Unit-III Apparatus Protection:

Md Irshad AhmadIrshad.ahmad@jit.edu.in

Electrical Engineering DepartmentSubject: NEE –602: SWITCHGEAR AND

PROTECTION

Unit-III Protection of

Transmission Line: Over current protection Distance protection, Pilot wire protection Carrier current protection Protection of bus Auto re-closing,

As the fault impedance is less than load impedance, the fault current is more than load current. If a short circuit occurs the circuit impedance is reduced to a low value and therefore a fault is accompanied by large current.

Over-current protection is that protection in which the relay picks up when the magnitude of current exceeds the pickup level.

The Over-current relays are connected to the system, normally by means of CT's

Over current protection

1.      High speed Over-current protection.

2.      Definite time Over-current protection.

3.      Inverse minimum time Over-current protection.

4.      Directional Over-current protection

(of above types).

Over-current relaying has following types:

Construction and working principle of instantaneous over current relay quite simple.

Magnetic core is wound by current coil. A piece of iron is so fitted by hinge support and restraining spring in the relay, that when there is not sufficient current in the coil, the NO contacts remain open. When current in the coil crosses a present value, the attractive force becomes sufficient to pull the iron piece towards the magnetic core and consequently the no contacts are closed.

Instantaneous Over Current Relay

Definite Time Over Current Relay This relay is created by applying intentional time delay after

crossing pick up value of the current. A definite time over current relay can be adjusted to issue a trip output at definite amount of time after it picks up. Thus, it has a time setting adjustment and pick up adjustment.

Inverse Time Over Current Relay Inverse time is a natural character of any induction type

rotating device. This means the speed of rotation of rotating art of the device is faster if input current is increased.

Inverse Definite Minimum Time Over Current Relay or IDMT O/C Relay

Inverse time characteristics can not be achieved, in an over current relay. As the current in the system increases, the secondary current of the current transformer is increased proportionally. The secondary current is fed to the relay current coil. But when the CT becomes saturated

Classification of Transmission Lines

Short Lines Length < = 80kM

Medium Lines Length < = 240kM

Long Lines Length > 240kM

Types of faults in the transmission system

Short circuit faults Frequency

Phase – Ground faults 85%Phase- Phase faults 8%Phase – Phase –Ground faults 5%3 Ph faults 2%

Open circuit faults

Broken conductorOpen jumper

Protection SchemeProtection Scheme for Transmission lines as per CBIP guidelines• Should have two independent high speed main protection schemes• Two stage over voltage protection• Sensitive IDMT directional E/F relays• Auto reclose relay suitable for 1 ph/3ph (with deadline charging and synchro check) reclosure.

Types of main Protections:

• Over Current Protection.

• Differential or Phase Comparison or Unit Protection.

• Distance Protection.

• Shall have min. of three independent zones with directional characteristics.• Shall be non switched type with separate measurement for both earth faults

and phase faults• Capable of 1phase and 3 phase tripping.• Capable of operation for close up faults and switch on to faults• Accuracy of better than 5% of reach setting for Zone 1, 10% for Zone-2 &3.• Shall have variable residual compensation.• Shall include power swing detection feature for selectively blocking.• Shall include fuse failure feature to monitor all types of fuse failures and block

distance protection.

Requirements of distance protection:

If = E/(ZS+ZL)

The reach of over current relay is function of Source Impedance which varies considerably, making it difficultto get fast and Selective tripping .

E ZS ZL

If

X XXXX

Over Current Protection

Phase Comparison Protection

Current Phase comparison type Suitable for operation with PLCC High sensitivity and selectivity for all types of

faults Capable of single and three pole tripping. Un effected By: Heavy load transfer Power swings CT saturation CT Phase errors

Distance ProtectionType of distance relaysReactance Suitable for short lines Not effected by fault resistance Effected by power swings Non directionalImpedance Suitable for medium lines Non directional Effected by fault resistanceMho Directional Least effected by power swings Less effected by fault resistance

Impedance Relay Characteristics

LoadArea

X

RZ1

Z2Z3

MHO relay characteristic The characteristic of a mho impedance element , when plotted on a R/X diagram, is a circle whose circumference pass through the origin .

Y

Y = relay characteristic angle

R

X

OFF set MHO characteristic Under close up faults, when the voltage is near to zero then MHO will not operate. The mho characteristic can be shifted towards origin for operation of close up faults. This is know as OFF set MHO.

Y

Y = relay characteristic angle

R

X

Load

Lenticular characteristics The characteristic of lenticular mho will be useful to provide maximum load transfer condition with maximum fault resistance coverage.

Y

Y = relay characteristic angle

Z-1

Z-2

Z-3

Z-3

R

Quadrilateral characteristic It is a basically a reactance relay superseded with controlled resistive reach.

