Power System Protection EE5940 -Lecture-1[1]

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Power System Protection EE 5940

By Pratap Mysore

Lecture 1- Fundamentals University of Minnesota

January 20, 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 [email protected]

mailto:[email protected]

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Reference Books

C.R. Mason, “The Art and Science of Protective Relaying”. GE Publication.Link-http://www.gedigitalenergy.com/multilin/notes/artsci/index.htmThis book was first published in 1956!

http://www.gedigitalenergy.com/multilin/notes/artsci/index.htm

http://www.gedigitalenergy.com/multilin/notes/artsci/index.htm

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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 IEEEFor IEEE members – free downloadwww.ieeexplore.orgSign in as a member – go to books and browse Title – Power System Protection-P.M.Anderson

http://www.ieeexplore.org/

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Major Power System Components

GeneratorsTransformersTransmission linesSwitching devicesLoads

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Power System One Line

GSU – Generator Step up Transformer

G

Generator

GSU transformer

Shunt Reactor

Transmission line

Shunt Capacitor

Distribution Transformer

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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 ClassificationLow Voltages – Up to 1000 VoltsMedium Voltage – 1 kV up to 100 kV High voltage (HV) – 100 KV up to 230 kVExtra HV (EHV) – 345 kV up to 765 kVUltra 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.

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Voltage RangeRange 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 RangeMax. voltage up to 345 kV +5% of the nominal.

500 kV – 550kV; 765 kV –800kV; 1100 kV –1200 kV.

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Overcurrent limitsConductors, 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. CTubular bus – 30 deg. Rise over 40 deg. AmbientTransformer – 65 deg. Rise over 40 degrees ambient

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Frequency

Typically very close to the nominal frequencyInterconnection guidelines within +/- 0.5 HZRegional requirements for corrective action for frequencies outside this limit

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Normal OperationAs 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 relayingShort 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 ConditionsSevere 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 RequirementsIsolate faulty equipment as soon as possible

G

Generator Transformer –GSU

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RedundancyWhat 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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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.

Backup Protection

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Other FailuresD.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 upIsolate faulty equipment as soon as possible

G

Generator Transformer –GSU

Primary protection

Secondary protection

Breaker failure Relay

TripTrip

Another relay looking from the transformer into the generator or the bus can also provide the back up function. Initiate

Breaker Failure

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Instrument TransformersCurrent 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

GGenerator

Transformer – GSU

Shunt Reactor

Transmission line

Shunt Capacitor

1- Generator 2- Transformer

3-Unit protection 4-Bus protection

5- Line protection 6- Shunt Capacitor

7- Shunt Reactor

1

23

4

5

6

7

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Zone BoundaryThe boundary is defined by

1. The fault interrupting device (Breaker in our example)

2. Location of the current transformer

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Zone Boundary Example

Typical Location of CT in

Bus relay

Zone of protection

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Overlapping Zones

GGenerator

Transformer – GSU

Shunt Reactor

Transmission line

Shunt Capacitor

1- Generator 2- Transformer

3-Unit protection 4-Bus protection

5- Line protection 6- Shunt Capacitor

7- Shunt Reactor

1

23

4

5

7

Prevents Blind spots

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Zone Boundary CT Location

Bus relay

Typical Location of CT in

Live tank Breaker

Blind zone

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Zone Boundary – Blind ZoneIf 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.

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Changing Zones of Protection Breaker maintenance/ Current Transformer (CT) problem

G

3Generator Transformer –GSU

Primary protection

Secondary protection

Original zone of protection

Expanded zone of protection

If CT is the problem – rewire CT to the line breakers 2 and 3 or rewire bus relay CT to transformer CT

Trip and initiate B/F of Bkrs 2 and 3

2

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Other Names for Clearly Defined Zone Protection

Merz-Price protectionUnit protectionDifferential protection

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Protection Where Zones Are Not Clearly Defined

G

Generator Transformer – GSU

Shunt Reactor

Transmission line

Shunt Capacitor


Feeder

Line Relay

Overcurrent relay

Impedance relay (Ratio of voltage to Current)

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Breaker Operation for Faults at Various Locations

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 22, 4,5 for failure of 32,3,6 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

G 1 23

5

6 8

4

710

Generator Transformer – GSU

9

Shunt Reactor

Transmission line L1

Shunt Capacitor

11 12


* depending on which

breaker fails

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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 FunctionsAuto restoration.Alarms to assist system operator’s decision.

