ISOP DC-DC Converters Equipped 5-Level ... - IEEE Xplore

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Huawei Energy Competence Center. Europe (HECCE). Huawei Technologies Dusseldorf GmbH. Munich/Nuremberg, Germany. Dept. of Industrial Engineering.
ISOP DC-DC Converters Equipped 5-Level Unidirectional T-Rectifier for Aerospace Applications Marco Di Benedetto, Alessandro Lidozzi, Luca Solero Department of Engineering Roma Tre University Roma, Italy

Petar J. Grbovic Huawei Energy Competence Center Europe (HECCE) Huawei Technologies Dusseldorf GmbH Munich/Nuremberg, Germany

Abstract – This paper deals with Input-Series Output-Parallel (ISOP) DC-DC converters to be used for capacitors voltage balancing in the 5-level unidirectional T-Rectifier for electric generating applications. The proposed generating unit is intended for aerospace applications, to this purpose the power electronic converter configuration is able to supply power to the electric loads at different voltage levels. The 5 level T-Rectifier output at ±270V feeds the ISOP DC-DC converters which output terminals are used to supply the 28V circuitry. Two different control strategies for the whole system are proposed, discussed and verified. First proposed architecture considers the AC-DC and DC-DC as coupled from the control point of view, introducing then the Couple Balancing Control Strategy (CBCS). A comparison is performed with respect to a completely decoupled regulation algorithm, where the AC-DC and DC-DC control loops are not nested, resulting in the Decoupled Balancing Control Strategy (DBCS).

I. INTRODUCTION Aircraft conventional generation of electrical networks is equipped with 400Hz fixed-frequency integrated drive generators. To the purpose of simplifying the constant-speed mechanical motion transfer, the variable-frequency electric drives are now preferred for aircraft generating units. Future trends in aircraft electrical networks consider permanentmagnet synchronous generator (PMSG) to be directly as a part of the engine; as a result, very high frequency of the EMF fundamental harmonic is expected. In such conceived generating units, for any operating condition set by the alternator input torque and speed and for a given rectified voltage level, the higher is the PMSG inductance, the lower will be the RMS value of the fundamental frequency current that circulate in the power switches and diodes of the controlled rectifier. Thereby, designing the PM generator with low value of the per-unit synchronous inductance is beneficial for the controlled rectifier in terms of reduced kVA rating and power loss. However, a low value of the synchronous inductance negatively affects the waveform of the alternator phase current, as for a given value of the switching frequency used in the controlled rectifier the lower is the alternator synchronous inductance, the higher is the total harmonic distortion (THD) of the alternator current waveform. As a consequence, the RMS value of the PM generator output current increases and this may offset the advantages envisaged

978-1-4673-7151-3/15/$31.00 ©2015 IEEE

Stefano Bifaretti Dept. of Industrial Engineering University of Rome Tor Vergata Roma, Italy

from the use of a low inductance alternator. In other words, the use of an electrical generator having low synchronous inductance reduces the fundamental frequency component of the alternator output current while increases the harmonic content in the same current. In order to retain the advantages resulting from the reduced value of the fundamental frequency component of the PMSG output current, multilevel power electronic converter topologies can be considered as appropriated [1]. This paper relates to the investigation of the 3-phase 5-level Unidirectional T-rectifier topology (5L T-RECT) to be used in pair with a high rotational speed permanent magnet synchronous generator (PMSG), being part of an isolated power supply for aircraft applications. The 5L T-RECT output voltage can be used to supply the distributed ±270V HVDC network; however, a Balancing Circuit (BC) is often required to balance the voltages across the multilevel converter output series capacitors. BC configurations can be properly chosen also in order to provide the auxiliary voltage level at 28V for the avionic systems, to this purpose an input-series outputparallel (ISOP) DC-DC output power stage is investigated in the paper. ISOP DC-DC configuration is recognized as promising in high input-voltage to low output-voltage static energy conversion systems. The capability in regulation both input and output variables is at the basis of a successful implementation of input-series output-parallel multi-converter topology. Several papers have been presented so far concerning ISOP DC-DC converter, in [2] a three control loops strategy is depicted for a modular DC-DC converter, in [3] a novel current sharing method is illustrated for paralleled converters, whereas in [4] a common duty-ratio technique is shown. Droop control analysis and validation is shown in [5] for ISOP converters configuration. Master/slave control strategy is illustrated in [6] for low-power, low-cost DC-DC power supply. Charge control with input feed-forward for input series full-bridge DC-DC converter is illustrated in [7] for a specific applications with analog control implementation. High performance, high reliability wireless input voltage and output current sharing is presented in [8]. Advancement in removing sensors for current sharing purpose is highlighted in [9], jointly with detailed small-signal stability analysis of the proposed method. Magnetic coupled ISOP converters are illustrated in [10]-[11].

