Description:A NOVEL CASCASED MULTILEVEL ISOLATED BIDIRECTIONAL DC-DC CONVERTER AND ITS OPERATION THEREOF

Field of the Invention:
The present invention relates to a novel cascaded multilevel isolated bidirectional DC-DC converter (1) capable of transferring power in both the directions. The proposed converter has higher power handling capacity which requires a single DC input to store the energy in multiple battery units and delivers smooth current waveform with lower total harmonics distortion (THD) in high frequency AC link conversion stage using the multilevel modulation scheme.

Background of the Invention:
The renewable energy is a key technology to meet future power system needs. To ensure the renewable power generation matches the power demand under all circumstances, energy storage system (ESS) is most commonly preferred. The challenges accrue when interconnection with renewable energy generation such as solar, wind and fuel cell is implemented [1-4].
An isolated bidirectional dual active bridge (DAB) DC-DC converter is the most suitable solution to achieve need-based smooth power exchange in renewable energy system [5-6]. It has several advantages such as bidirectional capability, high power exchange, zero-voltage switching (ZVS), symmetric structure and smaller size. The operational details and performance of DAB as compared to other DC-DC converter topologies have been reported in the literature [7-10].
To meet the requirement of integrating hybrid generation system to energy storage unit, the modular power converters are preferred as reported in [11-13]. The research in these papers was mainly focused on power balance of hybrid system interconnected with super capacitors and battery storage units. The single unit of DAB converter helps to avoid the mismatch between generation and load demand. In [14], for medium stage power conversion, DAB ensures precise and smooth power exchange between high voltage (HV) and low voltage (LV) dc voltage ports. To control the power transfer, single phase shift (SPS) modulation is commonly used in high power transfer applications owing to its simple control mechanism. However, it leads to circulating current and high current stress in DAB converter. To enhance the execution of DAB an extended phase shift (EPS) modulation and dual phase shift (DPS) modulation has been suggested in [15-21]. To reduce the switching and conduction losses of the DAB, transformer current is modulated in trapezoidal or triangular wave shapes using advanced modulation techniques such as trapezoidal modulation (TZM) and triangular modulation (TRM) [22] [23]. Further research has been achieved in modulation techniques to improve the efficiency over a wide operating range. Hybrid modulation techniques were suggested in [24]. With all these modulation techniques, efficiency improves greatly and gives better performances of DAB. However, all these modulation techniques result in non-sinusoidal inductor current, causing high total harmonic distortion (THD) in the high-frequency AC link stage. This is expected to result in more losses in the transformer. Furthermore, for the non-sinusoidal periodic condition, reactive power starts flowing in the reverse direction in the DAB converter. The flow of reactive or non-active power in DAB converter has been presented in [25]. For high power applications, reactive power flow causes instability in control, especially in renewable energy applications [26].

Summary of Invention:
Isolated bidirectional DC-DC converters are useful for smooth transfer of power in both the directions. Large scale energy storage and retrieval in batteries are important requirements in renewable power systems. A cascaded multilevel isolated bidirectional DC-DC converter capable of transferring power in both the directions has been developed herein. The proposed converter has higher power handling capacity which requires a single DC input to store the energy in multiple battery units. It delivers smooth current waveform with lower total harmonics distortion (THD) in high frequency AC link conversion stage using the multilevel modulation scheme. The performance of the proposed converter has been verified through the simulations and validated through the experimental work.
In large scale power transfer applications, galvanically isolated bidirectional dual active bridge (DAB) need to be connected in parallel/series to reduce the current/voltage handling capacity of HV-bridges of the multiple units DAB and to achieve low harmonic distortion AC current flow in high frequency link. In this invention, the HV bridges are connected in parallel and LV bridges are cascaded with separate isolated energy storage batteries to accomplish higher power transfer with low distortion AC current. The proposed configuration is said to be a cascaded multilevel isolated bidirectional DC-DC (CMIBDC) converter.

