TCE Relay Settings & Co-Ordination

8/12/2019 TCE Relay Settings & Co-Ordination

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TCE. M6-EL-PI-

G-RC-6507RELAY SETTINGS AND COORDINATION

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INITIALS SIGN INITIALS SIGN INITIALS SIGN INITIALS SIGN

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APP.BY UAK Sd/-

DATE 97-04-14

FORM NO. 020R2

DESIGN GUIDE NO. TCE. M6-EL-PI-G-RC-6507

FOR

RELAY SETTINGS AND COORDINATION


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CONTENTS

ITEM PAGE NO.

SCOPE

PART - I : RELAY CO-ORDINATION

1.0 RECOMMENDED PRACTICES

2.0 CALCULATION OF SHORT CIRCUIT LEVELS

PART - II : SETTING ON RELAYS ON TYPICAL FEEDERS IN POWER DISTRIBUTION NETWORKS

1.0 GENERAL

2.0 TRANSFORMER FEEDERS

3.0 MOTOR FEEDERS

4.0 TIE FEEDERS & BUS-COUPLERS/BUS-SECTIONS

5.0 MISCELLANEOUS PROTECTIVE RELAYS

6.0 APPENDIX / I / FIGS 1 TO 5


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REV NO DATE DESCRIPTION

R1 97.04.14 1. Document retyped


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1. SCOPE

This design guide presents preferred practices for relay settings and protectionco-ordination to achieve selective tripping in the electrical auxiliaries ofindustrial and power plants. Part-I of this guide details relay co-ordination

procedures while Part-II indicates methods of setting different types of relaysfor various protections.

PART-I : RELAY CO-ORDINATION

1.0 RECOMMENDED PRACTICES

The following points may be considered while co-ordinating operation ofdifferent relays.

1.1 The co-ordination starts from the extreme downstream protection, which maybe a fuse.

1.2 The co-ordination interval for the relay immediately above the fuse is decidedby the fuse positive tolerance, relay negative tolerance, relay overshoot and asafety margin. A minimum co-ordination interval of 0.2 sec. is to bemaintained between the relay and the fuse.

1.3 As far as possible, a co-ordination interval of 0.4 sec. is to be maintainedbetween two relays to ensure proper discrimination. This time includes thebreaker opening time, relay errors, relay overshoot and a safety margin.

1.4 For industrial plants, the operating time of the extreme upstream relay in theplant, considered along with its breaker opening time, at the incoming powersupply fault level, is governed by the maximum time permitted by theElectricity Board and equipment ratings at that fault level. The co-ordinationstarting from the extreme downstream relays shall ensure that this requirementis met.

1.5 For power plants the operating time of the extreme upstream relay isdetermined by the switchgear rating. Since the switchgear normally has a 1.0sec. rating, the maximum relay operating time should not exceed 0.9 sec. at therated fault level.

1.6 The following procedures can also be considered to simplify relay coordination:

1.6.1 Use of very inverse and extremely inverse time relays on downstream feeders.

1.6.2 Reduction of the co-ordination interval to 0.35 sec although this reduces thesafety margin.

1.6.3 Elimination of the co-ordination interval between two relays which will notcause power interruption to other loads. For example, in co-ordination of relayson the primary and secondary of transformers and co-ordination of relays on the

breakers at the sending and receiving end of a tie/radial feeder.


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1.6.4 The co-ordination interval between relays provided on incoming feeders and thebus coupler can be eliminated, in cases where the bus coupler is normally kept

open.

1.7 On any particular bus, amongst relays on various outgoing feeders, the relaywith the highest operating time is to be considered for co-ordination with therelay on the incoming feeder. It shall also be ensured that the relay on theincoming feeder does not operate for the starting condition of the largest motorwhen feeding all other normal loads.

1.8 The instantaneous relays on the primary side of the transformer feeder shall beset above the through fault level on the secondary side to prevent the relay fromoperating for a secondary side fault. Generally, the setting adopted is 1.3 timesthe through fault current in cases where relays with low transient over-reach areused, if not a setting of 2.7 shall be adopted. This value covers the CT error,

the relay error as well as the over-reach of the instantaneous relay.

