Three-Port Converters for Energy Conversion of PV-BES Integrated Systems—A Review

The integration of battery energy storage (BES) with photovoltaic (PV) systems is becoming economically attractive for residential customers. The conventional approach for the interconnection of PV and battery systems requires at least two separate power converters that results in multistage power conversion for some power flows. The dc-dc three port converters (TPCs) are an alternative solution. These converters have many topological variants and possess different operating principles, topological benefits and limitations, and complexities. This paper concentrates on the topological study of TPCs for integrated PV and BES systems applications in the power range from a few hundred watts to 350 kW. These are classified into three different categories based on their isolation features between the ports to establish a topological mapping of the reported TPCs. This provides a framework that systematically explores the full range of technical benefits and limitations of each TPC topology. This paper also examines the possible extension of the TPC topologies for grid-interactive PV-BES systems where bidirectional power flow capability is required between grid and BES systems. This extensive review will provide a useful framework and a strong point of reference for researchers for the selection of TPC topologies to meet the system requirements for PV and energy storage applications.


I. INTRODUCTION
PV is one of the major renewable energy sources (RES) [1], [2]. The transition towards a zero emission, dynamic and resilient power system could be effectively accelerated by integrating more RES and BES systems across the network [3]. According to the International Energy Agency (IEA) Photovoltaic Power Systems Programme (PVPS) report, the total global installed capacity of solar PV systems is 1160 GW with an annual installation rate of 220 GW in 2022 [4], [5]. A large part of these PV systems is being The associate editor coordinating the review of this manuscript and approving it for publication was Branislav Hredzak .
installed as small-scale rooftop PV installations at residential premises. Fig. 1 shows the global cumulative PV systems installation capacity over the ten year period from 2012-2022.
In Australia, PV installations are mostly residential in the form of rooftop solar PV, but there are also a growing number of commercial rooftop systems. The total global BES systems capacity in stationary applications is currently estimated to be 11 GWh and could increase to between 100 GWh and 167 GWh in 2030 [7]. These are largely residential BES systems. The operational flexibilities of the increased grid penetration of the RES could be potentially optimized by combining energy storage systems such as residential, commercial or community owned BES systems. As the unit  [4], [6].
cost of lithium-ion-based BES systems drops gradually, the penetration of integrated PV and BES systems can be significantly expanded across the network [8] to mitigate the erratic behavior of RES such as PV systems. An increasing trend of BES systems deployment in the energy sector is forecasted due to their significant price reduction by 2030 [8]. The BES systems are mostly rated against their output voltage and ampere-hour capacity. The ratings are standardized for commercial and industrial applications that may not match the PV systems ratings. To support the potential expansion of the PV and BES systems, efficient and improved converter topologies are essential. The integration of BES with PV systems can be achieved by multiple converter approaches. However, the growing concern is the cost of power electronics converters. The Little Box challenge is an example of an international contest to develop very compact and low cost conversion technologies [9]. Also, the deployment of multiple converters in energy conversion for PV and BES integrated systems reduces the overall efficiency due to multistage power conversion and increases total system cost [10], [13]. In addition, this requires an overarching control and communication system for power flow management. Communication delay and errors will adversely affect control performance [12], [14]. An alternative may be a multiport converter, which allows arbitrary energy flows between three or more ports and can be used to accommodate multiple energy sources and energy storage devices.
Ideally this should have features such as compactness, reduced cost, flexible centralized power flow control, and high efficiency and power density due to the reduced numbers of conversion stages [15]. The PV and BES (dc/ac) systems can be connected with loads/ac grids in a variety of ways using power converters. The four possible topological configurations for integrating PV and BES systems are shown in Fig. 2. Fig. 2(a) shows a topology for a dc-coupled PV and BES hybrid system. This topology requires two individual two port converters for the PV array and battery, and the systems are connected with the regulated dc bus. This is often termed a dc coupled BES system. This system requires a dc-ac converter to establish the power flow through the load/grid via an ac bus. The ac-coupled topology is shown in FIGURE 2. Basic topological structures for integrating PV and BES systems (a) two two-port dc-dc converters with a dc bus, (b) two two-port dc-ac converters with an ac bus, (c) a two-port dc-ac converter and an integrated ac battery system with an ac bus, (d) an integrated TPC with a dc bus. Fig. 2(b). This topology requires two dc-ac power converter stages that are directly tied to the ac bus to connect with the load and an ac grid. In this topology, bidirectional power flow between the ac grid and the dc BES system is possible. The ac-coupled topology in shown in Fig. 2(c). It is technically similar to Fig. 2(b). The difference is that an ac BES system is directly connected to the ac bus and has a built-in inverter. This is presented here as this topology is widely marketed as a packaged solution for residential customers who wish to retrofit batteries within a home with an existing solar system [16]. An integrated TPC topology is shown in Fig. 2(d), and it can provide a better interface with the complex system comprising of the PV and BES systems with the load and ac grids. It has advantages of higher system efficiency, lower cost, faster response, and compact packaging with centralized control compared with other topologies [12]. The topology could be a promising candidate for multiple RES integration [17], [18]. There are few review papers published in the literature [8], [19], and [20]. These review papers are primarily aimed at covering some topological aspects of TPC to integrate solar and energy storage systems, however, their topological variations and technical challenges to allow grid interactive operation, i.e., provision of bidirectional power flow between BES system and the grid are yet to review broadly to understand the full potential of TPC topologies. In this paper, the authors firstly provide a comprehensive review on the DC-DC TPC topologies and then extend it to their variants, and technical challenges to implement a bidirectional ac output port.
The major contribution of this comprehensive review is to supplement knowledge gaps of the existing reviews and extend the concepts of TPC topologies for grid interactive applications with a particular focus to implement a bidirectional ac output port to allow bidirectional power flow between the BES system and the grid. This review reveals that the bidirectional power flow between the BES system and grid can be achieved by making one of the ports of the TPC a bidirectional ac port. There are two approaches to realizing the ac port. The direct approach is to produce a topology that inherently offers a four-quadrant port. Alternatively, the ac port can be realized by combining a two-quadrant, bidirectional dc port with an inverter stage which may or may not be isolated. Both approaches are considered in this review. Firstly, this paper concentrates on a comprehensive review of the DC-DC TPC topologies expounded in recent publications for off-grid and grid-integrated systems. Secondly, this paper focus on a systematic review on the converter topologies with an ac port. A wide range of TPC topologies [14], [15], [21], [28] have been proposed for PV-BES applications. Depending on the electrical connection among the three ports, the existing TPC topologies can be categorized into three basic categories, namely non-isolated TPCs, partly isolated TPCs, and fully isolated TPCs [12]. However, it is important to further explore each of these topological variants to uncover the full range of benefits and limitations of these TPCs. The key aspect of this paper is that it covers these TPC topologies that are derived from a wide range of generic converter topologies including half bridge converters, interleaved halfbridge converters, full bridge converters, the buck, boost, and buck-boost converters, Cuk converters, sepic converters, zeta converters, forward converters, flyback converters, LLC and dual active bridge converters. It also provides a systematic comparison of each variant with quantitative analysis and identifies the favorable features of each topological variant as a sound foundation for future TPC applications. The paper is organized as follows. In Section II, the PV-BES integrated TPC topology and its power flow modes are presented. Section III discusses the reported TPC topologies. Section IV presents a detailed discussion on the reported TPC topologies. Section V presents the converter topologies with an ac port. A performance comparison of the TPC topologies with quantitative analysis is provided in Section VI. Finally, Section VII concludes the paper with details of the future research directions.