Y

Y = relay characteristic angle

Z-1&2

Z-1

Z-2

Z-3

Z-3

Zones of Distance Protection:

Z1Z2

Z3

BASIC SETTING PHILOSOPHY

ZONE –1 : 80 % of protected line ZONE –2 : 100 % of protected line + 20 % of shortest adj. line section or 100% + 50% of transformer impedance

ZONE –3 : 100% of protected line + 100 % of longest adj. line or 100 % + 100% of transformer impedance.

ZONE -4 : To cover close up back-up non-directional faults generally reverse reach will be provided in relays (10%).

X X X X X X

Parallel CompensationNecessity of parallel compensation:

For the fault on the parallel line, fault current also fed from healthy line and this current pass through ground. This current changes the mutual inductance and in turn causes relay measuring impedance to increase and is more than actual fault impedance.

This effect will be compensated by connecting neutral current of the line to parallel line.

This compensation will not work, if the parallel line neutral current is more than line neutral current.

ZONE-II CHARACTERISTICS Delayed tripping and non selective phase tripping.

Provide back up protection for part of adjacent line.

Trip the faulty line instantaneously using carrier aided tripping.Time delay is normally 500ms

ZONE-III CHARACTERISTICSThis provides back up protection for the adjacent lines or transformer

Time delay is normally 1500ms

ZONE –IV CHARACTERISTICSThis provides back up protection for the station faults

It is normally in the reverse direction

Time delay is normally 1500ms

Distance Schemes:

1 . P . U . R -- Permissive under reach scheme

2. P . O . R -- Permissive Over Reach scheme

3. BLOCKING SCHEME

4. WEAK END FEED

AUTORECLOSE – PHILOSOPHY

NEED FOR AUTO RECLOSE

1. REDUCING OUTAGE TIME 2. IMPROVED RELIABILITY 3. RESTORATION OF NETWORK STABILITY AND SYNCHRONISM

TYPES OF FAULTS

4. TRANSIENT FAULTS 5. SEMI PERMANENT FAULTS 6. PERMANENT FAULTS

TRANSIENT FAULTS -CHARACTERISTIC

Chracterised by disappearnance after Short dead time and aredisapper without any action being taken.

TYPES OF TRANSIENT FAULTS

1. Lightning strokes resulting in fashovers 2. Conductor swinging due to high winds3. Bird fault 4. Temporary contact with foreign objects like tree etc.

About 85 % of faults on transmission lines are transient in nature

DEAD TIME :

The time between the autoreclose scheme being energised and the operation of the contacts which energise the CB closing Circuit.

RECLAIM TIME :

The time following a successful closing operation measured from the instant the A/R relay closing contacts make, which must elapse before the autoreclose relay will initiate reclosingsequence in the event of a further fault.

CHOICE OF RECLAIM TIME The reclaim time must not be set to such a low value thatthe intended operating cycle of the breaker is exceededwhen two fault incidents occurs close together.

for example the reclaim time for a air blast circuit breakersmust allow time for air pressure to recover to its normal value.

CHOICE OF DEAD TIME Dead time for EHV system lower limit is decided by de-ionising time, upper limit is decided by transient stability and synchronism

Power SwingPower Swings are disturbances in system due to various reasons such as sudden load throw, bad synchronization etc

Power swings are characterized by slow power flow oscillations,resulting in swinging of voltages and currents, resulting in operating point movement into distance relay characteristics, in turn can cause tripping of distance relays.

Tripping during power swings is undesirable since no actual fault is present and moreover a line outage during power swing maycause further deterioration to system stability.

Detection of power swing will block the distance protectionZones 2,3,4. Normally tripping in Zone-I is not blocked even after detection of power swing.

X

R

Z1

Z2Z3

Power swing detection zones

Power Swing detection

Time taken by fault locus to cross the power swing detection zones is more than 40-50ms, then it is called power swing.

Fuse Failure FunctionAsymmetrical measuring voltage failure:

Substantial asymmetry of measured voltage, while the measured currents are in symmetry indicates fuse failAsymmetry of voltage detected by 3Uo or U2 > thresholdSymmetry in current detected by 3Io or I2 < thresholdDuring blocking of distance protection by fuse fail, the distance protection switched to emergency over current function automatically.

If the asymmetry in measured current is detected during blocking by FF function, then FF block will released.

Switch on to fault

This feature provide protection against energisation of the tr. line with fault or dead short.Distance protection will not provide protection in this case as voltage is not available for distance measurement.It can be activated by TNC switch or CB aux. binary input or internal detection of current rise.It provides instantaneous 3Ph trip and blocks auto reclose.

One and Half Breaker SchemeCkt-1 Ckt-2

Bus-1

Bus-2

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Stub Protection

DEF Protection

It provides back up protection for tr. line.It provides reliable protection for high resistance earth faults.It uses cross polarized voltage for directional discrimination.

Carrier current protection scheme is mainly used for the protection of the long transmission line.

In the carrier, current protection schemes, the phase angle of the current at the two phases of the line are compared instead of the actual current.

Phase angle of the line decides whether the fault is internal and external.

