Today, we will explain
the power factor compensation in case of harmonics distortion.
|
Linear and Non-Linear Loads
|
|
Electrical loads can be categorized to Linear and non-linear loads
as follows:
1- Linear loads
- Linear loads occur when the impedance is constant; which implies
the current is proportional to the voltage. a straight-line graph as shown
in the figure-1 below.
- Simple loads, composed of one of the elements (Resistors
only or inductors only or capacitors only) do not produce harmonics.
Fig.1: Linear loads
2- Non-linear loads
- Non-linear loads occur when the impedance is not constant; then
the current is not proportional to the voltage. as shown in the figure-2 below.
- Combinations of the components normally create non-linear loads and
harmonics.
Fig.2: Non-linear loads
- The non-linear loads (electronic systems) dramatically increase
harmonic noise on the line side of the power distribution plant which impacts
the whole electrical distribution system. Typical examples of non-linear
loads (harmonic sources) are:
|
1-
Electronic Switching Power Converters
|
Computers
|
|
Uninterruptible power supplies
(UPS)
|
|
Solid-state rectifiers
|
|
Electronic process control
equipment, PLC’s, etc
|
|
Electronic lighting ballasts, including light dimmer
|
|
Neon SCR controlled equipment
|
|
Reduced voltage motor controllers
|
|
DC drives
|
|
2- Arcing
Devices
|
Discharge lighting, e.g. Fluorescent, Sodium and Mercury
vapor
|
|
Arc furnaces
|
|
Welding equipment
|
|
Electrical traction system
|
|
3-
Ferromagnetic Devices
|
Transformers operating near saturation level
|
|
Magnetic ballasts (Saturated Iron
core)
|
|
Induction heating equipment
|
|
Chokes
|
|
4-
Appliances
|
TV sets, air conditioners, washing machines, microwave
ovens & vacuum cleaners
|
|
Fax machines, photocopiers,
printers
|
- Non-linear loads
inject non-sinusoidal currents into the network. These currents are formed by
a 50 Hz or 60 Hz fundamental component, (as well as a DC component in some
cases), plus a series of overlaid currents with frequencies which are
multiples of the fundamental frequency which known as harmonics.
|
|
Effects Of Harmonics On The
Electrical Power System
|
|
Harmonics have detrimental effect on the electrical power system
in a facility. Examples of Harmonic Distortion Problems are shown in below
table-1:uipment Consequences
|
Equipment
|
Consequence
|
|
Current Harmonic Distortion Problems
|
|
Capacitors
|
Blown fuses, reduced capacitor life
|
|
Motors
|
Reduced motor life, inability to fully load motor
|
|
Fuses/breakers
|
False/spurious operation, damaged components
|
|
Transformers
|
Increased copper losses, reduced capacity
|
|
Voltage Harmonic Distortion Problems
|
|
Transformers
|
Increased noise, possible insulation failure
|
|
Motors
|
Mechanical fatigue
|
|
Electronic loads
|
Mis-operation
|
Table-1: Negative Consequences of Harmonics on Equipment
If any of these conditions exist in your facility, an analysis
of your system must be done.
|
|
Harmonics Effects On Power Factor
Capacitors
|
|
- With non-linear loads it is extremely difficult to correct for
poor power factor without increasing existing harmonic distortion.
- The harmonics lead to a higher capacitor current, because the
higher frequencies are attracted to the capacitor. The impedance of the
capacitor decreases as the frequency increases. If the frequency of such a
resonating circuit is close enough to a harmonic frequency, the resulting
circuit amplifies the oscillation and leads to over-currents and
over-voltages.
- Capacitors themselves do not generate harmonics, but under
certain conditions they can amplify existing harmonics. Necessary precautions
must be undertaken when selecting the capacitors.
- To minimize the occurrence of harmonic resonance, the resonant
harmonic of the system including the capacitor should be estimated.
- The resonant frequency can be calculated from the
following formula:
f = fp √(Psc/Pc)
where:
f = resonant
frequency,
fp
= power frequency,
Psc
= short circuit power of the transformer (kVA),
Pc
= power of the capacitor (kVAR).
If the
frequency obtained is too close to that of a harmonic, the value of the
capacitor rating should be modified. Most common harmonic frequencies, 3rd,
5th, 7th, etc...
Example#1:
for the following transformer:
S = 630 KVA
Usc
= 6%
P = 500 KW
Qc =
275
kVAr
Calculate the resonant frequency.