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Relay Protection DesignsMostly 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.

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Relay InputsVoltage Current

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Electrical Parameters – Measuring principles

Voltage – over voltage/ under voltage or rate of changeCurrent – overcurrent or undercurrent or rate of changeDerived quantity – Impedance –V/I magnitude or rate of change; Direction based on phase angle relationship between V and IFrequency- Over or under frequency – magnitude or rate of changeVoltz per hertz - overfluxing

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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

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One Line Diagram – Protection DesignNeed 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.

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Alternate NamesMetering 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 DiagramProvides 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 Location

VS

Relay Location

RSLS RL

LL

VS (t) = Vm Sin(ωt); θ

- Fault initiation angle

R – total resistance ; L- Total Inductance; α

= R/L

ϕ - System angle ; ϕL – Line angle = tan-1(ωL/R)

ZL = √(RL2 + ωLL

2); Z = √(R2 + ωL2)

Voltage at Relay location,

VR = Vm (ZL /Z) [ sin (ωt + θ

- ϕ

+ ϕL ) – (Z/ ωL) Sin (θ

- ϕ) Sin (ϕL - ϕ) e- αt ]

Fault current, I(t) = (Vm /Z) [ sin (ωt + θ

- ϕ) –Sin (θ

- ϕ) e- αt ]

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Fault Current Waveform

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Relay InputsCurrent 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, ϕare close to each other.If ϕL = ϕS = ϕ; the system is considered as homogenous system.

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Relay Classification

ElectromechanicalStatic 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.

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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 RelaysTelephone Type RelaysHinged Armature Relays/ Clapper type RelaysPlunger RelaysInduction Disc relaysInduction cup/Cylinder Relays

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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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MaintenanceGEK-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

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Current Transformers – IEEE DocumentsIEEE 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

X1

X2

X3

X4

X5

IPrimary ISecondary

H1

•Sec. Winding Designation – X,Y, Z, U,W and V.

•Ratio 1200:5A

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Free Standing CTUsed in systems up to 800 kV.

AirOil

Nitrogen

Insulator

Primary winding

Secondary Core & winding

Tank

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Bushing CTInternally 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.

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Auxiliary CTUsed 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.

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Metering CTAccuracy 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:

High Accuracy class 0.15 is defined in C57.13.6 with burden class E-0.2 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

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

As defined in C57.13

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Relay Class CTsC-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 DesignationC-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

10A

Knee-Point

10% error at 100A

Voltage developed by CT

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Excitation CurveA 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.

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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 = 400VCT internal voltage at 100A (10A excitation current) = ~500V > 400V +48V.

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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 PerformanceCable- #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 AMicroprocessor Relays-1-5 VA at 5A. Max. Z=0.2 ohms.

Cable impedance

Cable impedance

Relay Burden

Excitation Current

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CT Transient PerformanceVoltage 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. offsetIf 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

ωt

Core FluxIf 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

http://www.pes-psrc.org/

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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.

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CT SelectionSelect 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 SelectionDetermine 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.

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Other Factors –CT SelectionMax. secondary current <Microprocessor relay A/D limit (Typical - 100Apk).Remanence (residual flux) in the core allows CT to saturate earlier.According to C37.110 Annex C Table C1 on 230 kV system, out of 141 CTs checked, 20% had up to 80% remanence, and 39% had up to 20% remanence. Based on this VC-Rating > (1+X/R) If ZB (1-Remanence/100) to avoid saturation.

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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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FaultsMajority 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 CalculatorExamine 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 RatingsTypically connected –Phase to ground.Standard Ratios as per Table 12 of IEEE C57.13-2008.40250V/ 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.

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VT BurdensDesignation BURDEN At 120V and at 69.3V basis

VA Power FactorW 12.5 0.1

X 25.0 0.7

M 35.0 0.2

Y 75.0 0.85

Z 200.0 0.85ZZ 400.0 0.85

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Capacitor Coupling Voltage Transformer

CCVTs are more economical at higher voltages instead of wound transformers.

C1

C2

Compensating Reactor

Transformer

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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 8Alstom – Chapter 2,3 and 6P.M.Anderson – Chapters 1 and 2

Power System Protection EE5940 -Lecture-1[1]

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