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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 Leuchter unob cz P Bauer TuDelft 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 Supercapacitor DC 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 IEEE3912 first 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 criteria s 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 S2 AC grid FC EGS PE B PE C PV dc ac acac 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 3913 a 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 grid C1 EGS 1 ac to dc dc to dc dc to ac ac to dc FC dc to dc PF dc to dc C2 PV dc to ac 2 3 4 6 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 C k 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 Vin Q1 D2 L1 C1 D1L2D3 C2 V1 Vout Fig 4 Two stage Boost dc to dc converter 7 D1 V V in out 1 in 2 1out V D1 1 V D1 1 V 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 3914 next 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 21 1 nn n n 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 Q1 Q2 L1 C1 L2 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 Inducto r Boost Design simple simple difficult for high power difficult for high power medium Gain of transfer volatge low higher then Boost medium mediu m 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 C k 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 C k 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 3915 They require a just one input induct