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Cancellation of Current Harmonics in Three Phase Four Wire System using Smart Filter
Miss.Roshani D Chaudhari PG Student
Electrical Engineering Dept SSGBCOET, Bhusawal [email protected]
Prof.Ajit P. Chaudhari Associate Professor Electrical Engineering Dept
SSGBCOET, Bhusawal [email protected]
Prof.Rajesh.C.Patil Asst Professor
Electrical Engineering Dept SSGBCOET, Bhusawal [email protected]
ABSTRACT
In this paper, we describe the new control topology for reduction of current harmonics in Three phase Four wire distribution system was presented in this thesis .Starting with incandescent light bulb every load today creates harmonics.
The issue of harmonics is of great concern to engineers, building designers and also in industrial applications because they do not just distort voltage waveforms, but they can also overheat the building wiring, overheat transformer unit and cause end-user equipment failure. Thus, giving attention towards power quality has become necessary today .The main objective of this project is to study and design smart filter using combination of fuzzy and PI controllers. In Smart filter only used single-phase inductances and capacitors, without using any transformer or special electromagnetic device. The simulation results based on MATLAB/ SIMULINK were performed to verify the effectiveness of smart filter. This filter reduced current harmonics more than previous methods of reducing current harmonics.
General Terms
Active power filters, hybrid power filters, passive power filters, power line filters, power system harmonics, reactive power control.
Keywords
PI, Fuzzy, Smart Filter.
1. INTRODUCTION
Current harmonics in distribution grids mostly result from the widespread usage of non- linear loads. Discharge lamps and power electronics based equipment are two frequent examples of nonlinear loads in residential, commercial, and industrial facilities. Currents harmonics also have a significant effect on medium-voltage (MV) and LV networks. Harmonics are integer multiple of fundamental frequency (i.e. 50 or 60 Hz) component which added together resulted in distorted waveform. For example second harmonic is two time of fundamental (i.e. 100 or 120 Hz), similarly for third harmonic it is thrice of the fundamental component (i.e. 150 or 180 Hz) and so on. Due to extreme use of power converters and other non-linear loads in industry it is observed that it deteriorates the power systems voltage and current waveforms. Static power converters such as single phase and three phase rectifiers, thyristor converters and large number of power electronic equipment are nonlinear loads which generate considerable disturbances in the ac mains. Mainly voltage harmonics and power distribution problems arise due to current harmonics produced by nonlinear loads. As nonlinear
currents flow through electrical system and the distribution- transmission lines, additional voltage distortion produce due to the impedance associated with the electrical network.
There are basically two types of active filters: the shunt type and series type. The shunt-connected active power filter, with a self-controlled dc bus used for reactive power compensation in power transmission systems. Shunt active power filters compensate load current harmonics by injecting equal-but opposite harmonic compensating current. Series active power filters were introduced by the end of the 1980s and operate mainly as a voltage regulator and as a harmonic isolator between the nonlinear load and the utility system. The series- connected filter protects the consumer from an inadequate supply voltage quality. The series active filter injects a voltage component in series with the supply voltage and therefore can be regarded as a controlled voltage source, compensating voltage sags and swells on the load side. Till now many control strategies have been developed but instantaneous active and reactive current (id-iq) component method and instantaneous active and reactive power (p-q) method are more popular methods. This project mainly concentrates on these two control strategies (id-iq and p-q) with PI controller.
Both methods are compared under distorted main voltage condition and it is found that id-iq control method achieve superior harmonic compensation performance. The id-iq control is based on a synchronous rotating frame derived from the mains voltages without the use of a phase-locked loop(PLL).By the id-iq control method many synchronization problems are avoided and a truly frequency-independent filter is achieved.
1.1. Harmonic Sources and Effects
A good assumption for most utilities in the United States is that the sine-wave volt- age generated in central power stations is very good. In most areas, the voltage found on transmission systems typically has much less than 1.0 percent distortion. However, the distortion increases closer to the load. At some loads, the current waveform barely resembles a sine wave. This has given rise to the widespread use of the term harmonics to describe distortion of the waveform.