II. PV-BES INTEGRATED TPC TOPOLOGY
The schematic of the PV-BES integrated TPC topology is shown in Fig. 3. This topology has three ports where port 1 connects the PV array and two bidirectional ports connect the load and battery respectively. For analysis of the PCS, a lossless power equation [29] is considered as below, where P PV (t) is PV instantaneous output power, P BES (t) is BES charging power and P ac (t) is instantaneous output power.
The power flow diagram of the PV-BES integrated TPC is presented in Fig. 4. In mode I, the PV array satisfies the load demand and surplus PV generation is stored at the BES  system. In mode II, the BES system absorbs full PV power when load power demand P ac (t) falls to zero. In mode III, the PV array and BES provide load power demand. If the BES system charging power P BES (t) falls to zero, the PV array supplies power to meet load demand in mode IV. Mode V allows the BES system to meet load power demand and/or charge using grid power when PV generation is unavailable. Mode VI allows the BES system to charge from grid while PV supplies power to the load. Operational modes V and VI require the TPC converter topology to have an ac output port capable of ensuring full four-quadrant ac operation to facilitate bidirectional ac power flow between the grid and battery [32]. The review is dominated by the TPC with three dc ports. An ac port is extremely rare. A bidirectional dc port can supply a non-isolated inverter to form an effective four quadrant ac port. The first part of this review will focus on TPCs with dc ports that can support this arrangement. TPCs with a direct ac port will also be examined.