1.9 Where inverse time relays with high-set instantaneous units are provided onoutgoing transformer/motor feeders, the IDMT relay on the incoming feedershall be co-ordinated with the operating time of the instantaneous relay to bringdown the bus fault clearing time. However, with the IDMT relay characteristicselected in this manner, for the incoming feeder, it should be ensured thatgrading is obtained with the outgoing feeder IDMT characteristic as well. Thisaspect has been further elaborated under item 2.1.3 of Part-II of this guide.

1.10 The operating time of the relay on an incoming feeder at that respectiveswitchgear fault level shall be such that the operating time of the immediate

back up relay, considered together with its breaker opening time, shall notexceed the short time rating of the switchgear, which is normally 1.0 second.Whenever there is a substantial difference between the system fault level andthe switchgear rating, the incoming feeder relay and the immediate back uprelay operating times, at the system fault level, are permitted to increase, basedon "I2T" criterion as may be necessary for co-ordination with downstreamrelays, for example, if the fault current at the switchgear bus is 'X' which ismuch lower than the switchgear 1.0 sec, rating, 'Y', then the relay operatingtime at the bus fault level 'X' can be increased to (Y)2x 1.0 second.

x

1.11 Current settings on directional relays when used for duplicate incoming feedersare to be set at 50% of the normal full load of the protected circuit and the timemultiplier set at 0.1, i.e., as low as possible. Care shall be taken to ensure thatthe continuous thermal rating of the coil is not preceded during power flow inthe reverse, i.e., non-operating direction.

2.0 CALCULATION OF SHORT CIRCUIT LEVELS

2.1 An impedance diagram of the Plant System is to be prepared showing the perunit impedance (considered with negative tolerances as per relevant standards)of all the circuit elements. Using network reduction techniques, the shortcircuit levels at various voltages of the system can be calculated. Design Guidefor Electrical Auxiliary System for Thermal Power Plants - TCE.M6-EL-Au-G-710-6009 may also be referred in this regard.

Motor contribution to the fault is to be included as follows :


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(a) For 415 V Motors (less than 175 kW rating)

Fault contribution = 6 x 1.2* x Motor rating in MVA 3(b) For 6.6 kV Motors

(i) For motors rated above 750 kWwith an rpm of 1500

OR

For motors rated above 175 kWwith an rpm of 3000 :

Fault contribution = 6 x 1.2* x Motor rating

1.5 in MVA

(ii) For motors of exceptionally largeratings such as BFP motors :

Fault contribution = 5 x 1.2* x Motor rating 1.5 in MVA

(iii) For other motors :

Fault contribution = 6 x 1.2* x Motor rating

3 in MVA

* The factor 1.2 relates t the 20% negative tolerance on the impedance.

2.2 Whenever there is a change in circuit parameters such as addition of motors oflarge ratings or changes in the transformer rating, etc., the fault calculationshave to be modified and co-ordination reviewed.


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PART - II : SETTING OF RELAYS ON TYPICAL FEEDERSIN POWER DISTRIBUTION NETWORKS______

1.0 GENERAL

The three main types of feeders normally encountered in power distributionsystems are :

1.1 Transformer Feeders : Which couple two switchboards at different voltagelevels with circuit breakers at both the sending (HV) and receiving (LV) ends.

1.2 Motors Feeders : Which are meant solely for the switching and protection ofmotors - either HV or LV.

1.3 Tie Feeders : Which couple two switchboards at the same voltage level withcircuit breakers at both the sending and receiving ends or with a breaker at thesending end and a switch at the receiving end. Power flow through thesefeeders is normally uni-directional, though under certain circumstances, bi-directional power flow may be permitted.

The various criteria to be kept in view while setting the individual relays fordifferent protections have been detailed under each type of feeder protection.Procedure for setting certain miscellaneous relays, common to a switchboard(like neutral displacement and under voltage relays on Bus voltage transformermodules) have also been indicated.

