1056 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 2, FEBRUARY 2017

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1 1056 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 2, FEBRUARY 2017 Unbalanced Control Strategy for A Thyristor-Controlled LC-Coupling Hybrid Active Power Filter in Three-Phase Three-Wire Systes Lei Wang, Chi-Seng La, Senior Meber, IEEE, and Man-Chung Wong, Senior Meber, IEEE Abstract This paper proposes a control strategy for a threephase three-wire thyristor-controlled LC-coupling hybrid active power filter (TCLC-HAPF), which can balance active power and copensate reactive power and haronic currents under unbalanced loading. Copared with TCLC-HAPF with conventional control strategy, active power filters and hybrid active power filters which either fail to perfor satisfactory copensation or require high-rating active inverter part for unbalanced copensation, a control strategy was proposed for TCLC-HAPF to operate with a sall rating active inverter part for a variety of loads with satisfactory perforance. The control idea is to provide different firing angles for each phase of the thyristor-controlled LC-coupling part (TCLC) to balance active power and copensate reactive power, while the active inverter part ais to copensate haronic currents. First, the required different TCLC ipedances are deduced. Then, independent firing angles referenced to the phase angle of voltage across TCLC are calculated. After angle transforations, final firing angles referenced to phase angle of load voltages are obtained. In this paper, a novel controller for TCLC-HAPF under unbalanced loading is proposed. Siulation and experiental results are provided to verify the effectiveness of the proposed controller in coparison with a state-of-the-art controller. Index Ters Active power, current haronics, hybrid active power filter (HAPF), reactive power, thyristor-controlled LCcoupling hybrid active power filter (TCLC-HAPF), unbalanced copensation. I. INTRODUCTION UNDER practical conditions, when unbalanced nonlinear inductive loads are connected to the three-phase utility distribution syste, a nuber of current quality probles, such as low power factor (PF), haronic pollution, and unbalanced currents will rise. If copensation is not provided to the distribution power syste, it will cause a series of undesirable Manuscript received April 26, 2015; revised July 4, 2015, August 31, 2015, and Deceber 23, 2015; accepted April 11, Date of publication April 20, 2016; date of current version Noveber 11, This work was supported in part by the Macau Science and Technology Developent Fund (FDCT) (FDCT 109/2013/A3) and in part by the Research Coittee of the University of Macau (MRG012/WMC/2015/FST, MYRG AMSV). Recoended for publication by Associate Editor B. Singh. (Corresponding author: Chi-Seng La.) L. Wang is with the Departent of Electrical and Coputer Engineering, Faculty of Science and Technology, University of Macau, Macau , China. C.-S. La is with the State Key Laboratory of Analog and Mixed Signal VLSI, University of Macau, Macau , China (e-ail: csla@ uac.o; c.s.la@ieee.org). M. C. Wong is with the Departent of Electrical and Coputer Engineering, Faculty of Science and Technology, and also with the State Key Laboratory of Analog and Mixed Signal VLSI, University of Macau, Macau , China. Color versions of one or ore of the figures in this paper are available online at Digital Object Identifier /TPEL consequences, such as additional heating and loss in the stator windings, daage on the overloaded phase power cable, reduction of transission capability, increase in transission loss, etc [1] [4]. Ipleentation of power filters is one of the solutions for power quality probles. In the early days, thyristor-based static var copensators (SVCs) are used. It can inject or absorb reactive power according to different loading situations [1] [4]. However, SVCs have any inherent probles including resonance proble, slow response, lack of haronic copensation ability, and self-haronic generation. Later on, the rearkably progressive concept of active power filters (APFs) was first proposed in 1976 for dynaically copensating reactive power and current haronics probles [5] [13]. However, APFs require high dc-link voltage levels (V dc > 2 V L L ) to perfor copensation, which drives up their initial and operational costs. Afterward, in order to reduce the cost of APFs, an LC-coupling hybrid active power filter (HAPF) with low dc-link operational voltage was proposed by Akagi and Srianthurong [14] in Unfortunately, HAPF has a narrow copensation range, which ay require a high dc-link operation voltage when it is operating outside its copensation range, thus losing its low inverter rating characteristic [14] [19]. Many control techniques have been proposed to iprove the perforance of the APFs and HAPFs and solve the unbalanced probles [5] [21]. The different current quality copensators and their unbalanced control ethods are suarized in Table I, and also copared in the following. Akagi et al. [5] first proposed instantaneous pq control ethod in order to eliinate the reactive power, haronic power, and unbalanced power of the loading instantaneously. In order to adapt instantaneous pq control ethod under different voltage conditions (distorted, unbalanced, etc), any other control techniques were further developed, such as dq control ethod [6], [7], [14] [17], pqr control ethod [8], [9], Lyapunov function-based control ethod [18], etc. However, those instantaneous power control ethods [5] [9], [14] [18] are dedicated to inverter/converter-based structures and their corresponding perforances are highly dependent on the coputation speed and the switching frequency of the digital controllers and the switching devices. On the other hand, another popular control ethod for APFs and HAPFs is to balance the syste by copensating the negative- and zero-sequence current coponents under unbalanced loading situation, as the oscillating power/voltages/currents can be analytically expressed as positive-, negative-, and zero-sequence coponents (+,, and 0 sequences) [10]. The ajor drawback of this control ethod is that the sequence coponents introduced by haronics are IEEE. Personal use is peritted, but republication/redistribution requires IEEE perission. See standards/publications/rights/index.htl for ore inforation.