III. REPORTED TPC TOPOLOGIES
Due to the many topological benefits of an integrated TPC over the conventional two converters for PV-BES systems, extensive research on TPC topologies is conducted and many topologies are proposed for the standalone and grid-connected renewable power systems. Most of the current literature on TPCs are concentrated on the standalone renewable power systems and very few of them are applied to grid-connected power systems. Depending on the electrical VOLUME 11, 2023 connection among the three ports, the existing reported TPC topologies can be categorized into three basic topologies [12], namely non-isolated TPCs, partly isolated TPCs and fully isolated TPCs. However, in addition to the isolation feature, these TPC topologies are further reviewed and analyzed based on their topological structure and operational characteristics such as the time-division concept, shared bus concept, single inductor concept and combining multiple generic converter concepts, etc., in the following sections.
A. NON-ISOLATED TPC TOPOLOGIES Fig. 5 shows the basic structure of a non-isolated TPC topology where all of the ports of the TPC are connected directly without any galvanic isolation. Several non-isolated TPC topologies have been proposed in the current literature [21], [33], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55]. These topologies can be further divided into variants based on topological structures. Fig. 6 shows a double input dc-dc converter which is derived using a single pole triple trough switch (SPST) for multiple DER applications [46]. The operation of this topology is controlled by the time-sharing concept of the active switches which entails that one, and only one, of the three SPST switches is on at any given time. When SPST switch S 1 is turned on, source V 1 delivers power to load whereas V 2 delivers power to load only when switch S 2 is on. Switch S 3 is used for freewheeling operation. With different switch realization, the topology can be configured to work on buck, boost and buck boost modes. The bidirectional power flow can be achieved by using four quadrant switches. Based on this time division concept, a multiple-input dc-dc converter [45] and a multiinput multioutput dc-dc converter [47] are proposed for EVs and other renewable energy sources. Based on the timesharing approach and switch configurations, these topologies can be operated in buck, boost and buck-boost modes.
Depending on the power flows, the concept of the dual input converter (DIC) and dual output converter (DOC) are adopted for derivation of non-isolated TPC topologies using a single inductor (SI) with multiple switches [33], [34], [37], [48], [56], [57], [58], [59]. Many of the non-isolated TPC has single quadrant output port (SQOP) and these are topologically limited to provide single quadrant operation for the output port. The six elementary dc-dc converters, namely buck, boost, buck-boost, Cuk, zeta and sepic could be useful  with the existing DOC and DIC topologies [60], [61] to provide a new energy transfer path to construct an isolated PWM TPC. A TPC topology based on single inductor with multiple switches [37] is shown in Fig. 7. The control degrees of freedom are not fully utilized in [37] and [62] and some desirable power flows are therefore missing. A similar topology is proposed using a single inductor with three switches and diodes in [48].
A family of buck, boost and buck-boost derived low cost, low power, compact non-isolated TPC topologies are proposed in [56] and [57] based on a simple and general cell consisting of two switches, two diodes. a BES system and an inductor. The four maximum control freedoms of choice for energy management between three ports are available through the two switches which extend the power flow capability of the TPC topology in [57]. This topology is fully functional in the DO and DI modes without the port voltage restrictions, unlike the topology proposed in [37] where there exists the port voltage restriction among the ports for these modes. In addition, this topology provides an additional SISO mode of operation where the PV can charge the battery only when the load requirement is idle which does not exist in [37]. The dynamics of these buck/boost/buck-boost derived single inductor TPCs (SI-TPCs) becomes complicated because of the various energy transfer modes within the multiport arrangement [44]. The differing modes result in different small signal models. A TPC topology proposed in [33] which is quite similar to the structures proposed in [56] and [57] with some additional components and the output port is also single quadrant type. As the solar and battery systems must operate over an extended voltage range, the major challenge with the non-isolated TPC derived from the basic buck/boost/buck-boost topology is to keep the port voltages within a suitable operating range to maintain its functionality. A compact non-isolated TPC with a reduced number of semiconductors and a common ground is proposed in [63] to address EMI issues where all three ports are grounded. This topology is based on the single inductor dual output (SIDO) converter [58], [59]. In this topology, a bidirectional path is established for the battery port, however, the output port still provides single quadrant operation. A non-isolated TPC topology is proposed based on a shared bus concept [50] as shown in Fig. 8. In this topology, the inductors form the dc-links. It provides a great improvement compared with the traditional common dc-bus-based solution as the bulky dc-link capacitor is eliminated. The topology can be realized as a three port bidirectional buck boost converter, and this provides single-stage conversion between any two of the three dc-buses. A similar shared bus concept is used to develop a modular multiple-input bidirectional dc-dc converter [49] to integrate multiple DERs at different voltage levels. The topology can be operated either in buck mode or boost mode and bidirectional power flow can be achieved. The total power required from the auxiliary sources can be shared between the battery system and the ultracapacitor bank. The power sharing is based on operating conditions such as charging current limitations and state of charge of the BES system and overall dynamics of the converter.
A family of TPC topologies are derived by combining two or more generic power converters in [40], [51], [52], [53], [54], and [55]. Fig. 9 shows a new bidirectional dc-dc boost converter topology based on double boost converters [51]. The topology is very simple, and it requires three controllable power switches and two inductors. A similar approach is used to develop an integrated single input multiple output converter using a few boost converters in [55]. The topology provides one high step-up and multiple step-down outputs at different voltage levels utilizing a lower number of switches compared to separate converters. This topology is more reliable due to its inherent shoot through protection and shows similar dynamic behavior as individual buck and boost converters. A similar concept is applied to propose a new dc-dc boost converter topology for mobile device  applications [54]. A TPC topology is proposed in [40] by integrating a buck-boost converter with a stacked dual halfbridge converter. This topology provides a low and a high voltage output at the output ports and both ports are bidirectional. Fig. 10 shows a TPC topology based on multiple buck converters for EV applications [52]. The topology provides multiple regulated outputs at different voltage levels. This topology requires a reduced number of switching components compared with the conventional separate buck converters. The dynamic behavior of this integrated topology is similar to the conventional buck converter which makes the controller design relatively simple.
Based on the concept of combining multiple dc-dc buck converters to develop a multioutput topology, a TPC topology is proposed for providing dual outputs with simultaneous bidirectional and unidirectional characteristics in [53]. One of the limitations of this topology is that it requires semiconductor power switches with current ratings higher than that required in the conventional solution comprised of two separate dc-dc buck converters.
A family of TPC topologies are proposed utilizing coupled inductor in [36], [38], and [43] as a voltage gain extension cell. Fig. 11 shows the TPC topology based on coupled inductors proposed in [38] and [43]. This topology provides voltage boosting for high step-up applications and the output port is bidirectional. The topological structure of the TPC proposed in [36] is quite similar to the topology proposed in [38] and VOLUME 11, 2023 FIGURE 11. Topology based on coupled inductors [38], [43]. [43] except the output port which is single quadrant in [36]. Like the topology in [38] and [43], this topology adopts the use of a coupled inductor to achieve a higher voltage ratio by adjusting its turns ratio. Unlike the topology in [38] and [43] that utilized complex two clamping circuits for recycling the leakage inductance energy, a lossless snubber circuit is used for the same purpose. However, this TPC topology is designed to work in DO, DI and SISO modes and the full control freedoms are also not utilized here. This structure is less complex than [38] and [43] and can achieve a higher overall efficiency within the operational modes.
A non-isolated dc-dc TPC topology based interleaved converter is proposed in [39]. The topology is derived from conventional buck and boost converters. The topology allows modular design for high power applications by interleaving multiple converter stages. Fig. 12 shows thee two stages interleaved topology that provides bidirectional power flow between the output port and battery port.
Partly isolated TPC topologies based on half bridge converter with synchronous rectification are proposed in [64], [65], and [67]. Fig. 14 shows the topology proposed in [64] for satellite applications where the output load port is single quadrant and is isolated from the PV and battery port by a HF transformer.
This converter is PWM modulated, however, the detailed analysis of the soft switching operation with synchronous rectification is not provided. Similar to this concept, a family of basic half-bridge converter based TPC topologies with post regulation, synchronous rectification and primary freewheeling with various implementations are proposed in [65]. This regulation is required to introduce an additional control degree of freedom to independently regulate the voltage of any two of the three ports of the TPC while the third one is used for power balance. A group of partly isolated TPC topologies are proposed in [14], [23], [29], [68], and [70] based on interleaved bidirectional half bridge converters and a secondary side bridge rectifier/converter.   15 shows a partly isolated TPC topology based on an interleaved converter and a secondary side bridge rectifier [14]. In these topologies, the primary side bidirectional interleaved converter is PWM modulated whereas the secondary side bridge rectifier/converter is phase shift modulated. These topologies are very effective in reducing the circulating current at the freewheeling stage and extending the soft switching range of the power switches. A bridgeless boost converter is used in the secondary side to reduce the input current ripple because of the 180 • phase shift between the input side switching legs in [70].
The TPC proposed in [23] and [68] are topologically the same except for an ac inductor and two diodes in [23] replacing the two power switches in the secondary side in [68]. The ac inductor further limits the circulating current at the freewheeling stage in the secondary side of the circuit as there are no dc inductors at the output circuits.
An interleaved bidirectional PWM converter is combined with a DAB converter to develop partly isolated TPC topologies in [66] and [73]. Fig. 16 shows the partly isolated TPC topology that consists of an interleaved PWM converter and a three-phase DAB converter.
The three-phase Y-Y connected transformer is used to allow bidirectional power flows with galvanic isolation and voltage matching between the different ports. A similar approach is used to develop a partly isolated TPC for EV battery systems in [73].
In this topology, a flying capacitor is added to the interleaved PWM converter to achieve the automatic current balancing for inductors in the PWM converter, eliminating an active current balancing control loop using current sensors. Furthermore, the added FC allows the MPC to operate with a nominal duty cycle of 0.5, improving the transformer utilization and reducing rms current. Some partly isolated TPC topologies are proposed in [69], [71], and [75] based on an interleaved bidirectional buck/boost converter and a full bridge LLC resonant converter in the primary side combined with a bridge rectifier in the secondary side.
These topologies feature low power component amounts, and a simple and symmetrical structure. These topologies inherently provide high power density, low circulating currents and zero voltage switching (ZVS) operation of all the primary side switches while ensuring the turn off with zero current switching (ZCS) operation of all diodes in the secondary side for the entire voltage and power range, leading to significantly reduced switching losses. This is very helpful for high frequency operation. The power flows of these topologies are managed by PWM and PFM approaches. Fig. 17 shows the partly isolated TPC topology proposed in [71]. Fig. 18 shows a new LCL based partially isolated TPC proposed in [74] and [76]. It features a smaller number of power switches than [69] and [71].
The output port of the aforementioned partly isolated topologies is single quadrant; however, the output side could be redesigned to have bidirectional power flow using four quadrant power switches. Fig. 19 shows the general structure of a fully isolated magnetically coupled TPC topology where the three ports are fully isolated from each other. A high-frequency transformer is normally used to provide isolation between the three ports. Several fully isolated TPC topologies are proposed in the current literature [12], [24], [26], [77], [87]. The fully isolated TPC topologies proposed in [25], [77], [81], and [85] are based on the DAB converter [88]. These TPC topologies  based on DAB converters produce triple active bridge (TAB) converters and each TAB converter consists of three full bridge converters. Fig. 20 shows a fully isolated TPC topology based on the DAB structure [25]. There are no topological differences in the TPC proposed in [81] and [85] compared with [25] and [88]; however, a shell-type planar HF transformer in [81] and a three-limb HF transformer in [85] are used as power transfer elements among the ports to analyze the performance of these transformers and providing soft switching operation. The TAB converters have advantages such as low device stress, bidirectional power flow capability, and fixed frequency operation.