2.0 TRANSFORMER FEEDERS

Protections, as listed below are normally provided on transformer feedersagainst abnormal conditions :

(a) Overcurrent protection(b) Unrestricted earth fault protection(c) Restricted earth fault protection(d) Standby or back-up earth fault protection(e) Differential protection(f) Gas and Oil surge protection(g) Over temperature protection.

Whether all the above, or a few selected protections are applied, depends on thetransformer rating and the system earthing. Application of these protectionshave been detailed in guide No. TCE.M6-PI-G-TF-6508, - "Protection ofTransformers".

2.1 Overcurrent Protection (50/51):

2.1.1 Purpose:

The purpose of this relay is to provide instantaneous protection to thetransformer against internal short circuits and faults on the transformer primaryterminals as well as back up time delayed over current protection on externaldownstream faults or excessive overloads.


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2.1.2 Type of Relay

An instantaneous over current relay (50) having a low transient over reach isused to provide the instanta-neous protection whereas a definite minimum timerelay having inverse characteristics (51) is used to provide time delayed back-up protection. Both the above relays may be mounted in a single unit orsupplied in different cases.

The instantaneous element is used on the primary side of the transformer only,as use of this relay on the secondary side would make co-ordination ineffective.Directional relays, both on the HV & LV sides, are used in case of Tietransformers where bi-directional power flow occurs.

2.1.3 Setting Procedure

A - IDMT - O/C Relay (51) on Transformer Secondary :

Assuming that the transformer secondary feeds a distribution switchboard, as isnormally the case, this relay on the LV side of the transformer has tonecessarily be co-ordinated with the relay on the outgoing feeder having thelargest operating time. If, there are large motors among the outgoing feeders,then the relay settings should also be so chosen as to avoid relay operationduring starting of the largest motor when all other feeders are supplying theirnormal loads. Meeting the above requirements and in the absence of any otherconstraint, the current setting of the relay should be as close as possible to thefull load rating of the feeder on the transformer secondary. The above wouldhold good when grading with outgoing fuse-switch feeders or with outgoing

feeders having circuit breakers with IDMT relays.

The method of setting the relay would therefore be to chose a higher currentsetting to avoid the motor starting inrush current and to choose a time multipliersetting to grade with the instantaneous over current element of the downstreamtransformer feeder. It may be noted that with this philosophy in setting,overload protection is not afforded by this relay,but much faster fault clearingtimes are achievable. Overload protection to the transformer is basically

provided by the over-temperature protection devices which sense thetransformer winding and oil temperatures. Relay characteristics illustrating theabove have been shown in Fig. 1.

B - IDMT-O/C Relay (51) on Transformer Primary :

While the current setting i.e., the plug setting multiplier (PSM) preferred on theprimary side would be just above the transformer full load current, it is usuallynot practical to choose such a low setting, as both the current (PSM) as well astime (TMS) settings have to be necessarily co-ordinated with the IDMT relay(51) on the transformer secondary. For reasons detailed above, the currentsetting may be quite high and as such this protection is considered as a back upand is expected to operate on both transformer internal faults as well as through(external) faults on the downstream side (Ref. Fig. 1).


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C - Instantaneous O/C Relay (50) on Transformer Primary :

It should be ensured, that in order to maintain co-ordination, this relay does notoperate for any fault on the secondary side of the transformer. The settingshould therefore be chosen so as to be above the secondary side short circuitcurrent (reflected on the primary side) but definitely well below the primaryside fault current. Also, in order to avoid spurious operation of this relay onoffset fault currents on the transformer secondary and magnetic inrush currents,an instantaneous over current relay with a low transient over-reach (5% or less)is recommended.

Generally, a setting of 1.3 times the through fault current is recommended tocover the relay and CT errors as well as the relay overreach in case of lowtransient over reach relays. If a low transient over-reach relay is not used, asetting of 2.7 times shall be adopted.

