Download UETTDRSO15A Operate and monitor system equipment (SCADA)

UETTDRTS01B
MAINTAIN NETWORK
PROTECTION & CONTROL
SYSTEMS
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3. Interdependent Protection and
their Applications
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TOPICS
• Inter-tripping schemes
• Bus-bar protection including breaker failure
backup and blocking schemes
• Pilot wire including phase comparison
• Distance protection scheme
• Reclosing and check-synchronising
• Tie-line protection and load shedding
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Inter-Tripping Schemes
• Inter-tripping is the controlled tripping of a circuit breaker
• The main use of this scheme is to Isolate both sides of
the faulty circuit
• This Schemes are used during the following
circumstances
– A feeder with a weak in feed at one end, insufficient to operate
the protection for all faults
– Feeder protection applied to transformer –feeder circuits
– Faults between the CB and feeder protection CT’s, when these
are located on the feeder side of the CB
– For high reliability EHV protection schemes, inter-tripping may be
used to give back-up
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Inter-Tripping Schemes
(Cont…)
• Direct tripping
– In direct tripping applications, inter-trip signals
are sent directly to the master trip relay
• Permissive tripping
– Permissive trip commands are always
monitored by a protection relay
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Inter-Tripping Schemes
(Cont…)
• Blocking scheme
–
Blocking commands are initiated by a
protection element that detects faults
external to the protected zone
• Purpose of Inter-tripping in transformer
–
In order to ensure that both the high and low
voltage circuit breakers operate for faults
within the transformer and feeder
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Inter-Tripping Schemes
(Cont…)
Application of
protection
signalling and
its relationship
to other
systems using
communication
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Inter-Tripping Schemes
(Cont…)
• Certain types of fault produce insufficient current
to operate the protection associated with one of
the circuit breakers. These faults are
– Faults in the transformer that operate the Buchholz
relay and trip the local low voltage circuit breaker
– Earth faults on the star winding of the transformer
– Earth faults on the feeder or high voltage delta
connected winding
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Bus-bar Protection
•
Bus-bars have often been left without specific
protection, for one or more of the following
reasons
–
–
–
The bus-bars and switchgear have a high degree of
reliability, to the point of being regarded as
intrinsically safe
It was feared that accidental operation of bus-bar
protection might cause widespread dislocation of
the power system
It was hoped that system protection or back-up
protection would provide sufficient bus protection if
needed
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Bus-bar Protection (Cont…)
• Bus-bar faults
– The majority of bus faults involve one phase and earth
• The special features of bus-bar protection are as
follows
• Speed: Bus-bar protection is primarily concerned
with
– Limitation of consequential damage
– Removal of bus-bar faults in less time than could be
achieved by back-up line protection
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Bus-bar Protection (Cont…)
• Stability
– The stability of bus protection is of paramount
importance
– Dangers exist in practice for a number of reasons.
These are
• Interruption of the secondary circuit of a current transformer
will produce an unbalance, which might cause tripping on
load
• A mechanical shock of sufficient severity may cause
operation, although the likelihood of this occurring with
modern numerical schemes is reduced
• Accidental interference with the relay, arising from a mistake
during maintenance testing, may lead to operation
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Bus-bar Protection (Cont…)
• A number of bus-bar protection systems
have been devised
–
–
–
–
–
System protection used to cover bus-bars
Frame-earth protection
Differential protection
Phase comparison protection
Directional blocking protection
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Bus-bar Protection (Cont…)
• Busbar blocking system
– Advantages
•
•
•
•
•
•
•
Very low or no cost system
Simple
Faster than faults cleared by back-up relays
Covers phase and ground faults
Adequate sensitivity–independent of no. of circuits
No additional CTs
Commissioning is simple–no primary current
stability tests
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Bus-bar Protection (Cont…)
– Disadvantages
• Only suitable for
simple busbars
• Additional relays
and control wiring
for complex
busbars
• Beware of motor infeeds to busbar
faults
• Sensitivity limited
by load current
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Busbar blocking system
Bus-bar Protection (Cont…)
•
System protection schemes
–
–
•
System protection includes over-current or distance
systems
It will inherently give protection cover to the busbars
Frame-earth protection (HOWARD
PROTECTION)
–
–
Frame leakage protection has been extensively
used in the past in many different situations.
