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75
CHAPTER 4
SHUNT ACTIVE POWER FILTER
Abstract— A synchronous logic based Phase angle control method pulse width modulation
(PWM) algorithm is proposed for three phase Shunt Active Power Filter (SAPF) for three-
phase systems to ensure low mains current harmonics and high energy efficiency by reducing
reactive power consumption, as well as maintaining a constant output DC-bus voltage.
Discussed the performance characteristics of SAPF topology with the synchronous logic
control algorithm that is based on the angle between the mains and the reference sine wave to
control the power factor and to inject a current where each harmonic current has the same
amplitude as that of the load current but in opposition of phase. However, this PWM
technique greatly simplifies the calculation process and is easier to implement with digital
processors. Designed and developed a three-phase SAPF system that is built and tested with
the inputs and output with load. The proposed PWM technique can be used in all applications
such as Inverters, Filters, UPSs, etc. The Simulation and experimental prototype performance
results of a three-phase SAPF with a power output of 3 kVA is presented to validate the
proposed control strategy and the results obtained.
4.1 Introduction
In the traditional approach, in order to suppress harmonics in power systems, passive power
factor correction techniques with line chokes and bulk capacitors are used but they are
neither convenient nor economical; they need bulky components and are not adaptive to
changing needs. However, the remarkable progress made in the field of the power electronic
devices made the systems design for harmonics compensator, named as Active Power Filters
76
(APFs) is a reality. These APFs eliminate the components of power that do not contribute to
the net transfer of energy from the source to the load. New systems and appliances can be
built with the unity power factor and low current harmonics front end rectifiers but large
number of systems that are already in operation need a special attention. Active power filters
can be divided in two classes: series type and shunt type active filters, as defined in [31],
from the system point of view. The combination usage of shunt active and passive filters has
already been in use to compensate large-rated loads input current harmonics. Active Power
Filters also help in meeting the IEEE 519-92, IEC-555 and European EN 61000-3-2/IEC
61000-3-4 standards for allowable harmonic contents of mains.
To prevail over the above drawbacks of the large number of systems that are already in the
field and in operation, power quality improvement filters are included as an inherent part of
the total power network system that produces high efficiency, reduced size and regulated
output. Various types of solutions to Power Quality problems with Active Power Filters are
shown in table 4.1.
Table 4.1: - Active Power Filter Solutions to Power Quality Problems.
Active Power Filter Solutions to Power Quality Problems
Active Filter Connection Load on AC Supply AC Supply to Load
Shunt -Current Harmonic Filtering.
-Reactive current compensation.
-Current unbalance.
-Voltage Flicker.
Nil
Series -Current Harmonic Filtering.
-Reactive current compensation.
-Current unbalance.
-Voltage Flicker.
-Voltage unbalance.
-Voltage sag/swell.
-Voltage unbalance.
-Voltage distortion.
-Voltage interruption.
-Voltage flicker.
-Voltage glitches.
77
An active harmonic power conditioner/compensator/filter is a device that uses at least one
static converter to meet the “harmonic compensation” function. This generic term thus
actually covers a wide range of systems, distinguished by:
• The number of converters used and their association mode,
• Their type (voltage source, current source),
• The global control modes (current or voltage compensation),
• Possible association with passive components (or even passive filters).
(a) (b)
Figure 4.1: - Active Power Filter (a) “shunt-type” and (b)“series type”.
Figure 4.2: - “Series/Parallel” type hybrid filter.
78
The only common feature of these active systems is that they all generate voltages and/or
currents that are in opposition (Phase) to the harmonics generated by non-linear loads. Figure
4.1(a) shows the simplest one that is normally known as “Shunt” (or “Parallel”) type
topology.
The “Series” type active harmonic conditioner shown in Figure 4.1(b) is rarely used. It works
by blocking the upstream harmonic voltage sources to provide better power to a sensitive
load on a troubled power network. However, in practice, this “upstream” technique is of little
interest since:
• The “quality” of the energy at the point of common coupling is satisfactory,
• Insertion of an equipment in “series” mode is not that easy because of the issues like
withstanding short-circuit current,
• It is more useful to look at the actual causes of voltage distortion (harmonic currents)
within the network.
Out of the numerous “Hybrid” alternatives the “Series/Parallel” type solutions have shown in
figure 4.2 the one that combines active and passive filtering is a very effective one for
harmonic cancellation that comes close to the high power converters. Provided that the
equipment is capable of injecting a current at any given time with the same amplitude as that
of the load current but exactly in opposition of phase, at that point of contact (coupling) the
current supplied by the mains is guaranteed to be purely sinusoidal as per the Kirchoff’s law.
