Auto-reclose Scheme
ZULKARNAIN BIN ISHAK
TECHNICAL EXPERT
ENGINEERING DEPARTMENT
TNB TRANSMISSION
1.0 Auto-reclose
Benefits of Auto-Reclosing
Fault Types
Auto-Reclose Terminology
Single Shot or Multi-Shot?
System Stability
3ph or 1ph A/R
Choice of Scheme
Synchronizing Check
Autoreclose
AUTO-RECLOSING
DISTRIBUTION
Generally <66kV
Mainly Applied on
Radial Systems
Continuity of Supply TRANSMISSION
Interconnected Supply
Sources
Stability
System Synchronism
Benefits of Auto-Reclosing
X Improved continuity of supply
Supply restoration is automatic (does not require human
intervention)
Shorter duration interruptions
Less hours lost to consumer
X Less frequent visits to substations
More unmanned substations
Reduced operating costs
Fault Types
¾Permanent Faults\
¾Self Clearing Faults
•Semi-Permanent Fault
•Transient Faults
Permanent Faults
Affected part of system that cannot be successfully re-
energised until fault has been rectified and damage
equipments has been repaired.
Transformers
Machines Most faults are
permanent
Cables (Underground)
Overhead Lines
Broken Conductors
Broken Insulators
Self Clearing Faults
No permanent damage to system.
Once cleared, the affected part of power system can be safely re-
energised.
Transient Faults
Cleared by immediate isolation of fault by circuit breaker.
X Insulator flashover due to transient overvoltage
Switching
Lightning
X Conductor clashing
Semi-Permanent Fault
Cleared within a few seconds of fault current interruption.
X Tree branches blown onto O/H Line
Fault Occurrence
Auto-reclose is confined to overhead lines and feeders.
Transient faults 80 to 85%
Semi-permanent faults 5 to 10%
Permanent 10%
Transient faults :- E.H.V. > H.V.
Semi-Permanent faults :- E.H.V. < H.V.
Auto-Reclose Terminology
High speed - C.B. reclose in less than 1 sec.
Low speed (delayed) - Reclose after more than 1 sec.
Repetitive scheme - Resets automatically after
successful A/R
Non-repetitive - Requires manual reset.
Single shot - Only 1 reclose attempt / fault
incidence
Multi shot - 2 or more reclose attempts
Single-shot Auto-reclose Scheme Operation
For A Transient Fault
Single-shot Auto-reclose Scheme Operation
For A Permenent Fault
Choice of Scheme
Need to consider :
X Dead Time
X Reclaim Time
X Number of Shots
Dead Time
Need to consider :
X Load
X Circuit Breaker
X Protection Reset Time
X Fault De-ionization Time
X System Stability
X System Synchronism
Dead Time – Circuit Breaker
X Minimum dead time = mechanism reset + closing time
May be as high as 0.5 secs.
Only significant for High Speed Auto-Reclose.
Dead Time - Load
Dead time has to be long enough to allow motor circuits to trip out on lost of
supply.
Sync: Minimum 0.3 secs.
Motors
Induction: not greater than 0.5 sec.
Usually 3 - 10 secs. (to allow all motors to be
disconnected)
Street Lighting : 1 - 2 secs.
Domestic : 10 secs - 3 minutes
Dead Time – Protection Reset
X Protection must fully reset during dead time.
X For high speed A/R :- Instantaneous reset required.
Dead Time – Fault De-ionization
X Time for ionised air to disperse imposes minimum system dead time.
X Time required depends on :-
System voltage
Cause and nature of fault
Weather conditions
Fault clearance time
X Difficult to calculate accurately
Approximately :-
De-ionising time = (10.5 + KV/34.5) cycles
For 66kV = 0.25 secs. (50Hz)
= 0.21 secs. (60Hz)
For 132kV = 0.29 secs. (50Hz)
= 0.24 secs. (60Hz)
X On distribution systems effect generally less important than C.B. operating
times.
