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Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
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Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer Saban Ozdemir a, Selami Balci b, Necmi Altin c,*, Ibrahim Sefa c a
Vocational School of Technical Sciences, Gazi University, Ankara, Turkey Ministry of National Education, Ankara, Turkey c Department of Electrical-Electronics Engineering, Faculty of Technology, Gazi University, Ankara, Turkey b
article info
abstract
Article history:
In this study, an isolated three-level DC-DC converter is proposed for high power and high
Received 15 November 2016
conversion ratio applications such as fuel cells. The proposed system consists of a single
Received in revised form
phase three-level inverter, a medium frequency transformer and a diode rectifier unit. In
28 January 2017
the proposed system, a DC supply voltage is converted to a medium frequency AC voltage
Accepted 23 February 2017
via a three-level inverter instead of the conventional two-level inverter. Since the three-
Available online xxx
level inverter generates an AC waveform with multiple steps, lower voltage harmonics and lower EMI levels than conventional two-level inverter are achieved. Thus, the three-
Keywords:
level inverter provides higher efficiency value. The medium frequency transformer en-
Medium frequency transformer
ables high voltage conversion ratio and provides galvanic isolation as well. The output
Three-level inverter
voltage of the medium frequency transformer is converted to the DC voltage and thus the
DC-DC converter
DC-DC conversion is achieved. According to simulation and experimental results, it is seen
Isolated converter
that the proposed DC-DC converter structure provides higher power density and higher efficiency values than conventional system. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Recently, number and power levels of the power electronics converters which become popular in residential, commercial and industrial applications have increased significantly. The key action in power electronics systems is switching, and higher switching frequency value provides more advantages. However, semiconductor power switches and inductive components limit their power levels. In general, while high switching frequency values can be applied in power converters which are under 10 kW, the medium frequency values are used for higher power levels. Therefore, high power
converter design is a challenge. Another problem for DC-DC converters is their limited conversion rates. Although nonisolated DC-DC converters are commonly used with their advantageous of being simple, their voltage conversion (step up or step down) ratio is limited. Therefore, isolated DC-DC converters which have a transformer embedded in converter structure are used in applications where high conversion ratio is required. They are also widely used in the structure of modern power systems such as electric vehicles and uninterruptible power supplies. The renewable energy sources such as PV modules and fuel cells generally generate lower voltage levels, and they require step up converters with high
* Corresponding author. Gazi University, Faculty of Technology, Department of Electrical-Electronics Engineering, 06500, Besevler, Ankara, Turkey. Fax: þ90 312 212 13 38. E-mail addresses: [email protected] (S. Ozdemir), [email protected] (S. Balci), [email protected] (N. Altin), isefa@gazi. edu.tr (I. Sefa). http://dx.doi.org/10.1016/j.ijhydene.2017.02.158 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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gain values to supply the loads and/or export generated energy to the grid. Recently, isolated DC-DC converter applications with embedded medium frequency transformer have been started to use in some kind of renewable energy source applications such as solar panels, wind turbines and fuel cells [1e3]. A medium frequency power converter consists of a transformer for different purposes. The size and volume of this transformer can be designed in a relatively small structure at medium frequency. In addition, next-generation grids and smart grids have become popular in recent years. In these systems, bi-directional and isolated DC-DC power converter topologies are designed for different purposes [4,5]. In such applications, power transformers have three functions: The first task is to provide safe operation with galvanic isolation via the magnetic coupling between the low voltage and high voltage windings. The second task is to adapt the supply voltage level to the load voltage level via conversion rate. Thus the voltage level obtained from different types of energy sources can be matched with the conversion ratio of the transformer. Third, the leakage flux in the medium frequency transformer primary side can be used as soft switching inductor element for series resonance circuit [6,7]. This type of DC-DC converters can also be designed as bi-directional. The isolated DC-DC converters are commonly discussed for different applications such as, renewable energy applications, traction applications and power supplies with their advantages [8e13]. The power converter with medium-frequency transformer topology which becomes popular in grid connected renewable energy applications is shown in Fig. 1. It is seen that, the main component in the structure of the converter circuit is isolated bi-directional DC-DC converter. This bi-directional DC-DC converter topology can be designed as single phase or threephase structure [1e4,11]. In this circuit, the input AC voltage level is converted to the DC voltage via a rectifier unit. Although, the rectifier unit is designed mostly in the form of diode rectifier type, it can be also designed as a controlled rectifier or a PWM rectifier. Then, obtained DC voltage is converted to a medium frequency AC voltage via a DC-AC inverter. This inverter supplies the primary windings of a medium frequency transformer. The secondary voltage of the medium frequency transformer is converted into DC voltage again via a rectifier circuit. Therefore, sinusoidal AC voltage generation is not required in inverter stage, and inverter is operated to generate medium frequency square wave voltage
waveform. Finally, in order to be connected in parallel to the grid, a DC/AC inverter is used for producing to the conditioned AC voltage on the grid conditions. Using the conventional two-level inverter in high power and high frequency power electronics applications is gathering number of problems such as limitations on current/ voltage values of semiconductor switches and high switching losses. In order to increase the power level of the converter, switches can be connected in series and/or parallel. However, the switches and circuits are not completely identical, and small time differences during the switching cycle may cause unbalances in voltage/current sharing. In order to overcome these problems, the multilevel inverter structure is developed. In these structure, it is possible to access high-power levels with standard power switches. Also, if the number of levels is increased then the harmonic components in output the voltage and/or current decrease [14e19]. Multilevel inverter structure has been studied in the literature for conventional inverters and converters. More recently, these type of converters have started to be studied in the DC-DC isolated converters. There are limited number of study in the literature about this subject. The core and winding losses of the medium frequency transformer are reduced by using multilevel inverter in isolated DC-DC converters. In addition, medium frequency power transformer design at high power level is another challenge in high power DC-DC converter design. Higher operating frequency values decrease the size of the transformer. However, the core material selection according to the operating frequency value has significant effect on both electrical and mechanical performance, volume and weight of the transformer. Soft magnetic materials such as amorphous, nanocrystalline and ferrites are common in medium and high frequency transformers [2]. These materials have different saturation flux density values at an operation frequency. Thus, the operation frequency and the core material become two most important design factors affecting the size and efficiency of the transformer [20]. The most important factor that determine the transformer performance is total power losses which compose of winding and core losses. The core losses are directly related with specific core loss value of the core material at the operation frequency. The operation frequency is also affects the AC resistance and thus winding losses of the transformer because of the skin and the proximity effects. In addition, non-sinusoidal excitation signals and harmonic components increase core losses [21,22]. Therefore, selection of both core and winding materials are
