Power Factor Correction Capacitors Sizing Calculations – Part Eighteen



Today we will explain the following points:

  • How to size the power and control cables used in PFC panels,
  • Effects of Power Factor Correction Capacitors on Generators,
  • Should capacitors be included in fault studies?



How to size Cables for PFC Panels?




Cables are used inside PFC panels for:

  1. Power circuit and
  2. Control circuit.


The selection of a proper and adequate cable for PFC panels depends on the following Points:

  • Voltage Rating: The type and thickness of insulation is determined by the voltage grade. It also helps in determining the minimum size of conductor that is suitable for loads.
  • Ampere Capacity: Current carrying capacity of the cable is selected based on the maximum current rating of each step.
  • Heating Conditions: Include the external thermal conditions which are responsible for determining the temperature increment of a cable.
  • Ambient temperature: The ambient temperature of the electrical room must not exceed 50˚C.
  • Presence of artificial cooling.
  • Installation method (trunking, duct, etc.).





First: Sizing Power Cables for Power Circuit




Power cables are used for interconnection between bus bar, step protection, switches, reactors and capacitors. The power cables for power circuit inside PFC panels must have the following characteristics:

  • The cables shall be multi strand, single core and PVC insulated.
  • It shall have a voltage class of 1100V grade. For a working voltage that is less than half the insulation voltage of the cable, i.e < 550V, these cables are considered to be class 2. Therefore they can be flanged directly to metal supports without use of any additional insulating material.
  • The ambient temperature of the electrical room must not exceed 50˚C.
  • The maximum permissible conductor temperature is 90˚C.


Power cables are sized based on the following methods:

  1. First Method: General Rule,
  2. Second Method: Manufacturers’ Tables,
  3. Third Method: Rule of Thumbs.





First Method: General Rule


NEC code article 460 stated that “The ampacity of capacitor circuit conductors shall not be less than 135 percent of the rated current of the capacitor” this means that:

I cable = 1.35 x In

Where:
I cable: designed power cable current capacity
In: the rated capacitor current

Some manufacturers recommend that the Power conductors must be oversize to carry continuous current of at least 1.5 times the rated capacitor current at a temperature of 50˚C

I cable = 1.5 x In

Reason to oversize the wire serving a capacitor bank:

  1. Capacitors can draw more than their rated current during overvoltage conditions. They also tend to be an attractive path for harmonic currents, which boost the RMS current.
  2. Note also that capacitors are not like typical power system loads that have some diversity (not on all the time). A fixed capacitor bank will draw full load current continuously. Even an automatically-switched capacitor bank can switch on and stay fully on for most of the day.


Example#1:

For 50 KVAR, 3 phase, 400V, 50Hz capacitor. Calculate the rated current? And the designed power cable current capacity?

Solution:

IN = KVAR x 103 / (√3 x UN)
IN = (50 × 1000) / (1.732 × 400)
IN = 72.16 A

I cable = 1.5 x In = 1.5 x 72.16 = 108.24 A





Second Method: Manufacturers’ Tables

Various manufacturers of cables distinguish cables based on cable diameter, current carrying capacity and type. Hence it is necessary to select cables suitably to meet the needs in APFC panels.

Note:

Selection of suitable lugs is very much important in order to achieve a proper joint. Proper crimping of these lugs to the cables should also be ensured. Only 2 lugs are permitted in one hole for termination (i.e. at either sides of the hole and not one above the other).

Below are the most used manufacturer’s tables for getting the size of cable, Fuse, Safety Switch And Circuit Breaker according to the used KVAR of the PFC Panel.


1- Tables as per NEC code (60 Hz):







Notes on above Tables:

  • 1 All feeder protection fuses are recommended to be time delay fuses and sized between 150–175% of the full load amperes of the capacitor current.
  • 2 All feeder protection breakers are recommended to be a minimum of 135% of the full load amperes of the capacitor current.
  • 3 80% rated circuit breakers should be derated for effective current rating shown above.
  • 4 Recommended size based on 90 ºC copper conductors applied at its 75 ºC rating.
  • 5 For conductor sizes involving multiple runs of 350 kcmil per phase or higher an external pull box may be required. Consult drawings for available space for termination.


