三相四线制电网部分调压调容无功补偿装置的设计
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Review of Power Electronic Concepts of Hybrid Power Source Jan Leuchter, *Pavol Bauer University of Defence, Czech Republic, *Delft University of Technology, The Netherlands Jan.Leuchterunob.cz, *P.BauerTuDelft.nl Abstract- In this paper, power supply unit based on the Engine-Generator Set (EGS) will be discussed. The recent trends lead to the use of more autonomous power sources to satisfy the maximum energy yield. These power units can be considered as comparatively sophisticated system, consisting of mechanical part, electromechanical energy conversion part, power electronics and control (feed-back) part. These systems usually request a power buffer with battery and bi-directional dc-dc converter. Two aspects will be considered, namely power electronics converter concept with central boost converter design. This paper also briefly shows experimental verification of these modern power sources. I. INTRODUCTION Engine-Generator Set (EGS), initially developed and produced mainly for military purposes, gradually found their use as power supplies for various machines and appliances to increase their high-power and high-energy density. Quite indispensable are the EGS in civil defence, crisis management forces, and naturally in armed and security forces, where mobility and maximum efficiency is required 1-6. The clear trends give us an idea about using more autonomous power sources which can archive the energy maximum yield. The system concept of autonomous sources is shown in Fig. 1. OH-H2 O2 Fuel Cell Water 1: Diesel Engine 2: PM synchronous generator DC AC SupercapacitorDC DC PE1 PE2 PE4 PE3 AC or DC grid Fig. 1. Hybrid power source There are four main system parts diesel engine coupled with PM generator; fuel-cell; solar array and power buffer with battery coupled together with supercacitor. The battery allows autonomous operation by compensating for the difference between power production and loads. In many of applications, the loads are not usually constant but rather variable. The investigations of operation in last years have shown that the majority of sets operate under low load which does not exceed more than 20 % of rated permanent load. On the other hand, very often for short time (seconds, minutes) the power required by the load is more then is nominal power. In these conditions the power source operates with low efficiency 1, 6. The dynamic behavior of such system is given usually by diesel engine or fuel-cell. Dynamic behavior of these is limited because the fuel injection is able to inject only finite mass of fuel and then the system cannot accelerate for all range of the loads. Consequently, suggested concept of power source must lead to power sources with battery buffer according to the Fig.1. Such system can get supply loads by means of power from battery without engine running, if the load required low power or system can deliver more power than is its nominal value. Other case of power generators are sources based on the fuel-cell technology. In the following we briefly discuss these. There are several types of fuel cells currently under development, each with its own advantages, limitations, and potential applications. The world research of fuel cells had been focused in the acidic types mainly with a polymer electrolyte membrane for a long time. Since a negative influence of the carbon dioxide on their operation the alkaline fuel cells were out of the main interest. Only in recent years it has been proved the doubts were not necessary, especially our use, when most of the time the fuel-cell is switched off and works as uninterruptible power supplies (UPS). On the contrary, their advantages have been revealed compared to the other types of fuel cells, particularly their high efficiency due to the fast kinetics of oxygen reduction in an alkaline environment. There is also possibility to use non-precious metal catalysts on both the anode and cathode since easily occurred electrochemical reactions, when precious metal catalysts are necessary in the others fuel cell types. In addition a construction structure of the alkaline fuel cell is considerably simpler. Their character allows us to use common and not expensive materials and electrolytes 8. Therefore, alkaline full-cell applies for planning use to build a power source which can achieve maximum efficiency of 60% what is considerably higher efficiency compared to diesel or petrol engines. In Fig. 1, you can see that output of fuel cell is connected to DC grid through the power electronics (PE3). Such concept of power electronics can be proposed just for 978-1-61284-972-0/11/$26.00 2011 IEEE3912first quadrant (Q-I) operation which represents just one direction of the power flow (Q1). On the