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However, in the highlighted papers, ISOP configuration and control strategy are considered completely decoupled from the input system, without any issue in having unidirectional or bidirectional DC-DC converters. ISOP main task is the voltage stabilization on the series connected input capacitors. Hence, considering the whole system point of view, i.e. AC-DC and DC-DCs, the proposed coupled control approach results advantageous for regulation and complete system integration. The analysis and the comparison are carried out in the present paper on the basis of a specific input rectifier that exhibits strongly non-symmetrical DC-side power transfer; this affects the ISOP shared control behavior as the balancing of the DCDC converters output current sharing is not recommended.

Figure 1. 5L T-RECT DC-link transferred power according to the modulation index for constant DC-bus voltage and with the balancing circuit shown in [12].

II. 5L T-RECTIFIER AND CAPACITORS BALANCING CIRCUIT The AC-DC Unidirectional T-Rectifier shown in Figure 1 is of interest for aerospace applications where 540V DC bus is required with the ±270V voltage levels; however, it exhibits an inherent DC-link capacitors unbalancing due to the asymmetrical AC-to-DC power transfer which is dependent on the operating conditions [12]. Figure 2 depicts the power distribution among DC-link capacitors when the rectifier is operated at constant DC-bus voltage, variable PMSG rotational speed and constant phase current; the calculated power is normalized with respect to the rectifier rated power. Accordingly, to obtain a balanced capacitors voltage, power must be transferred from the central capacitors (CB3 and CB2) to the external capacitors (CB4 and CB1) by the usage of a proper additional Balancing Circuit. However, the BC can have further tasks than the simple balancing of the 5L T-RECT output capacitors. The additional BC can integrate features that are required by some particular applications as in case of the aerospace systems. In case of requirements for also low output voltage level far from the rectifier DC-link rated value, isolated DC-DC converters can be used to provide both electrical isolation and high input-tooutput voltage ratio.

III. ISOP DC-DC CONVERTER CONFIGURATION AND CONTROL A general solution for the BC of the proposed power electronic converter topology is achievable when each Voltage Balancing Device shown in Figure 1 is realized with four DC-DC converters as shown in Figure 3. Each DC-DC converter has the basic scheme shown in Figure 4 including either unidirectional or bidirectional power flow capability. Fullbridge topology is modulated by the classical phase-shift method in order to assure zero DC voltage on the isolation transformer. ISOP converters share a common output link that must be regulated at the desired voltage, in order to supply the 28V onboard circuitry of aircrafts. In the proposed configuration, each

Figure 3. Integration of 5L T-RECT and ISOP system.

Figure 4. Simplified block scheme of the DC-DC converters used in the ISOP implementation. Figure 2. Configuration of the 5L T-RECT converter and block scheme of the capacitors voltage balancing system.

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DC-DC converter is also devoted to control its input voltage, in order to balance the voltage across the 5L T-RECT output capacitors. However, to this purpose, the applied control strategy disregards from the ISOP DC-DC converters specific hardware configuration (i.e. full-bridge, resonant, doubleactive bridge, etc…). Detailed description of the used DC-DC converter for the ISOP configuration is reported in [13]-[14]. A. Coupled Balancing Control Strategy (CBCS) The proposed coupled control strategy is based on the combined operation of both the 5L T-RECT and the ISOP converters as it is shown in Figure 5. Is this case, the ISOP DC-DC converters duty is mainly to balance the DC-link capacitors voltage, disregarding to the whole system output voltage. The AC-DC converter stage, which of course regulates the PMSG flux and torque, has an additional voltage loop devoted to regulate the provided 28V output voltage. When the requested output power increases, the corresponding output voltage falls, thus giving a positive torque error signal. The AC-DC voltage controller acts in order to increase the generator brake torque, which results in increased power that is transferred to the DC-link. As a consequence, due to the capacitors unbalance behavior previously depicted, the ISOP stage performs the necessary balancing by transferring power to the output link through four dedicated voltage control loops. According to the described control architecture, control loops for ISOP and AC-DC stages are completely nested. As a result, the ISOP regulation is necessarily faster than the AC-DC voltage control, in order to reduce as much as possible the delay in the AC-DC voltage regulation chain. The depicted CBCS forces the 5L T-RECT to operate at variable DC-link voltage, due to the absence of a direct regulation. In fact, the DC-link behavior is regulated by the ISOP stage following the variable reference that is selected according to the AC-DC input voltages (i.e. PMSG mechanical rotational speed). This allows to operate the controlled rectifier at almost constant modulation index and using all the available 5 levels. Moreover, the selected configuration for the DC-DC converters of the ISOP stage can be totally unidirectional as it is shown in Figure 6. This can be explained thanks to the variable DC-link operation that does not require power to be transferred from the middle capacitors (CB3 and CB2) to the external capacitors (CB4 and CB1) as in the balancing circuit shown in [12]. In fact, when the modulation index set-point, ma-ref, is selected above 0.5, AC-to-DC power is transferred also via the connections in parallel to CB4 and CB1, avoiding a bidirectional power flow. Mainly drawback of this control solution is its inability to feed loads connected to the highvoltage DC-bus, as highlighted in Figure 6, without the replacement of the top and the bottom DC-DC converters with a bidirectional topology. According to generator parameters, the DC-link reference voltage VDC-REF is on-line evaluated as

