Ripple Current Reduction of a Fuel Cell for a Single-Phase Isolated Converter using a DC Active Filter with a Center Tap

Transcription

1 Ripple Current Reduction o a Fuel Cell or a Single-Phase solated Converter using a DC Active Filter with a Center Tap Jun-ichi toh*, Fumihiro Hayashi* *Nagaoka University o Technology Kamitomioka-cho Nagaoka City Niigata, Japan itoh@vos.nagaokaut.ac.jp Abstract- This paper proposes a ripple reduction method in current without using any additional switching devices. The current ripple that has double requency component o the power supply is generated in the DC part when a single-phase PWM inverter is used or a grid connection. The current ripple causes short lietime or electrolytic capacitors, batteries and uel cells. The proposed circuit realizes a DC active ilter unction without increasing the number o the switching device because the energy buer capacitor is connected to the center tap o the isolation transormer. n addition, the buer capacitor voltage is controlled by the common mode voltage o the inverter. This paper describes the eatures o the proposed circuit, control strategy and experimental results. As a result, about 1/5 times o the ripple can be reduced.. NTRODUCTON Recently, green energy sources such as wind power systems, photovoltaic cells and uel cells, have been extensively studied in response to the global warming and environmental issues. The uel cell is an important technology or mobile applications and power grid distribution system since it does not draining CO. A uel cell system requires a grid interconnection converter to supply power to the power grid. A grid interconnection converter using an isolation transormer is preerred or power grid distribution system in terms o surge protection and noise reduction. Moreover, size reduction and high eiciency are also required essentially [1-6]. However, one problem with the uel cell system is the short lietime, which is dependent on the ripple current. Thereore, in order to extend the lietime, the uel cell ripple current must be reduced in the grid interconnection converter [7]. However, when single-phase PWM inverter is used or grid connection system, the grid requency happens to have double requency o the power ripple. This current ripple prevents long lietime o the uel cell. Thereore, in conventional grid connection inverters, large electrolytic capacitors are connected in parallel to the uel cell in order to reduce the ripple. However, the large-sized electrolysis capacitor causes big volume and expensive device cost. To reduce the current ripple, some methods without using a large-sized electrolytic capacitor have been proposed. For example, an active ilter is applied in DC link part [8-1]. The DC active ilter consists o a small capacitor as energy buer, a reactor to reduce the switching ripple and DC chopper. The DC chopper injects the ripple current to avoid power ripple. Capacitance can be lower since the terminal voltage o the capacitor can be widely changed. However, the numbers o the switching devices increased results the DC chopper is high cost and large volume. Likewise, other conigurations o the DC active ilter have the similar problems. This paper proposes a new circuit topology including a DC active ilter unction without extra switching devices. The proposed circuit consists o the isolated DC/DC converter and interconnection inverter, and achieved the DC active ilter unction by using the center tap o the isolation transormer. Besides, one eature o the proposed converter is that the primary side inverter in the DC/DC converter is controlled by the common mode voltage and dierential voltage, individually. The ripple current is suppressed by the common mode voltage control o the DC/DC converter and the main power low is controlled by the dierential mode voltage. At irst, this paper introduces the conventional and proposed circuit topologies with the principle o the current ripple suppression. Second, the control method o the proposed circuit is described. n addition, this paper indicates the design o the energy buer capacitor and transormer which the maximum power ripple can be accepted. Furthermore, experimental results are shown in order to conirm the validity o the proposed circuit.. PROPOSED CRCUT CONFGURATONS Figure 1 shows a conventional circuit that consists o the irst stage inverter or the medium requency link, an transormer, a diode rectiier, and a grid interconnection inverter. When the interconnection current and power grid voltage are sinusoidal waveorms, the instantiations power p o the grid interconnection is obtained by (1) at unity power actor p = sin( ωt) V sin( ωt) { 1 cos(ωt) } = V (1) where and V are the RMS value o the interconnection current and the grid voltage, and ω is the grid angular requency.

