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

Overcurrent Protection

Co-ordination

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

z Minimise system disruption due to the fault

F1 F2

F3 F3 F2

Overcurrent Protection

Fuses

z Simple

z Can provide very fast fault clearance

z <10ms for large current

z Limit fault energy

Pre Arc Time

Arcing Time

Prospective Fault Current

Total

Operating Time

Overcurrent Protection

Fuses - disadvantages

z Problematic co-ordination

z I_FA approx 2 x I_FB

z Limited sensitivity to earth faults

z Single phasing

z Fixed characteristic

z Need replacing following fault clearance

Overcurrent Protection

Direct Acting AC Trip

z AC series trip

z common for electromechanical O/C relays

51

I_F

Overcurrent Protection

Direct Acting AC Trip

z Capacitor discharge trip

z used with static relays where no secure DC supply is available I_F' Sensitive Trip Coil I_F 51 +

-Overcurrent Protection

DC Shunt Trip

z Requires secure DC auxiliary

z No trip if DC fails I_F' I_F DC BATTERY SHUNT TRIP COIL 51

Overcurrent Protection

Principles

z Current and time

z Cost

I_F1

I_F1

I_F2

Overcurrent Protection

Instantaneous Relays

z Current settings chosen so that relay closest to fault operates

z Problem

z Relies on there being a difference in fault level between the two relay locations

z Cannot discriminate if I_F1 = I_F2 50 B 50 A I_F1 I_F2

Overcurrent Protection

Definite (Independent) Time Relays

T_OP TIME

I_S Applied Current

Overcurrent Protection

Definite (Independent) Time Relays

z Operating time is independent of current

z Relay closest to fault has shortest operating time

z Problem

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

51

0.9 sec 0.5 sec

Overcurrent Protection

IDMT

z Inverse Definite Minimum Time characteristic

TIME

Applied Current (Relay Current Setting)

Overcurrent Protection

Disc Type O/C Relays

z Current setting via plug bridge

z Time multiplier setting via disc movement

z Single characteristic

z Consider 2 ph & EF or 3 ph plus additional EF relay

Overcurrent Protection

Static Relay

z Electronic, multi characteristic

z Fine settings, wide range

z Integral instantaneous elements

RESET A B C INST t I > Is INST t I > Is No Ph+ I n Vx V Hz 0.05 0 0 0 0 0 0 1 1 1 0.05 0.05 0.1 0.2 0.3 0.4 1 2 4 8 10 ∝ 0.05 0 0 0 0 0 0 1 1 1 0.05 0.05 0.1 0.2 0.3 0.4 1 2 4 8 10 ∝ 0.1 0.1 0.2 0.4 0.4 0.4 0.8 0 0 0 0.025 0 0 0 0 0 0 0 0 0 0 0 D LT1 S 1 V 1 E1 I t s I = Σ x I s s I = Σ x I s x t = Σ x t = Σ I_INST = Σ x I_s I_INST= Σ x I_s M CG G

Overcurrent Protection

Numerical Relay

z Multiple characteristics and stages

z Current settings in primary or secondary values

z Additional protection elements

Current Time I>1 I>2 I>3 I>4

Overcurrent Protection

Co-ordination Principle

z Relay closest to fault must operate first

z Other relays must have adequate additional

operating time to

prevent them operating

z Current setting chosen to allow FLC

z Consider worst case conditions, operating modes and current flows T I_S1 I_S2 Maximum Fault Level I R₂ R₁ I_F1

Overcurrent Protection

Co-ordination Example

Overcurrent Protection

IEC Characteristics

Current (Multiples of Is) 0.1 1 10 100 1000 1 10 100 Operating Time (s) VI EI SI LTI

Overcurrent Protection

Operating Time Setting - Terms Used

z Relay operating times can be calculated using relay

characteristic charts

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

Current (Multiples of Is) 0.1 1 10 100 1000 1 10 100 Operating Time (s)

