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> Distance Protection - January 2004
Distance Protection
J. Royle
> Distance Protection - January 2004
Distance Protection
Popular, widely used on Sub-Transmission and Transmission Systems
Virtually independent of Fault Current Level (ZS/ZL ratios)
Fast Discriminative Protection:- Zone 1 or ‘Aided’ Distance Scheme
Time Delayed Remote Back-Up:- Incorporated at little extra cost
> Distance Protection - January 2004
Advantages of Distance Protection
Measures Z, X or R correctly irrespective of System Conditions
Compare this with Instantaneous OvercurrentProtection:-
> Distance Protection - January 2004
Advantages of Distance Protection
F1
115kV 50
IF1ZS = 10 Ω
ZS = 10 Ω
ZL = 4 Ω
IF1 = 115kV/√3(5+4) = 7380A
∴ Is > 7380A
> Distance Protection - January 2004
Advantages of Distance Protection
Consider with one source out of service:-
IF2 = 115kV/√3 x 10 = 6640A
∴ Is <6640A
>7380A - IMPRACTICAL
F2
50
IF2
ZS = 10 Ω
> Distance Protection - January 2004
Simplified Line Diagram
XL = jWL XC = -jWC
at FN (50Hz) XC = large :-
LR R R RLLLCCC
RL
> Distance Protection - January 2004
Basic Principle of Distance Protection
ZLZS
Generation
DistanceRelay
IR
21 VR
> Distance Protection - January 2004
Impedance Seen By Measuring Element
jX
ZL
R
> Distance Protection - January 2004
Basic Principle of Distance Protection
LOADLRR
R Z Z V Zmeasured Impedance +=Ι
=
RelayPT.
Normal Load
IR ZLZS
VRVSZLOAD
> Distance Protection - January 2004
Basic Principle of Distance Protection
Fault
IRZS
VRVSZLOAD
ZL
ZF
Impedance Measured ZR = VR/IR = ZF
Relay Operates if ZF < Z where Z = setting
Increasing VR has a Restraining Effect ∴VRcalled Restraining Voltage
Increasing IR has an Operating Effect
> Distance Protection - January 2004
Plain Impedance Characteristic
jX ZL
R
TRIP STABLE
Impedance Seen At Measuring Location For Line Faults
> Distance Protection - January 2004
Impedance Characteristic Generation
Operate
IF
VF
Restrain
Spring
Trip
zF
Ampere Turns : VF IZ
Trip Conditions : VF < IFZ
jIX
IZ V1V2V3
IR
TRIP STABLE
Voltage to Relay = VCurrent to Relay = IReplica Impedance = Z
Trip Condition : S2 < S1where : S1 = IZ ≈ Z
S2 = V ≈ ZF
> Distance Protection - January 2004
Basic Principle of Distance Protection
IR
21 VR
I1/I2 ZP
V1V2
VFP
Relays are calibrated in secondary ohms :-
RATIOV.T.RATIO C.T. x Z Z
/VV/ x V
/ x /VVxV /V Z
PR
2121
FPFP
12FP12FP
RRR
=
ΙΙΙ
=ΙΙΙ
=Ι=
> Distance Protection - January 2004
Example
ZP = 4Ω; V1/V2 = 115kV/115V; I1/I2 = 600/5A
ZR(5) = 4 x 600/5 x 115/115x103 = 0.48Ω -5A Relay
ZR(1) = 2.4 Ω - 1A Relay
C.T. RATIOZR = ZP x V.T. RATIO
> Distance Protection - January 2004
Input Quantities for ∅-∅ Faults
FAULT VRESTRAINT IOPERATE
A - B VA - VB IA - IB
B - C VB - VC IB - IC
C - A VC - VA IC - IA
VRESTRAINT & IOPERATE are selected inside the relay
No setting adjustments are required apart from Z1 = Phase Replica Impedance
> Distance Protection - January 2004
Input Quantities for Phase to Earth Faults
FAULT VRESTRAINT IOPERATE
A - E VA ? IA ?
