Principle of Busbar Protection

Lattes, January 2005

Henri GRASSETMarketing Products

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Principle of Busbar Protection

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HV Application: Busbar Protection

I1 = I2 + I3

I1 - I2 - I3 = 0

Kirchhoff’s Principle applied to a healthy system

I1 I3I2

I = 0

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Kirchhoff’s Principle applied to a system with a fault

I3

I1 I2 + I3

I1 - I2 - I3 0 = IF

I1 I2

HV Application: Busbar Protection - 2

I 0

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Ii1

S1

Ii2

S2

Ii3

S3

Io1

Io2

Io3

Io4

Ii = Iin

Io = Ion

Ibias = Ii + Io

Idiff = Ii - Io

Import Ii

S

Simplified Substation Scheme

Export Io

Iin = Ion

Differential Bubar ProtectionPrinciple

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Stability is entirely due to a stabilising resistor in the circuit.

It is a simple, reliable and circulating current scheme (The CTs must have the same ratio & must be of high accuracy - class X)

The CT knee point voltage needs to be relatively high

Metrosil may be required

The CTs are usually not shared with other protections

The magnetising current can desensitise the scheme

The scheme can be very fast

Isolator contacts are needed to switch the full CT secondary current between the zones

Extending the scheme is quite simple

Buswire supervision can be offered with MVTP relays

High Impedance Principle(P122 & P141)

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Stability is entirely due to the bias characteristic of the scheme

CTs can have different ratios

Scheme bias characteristic can cater for lesser accuracy CTs (class 5P)

CTs with moderate knee point voltages can be used

Metrosils are never required

It is easier to share the CTs with other protection

Number of // circuits does not affect the primary operating current

Tripping is fast

Isolator contact are not needed to switch heavy currents.

Extending the scheme is simple

Self supervision and breaker fail protection is easier to integrate

Numerical scheme offers many fault, event and recording capabilities

Low Impedance/Biased Principle(MBCZ, P740)

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Large CTs to avoid saturation & ensure stability

Difficult to apply if the system topology is dynamic

Busbar

S1S1S1 S1P1

P2 S2 S2 S2 S2

Bias voltage image of the crossing current Differential elements

Flow control

CTs with different ratios

I1 I2 I3 I4

Interposing CTs required

Ud Rd

Ur

Differential Busbar ProtectionMerz-Price Principle

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Conventional SchemeArchitecture

BB5

BB3

BB2

BB1

Relay House/Marshalling Cubicles

1 or 5 ACT Cabling

DC cablingTrip/close orderCB and Isolator statusetc.

Relay Room

Interposing CT

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Zone 1

Zone 3 Zone 4

Zone 2

BS

BC1

F1 F2 F3 F4

BC2

Double Busbar Configuration