Relay

RELAY

When ever occurs a fault

Then the current should halt.

Otherwise, the fault current increases.

And the service continuity decreases.

Then the relay should act quick

And the circuit breaker should trip.

Thus the faulty section is disconnected

And the power system is protected.

RELAY( As per IEEE 100 -1984 )

Relay is an electric device that is designed to interpret input conditions in a prescribed manner and after specified conditions are met to respond, to cause contact operation or similar abrupt change in associated electric control circuits.

Inputs are usually electric, but may be mechanical, thermal or other quantities.

Limit switches and similar simple devices are not relays.

PROTECTIVE RELAYS

A relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action.

Fuses are also used in protection. But fuses are only over current protective device with a circuit opening fusible part that is heated and severed by the passage of the over current through it.

Thus protective relays and their associated systems are compact units of analog, discrete component and / or digital network connected through a power system for the purpose of sensing problems.

OutputAmplificationIntegrationtimings etc.

SensingSingle

orMultipleInputs

Single or

Multiple

Logic Representation of Electric Relays

The term PROTECTION does not indicate or imply that the protection equipment can prevent TROUBLE , such as fault and equipment failures. It cannot anticipate trouble.

The protective relays act only after an abnormal or intolerable condition has occurred with sufficient indication to permit their operation.

Thus protection does not mean PREVENTION, but rather minimising the duration of trouble and limiting the damage, outage time, and related problem that may otherwise result.

FIVE BASIC FACTS OF PROTECTIVE RELAYS

1. RELIABILITY :- To ensure that the protection scheme will perform correctly.

2. SELECTIVITY :- To ensure maximum continuity of service with minimum system disconnection.

3. SPEED OF OPERATION :- To ensure minimum fault duration and consequent minimum equipment damage.

4. SIMPLICITY :- To have minimum protective equipment and associated circuitry to achieve the protection objectives.

5. ECONOMY :- To provide maximum protection at minimum total cost.

CLASSIFICATION OF RELAYS

1. PROTECTIVE

2. REGULATINGGenerally don’t respond to system faultsunless faults are far too long.Associatedwith Tap changers of Transformers,Governors of generating equipments.

3. RECLOSING, SYNCHRONISM CHECK, SYNCHRONISING(PROGRAMMING)

These are related to computers & used in energising or restoring lines to service after an outage and in interconnecting pre-energised parts of system.

4. MONITORINGTo verify conditions in the poweror protective system (Alarm units)

5. AUXILLARYi ) Contact Multiplicationii) Circuit Isolation

Frequent Requirement of

Control System

i ) More outputs for multiple tripping, alarms and operating other equipment such as recording and data acquisition, lock out and so on.

ii ) Contacts that will handle higher currents or voltages in the secondary system.

iii ) Electrical and magnetic isolation of several secondary circuits.

TYPES OF RELAYS

ELECTRO MECHANICAL RELAY

These relays are based on the comparisonbetween operating torque/force and restoring torque/force.As the characteristic of such relay is limited,hence it can perform only one protective function. VA Burden of such relay is high.

ELECTRO MAGNETIC RELAY

These relays have a coil or an electromagnetenergised by coil. The coil is energised by the operating current or voltage. A plunger or rotating iron vane is subjected to the action of magnetic field produced by the operating current or voltage.These relays are operational both for AC andDC, because torque is proportional to I2 .These are fast operated relays due to smalllength of travel and light moving parts.

IDMT RELAY

The current setting is changed by plug setting to get desired number of turns in the coil.

Time multiplier setting is in the formof an adjustable back stop.

The desired time setting is obtained bychanging the relative position of contacts by adjusting the length oftravel of moving contacts.

STATIC RELAYS

In these relays, the sensing, comparison andmeasurements are made by electronic circuitshaving no moving part. These relays have low burden and versatile characteristics, thus incorporate several protective control and monitoring function in one compact unit.

Microprocessor based relays have several features such as :

i) Indication of operating values and thus no need of separate indicating instrument.ii)Reduction in number of relays as a single relay can perform even 10 different protective functions (e.g. AZ1114 ALIND RELAY)iii) Increased Reliability due to internal monitoring of own relay circuit.iv) Memory function can flash on the display the magnitude of current & instant of time at the moment of tripping.v) Extended range of application.

Fault clearing time is elapsed time between the instant of occurrence offault and the instant of final arc extinction.

F.C.T. = RELAY TIME +CB TIME

Relay Time = Instant of fault occurring to closure of trip circuit

CB Time = Closure of trip circuit to final arc extinction

Normally CB time is between 20 to 50 milliseconds (1 to 2.5 cycles)

AUTO RECLOSURE SEQUENCE

Fault Occurs

C.B. Trips

Auto ReclosureFeature

C.B. Recloses

CB Trips &Remains open

CB Remains closed

The auto reclosing of EHV lines is high speed and single shot i.e. only one reclosing is attempted

If fault persists If fault is cleared

S.N. NO. OF TIME IN OPERATION REMARKSCYCLES MILLI SEC

1 0 Fault occurs CB in closed positionProtective gear starts operating

2 0-2 0-40 Relay Time Fast relay functioning 3 2 40 Trip circuit closed Operating mechanism

starts to open4 2 to 4.5 40-90 Opening time of CB5 4.5 to 6 90-120 CB final arc extinction CB is of 4 cycles6 6 to 18 120-360 Dead time 12 cycles for De-ionization

CB remains open7 13.5 270 CB contacts starts

closing8 18 360 CB contacts touch

for reclose9 20 400 CB Reclosed CB closes in 0.4 sec

10 CB will remain closed, iffault has vanished.CB will open again, if faultstill persists & will remainlocked open.

Single shot auto reclosure complete

AUTO RECLOSURE SEQUENCE

Generally minimum time of 0.2 second must be allowed to elapse to enable the fault zone to become de-ionized completely. Hence a dead time of 0.3 second has been chosen for safe reclosure time.

