Post on 02-Feb-2016
description
Copyright © SEL 2015
Paralleling CTs for Line Current Differential Applications: Problems and Solutions
David Costello, Jason Young, and Jonas Traphoner
Schweitzer Engineering Laboratories, Inc.
Outline
• Review two fundamental principles
♦ Tapping multiratio CT derates CT
♦ Paralleling CTs to 87L inputs decreases security
• Compare 87L designs
• Discuss two case studies
• Present solutions
Equivalent Circuit for Paralleled CTsExternal Fault
I1I2 XMXM
RL RLRCT RCT
IF IF
CT BCT A
Criteria to Avoid SaturationModified for 87L Applications
F B STDX
I • Z 1 7.5VR
Fault Current ComponentsSymmetrical (AC) and Asymmetrical (DC)
R
tM M LV V
i t sin t sin eZ Z
Effect of Tapping 1200:5 Multiratio CTC800 at 300:5 Versus 1200:5
Time to Saturate
SAT
F S BSAT
VX 1I R RRT ln 1
XR
External CT Summation ConnectionNumber of CTs Is Greater Than Relay Inputs
1 2
IR IL1 + IR
IL1
Line
••••••
Modern Relay With Multiple CT Inputs
1 2
IR IL1 + IR
IL1
Line
••••••
1 2
Traditional Slope-Based 87L
IOP = KIRT
Restraining Region
Operating Region
IRT
K0
IOP
K2K1
Alpha Plane Characteristic
Re(k)
Im(k)
Operating Region
RestrainingRegion
Rad
ius
–1
Angle
External Fault Detector
–
+
–
+
DIOPP
DIRTP
DIRTR
DIOPRDIOP
DIRT
∆IOPR
∆IRTR
Advanced Relay Characteristics
Re(k)
Im(k
)
–4
–3
–2
–1
0
1
2
3
4
–4 –3 –2 –1 0 1 2
Case Study 1 One-Line Diagram
Substation Alpha
Substation Charlie
262524
21 22 23
Bus A Bus B
16
33
Line 1
Line 2
CGSubstation Bravo
•••
•••
Raw Event Data From Breaker 51Asymmetrical CG Fault
Relay and CT Connection87L With Tapped and Paralleled CTs
21 22
Bus A
Line 2
Primary 87L / 21 / 67
Secondary21 / 67
2000:5 2000:5800:5 800:5
2000:5800:5
2000:5800:5
Filtered 87L Event DataICL 1.5 Cycles After Fault Inception
Case Study 1 Summary
• CTs are paralleled to 87L relay – for external fault, relay measures only error or difference between CTs
• CTs are tapped down – this derates CT performance
• External asymmetrical fault (reclose) causes saturation and dissimilar CT performance
Case Study 2 One-Line Diagram
4 7
5 8
6 9
13 32
14 33
15 34
•••••• •••
TCC-6TCC-7
Substation PO-1
TCC-8TCC-9
Substation L
Substation SK
Raw Event Data From Terminal 14-15
Breaker Failure Relay DataIndividual CT 14-15 Signals
Difference or Error Between CTsBreaker 14 and 15 Data
Filtered Data for TCC-8BG Fault Evolves to ABG, IA Spikes
Breaker Failure Relay DataIndividual Breaker 32 and 33 Signals
Difference or Error Between CTsBreaker 32 and 33 Data
Difference Between CTsAnother, Less Obvious Case
Case Study 2 Summary
• CTs are paralleled to 87L relays
• CTs are tapped down
• External BG asymmetrical fault inception causes TCC-9 CT error and misoperation
• BG to ABG fault transition causes TCC-8 CT error and misoperation
Solution 1 – Add Short DelayAlso Delays Internal Fault Trips
Solution 2 – Use Full CT WindingCase Study 2 CTs at 300:5 and 1200:5 Taps
Solution 3 – Expand Restraining RegionNot Effective, Reduces Dependability
Im
Re
8.06.04.02.0
Solution 4 – Use Advanced Relay
Conclusions
• CTs paralleled to 87L relays
♦ Breaker-and-a-half and ring-bus applications
♦ Relays with one three-phase current input
♦ Measurement of only error or difference between CTs for external fault
• CTs tapped down
♦ Is sometimes done to meter low load currents better
♦ Derates CT performance during faults
Conclusions
• Two case studies
♦ Both show 87L misoperations for external faults
♦ Both use externally paralleled and tapped CTs
• Solutions involving setting changes
♦ Empirically derived settings may not work for all cases
♦ Delay also slows tripping for internal faults
♦ Increase in slope or Alpha Plane restraint decreases dependability
Conclusions
• Solution – use maximum CT winding
♦ Increase tap to improve CT performance
♦ Use short delay (1-cycle maximum) with improved CT taps if CT saturation is still possible
• Solution – install modern relay
♦ Relay should have two CT inputs, external fault detector, generalized Alpha Plane, and more
♦ Replay of original data proves secure
♦ Consider using full winding CTs
Questions?