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Application of Non-Directional Overcurrentand Earthfault Protection

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Non-Directional Overcurrent and Earth

Fault Protection

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Overcurrent ProtectionPurpose of Protection

Detect abnormal conditions

Isolate faulty part of the system

Speed

Fast operation to minimise damage and danger

Discrimination

Isolate only the faulty section

Dependability / reliability

Security / stability

Cost of protection / against cost of potential hazards

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Fuses

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Overcurrent ProtectionFuses

Simple

Can provide very fast fault clearance

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Overcurrent ProtectionFuses - disadvantages

Problematic co-ordination

IFA approx 2 x IFB

Limited sensitivity to earth faults

Single phasing

Fixed characteristic

Need replacing following fault clearance

Fuse A Fuse B

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Tripping Methods

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Overcurrent ProtectionDirect Acting AC Trip

AC series trip

common for electromechanical O/C relays

51

IF

Trip Coil

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Overcurrent ProtectionDirect Acting AC Trip

Capacitor discharge trip

used with static relays where no secure DCsupply is available

IF'

SensitiveTripCoil

IF

51

+

-

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Overcurrent ProtectionDC Shunt Trip

Requires secure DC auxiliary

No trip if DC fails

IF'IF

DCBATTERY

SHUNTTRIP COIL

51

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Overcurrent Protection

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Overcurrent ProtectionPrinciples

Operating Speed

Instantaneous Time delayed

Discrimination

Current setting

Time setting

Current and time

Cost

Generally cheapest form of protection relay

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IF1IF1IF2

Overcurrent ProtectionInstantaneous Relays

Current settings chosen so that relay closest tofault operates

Problem

Relies on there being a difference in fault levelbetween the two relay locations

Cannot discriminate if IF1 = IF2

50

B

50

A

IF1IF2

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Overcurrent ProtectionDefinite (Independent) Time Relays

TOP

TIME

IS Applied Current

(Relay Current Setting)

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Overcurrent ProtectionDefinite (Independent) Time Relays

Operating time is independent of current

Relay closest to fault has shortest operating time

Problem

Longest operating time is at the source wherefault level is highest

51

0.9 sec 0.5 sec

51

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Overcurrent ProtectionIDMT

Inverse Definite Minimum Time characteristic

TIME

Applied Current(Relay Current Setting)

IS

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Overcurrent ProtectionDisc Type O/C Relays

Current setting via plug bridge

Time multiplier setting via discmovement

Single characteristic

Consider 2 ph & EF or 3 phplus additional EF relay

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Overcurrent ProtectionStatic Relay

Electronic, multi characteristic

Fine settings, wide range

Integral instantaneous elements

RESET

A B C

INST

t

I > Is

INST

t

I > Is

No

Ph +

In

Vx V

Hz

0.05000000

111

0.050.050.10.20.30.4

124810

0.05000000

111

0.050.050.10.20.30.4

124810

0.10.10.20.40.40.40.8

000

0.02500000

000000

D

LT1

S1V1

E1 I

t

sI =

x IssI =

x Is

x t =

x t =

I =INST

x sII =INST

x sI

MCGG

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Overcurrent ProtectionNumerical Relay

Multiple characteristics and stages

Current settings in primary or secondaryvalues

Additional protection elements

Current

Time

I>1

I>2

I>3

I>4

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Overcurrent ProtectionCo-ordination Principle

Relay closest to faultmust operate first

Other relays must haveadequate additionaloperating time to

prevent them operating Current setting chosen

to allow FLC

Consider worst caseconditions, operatingmodes and currentflows

T

IS1IS2MaximumFault

Level

I

R2R1IF1

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Overcurrent ProtectionCo-ordination Example

C AB

0.01

0.1

1

10

Operatingtime(s)

Current (A) FLB FLC FLD

ED

C

B

DE

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Overcurrent ProtectionIEC Characteristics

SI t = 0.14(I0.02 -1)

VI t = 13.5(I2 -1)

EI t = 80(I2 -1)

LTI t = 120(I - 1)

Current (Multiples of Is)

0.1

1

10

100

1000

1 10010

Operating

Time(s)

VI

EI

SI

LTI

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Overcurrent ProtectionOperating Time Setting - Terms Used

Relay operating times can becalculated using relay

characteristic charts

Published characteristcs aredrawn against a multiple of

current setting or Plug SettingMultiplier

Therefore characteristics can beused for any applicationregardless of actual relay currentsetting

e.g at 10x setting (or PSM of 10)

SI curve op time is 3s


0.1

1

10

100

1000

1 10010

Operat

ingTime(s)

