Three-Phase UPQC Topology Based on Quadruple-Active-Bridge

In this study, a three-phase unified power quality conditioner (UPQC) topology based on a quadruple-active-bridge (QAB) is presented as an attractive solution to power quality issues. It is composed of three single-phase series converters, a three-phase shunt converter and a QAB converter located at the common DC-link. Since the bulky low frequency isolation transformers are replaced by the high frequency ones in QAB, it leads to higher power density, reduced weight and volume. The QAB converter could function as an energy router to provide multiple DC-link voltage access ports in the UPQC, which increase the flexibility of the UPQC. Finally, the MATLAB/Simulink simulation results and hardware-in-the-loop testing verify the correctness and effectiveness of the proposed topology and control method.


I. INTRODUCTION
Nowadays, the power quality (PQ) issues, including reactive and harmonic currents, sags/swells in the supply voltage, and distortion, are critical and not new [1], [2]. Especially, with the growing use of renewable energy sources (RES), such as inverter-based large-scale photovoltaic (PV) and wind integration, distributed generator (DG) system, vehicle to grid (V2G), the power quality issues become more severe [3]- [6].
In order to overcome those PQ issues, many power electronics-based compensation devices, such as static var generator (SVG), active power filter (APF) [7], [8], dynamic voltage restorer (DVR) [9], [10], unified power quality conditioner (UPQC) [11]- [13], are developed. Among these compensation devices, the topology of UPQC is relatively complex, which is formed with the back-to-back connection of a series-APF and a shunt-APF through a common DC-link. However, the UPQC is still considered as the most potential and versatile compensation device [13], because it possesses the ability of alleviating almost all the voltage-and currentrelated PQ issues.
Currently, many researchers have done a lot to study the topologies of UPQC with different aims [13]. In [14], [15], the UPQC integrated with DG system (UPQC-DG) is presented to relieve the load active power demand on the grid, The associate editor coordinating the review of this manuscript and approving it for publication was Nagesh Prabhu .
while the investment cost is relatively high. In [16], the interline UPQC (UPQC-I) is proposed to control the active power flow between two feeders and mitigate the PQ issues of the two feeders, but the current-and voltage-related PQ issues are not simultaneously mitigated in the same feeder. To improve the power level of UPQC, the modular UPQC [17], [18] based on cascade H-bridge and multilevel UPQC are proposed. To reduce the number of switches, a novel tenswitch topology for UPQC are proposed [19]. Although the UPQC with less switches has the advantages of decreased investment cost, it will inevitably increase DC-link voltage level. To avoid the use of transformers, the single-phase transformer-less UPQC and three-phase UPQC made up of three single-phase UPQCs (TSP-UPQC) without series transformers are proposed in [27] and [28], but the unbalanced grid currents cannot be compensated when mitigating the unbalanced grid voltage [28]. In [20], [21], some attention is put on the UPQC integrated with dual-active-bridge (DAB). The UPQC-DAB has the advantages of reduced cost, volume and weight, because the bulky low frequency (LF) isolation transformers are replaced by the high-frequency (HF) ones. However, in the DAB based TSP-UPQC, the unbalanced grid currents cannot be mitigated under unbalanced load, because it is not possible to realize the transfer of energy among each phase [21].
In this study, a quadruple-active-bridge (QAB) [22]- [24] based UPQC is proposed, which is formed with the back-to-back connection of three single-phase series converters and a three-phase converter through a QAB converter. Due to the lack of the bulky low frequency isolation transformers, the proposed UPQC-QAB has higher power density, reduced cost, volume and weight. Because the QAB converter functions as an energy router in the UPQC, it cannot only provide more power electronic interfaces for renewable energy sources, but also make it possible to mitigate the unbalanced grid currents caused by the unbalanced load. Besides, compared with DAB based TSP-UPQC, the number of switches and DC-link capacitances can be reduced. Although the three-phase UPQC-DAB could also remove the bulky low frequency isolation transformers, it could not provide more power electronic interfaces (only two DC ports) [29].
In summary, the main contributions of this study include: A three-phase UPQC topology based on the QAB is proposed, which can improve the power density and flexibility; The UPQC-QAB not only has the same functions as a conventional UPQC, but also provides some flexible power electronic interfaces for renewable energy source access. The other parts of this paper are organized as follows. Section II presents the topology and mathematical model of the UPQC-QAB. In Section III, the associated control schemes of the three single-phase series converters, the shunt converter and the QAB are proposed. In Section IV, the MATLAB/Simulink simulation results and hardware-in-theloop testing are presented. Finally, the conclusions are given in Section VI.