Objective of the invention:
The prime objective of the invention is to propose a novel Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1) comprising of a novel assembly of plurality of high-voltage bridge (2), low-voltage bridge (3), high frequency transformers (4) and energy storing units (5); wherein all said high frequency transformers (4) in the said assembly are conjugated with single coupling inductor (6); and wherein said converter (1) is provisioned to interconnect multiple renewable energy sources (7) with said energy storing units (5) and capable of exchange power between said interconnect multiple renewable energy sources (7) and said energy storing units (5)., and said high-voltage bridge (2) connected in parallel through the primary of said high-frequency transformers (4) and said low-voltage bridge (3) cascaded in series through secondary of said transformer (4), which are connected back-to-back via said inductor (6) to transfer the high power in a bidirectional way. The DC power can also be transferred via three-phase inverter (8) to the main AC grid (9). The common DC bus (10) collects the power from multiple renewable energy sources (7), act as single input to the Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1) and can feeds power to the main grid (9).
The another objective of the invention is to propose a novel cascaded multilevel isolated bidirectional DC-DC converter (1) used to interconnect multiple renewable energy sources (7) with multiple energy storing units (5) and capable of exchange power between said interconnect multiple renewable energy sources (7) and said multiple energy storing units (5).
Another objective of the proposed invention is to charge/discharge the energy in said multiple energy storage units using single DC input.
The another prime objective of the proposed cascaded multilevel isolated bidirectional DC-DC converter(1) is to achieve a wide range of DC power transmission with lower total harmonic distortions on AC side.
Another objective of the proposed invention is to transfer the power between multiple renewable energy sources (7) and multiple energy storing units (5) by controlling the pulse width modulation and phase angle shift between the two voltages ends of said coupled inductor (6).
Another objective of the proposed invention is to provide large scale energy storage applications with reduced voltage and power handling capability of each said high-voltage bridges (2) and low-voltage bridges (3), by connecting said multiple high-voltage bridges (2) in parallel through primary side of said transformer (4), and connecting same number of cascaded said low-voltage bridges (3) at secondary of said transformer (4).
Yet another objective of the proposed invention is to propose a Multi Phase Shift modulation technique to bring the harmonic distortions in AC current flow between HV and LV high frequency link in permissible limits for entire range of phase shift ratio compared to conventional modulation schemes and to accomplish sinusoidal current in high-frequency link stage.
The aforesaid objectives can be achieved by cascading the dual active bridge converter units by connecting said high-voltage bridges (2) in parallel and said low-voltage inverters (3) in series through isolated high frequency transformers (4). The configuration henceforth called cascaded multilevel isolated bidirectional DC-DC (CMIBDC) converter (1).