2 WANG et al.: UNBALANCED CONTROL STRATEGY FOR A THYRISTOR-CONTROLLED LC-COUPLING HYBRID ACTIVE POWER FILTER 1057 TABLE I CHARACTERISTICS OF DIFFERENT COMPENSATORS AND THEIR UNBALANCED CONTROL METHODS Fig. 1. Circuit configuration of a three-phase three-wire TCLC-HAPF. Notes: The shaded areas ean unfavorable characteristic. not taken into consideration. To solve this proble, the authors in [11] [13] and [19] cobine the above instantaneous control ethods with the +,, and 0 sequence control ethod, but the coputation steps increase a lot, thus significantly increasing the control coplexity. Recently, Czarnecki et al. [20], [21] proposed a power analysis control ethod based on the theory of currents physical coponent to copensate the reactive power in unbalanced three-phase four-wire syste. However, after this power analysis ethod copensation, the active power reains unbalanced, which eans the unbalance power cannot be copletely eliinated. With all the above control ethods, both APFs [5] [13], [20], [21] and HAPFs [14] [18] can effectively copensate the reactive power and haronic currents under unbalanced loading copensation. However, both APFs and HAPFs probably require high active inverter rating (high initial cost and switching loss) to perfor unbalanced current copensation due to the inductive coupling structures of APFs and the narrow copensation range liitations of HAPFs. In 2014, Rahani et al. [22] proposed the structure of a thyristor-controlled LC-coupling hybrid active power filter (TCLC-HAPF) which can operate with a sall rating active inverter part for reactive power and haronic current copensation in coparison to the conventional solutions [5] [21]. To control TCLC-HAPF, a state-of-the-art control ethod is proposed in [22] to reduce the steady-state error of the TCLC part and iprove the perforance of current tracking and voltage regulation of the active inverter part. However, the control ethod proposed in [22] was designed based on the assuption of balanced loading condition. If this control ethod is applied to TCLC-HAPF for unbalanced loading copensation, it either fails to perfor acceptable current quality copensation or requires a high-rating active inverter part for copensation, which results in increasing the syste initial cost, switching loss, and switching noise. Therefore, this paper pro- poses a hybrid unbalanced control ethod for the TCLC-HAPF, which can balance source side active power and copensate the reactive power and haronic currents with sall rating active inverter part. The control idea is to generate different firing angles to each phase of the TCLC in order to copensate reactive power and balance active power, and the active inverter part ais to copensate the haronic currents. As a result, the voltage rating of the active inverter part can be sall, and, consequently, the syste initial cost and switching noise can be significantly reduced. Moreover, the TCLC-HAPF can still aintain a wide copensation range with satisfactory perforance coparable to the conventional solutions [5] [21]. The proposed unbalanced controlled TCLC-HAPF is a cost-effective solution to copensate reactive power, haronic pollution, and unbalanced currents in distribution syste, especially for the ediu voltage-level distribution syste. In addition, given that ost of the loads in the distribution power systes are inductive, the following analysis and discussion will only focus on inductive loads [23]. In this paper, a brief introduction of research background and otivation is covered in Section I. In Section II, the circuit configuration of the TCLC-HAPF is presented and discussed. In Section III, the hybrid controller for the TCLC-HAPF under unbalanced loading is proposed and explained. Afterward, siulation and experiental results are given to verify the effectiveness of the proposed unbalanced control strategy for the TCLC-HAPF in coparison with the state-of-the-art control ethod [22] in Section IV. Finally, conclusion is drawn in Section V. II. CIRCUITCONFIGURATION OF A THREE-PHASE THREE-WIRE TCLC-HAPF The circuit configuration of a three-phase three-wire TCLC- HAPF is given in Fig. 1, where the subscript x denotes phase x = a, b, c. v sx,v x, and v invx are the syste voltage, load voltage, and inverter output voltage, respectively; L s is the syste inductance; i sx,i Lx, and i cx are the source, load, and copensated currents, respectively. C DC and V DC are dc-link capacitor and dc-link voltage; L c is the coupling inductor; L PF and C PF are the TCLC part inductor and capacitor.

3 1058 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 2, FEBRUARY 2017 TCLC ipedance is perfectly atched with the load ipedance, then the required V invxf can be equal to zero. In this paper, it is assued that the TCLC is controlled to be perfectly atched with the loading to siplify the following analysis; thus, V invxf =0, then the required TCLC ipedance can be calculated based on Fig. 2(b). In the following, the proposed hybrid control strategy for the TCLC-HAPF under unbalanced loading copensation will be presented and explained in three sections: Section III-A: TCLC part control strategy, which is based on the fundaental odel in Fig. 2(b), Section III-B: Active inverter part control strategy, and Section III-C: The overall hybrid controller for TCLC-HAPF. Fig. 2. Equivalent fundaental circuit odels of the TCLC-HAPF when: (a) V invx 0and (b) V invxf =0. In this topology, the TCLC part and the active inverter part can copleent each other s disadvantages. As the TCLC part offers the reactive power copensation range and provides a large voltage drop between load voltage and inverter voltage, the voltage rating of the active inverter part can be significantly reduced. On the other hand, the active inverter part can solve the inherent probles of using TCLC alone, such as inrush currents, resonance proble, noise of thyristors turning on/off, istuning of firing angles, and low-haronic copensation ability. Based on the circuit configuration as shown in Fig. 1, the unbalanced