Scholarly article on topic 'Power Quality Improvement Features for a Distributed Generation System using Shunt Active Power Filter'

Power Quality Improvement Features for a Distributed Generation System using Shunt Active Power Filter Academic research paper on "Electrical engineering, electronic engineering, information engineering"

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Abstract of research paper on Electrical engineering, electronic engineering, information engineering, author of scientific article — S. Premalatha, Subhransu Sekhar Dash, Paduchuri Chandra Babu

Abstract Electric utilities and end users of electric power are becoming increasingly concerned about meeting the growing energy demand. Renewable energy source (RES) form an invincible part of electric power distribution. Wind energy which forms one of the major available renewable energy sources. In this paper a grid interfacing inverter is designed which acts as a power converter to inject power generating from RES to grid and also act as Shunt Active Power Filter (SAPF) to minimize the harmonics, load neutral current. A new control approach named Unit Vector Template Generation (UVTG) is designed for SAPF and presented in this paper. The simulation has been done for three phase four wire (3P4W) distribution system.

Academic research paper on topic "Power Quality Improvement Features for a Distributed Generation System using Shunt Active Power Filter"

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Engineering

Procedia Engineering 64 (2013) 265 - 274 =

www.elsevier.com/locate/procedia

International Conference on Design and Manufacturing 2013 (IConDM 2013)

Power Quality Improvement Features for a Distributed Generation System using Shunt Active Power Filter

S.Premalathaa*, Dr.Subhransu Sekhar Dashb, Paduchuri Chandra Babuc

a Asst Professor, EEE, Velammal Engineering College, Chennai, India-600066 b HOD, EEE, SRM University, Chennai, India-603203 c Research Scholar, EEE, SRM University, Chennai, India-603203

Abstract

Electric utilities and end users of electric power are becoming increasingly concerned about meeting the growing energy demand. Renewable energy source (RES) form an invincible part of electric power distribution. Wind energy which forms one of the major available renewable energy sources. In this paper a grid interfacing inverter is designed which acts as a power converter to inject power generating from RES to grid and also act as Shunt Active Power Filter (SAPF) to minimize the harmonics, load neutral current. A new control approach named Unit Vector Template Generation (UVTG) is designed for SAPF and presented in this paper. The simulation has been done for three phase four wire (3P4W) distribution system.

© 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the organizing and review committee of IConDM 2013 Keywords: Renewable energy source, Shunt active power filter, Grid interfacing inverter, Harmonic distortion

1. Introduction

Electric utilities and end users of electric power are becoming increasingly concerned about meeting the growing energy demand. The growing concern regarding global heating and climate change has stimulated the development of environment friendly energy generation technologies. In the near future, because of the Kyoto protocol and other international accords, the electrical grid will include a very large number of small producers that use renewable energy sources like solar panels or wind generators, among other technologies. The power-electronic technology plays an important role in distributed generation and in integration of Renewable Energy Sources into the electrical grid [l]-[2]. To interface this RES power electronic converters are used. When the power electronic converters are used it introduces a lot of harmonics in the system. On the other hand, the increased use of sensitive electronic circuits by industries and households together with privatization and competition in electric energy systems, posed the power quality improvement as one of the major problems in electricity industry[3]-[5]. Harmonics causes distortion of source voltage, addition loss due to unwanted current flowing in the source and also it may lead to malfunctioning of protective relays, mains and other control units. So it is necessary to reduce the amount of harmonics. There are many techniques to reduce the effect of harmonics. One of those methods is to use SAPF which produces harmonic current of equal magnitude and opposite polarity to that of the harmonic current produced in the system such that it cancels the harmonic current in the system [3]-[8].In this paper SAPF is used considering its advantages of high speed response and flexibility in operation as it contains the power electronic devices. Also the same device is used to integrate the power from the RES to the distribution system. So the need of additional equipment is avoided. This paper proposes new control approach/structure named Unit Vector Template generation ((UVTG) that can be realized for SAPF to generate reference currents. This paper deals with Total Harmonic Distortion (THD) in the grid current and load current with the new control approach in which reference currents are generated which

