5.2_ACB_Releases

Electrical Standard Products

Switchgear Training Centre, Coonoor

Selection of Air Circuit Breakers & Various Types of Release Settings

– Case Study

by: K.Sivakumar, Switchgear Training Centre, Coonoor.

EXAMPLE OF ACB SELECTION

SELECTION OF ACB

‘11’, ‘21’ & ‘31’ IN THE SHOWN

SINGLE LINE DIAGRAM

SINGLE LINE DIAGRAM

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F1

F3

PCC

MCC BUSBAR

U/G CABLE

M2M1 M3 M4

11

21 22 23 24

31

50 HP 100 HP 30 HP 150 HP

) CB

SDF

STARTER

G

)

)

F2

12

13

STANDBY GENERATOR1250 kVA

TRANSFORMER1600 kVA

350 A 300 A 300 A

SYSTEM DATA

TRANSFORMER : 11/0.433 kV1600 kVA

STANDBY GENERATOR : UrG = 415 V1250 kVA

INDUCTION MOTORS : M1, IrM = 70 AM2, IrM = 135 AM3, IrM = 40 AM4, IrM = 200 A

AIR CIRCUIT BREAKER ‘11’REQUIRED CURRENT RATING

TRANSFORMER FL CURRENT = 1600 × 1000 3 × 433

= 2134 A

CONSIDERING 10% OVERLOADING, TOTAL CURRENT = 1.1 × 2134 = 2347 A

In OF CB ‘11’ 2347 A

AIR CIRCUIT BREAKER ‘11’

REQUIRED S/C BREAKING CAPACITY

Ics OF CB ‘11’ IK SUPPLIED BYTRANSFORMER AT F1

38.77 kA


REQUIRED S/C MAKING CAPACITY

Icm OF CB ‘11’ iP SUPPLIED BYTRANSFORMER AT F1

87.71 kA(Peak)


REQUIRED S/C MAKING & BREAKINGCAPACITY SAME AS FOR ACB ‘11’

REQUIRED CURRENT RATING SAMEAS FOR ACB ‘31’


REQUIRED CURRENT RATING

In OF CB ‘31’ (IrM)

(70 + 135 + 40 + 200)

445 A


REQUIRED S/C BREAKING CAPACITY

Ics OF CB ‘31’ IK SUPPLIED BYTRANSFORMER AT F3

15.67 kA


REQUIRED S/C MAKING CAPACITY

Icm OF CB ‘31’ iP SUPPLIED BYTRANSFORMER AT F3

27.70 kA(Peak)


Parameter RequiredAvailable

C‐Power (2500 C) U‐Power (2500 S)

In > 2347 A 2500 A 2500 A

Ics > 38.77 kA 60 kA 65 KA

Icm > 87.71 kA 132 kA 143 KA

Icw NA 60 kA for 1 s 55 KA for 1 s



C‐Power (630 S1) U‐Power (630 S)

In > 445 A 630 A 630 A

Ics > 38.77 kA 50 kA 55 kA

Icm > 87.71 kA 105 kA 121 kA

Icw NA 50 kA for 0.5 s 50 kA for 1 s



C‐Power (CN‐CS..E) U‐Power (630S)

In > 445 A 630 A 630 A

Ics > 15.67 kA 25 kA 55 kA

Icm > 27.70 kA 52.5 kA 121 kA

Icw NA 25 kA for 1 s 50 kA for 1 s

Release setting tutorial

C‐Power ACB & DN1 Release

AIR CIRCUIT BREAKER ‘31’Current Transformer Primary Current: 630ACT Taps available: 80% (504A) & 100% (630A)CT Tap selected: (In) = 80% (504A)

DN1 Thermal Release Setting (Ir):Settings Available: 75%, 90% & 100% of InSettings required: 445/504 = 0.88 InNearest setting available: 0.9 In (453.6A)This is only about 2% higher than the aggregate rated full load currents of the motors in this MCC, which is acceptable.