other hand, the fuel-cell concept requires start from batteries to supply additional units (air unit, water unit, and so on). In the same way, we can describe power electronics with photovoltaic cells (PV). There is also required concept of power electronics (PE2) operating in first quadrant. On the other hand, such power electronics must be familiar with MPPT tracking. Frankly, these sources with MPPT tracking can be more effectively applied with direct contact to the AC grid, where the optimal operating point can be easily managed. Other use of PV sources can be useful together with fuel-cell during electrolyses, where water decomposes into hydrogen and oxygen. Equally, the system of diesel engine coupled with permanent magnets generator (PMG) require power electronics (PE1), where operating is restricted to Q-I. The output power of PMG with variable voltage and frequency proportional to the speed is changed to the output power of the required controlled voltage and constant frequency. The structures of engine-generator-converter sets with VSCF technology are described in 6. Suppose the high-speed gas turbine can be also used for hybrid power sources. They have the benefit of a considerable size and weight reduction. When compared with common diesel engine, the typical high speed 40 kW source driven by gas turbine has the specific power output about 200 W/kg, while the classical 40 kW one, driven by diesel engine, yields only 20 W/kg. However, these high-speed turbines are usually useful for high power applications (100 kW and more). Power buffers are suitable use for a hybrid power source. Such power buffer, connected via an electronic converter (PE4), can improve the dynamic behavior of the system with diesel engine by means of injecting stored energy into the dc-link by the bi-directional dc-dc converter. And also power electronics must be able to change direction of power flow e.g. for battery charging. Therefore, two-quadrant control is required, usually first and second one (Q-I and Q-II) are necessary. Power buffer is suitable to cover high power peaks, which usually take several seconds. Usage of power buffer in these power sources can be resulting in reduced size of engine-generator set with diesel engines, which are bulky. Suppose the power buffers are suggested with power batteries to cover e.g. half of nominal power, whereas the engine-generator set can be then half. In this why is really important fact that more EGS operates very often under low load which does not exceed in average more than 20 30 % of the rated permanent load 1. Power buffer can reduce size and next using of power buffer, namely batteries, can be for saving power from PV generator, if the sun intensity is higher then is load requires to achieve MPPT tracking or if e.g. battery is required by diesel engine or fuel-cells during start-proces. II. THE CONCEPT OF THE POWER ELECTRONICS In the following we shall discuss the power electronics concept for each power source. There are many possible topologies to build up. Three different criterias can be investigated. The first one is cost of the system, where power electronics play an important role. Power electronics is expensive technology and high power can lead to cost-cutting in comparison with low separate power source. Second one is efficiency of source and the last one is power density of power electronics, which are also important. One of the ways to reduce cost and size is using just one central power electronic converter. That is possible if we reflect fact that is not always necessary use e.g. both engine-generator set and fuel-cells together at the same time. If the diesel engine is running, then average value of power can be covered by EGS set and by power buffer. On the other hand, using fuel-cells generator together with battery is necessary during military mission during 1 or 2 hours, when is it crucial to operate quietly without any kind of emission or during logistic complication. In this can be useful to use PV generator for electrolyses to make fuel from water and so on. On the other hand, the advantage of fuel-cells generators and batteries are that the lifetime performance is greatly limited in comparison with EGS and is heavily dependent upon the conditions which the cells are being used. The lifetime is affected by operating temperature, operating current, power and other factors and power flow profiles. The difference is too high between usual EGS and fuel-cell generators, e.g. it is typical that lifetime of alkaline fuel-cell is not higher then 2 000 hours, which is really not very high. Therefore, EGS are suitable to use in standard condition power generation to save expensive sources, namely fuel-cells sets. From this analyzes of usage is evident that power electronics concept