Distance_Protection_Part_3
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Distance Protection (3) (21Z) ZULKARNAIN BIN ISHAK TECHNICAL EXPERT ENGINEERING DEPARTMENT TNB TRANSMISSION
Directional Decision by Phase Angle Comparison (2)
OPERATION when S2 is within ±90° of S1 :Directional element is actually power measuring device which utilized the phase relationship between system voltage and current to determine the current flow direction.
S2
S2
S2
S2 S1
Power equation P =VI Cos (θ) 90>θ>270 = +ve value (Forward) 270>θ>90 = -ve value (Reverse)
S2
S2
S2
Directional Decision by Phase Angle Comparison (2)
RESTRAINT when S2 lags S1 by between 90° and 270° :S2 S2 S2
270>θ>90 = -ve value (Reverse)
S2
S1
S2 S2
S2
Contents – Part 3 9Distance Teleprotection Scheme 9Under-reach and Over-reach Effect 9Setting Calculation Example 9Voltage Transformer Supervision (VTS) Function 9Switch On To Fault (SOTF) Function 9Power Swing Blocking (PSB) Function
9.0 Distance Teleprotection Scheme XPermissive Underreach Scheme XPermissive Overreach Scheme XBlocking Scheme
Permissive Underreach Scheme
Teleprotection – Distance with Communication Signal X Communication signal provides various scheme to enhance the reliability of the distance protection relay. Scheme also knows as teleprotection scheme. X Distance tele-protection scheme X Permissive Under-reach (PUR) XPermissive Over-reach (PUR) XBlocking Scheme
Permissive Underreach Scheme X Applying basic step distance will caused the last 10-20% of line not covered by Zone 1 (Instantaneous tripping). X Fault within the last 10%-20 will be covered by Zone 2 - backup (delayed operation). X Need fast fault clearance. Z2G Z1G
H
K J
Z1H
G F
Z2G
Permissive Underreach Scheme X PUR utilized the underreach zone (Zone1) to issue permissive signal for instantaneous operation of the Zone 2 at remote end station. X Permissive Underreach (PUR) scheme accelerate the fault isolation. Z2G
J
Tx
Z1
Signal receive
Z2
Z2T 0
Z3
Z3T 0 0
T
Z1G
H
K
Z1H
Signal send
≥1
G F
Signal send
Trip
Trip
Z2G
Tx
≥1
Z1 Z2T 0
Z2
Z3T 0
Z3
&
& Rx
Rx
0
T
Signal receive
Permissive Underreach Scheme
A
B
C
D Fault
21
21
Send
Rx + Z2
A
B
C
D Fault
21
21 Rx + Z2
z Race between pick up of relay at D and resetting of signal send from relay at C, following opening of breaker at C z If signal send from C resets before relay D operates then aided tripping will not occur z To prevent this a delayed on drop off (normally 100ms) of the signal send is used in the scheme logic
Permissive Underreach Transfer Trip Advantages
X Only a simplex signalling channel required X Scheme is very secure as signalling channel only keyed for internal fault (Zone 1 initiation)
Permissive Underreach Transfer Trip - Disadvantages X If one terminal of the line is open then only Basic scheme logic will apply X If there is a weak infeed at one terminal then only Basic scheme logic will apply. i.e. sequential operation. (Slow fault clearance). X If signalling channel fails then only Basic scheme logic will apply. X Resistive coverage is governed by Zone 1 setting (may be limited on short lines)
Permissive Overreach Scheme
POR X Application of PUR is not possible for feeder where minimum Zone 1 setting will encroach into next adjacent line, e.g. with short line length. (Relay setting limitation.) Z2J Z1H H G F
Z1J K
L
J Z1K Z2K
X Fault at F will cause uncoordinated tripping, where circuit breaker H and K will trip simultaneously. X Permissive Overreach need to be used.
