fundamental of non directional overcurrent.pdf

Fundamentals OfNon-Directional

Overcurrent & Earth Fault Protection

Overcurrent Protection: Purpose of Protection

Detect abnormal conditions Isolate faulty part of the systemSpeed

Fast operation to minimise damage and dangerDiscrimination

Isolate only the faulty sectionDependability / reliability Security / stability Cost of protection / against cost of potential hazards

Overcurrent Protection Co-ordination

F3F2F1

Co-ordinate protection so that relay nearest to fault operates firstMinimise system disruption due to the fault

Overcurrent Protection Fuses

SimpleCan provide very fast fault clearance

<10ms for large currentLimit fault energy

Pre Arc Time

Arcing Time

Prospective Fault Current

TotalOperatingTime

t

Overcurrent Protection Fuses - disadvantages

Problematic co-ordination

IFA approx 2 x IFB

Limited sensitivity to earth faultsSingle phasingFixed characteristicNeed replacing following fault clearance

Fuse A Fuse B

Tripping Methods

Overcurrent Protection Direct Acting AC Trip

AC series tripcommon for electromechanical O/C relays

51

IF

Trip Coil

Overcurrent Protection Direct Acting AC Trip

Capacitor discharge tripused with static relays where no secure DC supply is available

IF'

SensitiveTripCoil

IF

51

+

-

Overcurrent Protection DC Shunt Trip

IF'IF

DCBATTERY

SHUNTTRIP COIL

51

Requires secure DC auxiliaryNo trip if DC fails

Overcurrent Protection

Overcurrent Protection Principles

Operating SpeedInstantaneousTime delayed

DiscriminationCurrent settingTime settingCurrent and time

CostGenerally cheapest form of protection relay

Overcurrent Protection Instantaneous Relays

Current settings chosen so that relay closest to fault operatesProblem

Relies on there being a difference in fault level between the two relay locationsCannot discriminate if IF1 = IF2

IF1IF1IF250

B

50

A

IF1IF2

Overcurrent Protection Definite (Independent) Time Relays

TOP

TIME

IS Applied Current(Relay Current Setting)

Overcurrent Protection Definite (Independent) Time Relays

Operating time is independent of currentRelay closest to fault has shortest operating timeProblem

Longest operating time is at the source where fault level is highest

510.9 sec 0.5 sec

51

Overcurrent Protection IDMT

TIME

Applied Current(Relay Current Setting)

IS

Inverse Definite Minimum Time characteristic

Overcurrent Protection Disc Type O/C Relays

Current setting via plug bridgeTime multiplier setting via disc movementSingle characteristicConsider 2 ph & EF or 3 ph plus additional EF relay

Overcurrent Protection Static Relay

Electronic, multi characteristicFine settings, wide rangeIntegral instantaneous elements

Overcurrent Protection Numerical Relay

Multiple characteristics and stagesCurrent settings in primary or secondary valuesAdditional protection elements

Current

Time

I>1

I>2

I>3

I>4

Co-ordination

Overcurrent Protection Co-ordination Principle

Relay closest to fault must operate firstOther relays must have adequate additional operating time to prevent them operating Current setting chosen to allow FLCConsider worst case conditions, operating modes and current flows

T

IS1IS2 MaximumFaultLevel

I

R2R1IF1

Overcurrent Protection Co-ordination Example

C AB

0.01

0.1

1

10O

pera

ting

time

(s)

Current (A) FLB FLC FLD

ED

C

B

DE

Overcurrent Protection IEC 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 10010O

pera

ting

Tim

e (s

)

VI

EI

SI

LTI

Overcurrent Protection Operating Time Setting -Terms Used

Relay operating times can be calculated using relay characteristic chartsPublished characteristcs are drawn against a multiple of current setting or Plug Setting MultiplierTherefore characteristics can be used for any application regardless of actual relay current settinge.g at 10x setting (or PSM of 10) SI curve op time is 3s Current (Multiples of Is)

0.1

1

10

100

1000

1 10010

Ope

ratin

g Ti

me

(s)

Overcurrent Protection Current Setting

Set just above full load currentallow 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 current flowing

PU/DO for static/numerical = 95%PU/DO for EM relay = 90%

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

Overcurrent Protection Current Setting

Current gradingensure that if upstream relay has started downstream relay has also started

Set upstream device current setting greater than downstream relay

e.g. IsR1 = 1.1 x IsR2

R1 R2 IF1

Overcurrent Protection Grading Margin

Operating time difference between two devices to ensure that downstream device will clear fault before upstream device tripsMust include

breaker opening timeallowance for errorsrelay overshoot timesafety margin

GRADING MARGIN

Overcurrent Protection Grading Margin -between relays

Traditionalbreaker op time - 0.1relay overshoot - 0.05allow. For errors - 0.15safety margin - 0.1Total 0.4s

Calculate using formula

R2R1

Overcurrent Protection Grading Margin -between relays

Formulat’ = (2Er + Ect) t/100 + tcb + to + ts

Er = relay timing errorEct = CT measurement errort = op time of downstream relaytcb = CB interupting timeto = relay overshoot timets = safety margin

Op time of Downstream Relay t = 0.5s0.375s margin for EM relay, oil CB0.24s margin for static relay, vacuum CB

Overcurrent Protection Grading Margin -relay with fuse

Grading Margin = 0.4Tf + 0.15s over whole characteristicAssume fuse minimum operating time = 0.01sUse EI or VI curve to grade with fuseCurrent setting of relay should be 3-4 x rating of fuse to ensure co-ordination

