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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