A laboratory model of a dual active bridge dc-dc converter for a smart user network
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The paper presents the application of a dual active bridge DC-DC converter for a smart user network that is a microgrid with a DC-powered local network (or DC bus). In such a microgrid the DC-DC converter connects a battery energy storage system to the DC bus. In a previous paper, the authors demonstrated that the DC-DC converter is able to assure a stable DC bus voltage to a reference value; performance was evaluated by means of numerical experiment using Simplorer. In this paper the DC-DC converter is further investigated using a 400W laboratory model of a smart user network; the laboratory results demonstrate that the converter provides a high level of reliability and resilience against disturbances.
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A laboratory model of a dual active bridge dc-dc converter for a smart user network
- 1. A laboratory model of a dual active bridge dc-dc converter for a smart user network • D. Menniti, N. Sorrentino, A. Pinnarelli, M.Motta, A. Burgio and P.Vizza
- 2. M.Motta - EEEIC 2015 2 Introducing to the Smart User Network (SUN) What’s the problem? The master converter The dual acvite bridge (DAB) coverter Basic principles Feedback control scheme Laboratory model The test system The case for the transient stability study The implemented feedback controller for the DAB converter Conclusion and future aim SUMMARY
- 3. M.Motta - EEEIC 2015 3 INTRODUCING TO THE SUN A SUN essentially is a private network connected to the LV network (the grid) by means of a bidirectional AC-DC power electronic interface (PEI). The local network of the SUN is DC-powered; the distributed energy resources (DERs), the energy storage systems (ESSs) and loads are all connected in parallel to the DC bus by means of appropriate power converters. What’s the problem ? Fixing the voltage of the DC bus (VDC) at a constant reference value (VDCref) is a key factor for the proper operation of the SUN. The master converter takes care of this task. The paper presents a dual active bridge (DAB) converter to charge/discharge the batteries which can be operated as master converter when the SUN operates in islanded mode. The smart user network (SUN) DC powered Local network
- 4. M.Motta - EEEIC 2015 4 BASIC PRINCIPLES OF A DAB A DAB is an isolated bidirectional DC/DC converter composed of two full-bridge DC/AC converters and an isolation high frequency (HF) transformer which provides a step-up voltage gain, a galvanic isolation and, furthermore, a leakage inductance which is the main energy transfer element. High frequency square voltage Direct voltage Direct voltage Inductor current 𝑃 = 𝑉𝐻1 𝑉𝐻2 2 𝑓𝑠 𝐿 𝑁 𝐷 (1 − |𝐷|)
- 5. M.Motta - EEEIC 2015 5 A DAB CONVERTER FOR A SUN The DAB converter which connects the battery ESS to the dc bus is a suitable substitute for operating as master converter. At this purpose a simple, conventional PI controller can be adopted to control the power converter: 𝐺 𝑠 = 𝐾𝑐 1 + 1 𝑇𝑖 𝑠 The tuning of PI controller coefficients, namely Kc and Ti in (3), was performed by the traditional trial-and-error method. For the considered laboratory test, the authors tuned these coefficients assuming: - 1% steady-state accuracy; - 200 ms settling time to restore the actual Vdc to within the 3% of Vref; - overshoot/undershoot of Vdc to within the 4% of Vref.
- 6. M.Motta - EEEIC 2015 6 THE TEST SYSTEM LABORATORY TEST To confirm the feasibility and the effectiveness of the DAB converter in controlling the dc bus voltage of a smart user network, a 400 W single-phase laboratory model was built; this model was also used to verify the good dynamic response of the DAB converter under transient condition due to a step change in power balancing.
- 7. M.Motta - EEEIC 2015 7 THE TEST SYSTEM LABORATORY TEST VBATT V 48 VH1, VH2 V 48, 480 Vref V 450 Kc V-1 0.0036 Ti s 0.0359 R1 220 R2 2000 L1 uH 10 L2 uH 50 C1 mF 1.32 C2 mF 1.10 N - 10 The DAB converter incorporates the inductor L2, the high frequency transformer Tr1 and two identical ac- dc converters, namely H1 and H2. The ac-dc converters H1 and H2 are two identical IGBT full bridge rectifiers (mod. IRAM136-3063B) with integrated gate drivers.
