Power System Protection EE 5940 By Pratap Mysore Lecture 1- Fundamentals University of Minnesota January 20, 2011 EE 5940 Jan 20, 2011; Copyright@2011
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Acknowledgements
University of Minnesota Center for Electric Energy (UMCEE). IEEE Twin Cities local chapter. Xcel Energy. HDR Inc.
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Instructor contact information
[email protected]
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Reference Books
C.R. Mason, “The Art and Science of Protective Relaying”. GE Publication. Linkhttp://www.gedigitalenergy.com/multilin/no tes/artsci/index.htm This book was first published in 1956! EE 5940 Jan 20, 2011; Copyright@2011
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Reference
“Network Automation & Protection Guide”, published by Alstom
Link changed due to acquisition- will be posted soon. Relevant IEEE documents – published papers, guides, standards and recommended practices.
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Reference Book from IEEE
For IEEE members – free download www.ieeexplore.org Sign in as a member – go to books and browse Title – Power System ProtectionP.M.Anderson
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Major Power System Components
Generators Transformers Transmission lines Switching devices Loads
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Power System One Line Transmission line
Generator
Distribution Transformer Shunt Reactor
G GSU transformer
GSU – Generator Step up Transformer
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Shunt Capacitor
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Normal Operating Conditions
Normal – Voltages are within the specified values.
ANSI C84.1 – 1995 (R2005) (100 Volts up to 230 kV). IEEE 1312-1993 (R2004) Re-designation of C92.2-1987 for voltages above 230 kV. (Short guide - only 6 pages).
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Voltage Classification Low Voltages – Up to 1000 Volts Medium Voltage – 1 kV up to 100 kV High voltage (HV) – 100 KV up to 230 kV Extra HV (EHV) – 345 kV up to 765 kV Ultra HV (UHV) – 1100 kV EHV and UHV defined in IEEE 1312
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Voltage Range
Electrical systems – designed and operated within a normal range (defined as Range A) and operating conditions leading to wider range (Range B) where corrective actions are required to bring back the system within normal range. EE 5940 Jan 20, 2011; Copyright@2011
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Voltage Range
Range A- Max. - +5% of the nominal and minimum varies –2.5% up to 5% – 120 V* within 114-126V ( Service range); 13.8 kV - 14.49 kV – 13.46 kV (97. 5% of 13.8 kV).
Utilization voltage range (voltage at the customer) – up to ~90% Vnom. * - State regulations may have different limits at low voltages.
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Voltage Range
Max. voltage up to 345 kV +5% of the nominal.
500 kV – 550kV; 765 kV –800kV; 1100 kV –1200 kV.
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Overcurrent limits
Conductors, pipes and wires, are rated to carry specified current that limits temperature rise over ambient or maximum temperature to design parameters. ACSR conductor –maximum 100deg. C Tubular bus – 30 deg. Rise over 40 deg. Ambient Transformer – 65 deg. Rise over 40 degrees ambient EE 5940 Jan 20, 2011; Copyright@2011
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Frequency
Typically very close to the nominal frequency Interconnection guidelines within +/- 0.5 HZ Regional requirements for corrective action for frequencies outside this limit EE 5940 Jan 20, 2011; Copyright@2011
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Normal Operation
As per C.R. Mason – “ Normal” operation assumes no failures, no mistakes of personnel nor “acts of god”.
The system design should take into account the failures, human errors and abnormal operating situations.
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Electrical Failures – Possible Solutions
Insulation failures due to lightning or switching transients or sustained overvoltages –Surge arrestors and over voltage relaying Short circuits resulting in excessive fault currents –fuses; relaying to detect such conditions. Relays operate isolating devices such as circuit breakers or circuit switchers. Circuit breakers are rated to interrupt short circuit currents.
