02-Non-Dir

Application of Non-Directional Overcurrent and Earthfault Protection

Non-Directional Overcurrent and Earth Fault Protection

Overcurrent Protection Purpose of Protection z Detect abnormal conditions z Isolate faulty part of the system z Speed z Fast operation to minimise damage and danger z Discrimination z Isolate only the faulty section z Dependability / reliability z Security / stability z Cost of protection / against cost of potential hazards

Overcurrent Protection Co-ordination

F1

F2

F3

z Co-ordinate protection so that relay nearest to fault operates first z Minimise system disruption due to the fault

Fuses

Overcurrent Protection Fuses z Simple z Can provide very fast fault clearance z Is

I > Is 0.1 0.1 0.2 0.4 0.4 0.4 0.8

0.05 0 0 0 0 0 0

0 0 0

1 1 1

0.025 0 0 0 0 0

0.05 0.05 0.1 0.2 0.3 0.4

0 0 0 0 0 0

1 2 4 8 10

∝

0.05 0 0 0 0 0 0

Is = Σ x Is

Hz V

Is =

Σ x Is RESET

1 1 1

Σ

I

INST

D

0.05 0.05 0.1 0.2 0.3 0.4

x t =

=

Σ x Is

1 2 4 8 10

∝

x t = Σ

I

INST

LT1

t S1 V1 E1

I

=

Σ x Is

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

Overcurrent Protection Numerical Relay

I>1 I>2 Time

I>3 I>4 Current

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

Co-ordination

Overcurrent Protection Co-ordination Principle z Relay closest to fault must operate first R1

R2

IF1

T

z Other relays must have adequate additional operating time to prevent them operating z Current setting chosen to allow FLC

IS2 IS1

Maximum Fault Level

I

z Consider worst case conditions, operating modes and current flows

Overcurrent Protection Co-ordination Example E

D

B

C

A

Operating time (s)

10

E D

1

C B 0.1

0.01

Current (A)

FLB

FLC

FLD

Overcurrent Protection IEC Characteristics 1000

t =

0.14 (I0.02 -1)

z VI

t = 13.5 (I2 -1)

z EI

t =

80 (I2

Operating Time (s)

z SI

100

10 LTI SI

1

-1)

z LTI t = 120 (I - 1)

VI EI

0.1 1

10

100

Current (Multiples of Is)

Overcurrent Protection Operating Time Setting - Terms Used

z Published characteristcs are drawn against a multiple of current setting or Plug Setting Multiplier z Therefore characteristics can be used for any application regardless of actual relay current setting z e.g at 10x setting (or PSM of 10) SI curve op time is 3s

1000

Operating Time (s)

z Relay operating times can be calculated using relay characteristic charts

100

10

1

0.1 1

100 10 Current (Multiples of Is)

Overcurrent Protection Current Setting z Set just above full load current z allow 10% tolerance z Allow relay to reset if fault is cleared by downstream device z consider pickup/drop off ratio (reset ratio) z relay must fully reset with full load current flowing z PU/DO for static/numerical = 95% z PU/DO for EM relay = 90% z e.g for numerical relay, Is = 1.1 x IFL/0.95

Overcurrent Protection Current Setting

z Current grading z ensure that if upstream relay has started downstream relay has also started

R1

R2

IF1

z Set upstream device current setting greater than

downstream relay e.g. IsR1 = 1.1 x IsR2

Overcurrent Protection Grading Margin

z Operating time difference between two devices to ensure that downstream device will clear fault before upstream device trips z Must include z breaker opening time z allowance for errors z relay overshoot time z safety margin

GRADING MARGIN

Overcurrent Protection Grading Margin - between relays

R1

R2

z Traditional z breaker op time

-

0.1

z relay overshoot

-

0.05

z allow. For errors

-

0.15

z safety margin

-

0.1

z Total z Calculate using formula

0.4s

Overcurrent Protection Grading Margin - between relays z Formula z t’ = (2Er + Ect) t/100 + tcb + to + ts z Er = relay timing error z Ect = CT measurement error z t = op time of downstream relay z tcb = CB interupting time z to = relay overshoot time z ts = safety margin z Op time of Downstream Relay t = 0.5s z 0.375s margin for EM relay, oil CB z 0.24s margin for static relay, vacuum CB

Overcurrent Protection Grading Margin - relay with fuse

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

Overcurrent Protection Grading Margin - relay with upstream fuse

Tf Tr I FMAX

z 1.175Tr

+

Allowance for CT and relay error

or z Tf = 2Tr + 0.33s

0.1 CB

+

0.1 Safety margin

=

0.6Tf Allowance for fuse error (fast)

Overcurrent Protection Time Multiplier Setting

z Used to adjust the operating time of an inverse characteristic z Not a time setting but a multiplier z Calculate TMS to give desired operating time in accordance with the grading margin

Operating Time (s)

100

10

1

0.1 1

100 10 Current (Multiples of Is)

Overcurrent Protection Time Multiplier Setting - Calculation

z Calculate relay operating time required, Treq z consider grading margin z fault level z Calculate op time of inverse characteristic with TMS = 1, T1 z TMS = Treq /T1

Overcurrent Protection Co-ordination - Procedure

z Calculate required operating current z Calculate required grading margin z Calculate required operating time z Select characteristic z Calculate required TMS z Draw characteristic, check grading over whole curve Grading curves should be drawn to a common voltage base to aid comparison

Overcurrent Protection Co-ordination Example

200/5

100/5 I

FMAX = 1400 Amp

B Is = 5 Amp

A Is = 5 Amp; TMS = 0.05, SI

z Grade relay B with relay A z Co-ordinate at max fault level seen by both relays = 1400A z Assume grading margin of 0.4s

