.79 0.85 50 0.42 0.46

LL

150

10 0.32 0.37 50 0.28 0.42

200

10 2.32 1.79 50 1.56 1.65

LLG

10

10 0.64 0.86 50 0.84 0.92

LLL

100

10 0.5 0.68 50 0.14 0.49