selective coordination study

8/21/2019 Selective Coordination Study

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Selective Coordination Study

Recommended Procedures

14

The following steps are recommended when

conducting a selective coordination study.

1. One-Line Diagram

Obtain the electrical system one-line diagram that identifies

important system components, as given below.

a. TransformersObtain the following data for protection and coordination infor-

mation of transformers:

- KVA rating

- Inrush points

- Primary and secondary connections

- Impedance

- Damage curves

- Primary and secondary voltages

- Liquid or dry type

b. Conductors - Check phase, neutral, and equipment

grounding. The one-line diagram should include infor-

mation such as:

- Conductor size

- Number of conductors per phase- Material (copper or aluminum)

- Insulation

- Conduit (magnetic or non-magnetic)

From this information, short circuit withstand curves can be

developed. This provides information on how overcurrent

devices will protect conductors from overload and short

circuit damage.

c. Motors

The system one-line diagram should include motor

information such as:

- Full load currents

- Horsepower

- Voltage

- Type of starting characteristic

(across the line, etc.)

- Type of overload relay

(Class 10, 20, 30)

Overload protection of the motor and motor circuit can be

determined from this data.

d. Fuse Characteristics

Fuse Types/Classes should be identified on the one-line

diagram.

e. Circuit Breaker Characteristics

Circuit Breaker Types should be identified on the one-line

diagram.

f. Relay Characteristics

Relay Types should be identified on the one-line diagram.

2. Short Circuit Study

Perform a short circuit analysis, calculating maximum

available short circuit currents at critical points in the

distribution system (such as transformers, main switchgear,

panelboards, motor control centers, load centers, and large

motors and generators.) (Reference: Bussmann Bulletin,Engineering Dependable Protection - EDPI.)

3. Helpful Hints

a. Determine the Ampere Scale Selection. It is most

convenient to place the time current curves in the center of

the log-log paper. This is accomplished by multiplying or

dividing the ampere scale by a factor of 10.

b. Determine the Reference (Base) Voltage. The best

reference voltage is the voltage level at which most of the

devices being studied fall. (On most low voltage industrial

and commercial studies, the reference voltage will be 208,

240, or 480 volts). Devices at other voltage levels will be

shifted by a multiplier based on the transformer turn ratio.

The best reference voltage will require the least amount ofmanipulation. Modern computer programs will automat-

ically make these adjustments when the voltage levels of

devices are identified by the input data.

c. Commencing the Analysis. The starting point can be

determined by the designer. Typically, studies begin with

the main circuit devices and work down through the

feeders and branches. (Right to left on your log-log paper.)

d. Multiple Branches. If many branches are taken off one

feeder, and the branch loads are similar, the largest rated

branch circuit should be checked for coordination with

upstream devices. If the largest branch will coordinate, and

the branch devices are similar, they generally will

coordinate as well. (The designer may wish to verify other

areas of protection on those branches, conductors, etc.)e. Don't Overcrowd the Study. Many computer generated

studies will allow a maximum of ten device characteristics

per page.

f. One-Line Diagram. A one-line diagram of the study

should be drawn for future reference.


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The following pages will analyze in detail the system

shown in Figure 11. It is understood that a short circuit

study has been completed, and all devices have adequate

interrupting ratings. A Selective Coordination Analysis is the

next step.

Examples of Selective Coordination Studies

15

M

JCN80EIFLA=42A

5.75% Z1000KVA

-Y480/277V

LOW-PEAK

KRP-C-1600SP

Fault X120,000A RMS Sym

Main Switchboard

1

LOW PEAK

LPS-RK-200SPLOW-PEAK

LPS-RK-400SPLOW-PEAK

LPS-RK-225SP

200A Feeder 400A Feeder 150KVA-Y

208/120V2% Z

LOW-PEAK

LPN-RK-500SPLOW-PEAK

LPS-RK-100SP

20A Branch

20A CB

20A CB

LP1

60HP 377A FLA

1600A Main Bus

PDP

13.8KV

Overcurrent Relay

#6 XLP

#3/0 THW

100A Motor Branch

#1 THW

250 kcmil2/ THW

#12 THW

This simple radial system will involve three separate

time current curve studies, applicable to the three feeder/

branches shown.

Figure 11


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Example

Time Current Curve #1 (TCC1)

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Device ID Description Comments

1 1000KVA XFMR 12 x FLA

Inrush Point @ .1 Seconds

2 1000KVA XFMR 5.75%Z, liquid

Damage Curves filled

(Footnote 1)

(Footnote 2)

3 JCN 80E E-Rated Fuse

4 #6 Conductor Copper, XLP

Damage Curve Insulation

5 Medium Voltage Needed for XFMR

Relay Primary Overload

Protection

6 KRP-C-1600SP Class L Fuse

11 LPS-RK-200SP Class RK1 Fuse

12 3/0 Conductor Copper THW


13 20A CB Thermal Magnetic

Circuit Breaker

14 #12 Conductor Copper THW


Footnote 1: Transformer damage curves indicate when it will be damaged,thermally and/or mechanically, under overcurrent conditions.

Transformer impedance, as well as primary and secondaryconnections, and type, al l wil l determine their damagecharacteristics.

Footnote 2: A -Y transformer connection requires a 15% shift, to the right,of the L-L thermal damage curve. This is due to a L-Lsecondary fault condition, which will cause 1.0 p.u. to flowthrough one primary phase, and .866 p.u. through the twofaulted secondary phases. (These currents are p.u. of 3-phasefault current.)

Notes:

1. TCC1 includes the primary fuse, secondary main fuse,

200 ampere feeder fuse, and 20 ampere branch circuit

breaker from LP1.

