TCE Relay Settings & Co-Ordination

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RELAY SETTINGS AND COORDINATION

DESIGN GUIDE NO. TCE. M6-EL-PI-G-RC-6507 FOR RELAY SETTINGS AND COORDINATION

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97-04-14 FORM NO. 020R2

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RELAY SETTINGS AND COORDINATION

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CONTENTS ITEM

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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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RELAY SETTINGS AND COORDINATION

DATE 97.04.14

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DESCRIPTION 1.

Document retyped

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1.

RELAY SETTINGS AND COORDINATION

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SCOPE This design guide presents preferred practices for relay settings and protection co-ordination to achieve selective tripping in the electrical auxiliaries of industrial and power plants. Part-I of this guide details relay co-ordination procedures while Part-II indicates methods of setting different types of relays for various protections. PART-I : RELAY CO-ORDINATION

1.0

RECOMMENDED PRACTICES The following points may be considered while co-ordinating operation of different relays.

1.1

The co-ordination starts from the extreme downstream protection, which may be a fuse.

1.2

The co-ordination interval for the relay immediately above the fuse is decided by the fuse positive tolerance, relay negative tolerance, relay overshoot and a safety margin. A minimum co-ordination interval of 0.2 sec. is to be maintained between the relay and the fuse.

1.3

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

1.4

For industrial plants, the operating time of the extreme upstream relay in the plant, considered along with its breaker opening time, at the incoming power supply fault level, is governed by the maximum time permitted by the Electricity Board and equipment ratings at that fault level. The co-ordination starting from the extreme downstream relays shall ensure that this requirement is met.

1.5

For power plants the operating time of the extreme upstream relay is determined by the switchgear rating. Since the switchgear normally has a 1.0 sec. rating, the maximum relay operating time should not exceed 0.9 sec. at the rated 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 the safety margin.

1.6.3

Elimination of the co-ordination interval between two relays which will not cause power interruption to other loads. For example, in co-ordination of relays on 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. ISSUE R1 FORM NO. 120 R1

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1.6.4

The co-ordination interval between relays provided on incoming feeders and the bus 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 relay with the highest operating time is to be considered for co-ordination with the relay on the incoming feeder. It shall also be ensured that the relay on the incoming feeder does not operate for the starting condition of the largest motor when feeding all other normal loads.

1.8

The instantaneous relays on the primary side of the transformer feeder shall be set above the through fault level on the secondary side to prevent the relay from operating for a secondary side fault. Generally, the setting adopted is 1.3 times the through fault current in cases where relays with low transient over-reach are used, 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 on outgoing transformer/motor feeders, the IDMT relay on the incoming feeder shall be co-ordinated with the operating time of the instantaneous relay to bring down the bus fault clearing time. However, with the IDMT relay characteristic selected in this manner, for the incoming feeder, it should be ensured that grading is obtained with the outgoing feeder IDMT characteristic as well. This aspect 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 respective switchgear fault level shall be such that the operating time of the immediate back up relay, considered together with its breaker opening time, shall not exceed the short time rating of the switchgear, which is normally 1.0 second. Whenever there is a substantial difference between the system fault level and the switchgear rating, the incoming feeder relay and the immediate back up relay operating times, at the system fault level, are permitted to increase, based on "I2T" criterion as may be necessary for co-ordination with downstream relays, for example, if the fault current at the switchgear bus is 'X' which is much lower than the switchgear 1.0 sec, rating, 'Y', then the relay operating time at the bus fault level 'X' can be increased to (Y)2 x 1.0 second. x

1.11

Current settings on directional relays when used for duplicate incoming feeders are to be set at 50% of the normal full load of the protected circuit and the time multiplier set at 0.1, i.e., as low as possible. Care shall be taken to ensure that the continuous thermal rating of the coil is not preceded during power flow in the 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 per unit impedance (considered with negative tolerances as per relevant standards) of all the circuit elements. Using network reduction techniques, the short circuit levels at various voltages of the system can be calculated. Design Guide for Electrical Auxiliary System for Thermal Power Plants - TCE.M6-EL-Au-G710-6009 may also be referred in this regard. Motor contribution to the fault is to be included as follows : ISSUE R1 FORM NO. 120 R1

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RELAY SETTINGS AND COORDINATION

(a)

For 415 V Motors (less than 175 kW rating)

(b)

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

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For motors rated above 750 kW with an rpm of 1500 OR For motors rated above 175 kW with an rpm of 3000 : Fault contribution = 6 x 1.2* x Motor rating 1.5 in MVA

(ii)

For motors of exceptionally large ratings 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 of large ratings or changes in the transformer rating, etc., the fault calculations have to be modified and co-ordination reviewed.