So, set the DN1 Thermal Release Setting (Ir) to: 0.9 In(453.6A)

AIR CIRCUIT BREAKER ‘31’DN1 Magnetic Release Setting (Is):

Settings available: 500% In (2520A) & 700% In(3528A)

This ACB ‘31’ is at the fag end of power distribution. As such, it need not have any discrimination with any downstream ACBs. So, this can be set to the minimum possible setting of 500% In (2520). At this setting, for the actual fault current of 15.67kA at “F3”, this ACB will trip in about 25ms. This is quite alright.

So, set the DN1 Magnetic Release Setting (Is) to: 500% of In (2520A).

AIR CIRCUIT BREAKER ‘21’Current Transformer Primary Current: 630ACT Taps available: 80% (504A) & 100% (630A)CT Tap selected: In = 80% (504A)

DN1 Thermal Release Setting (Ir):Settings Available: 75%, 90% & 100% of InSettings required: 445/504 = 0.88 InNearest setting available: 0.9 In (453.6A)

But, the DN1 Thermal Release of ACB ‘31’ is also set to the same level of 0.9 In (453.6A). Also, as both the releases are of the same DN1 type, their Thermal Curves are identical. Hence, there cannot be any time discrimination between these two ACBs.

AIR CIRCUIT BREAKER ‘21’So, only current discrimination can be effected between these two ACBs. To have an effective current discrimination, the DN1 Thermal Release of this ACB ‘21’ shall be set to the next available setting (i.e.) 100% In (504A). At this setting, the setting is 13.25% more than the aggregate of the rated currents of the group of motors fed by this MCC. This is not acceptable as it is more than 10%.

AIR CIRCUIT BREAKER ‘21’Alternately, the CT tap in this ACB can be set to 100% (In = 630A) and the DN1 Thermal Release can be set to 0.75 In (i.e.) 472.5A, which will offer a closer current discrimination with ACB ‘31’. Also, this will only be 6.2% higher than the aggregate rated currents, which is acceptable.

So, the final settings are:

CT tap selected: In = 100% (630A)

Set the DN1 Thermal Release Setting (Ir) to: 0.75 In(472.5A)


Settings available: 500% In (3150A) & 700% In(4410A)As both the DN1 Magnetic Releases have an MIT of 25ms, there cannot be time discrimination between these two ACB Magnetic Releases. Even if we set this release to the maximum possible setting of 700% In (4410A), we cannot avoid this ACB ‘21’ from tripping for the fault at “F3” as the fault current is 15.67kA. So, this release can be set to the minimum as a safety measure. So, set the DN1 Magnetic Release Setting (Is) to: 500% In (3150A).

AIR CIRCUIT BREAKER ‘11’Current Transformer Primary Current: 2500ACT Taps available: 80% (2000A) & 100% (2500A)CT Tap selected: In = 100% (2500A)

DN1 Thermal Release Setting (Ir):Settings Available: 75%, 90% & 100% of InSettings required: 2347/2500 = 0.9388 InNearest setting available: 0.9 In (2250A)So, set the DN1 Thermal Release Setting (Ir) to: 0.9 In (2250A)


Settings available: 500% In (12500A) & 700% In(17500A)

If we set this release to 500% In (12500A), then there is a chance that for a fault at “F3” (fault current is 15.67kA) this might trip first, which is not preferable. So, this release shall be set to: 700% In (17500A), in which case, this will not react for the “F3” fault current. Current discrimination is achieved thus.

So, set the DN1 Magnetic Release Setting (Is) to: 700% In (17500A).

AIR CIRCUIT BREAKER ‘11’However, we cannot avoid tripping of this ACB ‘11’ for a fault beneath ACB ‘21’ as the fault current then would be 38.77kA and the maximum possible setting is 17.5kA.

This, in fact, is the problem with conventional Thermal-Magnetic Releases. Effective Co-ordination cannot be had with upstream/ downstream releases. If one wants effective co-ordination, then one has to choose an advanced static release.

