Challenges One May Not Expect to Face When Applying...
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Challenges One May Not Expect to Face When Applying Digital relays
Miroslav Ristic – Pacific Gas and Electric Company
Harjeet Gill – Pacific Gas and Electric Company
Ilia Voloh – GE Grid Solutions
2019 Texas A&M Protective Relaying Conference
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Agenda
• Introduction
• Measurement ranges of the digital relays, maximum and minimum.
• Understanding specifications and functionality, Hysteresis, Accuracy, Timing, Testing Methods
• Understanding specific product behavior, RMS / Fundamental Phasor, Testing the Frequency and Distance Elements
• Conclusions
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Introduction
• Digital relays sometimes bring unexpected results to the users in the field, during relay operation or during testing.
• We’ll examine some important details of the digital relays being misinterpreted, or overlooked, which may cause the confusion to the user.
• We’ll examine why CT and VT sizing remains important, taking into consideration measurement ranges of the digital relays.
• We’ll also examine some confusions, stemming from relay not meeting specification claims.
• Paper is based on the practical experience of applying and testing digital relays-gives useful insights to engineers and technicians to understand better digital relay functionality, specifications and settings.
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Measurement ranges of the digital relays
• Unlike electromechanical and solid-state protective relays, digital
protective relays are used today not for protection purposes only,
but also for metering, system wide monitoring, synchrophasors
etc.
• Expectations are that relay are able to measure very low signal for
accurate current/power metering, but will be able to deal with a
high-level current during system fault as well.
AC Current AC Voltage
Minimum value of the measurement range
0.01-0.1 x In 0.01-0.1 x Vn
Maximum value of themeasurement range
20-100 x In 3-4 x Vn
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Measurement ranges of the digital relays
• Common practice to select CT class and ratio is based on the:
▪ Maximum load current
▪ Maximum fault current
▪ CT burden
▪ CT knee point voltage and excitation characteristics
• This is to ensure CT error does not exceed 10% for any
symmetrical current from 1 to 20 times rated secondary current
for IEEE class CT.
• Rarely CT transient (DC offset) performance is estimated using
appropriate tools and per IEC 61869-2:2012.
• Low end of CTs and relay range is not considered at all.
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Measurement ranges of the digital relays
Dilemma for the P&C engineer to select CT ratio
In
FaultLoad
ILMAX
Primary
Secondary
· Regular
practice CTR =
ILMAX/ IRn
· May cause
problem, when
IFMAX >> ILMAX
· Security for
fault CTR >
ILMAX/ IRn
· Loss of
sensitivity
during load
and low
current faults
InIn
· Metering
sensitivity CTR
< ILMAX/ IRn
· Security
jeopardized
during faults
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Measurement ranges of the digital relays
DC offset causing error even symmetrical current is same as relay 30 x In
range
54% of true mag
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Measurement ranges of the digital relays
• Possibility of miscoordination with downstream OC relays
• Possibility of bus differential misoperation during external fault,
where faulted feeder current may exceed conversion range but
other feeders, contributing to the fault current may not exceed
range.
• Distance protection will underreach.
• Possibility of directional OC maloperation, because phasors
angles are affected same as phasors magnitude.
• Generation of harmonics as a result of clipping, which can affect
functions using harmonics, such as 2nd harmonic inhibit in
transformer differential.
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Measurement ranges of the digital relays
• Relays range low end can impact operation as well.
• IEEE and IEC standards are silent on the accuracy requirements
for currents, below rated.
Accu
racy C
lass -
%
Rating Factor1.0 2.0 3.0 4.0
10%
0.60
0.30
0.15
0.15
0.30
0.60
100% 200% 300% 400%
0.3% Accuracy Region
0.6% Accuracy Region
Not defined
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Measurement ranges of the digital relays
• CTs experience inaccuracy at higher magnitudes of currents due
to saturation.
• CTs exhibit substantial magnitude and angular errors at low
currents, impacting sensitive protection and metering.
0
10
20
30
40
50
60
70
80
0 20 40 60 80 100
Mag
nit
ud
e e
rro
r (%
)
Current (x In)
0
1
2
3
4
5
6
0 20 40 60 80 100A
ng
le e
rro
r (d
eg
ree)
Current (% of In)
a) b)
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Measurement ranges of the digital relays
• Current below low end of the measurement range (cutoff) is
considered “noise” and is measured as zero current.
• This can compromise some relay functionality.
Fuse
Parameter Feeder 1 Feeder 2
Load > cutoff <cutoff
Voltage change on blown fuse
Detected Detected
Current change on blown fuse
Detected Not detected
Loss of potential (LOP) Not operated Operated
Feeder fault Trip No trip
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Understanding specifications and functionality
• Digital relays specifications need to be understood and not
overlooked.
Time
Current
Pickup setting
Dropout level
Protection response
Fault
Fault cle
ara
nce
Pick up/operateDropout Dropout
Hysteresis
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Understanding specifications and functionality
• Typically, hysteresis in digital relays is 2-3% and is defined in
product specifications.