Fig. 1 e Power converter topology with medium frequency transformer. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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very important steps in transformer design. In the past literature, medium frequency power transformer designs have been proposed with silicon steel alloys for 2 kHz and lower operating frequencies [14,23]. In another study, the performance of the medium frequency power transformers embedded into the power system of the electric train with silicon steel with 1 kHz and with nanocrystalline with 5 kHz frequency values. It is reported that, the volume and weight of the power system designed with nanocrystalline core materials are about eight times lower than the same system with the silicon core material [23]. Additionally, the performances of the medium frequency transformer with ferrite and nanocrystalline core materials have been compared and it is reported that the transformer with nanocrystalline core is advantageous in terms of efficiency and volume [14,24e27]. Although, ferrite materials can operate at higher frequency levels, they suffer from their limited sizes in high power transformer designs [26]. Since, the nanocrystalline material can be manufactured at larger sizes, they are suitable for high power medium frequency applications. In this study, a DC-DC converter with an embedded medium frequency transformer is designed and analyzed with two different inverter structures for fuel cell applications. The input and output voltage levels of the proposed system are 40 V and 120 V, respectively. This obtained via transformer turn ratio The medium frequency transformer is designed according to determined design parameters by using AnsysElectromagnetic Suite. Besides, three-level inverter circuit is designed and modelled in Ansys-Simplorer and then both transformer and inverter models are simulated together. A PI regulator is used to regulate the output voltage of the DC-DC converter. The simulation results are validated with experimental studies. It is seen from both simulation and experimental results that, the proposed isolated DC-DC converter with three-level inverter circuit has better performances than the DC-DC converter with two-level inverter in terms of output voltage ripple, transformer and total system efficiency values. Thus, it can be easily said that, proposed isolated DCDC converter system is suitable for renewable energy systems such as PV modules and fuel cells.
Multilevel inverters Parallel to the increasing power demands, power levels of the inverters tend to increase with each passing day. However, increasing the current/voltage levels of the standard semiconductor power switches, which are used in inverter circuits, increase the system cost. In addition, power switches cannot be manufactured at very high voltage/current values. Another problem with high-power semiconductor switches is their limited switching frequency values. Parallel connection of inverters to increase power levels are not economical. Another solution is connecting switches in series/parallel to increase the power level. However, switches and driver's circuits are not completely identical, and these small differences may cause switching voltage/current sharing problems. Therefore, load current/voltage is not shared equally by the series/parallel connected switches. Multilevel inverters are an
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economical and practical solution for high power applications. Although, the multilevel inverter requires more elements than ordinary inverter which is unappealing situation, it allows to design an inverter with standard and commonly used components. In the first years, the inclusion of more hardware than traditional inverters prevented the evolution of these inverters. Nowadays, multilevel inverter modules are currently being produced commercially; thus, low cost and compact inverter designs can be achieved. The multilevel inverter generates an AC waveform with multiple steps related with number of their levels. In addition to extending the output voltage range, it ensures lower voltage harmonics comparing to conventional inverter. So that, it provides better current and voltage waveforms. It has also better immunity against common-mode effect. In transformer-based systems, the transformer ratio can be reduced or the transformer requirement can be neglected. Furthermore, as a feature of multilevel structure, it has lower dv/dt and di/dt ratios, and therefore lower electromagnetic interference (EMI) levels than conventional two-level inverter can be achieved [28]. Moreover, for the same switching frequency, it requires a smaller filter. In multilevel inverters, the level concept is expressed as the number of output voltage level step values which changes according to the input level and number of switch. The symbolic status for the output voltage level and the switching states are shown in Fig. 2. If the number of output voltage levels is to be increased, the number of semiconductor switches and/or DC power supply need to be increased. Because of this, three-level inverters are used in many commercial low and medium voltage power applications. There are many multilevel inverter topologies introduced in the past literature. The most well-known topologies are the neutral point clamped (NPC), the cascaded Hbridge (CHB) and the flying capacitor (FC) inverter topologies as shown in Fig. 3 [29,30]. In the NPC structure, the neutral point is connected to the midpoint of the capacitors that divide the DC level into two parts. This is why the called as neutral point clamped. The disadvantage of this structure is the irregularities that occur when the voltages on the capacitor cannot be equalized. The FC structure is similar to the NPC structure. Here, the DC voltage is divided by the help of the capacitor connected to the inner switches. The disadvantage of this structure is the large capacity requirement of the capacitors that results bigger dimensions. The CHB structure is formed by serial connections of a classical two-level inverter. However, each two-level inverter structure must be supplied via an isolated source. The NPC inverter does not require an isolated power supply and their inverter modules are ready for commercial production. So that, it is preferred for many applications [29,30].
Proposed three-level isolated DC-DC converter It is well-known that, the size and the volume of transformer are affected significantly from the operating frequency. The size of the grid frequency transformer is very large when compared to the high-frequency transformer designed for the same power and voltage levels. Since, it is designed for the
Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Fig. 2 e Illustration of multilevel concept for an inverter leg, a) Two-level inverter b) Three-level inverter c) n-level inverter.
medium frequency range (400 Hz - 20 kHz), an isolated DC-DC converter's transformer size and volume are small [20]. Besides the electrical parameters, the core material is also affects mechanical parameters. The power capacity, both
winding and core losses, temperature rise and voltage regulation values are related with specific parameters of the core material. In addition, mechanical parameters such as power/ weight ratio (kVA/kg) are related with core saturation flux
Fig. 3 e Most used multilevel converter topologies, a) NPC converter b) FC converter c) CHB converter. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Table 2 e Designed medium frequency power transformer specifications.