POR X POR utilized the zone with overreach setting (e.g. Zone 2) to issue permissive signal for instantaneous operation of the distance zone at remote end station. X Distance relay at J and K will trip instantaneously for fault at F1. X Without permissive signal, Zone 2 (Z2K) will operate with delayed time (e.g. fault at F2). Z2J H G
F2
L
K J
F1 Z2K
Tx
Z1
Signal receive
Z2
Z2T 0
Z3
Z3T 0
≥1
Signal send
Trip
Signal send
Trip
Tx
≥1
Z1 Z2T 0
Z2
Z3T 0
Z3
&
& Rx
Rx
Signal receive
Permissive Overreach Scheme (CB Echo Logic) Z3 Z2 Z1
Z1 Z2 Z3
CB open
&
& Rx Tx
1
Rx Tx
Z1
& 1
Z2
T2
Z3
T3
Send Logic : Z2 Trip Logic : Rx + Z2 Open terminal echo : CB Open + Rx
1
&
CB open
.
Trip
Trip
Z1
1 .
T2
Z2
T3
Z3
Permissive Overreach Scheme (WI Echo Logic) Z4 Z2 Z1
Z1 Z2 Z4
CB open
&
Z4
&
Rx Tx
1
Rx Tx
1
& Z1 T2
Z3
T3
CB open
&
Z4
Send Logic : Z2 Trip Logic : Rx + Z2 Open terminal echo : CB Open + Rx Weak Infeed echo : Z4 + Rx
& 1
Z2
&
.
Trip
Trip
Z1
1 .
T2
Z2
T3
Z3
POR – Current Reversal
X Current reversal on the paralleled circuit, which following tripping one of the circuit breaker on the faulted circuit may cause mal-operation of distance on the healthy circuit that lead to unwanted tripping. H
H1
H2
K K1
K2
Weak source
Strong source Permissive signal
POR – Current Reversal
H
H1
H2
K K1
K2
Weak source
Strong source
Permissive signal
POR – Current Reversal H
H1
H2 Weak source
K K1
K2 Strong source
X Mal-operation of distance on the healthy circuit may happen if Zone 2 reach is set greater than 150% of the protected line impedance X It is essential for distance with POR scheme to have current reversal guard logic to prevent it from mal-operation.
POR- Current Reversal Guard
1. The timer is used to block the permissive trip and signal send
POR- Current Reversal Guard H
H1
K1
H2
K2
Weak source H
K
Strong source
H1
K1
H2
K2
Weak source
K
Strong source
2. The timer is energised if a signal is received and there is no operation of Zone 2 elements. 3. (tp) is delay on pick-up timer that usually set to allow instantaneous tripping for any internal faults, taking into account a possible slower operation of Zone 2.
POR- Current Reversal Guard
H
K
H1
K1
H2
K2
Weak source
Strong source
4. Pick-up of (tp) timer will blocked the ‘permissive trip’ and ‘signal send’ circuits by the time the current reversal takes place
POR- Current Reversal Guard
H
K
H1
K1
H2
K2
Weak source
Strong source
5. The timer is de-energised if the Zone 2 element operates or the 'signal received' element resets. 6. The reset time delay (td) of the timer is set to cover any overlap in time caused by Zone 2 elements operating and the signal resetting at the remote end, when the current in the healthy feeder reverses.
Permissive Overreach Transfer Trip - Advantages
X Provides better resistive coverage, especially on short lines, where MHO measuring elements are used X For cases where one line terminal is open, open breaker echo logic can be used X For cases of weak or zero infeed at one line terminal weak infeed logic can be used (reverse looking zone required)
Permissive Overreach Transfer Trip - Disadvantages X Duplex signalling channel required X Scheme is theoretically less secure then PUR as signalling channel is keyed for external faults X If signalling channel fails then only Basic scheme logic will apply
Distance Blocking Scheme
Blocking Scheme - Internal Fault
X Differ than permissive scheme for signal transmission. X Permissive scheme using forward zone to transmit signal to remote distance relay. X Blocking scheme using reverse Zone detection pickup to transmit blocking signal to remote distance relay.