C. FULLY ISOLATED TOPOLOGIES
The TAB converters are used for grid connected renewable energy systems in [84]. The topology is the same as the topologies proposed in [25] and [77], however, an additional  converter stage with LCL filter is connected to the dc-link at the output port to interface with the grid. A similar TAB converter topology is proposed in [79] for EV charging systems. An extra full bridge diode rectifier is connected with the active bridge in the EV charger port via a dc-link to interface with the grid [79]. An integrated fully isolated TPC topology based on a TAB converter is proposed in [86]. However, in this topology, two three-winding transformers and four full-bridges are utilized to establish all five possible power flows. In the output port, two isolated full bridges are connected in series for boost operation. The secondary windings are connected in a special way. The TAB converter is used to develop an isolated series resonant TPC topology in [26] and [78]. Fig. 21 shows a series resonant isolated TPC topology proposed in [26]. In this topology, the input side bridges are connected by a series LC resonant circuit. In [78], the load port consists of a full bridge diode rectifier and thus it cannot handle bidirectional power flow. An isolated LLC resonant dc-dc TPC for DC applications is proposed in [24] as shown in Fig. 22.
In the inner stage, three half bridges are connected with a three winding MF transformer through an LLC resonant stage, making a symmetrical LLC resonant converter. In the outer stage, additional buck/boost stages are connected at port 1 and port 3 for achieving the active power flow control.

