Claims:We Claim:
1. A hybrid active filter 100 for a three-phase supply 30a, 30b, 30c, the hybrid active filter 100 comprising:
a current controlled single-phase inverter 110 with two leg IGBT bridge structure, each leg 62, 64 connected to a common dc voltage bus which has energy storage capacitor in series;
a neutral current compensator 120 with three single-phase transformers 10, 12 and 14 connected in star with one end of the primary of each transformer 10, 12 and 14 is connected to one of the lines or phases of the incoming three-phase supply 30a, 30b, 30c, where load neutral is connected to one terminal of the series connection of three secondary windings 10b, 12b and 14b and also to the midpoint of the two series connected capacitors connected across the dc bus of the hybrid active filter 100 and the other end of the series connection of the windings is returned to the star connection of a three primary windings 10a, 12a and 14a and thereafter the same connection can be returned to the star connection of the incoming three -phase supply 30a, 30b, 30c and where the three single-phase transformers 10, 12 and 14 have 3:1 primary to secondary ratio, the neutral current compensator 120 is coupled adjacent to the single-phase inverter 110 for compensating current harmonics, dynamic reactive power and unbalanced load 40 current flowing in load neutral; and
a control block 80 for controlling pulse width modulation of the single-phase inverter 110, the control block 80 generates the pulses for driving the IGBTs used in each legs 62, 64.
2. The hybrid active filter 100 as claimed in claim 1, wherein the midpoint of the two IGBT legs 62, 64 are connected through inductors to two of the incoming three-phase supply lines or phases and with a stable output DC voltage and two DC capacitors 66 connected across it whose midpoint is returned to the remaining third line or phase of the incoming supply lines as one part.
3. The hybrid active filter 100 as claimed in claim 1, wherein the single-phase inverter 110 receives compensating current commands for current to be drawn from the supply lines or phases connected to them and the load and corresponding to necessary load current(s) harmonic compensation(s) and also corresponding to compensation of reactive part of the load current(s) for the those two supply lines or phases and where the midpoint of the two capacitors 66 across the dc bus generates summation of these two line or phase currents.
4. The hybrid active filter 100 as claimed in claim 1, wherein the Pulse Width Modulation of the single-phase inverter 110 is controlled by the control block 80 or control scheme which receives three-phase load currents and three-phase supply voltages as inputs for executing the control.
5. The hybrid active filter 100 as claimed in claim 1, wherein the single-phase inverter 110 uses control block 80 or control scheme and the Neutral Current Compensator (NCC) 120 together acting as the hybrid active filter 100 to produce compensating currents for load current harmonics and also the reactive part of the load currents and allow only active current part of the load currents to be drawn from the three–phase supply lines.
6. The hybrid active filter 100 as claimed in claim 1, wherein the single-phase inverter 110 draws a small in-phase current from its supply lines or phases and this corresponds to the power loss in IGBTs 62, 64, dc bus capacitors 66 and other losses such as in connecting cables or bus bars.
7. The hybrid active filter 100 as claimed in claim 1, wherein the hybrid active filter 100 is configurable with three-phase, star connected supply and three-phase, star connected unbalanced linear and non-linear load, thus for a four wire system.
8. The hybrid active filter 100 as claimed in claim 1, wherein the hybrid active filter 100 is configurable with three-phase, star connected supply and when load is unbalanced and delta connected.
9. The hybrid active filter 100 as claimed in claim 1, wherein the hybrid active filter 100 is configurable with three-phase, supply is star connected, load is unbalanced and star connected but neutral connection not returned to supply neutral point, directly or through earth.
10. The hybrid active filter 100 as claimed in claim 1, wherein the hybrid active filter 100 is configurable with three-phase, supply is delta connected, load is unbalanced and delta connected.
11. The hybrid active filter 100 as claimed in claim 1, wherein the hybrid active filter 100 is configurable with three-phase, supply is delta connected, load is unbalanced and star connected.
, Description:Field of the Invention
The present invention relates to a Hybrid Active Filter for Star or Delta connected unbalanced linear and non-linear loads. More specifically, the present inventions relates to a Hybrid Active Filter Star or Delta connected unbalanced linear and non-linear loads for (i) compensation of fundamental reactive power drawn by the loads, (ii) compensation of load current harmonic components, and (iii) and avoidance / elimination of unbalanced load current (also normally referred as Zero Sequence Current) flowing in neutral cable or bus bar into the supply when the load is Star connected as well as it is unbalanced, using reduced number of self-commutated active devices and a passive transformer based arrangement.
Background of the Invention
With the advancement in the field of semiconductor devices, especially the self-commutated power devices, and development of fast acting micro-controllers and Digital Signal Processors (DSPs), there has been substantial growth in use of power electronic converters for uninterruptible power supplies, motor drives, arc furnace, trolley cars, battery chargers, lighting appliances, power distribution and transmission networks, Railways, and many others. The applications of power converters have increased manifold to achieve better efficiency and less losses compared to classical methods used earlier. Also, there has been substantial increase in demand of power and it has been a major challenge for the utilities to maintain the uninterrupted supply and associated quality of power. All the power conversion applications have given rise to increased proliferation of non-linear loads compared to the linear loads. Typical applications like welding loads, fabrication shops, distributed loads within industrial establishments, LV distribution transformers, arc furnaces etc. are the examples of such non-linear loads.
Along with being non-linear in nature, the loads have been known for drawing fluctuating / dynamic (i) active and reactive power, (ii) current harmonics, and (iii) unbalanced line currents inclusive of fundamental and harmonic components from the utility sources. The dynamic variation in reactive power gives rise to supply voltage variation causing visible flicker many times. Increased voltage distortion beyond certain limit can cause loss of synchronization for other power electronic equipment, connected on same supply voltage bus, resulting in process shut down. It also can cause unwanted harmonic currents flowing from supply to other connected loads on the same supply bus. The harmonic currents not only cause increased upstream power loss but can cause unwarranted resonance with passive filters employed for harmonic current reduction especially on the same voltage bus. The supply voltage or network has a limited short circuit capacity and hence the reflected short circuit impedance (representing the short circuit capacity dependence on it) is an important element of the supply network system and the load connection. Thus, variation of voltage and distortion of the voltage, as explained above, are influenced by it directly.
The total current harmonic distortion is basically a function of the load current harmonic components demanded by such non-linear loads. There is always an increase in the voltage harmonics owing to the system short circuit impedance and the harmonic components of the non-linear loads. Hence, it has become necessary to maintain / reduce current harmonics experienced by the supply source below certain limits as per international standards (like IEEE 519 year 2014) to restrict supply voltage distortion for given short circuit capacity at the Point of Common Coupling (PCC).