Solution:
The
short-circuit power is: Ssc = S x 100 / Usc = 630 x 100
/ 6 = 10500 KVA
The resonance
frequency will therefore be:
f = fp √(Psc/Pc) = 50 x
√(10500/275) = 308.96 Hz
The system will resonate at order n = f/ fp = 6.18
Notes:
- In three-phase, low-voltage systems, harmonic values of 5, 7,
11, 13, 17, 19 etc. should be avoided as they correspond to the
characteristic harmonics of non-linear loads. This includes all of the odd
harmonics except for the multiples of 3. Examples of such devices are
variable-speed and variable-frequency ac drives, dc drives, three-phase
power-controlled furnaces and many other types of industrial equipment.
- In single-phase, low-voltage systems, generally exhibit the
following harmonics: 3, 5, 7, 9, 11, 13 etc. Note that this includes all of
the odd harmonics. Examples of such devices are those usually powered by
‘switch mode power supplies’, which include personal computers, fluorescent
lighting, and a myriad of other equipment found in the modern office. It also
includes equipment found in hospitals, TV and radio stations, and control
rooms of large processing plants. The harmonics from these devices are generally
richest at the third harmonic and continually decrease as the harmonic number
increases.
|
|
Harmonic Limits in Electric
Power Systems (IEEE 519-2014)
|
|
Harmonic limitations have been established by IEEE 519 2014 for
the following reasons:
- To limit the damage to power factor correction capacitors and
harmonic filter systems caused by excessive harmonics.
- To prevent series or parallel resonance in the electrical system.
- To keep the level of harmonics at the PCC (Point of Common
Coupling) from being excessive and distorting the system voltage and damaging
other equipment on the system.
The PCC is defined as the electrical connecting point or
interface between the utility distribution system and the customer's
electrical distribution system.
The harmonic voltage limitations set forth by IEEE 519-2014 for
Bus voltage V at PCC ≤ 1.0 kV are:
- Maximum Individual Frequency Voltage Harmonic: 5%
- Total Harmonic Distortion of the Voltage: 8%
For other voltage ratings, please see below table-2:
|
Bus voltage V at PCC
|
Individual harmonic (%)
|
Total harmonic distortion THD (%)
|
|
V ≤ 1.0 kV
|
5.0
|
8.0
|
|
1 kV < V
≤ 69 kV
|
3.0
|
5.0
|
|
69 kV <
V ≤ 161 kV
|
1.5
|
2.5
|
|
161 kV <
V
|
1.0
|
1.5a
|
Table 2: Voltage distortion
limits
Notes to Table-2:
- aA High-voltage systems can have up to 2.0% THD where the
cause is an HVDC terminal whose effects will have attenuated at points in the
network where future users may be connected.
- All values should be in percent of the rated power frequency
voltage at the PCC. Table-2 applies to voltage harmonics whose frequencies
are integer multiples of the power frequency.
|
|
Options to Reduce Harmonics
for PFCC
|
|
Harmonic
levels that exceed the recommended values set forth by IEEE 519-2014 should
be addressed through harmonic filtering. Failure to address these harmonic
issues may lead to problems on the electrical distribution system, such as
those detailed above. For PFCC, the resonant harmonics can be avoided in
several ways:
- Change the
applied KVAR to avoid unwanted harmonics,
- Add harmonic
filters,
- Add
blocking inductors (detuned reactors),
- Change the method of KVAR
compensation (harmonic filter, active filter, etc.).
|
|
1- Change the applied KVAR to
avoid unwanted harmonics
- Although this is the least expensive way to avoid resonant
harmonics, it is not always successful because typically some portion of the
applied KVAR is switched on and off as load conditions require.
- The
calculation of system harmonics should be repeated for each level of
compensation. Adjusting the size of the capacitor(s) may be necessary to
avoid the harmonic values.
|
|
2- Add Harmonic Filters
- In order to filter harmonics at a specific site, tuned harmonic
filters can be applied.
- A capacitor is connected in series with an inductor
such that the resonant frequency of the filter equals the harmonic to be
eliminated.
- Tuned filters should never be applied without a detailed analysis
of the system. The currents expected to flow in the filter are difficult to
predict and are a complex function of the system and load characteristics.
There
are 3 types of filters:
- Passive
filter,
- Active
filter,
- Hybrid
filter.
