The Effects of Waveform Distortion on Power Protection
Relays
Laith Al-Musawi / Andrew Waye / Dr. Nafia Al-Mutawaly, McMaster University, Canada
William Yu, OMICRON, USA
Introduction
With increased usage of Variable Frequency Drives
(VFDs), solid state inverters and other non-linear
loads, power quality is becoming a concern in
distribution network performance. Harmonics
produced by such loads can cause overheating of
neutral conductors, transformers, and motors.
Moreover, research has shown that high harmonic
content can have negative impacts on power
system performance, as protection relays are built
to operate at a nominal frequency (50/60 Hz) [1].
Previous research indicated that some relays may
mis-operate and trip under normal operating
conditions, or, conversely, fail to trip entirely in the
presence of harmonics [2]. False or missed relay
tripping may result in system failure, service
discontinuity or other economic losses.
This paper outlines work that has been conducted
to test various types of relays (electromechanical,
digital and Intelligent Electronic Devices) and their
performance in the presence of harmonics.
Although previous investigations addressed this
topic [2, 4], the findings did not comprehensively
quantify the impact of harmonics on relay
performance. The experiments discussed in this
paper were conducted using an OMICRON relay
test set (CMC 256). The tests conducted in this
study examined various levels of Total Harmonic
Distortion (THD) including individual harmonics up
to the 49th, superimposing harmonic profiles and
varying harmonic phase angles.
Test Methodology
General overview
Protective relay performance testing was
undertaken
on
both
conventional
(electromechanical) and modern (digital) relays. An
OMICRON CMC 256plus high-precision relay test
set was used to synthesize arbitrary waveforms and
capture resultant tripping times. Conventional (60
Hz) AC waveforms were applied directly to all relays
under test in order to determine the relay activation
baseline (time). To produce the same energy as that
of the fundamental frequency, a distorted waveform
was then synthesized (including both fundamental
and harmonic components) according to the
following RMS energy formula [3]:
A comparison between the tripping times of both
injected waveforms was calculated as shown in the
formula below:
The testing was repeated for all relays with data
collected, recorded (currents and harmonic
distortion levels) and plotted for detailed analysis.
Test Setup
With OMICRON’s current channels connected
directly to the relay (no current transformer), various
types of relays were evaluated (figure 1).
Fig. 1 Test Setup
The relay output (trip signal) was connected directly
to a OMICRON CMC 256plus digital input to
determine the tripping time. Electromechanical,
digital and Intelligent Electronic Device (IED) relays
were tested, as shown in Table 1.
Electromechanical
Digital
ABB CO-8
ABB REU 523
Westinghouse
CO-11
SEL 451 (IED)
Table 1 - Relays Tested
Results
The data obtained from various tests were
recorded, tabulated and graphed to reflect relay
performance. Experimental findings were organized
into five categories: inverse time characteristic,
© OMICRON 2013 – International Protection Testing Symposium
definite tripping time, relay current transformer (CT)
tap order, harmonic profiles and voltage distortion.
Inverse Time Characteristics
Electromechanical Relays
Figure 2 presents the impact of a given harmonic
order (marked points) and the variation of applied
harmonic magnitude (trend lines) on trip time.
Minimal impact was observed on relay performance
for Individual Harmonic Distortion (IHD) <10%.
However, when total harmonic distortion is 20% or
higher, two trends become apparent: the low order
harmonics have less impact on the tripping time,
while the higher order harmonics (>19th) have an
obvious impact.
Fig. 2
Fig. 3 Inverse Time Characteristics - Electromechanical
vs. Digital Relays
Definite Trip Time
Electromechanical Relays - Harmonic Order &
Phase Angle
Tripping time was impacted by harmonics;
specifically, it was found that the tripping time is
dependent on the harmonic order and phase angle.
From figure 4, it is evident that the 3rd and 5th
harmonics affect the device in different ways with
respect to tripping time, a variation which is
dependent on the harmonic phase shift relative to
the fundamental.
Inverse Time Characteristics Electromechanical Relays
Digital Relays
For digital relays, it was observed that respective
harmonic orders or magnitudes have little to no
effect on relay operation (the harmonics appear
invisible to the relay). Literature on the subject
indicates that digital filters eliminate harmonics
when converting from analog to digital (A/D)
quantities, as a filter in the relay’s digital signal
processor (DSP) stage removes frequencies
greater than 60Hz [4].
When comparing electromechanical versus digital
relay performance, it is apparent that digital relay
tripping time remains constant (independent of
harmonic content), whereas electromechanical
relay trip time is harmonic dependant (figure 3).
Fig. 4 Trip Time - Harmonic Order and Phase Angle
To quantify the effects of phase shifting the 3rd
harmonic, the IHD levels were increased as a
percentage of the fundamental, which resulted in an
observed increase in tripping time (figure 5).