Blocking Scheme - Internal Fault Z3 Z2 Z1
Send Logic : Z3 + Z2 Trip Logic : Rx + Z2
Z1 Z2
Z3
&
& Rx Tx
Rx Tx
&
& Z1
1
Z2
T2
Z3
T3
.
Trip
Trip
Z1
1 .
T2
Z2
T3
Z3
Blocking Scheme - External Fault Z3 Z2 Z1
Send Logic : Z3 + Z2 Trip Logic : Rx + Z2
Z1 Z2 Z3
&
& Rx Tx
Rx Tx
&
& Z1
1
Z2
T2
Z3
T3
.
Trip
Trip
Z1
1 .
T2
Z2
T3
Z3
Blocking Scheme - Advantages X Only simplex signalling channel required X Provides better resistive coverage than PUR on short lines where MHO elements are used X Fast tripping will still be possible at closed end of line for all fault positions with remote breaker open X Fast tripping will still be possible at strong infeed terminal for all fault positions where remote terminal has no or weak infeed
Blocking Scheme - Disadvantages
X Only 2 forward zones of protection available (unless relay has >3 Zones) X If signalling channel fails then only Basic scheme logic will apply X Current sensitivity is lower as tripping elements (Z2) are controlled by high set current level detectors (to ensure blocking elements (Z3/Z4) are more sensitive than tripping elements)
Permissive Schemes vs Blocking Schemes
X Permissive less reliable - require a signal from remote relay plus local operation to trip X Blocking less secure - require a signal from remote relay to prevent a trip X Permissive schemes are marginally faster and more sensitive (timer plus high set current elements on Blocking scheme)
10.0 UNDER and OVER REACH EFFECT XUnder-Reach and Over-reach Definition XCurrent / Source Infeed Effect Busbar Infeed between Relay and Fault Double Circuit
XMutual Coupling Effect
Under-Reach and Over-reach Definition
Under / Over Reach Effect X An impedance seen by the relay might differ from the calculated values due to errors such as: Those introduced by current and voltage instrument transformers, particularly under transient conditions. Inaccuracies in the line zero-sequence impedance data, and their effect on the zero sequence-compensation setting. The effect of infeed between the relay and the fault location, including the influence of different Z0/Z1 ratios of the various sources.
Under / Over Reach Effect Continued.
The phase impedance of the untransposed lines is not identical for all fault loops. The difference between the impedances for different phase-to-earth loops can be as large as 5-10%. The effect of a load transfer between the ends of the protected line. Especially when the fault resistance is appreciable; it must be recognized. Zero-sequence mutual coupling from parallel lines.
Under-Reach Definition
Impedance presented > apparent impedance %age Underreach = ZR - ZF x 100% ZR where ZR = Reach setting ZF = Effective reach
Over-Reach
Impedance seen < apparent impedance %age Overreach = ZF - ZR x 100% ZR where ZR = Reach setting ZF = Effective reach
Current / Source Infeed Effect
Infeed Effect Busbar Infeed between Relay and Fault ZA
IA
IA+IB
ZB
IB
Relay Location
VR = IAZA + (IA + IB) ZB IR = IA ZR = ZA + 1 + IB . ZB IA
(impedance seen by relay)
Relay experienced under reaching
Infeed Effect Busbar Infeed between Relay and Fault ∴ Relay with setting ZA + ZB will underreach with infeed. Relay with setting ZA + ZB + IB . ZB will measure IA correctly with infeed present but if infeed is removed the relay will overreach. Maximum allowable setting dictated by load impedance
Mutual Coupling Effect
Mutual Coupling A I B
B
Zm0 IA RELAY
F
X
Most of the multi circuit lines are double circuits that operates in parallel.