control strategy for the TCLC-HAPF will be proposed in next section. III. PROPOSED UNBALANCED CONTROL STRATEGY FOR TCLC-HAPF The purposes of the proposed unbalanced control strategy can be described as follows: the TCLC part is controlled to balance active power and copensate reactive power, while the active inverter part ais to copensate haronic currents. The equivalent fundaental circuit odels of the TCLC-HAPF for power analysis are illustrated in Fig. 2, where the subscripts f denotes the fundaental frequency coponent. In this paper, V sxf and V xf are assued to be pure sinusoidal without haronic coponents (V x = V sxf = V xf ) for siplicity. Fig. 2 is used to calculate the required ipedances and the corresponding firing angles for each phase of the TCLC part in order to balance and copensate active and reactive power. In Fig. 2(a), the active inverter can be treated as a controlled voltage source, and the required fundaental inverter voltage (V invxf ) depends on the TCLC ipedance [24] [26]. If the A. TCLC Part Control Strategy Fro the atheatical analysis as shown in Appendix A, it can be clearly illustrated that the three-phase source active power can becoe balanced once the reactive power is copensated. With this idea, the control target of the TCLC part is dedicated to balance and copensate the fundaental active and reactive power via the calculated required firing angles in this paper. The firing angles are deterined by the required TCLC ipedances (X af,x bf, and X cf ) for copensation which can be deduced by Oh s law. Referring to Fig. 2(b), in order to obtain the required TCLC ipedances (X af,x bf, and X cf ) for each phase, the virtual coon point (V nf ) is calculated first. Then, the TCLC ipedance of each phase can be obtained by X xf =(V x V nf )/I cxf, x = a, b, c, where I cxf is expressed in ters of each phase load power and V x. After that three independent firing angles α 0,x referenced to the phase angle of the voltage drop (V x V nf ) across the TCLC can be obtained. Since the control of the firing angles of the TCLC usually reference to the phase angle of the load voltage V x of each phase, an angle transforation process is required and also proposed in this paper. Based on the above discussion, there are three steps to find the final firing angles α x for controlling the TCLC, naely: 1) Calculation of V nf, 2) obtain the ipedances of X af,x bf, and X cf, and 3) find the final firing angles α x for each phase referenced to the phase angle of V x. In addition, a case study is provided in Appendix B to verify the proposed TCLC control ethod. 1) Calculation of V nf : Based on Fig. 2(b), the su of the copensating currents can be obtained by applying the Kirchhoff s circuit laws as I caf + I cbf + I ccf = V a V nf jx af + V b V nf jx bf + V c V nf jx cf =0 (1) where I caf, I cbf, and I ccf are vector fors of fundaental copensating currents. V a, V b, and V c are the vector fors of load voltages. Fro (1), V nf can be obtained as V nf = X bf X cf X af X bf + X bf X cf + X cf X af V a X cf X af + X af X bf + X bf X cf + X cf X V b af

4 WANG et al.: UNBALANCED CONTROL STRATEGY FOR A THYRISTOR-CONTROLLED LC-COUPLING HYBRID ACTIVE POWER FILTER 1059 X af X bf + X af X bf + X bf X cf + X cf X V c. (2) af With the expression of V nf, the TCLC ipedances can be obtained by Oh s law as discussed in the next part. 2) Obtain the Ipedance of X af,x bf, and X cf : Fro (1) and (2), the relationship aong the phase copensating currents I cxf, load voltages V x, and coupling ipedances X xf can be expressed as I caf I cbf I ccf (jx af ) V a V nf 0 (jx bf ) 1 0 V b V nf 0 0 (jx cf ) 1 V c V nf ( ) X j bf +X cf j X cf j X bf ( ) = j X cf X j af +X cf j X af ( ) j X bf j X af X j af +X bf V a V b (3) V c where V nf can be obtained fro (2) and = X af X bf + X bf X cf + X cf X af. The copensating apparent power S cx can be defined as S cx = P cx + jq cx = V x I cxf where I cxf is the conjugate of I cxf. Modifying S cx equation, the expression of the phase copensating currents can be given as I caf ((P ca + jq ca ) / ) V a I cbf ((P cb + jq cb ) / V ) b (4) ((P cc + jq cc ) / V ) c I ccf where the note denotes the conjugate. Va is set to be the reference phasor, so V a = V x 0, Vb = V x 120, and V c = V x 120, where = 1/2 j 3/2 and = 1/2+j 3/2, Vx is the root-ean-square (rs) value of load voltage which can be obtained in real tie as V x = v L / 3= va 2 + vb 2 + v2 c / 3 [5]. Q cx and P cx are the copensating reactive power and active power. Theoretically, after copensation, the source reactive power should becoe zero and the source active power should all be equal to (P La + P Lb + P Lc )/3. Therefore, the reactive power and active power generated/absorbed by the TCLC part can be expressed as ( Q cx = Q Lx and P cx = P Lx P ) La + P Lb + P Lc 3 (5) where x = a, b, c, Q cx and Q Lx are the copensating and load reactive power, and P cx and P Lx are the copensating and load active power. Cobining (1) (5), the relationship between TCLC ipedances and load power can be deduced as X af / X bf / = X cf / (Q ca Q cb Q cc ) /3 V 2 x (Q cb Q ca Q cc ) /3 V 2 x (Q cc Q cb Q ca ) /3 V 2 x (6) where = X af X bf + X bf X cf + X cf X af. Further siplifying (6), the coupling ipedances X af,x bf, and X cf can be obtained as X af X bf X cf 3 V 2 x (Q Lc Q Lb Q La ) 1 (Q Lb Q La Q Lc ) 1 (Q Lc Q Lb Q La ) 1 +(Q La Q Lb Q Lc ) 1 +(Q Lb Q La Q Lc ) 1 3 V 2 x (Q Lc Q Lb Q La ) 1 (Q La Q Lb Q Lc ) 1 (Q Lc Q Lb Q La ) 1 +(Q La Q Lb Q Lc ) 1 +(Q Lb Q La Q Lc ) 1 3 V 2 x (Q La Q Lb Q Lc ) 1 (Q Lb Q La Q Lc ) 1 (Q Lc Q Lb Q La ) 1 +(Q La Q Lb Q Lc ) 1 +(Q Lb Q La Q Lc ) 1. Fro (7), the TCLC ipedances are expressed in ters of the phase load reactive power Q Lx and rs voltage of phase load voltage V x. Then, the TCLC ipedances are used to find the firing angles to control the TCLC part, which will be discussed in the following part. 3) Find the Final Firing Angles α x Referenced to the Phase Angle of V x : The ipedance of the TCLC part can be considered as the X L c connected in series