II. TOPOLOGY AND MATHEMATICAL MODEL OF UPQC-QAB
In this section, the topology of the QAB-based UPQC is presented and descripted in detail. Besides, the mathematical models of the series converter, the shunt converter and the QAB are introduced.

A. TOPOLOGY OF UPQC-QAB
The circuit configuration of the three-phase UPQC-QAB is shown in Fig. 1, which is composed of three single-phase series converters, a three-phase shunt converter and a QAB converter. Three DC ports (ports a, b and c) of the QAB are connected with the three single-phase series converters, and the fourth one (port d) is connected with the shunt converter. In this topology, the QAB functions as an energy router in the UPQC, and it provides flexible power electronic interfaces for renewable energy sources. For example, ports a, b and c could be the interfaces for photovoltaic panels, where the DC-link voltages could be regulated conveniently. Moreover, the conventional bulky transformer is replaced by small highfrequency ones. Thus, in contrast to the conventional UPQC, the proposed UPQC-QAB has the advantages of higher power density and more flexibility.
Compared with other existing custom compensation devices such as DVR, STATCOM, three-phase UPQC, three single-phase UPQC, and three-phase UPQC-DAB, the features of the UPQC-QAB are listed in Table 1. As shown, apart from the merits of removing the LF transformer and providing more electronics interfaces, the proposed UPQC-QAB has the advantages of mitigation of voltage sag/swell, compensation of load reactive power, elimination of current harmonics and mitigation of unbalanced grid current simultaneously.

B. MODEL OF SERIES CONVERTER
In the UPQC-QAB, the series converter is composed of three single-phase H-bridges. Based on Fig. 1, the mathematic model of single-phase series converter can be expressed as follows: where v sex (x = a, b, c) are the injected voltages of series converter, v Sx and i Sx are the grid voltages and phase currents, respectively. i sex represent the inductance currents. v ix are the input voltages of series converter. L se and C se represent the filter inductance and capacitance, and R se is the equivalent resistance.

C. MODEL OF SHUNT CONVERTER
The shunt converter of the UPQC-QAB is a conventional three-phase converter. From Fig. 1, the mathematic model of the shunt converter is descripted as (2), where v Lx are the load voltages. v ox are the output voltages of the shunt converter. L sh and R sh represent the filter inductance and equivalent resistance.

D. MODEL OF QAB CONVERTER
The power circuit of the QAB converter is shown in Fig. 1. [C a C b C c C d ] T are the DC-link capacitances. Referring to port d and applying the Kirchhoff's law in ''Y'' equivalent circuit of the QAB (in Fig. 2(a)), the equivalent link inductance of port m and n [26] is calculated as, where m, n ∈{a, b, c, d} and m = n. [L a L b L c L d ] T are the equivalent leakage inductances referring to port d. Fig. 2(b) shows the '' '' equivalent circuit of the QAB. In this study, the phase-shift-modulation (PSM) is used to control the power flow between any two ports. Thus, the QAB is driven by the square-wave voltages generated by their corresponding full-bridge modules with a 50% duty cycle.
The power transferred from port m to port n can be expressed as follows, where f s is the switching frequency. D m and D n are the phase shift ratios of port m and n, respectively. v m and v n represent By utilizing Gyrator theory, the gyrator-based average model of QAB [25] can be achieved (in Fig. 2c). Furthermore, the associated gyration gain is given as (5), Thus, the mathematic model of the QAB is given as (6), where i pvx represent the output currents of PV array integrated to UPQC-QAB from DC port x. When i pvx is equal to 0, it means that the PV array is disconnected from port x. While i pvx is bigger than 0, it denotes that the PV array operates at UPQC-connected mode. From (4), the gyration gain g mn is a nonlinear function of the phase shift ratios D m and D n . As shown in (5), the original QAB system in Eq. (6) is considered as a three-input threeoutput nonlinear and strong cross-coupling system.