has three stage of power electronics, PE-A of EGS and fuel-cell generator (FC) with first quadrant operation, PE-B of power buffer with two-quadrant control and PE-C for photovoltaic generator. Concept can be seen in Fig. 2, where S1 and S2 are auxiliary power supplies way for starting aggregates. PE-A S1 S2AC gridFCEGSPE-B PE-C PVdcacacac acdc Fig. 2. Power electronics concept of hybrid power source III. THE REVIEW OF THE POWER ELECTRONICS The design of converter for power electronic interface in power management system of hybrid power source represents 3913a challenging task. In the following we shall discuss more the power circuits. From the analysis of power electronic concepts shown above, it is evident that the converter PE-A consists of three converters, ac-to-dc, dc-to-ac and dc-to-dc. The output of EGS is three-phase variable voltage that is first converted to a dc voltage. The trend is to use the inexpensive rectifier concept with diodes, as illustrated by the Fig. 3 between points 1 and 2. In such diode rectifier, the power flow can only be from the ac side into dc side. The dc output of rectifier is ripple free (Udc1) because the large capacitor is connected on the dc side. A serious drawback of these is output harmonic content. The advantage is in their simplicity. PWM rectifier techniques eliminate the selected harmonic from the output. Therefore, the concept with fully controlled PWM rectifier can help to improve the output shape of current, but these represent more expensive converter. ac gridC1 EGS 1 ac-to-dc dc-to-dcdc-to-ac ac-to-dc FC dc-to-dc PF dc-to-dc C2 PV dc-to-ac 2 3 46 5 7 8 9 10 Fig. 3. Power electronics of hybrid power sources with central Boost The dc-to-dc converters are a one of the most widely frequent components of all electronic circuits. Looking ahead to the application of this, we find that these converters are very often used with an electrical isolation transformer in the switch-mode dc power converters or without an isolation transformer. The fundamentals converters listed are shortly discussed in the following. Two very important configurations of dc-to-dc converter are often used for kW range: Buck, Boost concept. Nevertheless, there are more than 500 existing prototypes of these and can provide the some advantages like lower output voltage ripple in comparison with these fundamentals. E.g. configurations (Ck, Sepic or Luo-pump) using the capacitor as the primary element for storing and transferring energy from input to the output are often used for low voltage applications. The advantage of these configurations can be found as the possibility of transformer saturation due to dc offset is precluded by the series capacitor. An advantage of these circuits are that the input current are ripple free and it is possible eliminate the ripples completely by means of external inductor filter. That can be useful for concept with fuell-cell generator. These converters in their basic forms are capable of transferring energy only in one direction in I-Q. The concept of two-quadrant of dc-to-dc converter operates in I-Q and also in II-Q is shown in Fig. 3 (between point 7 and 8), which is connected with battery, where is necessary to change current flow during charging and discharging. A full-bridge converter topology is also capable of a bi-directional power flow. This converter operates in four quadrants and provides a good system topology, because the output current through these PWM full-bridge dc-dc converter does not become discontinuous. The advantage of this is in higher power rating. Again as previous with PWM rectifier, the main drawback of the concept with full-bridge is cost comparison with basic form of converter with Boost and Buck. Furthermore, more switches and transformer represents also lower reliability. Cascade concept of converter (Fig. 4) can help to implement the output voltage increasing with simpler structure. The two-stage boost circuit is set up from boost converter and adding the parts L2, D2, D3 and C2. Output voltage of the first-stage (V1) is given by Eq. 1 and the voltage across capacitor C2 is charged to Vout by Eg. 2. Higher stage can be designed by just multiple repeating of parts. These can be also useful for full-cell generator to transform output low voltage of FC to 300 dc-line (Udc1). The results of experimental verification and comparison with Boost are shown In Fig. 5. VinQ1D2L1C1D1L2D3C2V1Vout+ Fig. 4. Two-stage Boost dc-to-dc converter 7 D1VVinout= (1) in21outVD11VD11V= (2) The inputs and outputs voltage and current is shown in Fig. 5. The output voltage for Uin=20 V and duty 40% is 60 V. Therefore, these concept san achieve a higher voltage transformer gain in comparison with e.g. Boost (Fig. 6). The output voltage for the same conditions is just 41 