AC Motor Drive Fed by Renewable Energy Sources with PWM
J. Pavalam1; R. Ramesh Kumar2; R. Mohanraj3; K. Umadevi4
3
1
PG Scholar, M.E-Power electronics and Drives Excel College of Engineering and Technology
pavalam.eee@gmail.com
2
PG Scholar, M.E-Power electronics and Drives Excel College of Engineering and Technology
pbrameshkumar@gmail.com
Associative Professor, Head of the Department/ EEE , Excel College of Engineering and Technology
ciemohanraj@gmail.com
4
Excel College of Engineering and Technology
Uma1raj@gmail.com
Abstract - In this fast approaching nature of technology the need of Electricity
becomes a mandatory in developing technology. The need of Electricity increases
the power demand where the power demand met by the conventional sources of
energy has some disadvantage of pollution, this disadvantage can be decreased by
the use of the Renewable energy sources like Fuel Cell and available solar energy.
When a FUEL cell produces AC power, basically two stages are required for
conversion first a boosting stage and second is inversion stage. In this paper the
Boost inverter topology is achieved where in the conventional methods the normal
DC - AC power conversion method is used where as in this paper the PWM based
DC - AC inverter has been used which is useful in reducing the harmonics in the
output of the Inverter. The voltage controlled output is produced in the boost
inverter the current controlled output is taken from dc-dc bidirectional converter.
The Fuel cell cannot be relied as a whole so a Solar PV module is connected across
the Load so while the Sunlight days the PV arrays generate power and in the night
time the Fuel cell is used to generate power for the load. Since, the Fuel cell and
PV arrays can generate power in Partial load they are preferred than any other
sources. When the output from the Solar PV array is low or when the sunlight
available is not efficient in generating the power a automatic switch over is
provided in the junction between the Solar PV array and Fuel cell so that whenever
it happens the switch automatically switch over to another source. The simulation
results are presented to confirm the operational feature of the proposed system.
Key Words - PWM Boost-inverter; fuel cell; fuel cell power conditioning system;
renewable energy.
1
Introduction
Electricity is the most well-known energy transporter. An energy transporter is a substance or, system
that moves power in a functional form from one position to one more. Electricity was generated in
influence plants, in which a principal energy source is changed into electrical power. Case of widely
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vigor sources are fossil fuels, falling or flowing dampen and nuclear fission. An important drawback of
generating electricity from fossil fuels and nuclear fission.
Most common primary energy sources for electricity generation worldwide is the adverse
environmental impact, such as the greenhouse effect caused by the increase of the CO2 concentration
in the earth’s atmosphere and the nuclear waste problem. Further, fossil fuel and uranium reserves are
finite [1].
The output of the FUEL cell would be low more over the FUEL cell response would be very low when
compared to the other conventional sources. Thus, a battery backup unit is added with the FUEL cell
for improving the response time of the FUEL cell as well as to protect the FUEL cell from the over
voltage or Transient Voltages.
The double stage power production in FUEL cell is considered to be a bulky one thus it contains a
separate circuit for Boosting process and also it contains separate circuit for Inverting process. Thus, to
replace the Bulkier circuits with a simpler one the Single stage power processing is used in the FUEL
cell.
The objective of this paper is to propose an FC energy system with the lowest possible harmonics. In
particular, the proposed system, based on the boost-inverter with a backup energy storage unit, solves
the aforementioned problems, i.e., the low and variable output voltage of the FC and its slow response.
The boost-inverter utilizes two identical bidirectional boost converters with a PWM and delivers in a
single-stage boosting and inversion functions [2]. This results in a high power conversion efficiency,
reduced converter size, and low cost. Additionally, the backup unit supplies the low-frequency current
harmonics, hence minimizing the stresses on the FC, if it were to supply such currents. The control of
the boost-inverter is moderately complex to handle, and sliding-mode control or double-loop voltage
and current control schemes may be adopted in this system.[2]
2
Topology and Control Schemes
In our conventional method of Boost inverter the controlling method or triggering method used is the
normal pulse generation method. In this method of triggering the output may contains many harmonics
which may leads to poor power factor and also to a very low efficiency of the operating FUEL cell.