with the cobination of X C PF in parallel ( with a thyristor-controlled reactance (TCR) π X TCR (α 0,x ) = 2π 2α 0,x +sin2α 0,x X L PF ). Therefore, the expression of the TCLC ipedances (X af,x bf, and X cf )in ters of passive coponents and firing angles (α 0,x ) referenced to the phase angle of voltage across TCLC (V x V nf )isshown as πx L X PF C PF X af (α 0,a ) X C [2 π 2α PF 0,a +sin(2α 0,a )] πx L + X L c PF X bf (α 0,b ) πx L X PF C PF X C [2 π 2α PF 0,b +sin(2α 0,b )] πx L + X L c PF πx X cf (α 0,c ) L X PF C PF X C [2 π 2α PF 0,c +sin(2α 0,c )] πx L + X L c PF (8) where X L c, X C PF, and X L PF are the ipedances of the coupling inductor, the TCLC capacitor, and inductor, respectively. Equation (8) has a ter of 2α 0,x + sin(2α 0,x ) and it does not have a closed-for solution. Therefore, a look-up table (LUT) has been ipleented into the controller for perforing siulations and experients. Once the required coupling ipedance X xf (α 0,x ) is obtained fro (7), the precalculated firing angle α 0,x can be extracted based on the LUT of (8). As the control of the firing angles of the TCLC is usually referenced to the phase angle of the load voltage V x of each phase, an angle transforation process is required and proposed in the following. Fig. 3 shows the angle relationship between the phase angle of the load voltage V x and the phase angle of the voltage across TCLC (V x V nf ). Fro Fig. 3, there will be an angle difference (7)

5 1060 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 2, FEBRUARY 2017 Fig. 3. Phasor diagra of load voltage V x and the TCLC ipedance voltage V x V nf. for each phase between that of the load voltage V x and that of voltage across TCLC (V x V nf ), thus the above calculated firing angles α 0,x should have angle transforation if the control of the TCLC firing angles is referenced to the load voltage. To keep the sae ipedance X x in Section III-A2, an angle transforation process is required in order to obtain the final firing angle α x referenced to the phase angle of the load voltage V x. The final firing angles α x can be obtained as α a α b α c α 0,a α 0,b α 0,c φ a φ b φ c (9) where the phase angle φ x is the phase angle of the voltage across TCLC, and φ x can be calculated fro the expression of V nf in (2) as φ a φ b φ c ( tan 1 ( tan 1 ( tan 1 X cf X 3(X bf bf +X cf ) X af X 3(X cf af +X cf ) X bf X 3(X af bf +X af ) ) ) ) tan 1 θ [ 90,90 ]. (10) Based on (7) (10), the final firing angle α x can be obtained in ters of phase load reactive power, load voltages, and the passive coponent values of the TCLC. The phase load reactive power can be obtained with the help of single phase pq theory [28] and low-pass filters.the trigger signals to control the TCLC part are generated by coparing the calculated α x (9) with the phase angle of the load voltage θ x, which is instantaneously easured by the phase-lock loop. The TCLC part has two backto-back connected thyristors T x1 and T x2 for each phase, and they are triggered alternately in every half cycle. When θ x >α x, the gate pulse for thyristor T x1 can be generated. On the other hand, when θ x < 180 α x, the gate pulse for thyristor T x2 can be generated. In addition, the thyristors can turn on iediately when there is a trigger signal, while they can only be turned off when the inductor current goes to zero. A case study is included in Appendix B to verify the above analysis in Sections III-A1 III-A3. B. Active Inverter Part Control Strategy If only the TCLC part control is used, the haronic currents cannot be eliinated satisfactory. The purpose of the active inverter part is to instantaneously control the copensating current to track its reference, so that it can copensate the load haronic currents and significantly iprove the copensation ability and dynaic perforance of the TCLC part. In the following, the active inverter part control can be discussed in three sections: Section III-B1: Instantaneous power copensation control, Section III-B2: the dc-link voltage control, and Section III-B3: current PWM control. 1) Instantaneous Power Copensation Control: As the generalized instantaneous power theory [5] is valid for unbalanced three-phase systes, it is chosen to calculate the reference copensating current i cxref. The calculated i cxref contains the haronics, reactive power, unbalanced power, and the dc-link voltage regulating coponents. By controlling the copensating current i cx to its reference i cxref, the active inverter part can copensate the load haronic currents, iprove the reactive power copensation ability and dynaic perforance of the TCLC part, and also regulate the dc-link voltage to its reference value. The i cxref can be calculated through the well-known instantaneous pq theory [5] as i caref i cbref 3 1 vα 2 + vβ 2 1/2 3/2 i ccref 1/2 3/2 [ ] [ ] vα v β pαβ + dc p (11) v β v α q αβ + dc q where the dc p and dc q are the active and reactive coponents for dc-link voltage regulation, and the discussions of dc p and dc q are provided in the following Section III-B2. p αβ and q αβ is the instantaneous active and reactive power which include dc coponents p αβ and q αβ, and ac coponents p αβ and q αβ. p αβ and q αβ contain the fundaental active and reactive current coponents, respectively, while p αβ and q αβ contain haronic currents and negative-sequence coponents. p αβ is obtained by passing p αβ through a high-pass filter. p αβ and q αβ can be obtained as [ ] [ ] [ ] pαβ vα v β iα =. (12) q αβ v β v α i β In (11) and (12), the voltages (v α and v β ) and currents (i α and i β )inαβ plane are transfored fro abc fraes by [ ] [ ] v a vα 1 1/2 1/2 = 0 3/2 v b 3/2 v β [ iα i β ] = [ ] 1 1/2 1/2 0 3/2 3/2 v c i La i Lb i Lc where v x and i Lx are load voltage and current signals. (13)