V. So, cascade concept with higher voltage gain transfer is required by fuel-cell generators, because higher nominal voltage of FC produces higher cost of FC, lower FC reliability and mainly lower lifetime of FC. On the other hand, the input ripple voltage and current is higher, what incorrect e.g. just for concept with fuel-cell generator. The next drawback is higher number of components such as diode, inductors and mainly capacitors, what produce lower efficiency and lower reliability of power electronics. Many other circuits can be derived from these baseline topologies using fundamentals converters. Voltage converters with transformer isolation can provide voltage increasing as well with high efficiency. In order to show complete family dc-to-dc converters to achieved higher voltage transformer gain, we need to add 3914next developed converters tapped inductor converters. For instance, tapped inductor converters are the next most common converters of dc-to-dc converters based on the buck, boost or buck-boost topologies. The example of these converters based on Boost is shown in Fig. 7. Here the tapped inductor ratio is 211nnnn+= (3). The results of experimental verification of these can be seen in Fig 8. You can see that ripple of input voltage and current are lower in comparison with Boost and Two-stage Boost. Output voltage is slightly lower (32 V) in comparison with Boost. Ch1: Voltage IN Ch2: Current IN a) Ch1: Voltage OUT Ch2: Current OUT b) Fig. 5. Two-stage Boost dc-to-dc converter (L1 and L2 are 100 uH; C1=100 uF; C2=1000 uF, D=40 %; Uin=20 V; IIn=3.84 A, Uout=60.6 V and R=90 Ohm) Ch1: Voltage IN Ch2: current IN Fig. 6. Boost dc-to-dc converter (L1=100 uH; C1=100 uF; C2=1000 uF, D=40 %; Uin=20 V; IIn=1.4 A, Uout=40.96V and R=90 Ohm) Q1Q2L1C1L2+Vout+ Fig. 7. Tapped-Inductor Boost converter 7 Ch1: Voltage IN Ch2: current IN a) Ch1: Voltage OUT Ch2: current OUT b) Fig. 8. Tapped-Inductor Boost dc-to-dc converter (L1 and L2 are 100 uH; C1=100 uF; C2=1000 uF, D=40 %; Uin=20 V; IIn=1.4 A, Uout=33 V and R=90 Ohm) The final analyses of fundamentals concepts will be discussed in the following. Summary of concepts are shown in Table I. TABLE I CONVERTER COMPARISON Boost Two-stage Boost Cuk Sepic Tapped-Inductor Boost Design simple simple difficult for high power difficult for high power medium Gain of transfer volatge low higher then Boost medium medium higher then Boost and lower then two-stage Disturbances low high low low low The Buck or Boost dc-to-dc converter is simple concept with low gain of transfer voltage. This topology of converters can be used for high switching frequency, what is good from the view of power density. Boost can operate with low level of disturbances and with continuous current. The disadvantages are non-galvanic isolation between input and output. Whereas, the various types of dc-to-dc with isolation usually have high transfer voltage gain and high isolation between both sides what is advantage in the comparison with previous fundamentals dc-to-dc converters. The concept with Two-stage Boost converter can achieve higher transfer voltage gain in comparison with Boost concept, but with higher ripple of inputs. The other parameters are similar as a Boost. The Ck, Sepic concept operates with high efficiency, continuous input and output current, low level of disturbances, with high switching frequency and with low voltage and current ripple. Similarly is we can obtained the same features of the concept of Ck with transformer, where is a galvanic isolation of input and output. SEPIC concept are resistant for short-current, wild range of input voltage and both inductances can be at the same core. The main disadvantage of these are using capacitors for transferring energy from input to output and therefore is not applicable for kW. Therefore, the main concept for Hybrid power sources according Fig. 3 is basic Boost form of dc-to-dc converters. 3915They require a just one input inductor L and concept can be proposed for kW easily, what is main advantage besides simplicity. The next Boost advantages include low stress on the switches and high switching frequency, low level of disturbances. IV. BI-DIRECTIONAL CONVERTERS Bi-directional converters for power buffers were shown in Fig. 3 between points 7 and 8. Converters allow transfer of power in either direction according previous part of converter quadrants, where was shown the I-Q and II-Q is necessary for these, or the variant of all quadrants converters. Due to ability of reversing of current direction of flow and voltage polarity must me unchanged; they are being used in application of sources with battery. Achievable implementation of bi-directional converters can be with hard-switching technology, or with resonant or soft switching technology. All of these