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%

Overcurrent Protection

Current Setting

z

Current grading

z

ensure that if upstream relay has started

downstream relay has also started

z Set

upstream device current setting greater than

downstream relay

e.g. Is

= 1.1 x Is

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

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 0.4s

z Calculate using formula

R₂ R₁

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

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

z 1.175T_r + 0.1 + 0.1 = 0.6T_f

or

z T_f = 2T_r + 0.33s

Allowance for CT and relay error

CB Safety margin Allowance for fuse

error (fast)

T_f T_r

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

Current (Multiples of Is) 0.1 1 10 100 1 10 100 Operating Time (s)

Overcurrent Protection

Time Multiplier Setting - Calculation

z Calculate relay operating time required, T_req

z consider grading margin

z fault level

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

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

Is = 5 Amp; TMS = 0.05, SI I FMAX = 1400 Amp B A 200/5 100/5 Is = 5 Amp

Overcurrent Protection

Co-ordination Example

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) (14}0.02 _-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

Is = 5 Amp; TMS = 0.05, SI I_FMAX = 1400 Amp B A 200/5 100/5 Is = 5 Amp

Overcurrent Protection

Co-ordination Example

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

relay B OP time TMS = 1 = 0.14 x TMS = 0.14 = 3.52s

(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

Is = 5 Amp; TMS = 0.05, SI I FMAX = 1400 Amp B A 200/5 100/5 Is = 5 Amp

Overcurrent Protection

LV Protection Co-ordination

Overcurrent Protection

LV Protection Co-ordination

0. 1kA 10kA 1000kA

TX damage Very inverse M CCB (c o ld ) Relay 2 Relay 3 R e la y 4 F u s e

Overcurrent Protection

LV Protection Co-ordination

Overcurrent Protection

LV Protection Co-ordination

0. 1kA 10kA 1000kA

TX damage Long time inverse M CCB (c o ld ) Relay 2 Relay 3 Relay 4 F u s_e

ZA2135 R3 R2 R1 Block t > I > Start IF2 IF1 M _{(Transient backfeed ?)} Graded protection Blocked protection

Overcurrent Protection

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 I_line 0.866 I_f3∅ Turns Ratio = √3 :1 I_delta

Overcurrent Protection

z I_star = E∅-∅/2Xt = √3 E∅-n/2Xt z I_star = 0.866 E∅-n/Xt z I_star = 0.866 I_f3∅ z I_delta = I_star/√3 = I_f3∅ /2 z I_line = I_f3∅ I_line 0.866 I_f3∅ Turns Ratio = √3 :1 I_delta

Overcurrent Protection

z Grade HV relay with respect to 2-1-1 for

∅-∅ fault

z Not only at max fault level 51 HV Ø/Ø 51 LV I_f3∅ 86.6%I_f3∅

Overcurrent Protection

z Fast clearance of faults

z ensure good operation factor, I_f >> I_s (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

z Set HV inst 130% I_fLV

z Stable for inrush

z No operation for LV fault

z Fast operation for HV fault z Reduces op times required of upstream relays HV2 HV1 LV HV2 LV TIME CURRENT HV1 I_F(LV) I_F(HV) 1.3I_F(LV)

Overcurrent 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

z Combined with OC relays

E/F OC OC OC E/F OC OC

z Economise using 2x OC relays

Overcurrent Protection

z EF relay setting must be greater than normal

neutral current

z 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

z Solid earth

z 30% I_{full load} adequate

z Resistance earth

z setting w.r.t earth fault level

z special considerations for impedance earthing - directional?

Overcurrent Protection

z Settings down to 0.2% possible

z Isolated/high

impedance earth networks

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 B C E/F A

Overcurrent Protection

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 NO OPERATION OPERATION CABLE BOX CABLE GLAND CABLE GLAND/SHEATH EARTH CONNECTION E/F

Overcurrent Protection