Energy Sector © Siemens AG 2008 Distance Protection for transmission lines: part 1 Gustav Steynberg
Energy Sector© Siemens AG 2008
Distance Protectionfor transmission lines: part 1
Gustav Steynberg
Energy SectorEnergy AutomationPage 2 01/20/16© Siemens AG 2008
Why impedance protection?
Situation: Meshed network and two infeedsDirectional overcurrent time relays
0,6s
0,6s
0,3s
0,3s
0,6s
0,6s
0,3s
0,3s
non-selective trip
Energy SectorEnergy AutomationPage 3 01/20/16© Siemens AG 2008
Localization of short-circuits by means of an impedance measurement:
fault on the protected line
fault outside the protected line
Z1
relay A
selectivity
relay A
Z2
Basic principle of impedance protection
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Distance measurement (principle)
6 loops: 3 phase- phase loops and3 phase- ground loops
phase- phase -loop:
The same applies to the remaining loops
UL1-L2 = ZL ( IL1 - IL2)
Measured currentmeasured voltage
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ZL = RL + j XL
ZE = RE +j XE
IL1
IL2
IL3
IE
ZL
ZE
UL1 UL2 UL3
Energy SectorEnergy AutomationPage 5 01/20/16© Siemens AG 2008
phase-ground-loop: UL1 = L1 · ( RL + j XL )- E · ( RE +j XE)
L1, E measured currentUL1 measured voltage
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The same applies to the remaining loops
Distance measurement (principle)
IL1
IL2
IL3
IE
ZL
ZE
UL1 UL2 UL3
ZL = RL + j XL
ZE = RE +j XE
Energy SectorEnergy AutomationPage 6 01/20/16© Siemens AG 2008
Load and short-circuit impedances
ZL
ZLF1
ZLF2
RF RF
ZLoadDF1 F2
X
R
ZL
ZLF2
j SC1
j SC2
j L
RR
ZF1
ZF2
RR
ZLoad
ZLF1
Fault area
distance relayoperating characteristic
Increasing load
Fault in reverse direction Load area
Minimum Load Impedance:Minimum voltage 0,9 UnMaximum current 1,1 InMaximum angle 30°
Phase - Phase Fault
RR RF / 2
Phase - Earth Fault
RR RF /(1 + RE/RL)
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Principle of (analog) distance relaying
ISC
E
comparator
ZL
ZSC
ZReplica (line replica impedance)(corresponds to the set zone reach)
U1= k1 USC= k1 ISCZSC.
U2=k2 ISCZReplica
ZS
Relay design:operation if
U1< U2
i.e. ZSC< ZReplica
ZReplicaX
R
Ext. fault
Internal fault
A B
Energy SectorEnergy AutomationPage 8 01/20/16© Siemens AG 2008
Typical distance zone-characteristic
MHO-circle
shifted circle
polarisedMHO-circle quadrilateral
ZR
ZSC
ZSC'
externalfault
internalfault
X
R
X
R
ZS = 0
ZS small
ZS high ZS
RF
ZL
X
R
centre
ZSC'
ZSC
settable arc compensation
X
XA
ZSC-L Rarc
RRA
Energy SectorEnergy AutomationPage 9 01/20/16© Siemens AG 2008
Graded distance zones
time
D1 D2 D3
t1
t2
t3
Z1
Z2
Z3
distance
t = grading time
A CB D
Z1 = 0,85 ZAB
Z2 = 0,85 (ZAB + 0,85 ZBC)
Z3 = 0,85 (ZAB + 0,85 (ZBC + 0,85 ZCD))
Safety margin is 15 %: line error CT, VT error measuring error
Grading rules:
Energy SectorEnergy Automation01/20/16© Siemens AG 2008
2nd Zone: It must initially allow the 1st zone on the neighbouring feeder(s) to clear the fault.The grading time therefore results from the addition of the following times:
operating time of the neighbouring feeder mechanical 25 - 80 msstatic: 15 - 40digital: 15 - 30
+ circuit breaker operating time HV / EHV: 60 ms (3 cycles) / 40 ms (2 cycles) MV up to about 80 ms (4 cycles)
+ distance relay reset time mechanical: approx. 60-100 ms static: approx. 30 ms digital: approx. 20 ms.
+ errors of the distance relay internal timers mechanical: 5% of the set time, minimum 60-100 msstatic: 3% of the set time, minimum 10 msdigital: 1% of the set time, minimum 10 ms
+ distance protection starting time *) mechanical: O/C starter: 10 ms, impedance starter: 25 msstatic: O/C stater: 5 ms, impedance starter: 25 msdigital: generally 15 ms
+ safety margin (ca.) grading; mechanical-mechanical: 100 ms static/digital-mechanical or vice versa: 75 ms digital-digital or static-static 50 ms
*) only relevant if the set relay times relate to the instant of fault detection / zone pick-up. This is the case with all Siemens relays. There are other relays where the time is adapted by software to relate to the instant of fault inception. In the latter case the starting time has to be dropped.