- 8. M.Motta - EEEIC 2015 8 THE DYNAMIC RESPONSE LABORATORY TEST At the beginning of the laboratory test, both switches S1 and S2 are closed. The DAB feedback controller is disabled and the phase shift angle φ is forced to be equal 0 whereas the amplitude of Vb is regulated so to measure a Vdc equal to about 463.20 V. At time t1, the feedback control is activated and the DAB converter starts to compensate the error between Vdc and Vref.
- 9. M.Motta - EEEIC 2015 9 THE DYNAMIC RESPONSE LABORATORY TEST At the beginning of the laboratory test, both switches S1 and S2 are closed. The DAB feedback controller is disabled and the phase shift angle φ is forced to be equal 0 whereas the amplitude of Vb is regulated so to measure a Vdc equal to about 463.20 V.
- 10. M.Motta - EEEIC 2015 10 THE DYNAMIC RESPONSE LABORATORY TEST At time t1, the feedback control is activated and the DAB converter starts to compensate for the error between Vdc and Vref. At time t2, the switch S2 is opened so causing a deep variation in power balancing; now, the DAB converter is the only power source in the laboratory model and it has to provide the whole load demand, including power losses.
- 11. M.Motta - EEEIC 2015 11 THE DYNAMIC RESPONSE LABORATORY TEST At time t1, the feedback control is activated and the DAB converter starts to compensate for the error between Vdc and Vref. The good performance of the DAB converter in re-establishing the dc bus voltage at the reference value after the deep step change in power balancing is evident.
- 12. M.Motta - EEEIC 2015 12 CONCLUSION AND FUTURE AIM In this paper the application of a dual active bridge (DAB) converter in smart user network (SUN) has been presented. The DAB converter has been investigated using a 400W laboratory model of a SUN. The laboratory results have demonstrated that the DAB converter provides a high level of reliability and resilience against disturbances, safeguarding the SUN even against a sudden disconnection of the SUN itself from the grid. Indeed, when disconnection occurred, the dc bus voltage slightly decreased with respect the reference, converging again to the reference in a short settling time.
Editor's Notes
- When the SUN is grid connected, the grid participates to satisfy the local demand of electricity; in such a case, the PEI ensures for the power balancing in the SUN, absorbing and supplying electricity to the grid. When the main switch S is opened, the SUN operates in island mode and depends solely by the local DERs and local ESSs. Fixing the voltage of the DC bus (VDC) at a constant reference value (VDCref) is a key factor for the proper operation of the SUN; as a consequence, one of the power converters belonging to the SUN must be dedicated to this scope. This power converter is named master converter. The PEI is the preferable candidate to operate as master converter when the SUN is grid connected because it regulates a bidirectional power flow and, nearly ever, its rated power is higher than any other converter present in the SUN. Evidently, the PEI cannot operate as master converter when the SUN operates in island mode. In such a case, the DC-DC converter used for the battery ESS is suitable to functioning as master converter.
- Since 90’s, the dual active bridge (DAB) converter was presented as an attractive alternative for high-power applications; such a high-power, high-power-density bidirectional dc-dc converter shows very attractive features in terms of high efficiency, small filter components, low stress for both device and components, low switching losses thanks to the zero voltage switching. In order to operate the DAB converter, that controls both the direction and magnitude of power transmission, the most common technique is the phase shift angle technique; using this technique, the two converters H1 and H2 are switched at a fixed frequency with a duty cycle of 50% in order to generate two high frequency square-wave voltages, vH1 and vH2.
- The tuning of PI controller coefficients, namely Kc and Ti in (3), was performed by the traditional trial-and-error method. For the considered laboratory test, the authors tuned these coefficients assuming a 1% steady-state accuracy, a 200 ms settling time to restore the actual Vdc to within the 3% of Vref, an overshoot/undershoot of Vdc to within the 4% of Vref. . In such a case, in order to calculate the phase shift angle or, similarly, the phase-shift ratio D so to determine the power transmission required for maintaining the actual dc bus voltage close to the reference. The difference between the reference voltage and the measured voltage is the error e; such an error feeds the proportional-integral controller (PI controller) which returns the phase-shift ratio D useful for controlling the power transmission. Multiplying D by π the phase shift angle φ of vH1 with respect vH2 is obtained.
