Transmission line Protection

Transmission line protection

Since the impedance of a transmission line
is proportional to its length, for distance
measurement it is appropriate to use a relay
capable of measuring the impedance of a
line up to a predetermined point. Such a
relay is called distance relay ( 21 ).
 Transmission line protection
The basic principle of impedance measurement
( Z ) involves the comparison of the fault
current ( I ) with the voltage ( V ) “ seen ” by
the relay at the relaying point.
Zr = Vr / Ir
 Transmission line protection
Relay point

ZS Ir Z line

Vs Vr Zr Z load

Impedance seen by relay Zr = Zline + Zload

Transmission line protection

Operating condition :

Basic operation of distance relay

 Transmission line protection
Since the relay see current via CT and voltage
via VT, so actual impedance that relay seen is :

Z relay = Zr * CT ratio / PT ratio

 Transmission line protection
We use R-X diagram to represent the line
impedance:

Z = R + jX Ω
Relation between rectangular and polar form
Rectangular form Z = R + jX Ω
Polar form PƟ
R = P cos Ɵ
X = P sin Ɵ P =√ R + X
2 2

Ɵ = tan X/R
-1
 Transmission line protection
We use R-X diagram to represent the line
impedance:

Z = R + jX Ω
 Transmission line protection
jX
P1 Ɵ1
P2 Ɵ2 Z1=R1+jX1
Z2=R2+jX2

Ɵ2 Ɵ1
R
Load area

R-X diagram
Transmission line protection
jX

Plain impedance
 Transmission line protection
Plain
impedance
has no
direction !
Transmission line protection

Plain
impedance
with direction
Transmission line protection
jX

Mho

R
 Transmission line protection
jX

Offset mho
 Transmission line protection
Quadrilateral
 Transmission line protection
Stepped distance protection
- 3 zone of protection, zone1, zone2, zone3
- 3 difference tripping time
T3A Z3A
T2 Z2A T2
A Z1B B
Z1A
A B C

Z1B Z1C
T2B Z2C T2C
Z3C T3C
 Transmission line protection
Example criteria
Zone1 = 85 % line , instantaneous trip
Zone2 = 120 % line , delay trip T2
Zone3 = 100 % line+120 % next line
delay trip T3
Ex Calculation of 21
Data
- Base voltage =115 kV
- Base MVA = 100 MVA
- CT ratio = 800/5
- PT ratio 115/115 kV/V
- Conductor type : 477 MCM AAC – 590 A
Data
- Length of line AB = 70 km, line BC = 30 km, line BD = 50 km
- Impedance data :
AB : z1= z2 = 9.7 + j29.1 Ωp , z0 = 25.4 + j101 Ωp
BC : z1= z2 = 5.8 + j16.9 Ωp
BD : z1= z2 = 8 + j25 Ωp
C
21 D
A B
Multiply by 0.16, so
AB : z1= z2 = 1.55 + j4.65 Ωs , z0 = 4 + j16.1 Ωs
= 4.91 71.5° Ωs
BC : z1= z2 = 0.92 + j2.7 Ωs
BD : z1= z2 = 1.28 + j4 Ωs
Setting
Zone 1 = 85% line AB = 4.17 71.5° Ωs
Zone2 = 120% line AB = 5 71.5° Ωs , 0.5 sec
Zone3 = 100% line AB + 120% line AB = 9.17 71.5° Ωs ,1 sec
jX

zone3
zone2
B
zone1

71.5°
R
A
Earth fault compensate

Kn = ( z0 – z1 ) / 3z1

= 2.51 + j11.51 = 11.78 77.6°

4.67 + j13.97 14.73 71.5°

= 0.799 6.15°
 Transmission line protection
Zone1 can over trip due to :
- CT, PT error
- Impedance data and calculation error
- Relay error
So zone1 should not set 100 % line
Tele Protection
 Transmission line protection
Because zone 1 cannot clear fault at the end
of line ( 15 % ), fault must be cleared by
zone 2 with delay time T2. Too slow!
To solve this problem ‘ communication system ’
is required.
 Transmission line protection
Teleprotection scheme
1. Permissive underreach transfer trip ( PUTT )