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Overcurrent ProtectionCurrent Setting

Set just above full load current

allow 10% tolerance Allow relay to reset if fault is cleared by

downstream device

consider pickup/drop off ratio (reset ratio)

relay must fully reset with full load currentflowing

PU/DO for static/numerical = 95%

PU/DO for EM relay = 90%

e.g for numerical relay, Is = 1.1 x IFL/0.95

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Overcurrent ProtectionCurrent Setting

Current grading

ensure that if upstream relay has started

downstream relay has also started

Set upstream device current setting greater thandownstream relay

e.g. IsR1 = 1.1 x IsR2

R1 R2 IF1

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Overcurrent ProtectionGrading Margin

Operating time difference between two devices toensure that downstream device will clear fault beforeupstream device trips

Must include

breaker opening time

allowance for errors

relay overshoot time

safety margin

GRADINGMARGIN

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Overcurrent ProtectionGrading Margin - between relays

Traditional

breaker op time - 0.1 relay overshoot - 0.05

allow. For errors - 0.15

safety margin - 0.1

Total 0.4s

Calculate using formula

R2R1

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Overcurrent ProtectionGrading Margin - between relays

Formula

t = (2Er + Ect) t/100 + tcb + to + ts

Er = relay timing error

Ect = CT measurement error

t = op time of downstream relay

tcb = CB interupting time

to = relay overshoot time

ts = safety margin

Op time of Downstream Relay t = 0.5s

0.375s margin for EM relay, oil CB

0.24s margin for static relay, vacuum CB

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Overcurrent ProtectionGrading Margin - relay with fuse

Grading Margin = 0.4Tf + 0.15s over whole characteristic

Assume fuse minimum operating time = 0.01s

Use EI or VI curve to grade with fuse

Current setting of relay should be 3-4 x rating of fuse toensure co-ordination

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Overcurrent ProtectionGrading Margin - relay with upstream fuse

1.175Tr + 0.1 + 0.1 = 0.6Tf

or

Tf

= 2Tr

+ 0.33s

Allowance for CTand relay error

CB Safety margin Allowance for fuseerror (fast)

Tf

Tr

IFMAX

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Overcurrent ProtectionTime Multiplier Setting

Used to adjust the operatingtime of an inversecharacteristic

Not a time setting but a

multiplier

Calculate TMS to givedesired operating time inaccordance with the gradingmargin


0.1

1

10

100

1 10010

OperatingTime(s)

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Overcurrent ProtectionTime Multiplier Setting - Calculation

Calculate relay operating time required, Treq

consider grading margin

fault level

Calculate op time of inverse characteristic

with TMS = 1, T1

TMS = Treq /T1

O P i

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Overcurrent ProtectionCo-ordination - Procedure

Calculate required operating current

Calculate required grading margin

Calculate required operating time

Select characteristic

Calculate required TMS

Draw characteristic, check grading over wholecurve

Grading curves should be drawn to a commonvoltage base to aid comparison

O t P t ti

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Grade relay B with relay A

Co-ordinate at max fault level seen by both relays =1400A

Assume grading margin of 0.4s

Is = 5 Amp; TMS = 0.05, SI

IFMAX= 1400 Amp

B A

200/5 100/5

Is = 5 Amp

O t P t ti

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Relay B is set to 200A primary, 5A secondary

Relay A set to 100A If (1400A) = PSM of 14relay A OP time = t = 0.14 x TMS = 0.14 x 0.05 = 0.13(I0.02 -1) (140.02 -1)

Relay B Op time = 0.13 + grading margin = 0.13 + 0.4 = 0.53s

Relay A uses SI curve so relay B should also use SI curve

Is = 5 Amp; TMS = 0.05, SI

I

FMAX= 1400 Amp

B A

200/5 100/5

Is = 5 Amp


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Relay B Op time = 0.13 + grading margin = 0.13 + 0.4 = 0.53s

Relay A uses SI curve so relay B should also use SI curve

Relay B set to 200A If (1400A) = PSM of 7relay B OP time TMS = 1 = 0.14 x TMS = 0.14 = 3.52s

(I0.02 -1) (70.02 -1)

Required TMS = Required Op time = 0.53 = 0.15Op time TMS=1 3.52

Set relay B to 200A, TMS = 0.15, SI

Is = 5 Amp; TMS = 0.05, SI

IFMAX= 1400 Amp

B A

200/5 100/5

Is = 5 Amp


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Overcurrent ProtectionLV Protection Co-ordination