III. CONTROL SCHEMES OF UPQC-QAB A. CONTROL OF SERIES CONVERTER
To track the grid angular frequency ω accurately, an adaptive filter is utilized instead of the phase-looked-loop (PLL). The block diagram of the adaptive filter is shown in Fig. 4(a).
For the input signal x(n), the adaptive filter provides two signals x 1 (n+1) and its quadrature signal qx 1 (n+1).
where n represents the n-th sampling period, q represents the quadrature shift operator. T s is the sampling period. The parameter K s determines the steady-state accuracy and dynamic performance of the adaptive filter. And K s is chosen as 0.2. ω s is the resonant frequency, which has been proved that the ω s ≡ ω is the unique exponentially stable equilibrium in the proposed system in [30]. Besides, the detailed description, analysis and proof is shown in [30].
The adaptive law of the adaptive filter is given as where the parameter γ is given as 0.0001 in this study. In [30], by using the averaging theory, it has been proven that the control law of Eq. (9) has the exponential convergence property as long as γ is small enough and greater than zero.
Thus, the parameter γ should be relatively small to ensure the stability.
The control targets of series converters of UPQC-QAB is to maintain the load voltage at the desire value. The reference load phase-voltages v * Lx are given as where V * L and θ L are the reference amplitude value and phase angle of the load reference voltage.
Furthermore, the reference series injected voltage is given as, Fig. 4(b) shows the control block diagram for the single-phase series converter. To realize the dynamic voltage compensation of the negative sequence and harmonics components, the voltage loop design method based on proportional resonant (PR) controller is utilized. Finally, the outputs of PR controller i * sex and sensed currents i sex are sent to the inner current controller for the PWM signals generation of the single-phase series converter.

B. CONTROL OF SHUNT CONVERTER
The control objectives of the shunt converters are to 1) maximize the power factor of power grid, 2) mitigate the load current harmonics and 3) maintain the voltage v d at a desired value.
To obtain the desired grid current reference, the d-axis component of reference grid current is determined by (12),  [31]. The detailed discuss and analysis of MAF can be seen in [31]. The transfer function of MAF is given as (13), where T ω represents the window length of MAF. Fig. 4(c) shows the control block diagram for the shunt converter. After obtaining the reference grid current i * Sd , three-phase reference grid currents i * S,abc could be gained by utilizing the dq0/abc transformation. Finally, the errors between i * S,abc and i S,abc are sent to the PWM controller for the gate signals generation of the shunt converter.

C. CONTROL OF QAB CONVERTER
Because the QAB functions as an energy router in the UPQC-QAB, it aims to realize the active power flow between three single-phase series converters (port a, b, c) and threephase shunt converter (port d).
For convenience, (6) could be linearized around the operation point. Thus, the mathematic model of the QAB can be rewritten as follows by some manipulation. (14) where a, b, c, d and n = x).
Furthermore, (14) could be written as follows, where where ξ represents the disturbance. Thus, the reference phase shift ratios could be obtained by utilizing the PI controllers, which is given as (16) where K qp and K qi are the gains of the PI controller, respectively. v * x represents the reference voltages of the DC port. When the PV array is connected to the UPQC-QAB from DC port x, the reference port voltage v * x can be achieved by using the maximum power point tracking (MPPT) controller [15]. Otherwise, when PV array is disconnected, the reference port voltage is given as v xref . Fig. 5 shows the control block diagram of the QAB. As seen, the errors between the reference and measured DC-link voltages of port x are sent to the PI controllers to obtain the reference phase shift ratios [D * a D * b D * c ] T . Finally, the corresponding reference phase shift ratios are sent to the PSM controllers for the gate signals generation of the QAB.