6 WANG et al.: UNBALANCED CONTROL STRATEGY FOR A THYRISTOR-CONTROLLED LC-COUPLING HYBRID ACTIVE POWER FILTER 1061 Fig. 4. Proposed hybrid control block diagra for the TCLC-HAPF under unbalanced loads copensation. 2) DC-Link Voltage Control: The active inverter part can effectively control the dc-link voltage by feeding back the dc-voltage-controlled signals which include both reactive and active coponents (dc q, dc p ) [26] as shown in Fig. 4 dc q = k q (V DCref V DC ) (14) dc p = k p (V DCref V DC ). (15) In (14) and (15), k q and k p are the proportional gains of the dc-link voltage controller. Fro [25] [27], through the direct current hysteresis band PWM control, the dc voltage control reactive current coponent dc q is used to step change the dc-link voltage during the start-up process, while the active current coponent dc p is used to aintain the dc-link voltage as its reference due to the syste loss, in which the dc-link voltage control with feedback both active dc p and reactive dc q coponent [25] [27] can achieve both the start-up dc-link voltage self-charging function, aintaining the dc-link voltage and perfor dynaic reactive power copensation siultaneously [25] [27]. With the help of the three-phase instantaneous pq theory [5] and the ters of dc q and dc p, the dc-link voltage V DC can track its reference V DCref by transforing the dc voltage control signals into reference copensating currents through the above (11). Note that the dc-link control block requires a sall aount of active power to aintain dc-link voltage at its reference value. The active power for aintaining dc-link voltage is taken equally fro three phases, and it is relatively sall coparing with the copensating one, so that it will not affect the unbalanced loading copensation of the TCLC part. 3) Current PWM Control: The final reference and actual copensating currents i cxfinal and i cx will be sent to the PWM control, and then PWM trigger signals for controlling the active inverter switching devices can be generated. The PWM ethod, such as triangular carrier-based sinusoid PWM or hysteresis PWM can be applied. In this paper, the hysteresis current PWM [29] is selected due to its advantages of fast response, ease of ipleentation, and good current liiting capability, etc. C. Proposed Hybrid Controller for TCLC-HAPF Based on the above discussions, the proposed hybrid control block diagra for the TCLC-HAPF under unbalanced loads copensation is shown in Fig. 4. It consists of five ain control blocks: the TCLC part control block (discussed in Section III-A), the instantaneous power copensation control block (discussed in Section III-B1), the dc-link control block (discussed in Section III-B2), and the current PWM control block (discussed in Section III-B3). In the following, siulation and experiental results of the proposed control strategy will be presented in coparison with

7 1062 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 2, FEBRUARY 2017 the state-of-the-art control strategy in [22] in order to verify its effectiveness and superior copensating perforances under unbalanced loading condition. IV. SIMULATION AND EXPERIMENTAL RESULTS In this section, siulation and experiental results of the proposed TCLC-HAPF control strategy under unbalanced loading conditions will be presented and discussed in coparison with the results of the state-of-the-art control ethod in [22], in which the sae dc-link voltage is applied to both of the. A 110-V 5-kVA three-phase three-wire TCLC-HAPF experiental prototype is designed and constructed in the laboratory. The details of the TCLC-HAPF experiental setup and its testing environent are provided in Appendix C. The siulations are carried out by using PSCAD/EMTDC, and the syste paraeters used in siulations are the sae as the experients as shown in Table V of Appendix C. In addition, with reference to the IEEE standard [31], the acceptable total deand distortion (TDD) 15% with I SC /I L is in 100 < 1000 scale (a sall rating 110-V 5-kVA experiental prototype). The noinal rate current is assued to be equal to the fundaental load current at the worst case analysis, which results in THD = TDD 15%. Therefore, this paper evaluates the TCLC-HAPF current haronics copensating perforance by setting an acceptable THD 15%. Figs. 5 and 6 (siulation results) and Figs. 7 and 8 (experiental results) illustrate the wavefors of source currents, load currents (in Figs. 5, 6), copensating currents, capacitor (C PF ) currents, inductor (L PF ) currents, and dc-link voltage, source reactive and active power before and after copensation using the state-of-the-art control ethod [22] and the proposed control ethod. Figs. 9 and 10 (siulation results) and Figs. 11 and 12 (experiental results) and Table IV deonstrate the source current spectrus and phasor diagras of source voltages and currents before and after the state-of-the-art control ethod [22] and the proposed control ethod. For each haronic order of the current spectru as in Fig. 11, the three bars fro left to right represent phases a, b, and c, respectively. Moreover, Tables II IV suarize all the siulation and experiental results shown in the aforeentioned figures. By using the control ethod in [22], the siulated and experiental syste source PFs have been copensated to 0.93 and 0.95 (showing worst phase), respectively, as shown in Figs. 5 and 7, and Tables II and III. Fro Figs. 5(b), (c) and 7(b), (c), the source reactive power is copensated to 160 var, 9 var, and 237 var in the siulations and 130 var, 70 var, and 120 var in the experients. Besides, the siulated source active power (483, 662, and 698 W) and experiental source active power (510, 680, and 690 W) are not balanced after copensations. Moreover, fro Figs. 9(b) and 11(b) and Tables II and III, the siulated and experiental source current THD are copensated to 22.5% and 18.9% (showing worst phase), respectively. Fro Figs. 10(b) and 12(b), it can be seen that source voltage and current are not in phase (especially for phases a and c). In addition, as shown in Tables II and III, the siulated and experiental source current unbalanced factor (UBI fs )is Fig. 5. Siulation results of dynaic perforance by using the state-of-theart control ethod [22] before and after copensation: (a) Source currents (i sx ), load currents (i Lx ), copensating currents (i cx ), capacitor (C PF ) currents, inductor (L PF ) currents, and dc-link voltage (V DC ), (b) source reactive power Q sx, and (c) source active power P sx. 33.9% and 32.4%, respectively, after copensation. Fro Figs. 5 and 7, Figs. 9 12, Tables II IV, it is proved that the state-ofthe-art control ethod [22] cannot balance the source currents and provide good copensation perforances with the sall rating active inverter part under unbalanced loading condition. By applying the proposed control ethod, the siulated and experiental syste source PFs have been copensated to 0.99 or above in both the siulations and the experients, as shown in Figs. 6 and 8, and Tables II and III. Figs. 6(b), (c) and 8(b), (c) illustrate that the three-phase siulated and experiental source reactive power have been copensated to close to zero and the source active power is approxiately balanced after

