Power Factor Correction Capacitors Sizing Calculations – Part Fifteen



Today, we will explain The Main Components of PFC Panel and how to select each one of them.

1- The Main Components of PFC Panel



A Panel for power factor correction (PFC Panel) is constituted essentially from the following main components:

  1. A protective device;
  2. A switching device (contactor);
  3. One or more capacitors suitably connected;
  4. Resistors for capacitor discharge.
  5. A PF controller is used In case of an automatic PF compensation system to command switching in/off of the capacitors.




2- Types of Protection Devices in PFC Panel



The following are the Types of Protection Devices for PFC panels:

1- Over voltage protection
In the event of an over voltage, electrical stress on the capacitor dielectric and the current drawn by the capacitors will increase. The PFC equipment must be switched off in the event of over voltage with suitable over voltage relay.

2- Under voltage protection
In the event of under voltage, electrical stress on the capacitor dielectric and the current drawn by the capacitors will decrease. This condition is not harmful. But protection is provided based on customer request to protect system from under voltage.

3- Over Current protection
Over current condition is very harmful to all current carrying components. All the switchgears are selected on a higher maximum current carrying capacity. However suitable over current relays with alarm can be used for over current protection.

4- Short circuit protection
At the incomer level short circuit protection is provided by devices such as MCCB, ACB and SFU (switch fuse unit) suitably. At the step protection level, suitable fuses, MCB, MCCB can be used.

5- Thermal Overload protection
It is taken care by thermal overload relay. The PFC controller must be tripped in cases where internal ambient temperature exceeds the limits. Reactors are also provided with thermal switches, to trip in the case of temperature increase.

6- Earthing
Two earthing points are provided in the PFC panel for connecting to the earth bus. This will ensure the overall safety of operating personnel and equipment protection in case of earth faults.

7- Earth Leakage Relay
It has to be connected at power incoming side of the panel, Earth leakage relay is provided to safeguard the operator by tripping the incomer.

8- Timers
Capacitors require a minimum discharge time of approximately 60 seconds after they are switched off before they can be switched on again. This is to be set in the PFC controller. Capacitors are provided with discharge resistors.

Note:
Solid State switching (Thyristor Switches) can be used when fast switching in APFC Panels is needed.




First: How to select a Short Circuit Protection Device for a PFCC



Usually, the PFC panel has the following protection devices:

  1. Main Incomer,
  2. Outgoing Protection Devices.


1- The Main incomer:

It is used to disconnect the PFC panel in case of short circuit, over load and earth fault. The rating of the incomer protection device will vary depending on rating of the PFC equipment and the required fault level. SDF, MCCB or ACB can be used as incomer protection device.

2- The Outgoing protection devices:

The Outgoing protection devices is provided to protect the individual capacitor step in the PFC equipment. The outgoing protection may be HRC fuses, circuit breakers (MCB, MCCB) & SDF (switch disconnector fuse) depending on the rating of the individual capacitor steps, required fault level & customer requirement too.

Note: Use switching and protection devices designed for capacitor switching duty.

In article” Power Factor Correction Capacitors Sizing Calculations – Part Thirteenwe stated that there are 3 general methods of PF compensation according to the level of network harmonic pollution as follows:

  1. Standard compensation,
  2. Overrated compensation,
  3. Detuned compensation.


We will explain in below how to choose the protection device in each case. See Fig.1.



Fig.1





How to select Circuit Breakers for PFC capacitors
  


The circuit breakers are used for the protection and switching of capacitor banks in LV, they should have the following features:

  1. Withstand the transient currents that occur when connecting and disconnecting the banks. In particular, the instantaneous magnetic and electronic releases should not trip due to these peak currents,
  2. Withstand the periodic or permanent overcurrents due to the voltage harmonics and to the tolerance (+ 15%) of the rated capacity value,
  3. Perform a high number of no-load and on-load operations, also with high frequency,
  4. Be coordinated with any external device (contactors) Furthermore, the making and breaking capacity of the circuit breaker must be adequate to the short-circuit current values of the installation.





Case#1: For Standard Compensation

First: as per Standards IEC 60831-1 and 60931-1:

The capacitors shall normally function with an effective current value up to 130% of their rated current Icn (due to the possible presence of voltage harmonics in the network).

Icn = Qc/(√3.Un)

Where:
Icn is rated current of the connected capacitor bank (A)
Qc is the reactive power of the capacitor bank (KVAR);
Un is the rated line-to-line voltage (V);

a tolerance of +10% on the capacitance for banks up to 100 kvar and of 5% for banks exceeding 100 kvar is admitted (Amendment 1 of the above mentioned standards).