2.2 Unrestricted Earth Fault Protection (50N/51N)

2.2.1 Purpose

This relay provides protection in case of external earth faults in effectivelyearthed and low resistance earthed system.

2.2.2 Type of Relay

Sam as 2.1.2 except for the setting range which is lower.


Procedures defined in clause 2.1.3 for over current relays are generallyapplicable to this protection as well. However, for setting IDMT earth faultrelays, in the absence of any other constraint, the lowest current settingavailable may be generally chosen, keeping in view the co-ordinationrequirements detailed in 2.1.3 above and in Part-I of this guide.

2.3 Restricted Earth fault Protection (64):

2.3.1 Purpose:

This relay provides instantaneous earth fault protection to all internal faults onthe transformer winding to which it is applied. As it is a unit protection, thesetting of this relay does not require co-ordination with other protectionsystems. In low resistance earthed system, this protection also supplements thenormal differential protection, since it offers protection to a larger percentage ofthe transformer winding. This protection cannot be applied to high resistanceearthed (i.e., non-effectively earthed) systems.

2.3.2 Type of Relay

A high impedance, voltage operated relay is recommended for this application.


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This relay is normally set at the lowest current tap available, viz. 10%.However, to ensure that the relay does not maloperate on through faults, anadditional stabilizing resistor is connected in series with the relay. Theresistance ensures that the current in the relay circuit does not reach theoperating value even when the maximum voltage VRappears across the relaycircuit under through fault conditions. The procedure for calculating the valueof the stabilizing resistor is as follows :

(i) The maximum voltage that is likely to appear across the relay i.e., VRduring external faults is first calculated assuming the worst condition ofimbalance, i.e., the CT on one side saturating

VR = If(RCT+ RL), where

If = The secondary equivalent of the system fault current

RCT = The CT secondary winding resistance

RL = Maximum loop resistance of the CT secondary leads

(ii) Next, the total relay circuit impedance - RTis calculated at the relay tapselected (IR).

RT = VR

IR

(iii) The relay coil impedance RRat the chosen tap should be obtained fromthe manufacturers' catalogues.

(iv) The value of the additional stabilising resistor (RS) required would thenbe the total circuit impedance (RT) minus the relay coil impedance atthe selected tap (RR)

RS = RT- RR

As the current transformers designed for this protection would have a kneepoint voltage equal to at least 2 V

R, they would develop an adequate voltage,

higher than VR, to operate the relay in case of internal faults.

2.4 Standby or Backup Earth fault Protection (51SN):

2.4.1 Purpose:

This protection is usually provided on resistance earthed systems and isnormally set to protect the earthing resistor which is short time rated. This mayalso be applied to effectively earthed systems where this relay acts as a backupto the un-restricted E/F relay 51N in addition to providing protection fortransformer secondary winding faults.


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2.4.2 Type of Relay

An instantaneous overcurrent relay with timer is recommended for thisapplication. An IDMT relay with characteristics which offer adequateprotection to the resistor can also be considered.

2.4.3 Setting Procedure:

The current and time settings on this relay should protect the short-time ratedearthing resistor from damage. The current setting chosen should be less thanthe resistor continuous withstand value. The operating time of the relay should

be less than the withstand time of the resistor at the maximum system E/Fcurrent. In addition to matching the short-time rating of the resistor, the settingshould also co-ordinate with all downstream E/F relays. In effectively earthedsystems, the minimum current setting available on the relay may be chosen with

a time setting adequate to co-ordinate with all downstream earth fault relays.

2.5 Differential Protection (87):

2.5.1 Purpose:

This unit type protection is provided against phase to phase as well as phase toearth faults in both the transformer windings or at the transformer terminals.However, earth faults on the LV winding in high resistance earthed systemswould not be detected by this protection.

2.5.2 Type of Relay:

A percentage biased Differentials relay is recommended for this application.For transformers with ratings larger than 10 MVA, relay should in additionhave a 2nd harmonic restraint and 5th harmonic restraint or bypass feature asdetailed in the guide for transformer protection.