X
Mutual coupling effect produced by parallel circuits modify the sequence impedance parameters of the circuits.
X
Mutual coupling impedance resulting from positive and negative sequence current is generally negligible, in the order of 3% - 7% of the conductor self-impedance.
X
However, the zero-sequence mutual impedance can be as high as 50% to 70% of the self impedance.
Mutual Coupling X
Zero sequence mutual coupling can have a significant influence on the relay.
X
Mutual coupling causes distance relays to either underreach or overreach.
X
Only affects ground fault distance. A
Z0AB- Z0m
B
Z0m Z0AB- Z0m
a) Equivalent zero-sequence impedance for a single-phase-to-earth fault on the adjacent busbars with both parallel circuits in operation.
X
Mutual coupling reduce distance reach at one end but the same time proportionally increase at the opposite line end.
X
Therefore, reach reduction will not affect the operation of a permissive underreach scheme.
Mutual Coupling – Parallel Circuit Disconnected and Earthed Both Ends RELAY
F
IA
Z0AB- Z0m
B
Z0m Zm0
A
A
Im
Z0AB- Z0m B
b) Equivalent zero-sequence impedance circuit for a single-phase-to-earth fault on the adjacent busbars with both parallel circuit disconnected and earthed at both ends.
X
The distance protection will tend to overreach for single-phase-toearth faults on the protected line when the parallel circuit is disconnected and earthed on both ends.
X
The equivalent zero-sequence impedance is equal to the value.
Z 02AB − Z 02m ZE = Z 0 AB
Mutual Coupling – Parallel Circuit Disconnected and Not Earth. Im
A
B
A
Z0AB- Z0m
B
Z0m Zm0
Z0AB- Z0m
IA RELAY
F
c) Equivalent zero-sequence impedance circuit for a single-phase-toearth fault on the adjacent busbars with both parallel circuit disconnected and not earthed.
X
The line zero-sequence mutual impedance will not influence the measurement of the distance protection in a faulty circuit.
11.0 Distance Calculation Setting Example XMho and Off-set Mho Distance Characteristic Setting Example XQuadrilateral Distance Characteristic Setting Example
Mho and Off-set Mho Distance Characteristic Setting Example
Calculation Example – Mho Z1 & Z2 and Offset Mho Z3.
CT
VT
275kV System
1. Setting calculation for end A. 2. Line impedance i. +ve seq. impedance, ZL1 = (0.089 + j 0.324) ohm/km ii. zero seq. impedance, ZL0 = (0.204 + j 0.838) ohm/km
3. Distance relay at A using mho characteristic. 4. Minimum fault level at bus A is 1000MVA with both +ve and zero seq. are equal.
Calculation Example – Mho Z1 & Z2 and Offset Mho Z3. 6. Ignore the mutual effect. 7. CT primary current rating is equal to the protected line rating 8. Setting philosophy 1. Zone 1 = 80% of protected line. 2. Zone 2 =100% of protected line + 50% of shortest adjacent line. 3. Zone 3 = 100% of protected line + 120% of longest adjacent line. 4. Zone 3 Reverse = 20% of Zone 1 9. Protected line will experience additional 50% load current over its rated current during adverse system conditions with 5% system voltage drop.
Calculation Example – Mho Z1 & Z2 and Offset Mho Z3. 1. Calculate the secondary impedance factor. 2. Calculate the Zero Sequence Compensation factor KN for earth fault element. 3. Calculate the relay Zone reach setting to apply on the relay. i. Zone 1 ii. Zone 2 iii. Zone 3 iv. Zone 3Reverse 4. Calculate relay characteristic angle for i. Phase-phase distance element ii. Phase-earth distance element 5. Plot the relay characteristic using positive sequence impedance diagram. 6. Calculate minimum load impedance.