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PART - II : SETTING OF RELAYS ON TYPICAL FEEDERS IN POWER DISTRIBUTION NETWORKS______ 1.0

GENERAL The three main types of feeders normally encountered in power distribution systems are :

1.1

Transformer Feeders : Which couple two switchboards at different voltage levels 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 of motors - either HV or LV.

1.3

Tie Feeders : Which couple two switchboards at the same voltage level with circuit breakers at both the sending and receiving ends or with a breaker at the sending end and a switch at the receiving end. Power flow through these feeders is normally uni-directional, though under certain circumstances, bidirectional power flow may be permitted. The various criteria to be kept in view while setting the individual relays for different 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 transformer modules) have also been indicated.

2.0

TRANSFORMER FEEDERS Protections, as listed below are normally provided on transformer feeders against abnormal conditions : (a) (b) (c) (d) (e) (f) (g)

Overcurrent protection Unrestricted earth fault protection Restricted earth fault protection Standby or back-up earth fault protection Differential protection Gas and Oil surge protection Over temperature protection.

2.1

Whether all the above, or a few selected protections are applied, depends on the transformer rating and the system earthing. Application of these protections have been detailed in guide No. TCE.M6-PI-G-TF-6508, - "Protection of Transformers". Overcurrent Protection (50/51) :

2.1.1

Purpose : The purpose of this relay is to provide instantaneous protection to the transformer against internal short circuits and faults on the transformer primary terminals as well as back up time delayed over current protection on external downstream faults or excessive overloads.

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2.1.2

RELAY SETTINGS AND COORDINATION

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Type of Relay An instantaneous over current relay (50) having a low transient over reach is used to provide the instanta-neous protection whereas a definite minimum time relay having inverse characteristics (51) is used to provide time delayed backup protection. Both the above relays may be mounted in a single unit or supplied 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 Tie transformers 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 is normally the case, this relay on the LV side of the transformer has to necessarily be co-ordinated with the relay on the outgoing feeder having the largest operating time. If, there are large motors among the outgoing feeders, then the relay settings should also be so chosen as to avoid relay operation during starting of the largest motor when all other feeders are supplying their normal loads. Meeting the above requirements and in the absence of any other constraint, the current setting of the relay should be as close as possible to the full load rating of the feeder on the transformer secondary. The above would hold 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 current setting to avoid the motor starting inrush current and to choose a time multiplier setting to grade with the instantaneous over current element of the downstream transformer feeder. It may be noted that with this philosophy in setting, overload protection is not afforded by this relay,but much faster fault clearing times are achievable. Overload protection to the transformer is basically provided by the over-temperature protection devices which sense the transformer winding and oil temperatures. Relay characteristics illustrating the above 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 the primary side would be just above the transformer full load current, it is usually not practical to choose such a low setting, as both the current (PSM) as well as time (TMS) settings have to be necessarily co-ordinated with the IDMT relay (51) on the transformer secondary. For reasons detailed above, the current setting may be quite high and as such this protection is considered as a back up and 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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Instantaneous O/C Relay (50) on Transformer Primary :

It should be ensured, that in order to maintain co-ordination, this relay does not operate for any fault on the secondary side of the transformer. The setting should therefore be chosen so as to be above the secondary side short circuit current (reflected on the primary side) but definitely well below the primary side fault current. Also, in order to avoid spurious operation of this relay on offset 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 to cover the relay and CT errors as well as the relay overreach in case of low transient over reach relays. If a low transient over-reach relay is not used, a setting 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 effectively earthed and low resistance earthed system.