Harmonics problems counter many of the conventional rules of power system. Design and operation that consider only the fundamental frequency. Therefore, the engineer is faced with unfamiliar phenomena that require unfamiliar tools to analyze and unfamiliar equipment to solve. Harmonic distortion is not a new phenomenon on power systems. Concern over distortion has ebbed and flowed a number of times during the history of ac electric power systems. Scanning the technical literature of the 1930s and 1940s, one will notice many articles on the subject. At that time the primary sources were the transformers and the primary problem was inductive
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interference with open-wire telephone systems. The forerunners of modern arc lighting were being introduced and were causing quite a stir because of their harmonic content not unlike the stir caused by electronic power converters in more recent times. We can define harmonic as the frequency which is integer multiple of the fundamental frequency (i.e. 50 or 60 Hz). For example second harmonic is two time of fundamental (i.e. 100 or 120 Hz), similarly for third harmonic it is thrice of the fundamental component (i.e. 150 or 180 Hz) and so on.
In general, harmonic sources are given below:
•Converters, Devices which includes semi-conductor elements
• Generators, Motors, Transformers
• Lightening equipment working by gas discharge principle
• Photovoltaic systems, Computers, Electronic ballasts
• Uninterruptable power supplies, Switching power supplies
• Welding machines
• Control circuits
• Frequency converters
• Static VAR compensators
• Arc furnaces
• HVDC transmission systems
• Electrical Communication systems
• Static VAR compensators
1.2 Necessity
All electrical devices are prone to failure or malfunction when exposed to one or more power quality problems. The devices may be an electric motor or a transformer, a generator, a computer, a printer, communication paraphernalia or a house hold appliance. All these devices and other react adversely to power quality issues depending on the severity of problem. Both electric expediencies and end users of electric power are becoming increasingly anxious about the quality of electric power. Since the late 1980s the term power quality has become one of the most prolific buzzwords in the power industry. It is a brusque concept for a multitude of individual types of power system disturbances. The issues that fall under this curt are not necessarily new. A systematic approach is being used rather than handling them as individual problems this is what engineers are now attempting to deal with these issues.
1.3 Objectives
The main objective of this project is to study and design smart filter using combination of fuzzy and PI controllers.
The progeny of Newer-generation load equipment, with microprocessor based control and power
electronic devices, is more sensitive to power quality variations than was equipment used in the past.
The increasing emphasis on overall power system efficiency has resulted in continued growth in the application of devices such as high-efficiency, adjustable-speed motor drives and shunt capacitors for power factor correction to reduce losses. This is resulting in increasing harmonic levels on power systems and has many people concerned about the future impact on system capabilities.
End users have an increased awareness of power quality issues. Utility customers are becoming better informed about such issues as interruptions, sags, and switching transients and are challenging the utilities to improve the quality of power delivered.
Many things are now interconnected in a network.
Integrated processes mean that the failure of any component has much more important consequence.
1.4 Theme
The wide electrical network consisting generation, transmission, distribution and loads like residential, commercial and industrial. There are consider non-linear load also. So these have worst effect of harmonic distortion, it may loss efficiency of system; there are produce heat losses and many more bad affects. These systems may not be working in steady state. Therefore, the solutions of these problems Smart filters are designed for cancellation of harmonics in Three phase four wire system. In this project specially PI controller and combination of PI and Fuzzy controllers design for reduction of harmonics.
2 SYSTEM DESIGN
2.1 Problem Definition
From the previous literature review, it is observed that harmonic is as well-known problem of high complexity. To solve this problem
• To provide compensation for harmonic load current components.
• To study the control strategy of the Smart Filter for the reduction of harmonic currents of the voltage source type of non-linear loads.
• To develop the non-linear model of three-phase SH-APF.
• To develop the non-linear model of three-phase Smart Filter.
• To check how this technique is better than others in terms of reliability and efficiency.
2.2 Filters Design For Harmonic Reduction 2.2.1 Design FBS Power Filter
The general structure of the shunt power filter topology proposed in this thesis is shown in Fig.1
The pn-seq and the z-seq voltage components of the three- phase network that the filter is connected to are also represented in Fig..1. The FBS topology consists of three
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phase branches with three identical single-phase impedances Zf and one neutral branch with a fourth single-phase impedance Zn. In Fig..1, the FBS power filter is connected to a generic three-phase network in which pn-seq voltage component u12 = [uao, ubo, uco] and z-seq voltage components u0 have been represented separately for the sake of clarifying the superposition analysis presented in the following. When only pn-seq components are considered in the circuit of Fig.1. i.e., when it is assumed that u0 = 0, the center nodes at the source and filter sides (oo) are virtually connected, and hence, voo = 0. Therefore, the pn-seq impedance of the FBS power filter at a particular frequency Z12 is given by,
Fig.1.FBS power filter with generic branch impedances.
