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1 Current-Only Directional Overcurrent Relay Abhisek Ukil, Senior Member, IEEE, Bernhard Deck, Vishal H Shah Abstract— Overcurrent relays are widely used for protection of power systems, directional ones for transmission side, and nondirectional ones for distribution side. The fault direction may be forward (between relay and grid), or reverse (between relay and source), the normal power flow being from source to the grid. Known directional overcurrent relays rely on a reference voltage phasor for estimating the direction of the fault, requiring both current and voltage sensors. This increases the cost of the relays, prohibiting the utilization of such relays in the distribution side protection and automation, which is going to be a key part in the smart grid initiative. In this paper, a novel current-only directional detection possibility is highlighted. transformers for three-phase operations) are expensive. For distribution automation [2] towards a smarter grid, these type of relays have to be used in large numbers, making the cost an important factor. Also, the voltage polarization approach becomes unreliable when the fault is in too close proximity to the relay, because in this case, the relay is almost grounded by the short circuit, typically known as ‘close-in’ faults [1]. In this paper, a novel current-only detection possibility is highlighted. Index Terms— distribution automation, DA, DMT, IDMT, fault detection, fault direction, overcurrent, protection, smart grid. I. I NTRODUCTION D IRECTIONAL overcurrent relays [1] are widely used for protection of power transmission systems. Distributionside protection, e.g., for radial and ring-main units [2], mostly employs non-directional relays. Here, a fault generally means an overcurrent [3], mainly from a short circuit. The nondirectional relay typically monitors the overcurrent (from the set value) for a defined time to initiate the definite minimum time (DMT) or inverse definite minimum time (IDMT) type protection [1],[2]. Monitoring of overcurrent over time (typically 40-100ms) is required to differentiate faults from change in current due to loads, without allowing overcurrent too long. This is typically governed by IEC standards like 60255 [4] for lowset, highset, very highset stages, depending on various overcurrent magnitude and duration for non-directional relays. Considering a power line connecting an upstream power source to a downstream power distribution system (grid) (with the normal power direction from the upstream source to the downstream grid), the forward fault happens between the relay and the line, and the backward or reverse direction between the source and the relay. This is shown in Fig. 1. Known directional overcurrent relays rely on a reference voltage phasor (“voltage polarization”) [3], for estimating the direction of the fault. When a fault occurs, the fault current has a characteristic phase angle relative to the voltage phasor [5]. The fault direction is determined by judging the current phasor against the reference voltage phasor measured at a measurement location on the power line. This requires measurement of both current and voltage. Such overcurrent relays including a voltage measurement unit (typically requiring three voltage This work was supported by the ABB Corporate Research Funding. A. Ukil (corresponding author) is with ABB Corporate Research, Segelhofstrasse 1K, Baden-Daettwil, 5405, Switzerland (tel: +41 58 586 7034, fax: +41 58 586 7358, email: [email protected]). B. Deck is with ABB Sécheron S.A., MV, Baden, 5400, Switzerland (fax: +41 58 588 2063, email: [email protected]). V. H. Shah is with ABB Global Industries Services Ltd, Vadodara, India (email: [email protected]). Fig. 1. Overcurrent relay: forward (F) and reverse (R) fault. II. C URRENT- ONLY D IRECTIONAL D ETECTION A. Principle From Fig. 1, in the case of reverse or upstream fault (R), the fault current Irev flowing from the grid (G) to the lowest potential point at the fault location (R) is VG Irev = , (1) ZGR where, VG is the grid voltage, and ZGR is the line impedance between the grid (G) and the fault location (R). Similarly, in the case of forward or downstream fault (F), the fault current If wd flowing from the source (S) to the lowest potential point at the fault location (F) is VS If wd = , (2) ZSF where, VS is the source voltage, and ZSF is the line impedance between the source (S) and the fault location (F). The impedances ZGR and ZSF are not exactly known and may be different from one another. However, because the line is generally almost purely inductive with negligible resistance and capacitance, the impedances in denominator of (1 & 2) are almost purely imaginary (positive), making the current phasor with negative imaginary part. µ ¶ VS −VG Now, if Ipre = ZSG is the pre-fault current from the source (S) to the grid (G), then, the total post-fault current IR (sensed by the relay) in the case of reverse fault (R) is VG IR = Ipre − Irev = Ipre − . (3) ZGR Likewise, the total post-fault current IF (sensed by the relay) in the case of forward fault (F) is VS . (4) IF = Ipre + If wd = Ipre + ZSF 2 Please note the difference