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Abnormal Operating Conditions
Severe unbalance of generation and load leading to off nominal frequency operation;
Detection schemes to mitigate these situations such as frequency relays, out of step blocking/ tripping schemes, overload detection relays. Severe over/under voltage – voltage relays to isolate load/ equipment.
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Protective Relaying
detects short circuits and/or abnormal operating conditions that may affect the equipment/ the system. Isolate only the faulty equipment.
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Protection Requirements Isolate faulty equipment as soon as possible
G Transformer –GSU Generator
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Redundancy
What happens if the relay doesn’t operate?
Add a second set of relaying Make sure that some other relay clears the fault such as a relay looking into the generator from the system.
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Backup Protection
What happens if the breaker fails to operate?
Provide a relay to detect this condition and trip adjacent sources for the fault. This could be at the same location or at a remote location. EE 5940 Jan 20, 2011; Copyright@2011
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Other Failures
D.C. power is required for operating breakers and other auxiliary devices.
Battery failure: Solution is to provide remote back up or provide redundancy at the local station. D.C. circuits protected by either fuses or D.C. breakers. Provide separate circuits for primary and secondary trip paths.
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Generator Protection – Redundancy With Back up Isolate faulty equipment as soon as possible
G Transformer –GSU Generator
Trip Trip
Primary protection
Breaker failure Relay
Secondary protection Initiate Breaker Failure
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Another relay looking from the transformer into the generator or the bus can also provide the back up function.
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Instrument Transformers
Current transformer – scales down the primary current to manageable value for the relay. Ex: 2000 Amps is the nominal current of the generator ; 2000/5 can be the ratio. The relay input is 5Amps if the primary current is 2000 Amps.
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Instrument Transformers (Contd.)
Potential transformers – Step down the voltage to relay input range (66Volts to 120Volts)
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Relay Inputs
Current with nominal 5A rating. Voltage nominal –from 66V to 150 V (This could vary depending on the manufacturer).
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Zones of Protection 3
Transmission line
2
G Generator
7
4
Shunt Reactor
Transformer – GSU
1 1- Generator
5
2- Transformer
3-Unit protection 4-Bus protection 5- Line protection 6- Shunt Capacitor 7- Shunt Reactor 6 EE 5940 Jan 20, 2011; Copyright@2011
Shunt Capacitor
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Zone Boundary The boundary is defined by
1.
2.
The fault interrupting device (Breaker in our example) Location of the current transformer
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Zone Boundary Example Zone of protection
Bus relay
Typical Location of CT in
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Overlapping Zones Prevents Blind spots 3
Transmission line
2
G Generator
7
4
Shunt Reactor
Transformer – GSU
1 1- Generator
2- Transformer
5
3-Unit protection 4-Bus protection 5- Line protection 6- Shunt Capacitor 7- Shunt Reactor EE 5940 Jan 20, 2011; Copyright@2011
Shunt Capacitor
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Zone Boundary CT Location Live tank Breaker
Blind zone Bus relay
Typical Location of CT in
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Zone Boundary – Blind Zone
If the interrupting device (Breaker) is outside the CT boundary, there might be “ It is not my zone” situation – Blind Zone. One solution - Trip all zones associated with this breaker – Ex.- trip the bus relay when line relay picks up and trips the breaker. Modern relay Solution- Disable the CT input if the breaker is open – This will allow the bus relay to clear the fault after the line breaker opens. EE 5940 Jan 20, 2011; Copyright@2011
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Changing Zones of Protection
Breaker maintenance/ Current Transformer (CT) problem Original zone of protection
2
G Transformer –GSU
3
Generator Expanded zone of protection Primary protection Secondary protection
Trip and initiate B/F of Bkrs 2 and 3
If CT is the problem – rewire CT to the line breakers 2 and 3 or rewire bus relay CT to transformer CT
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Other Names for Clearly Defined Zone Protection
Merz-Price protection Unit protection Differential protection
