© 2019 JETIR May 2019, Volume 6, Issue 5 www.jetir.

org (ISSN-2349-5162)

Design of Shunt Passive Filter for Reducing

Harmonic in Variable Frequency Drives (VFDs):
A Review
Smita S. Desai, Krunalsinh H. Dattesh, Jayesh Pillai
Student, Assistant Professor , Assistant Manager
Electrical Department,
Parul University, Waghodia, India

Abstract : Variable Frequency Drives are used widely in process industries due to better speed regulation, quick response and
reduced wear and tear as compared with mechanical alternatives. However, based on topology of VFDs, harmonics injects in the
electrical power system which causes some harmful effects in power system such as heating in equipment and conductor
misfiring in variable speed drives and torque pulsation in motor. Hence, mitigation techniques are required to limit injection of
harmonics in the system. One of the mitigation techniques is application of passive filter. Installation of passive filter at input of
VFD, reduces harmonics, regulates voltage in steady and dynamic conditions and also improves power factor. However, passive
filter accompanies problems such as creating resonance to the network which needs to be calculated in harmonic study and
resolved during filter sizing. This paper involves procedures for sizing and comparison of different passive filters.

Keywords — Passive filter, Point of common coupling (PCC), Total demand distortion (TDD), Total harmonic distortion (THD), Individual
harmonic distortion (IHD);

I. INTRODUCTION
Harmonics are defined as periodic wave of voltage or currents having frequencies which are integral multiples of power
frequency in which the system is designed to operate [6]. Power quality covers a broad range of concerns such as swell, sag,
fluctuations and imbalances in voltage, interruptions, transients, harmonics, and power frequency variations.
When Current harmonics injected by VFDs flows through system impedance results in voltage harmonics and thus affects
the quality of power supply system. Harmonics are introduced with non-linear loads. The grid is generally connected with single
phase and three phase nonlinear loads, for example VFD, welding machines, rectifiers, VAR Compensators, Induction Furnaces,
PLC, Computer, CFL, Refrigerator, TV, SMPS and UPS. Current harmonic is result of distorted current of all these nonlinear
loads and the devices which are based on power electronics. When such harmonic current passes through impedances present in
power system devices such as transformer, Source and connected cable results in voltage drop at that harmonics. Distortion in
voltage is occurs due to addition of these voltages to the supply voltage. As impedances and harmonic current increases distortion
in voltage also increases. To lower the distortion in voltage requires lowering system impedance or reducing current harmonics.

Harmonic current and voltage can create below mentioned problems [5]
a) It increases losses in connected cables, lines and in equipment’s etc.,
b) Pulsating and reduced torque & vibrations in motors and other rotating equipment,
c) Due to increased stresses life of insulation of electrical equipment’s gets reduced.
d) In static and rotating equipment audible noise increases.
e) Mal-functioning of equipment happens which are sensitive to waveforms.
f) Voltage and current amplification occurs due to resonance.
g) Interference in Communication (proximity effect)

Guidelines for current and voltage harmonics on transmission and distribution system are given in IEEE standard 519[1, 2]. These
standards provide the limit to the voltage and current harmonic for transmission and distribution system.

There are different methods to reduce the harmonics as below

a) Active filters
b) Multi-pulse converter
c) Active front end model
d) Passive filters

Harmonics generated by nonlinear devices can be nullified by connecting active filters which adds equal and opposite
harmonics in the system. This method minimizes harmonics below required limit but it is expensive and reliability may be
problem.
Multi pulse converter system reduces harmonic, with this method 12 and 18 pulse drives can be used instead of 6 pulse.
But this method is costly for small capacity motors.
Harmonics can be reduced by using active front end system which provides close to sinusoidal current and hence has high
power factor. However, it has IGBTs in rectifier instead of diodes and hence, involves high cost.
Another method of reducing harmonics is Passive filter which has passive element (R, L and C). This method is reliable
and cheap. This paper focuses with different passive filters & it’s designing because this method is mostly used in industry.