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Understanding specifications and functionality
• Digital relays accuracy improvements.
Error typeRelay technology
E/M Static Digital
Magnitude overshoot 10% 5% 2%
Overshoot time 0.05s 0.03s 0.02s
Pickup error 5% 3% 1%
Timing error 7-15% 5-7% 3-4%
Coordination safety margin
0.1s 0.05s 0.03s
Typical coordination time 0.4s 0.3s 0.2s
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Understanding specifications and functionality
• Timing accuracy.
Time
Current
Pickup setting
Protection response
Magnitu
de ram
p u
p
t0“fault”
injection
Security delay
t1Internal
start
t2Start
(element pickup)
t3Operate
Inverse curve timing
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Understanding specifications and functionality
• Inject-to-operate:
Error=5.44% ->
FAIL.
• Pickup-to-operate
Error=2.25% ->
PASS.
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Testing methods and analysis
• In digital relays many elements use sequence-components, which
may surprise, but affect function under the test if overlooked.
• MHO function is comparing following quantities for phase
distance AB loop as an example:
𝐼𝐴 − 𝐼𝐵 × 𝑍 − 𝑉𝐴 − 𝑉𝐵 𝑎𝑛𝑑 𝑉𝐴 − 𝑉𝐵 𝑀
• Where 𝑉𝐴 − 𝑉𝐵 𝑀 is a memory voltage, which is actually… V1
rotated by 30°.
positive-sequence voltage is remaining stable during fault and
not as affected by fault transients, as self-polarized.
provides dynamic expansion of the characteristics
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Testing methods and analysis
• Dynamic expansion of the MHO characteristics is explained in
many publications, but when it comes to testing…
How much it expands?
What is the Pass/Fail criteria?
And resistive coverage I can expect for my application?
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Testing methods and analysis
• AB fault at 20% of the 385° transmission line with a fault
resistance of 1.2.
• Phase distance zone 1 with a 2.5585° reach didn’t operate at
all.
• Phase distance zone 2 MHO with a 4 85° reach setting picked
up initially, but dropped out after memory voltage expired,
resulting in NO zone 2 operation.Pre-fault FaultVa 66.4V0° 55.7V-38°
Vb 66.4V-120° 25.5V-115°
Vc 66.4V120° 66.4V120°
V1a 66.4V0° 46.5V-13°
Ia 0A0° 6.84A-28°
Ib 0A0° 6.84A152°
Ic 0A0° 0A0°
I1a 0A0° 3.95A-58°
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Testing methods and analysis
• For a phase-to-phase faults initially MHO expands to full Z1S
impedance and reduces expansion to Z1S/2 after memory expires.
• Source Z1S can be estimated using equation 𝑍1𝑆 = −∆𝑉1/∆𝐼1
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Understanding specific product behavior
• P&C engineers still today are thinking RMS measurements.
• But digital relays work on fundamental frequency phasors, where
some functions have a choice of RMS or phasor.
• Phasors are providing much more accurate measurement,
because they filter out DC component of the fault current and
harmonics.
• RMS measurements should include harmonics, DC offset and
other transients to give proper estimation of the impact on the
protected equipment.
• E/M and static relays were always using RMS values-digital relays
offer this option when there is a need to coordinate with older
generation relays.
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Understanding specific product behavior
• TOC using RMS
operates in 147ms.
• TOC using Phasor
operates in 189ms.
• Difference is 42ms
or 22.2%.
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Understanding specifications and functionality
• Testing of underfrequency, overfrequency and rate-of-change-of-
frequency requires some caution to avoid confusing results.
• Frequency elements are responding to the system frequency
change, which cannot change very rapidly in the conventional
power system due to system inertia.
• Step change in frequency is not a proper method to test accuracy
and timing, because digital relays may “reject” this measurement.
• Technician was testing underfrequency element with a 57Hz
pickup setting and 200ms delay.
• He applied a step change in the voltage and current signals from
60Hz to 54Hz – relay “failed” timing spec.
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Understanding specifications and functionality
0.3 0.35 0.4 0.45 0.5 0.55 0.6-200
-100
0
100
200
time
Voltages (
V)
0.3 0.35 0.4 0.45 0.5 0.55 0.640
45
50
55
60
65
time
f raw
, f filt
ere
d
raw freq
filtered freq
Va
Vb
Vc
Frequency change
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Conclusions• Attention must be paid to the measurement ranges of the
digital relay to ensure secure and dependable operation.
• Digital relays offer much better accuracy and timing
characteristics, compared with E/M and static relays.
• They also provide much better visibility into what signal relay
is responding to and what relay is measuring.
• Sometimes technical characteristics and functionality of
digital relays are not understood correctly, creating
confusions.
• Digital relays are using sequence components and applying
security checks to ensure correct operation.
• Some test methods are causing digital relays ‘failures’, where
it’s not really a case.
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Thank You
Questions?