Today, we will explain
The Main Components of PFC Panel and how to select each
one of them.
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1- The Main Components of
PFC Panel
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A Panel for
power factor correction (PFC Panel) is constituted essentially from the
following main components:
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2-
Types of Protection Devices in PFC Panel
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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.
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First:
How to select a Short Circuit Protection Device for a PFCC
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Usually, the PFC panel has the following 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.
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
Thirteen” we stated that there are 3 general methods of PF compensation
according to the level of network harmonic pollution as
follows:
We will explain in below how to choose the protection
device in each case. See Fig.1.
Fig.1
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How to select Circuit Breakers for
PFC capacitors
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The
circuit breakers are used for the protection and switching of capacitor banks
in LV, they should have the following features:
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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:
moreover:
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:
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
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Case#2: Overrated Compensation
The circuit breaker ratings in this case will be as follows:
Icmax = 1.5 x lcn
I3 ≥10.Icmax
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Case#3:
For Detuned Compensation
See table-1
for The circuit breaker ratings in this case:
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
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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:
Table-2
Notes:
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How To Select Fuses For PFC
Capacitors
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HRC fuses
must be used with the following ratings:
For detuned
type equipment, see below table-3.
Note:
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
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Second: How
To Select Switching Device (Contactor) For A PFCC
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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:
The
contactor must be protected against short-circuit by the protection device.
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There are two types of contactors used for switching each step:
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1- Capacitor Duty Contactor
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
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How
to limit inrush current for Parallel operation of capacitors in PFC panels
Fig.5
Fig.6
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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.
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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:
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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:
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Subject Of
Previous Article
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Article
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Glossary of Power Factor Correction Capacitors
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Types of Loads,
The Power
Triangle,
What is a power
factor?
Types of power
factor
Why utilities
charge a power factor penalty?
Billing Structure.
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What causes
low power factor?
Bad impacts
of low power factor,
Benefits of
Power Factor correction.
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How to make
Power Factor Correction?
Types of
Power Factor Correction Capacitors
Individual compensation
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Group compensation,
Central compensation,
Hybrid compensation.
Summary for Power Factor Correction
Capacitors Sizing Calculations Steps
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Step#1: Collect Monthly Billing Data
Step#2: Make Some Preliminary
Measurements For Current And Voltage
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Step#3: Fill the Economic Screening
Worksheet
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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
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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
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Power Factor Correction
Capacitors Sizing Calculations Steps For
New Designs
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Factors Affecting The Rated KVAR For a Capacitor
Calculation of the Capacitor KVAR Rating for Compensation at:
1-Transformer
2-Individual Motors
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3- Calculation Of The Capacitor KVAR Rating For Buildings
And Power Plants(Group Compensation)
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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
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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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