L2-2-Overcurrent protection.cleaned.ppt11111111

protection

1. OVER-CURRENT
PROTECTION
J.B. Ekanayake

2. Fuses
Simple
Can provide very fast fault clearance
<10ms for large current
Limit fault energy
Pre Arc Time
Arcing Time
Total
Operating
Time
t
In power
system where
we use fuses?

3. Fuses
Ring Main Unit
A striker pin to open the
switch ones the fuse
blows
Switch-
disconnector
Isolator
Ring Ring
Spur
Switch-fuse
Drop out fuse
The drop of the rod
gives an indication of
blown fuse

4. Definite (Independent) Time
Relay
Operating time is independent of current
Most down stream relay has the shortest
operating time  Time setting of R2 = t
1
Time setting of R1 =t
1 + Grading margin
51
R2
51
R1
F1
F2

5. Definite Time Relays
Grading margin
If t
1 = 0.1 sec and modern solid state relays
which are tripping vacuum or SF
6 switchgear
is used what is t
2?
If I
F1 = 1000 A; what could be I
F2? (a) 500A,
(b) 1000A or (c) 2000A
Problem -Longest operating time for the
highest fault current
CB tripping time Relay operating time
Errors Safety margin

6. Instantaneous Relays
Current settings chosen so that relay
closest to the fault operates
I
F1
I
F1
I
F2
50
B
50
A
I
F2
T
OP
TIME
I
S
Applied Current(Relay Current Setting)

7. Protection of distribution feeders
Calculate the fault current at the indicated locations
using per unit system with S
base of 250 MVA and V
base
of 11 kV, when both generators are in operation and
when one generator is switched off for maintenance.
What is the contribution of the motor during a fault?

1FI
A B C
2FI
4FI
3FI
11 kV
250 MVA
11/3.3 kV
4 MVA, 7%

j0.5 
j0.4 
300 A 200 A
M

300 A
Fuse

8. Example 2
A B C
M
Set to 8.56 kA Set to 5.56 kA
This relay sees primary current
Set to 1.95 kA
26.2 kA 8.56 kA5.56 kA5.23 kA
Set to 6.45 kA Set to 4.58 kA Set to 1.81 kA
13.1 kA 6.45 kA4.58 kA5.63 kA
When both generators are connected
When one generator is connected

9. Induction type relays

10. Induction type relays
Plug setting
Time setting
Fixed
contact
Moving
contact

11. Induction type relays

12. Inverse Definite Minimum
Time (IDMT) Relays
Plug setting not only changes the current at which disc
rotates but also set the speed of rotation of the disc
TIME
Applied Current
PS (Plug
Setting)
Minimum time
Initial position of the
trip contact can be
adjusted manually

13. IDMT relay

14. IEC Characteristics
Current (Multiples of Is)
0.1
1
10
100
1000
1 10010
O
p
e
r
a
t
i
n
g

T
i
m
e

(
s
)
VI
EI
SI
0.02
0.14
t TMS [ ]
1
s
PSM
 

13.5
t TMS [ ]
1
s
PSM
 

2
80
t TMS [ ]
1
s
PSM
 

Standard Inverse (SI)

Very Inverse (VI)
Extremely Inverse (EI)

15. Current and TM Setting
Example
Consider an over-current relay with a PS of
175% fed via a CT of ratio 300/5. Calculate the
operating time of the relay with TMS of 0.5 when
a fault current of 5000 A is flowing in the power
circuit assuming standard inverse curve.

16. Numerical relays
HARDWARE
Same for
any relay
SOFTWARE
Different from
relay to relay
Other relays are “comparison” type
Numerical relays are real time “Computational”
type

17. Block diagram
Anti-
aliasing
filter
Sample
and Hold
circuit
Anti-
aliasing
filter
Sample
and Hold
circuit
I/V
converter
Multiplexer
A/D
converter
Microprocessor
Timer
Memory
Input/
Output
Coms
F
r
o
m

V
T
s
F
r
o
m

C
T
s
M
O
V
Analog
Digital

18. Algorithm for DFT
Start
Compute cosine and
sine terms
Initialize the variables
Read the nth sample
A
o=A
0+sample(n)
A
1=A
1+sample(n) x Cos
B
1=B
1+sample(n) x Sin
n=n+1
Is
n>N
Calculate a
0, a
1
and b
1
Yes
No

19. OC Protection
Start
Read settings
Read samples
Perform DFT
and obtain I
rms
Is I
rms
>PS
No
Yes
Compute or
look-up time delay
Wait for time
equal to delay
Read samples
Read the peak
value
Is I
rms
>I
PU
No
Issue trip signal
Yes
Stop

20. Protection of radial feeders
A B
Relay B
Relay A
Grading
margin
T
im
e
Applied Current

21. Plug setting of IDMT relay
Normal loading
Permitted over
load
Minimum fault
current
Maximum fault
current
PS should be
in this region

22. Protection of distribution feeders
For a ph-to-ph fault, fault current through 300 A fuse
= 5625 A + 5 x 350 A
Operating time of 300 A fuse = 0.07 sec
Select grading margin of 0.4*t + 0.15 between the fuse and
CB, then minimum operating time of the relay at C
= 0.4 * 0.07 + 0.15
= 0.16 sec

1FI
A B C
2FI
4FI
3FI
11 kV
250 MVA
11/3.3 kV
4 MVA, 7%

j0.5 
j0.4 
300 A 200 A
M

300 A
Fuse

23. IDMT
example
1FI
A B C
2FI
4FI
3FI
11 kV
250 MVA
11/3.3 kV
4 MVA, 7%
j0.5 
j0.4 
300 A 200 A
Full load current (A)
Maximum fault
current (kA)
Minimum fault
current (kA)
Overload current (A)
CT ratio
Plug setting
PSM
Operating time for
TSM = 1
Operating time
TMS

24. Directional power relay
VOLTAGE
CURRENT

25. Protection of Parallel
feeders
B
C
A
D

26. Ring main circuit
A
B
C
D E
F
G
H
F1 F20.1 s
0.4 s
0.7 s
1.0 s
0.1 s
0.4 s
0.7 s