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II. PASSIVE HARMONIC FILTERS

Passive elements RLC are included in passive harmonic filter. Sizing of inductor and capacitor are done so they create
resonance at harmonic frequency which needs to be filtered. Series and shunt filter are the basic classification of the passive filter.
Series filter connected in series with the system from which harmonic needs to mitigate and it creates parallel resonance at tune
frequency hence it provides high impedance. Hence harmonic not flows through filter and system as well. As components in the
series filter are in line so needs to design it for full line current which makes the filter very expensive. Series filter is as shown in
below figure 1.

Figure 1 Series Filter

Shunt filters are mainly used in AC system. It is connected in parallel with the system or nonlinear load from which harmonics
are to be filtered which creates series resonance offering negligible impedance at which it is tune hence harmonic flows through
the filter and blocks the harmonic flowing through system. Shunt connected filter is low in cost compared to series as it is
designed for graded insulation level. Shunt filter observes the harmonic at which it is tune hence it is also called as tap filter [6].
Shunt filters are classified as single tune filter and high pass filter

Figure 2 (a) single-tune (b) first order high pass(c) second order high pass (d) third order high pass (e) c- type

Single tune filter is used to filter one harmonic while high pass filter is used for filtering harmonic above a certain range.
High pass filter are again classified as

a) First order high pass,

b) Second order high pass,
c) Third order high pass,
d) C-type filter.

Figure 2(a) shows Single tuned filter. This filter provides negligible impedance at tune frequency as it creates series
resonance at tune frequency. Power factor of the system can also improve as it contains the capacitor [4].
As shown from figure 2 (a) it contains only two components hence filter requires less maintenance. Quality factor of this
filter is high hence it reduces one harmonic very well. But for filtering more than one harmonic need to use more filter branches.
Due to high quality factor the filter is very sensitive to power frequency variation and component value variation. For accurate
tuning at site, require using tap on reactor, which increase the cost of filter.
First order filter as shown in figure 2 (b) consist of capacitor and resistor. Due to features of capacitor at high frequency it
offers minimum impedance. Current through capacitor is limited by resistor which is connected in series with capacitor.
Limitation of this filter is the size and cost as if needs to use it for higher frequency, then size and hence cost goes on increasing
as well as performance of it is poor for low frequency [4].
Second order filter is shown in above figure 2 (c) which is also called as damped high pass filter. Inductor (L) and
resistance (R) connected in parallel and this parallel combination is connected in series with capacitor (C). Below tuning
frequency it work as single tune filter and at high frequency works as 1 st order high pass filter. Resistance branch is bypasses by
inductor below tuning frequency as XL is low. And at higher frequency XL is high hence current diverts to resistance. L and C are
tune to the harmonic frequency hence at this frequency a notch can be seen [4].
Third order high pass filter shown in figure 2(d), it consists of two capacitor main capacitor (C 1) and auxiliary capacitor
(C2). Auxiliary capacitor C2 with resistance (R) connected in parallel with inductor (L) and the whole combination is connected in
series with main capacitor (C1). As similarly to second order high pass filter it works as single tune filter below tuning frequency
& first order high pass above it. At low frequency inductive reactance is small bypasses the RC branch and at high frequency
current diverts to RC branch at inductive reactance is high at high frequency. Due to use of auxiliary capacitor it provides low loss
at fundamental frequency than second order filter as well as provides very low impedance at tuning frequency similar to single
tune filter hence notch provided by filter at tuning is more dip compared to second order filter [4].
C type filter is as shown in figure 2 (e) It consists of capacitor connected in series with the parallel combination of L-C2&
resistor R as shown in figure 2 (e). L and C2 tune to the power frequency so this branch provides negligible impedance at power
frequency. Thus filter works as capacitor at power frequency bypassing the resistance branch hence power frequency loss is
reduced. For frequency more than power frequency the branch C 2+R and L resonant hence it works as single tune filter with
damping resistance. Inductive reactance becomes high at high frequency which makes circuit to works as first order high pass
filter [4].

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III. LIMIT FOR VOLTAGE & CURRENT HARMONICS AS PER STANDARD IEEE 519
IEEE standard 519 gives the acceptance criteria for voltage and current harmonics. Updated voltage distortion criteria as
per standard IEEE 519-2014 provided in below table. The values given at PCC shown in below table are mentioned in percentage
of power frequency voltage. Below table is applicable to harmonic voltages having frequencies which are integer multiple of
power frequency.