Fault Clearance Time
Minimised by :- (i) Fast protection (< 30msec)
eg. Distance
Pilot Wire
(ii) Fast circuit breakers
<50msec
Fast fault clearance reduces required fault arc de-ionising
time
Reclaim Time
Requirement:- A/R relay should not reset before
protection has had time to operate.
(Following reclosure for a permanent
fault).
Considerations:-
Supply continuity
Fault incidence / past experience
Switchgear duty (rating)
Switchgear maintenance
Reclaim Time for use with High Speed
Protection
When high speed protection is used to clear all faults :
Reclaim Time < 1 sec adequate
but, rarely used in practice, to relieve the duty on the circuit
breaker.
Reclaim Time < 5 secs
To relieve circuit breaker duty
Number of Shots (1)
Note:- Shots = reclose attempts
Usually :-
Transmission 1 shot
Additional shots not justified due to :-
Maintenance
System Disturbance
Damage
Sub-Transmission 1 or 2 shots
(2 or 3 if radial circuits)
Distribution 1, 2, 3 or 4 shots
Single Shot or Multi-Shot?
One Shot Scheme
X Relatively provides greatest improvement in supply
continuity.
X ∼ 80% of faults are transient.
X Minimum trip duty on circuit breakers.
X Important when high frequency of circuit breaker
maintenance required, eg. Oil C.B.
Circuit breaker duty cycle may prevent > 1 reclose attempt.
Multi-Shot Schemes
X Improved supply continuity.
-- > justified for distribution A/R.
X Helps prevent lockout due to successive flashovers
during severe thunderstorms.
Systems having relatively high levels of semi-permanent
faults.
Inst Trip
First shot A/R - Unsuccessful
IDMT trip - Fault Burns Clear
Second shot A/R - Successful
System Stability
Fault Shunts (1)
Z1 F1
E ZF
N1
ZF = Fault shunt
= Combined Impedance of -ve and zero sequence
network impedances for particular fault.
Fault Shunts (2)
Ø/E ZF = Z2 + Z0
Ø/Ø ZF = Z2
Ø/Ø/E ZF = Z2 . Z0
Z2 + Z0
3Ø ZF = 0 (short circuit)
Healthy ZF = ∝ (open circuit)
Power Angle Curves
A graph of P against δ, plotted from the above expression is known as a
Power/Angle curve.
Its amplitude is inversely proportional to the transfer reactance, which in
turn depends on system conditions.
Power
E E sin δ
Power Flow = 1 2
X
Load
Angle (δ)
Comparative Power Angle Curves
The effect of various system conditions, including different types of fault, can be
estimated using the concept of interconnected positive, negative and zero
sequence networks.
Power 3Ø Healthy
2Ø Healthy
1Ø Tripped
Ø/E Fault
Ø/Ø/E Fault
3Ø Fault
3Ø Tripped
Load Angle (δ)
Steady State
Initial operating conditions are at point A on curve.
Po represents the surplus of locally generated power at the sending end,
and the corresponding deficit at the receiving end.
Power
Normal
X Y Healthy
Operation
A
Sending end P0
Phase
δ0 Angle
Difference
During Fault
If a fault occurs, curve 2 applies, the operating point moves to B, with a lower power transfer
level P1.
Therefore a surplus of power P0-P1 at the sending end, and a corresponding deficit at the
receiving end.
The sending end machines start to speed up, and the receiving end machines
start to slow down, so phase angle δ increases, and the operating point moves
along curve 2 until the fault is cleared, when the phase angle is δ1.
Power
X Y
Normal
A
P0
P0 - P1 Ø/Ø/E Fault
P1 C
B
δ0 Phase Difference
δ1
Increased Power Level
The operating point now moves to point D on curve 3. There is still
a power surplus at the sending end, and deficit at the receiving end,
so the machines continue to drift apart and the operating point
moves along curve 3.
Power
Normal
Curve 3
X Y
P0 A E Faulted Feeder
Disconnected
D
P2
Ø/Ø/E Fault
C
B
δ0 Phase Difference
δ1
Increased Power Level
When operating point passes E, the transmitted power is more than generated sending end, so
there is a nett deficit at the sending end and a nett surplus at the receiving end.