- The laboratory model is illustrated in Figs. 4 and 5; it incorporates a dc voltage source, a DAB converter, a dc bus, some passive components, a microcontroller and diode bridge rectifier, all described in detail in the follow.
- According to the static electrical characteristics driver function of IRAM rectifiers, the minimum input voltage required by for logic "0" is 3 V; such a value is very close to 3.3 V that is the maximum output voltage of a PWM pin of the ATMEL evk1100. As a consequence, the voltage drops in wire connection between the ATMEL evk1100 and the IRAM drivers may cause a malfunctioning in DAB operation. In order to prevent the detrimental effects due to voltage drops, an interface circuit was built; by means of two logic gates LG along with an opto-coupler OC, such a circuit elevates and optically couples the PWM control signal of ATMEL evk1100 to IGBTs drivers, as illustrated in Fig. 7.
- In the same lapse, the voltage Vdc and the current Ibatt vary as depicted in Fig. 10; in particular, the dc bus voltage directly decreases from 463.20 V to 450 V while the current Ibatt firstly reverses its sign decreasing, almost instantly, from 0.45 A to about –1.6 A, then it increases again so converging to an average value of 0.15 A. Worth noting that the desired voltage steady-state accuracy is clearly achieved. The phase shift angle rapidly decreases to about -15 degrees, then it converges to a steady-state value of -5 degrees after 300 ms.
- In the same lapse, the voltage Vdc and the current Ibatt vary as depicted in Fig. 10; in particular, the dc bus voltage directly decreases from 463.20 V to 450 V while the current Ibatt firstly reverses its sign decreasing, almost instantly, from 0.45 A to about –1.6 A, then it increases again so converging to an average value of 0.15 A. Worth noting that the desired voltage steady-state accuracy is clearly achieved. The dc bus voltage directly decreases from 463.20 V to 450 V while the current Ibatt firstly reverses its sign decreasing, almost instantly, from 0.45 A to about –1.6 A, then it increases again so converging to an average value of 0.15 A. Worth noting that the desired voltage steady-state accuracy is clearly achieved.
- In the same lapse, the voltage Vdc and the current Ibatt vary as depicted in Fig. 10; in particular, the dc bus voltage directly decreases from 463.20 V to 450 V while the current Ibatt firstly reverses its sign decreasing, almost instantly, from 0.45 A to about –1.6 A, then it increases again so converging to an average value of 0.15 A. Worth noting that the desired voltage steady-state accuracy is clearly achieved. At this time the phase shift angle increases from -5 degrees to a maximum value of 18 degrees. After 500 ms the angle reaches a steady state value of 10.6 degrees.
- In the same lapse, the voltage Vdc and the current Ibatt vary as depicted in Fig. 10; in particular, the dc bus voltage directly decreases from 463.20 V to 450 V while the current Ibatt firstly reverses its sign decreasing, almost instantly, from 0.45 A to about –1.6 A, then it increases again so converging to an average value of 0.15 A. Worth noting that the desired voltage steady-state accuracy is clearly achieved. . Indeed, at time t2 the dc bus voltage slightly decreases from 450 V to 442 V, then it converges to the reference value in 260 ms. The steady state accuracy is about 0.66 % that is a value lower than 1%. As predictable, the current Ibatt relevantly changes in this lapse; in particular, it varies from 0.15 A to 3.3 A in just 180 ms, then it converges to 2.3 A in 320 ms.
- In this paper the application of a dual active bridge (DAB) converter in smart user network (SUN) has been presented. In a previous paper, the authors demonstrated that the DAB converter is able to assure a stable dc bus voltage to a reference value; the DAB performance were evaluated by means of numerical experiment using Simplorer. In this paper the DAB converter has been further investigated using a 400W laboratory model of a SUN.