introduction to system protection- protection basics

8/10/2019 Introduction to System Protection- Protection Basics

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INTRODUCTION TO SYSTEMPROTECTIONHands-On Relay School 2012


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CONGRATULATIONS

On choosing the field of system protection. It is anexciting, challenging profession.

System protection has changed considerably in thepast 20 years.

Many learning and growth opportunitieswill comeyour way in the future.


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What is SystemProtection?System protection is the art and science of detectingproblems with power system components and isolatingthese components.Problems on the power system include:

1.Short circuits2.Abnormal conditions3.Equipment failuresNERC defines the protection system as:

Current Approved Definition:

Protective relays, associated communication systems, voltage and currentsensing devices, station batteries and DC control circuitry.


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Purpose of System ProtectionProtect the publicImprove system stabilityMinimize damage to equipmentProtect against overloadsEmploy relay techs and engineers


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What Components (Equipment) Do We Protect?


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GeneratorsTransformers, ReactorsLinesBuses

Capacitors


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Some Basics

Current Transformer (CT)

A device which transforms the current on the power system fromlarge primary values to safe secondary values. The secondary

current will be proportional (as per the ratio) to the primary current.


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Some BasicsPotential Transformer (PT)A device which transforms the voltageon the power system from primaryvalues to safe secondary values, in aratio proportional to the primary value.


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I. Generator ProtectionA. Construction & Theory of Operation C:\Documents and Settings\clh6411\Desktop\hydroturbine.jpg


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Three Gorges Dam in ChinaLargest in the world(22,000MW, 26 Generators)

C:\Documents and Settings\clh6411\Desktop\800px-ThreeGorgesDam-China2009.jpgC:\Documents and Settings\clh6411\Desktop\Three Gorges Dam-1.jpg


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What can go wrong?A.Stator Winding Problems1. Winding-winding short2. Stator ground

I. Generator Protection


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What can go wrong?A.Stator Winding Problems

I. Generator Protection C:\IPCO\SysOp\HCPR_GEN2_JAN_8_2003\Pictures\Winding damage\P1100001.JPG


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How Do We Protect the Stator?

A. Differential Protection (what goes in must come out)

1. Detects phase-phase faults

B. Stator Ground Protection

1. 59N (95% of Stator)

2. Third Harmonic Voltage Method (100% of Stator)

3. Signal Injection (100% of Stator)

Generator Protection


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What can go wrong?

B. Rotor Problems

1. Loss of field

2. Field ground

a. First ground

b. Second ground

=TROUBLE


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Generator Protection E:\Picture1.jpgRotor


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Generator ProtectionHow Do We Protect theRotor?1.Loss of Fielda. Impedance


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How Do We Protect the Rotor?

2. Field ground

a. DC voltage relay (64F)

The field ground relay is connected from the negative sideof the field to DC ground. Detects voltage from the field toground.


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What else can go wrong?C. Abnormal Conditions1. Over/Under Frequency

2. Over Excitation3. Reverse Power4. Out of Step5. Unbalance Current


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Transformer Protection


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XFMR PROTECTIONPower transformers are expensive, and are a long lead-time item

(1 year or longer) so protection must be effective


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Construction

200681695059692


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Construction img0048Transformer Protection


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Construction image001Transformer Protection


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Transformer size and rating

. MVA: the capacity of the transformer in terms of millionvolt-amps. Size can range from less than 1 MVA to 500MVA and higher.

. Transformer rating (MVA) is determined in part by theamount of cooling employed. MVA rating increases withmore cooling. OA, FOA (stage 1), FOA (stage 2)



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What can go wrong?. Winding-to-winding faults. Winding-to-ground faults. Bushing faultsTransformer Protection


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Protection Methods

. Fuse

. Overcurrent

. Differential



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High Side Fuse



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Sysop03Transformer Damage Curve



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Overcurrent Relays



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Differential Protection:

What goes in must come out.. P-in = P-out



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Microprocessor Relays187T1-TWraps transformer187T1-BWraps transformer and busTransformer Protection


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187 T1-TZone ofProtectionTransformer Protection


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187 T1-B

Zone of

Protection



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Some terms you will be learning about this week:

Restraint

Operate

Slope

Inrush

2nd Harmonic



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LINE PROTECTION


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Transmission LineProtection

Transmission lines can vary in length from several hundred feet to severalhundred miles, and in voltage (line-to-line) from 46KV to 750KV.