2.5.3 Setting Procedure:

These relays are normally provided with a fixed operating value. However, incase relays with different settings of operating value are used, the lowest settingavailable may be adopted. A biased relay is used to prevent operation underthrough faults, as even under normal external fault conditions a certaindifferential current can flow through the operating winding of the relay due tothe following reasons :

(a) Transformer Tap Changing : If the transformer has a tap changer of+X%, then the maximum mismatch due to this would be X%, since CTratios are designed considering the transformer nominal tap.

(b) Mismatch between CT secondary currents and relay tap ratings.

(c) A certain degree of mismatch between the CT magnetisationcharacteristics of the CTs on the transformer primary and secondary.This value can be computed from the CT magnetisation characteristics.


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(d) Transformer Magnetising inrush currents during switching.

Maloperation due to the transformer magnetising inrush currents isavoided by features built into the relay design such as harmonicrestraint, time delays, etc. However, assuming that all the other threeunbalances are in the same direction, the total maximum possible

percentage unbalance is calculated by summating the individualunbalance caused by items a, b & c listed above and adding a margin of5% to this value. The next higher value of the percentage bias settingavailable on the relay should then be chosen.

2.6 Gas and Oil Surge Protection

2.6.1 Purpose

This relay provides protection against low oil level and transformer internalfaults, including incipient faults.

2.6.2 Type of Relay

A gas/oil flow actuated relay, commonly referred as the "Buchholz Relay" isnormally provided by the transformer manufacturer and is connected in the

piping between the transformer main tank and the conservator.


No setting is required to be carried out at site. The oil/gas surge rate and the

accumulated gas volume setting required to actuate the trip and alarm circuitsrespectively are preset at the factory depending on the capacity of thetransformer.

2.7 Over temperature Protection

The over temperature protection for both the windings and oil is to be set as perthe manufacturers' recommendation. This is normally preset at the factory.

3.0 MOTOR FEEDERS

The various protections provided on motor feeders of different ratings havebeen summarized in Table - 1. The setting procedure for each type of relay isdetailed below :

3.1 Bimetallic Thermal Overload Protection (49)

3.1.1 Purpose

To provide protection against overloading and to a certain extent, singlephasing to all motors upto 125 kW.

3.1.2 Type of Relay

Three phase bimetallic, temperature compensated thermal relay, either

operating directly off the motor current or through CT's for large motors. Therelay shall be hand reset type.


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The operating value of this relay shall be set at the motor full load current.

3.2 Locked Rotor or Stalling Protection (50 LR)

3.2.1 Purpose

These relays, provided for all motors above 100 kW, offer back up protection tomotors under stalling conditions. Thermal overload relays may also provide

protection under stalling conditions. In cases where the thermal protection isnot effective under stall conditions (i.e., where the thermal withstand charac-teristic of the motor lies below the relay operating characteristic) in either thecold or hot condition, this relay becomes the only protection under stalling

conditions.

3.2.2 Type of Relay

Instantaneous overcurrent relays having high drop off to pick up ratio (above80%) in R & B phases with one common timer (on delay type) is used for this

protection.


(a) The current setting chosen on each inst. O/C relay shall normally equaltwice the full load current of the motor. The common timer setting shall

be 1 to 2 secs. more than the starting time of the motor at the minimumpermissible voltage during starting, i.e., 80%.

(b) If however, on carrying out the relay application check, the relay hot orcold characteristic is found to cut the corresponding motor withstandcurve, at any point, which is say "X" times the full load current then thecurrent setting adopted shall be twice the full load current of the motor,or, (X x IFL) x 0.9 whichever is lower. The time setting chosen shall beas detailed under (a) above.

(c) In cases, where the locked rotor withstand time at 110% of rated voltageunder hot conditions is less than or nearly equal to the starting time ofthe motor, at 80%, of the rated voltage an arrangement shown in Fig. 2with a speed switch on the motor and an additional on-delay timer is to

be utilised.