B - E
C - E
> Distance Protection - January 2004
Neutral Impedance Replica Vectorial Compensation
Replica impedance circuit :-
Z1IRA
IRN
∑IZNZ1
N
Z1
ZN
Z1 = Phase replica impedance
ZN = Neutral replica impedance
IRA passes through Z1
IRN passes through ZN
ZT = Z1 + ZN
> Distance Protection - January 2004
Neutral Impedance Compensation
For a single phase to ground fault the total earth loop impedance is given by :- (Z1 + Z2 + Z0)/3 = ZT
ZT = (Z1 + Z2 + Z0)/3 = Z1 + ZN
ZN = (Z1 + Z2 + Z0)/3 - Z1
= (2Z1 + Z0)/3 - Z1
= - Z1 + Z0
= KN Z1
3 3
where KN = (Z0 - Z1) 3Z1
> Distance Protection - January 2004
Neutral Impedance Vectorial Replica Compensation
Line CT’sA
ZPH
B
C
IAZPH
ZPH IBZPH
ZPH ICZPH
ZN INZN
Set Z PH = Z F1
Set Z N = (Z F0- Z F1)3
Usually ∠ ZN = ∠ ZPH for OHL’s
> Distance Protection - January 2004
Neutral Impedance Replica Compensation
For cables ∠Z0 ≠ ∠Z1
∴ VECTORIAL COMPENSATION MUST BE USED
KN = Z0 - Z1 = KN ∠∅N
3Z1
> Distance Protection - January 2004
Characteristics
> Distance Protection - January 2004
Distance Characteristics
MHOR
Zn
jX
jX
R
Zs
Zn
CROSS-POLARISEDMHO
QUADRILATERAL
Zn
R
OFFSETMHO
jX
Zn′
ZnR
IMPEDANCE
jX
ZnR
LENTICULAR
jX
ZnR
POLYGON
Zn
R
> Distance Protection - January 2004
Self Polarised Mho Relays
Very popular characteristic
Simple
Less sensitive to power swings
Inherently directional
Operates for F1, but not for F2
Mho = 1/OHM
Settings :-
Z = reach settingϕ = characteristic angle
jX
R
F2
F1
Z
ϕ
OPERATE
RESTRAIN
> Distance Protection - January 2004
Neutral Impedance Replica Vectorial Compensation
Vectorial compensation allows for ∠ZN ≠ ∠ZPH which is especially important for cable distance protection where ∠ZN < ∠ZPH and ∠ZN is sometimes negative.
ZE = Earth-loop impedance for ∅ - earth fault on a cable
jX
R
ZE
ZPH ZN
> Distance Protection - January 2004
Offset Mho Characteristic
Normally used as backup protection
Operates for zero faults (close up faults)
Generally time delayed (as not discriminative)
jX
R
Z
-Z’
> Distance Protection - January 2004
Mho Relays
Directional circular characteristic obtained by introducing VPOLARISING
VF → self polarisedVSOUND PHASE → fully cross-polarisedVF + xVS.F. → partially cross-polarisedVPRE-FAULT → ‘memory’ polarised
Purpose for this is to ensure operation for close up faults where measured fault voltage collapses
> Distance Protection - January 2004
Quadrilateral Characteristic
Z
jX
ZR
RR
LoadL
1F
S
> Distance Protection - January 2004
Lenticular Load Avoidance Characteristic
jIX
IR
ba
Lenticularcharacteristic created from two offset Mho comparators
Aspect ratio = a/b
> Distance Protection - January 2004
Lenticular Characteristic
X
R
ab
Z3Aspect ratios a/b0.410.671.00
Load impedance area
Z3 reverse
> Distance Protection - January 2004
Forward Offset Characteristic
Z3
Z2
Z1
Rf
X
R
Load area
Forward blinder
Enhanced resistive coverage for remote faults
> Distance Protection - January 2004
Zones of Protection
> Distance Protection - January 2004
Zones of Protection
Z2A Z2C
Z3A Z3C
Time
T3
T2
Z1CZ1A
Z1B DCA
Z2BT2
Z1A = 80% of ZABZ2A = 120% of ZABZ3A(FORWARD) = 120% of ZAB + ZCD
B
> Distance Protection - January 2004
Zones of Protection
RA
D
CB
Z1A
Z2A
Z3A
jX
> Distance Protection - January 2004
Zone 1
FAST OPERATIONTrips circuit breaker without delay as soon asfault within Zone 1 reach is detected.