Similar to requirement of current harmonics below a certain limit, the second important factor has been demand for compensation of dynamic volt amperes reactive (VAr) leading to variation in the dynamic fundamental power factor, that is PF, (or true power factor TPF which is multiplication of fundamental power factor and current harmonic distortion factor DF). Therefore, it has become necessary to maintain the fundamental power factor close to unity and distortion factor closed to unity in order to reduce system voltage variation and supply voltage distortion, supply and upstream losses, and the electricity energy bills.
The third important factor for major consideration in electrical systems has been the flow of the neutral current caused by the unbalanced fundamental and / or harmonic loads, which is especially observed with varying linear and non-linear loads. The neutral current consists of unbalanced portion of the fundamental frequency component and the harmonic components (normally odd harmonic frequency components) of the linear and / or non-linear loads. It should be noted that AC power supply systems should not experience any dc offsetting current and even harmonics as per prescribed international standards. In fact, these components should be kept extremely low.
The presence of neutral current and especially, increase in its magnitude, gives rise to additional system losses and heating of the neutral cables. It also gives rise to the increased neutral to earth potential; resulting many times in mal-functioning of protection relays employed in the system and may also affect the loads connected on the same supply bus. The unbalance in supply currents also leads to reduced life of the supply transformers. Further, the unbalanced load current can flow from load to supply only in a four wire system or where supply and load both are Star connected and where the neutral gets connected to the star point of the supply directly or through earth. There is no neutral current flowing back to supply side in following cases which basically use three wire system.
Supply is delta connected, load is unbalanced and delta connected
Supply is delta connected, load is unbalanced and star connected
Supply is star connected, load is unbalanced and delta connected
Supply is star connected, load is unbalanced and star connected (but neutral connection not returned to supply neutral point, directly or through ground / earth).
Hence, it becomes necessary to provide a good solution which can resolve all the “following three major problems” of the power systems for a three or four wire system connected to a linear and / or non-linear load.
Compensation of load current harmonics to reduce them below a certain limit while flowing in supply
Compensation of load dynamic reactive power to reduce its burden on the supply
Avoidance / elimination of neutral current, carrying the unbalanced load current, flowing into the supply, specifically when the supply is Star connected and the linear and / or non-linear load is Star connected as well as it is unbalanced
The presently available active filters in industries use three-phase connection while working in parallel or in shunt with the load while providing the above necessary compensations. The invention covered here uses single-phase connection to provide the necessary solution to the major problems described above.
Objects of the Invention
An object of the present invention is to provide a Hybrid Active Filter for a three-phase “four” wire supply system (where supply is Star connected and linear and / or non-linear load is also Star connected) to achieve
Compensation of load current harmonics to reduce them below a certain limit while flowing in supply
Compensation of load dynamic reactive power to reduce its burden on the supply
Avoidance of the neutral current, carrying the unbalanced load current, flowing into the supply.
A further object of the present invention is to provide a Hybrid Active Filter for a three-phase “three” wire supply system (where supply is Star or Delta connected and linear and / or non-linear load is either Star connected or Delta connected) to achieve
Compensation of load current harmonics to reduce them below a certain limit while flowing in supply
Compensation of load dynamic reactive power to reduce its burden on the supply
Another object of the present invention is to provide a Hybrid Active Filter which can be used for various types of loads, such as unbalanced linear and / or non-linear loads drawing current harmonics from the supply, variable and dynamic power factor loads or loads drawing variable and dynamic reactive power from supply, and avoiding flow of any neutral current back in the supply.
Yet another object of the present invention is to provide a Hybrid Active Filter which is capable of compensating for the current harmonics, compensating the reactive power thus improving the power factor of the supply current, and avoiding the flow of neutral current carrying the unbalanced current of the load back into the supply by circulating the neutral current back in the system.
One more object of the present invention is to provide a Hybrid Active Filter which eliminates the dependency of neutral connection from the source side having Star connection making the HAF to be used with all possible three and four wire supply – load systems.
Still another object of the present invention is to provide a Hybrid Active Filter, which incorporates both the passive and active components thereby making it suitable for reliable use in both the industrial as well as power distribution side.
An added object of the invention is to improve overall system reliability due to reduction of one IGBT leg (two IGBTs) by using a single-phase Active Filter as a part of the Hybrid Active Filter as compared to a three leg (six IGBTs) normally used in a three-phase Active Filter.
Other objects and advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures which are not intended to limit the scope of the present disclosure.
Summary of the invention
According to the present invention, a “Hybrid Active Filter” is proposed which is a combination of a single-phase active filter connected to two-phase supply lines and a Neutral Current Compensator (NCC) for
Compensation of load current harmonics to reduce them below a certain limit while flowing in supply
Compensation of load dynamic reactive power to reduce its burden on the supply
Avoidance / elimination of the neutral current, carrying the unbalanced load current, flowing into the supply, specifically when the supply is Star connected and the linear and / or non-linear load is Star connected as well as it is unbalanced in a four wire system supplying power to a linear and / or non-linear load where neutral is “normally” connected to incoming supply Star point (supply Star connected).
This Hybrid Active Filter can also perform exactly same compensation, as mentioned above, for
Star connected supply supplying power to a Star connected load (three wire system)
Star connected supply supplying power to a Delta connected load (three wire system)
Delta connected supply supplying power to a Star connected load (three wire system)
Delta connected supply supplying power to a Delta connected load (three wire system) where connected load could be linear and / or non-linear.
Further, when there is three wire system, even if the load is unbalanced, the instantaneous summation of all the three-phase currents is always zero as a result of Kirchhoff’s Current Law (KCL) condition.
In the present invention, a two leg, single-phase IGBT active filter / inverter is used along with a NCC. The midpoint of the dc voltage bus of the inverter (which has two capacitor sections in series) is connected to the load side neutral and then connected to one terminal of the series connection of the NCC secondary side three windings.
Figure 3 explains the arrangement of the Hybrid Active Filter for a four wire system where the supply has been Star connected, load (linear and / or non-linear) has been Star connected and NCC has also been used for achieving the necessary compensation goals.
Consider the load as non-linear, with different active and reactive fundamental currents as well as current harmonics drawn by it. The three load currents can be defined as below.
iaL = iaLa + iaLr + ?iaLn
ibL = ibLa + ibLr + ?ibLn
icL = icLa + icLr + ?icLn
where subscript L represents load, subscript a after L represents active component, subscript after L represents reactive component, n represents harmonic number , and ? represents summation of current harmonic components of the load current. It is further rewritten as below.