In 25 KV AC Traction, time for single shot auto reclosure is set at 0.5 second i.e. before the opening of DJ of loco in 0.6 second.

OPERATING TIME OF RELAYS

(RDSO : - TI/PSI/PROTCT/CONVEN/2000 dated 1.9.2000)

Instantaneous OCR - Less than 15 ms

Mho Relay - Less than 50 ms *

Master Trip Relay - Less than 15 ms

Circuit Breaker - Less than 65 ms

* This value is at maximum torque angle of 75 0 and at 15 % less fault impedance than the relay setting.

Disadvantages of Electro mechanical relays

Bulky, poor accuracy, slow in operation

High VA burden, require frequent calibration

EMI and harmonics tend to effect

Integrated functions, multiple characteristics

and memory storage not possible

Fault waveform recording & analysis not

possible

Reasons of spurious trippingof relays

Growth of trees in section, sparking at ROB,

FOB, improperly set locomotive spark gaps

Excessive relay settings

Load encroachment of Inst OCR and

MHO Relay

Magnetic inrush currents

Operation of WPC for regenerative current

Reasons of spurious trippingof relays

Initial energisation of a transformer on NO

load causes currents going upto 5 times full

load rating of transformer depending upon

the instance of switching ON.

These though die out within 0.5 sec but are

sufficient to cause tripping of relays.

These inrush currents are rich in 2nd harmonics

(Contd.)

Type of relay Total relays(Characteristics) on I.R. Max Min AvgYTG 14(Lenticular) 12 29 5 10.3YCG14(Mho) 372 105 9 21.3AZ1114(Parallelogram) 23 16 2 5.1Others 37 40 5 19

Tripping of the relays / month

Statistics of distanceprotection relays on I.R.

Load encroachment Inst OCR

Inst OCR is set at 5 times the relays settings

and acts in < 15 ms.

Upto the set value of the current the relay shall

trip at higher time.

Typically the relay is set to 1000 to 1500 amp

to cover instantaneous faults of 5000 to 7500

amp

Thus the relay will trip at 1000 to 1500 amp

current in about 100 msec

This is load encroachment of inst OCR

0%

10%

20%

30%

40%

50%

1 2 3 4 5

Current in times of the relay setting

Re

lay

op

era

tin

g t

ime

in

m s

ec

Instantaneous OCR curves

Regenerative Current Handling

Regenerative currents if fed back to grid may

cause WPC tripping

These current fall in the 2nd quadrants

Selected area of WPC operation for 100 to

150 degree in the 2nd quadrant solves this

problem

At present only AZ 1114 latest version is

immunized for regenerative currents

X

R

Fault AreaWPC

Area

Load areaRegenerativearea

Regenerative Current Handling

Need for a Delta - I Relay

1. Conductor falling on rocky terrain.

2. Dry twig touching OHE

3. Monkey menace or bird faults on brackets

and 9-T insulators.

4. Bond not properly connected or open

5. Delta I can protect fault current upto 200 Amps

Panto flashover prevention relay

Tripping of one of the CB’s at a TSS & during

this state if a locomotive enters the TSS IOL

it can cause flashover at the OHE

Depending upon the intensity of the flashover

current, their can be catenary strands cutting

or parting of OHE

The solution to such a problem is to eliminate

the flashover by switching OFF the arc feeding

circuit breaker

Tripping of mainline CB for faults in yards

1. Normally big yard and loco sheds are known

to cause excessive tripping of the CB

2. Divisions sometimes provide separate CB to

feed such locations in order to avoid mainline

tripping

3. However, effective segregation is seldom

achieved due to the characteristics of the

relays provided.

Tripping of mainline CB for faults in yards

4. This problem can be solved by blocking the

mainline CB for a period of 100 ms from the

time of tripping of the yard OCR.

5. This shall block the mainline relay for yard

fault but for the mainline faults the system

shall remain unaffected

6. RDSO has cleared this scheme in 1998 for

implementation on Zonal Railways

(Contd.)

Remedial Measures

1. Relay settings to be done as per RDSO

standard guidelines

2. Tripping monitoring is an important tool

to judge section performance

3. Adequate relays are not available as spares.

10 % relays of each type with a minimum of

one relays of each type required

4. Availability of high speed auto reclosure to

ensure quick power supply availability

5. Changes in section like removal of BT-RC

change of CT and the transformer ratings

needs review of the relays settings.

Remedial Measures

6. Bonds connections to be ensured, tightened

for an effective return path for the faults.

7. Training of staff at the depot levels is must

8. To summarize

(a) Modern day Microprocessor based relays

are far more superior than the ancient

electromechanically technology, when

ever need is felt, replacement should be

done in a phased manner.

(b) Monitoring of the protective relays

performance as per their characteristics

should be closely watched by the

Sr.DEEs through regular and frequent

inspection.

(Contd.)