15418296 electrical distribution systems questions answers part i

8/8/2019 15418296 Electrical Distribution Systems Questions Answers Part I

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ELECTRICAL DISTRIBUTION SYSTEMS I.

Introduction to the course: this course is provided in a question & answer format and is divided into 5

chapters, it will become extremely useful to you:

If you want to know the parameters of the induction motors, their effect on the starting andnormal performance of the machine.

If you want to analyze an existing a.c. machine or evaluate a new one for its steady state or

transient state including starting and short circuit.

If you want to know the major components in the power distribution systems and how each

component is defined.

If you want to calculate the line and cable constants from the information found in standard

tables.

If you want to know the different types of breakers, starters and switchgear assemblies. Also, if

you want to know how they are defined.

If you want to do the necessary calculations to size the load breaking/interrupting/disconnecting

devices for normal and fault conditions.

If you want to know the types and sources of power line disturbances. Also, if you want to know

the effects of such disturbances on the major components of the power systems and the methods

of reducing these damaging effects.

If you want to know the effects of using local capacitors in increasing the circuit capacity and

how to size capacitors to improve the p.f. in motor circuits.

If you want to quantify the effect of direct and indirect lightning strokes on overhead distribution

systems.

If you want to know what are the data that have to be available in order to perform any of the

following studies: TRV, stability, load flow, fault/co-ordination, motor starting, reliability and

switching transients.

If you to know the procedure to calculate the following: basic values including base and p.u.

values, load flow, fault current/voltage sensitivity, motor starting, reliability studies for simple

radial systems.

If you want to know how steel properties, switchboard instruments, meters, relays are defined

and what are the major modules of PLC plus the building blocks of an office automation

system.

If you want to know the calculations to be performed if a major power transformer is to beprotected against short circuits or overloads.

If you want to know what are the essential programs and data for the power systems analyst (or

any other individual involved in electric power distribution and studies - for that matter).

Contents:

1. A.C. Electric machines parameters & performance.


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2. Overhead and undeground distribution systems components.

3. Switchgear, circuit breakers, MCC and starters.

4. Power line disturbances and power quality.

5. Power systems studies.

Lesson 1: AC electric machines parameters & performance.

1) What are the parameters of the induction motors and what are the tests to be performed on such

machines to be able to obtain the values of these parameters?

2) How can the performance of an induction motor be analyzed?

3) What are the important characteristics that are used in defining synchronous machines (in

general) and synchronous generators (in particular)? Sketch the equivalent circuit under the

different conditions of operation.

4) What are the data required from testing to calculate the parameters of synchronous motors?

5) Sketch the equivalent circuit of single phase transformer and list the required tests (to be

performed on transformers) to obtain the values of the parameters of the equivalent circuit?

6) A solved problem regarding the running & starting performance of an induction motor.

7) A solved problem regarding the characteristics of synchronous machines.

8) A solved problem regarding the parameters of single phase transformers.

9) A solved problem regarding the characteristics of 3-winding transformers.

10) Summarize the methods used in calculating the parameters of synchronous and induction

machines.

Lesson summary

References

1) What are the parameters of the induction motors and what are the tests to be

performed on such machines to be able to obtain the values of these parameters?

The parameters of the induction motors are: the stator resistance per phase, stator leakage

reactance/phase, rotor resistance/phase, rotor leakage reactance/phase, main flux susceptance

and conductance/phase. For motors under starting conditions the parameters are the same as

above except the values of the rotor resistance and reactance (referred to the stator) are higher

(due to skin effect) and lower (due to the skin effect & saturation), respectively. The tests to be

performed on such machines to be able to calculate the parameters of the machine are the no

load (open circuit) and locked rotor (short circuit) under full and reduced voltage. The reduced

voltage test is run to get the unsaturated reactance values (for rotor & stator). The data to be

collected from the no load test are: Primary voltage, the no load current and power at 75C (or 25

and corrected to 75); from the locked rotor: the voltage, current and power at 75C; from the

locked rotor (reduced voltage): the voltage and current. For the first 2 tests, the nominal motor

voltage is applied, if possible. Fig. 1 shows the equivalent circuit of a S.C.I.M.


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R1: stator resistance per phase, X1: stator leakage reactance/phase, R2': rotor resistance/phase

referred to stator, s: slip, X2': rotor leakage reactance referred to stator, Bm: main flux

susceptance, Gm: main flux conductance.

2) How can the performance of an induction motor be analyzed?

The performance of induction motors can be analyzed by studying the following points: heating

of winding/iron, efficiency of motor, power factor of machine, pull-out (maximum) torque,

starting torque, starting currents and the effect of the parameters on such points. a) Heating of

winding and iron: to reduce winding heating rotor and stator resistances have to be small.

Though for a high starting torque, the rotor resistance has to be high. To reduce iron losses, the

main flux has to be low. Note that the main flux and rotor current affect the torque.

b) Efficiency: to have a high efficiency motor, the windings (copper) and iron losses have to be

kept to the minimum possible.

c) Power factor: to achieve a high power factor machine, the leakage reactances (stator and

rotor) have to be low i.e. low reactive current. To have a high pullout torque, the flux has to be

high.

d) Maximum (pull-out) torque: to have a high pull-out torque in induction motors, primary

(stator) and secondary (rotor) reactances should be kept to a minimum, the rotor resistance will

only determine the slip of the maximum torque.

e) Inrush Current: reactances (and for small motors, the resistances) of the rotor and stator

windings have to be high to have a low inrush current.f) Starting torque: the rotor winding resistance has to be high to get a high starting torque, this

contravenes the efficiency requirement.

As can be seen from the above, the parameters of induction machines (i.e. their design) are a

compromise to achieve the optimum starting, pull-out and running performances required by the

different applications. When a motor is existing and the following data are available: motor HP,

terminal voltage, frequency, number of poles, stator resistance, windage losses, stray load losses

and the other motor parameters (or the results of the tests), the following are calculated: the


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rotor current referred to stator (I2'), starting torque (Tst) and the power transferred by the

rotating field to the rotor (Prot.f) for starting performance analysis. For the running

performance: the slip(s), stator and rotor currents, the developed torque and power, the

efficiency and the power transferred by rotating field at rated motor HP and at pull out

conditions or states are calculated.

3) What are the important characteristics that are used in defining synchronous machines(in general) and synchronous generators (in particular)? Sketch the equivalent circuit

under the different conditions of operation.

The characteristics of synchronous machines that define such machines are the no load, short

circuit, air gap and potier triangle.

To define a generator, the following characteristics to be investigated: no load and air gap, short

circuit and Potier triangle, load characteristics, external characteristics, regulation curve, short

circuit ratio and the determination of the direct axis reactance.

a) No load and air gap (unsaturated) ch/cs: it is expressed on a graph with the Y-axis

representing the armature (stator) e.m.f. (electromotive force) or the pole flux, with the X-axisrepresenting the field current (If) or the field m.m.f. (magnetomotive force in Ampereturn). The

air gap ch/cs is a straight line passing through the origin and the second point is the E (rated

stator voltage) value at a low field current (unsaturated conditions). The no load characteristics

under unsaturated condition coincides with the air gap characteristics - up to a point. At the

point where E is the rated induced in the armature by the flux produced from the resultant mmf

(Mr) the characteristics line is non-linear (bend inward) - saturated condition. The resultant mmf

will constitute of two portions: that part to drive the flux through the air gap and the other part

to drive the flux through the iron parts of the magnetic path. Fig.2 below shows the air gap, no

load & short circuit lines vs. If.

b) Short circuit characteristics (Potier triangle): the S.C. ch/cs represents the armature current

(Ia) as a function of the field current (If) or of the field mmf (MMF) with the armature

terminals short circuited. It is taken at synchronous speed of the generator. The S.C. ch/cs is a

straight line passing through the origin and the second point will have the X-axis equal to the

field MMF and the Y- axis equal the armature current. Potier triangle will have the following

important quantities: armature reaction mmf and the leakage reactance (impedance) of the

armature. The cos f =0 load characteristics is constructed from the no load ch/cs and Potier

triangle.

c) Load characteristics: it is the terminal voltage V as function of field current (If) or field mmf

(Mf) for constant load current (Ia) and power factor (p.f.). At a fixed load current, the field

current is required to sustain the no load voltage and it increases with the decrease of cos f, this

mainly to counteract the armature reaction. Potier triangle can be determined from the no load

ch/cs and two points on the cos f = 0 load ch/cs.

d) External characteristics: it is expressed as the terminal voltage (V) as function of the load

current (Ia) at constant If and p.f. For lagging current, the voltage drop increases as the power

factor decreases and vice versa for leading current.


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e) Regulation Curve: it describes the field current as function of either the load current with

constant p.f. or the p.f. at constant current (Ia) - provided that the terminal voltage is kept

constant. The field current increases with the increase in load current (p.f. is constant lets say at

1 or .8), the field mmf has to increase to compensate for the armature reaction increase.

f) Short circuit ratio: it is the ratio of the field current required to produce rated voltage on open

circuit to the field current required to produce rated current on short circuit. The saturated S.C.ratio is obtained from the no load curve and the rated armature current (field mmf to produce

rated V divided by the mmf producing the rated current). To get the unsaturated S.C. ratio,

divide the MMF value that produces the rated V from the air gap straight line (it is the extension

of the linear air gap line that is taken from the no load curve) by the MMF producing the rated

current under short circuit condition. A large SCR indicates a small armature reaction which

means that the machine is less sensitive to load variations. A small SCR means that the machine

is more sensitive to load variations.

g) Determination of direct axis synchronous reactance (Xd): it is obtained from the no load

linear portion (or air gap line) and S.C. characteristics. The field current induces the nominal

voltage on the air gap line. When the stator is short circuited and the field current is maintained

at the same level as with the open circuit condition, the induced emf in the stator is the same as

with the no load condition but in this case it is consumed by the drop due to the synchronous

impedance. The unsaturated short circuit ratio is used to calculate Xd, Xd = 1/SCR.