Determination of grading times(With numerical relays 250 ms is possible)
Energy SectorEnergy AutomationPage 11 01/20/16© Siemens AG 2008
SC
Current area forforward faults
SC
Current area for reverse faults
SC
USC
R
ZSC
Z'SC
Impedance area for forward faults
Impedance area forreverse faults
X
SC
Determination of fault direction
current / voltage diagram impedance diagram
Fault location Where is the fault ?
The impedance also shows the direction, but ....
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direction may be determined together with the impedance measurementbut: problems may arise in certain cases (e.g. close-in faults)
separate directional determination required!
Why impedance measurement and directional determination separately?
line characteristic
fault with arc resistancein forward direction
fault in forward direction
fault in reverse direction
close-in fault
X
R
A B
Impedance measurement and directional determination
Energy SectorEnergy AutomationPage 13 01/20/16© Siemens AG 2008
Alternatives for the directional measurement
faulty phase voltage
Vf
If
VL2
VL3
voltage memory(pre-fault voltage)
If
VL2VL3
VL1
healthy-phase voltage(phase to phase voltage)
If
Vf
VL2-L3 VL2VL3
~
~
~
~
~
~
~
~
~
ZlineZgrid relay
fault L1-E
Method 1 Method 2
VL1
VL1 Vf
Energy SectorEnergy AutomationPage 14 01/20/16© Siemens AG 2008
Directional measurementSummery of all 3 methods
uRI = uL2-
L3
uf = uL1
Distance measurement
Direction measurementwith voltage memory
Direction measurementwith unfaulted voltage
if(t)uL1
if
if
if
uL2-L3
uL1
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Measuringwindow
Energy SectorEnergy AutomationPage 15 01/20/16© Siemens AG 2008
Fault detection techniques
Over-current fault detectionVoltage dependant over-current fault detection
Voltage and angle dependantover-currentfault detection
I
U
I >>I > I >
R
X
Impedancefaultdetection
Not in 7SA522
Energy SectorEnergy AutomationPage 16 01/20/16© Siemens AG 2008
110 kV net SCC(3)" = 1500 MVA
40 MVAuSC = 15 %20 kV
400/1 A
l
I>start = 1,5 · IN = 600 A
D
OH-line95/15 Al/StZ'L = 0,483 /km
' · l)
10 20 30 40 50 60
I>start = 600 A
0,5
1,0
1,5
2,0
2,5
ISC(2) [kA]
l [km]
ISC(2) = UN · 1,1
2 · (ZS + ZS + ZL
reach of OC starterapprox. 32 km
N T
Reach of over-current fault detection
ph-ph fault as an example
There is a limitationto the reach
Energy SectorEnergy AutomationPage 17 01/20/16© Siemens AG 2008
II>>I>
UI>>
UI>
UN
Udigital
electro-mechanical
Powersystem
Relay
line
E
E
ZS
USC
ZSCISC
USC
SC
USC
G
G
Voltage controlled overcurrent fault detection
Energy SectorEnergy AutomationPage 18 01/20/16© Siemens AG 2008
Voltage and angle controlled overcurrent fault detection (U-I--starting)
50 %
100 %
U/UN
I/IN1 2 3
I> I> I>>
U(I>) U(I >>)
X X
R R
2
11
2
This method is used in Germany
Energy SectorEnergy AutomationPage 19 01/20/16© Siemens AG 2008
X
R
forwards
forwards
reverse
reverse
LoadLoad
Z1
Z2
Z4
Z3
Z1B
Z5
Line
Impedance zones of digital relays (7SA6 and 7SA52)
Distance zonesInclined with line angle Angle prevents overreach of
Z1 on faults with fault resistance that are fed from both line ends
Fault detection no fault detection polygon:
the largest zone determines the fault detection characteristic
simple setting of load encroachment area with Rmin and Load
Energy SectorEnergy AutomationPage 20 01/20/16© Siemens AG 2008
Zone grading chart, radial feeder
D
A
D
B
D
C
>>
D
>t
ZT
Z1
Z2
Z3
Z1 = 0.85 ZA-B
Z3 = 0.85 [ ZA-B + 0.85 (ZB-C+ 0.85 ZC-D) ]
Z2 = 0.85 (ZA-B + 0.85 ZB-C)
Grading accordingthe recommendationwith the safety margin of 15%.
Energy SectorEnergy AutomationPage 21 01/20/16© Siemens AG 2008
Ring feeder: with grading against opposite end
0.6
0.3
grading time(s)
The same grading from both sides
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Grading in a branched radial system
L2
L3
L4
L1Z2
Z1
Z3
The impedances of the Z2 and Z3 must be grading with the shortest impedance