Power Quality is an important problem a Power System has to handle to provide its consumers a reliable and economical supply. Three-phase four-wire distribution systems have been widely used in commercial and industrial installations. In the three-phase four-wire distribution systems, the neutral current carries the zero sequence current due to the unbalanced loading among the phase conductors. Due to the presence of power electronics load a triplen (third, ninth, etc) harmonics are introduced in the system.
This project uses a new harmonic suppression scheme for the neutral conductors of three-phase four-wire distribution systems. This new scheme consists of a series active filter, which is connected in series with the neutral conductor to suppress the harmonics in the neutral conductor. This new filtering technology scheme consists of a voltage source inverter in series with the series connected inductor and capacitor set. This scheme is implemented for a system feeding a non-linear loads and its performance is tested. The results show that the proposed scheme is suitable for practical applications.
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Electric Power becomes a most basic need in our day-to-day life. Electric utilities aim is to provide a consumer a good quality of reliable continuous power supply at economical cost. In three-phase four-wire distribution system, due to use of non-linear loads, excessive current will flow in the neutral conductor. In most three-phase power systems supplying single-phase loads, there will be some phase current imbalance and some neutral current. Under normal operating conditions with reasonably balanced load, the current in the neutral is expected to be 20% of the normal phase currents. Especially on three-phase four-wire distribution systems, the third-harmonic currents are increased. The excessive third harmonic currents cause overheating of the neutral conductors. Hence, voltage distortion on utility outlets and excessive neutral current on distribution lines have arisen and lead to a number of serious problems in the distribution system. These neutral currents are fundamentally third harmonics and odd multiples of 3rd due to non-linearity of loads.
The following are the Potential problems are directly related to excessive harmonic currents in the neutral conductor are
Wiring failure due to improper sizing of the neutral conductor.
Over heating of the transformer due to harmonic currents and insulation damages and failure.
Intermittent electrical noise from connections loosened by thermal cycling.
Excessive neutral to ground voltage due to a voltage drop caused by the neutral current. This common mode potential can result in the malfunction of sensitive electronic components.
Due to the presence of non-linear loads, the other loads connected to a Point of Common Coupling (PCC) also affected by harmonics shown in fig.1. Harmonics are injected to a line by use of non-linear loads. Harmonics is defined as "the current or voltage waveforms having frequencies that are whole integral multiplies of fundamental frequency i.e. multiplies of 50Hz or 60Hz".
These harmonics can be eliminated by using Passive filters (Combination of L and C). These types of filters have disadvantages of series resonance, parallel resonance, tuning of passive elements is difficult, etc.
Recently, harmonics suppression facilities based on Power Electronics techniques have proved to be important. This harmonics suppression can be done with the help of new emerging SERIES ACTIVE POWER FILTERS. This Series Active Filters (SAF) is connected in series with the neutral line to reduce the harmonics in the neutral line. This has more advantages when compared to the other configurations. The followings are the advantages of connecting series active filters in series with the neutral line.
It is very less complexity and less economical
It has small rating normally 4% of the load KVA
It acts as a harmonic isolator between the supply and the load
It is more economical for harmonic compensation of the large no-linear loads
It prevents supply-load interaction and resonance problem
It can adapt to changing the load condition
Figure 1. Point of common coupling
II. CONTROL STRATEGY OF THE ACTIVE FILTER
By using, the most efficient method of pulse Width Modulation (PWM) Control with in the inverter can control the output voltage of the inverter. The commonly used technologies are
Single Pulse Width Modulation
Multi Pulse Width Modulation
Always on Time
Marked to Standard
Sinusoidal Pulse Width Modulation
Modified Sinusoidal Pulse Width Modulation
The most commonly used method is Sinusoidal Pulse Width Modulation (SPWM). In this method, the output Voltage is controlled by producing the gating signal by comparing a sinusoidal reference signal with triangular carrier wave.
III. OVERALL BLOCK DIAGRAM AND THE DESIGN OF SERIES ACTIVE FILTERING TECHNOLOGY
Due to the development of Power Electronics technology, more and more Power Electronics appliances are widely used, which leads to the serious harmonic pollutions.
Figure 2. Overall block diagram
These kinds of harmonics can be eliminated by using Series Active Filtering technology. The development of new Series Active Filtering technology is presented in this chapter. The Series Active Filter is connected in series with the neutral conductor to suppress the harmonics, which often suppress the harmonics in the phase conductor as well. The concept of proposed Series Active Filtering technology, and its operating Principle, Control theory are also discussed in this chapter
IV. OVERALL BLOCK DIAGRAM OF THE SYSTEM
The overall block diagrams and the system configuration of the proposed series active filter are shown in the fig 2 and 3.
It consists of a three-phase source, which is connected to a three-phase non-linear diode bridge rectifier circuit. The Series Active Power Filter is connected in series with the neutral conductor. It consists of the voltage source inverter in series with the inductor and capacitor. The triggering for the inverter circuit is given through the control circuit. The inverter can be implemented by IGBTs operating in hard switched Pulse-Width Modulation (PWM) mode to provide sufficient bandwidth for the filtering function.
Figure 3 .Proposed series active filter
The proposed series active filter system can prevent overloading of the neutral conductor and the distribution transformer with one installation.
V. OPERATING PRINCIPLE
Three phase bridge rectifier with RC loads (non-linear loads) are connected to the three phase four wire distribution system as shown in fig 3. Due to the nature of the non-linear loads, harmonics are injected in to the system through the neutral currents. This harmonics can inject in to the phase conductors also as well.
Series Active Power Filter is connected in neutral conductor to suppress the harmonics. The Voltage Source Inverter (VSI) generates a compensating harmonics currents in to the neutral conductor through the inductor and capacitor sets connected in series with it.