Fig.2a below shows the equivalent circuit of the synchronous machine under the different

conditions.

Xs: stator leakage reactance, Xm: main flux reactance, Xf: field leakage reactance, Xd: damper

leakage reactance.


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4) What are the data required from testing to calculate the parameters of synchronous

motors?

The tests to be performed on the synchronous machine so that the parameters can be calculated

are: the open circuit (no load) characteristics, short circuit and the air gap line. From the first

test, two values are obtained: the nominal stator volt at the field current registered; from thesecond: 2 points are obtained (4 values): the nominal stator current and its corresponding field

current and the stator current (Ia) at the field current obtained from the open circuit test; from

the final test: the voltage at the field current equal to the one producing the nominal current in

the previous test.

5) Sketch the equivalent circuit of single phase transformer and list the required tests (to

be performed on transformers) to obtain the values of the parameters of the equivalent

circuit?

Fig. 3 below shows the equivalent circuit of a single phase transformer. The tests to be

performed on transformers so that the parameters, which are the primary and secondarywindings resistances and reactances plus the main flux susceptance and conductance, can be

calculated are: the no load (open circuited primary - H.V. - winding) test from which the open

circuit volt on l.v. side and the no load current are obtained; the S.C. (l.v. - winding short

circuited) from which 3 values are registered the S.C. losses in W (or kw), the rated S.C.

currentas seen from the primary side and the voltage circulating such current. The primary

winding resistance (d.c.) should also be measured by an ohmmeter at 25 C.

R1: primary winding resistance, X1: primary winding leakage reactance, R2': secondary

winding resistance referred to the primary winding, X2': secondary winding reactance referred

to the primary, Bm: main flux susceptance, Gm: main flux conductance, r2': load resistance

referred to the primary winding, x2': load reactance referred to the primary (in primary side

terms).

6) A 3 phase induction motor, 3 HP, 440/220 V, 60 HZ, 4-pole, 1750 rpm, has the following

no-load test results: Vnl

= 440 V, Inl

=2.36 A, Pnl

=211 W; locked rotor (full voltage) results:


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Ilr

=29 A, Vlr

=440 V, Plr

=13.9 KW; locked rotor (reduced voltage): Vlr1

=76 V, Ilr1

=4.25 A -

unsaturated reactance condition. Assume windage loss=44 W, stray load loss=48 W, skin

effect factors=1.3 for rotor winding resistance (referred to stator), .97 for rotor reactance

and stator resistance/phase = 2.26 W. Determine the 6 parameters of the induction motor,

its starting and running performance data, the transient reactance and the s.c. time

constant, the s.c. current after 1 and 2 cycles from fault inception. Calculate the duration

of the inrush current assuming J=2 lb.ft2

for the rotating parts.

GIVEN: Vprim

=440 V, Ioc

=2.36 A, Poc

=211 W, Vlr

=V1

=440 V, Ilr

=29 a, Plr

=13.9 KW,

Vlrr

=76 V, Ilrr

=4.25 A, HP=3, r1

@ 25 C=2.26 OHM, windage loss=44 W, stray load

loss=48W, skin effect for r2'

, x2'

=1.3, 0.97.

SATURATED REACTANCE:

Zl=440/[(3).5(29)]=8.73W, R

l=13920/3(29)(29)=5.48 W,

X=[(8.73)2-(5.48)2].5=6.8 W, r1 @ 75 C=2.26[(234+75)/(234+25)]=2.69 W,

r2'

=R1

-r1

(@ 75 C)=5.48-2.69=2.79 W, X1

=X2

=X/2=3.4 OHM.

UNSATURATED REACTANCE:

Z=76/[(3.5)(4.25)]=10.3 W, X=[(10.3)2-(5.48)2].5=8.72 W,

X1

=X/2=4.36 OHM, Saturation factor = 4.36/3.4=1.3,

including for skin effect: r2'

=2.79/1.3=2.14 OHM,

X2'

=4.36/.97=4.5 W, Xm

=[(V1

)-(Ioc

.X1

)]/Ioc

=[(440/3.5)-(2.36)(4.36)]/2.36 = 103 OHM,

Ph+e=Poc-3(Ioc)2

(r1 @ 75C)-windage losses=211-3(2.36)2

(2.69)-44=122W.

Assuming 1/2 the iron losses are due to the main flux. Ph+e

=122/2=61W

Gm

=Ph+e

/3(E1

)2

={61}/{3[(440/3.5

)-(2.36)(4.36)]2

}=3.43/10000 MHO

Rm

=Gm

.Xm

2=[(3.43)/(10000)][103]2=3.66 W

STARTING AND RUNNING PERFORMANCE:

Base values calculations: Vb

= 440/3.5b

=3x746/3x254=2.94 A, Zb

=254/2.94=86.5 OHM.

Pb

=254x2.94=746 W, Tb

=7.04(3)(unit power)/unit speed=7.04x3x746/1800=8.8 LB.ft.

Parameters for starting performance in p.u:r

1=2.69/86.5=.0311, r

2'=2.79/86.5=.0322, x

1=x

2'=3.4/86.5=.039 (saturated conditions).

Parameters for running performance in p.u:

r1

=.0311, r2'

=2.14/86.5=.0248, x1

=4.36/86.5=.0504, x2'

=4.5/86.5=.052 (unsaturated).

Parameters common to both:

Xm

=103/86.5=1.19, Rm

=3.66/86.5=.042 p.u.

Starting performance:


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r2'

(1-s)/s=0 as at standstill s=1=(ns-n)/n

s

Z1

=(r1

2+x1

2)=[(.03)2+(0.039)2]=0.05, tan-1=x1/r

1=51.4 Z

2'=[(.032)2+(.039)2]=.0506 /_50.6

Zm

=[(1.19)2+(.042)2] =1.19, tan-11.19/.042= 87.98

V1

=I1

.Z1

+ (I1

)/[(1/Z2'

)+(Ym

)]

V1

=I1

[.05 /_51.4+ (1)/(1/.0506 /_50.6)+(1/1.19 /_87.98)]; where Ym

= 1/Zm

, V1

=I1

.Zeq

,

V1

=1/_0

I1

=V1/{.05 /_51.4+{[(.0506)(1.19) /_138.58]/[.0506 /_50.6+1.19 /_87.98]}}

I2'

=I1

[Zm/(Z

2'+Z

m)]=9.66/_-50.4

Prot field

=I2'

2(r2'/s)= (9.66)2(.0322) = 3p.u.

Tst

=3p.u.=3x8.8=26.4 LB.ft

Running Performance:

s=.03, r2'(1-s/s)=.8, Z1 =.0593 /_58.3, Z2'=.825 /_3.6, Zm=1.19 /_87.98,

I1={1/_0}/{[.0593/_58.3]+[(.825 /_3.6 . 1.19 /_87.98)/(.825 /_3.6+1.19 /_87.98)]}

Input p.f.=cos 38.2=.785

I2'

=1.42/_-38.2[1.19 /_87.98/(.825 /_3.6+1.19 /_87.98)]=1.12 /_-5.74

Prot field

= I2'

2.(r

2'/s) = (1.12)

2(.0248/.03)=1.025 p.u.

Tdeveloped

=1.025 p.u.=1.025(8.8)=9.02 LB.ft.

Mechanical loss Torque=7.04(stray load loss+windage+main flux iron loss)/ns(Torque

base)

=7.04 (48+44+61)/1800 (8.8)=.068

Tnet developed

=1.025-.068=.957 p.u. x 8.8=8.4 LB.ft.

Power input=3(1)(1.42)( cos 38.2)=3.35 p.u.

Losses (stator-copper)=3(1.42)2(.0311)=.189 p.u.

Losses (rotor-copper)=3(1.12)2 x .0248=.093 p.u.

No load iron losses=122/746=.163 p.u.

Losses (stray load)=48/746=.064 p.u.

Losses (friction & windage)=.059 p.u.

Total losses=.568 p.u.

Pdeveloped=3.35-.568=2.78 p.u. x 746=2076 W=2.78 HPeff.=2.78/3.35=83%

slip = 3(1.12)2

(.0248)/[3.35-(.189+(.163/2))]=.03

snormal

=3/2.78=.0324

spullout

(@ max Torque)=[(1 + (X1/X

m))(r

2')/[X

1+ (1 + (X

1/X

m))X

2']

=[(1.0+.0504/1.19)(.0248)/[.0504+(1.042)(.052)]=.246 p.u.


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r2'

(1-s/s)=0.0758 (@ s=.246), Z2'

=r2'

+ r2'

(1-s/s) + jX2'

= {[r2'

+ r2'

(1-s/s)]2 + X2'

2}.5

=[(.0248+.0758)2

+(.052)2

].5

=.113/_27.3 tan-1

(.052)/(.0248+.0758) = 27.3

I1

=6.12/_-41, I2'

=5.85/_-36, Protating field

=I2'

2(r2'/s

p.o.)=3.44 p.u.

