1830IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 4, OCTOBER 2006 Ground Distance Relay Compensation Based on Fault Resistance Calculation M. M. Eissa, Senior Member, IEEE Abstract—The fault resistance introduces an error in the fault distance estimate, and hence may create an unreliable operation of a distance relay. A new compensation method based on fault resistance calculation is presented. The fault resistance calculation is based on monitoring the active power at the relay point. The compensated fault impedance measures accurately the impedance between the relay location and the fault point. The relay has shown satisfactory performances under various fault conditions especially for the ground faults with high fault resistance. This new compensation method avoids the under-reach problem in ground distance relays. Index Terms—Active power, distance protection, fault resistance, impedance measurement. I. INTRODUCTION ROTECTION of an important transmission line is most frequently performed using phase-and ground distance relaying techniques. Distance relays effectively measures the impedance between the relay location and the fault. If the resistance of the fault is low, the impedance is proportional to the distance from the relay to the fault. A distance relay is designed to only operate for faults occurring between the relay location and the selected reach point and remain stable for all faults outside this region or zone [1]. In developing distance relay equations, the fault under consideration is assumed to be an ideal (i.e., zero resistance) [2]–[8]. In reality, the fault resistance will be between two high-voltage conductors, whereas for ground faults, the fault path may consist of an electrical arc between the high-voltage conductor and a grounded object. The fault resistance introduces an error in the fault distance estimate and, hence, may create unreliable operation of a distance relay [9]. The impedance seen by the relay is not proportional to the distance between the relay and the fault in general, because of presence of resistance at the fault location. Some techniques for arcing faults detection and fault distance estimation are introduced in [10] and [11]. The techniques are based on the voltage and current at one terminal in the time domain. The overhead line parameters and arc voltage amplitude during the fault are given. The techniques have optimal application in the medium voltage networks and symmetrical faults. P Manuscript received May 25, 2005; revised October 30, 2005. Paper no. TPWRD-00309-2005. The author is with the Department of Electrical Engineering, Faculty of Engineering, Helwan University, Helwan, Cairo 11421, Egypt (e-mail:
[email protected]). Digital Object Identifier 10.1109/TPWRD.2006.874621 Fig. 1. Under reach of the distance relays. A distance relay is set to operate up to a particular value of impedance; for impedance greater than this set value the relay should not operate. This impedance, or the corresponding distance is known as the reach of the relay. A distance relay may under-reach because of the introduction of fault resistance as illustrated in Fig. 1. Relay at O is set for protection up to Z. If a fault at Z occurs such that fault resistance R is high and by adding this resistance the impedance seen by the relay OZ’ such that Z’ lies outside the operating region of the relay, then the relay does not operate. Fig. 1 shows the tripping polygonally characteristic in case of high fault resistance. Some techniques [12]–[17] are suggested for enhancing the high fault resistance problem. These techniques accommodate this problem by shaping the trip zone of the distance relay to ensure the apparent impedance is included inside the trip zone. In this paper, a new fault impedance compensation method based on fault resistance calculation is given. The fault resistance is calculated using the active power at the sending end. The relay uses a Fourier filter to derive the voltage and current phasors. The problem of under reach in ground distance relays is solved. The ground distance relay with this new compensated method will be demonstrated. The results will show that the fault impedance with high fault resistance is accurately zoned. II. DOUBLE-END-FED EARTH FAULTS Fig. 2 shows the phase current lags the phase current by the angle because of the transfer of power from to . Since the fault resistance can normally be neglected in the case 0885-8977/$20.00 © 2006 IEEE Downloaded from http://www.elearnica.ir EISSA: GROUND DISTANCE RELAY COMPENSATIONBASED ON FAULT RESISTANCE CALCULATION 1831 (2) where has been substituted for the sum has been substituted for the sum The impedance to the fault is given as and . Fig. 2. Relationship between the phase voltages and phase currents. (3) So the uncompensated fault impedance is (4) is a source of error in distance relays, so the actual fault impedance is Fig. 3. Diagram for illustrating the flow of sequence power quantities. (5) In the same manner and for a three-line-to-ground fault and symmetrical component circuit, the uncompensated fault impedance for the distance relays is (6) is a source of error in distance relays, so the actual fault impedance is (7) This paper aims to introduce compensated fault impedance for (5) and (7) based on fault resistance calculation from the active power and current measurements. Fig. 4. Symmetrical component circuit for a line-to-ground fault (A 0G fault). of phase-to-phase faults, it is sufficient to consider it only in the case of earth faults particularly since the tower-to-earth resistance, which under difficult ground conditions and the absence of a continuous earth wire, can reach significant values. Consider a single unbalanced fault from line-to-neutral on a system supplied through a grounded generator with pos, itive-, negative-, and zero-sequence impedances of , respectively, and with a generated positive sequence and and , respectively. Assume line-to-neutral voltages of that this system is supplying a fault resistance on phase whose impedance is as illustrated in Figs. 3 and 4. For a fault between phase A and ground, the symmetrical component connection diagram is shown in Fig. 4. The phase voltage and current can be expressed in terms of the symmetrical components, and the voltage of phase at the fault point can be set as (1) III. FAULT RESISTANCE CALCULATION The voltages at the fault point can be expressed by (8) (9) (10) (11) where (12) (13) The total phase quantities at the point of fault are readily obtained from the above sequence quantities with the following results: (14) (15) 1832 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 4, OCTOBER 2006 The sequence power quantities per phase at the fault are Also, the contribution current from the receiving end to the fault is a factor from the contribution current to the fault from the sending end (16) (17) (18) (24) Hence, (21) can be