ZA2118B

Relay 1Relay 2

Relay 3Relay 4Fuse

1

2

3

4

F

350MVA4 4

3 3

2

F

11kV

MCGG CB

ACB CTZ61 (Open)CTZ61

ACBMCCB

27MVA

20MVALoad

Fuse

2 x 1.5MVA11kV/433V

5.1%

K

1


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ZA2119

1000S

100S

10S

1.0S

0.1S

0.01S

0. 1kA 10kA 1000kA

TX damage

Veryinverse

MC

CB

(cold

)

Relay 2

Relay 3

R

elay4

Fuse


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ZA2120C

Relay 1Relay 2

Relay 3Relay 4Fuse

1

2

3

4

F

350MVA4 4

3 3

2

1

F

11kV

KCGG 142 CB

ACB (Open)KCEG 142

ACBMCCB

27MVA

20MVALoad

Fuse

2 x 1.5MVA11kV/433V

5.1%

K


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ZA2121

1000S

100S

10S

1.0S

0.1S

0.01S

0. 1kA 10kA 1000kA

TX damage

Long timeinverse

MC

CB

(co

ld)

Relay 2

Relay 3

Relay 4

Fuse


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ZA2135

R3

R2

R1

Block t >

I > StartIF2

IF1

M (Transient backfeed ?)

Graded

protection

Blockedprotection

Overcurrent ProtectionBlocked OC Schemes

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Delta / Star Transformers


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A phase-phase fault on one

side of transformerproduces 2-1-1 distributionon other side

Use an overcurrent elementin each phase (cover the 2xphase)

2 & EF relays can be usedprovided fault current > 4xsetting

Iline

0.866 If3

Turns Ratio

= 3 :1

Idelta

Overcurrent ProtectionTransformer Protection - 2-1-1 Fault Current


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Istar = E-/2Xt = 3 E-n/2Xt

Istar = 0.866 E-n/Xt Istar = 0.866 If3 Idelta = Istar/3 = If3 /2 Iline = If3

Iline

0.866 If3

Turns Ratio

= 3 :1

Idelta



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Grade HV relay withrespect to 2-1-1 for

- fault

Not only at maxfault level

51HV

/

51LV

If386.6%If3


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Use of High Sets


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Fast clearance of faults

ensure good operation factor, If >> Is (5 x ?)

Current setting must be co-ordinated to preventovertripping

Used to provide fast tripping on HV side of transformers

Used on feeders with Auto Reclose, prevents transientfaults becoming permanent

AR ensures healthy feeders are re-energised

Consider operation due to DC offset - transientoverreach

Instantaneous Protection


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Set HV inst 130% IfLV

Stable for inrush

No operation for LV fault

Fast operation for HVfault

Reduces op times

required of upstreamrelays

HV2 LVHV1

HV2

LVTIME

CURRENT

HV1

IF(LV) IF(HV)

1.3IF(LV)

Instantaneous OC on Transformer Feeders

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Earthfault Protection


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Earth fault current may be limited

Sensitivity and speed requirements may not be met byovercurrent relays

Use dedicated EF protection relays

Connect to measure residual (zero sequence) current

Can be set to values less than full load current

Co-ordinate as for OC elements May not be possible to provide co-ordination with

fuses

Earth Fault Protection


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Combined with OC relays

E/F OC OC OC E/F OC OC

Economise using 2x OCrelays

Earth Fault Relay Connection - 3 Wire System

Overcurrent ProtectionE th F lt R l C ti 4 Wi S t

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EF relay setting must begreater than normalneutral current

Independent of neutralcurrent but must use 3 OCrelays for phase to neutralfaults

E/F OC OC OC E/F OC OC OC

Earth Fault Relay Connection - 4 Wire System

Overcurrent ProtectionE th F lt R l C t S tti

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Solid earth

30% Ifull loadadequate

Resistance earth

setting w.r.t earth fault

level

special considerationsfor impedance earthing

- directional?

Earth Fault Relays Current Setting

Overcurrent ProtectionSensitive Earth Fault Relays

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Settings down to0.2% possible

Isolated/high

impedance earth networks For low settings cannot use residual connection, use

dedicated CT

Advisable to use core balance CT CT ratio related to earth fault current not line current

Relays tuned to system frequency to reject 3rd

harmonic

B

C

E/F

A

Sensitive Earth Fault Relays

Overcurrent ProtectionCore Balance CT Connections

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Need to take care with corebalance CT and armouredcables

Sheath acts as earth return

path

Must account for earth currentpath in connections - insulatecable gland

NO OPERATION OPERATION

CABLEBOX

CABLE GLAND

CABLE GLAND/SHEATHEARTH CONNECTION

E/F

Core Balance CT Connections

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