lead to an increase components which make decreasing total efficiency and lower reliability. The main points of our focus on these dc-to-dc converters are: high efficiency; high switching frequency; high voltage transfer gain between input and output; low current ripple; to allow bi-directional power control flow; galvanic isolation between input and output; low level of EMI; low stress on the switches. Therefore, the main goal is finding the middle ground of these previous points. The main proposed topology is a fundamental Buck-Boost concept from Fig. 9 (just one legs with Q1, Q2, D1 and D2). VHQ1D2Q2D1Q3D4Q4D3LfC1LL+ A CB Fig. 9. Multiple legs bi-directional Buck and Boost converter If current IL happens to flow into terminal A, it can find its way back to terminal B either via Q2 (if Q2 is closed) or via Q1 (if Q2 is open). Because one of the switches is always closed and it is evident that current IL can always circulate. The current IL can flow in either direction, but the polarity of the dc voltage is fixed, where voltage between points AB (VAB) is always positive. Therefore converters provide the bi-directional flow of power for battery charging and discharging. On the other hand, they require a bulky input inductor L to limit the current ripple, especially when high voltage transfer gain is required. The both modes (Buck and Boost) operate with high efficiency. The main disadvantage of this is non-galvanic isolation between input and output. To minimise the input inductor size and the current ripple, as well as to reduce the single switch current stress, the previous concept of dc-to-dc converter can be designed with multiple legs interleaving each other by means of an input coupling inductor according Fig. 9. The concept is based on the controlling the two identical legs with a phase shifting equal to one-half of the switching period. In this, the current ripple can be reduced, as well as Tapped-Inductor Boost, and the size of inductor then can be lower then concept with just one inductor. Such concept also reduces switch current stress. Additional advantage of multiple legs of bi-directional Buck and Boost converter is also using for kW fields of application. The bi-directional concept of dc-to-dc converter with Two-stage to improve voltage transfer gain based on the topology from Fig. 4 can desire gain improve, but the concept is so complicated and the output efficiency of this is out of the ordinary. However, when a very high dc voltage transfer gain is required, a high frequency transformer must be used. From the analyses of converter for Q-I and two-quadrant operation (Q-I +Q-II) of Hybrid power sources application that has been carry out, the simple concept of dc-to-dc converter based on Boost and Buck for kW provide good efficiency, low noise and low EMI. There are cost-effective solutions for power up-to kWs and the control a strategy is simple. The Bridge concepts can get more wide power ranging, where the main drawback of these Bridge converters are discontinuous output current, when operating as a boost converter. Consequently, the Bridge concept is more frequent for cases of higher voltage transfer gain request. More will be discussed at part focusing on power electronics of photovoltaic source. However, the simple Buck-Boost converter can be used and topology of converters provide the bi-directional flow of power for battery charging and discharging using just by one bulky inductor to have continues. On the other hand, they require really a bulky input inductor L to limit the current ripple, especially when high voltage transfer gain is required, what was mentioned above. Other advantages of the Buck-Boost concept include low stress on switches and high switching frequency. Therefore, both concepts are good candidate for these applications. V. PV CONVERTERS The dc-to-dc converters of PV (Fig. 3; converter dc-to-dc between 9 and 10) allow transform sun energy to electrical. The power electronics of photovoltaic generator are mainly based on technology of inverters to supply ac grid. The inverter is the most important components in PV generator to achieve the maximum yield of sun energy. The string technology of series connection of photovoltaic modules is one of the most using. These series connections were then connected in parallel, through string diodes. This PV topology includes some several limitation, such as power losses due to string diodes, extra losses between the PV modules and lower efficiency due to a centralized maximum power point tracking (MPPT), nonflexible a system design during time when owner want to extend a total power of photovoltaic sources. The newer technology, where several strings are interfaced with own dc-dc converter and then connected in parallel, can be more efficient. This