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CÁC BÁO CÁO BAO GỒM: 1. An Improved Control Strategy for Hybrid Series Active Filter dealing with Unbalanced Load (Thầy Nguyễn Xuân Tùng). 2. Xác định vị trí và dung lượng bù tối ưu trong lưới phân phối hình tia (Optimal Capacitor Placement and Sizing for Radial Distribution Networks). (Thầy Nguyễn Đức Huy). 3. Impacts of Inverterbased Distributed Generation Control Modes on Shortcircuit Currents in Distribution Systems. (Thầy Đào Văn Tú)
ĐẠI HỌC BÁCH KHOA HÀ NỘI BỘ MÔN HỆ THỐNG ĐIỆN eBook for You PHҪNIII BҦO Vӊ VÀ TӴ ĈӜNG HÓA TRONG Hӊ THӔNG ĈIӊN Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 201 eBook for You Paper An Improved Control Strategy for Hybrid Series Active Filter dealing with Unbalanced Load Nguyen Xuan Tung ∗ Non-member Goro Fujita ∗ Member Kazuhiro Horikoshi ∗∗ Member This paper presents an improved control strategy for hybrid series active power filter (HSAF) working with nonlinear and unbalance three-phase three-wire loads. An algorithm based on the Instantaneous Power Theory is introduced to precisely extract only harmonic component from supply current, even this current is contaminated with negative sequence component due to the imbalance of load. An improved control strategy based on that sequence extraction algorithm is proposed and investigated in detail by numerical simulation. The proposed control method has shown a better performance in mitigating harmonics, especially for the nonlinear and unbalanced loads. Keywords: Series active filter, Instantaneous power theory, Unbalanced load, Harmonic isolation. 1. Introduction The increasing use of power electronics-based loads (adjustable speed drives, switch mode power supplies, etc.) is responsible for the rise in harmonic distortion levels. These nonlinear loads appear to be prime sources of harmonic distortion in a power distribution system. Harmonics have a number of undesirable effects on the distribution system such as the excessive voltage distor- tion, increasing resistive losses or voltage stresses. In addition, the harmonic currents can interact adversely with a wide range of power system equipment such as capacitors, transformers, and motors, causing additional losses, overheating, and overloading. Because of the ad- verse effects that harmonics have on equipments, many solutions have been developed to deal with harmonic control (1)∼(3) . Besides conventional solutions such as passive filters, the hybrid series active power filters have proven to be an interesting alternative to compensate harmonics in power distribution systems. Compared to passive fil- ters, active filters provide superior filtering performance, more flexible operation and more compact. There are various series hybrid active power filter topologies re- ported in literature (4)∼(6) , but the most common one is shown in Fig. 1 Figure 1 shows the system configuration of series hy- brid active power filter (APF), in which the shunt pas- sive filter consists of one or more single-tuned LC fil- ters and/or a high pass filter (HPF). The hybrid se- ries APF is controlled to act as a harmonic isolator be- tween the source and nonlinear load by injection of a ∗ Shibaura Institute of Technology 3-7-5, Toyosu, Koto-ku, Tokyo 135-8548 ∗∗ Tohoku Electric Power Co.,Inc. 7-2-1, Nakayama, Aoba-ku, Sendai, Miyagi 981-0952 Fig. 1. Typical system configuration of hybrid se- ries active power filter controlled harmonic voltage source. It is controlled to offer zero impedance at the fundamental frequency and high impedance (ideally open circuit) at all undesired harmonic frequencies. This forces all harmonic load cur- rents to flow into the passive filter and decoupling the source and nonlinear load at all frequencies, except at the fundamental. Control algorithm of the series active power filter is mostly based on the Instantaneous Power Theory (7)∼(10) or the Synchronous Reference Frame (so-called dq transform) (11)∼(15) . Basically, all those existing con- trol schemes calculate the harmonic current reference signal based on the separation of fundamental positive sequence component and other “harmonic” components. In detail, the harmonic contents (i h ) are determined by excluding the fundamental component from measured supply current as presented in Eq. 1: [i h ]=[i s ] −[i fp ] ····························(1) here •i s : measured supply current. •i fp : fundamental positive sequence component of corresponding supply current. The high impedance imposed by the series active power filter is achieved by generating an appropriate voltage of IEEJ Trans. TEEE, Vol.125, No.1, 2005 1 Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 202 eBook for You the same frequency with that of the harmonic current component as shown below: [v F ]=K ×[i h ] ····························· (2) with K is the amplification factor. The performance of the active power filter depends mainly on the selected reference generation scheme. The reference current must reflex the desired compensation current, however, since the Eq. 1 is used, certainly the “harmonic” component here comprises all other current components those differ from fundamental positive se- quence current. Therefore this extraction method gives the true harmonic component if the load is assumed to be perfectly balanced. In a quite common situation, the