Brief description of the accompanying drawings:
Figure 1 depicts the view of the generalized block diagram of proposed cascaded multilevel isolated bidirectional DC-DC converter (1) comprise of multiple high-voltage bridges (2), multiple low-voltage bridges (3), multiple high frequency transformers(4), at least and more preferably two energy storing units(5), and an inductor(6), wherein said high-voltage inverters(2) are connected in parallel and said low-voltage bridges (3) are connected in series to the primary and secondary of said transformers(4), respectively, the said energy storing units(5) connected on the DC side of the said low-voltage bridges(3), and a common DC Bus(10) feeds the DC side of the said high voltage bridges(2), which can also be connected to a 3-phase inverter(8) whose ac side is considered output which is further connected to an AC Grid(9);
Figure 2 shows the circuit diagram of said cascaded multilevel isolated bidirectional DC-DC converter (1) for k = 2 (number of cascaded units), wherein L is the coupled inductor (6); VL is the voltage across the inductor; V1 and V2 are the input voltage and the voltage across battery storage unit, respectively; Ibat1 and Ibat2 are the current at the battery storage unit 1 and 2, respectively; C1, C2, C3 and C4 are the filter capacitors;
Figure 3 depicts the view of the AC side operating waveforms of said cascaded multilevel isolated bidirectional DC-DC converter (1) for (a) 0 = d = d1, (b) d1 = d = (d1 + d2), and (c) (d1 + d2) = d = 0.5; whereas figure on top shows net HV side voltage vHV/n across the common secondary of the transformer, middle figure shows LV side voltage vLV and bottom figure shows the inductor current IL.
Figure 4 shows the simulation results of HV and LV side voltages vHV/n, vLV, respectively in top figure; inductor current iL and battery currents (Ib1 and Ib2), for (a) 0 = d = d1, (b) d1 = d = (d1 + d2), (c) (d1 + d2) = d = 0.5, in bottom figure;
Figure 5 represents Total Harmonic Distortion for various modulation techniques at phase angle ? = 5° (a) SPS modulation, (b) EPS modulation, (c) DPS modulation (d) Proposed multi-phase shift (MPS) modulation.
Figure 6 depicts the view of the comparative graph of Total Harmonic Distortion under various modulation techniques with variation in phase angle ? between HV and LV side voltages;
Figure 7 shows (a) Snapshots of Transformers used in experimental setup, (b) Experimental results of transformer#1 for primary and secondary voltages, (c) Experimental results of transformer#2 for primary and secondary voltages;
Figure 8 depicts the view of the laboratory experimental set up of said cascaded multilevel isolated bidirectional DC-DC converter (1) comprising of said high-voltage bridges (2), said low-voltage bridges(3), said high frequency transformer (4) , said energy storage unit (5) and a processor/electronic programmable device (11).
Figure 9 depicts another view of the complete set up of said cascaded multilevel isolated bidirectional DC-DC converter (1) comprising of said high-voltage bridges (2), said low-voltage bridges(3), said high frequency transformer (4) and an inductor(6).
Figure 10 shows the Experimental Results of (a) Voltage at primary (vHV1 and vHV2) and secondary (vHV/n), (b) vHV/n, vLV and iL for the condition of 0 < d = d1 (c) vHV/n, vLV and iL for the condition of d1 = d = (d1 + d2) (d) vHV/n, vLV and iL for the condition of (d1 + d2) = d = 0.5;
Figure 11: Shows the zoomed view of experimental Results vHV/n, vLV and iL for the condition of 0 < d = d1;
Figure 12: Shows the experimental Results (a) vHV/n, vLV and battery current Ibat1(b) vHV/n, vLV and battery current Ibat2;