IV. DISCUSSION ON REPORTED TPC TOPOLOGIES
In this paper, the reported TPC topologies are grouped into three different categories based on their isolation arrangements namely non-isolated TPC, partly isolated TPC and fully isolated TPC. Further to this classification, each of these TPC topologies are individually analyzed and further subdivided into various topological variants based on their circuit configurations and operations. The key parameters such as power ratings, voltage levels at the ports, component counts, operating frequency and efficiencies of non-isolated, partly isolated and fully isolated TPC topologies are presented in Tables 1, 2, and 3 respectively to explore the pros and cons of these TPC topologies. This section focuses on important features of the individual categories that will form a very useful reference for future TPC design and implementation.

A. NON-ISOLATED TPC
A non-isolated TPC is generally derived from the basic converters, i.e., buck, boost, and buck-boost, that share switches and storage elements in each switching cycle [22]. The advantage of this type of topology is that it is smaller in size and weight due to the absence of a transformer for electrical isolation. This topology also offers high power density, high efficiency and high reliability due to lower component counts. Table 1 shows the details of the key parameters for the non-isolated TPC topologies. However, there are few major drawbacks as follow:

1) PORT VOLTAGE RESTRICTIONS
As these topologies are transformerless, they suffer from limitations on port voltage ranges when dealing with sources like PV and BES system that exhibit a wide voltage variation under certain conditions [15]. For example, the non-isolated topology proposed in [35] operates normally only when the PV and battery port voltages are higher than the load voltage.

2) VOLTAGE GAIN
In some power flows, the voltage gain of the non-isolated TPC becomes similar to the conventional boost converter that VOLUME 11, 2023  increases the voltage stress on the semiconductor switches. For example, the voltage gain of the TPC is the same as the boost converter when the TPC operates in dual output and dual input modes [37].

3) SINGLE QUADRANT OPERATION
The output ports of the non-isolated TPC topologies are frequently limited to single quadrant operations. These topologies are predominantly designed to have unidirectional diodes at the output ports. In some cases, output ports of these converter topologies could be made bidirectional using bilateral inversion techniques [89] that allow unidirectional diodes to be replaced with bidirectional active switches.

4) COMPLEXITY OF ACHIEVING FOUR QUADRANT OPERATION
None of the non-isolated TPCs have a four-quadrant port. The extension of the output port to four-quadrants requires a voltage reversal capability and this is not normally feasible as the impacts propagate to all ports given the absence of an isolation boundary.

5) OTHER COMPLEXITIES
To alleviate the port voltage restriction problems seen among the ports in [37], [56], and [57], a non-isolated TPC with variable structures can be used [15]. However, the trade-off could be a higher component count, higher cost and lower reliability. To reduce the voltage stress of the semiconductor devices, coupled inductors can be used as a voltage gain extension cell [38]. However, a problem with many coupled inductor designs is leakage inductance. The leakage inductance energy must be managed to maintain the converter efficiency and to limit device switching stresses. The non-isolated topologies may be applicable for relatively low power application where there is no need for isolation to comply with safety requirements [90].

B. PARTLY ISOLATED TPC
Amongst the three TPC topologies, the partly isolated topology has received the greatest interest and many of these topologies are available in the current literature. Partly isolated TPCs normally have isolation for the load port while PV and battery ports are connected via buck, boost or buck-boost converters that share common switches [14]. The partly isolated topology removes the port voltage restriction at the load port. However, the partly isolated TPC topology has the following shortcomings:

1) COMPONENT COUNT
This topology has a higher component count as compared to the non-isolated TPC topologies and the problem of port voltage matching will still appear at PV and battery ports.