REACH SETTINGCannot be set to 100% of protected line or mayoverreach into next section.Overreach caused by possible errors in :-
CTsVTsZLINE informationRelay Measurement
> Distance Protection - January 2004
Zone 1
PossibleOverreach
ZONE 1 = ZL
ZL
F
ZONE 1 = 0.8ZL
ZL
Possible incorrect tripping for fault at ‘F’
∴ Zone 1 set to ∼ 0.8ZL
> Distance Protection - January 2004
Zone 1 Settings for Teed Feeders
Z1C = 0.8ZACA
C
Z1A = 0.8ZABZ1B = 0.8ZBA
B
Z1C
Z1AZ1B
> Distance Protection - January 2004
Zone 1 Settings for Direct Intertrip Schemes
Z1AReceiveSend
Trip ‘B’
Z1BReceive Send
Z1B
Z1A
ZLA
B
> Distance Protection - January 2004
Zone 1 Settings for Direct Intertrip Schemes
Effective Zone 1 reaches at A and B must overlap.Otherwise :- No trip for fault at ‘F’
∴ Effective Z1A and Z1B must be > 0.5ZL
Settings for Zone 1 > 0.8ZL are o.k.
Z1B
Z1A
F
A
B
> Distance Protection - January 2004
Minimum Zone 1 Reach Setting
Dictated by :-
Minimum relay voltage for fault at Zone 1reach point to ensure accurate measurement.
Minimum voltage depends on relay design typically 1 → 3 volts.
> Distance Protection - January 2004
System Impedance Ratio :- SIR
SIR = ZS/Zn
where :- ZS = Source impedance behind relayZn = Reach setting
VRPA = Minimum voltage for reach point accuracy
Can be expressed in terms of an equivalent value of SIRMAX
SIRMAX = ZS MAXZn MIN
∴ Zn MIN ≡ ZS MAXSIRMAX
> Distance Protection - January 2004
Zone 2
Covers last 20% of line not covered by Zone 1.Provides back-up protection for remote busbars.
To allow for errors :-Z2G > 1.2 ZGH
Zone 2 is time delayed to discriminate with Zone 1 on next section for faults in first 20% of next section.
Z1H
Z2G
TIMEZ1G
G H
F
> Distance Protection - January 2004
Zone 2
Overlap only occurs for faults in first 20% of following line.Faults at ‘F’ should result in operation of Z1H and tripping of circuit breaker ‘H’.
If ‘H’ fails to trip possible causes are :-Z1H operates but trip relays fail.
Z2H may operate but will not trip if followed by the same trip relays.Fault must be cleared at ‘G’ by Z2G.
Z1H and trip relays operate but circuit breaker fails to trip.
Zone 2 on adjacent line sections are not normally time graded with each other
Z1G Z1H
Z2G Z2H
‘H’‘G’F
> Distance Protection - January 2004
Zone 2
No advantage in time grading Z2G with Z2H
Unless Z2H + trip relays energise a 2nd circuit breaker trip coil.
> Distance Protection - January 2004
Zone 2Z1H fails to operate.
Results in race between breakers ‘G’ and ‘H’ if Z2H and Z2G have the same time setting.
