Three-port dual-active-bridge (DAB) converter was studied to integrate three DC units into one converter. Two-inductor three-port DAB converter is proposed to eliminate the power correlation issue without additional control. A voltage balancer for bipolar low voltage DC (LVDC) distribution is proposed using Two-inductor three-port DAB converter.
Moreover, it has less current load in power switches than that of the conventional three-port DAB converter.
Introduction
The remainder of this thesis is organized as follows: Section II introduces the conventional three-port DAB converter and its operational principles and control strategy. Two-inductor three-port DAB converter can eliminate the power coupling that the conventional converter has without additional control. Dual-transformer DAB converter is also able to eliminate power coupling without additional control.
Finally, an isolated voltage balancer using ESS in bipolar low voltage DC (LVDC) distribution is proposed using the three-port two-inductor DAB converter in Section IV.
Conventional Three-port Dual-active-bridge (DAB) Converter
Theoretical Analysis
The zero voltage vn can be derived from the superposition principle as below: (Details of the derivation follow in APPENDIX.A). Since the neutral voltage vn can be derived, voltages applied across the inductances and gate currents can be calculated. Therefore, the operating waveforms of the conventional three-port DAB converter can be illustrated as depicted in Fig.
The delta-connected equivalent circuit with the primary port (Port 1) referenced parameters can be as depicted in Fig.
Control Strategy
The gate currents can be derived from their corresponding currents if the gate voltages being regulated are assumed to be constant. A system matrix representing the input-output relationship can be defined by setting the control variables ϕ12 and ϕ13 as the inputs and the gate currents I2 and I3 as the outputs. The gate currents I2 and I3 can be derived by dividing equations (7) and (8) by their corresponding voltages.
New control variables ϕ*12 and ϕ*13 can be defined by applying the decoupling matrix in the control loop.
Simulation Results
The maximum change rate of decoupling matrix elements is 8.3% with the design parameters mentioned above. Therefore, one decoupling matrix can be used, calculated at a nominal operating point, where P2 = 1 kW and P3 = 0 W in this case, since ESS is often inactive. 2.13 (a) and (b) show the simulation results without the decoupling control and with the decoupling control strategy, respectively.
The conventional three-port DAB converter and its control strategy are presented in this chapter.
Advanced Three-port DAB Converter
Two-inductor Three-port DAB Converter
- Proposed Converter Structure
- Theoretical Analysis
- Simulation and Experimental Results
However, the second port does not have power transfer inductance, which is the structural difference compared to the conventional three-port DAB converter. The same way applied to the conventional three-port DAB converter can also be used to analyze the proposed converter. Therefore, the two-inductor three-port DAB converter has less circulating current than the conventional three-port DAB converter, which can increase the overall power conversion efficiency.
A two-inductor, three-port DAB converter can have simpler control strategies since it is itself a decoupled system with its structure. It can effectively eliminate the port coupling problem that the conventional three-port DAB converter has. The equations and (18) are the linearized system matrix elements of the conventional three-port DAB converter.
It can also be applied to two-inductor three-port DAB converter in the same way. One control stage can be reduced in the proposed converter compared to the conventional three-port DAB converter. Hence, two-inductor three-port DAB converter can control each port independently without additional decoupling control strategies.
Therefore, the result shows that the two-coil three-port DAB converter is a power-separated system. 3.13 (a) shows the operating waveforms of port 3 remaining at zero power in a typical three-port DAB converter.
Dual-transformer Three-port DAB Converter
- Proposed Converter Structure
- Theoretical Analysis
- Zero Voltage Switching (ZVS) Capability Extentsion
- Magnetizing Inductance Design
- Simulation and Experimental Results
It can also make full use of the transformer magnetizing inductances used to compensate current for the ZVS operation. The ZVS limit can be obtained with respect to output power across the voltage gain M12 defined as n22V2/V1. A zero vector control strategy for the ZVS capability expansion is proposed for the conventional three-.
Magnetizing inductance, which can extend the performance of ZVS, was not considered in any of the three-port DAB converters due to the complex structure. However, in a two-transformer, three-port DAB converter, the magnetizing inductance can be fully utilized to achieve ZVS operation over the entire load range. The effect of the magnetizing current can extend one of the ZVS limits shown in equations (46) and (49) by two different ones.
3.23 (a) shows Case 1, where the two inductors are located in port 1 and the magnetizing inductance current compensates the ZVS operation for port 2 and port 3. To have the minimum conduction loss, each magnetizing inductance must compensate half of the current required for the ZVS -the operation in port 1. To practically guarantee the ZVS operation, the margin for the magnetizing inductance is also added (see APPENDIX.B).