IV. VERIFICATIONS
To elaborate and validate the proposed topology and control schemes, the MATLAB/Simulink simulations and hardwarein-the-loop testing have been carried out on the three-phase UPQC-QAB. Table 2 lists the parameters involved in the simulations. The switching frequencies of the control schemes of the series converter, the shunt converter and the QAB converter are assigned to be 20kHz, 20kHz and 100kHz. Besides, the UPQC-P method is utilized in this study, where the series converter only injects/absorbs the active power to/from the power grid [13].

A. SIMULATION RESULTS
In this section, the MATLAB/Simulink simulation results of the UPQC-QAB are illustrated. To comprehensively test the compensation performance of the UPQC-QAB, four case studies with different grid voltages and load scenarios are presented, where the detailed parameters are listed in Table 3.

1) CASE STUDY 1
Firstly, the performance of the UPQC-QAB prototype is tested in terms of mitigation of three-phase balanced voltage sag/swell, and compensation of the load reactive power. Fig. 6 shows the simulation waveforms of the UPQC-QAB under case study 1. As shown in Fig. 6 (a), three-phase balanced voltage sag happens between 0.12 and 0.16 s with a depth of 20%, and three-phase balanced voltage swell with a depth of 20% happens during 0.16s and 0.20s. With the compensation of series converters (in Fig. 6 (c)), the load voltage v L is always kept balanced and maintained at the desired value (seen in Fig. 6 (b)). From Fig. 6(d)-(f ), the grid current i S is balanced and in-phase with grid voltage by injecting the shunt current i sh . Besides, all the four DC-link voltages are at the corresponding desired values in Fig. 6(g)-(h). Fig. 7 shows the related simulation waveforms of load active/reactive power (P L /Q L ), the grid active/reactive power (P S /Q S ), the series injected active/reactive power (P se /Q se ) and the shunt injected active/reactive power (P sh /Q sh ). As shown, the load active power (8kW) is totally provided by the  grid, and the load reactive power (6kW) is totally provided by the shunt converter.

2) CASE STUDY 2
Secondly, the case study 2 is designed to test the capabilities of the UPQC-QAB in mitigation of unbalanced voltage sag, compensation of both reactive power, and elimination of current harmonics. Fig. 8 shows the simulation waveforms of the UPQC-QAB under case study 2. In Fig. 8(a), the single-phase (c-phase) grid voltage sag occurs during 0.12s and 0.16s with a depth of 40%, and three-phase unbalanced voltage sag occurs between 0.16s and 0.20s (the sag depths of v Sa , v Sb and v Sc are 20%, 15% and 25%, respectively). From Fig. 8(b)-(c), with the compensation of three single-phase series converters, the three-phase load voltages are almost kept unchanged. As seen  from Fig. 8(e), the load current contains lots of harmonics, where the total harmonic distortion (THD) is about 8.56%. Fig. 8(d) shows the grid current. As seen, the current is balanced and in-phase with grid voltage and its THD is 2.06%. Fig. 8(f ) shows the injected shunt current i sh through the shunt converter. Besides, all the DC-link voltages are around the desired values with small pulsations in Fig. 8(g)-(h). Fig. 9 shows the related simulation power waveforms of UPQC-QAB in case 2. As shown, since both the three-phase loads are unbalanced and the grid voltage are unbalanced, the series and shunt injected power are also unbalanced.

3) CASE STUDY 3
In this case, the UPQC-QAB operates under distorted grid voltage sag/swell and nonlinear loads.  The simulation waveforms of the UPQC-QAB under this case are shown in Fig. 10. As seen in Fig. 10(a), the grid voltage is distorted and unbalanced in the time interval (0.12s, 0.20s), where the total harmonic distortion is 25%. Due to the compensation of the UPQC-QAB (in Fig. 10(c)), the load voltage is kept balanced, and the THD is 0.74% (shown in Fig. 10(b)). From Fig. 10(d)-(f ), the grid current i S is balanced and in-phase with grid voltage by injecting the shunt current i sh through the shunt converter. And all the DC-link voltages at all DC ports are shown in Fig. 10(g)-(h).
The related simulation power waveforms of UPQC-QAB in case 3 are shown in Fig. 11(a)-(d). As shown, because the grid voltage are distorted, the series injected active/reactive power also have pulsations.

