Nov 15, 2010

How To Design Capacitor Bank


It is crucial for electrical engineer to know on how to design the capacitor bank. The purpose of installing the capacitor bank is to counter the resistive load (KVAR) in the electrical system. By doing so, the power factor can be maintain to your required value and not to mentioned, it will save you lots of dollars as well as few more advantages by maintaining the power factor.

The Simple Way In Designing The Capacitor Bank
The simple way in designing the capacitor bank is by using the ready-made software. Just click here to download this software (in Excel format).

You would see the file as below once you open the file.


The above calculation is to design the required capacitor bank for Main Switchboard No.1. All the parameters such as transformer size and voltage are based on my total load for Main Switchboard No.1. You can simply change those values according to your own loads.
How The Software Works?
This software is will automatically calculated the required capacitor bank for your system once you key-in the values inside those cells. First and foremost, you need to determine the parameters below 
a)      Transformer Size
b)      The Secondary Voltage (it might be 400V or 415V)
c)       The Loading Factor of the transformer
d)      The Maximum Demand (MD)
e)      The Uncorrected Power Factor (PF)
f)       The Targeted Power Factor that you want to achieve
g)      The Main Voltage of your system (400V or 415V)
h)      The Capacitor Voltage
i)        The Filtering Factor
j)        The Frequency of the electrical system (50hz or 60hz) and
k)      The No. of steps for capacitor bank
Once you determine the parameters, what you’ll need to do is to key-in those parameters inside those cells as indicates in the figure below.


Then, you’ll see that, the value required for your capacitor bank is automatically calculated.

Based on the figure above, the required capacitor bank is 536.42kVAr. Therefore, the value of the capacitor bank shall be slightly higher than the calculated kVAr value. For this calculation I’m using the kVAr value of 550kVAr. This is due to the total kVAr value is based on the ratio of 25kVAr.
Instead of 25kVAr, there are few selections of kVAr ratio that can be use;

Until this stage, you already knew your required kVAr value, the number of steps and you have done selection of kVAr ratio that will be use in designing the capacitor bank. The next step is to determine the selection of power ratio. Figure below is a few selection of power ration that can be used.

In this calculation, I’m using the power ratio #3. Since the capacitor bank is designed according to 8 numbers of steps. Therefore, the selectable power ratio will follow the sequence below;

The selection of the power sequence depending on your selection of kVAr unit and the total kVAr needed to achieve the targeted power factor. Once the power ratio and the actual kVAr finalized, then key-in those values into the respective cells as per shown below.

Finally, you have completed designing your capacitor bank. Looking into the figure below, the actual kVAr is 550kVAr compared to the calculated kVAr of 536.42kVAr.

The minimum breaker rating for the capacitor bank is 771.15 Amp. Therefore, the nearest breaker rating that can be selected is 800Amp. The selection of the Current transformer (CT) is 1500/5A based on the Maximum Demand of 900kW and the c/k value will be 0.051.
[c/k value = Value of the reactive current response threshold of the Controller in Ar (ampere reactif)]