2) COMPLEXITY WITH TOPOLOGICAL VARIATIONS
Partly-isolated TPC topologies using basic half bridge converters are simple and provide high power density. However, the switching losses are high because of the hard switching and high circulating currents caused by the freewheeling operation in the body diodes of the switches [99]. The power rating of these converters is also limited by the requirement for tight primary to secondary coupling. To reduce switching losses, a magnetizing inductor can be used in the design to provide the soft switching operation of the primary side switches. On the other hand, the partly isolated TPC topologies employing full-bridge structures can achieve soft switching operation and reduce the input current ripple. As the full bridge converters are controlled by a phase shift, the primary side phase shift (PSPS) based full bridge topologies with simple phase shift modulation strategy suffer from the limited soft switching operation range, high circulating current, high current ripple and the narrow voltage conversion range [66], [99], [100]. A full bridge interleaved bidirectional boost converter and a bridgeless boost rectifier based partly isolated TPC topologies reduce input current ripple; however, additional active switches are required for a phase shift to establish power flow control between ports that make driver circuit requirements more complex [70]. The body diodes of the MOSFETs experience hard switched conditions that add additional reverse recovery losses in the design [70].
LLC resonant converters can achieve ZVS and ZCS, however, much effort should be given to control the LLC converters as soft switching conditions and other parameters greatly depend on the resonant frequency. The partly isolated topologies are suitable for applications in which the low operating voltage of PV and battery need to be boosted to match the load side high voltage, which further feeds an inverter to generate an ac output.
The output ports of the majority of the partly isolated converters reviewed are topologically designed as either half bridge or full bridge configuration with output diode rectifiers. Putting aside likely impacts on switching stress or soft switching arrangements, the output ports of these converters can be potentially converted to bidirectional ports using bilateral inversion techniques [89] and replacing the diode with active switches. In some cases, and again putting switch stress issues to one side, a four-quadrant port may be possible by replacing the output diodes with four quadrant switches with bidirectional conduction and blocking ability.

C. FULLY ISOLATED TPC
The main advantage of fully isolated TPCs is that they provide independence of voltage levels at each port [82]. This style of TPC exhibits a symmetrical structure between ports that makes driving and control circuitry symmetrical for each port and thus makes the converter control issues less complex [25], [30]. The energy transfer can be established by the leakage inductor of the high-frequency transformers. In addition, the HF transformer provides full isolation and voltage matching among the different ports. However, the fully isolated full bridge structure has the following drawbacks:

1) COMPONENT COUNT
When compared with the non-isolated converters, the fully isolated structure will have a greater component count and the use of the high-frequency transformer will increase the overall size and therefore relatively reduce the power density and reliability [37].

2) CONDUCTION LOSSES AND ZVS RNAGE
The fully isolated TPC topologies based on a DAB/TAB with simple phase shift control cannot operate effectively under wide input voltage variation, leading to high conduction losses and a reduction of the soft switching range [101], [104].

3) RESONANT STRUCTURE
To minimize the losses, resonant TPC topologies can be used [24], [26], [78]. However, the crucial parts of these resonant TPC topologies are the design of resonant parameters. The operating point of the converter may change with the resonant frequency and the control implications associated with the resonant circuit make this topology very complex. However, these problems can be effectively alleviated by applying improved modulation and control strategies. TPC topologies based on the full bridge structure are an appropriate choice for interfacing with the ac grid and these are normally designed for relatively high power applications [25]. This requires one of the three ports of these TPCs to be bidirectional ac to allow full four-quadrant operation for grid interactive applications. Some isolated converters, such as those derived from the DAB, have a high degree of symmetry and naturally offer two quadrant ports. They are the most adaptable to four quadrant port operation through the introduction of fourquadrant switches.

V. TPC TOPOLOGIES WITH AC PORT
In Sections III and IV, the review examines a range of non-isolated, partly isolated, and fully isolated dc-dc TPC topologies and discusses the benefits and limitations of those topologies with their key parameters. In this section, the authors aim to explore the TPC topologies with a direct full four quadrant ac output port which is very essential for grid interactive PV-BES integrated systems. The review identifies different topologies such as the boost derived converter [115], [116], Z-source inverter [117], matrix converter [118], [119] and DAB with back-to-back converter that are used to implement the direct ac port which are discussed below.

A. BOOST DERIVED CONVERTER
A new TPC topology with a direct AC output port is proposed in [115]. This topology is shown in Fig. 23 which is a boost derived non-isolated converter system having two dc input ports and an output AC port. The topology consists of two switching legs where only three active switches are used for each leg. The proposed topology can support buck, boost, and ac conversions. However, the maximum modulation index for FIGURE 23. TPC topology with a direct AC port based on boost derived converter [115]. ac conversion is limited due to the buck and boost operation of the dc/dc parts of the converters.
A similar boost-derived converter topology with a direct AC output port is proposed in [116] for residential applications with simultaneous dc and ac loads. The topology is defined as a boost-derived hybrid converter (BDHC) as it is designed from a generic boost topology and it can provide simultaneous DC and AC outputs at the output ports [116]. The BDHC topology is realized by replacing the controlled switch of single-switch boost converters with a voltagesource-inverter bridge network. This converter gives extra flexibilities to interface with one of the dc ports to supply ac loads.

B. Z-SOURCE INVERTER
Z-source inverters can also be used as TPC with a direct ac output port. A new topology of the energy stored Quasi-Z-Source Inverter (qZSI) is proposed for PV power system applications in [117]. The topology is shown in Fig. 24. The qZSI topology is capable of simultaneously controlling the inverter output power, tracking MPP and managing the battery power regardless of the charging or discharging situation. The voltage boosting and inversion, and energy storage features are integrated in the qZSI.