1- Passive
filter
 |
| Passive filter |
- This
is an LC circuit tuned to a harmonic frequency to be filtered. This filter,
mounted on a bypass circuit, absorbs the harmonics and prevents them from
flowing in the power supply. However, for a significant reduction of the THD
on several harmonic orders, several branch filter circuits will be necessary.
- Passive
filters, which are defined on a case by case basis, according to a particular
harmonic to be filtered, are cost-effective and easy to be connected and put
into function.
2- Active
filter
- This
is an electronic power system designed to compensate either the harmonic
voltage or the harmonic currents generated by the load. This filter
re-injects, in phase-opposition, the harmonics present on the load's power
supply so that the current in the line becomes sinusoidal.
- The active
filter has many advantages as follows:
- Filtering
simultaneously dozens of harmonics and does not involve design costs for
dimensioning.
- They continue to
guarantee efficient harmonic compensation even when changes are made to the
installation.
- Auto-configuration to
harmonic loads whatever their order of magnitude
- Elimination of
overload risks
- Compatibility with
electrical generator sets
- Connection to any
point of the electrical network
- Several conditioners
can be used in the same installation to increase depollution efficiency (for
example when a new machine is installed)
- Active filters may
provide also power factor correction.
3- Hybrid
filter
- This
involves a combination of the two Passive & Active
filters described above for a broad power range.
 |
| Hybrid filter |
|
|
2.1
Choosing The Optimum Filter
Table-3 below shows the
criteria that can be taken into account to select the most suitable
technology depending on the application.
|
|
Passive filter
|
Active filter
|
Hybrid filter
|
|
Applications with total power of non-linear
loads (variable speed drive, UPS, rectifier…)
|
Industrial
|
Tertiary
|
Industrial
|
|
greater than 200
kVA
|
lower than 200 kVA
|
greater than 200
kVA
|
|
Power factor correction
|
√
|
No
|
√
|
|
Necessity
of reducing the harmonic distortion in voltage for sensitive loads
|
√
|
√
|
√
|
|
Necessity
of reducing the harmonic distortion in current to avoid cable
overload
|
√
|
√
|
√
|
|
Necessity
of being in accordance with strict
limits
of harmonic rejected
|
No
|
√
|
√
|
Table-3: Choosing the
optimum filter
|
|
2.2 Where to install your filter?
To
choose the most suitable location to connect a filter in an installation, you
must take into account:
- The
type of disturbance present on the installation, which defines the type of
filter to be installed.
- The
configuration of the installation:
- Existence
of capacitor banks
- Existence
of major loads causing disturbance
- Power
and location of the lighting and computer lines.
There
are 3 points in an installation where you can connect filtering equipment in
order to eliminate disturbances:
1- On
the low-voltage general switchboard (LVGS)
- When
the disturbances have been eliminated or attenuated directly at the level of
the loads or at the level of the secondary switchboards, the
remaining residual disturbances can be eliminated by connecting filtering
equipment on the general switchboard.
- In this way, it is possible
to ensure that the electrical signal is in a satisfactory state at the point
of connection with the energy supplier.
2- On
the secondary switchboard
- When
there are various low-power loads connected to the secondary distribution switchboard.
- Elimination of the disturbances prevents discharging of the lines connected
to the general switchboard.
3- On
the terminals of the load generating harmonics
- This
is the best solution for eliminating the disturbance directly at
the point where it is generated, thus preventing propagation to
all the lines in the electrical installation.
|
|
3- Add blocking inductors (detuned reactors)
- The most
common solution, as illustrated in the Std. IEC 61642, consists in connecting
in series an inductive reactance with the capacitor (detuning reactance); the
inductor shall be sized so that a resonance frequency which is below the
lowest frequency of the harmonic voltage in the network is achieved.
 |
| Detuned Reactors |
- Inductors added to the lines feeding the capacitor can be sized
to block higher than 4th harmonic currents. This method protects the
capacitor from the harmonics but does not eliminate the harmonics from the
system. A system study is required to determine correct ratings for the
capacitor and inductors.
|
|
3.1 Choice
Of Detuned Reactor Tuning Frequency
The detuned reactor, 400
V, 50 Hz range offers a wide selection of tuning frequencies: 135, 190 or 215
Hz. Tuning frequency of the reactor capacitor must be chosen according
to:
- The harmonic frequencies
present on the installation (tuning frequency must always be lower than the
harmonic spectrum)
- The remote control
frequencies, if any, used by electrical utilities,
- Presence of zero-sequence harmonics (3, 9, …),
- Need for reduction of the harmonic distortion level,
- Optimization of the capacitor and reactor components,
- Frequency of ripple control system if any.