The control strategy is to apply a current reference that is equal to all the harmonics content
of the current absorbed by non-linear load, then the current produced by the converter is in
phase opposition to the load and cancels all the harmonics at the point of common coupling
(PCC) as shown in figure 4.3, which is also known as Shunt Active Power Filter (SAPF).
This also comes under the classification of Flexible AC Transmission Systems (FACTS)
which is defined by the IEEE as "a power electronic based system and other static equipment
that provide control of one or more AC transmission system parameters to enhance
79
controllability and increase power transfer capability." The combination of “non linear load
currents plus active power filter currents” forms a linear load current to the mains power
supply. SAPF also supplies the reactive component of the load to make the source current to
be in phase with the voltage.
The SAPF thus forms a current source independent of mains supply impedance as clarified in
[14, 29, 31, 35], and the control algorithm as in [2], with the following characteristics:
• Band-width is sufficient to remove most of the harmonic components from the load
current,
• Response time is such that the harmonic compensation is effective in steady state as well
as in transient state,
• Has enough power to meet all harmonic components. However, all the harmonics
generated by the load are not necessarily compensated in total and permanently.
Provided that these three main objectives are simultaneously met, the SAPF forms an
excellent source solution as it is self-adaptive and there is no risk of interaction with power
network impedances. The aim of SAPF is not to re-phase the mains fundamental voltage and
current components but to make sure the current is linear. So, the insertion of SAPF has no
effect on the displacement power factor. Apart from controlling the harmonics in the source
currents, close to unity power factor ensures that the source is not loaded reactively. Here a
four quadrant, three-phase, six-switch and common link DC bus converter topology used.
Figure 4.4 shows a block diagram of the parallel active filter connection. It consists of a dc
link inverter and filter section. The filter inductor is used to convert the voltage source
inverter output to a current source capable of injecting harmonic currents to the load. The
configuration exhibited in Figure 4.5 uses load current feedback. The system is capable of
using utility source voltage and current signals wherein the source harmonic currents are
minimized.
Figure 4.3: - Equivalent circuit of Shun
Figure 4.4: - Shows the basic structure of Three
Figure 4.5: - Three
80
(a)
Equivalent circuit of Shunt Active Power Filter, (a) Single line diagram and principle of
compensation (filtering) (b) Three Phase
Shows the basic structure of Three-Phase Shunt Active Power Filter.
Three-Phase and neutral configuration of Shunt active power filter.
(b)
Single line diagram and principle of
Phase Shunt Active Power Filter.
active power filter.
81
The SAPF topology is implemented with six power switches (three legs) as shown in figure
4.5. Shunt active power filter (SAPF) is similar to a PWM unity power factor rectifier by
construction but differ it by a centre tap on the main filter capacitors / batteries which in turn
is connected to the neutral. The control logic types studied as in [3, 19] made improvements
by calculating the load current harmonics in real time by comparing with the fundamental,
which are applied as reference to the inverter control logic in order to cancel the harmonics
and to get a sinusoidal form for the mains supply current. SAPFs are gaining widespread
popularity and have received special attention in high-power high-voltage applications for
large capacity power applications as listed in Table 4.2 because of their excellent
performance and better line side power quality. This data is collected from the various device
manufacturers data sheets and also various application notes and presentations.
Table 4.2: - Commercial applications of Shunt Active Power Filters
Objective Rating Switching
Devices
Applications
Harmonic compensation
with or without reactive
power compensation
below100KVA IGBT
MOSFET
IGCT
GTO
Diode or thyristor rectifiers and cyclo-
converters for industry
Flicker compensation 100VA ~10 MVA IGBT Arc furnaces
Voltage regulation above10MVA GTO Sinkansen (the Japanese “bullet”
trains)
Designed and developed a three-phase SAPF system that was built and tested with the inputs
and output. The results confirm the theoretical analysis. The SAPF was designed to run over
a wide line-to-line supply voltage range of 160 to 520 Vrms, and 3 kVA nominal power
output. For an output Dc link voltage of 900 Vdc, the input phase current was approximately
5.0 Arms.
82
Experimental prototype system results of three-phase, twelve switches, PWM SAPF that was
designed to deliver an output of 3 kVA power are presented. Details on the formulated
problem design example are presented in section 4.2, including description of a power
section implementation with six power switches and six freewheeling diodes and two
filter/storage capacitors. Details of the SAPF analysis are described in Sections 4.3 including
Input current Analysis. Simulation details are presented in section 4.4. Simulation results are
presented in section 4.5 with nominal input while delivering total of 3 kVA harmonic
compensation. Details on the experimental performance, such as the input currents and
respective harmonics, DC link bus voltage and input voltages are given in Section 4.6. The
overall SAPF system performances with the synchronous logic control implementation are
concluded in Section 4.7.