C. MATRIX CONVERTER
Matrix converters are gaining more attention as alternative solutions to power converters with bulky dc-link capacitors. The matrix converter can provide size, weight and volume advantages for grid interconnection of the microgrids, generation systems, and loads [120]. Compared to traditional converters based on rectifier-inverters, the matrix converter system also provides several technical benefits [121], [122].
The key benefit is its inherent four quadrant power flow. Therefore, in relation to a DAB converter for the PV and BES port, the matrix converter system can be used to implement a full four-quadrant bidirectional ac output port for grid integration. In this case, a full bridge converter using four bidirectional switches is integrated with the DAB converter to implement the ac output port to allow four quadrant power flows. The realized TPC topology with a direct ac port based on a DAB and matrix converter is presented in Fig. 25. The commutation techniques and control strategies for a direct form single phase matrix converter system are challenging, specially to deal with the wide voltage range required to synthesize a low frequency sinusoidal output voltage. This requires a proper commutation method to ensure safe switching operation of the single phase matrix converter [121], [123].
A similar concept is used to implement a direct DC-AC conversion with high-frequency link in [118]. The topology is shown in Fig. 26. In this topology, the output port is realized by a matrix converter for direct dc-ac conversion and the input ports are derived from generic boost converters that can increase the dc input voltages, thus meeting the requirement of many distributed generation systems, such as PV and BES systems. The voltage boosting feature of this topology reduces the turn ratio of isolated HF link transformer. The boost inductor also reduces the input current ripple; therefore, the saturation of the HF transformer can be controlled. The component counts are reduced compared with the DAB derived topology. The topology becomes compact and that reduces the cost, size, and the volume of whole converter system.
A TPC topology with a direct ac port is proposed in [119] using an indirect matrix converter for applications in various motor drive systems including HEV. The direct dc-ac conversion is performed by an Indirect Matrix Converter (IMC) and the neutral point connection of a motor is utilized by connecting to an additional DC-DC converter. The DC link part of the IMC connects a boost-up type DC-DC converter and batteries to perform as a secondary power source to drive the motor.

D. BACK-TO-BACK CONVERTER WITH A DC-LINK CAPACITOR
In recent years, converters with non-polarized dc-link capacitors have been proposed because of size, cost and life-time advantages [124]. These could be a viable choice for the grid connected integrated PV-BES systems. The overall converter system can be developed by a triple active bridge (TAB) converter and an additional full bridge converter to connect one port to the grid as shown in Fig. 27. Therefore, the overall TPC will topologically have 16 active switches if the grid connection is single phase. However, single phase loads always produce a double frequency power ripple. This can appear on the dc-link capacitor and may propagate to the PV port [125], [127]. Consequently, the operating point of the MPPT changes due to this double frequency voltage ripple on the dc-link. There are control methods for this style of TPC topology to direct the ripple power away from the PV port. Table 4 shows the key parameters of TPC topologies having a direct AC output port.

VI. PERFORMANCE COMPARISON
The key parameters and discussion of the non-isolated, partly isolated, and fully isolated dc-dc TPC topologies along with TPC topologies having a direct AC output port are presented in Sections IV and V respectively. In this section, detailed comparisons of the reviewed topologies are presented based on their topological classification. The topological mapping presented in Table 5 shows that many different types of TPC topologies can be derived by the combination of a group of generic converters such as buck, boost, buck-boost, Cuk, sepic, zeta, forward, flyback, half bridge (HB), full bridge (FB), DAB, and LLC resonant. A comparative study of these converters reveals that the peak switch current for forward, flyback and half bridge converters is double that of full bridge converters. It is also evident that the number of switches required in forward, flyback derived isolated bidirectional DC converters are less than for the half-bridge and fullbridge converters that are of six and eight switches topology respectively [82]. However, the full bridge and half bridge converters offer natural voltage clamping for the switches because of their inverse diodes and the removal of the need to depend on well coupled winding structures as found in the forward and flyback applications. In addition, because of the higher number of switches, the power capacity is largest in the full-bridge topology compared with any other of the topologies, as the power transmission capacity is proportional to the number of switches if the rated voltage and current of the switches remain the same. The transformer utilization factor for the half bridge and full bridge converter is very good, whereas the forward and flyback converters can use only half of the B-H loop, limiting the duty cycles. These advantages are such that, in higher power converters, the full bridge is nearly universally applied.
The LLC converters that operate based on the variable frequency and phase shift control modulation are very attractive and becoming popular recently for industrial and PV applications [128], [129]. The LLC converters work well as a unidirectional two-port dc-dc converter in dc-dc power conversion systems. However, their control strategies seem to be very complex for applications requiring bidirectional power flow capability with at least one bidirectional port AC due to the characteristics of LLC networks [23]. A complex control scheme is required for the half bridge, LLC and fullbridge converters. It is evident from the literature that the full bridge topology offers minimal voltage and current stresses in the devices and minimum VA rating of the high frequency transformer and low ripple current levels in the output filter capacitor [88]. At the same operating voltage, the output filter circuits are expected to be smaller for full-bridge converter as the output ripple frequency is twice the switching frequency, whereas the ripple frequency is the same as the switching frequency for forward converters. Two active switches are conducted simultaneously for the full-bridge converter during half of the switching cycle, whereas one switch is used for the forward and flyback converters. Therefore, the conduction losses would be higher for the full-bridge converters. However, the benefits of soft switching operation over a wide range, bidirectional power capability, modularity and symmetric structure of DAB [130] are attracting a lot of attention from research communities, hence DAB is expected to be one of the core circuits for high-frequency link power conversion systems [82]. From the above discussion, it is apparent that DAB derived converters are strong candidates for power conversion in PV-BES integrated systems. There are effectively two choices. A converter can be assembled using a TAB and an additional inverter stage to interface one port to the grid. An alternative is to modify one port of the TAB with four quadrant switches to gain a matrix converter functionality to allow a direct grid connection. The improved modulation and control strategies method needs to be applied to DAB/TAB converters to achieve favorable switching conditions to improve the converter efficiency and reliability. The overall size, weight, and cost of the TPC can be minimized by the sophisticated design of the HF transformer link.