load current is usually unbalanced with the existence of fundamental negative sequence current. Consequently, that negative sequence current will present in the extracted harmonic compo- nent i h if Eq. 1 is still utilized although it is not a real “harmonic” component. In this case, the series active filter would have to handle not only the real harmonic current but also the undesirable fundamental negative sequence current. As a result, the controller would force the series active filter to generate the compensating volt- age at fundamental frequency and this could increase significantly the power rating of the PWM converter and also induce high voltage oscillations at double the sys- tem frequency in dc link (16) . Literature on series active filter shows that so far no attempt has been made to deal with unbalanced load and that is a disadvantageous point of the existing con- trol strategies. In this paper, an improved control strat- egy based on Instantaneous Power Theory is proposed which will ensure that the series active power filter work with only the harmonic components even the load is un- balanced. This paper firstly introduces the Instantaneous Power Theory, and then discusses the basic principle and scheme used to extract positive and negative sequence components. Next, control strategy is presented in de- tails. Finally, the numerical simulations are carried out to validate the feasibility and effectiveness of this pro- posal. 2. Instantaneous power theory and its ap- plication 2.1 Brief review of Instantaneous Power The- ory The Instantaneous Power Theory (16) is well uti- lized for control system of active filter. Control strategy based on this method provides fast response to changes in power system, good compensating performance and imposes a little computational burden (17) (18) . Figure 2 shows the calculation block of this theory: Firstly, three-phase voltages and load currents are trans- formed into the stationary α-β reference frame (Clarke transformation): v α v β = 2 3 × 1 −1/2 −1/2 0 √ 3 2 − √ 3 2 × ⎡ ⎣ v a v b v c ⎤ ⎦ (3) Fig. 2. Calculation block of Instantaneous Power Theory i α i β = 2 3 × 1 −1/2 −1/2 0 √ 3 2 − √ 3 2 × ⎡ ⎣ i a i b i c ⎤ ⎦ (4) Next, the instantaneous real power p and instantaneous reactive power q are calculated by: p q = v α v β −v β v α × i α i β ·············(5) According to Instantaneous Power Theory, p and q can be decomposed into average parts ¯p,¯q (dc parts) and oscillating parts ˜p,˜q asshowninEq.6: p =¯p +˜p q =¯q +˜q ····························(6) Where the ¯p,¯q are the dc components corresponding to the product of fundamental positive sequence quantities, and ˜p,˜q are the ac components corresponding to prod- uct of other components those differ from fundamental positive sequence components. By using a high-pass filter, the oscillating components ˜p,˜q can be extracted from p, q and then the reference harmonic current can be obtained as follow: i Fα i Fβ = 1 v 2 α + v 2 β × v α v β v β −v α × ˜p ˜q · (7) Next, those reference currents go through an inverse α-β transform to generate the reference current in conven- tional three-phase abc frame. 2.2 Consideration in case voltage and current are distorted and unbalanced If voltage and cur- rent are distorted (due to the presence of high order frequency harmonic components) and unbalanced (with the existence of the fundamental negative sequence com- ponent) then the resulted oscillating power components ˜p,˜q will be the cross products of not only the harmonic components but also the fundamental negative sequence components (16) . In other words, the oscillating power components ˜p,˜q contain the fundamental negative se- quence components. Consequently, if the reference current signals are gener- ated based on those power components then they would contain both negative sequence and harmonic compo- nents rather than only harmonic components as ex- pected. This fact raises a need to develop a method which can precisely extract only harmonic currents de- spite of the presence of the fundamental negative se- quence component. 2 IEEJ Trans. TEEE, Vol.125, No.1, 2005 Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 203 eBook for You An Improved Control Strategy for Hybrid Series Active Filter dealing with Unbalanced Load 3. Approach for determining reference current 3.1 Problem formulation and proposal All the existing control strategies for the series active power filter determine the reference currents i h by simply sub- tracting the fundamental positive sequence current from the supply current as below: [i h ]=[i s ] −[i fp ] ····························(8) This approach has shown many disadvantages as already mentioned in Sec. 1 since the reference current i h will contain the fundamental negative sequence current com- ponent if load is unbalanced. In order to overcome this drawback, the improved reference current extrac- tion method is proposed as follow: [i h ]=[i s ] −[i fp ] −[i fn ] ·····················(9) where i s ,i fp ,i fn are the measured supply current, fun- damental positive and negative sequence current com- ponents of corresponding supply current respectively. The improved reference current extraction method dif- fers from previous proposals since it eliminates not only the fundamental positive sequence current but also the fundamental negative