4.2 Design (Example)
The design and implementation of a shunt active power filter (SAPF) with reduced dc link
voltages as detailed in [35] was studied. Following Flowchart of the generalized design
methodology for SAPF is given in Figure 4.5 (a) and design example is used as an
improvement to the above reference:
1. Choose a suitable PWM switching pattern for the Inverter three phases. Enough lead and
lag angle range accommodated in order to keep the output dc link voltage constant even
under high input voltage fluctuations conditions.
2. Select the switching frequency of the SAPF bridge converter switches (MOSFET, IGBT
or GTO) based on voltage, current and power for the specified kVA rating. The selected
switching frequency is 50 kHz to compensate for higher harmonics of 29th order. So
83
MOSFETs are selected.
3. Select the Free-wheeling diodes for SAPF converter based on the voltage, current and
power for the specified load kVA. Fast Recovery Diodes are selected.
4. Calculate the input filter inductance based on voltage and the kVA rating.
5. Calculate the DC bus link capacitance based on voltage and the kVA rating of converter.
6. Calculate the input main inductance value for the specified output kVA (current) rating,
Lowest input voltage rating and selected constant switching frequency.
Flowchart of the generalized design methodology for SAPF:
I. Determine the required output power rating (Po), input voltage (Vin)
and required efficiency (η) of the system
II. Select a suitable sinusoidal switching pattern for the Inverter three-
phases
III. Determine the required lead and lag angles to keep the harmonic
content low even under high mains voltage and full output load
conditions
IV. Select the suitable Bi-Directional switches (Transistors and Diodes)
based on the type (MOSFET or IGBT), voltage and current based on
the voltage, current and power for the specified load kVA
V. Calculate the input filter inductance based on the voltage and kVA
rating, and desired per unit impedance
VI. Calculate the DC bus link capacitance based on voltage and the kVA
rating of converter
VII. Calculate input main coupling inductance based on the specified
output kVA (current), lowest input voltage and selected switching
frequency
VIII. Verify input current harmonics at various R, RL (fixed) and RL
(variable) loads
IX. Verify system efficiency at various output power levels with R, RL
(fixed) and RL (variable) loads
X. Finalize the design
Fig. 4.5 (a). Flowchart of the
generalized design
methodology for SAPF
Y
N
Y
N
I
II
IV
III
VIII
IX
X
V
VI
VII
84
In this section, the design prototype example of a SAPF is presented which is having the
following specifications:
Table 4.3: - Active Power Filter Specifications (4.8 Arms)
Type APF 3.0 kW
uRS (Nominal) Vrms 3 x 400 ± 15%, 47 to 53 Hz
Power Factor ≥ 0.95
THD of iR ≤10 %
Modulation type Sine PWM
Switching (modulation) frequency 50kHz
Rated VA @ 400 Vac 3.0 kVA
iR 5.0 Aac
Short term over load current in A ac 200% of iR for 0.5 sec. over 60 sec cycle time.
Estimated efficiency η 90
Operating Temp. °C 0 to +40; +40 to + 50 with derating
85
4.3 Shunt Active Power Filter Analysis
The proposed three-phase high-quality Shunt Active Power Filter (SAPF) is shown in figure
4.5. The four wires SAPF consists six power switches (three legs), six power diodes, two
capacitors in the DC side and three line inductances in the ac side. Figure 4.6 illustrates the
phasor diagrams for power exchanges at the Point of Common Coupling (PCC), converter
output current and voltage are shown in all four quadrants of the PQ plane.
Figure 4.6: - Phasor diagram for power exchanges in four quadrant operations.
A. Sinusoidal PWMs for Shunt Active Power Filters
The implementation of a SAPF with synchronous logic based phase angle control algorithm
based PWM is carried out by the same principle as that of a converter. Figure 4.7(a) shows a
reference control wave with frequency Fo and a carrier wave with frequency Fs i.e.,
86
switching frequency. The magnitude of the carrier wave has to be always kept greater than
the reference wave for better harmonics control. The carrier wave generates the gate drive
pulses as shown in Figure 4.7(b) for the power switches sR1 and sR1’ of the SAPF converter
R phase that generates voltage. And for the other two phases S and T, the reference
sinusoidal waves are displaced by an angle 2Π/3 and 4Π/3 respectively.