AIR CIRCUIT BREAKER ‘11’, ‘21’ & ’31’Final Release Settings with DN1 Thermal‐Magnetic Releases

Parameter ACB ‘11’ ACB ‘21’ ACB ‘31’

CT 2500A 630A 630A

CT Tap Selected (In) 100% (2500A) 100% (630A) 80% (504A)

DN1 Thermal Release Setting (Ir)

0.9 In (2250A) 0.75 In (472.5A) 0.75 In (453.6A)

DN1 Magnetic Release Setting (Is)

700% In (17.5kA) 500% In (3150A) 500% In (2520A)

Icw Required38.77kA for TOT of

HT CB 38.77kA for 65ms 15.67kA for 65ms

Icw Actual 60 kA for 1 s 50 kA for 0.5 s 25 kA for 1 s

ACB can tolerate the actual fault current for (2.39s) (831ms) (2.54s)

Final Release settings with DN1 Thermal‐Magnetic Releases

Hence it can be established that the selected ACBs are adequate for the application with respect to:

i) The Rated Currentii) The Breaking Capacityiii) The Making Capacityiv) The Short Time With stand Current

Hence, selection is Okay.

AIR CIRCUIT BREAKER ‘11’, ‘21’ & ’31’

Release setting tutorial

C‐Power ACB & SR 18/SR18G Release

AIR CIRCUIT BREAKER ‘31’Current Transformer Primary Current: 630ACT Taps available: 100% (630A)CT Tap selected: In = 100% (630A)Long Time Pick-up Current Setting (Ir):Settings available: 0.5 to 1.0 InSettings required: 445/630 = 0.71 InNearest setting available: 0.75 In (472.5A)This is only about 6.2% higher than the aggregate rated full load currents of the motors in this MCC, which is acceptable. So, set the Long Time Pick-up Current Setting (Ir) to: = 0.75 In (472.5A)The Long Time Delay Setting (Tr) is fixed at 2.5 sec at 6 Ir.

AIR CIRCUIT BREAKER ‘31’Short Time Pick-up Current Setting (Is):

Settings available: 2 to 10 Ir.

The consideration for this setting is that the release should not pick-up for the worst possible motor-start conditions. That is - with all other motors ON and the largest starting load is effected, then also this release should not pick-up.

There are 4 motors in the MCC. The worst condition starting analysis can be made as below:


Parameter M1 M2 M3 M4 Total Load on MCC

IrM 70A 150A 40A 200A

Ist 420A 300A 240A 400A

Condition

START ON ON ON 810A

ON START ON ON 610

ON ON START ON 660

ON ON ON START 460

So, the worst starting load is encountered, when M2, M3 & M4 are already ON and M1 is started, imposing a load of 810A on the MCC. Short Time Release shall not Pick-up for this.

AIR CIRCUIT BREAKER ‘31’So, the Short Time Pick-up Current setting shall be: 810/472.5 = 1.7 Ir. The nearest higher setting available is 2 Ir (i.e.) 945A.

So, set the Short Time Pick-up Current Setting (Is)to: 2 Ir (945A).

AIR CIRCUIT BREAKER ‘31’Short Time Delay Setting (Ts):Settings available: 20ms to 600ms

This ACB ‘31’ has to have time discrimination with downstream SCPDs, which in this case are the HRC Fuses in the Motor Feeders. As HRC Fuses are fast acting, the time delay setting of this release can be set to the minimum. However, to avoid spurious tripping of this release during transient motor starting in-rush conditions, the time delay can be set to 100ms, by which time the inrush would have subsided. So, set the Short Time Delay Setting (Ts) to: 100ms.

AIR CIRCUIT BREAKER ‘31’Ground Fault Pick-up Current Setting (Ig):

Settings available: 0.2 to 0.6 In.

If there are any unbalanced load distribution in the system, then this pick-up setting has to take allowance of that unbalance due to uneven distribution/utilisation of single-phase loads across all the three phases. But, in this MCC all the loads are Motor Loads, which are three-phase, three-wire. As such, no unbalance is anticipated. Hence, the pick-up current setting can be as minimum as possible.So, set the Ground Fault Pick-up Current Setting (Ig) to: 0.2 In (126A).