Table1-IEEE 519-2014 harmonic voltage limits

Limit for harmonic current for individual customer is given in standard IEEE 519 as shown in table 2.These standards give
the limit for injecting harmonic current in to the system so the resultant voltage distortion for all customers should be in limit.
Table - 2-IEEE 519 Harmonic current limits

In Table – 2, Point of common coupling (PCC)- is the location in the power system near to the user where the system owner
can give service to another user. This can possibly resolute by system owner. Customers High voltage step down transformer
secondary side and metering end are the probable locations of PCC.

Maximum demand load current (IL)- is given by average of monthly maximum demand in previous 12 months. The limit for
harmonic current in the IEEE standard is provided in the percentage of this current.
Short circuit ratio (SCR-Isc/IL) -is calculated as the short circuit current at the PCC (Isc) as fraction of the maximum demand
load current (IL). If customer size is small with respect to system then SCR is high. For such a system having high SCR limit for
injecting harmonic current is high for customer. Because individual customer can produce less system voltage distortion as
impedance of system is less [11]. Next section describes design of shunt passive filter for reducing harmonics.

IV. DESIGN OF SHUNT PASSIVE FILTER

A. DESIGN OF SINGLE TUNE FILTER -

The design of most widely used three filter namely single tune filter which is also called as notch filter, damped high pass
filter and C type filter are given in this section. The resonance frequency is given by following equation [6]
1
fresonant = = ffundamental√Xc⁄X
2π√LC L
Where,
Fresonant=resonance frequency
L=inductance of the filter
C=capacitance of the filter
XL=inductive reactance of the filter
XC=capacitive reactance of the filter
Ffundamental=Fundamental frequency

Step 1: Tune frequency for filter-Filter is tune to the slightly below harmonic frequency to allow for the tolerance in filter
elements and system impedance variation. Example for 5 th harmonic frequency the filter is tune to 4.7 th to allow for the tolerance.
Step 2: Size of capacitor bank –shunt passive filter also improve power factor & size of capacitor bank can be calculated as follow
𝑄𝑐 = P × {tan(cos−1 pfactual) − tan⁡(cos−1 pftargeted)
Where,
P = active power,
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Qc = reactive power to be compensated,

Pfactual = actual power factor,
Pftargeted = power factor to be improved.

Capacitor bank reactance (Xcapacitor) at power frequency is calculated as follow

kV 2
Xcapacitor =
Qc

Where, kV-system voltage

Step 3: size of filter inductive reactance –is calculated by using capacitive reactance to trap h harmonic as follows,
X
XL = capacitor
h2
XL
L=
2πffundamental

Resistance of reactor is calculated as follow

𝑋
𝑅=
𝑄𝑓
Where,
X-inductive or capacitive reactance at resonance
Qf -Quality factor

Typical values of Qf is in between 15-80 for filters used in industrial & commercial application [6].

B. DESIGN OF SECOND ORDER FILTER

The configuration of second order filter is as shown in figure 2(C) it consist of capacitor connected is series with parallel
combination of inductor, resistance. Hence it has 3 design equations from that the values of capacitor and inductor can be
calculated same way as specified in design of single tuned filter(step 2 & step 3), need to calculate value of damping resistor only
& it can be calculated from the quality factor. Quality factor (Q f) of second order filter is defined as the ratio of resistance to the
reactance (inductive/capacitive) of RC parallel circuit at tuned frequency. As Quality factor of this filter is reciprocal of quality
factor which is provided in design of single tune filter. Quality factor decides bandwidth that determines sharpness at the tuning
frequency & is given by
R
Qf =
X
Where,
X=XL or XC at tuned frequency

Hence damping resistor can be calculated as below

R = Qf × X

For second order or damped high pass filter value of quality factor varies between 0.5-5. Damped high pass filter with high
quality factor suppose Qf=5, filtering of harmonic at tune frequency is more definite. But impedance of filter increase gradually
for higher frequency hence filter is less effective for higher harmonic order. If the Q f is low as Qf=0.5, filtering of harmonic at
tune frequency is good and impedance of filter is constant for increasing value of frequency hence filtering of higher order
harmonic is also achieved well.
If Qf is kept very high to mitigate the tune harmonic very well suppose Qf from range10-50 in such a case resistance
increase and losses will become very high.

