A MINI PROJECT ON TRANSMISSION LINE PROTECTION

BY:MOHD MOIZUDDIN4-2, B.TECH EEE, SHADAN COLLEGE OF ENGINEERING & TECHNOLOGY

Classification of Transmission Lines

0Short Lines Length < = 80kM

0Medium Lines Length < = 240kM

0Long Lines Length > 240kM

Types of faults in the transmission system

0Short circuit faults Frequency

0Phase – Ground faults 85%0Phase- Phase faults 8%0Phase – Phase –Ground faults 5%03 Ph faults 2%

0Open circuit faults

0Broken conductor0Open jumper

Protection Scheme

0Protection 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.

0Types of main Protections:

• Over Current Protection.

• Differential or Phase Comparison or Unit Protection.

• Distance Protection.

0Requirements of 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.

• Max. operating time under given conditions shall be as follows

1. SIR: 4, Setting: 10Ohms, Fault location : 60% Trip contacts: 4, Fault resistance: 0

Max.Operating time including trip relays: 45ms

2. SIR: 15, Setting: 2 Ohms, Fault location : 60% Trip contacts: 4, Fault resistance: 0;

Max.Operating time including trip relays: 45 ms (3ph fault), 60ms (others)

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

0 Current Phase comparison type

0 Suitable for operation with PLCC

0 High sensitivity and selectivity for all types of faults

0 Capable of single and three pole tripping.

0 Un effected By:

0 Heavy load transfer0 Power swings 0 CT saturation0 CT Phase errors0

Distance Protection

0Type of distance relays0Reactance

0 Suitable for short lines0 Not effected by fault resistance0 Effected by power swings0 Non directional

0Impedance0 Suitable for medium lines0 Non directional0 Effected by fault resistance

0Mho0 Directional0 Least effected by power swings0 Less effected by fault resistance

Impedance Relay Characteristics

LoadArea

X

RZ1

Z2

Z3

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 .

= 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.

= 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.

= 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.

= 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

Terms associated with distance protection

Reach:

Reach is the impedance of the tr. line up to which the distance relay protects the line from the faults.

over reach Relay measured impedance less than the actual fault Impedance. i.e effective reach of the relay increases

Under reach

Relay measured impedance is more than actual impedance.i.e. Effective reach of the relay decreases

Parallel Compensation0Necessity of parallel compensation:

For the fault on the parallel line, fault current also fed from healthy line and 0 this current pass through ground. This current changes the mutual 0 inductance and in turn causes relay measuring impedance to increase and 0 is more than actual fault impedance.

This effect will be compensated by connecting neutral current of the line to0 parallel line.

This compensation will not work, if the parallel line neutral current is more0 than line neutral current.

ZONE - I CHARACTERISTICS Should isolate the faulted section instantaneously

Should cover protected circuit &fault resistance with some margin to take care of errors in measurement

Selective phase tripping for 1ph to ground faults for auto reclose of the breaker.

Fast operation with secured directional discrimination.

Should be stable in the presence of:

1. Fault resistance, arc resistance and tower footing resistance2. Power swings3. Heavily loaded condition4. In feed condition5. Errors in CT, CVT and CT saturation6. Transients in CT & CVT

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

CARRIER SCHEMES - P U R

CHANNEL

Z1A

Z2A

Z1B

Z2B

A

B

CARRIER

RELAYRELAY

Fault

Trip = Z1 + Z2.CR+Z3.T3+Z2.T2CS = Z1Under reaching zone sends carrier signal

CARRIER SCHEMES - P O R

RELAY

Z1A

Z2A

Z1B

Z2B

A

B

CARRIER RELAY

Fault

Trip = Z1+Z2.CR+ Z2.T2+Z3.T3CS = Z2 Over reaching zone sends carrier signal

WEAK END FEED

It is a condition which occurs on a line when no current infeed from the line terminal or when the current infeed is low due to weak Generation behind protection.