The following quotient of phasors:
Where U12 and I12 are the pn-seq voltage and current phasors affecting the FBS filter, respectively. Likewise, when only z- seq are considered in circuit of Fig 1 i.e., when u12=[0, 0,0], the z-seq impedance of the FBS power filter at a particular frequency Z0 is given by
Where U0 and I0 are the z-seq voltage and current phasors affecting the FBS filter, respectively. The single-phase impedances constituting the branches of FBS power filter are resonant cells, which could have several resonance frequencies. Connection of these resonant cells. According to FBS topology gives rise to two groups of resonance frequencies, i.e., one group for the pn-seq components and another one for the z-seq components. This implies that the shunt passive power filter with FBS topology is able to perform selective filtering of current harmonics by setting up low-impedance paths to explicit cur- rent components with specific frequencies and sequences. Even though the resonant cells composing the FBS filter branches can be really complex in some particular applications, a reasonably good filtering characteristic is obtained in practice when such resonant cells have a single resonance frequency. Simple LC resonant cells will be considered in this introduction of the FBS power filter to simplify further explanations. A regular implementation of the FBS power filter based on simple series LC resonant cells is presented in Fig.5.2.In this case, the phase and neutral impedances, Zf and Zn, are respectively given by,
Fig.2. FBS power filter based on simple series LC resonant cells.
According to FBS topology gives rise to two groups of resonance frequencies, i.e., one group for the pn-seq components and another one for the z-seq components. This implies that the shunt passive power filter with FBS topology is able to perform selective filtering of current harmonics by setting up low-impedance paths to explicit current components with specific frequencies and sequences. Even though the resonant cells composing the FBS filter branches can be really complex in some particular applications, a reasonably good filtering characteristic is obtained in practice when such resonant cells have a single resonance frequency.
2.2.2 FBS hybrid power filter
General Description of the FBS Hybrid Power Filter: A previously pre- sented FBS passive power filter can offer a fairly good behavior when applied to cancel out current harmonics in three-phase four-wire systems under optimal operating conditions. However, the filtering characteristic of the FBS passive power filter is affected by typical problems of any passive filter, i.e., its filtering capability depends on the value of the grid impedance; when there exists risk of resonance, retuning is necessary due to ageing and tolerances.
A solution to overcome drawbacks associated with passive filters consists of integrating a power converter into the filter structure. This filtering system is known as a hybrid power filters. A properly designed and well-controlled power converter can generate any voltage current relationship at its output, obviously provided that it works inside its operative range. Therefore, such power converter could be understood as a ”virtual impedance” integrated into the original structure of the passive filter. This virtual impedance improves the behavior of the original passive filter by increasing its capability for draining off current harmonics at frequencies different from the resonance ones, compensating drifts in the passive filter parameters, and damping oscillations due to resonance phenomena. Conventional three-phase three-wire hybrid filters are used to integrate a three-leg full-bridge voltage-source inverter (VSI) (without neutral connection) to improve the filter characteristic for pn-seq current harmonics.
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of an FBS Hybrid Power Filter
The control system of the FBS hybrid power filter is constituted by the four main blocks shown in Fig, namely.
The grid current processing block (F), which is in- charge of selecting current har- monics to be filtered (iS ) from the controlled grid current (iS );
The injected current controller, which sets a reference voltage for the VSI (ui ) in order to cancel out the selected current harmonics;
The dc-link voltage controller, which modifies the original reference voltage of the VSI by adding an extra term (uv ) in order to keep the dc-link voltage at its nominal value;
The modulator, which generates the switching signals of the VSI (sC ) from the final reference voltage of the VSI (uC ).