Figure 4.7(a): – Source Current (Red), Control wave (Green) and Carrier wave (Blue) for R-phase of the Three-
Phase H-Bridge SAPF Converter.
Figure 4.7(b): – Gate drive pulses for R-phase of the Three-Phase H-Bridge SAPF Converter.
87
4.4 System Simulation
The H-Bridge SAPF converter SIMULINK simulation gives DC link voltage of 900 Vdc and
average bridge rectifier DC voltage of 600 Vdc for SAPF converter, at nominal voltage
conditions. The mains current harmonics are much lower than the levels of the statutory
requirements. At various non-linear or/and reactive loads, it is possible to keep the mains
current harmonics low and power factor close to unity for input over voltage situations also.
In order to verify the input current harmonics of SAPF at different load levels while keeping
constant DC link voltage, simulations as shown in figures 4.8(a) and (b) have been
performed. Tabulated results by changing the loads on the output with R, RL (PF-fixed), and
RL (PF-variable) variable type. The control strategy has enough lag and lead angles “δ” to
keep the Dc link voltage stable, while keeping the mains current free of harmonics.
4.4.1 Simulation of SAPF with Synchronous Logic:
The model shown below in figure 4.8(a) gives the Simulation circuit of SAPF and figure
4.8(b) shows Synchronous Logic based Phase angle control scheme. Each leg has two power
switches and two power diodes are connected in parallel. IGBT’s are taken as the power
switches. The output voltage and current is taken as the output on the scope.
4.4.1.1 Model Parameters
These parameters are based on the per unit impedances for the given power level and output
ratings/specifications.
• Source: Voltage (Phase) – 230 Vac, Frequency – 50 Hz, Resistance- 0.1 Ω, Inductance - 1.0 mH
• Bridge Rectifier: Output voltage – 600 Vdc, Capacitance - 1000 µF, Load- R – 200 Ω and L -
0.01 mH
• Converter: Coupling Inductance - 1.0 mH, DC link Voltage – 900 Vdc, Switching Frequency –
50 kHz
88
Figure 4.8(a): - Simulation circuit of Shunt Active Power Filter (SIMULINK).
Figure 4.8(b): - Shunt Active Power Filter Control Scheme (SIMULINK).
89
4.5 System Simulation Results
Simulation studies were carried out to validate the Synchronous Logic based control
algorithm SIMULINK model to generate gate drive PWM for SAPF.
SAPF CONVERTER SYSTEM
The simulation results of SAPF system are shown below in figure 4.9 with Resistive (R)
Load. The three phase reference voltages uNR, uNS, uNT are calculated from the frequency
and amplitude of the source voltages as shown in figure 4.9(a). The control waveforms are
calculated from the load current waveforms and compared with the carrier waveforms to get
the drive signals of the power switches. Figure 4.9(b) shows the Load current, Source current
and converter current, 4.9(c) shows DC Capacitor link voltage, and 4.9(d) shows Bridge
Rectifier load (dc) current and voltage. The simulation and results are with Resistive (R)
Load. The input current is, iNR = 5.47 A (corresponding to 1 kW each phase) at 230 Vac
input voltage. Only harmonics 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th and 29th are considered.
Third and its multiples are negligible to consider.
4.5.1 Mains (Source) Voltage and Current
The mains (source) supply voltages and currents of R-Phase, Three-Phase, Four-wire, SAPF
are shown in figure 4.10(a) and figure 4.10(b) respectively.
90
(a)
(b)
(c)
(d) Figure 4.9: – SAPF converter; (a) Voltages (uNR, uNS, uNT) from source voltages, (b) Load current, Source
current and converter current, (c) DC Capacitor link voltage, (d) Bridge Rectifier load (dc) current and voltage.
91
(a)
(b) Figure 4.10: – Source waveforms of Three-Phase, Four wire, SAPF R-Phase; (a) Three Phase Voltages (b) Currents.
4.5.2 Source Current Harmonics
Suitable harmonic computation technique for active power filters as detailed in [45] was
studied. The source current harmonics of R-Phase, Three-Phase, Four wire, SAPF are shown
in figure 10(c).
Figure 4.10(c): – Simulation results source current harmonics of R-Phase Three-Phase Four wire SAPF.
0.000
0.050
0.100
0.150
0.200
0.250
0.300
5th
Harm.