Construction can be simple, such as a single wood pole with insulatorsatop a crossarm, with little spacing between the conductors and fromthe conductors to ground. At the other end of the scale are metallattice structures with bundled conductors (2 or more conductors perphase) with large spacing between conductors and betweenconductors and ground.


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Transmission LineProtection C:\HRS\2012 HRS\Introduction to System Protection\typical_transmission_structures.gif


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C:\HRS\2012 HRS\Introduction to System Protection\woodtower_transmission_lines.jpgC:\HRS\2012 HRS\Introduction to System Protection\transmission_lines_onpage.gif

C:\HRS\2012 HRS\Introduction to System Protection\46kv_trans_line.jpgC:\HRS\2012 HRS\Introduction to System Protection\flooded_power_lines.jpg


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Transmission Line Protection

How Do We Protect Transmission Lines?A.OvercurrentB.Directional OvercurrentC.Distance (Impedance)

D.Pilot1. DCB (Directional Comparison Blocking2. POTT (Permissive Overreaching Transfer Trip)E. Line Current Differential


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Overcurrent Protection

Non-Directional

Relay responds to overcurrent condition

Instantaneous (IOC) device #50

No intentional time delay

Time Overcurrent (TOC) device #51

Various curve types, including inverse, veryinverse, extremely inverse


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Transmission Line ProtectionOvercurrent Line Protection


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C:\Projects\traning\overcurrentgndblue.bmpAC Schematic


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Time Overcurrent Curves

C:\Projects\traning\u4toccurveblue.bmpTransmission Line Protection


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Transmission Line ProtectionDirectional OvercurrentProtection

Relay responds to overcurrent condition in theforward direction only (device #67, 67N,67NT)

Will not respond to reverse faults

Compares the current in the line versus aknown reference that will always be the

same (such as a voltage or polarizing currentsource)


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Transmission Line ProtectionDirectional Overcurrent Example


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Transmission Line ProtectionDistance ProtectionA distance relay measures the impedance of a line usingthe voltage applied to the relay and the currentapplied to the relay.When a fault occurs on a line, the current rises

significantly and the voltage collapses significantly.The distance relay (also known as impedance relay)determines the impedance by Z = V/I. If theimpedance is within the reach setting of the relay, itwill operate.


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Distance ProtectionElectromechanical distance relays use torque to restrain or operate

KD, GCY, etc. Device #21

Microprocessor distance relays use equations to restrain or operate

SEL, ABB, GE, Areva, etc. Device #11


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Distance Relay

CT and PT

Connections


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Transmission Line ProtectionDistance Protection

Typical zone reach

settings


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Distance ProtectionThe most common characteristic (orprotection shape) of distance relays isthe mho characteristic, a circular type

reach characteristic.Distance relays have a settablemaximum torque angle (mta), which isthe angle of the current compared tothe angle of the voltage at which therelay is most sensitive. In the drawingon the right, the mta is approximately75 degrees.


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Introduction to System Protection

Dependability: the certainty that a protection system will operate

when it is supposed to

Security: the certainty that a protection system will not operate

when it is not supposed to

Reliability = Dependability + Security

Terminology


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Transmission Line ProtectionPilot Relaying Scheme

A protection scheme which employs communications to send asignal from one station to another to allow high speed tripping(permission) or to prevent high speed tripping (blocking).

Pilot protection allows over-reaching zones of protection toensure full protection of the line as well as high speed tripping.


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Pilot Relaying Scheme

Directional Comparison Blocking (DCB)

A communications based protection scheme where high speedover-reaching tripping is allowed unless a block signal isreceived.


