# Power Factor Correction Capacitors Sizing Calculations – Part Twelve

As we explained before in article how to calculate the capacitor KVAR rating for compensation at:
1. Transformers,
2. Individual motors.

Today, we will explain how to calculate the capacitor KVAR rating for buildings and power plants (group compensation).

 Calculation Of The Capacitor KVAR Rating For Buildings And Power Plants

 To calculate the capacitor KVAR rating for buildings and power plants (group compensation), we have (2) methods: By using measuring devices, By using The Utility Company’s Invoice.

 Method#1: By Using Measuring Devices This method is used when the old P.F is unknown and there different methods to measure/calculate this old PF and then calculate the Capacitor KVAR Rating. These measuring methods are: Direct Measurement method:  Measurement of current and power factor    Indirect Measurement methods: which includes (2) sub methods: Measurements with recording of active and reactive power Measurement by reading meters

 Indirect Methods: Measurements With Recording Of Active And Reactive Power More reliable results are obtained with recording instruments. The parameters can be recorded over a longer period of time, peak values also being included. Required capacitor power rating is then calculated as follows: QC = QL – (P · tan Ø2) Where: QC = required capacitor rating QL = measured reactive power P = measured active power tan Ø2 = the corresponding value of tan Ø at the desired cos Ø (can be obtained from Table-1, (e.g. when cos Ø= 0.92 the corresponding tan Ø= 0.43)

 Indirect Methods: Measurement By Reading Meters The active and reactive current meters are read at the start of a shift. Eight hours later both meters are read again. If there has been a break in operation during this time, the eight hours must be extended by the duration of this break. Tan Ø can be calculated from the following equation: tan Ø = (RM2 – RM1) / (AM2 – AM1) Where: RM1 = reactive current meter reading at start RM2 = reactive current meter reading at finish AM1 = active current meter reading at start AM2 = active current meter reading at finish Using this calculated value of tan Ø and the desired cos Ø we can then obtain the factor k from Table-1. Then, The required capacitor power rating can thus be derived: QC = k* f* (AM2 – AM1 ) /8 Where: The parameter f is the ratio of the meter current transformers. Example#3: The following meter readings have been noted: active current meter (AM1)...115.3 KWH (AM2 )...124.6 KWH reactive current meter (RM1)...311.2 KVAR (RM2)...321.2 KVAR The meters work with 150/5 A current transformers. Calculate the KVAR rating for the power factor correction capacitors. Solution: tan Ø = (RM2 – RM1) / (AM2 – AM1) tan Ø = (3 2 1 . 2 – 3 1 1 . 2) /(124.6 – 115.3)  = 1.08 For a desired cos Ø of 0 .92 a factor k of 0.65 is obtained from Table-1. 150/5 A current transformers are used, so here the factor f = 150/5 = 30 The capacitor power rating is thus QC = k* f* (AM2 – AM1 ) /8 QC =0.65*30*(124.6 – 115.3 )/8 = 22.67 KVAR

 Method#2: By Using The Utility Company’s Invoice The required capacitor power rating can be determined relatively easily and accurately from the power supply company´s monthly invoice. 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. If regular and off-peak tariffs are measured separately, usually the regular tariffs are used for calculation purposes. It can be assumed that the capacitor power rating derived will be adequate to cover the reactive current circulating at night. In special cases, however, where the less expensive off-peak power is used predominantly, the off-peak consumption may not be neglected. First: In case of using Kilowatt-hour tariff the following procedure can be used to calculate the KVAR rating for the power factor correction capacitors: Step#1: Calculate Tan Ø from the following equation: tan Ø = reactive energy (regular) / active energy (regular )   Step#2: Use table-1 to find cos Ø1from the above calculated tan Ø, then find the multiplier factor kStep#3: Multiply KW by the K multiplier to get the proper Capacitor KVAR Rating. QC = k.P Example#4: The data derived from the utility company´s invoice are as follows: Active power 99 kW Active energy (regular tariff) 17820 kWh Reactive tariff (off-peak) 19840 kVArh Calculate the KVAR rating for the power factor correction capacitors. Solution: Step#1: Calculate Tan Ø from the following equation: tan Ø= reactive energy (regular) / active energy (regular )= 19840 KVARH / 17820 kWh = 1.11 Step#2: The actual value of cos Ø can now be obtained from Table 2, since the calculated tan Ø of 1.11 corresponds to a cos Ø1 of 0.67. A factor k of 0.68 is then obtained from Table 2 to produce a desired cos Ø2 = 0.92. Step#3: The required capacitor power rating is calculated from: QC = k.P = 0.68 x 99 kW = 67.32 KVAR In this case a standard capacitor rating of 75 kVAr must be selected. If a possible future expansion of the facility is also to be taken into account, then a somewhat larger capacitor, (e.g. 100 kVAr) could also be selected. Second: In case of using Demand tariff the following procedure can be used to calculate the KVAR rating for the power factor correction capacitors: Step#1: Use table-1 to find the multiplier factor k Step#2: Multiply KW by the K multiplier to get the proper Capacitor KVAR Rating. QC = k.P Note: In this case the utility company bases its invoice on the maximum amount of power drawn by the user during the given month. It is advisable to select a capacitor power rating that will achieve a cos Ø2 of 1. Example#5: The data derived from the utility company´s invoice are as follows: Maximum active power = 104 kW cos Ø1 = 0.62 Calculate the KVAR rating for the power factor correction capacitors. Solution: Step#1: From Table-1, with an uncorrected cos Ø1 = 0.62 and a desired cos Ø2 = 1, a factor k of 1.27 is read off. Step#2: Multiply KW by the K multiplier to get the proper Capacitor KVAR Rating. QC = k.P = 1.27 x 104= 132.08 KVAR For this duty a reactive power control relay with a capacitor power rating of 150 to 175 KVAR is arranged as a switched variable bank.

 Using Nomographs For Calculation Of Multiplier Factor K The multiplier factor K can be determined by using the above table-1, and also it can be determined by using the following Nomograph shown in Fig.2. Fig.2 You can follow the following procedure to determine the multiplier factor K from above nomograph: first trace a line segment from the value of the initial cosØ to the value of desired cosØ, Then, the intersection of the line with the middle graduated scale, gives the value of K.

 Important notes for sizing Capacitor KVAR Rating For Buildings And Power Plants When selecting automatic bank units, select the closest KVAR rating to the amount of KVAR desired based on present and future applications. If the desired rating is not listed, the next higher KVAR rating should be selected. When selecting fixed bank units, however, select the KVAR rating WITHOUT GOING OVER the desired capacitance level. In a three-phase system, the capacitor bank constituted by three capacitors having the same capacitance, can be delta- or star-connected. When selecting the connection modality, it is necessary to keep into account that with delta connection, each capacitance is subject to the supply line-to-line voltage, but, at the same level of generated reactive power, it has a value equal to 1/3 of the value it will have in case of star-connection: QcY = QcΔ CY = 3 ・ CΔ In the low voltage field, where insulation problems are less important, the delta connection is usually preferred for the capacitor bank, since it allows a smaller sizing of the capacitances of each phase.

In the next article, we will explain the Effects of Harmonics on Power Factor Capacitors. 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 Power Factor Correction Capacitors Sizing Calculations Steps For New Designs Factors Affecting The Rated KVAR For a Capacitor Calculation of the Capacitor KVAR Rating for Compensation at Transformer Calculation of the Capacitor KVAR Rating for Compensation at Individual Motors

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