Line ProtectionRoy Moxley – Siemens USA

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What is a Railroad’s Biggest Asset ?

• Rolling Stock

• Share-holders

• Relationships

• Shipping Contracts

• Employees (Engineers)

• Tracks and Right of Way

What is a Railroad’s Biggest Asset ?

• Rolling Stock

• Share-holders

• Relationships

• Shipping Contracts

• Employees (Engineers)

• Tracks and Right of Way

Transmission Lines are the Power System’s Largest Asset

Exposed to Weather

Self-Sustaining Arc

Clear Quickly to Avoid Permanent Damage

How Do We Detect and Isolate Faults on Transmission Lines ?

• Overcurrent

• Distance

• Differential

• Travelling Wave (?)

Overcurrent I>PU

1000 A

Advantages:•Simple•Reliable•Easy to Set

Overcurrent

900 A

Advantages:•Simple•Reliable•Easy to Set

Disadvantages:•Possible Overtripping•Non-Directional•Depends on Source Connected

Overcurrent

900 A

Advantages:•Simple•Reliable•Easy to Set

Disadvantages:•Possible Overtripping•Non-Directional•Depends on Source

Solutions:•Add Communications•Add Directional Polarizing•Change to Distance Protection

Distance IZ - V

3 Ω

Advantages:•Not Dependent on Source Current•Fixed Reach•Inherently Directional

Distance

3 Ω

Disadvantages:•Requires Potential•Limited Resistance Coverage•Impacted by Load Current and Power Swings•Reach is Not Perfect

Distance

3 Ω

Solutions:

• Monitor Loss of Potential• Use Reactance Element to Increase Resistance Coverage (limited)•Apply Power Swing Block / Trip and Load Cutout•Add Communications

Differential Iin ≠ Iout

Advantages:• Sensitive• Not Influenced by Load or Power Swings• Provides Unit Protection, Doesn’t Over Reach

Iin Iout

Differential

Disadvantages:• 100% Communication Dependent• CT Saturation Must be Accounted For• No Inherent Backup

Iin Iout

Differential

Solutions:• Monitor Communications• Detect and Account for CT Saturation• Provide Backup With No Communications or on Different Channel

Iin Iout

Traveling Wave

Advantages:• Sensitive• Minimal Settings• Fast• Precise Fault Location

Traveling Wave

Disadvantages:• 100% Dependent on Communications• 100% Dependent on Clocks• Difficult to Test• No Inherent Backup• Checkered Past

Solutions:

Yet to Determine

Every System Has Strengths and Weaknesses

Communications is King

Overcomes Weakness

Enhances Strengths

Two Basic Communication Schemes

Permissive:Permits a Trip Under Proper Conditions

I See a Fault I See a Fault too

I See a Fault I don’t Not see a Fault

Blocking:Prevents a Trip on External Fault

Permissive

I See a Fault I See a Fault too

Permissive

I See a Fault

?

What If the Remote End Doesn’t See the Fault ?What if the Signal Doesn’t get Through ?

BlockingI See a Fault

BlockingI See a Fault

What Happens if Block Signal is Late?

What Happens if Overreaching Zone sees past Blocking Zone?

Hybrid SchemeI See a Fault

Weak InfeedI See No Fault

Power Systems are Almost Never Simple

CA

DB

Line 1

Line 2

Which Breaker Would Trip First?

What Happens to the Fault Currents Then?

Power Systems are Almost Never Simple

CA

DB

Line 1

Line 2

Changed from forward to reverse Changed from reverse to forward

Timing Becomes Everything !

Three (or More) Terminals Save Money

BA

C

Blocking or Permissive ?

Sensitivity

On Short Lines Ground Resistance Can Be Higher than Line Impedence

But What Can Happen if we Just Increase Resistive Reach?

18 Ω

3 Ω

Load Flow and Sensitivity

Load Trajectory

For Sensitivity; Ground Overcurrent or Differential

18 Ω

3 Ω

DiffI

ResI

trip

area

restraint

area

1 2 3

N

iiII

0Diff

N

iiII

0Res

Differential Relays Have to Accommodate CT saturation

relay measures a distorted current signal (red curve)

relay calculates the fundamental frequency component (blue curve)

deviation between both curves (green area) is a criteria for signal distortion

relay determines restrained current depending this signal deviation

Restraining on Errors

AI

AI

BI

BI IRe

IImeach I is the summation of:Ii = ICT-Err.+ ISignal-Err.+ISync-Err.

DiffI

BI

AI

AI BII

Res_minRes III

min_Re sI Trip if differential current exceeds sum of measurement errors added by safety margin IRes_min

Protection / Communication topology

I2I1 R2R1

R5

I5

R6

I6

R3

I3

Standard application for two terminal line: Two terminal line with redundant communication channel Three terminal line with communication chain

topology

R4

I4

For terminal line - communication ring topology – transformer in the protected area

Six terminal line – ring topology

Special Conditions - Transformers in Zone

Wind Farm

Inrush ? 2nd Harmonic Restraint

Ratio and Angle Correction for differential

Reach for Distance Relay

Will the relay see through the transformer for ground faults?

Special Conditions – Tapped Loads and Infeed

Tapped Loads

2Ω 3Ω

Distance Without Infeed =

Distance With Infeed =

10,000 A

10,000 A

Special Conditions – Tapped Loads and Infeed

Tapped Loads

2Ω 3Ω

Distance Without Infeed =

Distance With Infeed =

10,000 A

10,000 A

5Ω

8Ω

Special Conditions Combined

Wind Farm

Tapped Loads

Major Industrials

Photo VoltaicDifferential – Multi-terminalDistance – Blocking SchemeDistance (no-Comms) – Sequential Tripping

Relay 1

Relay 6

Relay 2

Relay 5

Relay 3

Relay 4

Information sent to all Relays by Transmitting “Partial Summation”

Addressed Data Sent Until All Terminals Have All the Data

I1

I5

I4

I3

I2

I6

I2 +I3+I4+I5+I6

I2 +I1

Hot Standby

I6

I5 +I6

I4 +I5+I6

I3+I4 +I5+I6

I1I3+I2 +I1

I4+I3+I2 +I1

I5+I4+I3+I2 +I1

Relay 1

Relay 6

Relay 2

Relay 5

Relay 3

Relay 4

Set Reach to Inside Line With Maximum Infeed – But What About Faults Beyond That ?

Relay 1

Relay 6

Relay 2

Relay 5

Relay 3

Relay 4

Sequential Tripping Clears Strongest Source First – Best Stability Response

Note: Shape shows reach, not characteristic of the relay

Phu My

Industrial

Area

New Plant 720 MW total from Three Units

Real-World Application

Things Don’t Always End Up How You Expect !!

Who Me ?

Initial Plans Were Pretty Standard500 kV Station

GT 1 GT 2 ST 3

Everything Wasn’t Ready On Time –Changes Connections

220 kV Station

450 MVA

500 / 220 kV

450 MVA

500 / 220 kV

Initial Communications Layout

GT 1 GT 2 ST 3

As Installed Communications Layout

It’s OK

Contact

Roy MoxleySiemens Power Systems Protection Consultant

Phone: 509-288-0847

E-mail:roy.moxley@siemens.com