iaL = iaLa + iaLz
ibL = ibLa + ibLz
icL = icLa + icLz
where iLr + ?iLn (reactive component and the summation of current harmonics) is combined as iLz.
The single-phase two leg IGBT inverter (connected between phases B and C) receives commands to draw
iaLz – ibLz from the B phase and iaLz – icLz from the C phase delivering output current from dc bus midpoint as
(iaLz – ibLz) + (iaLz – icLz) = 2*iaLz - ibLz – icLz
The dc bus midpoint has been connected to the load neutral. The load neutral current
iLn = iaL + ibL + icL
= (iaLa + ibLa + icLa) + (iaLz + ibLz + icLz)
When this current is added to the output current from the dc bus midpoint, the total current entering the NCC secondary side series connection will be
inLs = (iaLa + ibLa + icLa) + (iaLz + ibLz + icLz) + 2*iaLz - ibLz – icLz
= (iLa?) + 3*iaLz
This current circulates through the primary Star connected windings of the NCC and distributes itself as 1/3rd in each primary winding. The part 1/3rd of 3*iaLz, that means iaLz, gets delivered in phase A satisfying the load component iaLz and relieving the supply of sourcing this component. The same 1/3rd current also flows in B and C phases which satisfies iaLz part of the command and hence drawn current of iaLz – ibLz and iaLz – icLz for phases B and C. The other parts of the commands or drawn currents of – ibLz and – icLz compensate the part of the load current + ibLz and + icLz for the phases B and C respectively and relieves the supply from sourcing these components of the load currents.
Thus, the supply is completely relieved of sourcing the load current parts iaLz, ibLz and icLz. Further, the 1/3rd part of the current iLa? which also flows through primary side Star connected windings or each winding circulates through the load as explained under NCC (as part of the definitions and explanations of the terms used).
As a final outcome, the arrangement explained in fig. 3, the supply or source supplies only fundamental active currents whose instantaneous summation always remains equal to zero because of NCC. The current harmonic compensation, dynamic reactive power compensation, and avoidance of unbalanced neutral current flowing into the supply satisfy a solution requirement of the Hybrid Active Filter given under this section.
It should be noted that the single-phase, two leg, IGBT based inverter works with a definite Pulse Width Modulation (PWM) technique to achieve the necessary current control. Further, in fig. 3, the connection between Star point of the supply and Star point of the three primary windings of the NCC can be done directly or through earth. There is no current flowing between the two points due to the inherent configuration of the NCC.
The above details summarize basic operation and / or principle of operation of the proposed Hybrid Active Filter. This is validated in following sections.
Brief Description of the Drawings
The figures described below give necessary information in relation to the invention and form an integral part for understanding of the invention. The drawings are illustrative in nature and not drawn to scale.
Figures 1a and 1b shows typical three-phase three wire and four wire system;
Figure 2 shows the Neutral Current Compensator as used in a three-phase four wire system;
Figure 3 shows the proposed Hybrid Active Filter (single-phase active filter / inverter and NCC) with Star connected supply and load;
Figure 4 shows an equivalent circuit of Star connected supply and star connected unbalanced linear and non-linear load;
Figure 5(a) shows interconnection details of the Hybrid Active Filter;
Figure 5(b) shows detailed circuit of the Hybrid Active Filter and its connection details;
Figure 5(c) shows the detailed control circuit diagram used for generation of control pulses (PWM operation) for Hybrid Active Filter in accordance with the present invention;
Figure 6(a) shows MATLAB / Simulink model for Star connected supply and Star connected unbalanced non-linear load with the Hybrid Active Filter;
Figure 6(b) shows MATLAB / Simulink model for detailed circuit of the Hybrid Active Filter and its connection details;
Figure 6(c) shows MATLAB / Simulink model for the detailed control circuit diagram used for generation of control pulses for Hybrid Active Filter;
Figure 7(a) shows MATLAB/Simulink results for Case 1 which relates to three-phase diode bridge Load + RL load + single-phase diode bridge load on all three phases;
Figure 7(b) shows MATLAB / Simulink results for the phase A source current waveforms with harmonic analysis for Case 1 (three-phase diode bridge Load + RL load + single-phase diode bridge load on all three-phases);
Figure 7(c) shows MATLAB / Simulink results for the phase B source current waveforms with harmonic analysis for Case 1 (three-phase diode bridge Load + RL load + single-phase diode bridge load on all three-phases);
Figure 7(d) shows MATLAB / Simulink results for the phase C source current waveforms with harmonic analysis for Case 1 (three-phase diode bridge Load + RL load + single-phase diode bridge load on all three-phases);
Figure 8(a) shows MATLAB/Simulink results for Case 2 which relates to RL load + single-phase diode bridge load on all three phases;
Figure 8(b) shows MATLAB / Simulink results for the phase A source current waveforms with harmonic analysis for Case 2 (RL load + single-phase diode bridge load on all three-phases);
Figure 8(c) shows MATLAB / Simulink results for the phase B source current waveforms with harmonic analysis for Case 2 (RL load + single-phase diode bridge load on all three-phases);
Figure 8(d) shows MATLAB / Simulink results for the phase C source current waveforms with harmonic analysis for Case 2 (RL load + single-phase diode bridge load on all three-phases);
Figure 9(a) shows MATLAB/Simulink results for Case 3 which relates to A phase: RL load; + B phase : RL load + single-phase diode bridge load; C phase : RL load + single-phase diode bridge load;
Figure 9(b) shows MATLAB / Simulink results for the phase A source current waveforms with harmonic analysis for Case 3 (A phase: RL load; + B phase: RL load + single-phase diode bridge load; C phase: RL load + single-phase diode bridge load);
Figure 9(c) shows MATLAB / Simulink results for shows the phase B source current waveforms with harmonic analysis for Case 3 (A phase: RL load; + B phase: RL load + single-phase diode bridge load; C phase: RL load + single-phase diode bridge load);
Figure 9(d) shows MATLAB / Simulink results for shows the phase C source current waveforms with harmonic analysis for Case 3 (A phase: RL load; + B phase: RL load + single-phase diode bridge load; C phase: RL load + single-phase diode bridge load);
Figure 10(a) shows MATLAB/Simulink results for Case 4 which relates to A phase : RL Load; + B phase : RL load; C phase : RL load + single-phase diode bridge load;
Figure 10(b) shows MATLAB / Simulink results for the phase A source current waveforms with harmonic analysis for Case 4 (A phase: RL Load; + B phase: RL load; C phase: RL load + single-phase diode bridge load);
Figure 10(c) shows MATLAB / Simulink results for shows the phase B source current waveforms with harmonic analysis for Case 4 (A phase: RL Load; + B phase: RL load; C phase: RL load + single-phase diode bridge load);
Figure 10(d) shows MATLAB / Simulink results for shows the phase C source current waveforms with harmonic analysis for Case 4 (A phase: RL Load; + B phase: RL load; C phase: RL load + single-phase diode bridge load);
Figure 11(a) shows MATLAB/Simulink results for Case 5 which relates to A phase: No Load; + B phase: No Load; C phase: single-phase diode bridge load;
Figure 11(b) shows MATLAB / Simulink results for the phase A source current waveforms with harmonic analysis for Case 5 (A phase: No Load; + B phase: No Load; C phase: single-phase diode bridge load);
Figure 11(c) shows MATLAB / Simulink results for shows the phase B source current waveforms with harmonic analysis for Case 5 (A phase: No Load; + B phase: No Load; C phase: single-phase diode bridge load);
Figure 11(d) shows MATLAB / Simulink results for shows the phase C source current waveforms with harmonic analysis for Case 5 (A phase: No Load; + B phase: No Load; C phase: single-phase diode bridge load);
Figure 12 shows a circuit of an experimental set up having star connected supply; and
Figure 13 shows recorded waveforms for the experimental set up shown in figure 12.