The normally closed contact of the speed switch provided shall open out at theset speed during starting.

(i) Timer TR 2shall be set as usual, i.e., more than the starting time of themotor.

(ii) Timer TR 1shall be set 1-2 secs. below the locked rotor withstand timeunder hot condition.


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(iii) The speed switch on the motor shall be set to operate at a speed attainedby the motor under normal starting conditions in a time less than the

setting of timer TR1.

With this arrangement, if the motor starts normally, the speed switch N/Ccontact provided in series with timer TR1would de-energise it before it couldoperate. If however, the motor stalls, the speed switch remaining in the closedcondition, timer TR1would operate and trip the motor before the locked rotorhot withstand time is reached. Timers TR1and TR2may either be integral withthe stalling protection relay or mounted separately.

3.3 Thermal Overload Protection (49)

3.3.1 Purpose

This relay provides protection to the motor against overheating due to eitheroverloading or presence of negative sequence currents under both hot and coldconditions.

3.3.2 Type of Relay

A thermal relay with inverse characteristics sensing and compensating for boththe positive and negative sequence components of the load current to simulate athermal image of the motor under hot and cold conditions is used. The relayshall sense currents from at least two phases.

A choice of characteristics shall be available with a wide range of time

constants to match the varied motor withstand curves encountered. As analternative, a inverse relay sensing only the total load current can be used with aseparate instantaneous negative sequence relay.


A. Current Setting

The current setting chosen, shall be calculated using the formula -

IRel= IFLx ISx R Ip P

where IRel= Current setting on relay

IFL = Full load current of the Motor

Ip = Rated CT primary current

IS = Rated CT secondary current

R = Overload factor of the Motor if any (For a CMR motor this shall be 1.0)

P = Pick up value of the relay in terms

of number of times the current setting.


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In case the relay has current settings available in steps the next highersetting (with respect to the calculated setting) shall be chosen. The

procedure indicated above is general in nature. Manufacturers'catalogues shall also be referred for particular recommendations, if any.

B. Choice of Time Constant

The time constant of the relay chosen shall be less than the timeconstant of the motor being protected. However, this shall be chosenafter carrying out the relay application check as described below.

C. Choice of Relay Characteristics

Relay application checks are to be carried out on all motors rated above125 kW. This check consists of plotting the motor withstand curves and

relay operating characteristics under both hot and cold conditionstogether with the motor starting current Vs time characteristics on thesame graph.

Normal relays available in the market have a choice of various operatingcharacteristics under both hot and cold conditions. As far as possible,the relay characteristics should be so chosen that both relay hot and coldcharacteristics :

(i) Lie completely below the corresponding motor withstand curves- upto the locked rotor value.

(ii) Follow the corresponding motor withstand characteristics asclosely as possible - with a safe time difference.

(iii) The relay hot operating characteristic lies above the motorstarting current Vs time characteristic.

If the above conditions are met then the stall protection is backedup by the thermal protection.

(a) Case (i)

The various motor and relay characteristics when dispositionedwith respect to one another as described above, offer the best

possible protection, but is only one of the many possibilities.This condition as described above is considered as case(i) andthe corresponding characteristics have been shown plotted inFig. 3.

However, as the withstand curves of motors vary widely withboth ratings and makes, the following additional possibilities canbe encountered, even after choosing the most optimum relaycharacteristic available on a particular make.

(b) Case (ii)

In this case either or both the relay characteristics, i.e., hot andcold, intersect with the corresponding motor withstand curves(Ref. Fig. 4).


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In this case the point of intersection - 'X' is determined after

plotting the curves. The thermal relay does not protect the motorbeyond this point of intersection and as such the locked rotorrelay, set as described under Clause 3.2.3 (b), would protect themotor under stall conditions.

(c) Case (iii)

Where the motor starting current Vs time characteristic intersectsor is very close to the relay operating characteristics (hot) (Ref.Fig. 5).

