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Overcurrent Protection

Grading Tutorials

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What is Overcurrent Protection Grading/

Objectives of Overcurrent setting co-ordination

To minimise equipment damage

To reduce process outage costs

To protect equipment from effects of its failures

To determine characteristic ratings and settings of overcurrent

devices

To provide information regarding optimum protection and

selectivity

Protection Co-ordination

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Time based discrimination

Current based discrimination

Current and time based discrimination -

Inverse time relays

Methods of Co-ordination

Protection Co-ordination

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Time based discrimination

F

Radial System

Source

t = 0.9s t = 0.5s t = 0.1s

~ 51 51 51

Highest times near to source where faultMVA is highest - disadvantage

Protection Co-ordination

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Fault current magnitude varies with location

Cannot distinguish between faults on either side

of the breaker

Might not trip if source fault level falls

Current Based Discrimination

~SOURCE B A

F1F2

Protection Co-ordination

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Suitable when there is sufficient intervening

impedance

Current Based Discrimination

~

SOURCEB A

F1F2

Protection Co-ordination

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Discrimination combining Current & Time

Inverse time o/c relays - time reduces withincreasing fault current

Faster clearance times

Discrimination easier to achieve

Protection Co-ordination

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Current-time curves

Current

Setting

Current

Time

Independent TimeCharacteristic

Inverse Time

Characteristic

Protection Co-ordination

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Long Time standby e/f

Definite Time

Inverse Time

Very Inverse Time

Extremely Inverse Time

Current

Time

Protection Co-ordination

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Protection Co-ordination

Where & when to Apply

When the available short circuit level in a plant

increases

When new loads are added or the existing

equipment ratings are increased.

Frequent indiscriminate trippings.

New installations

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Choice of Inverse Characteristics

K a

Standard Inverse 0.14 0.02

Very inverse 13.5 1.0Extremely inverse 80.0 2.0

Long - time inverse 120.0 1.0

Actual time, t = K x TMS

( I / IS )a

- 1

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Information required

SLD & System Fault levels

Transformer & Motor inrush currents

CT Ratios

System equipment sizes and loads

Equipment thermal and mechanical force limits

Protection Co-ordination

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Information required

SLD & System Fault levels

Transformer & Motor inrush currents

CT Ratios

System equipment sizes and loads

Equipment thermal and mechanical force limits

Protection Co-ordination

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Information required (Contd.)

Type of protective devices

Power, voltage ratings, impedance and connection details

of all transformers

Normal & emergency switching conditions

Short circuit data - subtransient reactance of all motor,generator and synchronous reactance of generator

Starting current and time of motors

Relay, direct-acting trip, fuse ratings and characteristics

Protection Co-ordination

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Plug Setting Multiplier

P S M = Fault Current

Pick-up Setting

Time Multiplier Setting

T M S = Required Operating TimeNormal Operating Time (at TMS = 1)

Terminology

Protection Co-ordination

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Grading MarginTo account for:

CB operating time - 0.1 s (5 cycles)

Relay Overshoot time - 0.15s

Relay timing errors - 0.15s

Final safety margin - 0.10s

Modern static relays and CBs - margin can be

lowered

Protection Co-ordination

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THANK YOU FOR YOUR ATTENTION!

ABCD