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 ABB, Nelamangala Bangalore

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Definitions

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Selective techniques

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1E-4kA 1E-3kA 1E-2kA 0.1kA 1kA 10kA 100kA 1E3kA 1E4kA

1E-3s

1E-2s

0.1s

1s

10s

100s

1E3s

1E4s

Time-Current curve LLL

-QF1, HD4/US 24.12.16 p210, PR521/DK

Ikmax=24.6kA

-QF2, E2B 2000 PR123-LSIG 2000A, RCQ

Transformer Inrush/ Thermal withstand

 Curve Definitions

MV-LV Coordination

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When a coordination of MV and LV circuit breakers are not carried properly, the resultant is

frequent tripping of MV circuit breaker even when a fault is on LV side downstream network.

For switching of MV circuit breakers, sometime support will be required from local utility

company depending on type electrical connection at site which leads to longer down time and

production losses.

The choice of the protection devices have to consider transient phenomena during which the

current may reach values higher than rated current of the Transformer and decays in few

seconds.

The protection devices should also guarantee that the transformer cannot operate beyond

point of maximum thermal overload under short circuit conditions. In accordance to IEC

60076-5 the Transformer is required to withstand value of short circuit current limited to 2

seconds.

The below diagram indicates rough profile of the Transformer inrush current curve and

corresponding point of withstand for a Transformer.

To achieve the protection coordination ensuring both curves (MV and LV curves) to be

between these two points and also establish a proper coordination becomes extremely

important and critical. Only with protection relays and protection releases which have

versatile functionality within can ensure proper coordination

Withstand point

Inrushcurrent curve

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For circuit breakers which are of current limiting type, the peak current and the energy let

through are made available by the manufacturers. The below curves indicates

  The maximum peak let-through current (Limitation curve)

  The let through I²t value (from the specific let-through energy curves)

Understanding Let Through Energy

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Versatile Discrimination option with ABB Circuit Breakers

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MCB to MCB Selectivity

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MCCB to MCB Selectivity

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MCCB to MCCB Selectivity

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 ACB to MCCB Selectivity

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Contact us for further information

 ABB LimitedDesign Institute88/3, 88/6, Basavanahalli Village562123, Bangalore North, Karnataka, INDIAemail: Ramprasad.satyam@in.abb.com

Training Module – Protection coordination and Selectivity