Tp.o.

=3.44x8.8=30.2 LB. ft.

Tdeveloped (@ 3HP & s=.0324)=5250 x 3/1800(1-.0324)=9.02 Lb.ft

Starting Torque/Normal Torque (rated) = 3/1.025 = 2.92

Pull out Torque/rated Torque = 3.44/1.025 = 3.36

Transient Analysis:

Transient reactance = .0504+[(.052)(1.19)/(.052+1.19)] = X" =. 1p.u.

P.F. = .785, efficiency = 83%, Ib = 2.94 A

I1

= 3x746/.785(.83)(440)(1.732) = 4.5 A = 1.53 p.u.

E1'

=1-[(.03+j.1)(1.53 x cos 38.2 - j1.53 x sin 38.2)] = .874 p.u. /_-6

Initial S.C. current = .874/.1 = 8.74 x 2.94 = 25.7 AT

o'= open circuit transient time constant = (X2+Xm)/2(pi)fr

2, where pi = 3.141592654

T' = To'

[X"/(X1

+Xm

)] = S.C. time constant

IS.C.

= Initial S.C.C. (e-t/T)

To'

= .052+1.19/2(pi)60(.0248)=.133 sec.

T' = .133(.1)/.124= .011 sec.

IS.C.

(after 1 cycle i.e. .0166 sec.) = 8.74(e.01666/.011) = 2 p.u.

I

S.C.

(after 2 cycles) = 8.74(.048) = .43 p.u.

Starting Time:

An approximate solution if the load and motor speed-torque curves are not available is as

follows:

t = J(rpm1-rpm2)2(pi)/60 g Tn

J = 2 lb ft2, rpm1 = 1740, rpm2 = 0, g = 32.2 ft/sec, Tst

= 26.4, Trated

(@ rated HP)= 9.02

Tn

= accelerating torque between rpm2 to rpm1 = Tst

+Trated

/2 = (26.4+9.02)/2 = 17.7 LB. ft.

t=.7 sec.

7) A synchronous machine rated 45 KVA, 3 phase, 220 Vl.l.

, was tested under open circuit

(no load) and short circuit (locked rotor), the following were the results: OPEN CIRCUIT:

Vl.l.

= 220 V, Ifield

= 2.84 A, SHORT CIRCUIT: Iarm.1

= 118 A (first point), Ifield1

= 2.2 A,

Iarm.2

= 152 A (second point), Ifield2

= 2.84 A, the machine was tested at a power factor = 0

from short circuited armature condition to no load condition. The no load and air gap

plus the short circuit curves can thus be plotted (as shown in question 3, fig.2 above) and
http://vepi.hostse.com/Ch1.htm#q3fig2http://vepi.hostse.com/Ch1.htm#q3fig2http://vepi.hostse.com/Ch1.htm#q3fig2http://vepi.hostse.com/Ch1.htm#q3fig2


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the characteristics of the machine can be obtained graphically. The air gap line is plotted

from: Ifield

= 2.2 A, Vl.l.

= 202 V at rated armature current = 118 A, assume s.c. load losses

(3-phase losses) = 1.8 KW @ 25 C, armature resistance @ 25 C = .0335 W/phase, field

resistance @ 25 C = 29.8 W/phase. Calculate the saturated and unsaturated reactance,

the effective armature resistance and the s.c. ratio. If this machine runs as a motor at V =

230 V, input power to armature = 45 KW, .8 p.f. leading current, field current = 5.5 A,

then calculate the efficiency of the motor.

Line to neutral voltage = 202/1.732 = 116.7V on the air gap line; for the same field current of

2.2a, on the S.C. line, the armature current = 118 amp.

The saturated reactance = Xs

The unsaturated reactance= Xs air gap

= 116.7/118 = .987 OHM, ratio of unsaturated reactance to

saturated reactance = .987/.836 = 1.18

Short circuit ratio = 2.84/2.2 = 1.29 (saturated), Xs(saturated)

= 1/1.29 = .775 p.u.

Air gap field current corresponding to 220v line - line armature voltage = (220/202)(2.2)=2.39

A

short circuit ratio=2.39/2.2=1.086 p.u. (unsaturated), Xs(unsaturated)

=1/1.086=.92 p.u.

The armature effective resistance:

S.C. load loss/phase = 1800/3 = 600W/ph.

ra(eff)

= 600/(118)2 = .043 W/phase

in p.u.: S.C. load loss = 1.8/45 = .04 p.u.

ra(eff)

in p.u. = .04/(1)2 = .04 p.u.

ratio of a.c./d.c. resistance =.043/0335=1.28 (skin effect and proximity)

Note: armature resistance of m/cs above few hundreds KVA is less than 0.01 p.u.

For the m/c operating as a motor with leading power factor:

Ia

= 45/(1.732)(.8) = 141 amp.

Assume stray load loss of 30% of S.C. load loss = .54 KW

Resistance for copper winding of armature at 75 C = 0.335[234+75/234+25] = .04 OHM/ph

Resistance for copper winding of field at 75 C = 29.8[(234+75)/(234+25)] = 35.5 OHM/ph

Armature Cu losses (assume ra

= .04 or .035W/ph) = 3 Ia

2 ra = 3(141)2 (.04)=2.38 KW

Field Cu losses = (5.5)2 (35.5) = 1.07 KW E = internal voltage of motor = V t - Ia ra =

(230/1.732)-(141)(.8+j.6)(.04) = 128.4-j3.4 Vphase

Vline to line

= 220 V at which core losses = 1.2 KW

Total losses = 2.38+1.07+.56+1.2+.91 = 6.12 KW

Input = 45+1.07 = 46.07 KW

Efficiency = 1-(6.12)/(46.07) = 86.7%


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8) Calculate the equivalent circuit parameters for the following single phase transformer.

The results of the open circuit test ( H.V. wdg open) are: Vo.c.

= 347 V, Io.c.

= 18 A, Po.c.

=

980 W. The results from the short circuit test (L.V. wdg shorted) are: Vs.c.

= 70 V, Is.c.

=

Irated

, Ps.c.

= 1050 W. The 2-winding transformer ratings are: 2400/347 V, 100 KVA,

primary winding resistance at 25 C= .24 W. Also, calculate the regulation and effeciency

at unity and .8 p.f. full load current.

Transformer ratio: 2400/346 V, Irated

= (100)/2.4 = 41.6 A

Transformation ratio = a = 2400/346 = 7, Zsc

= Vsc

/Isc

=70/41.6 = 1.68 OHM

Rsc

= Psc

/(Isc

)2 = 1050/(41.6)2 = .607 OHM

Xsc

= [(1.68)2-(.607)2].5 = 1.57 OHM Rsc

(at 75 C) = .607[(234+75)/(234+25)] = .723 OHM

r1

(at 75 C) =.24[234+75/234+25] = .28 OHM

r2' = Rsc-r1 = .723-.28 = .44 OHMX

1= X

2'= X

sc/2 = .785 OHM

From open circuit tests: (L.V. wdg. excited):

Ph+e

= Po.c.

-(Io.c.

/a)2r1

= 980-(18/7)2(.28) = 978 W

E1

= 2400-(Io.c.

/a)X1

= 2400-(18/7)(.785) = 2400 V

Xm

= 2400/18/7 = 934 OHM, gm

=Ph+e

/E1

2=.00017 MHO

rm

= gm

(Xm

)2 = .00017 (934)2 = 148 OHM

Zb = 2400/41.6 = 57.7 OHM, Zpu = 1.68/57.7 = .03 puX

pu= 1.57/57.7 = .027 pu, R

pu= .0125 p.u.

Regulation and efficiency:

Regulation at 1p.f.: Reg. = (.0125)+(.027)

9)A 3-winding transformer (primary, secondary, tertiary)- single phase- rated 7960 V

(1000 KVA), 2400 V (500 KVA) & 2400 V (500 KVA). The results of the s.c. tests are as

follows:

Test WDG Excited WDG Short Circuited Applied Voltage Value Current In Excited WDG

1 1 2 252 62.7

2 1 3 252 62.7

3 2 3 100 208

The 3 transformers are connected in a Y-D-D configuration on 13.8/2.4/2.4 KV. When the tertiary

windings are short circuited (3-phase), calculate the s.c. current and the voltage on the terminals of the

secondary windings of this bank.


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For primary winding: Vb

= 7960 V, Ib

= 1000/7.96 = 125.4 A

For secondary winding: Vb

= 2400 V, Ib

= 1000/2.4 = 416 A

For teritiary winding: Vb

= 2400, Ib

= 416 A

Test Z between terminals Vpu Ipu Zpu = Vpu/Ipu

1 1-2 252/7960=.0316 62.7/125=.5 .06322 1-3 252/7960=.0316 62.7/125=.5 .0632

3 2-3 100/2400=.0416 208/416=.5 .0832

Z1

= 1/2 [.0632+.0632-.0832] = .0216 pu

Z2

= 1/2 [.0832+.0632-.0632] = .0416 pu

Z3

= 1/2 [.0632+.0832-.0632] = .0416 pu

Isc

=1/(.0216+.0416)=15.8 p.u. (S.C. teritiary) = 15.8 x 125.4 = 1984 A

V2

= voltage on secondary bus while teritiary is short circuited = Is.c.

x Z3

- Isec

x Z2, Isec

is the load

current

= 15.8(.0416)-1(.0416)=62.4-4.16= .5824 p.u. To calculate the S.C. current assuming a base MVA = .5

MVA = 500 KVA:

Znew

= Zold

[KVAnew

/KVAold

] p.u.