Detailed Description of the Invention

Definitions and explanations of terms used

As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below.

Power Inverter: The expression inverter used in this specification is deemed to mean a converter which converts AC power into DC power.
Self-commutated power devices: The power converter uses a set of self-commutated power devices while converting the power from AC to DC or DC to AC. The expression self-commutated power devices relates to devices such as MOSFET (Metal Oxide Semiconductor Field Effect Transistor), MCT (MOS Controlled Thyristor), GTO (Gate Turn-off Thyristor), IGBT (Insulated Gate Bi-polar Transistor), IGCT (Integrated Gate Commutated Thyristor), and IEGT (Injection Enhanced Gate Transistor).
Normal Active Filter: The term active filter relates to an DC to AC power inverter (single-phase with two IGBT legs or three –phase with three IGBT legs) which allows compensation of load current harmonic components, but can also perform for compensation of (i) fundamental reactive power drawn by the loads, and (ii) unbalanced load current (also normally referred as Zero Sequence Current) that flows through neutral cable or a bus bar connected to supply neutral / star point, when the load is unbalanced, using a three leg / six self-commutated active devices, when connected in parallel with the linear or non-linear unbalanced. This is when the active filter operates in parallel with Star connected linear or non-linear, unbalanced load. However, the term active filter is also extendable to Delta connected linear or non-linear, unbalanced load. This normal active filter has midpoints of the legs connected to incoming three-phase lines or phases through boost inductors and the floating dc has dc capacitors. When neutral unbalance current compensation is required, the midpoint of dc capacitors (two capacitors in series across the dc bus) is connected to the load neutral. This normal active filter is controlled by its control electronics / control / block / control scheme generating a required Pulse Width Modulation for control of the compensating currents described above. Important aspect is that this normal active filter uses three leg construction and hence six self-commutated devices.
Hybrid active filter: The term hybrid active filter relates to a combination of an active filter along with a passive element such as a transformer while performing the same or similar function as that of the active filter. In this particular invention, a single-phase active filter or a single phase inverter makes use of a Neutral Current Compensator (NCC), which is made up of three single-phase transformers. This single-phase active filter or the single phase inverter uses only two leg construction and hence four self-commutated devices and the NCC to achieve compensation of load current harmonic components, (ii) fundamental reactive power drawn by the loads, and (iii) unbalanced load current (also normally referred as Zero Sequence Current) that flows through neutral cable or a bus bar connected to supply neutral / star point, when the load is unbalanced.
Three and four wire system: The supply available for a three-phase load could be three wire or four wire. When it is a three wire supply, the neutral which is the fourth wire is missing and is not available for the load connection. Thus, in this case, the load can be Delta connected or Star connected without neutral. In a four wire supply the neutral from supply is available for the load. Thus, in this case, the load can be Delta connected without using the neutral connection available or the load can be Star connected connecting the supply neutral (fourth wire) to the Star point of the load or through earth. Figure 1(a) and (b) explain the three and four wire system described here.
Neutral Current Compensator [1]: Figure 2 explains this particular compensator. This is applicable when it is a four wire system with neutral conducting the unbalanced portion of the load current. The load can be linear and / or non-linear. The NCC consists of three single-phase transformers connected as shown in fig. 2. The transformers have 3:1 winding ratio. The load neutral is connected to one end of the series connection of the secondary windings. The other end of the secondary series connection is returned to the Star point of the primary connection. The neutral current flowing through the series secondary connection is reflected as 1/3rd current in each primary and gets circulated in load. Thus, the Star point connection between primary side star windings and the supply side star winding does not carry any current. This means the supply does not carry any neutral current. Thus, the supply can also be Delta connected which can supply power to a star connected unbalanced load. Such a construction is not possible with Delta connected supply and Star connected unbalanced load unless NCC is used. This principle of NCC, as explained, has been made use of for the hybrid active filter detailed here.
Displacement factor or Fundamental power factor (PF or pf), current harmonics, current distortion, Distortion Factor (DF), True Power Factor (TPF), Total Current Distortion (ITHD), and Total Voltage Distortion (VTHD): These are known terminologies for electrical engineering. However, these are defined as and when they appear first time in this specification. However, to an extent these are defined here.
Displacement factor or Fundamental Power Factor (PF or pf): It is the cosine of the angle between fundamental voltage (more specifically phase to neutral supply voltage) and the corresponding fundamental phase current.
Current harmonics: Currents related to frequencies which are multiple of fundamental frequency (in this case 50 Hz). Normally these frequencies are odd integral multiples of fundamental frequency (such as 3,5,7,9, …… and so on) in AC supply systems. However, there are loads like arc furnace which can produce non-integral multiples of fundamental frequency.
Current Distortion: The fundamental current in an AC system is supposed to be sinusoidal. However, when current harmonics are drawn by the load, the current waveform deviates from its sinusoidal nature and is then referred as distorted current. The current distortion is due to superimposition of current harmonics on the fundamental current drawn by the load.
Distortion factor (DF): The ratio of RMS value of fundamental current and the RMS value of the distorted current is called as Distortion Factor. Thus it is the ratio I1/I.
True Power Factor (TPF): It is the multiplication of Displacement factor or Fundamental power factor (PF or pf) and the Distortion Factor (DF).
Total Current Distortion (ITHD): It is the ratio as expressed below.
ITHD = (v? In2)/I1 Equation (1)
Where In means RMS value of the nth current harmonic and I1 indicates RMS value of the fundamental current. Note here that n is ? 1 but varies from 2 to 8.
Total Voltage Distortion (VTHD): It is the ratio as expressed below.
ITHD = (v? Vn2)/V1 Equation (2)
Where Vn means RMS value of the nth harmonic voltage and V1 indicates RMS value of the fundamental voltage. Note here that n is ? 1 but varies from 2 to 8.

