Transmission Line Protection

Fundamentals of
Distance Protection
GE Multilin

Outline
• Transmission line introduction
• What is distance protection?
• Non-pilot and pilot schemes
• Redundancy considerations
• Security for dual-breaker terminals • Out-of-step relaying
• Single-pole tripping
• Series-compensated lines
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December 7, 2014

Transmission Lines
A Vital Part of the Power System:
• Provide path to transfer power between generation and load
• Operate at voltage levels from 69kV to 765kV
• Deregulated markets, economic, environmental requirements have pushed utilities to operate transmission lines close to their limits.

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December 7, 2014

Transmission Lines
Classification of line length depends on:
 Source-to-line Impedance Ratio (SIR), and  Nominal voltage
Length considerations:
 Short Lines: SIR > 4
 Medium Lines: 0.5 < SIR < 4
 Long Lines: SIR < 0.5
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December 7, 2014

Typical Protection Schemes
Short Lines

• Current differential
• Phase comparison
• Permissive Overreach Transfer Trip
(POTT)
• Directional Comparison Blocking (DCB)

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December 7, 2014

Typical Protection Schemes
Medium Lines

• Phase comparison
• Directional Comparison Blocking (DCB)
• Permissive Underreach Transfer Trip (PUTT)
• Permissive Overreach Transfer Trip (POTT)
• Unblocking
• Step Distance
• Step or coordinated overcurrent
• Inverse time overcurrent
• Current Differential
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December 7, 2014

Typical Protection Schemes
Long Lines

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Phase comparison
Directional Comparison Blocking (DCB)
Permissive Underreach Transfer Trip (PUTT)
Permissive Overreach Transfer Trip (POTT)
Unblocking
Step Distance
Step or coordinated overcurrent
Current Differential
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December 7, 2014

What is distance protection?
Intended
REACH point
F1
Z

I*Z
V=I*ZF
I*Z V

RELAY (V,I)

For internal faults:
> IZ – V and V approximately in phase (mho)
> IZ – V and IZ approximately in phase (reactance)
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December 7, 2014

What is distance protection?
F2

Intended
REACH point