Detailed description of the invention:
The generalized block diagram of proposed cascaded multilevel isolated bidirectional DC-DC converter for energy storage applications is shown in Figure 1. In this block diagram, the renewable energy sources (7) such as solar, wind and fuel cells feed the DC bus (10). The output of the DC bus act as a single input to said cascaded multilevel isolated bidirectional DC-DC converter is used to interconnect the renewable energy sources (7) and multiple energy storage batteries (5) to exchange power in both the directions. The DC side of the high-voltage bridges and low voltage bridges is considered input and AC side is considered output.
In present cascaded multilevel isolated bidirectional DC-DC converter, said multiple HV-bridges (2) are connected in parallel to the primary of each said high-frequency transformer (4) as shown in Figure 1. The secondary of each said transformer (4) is connected in series by cascading output of said LV-bridges (3). The multiple energy storage elements (5) are connected at the input side of the LV bridges (3). The power is transferred by controlling the pulse width modulation (PWM) plus phase angle shift between two voltage ends of said coupled inductor (6). The voltage across the secondary winding of said transformer (4) is vHV/n and the voltage across the AC side of said cascaded low-voltage bridges (3) is vLV. Here, n is turns ratio of said transformer (4). For large scale energy storage applications, multiple high-voltage bridges (2) can be connected in parallel through the primary side of said transformer(4), to reduce the voltage handling capability of each said high-voltage bridges (2). Likewise, the voltage handling capacity of low-voltage bridges (3) can be reduced by connecting same number of cascaded low-voltage bridges (3).
The number of devices used and maximum number of voltage levels obtained in proposed cascaded multilevel isolated bidirectional DC-DC converter (1) are, respectively, given by the following equations.
(1)
(2)
Where, k is the number of cascaded units comprising said k number of high-voltage bridges (2) and low-voltage bridges (3), connected in HV and LV side, respectively, of said cascaded multilevel isolated bidirectional DC-DC converter.
The operation of said Cascaded Multilevel Isolated Bidirectional DC-DC converter is described for k = 2 herewith. The multi-phase-shift (MPS) modulation technique is applied to said cascaded multilevel isolated bidirectional DC-DC converter to achieve near sinusoidal current at high-frequency AC link stage. The circuit diagram of said cascaded multilevel isolated bidirectional DC-DC converter for k = 2 is shown in Figure 2. Here, L is said coupled inductor (6); vL is the voltage across the inductor; V1 and V2 are the input voltage and the voltage across battery storage unit, respectively; ibat1 and ibat2 are the current at the battery storage unit 1 and 2, respectively; C1, C2, C3 and C4 are the DC side filter capacitors. Furthermore, there are three ways to control power transfer as follows:-
i) 0 = d = d1, ii) d1 = d = (d1 + d2), iii) (d1 + d2) = d = 0.5.
Where, d1 and d2 are the modulation phase shift ratios of vHV/n and vLV, respectively; d is the phase angle shift ratio between operating voltage waveform of vHV/n and vLV.
In order to achieve flexibility in control, three conditions are described here. The operating waveforms of said cascaded multilevel isolated bidirectional DC-DC converter(1) are shown in Figure 3 (a), (b) and (c), for the conditions, 0 = d = d1, d1 = d = (d1 + d2) and (d1 + d2) = d = 0.5, respectively. The range of d1, d2 and d3 lies between 0 and 1, whereas the summation of d1, d2 and d3 should not be greater than or equal to 0.5. Figure 3 clearly indicate the modulation phase shift ratios of d1, d2 and d3 under various conditions. The relation between phase shift ratios can be written as:-
(3)
The phase shift ratio d can be written as,
(4)
Where, ? = phase angle between vHV/n and vLV and the maximum power can be transferred when the phase angle is 90°.
The different peak currents of inductor are represented as iL1 to iL7 in Figure 3(a) to (c), for three conditions. The detailed operating waveform clearly shows that the various peak currents of inductor changes according to the voltage levels of vHV and vLV. It is important to note that the magnitude of peak inductor current is different in each condition, whereas, the wave shape of the inductor current almost remains same.
Simulation Results
An energy storage unit having 10 kW capacity has been simulated in PSCAD software. To accomplish the energy transfer, said cascaded multilevel isolated bidirectional DC-DC converter (1) has been connected with the battery storage unit (5). The rating and system parameters are listed in Table 1. The main objective of the proposed converter (1) is to achieve a wide range of DC power transmission with lower total harmonic distortions (THD) on AC side. To control the power flow in said cascaded multilevel isolated bidirectional DC-DC converter (1), there are three conditions as discussed earlier. The results obtained with these three conditions are presented in Figure 4. The voltage across the secondary of said high-frequency transformer (4) vHV/n and vLV is shown in Figure 4(a). Fundamentally, these two voltages have a phase shift of 15?, which lies in the range for the condition of 0 < d = d1. In this case, vLV lags vHV/n and the power gets stored in the battery storage units (5). The inductor current and current in battery storage unit (5) are shown in Figure 4(a). Similarly, the simulation results of vHV/n, vLV, iL, and battery storage current for the condition d1 < d = (d1+d2) and (d1 + d2) = d = 0.5, are shown in Figure 4(b) and (c), respectively. It can be observed from these results that the battery storage current is a part of inductor current which has a double frequency component.
Table 1
Simulation Setup Parameters
Parameter Simulation
Inductance (DAB converter) 10 µH
Switching Frequency 20 kHz
Nominal battery voltage 240 V
Nominal battery capacity 500 Ah
Transformer rating 20 kHz,
10 kVA
Transformer Turn ratio 50:1