For a thorough understanding of the present invention, reference is to be made to the following detailed description, including the claims, in connection with the above described drawings. Although the present invention is described in connection with exemplary embodiments, the present invention is not intended to be limited to the specific forms set forth herein. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but these are intended to cover the application or implementation without departing from the scope of the claims of the present invention. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.
The terms, such as “First” and “Second” do not denote any order, elevation or importance, but rather are used to distinguish placement of one element over another, and the terms such as “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms.
Referring to figures 1-3, the present invention provides Hybrid Active Filter (herein after referred as HAF 100) for three- phase, Star connected loads, in accordance with the present invention. Further, the HAF 100 can be used for various types of loads, such as unbalanced linear and / or non-linear loads drawing current harmonics from the supply, variable and dynamic power factor loads or loads drawing variable and dynamic reactive power from supply, and avoiding flow of any neutral current back in the supply. The HAF 100 is capable of compensating for the current harmonics, compensating the reactive power thus improving the power factor of the supply current, and avoiding the flow of neutral current carrying the unbalanced current of the load back into the supply by circulating the neutral current back in the system. HAF 100, hence, eliminates the dependency of neutral connection from the source side having Star connection. This makes the HAF 100 to be used with all possible three and four wire supply – load systems, as described below.
Supply is star connected, load is unbalanced and Star connected
Supply is star connected, load is unbalanced and delta connected
Supply is star connected, load is unbalanced and star connected (but neutral connection not returned to supply neutral point, directly or through ground / earth)
Supply is delta connected, load is unbalanced and delta connected
Supply is delta connected, load is unbalanced and star connected
The HAF 100 incorporates both the passive and active components thereby making it suitable for reliable use in both the industrial as well as power distribution side. It is a comparatively robust and reliable solution for power quality improvement as the device (IGBT) count is reduced compared to the presently available Active Filters. As covered under definition, the active component here is the single-phase active filter or the single-phase inverter which uses two legs consisting four self-commutated devices (in this case IGBTs) as against three legs consisting six self-commutated devices as used in a normal three-phase active filter. The passive component is a Neutral Current Compensator 120.
The HAF 100 is a combination of a single-phase active filter connected two-phase supply lines and a Neutral Current Compensator (NCC) 120 for
Compensation of load current harmonics to reduce them below a certain limit while flowing in supply
Compensation of load dynamic reactive power to reduce its burden on the supply
Avoidance / elimination of the neutral current, carrying the unbalanced load current 40, flowing into the supply, specifically when the supply is Star connected and the linear and / or non-linear load is Star connected as well as it is unbalanced in a four wire system supplying power to a linear and / or non-linear load where neutral is “normally” connected to incoming supply Star point (supply Star connected).
This Hybrid Active Filter 100 can also perform exactly same compensation, as mentioned above, for
Star connected supply supplying power to a Star connected load (three wire system)
Star connected supply supplying power to a Delta connected load (three wire system)
Delta connected supply supplying power to a Star connected load (three wire system)
Delta connected supply supplying power to a Delta connected load (three wire system)
where connected load could be linear and / or non-linear.
Further, when there is three wire system, even if the load is unbalanced, the instantaneous summation of all the three-phase currents is always zero as a result of Kirchhoff’s Current Law (KCL) condition.
In the present invention, a two leg, single-phase inverter 110 is used along with the NCC 120. The single-phase inverter 110 is having two leg IGBT bridge structure, with each leg 62, 64 connected to a common dc voltage bus which has energy storage capacitor in series. The midpoint of the dc voltage bus of the single-phase inverter 110 (which has two capacitor sections in series) is connected to the load side neutral and then connected to one terminal of the series connection of the NCC 120 secondary side three windings. The NCC 120 is coupled adjacent to the single-phase inverter 110 for compensating current harmonics, dynamic reactive power and unbalanced load current 40 flowing in load neutral.
Figure 3 explains the arrangement of the HAF 100 for a four wire system where the supply has been Star connected, load (linear and / or non-linear) has been Star connected and NCC 120 has also been used for achieving the necessary compensation goals.
Consider the load as non-linear, with different active and reactive fundamental currents as well as current harmonics drawn by it. The three load currents can be defined as below.
iaL = iaLa + iaLr + ?iaLn Equation (3)
ibL = ibLa + ibLr + ?ibLn Equation (4)
icL = icLa + icLr + ?icLn Equation (5)
where subscript L represents load, subscript a after L represents active component, subscript after L represents reactive component, n represents harmonic number , and ? represents summation of current harmonic components of the load current. It is further rewritten as below.
iaL = iaLa + iaLz Equation (6)
ibL = ibLa + ibLz Equation (7)
icL = icLa + icLz Equation (8)
where iLr + ?iLn (reactive component and the summation of current harmonics) is combined as iLz.
The single-phase inverter 110 with two leg IGBT 62, 64 (connected between phases B and C) receives commands to draw
iaLz – ibLz from the B phase and iaLz – icLz from the C phase delivering output current as
(iaLz – ibLz) + (iaLz – icLz) = 2*iaLz - ibLz – icLz Equation (9)
The dc bus midpoint has been connected to the load neutral. The load neutral current
iLn
= iaL + ibL + icL
= (iaLa + ibLa + icLa) + (iaLz + ibLz + icLz) Equation (10)
When this current is added to the output current from the dc bus midpoint as per equation (9), the total current entering the NCC 120 secondary side series connection will be
inLs = (iaLa + ibLa + icLa) + (iaLz + ibLz + icLz) + 2*iaLz - ibLz – icLz
= (iLa?) + 3*iaLz Equation (11)
This current circulates through the primary Star connected windings of the NCC 120 and distributes itself as 1/3rd in each primary winding. The part 1/3rd of 3*iaLz, that means iaLz, gets delivered in phase A satisfying the load component iaLz and relieving the supply of sourcing this component. The same 1/3rd current also flows in B and C phases which satisfies iaLz part of the command and hence drawn current of iaLz – ibLz and iaLz – icLz for phases B and C. The other parts of the commands or drawn currents of – ibLz and – icLz compensate the part of the load current + ibLz and + icLz for the phases B and C respectively and relieves the supply from sourcing these components of the load currents.
Thus, the supply is completely relieved of sourcing the load current parts iaLz, ibLz and icLz. Further, the 1/3rd part of the current iLa? which also flows through primary side Star connected windings or each winding circulates through the load as explained under NCC 120 (as part of the definitions and explanations of the terms used).
As a final outcome, the arrangement explained in fig. 3, the supply or source supplies only fundamental active currents whose instantaneous summation always remains equal to zero because of the NCC 120. The current harmonic compensation, dynamic reactive power compensation, and avoidance of unbalanced neutral current flowing into the supply satisfy a solution requirement of the HAF 100 given under this section.
It should be noted that the single-phase, two leg, IGBT based inverter works with a definite Pulse Width Modulation (PWM) technique to achieve the necessary current control. Further, in fig. 3, the connection between Star point of the supply and Star point of the three primary windings of the NCC 120 can be done directly or through earth. There is no current flowing between the two points due to the inherent configuration of the NCC 120.
Figure 4 shows a circuit diagram having Start connected supply and the unbalanced linear and non-linear load is also star connected (normally called as four wire system). Further, a load neutral is directly connected to supply neutral point and or load neutrals. Furthermore, in this arrangement the neutral connection in1 carries the unbalanced load current 40. The supply neutral point and the load neutrals may or may not be earthed (taken to earth). However, for safety and protection of power system, usually the Star point of the supply is earthed.
As shown in figure 5(a), the HAF 100 is connected to the system of fig. 4 having Star connected supply with Star connected unbalanced linear and non-linear load 40. This arrangement eliminates the neutral current flow back into the supply neutral point based on use of the NCC 120. This will be clear based on fig. 5(b). Further, the HAF 100 works with the unbalanced linear and non-linear load 40 where different load cases are considered for performance evaluation of the HAF 100.
Case 1 Three-phase diode bridge load + RL load + single-phase diode bridge load on all three-phases
Case 2 RL load + single-phase diode bridge load on all three-phases
Case 3 A phase: RL load; + B phase: RL load + single-phase diode bridge load; C phase: RL load + single-phase diode bridge load
Case 4 A phase: RL Load; + B phase: RL load; C phase: RL load + single-phase diode bridge load
Case 5 A phase: No Load; + B phase: No Load; C phase: single-phase diode bridge load
The supply side neutral, however, can remain earthed so that the Star point of the supply remains firmly held at earth potential.
The HAF 100 requires self-commutated power device (MOSFET, IGBT etc. and IGBTs in the present invention described) in coordination with NCC for its operation. The response of the HAF 100 is also instantaneous. The arrangement is shown in fig 5(b).
Figure 5(b) shows detailed circuit diagram of the HAF 100 in accordance with the present invention. The HAF 100 consists of single-phase inverter 110, formed with two leg IGBT bridge structure, each leg, 62 and 64, consists of series connected two IGBTs across the dc bus, with an energy storage capacitor being formed by two series connected DC dry film capacitors 66 to configure together as single phase inverter 110. The dc capacitors 66 are necessary for the single phase inverter 110 operation and also provide almost ripple free and stable dc voltage while helping the necessary energy storage. Further, as shown in fig. 5(b), the two middle terminals of the inverter IGBT legs 62 & 64 are connected to incoming two phases using series connected (normally called as boost inductors) and the midpoint of the dc capacitors 66 is returned to the third phase (in this cased phase A). The other end of each inductor (number) is returned to supply line or phase (in this case phase B and phase C). Also, it contains three single-phase transformers with three primary winding 10a, 12a and 14a as shown and three secondary windings 10b, 12b and 14b. Each of the primary windings 10a, 12a and 14a is magnetically coupled with the respective secondary winding 10b, 12b and 14b to configure three single-phase transformers 10, 12 and 14 bundled as unbalance current compensation circuit 50. This transformer arrangement is the Neutral Current Compensator 120.