8 WANG et al.: UNBALANCED CONTROL STRATEGY FOR A THYRISTOR-CONTROLLED LC-COUPLING HYBRID ACTIVE POWER FILTER 1063 Fig. 6. Siulation results of dynaic perforance by using the proposed control ethod before and after copensation: (a) Source currents (i sx ), load currents (i Lx ), copensating currents (i cx ), capacitor (C PF ) currents, inductor (L PF ) currents, and dc-link voltage (V DC ), (b) source reactive power Q sx,and (c) source active power P sx. copensation. Moreover, fro Figs. 9(c) and 11(c) and Tables II and III, the siulated and experiental source current THD have been copensated to 5.5% and 10.5% (showing worst phase), respectively. Fro Figs. 10(c) and 12(c), it can be seen that source voltage and current are in phase for all three phases. In addition, as shown in Tables II and III, the siulated and experiental source current unbalance factor (UBI fs ) is significantly reduced to 0.1% and 3.6% (fro 26.0% and 29.4%) after copensation. Fro Figs. 5 and 7, Figs. 9 12, and Tables II IV, they prove that the proposed ethod can siultaneously and effectively balance the source currents and provide satisfactory reactive power and current haronics Fig. 7. Experiental results of dynaic perforance by using the state-ofthe-art control ethod [22] before and after copensation: (a) Source currents (i sx ), copensating currents (i cx ), capacitor (C PF ) currents, inductor (L PF ) currents, and dc-link voltage (V DC ), (b) source reactive power Q sx,and(c) source active power P sx. copensation perforances with a sall rating active inverter part under unbalanced loading condition. In addition, the siulation results as shown in Figs. 5 and 6, Figs. 9 and 10, and Table II are consistent with experiental results shown in Figs. 7 and 8, Figs. 11 and 12, and Table III, which clearly verify the effectiveness and viability of the proposed unbalanced control strategy in coparison with the state-of-the-art one [22] for unbalanced loading copensation.

9 1064 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 2, FEBRUARY 2017 TABLE II SIMULATION RESULTS BEFORE AND AFTER TCLC-HAPF COMPENSATION BY USING THE STATE-OF-THE-ART CONTROLMETHOD [22] AND THEPROPOSED METHOD Q sx P sx PF i sxf THD isx UBI fs V DC (Var) (W) (A) (%) (%) (V) Before Cop. A B C Control Method [22] A B C Proposed Method A B C TABLE III EXPERIMENTAL RESULTS BEFORE AND AFTER TCLC-HAPF COMPENSATION BY USING THE STATE-OF-THE-ART CONTROLMETHOD [22] AND THE PROPOSED METHOD Q sx P sx PF i sxf THD isx UBI fs V DC (Var) (W) (A) (%) (%) (V) Before Cop. A B C Control Method [22] A B C Proposed Method A B C unbalanced loading is presented and discussed in details. Finally, siulation and experiental results are given to verify the proposed control ethod in coparison with the state-ofthe-art control ethod, which shows its superior copensating perforances under the unbalanced loading condition. APPENDIX Fig. 8. Experiental results of dynaic perforance by using the proposed control ethod before and after copensation: (a) Source currents (i sx ), load currents (i Lx ), copensating currents (i cx ), capacitor (C PF ) currents, inductor (L PF ) currents, and dc-link voltage (V DC ), (b) source reactive power Q sx,and (c) source active power P sx. V. CONCLUSION In this paper, a novel control strategy for a three-phase three-wire TCLC-HAPF is proposed, which can aintain it operating with a sall rating active inverter part and at the sae tie it can balance the active power and copensate the reactive power and haronic currents under unbalanced loading copensation. The design idea and operation steps of the proposed hybrid controller for the TCLC-HAPF under A. Balancing Three-Phase Fundaental Active Power by Reactive Power Copensation In this appendix, the atheatical analysis is provided to show how the active power can be controlled to be balanced when the reactive power is copensated. Fig. 13 shows the power flow analysis before TCLC-HAPF copensation. The phase fundaental apparent power is defined as S sx = P sx + jq sx = V sx I sxf where the note denotes the coplex conjugate. In Fig. 13, the su of I sxf can be expressed as [30]: I saf + I sbf + I scf = P sa + jq sa V saf + P sb + jq sb V sbf + P sc + jq sc V scf =0 (16)

10 WANG et al.: UNBALANCED CONTROL STRATEGY FOR A THYRISTOR-CONTROLLED LC-COUPLING HYBRID ACTIVE POWER FILTER 1065 Fig. 9. Siulated source current spectrus of phase a: (a) Before copensation, (b) after state-of-the-art control ethod copensation [22], and (c) after the proposed control ethod copensation. Fig. 10. Siulated phasor diagras of source voltages and currents: (a) Before copensation, (b) after the state-of-the-art control ethod copensation [22], and (c) after the proposed control ethod copensation. Fig. 11. Experiental source current spectrus: (a) Before copensation, (b) after the state-of-the-art control ethod copensation[22], and (c) after the proposed control ethod copensation. Fig. 12. Experiental phasor diagras of source voltages and currents: (a) Before copensation, (b) after the state-of-the-art control ethod copensation [22], and (c) after the proposed control ethod copensation. where I saf, I sbf, and I scf are the fundaental source current phasors. V saf, V sbf, and V scf are the fundaental load voltage phasors. V saf is set to be the reference phasor, so V saf = V xf 0, Vsbf = V xf 120, and V scf = V xf 120, where = 1/2 j 3/2 and = 1/2+j 3/2, V xf is the rs value of the source voltage. Siplifying (16), one can get (2P sa P sb 3Q sb P sc + ) 3Q sc + j (2Q sa Q sb + 3P sb Q sc ) 3P sc =0. (17)