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Protection Where Zones Are Not Clearly Defined Transmission line
G Generator
Line Relay
Distribution Transformer Shunt Reactor
Transformer – GSU
Overcurrent relay Impedance relay (Ratio of voltage to Current)
Shunt Capacitor
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Feeder
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Breaker Operation for Faults at Various Locations G
1
2 Transformer – GSU
Generator
Equipment
Normal clearing
Bkr Failure
Generator
1
2
GSU
1, 2
1,3,5 for Bkr 2 failure
Bus
2,3,5
1,3,5 for failure of 2 2, 4,5 for failure of 3 2,3,6 for failure of 5
L1
3, 4
2,5,4 or3, 7, 10
L2
5,6
2,3,6 or 8, 9,5
L3
7, 8
4,10,8 or7, 6,9
Capacitor
9
6,8
Shunt reactor
3,4
2,5,4 or 3, 7, 10
Dist. transformer
4,7,10
3 for Bkr 4 failure or 8 for Bkr 7 failure -Feeders Assumed radial
Feeder
11
10
3
Transmission line L1 Shunt Reactor
10
7
5
* depending on which breaker fails
4
Distribution Transformer
6
11 12
8 9 Shunt Capacitor
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Other Relay Terms
Sensitivity – Relay is sensitive enough to operate for faults.
Selectivity – Operate as intended. Reliability – Perform when called upon. Security – from operations point of View; undesired tripping Vs. failure to trip.
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Other Functions
Auto restoration. Alarms to assist system operator’s decision.
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Relay Protection Designs
Mostly based on past experiences. There are many ways to provide redundancy – Local or remote. Unlike physical layout designs, different solutions may not have any significant effect on the cost of the project. More of an art than science. EE 5940 Jan 20, 2011; Copyright@2011
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Relay Inputs
Voltage Current
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Electrical Parameters – Measuring principles
Voltage – over voltage/ under voltage or rate of change Current – overcurrent or undercurrent or rate of change Derived quantity – Impedance –V/I magnitude or rate of change; Direction based on phase angle relationship between V and I Frequency- Over or under frequency – magnitude or rate of change Voltz per hertz - overfluxing EE 5940 Jan 20, 2011; Copyright@2011
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Industry Accepted Device Numbers ANSI/ IEEE C37.2 Device number standard Examples: 52- Circuit breaker 87 – Differential relay 51 – time overcurrent relay 50 – Instantaneous over current relay 27 – Under voltage relay 59 – Over voltage relay 2- time delay relay EE 5940 Jan 20, 2011; Copyright@2011
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One Line Diagram – Protection Design
Need to know the layout of the substation or the power plant. Location of the breakers and CTs. What to use - Primary/ secondary or back up. A.C. connections. What to trip. EE 5940 Jan 20, 2011; Copyright@2011
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Alternate Names
Metering and relaying diagram. Based on past practices of Utilities/ Power plants. Varies from one company to another. Some utilities show CT and PT connections on one sheet and show the tripping logic on another sheet.
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M&R Diagram
Provides details on what protection is used. Shows which breakers are tripped. Serves as a good reference document to develop three line diagrams (Schematics).
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Example – Metering & Relaying Diagram
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Fault Current and Voltage at the Relay V (t) = V Sin(ωt); θ - Fault initiation angle Location S
R – total resistance ; L- Total Inductance; α = R/L
Relay Location VS
RS
LS
m
RL
LL
ϕ - System angle ; ϕL – Line angle = tan-1(ωL/R) ZL = √(RL2 + ωLL2); Z = √(R2 + ωL2)
Fault current, I(t) = (Vm /Z) [ sin (ωt + θ - ϕ) –Sin (θ - ϕ) e- αt ] Voltage at Relay location,
VR = Vm (ZL/Z) [ sin (ωt + θ - ϕ + ϕL ) – (Z/ ωL) Sin (θ - ϕ) Sin (ϕL- ϕ) e- αt ]
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Fault Current Waveform
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Relay Inputs
Current input may have D.C. offset. The amount of offset depends on the fault incidence point on the voltage waveform. Voltage inputs rarely have any offsets as line angle ϕL and system angle, ϕ are close to each other. If ϕL = ϕS = ϕ; the system is considered as homogenous system. EE 5940 Jan 20, 2011; Copyright@2011
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Relay Classification
Electromechanical Static Numerical/microprocessor (digital)
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Relay Chronology
Electro-mechanical relays – Significant portion still in service. – Major development in 1950s. Solid state – Transistor versions – Late 60’s. Op. amps/ CMOS Technology – early 1970s. Microprocessor relay – late 70- early 80s.