sequence current from the supply current to form the harmonic reference current. Because i s is already measured, the remaining task of determining the harmonic reference current based on new proposal is to extract the positive and negative se- quence components (two last components at right side of Eq. 9). In this paper, the sequence current compo- nent extraction implementation is completely based on the Instantaneous Power Theory. Basically, it includes following steps: •Generate an auxiliary voltage which contains only a pure fundamental positive or negative sequence voltage. •This auxiliary voltage will be used together with ori- gin supply current to create the instantaneous power components. •Implement filtering processes to achieve the desired power portions from those power components then applying inverse transformation to generate the cor- responding current. The role of the auxiliary voltage will be fully described in next section. 3.2 Positive sequence current extraction Con- sidering an auxiliary voltage that contains only funda- mental positive sequence component V +1 with phase an- gle φ +1 assumed to be zero then the α-β transform of this voltage results in: v +1α = √ 3V +1 sin (ω 1 t) v +1β = − √ 3V +1 cos (ω 1 t) ···········(10) Next step, this pure fundamental positive sequence volt- age is used together with the supply current to calculate the instantaneous power quantities p, q following Eq. 5. The supply current may contain fundamental negative sequence and high order harmonic components, however, the resulted dc components ¯p , ¯q of those power quanti- ties in this case are the cross product of only fundamen- tal positive components as shown below: ¯p =3V +1 I +1 cos (−δ +1 ) ¯q =3V +1 I +1 sin (−δ +1 ) ············(11) here •I +1 : the fundamental positive sequence current component of the measured supply current. •δ +1 : phase angle between the auxiliary positive se- quence voltage V +1 and the fundamental positive sequence current component I +1 . It is clear to see that only fundamental positive sequence voltage V +1 and current I +1 components contribute to average value ¯p and ¯q , the negative sequence compo- nents does not appear in those power quantities. Next, the low-pass filter is utilized to extract only those dc power components ¯p , ¯q . Once those power components in Eq. 11 is extracted then it is easy to obtain the posi- tive sequence current using the same definition as shown in Eq. 7. For extracting fundamental positive sequence compo- nent, the amplitude of v +1α and v +1β are not important and can be chosen arbitrarily due to the fact that they appear in both “direct” and “inverse”calculations (16) . For simplicity, they are set to unity hence Eq. 10 be- comes: v +1α =+sin(ω 1 t) v +1β = −cos (ω 1 t) ··················(12) 3.3 Negative sequence current extraction Similar procedure is employed to extract negative se- quence current, however, an auxiliary negative sequence voltage is considered instead of the auxiliary positive se- quence voltage. Theresultsofα-β transform of this auxiliary pure fun- damental negative sequence voltage is shown in Eq. 13. v −1α =+sin(ω 1 t) v −1β =+cos(ω 1 t) ··················(13) Here the amplitude of negative sequence voltage v −1α and v −1β are again selected to be unity and correspond- ing phase angles are assumed to be zero for simplifica- tion. The dc power components ¯p , ¯q resulted from the product of those auxiliary negative sequence voltages {v −1α ,v −1β } and the supply current are shown in Eq. 14 ¯p =3V −1 I −1 cos (−δ −1 ) ¯q =3V −1 I −1 sin (−δ −1 ) ············(14) here •I −1 : the fundamental negative sequence current component of the measured supply current. •δ −1 : phase angle between the auxiliary negative se- quence voltage V −1 and the fundamental negative sequence current component I − 1 . Again, only fundamental negative sequence voltage V −1 and current I −1 components show up in the average value ¯p ,¯q even the supply current is distorted and un- balanced. Therefore, if those dc power components are IEEJ Trans. TEEE, Vol.125, No.1, 2005 3 Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 204 eBook for You Fig. 3. Positive sequence current detection circuit extracted through a filtering process then the negative sequence current can be calculated using same definition as shown in Eq. 7. 3.4 Generation of auxiliary voltages and se- quence detection circuit The generation of the fundamental positive and negative sequence components is necessary for determining the harmonic reference cur- rent. An important part of generating auxiliary ref- erence voltage is the phase locked loop (PLL) circuit. The PLL circuit tracks continuously the fundamental frequency ω 1 of the measured system voltage. The PLL is designed to operate properly under distorted and unbalanced voltage wave forms. The frequency ω 1 is used in a sine wave generator to produce two quan- tities sin(ω 1 t)and−cos(ω 1 t) those correspond to the auxiliary fundamental positive sequence voltages v +1α & v +1β (mentioned in Eq. 12). The PLL circuit is al- ready well introduced in literature and it has good per- formance in handling this task (19) (20) (see Appendix for operation principle). Figure 3 shows the positive sequence detection circuit based on principle stated in Sec. 3.2. Similar circuit can be implemented for the negative sequence extraction if the output of PLL circuit are −sin(ω 1 t) and cos(ω 1 t) following the Eq. 13. 