It is a notable point that at smaller values of the phase shift ratio, the circulating current owing to the battery storage unit flowing in said cascaded multilevel isolated bidirectional DC-DC converter (1) is quite high. The simulated results of vHV/n, vLV, iL and battery storage currents are shown in Figure 4(b) and (c), for the condition d1 = d = (d1 + d2) and (d1+d2) < d = 0.5, respectively. The larger amount of deviation from the sinusoidal fundamental frequency produces unwanted heating and increases the losses in high-frequency transformer.
To verify the harmonic content in the transformer current up to the 15th order, the current harmonic spectrum for said cascaded multilevel isolated bidirectional DC-DC converter (1) for SPS, EPS, DPS and proposed Multi Phase Shift (MPS) modulation techniques are shown in Figure 5 (a), (b), (c) and (d), respectively at phase angle ? = 5°. It can be observed from Figure 6 that the current THD with the proposed multi-phase shift modulation technique is lesser than 6%. In all other modulation techniques the THD is greater than 6%. In order to further make a comparison of the THD of transformer current at different phase angle ?, it has been measured through simulations and plotted in Figure 6. With all other modulation methods, lower THD can only be achieved for higher phase angles. . However, the proposed MPS modulation with said cascaded multilevel isolated bidirectional DC-DC converter (1) brings the THD in permissible limits for entire range of phase angle, which is shown in Figure 6.
(a) Design Specification of Laboratory cascaded multilevel isolated bidirectional DC-DC converter (1) Converter Setup:
Following are the different components alongwith the detailed specifications of cascaded multilevel isolated bidirectional DC-DC converter (1):
High voltage and Low voltage bridge inverter (2) and (3):
It is used to convert DC into higher frequency AC output. To reduce the switching losses of HV and LV bridge inverter, IRFP250N-MOSFET device is used to form H-bridges for both and assembled in a single-PCB. It has the advantages of smaller rise and fall time which helps to reduce the switching losses while operating at higher frequency. Thick copper coating is formed on the track of copper strip on the reverse side of each H-bridge PCB to carry higher current. Each H-bridge is rated for 20 kW of power.

High frequency Transformer (4):
Two identical transformers (4) are used in said cascaded multilevel isolated bidirectional DC-DC converter (1). Both the secondary windings of said transformers (4) are connected in series through said LV bridges (3). The primary windings are connected in parallel via HV bridges (2). The transformer design specifications are given in Table 2.

Table 2: Design specifications
Core type Ferrite
Core area 15.7cm2
Number of turns and size, HV side 40
Number of turns and size, LV side 10

For the experimental setup, the turns ratio n of said transformer (4) is considered equal to 4.. The transformer coil has been chosen 5-10 mm2 depending upon the requirement of maximum current density. The calculation of diameter of the coil can be done by the following expression [27] and [28].
……….(5)
Where, S is the current density.
Inductor (6):
The ferrite ETD-16 can be used as core type for said inductor (6) [29]. It supports for wide frequency range of operation. The wire diameter of the inductor can calculated using equation-5 above.
Battery storage system/Energy storage unit (5):
Sealed and maintenance free lead acid batteries have been chosen with rating of 12V, 7Ah, 6 nos. The maximum energy of 0.5kVAh can be accumulated in energy storage unit (6).
Experimental results:
To verify the operation and validate the cascaded multilevel isolated bidirectional DC-DC converter (1) using multi-phase shift modulation, an experimental set up has been built-in the laboratory and implemented using FPGA 3AN Spartan processor. The experimental setup parameters are listed in Table 3. The key component of said converter (1) is said high frequency transformer (4), which has a direct impact on the performance of the said converter (1). Ferrite core is most popularly used as a core in high frequency applications. The picture of transformer#1 and #2 are shown in Figure 7(a). The experimental results are obtained for the primary and the secondary voltages of said transformers (4) as shown in Figure 7(b) and (c), respectively.
The built laboratory experimental set up and the complete pack of said cascaded multilevel isolated bidirectional DC-DC converter (1) is shown in Figure 8 and Figure 9 respectively. Three-level voltage of each primary winding and five-level voltage of cascaded secondary windings are shown in Figure 10(a) i.e., the three-level voltage is combined through said transformer (4) secondary to produce five-level output with the scaled magnitude of vHV/n. Thereafter, it supplies the voltage to said inductor (6) at one end. The other end of said inductor (6) is supplied back from said cascaded low-voltage bridge (3) with five-level output. Figure 10(a) shows, the primary voltages of transformer #1 and #2 and the combination of secondary voltage (vHV/n) of said transformer(4). Here, the primary coils are connected parallel while the secondaries are connected in series. As discussed earlier, there are three modes to control the power transfer in said cascaded multilevel isolated bidirectional DC-DC converter (1). Figure 10(b) shows, vHV/n, vLV and inductor current iL at a phase angle of 15?, i.e., for the condition of 0 < d = d1. It confirms that said converter (1) produces near sinusoidal inductor current. Similarly, the waveforms of vHV/n, vLV and iL for the condition d1 < d = (d1+d2) and (d1+d2) < d = p/2, have also been obtained. These results have been captured at the phase angles of 50? and 70?, between the voltages vHV/n and vLV, which is shown in Figure 10(c) and 10(d), respectively. In order to comprehend, how each interval of inductor current gets influenced by five levels of vHV/n and vLV, with the zoomed view captured in Figure 11. The currents at battery #1 and #2 are captured and shown in Figure 12.
The ripple current is more under light load conditions and circulation current flows for SPS, EPS and DPS modulation techniques. As a consequence, higher ripple current flows at the output DC side in the batteries. Several optimization algorithms have been proposed in the literature, to reduce the circulating current. However, the circulation current cannot be eliminated for the entire range of power transfer range [12-15]. With the proposed MPS modulation strategy, the peak current of the inductor gets reduced with minimum circulating current. Therefore, it transfers the power with lesser ripple in battery current. The experimental results of current at battery #1 and battery #2 are shown in Figure 12 (a) and (b), respectively.
Table 3
Experimental Setup Parameters
Parameter Experiment
Inductance (DAB converter) 200 µH
Switching Frequency 20 kHz
Nominal battery voltage 12 V
Nominal battery capacity 9 Ah
Transformer rating 20 kHz,
1 kVA
Transformer Turn ratio 4:1
The aforementioned analysis conclusively yields that there is a good correlation between theoretical study, simulated results and experimental results. The large amount of power can be accomplished in the battery storage unit at higher phase shift ratio.