Therefore, a capacitor bank can absorb a maximum current Icmax of:



where:
Icmax: max current absorbed by the capacitor bank
Qc is the reactive power of the capacitor bank (KVAR);
Un is the rated line-to-line voltage (V);
Icn is rated current of the connected capacitor bank (A)


Therefore:
  • The rated current of the circuit-breaker shall be greater than Icmax
  • The overload protection setting shall be equal to Icmax
  • The connection of a capacitor bank, similar to a closing operation under short circuit conditions, is associated with transient currents with high frequency (1 to 15 kHz), of short duration (1 to 3 ms) and with high peak (25-200Irc)


moreover:
  • The circuit-breaker shall have adequate making capacity
  • The setting of the instantaneous short-circuit protection must not cause unwanted trips


For the second condition, it is generally given that:

for thermal magnetic trip units, by setting the magnetic protection I3 at values not lower than 10.Icmax

I3 ≥ 10. Icmax

Where:
I3: magnetic protection current

Notes:
  • For electronic releases, the instantaneous short-circuit protection shall be deactivated,
  • For electronic trip units, by setting in OFF the instantaneous protection against short-circuit (I3 = OFF).


Example#1:

For 150kvar/400v – 50Hz Capacitor, calculate the circuit breaker ratings?

Solution:

Un = 400V; Qc = 150kvar
Icn = Qc/(√3.Un) = 150000/400√3 = 216A
Qc ˃ 100 KVAR, then:
Circuit Breaker Rating = 216 x 1.365 = 294.84A
Select a 300A Circuit Breaker.
Circuit Breaker thermal setting = 216 x 1.5 = 294.84 A
Select a Circuit Breaker of 300A with Thermal Setting at 324A and Magnetic Setting ( Short Circuit = 10.Icmax = 10 x 294.84) at 2984.5 A





Case#2: Overrated Compensation

The circuit breaker ratings in this case will be as follows:

Icmax  = 1.5 x lcn

I3 ≥10.Icmax




Case#3: For Detuned Compensation

See table-1 for The circuit breaker ratings in this case:

Relative impedance (%)
Tuning order
Icmax
I3
5.7
4.2
1.31 x lcn
≥10.Icmax
7
3.8
1.19 x lcn
≥10.Icmax
14
2.7
1.12 x lcn
≥10.Icmax

Table-1

Example#2:

For 20kvar/400v – 50Hz Harmonic Range with 7% Detuned Reactor. Calculate the circuit breaker ratings?

Solution:

Un = 400V; Qc = 20 kvar
Icn = Qc/(√3.Un) = 20000/(400.√3) = 28.86A
Circuit Breaker thermal setting = 28.86 x 1.19 = 34.34 A
Magnetic Setting ( Short Circuit ) = 34.34 X 10 = 343.4 A
Select a Circuit Breaker of 40A ( or next available appropriate range) with
Thermal Setting at 34.34 A and Magnetic Setting ( Short Circuit ) at 343.4A




Second: as per NEC

Requirements for Capacitors & their protective devices in NEC:

1- Nameplate kvar: Tolerance +15, –0%.

2- Discharge resistors: Capacitors rated at 600 V and less must reduce the charge to less than 50 V within 1 minute of de-energization. Capacitors rated above 600 V must reduce the charge within 5 minutes.

3- Continuous operation: Up to 135% rated (nameplate) kvar, including the effects of 110% rated voltage
(121% kvar), 15% capacitance tolerance and harmonic voltages over the fundamental frequency (60 Hz).

4- Dielectric strength test: Twice the rated AC voltage (or a DC voltage 4.3 times the AC rating for non-metallized systems).

5- Overcurrent Protection:

  • The NEC, Section 460.8(c)(3), requires the disconnecting means to be rated not less than 135% of the rated capacitor current (for 600 V and below).
  • Circuit breakers and switches for use with a capacitor must have a current rating in excess of rated capacitor current to provide for overcurrent from overvoltages at fundamental frequency and harmonic currents. The following percent of the capacitor-rated current in Table-2 should be used as a general guideline :


Switching/protection device
Rating
Fused and unfused switches
165%
Molded-case breaker or equivalent
150%
Power circuit breakers
135%
Insulated case circuit breakers
135%
Contactors
150%

Table-2

Notes:

  • When capacitor is connected to the load side of the motor overcurrent protection, fused disconnects or breaker protection is not required. Fuses are recommended for all other indoor applications.
  • Does not preclude NEC requirement for overcurrent protection in all three ungrounded conductors.
  • Since power capacitors for industrial service are designed for use in an ambient temperature of 46°C (115°F) maximum, the cables and disconnecting devices should also be selected for this ambient operation.





How To Select Fuses For PFC Capacitors



HRC fuses must be used with the following ratings:

  • 1.6 x ln for standard compensation
  • 1.6 x ln for overrated compensation


For detuned type equipment, see below table-3.