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kVA/kg
Parameter
Value
Input Voltage Level Output Voltage Level Nominal Power Operating Frequency Flux Density Conversion Ratio Current Density Number of Turns of PrimereSecondary Winding Winding Material
2
0
Vitroperm250F 2605sa1
Ferrite-N87
6.5%SiFe
Fig. 4 e Power density (kVA/kg) ratings of different core materials.
density and specific core losses besides operation frequency. The core material type also affects the winding volume; therefore, determination of the core material has significant effects on transformer performance [20]. Different soft magnetic materials can be used as core material of medium and high frequency transformer. Cost and accessibility of the core material also affect the design strategy of the transformer. On the other hand, the shape of the excitation voltage waveform at high switching frequency value increases both the core and the coil losses. This situation deteriorates the efficiency and results in negative effects such as increasing temperature. Therefore, high frequency transformers are designed by using soft magnetic materials as core material due to the lower specific core loss values. Medium frequency power transformer's core material type affects both mechanical and electrical performance. Soft magnetic materials, such as amorphous, ferrite, nanocrystalline are usually used in medium frequency transformer design. The operating frequency, the desired current density and the specific core loss values which are main variables determine the core material. The power/weight ratio (kVA/kg) of a transformer for a certain power and frequency value with different core materials such as non-oriented silicon steel, ferrite, nanocrystalline and amorphous is given in Fig. 4. This values are obtained for 35 kVA power and 10 kHz operation frequency values by using manufacturer datasheet values,
40 V 120 V 5 kVA 10 kHz 0.3 T 1:3 2 A/mm2 12e36 Aluminum foil (100 0.4 mm) Nanocrystalline 0.2 4
Core Material Windows Usage Coefficient Waveform Coefficient
and properties of 6.5% Si-Fe (as non-oriented silicon steel), N87 (as ferrite), 2605sa1 (as amorphous) and Vitroperm250F (as nanocrystalline) are used [31e33]. As it can be seen from Fig. 4, non-oriented silicon steel core material has worst performance at 10 kHz operation frequency. Therefore, this material is usually used in transformers whose operation frequency is below 2 kHz. Although saturation flux density of the amorphous material is higher than nanocrystalline and ferrite materials, their specific core loss value is also higher than these materials. The ferrite material can be used in high frequency values, but their saturation flux density is 2.5 times lower than nanocrystalline material. This results bigger core size. In addition, their limited size limits their usage in high power applications. Furthermore, there is significant difference between the performances of the nanocrystalline and amorphous core materials in terms of noise at high frequency values. Therefore, amorphous material is not useful for over 4 kHz operation frequencies. Specific properties of these core materials are given in Table 1 to compare them easily [20,31e34]. As it is seen, among these materials, the nanocrystalline core material provides optimum performance in terms of weight, volume and efficiency, allows more efficient and compact medium frequency transformer designs. In this study, a medium frequency power transformer that is sized according to the specifications given in Table 2, is designed. The transformer is modeled in Ansys-
Table 1 e General specific properties of the soft magnetic material. Specific Parameters
Material Code Saturation Flux (Tesla) Permeability (m) Coercivity Force (A/m) Mass Density (kg/m3) Curie Temperature ( C) Thickness (mm) Core Loss (W/kg), (at 10 kHz; 0.1 T) Magnetostriction Coefficient (ppm)
Material Type Nanocrystalline
Amorphous
Ferrite
Si-Fe
Vitroperm250F 1.23 5000e7000 0.8 7200 510 0.018 0.35 0.1
2605sa1 1.56 1300e2000 4 7180 395 0.025 2.5 27
N87 0.39 1800e2200 21 4850 210 block 1.5 0.6
6.5%SiFe 1.87 800e1000 30 7650 700 0.05 8 3
Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Electromagnetic Suite software. Three-dimensional view of the designed transformer is shown in Fig. 5. In the medium frequency transformer mechanical sizing, Ap which is multiplication of core cross-sectional area (AC) and window region area (Wa) is used as base parameter. In addition, other parameters such as transformer nominal power (VA), operating frequency (f), amplitude of the flux density (Bm), current density of windings (J), windows usage coefficient (Kcu), and exciting voltage waveform coefficient (Kf) are other used parameters. According to the given power level, Eq. (1) can be used to determine the core size. This equation is a very useful mathematical expression to determine the power capacity of the transformer [35]. Ap ¼ Ac ,Wa ¼
Fig. 5 e The designed medium frequency transformer with nanocrystalline core.
VA,104 Kf Kcu Bm fJ
(1)
The number of turns of primary and secondary winding, and thus transformer conversation ratio of the transformer should be defined to obtained desired converter output voltage. The voltage induced at transformer windings via electromagnetic induction principle can be calculated with Eq. (2) [14]. vðtÞ ¼ N
dfðtÞ dBðtÞ ¼ NAc dt dt
(2)
Fig. 6 e Isolated DC-DC converters circuits, (a) with two-level inverter (b) with three-level inverter. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Fig. 7 e Switching signal of the two-level inverter.
The maximum flux value over a period is defined with Eq. (3), because of the switch dead time (T0) [14]. Bm ¼
1 E T T0 2 NAc 2
(3)
frequency (f), voltage form factor (Kf), core cross-sectional area (Ac) [2,14]. N¼
E T0 1 Kf fAc Bm p
(4)
Thus, definition of the number of turns (N) is obtained as given in Eq. (4) in terms of DC supply voltage, switching
Fig. 8 e Switching signal of the three-level inverter. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Simulation and experimental results The proposed three-level isolated DC-DC converter for high power applications is modeled and simulated in AnsysSimplorer software. The proposed system consists of the DC power supply which is located instead of fuel cell, three-level inverter, driver circuitry for semiconductor power switches, a medium frequency transformer, an uncontrolled AC-DC rectifier and the load. In order to compare the inverter performance, same system is also designed with a single phase two-level inverter. The other parts of the system are kept same. The DC-DC converter circuit topologies with two-level and three-level inverters that used in the comparison are shown in Fig. 6. Simulations of the power electronics circuit are performed in Ansys-Simplorer software. The designed transformer in Ansys-Maxwell is linked to the Simplorer, and both the power electronics circuit and the designed transformer are co-simulated together. A PI controller is used to regulate the output voltage of the converter. Both the electrical and the electromagnetic performance of converters and medium frequency power transformer are investigated. The two-level inverter input supply voltage level is 40 V DC which is common minimum voltage level in fuel cell systems.