Z3new

= .5(.0416) = .0208, Z1new

= .5 (.0216) = .0108, Isc

= 1/.5 (.0632) = 1/(.0208+.0108) = 31.6 p.u.,

Ibase

= 500/7.96 = 62.8 A, Isc

= 31.6(62.8) = 1984 A

10. Summarize the methods used in calculating the parameters of synchronous and induction

machines.

Synchronous machines:

Positive sequence (synchronous parameters): The air gap, no load and short circuit characteristics: the

machine is run at synchronous speed in the proper direction. The three phase armature terminals are

kept open and line to line voltage readings are taken at different field current (a rheostat) in the field

winding is used to vary the field current). The excitation is reduced to a minimum and the 3 phase are

shorted and the field reading plus the current flowing in one of the lines are taken (to draw the short

circuit unsaturated characteristics line passing through the origin).

Subtransient reactance: short circuit the field through an ammeter, apply a single phase voltage to any

of the three line terminals, rotate the rotor by hand. At the position indicating maximum field current in

the field ammeter, half the voltameter reading in the armature circuit divided by the armature current in

one line (using an ammeter in one of the line terminals) is equal the subtransient direct axis reactance.

At the minimum field current reading, half the armature voltage divided by the armature current will

give the subtransient reactance in the q-axis.

Negative Sequence reactance: the field is shorted on itself, two phases are connected to each other

through an ammeter and a single phase voltage is applied to this armature configuration. The field is


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rotated at rated speed and the voltage applied to the armature will correspond to approximately

circulating the rated machine current. The negative sequence impedance = V/1.7321( I). This method

can also be used with induction m/cs.

Zero sequence impedance: zero sequence impedance is much smaller than the positive and negative

and in theory is close to zero. The machine is at stand still, the field is open, the six terminals ofthe

machine are available outside the machine and the three windings are connected in series. A reducedvoltage is applied across the 3 connected windings and the zero sequence impedance will be equal to

one third the voltage read on the voltameter divided by the circulated current read on the ammeter in

the armature circuit.

Induction Machine:

Steady state reactance: from the three tests, no load (open circuit), locked rotor (short circuit) - full

voltage (for saturated reactance) and locked rotor (short circuit) - reduced voltage (for unsaturated

reactance), the parameters are obtained.

Transient reactance: is calculated from the parameters of the machine obtained from the above tests. X"

= [X2(Xm)/(X2+Xm)]+X1.

SUMMARY:

In this chapter, the parameters and performance of a.c. electric machines were presented. The tests

performed on such machines to obtain their parameters were given, the parameters and their effects on

the machines performance were covered, too. Numerical examples were given to show and clarify the

interrelations between the machine parameters and the performance. The induction machines

parameters are: stator resistance & reactance, the rotor resistance and reactance, the main flux

susceptance and conductance. These parameters affect the machine during starting (inrush current &

starting torque), at nominal load (full load current & nominal torque), and for the maximum (pull-out)

machine torque. They, also, affect the losses (heating of iron & winding), efficiency and power factor .

The characteristics of synchronous machines in general and generators in particular presented in this

chapter were: the air gap and no load (armature emf or pole flux vs. mmf or field current), short circuit

(armature current vs. field current), load ch/cs (terminal voltage vs. field current provided that the load

current and the p.f. are kept constant), external (voltage vs. load curent with constant field current &

p.f.), regulation (field current vs. load current with constant p.f. or field current vs. p.f. with constant

load current, voltage is kept constant in either case), short circuit ratio (field current producing nominal

voltage under open circuit condition vs. field current producing rated current under - armature - short

circuit condition) and direct-axis synchronous reactance determination (which is equal to 1 divided by

the SCR). For the transformers, 2 & 3 winding parameters were presented and their effect on

regulation/efficiency of 2 windings transformers and for 3 windings short circuit plus voltage

sensitivity of the unfaulted bus were given and emphasized by numerical examples.

REFERENCES:

1. Greenwood, Allan "Electrical Transients in Power Systems", Wiley.

2. Granger & Stevenson "Power System Analysis", McGraw Hill.


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3. Tuma, Jan "Engineering Mathematics Handbook", McGraw Hill.

4. Fitzgerald, Kingsley & Kusko "Electric Machinery", McGraw Hill.

5. Kheir, "Automating Power Systems Analysis", Kheir

Home page of VePi

Lesson 2: Overhead and underground distribution systems components.

1) How would EPR (ethylene propylene rubber) cables, up to 35 KV, be classified?

2) How would underground XLPE (cross linked polyethylene) cables classified?

3) What are the defining parameters of cables?

4) What are the factory and site tests to be performed on cables?

5) What are the defining parameters for low voltage secondary cables?

6) What are the different types of transformers found in distribution systems?

7) What are the different types of overhead switches, padmounted switchgear and those of

lightninig arresters?

8) What are the standards that govern distribution transformers? How are distribution

transformers defined?

9) How are wooden and concrete poles defined?10) What are the different applications of oil switches and what are their defining parameters?

11) How are overhead air switches classified?

12) How are padmounted switchgear defined?

13) What are the important parameters by which lightninig arresters are defined?

14) How would copper conductors be defined?

15) How would ACSR (aluminum conductor steel reinforced) and ASC (aluminum stranded
http://www.vepi.hostse.com/http://www.vepi.hostse.com/http://www.vepi.hostse.com/


16/62

conductor) be defined?

16) How would AASC (aluminum alloy stranded conductor) and self-dampening/compact

ACSR be defined?

17) What are the different types of cable splices and terminations?

18) What are the different types of connectors and elbows?

19) What are the design tests performed on the separable connectors?20) What are the different types of insulators, their material and characteristics?

21) What are the functions of DAC (distribution automation and control)? Sketch the typical

underground distribution arrangements and the overhead distribution system.

22) What are the major information obtained from a distribution system SCADA? Show a block

diagram of the major components of a SCADA (system supervisory control & data acquisition).

Sketch a diagram to show a typical automated distribution system with power and

communication lines.

23) What are the basic modules in a PLC (programmable logic controller) system? Sketch a

front plate of 2 of the basic modules. Sketch a block diagram showing the interrelation of the

major modules.

24) A solved problem regarding constants of overhead conductors?

25) A solved problem regarding constants of underground cables?

26) A solved problem regarding reflected & refracted powers at a conductor/cable junction?

27) A solved problem regarding power transformer protection?

Lesson summary

References

1) How would EPR (ethylene propylene rubber) cables, up to 35 KV, be classified?

The classification of EPR (up to 35KV) cables is as follows: the voltage class, the conductor

material/size (which is function of the normal/overload/short circuit current values and the

installation method/configuration), the insulation thickness (whether 100% or 133%), jacketed

or unjacketed, neutral size (either full or 1/3 rating), cable in conduit configuration/direct buried

or concrete encased conduits.

2) How would underground XLPE (cross linked polyethylene) cables classified?

The classification of XLPE (up to 46KV) cables is as follows: the voltage class, the conductor

material/size, insulation thickness, jacketed or unjacketed, neutral (concentric neutral and rating

full or 1/3 main conductor) or shielded (Cu tape), single or 3-conductor cables, jacket-type

(whether encapsulated or sleeved), the use of strand-fill or water blocking agent between the

insulation and jacket.

3) What are the defining parameters of cables?

The defining parameters can be classified broadly into dimensional, insulation material

properties and current carrying capacity. For dimensional parameters, conductor size/number of

strands/type of strands, diameter over conductor, diameter over insulation, diameter over

insulation screen, number and size of neutral conductors or tape details (thickness, width & lap

type) and diameter over the jacket are the defining data. Other important data are: weight/1000


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ft length, size of reels and length/reel. The insulation/jacket defining parameters are: before and

after aging tensile strength and elongation, hot creep elongation/set, dielectric constant,

capacitance (SIC) during and after the stability period, insulation resistance constant, water

absorption properties. The last set of defining parameters are the current levels at the nominal

voltage under the different operating conditions which are function of: the layout and proximity

of current carrying cables, the method of laying/pulling of cables, provision of future additionalloads with their corresponding maximum allowable voltage drop and finally the maximum

acceptable temperature rise & duration for the cable insulating material.

4) What are the factory and site tests to be performed on cables?

The different types of tests that are performed on cables at the factory are: partial discharge, DC

resistance of central conductor, AC high voltage dielectric withstandability, DC high voltage

withstandability, insulation resistance, physical dimensions of cable components, cold bend, low

temperature impact, jacket integrity, water penetration and high temperature drip test for the

strand fill (if applicable). For conductor shield the tests are: volume resistivity, elongation at

rupture, void and protrusions, irregularities verification. For the insulation are: tensile strength(aged and unaged)/elongation at rupture (aged and unaged)/dissipation factor (or power

factor)/hot creep (elongation and set)/voids and contamination/solvent extraction (if applicable).

For the insulation shield are: volume resistivity/elongation at rupture/void and protrusions

irregularities/strippability at room temperature and at -25C/water boil test. For the jacket are:

tensile strength, elongation at rupture (aged and unaged), absorption coefficient (of water), heat

shock and distortion. On the cable the following tests may be performed: structural stability and

insulation shrink back for certain insulation materials. The tests performed on site are: visual

inspection, size/ratings verification and D.C. withstandability tests at voltage level below those

used in the factory.