The sending end machines start to slow down and the receiving end machines start to speed
up.
However, the sending end machines are still running faster than the receiving end, so the
phase angle continues to increase, though at a decreasing rate.
Power
Normal
P2' F
Curve 3
X Y A E Faulted Feeder
P0 Disconnected
P2 D
Ø/Ø/E Fault
C
B
δ0 δ2 Phase Difference
δ1
Increased Power Level
Eventually, at point F, the machines are again rotating at the same
speed, so the phase angle stops increasing. According to the
“Equal Area Criterion”, this occurs when area 2 is equal to area 1.
Power
Normal
P2' F
Curve 3
X Y A E Faulted Feeder
P0 Disconnected
P2 D
Ø/Ø/E Fault
C
B
δ0 δ2 Phase Difference
δ1
Increased Power Level
At F there is still a nett sending end power deficit and receiving end power surplus, so
the sending end machines continue to slow down, and the receiving end machines
continue to speed up.
Phase angle starts to decrease, and the operating point moves back towards E.
Power
Normal
P2' F
X Y Curve 3
A E Faulted Feeder
P0 Disconnected
P2 D
Ø/Ø/E Fault
C
B
δ0 δ1 δ2 Phase Difference
Increased Power Level
As the operating point passes E, the nett sending end deficit again becomes a surplus, and the
receiving end surplus becomes a deficit, so the sending end machines begin to speed up and the
receiving end machines begin to slow down.
After some time, due to losses the oscillation is damped, and the system eventually settles at
operating point E. The system is therefore stable
Power
Normal
P2' F
X Y Curve 3
A E Faulted Feeder
P0 Disconnected
P2 D
Ø/Ø/E Fault
C
B
δ0 δ1 δ2 Phase Difference
Equal Area Criteria
With higher initial power transfer level Po' at such a value that the area enclosed by curve 3
above the Po' line is only just greater than the area enclosed by the locus of the operating point
below the Po' line.
In this case, the two sets of machines have the same speed just before the operating point
reaches G, and the operating point then starts to return along curve 3, with the system
eventually settling to stable synchronous operation at point E.
Power
Normal
Curve 3
Faulted Feeder
Disconnected
E G
P0'
P0 A
D
C Ø/Ø/E Fault
B
δ2 Phase Angle
Difference
Equal Area Criteria
If the initial power transfer level is above Po’, the area enclosed by curve 3 above the Po’’ line is
less than the area enclosed by the locus of the operating point below the Po’’ line, so when the
operating point passes G, the phase angle difference is still increasing.
The local sending end power deficit becomes a surplus, causing the machines to speed up, and
the local receiving end power surplus becomes a deficit, causing the machines to slow down;
therefore the two sets of machines continue to drift out of synchronism and the system has
become unstable.
Power
Normal
Curve 3
Faulted Feeder
P0’’ G Disconnected
E
P0'
P0 A
D
C Ø/Ø/E Fault
B
δ3 Phase Angle
Difference
Transient Fault – Successful A/R
At higher power transfer level, it is assumed that after a certain time following fault
clearance, the faulted feeder is successfully reclosed, restoring the system to its
original healthy state, so curve 1 applies and the operating point moves from E to G.
The maximum level of Po'' for the system to have transient stability, is that value
which will make area 2 on Figure 5 just greater than area 1.
it is clear that the application of high speed auto-reclosing enables the system to
operate at a higher power level while retaining transient stability.
Power Normal
G
P0'' A H
F
E Faulted Feeder
D
X Y Disconnected
B C Ø/Ø/E Fault
Phase Angle
Difference
3ph or 1ph A/R
Single Feeder – 3ph A/R
For single circuit interconnector, the power transfer level is zero while the feeder
is open, i.e. curve 3 lies along the ‘δ’ axis, and the limiting power transfer level for
transient stability is zero if auto-reclosing is not applied.