VII. RECOMMENDATION AND FUTURE RESEARCH DIRECTIONS
The reported TPC topologies are categorized into three different arrangements based on isolation between the ports i.e., non-isolated topologies, partly isolated topologies and fully VOLUME 11, 2023     isolated topologies. The key parameters of these topologies such as power ratings, port voltages, switching frequencies, component counts and values, average efficiencies are shown in Tables 1-3 respectively. These TPC topologies are further classified based on their topological configurations as presented in Table 5. The detailed comparison of key features and limitations, control complexities and important advantages and disadvantages of the individual TPC arrangements are discussed and presented in Table 5. It is evident from Tables 1-4 that the choice of the TPC topologies depends on systems requirements and applications. For example, it is not an essential design requirement for TPC topologies to have bidirectional power flow capabilities between BES and load if the TPC topologies are intended for use in standalone power systems employing PV and BES. In an application where there are no isolation requirements between the load and BES terminal, the non-isolated topologies might be an appropriate choice. From the comparative study and above discussion, it can be established that non-isolated topologies are highly efficient and cost effective and are well suited for low power applications where there is no isolation needed, whereas isolated topologies are well suited for medium and high power applications where isolation requirements are necessary. Among two different isolated TPC arrangements, the fully isolated TPC based on the DAB/TAB derived configuration seems to be a suitable topology and can be reconfigured to use in grid-interactive PV-BES integrated systems. The review also explored converter topologies with a direct AC port in Section V. It identified different topologies such as the boost derived converter, Z-source inverter, matrix converter and DAB with back-to-back converter. The boost derived topology and Z-source inverter might be suitable for low power applications, however, these topologies cannot be used where an isolation requirement exists. Considering these factors, DAB derived topologies seem to be an appropriate topology. This review also examines and discusses the options to reconfigure a DAB/TAB for full four quadrant ac output port operations for grid interactive systems. The study found that the direct full four quadrant ac output port for the grid integrated PV-BES system is implemented by either a direct form matrix converter approach or DAB and a back-to-back converter with a small dc-link capacitor. However, there are some complexities in the modulation and commutation process of a matrix converter. Therefore, the future research must address the challenges in modular, cost-effective, and highly efficient converter design. The future research should also address the challenges to develop innovative control strategies for converters to minimize the losses and improve the power density. For isolated topologies, the research challenge is to meet compact converter design requirements with technological innovation in HF transformer designs, lower component counts, and fast semiconductor switches to meet the lower cost and higher efficiency targets. Table 6 provides the best recommendations for the choice of TPC topologies based on system requirements and applications.

VIII. CONCLUSION
The use of PV and BES systems is gradually increasing in residential applications. The effective utilization of the PV and BES systems requires enabling power converter technologies. TPCs are an alternative means to implement single stage power electronics conversion to support PV and BES system utilization and integrations, having fundamental features such as the MPPT option for PV and charging/discharging capabilities for battery port. In this paper, a comprehensive review is carried out on a variety of dc-dc TPC topologies that are reported in recent publications. This paper provides a framework that systematically explores the full range of technical benefits and limitations of each TPC topology. This extensive review with its thorough discussion provides a useful framework and a strong foundation for researchers working on future TPC topology developments.  He has over 20 years of experience in academia, industry, and international standardization committees, including eight years in two large research and development centres working on power electronics and power quality projects.
He has received several awards, such as an Australian Future Fellowship, John Madsen Medal, Symposium Fellowship, and Early Career Excellence Research Award. He has published four books, over 280 journals and conference papers, five patents, and over 40 technical reports. He is also the Task Force Leader (the International Project Manager) of Active Infeed Converters to Develop the First International Standard IEC 61000-3-16 within the IEC Standardization SC77A. He is a Senior Editor of IEEE ACCESS journal, a Guest Editor and an Associate Editor of the IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS, an associate editor of IET journal, and an editorial board member of several international journals. His research interests include power electronics topology, control and applications, power quality and regulations, and pulsed power applications.
OMAR FARROK (Member, IEEE) received the B.Sc., M.Sc., and Ph.D. degrees from the Rajshahi University of Engineering & Technology, Rajshahi, Bangladesh, in 2006, 2009, and 2016, respectively, all in electrical and electronic engineering (EEE). He was promoted to a Full Professor with the Department of EEE, Ahsanullah University of Science and Technology, Dhaka, Bangladesh, in 2020. He has authored or coauthored more than 80 technical papers in the international journals and conference proceedings, including 12 IEEE TRANSACTIONS/journal articles. His research interests include permanent magnet machines, linear electrical generator, magnetic material, renewable energy systems, oceanic wave energy converter, electrical machine and drive, electromagnetics, and power electronics. He is also a Life Fellow of the Institution of Engineers, Bangladesh. He was a recipient of six best paper awards from the IEEE international conferences. He is elected as the Chair, the Co-Chair, and nominated as a member of several technical committees formed by the Bangladesh Government under the Ministry of Industries and others. He was a Professional Engineer with the Board of Bangladesh Professional Engineers Registration Board, in 2017. VOLUME 11, 2023