string topology (called a Multi-string) can be beneficial, compared with the centralized system, since every string can be 3916controlled individually and maximum power point tracking can be applied individually. Flexible design with higher efficiency is hereby achieved. On the other hand the separate dc-dc modules compared with the centralized dc-dc module leads to higher cost and higher retail prices. Therefore, the converter required the maximum efficiency. The main two concept of PV converter are shown in Fig. 10. Good grid compatibility can be achieved by inverters with transformer according Fig. 10a. The switching follows 50 Hz and then the results are bulky due to transformer. Higher switching frequency improve size and weigh through high-switching frequency transformer, nevertheless the output efficiency is none too. If maximizing the energy yield the tranformerless inverter is the more suitable concept, see Fig. 10b. C1Q1Q2Q3Q4K1 PVinput ac gridoutput a) C1Q1Q2Q3Q4C2L1D1Q5C3L2D2Q6K1b) Fig. 10. Transformer and Multiple legs dc-to-dc concept of the PV The concept of PV based on delivering power to dc grid is also possible, but the solution of MPPT is slightly more difficult and therefore the concepts of converters from Fig. 10 that can operate with MPPT and deliver power to the ac grid concept is one of the most widely used. Problem arises if the power from PV is higher then load request, then MPPT point cannot be set and then we operate with lower efficiency then we could. To improve these patches, the concepts of power electronic according Fig. 11 with PV generator delivering power into dc grid can be get better. VI. INVERTERS (DC-TO-DC) The dc-to-ac converter (Fig. 3 and 11; converter dc-to-dc between 3 and 4) enables us to produce a sinusoidal ac output or rectify output voltage from PV. Therefore, for power electronic concept according Fig. 4 the dc-to-ac converter must be bi-directional, because power flow in both directions is required. P-phase, 6-pulse controllable PWM inverters is one of the most widely used units in power electronic, due to its practical importance. These can run in 4-quadrant, which is operating at a constant switching frequency of several kHz. They can operate in bi-directional mode, if rectifier mode is necessary. That is typical, for example, during braking of induction motors. ac gridC1EGS1ac-to-dcdc-to-dc dc-to-ac ac-to-dc FCdc-to-dcPFdc-to-dc C2 PVdc-to-dc23 46578 910 Fig. 11. Power electronics of hybrid power sources For our case is rectifier mode can be useful during setting of MPPT if the power of PV is higher then load and then rectifier mode of converter supplied battery through a converter. It should be noted that such bi-directional converter require inductance L, what is real drawback. To obtain a good value of L to improve efficiency is necessary converter design around for 200 kHz what is expensive technology. Therefore, the concept from Fig. 3 is better replaced by concept from Fig. 11. Fig. 12 shows the converter which is based on the multi-input dc-to dc Boost converters. 654321Udc1Udc278910EGS PV 1 PV n SC Fig. 12. The 10 kW power electronics (PV. Photovoltaic panels, SC. Supercapacitor) VII. A DESIGN OF 10 KW POWER CONVERTER A simplified block diagram of power electronics with EGS with variable speed control (6 kW), n x PV generator (1.5 kW), and supercapacitor is shown in Fig. 12. As a consequence of varying the engine speed when using the optimum variable speed control, both the output voltage and 3917the output frequency of the generator vary and must be regulated to a constant value as required by the load. Therefore, a power electronic converter with stage of ac-to-dc; dc-to-dc and dc-to-ac is required to regulate the output voltage and frequency (3 phase, 400 V, 50 Hz). Diesel engine and synchronous generator with permanent magnets has been widely use on many power related systems. Our experimental model has been selected for a 6 kW EGS with a diesel engine (HATZ 1D40) with an output power of 7.7 kW at 3600 rpm and 3.8 kW at 1500 rpm. The engine is slightly oversized (as regards power output) to facilitate dynamic changes of the load. Permanent-magnet synchronous generators is the 12-pole, synchronous generator is connected to the diesel engine by means of a mechanical clutch. The output voltage of the generator has an unregulated frequency as it is generated by a varying engine speed. The ac output voltage of the SGPM is in the range of 100 V to 450 V at a frequency ranging from 100 Hz to 300 Hz. The power electronics represents circuit: ac-to-dc, where Vin is AC input and Vdc1 is DC output; dc-to-dc, where Vdc1 is DC input and Vdc2 is DC output; and dc-to-ac stage. Test results of power electronics can be seen in Fig. 13. The dc-to-dc converter with feedback control steps the voltage up from variable output voltage of uncontrolled rectifier to a constant value of Vdc2=570 V. Vin Iin Vac-Vdc- Vdc1 Vdc2 Fig. 13. The input and output of ac-dc; dc-dc and dc-ac converter during 1453 RPM speed For instance, the results of efficiency test results of power electronics, Boost (blue color), and Two-stage Boost concept (red color) are shown in Fig. 14. 