However, the conventional DC power supply is used in the study instead of fuel cell. The three-level inverter input voltage level is selected as twice to achieve the same power level. The switching frequency of inverters are 10 kHz. The transformer conversion ratio is 1:3 and this allows to transformer secondary voltage amplitude to be about 120 V. The switching signals for two-level inverter and three-level inverter are depicted in Fig. 7 and Fig. 8, respectively. Output voltage waveforms of both two-level and three-level inverters for 10 kHz switching frequency are shown in Fig. 9. This voltage levels are also primary voltages of the medium frequency transformer. As seen from the figure, the waveform of Fig. 9 (a) has a square wave structure while the waveform of Fig. 9 (b) has stepped. Thus, harmonic components of the three-level inverter supplied transformer's input voltage and current are lower due to this stepped structure. The total harmonic distortion (THD) level of three-level inverter primary voltage is calculated as 37% while the THD level of twolevel inverter primary voltage is 48%. Transformer primary input current waveforms of the twolevel and the three-level inverter conditions are given in Fig. 10. The peak values of the transformer primary current and the secondary current are about 189 A and 63 A, respectively.
Fig. 9 e Inverter output waveforms, (a) with two-level inverter (b) with three-level inverter. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Fig. 10 e Medium frequency transformer primer winding current waveforms, (a) with two-level inverter (b) with three-level inverter.
Fig. 11 e Transformer primary current harmonic spectrum, (a) with two-level inverter (b) with three-level inverter.
Fig. 12 e Converters output voltage ripples (a) with twolevel inverter, (b) with three-level inverter.
Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Table 3 e Comparison of two-levels and three-level inverter parameters. Parameters Converter Efficiency Transformer Efficiency Transformer Losses Total Losses Converter Output Voltage Ripples Transformer Input Current THD Transformer Input Voltage THD
Two-level inverter
Three-level inverter
81% 94% 450 W 1078 W 0.77 V 32% 48%
92% 96% 328 W 447 W 0.45 V 28% 37%
Harmonic spectrums of the inverters output current are shown in Fig. 11. The fundamental wave frequency of the harmonic spectrum is 10 kHz and the harmonic components have been computed up to 51. The total harmonic distortion (THD) of the current components are obtained up to these values. The THD level of three-level inverter primary current is obtained as 28% while the THD level of two-level inverter primary current is 32%. As seen from the figures, the three-level inverter output current, which is applied to the medium frequency transformer, contains less harmonic components when compared to the two-levels inverter current harmonics. The converters output voltage ripples for the two-level and three-level inverter based converters, are given in Fig. 12. The converters output voltage ripples are compared at 120 V DC voltage level. According to the simulation results, the amplitude of the output voltage ripple of the three-level based DCDC converter is smaller than two-level inverter based DC-DC converter. Also, three-level inverter based DC-DC converter ripple waveform smoother than two-level inverter based DCDC converter output voltage ripple. The data obtained in the simulation are shown in Table 3 comparatively. The nanocrystalline core material provides higher efficiency and reduced volume and weight. Besides, the primary current and voltage harmonics are reduced by using
three-level inverter, and thus the core and winding losses are reduced when compared with two-level inverter based system. It can be seen from the table that; efficiency of the medium frequency transformer is improved from 94% to %96 by using three-level inverter circuit. With contribution of the transformer efficiency, the three-level inverter based DC-DC converter efficiency is better than the two-level inverter based DC-DC converter structure. The three-level structure efficiency is measured as 92% while the two-level structure efficiency is measured as 81%. Amplitude of the converter output voltage ripple of the three-level inverter base DC-DC converter is also better than the DC-DC converter with conventional two-level inverter. All of these additives, we can say that three-level inverter based converter show better performance than two-level inverter based converter. The experimental studies are performed to validate analysis and simulation results. The proposed system consists of the three-level NPC inverter, medium frequency transformer with nanocrystalline core, uncontrolled rectifier unit and resistive load. The input DC voltage and the switching frequency of the isolated DC-DC converter is 48 V and 10 kHz, respectively. The primary and the secondary voltages of the designed transformer are given in Fig. 13. It is seen that, while the primary voltage is about 48 V, it is step up to 120 V by transformer, and these results are greatly meet with simulation results. It can be easily observed that the primary and secondary voltages of the proposed three-level inverter system have three voltage steps and they are closer to the sinusoidal waveform than conventional two-level inverter. Thus the primary current of the transformer is also closer to the sinusoidal waveform as given in Fig. 14. The FFT analysis of the transformer primary current is also depicted in the figure. It is seen that, the fundamental component of the current is at 10 kHz, and magnitudes of the harmonics components are limited. Since the high frequency harmonics components are limited, it can be easily said that, the core losses are also
Fig. 13 e The primary and the secondary voltage waveforms (Ch. 1 is secondary voltage and Ch. 2 is primary voltage). Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Fig. 14 e The primary current (Ch. 4) and its FFT (Math channel) waveforms.
reduced when compared to conventional two-level inverter fed condition.
of the two-level and three-level inverter based isolated DC-DC converters are measured as 0.77 V and 0.45 V respectively. Consequently, proposed three-level inverter based DC-DC converter is more suitable for fuel cell supplied systems.