11 1066 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 2, FEBRUARY 2017 TABLE IV EXPERIMENTALHARMONICCURRENTVALUE OF EACH HARMONICORDER:BEFORECOMPENSATION,AFTER THESTATE-OF-THE-ART CONTROLMETHOD COMPENSATION [22], AND AFTER THE PROPOSED CONTROL METHOD COMPENSATION Before Copensation After control ethod [22] copensation After proposed ethod copensation Phase a Phase b Phase c Phase a Phase b Phase c Phase a Phase b Phase c Haronic Order (%) (A) (%) (A) (%) (A) (%) (A) (%) (A) (%) (A) (%) (A) (%) (A) (%) (A) THD B. Case Study of Section III-A In this case study, referring to Fig. 2(b), the rs value of load voltage is given as V x = 110 V, where V a = V x 0, V b = V x 120, and V c = V x 120, and the unbalanced phase load apparent power is assued to be P La + jq La j438 P Lb + jq Lb j203. (21) P Lc + jq Lc j429 Fig. 13. Power flow analysis before copensation. In (17), both real part and iaginary part needs to be zero. Thus 2P sa P sb 3Q sb P sc + 3Q sc =0 2Q sa Q sb + 3P sb Q sc 3P sc =0. (18) If the source reactive power is copensated to be zero by the TCLC-HAPF as Q sa = Q sb = Q sc =0. (19) By substituting (19) into (18), the relationship of the threephase source active power can be obtained as P sa = P sb = P sc. (20) Based on the analysis of (16) (20), it can be concluded that the source active power can becoe balanced once the reactive power is copensated by the TCLC-HAPF. This idea can also be verified by the case study provided in the Appendix B. Fro (7) and (21), the required TCLC ipedances for copensating the above unbalanced loading can be obtained as X af X bf X cf 3 V x 2 (Q Lc Q Lb Q La ) 1 (Q Lb Q La Q Lc ) 1 (Q Lc Q Lb Q La ) 1 +(Q La Q Lb Q Lc ) 1 +(Q Lb Q La Q Lc ) 1 3 V x 2 (Q Lc Q Lb Q La ) 1 (Q La Q Lb Q Lc ) 1 (Q Lc Q Lb Q La ) 1 +(Q La Q Lb Q Lc ) 1 +(Q Lb Q La Q Lc ) 1 3 V x 2 (Q La Q Lb Q Lc ) 1 (Q Lb Q La Q Lc ) 1 (Q Lc Q Lb Q La ) 1 +(Q La Q Lb Q Lc ) 1 +(Q Lb Q La Q Lc ) = (22) With the obtained TCLC ipedances above, the final firing angles can be found through (8) (10) as α a α b α c α 0,a α 0,b α 0,c φ a φ b φ c

12 WANG et al.: UNBALANCED CONTROL STRATEGY FOR A THYRISTOR-CONTROLLED LC-COUPLING HYBRID ACTIVE POWER FILTER 1067 = (23) Then the corresponding source active power P sx and reactive power Q sx after TCLC copensation can be calculated by using the above X af,x bf, and X cf values or siulated by using α a,α b, and α c, which will be discussed in the following. If the conditions P sa = P sb = P sc and Q sa = Q sb = Q sc =0are satisfied, it eans that TCLC part can balance the active power and copensate reactive power of the unbalanced loading in (21). Thus, the validity of the proposed ethod can be verified. With the required TCLC ipedances in (22), load voltages V x and the help of (3), the copensating currents I cxf can be calculated as where I caf I cbf I ccf j ( X b +X c j X cf j X b V a V b V c ) j j X cf ( X af +X cf j X af ) j j X bf j X af ( X af +X bf ) , (24) X a / X b / X c / = X af X af X bf +X bf X cf +X cf X af X bf X af X bf +X bf X cf +X cf X af X cf X af X bf +X bf X cf +X cf X af The phase load currents I Lxf can be calculated as I Laf I Lbf I Lcf [(P La + jq La ) / ] V a [(P Lb + jq Lb ) / V ] b [(P Lc + jq Lc ) / V ] c = (25) Fig. 14. Siulated source reactive power Q sx and active power P sx before and after TCLC copensation: (a) Q sx and (b) P sx. Fro (24) and (25), the source currents after TCLC copensation can be calculated as I saf I Laf I caf I sbf I Lbf + I cbf (26) I scf I Lcf I ccf Moreover, the active power and reactive power at source side can be calculated as P sa + jq sa V ai saf j0 P sb + jq sb V bi sbf j0. (27) P sc + jq sc V ci scf j0 In (27), as P sa = P sb = P sc and Q sa = Q sb = Q sc =0,it can be clearly shown that the calculated TCLC ipedances in (22) can balance and copensate the active and reactive power. Moreover, under unbalanced loading as shown in (21), Fig. 14 shows the siulated Q sx and P sx before and after copensation by applying the proposed TCLC control ethod with the firing angles α x obtained in (23). In Fig. 14, Q sx have been copensated to zero while P sx are balanced after the TCLC copensation. Besides, the siulation results are consistent with the theoretical results. Therefore, fro the results shown in (27) and Fig. 14 of the case study, it can be verified that the proposed TCLC control ethod can balance active power and copensate reactive power. C. Experiental Setup for a 110-V 5-kVA Three-Phase Three-Wire TCLC-HAPF Experiental Prototype An 110-V 5-kVA three-phase three-wire TCLC-HAPF experiental prototype is designed and constructed in the laboratory and the testing environent is shown in Fig. 15. The digital control syste of the TCLC-HAPF consists of two paralleled digital signal processors (DSPs) TMS320F2812s, and the basic settings of both DSPs are the sae. The sapling frequency of the control syste is 25 khz in both siulation and experient (for both DSPs). For every 1/25 khz(s) period, the tier will provide a signal to process analog to digital (A/D) conversion and the corresponding interrupt. After processing the proposed control strategy, the output PWMs signal will be generated and the axiu PWM switching frequency is 12.5 khz. The SanRex PK110FG160 thyristors are used for the TCLC part, while the Mitsubishi IGBT intelligent power odules PM300DSA60 are eployed as the switching devices of the inverter. To capture the experiental results, the voltage and current wavefors are easured by an oscilloscope Yokogawa DL750 and

