A Two-Input Dual Active Bridge Converter for a Smart User Network Using Integrated Power Modules D. Menniti, N. Sorrentino, A. Pinnarelli, M.Motta, A. Burgio and P.Vizza

SUMMARY Introducing to the Smart User Network (SUN) The dual acvite bridge (DAB) converter The proposed novel topology for a DAB Laboratory model, test and results Conclusion and future aim

INTRODUCING THE SUN Smart User Network DC powered (SUN) Local network The PEI A Smart User Network (SUN) is a private microgrid; a bidirectional ac-dc power electronic interface (PEI) connects the SUN to the grid. All distributed energy resources, energy storage systems and loads are connected in parallel a local Direct Current bus (DC bus). A DC bus voltage (VDC) stable to a reference value (VDCref) is a key factor for the proper operation of the SUN; one of the power converters belonging to the SUN must be devoted to this scope. This power converter is named the master converter; the PEI is the preferable candidate because, most likely, its rated power is the highest with respect the others converters.

The Dual Active Bridge Converter Inductor current Battery side DC bus side Direct voltage Direct voltage High frequency square voltage When the SUN operates in island mode, the dc-dc power converter used for the battery ESS is suitable to functioning as master converter in place of the PEI. Why using a DAB converter? Because its high power density, high efficiency, low switching losses, bidirectional dc-dc power flow, isolation high frequency (HF) transformer. Why using it as master converter? Effectiveness, fast response, bidirectional power flow, robust and unaffected controllers.

What is the problem? The peak current is a key factor in designing robust DAB converter: the greater the peak current, the greater the total device rating. Moreover, the high pulsed collector currents subject IGBTs to high electrical and thermal stress, they force the IGBTs to operate outside the linear region with a significant rise in conduction losses and possible device disruption, burnout, overheating or short circuit

As an example….. Let us consider: V1=72Volt, V2=8*72Volt, L=20μH, the required power is 1 kW so D=0.10 and IRMS=13.88A. The peak current Ipeak is about 73.10A, see a. N is the HF transformer ratio fs is the switching L is the leakage inductance D is the phase-shift ratio defined as D=(ϕ/π) ϕ is the phase shift angle between v1 and v2

What is the problem? The peak current of 73A (see a in figure) can be reduced up to 27.77A adopting L=54μH and setting D=0.5 (see b in figure). But D=0.5 corresponds to the maximum of the DAB power transmission characteristic; typical values for the max(D) are in the range of 0.25÷0.33 [15]. Setting D=0.30, the operating point to c (see figure) where L=46μH and Ipeak=32A. In order to achieve a deeper reduction of the peak current, many DAB converters have to be used; for example, the peak current reduces to 10.96A if three DAB converters are used, using L1= L2= L3 = 135μH and setting D=0.3 (see d in figure).

The proposed DAB topology Battery side DC bus The authors propose a novel topology for a DAB converter which deeply reduces the peak current so achieving an higher robustness and reliability. A 1kW prototype of a DAB converter implementing the proposed topology was built using integrated power modules in place of discrete ones.

The proposed DAB topology Battery side DC bus side Damping resistor The proposed topology is congenial to the use of integrated power modules (IPM) in place of discrete ones.

LABORATORY MODEL A laboratory prototype of a SUN, along with a DAB converter implementing the proposed topology, has been built and tested in the laboratory. The good dynamic response of the DAB converter under transient condition has been tested considering a deep step change in power balancing. The DAB rated power is about 1kW, three inductors L=150μH have been adopted, the phase shift ratio has been set to setting D=0.3 so the peak current is limited to about 8.74 A that is 2.31 times the RMS current.

LABORATORY MODEL R IRAM SUN L1, L2 and L3

Laboratory Test At the 0s, the PEI drawn a grid current so to supply the load, batteries do not charge/discharge, Stirling engine is in stand-by. At 10s the PEI disconnects the grid, the DAB converter supplies the load. At 25s the grid is re-connected. At 38s the Stirling Engine starts generating 350W, the braking resistor dissipates 186W. The PEI does not inject a current into the grid, the DAB braking resistor is used to set the DC bus to 415V, a value intentionally higher than 400V.

DAB peak current Voltage VL The RMS current is 1.66A whereas the peak current is in the range 2.81÷3.18A. The peak current is about 1.69÷1.91 times the RMS current, so confirming the effectiveness of the proposed topology for a DAB converter in reducing the peak of the inductor current. Current IL

CONCLUSION AND FUTURE AIM For a Dual Active Bridge converter, peak current represents … a cost effective solution to limit the peak current …. is the adoption of a further inductor placed in series with the transformer is a.. A deeper reduction of the peak current is achieved by adopting the novel topology for a DAB converter proposed in the paper. Such a novel topology reduces costs, size, devices and associated circuits; moreover, the proposed topology is congenial to the use of integrated power modules in place of discrete ones.

CONCLUSION AND FUTURE AIM A 1kW prototype of a DAB converter implementing the proposed topology was built and tested in the laboratory; the results of the laboratory test demonstrated the good dynamic response of the DAB converter in ensuring a stable DC bus voltage to the reference value when a deep step change in power balancing occurs in a smart user network. The use of IPM modules also on the HV side gives the opportunity to control the power flow in the MG also in an emergency condition, allowing an effective and reliable stand-alone operation. The test system verifies that the architecture of the SUN is so expandable in a more powerful and diversified system.

Thank you! michele.motta@unical.it University of Calabria, Italy

A 1kW prototype