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Pilot Relaying SchemePermissive over-reaching transfer trip (POTT)A communications based protection scheme where high speedover-reaching tripping is allowed only if a permissive signal is

received


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Relay

BLOCKING SCHEMEOPERATING PRINCIPLE

Relay

STATION A

STATION BTransmission Line Protection

BLOCKING SCHEME OPERATING PRINCIPLE


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RelayBLOCKING SCHEMEOPERATING PRINCIPLE


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BLOCKING SCHEME OPERATING PRINCIPLERelay

DO NOTTRIP!!!External Fault



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RelayPERMISSIVE SCHEMEOPERATING PRINCIPLE

Relay

STATION A

STATION B

Permissive Scheme



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Relay

Permissive scheme internal fault

C:\Documents and Settings\clh6411\Local Settings\Temporary Internet Files\Content.IE5\U3WPAPQL\MCj03638360000[1].wmf

C:\Documents and Settings\clh6411\Local Settings\Temporary Internet Files\Content.IE5\2MNXHIZG\MCj03478690000[1].wmfRelay

STATION B

STATION A



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Relay

C:\Documents and Settings\clh6411\Local Settings\Temporary Internet Files\Content.IE5\U3WPAPQL\MCj03638360000[1].wmfC:\Documents and Settings\clh6411\Local Settings\Temporary Internet Files\Content.IE5\2MNXHIZG\MCj03478690000[1].wmf

Relay

STATION B

STATION APermissive scheme internal fault



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Relay C:\Documents and Settings\clh6411\Local Settings\Temporary Internet Files\Content.IE5\U3WPAPQL\MCj03638360000[1].wmfC:\Documents and Settings\clh6411\Local Settings\Temporary Internet Files\Content.IE5\2MNXHIZG\MCj03478690000[1].wmfRelayYou Can Go Ahead

and Trip If You WantTo!

You Can Go Aheadand Trip If You WantTo!

High Speed Tripping Takes Place at Station A and BSTATION B

STATION A

Permissive scheme internal fault



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Relay


Relay

STATION BSTATION A

Permissive scheme external fault



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Relay


Relay

STATION B

STATION APermissive scheme external fault



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Relay C:\Documents and Settings\clh6411\Local Settings\Temporary Internet Files\Content.IE5\U3WPAPQL\MCj03638360000[1].wmfC:\Documents and Settings\clh6411\Local Settings\Temporary Internet Files\Content.IE5\2MNXHIZG\MCj03478690000[1].wmfRelayYou Can Go Ahead

and Trip If You WantTo!

No High Speed Tripping Takes Place Because the Fault

Is Reverse to the Relay at Station B.

STATION BSTATION A

Permissive scheme external fault



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BLOCKING VS. PERMISSIVE

BlockingIncreased dependability because if the carrier fails, the protection will tripanyway.Decreased security because if the carrier fails, the protection will trip for an

out of section fault.PermissiveIncreased security because if the communication fails, the protection will nottrip high speed.Decreased dependability because if the comm fails, the protection will nottrip high speed for an in section fault.



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LINE DIFFERENTIAL

No fault or external fault, current at each end is balancedThe current going into the line is going out at other endTransmission Line Protection


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Line differential

Internal fault, relay trip is processed



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Bus Protection

Bus Differential:

Current into bus must equal current out of bus


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Bus Fault

Bus Protection


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Single bus with XFMR

Bus Protection


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Double bus,breaker-and-a-half

Bus Protection


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Double bus,double breaker

Bus Protection


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Capacitor Protection

Purpose of capacitors:Shunt capacitors raise the voltage on a bus or line to a higherlevel, thus helping keep the voltage at desired levelSeries capacitors cancel out the inductive reactance of a line,

thus making the line appear shorter increasing load flow on theline.


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Capacitors connected in parallel add


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Capacitors connected in series sum like they are in parallel


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Capacitors are connected in series and parallel combination

to obtain the desired total capacitance for the bank

CDWL C131.JPG


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Capacitor ProtectionVoltage across cap bank is determined by current flow and impedance

(capacitive reactance) of bank.

If a capacitor fuse blows or if a capacitor shorts, the voltage drop across

the bank changes due to a change in capacitive reactance of the bank.

A voltage relay detects the higher voltage and trips the breaker


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Introduction to System Protection

If you are still awake, nudge your sleeping neighbor and tell him/her that

the lecture is over and it is almost time for the next lecture, which may actually

prove to be an interesting and informative lecture (unlike this one).

If you are not awake, may you dream that you are on a Hawaiian beach

And then wake up in Pullman!

????? QUESTIONS ?????


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