00,20,40,60,811,2020406080100D (%)Efficiency (%) Fig. 14. Test results of efficiency: Boost and Two-stage Boost 1 kW with L=250 uH. The results of our PV generator (1.2 kW), which is set up by 6x polycrystalline modules MPE PS 05 with peak power 205 W and output voltage 27 V and short current 7.6 A. The output voltage is around 140 -150 V of the dc-to-dc converter is controlled to achieve MPPT. Output PV converter is set up as 570 V. An power buffer, connected via an electronic converter, see Fig. 12 between points 7 and 8, can power supplies or charge through the dc-link by means of more quadrant dc-dc converter (namely I and II. Q). This concept is based on the delivery of peak power or to charge power of PV generator to operate with MPPT if the load request lower then PV can produce. The requested energy W is given by the maximal required power P and the average time of the regulation TR, eq. (4). First of all, rate of the handling storage energy in buffer is given by power of one our PV generator providing peakly 1.2 kW and by 6 kW EGS concept with optimum variable speeds. Such sources operates very often under low load which does not exceed in average more than 20 % of the rated permanent load and on other hand they peakly (2 s) require more then nominal load. Therefore, our requirement of power buffer for 10 kW source are bring 6kW during 10 s into dc line and saving power of PV generator approximately 1 kW during 30 minutes. So, the power buffer must provide more then 60 kJ in order to satisfy the peak power. The storage by supercapacitor requires for 60 kJ and 150 V the output capacity 7 F and 213 F for 1800 kJ according eq. (5). kJ1800)6030(1000andkJ60106000TPWR=? (4) F7215015060000022UUW2C222nom2nom= (5) The capacity 7 F is easy to build it but 213 F is too high for superacapacitor technology. Therefore, Ni-Mh batteries have been selected for our experimental set up. These batteries are some of the most cost-efficient energy storages available today, where the energy is stored electrochemically. The advantages of these batteries are good electrical behavior and low initial cost. However, they have lower power density and larger size and weight, low cycle life, relative high ESR, when compared to supercapacitors or Li-Ion. The analyses outline in 13, 14 has shown that: batteries are capable of supplying efficiently pulsating loads, with deep discharges. But the life-time is rapidly reducing by this operation (means - high peaks loading). Supercapacitors can deliver constant high power for short time with high efficiency, and then supercapacitors are very interesting solution in the area of energy buffers. The set up of battery-supercapacitor mixed system with electronics according Fig. 10 to achieve both advantages of batteries and converters has been used and set up. Multiple legs bi-directional converter can obtain higher voltage gain without transformer. On the other hand, the main advantage of this concept can be found undisputed evidence of using one design of inductor for multiple-input converter according Fig. 12. Magnetic circuit which was designed for central dc-to-dc converter is given: U core; 1924 mm2 (37x52 mm) and profile a=19 mm; BMax=0.3 T; permeability rFE=1000 a path length lFE=0.28m. Then number of inductor winding is N=200 and 3918the airgap are lv= 0.001 (m). Fig. 15a shows the picture of our boost converter inductances and results of our experimental model of 10 kW power electronics is shown in Fig. 15b. a) b) Fig. 15. Experimantal set up of 10 kW power electronics The fuel cell was not used in our experimental set up, because the prize of e.g. 1 kW is too high. Testing was done with Boost, Two-stage and Tapped-inductor power electronics for small fuel cell 10 W. Experiment shows that ripple of input voltage and current are lower for tapped-inductor in comparison with Boost or Two-stage dc-to-dc converter. VIII. CONCLUSION The review of power electronics converter concept has been shown of hybrid power sources. Two aspects of power electronics converter were shown, multi-input converters and optimal architecture. The concept of 10 kW power electronics with central Boost converter was developed and a design of inductor was described. Boost, Buck-Boost, Two-stage boost dc-to-dc converter, Tapped-Inductor and Multiple-legs of bi-directional converter were designed and com
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