Conclusions In this study, an isolated three-level DC-DC converter for high power and high conversion ratio applications with ratings of 40 V/120 V, 5 kW is designed and implemented. The proposed system consists of a DC power supply, a single phase threelevel inverter, a medium frequency transformer and a diode rectifier unit. The nanocrystalline is used as core material of the medium frequency transformer that has 1:3 conversion ratio. In the proposed system, a DC supply voltage is converted to a medium frequency AC voltage. This conversion is made both via three-level inverter and conventional two-level inverter. The switching frequency power and voltage levels are kept constant for two inverter topologies. Simulation studies are performed in Ansys-Simplorer software. Losses of the medium frequency transformer are calculated in Ansys Electromagnetic Suite for both inverter topologies. In addition, converter efficiency, transformer primary voltage and current harmonics and output voltage ripples are compared. In addition, simulation results are validated with experimental studies. It is seen from obtained results that THD value of primary voltage is reduced from 48% to 37% and THD value of primary current is reduced from 32% to 28% by using threelevel inverter. The three-level inverter structure ensures lower voltage harmonics in medium frequency transformer input, since it generates an AC waveform with multiple steps. In addition, both transformer and inverter loses are reduced and efficiency of the system is improved from 81% to 92%. Furthermore, the medium frequency transformer enables high voltage conversion ratio and galvanic isolation as well, and, the operating range of the DC-DC converter can be changed via turn ratio of medium frequency transformer for different design requirements. Besides, output voltage ripples
references
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Available online at www.sciencedirect.com
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Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer Saban Ozdemir a, Selami Balci b, Necmi Altin c,*, Ibrahim Sefa c a
Vocational School of Technical Sciences, Gazi University, Ankara, Turkey Ministry of National Education, Ankara, Turkey c Department of Electrical-Electronics Engineering, Faculty of Technology, Gazi University, Ankara, Turkey b
article info
abstract
Article history:
In this study, an isolated three-level DC-DC converter is proposed for high power and high
Received 15 November 2016
conversion ratio applications such as fuel cells. The proposed system consists of a single
Received in revised form
phase three-level inverter, a medium frequency transformer and a diode rectifier unit. In
28 January 2017
the proposed system, a DC supply voltage is converted to a medium frequency AC voltage
Accepted 23 February 2017
via a three-level inverter instead of the conventional two-level inverter. Since the three-
Available online xxx
level inverter generates an AC waveform with multiple steps, lower voltage harmonics and lower EMI levels than conventional two-level inverter are achieved. Thus, the three-
Keywords:
level inverter provides higher efficiency value. The medium frequency transformer en-
Medium frequency transformer
ables high voltage conversion ratio and provides galvanic isolation as well. The output
Three-level inverter
voltage of the medium frequency transformer is converted to the DC voltage and thus the
DC-DC converter
DC-DC conversion is achieved. According to simulation and experimental results, it is seen
Isolated converter
that the proposed DC-DC converter structure provides higher power density and higher efficiency values than conventional system. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Recently, number and power levels of the power electronics converters which become popular in residential, commercial and industrial applications have increased significantly. The key action in power electronics systems is switching, and higher switching frequency value provides more advantages. However, semiconductor power switches and inductive components limit their power levels. In general, while high switching frequency values can be applied in power converters which are under 10 kW, the medium frequency values are used for higher power levels. Therefore, high power
converter design is a challenge. Another problem for DC-DC converters is their limited conversion rates. Although nonisolated DC-DC converters are commonly used with their advantageous of being simple, their voltage conversion (step up or step down) ratio is limited. Therefore, isolated DC-DC converters which have a transformer embedded in converter structure are used in applications where high conversion ratio is required. They are also widely used in the structure of modern power systems such as electric vehicles and uninterruptible power supplies. The renewable energy sources such as PV modules and fuel cells generally generate lower voltage levels, and they require step up converters with high
* Corresponding author. Gazi University, Faculty of Technology, Department of Electrical-Electronics Engineering, 06500, Besevler, Ankara, Turkey. Fax: þ90 312 212 13 38. E-mail addresses: [email protected] (S. Ozdemir), [email protected] (S. Balci), [email protected] (N. Altin), isefa@gazi. edu.tr (I. Sefa). http://dx.doi.org/10.1016/j.ijhydene.2017.02.158 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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gain values to supply the loads and/or export generated energy to the grid. Recently, isolated DC-DC converter applications with embedded medium frequency transformer have been started to use in some kind of renewable energy source applications such as solar panels, wind turbines and fuel cells [1e3]. A medium frequency power converter consists of a transformer for different purposes. The size and volume of this transformer can be designed in a relatively small structure at medium frequency. In addition, next-generation grids and smart grids have become popular in recent years. In these systems, bi-directional and isolated DC-DC power converter topologies are designed for different purposes [4,5]. In such applications, power transformers have three functions: The first task is to provide safe operation with galvanic isolation via the magnetic coupling between the low voltage and high voltage windings. The second task is to adapt the supply voltage level to the load voltage level via conversion rate. Thus the voltage level obtained from different types of energy sources can be matched with the conversion ratio of the transformer. Third, the leakage flux in the medium frequency transformer primary side can be used as soft switching inductor element for series resonance circuit [6,7]. This type of DC-DC converters can also be designed as bi-directional. The isolated DC-DC converters are commonly discussed for different applications such as, renewable energy applications, traction applications and power supplies with their advantages [8e13]. The power converter with medium-frequency transformer topology which becomes popular in grid connected renewable energy applications is shown in Fig. 1. It is seen that, the main component in the structure of the converter circuit is isolated bi-directional DC-DC converter. This bi-directional DC-DC converter topology can be designed as single phase or threephase structure [1e4,11]. In this circuit, the input AC voltage level is converted to the DC voltage via a rectifier unit. Although, the rectifier unit is designed mostly in the form of diode rectifier type, it can be also designed as a controlled rectifier or a PWM rectifier. Then, obtained DC voltage is converted to a medium frequency AC voltage via a DC-AC inverter. This inverter supplies the primary windings of a medium frequency transformer. The secondary voltage of the medium frequency transformer is converted into DC voltage again via a rectifier circuit. Therefore, sinusoidal AC voltage generation is not required in inverter stage, and inverter is operated to generate medium frequency square wave voltage