7th
Harm.
11th
Harm.
13th
Harm.
17th
Harm.
19th
Harm.
23rd
Harm.
25th
Harm.
29th
Harm.
100W
200W
400W
600W
800W
1000W
92
Table 4.4: – SAPF Simulation Input current and voltage at different Resistive loads.
Table 4.5: – SAPF Simulation Input current harmonics at different Resistive loads.
93
4.6 Experimental Results
In order to verify the SAPF concept, a prototype of a three-phase four wire Shunt Active
Power filter with proposed synchronous logic based phase angle control with source voltage
and load current sensing was built with following specifications: Maximum output of 5A per
Phase current harmonic compensation.
• Source: Voltage (L-N) – 230 V, Frequency – 50 Hz, Inductance - 1.0 mH
• Bridge Rectifier: Output voltage – 600 Vdc, Capacitance - 1000 µF, Load- R – 120 Ω
and L - 0.01 mH
• Converter: Coupling Inductance - 1.0 mH, DC link Voltage – 900 Vdc, Switching
Frequency – 50 kHz
The real-lab prototype for the three phase with neutral SAPF system configuration is shown
in figure 4.5 is shown in figures, 4.11(a), Logic section and Power sections, 4.11(b),
complete assembly of SAPF. The power supply, some line filters and output load sections are
not shown due to space constraints. Component values are based on the kVA rating, per unit
impedance, switching frequency and output rating. The experimental condition is as follows:
three-phase Source inductance is 1.0 mH; SAPF converter coupling inductance is 1.0 mH;
Bridge rectifier output inductance, 0.01 mH and capacitance, 1000 µF. Six main bi-
directional switches in parallel with six ultra fast recovery diodes used to build three-phase
full bridge SAPF converter. SAPF converter phases uNR, uNS and uNT are implemented
with MOSFETs as shown in figure 4.5. The Bridge rectifier output load resistance is 120
ohms. The switching frequency is 50 kHz. Figure 4.12 shows the experimental waveforms.
Three Phase Input Current waveforms (Ch1 – Ch4) iNR, iNS, iNT and iN when a resistive
load of 3 kW applied on output of bridge rectifier.
94
Figure 4.11(a): – Real-Lab prototype set up of the three-phase four wire SAPF Logic and Power sections.
Figure 4.11(b): – Real-Lab prototype set up of the three-phase four wire SAPF Converter.
Ch1: 3A, Ch2: 3A, Ch3: 3A, Ch4: 3A; Scale: 5 ms; Trigger: Line
Figure 4.12: – Three Phase Source Current waveforms (Ch1 – Ch4) iNR, iNS, iNT and iN at 230 V when total
Resistive Load of 3 kW applied on Bridge Rectifier Output.
95
Readings have been taken in order to verify the utility mains current harmonics at different
load levels. Recorded readings of source current harmonics; bridge rectifier output voltages
and efficiency, by changing the loads on the output with R, RL(fixed PF) and RL (variable
PF) type and presented in figures 4.13(a), 4.14(a), 4.15(a); 4.13(b), 4.14(b), 4.15(b) and 4.16
respectively. The measured power factor was 0.97 when the DC-Link voltage was 900 Vdc.
Bridge rectifier output voltage is 600 Vdc (+300 and -300 referenced to center point) and
currents are multiplied by 50 to make them easily readable. Only harmonics 5th, 7th, 11th, 13th,
17th, 19th, 23rd, 25th and 29th are considered. Third and its multiples are negligible to consider.
The values of the displacement power factor (DPF), power factor (PF), and current total
harmonic distortion (THD), respectively for the load and power supply source are presented
in Table 4.4. The values of THD are for the phase and neutral currents. The values of DPF
and PF are presented to each of the phases.
Table 4.6: – SAPF DPF, PF and currents THD values at Load and source with crest factor loads.
It is confirmed after comparing the results that the SAPF compensates the PF and the
harmonic currents, turning them into near unity and near zero respectively at source supply.
The source current spectrum, rectifier output voltages and current are shown in Figure 4.13
with resistive (R) Load. The input current is, iNR = 5.0 A at 232 Vac input voltage. The
efficiency at rated power is 82%.
96
Table 4.7: –SAPF Input current and voltage at different Resistive loads.
Table 4.8: – SAPF Input current harmonics at different Resistive loads.
97
(a)
(b)
Figure 4.13: - SAPF with Resistive Load, (a) Source current harmonics at different load levels, (b) Bridge
Rectifier output Voltage and Current with 3 kW load.