There are several variations of frame leakage
schemes available, providing bus-bar protection
schemes with different capabilities
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Bus-bar Protection (Cont…)
Single zone frame-earth protection
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Current distribution for external fault
Bus-bar Protection (Cont…)
Three zone frame earth scheme
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Frame-earth scheme: Bus section
breaker insulated on one side only
Bus-bar Protection (Cont…)
• For satisfactory operation of supervisory
protection schemes audible and visual
alarms are used. These alarms are used
for
–
–
–
–
–
Bus-bar faults
Bus-bar protection in service
Bus-bar protection out of service
Tripping supply healthy
Alarm supply healthy
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Bus-bar Protection (Cont…)
• Numerical Bus-bar protection schemes
• Differential protection requires sectionalizing the
busbars into different zones
– High impedance bus zone
– Advantages
•
•
•
•
Relays relatively cheap – offset by expensive CTs
Simple and well proven
Fast–15…45 m secs
Stability and sensitivity calculations easy, provided data is
available
• Stability can be guaranteed
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Bus-bar Protection (Cont…)
– Disadvantages
•
•
•
•
•
•
•
•
•
•
•
•
Very dependent on CT performance
CT saturation could give false tripping on through faults
Sensitivity must be decreased
DC offset of CTs unequal–use filters
Expensive class X CTs–same ratio–Vknp = 2 times relay setting
Primary effective setting (30...50%)
Limited by number of circuits
Z grounded system difficult for ground fault
Duplicate systems–decreased reliability
Require exact CT data
Vknp, Rsec, imag, Vsetting
High voltages in CT circuits (+/− 2.8 kV) limited by volt dependent
resistors
• Biased medium impedance differential
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Bus-bar Protection (Cont…)
– Advantages
•
•
•
•
•
•
•
•
•
•
•
•
•
High speed 8…13 m secs
Fault sensitivity +/− 20%
Excellent stability for external faults
Normal CTs can be used with minimal requirements
Other protection can be connected to same CTs
No limit to number of circuits
Secondary voltages low (medium impedance)
Well proven 10000 systems worldwide
Any busbar configuration
No need for duplicate systems
Retrofitting easy
No work on primary CTs
Biasing may prevent possibility of achieving a sensitive enough
ground fault setting of Z grounded systems
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Bus-bar Protection (Cont…)
– Disadvantages
• Relays relatively expensive
• Offset by minimal CT requirements
• Relays with auxiliary CTs require a separate panel
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Bus-bar Protection (Cont…)
• Available protection functions are:
– backup over-current protection
– breaker failure
– dead zone protection
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Bus-bar Protection (Cont…)
Architecture for numerical protection scheme
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Pilot wire Including Phase
Comparison
• Following are the reasons for not using current
differential relaying
– The likelihood of improper operation owing to CT
inaccuracies under the heavy loadings that would be
involved
– The effect of charging current between the pilot wires
– The large voltage drops in the pilot wires requiring
better insulation
– The pilot currents and voltages would be excessive for
pilot circuits rented from a telephone company
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Pilot wire Including Phase
Comparison (Cont…)
• Purpose of a pilot
Transmission-line sections for illustrating the purpose of a pilot
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Pilot wire Including Phase
Comparison (Cont…)
• D-C wire-pilot relaying
Schematic illustration of a
d-c wire-pilot relaying
equipment
D = voltage-restrained
directional (mho) relay
O = over-current relay
T = auxiliary tripping relay
S = auxiliary supervising
relay
PW = pilot wire
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Pilot wire Including Phase
Comparison (Cont…)
Schematic illustration of a
d-c wire-pilot scheme where
information is transmitted
over the pilot
D = voltage-restrained
directional (mho) relay
B = auxiliary blocking relay
O = over-current relay
TC = trip coil
PW = pilot wire
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Pilot wire Including Phase
Comparison (Cont…)
• A-C wire-pilot relaying
Schematic illustration of the circulating-current principle of a a-c wirepilot relaying
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Pilot wire Including Phase
Comparison (Cont…)
Schematic illustration of the opposed-voltage principle of a-c wirepilot relaying
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Pilot wire Including Phase
Comparison (Cont…)
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Effect of
Shorts
Effect of Open
Circuits
Opposed
voltage
Cause tripping
Block tripping
Circulating
current
Block tripping
Cause tripping
Pilot wire Including Phase
Comparison (Cont…)
Schematic connections of a circulating-current a-c wire-pilot relaying equipment
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Pilot wire Including Phase
Comparison (Cont…)
Schematic
connections of an
opposed-voltage ac wire-pilot relaying
equipment
P = current
polarizing coil
R = voltage
restraining coil
O = current
operating coil
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Distance Protection Schemes
•
•