5) What are the defining parameters for low voltage secondary cables?

The defining parameters for l.v. secondary cables are: material of phase conductor, number of

strands, class of strand, type of conductor (ie. concentric, compact or compressed), conductor

size, insulation thickness, over all diameter per cable, overall diameter per assembly (ie. triplex

or quadruplex), the neutral conductor size (equal to the phase or reduced), if applicable,

insulation and jacket materials, jacket thickness and the weight per assembly per 1000 ft length.

6) What are the different types of transformers found in distribution systems?

The different types of transformers found in distribution systems are: Power (up to 10MVA)

liquid filled (oil), power (over 10 up to 100 MVA) oil filled with radiators/fans (one or 2 sets),

single phase distribution transformers/oil filled (with or without radiators/fans) up to 500 KVA,

three phase distribution transformers/oil filled (with or without radiators/fans) up to 1.5MVA,

dry type power transformers/3 phase 300 KVA to 2MVA or silicone filled or epoxy resin

insulated for indoor installations. All oil filled transformers are installed outdoor unless a

special layout with fire proof (resisting) material and appropriate barriers are used, then indoor

installation is possible. Distribution transformers can be of the pole mounted, vault or

padmounted type. The primary voltage of power transformers can be as high as 750 KV, though


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the most common are 345KV, 220KV, 115KV, for distribution transformers as high as 72KV

though the most common are 34.5, 25KV (27.6KV), 15KV.

7) What are the different types of overhead switches, padmounted switchgear and those of

lightninig arresters?

The different types of overhead switches are: either single or three phase, either manually

operated or electric/manual operated, either local control or remote/local control, oil insulatedor air or SF6. The different types of padmounted switchgear are: either manually or

manually/motor operated, controlled locally or locally/remotely, air or oil or SF6/vacuum

insulated, protective devices are either fuses or electronic devices. The configuration will,

generally, have four compartments with any combination of fuse or interrupter, switch, solid or

empty compartment. The different types of lightning arresters are: station, intermediate,

distribution (heavy duty, normal or light duty) and may be riserpole type.

8) What are the standards that govern distribution transformers? How are distribution

transformers defined?

The standards that govern distribution transformers are: CSA "Single phase & three phasedistribution transformers" Std. C2, CSA "Dry type transformers" C9, CSA "Guide for loading

Dry-type distribution and power transformers" C9.1 and CSA "Insulating oil" C50. The

distribution transformers are defined as follows: the voltage ratings (insulation class level of

primary -h.v.- winding, the primary and secondary windings rated voltage), short circuit

capability for a fault on the bushings of the transformer (current value and its corresponding

duration), dielectric test values (applied voltage for 1 minute, full wave and chopped BIL and

time to flash over for the chopped), outdoor transformer bushings ratings (defined by their

insulation class, 60HZ 1 minute/dry, 10 second/wet dielectric withstandability, the full wave and

chopped BIL), audible sound levels and induced voltage tests.9) How are wooden and concrete poles defined?

The wooden poles are defined as follows: the class (1 to 7-4500LB to 1200 minimum horizontal

breaking load when applied 2 ft. from pole top), the minimum circumference at pole top level

(27" to 15"), length of pole (25 to 110 ft, generally), minimum circumference at the ground level

(distance from butt), the wood species (Western Red Cedar, Southern Yellow Pine, Douglas Fir,

Western Larch), the treatment against attack from fungi and insects (eg. creosote oil, ammonical

copper fatty acid, pentachlorphenol or chromated copper arsenates) and the weight per pole.

The concrete poles are defined accordingly: ultimate load (class A to J, 600 LB to 4500 LB,

respectively), the length, the manufacturing process (regular or prestressed class), the steel

reinforcing rods (cage) tensile strength, the diameter, the raceway diameter, spacing and

diameter of holes in the pole, grounding bars (galvanized or coated) surface treatment.

10) What are the different applications of oil switches and what are their defining

parameters?

The different applications of overhead oil switches in utility distribution systems are: general

purpose for inductive and resistive loads & capacitor (capacitive current switching). The

defining parameters are: the rated maximum voltage, the basic impulse level, the dielectric


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withstand, continuous current, inductive load switching, capacitive switching current, making

current, momentary current, short time current rating; for the control circuit: nominal and range

of operating voltage, trip coil current. The weight, dimensions, oil volume and speed of

operation for the switch are also important defining data. The other two devices that may use oil

as the switching medium are the sectionalizes and reclosers.

11) How are overhead air switches classified?The following is the classification of the air insulated switches according to their breaking type:

side break switches, vertical break and double break. The different types of mountings for such

switches are: upright, vertical, triangular, tiered outboard mounting and pedestal. The insulators

of the switch may be epoxy or porcelain, the base is insulated or steel.

12) How are padmounted switchgear defined?

Padmounted switchgear can be defined (specified) accordingly: the insulating material used i.e.

air insulated, oil or gas, the nominal voltage class, maximum operating voltage, the basic

impulse level, the current ratings for the different sides i.e. continuous current, load interruption

(resistive, inductive including no load transformer magnetizing and capacitive including cablecharging), momentary, fault close, the dimensions of the gear, the opening for the cable entry,

properties of steel work (like thickness -gauge, surface treatment and finish), the weight and the

assembly voltage withstandability tests (A.C. and D.C.). The speed of operation (current time

curves) for the fuses or protective devices had to be specified.

13) What are the important parameters by which lightning arresters are defined?

The important parameters by which L.A. are defined are: duty cycle voltage, impulse test crest

voltage, power frequency voltage (dry and wet - for outdoor installations), impulse current

rating, maximum continuous operating voltage, switching surges capability, high current/short

time and low current/long duration rating, material of housing, design of internals ie. gapped or

gapless elements (non-linear resistance material).

14) How would copper conductors be defined?

The construction of copper conductors is defined as follows: cross section area, class of

conductor (indication of degree of flexibility), number of wires, diameter of wire, tensile

strength, elongation, diameter of conductor, type (concentric lays, compact or compressed) and

weight per 1000 ft.

15) How would ACSR (aluminum conductor steel reinforced) and ASC (aluminum

stranded conductor) be defined?

The defining parameters for aluminium conductor steel reinforced designs are: the Al area, the

total conductor area, steel/Al area ratio, number of Al wires, diameter of Al wire, area of steel

wire, diameter of steel wire, diameter of core (steel), diameter of conductor, tensile strength,

AWG size, total conductor weight per 1000 ft. and the ratio of Al weight to the total weight. The

defining parameters for aluminium stranded conductors are: the aluminum conductor area, the

quantity (number) of Al wires, diameter of each wire, the diameter of the conductor, the tensile

strength, the elongation and the total weight of conductor per 1000 ft.

16) How would AASC (aluminum alloy stranded conductor) and self-dampening/compact


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ACSR be defined?

The defining parameters for Aluminium alloy stranded conductors are: Al alloy area and the

equivalent Al area, number of Al alloy wire, diameter per wire, overall diameter of conductor,

AWG/KCMIL, weight/1000 ft, tensile strength and elongation. The defining parameters for self-

dampening conductors and compact ACSR are: aluminium area, total conductor area, steel to Al

area (ratio), number of Al wires, number of steel wires, core (steel) diameter, overall conductordiameter, conductor weight/1000 ft length, ratio of Al weight to total weight, tensile strength

and elongation.

17) What are the different types of cable splices and terminations?

The different types of cable splices are: tapped, heat shrinkable and cold shrinkable. The major

components of a splice are: cable adapters, splice housing, conductor contact, conductive insert,

retaining rings/tube, interference fit and grounding eye. The different types of cable

terminations are: the fully taped, moulded stress cone and tape, one piece moulded cable

termination, porcelain terminators, heat shrinkables and potheads.

18) What are the different types of connectors and elbows?The different types of connectors are: the mechanical (for Al and/or Cu conductors), the

compression, the wedge (to connect main conductors to taps), hot line clamps (the main

overhead to equipment connection) and the stirrups (wedged or bolted). The two types of

separable connectors (elbows) are the dead break and load break. The major components of

elbows are: the connector, the moulded insulating body, cable adapter, the test point, the semi-

conducing shield, semi-conducting insert, grounding tabs the pulling eye, the probe (for load

break, it is field replaceable with abelative material arc follower).

19) What are the design tests performed on the separable connectors?

The design tests performed on the elbows are: partial discharge inception and extinction levels(corona), withstand power frequency voltage capability (a.c. and d.c.), impulse voltage

withstand level, short time current rating, switching test, fault closure rating, current cycling for

insulated and uninsulated connectors, cable pullout from elbow (connector), operating force,

pulling eye operation, test point cap pulling test, shielding test, interchangeability, accelerated

thermal and sealing life, test point capacitance (voltage presence indication) test.

20) What are the different types of insulators, their material and characteristics?

The different types of insulators are: the pin, the suspension and the post (vertical and

horizontal). The different insulators materials are: porcelain, glass, fibreglass, polymer and

silicone. The properties of insulators can be broadly classified into: mechanical, electrical,

environmental and maintenance. The mechanical can further be classified into: different loads

the insulators is subjected to due to weights of supported components, short circuit, ice, etc.

