Today, we will start explaining Power Factor Correction Capacitors
Sizing Calculations Steps in detail for the new designs as in the previous
article “Power Factor Correction Capacitors Sizing Calculations – Part Five”, we only listed the
Power Factor Correction Capacitors Sizing Calculations Steps.
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4- Power Factor Correction Capacitors Sizing Calculations
Steps
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Now, we are going to
explain the Power Factor Correction Capacitors Sizing Calculations Steps for
Different Cases of Installations:
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4.2 Power Factor Correction Capacitors Sizing Calculations
Steps
For
New Designs
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Power Factor Correction
Capacitors Sizing Calculations Steps
For New Designs will include the following steps:
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Step#1:
Review One-Line Diagram
and Develop a Fixed Capacitor Scheme with Group Compensation
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1- The one-line diagram
must show the main structure of the electrical system from the utility
interconnection up to at least the main motor control centers and larger
panels which is called Main Low Tension Panels (MLTP).
2- The feeding system of
the MLTP panels usually 1 out of 3 which divide the MLTP to two parts
Fig.1
3- The lowest cost installations are
generally fixed capacitors with few additional accessories such as automatic
switches and harmonic filters. Therefore, the philosophy of this process is
to start with a suitable fixed capacitor scheme and then modify it until the
voltage and harmonic constraints are met. In many cases, it will be possible
to meet the constraints without resorting to these additional items.
4- On the new designs
always we recommend to put one capacitor bank on each part of the MLTP or
MCC. This means that the group compensation is usually the most
appropriate type of compensation for the new designs (see Fig.2). This will generally achieve a good capacitor
scheme with respect to losses, although, it may be not optimal.
Please check
the title "group compensation" in article "Power Factor Correction Capacitors Sizing Calculations – Part Five"
Fig.2
5- The rating of the
capacitors is calculated using the designed PF (this will be considered as
the old PF in calculations), the calculations of the capacitors rating will
be explained in next articles. Below figure-3 shows an example of a one-line
diagram where the group compensation is applied to each part of the MLTPs and
marked with red circles.
Fig.3: One-Line Diagram for New
Designed Project
You can download PDF
copy from this One-Line Diagram from this link:
Note:
You can Supply the
one-line diagram to suppliers/manufacturers to assist with the application to get an idea of the costs and the optimal
distribution of the capacitor banks.
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Step#2:
Use the detailed loads schedules to calculate the Design PF (old PF)
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This step is used to
calculate the Designed PF (old PF) at each part of MLTPs by doing the
following steps:
Example#1:
A new factory had been designed and you asked to determine the
designed PF (uncorrected power factor of total load or old PF) given
the following loads of one MCC:
Solution:
Step#1: You must have a schedule of load in MS
Excel or you can enter loads into a new excel file,
Step#2: Calculate the true power in KW for each
group of typical loads by multiply load kW * number of * load factor,
To do this type in cell F3 the
following equation ( =C3*D3*E3) and press “enter”.
Then highlight cell, click onto
bottom right corner and drag to bottom of column to obtain results for rest
of “total load” column.
Step#3: Enter power factor manually. If power
factors not given, use 0.98 for VSDs and 0.85 for DOL motors, star-delta
started motors and soft started motors etc. Ignore Trace heating as resistive
(unity power factor) and sight glass light,
Step#4: Calculate the reactive power kVAR for each
group of typical loads using the equation kVAR = kW*tan Φ = kW*tan(acos(PF)),
To obtain the angle Φ, we obtain the
cos-1 of cosΦ . In excel, ACOS = cos-1 so
kW*tan(acos(PF)) in excel is equivalent to kW*tanΦ.
In row 3 above, total kW = F3 and PF
= G3. Therefore, kVAR of 0.37kW VSDs is =F3*TAN(ACOS(G3)).
After typing =F3*tan(acos(G3)) into
cell H3, press “enter”. Then highlight cell, click onto bottom right corner
and drag to bottom of column as done previously.
Step#5: Calculate the total of KW by summing kw
column and calculate the total kVAR by summing the KVAR column,
Highlight cell at bottom of TOTAL kW
column and then click SAutoSum and enter. Then highlight cell at
bottom of kVAr column and click SAutoSum
and enter.
Step#6: Calculate the total KVA from the equation
kVA = √(kW2 + kVAR2),
The cell F33 shows the (total kW)
while the cell H33 shows the (total kVAR). Type in cell F35 the following
equation (=SQRT(F33Ù2+H33Ù2)) then press enter to get the total KVA.
Step#7: Calculate the designed PF (uncorrected
power factor of total load or old PF) from the equation PF = kW /kVA.
Type in cell F36 the following
equation (=F33/F35) then press enter to get the total PF value which is the
designed PF (uncorrected power factor of total load or old PF).
Hence, the designed PF (uncorrected power factor of total load or old PF) = 0.9005617 which will be used later in the calculation of capacitors rating.
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Step#3:
Checking the "No Load" Voltage Rise
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This step is used to check that if the Voltage Constraints Met or
not. Please review the same step in article "Power Factor Correction Capacitors Sizing Calculations – Part Nine" for existing
installations.
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Step#4:
Select Capacitor Switching Options
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Two types of capacitor switching
schemes are available as follows:
You can review article " Power Factor Correction Capacitors Sizing Calculations – Part Four" to know when and why we need to use one of
the above switching schemes.
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In the
next article, we will start explaining different
method of calculations for PF correction capacitor Ratings. Please, keep
following.
The
previous and related articles are listed in below table:
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Subject Of
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