C. DESIGN OF C-TYPE FILTER

It consists of main capacitor C1 connected in series with parallel combination of C2+L & resistance R. Hence four design
equations are presents. The value of main capacitor & inductor can be calculated same way as in single tuned filter. The second
capacitor tuned to inductor at power frequency to reduce power frequency loss & is calculated as below,
Q
C1 = 2
V × 2πf
1
L=
[(2πfr)2 ∗ C1]
R = 𝑄𝑓 × 2πfr × L
1
C2 =
(V × 2πf)2 × L
Where,
Q = reactive power produced by filter at power frequency
V = voltage at which filter is to be installed
f = power frequency
fr = tuning frequency
Qf=quality factor of the filter
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V. HARMONIC FILTER RATING -

After completing harmonic analysis and sizing components of shunt passive filter accordingly the capacitor ratings peak
voltage, current, RMS voltage and kVAR needs to check. These ratings should be in limit individually provided in IEEE standard
18-1992 shunt power capacitor.
IEEE std 18-1992 “shunt power capacitor states that
With the harmonics in the system capacitor can be operated continuously on the condition that the following have to be satisfied
1) Peak current-should not exceed 130% of rated current.
𝐼𝑃𝐸𝐴𝐾 < 1.8⁡𝐼𝐹𝑢𝑛𝑑
Where, IFund = fundament capacitor current
𝐼𝑅𝑀𝑆
= √1 + 𝑇𝐻𝐷 𝑖 2 ≤ 1.8
𝐼𝐹𝑢𝑛𝑑

2) RMS voltage –should not exceed 110% of rated

𝑉𝑅𝑀𝑆 < 1.1⁡𝑉1
Where, V1=fundamental capacitor voltage
𝑉𝑅𝑀𝑆
= √1 + 𝑇𝐻𝐷 𝑣 2 ≤ 1.1
𝑉1

3) Peak voltage –should not exceed 120% of rated

𝑉𝑝𝑒𝑎𝑘 < 1.2⁡𝑉1
Where, V1=fundamental capacitor voltage
∑ 𝑉𝑐ℎ < 1.2𝑉1
ℎ

4) Reactive power should not exceed 135% of rating

𝑄𝑐 < 1.35𝑄𝑐1
Where Qc1=fundamental reactive power generated by the capacitor
𝑄𝑐 𝑉 2 1 𝐼 2 𝐼 2
= ∑ ℎ ( ℎ) = ∑ ( ℎ) = ∑ ℎ𝑝𝑢 ≤ 1.35
𝑄𝑐1 𝑉1 ℎ 𝐼1 ℎ
ℎ ℎ=1 ℎ=1
Filter tune to the certain harmonic frequency will observe parts of other harmonic frequency hence this factor should also be
considered this standard reflect that fact. Capacitor rated voltage must be higher than the bus bar voltage at which filter is placed
hence filter can operate successfully in case of over voltages in the system and during unbalance condition of capacitor bank
(filter).

Table - 3 Filter duty limit IEEE Standard 18-1992

Duty Limit %
Peak voltage 120
RMS voltage 110
Peak current 180
Kvar 135

VI. COMPARISION
Table - 4 Comparison of different types of filter
Criteria Single tuned Second order filter C-Type filter
filter
Filtering of harmonics single single harmonic and single harmonic and above
harmonic above certain range certain range

No of component 2 3 4
Design Complexity Simple Simple Complex

Losses at power frequency Less High less compare to second order &
high compare to single tuned
Quality factor high (15-80) Low (0.5-5) Low (0.5-5)

Sensitive to fundamental High Low Low

frequency & component value
variation

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VII. CONCLUSION
In this paper reviewed different shunt passive filter for reducing harmonics such as single tuned filter, second order high pass
filter & C-type filter. The design of each method is presented. And finally comparison conducted on different point such as
performance, design complexity and losses.
Though passive filter is having low cost, simplicity, reliability & efficiency it has limitations such as, resonance problem, fixed
compensation character, possible overload and poor dynamic behaviour.

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