13 1068 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 2, FEBRUARY 2017 Fig. 15. Experiental setup: (a) 110-V 5-kVA TCLC-HAPF experiental prototype and its testing environent and (b) testing loads. Fig. 16. power. Relationship between the firing angle and the copensating reactive TABLE V TCLC-HAPF EXPERIMENTAL PARAMETERS FOR POWER QUALITY COMPENSATION Paraeters Physical values Syste paraeters v x, f 110 V, 50 Hz TCLC-HAPF paraeters L c,l PF,C PF 5H(Q= 18, ESR = 0.09 Ω) 30 H(Q = 23, ESR = 0.41 Ω) 160 μf(q = 310, ESR = 0.06 Ω) C DC,V DC 5F,60V Q cx range [ 630 var, 600 var] Notes: Q stands for the quality factor and ESR stands for equivalent series resistance. the current haronic spectrus for three phase are easured by a power quality analyzer Fluke 435. The syste and TCLC-HAPF paraeters for experients are suarized in Table V, and the photograph of TCLC-HAPF experiental prototype with its testing environent and testing loads is shown in Fig. 15. The copensating reactive power Q cx range in ter of TCLC ipedance X TCLC (α x ) or firing angle α x can be expressed as Q cx (α x )= = V 2 x X TCLC (α x ) V 2 x πx L PF X C (28) PF X C PF [2 π 2α x +sin(2α x )] πx L + X L c PF where V x is the rs value of load voltage, X L c, X L PF, and X C PF are the ipedance of Lc, L PF, and C PF, respectively. When both thyristors are turned off for the whole fundaental period (firing angle α x = 180 ), the TCLC part can be considered as an LC filter (L c and C PF ). In this case, the TCLC is providing the axiu capacitive copensating reactive power Q cx(maxcap). On the other hand, when one of the thyristors is turned on for half of a fundaental period alternately (firing angle α x =90 ), the TCLC part can be considered as the L c in series with the parallel cobination of L PF and C PF. At this case, the TCLC filter provides the axiu inductive copensating reactive power Q cx(maxind) as α x =90. 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Quintela, J. M. G. Arevalo, and R. C. Redondo, Power analysis of static var copensators, Electr. Power Syst. Res., vol. 30, no. 6, pp , [31] IEEE Recoended Practices and Requireents for Haronic Control in Electrical Power Systes, IEEE Standard , Lei Wang received the B.Sc. degree in electrical and electronics engineering fro the University of Macau (UM), Macao, China, in 2011, and the M.Sc. degree in electronics engineering fro the Hong Kong University of Science and Technology, Hong Kong, in Since 2012, he has been working toward the Ph.D. degree in electrical and coputer engineering at the Power Electronics Laboratory, UM. His research interests include power electronics, power quality and distribution flexible ac transission syste, power quality copensation, and renewable energy. Mr. Wang received the chapion award in the Schneider Electric Energy Efficiency Cup, Hong Kong, in Chi-Seng La (S 04 M 12 SM 16) received the B.Sc., M.Sc., and Ph.D. degrees in electrical and electronics engineering fro the University of Macau (UM), Macao, China, in 2003, 2006, and 2012, respectively. Fro 2006 to 2009, he was an E&M Engineer with UM. In 2009, he siultaneously worked as a Laboratory Technician and started to pursue his Ph.D. degree and copleted within three years. In 2013, he was a Postdoctoral Fellow with The Hong Kong Polytechnic University, Hong Kong. He is currently an Assistant Professor at the State Key Laboratory of Analog and Mixed-Signal VLSI, UM. He has coauthored two books: Design and Control of Hybrid Active Power Filters (New York, NY, USA: Springer, 2014) and Parallel Power Electronics Filters in Three-phase Four-wire Systes Principle, Control and Design (New York, NY, USA: Springer, in press), one U.S. patent, two Chinese patents, and ore than 50 technical journals and conference papers. His research interests include integrated power electronics controllers, power anageent integrated circuits, power quality copensators, sart grid technology, renewable energy, etc. Dr. La received the Macao Science and Technology Invention Award (Third-Class) and an R&D Award for Postgraduates (Ph.D.) in 2014 and 2012, respectively. He also received the Macao Governent Ph.D. Research Scholarship in , the Macao Foundation Postgraduate Research Scholarship in , and the third RIUPEEEC Merit Paper Award in In 2007, 2008, and 2015, he was the Gold Officer, Student Branch Officer, and Secretary of the IEEE Macau Section. He is currently the Vice-Chair of the IEEE Macau Section and Secretary of the IEEE Macau PES/PELS Joint Chapter. He was the Local Arrangeent Chair of TENCON 2015 and ASP-DAC Man-Chung Wong (SM 06) received the B.Sc. and M.Sc. degrees in electrical and electronics engineering fro the University of Macau (UM), Macao, China, in 1993 and 1997, respectively, and the Ph.D. degree in electrical engineering fro Tsinghua University, Beijing, China, in He was a Visiting Fellow with Cabridge University, U.K., in He is currently an Associate Professor at the Departent of Electrical and Coputer Engineering, UM. He has coauthored two Springer books, ore than 100 journal and conference papers, and six patents (China and USA). His research interests include power electronics converters, pulse with odulation, active power filters, hybrid active power filters, and hybrid power quality copensator for high-speed railway power supply syste. Recently, an industrial power filter platfor was developed and installed in a practical power syste based on his research results. Prof. Wong received the Macao Young Scientific Award fro the Macau International Research Institute in 2000, the Young Scholar Award of UM in 2001, the Second Prize for the Tsinghua University Excellent Ph.D. Thesis Award in 2003, and the Macao Science and Technology Invention Award (Third-Class) in 2012 and 2014, respectively. He supervised several students to receive erit paper awards in conferences and chapions in student project copetitions. He was several conference coittee ebers and General Chair of the IEEE TENCON 2015 in Macau. In , he was an IEEE Macau Section Chair. Recently, he is a North Representative of the IEEE Region 10 Power and Energy Society and the IEEE Macau PES/PELS Joint Chapter Chair.