Due to the voltage gain M12 being greater than unity and M13 being less than unity, Port 2 has the ZVS operation, but Port 1 and Port 3 have the hard switching operation. Due to the voltage gain M12 and M13 being greater than unity, Port 2 and Port 3 have the ZVS operation, but Port 1 has the hard switching operation. The proposed method brings lower efficiency as Double transformer three-port DAB converter can have the ZVS operation under heavy load condition and the proposed method has higher core loss.
In the experimental results of ignition transition, the proposed magnetization design method effectively extends the operation of ZVS for different cases of voltage increase.
Isolated Voltage Balancer for Bipolar LVDC Distribution
Proposed Voltage Balancer Structure
Electrical isolation between the ESS and the grid is recommended to meet safety requirements as well as to protect operators during maintenance [36], [37]. However, the power supply assembly has between three ports due to the cross-coupled loops formed by its structure, which may cause control complexity to regulate the two poles of the DC bipolar LVDC distribution system. A two-inductor three-port DAB converter is used with the advantages of galvanic isolation and bidirectional power control capability.
Therefore, using a simple control algorithm, the converter can regulate the bipolar DC pole voltage levels at the grid-connected voltage balancing fault condition. In addition, it has less current voltage, which can improve the overall power conversion efficiency compared to the conventional converter. By using the proposed converter, the ESS can balance the bipolar DC voltage level for a longer time with the same amount of energy during AC grid-connected voltage balancer failure.
As mentioned earlier, the ESS must be accessible to a pair of DC DC poles to maintain the same voltage level in both DC poles even during grid-connected voltage stabilizer faults. A two-inductor three-port DAB converter connected to bipolar DC half-wires is shown in the figure. A full bridge is used for the ESS gate to handle the electrical energy provided by both poles of the DC current.
A half bridge is used for each DC pole gate since they handle less power than the ESS gate receives. The transformer provides galvanic isolation between the ESS and the grid and regulates the voltage difference between the ports.
Theoretical Analysis
- Voltage Balancing Operation
- Current Stress Reduction
The balancing strategy can be achieved by setting the gate reference voltage stage without an inductor. Otherwise, the off-diagonal elements G12 and G21 cannot become zero, which leads to difficulties in voltage balancing control. The current stress of the proposed converter at each gate is defined as the peak current value.
It shows that the proposed converter has less current stress than that of the conventional converter. As a result, there is no interference in the operation of the voltage regulation between the two DC poles. The peak current difference is from the leakage path formed by L23 located in the conventional converter.
Furthermore, the proposed converter has only two inductors, resulting in lower conduction and core losses than those of the conventional converter that has three inductors. The advantages of the proposed converter, which are the simple control of the voltage balancing capability and the reduced current load, are analyzed theoretically. In the current voltage comparison, the largest reduction in current voltage is 30.6% in the proposed converter.
Using a 3 kW prototype dual-transformer, three-port DAB converter, the performance and effectiveness of the converter are verified with the experimental steady-state operational waveforms and the dynamic response waveform. A voltage balancer for bipolar LVDC distribution is proposed that uses a two-inductor, three-gate DAB converter to balance the bipolar voltage levels in the event of grid-tied voltage balancer faults.
Experimental Results
Conclusion
Elimination of the power coupling issue and reduction of circulating current in one port idler is theoretically analyzed and discussed in Two-inductor three-port DAB converter. The operational principles and the absence of the power coupling issue of Double transformer three-port DAB converter are explained and the magnetizing inductance design methodology is also proposed to achieve the full ZVS operation. It can reduce the power density of the inverter, but has a greater consumption to connect more than three ports.
The experimental results indicate that the converter balances each bus voltage under multiple load conditions, shows that it independently regulates each bus without interference, and shows that it has reduced current voltage compared to the conventional converter. Electric Energy Delivery and Management (FREEDM) System: The Energy Internet," in Proceedings of the IEEE, vol. A high power density three-phase soft-switched DC/DC converter for high power applications." In: Conference minutes from the IAS annual meeting.
Dunford, "Stability Analysis of Bidirectional Isolated Full-Bridge DC-DC Converter with Triple Switching Control," in IEEE Transactions on Power Electronics, vol. Decoupled Power Control of Three-Port Active Two-Bridge DC-DC Converter for DC Microgrid Systems. Sheng, “Circulating Current and ZVS-on of a Dual Active Bridge DC-DC Converter: A Review,” in IEEE Access, vol.
Steinmetz, "On the Law of Hysteresis," in Transactions of the American Institute of Electrical Engineers, vol. The RMS value of the MOSFET current and Rds(on) is the on-resistance of the MOSFET as presented in the datasheet. 2) Switching Loss of MOSFET: The switching loss of MOSFET consists of on loss and off loss.