4. Control strategy 4.1 Operation principle of series active filter as harmonic current isolator It is well known that series active filters correct current system distor- tion caused by non-linear load by synthesizing an active impedance presenting a zero impedance at fundamen- tal frequency and a high resistance K between load and source at all harmonics frequencies. By inserting a high resistance K, the series active filter forces the high fre- quency current flow mainly through LC passive filter connected in parallel to load (2) (21) . The equivalent single phase circuit for harmonic com- pensation is shown in Fig. 4. In this figure, non-linear load is represented by a harmonic current source I h and source voltage is represented by harmonic voltage source V sh . The series active filter is equivalent to a controlled voltage source V c and shunt passive filter becomes an equivalent impedance Z F . If the series active filter is controlled as V c = K ×I sh (equivalent to a resistor of K ohm) then the I sh can be calculated as: I sh = V sh Z S + Z F + K + Z F Z S + Z F + K × I h ··(15) Fig. 4. Equivalent circuit for harmonic compensation Fig. 5. Proposed control circuit for series active filter If the gain factor K is set sufficiently large as K (Z S + Z F ) then neither harmonic current flow from load to ac source nor from ac source to load side. 4.2 Control Scheme The proposed control cir- cuit is shown in Fig. 5, where the three-phase load cur- rent is measured and transformed into stationary α-β frame. The PLL circuit generates the auxiliary posi- tive and negative sequence voltages (corresponding to Eq. 12 & 13). Those currents and auxiliary voltages in stationary α-β frame are supplied to positive and neg- ative sequence extraction blocks (extraction principle is detailed in Sec. 3.2 & 3.3). The output positive and neg- ative sequence currents in α-β frame are passed through the inverse α-β transform to give the positive and neg- ative sequence currents in conventional three-phase abc frame. Harmonic current is extracted from measured supply current after subtracting the fundamental positive and negative sequence components as stated in Eq. 9. Next, each extracted harmonic current is passed through a gain block with the amplification factor of K to form reference voltage v F (Eq. 2). Finally, this reference voltage v F is applied to the gate control circuit for each PWM converter. 5. Simulation setup Simulation is setup as following: •The investigated system is three-phase, three-wire system then zero sequence component does not ex- ist. •Source’s line to line voltage is 200V (50Hz). Source impedance is Z s =0.02∠80 pu (with the system base of U base = 200V and S base =20kV A). •Active filter rating capacity is 700VA and it is ac- tivated at 0.5 [s] during simulation. •PWM converter’s switching frequency is set at 4 IEEJ Trans. TEEE, Vol.125, No.1, 2005 Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 205 eBook for You An Improved Control Strategy for Hybrid Series Active Filter dealing with Unbalanced Load Fig. 6. Representative diagram for simulation Table 1. Parameter of shunt passive filter Inductance [mH] Capacitance [μF] 5 th filter 1.2 340 Q=14 7 th filter 1.2 170 Q=14 High-pass 0.26 300 R=3Ω 15 kHz. The dc source’s voltage is V dc = 200V . •Coupling transformer’s turn ratio n = 1 : 20. The ripple filter connected at the output of PWM con- verter consists of a series inductor L r =1mH and a shunt capacitor C r =0.33μF . •Passive filters are tuned to the most dominant 5 th , 7 th harmonics and a high pass filter with total ca- pacity of 10kVA (parameters are given in Table 1). •The unbalanced and nonlinear load is represented by a combination of a 20 kVA three-phase thyristor rectifier and a linear single phase load (17.5Ω) as shown in Fig. 6. This setup gives a total load cur- rent of about 60 [A] RMS with the unbalance factor I 2 /I 1 ∼ 10%. •Current is measured in ampere [A]. 6. Result and discussion 6.1 Effectiveness of series active filter equipped with improved control algorithm Figure 7 gen- erally shows the effect of active power filter equipped with improved control strategy on the nonlinear and un- balanced load. For detail: •Figure 7a shows that: Once active filter is acti- vated at 0.5 [s] then it almost immediately takes ef- fect to reduce the harmonic contents injecting into the source, the source current almost becomes si- nusoidal. Moreover, the active filter not only suc- cessfully mitigates the harmonic contents but also preserves perfectly the imbalance characteristic of load as expected. •Figure 7b & 7c present the spectra of source current before and after active filter is activated. Evidently, the harmonic contents significantly drops at all har- monic frequencies. Thus, this is again to numer- ically confirm the effectiveness of the series active filter. The fundamental positive and negative sequence cur- rents extracted from measured source current are also shown in Fig. 8 & Fig. 9. Noticeably, the positive and negative sequence current remain constant, even while active filter is operating. These results confirm the ad- vantage of improved control strategy: the active filter does not alter the load imbalance characteristic. In