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Estimation of fault Location on UHV transmissionline using synchronized PMU measurements
K Ravishankar, D ThukaramDepartment of Electrical EngineeringIndian Institute of Science, Bangalore
AbstractIn this paper an accurate fault location algorithmfor adaptive fault detection/location on UHV transmission systemusing two end synchronized Phasor Measurement Unit (PMU)measurements is presented. The algorithm utilizes synchronizedmeasurements of voltages and currents from both ends of aline. The Electromagnetic Transients (EMT) program has beendeveloped to simulate a 765 kV Indian transmission System inwhich the distributed parameter line model is used. Simulationstudies have shown that the developed algorithm yields accurateresults independent of fault types and is insensitive to thevariation of source impedance, fault impedance, and line loading.The accuracy of fault location estimation achieved can be up to99.9% for many simulated cases. The technique will be verysuitable for implementation in an integrated digital protectionand control system for UHV transmission system. Illustrativeresults are presented for various type of faults.
An accurate fault detection/location technique is of specialimportance in improving power system reliability includingrelaying, analysis for line inspection, and routine maintenance.Fault locator facilitates repair and restoration by immediateknowledge of the location and the nature (type and measuringdata) of the fault. A locator is also useful for transient faults,pointing to a weak spot that is threatening further trouble. Thelocator allows rapid arrival at the site before the evidence isremoved or the trail becomes cold. Also, the knowledge thatrepeat faults are occurring in the same area can be valuable indetecting the cause. Weak spots that are not obvious may befound because a more thorough inspection can be focused inthe limited area defined by the fault locator. Distance relaysfor transmission-line protection provide some indication of thegeneral area where a fault occurred, but they are not designedto pinpoint the location .
Fault location schemes widely used are based on operatingprinciples of the travelling wave propagation on transmissionline. One approach is to observe the time at which a locatorof each terminal detects the first incoming fault surge from afaulted point. Another one is to transmit an electrical pulse intothe line, and to measure the period from emission to return ofthe pulse, thus known as a pulse radar method. This approachis based either on the travel time measurements using corre-lation techniques  or on the reconstruction of voltages andcurrents at the fault location. The traveling wave techniquesoffer some advantages but the computational complexity isincreased. The identification of the desired signal becomes
the essential problem of the usual kind of travelling waveprotection scheme. An adaptive travelling wave protectionalgorithm using two cross-correlation functions aid effectivelyin the identification of travelling wave. Fault location methodsutilizing travelling waves are independent of network configu-rations and devices installed in the network. But it is not easyto identify the local maximum of cross-correlation function forcalculating fault location. The other limitations of travellingwave techniques are that they require a very high samplingrate and cost.
Takagi et al.  proposed an approach based on thesuperposition theorem using the Laplace and the Fouriertransforms. The results obtained are quite accurate as long asthe assumptions are satisfied. A time domain representationof a transmission line model has also been considered byKezunovic et al. . Data samples are considered as beingavailable from one end only. The voltage at the other end isestimated using pre-fault data. While one-terminal algorithmsprovide usefully accurate results, certain errors will remaindue to the inherent assumptions which are required in thealgorithms.The measurements from both ends provides suf-ficient number of equations to find the location of the fault. Thukaram et al. applied Artificial Intelligence techniquesto fault location ,  in absence of full information of fault.As the digital relays and communication systems provide theopportunity to perform fault locating using data from bothends in transmission lines, the fault location can be estimatedwith minimal assumptions and sources of errors.
In this paper, new fault detection/location technique usingsynchronized fundamental voltage and current phasors at bothends of transmission line is presented. Using the accuratetiming signal provided by Global Positioning System (GPS),  as common time base for measuring instrumentslocated at both ends of line, we can highly promote theaccuracy of synchronized measurements and reduce the costof equipment greatly. Because of low frequency of the timingsignal of GPS, it can not be used as sampling signal directly.This means that a timing device is needed to do frequencymultiplication/division task. The fault detection/location tech-nique incorporated with line parameter estimation forms anadaptive technique. The fault detection/location technique cancope with various factors associated with the accuracy of faultdetector/locator mentioned above. A new and unique algorithmto estimate and eliminate the decaying dc component in a
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010 722
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
fault current signal is presented.The dc-removed current signalwas obtained by eliminating the dc component from the faultcurrent at each sampling instant. The algorithm can estimatethe dc component exactly from fault currents and is used forphasor extraction .
Electromagnetic Transients (EMT) program has been de-veloped to simulate a 765 kV Indian transmission Systemusing Dommels method . The performance of the faultdetection/location algorithms are evaluated by simulating the765 kV Indian transmission System with respect to variousparameters such as fault resistance, source impedance varia-tion, line loading and fault incidence angle through the EMTPgenerated data.
II. CONFIGURATION OF FAULT LOCATOR FOR UHVTRANSMISSION SYSSTEM USING PMU MEASURMENTSThe overall diagram of the fault locator for UHV transmis-
sion sysstem using PMU measurments is shown in Fig. 1. Thephasor measurement units are installed at both ends (sendingend S and receiving end R) of the transmission line. Thethree phase voltages and three phase currents are measuredby PMUs located at both ends of line simultaneously. Sincethe Global Synchronism Clock Generator (GSCG) has beenequipped in PMU to provide an extremely accurate and reli-able external reference clock signal, it can guarantee samplingsynchronization to an accuracy of better than 1 sec.
The EMTP simulated on 765 kV typical of an Indiantransmission System. Voltage and current waveforms wereconsidered to be directly taken as the synchronized sam-pled data (voltages and currents) from substations Anparaand Unnao. A new DFT method is used to extract close-infundamental phasors.
Display fault distance
Synchronized phasors I I ,V VS SR R
PMU at Substation S PMU at Substation R
Calculate Num, Den and p
Fig. 1. Fault locator for UHV transmission sysstem using PMU measurments
III. COMPUTATION OF FAULT DETECTION/LOCATIONINDEX
A. Single-phase Case:The index using the synchronized voltage and current sam-
ples at both ends of a transmission line to calculate the locationof the fault is presented in this section.
Consider an un-faulted single-phase (two-conductors in freespace) transmission line shown in Fig. 2. Under sinusoidalsteady state condition both voltage and current measured ata distance x km away from receiving end obey two lineardifferential equations
A homogeneous transmission line
Faulted transmission line
Fig. 2. Transmission line under pre-fault and during fault conditions
dx= (R+ jL) (1)
dx= (G+ jC) (2)
Solving equations (1) and (2) by using boundary conditionswe get
V (x) =(VR + IRZC)e
I(x) =(VR + IRZC)e
where x is distance measured from receiving end
V (x) =(VS ISZC)ex
(VS + ISZC)ex
I(x) =(VS ISZC)ex
(VS + ISZC)ex
where x is distance measured from sending end
where ZC =
(R+ jL)/(G+ jC)
(R+ jL)(G+ jC)
In case of a fault occurrence at the point F which is x = pdkm away from receiving end R on a transmission line S-Rshown in Fig. 2. d is the total length of the transmission line,and p is the per unit distance from receiving end to the faultand is also used as a fault detection/location index. When faultoccurred at the point F, the transmission line is thus dividedinto two homogeneous parts. Each section acts as independenttransmission line. Then the voltage VF at the point F usingboth ends boundary conditions is given by
VF (x) =(VR + IRZC)e
VF (x) =(VS ISZC)e(1p)d
(VS + ISZC)e(1p)d
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010 723
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDI