03-Non-Dir. Overcurrent.ppt

OC

2. Application of Non-Directional Overcurrent
and Earthfault Protection

3. Non-Directional Overcurrent and Earth
Fault Protection

4. Overcurrent Protection
Purpose 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

5. Overcurrent Protection
Co-ordination
Co-ordinate protection so that relay nearest to
fault operates first
Minimise system disruption due to the fault
F1 F2
F3F3
F2F1

6. Fuses

7. Overcurrent Protection
Fuses
Simple
Can provide very fast fault clearance
<10ms for large current
Limit fault energy
Pre Arc Time
Arcing Time
Prospective Fault Current
Total
Operating
Time
t

8. Overcurrent Protection
Fuses -disadvantages
Problematic co-ordination
I
FAapprox 2 x I
FB
Limited sensitivity to earth faults
Single phasing
Fixed characteristic
Need replacing following fault clearance
Fuse A Fuse B

9. Tripping Methods

10. Overcurrent Protection
Direct Acting AC Trip
AC series trip
common for electromechanical O/C relays
51
I
F
Trip Coil

11. Overcurrent Protection
Direct Acting AC Trip
Capacitor discharge trip
used with static relays where no secure DC
supply is available
I
F'
Sensitive
Trip
Coil
I
F
51
+
-

12. Overcurrent Protection
DC Shunt Trip
Requires secure DC auxiliary
No trip if DC fails
I
F'
I
F
DC
BATTERY
SHUNT
TRIP COIL
51

13. Overcurrent Protection

14. Overcurrent Protection
Principles
Operating Speed
Instantaneous
Time delayed
Discrimination
Current setting
Time setting
Current and time
Cost
Generally cheapest form of protection relay

15. I
F1
I
F1
I
F2
Overcurrent Protection
Instantaneous 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 I
F1= I
F2
50
B
50
A
I
F1
I
F2

16. Overcurrent Protection
Definite (Independent) Time Relays
T
OP
TIME
I
S Applied Current
(Relay Current Setting)

17. Overcurrent Protection
Definite (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
51
0.9 sec 0.5 sec
51

18. Overcurrent Protection
IDMT
Inverse Definite Minimum Time characteristic
TIME
Applied Current
(Relay Current Setting)
I
S

19. Overcurrent Protection
Disc 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

20. Overcurrent Protection
Static Relay
Electronic, multi characteristic
Fine settings, wide range
Integral instantaneous elements

21. Overcurrent Protection
Numerical 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

22. Co-ordination

23. Overcurrent Protection
Co-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
I
S1
I
S2
Maximum
Fault
Level
I
R
2R
1
I
F1

24. Overcurrent Protection
Co-ordination Example
C AB
0.01
0.1
1
10
Operating time (s)
Current (A) FL
BFL
C FL
D
E
D
C
B
DE

25. Overcurrent Protection
IEC Characteristics
SI t = 0.14
(I
0.02
-1)
VI t = 13.5
(I
2
-1)
EI t = 80
(I
2
-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

26. Overcurrent Protection
Operating 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
Current (Multiples of Is)
0.1
1
10
100
1000
1 10010
Operating Time (s)

27. Overcurrent Protection
Current 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 I
FL/0.95

28. Overcurrent Protection
Current Setting
Current grading
ensure that if upstream relay has started
downstream relay has also started
Set upstream device current setting greater than
downstream relay
e.g. Is
R1 = 1.1 x Is
R2
R
1
R
2
I
F1

29. Overcurrent Protection
Grading 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

30. Overcurrent Protection
Grading 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
R
2R
1

31. Overcurrent Protection
Grading 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

32. Overcurrent Protection
Grading 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

33. Overcurrent Protection
Grading Margin -relay with upstream fuse
1.175T
r+ 0.1+ 0.1 = 0.6T
f
or
T
f= 2T
r + 0.33s
Allowance for CT
and relay error
CB Safety margin Allowance for fuse
error (fast)
T
f
T
r
I
FMAX

34. Overcurrent Protection
Time 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
Current (Multiples of Is)
0.1
1
10
100
1 10010
Operating Time (s)

35. Overcurrent Protection
Time Multiplier Setting -Calculation
Calculate relay operating time required, T
req
consider grading margin
fault level
Calculate op time of inverse characteristic
with TMS = 1, T
1
TMS = T
req
/T
1

36. Overcurrent Protection
Co-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

37. Overcurrent Protection
Co-ordination Example
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
I
FMAX
= 1400 Amp
B A
200/5 100/5
Is = 5 Amp

38. Overcurrent Protection
Co-ordination Example
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
(I
0.02
-1) (14
0.02
-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

39. Overcurrent Protection
Co-ordination Example
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
(I
0.02
-1) (7
0.02
-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
I
FMAX
= 1400 Amp
B A
200/5 100/5
Is = 5 Amp

40. Overcurrent Protection
LV Protection Co-ordination
ZA2118B
Relay 1
Relay 2
Relay 3
Relay 4
Fuse
1
2
3
4
F
350MVA
4 4
3 3
2
F
11kV
MCGG CB
ACB CTZ61 (Open)CTZ61
ACB
MCCB
27MVA
20MVALoad
Fuse
2 x 1.5MVA
11kV/433V
5.1%
K
1

41. Overcurrent Protection
LV Protection Co-ordination
ZA2119
1000S
100S
10S
1.0S
0.1S
0.01S
0. 1kA 10kA 1000kA
TX damage
Very
inverse

42. Overcurrent Protection
LV Protection Co-ordination
ZA2120C
Relay 1
Relay 2
Relay 3
Relay 4
Fuse
1
2
3
4
F
350MVA
4 4
3 3
2
1
F
11kV
KCGG 142 CB
ACB (Open)KCEG 142
ACB
MCCB
27MVA
20MVALoad
Fuse
2 x 1.5MVA
11kV/433V
5.1%
K

43. Overcurrent Protection
LV Protection Co-ordination
ZA2121
1000S
100S
10S
1.0S
0.1S
0.01S
0. 1kA 10kA 1000kA
TX damage
Long time
inverse

44. ZA2135
R
3
R
2
R
1
Block t >
I> Start
IF2
IF1
M(Transient backfeed ?)
Graded
protection
Blocked
protection
Overcurrent Protection
Blocked OC Schemes

45. Use of High Sets

46. Fast clearance of faults
ensure good operation factor, I
f>> I
s(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 Protection
Instantaneous Protection

47. Set HV inst 130% I
fLV
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
I
F(LV)I
F(HV)
1.3I
F(LV)
Overcurrent Protection
Instantaneous OC on Transformer Feeders

48. Earthfault Protection

49. 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 Protection
Earth Fault Protection

50. Combined with OC relays
E/F
OC OC OC E/F
OC OC
Economise using 2x OC
relays
Overcurrent Protection
Earth Fault Relay Connection -3 Wire System

51. 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 Relay Connection -4 Wire System

52. Solid earth
30% I
full load
adequate
Resistance earth
setting w.r.t earth fault
level
special considerations
for impedance earthing
-directional?
Overcurrent Protection
Earth Fault Relays Current Setting

53. Settings down to
0.2% 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
Overcurrent Protection
Sensitive Earth Fault Relays

54. 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/SHEATH
EARTH CONNECTION
E/F
Overcurrent Protection
Core Balance CT Connections