Further, the unbalanced current compensation circuit 50, is coupled as close as possible with single phase inverter 110. One end of each of the primary windings 10a, 12a and 14a is connected to respective phase of a three-phase 30a, 30b and 30c supply. For the purpose of explanation only, the primary winding 10a is connected to a phase 30a and the primary winding 12a is connected to a phase 30b and to one leg of inverter 60a, and the primary winding 14a is connected to a phase 30c and to one leg of inverter 60b.
Further, the secondary windings 10b, 12b, and 14b of the respective transformer 10, 12 and 14 are connected with each other in series. One end of the secondary windings 10b, 12b, and 14b connected in series is connected with the neutral load connection of the unbalance load 40 and the mid-point of the series connected AC capacitor 60c also coupled at same point 30d. Other end of the secondary windings 10b, 12b, and 14b in series is connected to another common end of the primary windings 10a, 12a, and 14a with star connection. Specifically, the primary windings 10a, 12a, and 14a and the secondary windings 10b, 12b, and 14b ratio for each of the transformer 10, 12, and 14 is 3:1 (for example, if there are 240 Volts on the primary winding there will be 80 Volts on the secondary winding). Further, a common connection from the primary windings 10a, 12a, and 14a and the secondary windings 10b, 12b, and 14b is connected to supply neutral point or to the earth. This is how the NCC 120 is integrated with the two leg IGBT based single phase inverter 110.
Due to internal coupled connection, the HAF 100, it is assured that the theoretically that the voltage across secondary series connection of unbalance current compensator (NCC) windings 50 is close to zero Volts as the instantaneous sum of three-phase balanced voltages is always zero volt. This is reflected as 1/3 identical (value / magnitude and phase) current in the primary three-phase windings 10a, 12a and 14a. The current circulates through the unbalanced load 40 and is then returned through the load neutral completing the loop as well as adhering to the famous Kirchhoff’s Current Law (KCL). The Active filter (part of HAF) is the single-phase inverter 110 which is widely recognized in literature as single-phase Pulse Width Modulated, current controlled, Voltage Source Inverter (VSI). The Pulse Width Modulation employed in this invention, however, is specific as this pulse width modulation decides the currents drawn from the supply lines or the phases by the single-phase inverter 110. The dc bus voltage in this single-phase inverter 110 is kept suitably higher than peak of the line to line or phase to phase voltage as is necessary for its operation. The IGBT legs 62 and 64 in addition with capacitor arm 66 compensates for the harmonics and the reactive currents whereas the differential unbalance phase currents circulate through secondary windings 10b, 12b, and 14b. The control block 80 in fig. 5(b) is detailed as control scheme in fig 5(c) which generates the pulses for driving the IGBT legs 62 and 64.
The single-phase inverter 110 receives compensating current commands for current to be drawn from the supply lines or phases, connected to them and the load and corresponding to necessary load current(s) harmonic compensation(s) and also corresponding to compensation of reactive part of the load current(s) for the those two supply lines or phases and where the midpoint of the two capacitors across the dc bus generates summation of these two line or phase currents. Pulse Width Modulation of the single-phase inverter 110 is based on achieving the above and is controlled by the control block 80 or control scheme.
The NCC 120 and the single-phase inverter 110 together produce compensating currents for load current harmonics and also the reactive part of the load currents and allow only active current part of the load currents to be drawn from the three–phase supply lines.
The control block 80 in fig. 5(b) and hence the control scheme in fig. 5(c) achieves it exactly.
The control block 80, is specially designed so as to achieve reduction in IGBT legs 62 and 64, consisting of series connection active devices for the single-phase inverter 110. The control technique can be based on the input source current sensing or else the load current sensing. The document presented here is focused on the load current sensing for the ease but it is not limited to the use of load currents only. Same can be implemented with source current sensing. The output of the control block 80, is generation of the gate pulses; g1, g2, g3 & g4 for driving the IGBTS S1, S2, S3 and S4.
Referring now to figure 5(c), the control block 80 gets inputs from load currents, source voltages and for generating the intended output by undergoing various mathematical operations. Initially, the three input voltages are sensed and processed by a phase lock loop 80(a), to track the phase angle and the track the frequency of the input fundamental voltages. The tracked phase angle of the reference voltage is fed as input to calculate the sine and cosine tables 80(b).
The three-phase load currents are sensed, amplified and converted into two phase components by using alpha-beta transformation matrix 80(c) as mentioned below.
(Io(a) Io(ß) Io(0) )=(2/3 -1/3 -1/3 0 1/v3 -1/v3 1/3 1/3 1/3 )(Io(a) Io(b) Io(c) )
Further the two phase stationary frame components are converted into rotating frame components using Park’s transformation 80(d) as mentioned in equation below using the matrix multiplication of 80(b) and 80(c)
(Io(d) Io(q) Io(0) )=(?cos cos (wt) ?sin sin (wt) 0 -?sin sin (wt) ?cos cos (wt) 0 0 0 1 )(Io(a) Io(ß) Io(0) )
Using the Park Transformation, Id and Iq along with zero sequence components are obtained. Here Id is responsible for the active component or direct component of the load currents whereas Iq is responsible for the reactive contribution or quadrature component of the load currents. The Id has two components; one is active (DC) component and other is Harmonic (AC) component. Here as it is intended to compensate for the harmonic currents, the (AC) part of the Id is extracted using filtering techniques.
Io(d)=Io(d)ac+ Io(d)dc
Out of these 2 components, the interest is in only harmonic component. Thus, extracting only ac component using high pass filter 80(e).
From 80(e), a new equation comprising of Io (dac; q; 0) is formed in matrix form. This new 3 X 1 comprised of the ac component of Id; complete part of the reactive component Iq and the zero-sequence component of I0.
[Io(d)ac Io(q) Io(0) ]= I(dq0)
The newly formed matrix is used to calculate the compensating currents in two phase stationary reference frame using Inverse Park’s Transformation 80(f).
(Ic(a) Ic(ß) Ic(0) )=(?cos cos (wt) -?sin sin (wt) 0 ?sin sin (wt) ?cos cos (wt) 0 0 0 1 )(Io(d) Io(q) Io(0) )
Three-phase compensating currents are calculated back from the two-phase stationary reference frame components using inverse Clarke’s Transformation 80(g) as shown in equation below.
(Ioc(a) Ioc(b) Ioc(c) )=(1 0 1 -1/2 v3/2 1 -1/2 -v3/2 1 )(Io(a) Io(ß) Io(0) )
This is the most important part and a novel concept of calculating the final compensation signal for B and C phase. The final compensating currents are calculated from components of compensating currents obtained from 80(g). The final compensating currents 80(h) are calculated as shown in equations below.
icb= -Ioc(a)+Ioc(b)
icc= -Ioc(a)+Ioc(c)
The active component, of the compensating current signal, drawn by HAF is obtained by subtracting the actual Vdc voltage across the AC capacitor Arm 66; and the reference value set for the intended Vdc 80(i). This error is amplified in proportion to the reference 80(j) and then multiplied by the unit source voltage vectors 80(j) and 80(k). The final compensating signal are compared with the feedback signals 80(m) and 80(n) in order to make the two Leg single-phase inverter 110 generate the required compensating currents as shown in equations below. In these equations, the active current taken by the single-phase inverter 110 is added to the final compensating currents as shown in equations below.
Error_b= -(-Icb+?If?_(dc_b) )+?If?_actual (b)
Error_c= -(-Icc+?If?_(dc_c) )+?If?_actual (c)
The compensating signals are once again amplified 80(o) and 80(p) in proportion to the actual filter currents so that the HAF generates the currents with reference to the compensating currents generated by the proposed algorithm 80. Finally, the reference compensating currents are compared with the high frequency triangular wave 80(q) to generate the final gate pulses g1, g2, g3 and g4.
The single-phase Active Filter / the single-phase inverter 110 (part of Hybrid Active Filter) draws a small in-phase current from its supply lines or phases (in this case from phases B and C) which is called as the loss component of the currents drawn by the single-phase Active Filter / the single-phase inverter 110. This corresponds to the power loss in IGBTs, dc bus capacitors and other losses such as in connecting cables or bus bars. The loss component obviously varies based on the input currents drawn by the single-phase Active Filter / the single-phase inverter 110, though its magnitude may be small. This in-phase current, however, is necessary for the proper operation of the single-phase Active Filter / the single-phase inverter 110 and due to which the dc bus voltage is maintained constant
All above as functional control scheme is achieved by what is called as control algorithm in case of a digital controller embedded in its digital codes (as in case of present invention). The same can be done or implemented using analog circuits though it becomes complex and bulky.
Most important aspect to be noted is that the control scheme (which can be implemented by an analog digital controller or a hybrid controller) switches the IGBTs in the single-phase Active Filter / the single-phase inverter 110 (part of HAF) shown in fig. 5(b) in such a way that the dc bus voltage across the dc capacitors 66 is reflected as an AC modulated voltage at the midpoint of the two IGBT legs 62, 64 and it controls the two input currents and output current at dc capacitor 66 midpoint of the single-phase Active Filter / the single-phase inverter 110 (part of HAF). This along with NCC 120 allows the HAF 100 to achieve necessary requirements, that is
Compensation of load current harmonics to reduce them below a certain limit while flowing in supply
Compensation of load dynamic reactive power to reduce its burden on the supply
Avoidance of the neutral current, carrying the unbalanced part of current harmonics in load currents, reactive and active part of the load currents at fundamental supply frequency , flowing into the supply star point or supply neutral
Further, at least one isolator switch with semiconductor fuses may be connected to the HAF 100 for protection against overload protection of the single phase inverter 110. This isolator is connected at input of HAF 100 for switching “ON” and “Off” the HAF 100. In an embodiment, the isolator can be replaced by a MCCB or an ACB (air circuit breaker), which may be obvious to a person skilled in the art.