Figure 4.14 shows the source current spectrum, rectifier output voltages and current with
Resistive and Inductive (RL) Load with a fixed power factor. The input current is, iNR =
5.14 A at 231 Vac input voltage. The efficiency at rated power is 80%.
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
0.400
5th
Harm.
7th
Harm.
11th
Harm.
13th
Harm.
17th
Harm.
19th
Harm.
23rd
Harm.
25th
Harm.
29th
Harm.
100W
200W
400W
600W
800W
1000W
-400
-300
-200
-100
0
100
200
300
400
I-R-Load V1 - R
V2 - R
98
Table 4.9: –SAPF Input current and voltage at different RL loads (Fixed-PF).
Table 4.10: – SAPF Input current harmonics at different RL loads (Fixed-PF).
99
(a)
(b)
Figure 4.14: - SAPF with Resistive and Inductive (RL) Load with a fixed rated power factor, (a) Source current
harmonics at different load levels, (b) Bridge Rectifier output Voltage and Current with 3 kW load.
Figure 4.15 shows the source current spectrum, rectifier output voltages and current with
Resistive and Inductive (RL) Load with variable power factor. The input current is, iNR =
5.08 A at 231 Vac input voltage. The efficiency at rated power is 81%.
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
0.400
0.450
5th Harm.
7th Harm.
11th Harm.
13th Harm.
17th Harm.
19th Harm.
23rd Harm.
25th Harm.
29th Harm.
100W
200W
400W
600W
800W
1000W
-400
-300
-200
-100
0
100
200
300
400
I-RL- (PF-Fixed) V1 - RL-(PF-Fixed)
V2 - RL-(PF-Fixed)
100
Table 4.11: –SAPF Input current and voltage at different RL loads (Variable-PF).
Table 4.12: – SAPF Input current harmonics at different RL loads (Variable-PF).
101
(a)
(b)
Figure 4.15: - SAPF with Resistive and Inductive (RL) Load with variable power factor, (a) Source current
harmonics at different load levels, (b) Bridge Rectifier output Voltage and Current with 3 kW load.
Figure 4.16: - Efficiencies of SAPF with various load levels.
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
0.400
5th Harm.
7th Harm.
11th Harm.
13th Harm.
17th Harm.
19th Harm.
23rd Harm.
25th Harm.
29th Harm.
100W
200W
400W
600W
800W
1000W
-400
-300
-200
-100
0
100
200
300
400
I-RL-(PF-Variable)
V1 - RL-(PF-Variable)
V2 - RL-(PF-Variable)
0
10
20
30
40
50
60
70
80
90
100 200 400 600 800 1000
ƞ R Load
ƞ RL Fixed
ƞ RL Variable
102
The efficiencies shown in figure 4.16 are with Resistive, Resistive and Inductive (RL) Load
with fixed power factor and RL with variable power factor from unity to rated.
Measurements were recorded by changing the loads on the output of the setup. The
efficiency varied based on the type of load and also percentage of the load from 58% to 82%.
4.7 Summary
More reactive power and more current harmonics means more losses, lower efficiency, more
electric energy to be produced and correspondingly more environmental pollution and
particularly more carbon dioxide put in to the atmosphere. Cancellation of harmonics and
reduction of reactive power in the mains represents not only an economic sense but also a
direct solution for sustainable development. A number of possible SAPF were presented. It
was demonstrated that both voltage and current harmonics can be cancelled as well as the
reactive component of the load current by using proper control logic. The phase angle control
was based on specialized logic developed for converter applications, providing a simple and
cost effective solution.
The proposed three-phase, four wire, six switch H-Bridge, SAPF converter was investigated
and simulated; source current distortion that usually generated by diode bridge rectifier and
capacitive filter was controlled. A new three-phase synchronous logic based phase angle
control is introduced and simulations show that it can produce very low DC link voltage
ripple and very low source current harmonics with unity power factor. The resulting input
current harmonics were below the limits of IEC 61000-3-4 standard for various load levels.
Experimental SAPF converter of 1 kVA per phase, with six switches H-Bridge was built to
103
verify the concept. Near unity power factor was measured in all three phases. The source is
having lower current harmonics but more number of conducting devices (Converter and
Rectifier) resulted in lower efficiency. The proposed controller is implemented by sensing
source voltages, load currents and DC link voltage. The SAPF converter controller is very
simple and reliable. The input inductance size and value is reduced when compared to the
passive filter circuit. The experimental results confirm the prototype circuit’s behavior.