Overcurrent Protection Grading Margin -relay with upstream fuse

1.175Tr + 0.1 + 0.1 = 0.6Tf

orTf = 2Tr + 0.33s

Allowance for CT and relay error

CB Safety margin Allowance for fuse error (fast)

Tf

Tr

IFMAX

Overcurrent Protection Time Multiplier Setting

Used to adjust the operating time of an inverse characteristicNot a time setting but a multiplierCalculate TMS to give desired operating time in accordance with the grading margin

Current (Multiples of Is)

0.1

1

10

100

1 10010

Ope

ratin

g Ti

me

(s)

Overcurrent Protection Time Multiplier Setting - Calculation

Calculate relay operating time required, Treq

consider grading marginfault level

Calculate op time of inverse characteristic with TMS = 1, T1

TMS = Treq /T1

Overcurrent Protection Co-ordination - Procedure

Calculate required operating currentCalculate required grading marginCalculate required operating timeSelect characteristicCalculate required TMSDraw characteristic, check grading over whole curve

Grading curves should be drawn to a common voltage base to aid comparison


Grade relay B with relay ACo-ordinate at max fault level seen by both relays = 1400AAssume grading margin of 0.4s

Is = 5 Amp; TMS = 0.05, SI

IFMAX= 1400 Amp

B A

200/5 100/5

Is = 5 Amp


Relay B is set to 200A primary, 5A secondaryRelay 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 = 0.53sRelay A uses SI curve so relay B should also use SI curve

Is = 5 Amp; TMS = 0.05, SI

IFMAX= 1400 Amp

B A

200/5 100/5

Is = 5 Amp


Relay B Op time = 0.13 + grading margin = 0.13 + 0.4 = 0.53sRelay A uses SI curve so relay B should also use SI curveRelay 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 = 0.15

Op time TMS=1 3.52Set relay B to 200A, TMS = 0.15, SI

Is = 5 Amp; TMS = 0.05, SI

IFMAX= 1400 AmpB A

200/5 100/5

Is = 5 Amp

Overcurrent Protection LV Protection Co-ordination

Relay 1Relay 2Relay 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


1000S

100S

10S

1.0S

0.1S

0.01S0. 1kA 10kA 1000kA

TX damage

Veryinverse


Relay 1Relay 2Relay 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


1000S

100S

10S

1.0S

0.1S

0.01S0. 1kA 10kA 1000kA

TX damage

Long timeinverse

Overcurrent Protection Blocked OC Schemes

R3

R2

R1

Block t >

I > StartIF2

IF1M (Transient backfeed ?)

Gradedprotection

Blockedprotection

Delta/Star Transformers

Overcurrent Protection Transformer Protection - 2-1-1 Fault Current

A phase-phase fault on one side of transformer produces 2-1-1 distribution on other sideUse an overcurrent element in each phase (cover the 2x phase)2∅ & EF relays can be used provided fault current > 4x setting

Iline

0.866 If3∅

Turns Ratio = √3 :1

Idelta


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


Grade HV relay with respect to 2-1-1 for ∅-∅ faultNot only at max fault level

51

HV

Ø/Ø51

LV

If3∅86.6%If3∅

Use of High Sets

Overcurrent Protection Instantaneous Protection

Fast clearance of faultsensure good operation factor, If >> Is (5 x ?)

Current setting must be co-ordinated to prevent overtrippingUsed to provide fast tripping on HV side of transformersUsed on feeders with Auto Reclose, prevents transient faults becoming permanent

AR ensures healthy feeders are re-energisedConsider operation due to DC offset - transient overreach

Overcurrent Protection Instantaneous OC on Transformer Feeders

Set HV inst 130% IfLV

Stable for inrushNo operation for LV faultFast operation for HV faultReduces op times required of upstream relaysCURRENT

HV2 LVHV1

HV2

LVTIM

E HV1

IF(LV) IF(HV)1.3IF(LV)

Earthfault Protection

Overcurrent Protection Earth Fault Protection

Earth fault current may be limitedSensitivity and speed requirements may not be met by overcurrent relays

Use dedicated EF protection relaysConnect 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

Overcurrent Protection Earth Fault Relay Connection - 3 Wire System

Combined with OC relays Economise using 2x OC relays

E/F OC OC OC E/F OC OC

Overcurrent Protection Earth Fault Relay Connection - 4 Wire System

EF relay setting must be greater than normal neutral current

Independent of neutral current but must use 3 OC relays for phase to neutral faults

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

Overcurrent Protection Earth Fault Relays Current Setting

Solid earth30% Ifull loadadequate

Resistance earthsetting w.r.t earth fault levelspecial considerations for impedance earthing - directional?

Overcurrent Protection Sensitive Earth Fault Relays

Settings down to0.2% possibleIsolated/highimpedance earth networksFor low settings cannot use residual connection, use dedicated CTAdvisable to use core balance CTCT ratio related to earth fault current not line currentRelays tuned to system frequency to reject 3rd harmonic

BC

E/F

A

Overcurrent Protection Core Balance CT Connections

Need to take care with core balance CT and armoured cablesSheath acts as earth return pathMust account for earth current path in connections - insulate cable gland

NO OPERATION OPERATION

CABLEBOX

CABLE GLAND

CABLE GLAND/SHEATHEARTH CONNECTION

E/F

fundamental of non directional overcurrent.pdf

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