Power Factor Correction Capacitors Sizing Calculations – Part Six



Today, we will continue explaining Power Factor Correction Capacitors Sizing Calculations Steps in detail as in the previous article “Power Factor Correction Capacitors Sizing Calculations – Part Five”, we only listed the Power Factor Correction Capacitors Sizing Calculations Steps.

So, today we will start explaining the following steps for Power Factor Correction Capacitors Sizing Calculations For Existing Installations:
  • Step#1: Collect monthly billing data, 
  • Step#2: Make some preliminary measurements for current and voltage.




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.






4.1 Power Factor Correction Capacitors Sizing Calculations Steps
For Existing Installations




the Power Factor Correction Capacitors Sizing Calculations Steps For Existing Installations include two phases as follows:

  1. Preliminary Evaluation Phase,
  2. Design Phase.


Fig.1 shows the phases of Power Factor Correction Capacitors Sizing Calculations For Existing Installations.



Fig.1



First: Preliminary Evaluation Phase



The preliminary evaluation is performed to determine if the application of power factor correction capacitors is likely to be economical or not. Fig.2 shows a typical flow chart
for the preliminary evaluation process.


Fig.2



Step#1: Collect Monthly Billing Data



This step is used to obtain the data necessary for determining the total amount of capacitance needed and the savings possible.
In this step, we will learn:
  • How to collect monthly billing data for calculating The required capacitor power rating?
  • What is The energy charge (Kilowatt-hour tariff),
  • What is The demand charge (Demand tariff),
  • What to use; Actual demand, peak demand or the average maximum demand for determining the total amount of capacitance needed?
  • What is The power factor penalty,
  • How to calculate the power factor penalty?
  • How to calculate the power factor for an existing installation?


The data collected from this step are used for the Economic Screening Worksheet in the second major step of this design process.



How to collect monthly billing data for calculating The required capacitor power rating?

  • Preferably, all bills for the previous year should be collected, If power consumption is constant throughout the year, the annual electricity consumption or any desired monthly invoice (but not for the month in which the annual shutdown occurs), may be taken as a basis. If seasonal variations are apparent, an invoice from the "high season" must of course be selected.
  • Use the highest demand month unless there is reason to believe that it is an anomaly and will not be repeated.
  • You may want to plot out these factors for a year of usage, or more, to better understand the trend in the load.
  • If regular and off-peak tariffs are measured separately, usually the regular tariffs are used for calculation purposes.
  • The key data required are maximum demand, power factor, typical energy usage, and power factor penalty or demand charge.


The charges included in Industrial end user bills generally have three main parts as follows:

  1. The energy charge,
  2. The demand charge,
  3. The power factor penalty

  
There are also taxes and other miscellaneous charges, but these do not have a significant impact on the economic justification of power factor correction capacitors.




A. The energy charge (Kilowatt-hour tariff)

  • The energy charge is determined based on the active power by multiplying the number of kilo-watt-hours (kWh) of energy consumed in the month times the energy rate ($/kWh).
  • While The Reactive energy is invoiced as separate items. In most power supply contracts, no charge is made for reactive energy if its magnitude is up to 50% of the active energy. Only amounts that exceed this figure must be paid for. This corresponds approximately to a PF of 0.9. It is recommended, however, to use a slightly higher figure, e.g. 0.92, for calculation purposes, in order to have a small margin in reserve in the capacitor power rating.






B. The demand charge (Demand tariff)

  • The demand charge is more complicated. It is typically based on the peak kW demand over a given 15-, 30-, or 60-minute interval during the month and then multiplied by the demand charge rate ($/kW).
  • In this case, the utility company bases its invoice on the maximum amount of power drawn by the user during the given month. If it is not the active power but instead the apparent power, which is measured for this purpose, it is advisable to select a capacitor power rating that will achieve a PF of 1.


What to use; Actual demand, peak demand or the average maximum demand for determining the total amount of capacitance needed?

  • You do not necessarily want to use the actual demand recorded on the bill to determine the amount of capacitors. Many loads peak sharply a few times per day for a brief time period then settle down to about 80% of that value. Determining the amount of capacitors based on the absolute peak will generally result in too many capacitors.
  • If you, or your utility, have demand interval data, use that to determine the average demand at heavy load. You may also use a value from the preliminary measurements taken during a heavy load period.
  • You may choose to use the peak demand during the preliminary screening. If that shows that the installation is likely to be economical, then it will certainly be economical for fewer capacitors.


Therefore, when you design the actual installation, use the average maximum demand determined from the detailed plant survey.




C. The power factor penalty

In addition, many utilities assess a penalty to the demand if the power factor is lower than a predetermined value (typically 0.95). There are two common formulae in use for determining the billed demand when the power factor, PF, is lower than 0.95, lagging:

KW billed = KW actual x (0.95 / PF)
KW billed = KW actual x (1+0.95 –PF)

Both of these are applied only when PF is less than 0.95, lagging. Otherwise, the billed demand is the same as the actual demand.

How to calculate the power factor penalty?

The difference between the amount paid for the billed demand and the amount that would be paid for the actual demand is often termed the power factor penalty. This quantity is generally responsible for the bulk of the justification for capacitors.

Compute the bill with the normal power factor, and then re-compute the bill assuming the power factor has been corrected sufficiently to avoid an extra charge. The difference is the power factor penalty.

