bus-diff
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Bus Differential Protectionand Simulation
EE 4223/5223
April 6, 2007
Andrew Kunze
Based on Team ITC 2005/2006 Senior Design Project
Contents
• Project Description
– Differential Protection
– Relays
– Current Transformers
• Settings Calculations
– SEL-551C
– SEL-587Z
• ATP Simulation
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Sponsor
• International TransmissionCompany
– Novi, MI
– 2700 miles of transmission lines in 13counties in southeasternMI
– www.itctransco.com
Dependability and Security • Reliability – tripping breakers to protect
from a fault
• Security – tripping only the necessary
breakers
• Relay protection is finding a balance
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Differential Relay Protection
• Relay trips by the difference between the
current (measured by CTs) going into and
out of the zone of protection
Differential Relay Protection
One-line Current Path under Normal Operation (I1=I2)
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Differential Relay Protection
One-line Current Path under Fault Conditions (I1≠I2)
Existing Protection Scheme
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IAC 55B
• General Electric
electromechanical relay
– Instantaneous time-
overcurrent relay
– Used by ITC to trip
relay when
differential current
exceeds maximumvalue
SEL Relays
SELSEL--587Z High Impedance587Z High Impedance
Differential RelayDifferential Relay
SELSEL--551C Overcurrent Relay551C Overcurrent Relay
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Relay Characteristics• SEL-551C
– Near direct replacement for existing IAC55B relays
– Less expensive than the SEL-587Z
– Can be used with auxiliary CTs
– Significantly slower trip time under some circumstances
• SEL-587Z – Faster fault detection & trip time
– Provides greater degree of protection than overcurrent relays
– High Impedance generates voltages up to 2kV (MOV)
– Requires lock out relay protection – Requires dedicated main CTs
•Recommends lockout relay contacts inparallel with the587Z CT inputs
•Allows the relayto be shorted outon the circuitafter fault isdetected
•No more then 4cycles (67ms) of
fault current throughthe 587Z
•Breakers not reliedon to interrupt thefault current cominginto the relay
Relay Specifications
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Settings Calculations
• Information provided by ITC for Bus 102
of the Milan substation
• SEL-551C
– Instantaneous Current Setting
– Time-Overcurrent Pickup and Time Dial
• SEL-587Z
– Voltage setting
Settings Considerations• Minimum Internal Fault
– Single-line to ground
– Relay must trip for internal faults
• Maximum External Fault – Fault outside zone of protection causes CT
saturation – Differential current on secondary
– Relay should not trip for external faults
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CTs
• Mistubishi Electric
• Mulit-ratio 3000:5
• C800 Accuracy
CT Saturation• Causes of CT saturation
– Primary winding of CT has a DC component
– Primary winding’s current is too high and core
flux saturates
• Primary and secondary windings lose their
linearity causing an error current
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SEL-551C Saturated Circuit
Calculate Instantaneous Setting
• The current through the relay is:
= 6.27A
= 10 A
= 6000 A
N
I
R R
R I F
CT
CT E ⋅
+= )(
5.7
600
17877)
5.70.2
0.2(
A I E ⋅
Ω+Ω
Ω=
E s I K I ⋅=
10600min ⋅=⋅= S I N I
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TOC Curve
Time-Overcurrent Pickup• Using ITC’s standards, time-overcurrent pickup is set at
10% of the maximum external fault
A APU 180017877%10 ≈⋅=
F I PU ⋅= %10
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Time Dial Calculation
• TD = Time dial setting
• tp = Trip time at multiple of pick-up (12 cycles = 0.2
secs)
• M = Multiple of pick-up (3600/1800 = 2)
TD = 0.8
)1
00342.000262.0(
02.0−
+⋅= M
TDt p
)12
00342.000262.0(2.0
02.0−
+⋅= TD
SEL-587Z Saturated Circuit
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Excitation Current
• Using the graph
for the CT the
value of the
excitation current
(Ie) can be found
given Vs = 112 V
A I e 015.0=
Current Through the SEL-587Z
• The current through the relay (Ir ) can be found by:
R
V I sr =
Ω=
2000
112V I r
=r I 0.056 A
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Current Through the MOV
V V MOV 1000<
When Vmov < 1000 V
the current through
the MOV (Im) is 0 A
Minimum Current • Using the determined values, the minimum primary
differential current (Imin) is:
• Minimum Internal Fault = 4168 A
N I I nI I mr e×++= )(min
600)0056.0015.03(min ×++×= I
=min I 61 A
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ATP Simulation
• Use ATP to simulate CTs
• Determine voltage ‘seen’ by the relay for
internal fault conditions
• Apply simulated waveforms to the relay for
testing
CT Simulation
• Ideal transformer • Type 93 non-linear inductor
• Series resistance
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CT Characteristics
4.502208.3731001.2
3.75154.241201
3.60117.581100.96
2.0260.0690.050.54
0.870.0320.0240.232
0.4350.020.0140.116
0000
λ (Wb-T PK)I (A PK)I (A RMS)E (kV)
Non-linear InductanceCT
ATP Simulation
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Minimum Internal Fault (4 kA)
Maximum Internal Fault (20 kA)
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Thevenin Equivalent
Fault Contributions
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Line Impedances
Calculated Line Impedances at 120 kVCalculated Line Impedances at 120 kV
11.430.761 + j8.7028.735 /_ 8513738 /_ -85
3.7313.233 + j49.38751.129 /_ 752347 /_ -75
4.337.102 + j30.76131.571 /_ 773801 /_ -77
X/RR + jX ()Z ()I (A)
DC Offset
o
90min −=θ α
o180max −=θ α R
X 1tan−=θ
• DC offset determined by initial angle
• Use X/R ratio of line carrying most fault
current
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Minimum DC Offset
Minimum DC Offset
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Maximum DC Offset
Maximum DC Offset
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Doble Limitation
Relay Voltage
• Doble Simulator maximum output – 300 V
• Resulting wavform sinusoid with decaying DC offset
• Relay trips properly on slightly saturated waveform
Conclusion
• Differential schemes protect bus from internalfaults
• Relay settings must balance reliability withsecurity
• ATP can simulate CT saturation to understandfault conditions
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References• [1] Blackburn, J. Lewis. Protective Relaying: Principles
and Applications. 2nd ed. New York: Marcel Dekker, Inc.,1998.
• [2] Dr. Mork, Bruce. Personal communication onsubstation layouts and relays. Michigan TechnologicalUniversity. Fall 2005.
• [3] General Electric. Short-Time Overcurrent Relays.
• [4] Mitsubishi Electric Power Products, Inc. Factory TestReport 120-SFMT-40J Gas Circuit Breaker. Oct 4, 2004.
• [5] Schweitzer Engineering Laboratories. SEL-587ZInstruction Manual
• [6] Schweitzer Engineering laboratories. SEL-551CInstruction Manual
Questions or Comments?
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