Even though the FUEL cell processing stage is reduced, due to the presence of harmonics in the output
the output voltage becomes inefficient. The conventional equivalent circuit of the FUEL CELL is
shown in the Fig.1
The popular FC energy system consists of two power converters: the main boost-inverter (Normal)
and the extra backup unit, as shown in Figs. 2 and 3. The output of the boost-inverter is connected to
the load, while the input side is supplied by the FC and the backup unit, and both are connected to the
same unregulated dc bus. The backup unit incorporates a current-mode-controlled bidirectional boost
converter with battery-based energy storage to support the FC power generation and two voltagecontrolled boost converters making up the boost-inverter stage.
The FC energy system must dynamically adjust to varying input voltage while maintaining constant
power operation. Voltage and current limits, which have been provided from manufacturer in Fig. 4,
need to be imposed at the input of the converter to protect the FC from damage due to excessive
loading and transients.
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Fig.1: Equivalent circuit of FUEL CELL
The power has to be ramped up and down so that the FC can react appropriately, avoiding transients
and extending its life. The converter also has to meet the maximum ripple current requirements of the
FC.
3
Modes of Control
There are two modes of control available practically for robust control of the Boost inverter. Namely:
1.
Sliding mode control
2.
Double loop control scheme.
The Sliding mode control theory is applied to a sinusoidal output voltage boost inverter with linear
load. The boost inverter can be used in UPS design, where a second power conversion stage is not
needed. [8]
A sliding mode controller applied to the DC - AC boost converter achieved stability with respect to
load parameter variation and good static behavior. The controller has a fast dynamic response, since all
control loops act concurrently, and the robustness inherent to sliding mode control. The fig 5 shows
the sliding mode control of Boost Inverter.
In this case, the boost inverter operate with variable frequency, switching frequency varies depending
on the working point. By means of this controller, the converter generates a sinusoidal output voltage
with a total harmonic distortion lower than 2% [7].
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Fig.2: FC energy system consisting of the boost-inverter and a backup unit.
Boost DC-AC inverter naturally generates in a single stage an AC voltage whose peak value can be
lower or greater than the DC input voltage. The main downside of this construction deals with its
influence. Boost inverter consists of Boost DC-DC converters that have to be controlled in a variableoperation point condition. The sliding mode control has been planned as an option to control the
output voltage of the inverter. Nevertheless, it do not openly control the inductance averaged-current.
The double-loop regulation scheme that consists of a new inductor current control inner loop and an
also new output voltage control outer loop. These things contain compensations in order to survive
with the boost variable operation point condition and to achieve a high robustness to both input
voltage and output current disturbances. The double loop regulation control strategy achieves a very
high unswerving performance, even in complicated momentary situations such as nonlinear masses,
quick load changes, short circuits, etc., which sliding mode be in charge of cannot handle with. The
Figs. 6 and 7 shows the current controlled loop and Voltage controlled loop of Double loop control
scheme respectively.
Fig. 3: Block Diagram of Existing FC system with Normal Boost Inverter and Battery Backup unit
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Fig.4: Illustration of the beginning of life (BOL) polarization characteristics of the Nexa1.2-kW
PEMFC power module: voltage-current and power-current characteristics with parasitic power
graph
Fig.5: Sliding mode control of Boost Inverter
Fig.6: Current controlled Double loop control scheme
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Fig.7: Voltage controlled Double loop control scheme
4
4.1
Proposed FC System
Description of the system
The proposed FC system consists of two power converters: the main PWM Boost inverter and extra
Back up unit. The input of the PWM Boost inverter is connected to the output of the FC system. The
Output of the FC system consists of a Battery backup unit in parallel to it where it is used for
increasing the output response of the FC system.