Power Factor Penalty = (KW billed - KW actual) X Cost

Cost in units of $/KW


Note:
  • Some billing schedules are more complicated than this. For example, it is also common for the demand charge to be included with the first block of energy, which is charged at a different rate than the remaining energy usage.






How to calculate the power factor for an existing installation?

  • The power factor used in billing is generally an average power factor determined over the entire month, although a few utilities bill interval-by¬ interval.
  • There are two usual procedure for determining the power factor for existing buildings by using one of the following measuring devices:

  1. the kilo-var -hours (kvarh) meter as well as the kilo-watt-hours (kWh) meter,
  2. A clamp on power factor meter.



Method#1: The kilo-var-hours (kvarh) meter as well as the kilo-watt-hours (kWh) meter.

KWH / KVARH Meter
  • This may be done by two separate meters or may be contained within one electronic meter. The kvarh are then combined with the kWh to obtain an equivalent kilo-volt-ampere-hours (kVAh):


kVAh = 
Ö( kWh2 + kvarh2 )

The average power factor is then:

PF = kWh / kVAh

  • The kvarh meter is usually “detented” so that it only records lagging vars; that is, the vars drawn by motors. No credit is given for leading vars.


Note:

  • Many utilities are now considering billing for kvarh similarly to kWh. Existing meter technology can separately track leading and lagging kvarh. This provides the opportunity to have flexible rate structures to create more incentives for industrial end users to control var consumption and production.


Method#2: A clamp on power factor meter

Clamp on Power Factor Meter

  • Since each load has its own power factor, the measurements should start with each individual machine and move upward to each distribution panel and finish at the feeder and then to transformer as shown in the Fig.3 below.



Fig.3
  • Measuring power factor is a costly procedure when it is required to shut the load down and connect in a metering system to measure the current, voltage and power. So as to avoid the costly shutdown and time consuming measurement, it is preferable to use a clamp on power factor meter.
  • To connect the meter, the voltage leads are first connected to the meter and then clipped to the phases supplying the load. The clamp-on current transformer is then clamped on to the phase supplying the load. To select the appropriate clamp on CT, a conventional clamp tester is used to measure the load current. The voltage is also measured. Now using the clamp-on, power factor meter with appropriate CT, the power factor reading is noted.





Step#2: Make Some Preliminary Measurements For Current And Voltage




This step is used to get a rough idea of how heavily loaded the plant cables and transformers are so that the loss savings can be better estimated. The measurements are also used to help identify potential harmonic problems.
In this step, we will learn:
  • What are the Measurements used for Calculation of the cable capacity factor?
  • What are the Measurements used for Calculation Transformer Losses?
  • How to determine of the need for harmonic study?





Measurements used for Calculation of the cable capacity factor

Measure the currents on several of the more significant feeds within the plant. Compare the current measurements to the ampacity of the cables and estimate a typical loading factor for the plant's cables. The ratio of measured current to ampacity is the cable capacity factor needed for the Loss Savings Worksheet.


Cable Capacity Factor = Measured Current / Cable Ampacity




Measurements used for Calculation Transformer Losses

Transformers can account for a large percentage of the losses. If the plant has step-down transformers on the load side of the utility meter, the average load kVA flowing through these transformers will be determined from the following formula:


KVA 3-phaseÖ3 V line-line I line

Where the line-line voltage is given in kV (rms) and the current, I, is the rms line current reading on the same side of the transformer.

 So, we need to measure the rms current flowing into the transformer from either the primary or secondary side of the transformer, depending on which is the most convenient.

Notes:

  • Remember to compare your measurement with the rated current on the side you measured.
  • Consider only transformers for which capacitors will be placed on the secondary side. Capacitors don't help losses unless they are placed on the secondary side to reduce the current.
  • Exclude transformers on the utility side of the meter and transformers that will not have capacitors installed on their secondary circuits.





Determination of the need for harmonic study

  • At this point in the design process, we are simply interested in determining if there is sufficient cause to include harmonic studies in the cost justification and we are looking for effects of harmonic sources in the vicinity of the plant, either within the plant or in neighboring plants.
  • This can be done by measuring at least the main bus voltage and the total load current with an instrument capable of measuring both total harmonic distortion (THD) and true rms values.
  • The measured readings can be an indication for Harmonics possibility as follows:



Measured Bus voltage distortion %
Harmonics possibility
approximately 1%
is not necessarily an indication that there are harmonic sources to be concerned
more than 2%
There is a strong possibility that there are significant harmonic sources in the area that could impact a capacitor installation.


Notes:

  • This is not to be confused with the 5% upper limit set by some standards. While it is often acceptable by standards to have a voltage distortion of as much as 5%, lower values of distortion can cause problems with capacitors if the conditions are right.



In fact, there is a possibility of problems for low values of voltage distortion less than 2% if the system gets into resonance, particularly, if the predominant harmonics are higher than the fifth.


Measured current distortion %
Harmonics possibility
10% or more
This reading indicates the presence of harmonic sources that could conflict with capacitor installation.


Also, if there are any existing capacitors in the installation, a good predictor of potential harmonic problems is the current in the capacitors.

Capacitor rms current
Harmonics possibility
exceeds 120- 130% of the rated capacitor current
it is likely that there are significant harmonic sources.


Again, in this step, we are not necessarily interested in characterizing the harmonic sources, but only wanting to know of their existence so that we can take them into account in the economic screening by assuming extra costs will be incurred.





In the next article, we will continue explaining other steps for Power Factor Correction Capacitors Sizing Calculations for Existing Installations. 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








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