Can only be overcome by time grading Z2G with Z2H.
Problem with this :-
Zone 2 time delays near source on systems with several line sections will be large.
End zone faults on lines nearest the infeed source point will be cleared very slowly.
Z1G Z1H
Z2G
Z2H
‘H’‘G’
> Distance Protection - January 2004
Maximum Allowable Zone 2 Reach to Allow for Equal Zone 2 Time Settings
Z2A must not reach beyond Z1B
i.e. Z2A(EFF) MAX must not reach further than Z1B(EFF) MIN
Z1BSETTING = 0.8ZL2Z1B(EFF) MIN = 0.8 x 0.8ZL2 = 0.64ZL2
∴ Z2A(EFF) MAX < ZL1 + 0.64ZL2∴ 1.2 Z2ASETTING < ZL1 + 0.64ZL2
Z2ASETTING < 0.83ZL1 + 0.53ZL2
Z2A (EFF) MAX
Z1B (EFF) MIN
ZL2ZL1 BA
> Distance Protection - January 2004
Zone 2 Time Settings on Long Line Followed by Several Short Lines
Z3H
Z2G
Z3J
Z2HZ2J
Z1H Z1JZ1G
‘H’ ‘J’‘G’F
Z2G reaches into 3rd line section.
To limit remote back-up clearance for a fault at ‘F’, the time setting of Z2G must discriminate with Z3H.
> Distance Protection - January 2004
Zone 3
Provides back-up for next adjacent line.Provides back-up protection for busbars (reverse offset).Actual Zone 3 settings will be scheme specified, i.e. permissive or blocking schemes.Many modern relays have more than 3 Zones to allow the use of three forward and an independent reverse zone.
HG K
Z1G Z1HZ2G
Z3G REV Z3G FWD
Time
Typical settings : Z3FWD > 1.2 x (ZGH + ZHK)Z3REV 0.1 to 0.25 of Z1G
> Distance Protection - January 2004
Zone Time Coordination - Ideal Situation
Zone 1 :- tZ1 = instantaneous (typically 15 - 35mS)
Zone 2 :- tZ2 = tZ1(down) + CB(down) + Z2(reset) + Margine.g. tZ2 = 35 + 100 + 40 + 100 = 275mS
Zone 3 :- tZ3 = tZ2(down) + CB(down) + Z3(reset) + Margine.g. tZ3 = 275 + 100 + 40 + 100 = 515mS
Note: Where upper and lower zones overlap, e.g. Zone 2 up sees beyond Zone 1 down, the upper and lower zone time delays will need to be coordinated, e.g. tZ2(up) to exceed tZ2(down).
> Distance Protection - January 2004
Under / Overreach
> Distance Protection - January 2004
Under-Reach
Impedance presented > apparent impedance
%age Underreach = ZR - ZF x 100%ZR
where ZR = Reach settingZF = Effective reach
> Distance Protection - January 2004
IA IA+IB
Relay Location IB
ZA ZB
VR = IAZA + (IA + IB) ZB
IR = IA
ZR = ZA + ZB + IB . ZB
IA
Underreaching Due to Busbar Infeed between Relay and Fault
> Distance Protection - January 2004
Underreaching Due to Busbar Infeed between Relay and Fault
∴ Relay with setting ZA + ZB will underreach withinfeed.
Relay with setting ZA + ZB + IB . ZB will measureIA
correctly with infeed present but if infeed is removed the relay will overreach.
Maximum allowable setting dictated by load impedance
> Distance Protection - January 2004
Under-Reach
What relay reach setting is required to ensure fault at F is at boundary of operation ?
Impedance seen for fault at F= ZG + IG + IP . ZK
IGLimit of operation is when Impedance Seen = Reach Setting
∴ Reach setting required= ZG + IG + IP . ZK
IG
IP
ZK FIG+IP
ZG IG
RELAY
> Distance Protection - January 2004
Over-Reach
Impedance seen < apparent impedance
%age Overreach = ZF - ZR x 100%ZR
where ZR = Reach settingZF = Effective reach
> Distance Protection - January 2004
Mutual Coupling
> Distance Protection - January 2004
Mutual Coupling
Mutual coupling causes distance relays to either underreach or overreach.
