BUS BAR PROTECTION

By AJIT PRATAP SINGH

Profile Passed B.Tech in Electrical Engg.(with honors) from IT-

BHU, Varanasi in 1994. Worked with Telco,Jamshedpur from July 1994 till

Dec’95. Joined NTPC in Dec’95. Worked in Electrical Erection and carried out erection of

Switchyard & Generator Protection, Switchyard Equipments, Excitation system, Generator Bus-Duct for two and half years from year Jan ’97 till July’99 at NTPC Unchahar.

Carried out commissioning & maintenance of Switchyard & Generator Protection, Switchyard Equipments, Excitation system for about four years from Aug’99 till April’03 at NTPC Unchahar.

Working in the Protection Group at NTPC,EOC since May’03

FUNDAMENTALS

WHAT? WHERE? WHY? WHEN? HOW?

What is Bus-Bar Protection?

It’s a protection system which primarily protects bus-bars and associated equipments of transmission or distribution network substations/ switchyards from phase to phase or phase to earth faults.

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Where Bus-Bar Protection?

Transmission or distribution network substations / switchyards typically starting from 33KV and above.

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Why Bus-Bar protection?

Any substation is a pooling point or Node of several power sources.

These power sources provide electricity to various consumers ranging from domestic to Industrial.

For a fault in a substation bus- bars, all the sources feed this fault, which may be to the tune of 40KA even at 400KV Voltage level.

Why Bus-Bar protection?contd

If this fault is not cleared immediately: Associated equipments and bus-

bars may get seriously damaged Network may get unstable and

result into cascade tripping and grid collapse.

HUGE NATIONAL LOSS!!! back

When Bus-Bar Protn Operates?

MUST operate very fast when fault is internal (operating time of 15 ms at 5 times set value).

MUST NOT operate for external or Through fault.

MUST be very stable during normal operating conditions

When Bus-Bar Protn Operates?

contd

Internal Fault

Substation to be protected

External/Through Fault

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How Bus-Bar Protn operates ?

KIRCHOFF’s CURRENT LAW

KIRCHOFF’S CURRENT LAW

At every node, the sum of all currents entering a node must be equal zero. What this law means physically is that charge cannot accumulate in a node; what goes in must come out.

i.e. I1 + I2 - I3 = 0Or I1 + I2 = I3

Node

KIRCHOFF’S CURRENT LAWcontd

I1

I2

I3

I4

I1 + I2 = I3 + I4

If

If = I1 + I2 + I3 + I4

Substation to be protected

How Bus-Bar Protn operates ?

contd

Main-1 Main-2

Trip Bus

Substation to be protected

Operating Principle

Differential

Types of Bus Bar Protection

High Impedance Differential protection

Low impedance Differential protection

High Impedance Differential

I1 I1

i1 i1’

i1 – i1’

Equipment to be protectedI2

i2i1 + i2Stabilising

Resistance

Operating relay

High Impedance Differential

contd

VOLTAGE OPERATED CURRENT OPERATED ALL CT CONNECTIONS ARE LOOPED IN

THE YARD AND SINGLE CABLE TAKEN TO THE RELAY

AUGMENTATION IS EASY THROUGH FAULT STABILITY ACHIEVED

THROUGH STABILISING RESISTORS CONNECTED IN THE RELAY CIRCUIT.

High Impedance Differentialcontd

Settings Vs = If(Rct+2 Rl) Vk not less than 2 x Vs Effective current setting

Ir = Is+ nxIeIs= relay circuit current settingIe= Magnetising curentN= No of CTs in parallelI pry = Ir X turns ratio

CT Matching CT ratio to avoid spill

current during healthy state Less Rct/ less Ie/high CT ratio

Routing of CT connection Looped at the yard itself to ensure

minimum loop resistance and thus a minimum setting voltage and a minimum Vk for a given stability limit

High Impedance Differentialcontd

Low Impedance Differential

I1 I1

i1 i1’

i1 – i1’

Equipment to be protectedI2

i2

i1 + i2

Biasing/Restraining element

Differentialelement

Low Impedance Differential

Eqpt to be protected

Low Impedance Differentialcontd

Characteristics

Id= I1+I2+……Ib=ampl(I1)+ampl(I2)+……

Low Impedance Differentialcontd

Uses Biased Differential Principle. Stability for through fault achieved

by restraining quantities proportional to the feeder current.

Low Impedance Differentialcontd

CT wires directly to the relay CT mismatch (typ of the order of

1:5 ) can be accommodated. More suitable for numerical

integrated protection systems as the CTs can be shared for many functions

Standard Practice

400 kV Duplicated main protection per zone

(bus) Two prot connected to different CT

cores Preferably, both main protections on

different principles. Two out of two principle(main 1 & 2)

for tripping.

Standard Practicecontd

220 kV / 132 kV One main protection per zone (bus) Single CT core is switched to

respective main zone through switching relays

Common check zone for all the buses connected to an independent CT core other than main protection

Tripping based on operation of both main and check zone

LOGIC OF MAIN AND TRANSFER BUS SYSTEM

Numerical Bus Bar protection Centralized: all functions in a centre

unit Decentralized : Peripheral units attached to

each bay and a central unit for scheme logic.

Have many zones of protection in the same relay

Many added functions like LBB, Feeder back up protection

Event logging Disturbance recording

Numerical Bus-Bar Protection

Central Unit

Peripheral Units

Centralized Bus bar Protection

Bus Bar Protection

Unit ProgrammingUnit

Hardwires

Decentralized Bus bar Protection

Peripheral Units

CentralUnit

ProgrammingUnit

Hardwires

Optical fibrecables

THANKS