The resonant cells of the FBS hybrid power filter of Fig.3.4 offers very low impedance to pn-seq and z-seq currents at the tuning frequencies f12 and f0, respectively. Therefore, a low dc link voltage only about 10 per of the grid voltage is necessary in the VSI to inject into the grid significant levels of harmonic currents at frequencies f12 and f0. However, impedance offered by the resonant circuits grows as frequency goes far away from the resonance ones. As a positive consequence, the current ripple injected by the FBS hybrid power filter into the grid at the switching frequency is very low. However, this also implies that the FBS hybrid power filter can only compensate a limited range of the pn-seq and z- seq current harmonics. For this reason, the grid current processing block (F) extracts those individual frequencies that are suitable to be filtered (iS ) from the grid current (iS ) by using any signal filtering technique. As previously mentioned, the harmonics compensation range can be extended by increasing the dc-link voltage level, which increases the VSI rating, and consequently, its cost. Currents signals at the input of the current processing block can be sensed either upstream or downstream of the point of common coupling (PCC) between the FBS power filter and the grid. The transfer function of the control system depends on both the current sensing point and the type of injected current controller. This current controller can work on either synchronous or static reference frames using either conventional synchronous PI or stationary resonant controllers, respectively.The dc-link voltage controller generate a reference voltage in phase with the current at the fundamental grid frequency flowing through the VSI. Interaction of both voltage and current generates an exchange of active power between VSI and the grid intended to keep the energy stored into the dc-link stored energyand so the dc-link voltage close to its nominal value.The control diagram of Fig. 5.4 is implemented by using light algorithms, as described hereafter. Hence, the grid current processing block (F) consists of three very narrow notch filters (NFs) one per phase canceling out the fundamental frequency component from the current harmonics to be compensated by the filter. The injected current controller was implemented by a simple proportional regulator per phase.
u
C= Z
V.i
S, Z
V= u
kF
(3.3) Where u is the gain of the VSI.The grid impedance given by
Z
s= R
s+ jL
sw(3.4) Where RS and LS are the resistance and inductance presented by the grid, respectively. Transfer function of 5.16 reveals that integration of a VSI controlled by the control law of 5.15 into the FBS hybrid filter structure is equivalent to inserting a virtual impedance ZV upstream of the PCC between the hybrid power filter and the grid.
2.2.4 Performance Study of The Three-phase Four-wire FBS hybrid power filter
The implementation of the three-phase four-wire FBS hybrid power filter is shown in Fig.2.6To evaluate the performance of the proposed hybrid power filter, three single- phase diode rectifiers injecting current harmonics at the load side (iL) are considered.
Fig.3. Three-phase four wire FBS hybrid power filter.
In this particular implementation, a conventional three-leg full-bridge VSI with the negative rail of the dc-bus connected to the neutral conductor is used for injecting both pn-seq and z-seq currents into the three-phase four-wire grid. This interesting VSI topology was applied in to a conventional three-phase four-wire hybrid power filter with only one common resonance frequency for both the pn-seq and the z- seq components.
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Fig.4. Control of the three-phase four-wire implementation of the FBS hybrid power filter.
3. DESIGN SMART FILTER USING FUZZY AND PI CONTROLLER.
3.1. Construction of PI Controller
Figure 5 shows the internal structure of the control circuit.
The control scheme consists of a PI controller, a limiter, and a three phase sine wave generator for reference current generation and generation of switching signals. The peak value of reference currents is estimated by regulating the DC link voltage. The actual capacitor voltage is compared with a set reference value. The error signal is then processed through a PI controller, which con- tributes to zero steady error in tracking the reference current signal. The output of the PI controller is considered as peak value of the supply current (Imax), which is composed of two components: (a) fundamental active power component of load current, and (b) loss component of APF to maintain the average capacitor voltage to a constant value. The peak value of the current (Imax) is multiplied by the unit sine vectors in phase with the respective source voltages to obtain the reference compensating currents. These estimated reference currents (Isa*, Isb*, Isc*) and sensed actual currents (Isa, Isb, Isc ) are compared at a hysteresis band, which gives the error signal for the modulation technique. This error signal decides the operation of the converter switches. In this current control circuit configuration, the source/supply currents Isabc are made to follow the sinusoidal reference current Iabc, within a fixed hysteretic band. The width of hysteresis window determines
Fig. 5. Conventional PI Controller
the source current pattern, its harmonic spectrum and the switching frequency of the devices. The DC link capacitor voltage is kept constant throughout the operating range of the converter. Each phase of the converter is controlled independently. To increase the current of a particular phase, the lower switch of the converter associated with that particular phase is turned on. To decrease the current the upper switch of the respective converter phase is turned on.
3.2. Construction of Fuzzy Controller
Fig. 6 shows the internal structure of the control circuit. The control scheme consists of a fuzzy controller, a limiter, and three phase sine wave generator for reference current generation and generation of switching signals. The peak value of reference currents is estimated by regulating the DC link voltage. The actual capacitor voltage is compared with a set reference value. The error signal is then processed through a Fuzzy controller, which contributes to zero steady error in tracking the reference current signal. A fuzzy controller converts a linguistic control strategy into an automatic control strat- egy, and fuzzy rules are constructed by expert experience or knowledge database.