Fig. 5 Trip Time - 3rd Harmonic Distortion Percentage
© OMICRON 2013 – International Protection Testing Symposium
Digital Relays - Harmonic Order & Phase Angle
Harmonic order had limited (and minimal) impact on
the performance of the digital relays; however,
changing the phase angle of the 3rd harmonic
resulted in a noticeable impact on tripping time.
Figure 6 shows that the 5th and 7th order harmonics
were not detected by the relay, however the 3rd
order harmonic (at a 120-270 degree phase shift)
resulted in increased tripping time. This can be
attributed to the software algorithm for the DSP
stage of the relay.
Harmonic Profiles
Electromechanical Relays
Mixed harmonic distortion had a minimal impact on
the tripping time of the electromechanical relays
tested, as shown in figure 8.
Fig. 8 Mix Distortion - Electromechanical Relays
Digital Relays
Fig. 6 Trip Time - Harmonic Phase Shifting
There was no impact on digital relay tripping time
due to mixed harmonic distortion (figure 9).
Relay CT Tap Order
Electromechanical Relays
Tap order had a minimal impact on the performance
of the relays tested.
Fig. 9 Mix Distortion - Digital Relays
Voltage Distortion
Fig. 7 Tap Order - Electromechanical Relays
Digital Relays
As there are no tap connections on digital relays,
there can be no impact to relay performance based
on the tap setting.
Digital Relays - Undervoltage
Harmonics of all orders (20% total distortion)
resulted in essentially the same impact to relay
tripping time in an undervoltage condition. Figure 10
shows that a per-unit trip time of 1 (for fundamental
voltage only) was observed as expected.
Harmonics beyond the fundamental (up to the 49th)
resulted in a per-unit trip time of less than 0.3, which
indicates that voltage harmonics are completely
filtered.
© OMICRON 2013 – International Protection Testing Symposium
Fig. 12 Electromechanical Relay’s Induction Disk [2]
Fig. 10 Voltage Distortion - Digital Undervoltage
Digital Relays - Overvoltage
Harmonics of all orders (20% total distortion)
resulted in essentially the same impact to relay
tripping time in an overvoltage condition. Figure 11
shows that a per-unit trip time of 1 (for fundamental
voltage only) was observed as expected.
Harmonics beyond the fundamental resulted in a
per-unit trip time of greater than 8. The relay takes
longer to trip as the potential transformer (PT) is
designed to detect only fundamental frequencies
(50/60Hz).
With fundamental pick-up current applied, torque of
sufficient magnitude is generated to overcome the
spring restraint, causing disk rotation [2]. This
torque results from interaction between disc
currents produced by each pole flux and the other
two pole fluxes, with all forces in the same direction.
Additionally, the total flux will decrease in inverse
proportion to frequency (harmonics on input current)
resulting in a net impact on the centre pole. As
harmonics are added, the induced lag coil current
remains largely unchanged, causing the third and
centre pole fluxes to draw closer in phase, resulting
in:
 Increase of pickup current
 Reduction in disc rotation speed
 Inconsistent tripping times
In contrast, digital relays operate on the concept of
capturing current and voltage waveforms, filtering
the waveforms and processing them using Discrete
Fourier Transform (DFT) methods. As shown in
figure 13, the waveform is sampled every quarter
cycle to ensure proper capture of the waveform
energy (area under the curve).
Fig. 11 Voltage Distortion – Digital Overvoltage
Discussion
Electromechanical relay induction discs consist of a
three-pole electromagnet manufactured to operate
on a nominal frequency of 50/60Hz. The centre pole
is energized by the flow of mains current, while the
outer pole is equipped with a lag coil, and the
remaining pole receives flux generated by the other
two poles (figure 12).
Fig. 13 Sampling in Digital Relays
© OMICRON 2013 – International Protection Testing Symposium
Using the resulting samples, the relay DSP stage
computes the magnitude of each harmonic based
on the following equation:
Where,
m is sample number
n is harmonic order
N is total number of samples
X(m) is the signal magnitude at sample m
Data acquisition and filtering stages are essential
parts of a digital relay. The presence of harmonic
pollution on input signals may result in the
malfunction of digital algorithms and therefore in the
mis-operation of the relay [5]. Since microprocessor
controlled relays implement low pass filters before
digital filtering, digital relays do not respond to
higher order harmonics [6]. As a result, they are
incapable of including energy within the higher
frequency harmonics into their trip algorithms [6].