2.2.2

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

2.2.3

Setting Procedure Procedures defined in clause 2.1.3 for over current relays are generally applicable to this protection as well. However, for setting IDMT earth fault relays, in the absence of any other constraint, the lowest current setting available may be generally chosen, keeping in view the co-ordination requirements 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 on the transformer winding to which it is applied. As it is a unit protection, the setting of this relay does not require co-ordination with other protection systems. In low resistance earthed system, this protection also supplements the normal differential protection, since it offers protection to a larger percentage of the transformer winding. This protection cannot be applied to high resistance earthed (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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2.3.3

RELAY SETTINGS AND COORDINATION

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Setting Procedure 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, an additional stabilizing resistor is connected in series with the relay. The resistance ensures that the current in the relay circuit does not reach the operating value even when the maximum voltage VR appears across the relay circuit under through fault conditions. The procedure for calculating the value of the stabilizing resistor is as follows : (i)

(ii)

The maximum voltage that is likely to appear across the relay i.e., VR during external faults is first calculated assuming the worst condition of imbalance, 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

Next, the total relay circuit impedance - RT is calculated at the relay tap selected (IR). RT = VR IR

(iii)

The relay coil impedance RR at the chosen tap should be obtained from the manufacturers' catalogues.

(iv)

The value of the additional stabilising resistor (RS) required would then be the total circuit impedance (RT) minus the relay coil impedance at the selected tap (RR) RS = RT - RR

As the current transformers designed for this protection would have a knee point voltage equal to at least 2 VR, 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 is normally set to protect the earthing resistor which is short time rated. This may also be applied to effectively earthed systems where this relay acts as a backup to the un-restricted E/F relay 51N in addition to providing protection for transformer secondary winding faults.

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2.4.2

RELAY SETTINGS AND COORDINATION

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Type of Relay An instantaneous overcurrent relay with timer is recommended for this application. An IDMT relay with characteristics which offer adequate protection 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 rated earthing resistor from damage. The current setting chosen should be less than the 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/F current. In addition to matching the short-time rating of the resistor, the setting should also co-ordinate with all downstream E/F relays. In effectively earthed systems, 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 to earth faults in both the transformer windings or at the transformer terminals. However, earth faults on the LV winding in high resistance earthed systems would 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 addition have a 2nd harmonic restraint and 5th harmonic restraint or bypass feature as detailed in the guide for transformer protection.

2.5.3

Setting Procedure : These relays are normally provided with a fixed operating value. However, in case relays with different settings of operating value are used, the lowest setting available may be adopted. A biased relay is used to prevent operation under through faults, as even under normal external fault conditions a certain differential current can flow through the operating winding of the relay due to the 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 CT ratios 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 magnetisation characteristics of the CTs on the transformer primary and secondary. This value can be computed from the CT magnetisation characteristics.

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(d)

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Transformer Magnetising inrush currents during switching. Maloperation due to the transformer magnetising inrush currents is avoided by features built into the relay design such as harmonic restraint, time delays, etc. However, assuming that all the other three unbalances are in the same direction, the total maximum possible percentage unbalance is calculated by summating the individual unbalance caused by items a, b & c listed above and adding a margin of 5% to this value. The next higher value of the percentage bias setting available 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 internal faults, including incipient faults.

2.6.2

Type of Relay A gas/oil flow actuated relay, commonly referred as the "Buchholz Relay" is normally provided by the transformer manufacturer and is connected in the piping between the transformer main tank and the conservator.

2.6.3

Setting Procedure 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 circuits respectively are preset at the factory depending on the capacity of the transformer.

2.7

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

3.0

MOTOR FEEDERS The various protections provided on motor feeders of different ratings have been summarized in Table - 1. The setting procedure for each type of relay is detailed below :

3.1

Bimetallic Thermal Overload Protection (49)

3.1.1

Purpose To provide protection against overloading and to a certain extent, single phasing 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. The relay shall be hand reset type. ISSUE R1 FORM NO. 120 R1

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3.1.3

RELAY SETTINGS AND COORDINATION

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Setting Procedure 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 to motors under stalling conditions. Thermal overload relays may also provide protection under stalling conditions. In cases where the thermal protection is not effective under stall conditions (i.e., where the thermal withstand characteristic of the motor lies below the relay operating characteristic) in either the cold 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 (above 80%) in R & B phases with one common timer (on delay type) is used for this protection.

3.2.3

Setting Procedure (a)

The current setting chosen on each inst. O/C relay shall normally equal twice 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 minimum permissible voltage during starting, i.e., 80%.