VDC − REF =

(

3ωel λ f

)

ma − ref

where λf is the generator peak-flux and, finally, ωel is the electrical machine electrical speed in rad/s and ma-ref is the desired rectifier modulation index. With reference to Figure 5, controllers’ bandwidth should be carefully selected. Fastest

loop is related to the DC-DC converters current control as well to the 5L T-RECT dq-axes current regulation. The DC-bus capacitor balancing control loop, which is operated by the ISOP stage, is nested inside the outer VOUT low-voltage loop; hence, it requires a higher bandwidth than the output voltage

Figure 5. Block scheme for the proposed Coupled Balancing Control Strategy.

loop controller. B. Decoupled Balancing Control Strategy (DBCS) When the decoupled control is used for the 5L T-RECT and the balancing circuit, two separate regulation loops can be considered. In this case, the DC-link voltage is stabilized by the boost-rectifier, whereas the ISOP stage controls both the output voltage VOUT and the DC-link capacitors voltage distribution. This double voltage control action requires a slight modification of the conventional regulation loops, introducing a new variable ΔIx (x: 4, 3, 2, 1) that acts as a priority indicator for each DC-DC converter being part of the ISOP power stage. DBCS allows the rectifier to operate at variable modulation index, having the drawback of strongly asymmetrical AC-toDC transferred power dependent on the converter modulation index. According to Figure 2, an equal DC-link capacitors voltage distribution requires that the energy is transferred from the central capacitors to the external ones as it is depicted in Figure 7. In this case, DC-DC converter requirements are different from the CBCS. Inner converters, which stabilize voltage across CB3 and CB2 can be unidirectional being power transferred from input to output; whereas, outer DC-DC systems that regulate the voltage across CB4 and CB1 must be bidirectional. In fact, they have to transfer power from the output to the input capacitors CB4 and CB3 in order to achieve the capacitors voltage balanced distribution. In this case, the magnetic coupling between two adjacent DC-DC converters can be suggested to directly transfer energy using only the

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added due to the DC-link asymm metrical power distribution.

Figure 7. Coupled Balancing scheme with variable D DC-link voltage and constant modulation index.

Figure 9. Block scheme for the Decoupled d Balancing Control Strategy

These controllers are able to modify the voltage loop phaseshift reference according to the meaasure of the voltage of each DC-link capacitor which must be reg gulated at VDC/4. IV. HARDWARE REALIZAT TION AND RESULTS Figure 6. Decoupled Balancing scheme and power flow with constant DC-link voltage and variable modulation index.

primary side of the converter configuration off Figure 4, hence, increasing the overall efficiency. This event occurs when the modulation index is below 0.5, as previouslyy shown in Figure 2, in that case there is no transferred power accross CB4 and CB1. Being the DC-link voltage kept constant by the AC-DC converter, these capacitors must be charged uusing the DC-DC converters output common link. Again with reeference to Figure 7, the DBCS, due to the usage of two bidirrectional DC-DC converters, allows to supply directly loads thhat are connected to the high-voltage bus, without any additional requirements. As it is shown in Figure 8, the proposed deccoupled balancing scheme requires two separate control loopss. Fastest control loops can be assigned to the single controllerr used to regulate the DC-DC output common bus at VOUUT. Finally, four additional voltage regulators with reduced baandwidth must be

The 5L T-RECT prototype has beeen built with proper gate drivers and AC current sensor as sh hown in Figure 9, where the TMS320F28335 based control boaard that sits on top of the middle phase-leg has been provisio onally removed. Figure 10 depicts the prototype of a single DC-DC D converter, where all the required components have been integrated on a single PCB.

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Figure 8. 5L T-RECT prototype.

Table I illustrates the main parameters of the whole generating unit: permanent magnet machine, 5L T-R RECT and ISOP

region as shown l converter is forced to work in the low-losses in [12]. DC-bus capacitors voltage and output voltage, in the 14. It can be noticed F same operating point, are shown in Figure that the proposed ISOP coupled co ontrol strategy is perfectly able to balance the capacitors voltag ge and to regulate the 28 V

Figure 13. Prototype of the unidirectional Fulll-Bridge DC-DC converter.

system. TABLE I - 5L T-RECT, PM-GENERATOR AND ISOP SYSSTEM PARAMETERS

Rated mechanical input power [W] Rated speed [rpm] Rated phase EMF [Vrms] Rated phase current [Arms] Pole-pairs Rated DC-bus voltage [V] Switching frequency [kHz] ISOP switching frequency [kHz] ISOP output voltage set-point [V]

16500 3500 250 22 5 800 12 20 28

Figure 10. Normalized DC-link powerr distribution via converters connected across CB4…CB1, and with respeect to the generator mechanical speed for CBCS.