Modeling and Simulation Results (MSR)

Figure 6(a) shows MATLAB/Simulink model for star connected supply and star connected unbalanced, non-linear, and reactive power load with the HAF 100 in accordance with the present invention. The supply considered here is three-phase, 50 Hz, 415 V. The supply can be either star connected, or delta connected delivering a 300 kVA load but not limited to this load rating delivery.

The Modeling and Simulation Results (MSR) are presented here for following cases as in Table -1

Case No. Load Conditions
Phase A Phase B Phase B
1 3-ph Diode Bridge NL load + Balance RL load + single-phase diode bridge load: Load: 100 kW 3-ph Diode Bridge NL load + Balance RL load + single-phase diode bridge load: Load:100 kW 3-ph Diode Bridge NL load + Balance RL load + single -phase diode bridge load: Load: 100 kW
2 Balance RL load + single-phase diode bridge load: Load: 65 kW Balance RL load + single -phase diode bridge load: Load: 65 kW Balance RL load + single-phase diode bridge load: Load: 65 kW
3 Balance RL load Balance RL load + single -phase diode bridge load Balance RL load + single-phase diode bridge load
4 Balance RL load Balance RL load Balance RL load + single-phase diode bridge load
5 No load No load Single-phase diode bridge load
Table -1

The modeling and simulation results (MSR) are presented here for following cases.
Case 1
Star connected supply, Star connected load (as described in table above) and HAF 100 are connected as explained in fig. 5(a). The MATLAB / Simulink model is given in figs. 6(a), (b) and (c) and the simulation results are given fig. 7(a).
Specifically, fig. 7(a) shows:
Ch1: Three-Phase Input Voltages Ch4: Three-Phase HAF currents
Ch2: Three-Phase Input Currents Ch5: DC bus voltages
Ch3: Three-Phase Load Currents Ch6: Input Neutral Current

Figure 7(b) shows the phase A source current waveforms with harmonic analysis
Figure 7(c) shows the phase B source current waveforms with harmonic analysis
Figure 7(d) shows the phase C source current waveforms with harmonic analysis
Case 2
Star connected supply, star connected load (as described in table above) and HAF 100 are connected as explained in fig. 5(a). The MATLAB / Simulink model is given in figs. 6(a), (b) and (c) and the simulation results are given fig. 8(a).
Specifically, fig. 8(a) shows:

Ch1: Three-Phase Input Voltages Ch4: Three-Phase HAF currents
Ch2: Three-Phase Input Currents Ch5: DC bus voltages
Ch3: Three-Phase Load Currents Ch6: Input Neutral Current