Calculation Example – Mho Z1 & Z2 and Offset Mho Z3. 1. Calculate the secondary impedance factor. S.I.F. =
C.T. RATIO V.T. RATIO
S.I.F. =
600/1 275kV/110V
S.I.F. =
0.24
So Line +ve seq. impedance (secondary), ZL1 = (0.02136 + j 0.07776) ohm/km = 0.0806∠74.64˚ Line zero seq. impedance, ZL0 = (0.04896 + j 0.20112) ohm/km = 0.207∠76.32˚(secondary)
2. Calculate the Zero Sequence Compensation factor KN for earth fault element. KN = KN =
1 Z0 – 1 3 Z1 0.523∠2.75˚
Calculation Example – Mho Z1 & Z2 and Offset Mho Z3. 3. Calculate the relay Zone reach setting to apply on the relay (secondary) i.
Zone 1 = 80% of protected line. = 0.8 x 50 x 0.0806 = 3.224 Ω
ii.
Zone 2 =100% of protected line + 50% of shortest adjacent line = (50 x 0.0806) + (0.5 x 90 x 0.0806) = 7.657 Ω
iii.
Zone 3 =100% of protected line + 120% of longest adjacent line = (50 x 0.0806) + (1.2 x 90 x 0.0806) = 12.7348 Ω
iv.
Zone 3R = 20% of Zone 1 = 0.2 x 3.224 = 0.6448 Ω
Calculation Example – Mho Z1 & Z2 and Offset Mho Z3. 4. i. Phase-phase distance element ∠Ø-Ø =
74.64˚
ii. Phase-earth distance element Angle for earth loop impedance can be calculate = Z1KN = 0.0806∠74.64˚ x 0.52∠2.7˚ = 0.042∠77.34˚
∠Ø-E = 77.34˚
Calculation Example – Mho Z1 & Z2 and Offset Mho Z3. 5. Plot the relay characteristic using positive sequence impedance diagram.
6. Calculate minimum load impedance. ZLmin =
0.95x 275kV √3 x 1.5 x 600
=
167.59Ω primary
=
40.22Ω secondary
Quadrilateral Distance Characteristic Setting Example
Calculation Example – Numerical relay with Quadrilateral Characteristic .
CT
VT
275kV System
1. Setting calculation for end A. 2. Line impedance i. +ve seq. impedance, ZL1 = (0.089 + j 0.324) ohm/km ii. zero seq. impedance, ZL0 = (0.204 + j 0.838) ohm/km
3. Distance relay at A using quad. characteristic. 4. Minimum fault level at bus A is 1000MVA
Calculation Example – Numerical relay with Quadrilateral Characteristic . 6. Ignore the mutual effect. 7. CT primary current rating is equal to the protected line rating 8. Setting philosophy 1. Zone 1 = 80% of protected line. 2. Zone 2 =100% of protected line + 50% of shortest adjacent line. 3. Zone 3 = 100% of protected line + 120% of longest adjacent line. 4. Zone 3 Reverse = 20% of Zone 1 9. Transmission tower construction having 7 meters minimum separation between phase conductor and 4 meters separation between phase and tower body. The maximum tower footing resistance allowed is 10Ω.
Calculation Example – Mho Z1 & Z2 and Offset Mho Z3. 1. Calculate the secondary impedance factor. 2. Calculate the Earth Impedance Ratio factor XE/XL; RE/RL for earth fault element. 3. Calculate the relay Zone reach setting to apply on the relay. i. Zone 1 ii. Zone 2 iii. Zone 3 iv. Zone 3Reverse 4. Plot the relay characteristic using positive sequence impedance diagram.