4) CASE STUDY 4
Finally, the performance of the UPQC-QAB integrated with PV arrays is tested, and the grid voltage and load scenarios are VOLUME 9, 2021 the same with case study 1. The related simulation waveforms are shown in Fig. 12. The voltage at the maximum power point is 387V and the current at the maximum power point is given as 8A.
As shown in Fig. 12(a)-(b), the load voltage v L is always kept balanced and at the desired value because of the compensation of UPQC-QAB. The grid current i S (shown in Fig. 12(c)) is also balanced and in-phase with grid voltage. Fig. 12(e) shows the output current of PV array i pv . As seen, due to the integration with PV array, the grid current decreases, which means that the power grid and the PV array supply the load active power together. Besides, the DC voltage v b of port b (which connects to the PV array) is still kept stable and v d has a smaller pulsation (around the voltage 387 V at the maximum power of PV array) in contrast to case study 1 (in Fig. 12(f )-(g)).
The related simulation power waveforms of UPQC-QAB in case 4 are shown in Fig. 13(a)-(f ). As shown, the active power generated by PV arrays is about 3kW and supplies the load from the shunt converter. Thus, the active power is decreased to 5kW from 8kW. Besides, due to reduced grid active power, the grid currents and series injected active power will be reduced when compared with case 1.

1) OPERATION UNDER NONLINEAR LOADS AND BALANCED GRID VOLTAGES SAG
The waveforms of the UPQC-QAB under nonlinear load and balanced grid voltage sag are illustrated in Fig. 14. As shown, the grid voltage v S changes from the normal state to 20% sag state, the load voltage v L is almost kept unchanged by injecting the voltage v se through the series converter. Besides, due to the compensation of shunt converter, the grid current i S is always balanced and in-phase with grid voltage, and the current harmonics are also mitigated. Moreover, four DC-link voltages (v a , v b , v c and v d ) are at the corresponding desired values.   Fig. 15 illustrates the waveforms of the UPQC-QAB under nonlinear load and unbalanced grid voltage. As shown, the grid voltage v S changes from the normal state to the a-phase voltage sag state, the load voltage v L is maintained balanced and at the rated value by injecting the voltage v se . By injecting the current i sh through the shunt converter, the grid current i S is maintained balanced and in-phase with grid voltage.

3) OPERATION UNDER NONLINEAR LOAD AND DISTORTED GRID VOLTAGES
The waveforms of the UPQC-QAB under nonlinear load and distorted grid voltage are illustrated in Fig. 16. As shown, the grid voltage v S changes from the normal state to the distorted state, the load voltage v L also remains unchanged by injecting the voltage v se . By injecting the current i sh through the shunt converter, the grid current i S is maintained balanced and in-phase with grid voltage.

V. CONCLUSION
In this study, a topology of UPQC integrated with the QAB is proposed. Due to the removal of the bulky low frequency isolation transformers and the use of the QAB, the UPQC-QAB has the advantages of higher power density and more flexibility. Besides, the unbalanced grid currents caused by the unbalanced load can be mitigated compared with three-phase UPQC made up of three single-phase UPQCs. The effectiveness of the UPQC-QAB were evaluated under distorted grid voltage, balanced/unbalanced grid voltage sag/swell, unbalanced and nonlinear load conditions. Because of the lower investment cost, smaller size and more power electronic interfaces, the UPQC-QAB may be widely used in residential areas or families. Furthermore, under the topology of proposed UPQC-QAB, some promising work can be done in the future, including: 1) the optimal VA capacity design of the UPQC-QAB; 2) the optimal operation of the UPQC-QAB; and 3) the UPQC-QAB integrated with renewable energy sources (photovoltaic panels, fuel cell, battery and so on). She is currently an Associate Professor with the College of Electrical and Information Engineering, Hunan University, Changsha. Her research interests include wind energy conversion systems, Information processing, and motor control technology.
SHAONAN GONG was born in Shanxi, China, in 1996. He received the B.S. degree in electrical engineering from Yanshan University, Qinhuangdao, China, in 2019. He is currently pursuing the M.S. degree with the College of Electrical and Information Engineering, Hunan University, Changsha, China.
His research interest includes modeling and control of power electronics converters.