Figure 8(b) shows the phase A source current waveforms with harmonic analysis
Figure 8(c) shows the phase B source current waveforms with harmonic analysis
Figure 8(d) shows the phase C source current waveforms with harmonic analysis
Case 3
Star connected supply, star connected load (as described in table above) and HAF 100 are connected as explained in fig. 5(a). The MATLAB / Simulink model is given in figs. 6(a), (b) and (c) and the simulation results are given figure 9(a).
Specifically, fig. 9(a) shows:
Ch1: Three-Phase Input Voltages Ch4: Three-Phase HAF currents
Ch2: Three-Phase Input Currents Ch5: DC bus voltages
Ch3: Three-Phase Load Currents Ch6: Input Neutral Current
Figure 9(b) shows the phase A source current waveforms with harmonic analysis
Figure 9(c) shows the phase B source current waveforms with harmonic analysis
Figure 9(d) shows the phase C source current waveforms with harmonic analysis
Case 4
Star connected supply, star connected load (as described in table above) and HAF 100 are connected as explained in fig. 5(a). The MATLAB / Simulink model is given in figs. 6(a), (b) and (c) and the simulation results are given figure 10(a).
Specifically, fig. 10(a) shows:
Ch1: Three-Phase Input Voltages Ch4: Three-Phase HAF currents
Ch2: Three-Phase Input Currents Ch5: DC bus voltages
Ch3: Three-Phase Load Currents Ch6: Input Neutral Current

Figure 10(b) shows the phase A source current waveforms with harmonic analysis
Figure 10(c) shows the phase B source current waveforms with harmonic analysis
Figure 10(d) shows the phase C source current waveforms with harmonic analysis
Case 5
Star connected supply, star connected load (as described in table above) and HAF 100 are connected as explained in fig. 5(a). The MATLAB / Simulink model is given in figs. 6(a), (b) and (c) and the simulation results are given fig. 11(a).
Specifically, fig. 11(a) shows:

Ch1: Three-Phase Input Voltages Ch4: Three-Phase HAF currents
Ch2: Three-Phase Input Currents Ch5: DC bus voltages
Ch3: Three-Phase Load Currents Ch6: Input Neutral Current
Figure 11(b) shows the phase A source current waveforms with harmonic analysis
Figure 11(c) shows the phase B source current waveforms with harmonic analysis
Figure 11(d) shows the phase C source current waveforms with harmonic analysis
From these results it is clear that there is reduction in harmonic content in the source current proving that the first claim of the current harmonic compensation by the HAF 100 is fulfilled. Table -2 given below also confirms the same where it shows that even when only one phase is loaded with 62.17% ITHD, the supply current ITHD is between 14 to 15.76% which quite low.
The HAF 100 also helps to improve the power factor close to unity at the source side; which is quite evident from the results. Hence the claim of the power factor improvement also holds true.
There is also “no” or “zero” neutral current returned to supply neutral point for all the cases stated earlier and explained in conjunction with fig. 5 (a). Hence the claim related to avoidance / elimination of neutral current flowing into supply star point also holds true.

Sr No. Load Condition Source Current THD% (With HAF) Source Current THD% (Without HAF)
A-Phase B-Phase C-Phase Transition Time I(A-phase) I(B-phase) I(C-phase) I(A-phase) I(B-phase) I(C-phase)
Case 1 3? Diode Bridge+RL Load+1? Diode Bridge 3? Diode Bridge+RL Load+1? Diode Bridge 3? Diode Bridge+RL Load+1? Diode Bridge t= 3 Sec 4.00 4.30 4.41 26.88 26.88 27.00
Case 2 RL Load+1? Diode Bridge RL Load+1? Diode Bridge RL Load+1? Diode Bridge t= 3.3 Sec 1.96 1.93 2.23 38.72 38.72 38.93
Case 3 RL Load RL Load+1? Diode Bridge RL Load+1? Diode Bridge t= 3.5 Sec 4.49 3.93 4.38 0.10 37.06 38.93
Case 4 RL Load RL Load RL Load+1? Diode Bridge t= 3.7 Sec 6.08 6.77 6.50 0.01 0.10 38.93
Case 5 No Load No Load 1? Diode Bridge t= 3.9 Sec 15.76 14.05 14.36 1.22 2.58 62.17
Table -2
Experimental Results

Figure 12 shows the experimental set up. The experimental set up consists of the HAF 100 working with a real time unbalanced non-linear load supplied through 11kV / 415 V, Delta-Star, 2 MVA transformer. The supply is star connected and the unbalanced non-linear load is also star connected (similar to system shown in fig. 5(a)). For the sake of brevity, the experimental set up shown in figure 12, which is similar to the system shown and described along with fig. 5(a), is not explained in detail. The system shown in fig. 12 further includes first an isolator with semiconductor fuse connected before the HAF 100. This isolator is used for switching ON and OFF and the semiconductor fuses are used for protection of the HAF 100. There are individual isolators connected before each type of load for switching ON and OFF of these loads to create various conditions. The isolator breaker can be replaced by a MCCB or an ACB (air circuit breaker), which may be obvious to a person skilled in the art.
Figure 13 shows the recorded waveforms for the three supply currents, and the load current for one phase. From the waveforms it is clear that the current harmonic components have been compensated along with the unbalance and reactive current compensation.
Specifically, fig. 13 shows:
Ch1: Phase “A” supply current
Ch2: Phase “B” supply current
Ch3: Phase “C” supply current
Ch4: Phase “A” load current
From the modeling and simulation results and the experimental results illustrated and described above, it is clear that the proposed HAF 100 works satisfactorily for the different cases of Star connected supply feeding Star connected load (linear and / or non-linear). It meets the objectives as mentioned under “objects of the invention”.
It is also clear that apart from the four wire system where Star connected supply feeds Star connected unbalanced linear and / or non-linear load, the HAF can satisfactorily work with a three-phase “three” wire supply system (where supply is Star or Delta connected and linear and / or non-linear load is either Star connected or Delta connected) as given below.
Supply is delta connected, load is unbalanced and delta connected
Supply is delta connected, load is unbalanced and star connected
Supply is star connected, load is unbalanced and delta connected
Supply is star connected, load is unbalanced and star connected (but neutral connection not returned to supply neutral point, directly or through earth)
Therefore, the present invention has an advantage of providing HAF 100 which considerably compensates or attenuates the current harmonic components of the non-linear loads flowing in the supply and hence reduces the impact of current harmonics that are generated by the non-linear loads, on the supply system. The results are found to be within limits of IEEE 519 (2014) standard which is a widely accepted harmonic standard. The considerable reduction of the current harmonics helps reducing heating of transformers, cables and also avoids the malfunctioning of sensitive loads.
The HAF 100 helps to improve the input power factor on the source side by compensating the reactive components generated by the leading or lagging loads (which improves the optimum use of supply capacity),
The HAF 100 helps to work the system with no or poor source neutral conditions, thereby improving the overall system performance and operational reliability. It reduces the neutral losses and dependence on neutral, especially when the supply is Star connected and for three-phase star connected loads with neutral connected to supply star point.
The HAF 100 has an added advantage of reduction of active devices along with the passive components; which makes is more reliable and robust solution compared to the present day solutions available for satisfying same objectives defined earlier.
The HAF 100 is more economical in construction in compared to any active device based solution provided at present due to reduction of active devices, improved reliability while meeting all combined objectives defined earlier.
The HAF 100 rating depends upon system or supply voltage and maximum amount of the currents to be compensated and can always be suitably designed for given supply and load conditions.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the scope of the claims of the present invention.