In this case the DC supply to the relay shall be wired to the relaythro' a 52A contact as shown in Fig. 5. With this arrangement,

which is applicable to static relays only, it would be ensured thatthe relay cold characteristic would be applicable even in case ofa hot restart. In this case, it is preferable to have relay cold curve

below motor hot curve. Even if it is not so a certain degree ofoverload protection will be offered and stall protection wouldstill be available in this condition, this is acceptable since thiscondition lasts only for a short duration until the relay reachesthermal equilibrium once again. However, acceptability of thisscheme shall be checked out for each make of relay.

3.4 Short Circuit Protection (50)

3.4.1 Purpose

This relay provides protection against interphase winding short circuits andterminal/cable faults.

3.4.2 Type of Relay

Instantaneous overcurrent relays, one on each phase having low transient over-reach (less than 5%) to prevent pick up during transient inrush currents whenstarting the motor.


The relay shall be set at 1.5 times the starting current. The additional factor for0.5 takes care of the CT & relay errors, transient over-reach of the relay andtolerance on the starting current.

3.5 Earth fault Protection (50 N)

3.5.1 Purpose

This relay provides protection to the motor against leakage current to ground.

3.5.2 Type of Relay

(a) On effectively earthed systems or low resistance earthed systems asingle pole instantaneous over-current relay immune to startingtransients shall be used.


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(b) On high resistance earthed systems, an instantaneous overcurrent relay

having a sufficiently low pick up value depending on the system earthfault current and having a timer with setting of 1.0 sec. shall be usedwith a core balance CT having a ratio depending on the total maximumfault current.


(a) Effectively earthed/Low Resistance earthed systems

The relay shall be set at 20% of the rated value in case of solidly earthedsystems and at 10% in case of system grounded through a lowresistance.

(b) High Resistance Earthed Systems

The maximum Earth fault current likely to flow due to the systemcapacitance is calculated. The relay is set at a value lower than 50% ofthe secondary equivalent of the fault current. The setting shall however,

be higher than the fault current contributed by the capacitance of thelargest motor on the bus to prevent the core balance relay on that

particular feeder from operating. The CBCT manufacture shall beinformed of the relay setting and total circuit burden. The performanceof the CBCT shall be guaranteed by the CBCT manufacturer at theminimum operating current.

3.6 Differential Protection (87)

3.6.1 Purpose

This relay, used for motors having a rating above 1500 kW provides fast acting,unitised protection to the motor against internal phase faults and ground faultsfor motors connected to solid by earthed or low resistance earthed systems. Forhigh resistance earthed system this protection will not sense earth faults.

3.6.2 Type of Relay

Three single pole, high impedance voltage operated, instantaneous over currentrelays are used with a stabilising resistor in series with each relay.


The relay shall normally be set at 20% of the full load current. The stabilisingresistor shall be set in a manner identical to that detailed under Clause 2.3.3.However, in this case the maximum starting current should be consideredinstead of the system fault current.

3.7 Overload Alarm Relay (50-OLA)

3.7.1 Purpose

To provide an audio-visual alarm in case the motor is overloaded continuouslyto enable the operator to take suitable measures, if possible, to avoid ultimate


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tripping of the motor on operation of thermal protection. This feature isprovided on motors above 175 kW.

3.7.2 Type of Relay

Single pole instantaneous overcurrent relay with a high drop off to pick up ratio(of the order of 9% or more) together with a on-delay timer. This relay shallhave a range of 70-130%, preferably continuously adjustable or adjustable insteps of 5%.


The current setting adopted should lie in the region of 110 to 120% of the motorfull load current. The timer should be set to operate at a value larger than thestarting time at the minimum permissible voltage (80%).

3.8 Other Protective Devices for Motors

Motors above 175 kW are also provided with the following protective devices :

(a) Water flow monitor (only for CA CW motors)

(b) Lube Oil pressure monitor (when forced lubrication is provided)

(c) Bearing temperature alarm/trip

(d) Winding temperature alarm/trip

As items (a), (b) and (c) above do not require any setting as such, these are notdiscussed in this guide.