The main elements of the carrier channel are a transmitter, receiver, coupling equipment, and line trap

Carrier Current Protection of Transmission Lines

In Absence Of BUSBAR Protection, Fault Clearance takes place

in Zone II of Distance Relay by Remote End Tripping

This Means Slow & Unselective Tripping and wide spread blackout

NEED FOR BUS BAR PROTECTION

Minimizing damage at fault location

Maintaining system stability

Localizing isolation to avoid wide spread disruption

Delayed clearance create shock to inter connected equipment like Generator shaft and windings of Transformer

NEED FOR BUS BAR PROTECTION

High speed operation

Selectivity – shall isolate the faulty bus bar only

Stability – stable for through faults upto 40 KA fault level

Reliability – Check feature

Applicable for any type of bus bar protection

Shall provide zone indication

REQUIREMENT OF BUS BAR PROTECTION

Continuous supervision for CT Secondaries against any possible Open Ckt.

In case of detection of any Open Circuiting of CT secondary, after a time delay, the affected zone of protection shall be rendered(provide) in-operative and an alarm initiated.

REQUIREMENT OF BUS BAR PROTECTION

Bus bar protection must be provided in all new 400kV and 220kV Substations as well as Generation Switch Yards

For existing substations, provision of bus bar protection is considered a must at 400kV level and for 220kV level it is essential at substations having multiple feed.

In case of radially fed 220kv substations, having more than one bus it is desirable to have bus bar protection but is not a must

RECOMMENDATIONS FOR PROVIDING BUS BAR PROTECTION

NEED FOR BUSBAR PROTECTION

In Its Absence Fault Clearing Takes Place In Zone-2 Of Distance Relay By Remote End Tripping. this means slow and unselective tripping and wide spread blackout.

EFFECT OF DELAYED CLEARENCE

Greater damage at fault point.

Indirect shock to connected equipment like shafts of generator and windings of transformer.

BASIC THEORY

Kirchoff’s current law states that the sum of the currents entering a given node must be equal to the currents leaving at that node.

BASICS OF BUSBAR PROTECTION

TYPES OF BUS BAR PROTECTION

Low Impedance High Impedance

Both use Circulating Current Differential Principle - To isolate the entire bus bar section by disconnecting all the feeders connected to the bus.

HIGH SPEED DIFFERENTIAL PROTECTION

Simple Current Differential Circuit:

Simple Current Differential Circuit with Bus Fault

Simple Current Differential Circuit with Close in Line Fault

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AUTO-RECLOSING It is placed on over head transmission line and nearer

to the CB . It is also one of the protective relay.

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What is Auto – Reclosing ?

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Introduction to auto-reclosing scheme

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Types Of Faults

• Transient Faults• Semi Permanent faults• Permanent Faults

Utility Substation Customers

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STEPS TO BE FOLLOWED IN AUTORECLOSING

Visual Check

Check of thePower Supply

Test of the Auto-re-closure settings

Functional Testing (involving operation of

Protective Relays & Circuit Breaker(s)

INSTALLATION

OPERATION

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Scheme Of Operation

• Live bus\ Dead Line Charging• Other end closing under synchronism check relay• Live bus\ Live line charging

Types of Auto Reclosing1. Medium voltage auto-reclose

• Aim of continuity of supply• the obvious advantages are continuous supply except for short duration when tripping and re-closure operations are being performed.• high speed protection decreases the amount of damage incurred and thus increases the probability of auto reclosure.

2. High voltage auto-reclose• main considerations are of stability and synchronizing.• the fault levels associated are extremely high.• High speed re-closure in high voltage circuits improves the stability to a considerable extent on single-circuit ties.

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FACTORS TO BE CONSIDERED FOR AUTO RECLOSING SYSTEM• Protection Characteristics• De- Ionization of Fault Arc• Circuit Breaker Characteristics• Reclaim Time• Number of Shots

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SINGLE PHASE AUTO – RECLOSING

ADVANTAGES:-•Maintenance of System Integrity• Reduced switching overvoltage due to reclosing• Reduced shock to generators.• Sudden changes in mechanical output are less

• Longer De – ionization time

DISADVANTAGE:-

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1. Densely interconnected systems.

Where the Loss of a single line is unlikely to

cause loss of synchronism between two ends

2. Dead Time is allowed to be long enough for any power swings due to fault and tripping allowed to decay before reclosing

a) Less shock to system than with high speed A/R

b) Problems of arc de – ionization times, and circuit breaker operating characterstics are eliminated.

Delayed Auto-Reclose Application

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Operating feature of Auto reclosing schemes

• Dead Timer• Anti Pumping Devices• Reclaim timer• CB Lockout• Manual Closing• Multi Shot Scheme

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Auto - Reclose Blocking• Receipt of transfer Trip• Manual Trip• Breaker failure• Three phase Fault• Faults On Buses• Under frequency/ Under voltage load shedding trips

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Benefits Of Auto - Reclosing

• Minimizing the interruption of supply.• Maintenance of system stability and Synchronism.• Minimum Outage and least expenditure of manpower.• Ability to run substation unattended.