The maximum limiting power transfer for transient stability for a particular fault
will be obtained when the area enclosed by the locus of the operating point below
the power transfer line is as small as possible. This may be achieved by clearing
the fault as quickly as possible, and reclosing the circuit in a short a time as
possible
Power
Normal
X Y
P3Ø(A/R)
Ø/E Fault
Line
Open 3Ø Phase
Angle
δ Difference
High Speed 1Ø A/R Single
Interconnector
Power transfer is zero when all three poles of a single circuit interconnector are open. Fast three-
pole tripping and auto-reclosing allows a substantial power transfer limit, while retaining
transient stability for single phase to ground faults, which are the most common type of fault on
overhead transmission circuits.
However, if only the faulted pole is tripped and then reclosed, a higher power transfer limit can
be obtained, due to the power transfer through the two healthy poles. This is illustrated in
Figures below.
Power Normal Power
Normal
P1Ø(A/R)
1Ø Open
P3Ø(A/R)
Ø/E Fault
Ø/E Fault
Line Open 3Ø
Phase
Angle
δ Differen
Phase Angle δ
Difference
Power transfer limit for stability following Power transfer limit for stability following
successful high speed 1Ø auto-reclose. successful 3Ø auto-reclose.
1Ø Auto-Reclose Advantages (over 3Ø A/R)
1. Higher power transfer limit.
2. Reduced power swing amplitude.
3. Reduced switching overvoltages due to reclosing.
4. Reduced shock to generators.
Sudden changes in mechanical output are less
Choice of Scheme
Choice of Scheme (1)
High Speed Auto-Reclose
1. Single transmission links.
2. 3Ø A/R.
3. 1Ø A/R for 1Ø-E/Fs
3Ø A/R for multiphase faults.
4. 1Ø A/R for 1Ø-E/Fs
Lockout for multiphase faults.
Choice of Scheme (2)
Delayed 3Ø Auto-Reclose
1. Densely interconnected systems.
Ð
Minimal power transfer level reduction
during dead time
2. Allow power swings due to fault and
tripping to decay
Ð
Less shock to system than with high
speed A/R
1Ø Auto-Reclose Factors Requiring
Consideration
1. Separate control of circuit breaker poles.
2. Protection must provide phase selection.
3. Mutual coupling can prolong arcing and require
de-ionising time.
4. Unbalance during dead time
(i) Interference with communications
(ii) Parallel feeder protection may maloperate
5. More complex and expensive than 3Ø A/R
High Speed Auto-Reclose (H.S.A.R.) (1)
Protection
High speed < 2 cycles
Fast clearance at each line end.
Current Differential
Distance schemes with signalling
Direct intertrip
Phase selection required for 1Ø A/R – important
of Fault Detector / Phase Selector element if
using Distance Protection.
High Speed Auto-Reclose (H.S.A.R.) (2)
Dead Time (short as possible)
Circuit breaker minimum ‘open - close’
time
∼ 200 - 300 msecs.
Same dead time at each line end.
De-ionising time
1Ø A/R longer → special steps
Delayed Auto-Reclosing (D.A.R.) (1)
Protection
High speed not critical for system stability
↓
desirable to limit fault damage
↓
improves probability of successful A/R
Dead Time
Allow for power swings and rotor oscillations to
die down.
Different settings for opposite feeder ends.
Typically 5 to 60 secs.
Synchronizing Check
Synchronism & Voltage Check
Radial single feeder, 3Ø tripping caused lost interconnection between both
line ends. Re-closing require voltage check on the least dead time end and
synchro check on the other end.
On interconnected systems - little chance of complete loss of synchronism
after fault and disconnection of a single feeder.
Phase angle difference may change to cause unacceptable shock to system
when line ends are re-connected.
VB
VL VB
VL
VL = 0 Dead time = 3.0 sec. Dead time = 3.5 sec. VL = live
VB = live VB = live
∴ Dead Line Charge ∴ Synch Check
Check Synchronising
Used when system is non radial.
Check synch relay usually checks 3 things:
1) Phase angle difference
2) Voltage
3) Frequency difference
