dc-dc converter stage of fuel cell based grid integration

8/11/2019 DC-DC converter stage of fuel cell based grid integration

1/22

1

CHAPTER 1

Fuel CellAs the fossil fuels are getting exhausted and also due to the troubles caused by the by products of fossilfuels, we are forced to search for an alternate source of energy. Fuel cells are being considered one of themost promising sources of clean energy in future. Fuel cells have higher efficiency than conventional

power plants and they produce less noise and they have no issues of polluting the environment by their by products. Thermal energy produced by them during the process of electricity production can be used forheating.

Fuel cell is a device which produces electricity through the electrochemical process and is very similar tothe battery. The main difference between battery and fuel cell is that fuel cell can continue supplying

electricity as long as fuels are being supplied to the device.They have been used commercially in space shuttles to provide electricity. These days they areincreasingly being used in automobiles to supply energy for electric cars. In future their use can be spreadto many other areas like utility vehicles, trains, motorcycles and even in power plants.

Figure 1. Schematic of a Fuel Cell

Fuel cells provide dc voltage which has a wide variation and the magnitude of voltage is also very low.So, to get a constant output AC voltage at the level which most of our residential load require (which is120V/240V) we need a converter which can take input voltage over a wide range and convert it to aconstant high voltage DC output which can then be finally converted into 120 V/240 V AC with the helpof rectifiers. The constant high voltage DC output is generally 400 V.

Response time of the fuel cell is very slow during changes in power demand of the load. The fuel cellcant provide the changed power demand im mediately since the response time is from few seconds to fewminutes. To circumvent this problem a battery or a high voltage rating capacitor is required to provide


2/22

2

energy to the load during which fuel cell can make necessary adjustment to increase or decrease its

power to the level required by the changed load,

A DC-DC converter design has been proposed by authors M.H.Todorovic, L. Palma, P.Enjet in the papertitled Design of a Wide Input Range DC -DC Converter with a Robust Power Control Scheme Suitable for

Fuel Cell Power Conversion. The DC -DC converter proposed in this paper consists of two stages whichboosts the widely varying dc output voltage of fuel cell into a constant voltage DC output and alsoinvolves a ultracapacitor to improve the dynamics of the converter in case of sudden power demand

changes by the load.

A detailed analysis of the above mentioned converter has been given in this report. Given converter has

also been simulated in PLECS software module to obtain and verify the outputs.


3/22

3

CHAPTER 2

Converter Design

Figure 2. Complete Block Diagram of the System

Above given block diagram for the converter has been proposed for a 5 KW fuel cell power conditioner.The DC input voltage varies from 42 V to 60 V and the DC output voltage is kept at 400 V.

The input DC voltage from the fuel cell is first boosted to the value of 80 V DC using a three-level boostconverter. The 80 V DC output then converted into 400 V DC with the help of two inductor isolated boostconverter. An ultracapacitor is also placed after the three-level boost converter to improve the responsetime of the DC-DC converter.

The efficiency of the high frequency transformers which are placed in the converter depend on theconsistency of the input voltage. The transformer should be such that it should support the minimum inputvoltage to get the desired output. Since most of the time input voltage is much larger than minimumvalue, transformers work on very less efficiency. To better utilize the transformer, in this converter, athree-level boost converter has been provided which always provides 80 V DC to the transformer, one moreadvantage of using this converter topology is that since input voltage of the DC-DC converter is higher,the current ratings of switches can be much lower, which will reduce the cost of the system.

DC-DC converter topology

As has been discussed before, we need a converter which can boost and regulate the widely varying

output voltage of fuel cell as well as provide high efficiency and galvanic isolation. A detailed schematicof the converter topology is given below:


4/22


5/22

5

In the case where input voltage is higher that half of the output voltage, the inductor charging voltage can be chosen to be V in-Vo/2 and the discharging voltage will be V o-V in.

In the case of converter described in this report, the input voltage is always higher than half of the outputvoltage because when maximum power is delivered the input voltage is equal to its nominal value of 42V.Hence the boost inductor charging voltage can be shown to be V

capacitor -V

o/2 and the discharge voltage to

be V capacitor -V in which results in current waveform shown below.

Figure 4. Gating Signals of the parallel three level boost converter [1]

The duty cycle of the first stage of the converter is given by

In the mode of operation where output voltage is higher than half of output voltage, the current ripple ismaximum for D=0.5, which corresponds to the input voltage of Vin = 0.75V capacitor . The maximum ripplecurrent is given by

The maximum ripple obtained corresponds to the one fourth of the maximum current ripple ofconventional boost converter.