waveform. Finally, in order to be connected in parallel to the grid, a DC/AC inverter is used for producing to the conditioned AC voltage on the grid conditions. Using the conventional two-level inverter in high power and high frequency power electronics applications is gathering number of problems such as limitations on current/ voltage values of semiconductor switches and high switching losses. In order to increase the power level of the converter, switches can be connected in series and/or parallel. However, the switches and circuits are not completely identical, and small time differences during the switching cycle may cause unbalances in voltage/current sharing. In order to overcome these problems, the multilevel inverter structure is developed. In these structure, it is possible to access high-power levels with standard power switches. Also, if the number of levels is increased then the harmonic components in output the voltage and/or current decrease [14e19]. Multilevel inverter structure has been studied in the literature for conventional inverters and converters. More recently, these type of converters have started to be studied in the DC-DC isolated converters. There are limited number of study in the literature about this subject. The core and winding losses of the medium frequency transformer are reduced by using multilevel inverter in isolated DC-DC converters. In addition, medium frequency power transformer design at high power level is another challenge in high power DC-DC converter design. Higher operating frequency values decrease the size of the transformer. However, the core material selection according to the operating frequency value has significant effect on both electrical and mechanical performance, volume and weight of the transformer. Soft magnetic materials such as amorphous, nanocrystalline and ferrites are common in medium and high frequency transformers [2]. These materials have different saturation flux density values at an operation frequency. Thus, the operation frequency and the core material become two most important design factors affecting the size and efficiency of the transformer [20]. The most important factor that determine the transformer performance is total power losses which compose of winding and core losses. The core losses are directly related with specific core loss value of the core material at the operation frequency. The operation frequency is also affects the AC resistance and thus winding losses of the transformer because of the skin and the proximity effects. In addition, non-sinusoidal excitation signals and harmonic components increase core losses [21,22]. Therefore, selection of both core and winding materials are
Fig. 1 e Power converter topology with medium frequency transformer. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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very important steps in transformer design. In the past literature, medium frequency power transformer designs have been proposed with silicon steel alloys for 2 kHz and lower operating frequencies [14,23]. In another study, the performance of the medium frequency power transformers embedded into the power system of the electric train with silicon steel with 1 kHz and with nanocrystalline with 5 kHz frequency values. It is reported that, the volume and weight of the power system designed with nanocrystalline core materials are about eight times lower than the same system with the silicon core material [23]. Additionally, the performances of the medium frequency transformer with ferrite and nanocrystalline core materials have been compared and it is reported that the transformer with nanocrystalline core is advantageous in terms of efficiency and volume [14,24e27]. Although, ferrite materials can operate at higher frequency levels, they suffer from their limited sizes in high power transformer designs [26]. Since, the nanocrystalline material can be manufactured at larger sizes, they are suitable for high power medium frequency applications. In this study, a DC-DC converter with an embedded medium frequency transformer is designed and analyzed with two different inverter structures for fuel cell applications. The input and output voltage levels of the proposed system are 40 V and 120 V, respectively. This obtained via transformer turn ratio The medium frequency transformer is designed according to determined design parameters by using AnsysElectromagnetic Suite. Besides, three-level inverter circuit is designed and modelled in Ansys-Simplorer and then both transformer and inverter models are simulated together. A PI regulator is used to regulate the output voltage of the DC-DC converter. The simulation results are validated with experimental studies. It is seen from both simulation and experimental results that, the proposed isolated DC-DC converter with three-level inverter circuit has better performances than the DC-DC converter with two-level inverter in terms of output voltage ripple, transformer and total system efficiency values. Thus, it can be easily said that, proposed isolated DCDC converter system is suitable for renewable energy systems such as PV modules and fuel cells.
Multilevel inverters Parallel to the increasing power demands, power levels of the inverters tend to increase with each passing day. However, increasing the current/voltage levels of the standard semiconductor power switches, which are used in inverter circuits, increase the system cost. In addition, power switches cannot be manufactured at very high voltage/current values. Another problem with high-power semiconductor switches is their limited switching frequency values. Parallel connection of inverters to increase power levels are not economical. Another solution is connecting switches in series/parallel to increase the power level. However, switches and driver's circuits are not completely identical, and these small differences may cause switching voltage/current sharing problems. Therefore, load current/voltage is not shared equally by the series/parallel connected switches. Multilevel inverters are an
3
economical and practical solution for high power applications. Although, the multilevel inverter requires more elements than ordinary inverter which is unappealing situation, it allows to design an inverter with standard and commonly used components. In the first years, the inclusion of more hardware than traditional inverters prevented the evolution of these inverters. Nowadays, multilevel inverter modules are currently being produced commercially; thus, low cost and compact inverter designs can be achieved. The multilevel inverter generates an AC waveform with multiple steps related with number of their levels. In addition to extending the output voltage range, it ensures lower voltage harmonics comparing to conventional inverter. So that, it provides better current and voltage waveforms. It has also better immunity against common-mode effect. In transformer-based systems, the transformer ratio can be reduced or the transformer requirement can be neglected. Furthermore, as a feature of multilevel structure, it has lower dv/dt and di/dt ratios, and therefore lower electromagnetic interference (EMI) levels than conventional two-level inverter can be achieved [28]. Moreover, for the same switching frequency, it requires a smaller filter. In multilevel inverters, the level concept is expressed as the number of output voltage level step values which changes according to the input level and number of switch. The symbolic status for the output voltage level and the switching states are shown in Fig. 2. If the number of output voltage levels is to be increased, the number of semiconductor switches and/or DC power supply need to be increased. Because of this, three-level inverters are used in many commercial low and medium voltage power applications. There are many multilevel inverter topologies introduced in the past literature. The most well-known topologies are the neutral point clamped (NPC), the cascaded Hbridge (CHB) and the flying capacitor (FC) inverter topologies as shown in Fig. 3 [29,30]. In the NPC structure, the neutral point is connected to the midpoint of the capacitors that divide the DC level into two parts. This is why the called as neutral point clamped. The disadvantage of this structure is the irregularities that occur when the voltages on the capacitor cannot be equalized. The FC structure is similar to the NPC structure. Here, the DC voltage is divided by the help of the capacitor connected to the inner switches. The disadvantage of this structure is the large capacity requirement of the capacitors that results bigger dimensions. The CHB structure is formed by serial connections of a classical two-level inverter. However, each two-level inverter structure must be supplied via an isolated source. The NPC inverter does not require an isolated power supply and their inverter modules are ready for commercial production. So that, it is preferred for many applications [29,30].
Proposed three-level isolated DC-DC converter It is well-known that, the size and the volume of transformer are affected significantly from the operating frequency. The size of the grid frequency transformer is very large when compared to the high-frequency transformer designed for the same power and voltage levels. Since, it is designed for the
Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Fig. 2 e Illustration of multilevel concept for an inverter leg, a) Two-level inverter b) Three-level inverter c) n-level inverter.
medium frequency range (400 Hz - 20 kHz), an isolated DC-DC converter's transformer size and volume are small [20]. Besides the electrical parameters, the core material is also affects mechanical parameters. The power capacity, both
winding and core losses, temperature rise and voltage regulation values are related with specific parameters of the core material. In addition, mechanical parameters such as power/ weight ratio (kVA/kg) are related with core saturation flux
Fig. 3 e Most used multilevel converter topologies, a) NPC converter b) FC converter c) CHB converter. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Table 2 e Designed medium frequency power transformer specifications.