Conclusions
The tripping times of electromechanical relays,
based on inverse time characteristics, were
observed to be dependent on harmonic amplitude
and order. IHD values of <10% were found to have
little to no impact on relay performance; this can be
attributed to the minimal magnetic flux introduced in
the induction disk. IHD values of >20% resulted in
an observable impact due to the generation of nonlinear magnetic flux. Harmonics greater than the
19th resulted in a significant variation in tripping time
(figure 2). In contrast, digital relay tripping times
remain constant regardless of harmonic amplitude
and order due to filtering implemented within the
relay; however, the tripping time of the digital relay
did appear to have a constant offset in the presence
of harmonics (figure 3).
Definite tripping times on electromechanical relays
were observed to be affected by the phase angles
of lower-order harmonics (3rd and 5th). Little to no
impact was observed with higher order harmonics
or phase angles (figures 4 and 5). Digital relays
experience minimal impact in the presence of 5th
order harmonic phase-shifting, however a
noticeable impact is observed with the 3rd order
harmonic (figure 6). The phase angle imparts more
impact for low order harmonics (3rd and 5th) whereas
higher harmonics have minimal impact. Phase
shifting the 3rd harmonic to 150-180 degrees will
introduce a destructive effect on the fundamental;
the inverse phenomenon applies to the 5th harmonic
introducing an additive effect on the fundamental.
However, with higher harmonics, an additive or
destructive action due to phase shifting has
relatively less impact on the fundamental.
Electromechanical relays’ CT tap order had little to
no impact on the relay’s performance; changing the
tap will not modify the magnetic flux induced by the
harmonics. Deviations in captured tripping times
were apparent, but these increases were not
attributable to the CT tap position (figure 7).
With undervoltage distorted waveforms, the relay
did not detect the presence of harmonics, treating
the signal as a fundamental with a lower magnitude
resulting in reduced tripping time (figure 10).
Conversely, overvoltage distorted waveforms were
observed to increase the tripping time as the relay
cannot detect the harmonics energy (figure 11).
However the results indicate that this change is
irrespective of the harmonic order for both under
and overvoltage conditions, as the resulting trip time
change was constant.
To the extent that the two types of relays tested
yielded inconsistent tripping times (for the same
energy input), it is difficult to identify a single solution
as to how grading should be applied when
harmonics are present in protection systems.
Given that digital relays filter out harmonics (except
for the 2nd and 4th harmonics in the case of inrush
current for transformer energization) a parallel
harmonic detection mechanism would be required
to improve digital relay performance in the presence
of high harmonic content. Electromechanical relays
however, do not exhibit predictable performance
changes when influenced by harmonics, and as
such cannot be effectively modified to reliably detect
harmonic content. This remains a concern,
considering the widespread implementation of
electromechanical relays throughout North America
at this time.
Future work
Data recently collected (waveforms and harmonic
profiles) from typical modern homes reveal the
presence of significant harmonics and waveform
distortion (figures 14 and 15). As such, the need for
further investigation of harmonic distortion’s effects
on the distribution system is clearly needed.
Fig. 14 Data Collected from a Typical Modern Home
© OMICRON 2013 – International Protection Testing Symposium
About the Author
Fig. 15 Typical Modern Home’s Harmonic Profile
Laith Al-Musawi received
his BTech. degree in Energy
Eng. Tech. from McMaster
University, Canada in 2013.
He is currently enrolled in an
MSc
in
Electrical
Engineering at McMaster
University. He is also
working as a Research Assistant in the Engineering
Technology department at McMaster University. His
research interests include protection and control of
power systems, power quality, and instrumentation.
Acknowledgment
The authors wish to thank the Natural Sciences and
Engineering Research Council (NSERC) programs
which helped to support this publication.
References
[1]
Donohue, Paul; Islam, Syed: The Effect of
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Current
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Solid-State
Overcurrent
Relay
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IEEE
Transactions on Industry Applications, Vol.46,
No.6, November/December 2010
[2]
A. Elmore, W.; A. Kramer, Cheryl; E. Zocholl,
Stanley: Effect of Waveform Distortion on
Protective Relays. IEEE Transactions on
Industry
Applications,
Vol.29,
No.2,
March/April 1993
[3]
Y., Recep; G., Kayhan; B., Altug; K., Celal; U.,
Mehmet: Analysis of Harmonic Effects on
Electromechanical Instantaneous Overcurrent
Relays with Different Neural Networks
Models.Yildiz Technical University
[4]
E. Zocholl, Stanley; Benmouyal, Gabriel: How
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Saturation, and Other Wave Distortions. The
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Conference, 1997
[5]
Zamora, I.; Mazón, A.J.; Valverde, V.; San
Martín, J.I.; Buigues, G.; Dyśko, A.: Influence
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[6]
Medina, A.; Martínez-Cárdenas, F.: Analysis of
the Harmonic Distortion Impact of the
Operation of Digital Protection Systems. IEEE
© OMICRON 2013 – International Protection Testing Symposium
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