2 Thus, the instantaneous power has a ripple that is double requency o the power grid requency. To reduce the ripple power o the DC power source, such as a uel cell, battery or photovoltaic cell, large electric capacitors, C DC1 and C DC, are used in the converter, as shown in Figure 1. The use o large electrolytic capacitors precludes reduction in size and cost. Figure shows the other conventional circuit using a DC active ilter, which is constructed with a DC chopper and an energy buer capacitor C. The capacitor C is used as an energy buer to absorb the ripple power. The inductor L can suppress the switching current. The voltage o the capacitor C is controlled at the double requency o the power grid requency. As a result, the ripple power does not appear in V dc, despite the use o small capacitors C DC1 and C DC. However, the problem o this method is that the number o the switching elements had to increase. Figure 3 shows the proposed circuit that combines the irst stage inverter and DC active ilter unction. The energy buer capacitor C is connected to the center tap o the medium requency transormer. The zero vector o the ull bridge irst stage inverter is used to control the center tap potential voltage. n addition, the leakage inductance o the transormer is used to suppress the switching current instead o L. Table 1 shows comparison o the number o the switching device and capacitor capacity among the conventional circuit, the conventional circuit with the DC active ilter and the proposed circuit. The proposed circuit does not require an additional switching device and an inductor, in comparison with the conventional circuit with the DC active ilter. Note that the current rating o the power device in the irst stage inverter and transormer are slightly larger than that o the conventional circuit because the DC active ilter current lows in the irst stage inverter and the transormer.. CONTROL METHOD The irst stage inverter in this proposed circuit has two roles which are perormed as a DC/DC converter and a DC active ilter. These roles are achieved by controlling the common and dierential mode voltage in the irst stage inverter. This chapter explains the principle o the proposed control method and the design method or the buer capacitor and the transormer. A. Switching pattern generation method Figure 4 illustrates the two switching modes o the irst stage inverter in the proposed circuit. n the dierential mode, the terminal voltage o the transormer is controlled as shown in Figs. 4(a) and (b), and in the common mode, the center tap voltage is controlled as shown in Figs. 4(c) and (d). n other words, the inverter outputs the zero voltage vectors ( and 11 are two) in common mode operation. When the zero voltage vectors are selected, the line to line voltage o transormer is zero. However, the center tap voltage is either V dc or zero, depending on the zero vector o Figs. 4(c) or (d), respectively. Thus, by controlling the ratio o the zero vectors, the center tap voltage can be controlled. t should be noted that the i in V dc C DC1 i com Fig. 1. Conventional circuit. L C AC-link 1 khz C DC Fig.. Conventional circuit with DC active ilter. Fig. 3. Proposed circuit. Table1. Comparison device number and capacitance o capacitor. Device DC Link Number Power Grid V 5Hz L s i s Capacitor Conventional Circuit 8 large Conventional circuit With DC active ilter 1 small Proposed Circuit 8 small output switching pattern must included the zero vector period. Thereore, the voltage transer ratio o the irst stage inverter is limited by the DC active ilter control. As a result, the terminal voltage o the transormer is decreased. Figure 5 shows the control block diagram o the proposed circuit. To suppress the ripple current o the uel cell, all the ripples current are provided by the energy buer capacitor. Thereore, the capacitor current command i com * is obtained by calculating the power ripple. The grid interconnection control can be applied to the conventional control method, which uses an automatic current regulator (ACR) or the interconnection current command.