A. Coupled Balancing Control Strategy (CBCSS) Results related to the power distribution for the Coupled Balancing Control strategy are shown in Figuure 11 and Figure 12. Figure 11 depicts the power transferredd by the DC-DC converters to the output, normalized with reespect to the DClink maximum power. It can be noticed that the power managed by the inner DC-DC converters, viaa the connections across CB3 and CB2, starts from low values around 6%, increasing linearly to reach a value close to 322% at AC-DC full power. In this mode of operation, beinng the AC-DC modulation index kept constant, also outer D DC-DC converters participate to the power transfer. Power acrooss the converters connected in parallel to CB4 and CB1 increasess according to the generator mechanical speed and it reaches the rated value close to 18% of the system power rating. The achieeved investigation suggests important information concerning the size of the ISOP DC-DC system. In fact, the outer connverters must be designed with reference to the 18% of the AC-DC rated power, whereas, the inner converters should be sized with reference to the 32% of the rectifier full power. Operatiing the CBCS at variable DC-link voltage, in order to fully expploit the available 5L T-RECT levels, Figure 12 shows the normalized DC voltage with respect to the operating modulattion index, which is direct function of the generator rotational sppeed. Monitored electrical waveforms are shown iin the next plots. Figure 13 depicts the filtered line-to-line voltaages and the DClink voltage, when the generator is operated att 2000 rpm. In the CBCS the DC-link voltage is variable in ordeer to fully exploit the all available five-levels. In this case, the 5L T-RECT

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Figure 11. DC-bus voltage with respect to generator mechanical speed for CBCS.

Figure 12. Line-to-line voltages and DC C-link voltage at steady-state conditions for the CBCS.

Figure 15. Normalized DC-link power distribution via the converters connected across CB4…CB1, and with respect to the generator mechanical speed for DBCS. Figure 14. DC-link capacitors voltage and ISOP output voltage.

output. B. Decoupled Balancing Control Strategy (DBCS) Figure 15, which is obtained under the assumption that loads are applied to the low-voltage link, depicts the DC-DC converters normalized power distribution with respect to the generator mechanical speed (i.e. AC-DC modulation index, ma). The DC-link voltage is kept constant through the control action of the rectifier voltage loop. When ma is below 0.5, the power managed by the outer DC-DC converters is close to zero. This can be explained considering that the DC-DC converters have to transfer only the power required to charge the capacitors CB4 and CB1. It can be noticed that DC-DC converters size is strongly related to the generating unit specific application and mission profile. When the PMSG is operated at constant speed, rectifier input voltage is almost constant; consequently 5L T-RECT and ISOP size can be obtained directly from Figure 11 and Figure 15 with respect to the system rate power and the rectifier modulation index which is, in this case, almost constant. When the gen-set has to operate at variable speed and then variable input voltage, the system size could be different, being the gen-set operated also in the region characterized by a low modulation index and then with an AC to DC high boost factor. As previously shown in the case of the Coupled Balancing Control Strategy, steady-state behavior when using DBCS is depicted in the following figures. Figure 16 illustrates the line-to-line voltages and the DC-link voltage when the generator operates at 2000 rpm. It can be noticed that the 5L T-RECT does not use all the available five levels due to the modulation index that is dependent by the constant DC-link voltage and the line-to-line voltages. However, in this case also, the capacitors voltage is perfectly balanced by the proposed ISOP control configuration as shown in . It shows also the achieved output voltage VOUT regulated at the desired reference sets to 28 V.

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Figure 16. Line-to-line voltages and DC-link voltage at steady-state conditions for the DBCS.

Figure 17. DC-link capacitors voltage and ISOP output voltage.

[5]

CONCLUSIONS ISOP DC-DC converters have been proposed to be used for the DC-link capacitors active voltage balancing in the 5L T-RECT multilevel unidirectional rectifier. Considering the aircraft applications, two control strategies have been investigated and the achieved results have been depicted. The Coupled Balancing Control Strategy forces the 5L T-RECT to operate at constant modulation index, allowing the usage of the all the available five levels. On the contrary, the Decoupled Balancing Control Strategy acts in order to control the DC-link voltage at a constant level, thus decoupling the 5L T-RECT and the ISOP controllers. According to the achieved analysis and results, ISOP DC-DC converters size can be obtained straightforwardly with respect to the generating unit operating conditions.

[6]

[7]

[8]

[9] [10]

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