* S. Premalatha Tel. :+91-944-578-0457; E-mail address: premsbala@gmail.com

1877-7058 © 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the organizing and review committee of IConDM 2013 doi: 10.1016/j.proeng.2013.09.098

are compared with actual grid currents to generate switching signals for the SAPF. Thus the proposed control strategy effectively reduces the THD in the grid and load currents. Thus the SAPF acts as the grid interfacing inverter for interfacing RES to the electrical grid without additional power conditioning equipment.

2. Proposed 3P4W Distribution System with SAPF

Generally, a 3P4W distribution system is realized by providing a neutral conductor along with three power conductors from generating station or by utilizing a A-Y transformer at distribution level. Fig.l shows a 3P4W network in which neutral conductor is provided by utilizing A-Y distribution transformer. The proposed system consists of RES connected to the delink of a grid-interfacing inverter. The voltage source inverter is a key element of a distributed generation system as it interfaces the RES to the grid and delivers the generated power. The RES may be a DC source or an AC source with rectifier coupled to dc-link. Usually, the fuel cell and photovoltaic energy sources generate power at variable low dc voltage, while the variable speed wind turbines generate power at variable ac voltage. Thus, the power generated from these renewable sources needs power conditioning (i.e., dc/dc or ac/dc) before connecting on dc-link. The dc-capacitor decouples the RES from grid and also allows independent control of converters on either side of dc-link. In the system, the distribution transformer is used to step down the grid voltage to the required domestic voltage. The system supplies the set of three-phase and single-phase loads. As it is a Distributed generation system it also contains the RES (wind). The output of the RES is connected to the dc-link capacitor which in turn connected to the grid interfacing inverter. The dc-link plays an important role in transferring this variable power from RES to the grid. The above mentioned grid interfacing inverter also acts as the shunt active filter which reduces the harmonics in the system. It contains the fourth leg for the compensation of neutral current which reduces its magnitude to zero practically. The power electronic devices in the shunt active power filter needs the control pulses for its operation and the controller in the system provides those pulses for the eight devices as shown. The output of the inverter is interfaced at the PCC via the coupling inductors.

Fig.l.Proposed 3P4W System with SAPF (Grid Interfacing inverter)

2.1. Controller for SAPF

The control strategy is basically the way to generate reference signals for Shunt Active Power Filter. The compensation effectiveness of the Shunt Active Power Filter depends on its ability to follow with a minimum error and time delay to calculate the reference signals to compensate the distortions, unbalanced voltages or currents or any other undesirable condition. In the following section an approach based on UVTG is explained to extract the reference current signals Shunt

Active Power Filter. The control diagram of grid- interfacing inverter for a 3P4W system is shown in Fig. 2

Fig.2. Controller for Grid Interfacing Inverter

2.2. Extraction of Unit Vector Templates

The extraction of 3 phases voltage reference signals are based on Unit Vector Template Generation. A Phase Locked Loop (PLL) is used to extract the pure sinusoidal signal at fundamental frequency. The PLL gives signal in terms of sine and cosine functions. Here only sine terms are considered. As we know the supply voltage peak amplitude in advance, we can generate the unity supply voltage signals. To get the unity terminal voltage vector UB4_abc, the terminal voltages are sensed and multiplied by a gain equal to 1/Vm where Vm is the peak amplitude of fundamental terminal voltage. These unity voltage vectors are then taken to PLL. Thus the output of PLL is equal to unity terminal voltage at fundamental frequency only. The output of the PLL gives Ua (unit vector for phase A) which can be expressed mathematically as

U a = sin(fflO

The unit vectors for phase B and phase C can be obtained by proper phase shifting. They can be expressed as

Ub = sin(fflf-120) Uc = sin(fi# +120)

(2) (3)

The extraction of Unit Vector Template is shown in Fig.3.