Calculation Example – Numerical relay with Quadrilateral Characteristic . 1. Calculate the secondary impedance factor. S.I.F. =
C.T. RATIO V.T. RATIO
S.I.F. =
600/1 275kV/110V
S.I.F. =
0.24
So Line +ve seq. impedance, ZL1 = (0.02136 + j 0.07776) ohm/km = 0.0806∠74.64˚ (secondary) Line zero seq. impedance, ZL0 = (0.04896 + j 0.20112) ohm/km = 0.207∠76.32˚(secondary)
Earth Impedance Ratio 2. Earth impedance ratio:
RE 1 ⎛ R0 ⎞ = ⋅ ⎜⎜ − 1⎟⎟ RL 3 ⎝ R1 ⎠ 1 ⎛ 0.04896 ⎞ = ⋅⎜ − 1⎟ 3 ⎝ 0.02136 ⎠ = 0.4307Ω XE 1 ⎛ X0 ⎞ = ⋅ ⎜⎜ − 1⎟⎟ X L 3 ⎝ X1 ⎠ 1 ⎛ 0.20112 ⎞ = ⋅⎜ − 1⎟ 3 ⎝ 0.07776 ⎠ = 0.5288Ω
Calculation Example – Numerical relay with Quadrilateral Characteristic . 3. Calculate the relay Zone n reactance Xn reach setting to apply on the relay (secondary) i.
Zone 1 = 80% of protected line.
= 0.8 x 50 x 0.0806 = 3.224 ∠74.64˚ Ω = 0.854 + j 3.1088 Ω X1 = 3.1088 Ω ii.
Zone 2 =100% of protected line + 50% of shortest adjacent line = (50 x 0.0806) + (0.5 x 90 x 0.0806) = 7.657 ∠74.64˚ Ω =2.0282 + j 7.3835 Ω X2 = 7.3835 Ω
Calculation Example – Numerical relay with Quadrilateral Characteristic . iii.
Zone 3 =100% of protected line + 120% of longest adjacent line = (50 x 0.0806) + (1.2 x 90 x 0.0806) = 12.7348 ∠74.64˚ Ω = 3.3475 + j 12.287 Ω X3 = 12.287 Ω
iv.
Zone 3R = 20% of Zone 1 = 0.2 x 3.224 = 0.6448 ∠74.64˚ Ω = 0.1708 + j 0.6218 Ω X3R = 0.6218
Phase-phase Resistive Reach Setting For resistance setting in relation to overhead lines, consideration of arc resistance is most important for phase to phase fault. The arc resistance can be calculated using the empirical formula derived by A.R. van C. Warrington
Phase-phase Rarc
28710 ⋅7 Rarc = 1.4 2099 = 4.489Ω ≈ 5.0Ω
Phase-earth Rarc
28710 RarcE = ⋅4 1.4 2099 = 2.6Ω
Phase-phase Resistive Reach Setting Only half of the arc resistance value was used, because it is additive to the loop impedance and only half should be added to the impedance per phase.
1 RF 1 = R1Line + ⋅ Rarc 2 1 = 0.854 + ⋅ 5.0 2 = 3.354Ω 1 RF 2 = R2 Line + ⋅ Rarc 2 1 = 2.0282 + ⋅ 5.0 2 = 4.5282Ω
Phase-phase Resistive Reach Setting 1 RF 3 = R3 Line + ⋅ Rarc 2 1 = 3.3475 + ⋅ 5.0 2 = 5.8475Ω
1 RF 3 R = R3 RLine + ⋅ Rarc 2 1 = 0.1708 + ⋅ 5.0 2 = 2.6708Ω
Phase-earth Resistive Reach Setting For earth fault, a separate resistance can be set.