Overcurrent Protection Co-ordination Example

200/5

100/5 I

FMAX = 1400 Amp

B Is = 5 Amp

A Is = 5 Amp; TMS = 0.05, SI

z Relay B is set to 200A primary, 5A secondary z 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) z Relay B Op time = 0.13 + grading margin = 0.13 + 0.4 = 0.53s z Relay A uses SI curve so relay B should also use SI curve

Overcurrent Protection Co-ordination Example 200/5

B Is = 5 Amp

100/5

A

I FMAX = 1400 Amp

Is = 5 Amp; TMS = 0.05, SI

z Relay B Op time = 0.13 + grading margin = 0.13 + 0.4 = 0.53s z Relay A uses SI curve so relay B should also use SI curve z Relay B set to 200A ∴ If (1400A) = PSM of 7 0.14 = 3.52s relay B OP time TMS = 1 = 0.14 x TMS = (I0.02 -1) (70.02 -1) z Required TMS = Required Op time = 0.53 = 0.15 Op time TMS=1 3.52 z Set relay B to 200A, TMS = 0.15, SI

Overcurrent Protection LV Protection Co-ordination 11kV MCGG

4

CTZ61

3

CB 2 x 1.5MVA 11kV/433V 5.1% ACB

4

CTZ61

3

350MVA

(Open)

2

ACB 1 1 2 3 4 F

Relay 1 Relay 2 Relay 3 Relay 4 Fuse

27MVA Fuse Load

ZA2118B

MCCB

F

K 20MVA

Overcurrent Protection LV Protection Co-ordination 1000S

MCCB (cold)

10S

TX damage

Fuse

100S

Very inverse

1.0S Relay 3

Relay 2

Relay 4

0.1S 0.01S 0. 1kA ZA2119

10kA

1000kA

Overcurrent Protection LV Protection Co-ordination 11kV KCGG 142

4

CB

4

350MVA

2 x 1.5MVA 11kV/433V 5.1% KCEG 142

3

ACB

3

(Open)

2

ACB 1 1 2 3 4 F

Relay 1 Relay 2 Relay 3 Relay 4 Fuse

27MVA Fuse Load

ZA2120C

MCCB

F

K 20MVA

Overcurrent Protection LV Protection Co-ordination 1000S Long time inverse

100S Fuse

TX damage

1.0S

MCCB (cold)

10S

Relay 3

0.1S

Relay 2

Relay 4

0.01S 0. 1kA ZA2121

10kA

1000kA

Overcurrent Protection Blocked OC Schemes

Graded protection R3 R2 IF2

R1

Block t > I > Start

IF1 M ZA2135

(Transient backfeed ?)

Blocked protection

Delta / Star Transformers

Overcurrent Protection Transformer Protection - 2-1-1 Fault Current Turns Ratio = √3 :1

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

Iline Idelta

0.866 If3∅

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

Turns Ratio = √3 :1

z Istar = E∅-∅/2Xt = √3 E∅-n/2Xt z Istar = 0.866 E∅-n/Xt z Istar = 0.866 If3∅

Iline

z Idelta = Istar/√3 = If3∅ /2 Idelta

0.866 If3∅

z Iline = If3∅

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

51

51

HV

LV

z Grade HV relay with respect to 2-1-1 for ∅-∅ fault z Not only at max fault level

86.6%If3∅

If3∅

Ø/Ø

Use of High Sets

Overcurrent Protection Instantaneous Protection

z Fast clearance of faults z ensure good operation factor, If >> Is (5 x ?) z Current setting must be co-ordinated to prevent overtripping z Used to provide fast tripping on HV side of transformers z Used on feeders with Auto Reclose, prevents transient faults becoming permanent z AR ensures healthy feeders are re-energised z Consider operation due to DC offset - transient overreach

Overcurrent Protection Instantaneous OC on Transformer Feeders

HV2

HV1

LV

z Stable for inrush

HV2 TIME

z Set HV inst 130% IfLV z No operation for LV fault

HV1 LV

z Fast operation for HV fault

IF(LV)

IF(HV)

1.3IF(LV)

CURRENT

z Reduces op times required of upstream relays

Earthfault Protection

Overcurrent Protection Earth Fault Protection

z Earth fault current may be limited z Sensitivity and speed requirements may not be met by overcurrent relays z Use dedicated EF protection relays z Connect to measure residual (zero sequence) current z Can be set to values less than full load current z Co-ordinate as for OC elements z May not be possible to provide co-ordination with fuses

Overcurrent Protection Earth Fault Relay Connection - 3 Wire System

E/F

OC

OC

OC

z Combined with OC relays

E/F

OC

OC

z Economise using 2x OC relays

Overcurrent Protection Earth Fault Relay Connection - 4 Wire System

E/F

OC

OC

OC

z EF relay setting must be greater than normal neutral current

E/F

OC

OC

OC

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

Overcurrent Protection Earth Fault Relays Current Setting

z Solid earth z 30% Ifull load adequate

z Resistance earth z setting w.r.t earth fault level z special considerations for impedance earthing - directional?

Overcurrent Protection Sensitive Earth Fault Relays A B C

z Settings down to 0.2% possible z Isolated/high impedance earth networks

E/F

z For low settings cannot use residual connection, use dedicated CT z Advisable to use core balance CT z CT ratio related to earth fault current not line current z Relays tuned to system frequency to reject 3rd harmonic

Overcurrent Protection Core Balance CT Connections

OPERATION

NO OPERATION

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

CABLE GLAND CABLE BOX

E/F

CABLE GLAND/SHEATH EARTH CONNECTION