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kVA/kg
Parameter
Value
Input Voltage Level Output Voltage Level Nominal Power Operating Frequency Flux Density Conversion Ratio Current Density Number of Turns of PrimereSecondary Winding Winding Material
2
0
Vitroperm250F 2605sa1
Ferrite-N87
6.5%SiFe
Fig. 4 e Power density (kVA/kg) ratings of different core materials.
density and specific core losses besides operation frequency. The core material type also affects the winding volume; therefore, determination of the core material has significant effects on transformer performance [20]. Different soft magnetic materials can be used as core material of medium and high frequency transformer. Cost and accessibility of the core material also affect the design strategy of the transformer. On the other hand, the shape of the excitation voltage waveform at high switching frequency value increases both the core and the coil losses. This situation deteriorates the efficiency and results in negative effects such as increasing temperature. Therefore, high frequency transformers are designed by using soft magnetic materials as core material due to the lower specific core loss values. Medium frequency power transformer's core material type affects both mechanical and electrical performance. Soft magnetic materials, such as amorphous, ferrite, nanocrystalline are usually used in medium frequency transformer design. The operating frequency, the desired current density and the specific core loss values which are main variables determine the core material. The power/weight ratio (kVA/kg) of a transformer for a certain power and frequency value with different core materials such as non-oriented silicon steel, ferrite, nanocrystalline and amorphous is given in Fig. 4. This values are obtained for 35 kVA power and 10 kHz operation frequency values by using manufacturer datasheet values,
40 V 120 V 5 kVA 10 kHz 0.3 T 1:3 2 A/mm2 12e36 Aluminum foil (100 0.4 mm) Nanocrystalline 0.2 4
Core Material Windows Usage Coefficient Waveform Coefficient
and properties of 6.5% Si-Fe (as non-oriented silicon steel), N87 (as ferrite), 2605sa1 (as amorphous) and Vitroperm250F (as nanocrystalline) are used [31e33]. As it can be seen from Fig. 4, non-oriented silicon steel core material has worst performance at 10 kHz operation frequency. Therefore, this material is usually used in transformers whose operation frequency is below 2 kHz. Although saturation flux density of the amorphous material is higher than nanocrystalline and ferrite materials, their specific core loss value is also higher than these materials. The ferrite material can be used in high frequency values, but their saturation flux density is 2.5 times lower than nanocrystalline material. This results bigger core size. In addition, their limited size limits their usage in high power applications. Furthermore, there is significant difference between the performances of the nanocrystalline and amorphous core materials in terms of noise at high frequency values. Therefore, amorphous material is not useful for over 4 kHz operation frequencies. Specific properties of these core materials are given in Table 1 to compare them easily [20,31e34]. As it is seen, among these materials, the nanocrystalline core material provides optimum performance in terms of weight, volume and efficiency, allows more efficient and compact medium frequency transformer designs. In this study, a medium frequency power transformer that is sized according to the specifications given in Table 2, is designed. The transformer is modeled in Ansys-
Table 1 e General specific properties of the soft magnetic material. Specific Parameters
Material Code Saturation Flux (Tesla) Permeability (m) Coercivity Force (A/m) Mass Density (kg/m3) Curie Temperature ( C) Thickness (mm) Core Loss (W/kg), (at 10 kHz; 0.1 T) Magnetostriction Coefficient (ppm)
Material Type Nanocrystalline
Amorphous
Ferrite
Si-Fe
Vitroperm250F 1.23 5000e7000 0.8 7200 510 0.018 0.35 0.1
2605sa1 1.56 1300e2000 4 7180 395 0.025 2.5 27
N87 0.39 1800e2200 21 4850 210 block 1.5 0.6
6.5%SiFe 1.87 800e1000 30 7650 700 0.05 8 3
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Electromagnetic Suite software. Three-dimensional view of the designed transformer is shown in Fig. 5. In the medium frequency transformer mechanical sizing, Ap which is multiplication of core cross-sectional area (AC) and window region area (Wa) is used as base parameter. In addition, other parameters such as transformer nominal power (VA), operating frequency (f), amplitude of the flux density (Bm), current density of windings (J), windows usage coefficient (Kcu), and exciting voltage waveform coefficient (Kf) are other used parameters. According to the given power level, Eq. (1) can be used to determine the core size. This equation is a very useful mathematical expression to determine the power capacity of the transformer [35]. Ap ¼ Ac ,Wa ¼
Fig. 5 e The designed medium frequency transformer with nanocrystalline core.
VA,104 Kf Kcu Bm fJ
(1)
The number of turns of primary and secondary winding, and thus transformer conversation ratio of the transformer should be defined to obtained desired converter output voltage. The voltage induced at transformer windings via electromagnetic induction principle can be calculated with Eq. (2) [14]. vðtÞ ¼ N
dfðtÞ dBðtÞ ¼ NAc dt dt
(2)
Fig. 6 e Isolated DC-DC converters circuits, (a) with two-level inverter (b) with three-level inverter. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Fig. 7 e Switching signal of the two-level inverter.
The maximum flux value over a period is defined with Eq. (3), because of the switch dead time (T0) [14]. Bm ¼
1 E T T0 2 NAc 2
(3)
frequency (f), voltage form factor (Kf), core cross-sectional area (Ac) [2,14]. N¼
E T0 1 Kf fAc Bm p
(4)
Thus, definition of the number of turns (N) is obtained as given in Eq. (4) in terms of DC supply voltage, switching
Fig. 8 e Switching signal of the three-level inverter. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Simulation and experimental results The proposed three-level isolated DC-DC converter for high power applications is modeled and simulated in AnsysSimplorer software. The proposed system consists of the DC power supply which is located instead of fuel cell, three-level inverter, driver circuitry for semiconductor power switches, a medium frequency transformer, an uncontrolled AC-DC rectifier and the load. In order to compare the inverter performance, same system is also designed with a single phase two-level inverter. The other parts of the system are kept same. The DC-DC converter circuit topologies with two-level and three-level inverters that used in the comparison are shown in Fig. 6. Simulations of the power electronics circuit are performed in Ansys-Simplorer software. The designed transformer in Ansys-Maxwell is linked to the Simplorer, and both the power electronics circuit and the designed transformer are co-simulated together. A PI controller is used to regulate the output voltage of the converter. Both the electrical and the electromagnetic performance of converters and medium frequency power transformer are investigated. The two-level inverter input supply voltage level is 40 V DC which is common minimum voltage level in fuel cell systems.