REF 1 = R1Line + RarcE + Rtower _ footing = 0.854 + 2.6 + 10 = 13.454Ω
REF 2 = R2 Line + RarcE + Rtower _ footing = 2.0282 + 2.6 + 10 = 14.6282Ω
Phase-earth Resistive Reach Setting
REF 3 = R3 Line + RarcE + Rtower _ footing = 3.3475 + 2.6 + 10 = 15.9475Ω
REF 3 R = R3 RLine + RarcE + Rtower _ footing = 0.1708 + 2.6 + 10 = 12.7708Ω
Distance Zone Characteristic
11.0 VOLTAGE SUPERVISION (VTS) Voltage Supervision -ve and 0 sequence detection External Binary Initiation VTS Logic Diagram
Voltage Supervision X Distance protection measures system voltage that derive from Voltage Transformer as one of its input for operations. Numerical Distance Relay with other build in functions z z z z
Under/Over voltage protection function Under/Over frequency protection function Synchrocheck Dead Breaker Logic
X MCBs or fuses are normally used to protect the secondary wiring between VT’s secondary terminal and relay terminal. X Secondary voltage signal may experienced disturbance or interruption in the form of total lost or voltage reduction Loose connection / broken conductor VT MCB tripped / VT Fuse blown
Voltage Supervision External Binary Initiation
Distance Relay CT To relay Terminal Block
VT MCB VT
+
From VT
+
Voltage Supervision X Distance protection using voltage input to determine the fault direction and its impedance. Disturbance on the voltage signal will cause distance relay to mal-operate. X Distance employ VT Supervision (VTS) to monitor the voltage condition inputs to the relay. X On detection of VT failure, tripping of the distance relay can be blocked and/or an alarm can be given.
Voltage Supervision
X VTS can works base on the following methods, It can operate on the basis of the zero-sequence measuring quantities: z
a high value of voltage 3U0 without the presence of the residual current 3I0
It can operate on the basis of the negative-sequence measuring quantities: z
a high value of voltage 3U2 without the presence of the negative-sequence current 3I2
It can operate only on initiation using external binary signals from the miniature circuit breaker.
Voltage Supervision -ve and 0 sequence detection X -ve and zero seq. always exist in the system due to different non-symmetries in the primary system differences in the CT & VT
X –ve seq. and zero seq. setting must always be set with a safety margin of 10 to 20%, depending on the system operating conditions. X The negative sequence detection algorithm, used in isolated or high-impedance earthed networks. V2m > (3·V2 Setting) and I2m > (3·I2Setting)
X The zero sequence detection algorithm, used in directly or low impedance earthed networks V0m > (3·V0 Setting) and I0m > (3·I0 Setting)
Voltage Supervision
Example: VTS Logic Diagram
Example: VTS Logic Diagram
12.0 SWITCH ON TO FAULT
SOTF X Overhead line in the transmission always have a dedicated VT installed on the line side of circuit breaker. X Energizing dead overhead line will possibly experience switch on to a 3-phase short circuit / fault – due to earth wire not removed following line works. CT
CT
VT
21Z
VT
21Z
SOTF X All voltages collapse totally if the short circuit is close of the VT terminal No voltage measured by the relay No polarizing voltage No memory polarizing voltage since no voltage before and after the fault
X Directional zones of distance relay unable to determine fault direction and cannot operate. X Leaving fault to be cleared by other backup non-directional zone, thus causes slow fault clearance. X Switch-On-To-Fault SOTF feature is employed by distance relay as a complementary function for fast fault isolation which may occur when energizing a faulted line.
SOTF X Using switch-onto-fault (SOTF-) function, a fast trip is achieved for a fault on the whole line, when the line is being energized. X SOTF apply either or both i. non-directional (ND) instantaneous overcurrent element ii. ND distance zone (without time delay)
to achieve fast tripping/ fault clearance. X The ND distance protection zone used together with the switchonto-fault function shall be set to cover the entire protected line. X The ND instantaneous overcurrent shall be set more than the maximum loading condition during emergency situation e.g. 200% of the line rating.
SOTF X SOTF function will be primed after the line was identified as dead through either line voltage check (under voltage) or circuit breaker open position check. X The SOTF protection issues a three-phase tripping to the circuit breaker for the operation of the ND an overcurrent element or ND distance measuring elements for normally less than 1 second after the circuit breaker is closed and/or after the under voltage dead line detector resets.