Fig.3. Extraction Of Unit Vector Template

2.3. Reference Current Signal Generation

The shunt APF is used to compensate for current harmonics as well as to maintain the dc link voltage at constant level. To achieve the aforementioned task the dc link voltage is sensed and passed through the first-order low pass filter to eliminate the ripples present on the dc-link voltage, VdC, and then it is compared with the reference dc link voltage. This gives the dc-link voltage error. The dc-link voltage error at the nth sampling instant is given as

V dcerr(n) = V*dc(n)-V dc(n) (4)

The error is then processed by a discrete PI controller. The output of the PI controller then will be the peak amplitude of fundamental input current, Im, which must be drawn from the supply in order to maintain dc link voltage at constant level and to supply losses. The output of this discrete Pi-controller at nth sampling instant can be expressed as

Im{n) = Im(n-\)-KPVdc(Vdcerr{n)-Vdcerr(n-\)) + KlVdcVdcerr{n) (5)

This peak amplitude, Im, is then multiplied with unit vector templates giving reference current signals for shunt APF, as shown in Fig.4.

Fig.4. Reference Current Signal Generation The instantaneous values of reference three-phase grid currents are computed as

la~Im Ua I*b = Im-Ub

(6) (7)

Ic = Im~U

The neutral current, present if any, due to the loads connected to the neutral conductor should be compensated by forth leg of grid-interfacing inverter and thus should not be drawn from the grid. In other words, the reference current for the grid neutral current is considered as zero and can be expressed as

* « /« = 0

2.4. Gating Signal Generation

After extracting the reference current signals for Shunt Active Power Filter, the next step is to force the inverters to follow these reference signals. This can be done by switching the inverter IGBTs in a proper manner. To have the required gating signals, the hysteresis controller is used. The Hysteresis current control is a method of controlling voltage source inverter so that an output current is generated which follows a reference current waveform. This method controls the switches in an inverter asynchronously to ramp the current through an inductor up and down so that it follows a reference.

This method requires a current feedback. The sensed current is compared to the hysteresis limits and the result of the comparison is used to control the switching sequence in the inverter. The advantage of this method is low switching losses and very high speed.

The reference grid currents (la ,1b Jc aild In ) are compared with actual grid currents (Ia, lb, Ic and In ) to compute the current errors as

' aerr -

■ cerr"

a~ I a (11)

* (12)

* c~Ic (13)

These current errors are given to hysteresis current controller. The hysteresis controller then generates the switching pulses (PltoP8) for the gate drives of grid-interfacing inverter as shown in Fig.5.

: Controlfer Fig.5. Hysteresis Current Controller

2.5. Inverter Design

The inverter output voltages can be modelled in terms of instantaneous dc bus voltage and switching pulse for inverter

r (PI - PA) r Vinva ~ 2 V dc (13)

(P3-P6) Vinvb= 2 V dc (14)

r (P5-P2) r Vinvc ~ 2 ^ dc (15)

r (P7-P8) r Vinvn ~ 2 ^ dc (16)

where Vinva, Vinvb, Vinvc and V^, inverter. are the three-phase ac switching voltages generated on the output terminal of

Similarly the charging currents Iinvad, Iinvbd, Iinvcd and IInvrid on dc bus due to the each leg of inverter can be expressed as Iinvad = Iinva(pl-p4) (17)

Iinvbd = Iinvb(p3-p6) (18)

Iinvcd = I invc (p5 ~ p2) (19)

I invnd = I invn (p7 ~ p%) (20)

The switching pattern of each IGBT inside inverter can be formulated on the basis of error between actual and reference current of inverter, which can be explained as:

If Iinva< (Iinva* - h<,bo), then upper switch SI will be OFF (P1=0) and lower switch S4 will be ON (P4=l) in the phase "a" leg of inverter.