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What Has Changed?
Electro-mechanical relays – single function relays. One relay for each function. Solid state technologies – combined two or three functions. Microprocessor based relays – Multifunction relay with emphasis on fault records, control features. Latest Technology – Wide area protection, Synchro-phasors, interoperability standards. EE 5940 Jan 20, 2011; Copyright@2011
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What Is Hard To Change?
All protection application philosophies, based on electromechanical relay concepts, are still prevalent - several multifunction relays used as multiple single function relays. Cost of implementation of new concepts is easy but, prevalent legacy approach blocks such changes citing additional maintenance overheads.
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Electromechanical Relays Types of electromechanical Relays
Telephone Type Relays Hinged Armature Relays/ Clapper type Relays Plunger Relays Induction Disc relays Induction cup/Cylinder Relays EE 5940 Jan 20, 2011; Copyright@2011
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Telephone Type Relay
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Induction Disc Type Relay
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Induction Cup Relay
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Maintenance
GEK-99350 –Adjustment Techniques for Electromechanical relays by GE
http://pm.geindustrial.com/FAQ/Documents /PVD/GEK-99350.pdf
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Current Transformer (CT) Steps down high currents to relay input levels. Applications – Metering and Relaying. Types
Free standing CT. Bushing CT. Auxiliary CT. Optical EE 5940 Jan 20, 2011; Copyright@2011
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Current Transformers – IEEE Documents
IEEE C57.13-2008, “IEEE standard requirements for instrument Transformers”. IEEE C57.13.1-2006, “ IEEE guide for Field Testing of Relaying Current Transformers. IEEE C57.13.3-2005, “ IEEE guide for Grounding of Instrument Transformer secondary circuits and Cases”. IEEE C57.13.5-2009, “ IEEE Standard for Performance and Test Requirements for Instrument Transformers of a Nominal System voltage of 115 kV and above”. IEEE C57.13.2 –2005 – This standard covers tests required for CT from 600V up to 38kV. IEEE C37.110-2007 “IEEE Guide for the Application of Current Transformers for Protective Relaying Purposes”.
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Polarity Marking IPrimary
H1 X1
ISecondary
X2
•Sec. Winding Designation – X,Y, Z, U,W and V.