The most important and versatile family of
relays is the distance-relay group
It includes the following types
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–
–
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–
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Impedance relays
Reactance relays
Mho relays
Angle impedance relays
Quadrilateral relays
Elliptical and other conic section relays
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Distance Protection Schemes
(Cont…)
Basic principle of operation
I f  E / Z f  Z s 
V f  E  Z f /( Z f  Z s )
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Distance Protection Schemes
(Cont…)
Balanced beam principle
Bridge comparator
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Plain impedance characteristic
Distance Protection Schemes
(Cont…)
• Tripping characteristics
MHO characteristic
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Offset MHO characteristic
Distance Protection Schemes
(Cont…)
• Application onto a power line
Zone 1 MHO characteristic
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3 Zone MHO characteristics
Distance Protection Schemes
(Cont…)
• Effect of load current
Load encroachment
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Effect of arc resistance
Distance Protection Schemes
(Cont…)
• Different shaped characteristics
Lenticular characteristic
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Distance Protection Schemes
(Cont…)
Figure-of-eight
characteristic
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Trapezoidal
characteristic
Reclosing & Check
Synchronising
• Reclosers
– Adjacent reclosers can be coordinated more closely
since there are no inherent errors
• Self contained pole mounted ARC
– The following are the system service conditions which
are suitable for using AR on non-effectively earthed
and effectively earthed networks
•
•
•
•
Nominal system voltage : 11 kV
Maximum system voltage : 15 kV
Continuous current rating : 630 A
Short circuit-breaking capacity: 12.5 kA
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Reclosing & Check Synchronising
(Cont…)
• Rated power frequency withstand voltage :
50kVrms
• Impulse withstand voltage : 125 kVp
• System frequency : 50 Hz; + 1.5
• Number of phases : 3
• Insulation medium : Solid dielectric
• Operating mechanism :Magnetic actuator
• Ambient temperature (minimum) : 5 °C
• Ambient temperature (maximum) : 50 °C
• Max. Relative humidity. : 100%
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Reclosing & Check
Synchronising (Cont…)
• Check Synchronising
– Faults on overhead lines fall into one of three
categories:
• Transient
• Semi-permanent
• Permanent
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Reclosing & Check
Synchronising (Cont…)
Single-shot auto-reclose scheme operation for a transient fault
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Reclosing & Check
Synchronising (Cont…)
Operation of single-shot auto-reclose scheme on a permanent fault
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Reclosing & Check
Synchronising (Cont…)
•
Application of auto-reclosing
–
The most important parameters of an auto-reclose scheme are
•
•
•
•
Dead time
Reclaim time
Single or multi-shot
Advantages of using Auto-reclosing on HV distribution Networks
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–
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Reduction to a minimum of the interruptions of supply to the
consumer
Instantaneous fault clearance can be introduced, with the
accompanying benefits of shorter fault duration and fewer permanent
faults
Improved supply continuity
Reduction of substation visits
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Reclosing & Check
Synchronising (Cont…)
• Several factors affect the selection of
system dead time as follows
–
–
–
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System stability and synchronism
Type of load ,CB characteristics
Fault path de-ionisation time
Protection reset time
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Reclosing & Check
Synchronising (Cont…)
• Factors affecting the setting of the reclaim
time are
–
–
Type of protection
Spring winding time
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Reclosing & Check
Synchronising (Cont…)
• Auto-reclosing on EHV transmission lines
Effect of high-speed three-phase auto-reclosing on system stability for a weak system
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Reclosing & Check
Synchronising (Cont…)
• De-Ionisation of Fault Arc
– The de-ionisation time of an
uncontrolled arc, in free air
depends on
• The circuit voltage
• Conductor spacing
• Fault currents
• Fault duration
• Wind speed
• Capacitive coupling from
adjacent conductors
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Line
voltage
(kV)
Minimum deenergisati
on time
(seconds)
66
0.2
110
0.28
132
0.3
220
0.35
275
0.38
400
0.45
525
0.55
Fault-arc de-ionisation times
Reclosing & Check
Synchronising (Cont…)
• The advantages of single-phase autoreclosing are
– The maintenance of system integrity
– On multiple earth systems, negligible
interference with the transmission of load
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Reclosing & Check
Synchronising (Cont…)
Typical
three
zone
distance
scheme
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Reclosing & Check
Synchronising (Cont…)
Delayed autoreclose
scheme logic
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Reclosing & Check
Synchronising (Cont…)
•
The synchronism
check relay element
commonly provides
a three-fold check
