03 Non Dir. Overcurrent

Application of Non-Directional Overcurrent and Earthfault Protection

J.W. WrightEngineering Manager

Non-Directional Overcurrent and Earth Fault Protection

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

Overcurrent ProtectionCo-ordination

Co-ordinate protection so that relay nearest to fault operates first

Minimise system disruption due to the fault

F1 F2 F3F3F2F1

Fuses

Overcurrent ProtectionFuses

Simple Can provide very fast fault

clearance <10ms for large current

Limit fault energy

Pre Arc TimeArcing Time

Prospective Fault Current

TotalOperatingTime

t

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

Tripping Methods

Overcurrent ProtectionDirect Acting AC Trip

AC series trip common for electromechanical O/C relays

51

IF

Trip Coil

Overcurrent ProtectionDirect Acting AC Trip

Capacitor discharge trip used with static relays where no secure

DC supply is available

IF'

SensitiveTripCoil

IF

51

+

-

Overcurrent ProtectionDC Shunt Trip

Requires secure DC auxiliary No trip if DC fails

IF'IF

DCBATTERY

SHUNTTRIP COIL

51

Overcurrent Protection

Overcurrent ProtectionPrinciples

Operating Speed Instantaneous Time delayed

Discrimination Current setting Time setting Current and time

Cost Generally cheapest form of protection

relay

IF1IF1IF2

Overcurrent ProtectionInstantaneous Relays

Current settings chosen so that relay closest to fault operates

Problem Relies on there being a difference in fault

level between the two relay locations Cannot discriminate if IF1 = IF2

50

B

50

A

IF1IF2

Overcurrent ProtectionDefinite (Independent) Time Relays

TOP

TIME

IS Applied Current(Relay Current Setting)

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 where fault level is highest

510.9 sec 0.5 sec

51

Overcurrent ProtectionIDMT

Inverse Definite Minimum Time characteristic

TIME

Applied Current(Relay Current Setting)

IS

Overcurrent ProtectionDisc Type O/C Relays

Current setting via plug bridge

Time multiplier setting via disc movement

Single characteristic Consider 2 ph & EF or 3 ph

plus additional EF relay

Overcurrent ProtectionStatic Relay

Electronic, multi characteristic Fine settings, wide range Integral instantaneous elements

RESET

A B CINST

t

I > Is

INST

t

I > Is

NoPh+

I nVx 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

DLT1

S1V1E1 I

t

sI =

x Is

sI =

x Is

x t =

x t =

I =INST x sI

I =INST x sI

MCGG

Overcurrent ProtectionNumerical Relay

Multiple characteristics and stages Current settings in primary or secondary values Additional protection elements

Current

Time

I>1

I>2

I>3

I>4

Co-ordination

Overcurrent ProtectionCo-ordination Principle

Relay closest to fault must operate first

Other relays must have adequate additional operating time to prevent them operating

Current setting chosen to allow FLC

Consider worst case conditions, operating modes and current flows

T

IS1IS2MaximumFaultLevel

I

R2R1

IF1

Overcurrent ProtectionCo-ordination Example

C AB

0.01

0.1

1

10Op

erat

ing

time

(s)

Current (A) FLB FLC FLD

ED

CB

DE

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 10010O

pera

ting

Tim

e (s

)

VI

EI

SI

LTI

Overcurrent ProtectionOperating Time Setting - Terms Used

Relay operating times can be calculated using relay characteristic charts

Published characteristcs are drawn against a multiple of current setting or Plug Setting Multiplier

Therefore characteristics can be used for any application regardless of actual relay current setting

e.g at 10x setting (or PSM of 10) SI curve op time is 3s


0.1

1

10

100

1000

1 10010

Ope

rati

ng T

ime

(s)

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 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 ProtectionCurrent 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 R2IF1

Overcurrent ProtectionGrading Margin

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

Must include breaker opening time allowance for errors relay overshoot time safety margin

GRADING MARGIN

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

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

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

to ensure co-ordination

Overcurrent ProtectionGrading Margin - relay with upstream fuse

1.175Tr + 0.1 + 0.1 = 0.6Tf

or Tf = 2Tr + 0.33s

Allowance for CT and relay error

CB Safety margin Allowance for fuse error (fast)

Tf

Tr

IFMAX

Overcurrent ProtectionTime Multiplier Setting

Used to adjust the operating time of an inverse characteristic

Not a time setting but a multiplier

Calculate TMS to give desired operating time in accordance with the grading margin


0.1

1

10

100

1 10010

Ope

rati

ng T

ime

(s)

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

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 whole

curve

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


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


Relay B is set to 200A primary, 5A secondary Relay A set to 100A If (1400A) = PSM of 14

relay 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

IFMAX = 1400 Amp

B A

200/5 100/5

Is = 5 Amp


Relay B Op time = 0.13 + grading margin = 0.13 + 0.4 = 0.53s Relay A uses SI curve so relay B should also use SI curve Relay B set to 200A If (1400A) = PSM of 7

relay 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 3.52 Set 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 ProtectionLV Protection Co-ordination

ZA2118B

Relay 1Relay 2Relay 3Relay 4Fuse

1

23

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


ZA2119

1000S

100S

10S

1.0S

0.1S

0.01S0. 1kA 10kA 1000kA

TX damage

Veryinverse

MC C B ( co ld )

Relay 2

Relay 3Relay 4

Fuse


ZA2120C

Relay 1Relay 2Relay 3Relay 4Fuse

1

23

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


ZA2121

1000S

100S

10S

1.0S

0.1S

0.01S0. 1kA 10kA 1000kA

TX damage

Long timeinverse

MC C B ( co ld )

Relay 2

Relay 3

Relay 4

Fuse

ZA2135

R3

R2

R1

Block t >

I > StartIF2

IF1M(Transient backfeed ?)

Gradedprotection

Blockedprotection

Overcurrent ProtectionBlocked OC Schemes

Use of High Sets

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

Current setting must be co-ordinated to prevent overtripping

Used to provide fast tripping on HV side of transformers

Used on feeders with Auto Reclose, prevents transient faults becoming permanent AR ensures healthy feeders are re-energised

Consider operation due to DC offset - transient overreach

Overcurrent ProtectionInstantaneous Protection

Set HV inst 130% IfLV

Stable for inrush No operation for LV fault Fast operation for HV fault Reduces op times required

of upstream relays

HV2 LVHV1

HV2

LVTIME

CURRENT

HV1

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

Overcurrent ProtectionInstantaneous OC on Transformer Feeders

Earthfault Protection

Earth fault current may be limited Sensitivity and speed requirements may not be met

by overcurrent 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

Overcurrent ProtectionEarth Fault Protection

Combined with OC relays

E/F OC OC OC E/F OC OC

Economise using 2x OC relays

Overcurrent ProtectionEarth Fault Relay Connection - 3 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 ProtectionEarth Fault Relay Connection - 4 Wire System

Solid earth 30% Ifull load

adequate

Resistance earth setting w.r.t earth fault

level special considerations for

impedance earthing - directional?

Overcurrent ProtectionEarth Fault Relays Current Setting

Settings down to0.2% possible

Isolated/highimpedance 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

BC

E/F

A

Overcurrent ProtectionSensitive Earth Fault Relays

Need to take care with core balance CT and armoured cables

Sheath acts as earth return path

Must account for earth current path in connections - insulate cable gland

NO OPERATION OPERATION

CABLE BOX

CABLE GLAND

CABLE GLAND/SHEATHEARTH CONNECTION

E/F

Overcurrent ProtectionCore Balance CT Connections

Documents

03 Non Dir. Overcurrent