X3 X4 X5
•Ratio 1200:5A EE 5940 Jan 20, 2011; Copyright@2011
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Free Standing CT
Used in systems up to 800 kV. Air Oil
Nitrogen
Insulator Primary winding
Secondary Core & winding Tank
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Bushing CT Internally or Externally Mounted on the Bushing
CT rating- If primary is not an integral part of the CT, The CT should be rated for the equipment ratings. Ex: Transformer or breaker bushing CT. EE 5940 Jan 20, 2011; Copyright@2011
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Auxiliary CT Used for Ratio Matching
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Terminology Related to CTs
Rating Factor – Specifies the maximum continuous Primary current carrying capability. –1.0, 1.33, 1.5, 2.0, 3.0 ad 4.0 are the preferred rating factors as per the standard. Ex: 2000/5 R.F: 2.0; The maximum rating on the primary is 4000A. EE 5940 Jan 20, 2011; Copyright@2011
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Metering CT
Accuracy is defined at a rated connected load ( referred to as Burden)
Example:
Burden Designation
Resistance (Ω)
Inductance (mH)
Impedance (Ω) @ 60 HZ
Power factor
B-0.1
0.09
0.116
0.1
0.9
B-0.5
0.45
0.58
0.5
0.9
B –1.8
1.62
2.08
1.8
0.9
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Metering Accuracy Class: As defined in C57.13 Metering Accuracy Class
Accuracy 10% rated current
At rated current
0.3
0.994 - 1.006
0.994 - 1.006
0.6
0.988 – 1.012
0.994-1.006
1.2
0.976 – 1.024
0.988 – 1.012
High Accuracy class 0.15 is defined in C57.13.6 with burden class E0.2 and E-0.04 at unity power factor. The CT will confirm to the Accuracy class at higher currents if Rating Factor is greater than 1.0 EE 5940 Jan 20, 2011; Copyright@2011
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Relay Class CTs
C-Class – 3% at rated current and 10% at 20 times the rated current. This is based on designated burden. Burden Designation
Resistance (Ω)
Inductance (mH)
Impedance (Ω) @ 60 HZ
Power factor
B-1.0
0.5
2.3
1.0
0.5
B - 2.0
1.0
4.6
2.0
0.5
B - 4.0
2.0
9.2
4.0
0.5
B – 8.0
4.0
18.4
8.0
0.5
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Relay CT Designation
C-Class low leakage flux, The ratio can be calculated.
T- Class – high leakage ratio cannot be calculated and has to be determined by tests. X- Class – 1% accuracy at rated current and user defined accuracy at 20 times the rated current. Refer C57.13 to specify.
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Excitation Curve Voltage developed by CT
Knee-Point 10% error at 100A
10A EE 5940 Jan 20, 2011; Copyright@2011
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C400 CT Excitation Curves
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Excitation Curve
A finite amount of current is used to establish flux in the magnetic core – Excitation or Magnetizing current . C400 – CT can push 100 A into 4 ohms burden at 10% accuracy. From excitation curve, CT develops ~500 V at 100 A with 10A excitation current. EE 5940 Jan 20, 2011; Copyright@2011
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Excitation Curve
Internal voltage drop at 100A = 0.002 ohms x 240 turns x 100A = 48V; CT terminal voltage = 4 X 100A = 400V CT internal voltage at 100A (10A excitation current) = ~500V > 400V +48V. EE 5940 Jan 20, 2011; Copyright@2011
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Knee- Point Voltage
45 deg. Line intersection with the excitation curve.
IEC defines the point as the voltage at which 10% increase in voltage results in 50% increase in excitation current. IEC Classification – 5P20, 10P20 – 5% error at 20 times the nominal current or 10% error at 20 times the nominal current.
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CT – Steady State Performance
Cable impedance Excitation Current
Relay Burden Cable impedance
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Cable- #10 gauge – 1.0 ohms per 1000 ft. E/M relay burden generally published as Volt Ampere (VA) – Depends on the relay and the setting – as high as 15.68 ohms – 3.92 VA @0.5 A Microprocessor Relays1-5 VA at 5A. Max. Z=0.2 ohms. 76
CT Transient Performance Voltage developed across the secondary of a CT is given by VS = (Zburden + 2Rcable +RCT) If, if there is no D.C. offset
VS = (1+X/R) (Zburden + 2Rcable +RCT) If, with D.C. offset If is the primary current/ CT ratio. X,R - reactance and resistance of the primary system. The CT voltage rating > VS to avoid CT saturation.
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Current Transformers References -Books and Papers
Stanley E. Zocholl, “Analyzing and Applying Current transformers” SEL Publication, 2004. IEEE WG, “Transient response of Current Transformers”, IEEE Trans. PAS, VOL.PAS-91, 1977. Arthur Wright, “ Current Transformers, their transient and steady state Performance”, Chapman and Hall, 1968. Brian D. Jenkins, “Introduction to Instrument-Transformers”, CRC Press, 1967.
