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UETTDRTS01BMAINTAIN 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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• 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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Application of protection signalling and its relationship to other systems using communication

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• 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 in-feeds to busbar faults

• Sensitivity limited by load current

Busbar blocking system

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Bus-bar Protection (Cont…)• System protection schemes

– System protection includes over-current or distance systems

– It will inherently give protection cover to the bus-bars • 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 Current distribution for external fault

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Three zone frame earth schemeFrame-earth scheme: Bus section breaker insulated on one side only

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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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– 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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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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• D-C wire-pilot relayingSchematic 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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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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• A-C wire-pilot relaying

Schematic illustration of the circulating-current principle of a a-c wire-pilot relaying

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Schematic illustration of the opposed-voltage principle of a-c wire-pilot relaying

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Effect of Shorts

Effect of Open Circuits

Opposed voltage

Cause tripping

Block tripping

Circulating current

Block tripping Cause tripping

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Schematic connections of a circulating-current a-c wire-pilot relaying equipment

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Schematic connections of an opposed-voltage a-c wire-pilot relaying equipment P = current polarizing coilR = voltage restraining coilO = 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– 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

sff ZZEI /

)/( sfff ZZZEV

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Balanced beam principle

Bridge comparator Plain impedance characteristic

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• Tripping characteristics

MHO characteristic Offset MHO characteristic

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• Application onto a power line

Zone 1 MHO characteristic 3 Zone MHO characteristics

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• Effect of load current

Load encroachment Effect of arc resistance

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• Different shaped characteristics

Lenticular characteristic

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Figure-of-eight characteristic

Trapezoidal characteristic

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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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Single-shot auto-reclose scheme operation for a transient fault

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Operation of single-shot auto-reclose scheme on a permanent fault

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• 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

– 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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• Several factors affect the selection of system dead time as follows– System stability and synchronism – Type of load ,CB characteristics – Fault path de-ionisation time – Protection reset time

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• Factors affecting the setting of the reclaim time are– Type of protection– Spring winding time

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• 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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• 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

Line voltage (kV)

Minimum de-

energisation time

(seconds)

66 0.2110 0.28132 0.3220 0.35275 0.38400 0.45525 0.55

Fault-arc de-ionisation times

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• The advantages of single-phase auto-reclosing are– The maintenance of system integrity– On multiple earth systems, negligible

interference with the transmission of load

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Typical three zone distance scheme

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Delayed auto-reclose scheme logic

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• 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– Generator rejection– Load rejection– Under frequency load shedding– System separation– Turbine valve control

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Tie-Line Protection and Load Shedding (Cont…)

– Load and generator rejection– Stabilizers– HVDC controls– Out of step relaying– Discrete excitation control– Dynamic braking– Generator runback– VAR compensation

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• 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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• Estimation of rate of change of frequency (df/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

PfDx dtdf

f2H

0

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• The overload may be differentiated by the method that is used to detect and respond to condition– 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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• Methods of overload detection and load shedding– 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

– 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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• Pre-disturbance operating conditions– 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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• Post-disturbance operating conditions– 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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• Nature and duration of the disturbance– 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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• 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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• 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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Load Shedding scheme function Block diagram

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