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Impact of Voltage Transients and System Impedance Ratio on Zone
1 Distance Relay Reach
56th Minnesota Power Systems ConferenceNovember 3, 2020
Pratap G. Mysore, P.E., Pratap Consulting Services, LLC
&
John U. Berzins P.E., Xcel Energy, MN
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Source to Line Impedance Ratio - SIR
2
§Also defined as the system impedance ratio§ for a fault at the
end of the relay reach, ZL, the voltage at the relay
location,
§ VR = !
(#$%#&)∗ 𝑍𝐿 = = (!"
!#%)= (
(*+,%))--- (1)
§ where, E is the phase to ground voltage§ and ZS is the
Impedance behind the relay location
E
ZLZS
Relay Location
VR
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SIR
3
§Voltage at the Relay Inversely proportional to SIR
§ Three phase and phase-phase Fault, ZS = ZS1; ZL = ZL1
§ Single line to ground fault,ZS = (2ZS1+ZS0) and ZL =
(2ZL1+ZL0).
VR =(
[!!"!#"∗[ $%&$%' %)]
where, p = 0!"0!#
and q = 0$"0$#
§ SIR –depends on fault type
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Voltage magnitude on Distance Relay
4
§ Fault Program – Best way to figure out the lowest voltage for
end zone fault.
§ SIR = [E/V -1]
§Relay manufacturers provided operating times based on the fault
location with reference to setting and at various SIRs.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.01 0.1 1.0 10 100
Boundary
SIR ZS/ZL
Faul
t Loc
atio
n (P
U o
f the
Rea
ch
Setti
ng)
20 mS
15 ms
10
20
10 20 30 40 50 60 70 80 90 100
Fault location (Percentage of the reach setting)Re
lay
oper
atin
g Ti
me
(ms)
Microprocessor Relay
Electro Mechanical Relay
Static
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Distance relay
5
§Operating Quantity: (IZ-V) also known as Compensated
voltage;
§ I – Current§ Single Line to Ground Fault(A-G): IA+K0IN, K0=
Zero Seq. Compensation Factor§ Line –Line Fault (B-C): (IB-IC)§
Three Phase – Same as Phase-Phase fault
§V –Voltage§ A-G Fault: VA§ B-C Fault: (VB-VC)§ Three Phase
fault: same as line to line
§Reference quantity : § Voltage input (Mho)§ Curent based input
(Quadrilateral)
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Variation of (IZ-V)
6
0
20
40
60
80
100
0 0.2 0.4 0.6 0.8 1 1.2(IZ-V
) -%
of N
omin
al V
Fault Location, M
(IZ-V) for fault location, M (M as PU of the Reach Setting)
IZ-V for SIR=0.1 IZ-V for SIR=1
V-IZ for SIR =5 V-IZ for SIR =10
V-IZ for SIR =20 V-IZ for SIR =30
§ (IZ-V) = ("#$)(&'()")
𝐸; M -fault location from the relay.
§ (IZ-V) =IZ for a fault at the relay location; M=0
§ Decreases as fault location is moved away from the relay
location.
§ Positive up to the Reach.§ Negative beyond the Reach.§ Will be
Zero at the reach; M=1§ Slope decreases with the
increase in SIR
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(IZ-V) at 80% of the Reach set to 80% of the Line Impedance
7
SIR 0.1 1 5 10 20 30%(IZ-V) 18.2% 10% 3.3% 1.8% 0.95% 0.65%
§ Relay Zone 1set to 80% of the line Impedance.
Observation:§ Fault at 80% of Zone-1 reach (64% of the line
impedance).§ Differential voltage between the 80% of the reach and
the
reach location reduces as SIR increases.§ If the error exceeds
these numbers, zone-1 operates for faults
beyond the protected line section.
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Current Error in (IZ-V)
8
§CT Errors: C-Class CTs are used for protection; Errors
specified at
rated burden
§ 10% at 100A and 3% at 5A (Relay always sees less current than
the
actual Value).
§ CT saturation: Relay always estimates lower currents
§ DC offsets – Replica Impedance, Digital mimic filters
§ Relay measuring errors
Assume worst case CT error: 5%
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Impedance Error in(IZ-V)
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§ Z: Setting error 3%; Impedance calculation error: 5%
§ Total error of IZ: (1+0.05)*(1+0.08) -1 = 0.134; Error:
13.4%
§ The relay will overreach by 13.4%.
