This literature review is for the ongoing research on DC Nano grid. Integrating single phase AC to the grid and supplying AC loads are the future extension for this Nano grid research. Here dual active bridge converter cascaded with inverter topology related literatures has been reviewed to find out the feasibility of introducing AC supply and AC loads to Nano grid with 48V DC bus using DAB converter.

**Control Scheme for a Bidirectional Converter in a Self-Sustaining Low-Voltage DC Nanogrids**^{[1]}[edit | edit source]

Saravana Ilango Ganesan, and Dinesh Pattabiraman et.al. developed a control scheme for two stage DAB and inverter to interface between Nano grid and AC network.

- Shown optimal capacitor link voltage range for the power transfer in both directions between AC network and DC Nano grid.
- Designed a 5kVA converter in hardware and verified the operation.
- Power flow is controlled by varying the duty cycle of the primary and secondary connected inverter.
- The power flow was controlled in three different modes decided by energy management system. The modes are power surplus, deficit and idle mode.
- Power flow is automatically controlled based on DC bus bar control.
- The three phase inverter has double loop controller. The outer loop regulates the grid currents and the outer loop controls the capacitor link voltage.
- During hardware implementation they have used 5kVA transformer of 10 kHz and IGBT as switches. And used fixed switching frequency 10kHz.

**A Grid-Connected PV Interface System Based On The DAB-Converter**^{[2]}

Mostafa I. Marei and Hadi El-Helw et. al. presented a direct interface of DAB converter and inverter between PV array and grid.

- The PV voltage is regulated by PI controller which regulate the phase shift angle of DAB and the inverter regulates the DC link voltage at a constant value to transferring the power to the grid.
- In the
**Control Scheme for a Bidirectional Converter in a Self-Sustaining Low-Voltage DC Nanogrid**DAB, both of the H bridges have same frequency (1kHz) signal of 50% duty cycle and the phase shift controls the power flow through the transformer as well as the direction of power flow. - They se
**Control Scheme for a Bidirectional Converter in a Self-Sustaining Low-Voltage DC Nanogrid**t the phase shift limit (21^{0}) for the DAB which is found from the characteristics curve of DAB operating at the maximum power point voltage of PV, 110V.

**Active PoweControl Scheme for a Bidirectional Converter in a Self-Sustaining**
Low-Voltage DC Nanogrid r and DC Voltage Coordinative Control for Cascaded DC–AC Converter with Bidirectional Power Application^{[3]}

Yanjun Tian and Zhe Chen et. al. proposes a coordinated control method for dual active bridge cascaded with inverter topology to reduce fluctuation of DC link voltage for reducing switch stress as well as to the improve the dynamic performance.

- This proposed method coordinates the DC link voltage with the power, thus the variation of DC link voltage variation due to power variation is suppressed significantly.
- In this control method the DAB and inverter both control dc-link voltage and output AC power simultaneously.
- If the Capacitor link voltage is above the reference, then the DAB will reduce the output AC power to reduce the capacitor link voltage on the other hand the inverter can increase output power in order to decrease the dc link voltage and avoids cross regulation issue.

**High-Efficiency Bidirectional DAB Inverter Using a Novel Hybrid Modulation for Stand-Alone Power Generating System with Low Input Voltage**^{[4]}[edit | edit source]

Yong-Won Cho and Woo-Jun Cha et. al. proposed a novel hybrid modulation technique for DAB Inverter topology of a low voltage DC grid.

- Achieved high efficiency of 94.2% by implementing ZVS turn on of the switches in both full bridges.
- To eliminate the drawbacks of phase shift switching of poor light load efficiency, large reactive current, and reduced range of soft switching during wide variation of input output voltages, a novel hybrid modulation combining PSM and variable frequency control is proposed.
- Transformer with high turn ratio has been used which has high leakage inductance L
_{lk,}eliminates the requirement of a series inductor. - Synchronous rectifier is used as inverter switch and Since all switches in SR operate at the grid frequency f
_{L}, the switching losses of those switches are very small. - The variable frequency control makes the nonlinear relation between iu and ϕ into a linear relationship.

**Impedance Modeling and Verification of a Dual Active Bridge (DAB) DC/DC Converter Enabled DC Microgrid in FREEDM System**^{[5]}

Qing Ye, Ran Mo and Hui Li analyzed the impedance characteristics of DAB connected used in DC microgrid.