in sign in (3) and (4), which is due to the fault current in reverse case Irev being directed reverse to the pre-fault current compared to If wd . This sign difference is visible in the current-phasor diagram of Fig. 2-(a). In Fig. 2(a), the short-circuit current phasors −Irev and If wd by which the current phasor may jump have mutually opposite sign due to the sign difference in (3 & 4), and due to ZGR and ZSF being both imaginary with positive imaginary part. Hence, a positive phase angle change (counter clockwise) may indicate fault in the reverse direction, while a negative (clockwise) in the forward direction. Hence, it is possible to judge the direction of the post-fault current (reverse and forward) with respect to Ipre only, without requiring any bus voltage. Fig. 2-(b) shows the lagging current phasor of Ipre with respect to the grid to source voltage difference, for an inductive line. B. Detection Methods Fig. 3 shows the possible detection methods. The input is only current samples from current transformers (CTs), Rogowski coils, etc. These are then transformed into suitable feature vectors, like time-domain (e.g., average, rectified samples, etc), frequency-domain (e.g., magnitude, phase angles from discrete Fourier transform (DFT), symmetrical components, etc), mixed time-frequency domain (e.g., wavelet transform [6] coefficients). The feature vectors are then utilized by the decision block to determine the direction. The basic idea is that under normal condition, there is not much change in the current phase angle, while that changes during fault. Let’s say, we continuously estimate the current phase angle via DFT at each cycle n, n + 1 as φn , φn+1 , and consider the difference ∆φ = φn+1 − φn . ∆φ ≈ 0 under normal condition, while at forward or reverse fault, it will show big angle changes, with negative and positive polarity. DFT is particularly suitable for tackling the total harmonic distortions (THD) [5] (see II.D). The recursive method for DFT [5] allows for reduced computation. Non-DFT based phasor estimation, e.g., Kalman filter, recursive least squares [5] might be too slow for online protection, typically in the range of 20–100ms [2]. Blackbox methods like neural networks (NN) [7] as decision logic might not be preferred over the deterministic methods like the DFT. However, NN might provide cheaper computation if DFT, Kalman filters cannot be supported. Ensemble of decisions is also possible. sensitivity. For example, if the sampling frequency is say 1 kHz, then for 50 Hz system, the minimum angle change 360◦ = 18◦ per sample. sensitivity (per cycle) would be 1000/50 iv. Accuracy of the phasor computation [5] approaches might be influenced by realistic problems like frequency deviations, inherent unbalances in the systems, noise and other measurement uncertainties [2], etc. Fig. 2. Phasor diagram for (a): reverse and forward fault, (b) pre-fault current. Fig. 3. Current-only directional detection methods. C. Limitations D. Tackling of Harmonics, Noise Besides using low-noise, good accuracy sensors, noise/jitter, offset correcting filters, useful approach to deal with noise in power systems is to use the DFT-based phasor computation [5], considering only the phase angle of the fundamental frequency (e.g., 50 or 60Hz). This is essentially harmonic filtering. Any frequency deviation can be tackled by keeping the number of samples per period (over which the DFT is computed) constant. This can be done by estimating the actual line frequency (using frequency tracking methods) and then adjusting the sampling frequency (by changing the ADC interrupt timings) to keep the samples/cycle ratio constant. Based on the detection methods, prototype development using ABB relays are underway. i. The direction information estimated during fault, is judged with respect to the pre-fault current according to basic theory. If the direction of the pre-fault current changes during normal condition, the direction definitions (forward or reverse) would be vice versa. From the theory, the relay would not be able to judge that automatically. ii. To judge the direction, as baseline information, the current-only relay has to see valid pre-fault current for certain duration. This is not any imposing condition, as the pre-fault current flows, and the relay sees that before fault inception. iii. Depending on the sampling frequency, there might be limitation on the minimum fault angle change detection [1] W. A. Elmore, Protective Relaying Theory and Applications, 2nd ed. Marcel Dekker: New York, 2003. [2] P. M. Anderson, Power system protection, McGraw-Hill: New York, 1999. [3] J. Horak, “Directional overcurrent relaying (67) concepts,” In Proc. 59th IEEE Conf. Protective Relay Engineers, 2006. [4] Int. Eletrotechnical Commission (IEC), “Standard for measuring relays and protection equipment,” no. 60255, 2008. [5] A. G. Phadke, J. S. Thorp, Synchronized Phasor Measurements and Their Applications, Springer: New York, 2008. [6] I. Daubechies, Ten Lectures on Wavelets, Society for Industrial and Applied Mathematics: Philadelphia, 1992. [7] A. Ukil, Intelligent Systems and Signal Processing in Power Engineering, Springer: Heidelberg, 2007. R EFERENCES