Answer / shashikant

CALCULATIONS AND TABLES

Calculation and selection of required capacitor rating

Qc = P * {tan [acos (pf1)] - tan [ acos (pf2)]}

Qc = required capacitor output (kVAr)
pf1 = actual power factor
pf2 = target power factor
P = real power (kW)

The table below shows the values for typical power factors
in accordance with the above formula


TARGET POWER FACTOR
0.7 0.75 0.8 0.85 0.9 0.92 0.94 0.96 0.98 1
Actual Power Factor
0,40 1.27 1.41 1.54 1.67 1.81 1.87 1.93 2 2.09 2.29
0,45 0.96 1.1 1.23 1.36 1.5 1.56 1.62 1.69 1.78 1.98
0.50 0.71 0.85 0.98 1.11 1.25 1.31 1.37 1.44 1.53 1.73
0,55 0.5 0.64 0.77 0.9 1.03 1.09 1.16 1.23 1.32 1.52
0,60 0.31 0.45 0.58 0.71 0.85 0.91 0.97 1.04 1.13 1.33
0,65 0.15 0.29 0.42 0.55 0.68 0.74 0.81 0.88 0.97 1.17
0,70 0 0.14 0.27 0.4 0.54 0.59 0.66 0.73 0.82 1.02
0.75 0 0.13 0.26 0.4 0.46 0.52 0.59 0.68 0.88
0,80 0 0.13 0.27 0.32 0.39 0.46 0.55 0.75
0.85 0 0.14 0.19 0.26 0.33 0.42 0.62
0,90 0 0.06 0.12 0.19 0.28 0.48


The required capacitor output may be calculated as follows:
select the factor (matching point of actual and target
power factor) k
calculate the required capacitor rating with the formula:

Qc = k * P

Example:
actual power factor = 0.70, target power factor = 0.96,
real power = P = 500kW,

Qc = k * P = 0.73 * 500kW = 365 kVAR

Answer / cbc

1. PF=Real Power/Apparent Power or PF=KW/KVA

2. a.) (KVAR1)*2 = (KVA1)*2 - (KW1)*2
b.) (KVAR2)*2 = (KVA2)*2 - (KW1)*2
c.) REQ'D KVAR= KVAR1-KVAR2

Answer / ezzuddin

SORRY FOR THE ABOVE WRONG INFO AND The correct answer AS
STATED below is the proof of the formula:

KVAR= MAX Demand X [TanQ1 - TANQ2];
TanQ1= [Sqrt(1–(CosQ1)^2)]/CosQ1
TanQ2= [Sqrt(1–(CosQ2)^2)]/CosQ2

The proof:
Pythagoras Theorem states that in a right-angled triangle
the square of the hypotenuse is equal to the sum of the
squares of the other two sides.
so that:
C is the hypotenuse.
S is the opposite.
P is the adjacent.
TanQ= opposite/adjacent = S/P -----> S=P x(TanQ;)
Pf=CosQ;= adjacent/hypotenuse = P/C ---->C=P/CosQ;
Pythagoras said that:
C^2= P^2 + S^2
P^2/(CosQ)^2= P^2 + (P^2 x (TanQ)^2)
by divide P^2:
1/(CosQ)^2 = 1 + (TanQ)^2
by changing the both sides:
(TanQ)^2 = (1/(CosQ)^2) - 1
(TanQ)^2 = (1/(CosQ)^2) - ((CosQ)^2/(CosQ)^2)
by Taking the (1/(COSQ)^2) Common factor:
(TanQ)^2 = {1/(CosQ)^2)} x ( 1 - (CosQ)^2)
by taking the square root:

TanQ; = [Sqrt(1–(CosQ)^2)]/CosQ;

so that:
KVAR = KW x [TanQ1 - TANQ2]

example:
PF1 = CosQ1 = 0.8
PF2 = CosQ2 = 0.92
TanQ1 = 0.75
TanQ2 = 0.426
P=1000 KW
Max Demand = 1000x0.8= 800 kW
Capacity of the required Capacitor= 800 x(0.75 - 0.426)=
264 KVAR
We proposed automatic capacitor= 300 KVAR

Answer / sm

pf is the cosine of the angle b/w voltage phasor and current phasor. To maintain unity feedback the capacitor bank value should be such that it compensates the inductive reactance value in circuit i.e., X becomes zero. Thus, the circuit becomes purely resistive and pf is unity.

Answer / rukmagoud

Required Capacitor bank and required KVAR of the bank to be
installed to improve power factor from say x to y is
KVAR=KW(TANx-tany).This can be better understood from power
triangle.

RUKMAGOUD
APGENCO
HYD

Answer / udayak25

in short I tell u -
kVA
kVAR = -----
0.432

it gives u a correct value of capacitor bank.

-Uday

Answer / j. nagarajan

The power factor is the ratio of voltage to current at
120Deg Phase shift.

Cos phi = KVA/KW
The active power factor in our country will be 0.8 only, ie
only 80% of power consumed and balance is lost due to
various lossed in the distribution system.

To calculate Cap. bank the formula is
KVAR = Power tan (cos phi(ext)-tan cos phi (exp)

where cos phi(ext) - existing power factor & exp is
expecting power factor.

It is not possible to brought to unity powerfactor is
utilising 100% of power to the application. Max. of 95% can
be possible for which the power factor value will be 0.95

After calculating cap. banks, say 100kvar, it can be
seperated like below:

1:1:1:1 (where 1=25kvar)
1:2:1(where 2=50kvar)... and so on.

- J. Nagarajan

Answer / hassan azzam

dear all your formula is correct but this for existing
system , wher if you dont have mesured value how to estimat
there use my estimation
Qc (Kvar) = connected load( kw)* 0.8 * 0.75 * 0.421
Qc (Kvar) = connected load (kw) * 0.2526
this is for future load or testing and commissioning perid

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