described as (25) The total power quantities may be obtained by combining the sequence quantities as The total power in the fault is described as (26) Also, from the single-phase solution, the total power quantities equal From (25) and (26), the real part of the fault resistance is given as (27) (19) From (19), it can be concluded that the total power in the fault and ); resistance equals the power in phase “ ” ( (Fig. 3). So is defined as the instantaneous power measured at the where sending end and is determined as [19] and [20]. According to the above explanation, the compensated fault impedances for (5) and (7) are described, respectively, as (28) (20) and are the power in the fault resistance from the where two sources for phase . So (20) can be expressed as (21) With minimal load flow at the time of the fault [18] and the electromotive-force (emf) constant at the sending and receiving and ), the current contribution at the receiving ends ( at the sending end. end is almost in phase with the current Thus, the phase relationship between the fault currents ( and ) and the voltages ( and ) can be described by ( ) and ( ). From the above explanation, it can be concluded that the reis lation between the total power at the receiving end directly proportional with a factor of the total power at the sending end . So (22) where is defined as the distribution factor of the generated power at the receiving end with respect to the generated power at the sending end. Consequently, the contribution power to the fault from the receiving end is also a factor of the contribution power to the fault from the receiving end. So (23) and (29) Equations (28) and (29) are the compensated fault impedance calculation for the single and three earth faults, respectively. IV. SIMULATION RESULTS The power system used for testing the proposed new method is a part of a 500-kV power system shown in Fig. 5. The system includes two generating stations. A distance relay is located at buses and as shown in Fig. 5. The voltage and current signals are the inputs to the relays, and 300 km is the line length. The results described on the R-X diagram (transient impedance trajectory). The relay is set to protect 90% of the line. It forms the first zone of the relay, corresponding to a maximum reach of about 0.486 p.u., and has an arcing reverse of about 150% [21]–[23]. The arcing reverse is the resistive allowance of the trip area as a ratio of the inductive reactance. The reach of the second zone is set at 120% of line-1. The power system is modeled and different symmetrical and unsymmetrical faults with solid and fault resistance are simulated using the Electromagnetic Transients Program (EMTP). The voltage and current signals are measured at the relay locations using a sampling frequency of 5000 Hz. The results obtained from the tests are given here. The uncompensated and compensated fault impedances given in (4) and (28) have been EISSA: GROUND DISTANCE RELAY COMPENSATIONBASED ON FAULT RESISTANCE CALCULATION 1833 Fig. 5. Single-line diagram of the 500-kV power transmission system. Fig. 8. Fault impedance trajectory for C from relay-S (F 2). Fig. 6. Fault impedance trajectory for 200 at 50 km from relay-S (F 1). 0 G fault with a solid fault at 150 km C 0 G fault with fault resistance = Fig. 9. Fault impedance trajectory for B 0 G fault with fault resistance = 200 at 320 km from relay-S (external fault at F 3). Fig. 7. Fault impedance trajectory for 100 at 150 km from relay-S (F 2). C 0 G fault with fault resistance = described as the single-phase earth faults while the uncompensated and compensated fault impedances given in (6) and (29), respectively, have been described as the three-phase earth faults. The performance of the proposed technique was evaluated for different types of internal and external faults, source impedance, and fault resistance. Results showed faults are taken with a fault resistance ranging from 0 to 300 . The value of compensated and uncompensated fault impedances seen by the phase to ground relay element is depicted in Figs. 6–9. It is observed that if the compensated fault impedance is used, the relay of fault is located exactly in its proper zone. Whereas, if the uncompensated impedance is used the fault impedance is misoperated and located out of its zone or inaccurately located in its zone. Figs. 10 and 11 show the fault trajectory for the 3L-G fault (internal and external) protected zone. As seen in the figures, the compensated fault impedance is properly identified as the zone of fault and, thus, avoids misoperation in case of 3L-G faults through high fault resistances. 1834 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 4, OCTOBER 2006 fault impedance compensation based on fault resistance calculation. The problem of under reach in ground distance relays is solved. The investigation showed that the fault resistance detection could reach 300 . The results showed that the relay operates correctly for faults simulated within the first, second, and third zones. The suggested technique gives the solutions for the symmetrical and unsymmetrical faults. Fault impedance is accurately calculated; this will improve the relay selectivity. The techniques can be used for medium and long lines. Fig. 10. Fault impedance trajectory for 3L 200 at 150 km from relay-S (F 2). 0 G fault with fault resistance = Fig. 11. Fault impedance trajectory for 3L-G fault with fault resistance 200 at 370 km from relay-S (external fault at F 4). = The suggested technique gives the solutions for the symmetrical and unsymmetrical faults with solid and fault resistances. The presented technique does not depend on the line lengths, so it can be applied on the long lines. The fault conditions such as a dc component, sampling frequency, and point on wave do not have an effect only on the relay performance. Moreover, the compensated fault impedance accurately measures the impedance between the relay location and the fault point. So, the protective system will be very selective. 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Eissa (M’96–SM’01) was born in Helwan, Cairo, Egypt, on May 17, 1963. He received the B.Sc. and M.Sc. degrees in electrical engineering from Helwan University, Cairo, in 1986 and 1992, respectively, and the Ph.D. degree from the Research Institute for Measurements and Computing Techniques. Hungarian Academy of Science, Budapest, Hungary, in 1997. Currently, he is an Associate Professor with Helwan University. In 1999, he was invited to be a Visiting Research Fellow at the University of Calgary, Calgary, AB, Canada. His research interests include digital relaying, application of wide-area networking to power systems, and wavelet applications in power systems. Dr. Eissa received the Egyptian State Encouragement Prize in Advanced Science in 2002 and the best research in the advanced engineering science from Helwan University in 2005. 1835