–
–
–
•
Phase angle
difference
Voltage
Frequency
difference
Auto-close scheme
Standby transformer with auto-closing
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Tie-Line Protection and Load
Shedding
•
Special protection schemes
–
•
System protection schemes are protection strategies designed
to detect a particular system condition that may cause unusual
stress to the power system
The most common types of system protection schemes
are as follows
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–
–
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Generator rejection
Load rejection
Under frequency load shedding
System separation
Turbine valve control
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Tie-Line Protection and Load
Shedding (Cont…)
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Load and generator rejection
Stabilizers
HVDC controls
Out of step relaying
Discrete excitation control
Dynamic braking
Generator runback
VAR compensation
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Tie-Line Protection and Load
Shedding (Cont…)
• The main objectives of using system
protection schemes are
– Operation of power systems closer to their
limits
– Increase power system security particularly
during extreme contingencies
– Improve power system operation
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Tie-Line Protection and Load
Shedding (Cont…)
• Estimation of rate of change of frequency (df/dt)
2H df
  Dx f  P
f 0 dt
•
•
•
•
•
•
•
•
H = System Inertia Constant on system base (seconds)
f o = Frequency at the time of disturbance (Hz)
df/dt =Rate of change of frequency (Hz/Sec)
Δ P =( PL-PG)/PG , Power change (per unit in system load base)
PL =Load prior to generation Loss in MW
PG =System Generation after Loss in MW
D =Power/frequency characteristic of the system in pu/Hz
Δf =Change in frequency
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Tie-Line Protection and Load
Shedding (Cont…)
• The overload may be differentiated by the
method that is used to detect and respond to
condition
–
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Dispersed frequency monitoring
Dispersed voltage monitoring
Utility Scale SCADA System Configuration Monitoring
Industrial Scale System Configuration Monitoring
Local equipment overload monitoring
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Tie-Line Protection and Load
Shedding (Cont…)
•
Methods of overload detection and load shedding
–
–
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By Frequency Monitoring
By Voltage Monitoring
By SCADA System Monitoring
•
•
–
•
Application to Utility Scale Systems
Application to Smaller Industrial Facilities
By Current and/or Power Monitoring
Drawbacks of breaker interlock load shedding
–
–
–
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Load shedding based on worst-case scenario
Only one stage of load shedding
Almost always, more load is shed than required
Modifications to the system are costly
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Tie-Line Protection and Load
Shedding (Cont…)
• Pre-disturbance operating conditions
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Total system load demand
Total system power exchange to the grid
Generation of each on-site unit
Spinning reserve for each on-site unit
Control settings for each running unit
System configurations
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Tie-Line Protection and Load
Shedding (Cont…)
• Post-disturbance operating conditions
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–
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New system load demand
Remaining generation from on-site generation
Spinning reserve for each remaining unit
Time duration to bring up the spinning reserve
New system configurations
Status, settings and loading conditions of the
remaining major rotating machines
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Tie-Line Protection and Load
Shedding (Cont…)
• Nature and duration of the disturbance
–
–
–
–
–
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Electrical and/or Mechanical faults
Complete or partial loss of power grid connection
Complete or partial loss of on-site generation
Load addition (impact)
Location of disturbance
Duration of disturbance and its termination
Subsequent system disturbances
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Tie-Line Protection and Load
Shedding (Cont…)
• System transient response to a
disturbance
–
–
–
–
System frequency response
System voltage response
Rotor angle stability of each remaining unit
Operation of protective devices
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Tie-Line Protection and Load
Shedding (Cont…)
• Load shedding system can be designed to meet the following
objectives
– Map a complex, highly nonlinear, nonparametric, load shedding
problem, to a finite space with a limited number of data collection points
– Automatic recall of system configuration, operating condition, and
system response to disturbances
– Pattern recognition capability to predict system response to disturbances
– Systems knowledgebase trainable by user defined cases
– Self-learning capability to new system changes
– Make prompt decisions regarding which loads to shed based on the
online status of sheddable loads
– Shed the minimum amount of load to maintain system stability
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Tie-Line Protection and Load
Shedding (Cont…)
Load Shedding scheme function Block diagram
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