However, the conventional DC power supply is used in the study instead of fuel cell. The three-level inverter input voltage level is selected as twice to achieve the same power level. The switching frequency of inverters are 10 kHz. The transformer conversion ratio is 1:3 and this allows to transformer secondary voltage amplitude to be about 120 V. The switching signals for two-level inverter and three-level inverter are depicted in Fig. 7 and Fig. 8, respectively. Output voltage waveforms of both two-level and three-level inverters for 10 kHz switching frequency are shown in Fig. 9. This voltage levels are also primary voltages of the medium frequency transformer. As seen from the figure, the waveform of Fig. 9 (a) has a square wave structure while the waveform of Fig. 9 (b) has stepped. Thus, harmonic components of the three-level inverter supplied transformer's input voltage and current are lower due to this stepped structure. The total harmonic distortion (THD) level of three-level inverter primary voltage is calculated as 37% while the THD level of twolevel inverter primary voltage is 48%. Transformer primary input current waveforms of the twolevel and the three-level inverter conditions are given in Fig. 10. The peak values of the transformer primary current and the secondary current are about 189 A and 63 A, respectively.
Fig. 9 e Inverter output waveforms, (a) with two-level inverter (b) with three-level inverter. Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Fig. 10 e Medium frequency transformer primer winding current waveforms, (a) with two-level inverter (b) with three-level inverter.
Fig. 11 e Transformer primary current harmonic spectrum, (a) with two-level inverter (b) with three-level inverter.
Fig. 12 e Converters output voltage ripples (a) with twolevel inverter, (b) with three-level inverter.
Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Table 3 e Comparison of two-levels and three-level inverter parameters. Parameters Converter Efficiency Transformer Efficiency Transformer Losses Total Losses Converter Output Voltage Ripples Transformer Input Current THD Transformer Input Voltage THD
Two-level inverter
Three-level inverter
81% 94% 450 W 1078 W 0.77 V 32% 48%
92% 96% 328 W 447 W 0.45 V 28% 37%
Harmonic spectrums of the inverters output current are shown in Fig. 11. The fundamental wave frequency of the harmonic spectrum is 10 kHz and the harmonic components have been computed up to 51. The total harmonic distortion (THD) of the current components are obtained up to these values. The THD level of three-level inverter primary current is obtained as 28% while the THD level of two-level inverter primary current is 32%. As seen from the figures, the three-level inverter output current, which is applied to the medium frequency transformer, contains less harmonic components when compared to the two-levels inverter current harmonics. The converters output voltage ripples for the two-level and three-level inverter based converters, are given in Fig. 12. The converters output voltage ripples are compared at 120 V DC voltage level. According to the simulation results, the amplitude of the output voltage ripple of the three-level based DCDC converter is smaller than two-level inverter based DC-DC converter. Also, three-level inverter based DC-DC converter ripple waveform smoother than two-level inverter based DCDC converter output voltage ripple. The data obtained in the simulation are shown in Table 3 comparatively. The nanocrystalline core material provides higher efficiency and reduced volume and weight. Besides, the primary current and voltage harmonics are reduced by using
three-level inverter, and thus the core and winding losses are reduced when compared with two-level inverter based system. It can be seen from the table that; efficiency of the medium frequency transformer is improved from 94% to %96 by using three-level inverter circuit. With contribution of the transformer efficiency, the three-level inverter based DC-DC converter efficiency is better than the two-level inverter based DC-DC converter structure. The three-level structure efficiency is measured as 92% while the two-level structure efficiency is measured as 81%. Amplitude of the converter output voltage ripple of the three-level inverter base DC-DC converter is also better than the DC-DC converter with conventional two-level inverter. All of these additives, we can say that three-level inverter based converter show better performance than two-level inverter based converter. The experimental studies are performed to validate analysis and simulation results. The proposed system consists of the three-level NPC inverter, medium frequency transformer with nanocrystalline core, uncontrolled rectifier unit and resistive load. The input DC voltage and the switching frequency of the isolated DC-DC converter is 48 V and 10 kHz, respectively. The primary and the secondary voltages of the designed transformer are given in Fig. 13. It is seen that, while the primary voltage is about 48 V, it is step up to 120 V by transformer, and these results are greatly meet with simulation results. It can be easily observed that the primary and secondary voltages of the proposed three-level inverter system have three voltage steps and they are closer to the sinusoidal waveform than conventional two-level inverter. Thus the primary current of the transformer is also closer to the sinusoidal waveform as given in Fig. 14. The FFT analysis of the transformer primary current is also depicted in the figure. It is seen that, the fundamental component of the current is at 10 kHz, and magnitudes of the harmonics components are limited. Since the high frequency harmonics components are limited, it can be easily said that, the core losses are also
Fig. 13 e The primary and the secondary voltage waveforms (Ch. 1 is secondary voltage and Ch. 2 is primary voltage). Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158
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Fig. 14 e The primary current (Ch. 4) and its FFT (Math channel) waveforms.
reduced when compared to conventional two-level inverter fed condition.
of the two-level and three-level inverter based isolated DC-DC converters are measured as 0.77 V and 0.45 V respectively. Consequently, proposed three-level inverter based DC-DC converter is more suitable for fuel cell supplied systems.
Conclusions In this study, an isolated three-level DC-DC converter for high power and high conversion ratio applications with ratings of 40 V/120 V, 5 kW is designed and implemented. The proposed system consists of a DC power supply, a single phase threelevel inverter, a medium frequency transformer and a diode rectifier unit. The nanocrystalline is used as core material of the medium frequency transformer that has 1:3 conversion ratio. In the proposed system, a DC supply voltage is converted to a medium frequency AC voltage. This conversion is made both via three-level inverter and conventional two-level inverter. The switching frequency power and voltage levels are kept constant for two inverter topologies. Simulation studies are performed in Ansys-Simplorer software. Losses of the medium frequency transformer are calculated in Ansys Electromagnetic Suite for both inverter topologies. In addition, converter efficiency, transformer primary voltage and current harmonics and output voltage ripples are compared. In addition, simulation results are validated with experimental studies. It is seen from obtained results that THD value of primary voltage is reduced from 48% to 37% and THD value of primary current is reduced from 32% to 28% by using threelevel inverter. The three-level inverter structure ensures lower voltage harmonics in medium frequency transformer input, since it generates an AC waveform with multiple steps. In addition, both transformer and inverter loses are reduced and efficiency of the system is improved from 81% to 92%. Furthermore, the medium frequency transformer enables high voltage conversion ratio and galvanic isolation as well, and, the operating range of the DC-DC converter can be changed via turn ratio of medium frequency transformer for different design requirements. Besides, output voltage ripples
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Please cite this article in press as: Ozdemir S, et al., Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.158