Tuning frequency of the reactor capacitor can be selected from the below table-4:
|
harmonic generators (Gh)
|
remote
control frequency (Ft)
|
|
|
none
|
165 < Ft≤ 250 Hz
|
250 < Ft≤ 350 Hz
|
Ft > 350 Hz
|
|
three-phase:
variable
speed drives, rectifiers,
UPS,
starters
|
tuning frequency
135 Hz
190 Hz
215 Hz *
|
tuning frequency
135 Hz
|
tuning frequency
190 Hz
|
tuning frequency
215 Hz
|
|
single-phase (Gh 1Ph > 10 % Sn):
discharge
lamps, lamps with electronic ballast,
fluorescent
lamps, UPS,
variable
speed drives, welding machines
|
tuning frequency
135 Hz
|
tuning frequency
135 Hz
|
tuning frequency
135 Hz
|
tuning frequency
135 Hz
|
Table-4:
DR, 400 V, 50 Hz tuning frequency selection
* Recommended tuning
frequency, allowing a greater reduction in 5th order harmonic pollution than
the other tuning frequencies.
Gh 1Ph: power of
single-phase harmonic generators in kVA.
Relation between tuning
frequency, tuning order and relative impedance:
The most common values of relative impedance are 5.7, 7 and 14%.
(14% is used with high level of 3rd harmonic voltages).
The tuning frequency can be expressed by the relative impedance of
the reactor (in %), or by the tuning order, or directly in Hz.
|
Relative
impedance (%)
|
Tuning
order
|
Tuning
frequency
@50Hz
(Hz)
|
Tuning
frequency
@60Hz
(Hz)
|
|
5.7
|
4.2
|
210
|
250
|
|
7
|
3.8
|
190
|
230
|
|
14
|
2.7
|
135
|
160
|
|
|
Power
Factor Compensation In Case Of Harmonics
|
|
Generally,
and from the past articles, we can say that we have 3 general methods of PF
compensation
according to the level of network harmonic pollution as
follows:
- Standard compensation,
- Overrated
compensation,
- Detuned
compensation.
The selection
between the above methods can be made based on the following two criteria:
- From the Gh/Gn ratio,
- From the THD(I) current
total harmonic distortion measured.
|
|
1- From
the Gh/Gn ratio
Considering the following
parameters:
- Sn : transformer apparent
power
- Gh : apparent power of loads
generating harmonics (variable speed motors, static converters, power
electronics, etc.)
- Qc : compensation equipment
power
- U : network voltage
So, the
following table can be applied:
|
Gh/Gn
value
|
Compensation
equipment type
|
|
< 15%
|
standard type
|
|
15 : 25%
|
Overrated type
|
|
> 25%
|
detuned type *
|
|
> 60%
|
detuned type + harmonics filtering **
|
* Please check that the
de-tuned capacitor bank does not interfere with telecommunication frequency
used by the utilities
** requires a harmonic
filtering study.
Example#2:
Select the best
compensation method For The following parameters:
U = 400 V
Sn = 800 kVA
In the following cases:
Case#1: P = 450 kW and Gh
= 50 kVA
Case#2: P = 300 kW and Gh
= 150 kVA
Case#3: P = 100 kW and Gh
= 400 kVA
Solution:
For case#1:
Gh/ Sn = 6.2 %
Use Standard type
equipment
For case#2:
Gh/ Sn = 18.75 %
Use Overrated type
equipment
For case#3:
Gh/ Sn = 50 %
Use Detuned type
equipment
|
|
2- From
the THD(I) current total harmonic distortion measured
Considering the following
parameters:
Sn = transformer apparent
power
S = load in kVA at the
transformer secondary at the time of measurement
So, the
following table can be applied:
|
THD(I)
x S/ Sn < 5 %
|
Use
Standard type equipment
|
|
5
% < THD(I) x S/ Sn < 10 %
|
Use
Overrated type equipment
|
|
10
% < THD(I) x S/ Sn < 20 %
|
Use Detuned type
equipment
|
Notes:
- Harmonics must be
measured at the transformer secondary, at full load and without capacitors.
- Apparent power must be
taken into account at the time of measurement.
|
In the
next article, we will explain the (3) Famous 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:
- Transformer
- Individual Motors
|
|
|
- 3- Calculation Of The Capacitor KVAR Rating For Buildings
And Power Plants(Group Compensation)
|
|
No comments:
Post a Comment