other words, the active filter does work with only harmonic 0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540 0.550 0.560 -150 -100 -50 0 50 100 150 Isa Isb Isc (a) Harmonic mitigation effect with improved control strategy for unbalanced load Current S p ectrum 5.0 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 2 2 23 24 25 26 27 28 29 30 31 (b) Current spectra: Before activation of active filter Current S p ectrum 5.0 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 2 2 23 24 25 26 27 28 29 30 31 (c) Current spectra: After activation of active filter Fig. 7. Waveshapes and sp ectra of source current before and after active filter is activated 0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540 0.550 0.560 -150 -100 -50 0 50 100 150 Iap Ibp Icp Fig. 8. Extracted positive sequence current 0.460 0.480 0.500 0.520 0.540 0.560 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 Ian Ibn Icn Fig. 9. Extracted negative sequence current current components as designed. For nonlinear and balanced load, the active filter equipped with new control strategy still works very well as it can be seen in Fig. 10, the harmonic contents are mostly eliminated and the inherent load characteristic remains untouched. 6.2 Effectiveness comparison over series active filters equipped with improved and previous con- IEEJ Trans. TEEE, Vol.125, No.1, 2005 5 Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 206 eBook for You 0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540 0.550 0.560 -150 -100 -50 0 50 100 150 Isa Isb Isc Fig. 10. Harmonic mitigation with improved con- trol strategy for balanced load 0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540 0.550 0.560 -150 -100 -50 0 50 100 150 Isa Isb Isc (a) Result with improved control strategy 0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540 0.550 0.560 -150 -100 -50 0 50 100 150 Isa Isb Isc (b) Result with previous control strategy Fig. 11. Waveshape comparison in cases with im- proved and previous control strategies trol algorithms For comparison purpose, the active filter which is equipped with the previous control strat- egy is also simulated. Simulation studies for comparison are setup based on follow assumptions: •In previous control strategy, the negative sequence component is not excluded from the reference har- monic current. •On the contrary, for the improved control strategy, the negative sequence component is excluded from the reference harmonic current. •Comparisons are carried out with unbalanced loads since the improved control strategy is proposed to help the series active filter performs better under unbalanced loading conditions. •All other conditions remains the same for both cases. Figure 11 & 12 compare the current waveshapes and spectra in cases the active filters utilize the improved and previous control strategies. Under the same test- ing conditions, the active filter with improved control strategy shows a better performance. This conclusion can be clarified in below discussion: •The active filter equipped with previous control strategy will have to handle both harmonic and the fundamental negative sequence current components, consequently the load will be forced to be balanced as shown in Fig. 11b and the PWM converter might be easily overloaded. Besides exposing the active Current S p ectrum 5.0 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 2 2 23 24 25 26 27 28 29 30 31 (a) With improved control strategy Current S p ectrum 5.0 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 2 2 23 24 25 26 27 28 29 30 31 (b) With previous control strategy Fig. 12. Current spectrum comparison in cases with improved and previous control strategies filter to overload condition, degrading the harmonic mitigation effect, that balancing the inherent unbal- anced load does not bring any benefit for customer who invested money for that active filter. •In contrast, the active filter employing the improved control method does not need to handle the negative sequence current then it can devote all of it capacity for harmonic mitigation function. As a result, the harmonic mitigation efficiency in this case is higher. This higher efficiency is illustrated in Fig.12a as the harmonic contents shown there are much more lower than those in Fig.12b. In other words, the active fil- ter with the improve control algorithm has a better performance. Figure 13 shows the compensating voltages generated by the series active filters (the blue and red lines show instantaneous and RMS values respectively), those fig- ures are for the output volt-ampere comparison purpose. Figure 13b presents the output voltage of series active filter which is equipped with previous control algorithm. Apparently, looking at voltage waveform, one may see the presence of the 50Hz component. That is reason why the output voltage in RMS value is almost double of that in Fig. 13a. This phenomenon is due to all the previous control algorithms do not exclude the negative sequence component which may occur if load is unbal- anced. For numerical detail comparison: •In case the active filter employs improved control al- gorithm, the compensating voltage presents a RMS value of only about 3.4V . This means that the volt-ampere rating of the series active filter is only 3.4V ×60A×3 = 612VA. Assuming that the active filter rating