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Asymmetrical Fault Current –Core Flux Waveform Core Flux ωt
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If the fault is cleared before the steady state is reached, core may have high remanent flux.
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IEEE PSRC Documents IEEE C37.110-2007, “IEEE Guide for the Application of Current Transformers used for Protective Relaying Purposes”. CT saturation calculator. http://www.pes-psrc.org/. published reports
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CT Selection
Maximum continuous rating of the primary circuit. 2000/5 RF = 2.0 – Primary can carry 4000 A continuously. This means that all loads connected to the secondary side should be capable of carrying 10A. EE 5940 Jan 20, 2011; Copyright@2011
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CT Selection
Select primary rating based on the load current carrying capability.
If load rating is 5A is max. rating and if the primary needs to be 3000A, select 3000/5A.
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CT Selection
Determine the max. fault current. Determine the X/R of the system. Determine the burden – Add cable impedance and the connected relay impedance. Select the C Class (100, 200, 400 or 800) so that C-Class V > (1+X/R)*If*(ZBurden)) to avoid saturation. It may not be possible to avoid saturation. EE 5940 Jan 20, 2011; Copyright@2011
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Other Factors –CT Selection
Max. secondary current
(1+X/R) If ZB (1-Remanence/100) to avoid saturation. EE 5940 Jan 20, 2011; Copyright@2011
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CT Ratings- at Tapped Ratios
C-class Excitation curves are provided for all lower ratios. The Rating Factor higher than 1.0 leaves to confusion on the ratings. EX- 2000/5 RF 2.0 Ratings, 4000A and 10A. At 1500/5, primary rating is 3000A based on secondary limit of 10A. At 800/5, Primary rating is 1600A based on 10A limit of the secondary.
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Will CTs Always Saturate?
It depends on the burden, X/R and C-rating and also on the point of fault incidence.
C37.110 and the referenced paper provide time to saturate equations and curves. Use Excel spread sheet on IEEE-PSRC website.
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Faults
Majority of faults are single line to ground. D.C offset is maximum if θ - ϕ =900 System angle, ϕ is around 700 –850 Fault should occur around zero on the voltage waveform. Most of the faults due to insulation failure are around the voltage peak. D.C. offset Probability is low for line to ground faults. On three phase faults at least one of the phases will have significant offset.
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CT Saturation Calculator
Examine the time to saturate, effective current calculated by the relay
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Potential Transformer
Steps down the voltage to 120V or less. C57.13 specifies the ratings. Metering Accuracy –0.3, 0.6, 1.2 at 90% to110% of the nominal rating. C57.13.6 – 0.15 Accuracy Class specified.
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Voltage Transformer Ratings
Typically connected –Phase to ground. Standard Ratios as per Table 12 of IEEE C57.132008. 40250V/ 115V/67V on 69 kV system – Secondary has a tap. Relays (legacy relays) are normally connected to 67 V tap. Modern relays can withstand at least up to 150V. EE 5940 Jan 20, 2011; Copyright@2011
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VT Burdens Designation
BURDEN At 120V and at 69.3V basis VA
Power Factor
W
12.5
0.1
X
25.0
0.7
M
35.0
0.2
Y
75.0
0.85
Z
200.0
0.85
ZZ
400.0
0.85
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Polarity H1 X1 X2 X3
Y-Winding
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Capacitor Coupling Voltage Transformer
CCVTs are more economical at higher voltages instead of wound transformers.
C1 Compensating Reactor Transformer
C2
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CCVT Transient Performance
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CCVT Transient Performance
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CCVT –Effect of Load on Transients
Transients are higher if CCVT is loaded. Fault Point on the waveform also has an effect.
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Further Reading/ References
C.R. Mason- Chapters 1,2,7 and 8 Alstom – Chapter 2,3 and 6 P.M.Anderson – Chapters 1 and 2
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Questions?
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