Power Factor Correction Capacitors Sizing Calculations – Part Ten



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.


4- Power Factor Correction Capacitors Sizing Calculations Steps



Now, we are going to explain the Power Factor Correction Capacitors Sizing Calculations Steps for Different Cases of Installations:
  1. For Existing Installations,
  2. For New Designs.





4.2 Power Factor Correction Capacitors Sizing Calculations Steps
For New Designs




Power Factor Correction Capacitors Sizing Calculations Steps
For New Designs will include the following steps:

  • Step#1: Review One-Line Diagram and Develop a Fixed Capacitor Scheme with Group Compensation,
  • Step#2: Use the detailed loads schedules to calculate the Design PF (old PF) for Each Part of MLTPs,
  • Step#3: Checking the "No Load" Voltage Rise,
  • Step#4: Select Capacitor Switching Options.




Step#1: Review One-Line Diagram and Develop a Fixed Capacitor Scheme with Group Compensation



  • This step is used to make judgments about the best locations of power factor correction capacitors on the designed one-line diagram.
  • Review the one line diagram of the new design to determine the most appropriate locations of power factor correction capacitors taking into consideration the following notes:

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  (see Fig.1).


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: One-Line Diagram


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.



Step#2: Use the detailed loads schedules to calculate the Design PF (old PF)



This step is used to calculate the Designed PF (old PF) at each part of MLTPs by doing the following steps:

  • 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,
  • 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)),
  • Step#5: Calculate the total of KW by summing kw column and calculate the total kVAr by summing the KVAR column,
  • Step#6: Calculate the total KVA from the equation kVA = √(kW2 + kVAr2),
  • Step#7: Calculate the designed PF (uncorrected power factor of total load or old PF) from the equation PF = kw/kVA.



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:


Variable speed drives VSD
size
No of
Load factor
VSD 0.37 KW
2
0.75
VSD 0.55 KW
1
0.85
VSD 0.75 KW
5
0.9
VSD 1.1 KW
4
0.75
VSD 1.5 KW
2
0.85
VSD 4.0 KW
4
0.9
VSD 5.5 KW
2
0.75
VSD 7.5 KW
4
0.85
VSD 18,5 KW
1
0.9
VSD 22 KW
2
0.75
VSD 37 KW
3
0.85
VSD 75 KW
1
0.9
Direct Online Starters DOL
DOL 0.75 KW
6
0.9
DOL 1.1 KW
11
0.75
DOL 1.5 KW
5
0.85
DOL 2.2 KW
1
0.9
Soft starters S/S
S/S 5.5 KW
9
0.75
S/S 7.5 KW
4
0.85
S/S 11 KW
1
0.9
S/S 15 KW
5
0.75
S/S 18.5 KW
4
0.85
S/S 132 KW
3
0.75
Special starters
Separator – star/delta – 25 KW
2
0.75
Sight glass light – 24VDC , LED
1
0.9
Trace heating (meters)
10
0.75
Forward/reverse – 0.75 KW
2
0.85
2 speed - 1.1 KW
2
0.9

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.




Step#3: Checking the "No Load" Voltage Rise



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.




Step#4: Select Capacitor Switching Options



Two types of capacitor switching schemes are available as follows:

  1. Single switching control,
  2. Selective switching control.


You can review article " Power Factor Correction Capacitors Sizing Calculations – Part Fourto know when and why we need to use one of the above switching schemes.




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:
Subject Of Previous Article
Article
  • Glossary of Power Factor Correction Capacitors


  • Types of Loads,
  • The Power Triangle,
  • What is a power factor?
  • Types of power factor
  • Why utilities charge a power factor penalty?
  • Billing Structure.




  • What causes low power factor?
  • Bad impacts of low power factor,
  • Benefits of Power Factor correction.



  • How to make Power Factor Correction?
  • Types of Power Factor Correction Capacitors
  • Individual compensation



  • Group compensation,
  • Central compensation,
  • Hybrid compensation.
  • Summary for Power Factor Correction Capacitors Sizing Calculations Steps




  • Step#1: Collect Monthly Billing Data
  • Step#2: Make Some Preliminary Measurements For Current And Voltage



  • Step#3: Fill the Economic Screening Worksheet



  • Step#4: Make Preliminary Measurements For Harmonics
  • Step#5: Repeat the Economic Screening Worksheet
  • Step#6: Compare the Savings with the Probable Cost of Capacitors' Installation
  • Second: Design Phase
  • Step#1: Performing a Detailed Plant Survey
  • Step#1.A: Review the one line diagram
  • Step#1.B: Take into consideration the loads that produce harmonics
  • Step#1.C: collect sufficient data Inventory by using measuring instruments





  • Step#2: Select Economical Capacitor Scheme
  • Step#3: Checking the "No Load" Voltage Rise
  • Step#4: Select Capacitor Switching Options
  • Step#5: Check the Harmonic Distortion and make Harmonic Mitigation Options
  • Step#6: Use the Economic Screening Worksheet again









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