2- Tables as per VDE code (50 Hz):








Third Method: Rule of Thumbs

Rule of thumbs is used for sizing power cables for power circuit in PFC panels are as follows:

  • Minimum of 4 mm² power cable is considered for connection capacitor step up to 10 kvar.
  • Maximum of 35 mm² cable is considered for 50 kvar step.
  • And for higher rated steps of 75 or 100kvar, 2 numbers of 35 mm² cable are generally used in parallel.





Second: Sizing Control Cable for Control Circuit



Control cable for control circuit inside PFC panels must have the following characteristics:

  • It should have a voltage class of 1100/660V grade or more.
  • The cables must be multi strand, single core and PVC insulated.


Note: For Current Transformer connection the lug used should be ring type.

The following cable cross sections are recommended for control wiring:

  • At least 1.5 mm² copper wires for the auxiliary voltage circuits 230 V AC.
  • At least 2.5 mm² copper wires for current circuits- CT secondary connection and for earthing.






Power Factor Correction Capacitors and Generators



Introduction

  • PF correction capacitors are used to correct the low lagging power factor caused by inductive loads such as motors to reduce the utility suppliers tariff charges.
  • Problem arises when PF correction banks remain connected when the inductive load is switched off. This presents a leading power factor load to the alternator causing the terminal voltage of the alternator to rise.
  • Capacitor banks controlled by automatic power factor regulators are not a problem since the regulator will only switch in the required number of capacitors to correct the lagging power factor.
  • A capacitor bank will reduce the current from the generator but does not reduce its size since the kW load does not change.






Power Factor & Generator Set KVA

  • Generator sets are rated in kVA at 0.8 power factor lagging. This 0.8 power factor is not the load power factor. It is a nominal power factor used to calculate the kW output of an engine to supply the power for a particular alternator kVA output.
  • Alternators are therefore designed to supply their rated kVA at 0.8 lagging power factor.
  • At other power factors the generator set and alternator have certain output limitations as defined by the Generator Set Capability Curve.


Example#2:

Alternator output (kVA): 100kVA
So, Engine power output (kW): 100kVA x 0.8 = 80Kw


Any changes in the rated PF 0.8 value lagging of the generator will affect the generator operation as follows:

In case of Lagging power factor <0.8:

lagging power factor below 0.8 results in:
  • The engine having surplus kW to power the alternator in excess of its 100% kVA rating.
  • The heating of the rotor winding at 100% of alternator kVA output.


In case of Lagging power factor >0.8 :

  • Lagging power factor above 0.8 results in the engine not having sufficient kW to power the alternator to 100% of its kVA rating.



In case of Leading power factor:

  • Problem arises when PF correction banks remain connected when the inductive load is switched off. This presents a leading power factor load to the alternator causing the terminal voltage of the alternator to rise. Leading power factor results in alternator stator end iron heating and the alternator automatic voltage control system becoming unstable.





Should Capacitors Be Included In Fault Studies?



  • The simple answer is, No.
  • The reasoning is relatively simple. An energized capacitor will very quickly discharge into other loads (or the fault) within ¼ of a 60 Hz electrical cycle at a frequency much higher than the fundamental fault current. This is significantly different than motors operating on the system that will “generate” back into the faulted system and contribute to the overall fault current.
  • Therefore, since capacitors do not offer a significant source power during a fault, they can be ignored. This is the standard used in commercially available power system modeling programs.







End of the Course



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:
1-Transformer
2-Individual Motors

3- Calculation Of The Capacitor KVAR Rating For Buildings And Power Plants(Group Compensation)

Harmonics Effects On Power Factor Capacitors
Harmonic Limits in Electric Power Systems (IEEE 519-2014)
Options to Reduce Harmonics for PFCC
Power Factor Compensation In Case Of Harmonics

Power Factor Correction Capacitors Calculators:
1- Arteche Reactive Power and Harmonic Resonance Point Calculator,
2- Eaton Power Factor Correction Calculator,
3- AccuSine Sizing Spreadsheet,
4- Square-D (Schneider Electric) Calculator.

The Main Components of PFC Panel
How to select Circuit Breakers for PFC Panel
How to select Fuses for PFC Panel
How to select Contactors for PFC Panel
How to select a capacitor for PFC Panel and Capacitors’ rules
Capacitor compensation with a detuned reactor
How to Select a Detuned Reactor

How to Select a Discharge Resistor
How to Calculate the Value of Discharge Resistor
How to Select Power Factor Controller
How to Calculate the Number of Physical Steps, Electrical Steps and Electrical Control.







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