- They used DAB for grid connectivity as well as to integrate energy storage devices to the DC microgrid.
- The output impedance of the DAB converters together with inverter are derived considering energy control flexibility.
- This impedance characteristics of DAB can be used in the future to improve system stability and formulate design methodology.
- Droop control is used for both AC grid connectivity and Energy storage devices. Under normal droop control the battery can’t be discharge at its full limit though the bus voltage is below the predetermined value. Therefore, a constant current method used to enable battery to discharge at its maximum capability.
- The DAB has three control loop which are droop control, outer voltage control and inner current control.
- While varying perturbation frequency from 1Hz to 10Hz, the output impedance behavior of DAB changes from inductive to capacitive.
- the SST DAB converter output impedance and DESD DAB converter output impedance in droop control mode has different characteristics, which can cause low frequency oscillation.

**Flyback Mode for Improved Low-Power Efficiency in the Dual-Active-Bridge Converter for Bidirectional PV Microinverters with Integrated Storage**^{[6]}

Shahab Poshtkouhi and Olivier Trescases introduced a novel DAB switching scheme for bidirectional PV micro inverters.

- For low power mode, the DAB is modified in such a way it can operate similar to a conventional two transistor fly back converter.
- The fly back mode has better power regulation capability and stable operation for low power level compared to conventional DAB.
- To avoid interfacing with the audible range operating frequency 20KHz is selected.
- But this mode has more conduction losses and more core losses which is eliminated reducing the switching frequency.

**Hardware Design and Demonstration of a 100kW, 99% Efficiency Dual Active Half Bridge Converter Based on 1700V SiC Power MOSFET**^{[7]}

Wei Xu, Zhicheng Guo and S. Milad Tayebi et. al. designed a 1.5kV DC 100kW bidirectional DAHB using SiC MOSFET with efficiency of 99%.

- 100kW medium frequency transformer is used and the parallel concentric winding structure increases current carrying capability and reduces leakage inductances to 2.2uH.
- Switching frequency set between 15-50kHz.
- Lossless snubber capacitor can significantly reduce the turn off loss especially heavy load operation.
- ZVS turn on over full voltage range for DC/AC operation mode.
- And optimized PCB based bus bar design significantly reduced the voltage overshoot across devices.

- ↑ S. I. Ganesan, D. Pattabiraman, R. K. Govindarajan, M. Rajan, and C. Nagamani, “Control Scheme for a Bidirectional Converter in a Self-Sustaining Low-Voltage DC Nanogrid,”
*IEEE Transactions on Industrial Electronics*, vol. 62, no. 10, pp. 6317–6326, Oct. 2015, doi: 10.1109/TIE.2015.2424192. - ↑ M. I. Marei, H. El-Helw, and M. Al-Hasheem, “A grid-connected PV interface system based on the DAB-converter,” in
*2015 IEEE 15th International Conference on Environment and Electrical Engineering (EEEIC)*, Jun. 2015, pp. 161–165. doi: 10.1109/EEEIC.2015.7165534. - ↑ Y. Tian, Z. Chen, F. Deng, X. Sun, and Y. Hu, “Active Power and DC Voltage Coordinative Control for Cascaded DC–AC Converter With Bidirectional Power Application,”
*IEEE Transactions on Power Electronics*, vol. 30, no. 10, pp. 5911–5925, Oct. 2015, doi: 10.1109/TPEL.2014.2375573. - ↑ Y.-W. Cho, W.-J. Cha, J.-M. Kwon, and B.-H. Kwon, “High-Efficiency Bidirectional DAB Inverter Using a Novel Hybrid Modulation for Stand-Alone Power Generating System With Low Input Voltage,”
*IEEE Transactions on Power Electronics*, vol. 31, no. 6, pp. 4138–4147, Jun. 2016, doi: 10.1109/TPEL.2015.2476336. - ↑ Q. Ye, R. Mo, and H. Li, “Impedance modeling and verification of a dual active bridge (DAB) DC/DC converter enabled DC microgrid in FREEDM system,” in
*2016 IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia)*, May 2016, pp. 2875–2879. doi: 10.1109/IPEMC.2016.7512754. - ↑ S. Poshtkouhi and O. Trescases, “Flyback Mode for Improved Low-Power Efficiency in the Dual-Active-Bridge Converter for Bidirectional PV Microinverters With Integrated Storage,”
*IEEE Transactions on Industry Applications*, vol. 51, no. 4, pp. 3316–3324, Jul. 2015, doi: 10.1109/TIA.2015.2409179. - ↑ W. Xu
*et al.*, “Hardware Design and Demonstration of a 100kW, 99% Efficiency Dual Active Half Bridge Converter Based on 1700V SiC Power MOSFET,” in*2020 IEEE Applied Power Electronics Conference and Exposition (APEC)*, Mar. 2020, pp. 1367–1373. doi: 10.1109/APEC39645.2020.9124401.