2. Analysis will begin at the main devices and proceed

down through the system.

3. Reference (base) voltage will be 480 volts, arbitrarily

chosen since most of the devices are at this level.

4. Selective coordination between the feeder and branch

circuit is not attainable for faults above 2500 amperes that

occur on the 20 amp branch circuit, from LP1. Notice the

overlap of the 200 ampere fuse and 20 ampere circuit

breaker.

5. The required minimum ratio of 2:1 is easily met between

the KRP-C-1600SP and the LPS-RK-200SP.


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Example


17

CURRENT IN AMPERES X 10 @ 480V

TIMEIN

SECONDS

FLA

11

2

2

3

5

12

14

13

1

6

#12 DAMAGE

3/0 DAMAGE

20A MCCB

XFMRDAMAGE

4

JCN 80E

MV OLR

1000KVA5.75%Z-Y480/277V

JCN80E

13.8KV

KRP-C-1600SP

LPS-RK-200SP

200AFeeder

#6 XLP

#3/0 THW

OvercurrentRelay

20A CB

#12 THW20A CB

KRP-C-1600SP

TXINRUSH

600

400

300

200

100

80

60

40

30

20

10

8

6

4

3

2

1

.8

.6

.4

.3

.2

.1

.08

.04

.06

.03

.02

.01

800

1000

1 2 3 4 6 810

20

30

40

60

80

100

200

300

400

600

800

1000

2000

3000

4000

6000

8000

10

,000

LPS-RK-200SP

#6 DAMAGE


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Example


18

Notes:


400 ampere feeder fuse, 100 ampere motor branch fuse,

77 ampere motor and overload relaying.


down through the system.

3. Reference (base) voltage will be 480 volts, arbitrarily


Device ID Description Comment


Inrush Point @ .1 seconds



(Footnote 1)

(Footnote 2)






Protection



22 Motor Starting Curve Across the Line

Start

23 Motor Overload Relay Class 10

24 Motor Stall Point Part of a Motor

Damage Curve

25 #1 Conductor Copper THW






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Example


19

KRP-C-1600SP

LPS-RK-400SP

LPS-RK-100SP

60HP

400A Feeder


TIMEIN

SECOND

S

MV OLR

2

2

3

5

1

6

4

21

2225

23

24

FLA

MS

JCN80EMTR START

LPS-RK-100SP

MTR OLRXFMR DAMAGE

M

#1 THW

TXINRUSH

#6 XLP

13.8KV

OvercurrentRelay

JCN 80E

1000KVA5.75%Z-Y480/277V

600

400

300

200

100

80

60

40

30

20

10

8

6

4

3

2

1

.8

.6

.4

.3

.2

.1

.08

.04

.06

.03

.02

.01

800

1000

1 2 3 4 6 810

20

30

40

60

80

100

200

300

400

600

800

1000

2000

3000

4000

6000

8000

10

,000

KRP-C-1600SP

#1 DAMAGE

#6 DAMAGE


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Example


20

Notes:


225 ampere feeder/transformer primary and secondary

fuses.


down through the system.3. Reference (base) voltage will be 480 volts, arbitrarily


4. Relative to the 225 ampere feeder, coordination between

primary and secondary fuses is not attainable, noted by

overlap of curves.

5. Overload and short circuit protection for the 150 KVA

transformer is afforded by the LPS-RK-225SP fuse.

Device ID Description Comment


Inrush Point @ .1 seconds



(Footnote 1)

(Footnote 2)






Protection



32 150 KVA XFMR 12 x FLA

Inrush Point @.1 Seconds

33 150 KVA XFMR 2.00% Dry Type

Damage Curves (Footnote 3)

34 LPN-RK-500SP Class RK1 Fuse

35 2-250kcmil Conductors Copper THW





Footnote 3: Damage curves for a small KVA (


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Example


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600

400

300

200

100

80

60

40

30

20

10

8

6

4

3

2

1

.8

.6

.4

.3

.2

.1

.08

.04

.06

.03

.02

.01

TIMEIN

SECOND

S

800

1000

4

5

2

FLA FLA

MV OLR

XFMR DAMAGE

JCN80E

XFMR DAMAGE 33

35

LPS-RK-225SP

LPN-RK-500SP

KRP-C-1600SP

TXINRUSH

TXINRUSH

132

6

#6 XLP

JCN 80E

13.8KV

OvercurrentRelay

250 kcmil2/ THW

31

34

KRP-C1600SP

23

1000KVA5.75%Z-Y480/277V

150KVA2.0%Z-Y208/120V

1 2 3 4 6 810

20

30

40

60

80

100

200

300

400

600

800

1000

2000

3000

4000

6000

8000

10

,000

LPS-RK-225SP

LPN-RK-500SP

2-250 DAMAGE

#6 DAMAGE


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Unnecessary power OUTAGES, such as the

BLACKOUTS we so often experience, can be stopped by

isolating a faulted circuit from the remainder of the system

through the proper selection of MODERN CURRENT-

LIMITING FUSES.

Time-Delay type current-limiting fuses can be sizedclose to the load current and still hold motor-starting

currents or other harmless transients, thereby

ELIMINATING nuisance OUTAGES.

The SELECTIVITY GUIDE on page 10 may be used for

an easy check on fuse selectivity regardless of the short-

circuit current levels involved. Where medium and high

voltage primary fuses are involved, the time-current

characteristic curves of the fuses in question should be

plotted on standard NEMA log-log graph paper for proper

study.

The time saved by using the SELECTIVITY GUIDE will

allow the electrical systems designer to pursue other areas

for improved systems design.

Conclusions

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selective coordination study

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