capacity is chosen as 700VA, then this figure presents only a small portion as about 3.5% of the load rating 20kV A. •On the contrary, if the previous control algorithm is utilized, then the output compensating voltage in 6 IEEJ Trans. TEEE, Vol.125, No.1, 2005 Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 207 eBook for You An Improved Control Strategy for Hybrid Series Active Filter dealing with Unbalanced Load 0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540 0.550 0.560 -20.0 -16.0 -12.0 -8.0 -4.0 0.0 4.0 8.0 12.0 16.0 20.0 V_active_filter VfilterRMS (a) With improved control strategy 0.460 0.470 0.480 0.490 0.500 0.510 0.520 0.530 0.540 0.550 0.560 -20.0 -16.0 -12.0 -8.0 -4.0 0.0 4.0 8.0 12.0 16.0 20.0 V_active_filter VfilterRMS (b) With previous control strategy Fig. 13. Voltage generated by series active filters Zs = 0.02 p u THDv (%) 1.06281 02 (a) Z s =0.02pu Zs = 0.1 p u THDv (%) 2.01309 02 (b) Z s =0.1pu Zs = 0.2 p u THDv (%) 2.30476 02 (c) Z s =0.2pu Fig. 14. Total harmonic distortion (THD) of source voltages this case is about 8V as shown in Fig. 13b. Numer- ically, the required volt-ampere output of the active filter now is up to 8V ×60A ×3 = 1440VA.Inthis case, the active filter is about 1440VA/700VA = 2.05 times overloaded. As a result, this will seriously damage the converter. Apparently, the percentage of overload depends on the load unbalance factor. 6.3 Influence of source side impedance on har- monic mitigation effect From Fig. 4, it is easy to see that the harmonics generated by nonlinear load are injecting into source. As consequence, the source volt- age will be distorted depending on the value of source impedance. If the source impedance is low, then the voltage distortion is low and vice versa. Since this con- trol algorithm utilizes the quantities calculated from both voltage and current, then it is necessary to examine the influence of voltage distortion (or source impedance) on the compensation effect. All above simulations run with source impedance set at 0.02pu, then now two more worse scenarios are exam- ined: source impedances are set higher at 0.1pu and 0.2pu. Consequently, the results in Fig. 14 show that total harmonic distortion (THD) factors of source volt- ages are 1.06%, 2.01% and 2.3% respectively. Thus, the higher the source impedance, the worse the voltage dis- tortion. Figure 15 show the corresponding THDs of source cur- rents after compensation. Noticeably, after series active filter was started, the remain amounts of harmonic con- Zs = 0.02 p u THDi (%) 0.869893 02 (a) Z s =0.02pu Zs = 0.1 p u THDi (%) 0.631617 02 (b) Z s =0.1pu Zs = 0.2 p u THDi (%) 0.497768 02 (c) Z s =0.2pu Fig. 15. Total harmonic distortion (THD) of source currents tents tend to go down for three cases. This fact may be discussed as below: •Since the source impedance are set increasingly from 0.01pu to 0.2pu , the source becomes weaker and its voltage actually get distorted. However, on the con- trary, the higher the source impedance, the smaller amount of harmonic currents injected into source. That is reason why the harmonic contents after com- pensation is reduced correspondingly for three cases. •Despite the distortion of voltage, the series active fil- ter still work effectively. This point proves that the Instantaneous Power Theory used in the sequence extraction algorithm can work well under the volt- age distortion conditions. 7. Conclusions The improved control strategy for hybrid series active filter is already proposed and investigated in detail. This control method bases on sequence component elimina- tion algorithm to obtain only the harmonic content from a distorted and unbalanced current set. Consequently, this control strategy does help the series active filter to improve its performance, especially when load is unbal- anced. Main conclusions can be recognized as follow: •Only harmonic component is precisely extracted to form the reference current for the series active filter, therefore, the active filter work with only harmonic component as expected, even when load is unbal- anced. •The proposed control strategy enhances the stabil- ity of control process since the imbalance of load will not have any affect on the reference signal. •Especially, the active filter is protected away from severer overload conditions because it does not have to deal with the fundamental negative sequence component which may occur if load is unbalanced. •The proposed control strategy is well tailored to suit with all operating conditions such as serving bal- anced or unbalanced loads. In other words, it can apply for the series active filter working with any generic loads. Finally, all the simulation results have successfully vali- dated the effectiveness and feasibility of this proposal. References ( 1 ) Z. Salam, and T. P. Cheng, and A. Jusoh, “Harmonics Miti- gation Using Active Power Filter: A Technological Review”, IEEJ Trans. TEEE, Vol.125, No.1, 2005 7 Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 208 eBook for You Elektrika Journal of Electrical Engineering, (2006) ( 2 ) F. Z. Peng, and H. Akagi, and A. Nabae, “A New Approach to Harmonic Compensation in Power Systems”, IEEE Industry Applications Society Annual Meeting, (1988) ( 3 ) M. El. Habrouk, and M. K. Darwish, and P. Mehta, “Active Power Filters: A Review”, IEE Proceedings Electric