(normal, design, cyclic, torsional, overloads - exceptional), safety factors, single or multiple

insulator assemblies and aging effect on strength of insulator. The electrical parameters defining

the insulators are: BIL, power frequency withstandability (dry, wet and flashover level), leakage

distance, power arcs effect, performance under steep front voltage wave, clearances and

performance under contamination. The environmental characteristics can be further broke down


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into: insulator ageing under ultra-violet rays and dry arcing, type of contamination, radio

interference voltage, washing requirements, corrosive environments and temperature range.

21) What are the functions of DAC (distribution automation and control)? Sketch the

typical underground distribution arrangements and the overhead distribution system.

Distribution automation and control functions can be classified into: load management, real

time operational management and remote metering. The first function may be subclassified into:discretionary load switching, peak load pricing, load shedding and cold load pick-up. The

second function is subclassified into: load reconfiguration, voltage regulation, transformer load

management, feeder load management, capacitor control, fault indication/location/isolation,

system analysis/studies, state/condition monitoring and remote connect/disconnect of services.

Fig. 4 shows the underground arrangements and fig. 5 shows the typical overhead system.


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22) What are the major information obtained from a distribution system SCADA? Show a

block diagram of the major components of a SCADA (system supervisory control & data

acquisition). Sketch a diagram to show a typical automated distribution system with power

and communication lines.

The major information obtained from a SCADA in a power distribution system are: indications

(eg. state change like opening or closing of circuit breakers, load break switches, reclosures,

disconnects, operation of a relay or fault indicators) of events or alarms, levels (eg. oil level, tap

changer position, reading from pressure gauges), pulses (eg. energy meter counters),

measurands (eg. current, voltage, power reading, temperature of oil or windings, leakage

current). Fig. 6 shows a typical SCADA and fig. 7 shows a single line for an automated system.


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23) What are the basic modules in a PLC (programmable logic controller) system? Sketch

a front plate of 2 of the basic modules. Sketch a block diagram showing the interrelation of

the major modules.

The basic modules of a PLC system are: the processor, the input/output (they can further be

classified into digital and analog), process control (proportional/integral/derivative), stepper

motor, interface modules (they can be further classified into: local and remote, local and remote

transfer, network, network transfer, multimedia network interface, peripheral devices (they

include loader/monitor, process control stations, CRT programmers, hand held programmers,

tape loader). Fig. 8 shows the front plates of the local and remote interface units, fig. 9 shows a

typical block diagram including the racks, interface/ input/ output/ network interface modules.


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24)For an overhead conductor with size = 556.5 MCM, aluminum core with 19 strands

and operating voltage of 25 KV, calculate the reactance at 60 c/s and 50HZ per 1000 ft, the

resistance (AL1350) per 1000 ft, the capacitance, charging current/1000ft and surge

impedance.

Given: Conductor details: 556.5 MCM/19 strands/Al 1359.

Nominal voltage: 25 KV, frequency: 60 & 50 c/s, length 1000 ft, conductor diameter: .855"

(from tables), area: .437 inch2 (from tables or 7.854(1000)(556.5)/(10+7) = .437 in.2)

L = (2)/(10+7)[ln(d/r')] H/m = (2/10+7)ln 12(2)/.855(.7788) (at 1 ft spacing)

L= 7.17(1000)/3.28(10+6) = 2185/10000000 H/1000ft.

XL

= 2 pi 60 L = .082 OHM @ 60 c/s and XL

= .068 OHM @ 50 c/s , where pi=3.141592654

R = .0927 microOHM ft (1000)/.437/(12)(12) = .0305 - ignoring skin effect, taking into account


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approximate skin effect then R = .0305(1.15) = .035 OHM

C = 1/ (10+9

)(18)(ln d/r)F/m = 1/ (10+9

)(18)(ln 12/.855/2) = .0166/10+9

C = .0166(1000)/3.28(10+9) F/1000ft = 5.08(10-9) farad/1000ft

XC

= 1/2 pi f C = 5.22 (10+5) OHM,

charging current = 25000/1.732(5.2)(10

+5

) = .028 amp/1000ftZ = surge impedance under these conditions = (L/C).5)= 207 OHM

25) For an underground XLPE cable, size = 250 MCM, aluminum 1350 core with 37

strands and operating on 25 KV system, calculate the reactance at 60 & 50 HZ per 1000 ft,

resistance per 1000 ft, capacitance, surge impedance, charging current/1000 ft, speed of

propagation of the wave and the insulation resistance.

Given: cable data: 250 MCM, XLPE, AL1350, 37 strands, 25 KV, 1000 ft length, conductor

dia.: .575 in, area = .196 in2), dia. over insulation: 1.16 ", e = 3.5, where e is the dielectric

constant

L = (2/10+7)[ln 12(2)/(.575)(.7788) = 2427/(10+7) H/1000 ft at 1 ft spacing

XL

= .092 OHM @ 60 c/s and .076 OHM @ 50 c/s

R/1000 ft = .0927(1000)(144)/.196 = .068 OHM (1.15) = .078 OHM

C = 3.5/)10+9

)(18)(ln 1.16/.575) = .277/(10 )F/m(1000)/3.28 = 84/(10+9

) F/1000ft

L at insulation neutral or sheath = 2 ln 1.16/.575(.7788)/(10+7) = 2 (10-7) H/m

Z = surge impedance = (L/C).5

) = 27 OHM, XC

= 1/2 pi C 60 = 31578 OHM/1000ft

Charging current for 1000 ft cable length =(25000/1.732)(31578) = .46 amp.

v = speed of propagation = (1)/(LC).5

) = 1.34(10+8

) m/sec.

Volumetric insulation resistance = (ra) (ln 1.16/.575)/(2pl)(12)(2.54), where ra is the resistivity

or specific resistance of the dielectric

assume ra = 6(10+14), Rvolumetric

= 6(10+14)(.7)/191511 = 2193 MegaOHM/1000 ft.

26) If the overhead conductor and the underground cable of problems 24 & 25 are

connected in series and a voltage wave of 25 KV is travelling through the overhead portion,

calculate the reflected and refracted powers at the junction point.

Zline

= 207, Zcable

= 27, KV = 25

reflacted voltage = (27-207)25/1.732(207+27) = -11.1 KV

refracted voltage = (2)(27)(25)/1.732(207+27) = 3.33 KV

refracted power = 3 (Vph)(Vph)/Zcable = 3(3.33)(3.33)/27 = 1232 KW

reflected power = 3(-11.1)(-11.1)/207 = 1785 KW

27) Provide the differential and gas accumulation/sudden release protection to a 100 MVA

power transformer, 220/25KV with +/-16% tap changer. Assume that the available relays

have a pick-up setting between 20-50% of relay rating with an adjustable slope of 20-50%

and another with fixed slope and restrained pick-up between 20 and 50% and


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unrestrained pick-up of 8, 13, 20 x relay nominal current. The pressure gas relays have two

settings, for the trip 5.2 - 17.2 KPa and for the alarm 200-400 CC. The tap changer gas

pressure trip can be set between 35-390 KPa.

I excitation = 5% of full load primary, C.T. error = 2.5%, relay rating = 5A, C.T. primary

current = 1.5 x full load current of transformer.

I primary = 100 (1000)/(220)(1.732) = 262 AI secondary = 262 x 220/25 = 2300 A

Using a 1200/800/200:5A C.T. on the primary side (the C.Ts are delta connected), 3500 : 5A

C.T. on secondary side (the C.Ts are wye connected) of the power transformer. Turns ratio of

C.T. on primary winding = 800/5 = 160, on secondary = 3500/5 = 700. Relay current due to

primary C.T. at f.l. = (262/160) 1.732 = 2.8 amp, relay current due to secondary C.T. at f.l. =

2300/700 = 3.28 amp, relay current ratio = 3.28/2.8 = 1.17. Mismatch at midpoint changer and

full load = 17%.

At 220 + 16% = 255KV tap, maintaining secondary voltage at 25KV, primary full load current

= 100 (1000)/255 (1.732) = 226 amp, relay current = 226/160(1.732) = 2.45amp.,voltage of 25KV on the secondary while primary = 220 - 16 % = 185 KV primary full load current =

100(1000)/185(1.732) = 312 amp., relay current = 312/160 (1.732) = 3.4 A

Mismatch for + 16% = 3.28/2.45, mismatch for -16% = 3.28/3.4 which are 34 % and -4 %,

respectively. The maximum mismatch = 34 %, add 6 % as safety margin. Thus the slope

adjustment = 40 % (range is 20 to 50 %). The pick-up level under full load current =

inacccuracies + esciting current + allowance for the limited restraint at emergency load through

currents = 2.5(5/100) + (1.732)(5(262)/160)(100) + (3.28-2.45) = .125 + .14 + .83 = 1.1 amp, the

pick-up setting = 40%(5) = 2 A (range 20 to 50 %).

The unrestrained instantaneous triping current = 13 x 5 = 65 amp. secondary relay current.

VERIFICATIONS:

Assuming a 100 MVAbase and 220/25 KVbase, Ibase (@ primary side) = 100/1.732)(220) =

262 amp., Ibase (@ secondary side) = 2300 amp.

Assuming an infinite source, 11% impedance transformer, a 3-phase fault on the secondary of

the transformer beyond the differential protection zone will produce 9 p.u. fault current (1/.11),

Iprimary = 2358 amp., Isec. = 20700 amp. The current from the primary side into the relay =

(1.732)2358/160 25.5 amp., from the secondary side = 20700//700 = 29.6 amp. A mismatch of

29.6/25.5 = 16 %.