If Iinva> (Iinva* - hb), then upper switch SI will be ON (Pl=l) and lower switch S4 will be OFF (P4=0) in the phase "a" leg of inverter.

where hb is the width of hysteresis band. On the same principle, the switching pulses for the other remaining three legs can be derived

3. Simulation Results

Simulation results for the proposed SAPF-based 3P4Wtopology are shown. MATLAB/Simulink is used as a simulation tool. The system consists of the composite load which contains a three-phase non-linear load and a single-phase non-linear load in each phase. In every case the non-linear load is a bridge rectifier feeding a RL load. The non-linear load connected to the source injects current harmonics, which distorts the source current. This disturbs the quality of power supplied by the source. The current harmonics are suppressed by injecting the compensating currents using Shunt Active Power Filter. The unit vectors generated and reference current for three phase are shown in Fig. 6 and Fig. 7 respectively

Fig.6 Unit Vectors Generated for Phases A, B and C

Fig.7 Reference Current for the Three Phases A, B and C

; ||5 J! J [J! j i,

Fig.8 Inverter Output Voltage and Current for Three-Phases A, B and C

The grid current for the three-phase A, B and C are shown in Fig.9. At tH).2s, the grid interfacing inverter is interfaced with the system. It is noticed that before t^0.2s the grid currents are equal to load current which is non-sinusoidal. After t=0.2s the grid currents are found to be sinusoidal due to the action of the Shunt Active Power Filter.

Fig.9 Grid Currents for Three-Phases A, B and C

Fig. 10 Harmonic Spectrum for grid Current Three-Phases A, B and C

The load voltages for the three-phase A, B and C are shown in Fig.l1

Fig. 11 Load Voltages for the Three Phases A, B and C

The load current for the three-phases are shown in Fig. 12. It is found that even after interfacing the inverter at t=0.2s, the distortion in the current due to non-linear load is maintained at the same level as before interfacing it.

Fig. 12 Load Current for the Three-Phases A, B and C

The neutral current is shown in Fig. 13. It is seen that after interfacing the Shunt Active Power Filter at t=0.2s, the magnitude of the neutral current is reduced.

m mi su ¡TI

InL .rHL'f-.jnl

Fig. 13 Neutral Current

The FFT analysis is done for the three phase grid currents and load currents to calculate the percentage of THD before and after the interfacing of the SAPF. Before interfacing the SAPF the current profile is same for both the grid current and load current. The THD of the grid current and load current before and after the interfacing of the SAPF is given in Table 1 and Table 2 respectively. It is seen that the THD is high before interfacing the SAPF in the grid current and load current. After connecting the SAPF the THD in source current is reduced and at the load side it is maintained at the same level. The magnitude of the neutral current is found to be reducing after interfacing SAPF; it is shown in Table 3.

Table 1. Percentage THD for Grid Currents.!)

Phase Sequence Before interfacing SAPF (in %) After Interfacing SAPF (in %)

A 9.42 6.18

В 9.41 6.86

С 9.43 7.51

Table 2. Percentage : THD for Load Currents.

Phase Sequence Before interfacing SAPF (in %) After Interfacing SAPF (in %)

A 9.42 9.93

В 9.41 8.99

С 9.43 10.03

Table 3. Magnitude of Neutral Current

Before interfacing SAPF (in %) After Interfacing SAPF (in %)

1.26 0.28

6. Conclusion

This paper has presented a novel control of an existing grid interfacing inverter to improve the quality of power at PCC for a3-phase 4-wire DG system. It has been shown that the grid-interfacing inverter can be effectively utilized for power conditioning without affecting its normal operation. The grid-interfacing inverter with the proposed approach can be utilized to inject real power generated from RES to the grid, and/or, operate as a shunt Active Power Filter (APF). This approach thus eliminates the need for additional power conditioning equipment to improve the quality of power at PCC. Extensive MATLAB/Simulink simulation results have validated the proposed approach and have shown that the grid-interfacing inverter can be utilized as a multi-function device.

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