Power Applications, (2000) ( 4 ) F. Z. Peng, and H. Akagi, and A. Nabae, “Compensation Characteristics of The Combined System of Shunt Passive and Series Active Filters”, IEEE Transactions on Industry Ap- plications, (1993) ( 5 ) F. B. Libano, and D. S. L. Simonetti, and J. Uceda, “Fre- quency Characteristics of Hybrid Filter Systems”, Power Electronics Specialists Conference, (1996) ( 6 ) J. Turunen, and M. Salo, and H. Tuusa, “Comparison of Three Series Hybrid Active Power Filter Topologies”, International Conference on Harmonics and Quality of Power, (2004) ( 7 ) B. Han, and B. Bae, and H. Kim, and S. Baek, “Combined Operation of Unified Power Quality Conditioner With Dis- tributed Generation”, IEEE Transactions on Power Deliv- ery, (2006) ( 8 ) S. P. Litran, and P. Salmeron, and R. S. Herrera, and J. R. Vazquez, “New Control Strategy to Improve Power Quality Using A Hybrid Power Filter”, International Conference on Renewable Energies and Power Quality, (2008) ( 9 ) H. Akagi, and Y. Kanazawa, and A. Nabae, “Instantaneous Reactive Power Compensators Comprising Switching Devices without Energy Storage Components”, IEEE Transactions on Industry Applications, (1984) (10) Q. Wang, and W. Yao, and J. Liu, and Z. Wang, “Voltage Type Harmonic Source and Series Active Power Filter Adopt- ing New Control Approach”, Industrial Electronics Society Conference IECON, (1999) (11) S. Bhattacharya, and D. Divan, “Synchronous Frame Based Controller Implementation for A Hybrid Series Active Filter System”, Industry Applications Conference, (1995) (12) G. D. Marques, and V. F. Pires, and M. Malinowski, and M. Kazmierkowski, “An Improved Synchronous Reference Frame Method for Active Filters”, The International Conference on Computer as a Tool, (2007) (13) S. Bhattacharya, and D. Divan, “Design and Implementation of A Hybrid Series Active Filter System”, Power Electronics Specialists Conference, (1995) (14) K. Karthik, and J. E. Quaicoe, “Voltage Compensation and Harmonic Suppression Using Series Active And Shunt Passive Filters”, Canadian Conference on Electrical and Computer Engineering, (2000) (15) B. R. Lin, and B. R. Yang, and T. L. Hung, “Implementa- tion of A Hybrid Series Active Filter for Harmonic Current and Voltage Compensations”, International Conference on Power Electronics, Machines and Drives, (2002) (16) H. Akagi, and E. H. Watanabe, and M. Aredes: Instantaneous Power Theory and Applications to Power Conditioning, John Wiley & Sons, Inc. (2007) (17) D. Chen, and S. Xie, “Review of The Control Strategies Applied to Active Power Filters”, IEEE International Con- ference on Electric Utility Deregulation, Restructuring and Power Technologies, (2004) (18) G. W. .Chang, and T. C. Shee, “A Comparative Study of Active Power Filter Reference Compensation Approaches”, Power Engineering Society Summer Meeting, (2002) (19) S. A. O. Silvia, and P. F. Donoso-Garcia, and P. F. Seixas, “A Three Phase Line Interactive UPS System Implementation with Series-Parallel Active Power Line Conditioning Capaci- ties”, IEEE Transactions on Industry Applications, (2002) (20) F. F. Ewald, and M. A. S. Masoum: Power Quality in Power Systems and Electrical Machines, Academic Press (2008) (21) L. Xu, and E. Acha, and V. G. Agelidis, “A New Synchronous Frame-Based Control Strategy for A Series Voltage and Har- monic Compensator”, Applied Power Electronics Conference and Exposition, (2001) Appendix Phase locked loop (PLL) circuit The PLL is one of components of the sequence cur- rent detection circuit. It detects fundamental angular frequency (ω 1 ) and generates synchronous sinusoidal sig- nals those correspond to auxiliary positive sequence volt- ages under sinusoidal as well as highly distorted and un- balanced source voltages. The PLL circuit introduced app. Fig. 1. Phase locked loop circuit here is based on the instantaneous active power expres- sion: p 3Φ = v a i a + v b i b + v c i c = v ab i a + v cb i c =¯p 3Φ +˜p 3Φ (A1) The current feedback signals i a (ωt)=sin(ωt)and i c (ωt)=sin(ωt + 120 0 ) are generated by sine genera- tor circuits through the time integral of the output ω of the PI controller. The PLL can reach stable point of operation only if the input p 3Φ of PI controller has, in steady state, a zero average value, that is, ¯p 3Φ =0. Moreover, the low frequency oscillations ˜p 3Φ should be minimized. Because i a (ωt)andi c (ωt) contain only positive-sequence components and have unity magnitudes, therefore, the average three phase power p 3Φ =¯p 3Φ is given by: ¯p 3Φ =3V +1 I +1 cos(φ + + δ + ) ··············· (A2) where φ + and δ + are the initial phase angles of the fun- damental positive sequence voltage and current i a (ωt) respectively. Now the PLL can reach stable point of op- eration only if the input p 3Φ of PI controller has a zero average value, that is equivalent to: ¯p 3Φ =3V +1 I +1 cos(φ + + δ + )=0··········· (A3) The above condition is satisfied since φ + + δ + =90 0 . This means that the auxiliary currents i a (ωt)andi c (ωt) become orthogonal to the fundamental positive sequence component V +1 of the measured voltages v a and v c re- spectively. Therefore, the generated auxiliary voltage v +1a = sin(ωt −90 0 ) is in phase with fundamental pos- itive sequence voltage V +1 . Similar relations hold for v +1b and v +1c . Nguyen Xuan Tung (Non-member) received the B.E. de- gree in electrical engineering from Hanoi Uni- versity of Technology, VietNam in 1999 and the M.E degree from Curtin University of Technology, Australia in 2005. He has been pursuing PhD degree in Shibaura Institute of Technology, Japan since 2007. His interests are about protective relay system and power quality issue in power distribution system. 