Assuming the tap changer to be at 220KV + 16% = 255 KV and the impedance = 14%, the short

circuit fault current of a 3-phase fault = 1/.14 = 7.16 p.u., full load primary current = 232 amp.,

SCC on the primary side = 1661 A, SCC on the secondary side = 16468 A, relay current from

primary side = 1661(1.732)/160 = 18 A, relay current from the secondary side = 16468/700 =

23.5, the mismatch = 30%.

Assuming the tap changer at 220 KV - 16% = 185 KV and the impedance = 8%, a 3-phase fault


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current = 1/.08 = 12.5 p.u., primary current = 312 A, SCC on primary side = 3900 A, SCC on

secondary side = 28750 A, relay current from prim. side = 42.2 A, relay current from sec. side =

41.1 A, mismatch = 3%

GAS RELAYS:

Main tank alarm = 200 cc, main tank trip = 17 KPa above static head at relay level, tap changer trip =

100 KPa.

SUMMARY:

In this chapter, the defining parameters, classifications, tests and typical configurations of distribution

systems components were presented. The general properties of medium voltage EPR & XLPE cables

were given plus factory & site tests. For low voltage secondary cables, the defining parameters were

listed. For transformers, a broad classification was given. The defining parameters and the CSA

standards that govern the ratings, design, manufacturing and testing of distribution transformers were

presented. Other components found in overhead and underground distribution systems were covered

from their types and defining parameters point of view. These components are: lightning arresters,conductors, terminations, splices, connectors, elbows and insulators. Distribution systems automation

and SCADA were presented by covering the functions and data collected from such systems. Typical

systems were given to clarify this topic. The major modules found in a typical PLC (programmable

logic controllers) installation were presented. The numerical examples at the end of this chapter showed

how the line and cable constants (inductance, capacitance, resistance, inductive/capacitive reactances,

surge impedance, charging current and propagation speed) and the effect of the surge impedance on

travelling waves are calculated. They also demonstrated a method to select/adjust/verify the settings of

relays (differential & gas) used in the protection of power transformers.

REFERENCES:

1. Wildi, T "Electrotechnique", Les Presses de l'Universit Laval.

2. CSA, "Canadian Electricity Code", Part 1, std C22.1.

3. CSA, "Single phase & three phase distribution transformers", std C2.

4. Gonen "Electric power distribution systems engineering", McGraw Hill.

5. Kurtz, "The lineman's & cableman's Handbook", McGraw Hill.

6. Perry, "Chemical Engineers handbook", McGraw Hill.

7. ICEA S-66-524, "Cross linked thermosetting polyethylene insulated wire & cable for the

transmission & distribution of electrical energy".8. ASTM 2.03, "Non-ferrous metal products - electrical conductors.

9. Brady, "Materials handbook", Mcgraw Hill.

10.Kheir, "Computer Programming for Power System Analysts", Kheir.

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Lesson 3: Switchgear, circuit breakers, MCCS and contactors.

1) What are the defining parameters for low voltage circuit breakers?

2) What are the defining parameters for medium voltage circuit breakers?

3) What are the different types of interrupting media used in m.v.c.b.? What are the commonproperties for such quenching media?

4) What are the different types of switchgear assemblies and what are the subclassification of

each type? What are the standards governing switchgear assemblies & circuit breakers? What

are the types of batteries found in such equipment? Give a brief description of each type.

5) What are the defining parameters for low voltage alternating current magnetic

contactors/starters?

6) What are the defining parameters of l.v. combination starters?

7) What are the defining parameters for full voltage 2-speed starters?

8) What are the defining parameters of low voltage, reduced voltage starter units?9) What would a table defining the motor protection circuit breakers have as headings?

10) What would a table defining motor protection fuses have as headings?

11) What are the headings of a table defining m.v. controllers?

12) What are the general properties of constructional and stainless steels?

13)What are the different types of switchboard instruments and the different mechanisms? What

are the defining parameters for such devices?

14) What are the defining parameters for KWH, KVAR and solid state meters?
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15) What are the different constructions of protective relays? Give the defining parameters for

the different types of the following relays: overcurrent, over/undervoltage, differential and

distance.

16) A solved problem regarding the sizing of the breaking devices of a switchgear assembly.

Lesson summary

References1) What are the defining parameters for low voltage circuit breakers?

The defining parameters of low voltage circuit breakers are: the rated voltage, the circuit breaker

current rating, symmetrical interrupting capacity (with instantaneous and delayed integral

protective relays), close and latch rating for a second, the sensors current ratio, the functions

of protection on the integral overcurrent protective device, electrically/manually operated or

manually only, the weight and dimensions of circuit breakers, the indicators of the integral

protective device (if available). For electrically operated breakers, the range of operating

voltages and currents plus the nominal values for the solenoids/motors/trip coils are important

parameters, too.2) What are the defining parameters for medium voltage circuit breakers?

The defining parameters of medium voltage circuit breakers are: the voltage ratings (nominal,

maximum and minimum), the 3-phase MVA breaker rating, the rated current, the K factor

(Max./Min. ratio), symmetrical interrupting ratings (at maximum, nominal and minimum

voltage) in KA, the asymmetrical factor, the short time rating, the close and latch, the insulation

level (power frequency, impulse level), the weight, the dimensions, the interrupting medium, the

TRV capability, any arcing medium monitoring devices, circuit breaker closing time, tripping

time, interrupting time, spring charging time, the control voltages (nominal and range), the

spring charging current, close coil current requirement, the trip coil current rating and surges

switching capabilities.

3) What are the different types of interrupting media used in m.v.c.b.? What are the

common properties for such quenching media?

The interrupting media used in medium voltage circuit breakers are: air, oil SF6 and vacuum.

The general properties of fluids used in arc extinguishing chambers in m.v. c.b. are: high

dielectric strength of the gas or liquid, thermally and chemically stable, non-inflammable, high

thermal conductivity, low dissociation temperature, short thermal time constant, should not

produce conducting material during arcing. Gases used so far in m.v. c.b. can be classified into

simple (air) or electronegative (SF6).

4) What are the different types of switchgear assemblies and what are the subclassification

of each type? What are the standards governing switchgear assemblies & circuit breakers?

What are the types of batteries found in such equipment? Give a brief description of each

type.

The different types for switchgear assemblies are: indoor and outdoor. The subclassification for

the indoor is: standard, sprinkler proof, arc proof, dust proof, seismic proof, metalclad

construction vs. metal enclosed, enclosure with or without a drip hood; for the outdoor is: walk


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in, walk in with working area, walk-in double row, non-walk in with or without working area,

enclosure for cable/bus entry or transformer throat, enclosure with thermal insulation/isle

heaters and finally indoor cubicle design installed in an outdoor house.

The standards that govern circuit breakers/switchgear design testing and application are: ANSI

C37 series including .09 "Test procedure for A.C. high voltage circuit breakers rated on a

symmetrical current basis", .04 "Rating structure for A.C. high voltage circuit breakers rated ona symmetrical current basis", .06 "Preferred ratings and related required capabilities for A.C.

high voltage circuit breakers rated on a symmetrical current basis", .20 "Switchgear

assemblies", IEC 56 "High voltage A.C. circuit breakers", IEC 60 series "High voltage test

techniques", IEC 694 "Common clauses for high voltage switchgear and controlgear",

C22.2#31" switchgear assemblies" and CAN3-C13 "Instrument transformers.".

The different types of stationary batteries used in conjunction with these equipment are: lead

acid and nickel cadmium. The first has three possibilities of positive plates which are: the

pasted, multitubular and plante type. The negative plates will be of the pasted type, the grid for

the plante and multitubular is made of lead antimony and that for the pasted is made of either

lead antimony or lead calcium. The active material in the +ve plate is lead oxide and in the -ve

plate is sponge lead. The electrolyte is a solution of diluted sulphuric acid with specific gravity

of approximately 1.2. For the nickel-cad batteries, the plates may be of the pocket or the sintered

type. The active material (nickel hydrated for the +ve plate and cadmium sponge for the -ve) is

placed in nickel plated steel holders. The electrolyte is a solution of potassium hydroxide diluted

in water with a specific gravity of 1.16 to 1.19 at 25 C.

5) What are the defining parameters for low voltage alternating current magnetic

contactors/starters?

The defining ratings for low voltage alternating current magnetic contactors/starters are: the

NEMA size, the voltage rating, the maximum HP for single phase and three phase motors (for

both nonplugging/nonjogging and plugging/jogging applications), the continuous current rating

of the contactor/starter, the service limit, transformer switching capability rating for single and

three phase applications, the capacitive switching capability (in volt and KVAR), the

dimensions, the weights, the overload protective element type and rating.

6) What are the defining parameters of l.v. combination starters?

The defining parameters of low full voltage combination starter units: the starter size, the

maximum motor HP at the different standard voltages (200v, 230, 460 and 575v), whether the

unit is reversing or non-reversing, the fuse or circuit breaker size (used as a protection against

short circuit or protection/load break device), the size of the unit in inches or space factor, the

weight of the unit, method of attachment to riser bus bars of MCC (bolt-on or plug in) and size

plus type of motor overload protection element/relay.

7) What are the defining parameters for full voltage 2-speed starters?

The defining parameters for full voltage 2-speed starter units are: the starter size, the HP

(maximum) at the different nominal voltages, the circuit breaker or fuse size for the short circuit

protection, the dimension in inches or space factor for the 1-winding and 2-winding motor


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starter unit, unit weight, method of attachment to MCC and the type plus size of o/c protection

device.

