Bus Bar Protection REV 1

7/28/2019 Bus Bar Protection REV 1

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BUS BAR PROTECTION AND

LBB PROTECTION.

By

Y. k. Pandharipande.


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BUS BAR PROTECTION AND LBBPROTECTION

PERFORMANCEREQUIREMENTS

Stability

High Speed Operation

Selectivity

Reliability

Sensitivity

Suitable for Use with moderate CT Ratings.

Extendable to changes in Bus Bar configuration & additionof Circuits.


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BUS BAR PROTECTIONCIRCULATING CURRENT DIFFERENTIAL PRINCIPLE

CURRENT DISTRIBUTION EXTERNAL FAULT

CURRENT DISTRIBUTION INTERNAL FAULT


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BUS BAR PRIOTECTION

A. HIGH IMPEDANCETYPE

Relay Branch made High Impedance to limit differential currentdue to unequal CT saturation on external fault.

Requires exclusive CT core of identical ratio and rating.

Simple in design and execution.


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BUS BAR PROTECTION

B. LOW IMPEDANCE (BIASED) TYPE

Through current Bias increases differential pick-up on externalfault to ensure stability.

Tolerant to CT mismatch and can use shared core. Comparatively more complex in design and execution. Practical schemes provide CT saturation detectors to enhance

stability.


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BUS BAR PROTECTION (87)HIGH IMPEDANCE SCHEME

If B CT fully saturates on external fault,Vs = If/n (Rct + 2Rl) Volts.

Relay Branch Impedance = Vs/Is, where Is = Setting current of Relay.

External Stabilising Resistor RST = Vs/Is VA /Is, where VA = RelayVA burden at setting current.


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BUS BAR PROTECTION (87)HIGH IMPEDANCE SCHEME- Typical Setting Workout.

Max. Through fault Current (If) = 20,000A (Prim) / 20A(SEC)

Vs = 20 (5 + 1) = 120V

Stabilising Resistor (RST) = 120/0.2 1.0/0.2 = 600 - 25 = 575 ,(assuming relay VA burden = 1 VA)


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BUS BAR PROTECTION

HIGH IMPEDANCE SCHEME Typical Setting Workout.

Primary Fault Current setting (assuming number of circuitsconnected to Bus Bar, N=10)

= CT Ratio (Is + N. Im)

= 1000 (0.20 + 10 x 0.03) = 500A.


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BUS BAR PROTECTION (87)HIGH IMPEDANCE SCHEME Need For Metrosils.

Peak Voltage across CT Secondary for Max. Internal Fault= 2 2 [Vk x (Vp Vk)]

Vk = 240V, Vp = 20 (575 + 25) = 12000V

Peak Voltage = 22 [(240 x (12000-240)] = 5300V


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BUS BAR PROTECTION

HIGH IMPEDANCE SCHEME Need For Metrosils.

If Peak voltage exceeds 3000V threatening secondary

insulation, Metrosils (Non-linear resistors) are used to limitvoltage to safe value (within 1000V) on internal fault.

Metrosil offers high resistance at lower voltage levelswhich drastically reduces at higher voltages, therebyquenching the voltage across relay branch.

Typical Metrosil EquationV = C x I ( C = 900 , = 0.25)


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BUS BAR PROTECTION

HIGH IMPEDANCE SCHEME -CT SUPERVISION (95)

Required to detect CT open circuit on load to prevent maloperation. Connected for alarm & short CT bus wires on operation. Time delayed to avoid operation on internal / external faults (3Sec) Voltage relay with sensitive setting (2 -14V) is used. Normally set to detect primary unbalance of 25A or 10% of least loadedcircuit. Also should not operate with normal unbalance on load.


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BUS BAR PROTECTION CT SWITCHING.


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TYPICAL 2 ZONE SCHEME WITH INDEPENDANT CHECK


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TYPICAL 2 ZONE SCHEME WITH INDEPENDANT CHECK

D.C. CIRCUITS.

20K resistor on negative side used for cathodic protection.


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LOCAL BREAKER BACKUP (LBB) PROTECTION

NEED

Provides local isolation when breaker gets stuck onfault.

Ensures comparatively faster clearance than remotebackup.

Avoids wide spread interruption and enables fastrestoration.


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LBB PROTECTION OPRATING PRINCIPLE & CT CKTS.


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LBB PROTECTIOND.C. CKTS USING BUS BAR PROTECTION TRIP LOGIC.

LBB CURRENT CHECK RELAY (50)Current Setting Range 20-320% or 5 -80%In (Recommended setting - 5%

for Generator & 20% for Other Circuits)

LBB TIMER (2/50) Range o.11.0 S (Recommended 0.20S)


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LBB PROTECTION TIMER SETTING CRIETERIA

TLBB = TCB + TDO + TM TPU,

Where TCB = C.B. Trip Time, TDO= Current Check relay DO time,TM = Safety margin, TPU = Current Check relay PU Time

Normally,200ms delay recommended which is faster thanremote backup and allows sufficient time for normal C.B.

tripping.

Documents

Bus Bar Protection REV 1