Note:
  • When two steps are protected by the same set of fuses, the coefficient becomes 1.4 x In for standard. (Icn: sum of the currents of 2 steps).


For, 400 V/50 Hz Capacitors
Relative impedance (%)
Tuning order
Fuse
Icmax
5.7
4.2
1.44 x lcn
7
3.8
1.3 x lcn
14
2.7
1.23 x lcn

Table-3

Example#3:

For 50 kvar / 400 V - 50 Hz – standard. Calculate the fuse rating?

Solution:

Icn = Qc/(√3.Un) = 50000/400√3 = 72 A
Fuse rating = 1.6 x Icn = 1.6 x 72 = 115 A
Select next available appropriate range of fuse = 125 A

Example#4:

50 kvar / 400 V - 50 Hz - DR (tuning 4.3). Calculate the fuse rating?

Solution:

Icn = Qc/(√3.Un) = 50000/400√3 = 72 A
Fuse rating = 1.44 x Icn = 1.44 x 72 = 104 A
Select next available appropriate range of fuse = 125 A





Second: How To Select Switching Device (Contactor) For A PFCC



Capacitors or capacitors banks are usually switched by a contactor which shall be chosen so that it can operate properly; the contactor shall be sized so that:

  1. It can sustain a current equal to the Icmax of the capacitor bank;
  2. It can sustain without damages the inrush current of the capacitors.


The contactor must be protected against short-circuit by the protection device.




There are two types of contactors used for switching each step:

  1. Capacitor Duty Contactor,
  2. Power Contactor.





1- Capacitor Duty Contactor

  • Capacitor duty contactors are recommended in APFC equipment to minimize inrush currents experienced during the switching of capacitors. These contactors have special early make contacts with series resistances which dampens the inrush currents. Capacitor duty contactors are rated based on nominal reactive power.
  • Capacitor duty contactors are normally rated up to 60kvar. When higher rated steps exist such as 75kvar, 100kvar and above, the following methods can be adopted.


Method#1: for steps up to 120kvar

Capacitor Duty Contactors can be used in parallel to switch the Steps.

Example#5:

To switch 100kvar step, using capacitor duty contactor, there are two options as follows:

Option A: Provide two steps of 50kvar each, (having 50kvar switch, and 50kvar reactor and 50kvar capacitors) and the connection of relay in such a way to operate both together. See fig.2


Fig.2

Option B: Provide single step using two capacitor duty contactors in parallel with one 100kvar reactor and one 100kvar capacitor (4x25kvar / 2x50kvar) See fig.3



Fig.3

In option B, if one contactor malfunctions, the other contactor gets over loaded and there is possibility for damage in second contactor also. However if both contactors are in good conditions and operate exactly at the same moment, theoretically there will be no problem. But the contactor switching may not occur exactly at the same time, and a very small time difference may cause over loading of contactors.

Method#2: for steps more than 120kvar

Contactors of AC3 Duty can be used in this case but capacitors need to be put in series with inductor coil.

Example#6:

To switch 120 kvar step, using AC3 Duty contactor, provide four steps of 30kvar capacitor connected to an inductor coil in series. see Fig.4


Fig.4




How to limit inrush current for Parallel operation of capacitors in PFC panels

  • At capacitor switching while one or more capacitors are connected to the system, the switching capacitor will see a high inrush current. This is due to the current flow from the already connected capacitor(s) (which will act as a source) through the least impedance path set by the switched capacitor along with the current from the main source. This means that when the number of capacitors in parallel increases, the amount of inrush current also increases. Inrush current will damage the capacitor as well as the switching device.
  • In order to prevent inrush current, it is required to use current limiting devices such as Capacitor Duty Contactors or Inductor Coil.
  • See fig.5 for the conventional single stage and multi stage capacitor switching and switching using normal power contactor and compare it with fig.6 when using a special capacitor duty contactor.



Fig.5



Fig.6




2- Power Contactor

When normal Power contactors are used, and when there are no reactors in series, a suitable inductor coil has to be connected in series with the contactor for limiting the inrush currents. Power contactors are rated based on the rated operational current.




Selection Factors For Contactors In PFCC

Contactors will have to be chosen with care, keeping in mind the following factors which can influence their performance:

  1. Contactor shall be re- strike free and adapted for capacitors
  2. The rated voltage of the contactor shall be equal to or higher than the maximum network voltage with the power factor correction installation.
  3. The contactor shall be designed for continuous current (including harmonics) which can pass the power factor correction installation and/or filter at maximum source voltage, maximum frequency and extreme tolerances of the components, especially capacitors and reactors.
  4. Capacitor duty contactors are rated based on nominal reactive power and Power contactors are rated based on the rated operational current.




In the next article, we will explain how to choose Capacitors, Resistors, PF Controllers and Cables for Power Factor Correction Capacitors Calculators. 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:
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.





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