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Voltage Errors in (IZ-V)
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§Wound PT: Errors defined between 90% -110% voltage range
§Accuracy class: 0.3% -1.2%
§No data – V< 90%
§Check with manufacturers
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Capacitive Voltage Transformers
11
§Most common source of voltage input
L
C1
C2
Ferro Resonance Suppression Filter
Active Passive
CVT Errors§ Steady State 0.3% error from 5-100% VNOM§ Steady
state error 0.6% at 2% VNOM§ Transient Errors:10%§ always defined
at one cycle after the disturbance.§ Special Class: 0.4% at 0.5
Cycle and 0.2% at one
cycle.
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Relay Protection Design- Signal to Noise Ratio
12
§ Today many utilities still used the 66.4V taps on CVTs for
line protection needs § Due to limitations on older
electromechanical relays§ Microprocessor relays have an upper limit
above 250V
§ The use of full tap increases the signal to noise ratio of the
relay terminal voltage by 1.73 (assumes CVT with 115V/66.4V
taps)
§ Example: For a line with SIR of 20, for end zone fault, the
voltage at the relay would be 0.048 PU§ Using the CVT’s 66.4V tap:
the actual secondary voltage would be 3.1 V § Using the CVT’s 115V
tap: the actual secondary voltage would be 5.5 V
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PSCAD Line Model Study
13
Modeled 230kV 2-mile line with an infinite system
The source impedance behind the relay location was varied to
provide SIR from 1 to 40 in steps of 5
Zone 1 phase and ground distance elements were set to 85% and
125% of the line impedance
Single line to ground fault and three phase faults were
simulated at the end of the line
Fault incidence angles were varied from 00 to 1500 for each SIR
value for all fault types
Potential Transformer
PTR 2000
0.85*ZL – Zone 1 reach Impedance
0.15*ZL –Line impedance beyond Zone 1
Ground Fault Switch (1-Ph or 3-Ph) with
variable fault initiating angle
*- Wound PT, CCVT with Active filter and CCVT with Passive
filter were used
Variable Source Impedance
CTR2000:5
NOTE:- Line impedance Z1: 0.775@ 83.180 ohms/mile; Z0:[email protected]
ohms/mile- line length considered: 2miles - Source impedance varied
based on SIR from 1 to 40 in discreet steps.-Homogenous system
considered- source and line angles are considered same.
Strong Remote Source Impedance,
Zpos=Z0 = 0.5168Ω@830
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Simulated Case Study 1
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Remote end three phase fault at an SIR of 40
The faults inception was at a voltage zero crossing
CVT with active ferro-resonance suppression filter
No transient detection logic or with no zone-1 time delay
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Simulated Case Study 2
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Remote end three-phase fault, an SIR of 40, with an active
CVT
No transient detection logic or with any zone-1 time delay
Transient impedance trajectory would be inside the zone-1 mho
for roughly a ¼ cycle before swinging out again
Actions prevented the relay from overreaching:
Addition of a 1 cycle delay on the zone-1 elements prevent an
over operation
Pulling back the reach to 80% or less for SIRs at or above
30
Enabling transient detection logic in the relay
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Simulated Case Study 3
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Remote end single line to ground (SLG), an SIR of 40, with an
active CVT
No transient detection logic or with any zone-1 time delay
enabled
Zone-1 over operated when set and tested at 60%
Zone-1 element picks for roughly 33ms after the fault's
initiation.
Zone-1 element remained picked up for about 6.5ms
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ACTUAL CASESTUDY 1
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Actual 345kV, 21.4-mile long line with a remote SLG fault
Zone-1 ground element reach set at 80%
The voltage input was from an active CVT, 66.4V tap
SIR is calculated from pre-fault and fault voltage magnitude SIR
= 206.667/38.59 -1 = 4.35
Ground zone-1 mho was picked up less than a ¼ cycle
Roughly 1 cycle after the fault initiation, the impedance was
around 98.9% (12.82 Ωpri)
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ACTUAL CASESTUDY 2
18
Actual 345kV, 20.8-mile long line with a remote line to line
fault
Sub-cycle static relays with zone-1 phase element reaches set at
83%
The voltage input was from an active CVT, 66.4V tap
SIR is calculated from pre-fault and fault voltage magnitude SIR
was calculated to be 2.17
These static relays have no settable zone delay
The zone-1 reaches were reduced to 70%
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Conclusion
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§ CVT generate transient for one to two cycles after a
disturbance due to stored energy in capacitance.
§ In systems with high source to line impedance ratios, CVT
transients may dominate during the transient period
§ CVT designs with lower capacitance values and with active
ferro-resonance suppression filters tend to increase overreach
issues
§ Use of full secondary voltage instead of tapped value improves
signal to noise ratio.
§ CVT transient mitigation:§ Add intentional delays to zone-1
elements or/and reduction in zone 1
reach – This could be a setting in the relay or an added delay§
Reduction of zone-1 reach may be necessary.