8 IEEJ Trans. TEEE, Vol.125, No.1, 2005 Tổng hợp các bài báo khoa học giai đoạn 2007-2012 Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 209 eBook for You [...]... source is strong and possibly the distribution line sections are short Figure 15, 16 and 17 show the absolute value of node voltages with respect to the source voltage of 6750 (V) In this simulation, the downstream node voltages sometimes are higher than the source voltage due to the lead223 B mụn H thng in - i hc Bỏch Khoa H Ni 6 IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx Tng hp cỏc bi bỏo khoa hc giai... circuit complexity However, another concern Shibaura Institute of Technology 3-7-5, Toyosu, Koto-ku, Tokyo 135-8548, Japan 211 B mụn H thng in - i hc Bỏch Khoa H Ni IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx Typical topology of DVR 1 Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 Fig 2 Equivalent diagram of investigated system Fig 4 Relationship between compensated fault current and corresponding injected voltage... current At this time, relationship as shown in Eq 1 has been 212 B mụn H thng in - i hc Bỏch Khoa H Ni 2 IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx eBook for You US + UDV R = If (c) ì Zf = U(c) ã ã ã ã ã ã ã ã ã ã ã (2) Fig 3 Relationship between fault current and voltage drop on fault impedance Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 Fault Current Limiting Function of Dynamic Voltage Restorer Utilizing... storage as shown in Fig 9 Main components of DVR model include: Coupling transformer 214 B mụn H thng in - i hc Bỏch Khoa H Ni 4 IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx eBook for You with = angle(Vinjected , Icompensated ) Maximum voltage can occur across capacitor is: Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 Fault Current Limiting Function of Dynamic Voltage Restorer Utilizing Signals from Existing... post-fault in which fault current limiting function of DVR shows its eect (in practice, time margin between FPS and LPS is nor215 B mụn H thng in - i hc Bỏch Khoa H Ni IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx 5 eBook for You 2 Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 CURRENT (in Ampere) 1.0k Ia Ib CURRENT (in Ampere) Ic 1.0k 0.3k y 0.0 0.0 -0.3k -0.3k -0.5k -0.5k -0.8k -0.8k -1.0k -1.0k 0.000 0.050... after 0.1 [s] (at the same time FPS reset to standby), DVR immediately reverse its action this leads to DC link voltage goes down suddenly 216 B mụn H thng in - i hc Bỏch Khoa H Ni 6 IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 Fault Current Limiting Function of Dynamic Voltage Restorer Utilizing Signals from Existing Protective Relays CURRENT (in Ampere) Ia Ib Ic (5)... Tohoku Electric Power Co.,Inc 7-2-1, Nakayama, Aoba-ku, Sendai, Miyagi 981-0952 218 B mụn H thng in - i hc Bỏch Khoa H Ni IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx 1 eBook for You Keywords: Phase Loading Imbalance, Discrete Optimization, Passive Compensation, Distribution System Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 Diagram of unbalanced load and compensator Fig 2 the feeder including mutual coupling... of using only either capaci- tive or inductive element as a compensator One of the factors which electrical utility consider 219 B mụn H thng in - i hc Bỏch Khoa H Ni 2 IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx eBook for You Fig 1 Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 Phase Load Balancing In Distribution Power System Using Discrete Passive Compensator Diagram of single-phase compensations Table 1... compensator is capacitive then the required vector can be decomposed into the two nearest component vectors on CA and BC axes with 220 B mụn H thng in - i hc Bỏch Khoa H Ni IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx 3 eBook for You Fig 3 Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 Representative feeder as seen in practice Algorithm must produce as more accurate results as possible in comparison with other proposed... utility company) The compensators are selected to be all inductive due to existing leading power factors of loads; if ca222 B mụn H thng in - i hc Bỏch Khoa H Ni IEEJ Trans TEEE, Vol.xxx, No.xx, xxxx Phase load balancing algorithm 5 Tng hp cỏc bi bỏo khoa hc giai on 2007-2012 Negative current through Line 2 16 Before Fig 8 Percentage (%) 14 Case study of feeder with three loads After 12 10 8 6 4 2 Negative . even while active filter is operating. These results confirm the ad- vantage of improved control strategy: the active filter does not alter the load imbalance characteristic. In other words, the. only the harmonic components even the load is un- balanced. This paper firstly introduces the Instantaneous Power Theory, and then discusses the basic principle and scheme used to extract positive. customer who invested money for that active filter. •In contrast, the active filter employing the improved control method does not need to handle the negative sequence current then it can devote