Positive and negative sequence has no impact.
Zero sequence mutual coupling can have a significant influence on the relay.
Only affects ground fault distance.
> Distance Protection - January 2004
Mutual Coupling Example Under Reach
Z2 ‘Boost’ G/F
Z2 PH
Zmo
> Distance Protection - January 2004
Mutual Coupling Example Over Reach
Z2 ‘reduced’ G/F
Z2 PH
> Distance Protection - January 2004
Mutual Coupling Example Over Reach
Z1 G/F (optional)Z1 G/F (normal)
Zmo
> Distance Protection - January 2004
Ancilliary Functions
> Distance Protection - January 2004
Switch on to Fault (SOTF)
XXX
Fast tripping for faults on lineenergisation, even where line VTsprovide no prefault voltage memory
> Distance Protection - January 2004
Voltage Transformer Supervision
A VT fault and subsequent operation of VT fuses or MCB’s results in misrepresentation of primary voltagesRelay will remain stable as the current phase selector will not pick upSubsequent system fault may cause unwanted / incorrect trippingVTS operating from presence of V0 with no I0 or V2 with no I2 is used to block relay if required
> Distance Protection - January 2004
VT Supervision
Under load conditionsLoss of 1 or 2 phase voltagesLoss of all 3 phase voltages
Upon line energisationLoss of 1 or 2 phase voltagesLoss of all 3 phase voltages
Digital input to monitor MCB
Set to block voltage dependent functions
> Distance Protection - January 2004
Zone 1 Mho Relay
Will not operate for load or stable power swingØ1, Ø2, Ø3, = Angles between system voltages at ‘K’ and ‘L’Ø increases as power swing approaches relay at G‘J’ is point where power swing enters relay characteristicAt ‘J’ the angle between voltages at ‘G’ & ‘H’ is 90°Normal limit of angle between voltages at ‘G’& ‘H’ for load is of the order of 30°
L
KZS
HH
Z1Ø3 J
G
ZS
G
Ø1
Power Swing Locus
Ø2 LOAD
> Distance Protection - January 2004
Comparison between Stability of Mho and Quadrilateral Impedance Elements during a Power Swing
jX
Power Swing Locus
R
θ
> Distance Protection - January 2004
Illustration of Basic Power Swing Blocking System
jX
Power Swing Locus
R
Z3
ZP
> Distance Protection - January 2004
Power Swing Blocking
A power swing will result in continuous change of current
Continuous output from the relay superimposed current element can be used to block for a power swing
Using this method the relay is able to operate for faults occurring during a power swing
> Distance Protection - January 2004
Directional Earth Fault Protection (DEF)
High resistance ground faults
Instantaneous or time delayed
IEC and IEEE curves
Single or shared signalling channel
> Distance Protection - January 2004
Transformer Feeders
> Distance Protection - January 2004
Transformer Feeders
Zone 1 = ZL + 0.5ZTT1 = Instantaneous
Zone 2 = 1.2 (ZL +ZT)T2 = Co-ordinate with downstream protection
Zone 3T3
- Back-up use as appropriate
ZL
ZT
21
> Distance Protection - January 2004
Low Voltage VT, High Voltage CT
* 1 VT may be required to account for phase shift.Example 1
ZT = 10Ω , ZL = 1ΩSet relay Z1 = 0.8 x (ZT + ZL) = 8.8Ω∴ Z1 does not reach through transformer.Example 2
ZT = 10Ω , ZL = 1ΩZ1 = ZT + 0.8ZL = 10.8Ω
with 20% error = 12.96Ω - overreach problem
ZT
21
ZL