The proposed Fuel Cell has the power regulation by the action of the PWM Boost Inverter. The Boost
inverter present in the output side of the FC would consists of two inverter where the one converter is
Boost converter and another one is normal DC - AC inverter. The input of the PWM inverter consists
of 30 V as input and 250 V as an output. The PWM inverter is used for reducing the ripples present in
the output of the FC.
4.2
Backup Unit
The functions of the backup unit should be divided into two parts. First, the backup unit is designed to
support the slow response of the FC and is shown in Fig. 3. Second, in order to protect the FC system,
the backup unit provides low-frequency AC current that is required from the boost-inverter operation.
The backup unit comprises a current-mode-controlled bidirectional boost converter and a battery as the
energy storage medium. For instance, when a 1-kW load is added from a no-load condition, the
backup unit immediately provides the 1-kW power from the battery to the load as shown in Table I.
On the other hand, when the load is disconnected suddenly, the surplus power from the FC could be
recovered and stored into the battery to increase the overall efficiency of the energy system.
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Table 1: Backup unit operation
P3 increase
P3 Decrease
Normal
P1+P2→ P3
P1P2+P3
P1=P3
Discharge
↓
Charge
↓
Normal
Charge
↓
Normal
Normal
Two generic 12-V lead acid batteries are introduced in this unit for energy storage to deal with the
need to provide fast response and a relatively low cost solution. [10]
The proposed backup unit performs properly not only the support function for the FC module during
transients but also is used as storage when any surplus power delivered by the FC is recovered.
In order to control the output current of the backup unit, the inner current control loop of the boostinverter is used. The reference of ILb1 is taken from Idc through a high-pass filtering and the
demanded current Idemand relating the load change.
Fig. 8: Proposed Block Diagram
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Fig.9: Proposed Circuit
The proposed system uses the output of the FC which is guarded by the Backup unit and the output of
the FC is fed to the Unregulated DC Bus. [15] The output of the FC system is always a DC one so to
convert it to AC the Boost inverter is used with the SPWM technique.
Fig.10: Proposed Simulation Model
4.3
PWM inverter
In this method, pulse over a half cycle, of unequal widths is generated. Pulse thickness is a sinusoidal
function of the pointed location of each sequence. This is completed by comparing a sinusoidal signal
of the same occurrence as inverter yield against a triangular transporter occurrence wave [50, 51, 52,
and 53]. This practice is primarily used because of its plainness and easiness of realization. The
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amount of signal per round is being decisive by the ratio of the triangular transporter occurrence to that
of the modulating sinusoid, fr. This is shown in the figure 11. This method is known as SPWM. [1]
The waveform in figure 10 is a 2 - level waveform with the inverter output changing from +V to -V
Fig.11: Sinusoidal modulation
The fundamental component of the PWM output waveform with N chops per quarter cycle is:
=
4.4
4
[2
Π
(−1)
− 1]
Sismulation Result
Fig.12: Output waveform
The output of the simulation results is shown in the above figure 12. In this the x axis consists of Time
and the Y axis consists of Voltage.
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Fig.13: Current and Voltage Output waveform
Fig.14: Circuit output waveforms
The voltage V1, V2 are the voltages measured across the capacitor C1 and C2 and the Input voltage is
30 V and the output observed in the FC system is about 230 V
Fig.15: Motor Torque and Speed
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Conclusion
In this paper the efficiency in FC is increased by the action of SPWM technique. In this the sinusoidal
signal is taken as reference signal and the sample signal is taken into account and compared for the
production of the firing angle for the Boost Inverter. By using this power generated from FC can be
increased for considerable amount. The technique used here would increase the output of the FC and
also increases the efficiency of the FC. The system proposed here can be used to reduce the power
demand in the society and thus reducing the economic set back to the nation. Due to the increase in the
power demand the output from the FC can be combined with the output of the PV arrays in the grid
where the power generated from both the sources can be coupled and used for power generation. The
PV arrays used as a source can be used while the solar energy is sufficient and the FC can be used to
produce power when PV array fails or when PV arrays’ efficiency is low.
The inverter design for the PV arrays and the FC would be different so the inverters used in the model
would be designed separately for each source.
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