In case where RTD's for winding temperature monitoring are used with remotesensing/monitoring devices which require to be set, th trip temperature settingshall be bout 10C less than the withstand temperature of the class of insulationused in the motor.

4.0 TIE FEEDERS AND BUS COUPLERS/BUS SECTION

A tie feeder is a connection between two individual switchboards with circuitbreakers or circuit breakers and switches at the sending and receiving ends,whereas a bus coupler or a bus sectionalising breaker couples the two sectionsof the same switchboard.

The protections normally provided on these feeders are:

(a) Inverse definite minimum time overcurrent relay with normalcharacteristic or very inverse characteristic*

*(Only in cases where there is a large variation in fault current at thesending and receiving end switchboards due to the large impedance ofthe connecting cable).

(b) Inverse definite minimum time earth fault relay (only in case ofeffectively earthed or low resistance earthed systems).


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The type and procedure for setting these relays are identical to that detailedunder the corresponding protections for transformers under Clause 2.0.

5.0 MISCELLANEOUS PROTECTIVE RELAYS

5.1 Neutral Displacement Relay

5.1.1 Purpose

This relay is used to sense earth faults in systems earthed through a highresistance. As this protection may be sensitive to earth faults throughout thesystem, it cannot be used for tripping any feeder in particular but is connectedto give an alarm only.


Recommended voltage settings :

(a) U/V relay for motors - 80% (nominal voltage)

(b) For initiating autochangeover - 20% (nominal voltage)

(c) For monitoring the busonto which the load getstransferred during autochangeover - 80% (nominal voltage)

Recommended time setting : 1 second.


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TABLE - 1 : SUMMARY OF PROTECTIONS PROVIDED FOR MOTORS

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Sl. Motor Motor Type of ___________________Type of Protection Provided_______________________

No Rating Vol- Switch- HRC Bimet- Locked Thermal Short Earth Under Differ- Over-

(kW) tage ing Fuses allic Rotor Over- Circuit Fault Voltage ential load

(V) Device Thermal Relays load Protec- Relays Protec- Relays Alarm

Relays Relays tion tion Relay

Relays

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1 2 3 4 5 6 7 8 9 10 11 12 13

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1. 0-99 415 Contactor 3 Nos. 1 No. - - - - - - -

(3 Phase)

2. 100-125 415 Contactor 3 Nos. 1 No. 2 Nos. - - - - - -

(3 Phase)

3. 126-175 415 Circuit - 3 Nos. 2 Nos. - 3 Nos.

(Alt-1) Breaker (1 per (1 per

Phase, Phase,

inbuit inbuilt

with with

breaker) breaker)

4. 126-175 415 Circuit - - 2 Nos. 1 No.* 3 Nos. (Alt-2) Breaker (1 per

Phase)

5. 176-1500 6600/ Circuit - - 2 Nos. 1 No.* 3 Nos. 1 No. Provided - 1 No.

11000 Breaker (1 per (from bus

Phase) u/v relay)

6. Above 6600/ Circuit - - 2 Nos. 1 No.* 3 Nos. 1 No. Provided 3 Nos. 1 No.

1500 11000 Breaker (1 per (from bus (1 per

Phase) u/v relay) Phase)

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NOTES : (TABLE - 1)

1. For further particulars refer Design Guide No. TCE.M6-EL-PI-G-M-6504 -'Control and Protection of Medium Voltage, Squirrel Cage Motors' andStandard Document TCE.M2-EL-CW-D-2500 for 415 V motors.

2. For details of Control and Protection of 6.6 kV motors refer Design

Guide No. TCE.M6-PI-20412 "HV Squirrel Cage Induction Motor Protection".


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*3. One relay sensing currents from at least 2 phases.

4. Two numbers Locked Rotor and Thermal Relays indicated shall be connectedto the R & B phases.


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TCE Relay Settings & Co-Ordination

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