3 (a) Dierential mode 1 (b) Dierential mode (c) Common mode 1 (d) Common mode Fig. 4. Operation modes o the proposed circuit. Fig. 5. Control block diagrams. The DC active ilter voltage command v * com is obtained by the P regulator in the current regulator. The dierential mode voltage command v * di is set to 1 as the maximum value, in order to obtain the maximum terminal voltage o the transormer. The eature o the proposed circuit control is that the DC active ilter voltage command v * com is added to the dierential voltage command v * di as the common mode voltage. The output voltage commands v * 1 and v * or each leg in the irst stage inverter are obtained by (). * 1 * * v1 = ( vcom + vdi ) () * 1 * * v = ( ) vcom vdi B.. Design o the buer capacitor or the DC active ilter operation The buer capacitor is used as an energy storage element o the active ilter. The capacitor C has to absorb the power ripple or hal cycle o the power grid. Thus, the required storage energy W C is given by (3) rom (1) and the capacitor energy W is obtained by (4) T / 4 W C = cos ωtdt = (3) ω 1 W = C ( V max V min ) (4) where P in is the input power, ω is angle requency o the power grid, V max and V min are the maximum and minimum voltage o C, respectively. Thereore, the required capacitance C is given by (5) rom (3) and (4). C = = (5) ω(( Vc + ΔVc / ) ( Vc ΔVc / ) ) ωvc ΔVc where V c and ΔV c are the average voltage and the variation voltage o C, respectively. Figure 6 shows the relation between the capacitance and the voltage variation ΔV c o the capacitor to compensate the power ripple at 1 kw and V c o 15 V, according to (5). The capacitance can be reduced greatly by the capacitor voltage variation. t is noted that reactor L is set to decrease the switching ripple. That is, the reactor L depends on the switching requency o the irst stage inverter. The leakage inductance o the transormer is used as L. C. Design o the transormer. The major ocus in the design or the transormer is the current capacity. This is because the transormer has two unctions; irst is to be a DC active ilter and second is to be an isolation transormer. The transormer current, tans,is equaled to the sum o the active ilter current com and the current di which is according to the output power is shown in (6). com trans = di + (6) Note that in (6) the active ilter current is divided by because the transormer winding is connected in parallel to the

4 center tap in the common mode circuit as shown in Figs.4 (c) and (d). The common mode voltage controls the capacitor voltage variation and or the dierential mode voltage, it controls the transmission power to the power grid. The common mode voltage should be changed widely rom the viewpoint o capacitance suppression as shown in (5). However, the period o the dierential mode becomes short when the common mode voltage is widely changed. Figure 7 shows the dierential mode current waveorms and the terminal voltage waveorms o the transormer in the case o common mode command is and.3 p.u respectively, given at the same output power. The dierential mode current increases when the common mode voltage increases, because the power transmission period is shaved o by the common mode voltage. Thereore, the duty ratio D di or the dierential mode can be constrained by (7). D di + D com = 1 (7) where D com is the duty ratio or the common mode voltage. The duty ratio or the common mode voltage is obtained by (8) because the average voltage V c o C is hal o the DC voltage. ΔVc ΔVc Dcom = = (8) V c where V dc is the DC voltage o the irst stage inverter. Also, the dierential mode current di is obtained by (1) using (7) and (8). di = = (9) Ddi _ max ΔVc On the other hands, the common mode current com is shown in (1) because the maximum value o the power ripple is the double o the input power. com = (1) Finally, the transormer current trans is obtained by (11) rom (6), (9), and (1). com trans = di + = + (11) ΔVc t is noted that com in (1) is the peak value o the common mode current. Thereore, RMS value trans(rms) o the transormer current, which is used to decide the thickness o transormer winding, is obtained by (1). com trans( RMS ) = di + = + (1) ΔVc Figure 8 presents the relation between the transormer current