If the fault current infeed is too low to operate the protection at the week end, the following might occur, depending on The selected communication scheme.

In permissive schemes or blocking schemes CB at week infeedend may fail to trip instantaneously, due to no relay operationin the weak end.

In permissive over reach scheme, if the fault is in Zone 2 from stronger end, both CBs may fail to trip instantaneously due tono relay operation in the weak end.

WEAK END FEED LOGIC IN P.O.R

Fault in Z2A+UV relay + ReceiptOf echo signal etc = Trip breaker A + Send echo signal

Fault in Z1B= Trip CB BFault in Z2B=Sends Echo signal

Z1A

Z2A

Z1B

Z2B

A

B

CARRIER RELAYRELAY

FaultWeakEnd

AUTORECLOSE – PHILOSOPHY

NEED FOR AUTO RECLOSE

1. REDUCING OUTAGE TIME

2. IMPROVED RELIABILITY

3. RESTORATION OF NETWORK STABILITY AND

SYNCHRONISM

TYPES OF FAULTS

1. TRANSIENT FAULTS

2. SEMI PERMANENT FAULTS

3. 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

SEMI PERMANENT FAULTS

This type of faults requires more than one De energised interval before it disappears. Such faults are prevalent on EHV lines traversing forest.

An example is a tree falling on the line and getting burnt up by the arc when the line is re energised.

10% of the reclosures are successful with second shot. However this can cause unnecessary wear on EHV CBs.Therefore second shot is not recommended for EHV Systems.

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 Swing

0Power Swings are disturbances in system due to various reasons 0such as sudden load throw, bad synchronization etc

0Power swings are characterized by slow power flow oscillations,0resulting in swinging of voltages and currents, resulting in 0operating point movement into distance relay characteristics, 0in turn can cause tripping of distance relays.

0Tripping during power swings is undesirable since no actual fault 0is present and moreover a line outage during power swing may0cause further deterioration to system stability.

0Detection of power swing will block the distance protection0Zones 2,3,4. Normally tripping in Zone-I is not blocked even after detection of power swing.

X

R

Z1

Z2

Z3Power 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 Function0Asymmetrical measuring voltage failure:

Substantial asymmetry of measured voltage, while the measured

0 currents are in symmetry indicates fuse fail

Asymmetry of voltage detected by 3Uo or U2 > threshold

Symmetry in current detected by 3Io or I2 < threshold

During blocking of distance protection by fuse fail, the distance 0 protection switched to emergency over current function automatically.

0 If the asymmetry in measured current is detected during blocking by FF

0 function, then FF block will released.

Switch on to fault

0This feature provide protection against energisation of the tr. line with fault or dead short.0Distance protection will not provide protection in 0this case as voltage is not available for distance measurement.0It can be activated by TNC switch or CB aux. binary input or internal detection of current rise.0It provides instantaneous 3Ph trip and blocks auto reclose.

One and Half Breaker Scheme

Ckt-1 Ckt-2

Bus-1

Bus-2

21

Stub Protection

DEF Protection

0It provides back up protection for tr. line.0It provides reliable protection for high resistance earth faults.

0It uses cross polarized voltage for directional discrimination.

Local breaker back up protection

0It is the secondary protection

0To provide back up isolation during failure of breaker to open.

0It opens source to that breaker (i.e other end breakers, bus bar, etc.)

0It will be triggered by operation of any primary protection (like distance, DEF, bus bar, etc..)

0It sends direct trip command to other end.

Direct trip Scheme

0 It is required to trip other end breakers without any 0 checking the status at other end during following 0 conditions:

• Operation of over voltage protection.• Operation of bus bar with tie breaker open.• Manual tripping of both the breakers (main&Tie)• Operation of LBB

On receipt of command through PLCC at other end breakers will trip directly.

Over Voltage Protection

0It will have 2 stages

0Stage-I:

0Setting: 110%0Time delay: 5 Sec.

0Stage –II

0Setting: 140%

0Time delay: Instantaneous.