Firstly, input voltage Vdc and the input reference voltage Vdc-ref have been placed of the angular velocity to be the input variables of the fuzzy logic controller. Then the output variable of the fuzzy logic controller is presented by the control current Imax. To convert these numerical variables into linguistic variables, the following seven fuzzy levels or sets are chosen NB (negative big), NM (negative medium),
Fig.6.Conventional Fuzzy Controller.
NS (negative small), ZE (zero), PS (positive small), PM (positive medium), and PB (positive big). The fuzzy controller is characterized as follows:
• Seven fuzzy sets for each input and output.
• Fuzzification using continuous universe of discourse.
• Implication using Mamdani’s ’min’ operator.
• De-fuzzification using the ’centroid’ method.
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Fuzzification: The process of converting a numerical variable (real number) con- vert to a linguistic variable (fuzzy number) is called fuzzification.
De-fuzzification: The rules of FLC generate required output in a linguistic vari- able (Fuzzy Number), according to real world requirements, linguistic variables have to be transformed to crisp output (Real number).
Database: The Database stores the definition of the membership Function re- quired by fuzzifier and defuzzifier.
4. PERFORMANCE ANALYSIS
4.1 Simulation models of smart filter with combination of PI and Fuzzy controller
4.1.1 Simulation model of smart filter topology with combination of pi and fuzzy controller
Below figure 7 shows Simulation model for smart filter in which we use combination of PI and Fuzzy controller.
Due to combination of PI and Fuzzy performance are improved.
Fig.7. Simulation model of smart filter topology with combination of pi and fuzzy controller.
3.1.2 Generator part of Simulation model of smart filter topology with combination of pi and fuzzy controller
Fig.8. G part of Simulation model of smart filter topology with combination of pi and fuzzy controller.
3.1.3 Load part of Simulation model of smart filter topology with combination of pi and fuzzy controller.
Fig.9.L part of Simulation model of smart filter topology with combination of pi and fuzzy controller.
3.1.4 Filter part of Simulation model of smart
filter topology with combination of pi and
fuzzy controller.
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Fig.10. F part of Simulation model of smart filter topology with combination of pi and fuzzy controller.
3.2 MATHEMATICAL CALCULATION:
Shunt capacitor calculation
√ ⁄
| |
= Source Voltage = 400v
= Load current = 20.5A T = Source Time = 0.16Sec
= Change in voltage across capacitor 500v
= 1200
√
| | =7. 29 F
=7. 29 F = 800 F = 800 F
Interfacing Inductor
Max frequency of switching ripple.
= Peak to peak switching ripple between non pulsating current
= 1.875
Experiment Results and Discussion
Following are the standard grid parameters with considering line voltage, line resistance, line inductance, frequency.
Table 1: Grid Parameter.
Sr.
No.
Parameter Value
1 Line Voltage 400 volt
2 Line Resistance 240mΩ
3 Line Inductance 3.2mH
4 Frequency 50Hz
Following are the load parameters considering load resistance, load inductance, actual load.
Table.2.Load Parameter.
Sr. No. Parameter Value
1 Load Resistance 200Ω
2 Load Inductance 0.1H
3 Load 10kw
This is the output table of Smart filter, there are total harmonics distortion calculated with PI controller and combination of PI and Fuzzy controller
Table 3. Output Table.
Sr.No. THD with PI Controller THD of PI and Fuzzy
1 8.5 2.43
3.3 GRAPHICAL REPRESENTATION
These are the graph of FFT analysis for source current where Total harmonic distortion (THD) 2.43% and result is shown by simulation. From point 0.05 all harmonic distortion eliminated.
Fig.11.FFT analysis for source current.
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Fig.12. Simulation Result of Source Current
Fig 13 & 14 shows the graph and simulation of load voltage with total harmonic distortion is 0.22%.
Fig.13.FFT analysis for Load voltage.
Fig.14.Simulation Result of load Voltage.
CONCLUSION
In this dissertation two controllers are developed and verified with three phase four wire system. Even though both controllers are capable to compensate the cur- rent and voltages harmonics in the 3 phase 4-wire system, it is observed that the fuzzy controller shows more dynamic performance over conventional PI controller.Source volt- age and current THD is satisfactory reduced by using Smart filter which are check by simulink/matlab software.
According to table No.3 which is the output table of THD with PI controller and THD with combination of PI and Fuzzy controller. The PI controller has 8.5 THD and the combination of PI and Fuzzy controller has 2.43 THD. So combination of PI and Fuzzy controller has more better performance.
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