and the capacitor voltage variation at V c =15 V, P in = 1 kw and V dc = 3 V. As the capacitor voltage variation becomes large, a smaller active ilter capacitor C can be achieved. However, as the larger the capacitor voltage variation becomes, it results the system required a large current capacity transormer. Capacitance [μf] Capacitor Voltage ΔVc[V] Fig. 6. Required capacitance or power ripple compensation. (P in :1 kw,v dc :3 V,V c :15 V) V * = t V * =.3 p.u. Fig. 7. The current and voltage wave pattern by the dierence o the common mode duty ratio at the constant power. Trans Current trans[a] Capacitor Voltage ΔVc[V] Fig. 8. The current capacity o the transormer which is necessary or compensation (P in :1 kw, V dc :3 V, V c :15 V) Capacitance [μf] Trans Current trans[a] Fig. 9. The transormer current capacity and capacitance which is necessary or compensation.(p in :1 kw, V dc :3 V, V c :15 V) n addition, the relation between the transormer current and the capacitance is represented by (13) rom (5), and (11). ( trans ) C = (13) ω V V (P V ) c dc in dc trans t

5 Figure 9 will urther explain the relation between the transormer current and the capacitance V c = 15 V, P in = 1 kw and V dc = 3 V. As can see rom Fig. 9, a choice o picking up a smaller capacitance results a larger current capacity transormer is chosen. For example, or a 1 kw system, i a 15 μf capacitor is chosen, then the transormer which has current capacity o more than 1A needs to be considered. n other words, the proposed method requires a at winding transormer in comparison with that o the conventional circuit. Note that the number o turns in a winding or the transormer is calculated rom (14) as same as the conversional transormer. N = (14) 4 SB where, N is the number o turns in a winding, is the switching requency o the irst stage inverter, B is the lux density o the core, S is the section area o the core. V. EXPERMENTAL RESULTS The proposed converter was tested under experimental conditions which are shown in Table to conirm the validity o the proposed circuit operation. The auxiliary inductor is connected to the center tap o the transormer, because the leakage inductance o the transormer is not suicient to reduce the switching ripple current. Figure 1 shows the operation waveorms o the conventional circuit without the DC active ilter. The sinusoidal grid current waveorm and unity power actor are obtained; however, the DC input current has a large ripple current component o 1 Hz. Figure 11 shows the operation waveorms o the proposed converter. The ripple o the DC input current is suppressed to % o that with the conventional circuit, indicating that the DC active ilter unction is eective. Figure 1 shows the operation waveorms o the conventional circuit using a large electrolytic capacitor o μf. This circuit was tested in order to determine the reduction in the DC ripple current by using a large electrolytic capacitor. Although a large electrolytic capacitor, which is times that shown in Figure 11, is used, the DC input current did not reduce signiicantly. Figure 13 shows the DC input current total harmonic distortion (THD) o the conventional and proposed circuit, which is deined by (14). n DC _ THD = (15) DC where n is harmonic components and dc is DC component. The major harmonic component in the input current is 1 Hz. n a conventional circuit, the DC input current THD increases accordingly to the increment o the output power. n contrast, the DC input current THD decreases despite the increment o the output power in the proposed circuit. That is, Table. Experimental parameters. Output power 1 kw Grid requency 5 Hz Grid voltage V AC Link requency 1 khz Active ilter inductor 5 mh Energy buer capacitor 44 μf DC link capacitor (Proposed circuit) 11 μf DC link capacitor (Conventional circuit) μf DC nput Voltage DC nput Current Grid Voltage Grid Current 5 V/div 5 A/div 5 V/div 1 A/div 1 ms/div Fig.1. Operation waveorms o the conventional circuit without a DC active ilter. DC nput Voltage DC nput Current Grid Voltage Grid Current 5 V/div 5 A/div 5 V/div 1 A/div 1 ms/div Fig.11. Operation waveorms o the proposed circuit. Fig.1. Operation waveorms o the conventional circuit with a large electrolytic capacitor C DC = μf.