Secondary boosting converter

The second stage boosting is done by a converter consisting of two inductors to bring the voltage to itsfinal value. This converter has two coupled inductors and the alternating operation of switches to get a

primary voltage gain. The galvanic isolation between the input and the output of the converter, along withadditional boosting, is provided by the transformer.

The voltage gain of the converter is obtained from

Where n1 and n2 are the primary and secondary total turns ratio of the transformer and D is theconverters duty cycle .


6/22

6

Figure 5. Illustration of the working of the two inductor isolated boost converter [1]

A synchronous rectifier is used on secondary side to rectify the voltage and increase the efficiency of

converter. The gating signals for output rectifier switch s1 and s2 are complimentary of the gatingsignals for the primary side switches s1 and s2. Operation of the converter can be divided into four stagesas shown in the above figure. The first stage is shown in a part of the figure, where two primary switchess1 and s2 are closed and rectifier switches s1 and s2 are kept open. In this case the current I1 and I2through inductors L1 and L2 charge the inductor and the load is being supplied by the energy stored in thecapacitor. In the second state switch s1 is opened and switch s2 is kept closed, during this state inductorsL1 and L2 supply the required energy to the primary side to transformer T2. Voltage across the lowertransformer secondary winding is positive with res pect to ground so switch s1 is closed to supply theload and charge the output capacitor. During the third state, s1 is closed again to allow the inductorscurrent to build up and the load is supplied by the output capacitor. In the fourth state switch s2 is openedwhile s1 is kept closed allowing current to flow in the opposite direction as in the second state. This hasthe effect of producing a positive voltage across the secondary winding, thus s2 is closed while s1 iskept open to supply the load and output capacitor.


7/22

7

CHAPTER 3

Simulation of Parallel Three Level Boost Converter

Aim: To step up 40 V input DC voltage to 80 V DC output voltage using a parallel three level boostconverter. The PLECS schematic is shown below

Figure 6. PLECS schematic of parallel three level boost converter

The converter duty cycle required for the above requirement is

Thus the duty cycle for the gating signal of each switch is less than 50%. The duty cycle of each switchcomes out to be 0.95/2 = 0.475. The switching frequency is 100 kHz.

Considering a peak to peak ripple of 1A in the inductor current, the value of the two inductance iscalculated as


8/22

8

The plot of output voltage and inductor current vs time is given below

Figure 7. Plot of inductor current and output voltage waveforms

Figure 8. Gating signals for the two switches

Thus it can be seen that for DC input voltage of 42 V, DC output voltage of 80 V is obtained.


9/22

9

CHAPTER 4

Simulation of Isolated Two Inductor Boost Converter

Aim: To step up 80 V input DC voltage to 400 V DC output voltage using a isolated two inductor boostconverter.

The PLECS schematic is shown below

Figure 9. PLECS schematic of isolated two inductor boost converter

The converter duty cycle required for the above requirement is

Where, the ratio of n1/n2 is 0.5. This duty cycle is the overlapping time of the gating signals of the twoswitches S1 and S2. Switches S1 and S2 are complementary to S1 and S2 respectively. The switchingfrequency of the converter is 20 kHz. To achieve an overlapping time of 20% between the two gatingsignals, the duty ratio of the gating signals of the individual switches comes out to be 0.6.

Considering a zero to peak ripple of 0.5A in the inductor current, the value of the two inductances iscalculated as


10/22

10

Where I ripple is the zero to peak ripple and D is the individual duty cycle of the two gating signals of theswitches.

The plots of the simulation is given below

Figure 10. Plot of secondary current and output voltage waveforms

Figure 11. Inductor current and Primary voltage waveforms


11/22

11

Figure 12. Gating signals of switches S1 and S2

Figure 13. Illustration of the ripple in the output voltage

If the output resistance is increased then the ripple in the output voltage is also decreased. For examplesimulating the above PLECS schematic with a 100 ohm output resistance the peak to peak ripple in theoutput voltage is just 0.2 V peak to peak.



12/22

12

CHAPTER 5

Simulation of the full DC-DC Converter

Aim: To step up 42 V input DC voltage to 400 V DC output voltage using a parallel three level boostconverter as first stage and a isolated two inductor boost converter as the second stage.