(b)

If however, on carrying out the relay application check, the relay hot or cold characteristic is found to cut the corresponding motor withstand curve, at any point, which is say "X" times the full load current then the current 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 be as detailed under (a) above.

(c)

In cases, where the locked rotor withstand time at 110% of rated voltage under hot conditions is less than or nearly equal to the starting time of the motor, at 80%, of the rated voltage an arrangement shown in Fig. 2 with 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 the set speed during starting. (i)

Timer TR2 shall be set as usual, i.e., more than the starting time of the motor.

(ii)

Timer TR1 shall be set 1-2 secs. below the locked rotor withstand time under hot condition.

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(iii)

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The speed switch on the motor shall be set to operate at a speed attained by 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/C contact provided in series with timer TR1 would de-energise it before it could operate. If however, the motor stalls, the speed switch remaining in the closed condition, timer TR1 would operate and trip the motor before the locked rotor hot withstand time is reached. Timers TR1 and TR2 may either be integral with the 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 either overloading or presence of negative sequence currents under both hot and cold conditions.

3.3.2

Type of Relay A thermal relay with inverse characteristics sensing and compensating for both the positive and negative sequence components of the load current to simulate a thermal image of the motor under hot and cold conditions is used. The relay shall 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 an alternative, a inverse relay sensing only the total load current can be used with a separate instantaneous negative sequence relay.

3.3.3

Setting Procedure A.

Current Setting The current setting chosen, shall be calculated using the formula IRel = IFL x IS x 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. ISSUE R1 FORM NO. 120 R1

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In case the relay has current settings available in steps the next higher setting (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 time constant of the motor being protected. However, this shall be chosen after 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 above 125 kW. This check consists of plotting the motor withstand curves and relay operating characteristics under both hot and cold conditions together with the motor starting current Vs time characteristics on the same graph. Normal relays available in the market have a choice of various operating characteristics under both hot and cold conditions. As far as possible, the relay characteristics should be so chosen that both relay hot and cold characteristics : (i)

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

(ii)

Follow the corresponding motor withstand characteristics as closely as possible - with a safe time difference.

(iii)

The relay hot operating characteristic lies above the motor starting current Vs time characteristic. If the above conditions are met then the stall protection is backed up by the thermal protection.

(a)

Case (i) The various motor and relay characteristics when dispositioned with 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) and the corresponding characteristics have been shown plotted in Fig. 3. However, as the withstand curves of motors vary widely with both ratings and makes, the following additional possibilities can be encountered, even after choosing the most optimum relay characteristic available on a particular make.

(b)

Case (ii) In this case either or both the relay characteristics, i.e., hot and cold, intersect with the corresponding motor withstand curves (Ref. Fig. 4). ISSUE R1 FORM NO. 120 R1

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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 motor beyond this point of intersection and as such the locked rotor relay, set as described under Clause 3.2.3 (b), would protect the motor under stall conditions. (c)

Case (iii) Where the motor starting current Vs time characteristic intersects or 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 relay thro' a 52A contact as shown in Fig. 5. With this arrangement, which is applicable to static relays only, it would be ensured that the relay cold characteristic would be applicable even in case of a 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 of overload protection will be offered and stall protection would still be available in this condition, this is acceptable since this condition lasts only for a short duration until the relay reaches thermal equilibrium once again. However, acceptability of this scheme 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 and terminal/cable faults.

3.4.2

Type of Relay Instantaneous overcurrent relays, one on each phase having low transient overreach (less than 5%) to prevent pick up during transient inrush currents when starting the motor.

3.4.3

Setting Procedure The relay shall be set at 1.5 times the starting current. The additional factor for 0.5 takes care of the CT & relay errors, transient over-reach of the relay and tolerance 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 a single pole instantaneous over-current relay immune to starting transients shall be used. ISSUE R1 FORM NO. 120 R1

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(b)

3.5.3

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On high resistance earthed systems, an instantaneous overcurrent relay having a sufficiently low pick up value depending on the system earth fault current and having a timer with setting of 1.0 sec. shall be used with a core balance CT having a ratio depending on the total maximum fault current.

Setting Procedure (a)

Effectively earthed/Low Resistance earthed systems The relay shall be set at 20% of the rated value in case of solidly earthed systems and at 10% in case of system grounded through a low resistance.