6 nput Current THD [%] Output Current THD [%] Fig.13. nput DC Current distortion actor Fig.14. THD o Grid connection Current. Thereore, these experimental results conirmed that the proposed converter is valid or the reduction o the DC input ripple current in the DC power supply, without the need o large electrolytic capacitors. V. CONCLUSONS A novel single-phase isolated converter was proposed or grid interconnection application. The ripple current in a DC power supply, such as a uel cell, battery or photovoltaic cell, can be reduced by a proper operation o the DC active ilter. The main eature o the proposed circuit is that it does not require additional switching devices, because the zero vector o the irst stage inverter is controlled as the DC active ilter., A 1kW prototype was constructed based on the proposed circuit and the experimental results were obtained as ollow, 1) The ripple current can be decreased to % lower than a conventional circuit. ) The proposed circuit shows a degree o eectiveness in a high power application. 3) The requirement o the total electrolytic capacitor in the system decreases to 1/4 times. 4) The DC active ilter operation in the proposed method circuit does not intererence the grid interconnection current control. n uture, the optimization o the transormer and constructing a high power prototype will be carried out. REFERENCES Fig.15. Eiciency and Grid connection power actor. the proposed circuit is suitable or the high power application due to eective in high output power region. Figure 14 shows the THD o the grid interconnection current o the conventional and proposed circuit. Almost the same current THD values were obtained. The proposed circuit can achieve the same perormance level as the conventional does. Figure 15 shows the eiciency and the grid interconnection power actor o the conventional and proposed circuit. The eiciency o the conventional circuit is higher than the proposed circuit. One o the reasons power loss had increased is because o the increasing current in the transormer. Thereore, the eiciency o the proposed circuit can be improved i the design o the transormer has been optimized. Note that the proposed converter has good perormance as the grid interconnection converter because both power actors o the proposed circuit and the conventional circuit were 99%. [1] S.Sumiyoshi, H.Omuri, Y.Nishida "Power Conditioner Consisting o Utility nteractive nverter and Sot-Switching DC-DC Converter or Fuel-Cell Cogeneration System"in Proc. EEE PCC'7 Nagoya, 7, pp [] R.Nojima,.Takano, Y.Sawada "Transient perormance o a new-type hybrid electric power distributed system with uel cell and SMES"in Proc. EEE ECON'1, 1, pp j. [3] H.Cha, J.Choi, B.Han"A New Three-Phase nterleaved solated Boost Converter with Active Clamp or Fuel Cells" in Proc. EEE PESC'8, pp [4] L.Danwei, L.Hui "A Three-Port Three-Phase DC-DC Converter or Hybrid Low Voltage Fuel Cell and Ultracapacitor" in Proc. EEE ECON'6, 6, pp [5] P.T.Krein, R.Balog "Low Cost nverter Suitable or Medium-Power Fuel Cell Sources" in Proc. EEE PESC',, pp [6] B.BOUNEB, D.M.GRANT, A.CRUDEN, J.R.McDONALD "Grid Connected nverter Suitable or Economic Residential Fuel Cell Operation" in Proc. EEE EPE'5, 5, 386 [7] D.Polenov, H.Mehlich, J.Lutz "Requirements or MOSFETs in Fuel Cell Power Conditioning Applications" in Proc. EEE EPE-PEMC'6, 6, pp [8] F. Perumo, A. Tenconi, M. Cerchio, R. Bojoi, G. Gianolio, "Fuel Cell or Electric Power Generation: Peculiarities and Dedicated Solutions or Power Electronic Conditioning Systems, " EPE Journal Vol.16, pp.44-5, Feb. 6. [9] M.Pereira, G.Wild, H.Huang, K.Sadek "Active Filters in HVDC Systems: Actual Concepts and Application Experience" Power System Technology. PowerCon. nternational Conerence, pp [1] M.Saito, N.Matsui "Modeling and Control Strategy or a Single-Phase PWM Rectiier using a Single-Phase nstantaneous Active/Reactive Power Theory" in Proc. EEE NTELEC'3,pp [11] X.Ma, B.Wang, F.Zhao, G.Qu, D.Gao, Z,Zhou "A High power Low Ripple High Dynamic Perormance DC Power Supply Based On Thyristor Converter And Active Filter" in Proc. EEE ECON',pp138-1