The generated harmonic currents and the harmonic currents flows in the neutral conductor cancel each other without affecting the fundamental part of the neutral current.
VI. EQUIVALENT CIRCUIT OF THE SYSTEM
Fig. 4 shows the equivalent circuit diagram of a proposed series active filter
Figure 4. Equivalent circuit diagram
Vsa,Vsb,Vsc are the source voltage of the phase A, phase B, and phase C Respectively
VAF, represents the series active filter inverter output voltage
VLa,VLb,VLc are modelled load voltage source (Balanced or unbalanced)
ZL Load impedance
Zs Source impedance
ia, ib, ic and in are the phase currents and neutral current respectively
VAF = Kh.G.In, (1)
Kh is a gain
G = 0.0 for fundamental frequency
1.0 for harmonic frequencies
The relationship of currents and Voltages can be expressed as
Vsa-ZsIa-ZLIa-VLa+VAF+ZnIn = 0 (2)
Vsb-ZsIb-ZLIb-VLb+VAF+ZnIn = 0 (3)
Vsc-ZsIc-ZLIc-VLc+VAF+ZnIn = 0 (4)
By adding above equations
VLa-VLb-VLc+3VAF+3ZnIn = 0 (5)
Since (Ia+Ib+Ic = In)
Vsa+Vsb+Vsc = 0 for balanced conditions (6)
Where, VAF = Kh.G.In
In = -(VLa+VLb+VLc)/ (Zs+ZL+3Zn+3KhG) (8)
VII. DESIGN OF CONTROLLER FOR SERIES ACTIVE FILTER
Fig.5 shows the control block diagram of the proposed Active Power Filter. An active filter is placed in series connection with the neutral conductor of the three-phase four-wire system. A Hall-Effect sensor provides the measurement of the neutral current in the system controller.
The measured neutral current In from the system containing both harmonics and fundamental components is multiplied by sin (Ï‰ot) and cos (Ï‰ot) respectively, to extract the fundamental components of In, where Ï‰o is the frequency of the utility grid.
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The fundamental components of In is converted in to DC and the harmonics are converted in to AC after the multiplication process. Sin (Ï‰ot) and Cos (Ï‰ot) are synchronized to the utility by a Phase-Lock-Loop (PLL) circuit.
Low Pass Filter (LPF) cut off at 5Hz is applied to extract the DC components. The output of the Low Pass Filter is a DC components, which are multiplied by sin (Ï‰ot) and cos (Ï‰ot) respectively and then summed to synthesize the fundamental components of In (represents by In, f). The Scaling factor of 2 is for normalization.
The initially measured current In is subtracted with the fundamental components of In. i.e. In,f.
Then the harmonic component of neutral current is obtained. The Voltage command of the active filter inverter is generated by
Figure 5. Controller of series active filter
Where In,h represents the harmonic components.
Vinv represents a high gain (Kh) for the current harmonics in the neutral conductor, therefore will be suppressed by the active filter inverter while the fundamental components In,f is not affected. The remaining In,f will not overload the neutral conductor which is usually sized the same as phase conductor.
The Voltage command Vinv is compared to the triangular carrier to generate the PWM gating pulse. The active filter inverter switches at 20 KHz to provide sufficient bandwidth for the desired filtering characteristics.
Figure 6. Output waveforms without filter
Depends upon the PWM gating pulses, the inverter circuit will inject the negative harmonic currents to the neutral line, as a result only fundamental components of the neutral current will flows through the line.
VIII. CIRCUIT DIAGRAM AND SIMULATION RESULT
The Circuit diagram for with filter is shown in fig 8. In this active filter is connected in series with the neutral line. The load taken for the simulation is parallel RC load. Simulation is done for both balanced and unbalanced conditions.
IX. SIMULATION RESULTS FOR THE PROPOSED FILTER
The system chosen is 230V, 50Hz distribution system feeding a diode rectifier with parallel R and C loads. The Simulation is carried out for a period of 200ms.
The output waveforms of the phase currents and neutral currents for both balanced and unbalanced conditions without filter for R and RC loads are shown in fig.6. Form the figure, obviously shown that the neutral current waveforms without filters are distorted because of the harmonic and phase current are in unbalanced condition.
Figure 7. Output waveforms with filter
The waveforms of the phase currents and the neutral currents for both balanced and unbalanced conditions with filter for R and RC loads are shown in fig.7. It clearly shows that the neutral current distortions are much reduced.
Figure 8.The simulation model of the proposed system
X. EFFECTIVENESS OF THE FILTER
The Total Harmonic Distortion (THD) of the output waveforms without and with filter is analyzed using Fast Fourier Transform. From the fig 9 it is clearly shown that THD of the neutral current is 24.29%.
Figure 9. FFT Analyses for Neutral current waveform without filters.
After the series active filter is connected in the neutral conductor the THD are much reduced to 1.24% and it's shown in fig 10.
Figure 10. FFT Analyses for Neutral current waveform with filters
 M.Aredes, J. Hafner, and k.Heumann, "Three-phase four wire shunt active filter control strategies," IEEE Transactions on Power Electronics.,vol. 12,no.2.pp.311/318.March 1997.
 P.T.Cheng, C.C.Hou, and Y.F.Huang, "Overload Prevention," IEEE Transactions on Industry Applications,vol.10 ,no.6,pp.26-34,Nov/Dec.2004
The model of the proposed new Series Active Power Filter is realized. The developed new Series Active Power Filtering technology is implemented for a system feeding a non-linear load. It is simulated using the highly developed graphic tool SIMULINK available in MATLAB. The results reveal that the proposed new Series Active Filtering technology is simple and effective and is suitable for practical applications.