2. Permissive overreach transfer trip ( POTT )

 Transmission line protection
1. Permissive underreach transfer trip ( PUTT )
- Send carrier by zone 1
- High speed trip ( by pass T2 ) when
zone 2 start and carrier received
 Transmission line protection
2. Permissive overreach transfer trip ( POTT )
- Send carrier by zone 2
- High speed trip ( bypass T2 ) when
zone 2 start and carrier received
 Transmission line protection
Benefit of teleprotection
- clear fault 100% line as fast as zone 1
- on more over trip
- also initiate recloser as zone 1
 Transmission line protection
Other functions in distance relay
* Power swing blocking
* Fuse failure
* Switch onto fault ( SOTF )
 Transmission line protection
Power Swing Blocking
distance relay operate by detect impedance in its
zone, and sometime voltage and current in the
system are disturb by fault. System impedance
also change and if impedance move into relay’s
zone, It’s trip. Wrong operation!
 Transmission line protection

To prevent this situation, relay use PSB. By

detect rate of change of the impedance, relay will
know which one is fault which one is power swing
and block itself to trip…
 Transmission line protection
Fuse failure
Distance relay calculate impedance by the
ratio of voltage to current. If voltage goes to zero,
impedance will be zero also. Zero impedance
means fault is very close to distance relay and
should trip the transmission line.
 Transmission line protection
PT fuse blows can make distance relay see
‘zero impedance’ in spite of no fault in high
voltage system. Distance relay use ‘ zero sequence
concept’ to protect itself from misoperation
 Transmission line protection
Switch OnTo Fault ( SOTF )
For safty in transmission line maintenance, the
line should be grounded for all 3 phases. After
finish the job, sometime ground are forgotten to
remove from line. When CB is closed, it closed to
fault.
 Transmission line protection
Distance relay use healthy voltage for
reference, so when close into 3 phase fault , no
voltage reference at all. It’s possible that all the
zones are not trip!. Memory feature is now used
to make high speed trip instead. It’s SOTF….
 Transmission line protection
Auto recloser relay ( 79 )
- Close circuit breaker after tripped by distance relay
( only trip by high speed zone )
- Single or multi shots
- Single or three poles
- Dead time and reclaim time should be set properly
- Helpful for temporary fault
Trip & reclose

Dead time
 Transmission line protection
Synchrocheck relay ( 25 )
- Supervise recloser relay befor close circuit breaker
by check voltage level, frequency, and phase angle
at both sides of circuit breaker ( sync. Function ,
BH-LH or LL-LB)
- Only check voltage level for charge line function
( voltage check Function, BH-LD or DL-LB )
Back up Protection
 Back up Protection
In protection system, each equipment
should have 2 sets of protective relay. One
set we call ‘primary protection’ and the
another is call ‘back up protection’.
 Back up Protection
Example :
Transformer
Primary – 87K, self protection, Back up – 51T, BF
Feeder
Primary – 51/51G, Back up – 51T, BF
 Back up Protection
Transmission line
Primary – zone1, Back up – zone2, zone3
and others distance relay ( 115 kV ), BF
Primary – protection primary, Back up –
protection back up and others distance
relay ( 230 kV ) , BF
 Back up Protection
Bus bar
Primary – 87B, Back up – zone2, zone3 of
others distance relay at remote end
substation ( 115 kV ), BF
 Back up Protection
Principle of Breaker failure
Measure the duration of fault current from the
instance at which any relay operates to trip
circuit breaker. If current is still flowing after
preselected time delay, it is considered that the
circuit breaker has failed to trip.
 Back up Protection
Principle of Breaker failure
Normally breaker failure timer should less than
zone 2 timer of distance relay at remote end
substation to limit the tripping area only in
substation that breaker fail.
 Back up Protection
Element of Breaker failure
1. Main protection operate
2. Current detector operate ( 50BF)
3. Breaker fail timer operate ( 62BF)
4. On this function by cut off switch ( BFCO)
 Back up Protection
Function of Breaker failure
When breaker fail to trip, the tripping and
interlocking of all other circuit breakers
connected to the failed circuit breaker will be
initiated. Another lockout relay, 86BF, is
required.