(b)

High Resistance Earthed Systems The maximum Earth fault current likely to flow due to the system capacitance is calculated. The relay is set at a value lower than 50% of the secondary equivalent of the fault current. The setting shall however, be higher than the fault current contributed by the capacitance of the largest motor on the bus to prevent the core balance relay on that particular feeder from operating. The CBCT manufacture shall be informed of the relay setting and total circuit burden. The performance of the CBCT shall be guaranteed by the CBCT manufacturer at the minimum 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 faults for motors connected to solid by earthed or low resistance earthed systems. For high 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 current relays are used with a stabilising resistor in series with each relay.

3.6.3

Setting Procedure The relay shall normally be set at 20% of the full load current. The stabilising resistor 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 considered instead 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 continuously to enable the operator to take suitable measures, if possible, to avoid ultimate ISSUE R1 FORM NO. 120 R1

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tripping of the motor on operation of thermal protection. provided on motors above 175 kW. 3.7.2

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This feature is

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 shall have a range of 70-130%, preferably continuously adjustable or adjustable in steps of 5%.

3.7.3

Setting Procedure The current setting adopted should lie in the region of 110 to 120% of the motor full load current. The timer should be set to operate at a value larger than the starting 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 not discussed in this guide. In case where RTD's for winding temperature monitoring are used with remote sensing/monitoring devices which require to be set, th trip temperature setting shall be bout 10°C less than the withstand temperature of the class of insulation used in the motor. 4.0

TIE FEEDERS AND BUS COUPLERS/BUS SECTION A tie feeder is a connection between two individual switchboards with circuit breakers or circuit breakers and switches at the sending and receiving ends, whereas a bus coupler or a bus sectionalising breaker couples the two sections of the same switchboard. The protections normally provided on these feeders are: (a)

Inverse definite minimum time overcurrent relay with normal characteristic or very inverse characteristic* *(Only in cases where there is a large variation in fault current at the sending and receiving end switchboards due to the large impedance of the connecting cable).

(b)

Inverse definite minimum time earth fault relay (only in case of effectively earthed or low resistance earthed systems). ISSUE R1 FORM NO. 120 R1

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The type and procedure for setting these relays are identical to that detailed under 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 high resistance. As this protection may be sensitive to earth faults throughout the system, it cannot be used for tripping any feeder in particular but is connected to give an alarm only.

5.2.3

Setting Procedure Recommended voltage settings : (a)

U/V relay for motors

- 80% (nominal voltage)

(b)

For initiating auto changeover

- 20% (nominal voltage)

(c)

For monitoring the bus onto which the load gets transferred during auto changeover - 80% (nominal voltage) Recommended time setting : 1 second.

ISSUE R1 FORM NO. 120 R1

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TABLE - 1 : SUMMARY OF PROTECTIONS PROVIDED FOR MOTORS --------------------------------------------------------------------------------------------------------------------------------------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 --------------------------------------------------------------------------------------------------------------------------------------1 2 3 4 5 6 7 8 9 10 11 12 13 --------------------------------------------------------------------------------------------------------------------------------------1.

0-99

415

Contactor 3 Nos. 1 No. (3 Phase)

-

-

-

-

-

-

2. 100-125 415

Contactor 3 Nos. 1 No. (3 Phase)

2 Nos.

-

-

-

-

-

-

3. 126-175 415 (Alt-1)

Circuit Breaker

-

3 Nos. (1 per Phase, inbuit with breaker)

2 Nos.

-

4. 126-175 415 (Alt-2)

Circuit Breaker

-

-

2 Nos.

3 Nos. (1 per Phase, inbuilt with breaker)

1 No.* 3 Nos. (1 per Phase)

5. 176-1500 6600/ Circuit 11000 Breaker

-

-

2 Nos. 1 No.*

6. Above 6600/ Circuit 1500 11000 Breaker

-

-

2 Nos. 1 No.*

3 Nos. 1 No. Provided (1 per (from bus Phase) u/v relay)

1 No.

3 Nos. 1 No. Provided 3 Nos. 1 No. (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' and Standard 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". ISSUE R1 FORM NO. 120 R1

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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 connected to the R & B phases.

ISSUE R1 FORM NO. 120 R1