AIR CIRCUIT BREAKER ‘31’Ground Fault Time Delay Setting (Tg):Settings available: 100ms to 400ms

This ACB ‘31’ has to have time discrimination with downstream GFPDs. But, as the downstream feeders are conventional motor starters only, no specific ground fault protection is assumed to be provided. So, this release need not have any time discrimination. However, to avoid spurious tripping during motor start/cold-in-rush conditions, there can be a small time delay of 100ms, by which time any unbalance would have subsided.So, set the Ground Fault Time Delay Setting (Tg) to: 100ms.

AIR CIRCUIT BREAKER ‘21’Current Transformer Primary Current: 630ACT Taps available: 100% (630A)CT Tap selected: In = 100% (630A)Long Time Pick-up Current Setting (Ir):Settings Available: 0.5 to 1.0 InSettings required: 445/630= 0.7061 InNearest setting available: 0.75 In (472.5A)But, the Long Time pick-up Current Setting of ACB‘31’ is also set to the same level of 0.75 In (472.5A).Also, as both the releases are of the same SR 18type, their Thermal Curves are identical, as the timedelay setting for Long Time Pick-up is fixed at 2.5sec. @ 6 Ir. Hence, there cannot be any timediscrimination between these two ACBs.

AIR CIRCUIT BREAKER ‘21’So, only current discrimination can be effectedbetween these two ACBs. To have an effectivecurrent based co-ordination, the Long Time Pick-upCurrent Setting of this ACB ‘21’ Release shall be setto the next available setting (i.e.) 80% In (504A). Atthis setting, the setting is 13.26% more than theaggregate of the rated currents of the group of motorsfed by this MCC. This is acceptable, in view of co-ordination requirements.

So, set the Long Time Pick-up Current Setting (Ir) to: 0.80 In (504A).The Long Time Delay Setting (Tr) is fixed at 2.5 sec at 6 Ir.


Settings available: 2 to 10 times Ir

Considering motor start duty requirements as in the case of ACB ‘31’ this ACB Short Time Pick-up Current Setting also has to be more than 810A. The setting required: 810/504 = 1.60 Ir. The nearest setting available is 2 Ir (i.e.) 1008A.

So, set the Short Time Pick-up Current Setting (Is) to: 2 Ir.


This ACB ‘21’ has to have time discrimination with ACB ‘31’ Short Time Release. The minimum impulsion time of ACB ‘21’ short time release must be more than the TOT of ACB ‘31’ short time release. The time setting should be:MIT of ACB ‘31’ Release + BOT of ACB ‘31’ + A safety margin of about 100ms = 100 + 40 + 100 = 240 ms.The nearest higher setting available is: 260ms.

So, set the Short Time Delay Setting (Ts) to: 260ms.


Ground Fault Pick-up Current Setting (Ig):

Settings available: 0.2 to 0.6 In.

Similar to ACB ‘31’, this is also feeding to motor loads only. Hence, no load related unbalance is anticipated. GF Pick-up current setting can be as minimum as possible.

So, set the Ground Fault Pick-up Current Setting (Ig) to: 0.2 In (126A).


This ACB ‘21’ has to have time discrimination with ACB ‘31’ GF Release. The minimum impulsion time of ACB ‘21’ GF release must be more than the TOT of ACB ‘31’ for a GF. The time setting should be:MIT of ACB ‘31’ GF Release + BOT of ACB ‘31’ + A safety margin of about 100ms = 100 + 40 + 100 = 240 ms. The nearest higher setting available is: 300ms.

So, set the Ground Fault Time Delay setting (Tg) to: 300ms.