8) What are the defining parameters of low voltage, reduced voltage starter units?

The defining parameters of low voltage reduce voltage starter units (applicable to part winding

and auto-transformer units): the starter size, the maximum motor HP at the different rated

voltages, the circuit breaker or fuse rating, the dimensions or space factor for the units, theweight, the size plus type of the o/l element, the installation method in the MCC.

9) What would a table defining the motor protection circuit breakers have as headings?

The table will have the following headings: 3-phase motor HP, the motor full load current at the

different nominal voltages, the circuit breaker continuous current, the different adjustments (eg.

7x/11x/13x), the weight and dimension, the adjustable range.

10) What would a table defining motor protection fuses have as headings?

The fuses for motor protection table will have the following: the maximum motor HP rating, the

motor full load current rating, the fuse size for the different voltage classes, the fuse type (50KA

interrupting fuse, HRC-200 KA I.c., code fuse - 10KA or size L over 600a - 200KA), the fuseweight.

11) What are the headings of a table defining m.v. controllers?

The table for a medium voltage controller (contactor and fuse) will have the following headings:

the contactor maximum continuous current, the interrupting capacity (at the specified KV), the

designation, the voltage rating and range, the interrupting capacity of the fuse (in KA and MVA

@ the rated voltage), the maximum HP motor rating for the motor design/p.f./voltage/controller

current rating, the dielectric withstand voltage, the controller -type (full voltage, reversing vs

non-reversing, reduced voltage - auto transformer vs reactor). The m.v. fuses for controllers can

be defined when the following values are given: the motor locked rotor current, motor full loadcurrent x service factor, the maximum continuous current rating of the fuse inside the

compartment, the fuse size, the peak current let through characteristics.

12) What are the general properties of constructional and stainless steels?

The mechanical properties of constructional steels are: the ASTM designations, the thickness

range, yield point, elongation as a percent of the length (eg. in 8 inches), tensile strength and

weldability. Those for stainless steel are: A1SI designation, condition, .2% yield point,

elongation in 2 inches length, tensile strength and area reduction.

13)What are the different types of switchboard instruments and the different

mechanisms? What are the defining parameters for such devices?The different types of switchboard instruments: voltmeters, wattmeters (single phase and

polyphase), varmeters, power factor meters, frequency meters, ammeters. The different

mechanisms are: taut band suspension, repulsion vane, electrodynamic, D'Arsonval/zener diode.

The defining parameters: impedance, input resistance, inductance, voltampere/W/RVA/p.f. of

mechanism (burden), mechanism type (self-contained or transformer), instrument transformer

ratio, scale and units/scale division.

14) What are the defining parameters for KWH, KVAR and solid state meters?


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The defining parameters of KWH meters are: type of meter and number of elements, connection

method, weight/dimension, burden data for each type/elements number (phase, potential circuit

and current circuit), KVA attachment data (meter, volt, scale/multiplier, amp range), disk

constant-WH/disk revolution, register ratio. Those for KVAR are: burden (potential and current

circuits), weight and dimensions (for the different designs), temperature rise, ratings and

enclosures for the different types/numbers of elements. Those for solid state meters are:potential input ratings (input voltage, impedance, burden, overload capability, current input

ratings (burden, input current and impedance, overload), weight & dimensions, control power

requirements, communication capability (protocol, baud rate, standards), measured parameters

(measurands).

15) What are the different constructions of protective relays? Give the defining

parameters for the different types of the following relays: overcurrent, over/undervoltage,

differential and distance.

Relays can be classified based on their construction into electromechanical (magnetic induction,

magnetic attraction, thermal and D'Arsonval), solid state and microprocessor/digital based.The defining parameters for electromechanical based o/c relays are: weight and dimensions,

type of relay time current characteristics curves (inverse definite minimum time, short time,

very inverse, extremely inverse), current tap range, time dial range, operating time, burden and

thermal ratings (continuous current, 1 sec. rating and power factor, at tap setting, at 3 times tap,

at 10 times tap and at 20 times tap). Those for o/c solid state relays are: weight and dimensions,

current rating, frequency, d.c. supply voltage and burden, burden on C.T., setting range for the

instantaneous and time delay functions, time multiplier setting, operating times for time delay

and instantaneous functions. The parameters for the microprocessor based are: weight and

dimensions, rated current, setting range, rated current for the ground fault unit and the setting

range, operating times, burden, overload capacity, control power voltage and burden, resetting

times and contacts ratings.

The defining parameters for the over/undervoltage electromechanical relays are: the weight and

dimensions, the continuous input voltage rating, the short time (eg. 2 minute) voltage rating, the

taps' range, burden, time dial range. Those, for the solid state are: weight and dimension, input

voltage ratings, pick-up range, drop-out setting range, time delay setting (pick-up and drop- out),

control (auxiliary) supply voltage and range.

The defining parameters for a electromechanical differential relays are: the weight and

dimensions, the application i.e. transformer differential or motor or generator differential

protection, number of phases i.e. single or three phase, number of operate and restraint circuits,

minimum trip current, burden, operating time, type i.e. fixed or variable percentage (biased)

relay, current inputs.

The defining parameters for solid state percentage (biased) relays with instantaneous tripping

and harmonics restraint: weight and dimensions including interposing relays (if any), the

currents input, overload capacity, frequency, interposing C.T. ratio (if any), burden, operating

current, restraint current settings, unrestrained current setting range, operate time, reset ratio,


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order of harmonics restrained i.e. 2nd or 5th, auxiliary power voltage and power consumption,

burden of operating and restraint circuits.

The defining parameters for the electromechanical distance relays are: the weight and

dimensions, characteristics type i.e. impedance/reactance/admittance/angle impedance

(OHM)/offset MHO/modified impedance/complex/elliptical/quadrilateral, relay reach, taps,

variation in reach and maximum torque angle over the range of taps, burden on instrumenttransformers. Those for solid state relays, the parameters are: weight and dimensions, input

circuits, burden, control voltage and consumption, characteristics torque (angle), reset ratio.

16) A unit substation is connected to the utility line through a 25KV circuit breaker which

feeds a 5 MVA transformer, 5% impedance 3-phase, 25KV/600v. The low voltage winding

of the transformer is connected through a 600v main breaker to a low voltage switchgear.

The switchgear has 3 circuit breakers feeding each a 400 HP motor, .25 reactance, 1800

RPM. A fourth circuit breaker is feeding a 750 KVA, 4.5% impedance 600/208v and the

last breaker in this lineup feeds the other 600/347v loads (2 MVA). For a fault (L-L-L) on

the main bus, calculate the fault current with and without motor contribution and give the

contribution of each motor. Calculate also the motor breaker fault current for a fault on

the motor terminals. Assume on infinite source. Give the breakers' size on the low voltage

side of transformer, give a typical unit substation layout.

MVAbase

= 5, KVbase

(primary side of power transformer) = 25, KVbase (secondary side) = .6,

Ibase = 5 (1000)/1.732(.6) = 4811 amp= full load current. Full load current on primary side =

4811(600)/25000 = 115 amp. Fault current for 3-phase fault on main bus = 1/.05 = 20 p.u. =

20(4811) =96 KA (from suply side).


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Xmotor

= .25 (5000/350) = 3.5 p.u. (assuming motor p.f. = .85)

3 motors cotribution for a 3-phase fault on main bus = 1/1.16 = .85 p.u. = .85(4811 = 4089 amp.

Total fault current = 96 + 4 = 100 KA

Contribution per motor = .283 (4811) = 1370 amp.

For L-L-L fault on motor terminals, the fault current through the motor breaker = 2(1.37) + 96 =

98.7 KA.Motor full load current = 400(746)/1.732(.85)(.9)(600) = 375 amp. (assuming a motor efficiency

of .9)

F.L. current to 750 KVA transformer = 750 (1000)/1.732(600) = 722 amp.

F.L. current to the 2 MVA load = 2000/1.732(.6) = 1924 amp.

For the main low voltage circuit breaker:

Continuous curent (frame size) = 5000 amp., I.C. = 120 KA

For the motors circuit breakers (fused or current limiting):

Continuous current = 800 Amp, I.C. = 100 KA , Sensors ratio + 400/5 amp.

For the 750 KVA transformer (fused or current limiting):Continuous current = 1600 amp., I.C. = 100 KA, Sensor = 800/5 amp.

For the 2 MVA load (fused or current limiting):

Continuous current = 2000 amp., I.C. = 100 KA, Sensor = 2000/5 amp.

Fig. 11 shows the typical layout.

SUMMARY:


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In this chapter, circuit breakers, switchgear assemblies, starters and controllers were presented

from the definition, standards and brief description points of view. For low voltage circuit

breakers, the coverage included the ratings of the power and electric control circuits. For

medium voltage circuit breakers and switchgear assemblies, the coverage included the breakers

interrupting media, breakers ratings, governing standards, different types of switchgear

assemblies and types of stationary batteries found in such assemblies. For l.v. starters, thecoverage included the defining parameters for the following devices: contactors, combination

starters (full voltage/single speed, full voltage/two speed, autotransformer/part winding reduced

voltage), motor protection fuses and magnetic element only circuit breakers). For medium

voltage starters, the coverage included the contactors and fuse defining parameters. Certain

related miscellaneous topics were also covered briefly like the gene

15418296 electrical distribution systems questions answers part i

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