The PLECS schematic of the full DC-DC converter is given below,

Figure 14. PLECS schematic of the full DC-DC converter

In this schematic the parallel three level boost converter and the isolated two inductor boost converter areconnected in series through an ultracapacitor. The value of the ultracapacitor is taken as 100 . Thevoltage rating of the ultracapacitor is very high. The first stage boosts the fuel cell output voltage to 80 V.The second stage boosts the 80 V to 400 V. The output resistance of this DC-DC converter is taken as 100ohms in this case in order to decrease the ripple in the output voltage which is 400 V.


13/22

13

The plots for the simulation of the above PLECS schematic is given below

Figure 15. Output voltage and secondary current waveforms

Figure 16. First stage inductor current, capacitor current and output voltage (ultracapacitor voltage)waveforms


14/22

14

Figure 17. Second stage inductor current and primary voltage waveforms

Figure 18. Second stage secondary current and its CCA waveform


15/22

15

Figure 19. First stage switch Sb1 current and its CCA waveforms

Figure 20. Second stage switch S2 current and its CCA waveforms



16/22

16

CHAPTER 6

Simulation of the full DC-DC Converter with Duty Cycle Control

Aim: To step up wide varying range of input DC voltage (42 60 V) to 400 V DC output voltage using a parallel three level boost converter as first stage and a isolated two inductor boost converter as the secondstage.

The theory is that by controlling the output voltage of the first boosting stage and keeping it a constant 80V DC, any change in input voltage would produce a constant 400 V DC output voltage. This can beimplemented by controlling the duty cycle of the gating signals of each switch in the first or primary

boosting stage. In this simulation the duty cycle of the gating signals of each switch (Sb1, Sb2, Sb3 andSb4) of the first stage, parallel three level boost converter, is controlled by mathematically implementingthe expression of duty cycle of the three level boost converter in PLECS. The expression for the firststage converter duty cycle is again given below

The PLECS schematic of the full DC-DC converter is given below,

Figure 21. PLECS schematic of the full DC-DC converter with duty cycle control


17/22

17

The simulation plots are given below

Figure 22. Output voltage, input voltage and ultracapacitor voltage waveforms

Figure 23. Duty cycle variation with varying input voltages


18/22

18

Figure 24. Transients in output voltage and ultracapacitor voltage during input voltage variation

Thus it can be seen that for DC input voltage of 42 V 60 V, DC output voltage of 400 V is obtaineddespite the variations in input voltages.


19/22

19

CHAPTER 7

Controller Design

The output voltage of the DC-DC converter should be a constant 400 V DC. In order to design thecontroller for the full DC DC converter, first a small perturbation is given to the system and thefrequency response of the system is observed. The PLECS schematic to achieve the frequency response ofthe system when a perturbation is induced is given below

Figure 25. PLECS schematic of the full DC-DC converter to obtain the frequency response

The bode plots obtained from the above simulation is given below


20/22

20

Figure 26. Bode plots

The cross over frequency is chosen as 10 kHz, which is one-tenth of the switching frequency of the primary boosting stage.

Using the K- factor method to design the controller, the phi system at 10 kHz is -70 degrees. The phi boost required is given by,

Thus the boost required is 40 degrees. Thus the controller required is type II controller.

For type II controller,

( )

Thus the type II controller is of the form,

The value of K c is obtained by observing the magnitude of the system at crossover frequency. Thus K c isinverse of the gain obtained above.


21/22

21

CONCLUSIONThe proposed new DC DC converter topology has been successfully simulated in PLECS. Theconverter is able to step up the voltage from 42 V 60 V DC to 400 V DC. Simulation plots of the

parallel three level boost converter, the isolated two inductor boost converter and the full DC DCconverter are shown. Simulation results indicate that the system has a good performance andcharacteristics under changes in input voltage which models the variation in fuel cell voltage according tothe changes in load. In this variation in fuel cell voltage, the output voltage is held constant at 400 V bythe DC DC converter. The transfer function of the controller of the system has also been calculated inthis project work. This controller will ensure that the output voltage remains at 400 V DC for any changesin the system.


22/22

22

References[1] M. H. Todorovic, L. Palma, P. Enjeti , Design of a wide input range DC -DC converter with a robust

power control scheme suitable for fuel cell power conversions, Nineteenth Annual IEEE AppliedPower Electronics Conference and Exposition, APEC 2004, vol. 1, pp. 374 379.

[2] J. Yungtaek, M . Jovanovic, New two inductor boost converter with auxiliary transformer,Proceedings of the APEC, March 2002, vol. 2, pp. 10 14.

dc-dc converter stage of fuel cell based grid integration

Documents