AIR CIRCUIT BREAKER ‘11’Current Transformer Primary Current: 2500ACT Taps available: 100% (2500A)CT Tap selected: In = 100% (2500A)

Long Time Pick-up Current Setting (Ir):Settings available: 0.5 to 1.0 InSettings required: 2347/2500 = 0.9388 InNearest setting available: 0.95 In (2375A)

So, set the Long Time Pick-up Current Setting (Ir) to: 0.95 In (2375A)

The Long Time Delay Setting (Tr) is fixed at 2.5 sec at 6 Ir.


Settings available: 2 Ir (4750) to 10 Ir (23750A)

As time based discrimination is possible with the short time releases of ACBs, ‘11’, ‘21’ & ’31’, we can set this current setting in the minimum so that it can even react for a fault at “F3”. As a safe measure, we can set this to the minimum setting of 2 Ir (4750A).

So, set the Short Time Pick-up Current Setting (Is) to: 2 Ir (4750A).


This ACB ‘11’ has to have time discrimination with ACB ‘21’ Short Time Release. The minimum impulsion time of ACB ‘11’ short time release must be more than the TOT of ACB ‘21’ for a Short Circuit. The time setting should be:MIT of ACB ‘21’ Release + BOT of ACB ‘21’ + A safety margin of about 100ms = 260 + 40 + 100 = 400 ms. The nearest setting available is: 400ms.

So, set the Short Time Delay Setting (Ts) to: 400ms.

AIR CIRCUIT BREAKER ‘11’Ground Fault Pick-up Current Setting (Ig):Settings available: 0.2 to 0.6 In.

We are not sure of the type of loads that areconnected to this PCC. There can be single-phaseloads too. However, as a safe practice, we will keepthis release at the minimum of the setting (i.e.) 0.2 In(500A). If the ACB trips on GF during regularoperation, the neutral current can be measured andthis setting be kept slightly above the maximummeasured neutral current under fully-loaded, healthyoperating conditions. However, for the time being, setthe Ground Fault Pick-up Current Setting (Ig) to : 0.2In (500A)


This ACB ‘11’ has to have time discrimination with ACB ‘21’ GF Release. The minimum impulsion time of ACB ‘11’ GF release must be more than the TOT of ACB ‘21’ for a GF. The time setting should be:MIT of ACB ‘21’ GF Release + BOT of ACB ‘21’ + A safety margin of about 100ms = 300 + 40 + 100 = 440 ms. The nearest setting available is: 400ms.

So, set the Ground Fault Time Delay Setting (Tg) to: 400ms.

AIR CIRCUIT BREAKER ‘11’, ‘21’ & ’31’Final Release Settings with SR 18/ SR 18G Releases

Parameter ACB ‘11’ ACB ‘21’ ACB ‘31’

CT 2500A 630A 630A

CT Tap Selected (In) 100% (2500A) 100% (630A) 100% (630A)

LTPU (Current) (Ir) 0.95 In (2375A) 0.80 In (504A) 0.75 In (472.5A)

LTPU (Time Delay) (Tr) 2.5 s @6 Ir 2.5 s @6 Ir 2.5 s @6 IrSTPU (Current) (Is) 2 Ir (4750A) 2 Ir (1008A) 2 Ir (945A)

STPU (Time Delay) (Ts) 400ms 260ms 100ms

GFPU (Current) (Ig) 0.2 In (500A) 0.2 In (126A) 0.2 In (126A)

GFPU (Time Delay) (Tg) 400ms 300ms 100ms

Icw Required38.77kA for TOT of

HT CB 38.77kA for 700ms 15.67kA for 400ms

Icw Actual 60 kA for 1 s 50 kA for 0.5 s 25 kA for 1 s

ACB can tolerate the actual fault current for (2.39s) (831ms) (2.54s)

Final Release settings with SR 18/SR 18G Releases

Hence it can be established that the selected ACBs are adequate for the application with respect to:

i) The Rated Currentii) The Breaking Capacityiii) The Making Capacityiv) The Short Time With stand CurrentHence, selection is Okay.

AIR CIRCUIT BREAKER ‘11’, ‘21’ & ’31’

Thank You

stc‐[email protected]@LNTEBG.com

Documents

5.2_ACB_Releases