SOTF Delay pickup timer ON
UVPP
I>
ND- Zone
Delay drop-off timer SOTF reset time
& t
UVPE CB-OP
Dead line check
0
0
t
ON
1 ON
UV or CB or Both
SOTF Trip & ≥1
ON
&
Example SOTF Logic
13.0 POWER SWING BLOCKING (PSB) Power Swing Characteristic Requirement on Power Swing Blocking Power Swing Detection Power Swing Impedance Locus Power Swing with Fault Condition Zone Blocking Selection
Power Swing Characteristic X Power system faults, line switching, generator disconnection and loss of large blocks of load result in sudden change of electrical power, whereas the mechanical power input to the generators remains relatively constant. X These system disturbances cause oscillations in machine rotor angles and can result in severe power swings. X Power swing - load impedance seen by a distance relay move (relatively slowly) from the load area into the distance protection operating characteristic. jX Power Swing Locus θ R
Requirement on Power Swing Blocking X This phenomenon appears to the distance protection measuring elements like a three phase fault condition and may result in tripping if no countermeasure is applied. X Most power swings are transient conditions from which the power system can recover after a short period of time, and distance protection tripping is therefore highly undesirable in such cases. X Distance protection shall provides a power swing blocking function (PSB) to prevent unwanted tripping during a power swing.
Power Swing Detection X Power swings blocking function measure the impedance rate of change dZ/dt or ΔZ/Δt and compare with a threshold value to distinguish between short-circuit and power swings. X Simples method is determine the elapsed time required by the impedance vector to pass through a zone limited by two impedance characteristic. X Thus, for such requirement Power Swing characteristic is provided. X Power Swing Characteristic shall encompass the distance zones including the starting or fault detection characteristic with a fixed distance of ΔZ. The time different is measured. X Once power swing is detected, the blocking signal must be maintained until the load impedance vector exist the starting / outer PS characteristic. X It is also possible to remove the blocking signal after fixed/set time delay.
Power Swing Impedance Locus X (1) A short-circuit, the impedance vector abruptly changes from the load condition into this fault detection range. X (2) A power swing, the apparent impedance vector initially enters the outer characteristic of power swing and only later enters the fault detection range. X (3) It is also possible that a power swing vector will enter the area of the power swing range and leave it again without coming into contact with the fault detection range X (4) If the vector enters and passes through and leaving on the opposite side of the power swing characteristic, then the sections of the network seen from the relay location have lost synchronism
Power Swing Vector with Quad Distance Characteristic X
ΔZ
PS outer zone PS inner zone
Zone 3 FD
4
Zone 2
ΔZ
3
Zone 1
2 1 R Load Impedance Area
Zone 4
ΔZ
Load Impedance
ΔZ
Area
Power Swing Vector with MHO Distance Characteristic jX PS Inner Zone
PS Outer Zone
Z3
4 Z2
3
Z1
2 1 ΔZ
Load Impedance Area
R
Load Impedance Area
Power Swing with Fault Condition X Blocking of the distance protection naturally involve risk, which genuine fault during PS blocking will cause no tripping. X Power swing is generated by balance symmetrical threephase system conditions. X Relay using unbalance system conditions or earth fault current to remove the blocking condition immediately, thus allow distance zones to operate for unsymmetrical fault. X For three-phase fault, there will be no residual current. However, if a current jump detected due to three phase short circuit during PS vector in side the starting zone, the blocking condition is remove immediately.
Power Swing Zone Blocking Selection X It is also possible to select which zones of distance will be blocked by PSB function, i.e. either all zones, only first zone or all zones except first zone. X Sometime first zone is not blocked when it is required to trip for unstable system conditions. X Blocking of the higher zones is not required when no slow PS is expected in the system, i.e. power swing vector exists the relevant zone before the set zone time is expired. X Blocking of all zones normally recommended as the course and frequency of power swing depend on the system arrangement and therefore never accurately be predicted in advance.
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