Benefits of the Invention:
--The proposed invention is useful to accumulate larger amount of energy with controlled charging/discharging of batteries. With better power regulating capability the proposed said cascaded multilevel isolated bidirectional DC-DC converter (1) is suitable for integration of renewable energy systems to energy storage.

--The power handling capacity of the proposed said cascaded multilevel isolated bidirectional DC-DC converter (1) is higher as compared to conventional dual active bridge isolated bidirectional converter, with lesser battery ripple current and lower THD current in high frequency transformer. This increases the battery life and reduces the transformer losses.

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Claims:We Claims:

1. A novel Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1) comprising of a novel assembly of plurality of high-voltage bridges (2), low-voltage bridges (3), high frequency transformers (4) and energy storing units (5); wherein all said high frequency transformers (4) in the said assembly are conjugated with single coupling inductor (6); and wherein said converter (1) is provisioned to interconnect multiple renewable energy sources (7) via common single DC bus (10) with said energy storing units (5) and capable of exchange power between said interconnect multiple renewable energy sources (7) and said energy storing units (5).

2. A novel Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1), as claimed in claim 1, wherein the number of HV bridges, LV bridges and high frequency transformers in the assembly of said Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1) is determined by the number of levels required on the AC side of the said converter (1) and is equal to (Nstep-1)/2, wherein Nstep represents the number of levels required on the AC side of the said converter (1).

3. A novel Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1), as claimed in claim 1, wherein said high-voltage bridges (2) connected in parallel through the primary of said high-frequency transformers (4) and said low-voltage bridges (3) are cascaded in series through secondary of said transformer (4), which are connected back-to-back via said inductor (6) to transfer the high power in a bidirectional way.

4. A novel Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1), as claimed in claim 1 and claim 3, wherein a provision of a Multi-Phase Shift (MPS) Modulation technique has been employed to control the power transmission in said converter (1) and to bring the harmonic distortions on AC current flow between HV and LV high frequency link in permissible limits for entire range of phase angle compared to conventional modulation schemes and to accomplish near sinusoidal current in high-frequency AC link stage.

5. A novel Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1), as claimed in claim 1 and claim 3, wherein a common single DC bus is capable to charge/discharge the energy in said energy storage units (5).

6. A novel Cascaded Multilevel Isolated Bidirectional DC-DC Converter (1), as claimed in claim 1 and claim 3, wherein the AC current of said transformer (4) is close to sinusoidal waveshape with low total harmonic distortion (THD) in said converter (1) and hence the battery charging/discharging current have lower ripple component.