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IEEE NECEC November 8, 2007 St. John’s, NL
Power electronics and control of a four inputs
hybrid power system
Reaz Ul Haque M. T. Iqbal John Quaicoe
Faculty of Applied Science and Engineering
Memorial University of Newfoundland
St. John’s, NL, A1B3X5
Abstract- A hybrid power system consists of two or more power
sources working in parallel. Such a system needs power
electronics to extract maximum power and to control the flow of
power from each renewable power source to the users load. We
consider a small hybrid power system consisting of two wind
turbines, a PV array and a pico-hydro system. A parallel
combination of DC-DC converters connected between renewable
energy sources and a common DC bus is used to control the
power flow in the hybrid power system. In this paper we present
a power electronic and control solution for such four inputs small
hybrid power system with a common 48V DC bus. System power
electronics design and proposed control strategies for each
renewable power inputs are presented. Design and development
progress of the proposed power electronics and control
arrangement is included in the paper.
Index Terms—Hybrid Energy System, Multi-input converter,
supervisory controller.
I. INTRODUCTION
Applications of renewable energy have increased
significantly during the past decade. Hybrid power systems
employing two or more renewable energy sources are the best
option for many remote and grid connected sites. Such
systems need power electronics and control arrangement to
extract maximum power from the available renewable energy
resources and deliver a reliable power to the user load. Hybrid
power system power electronics is basically a number of
power converters working in parallel. Each converter in such
an arrangement has its own PWM controller. In addition to
that there is a system supervisory controller that takes system
measurements and control power flow with in the system.
Supervisory controller can also log system performance data
and take commands from a remote dispatch center.
Different circuit topologies for multi-input dc/dc converters
and control algorithm have been proposed to combine
different types of clean energy sources to obtain regulated dc
output voltage. Different dc sources can be put in series [1, 2]
or can be paralleled [3, 4] by using coupled transformer to
achieve desired load voltage. The control strategies for
paralleled dc sources are based on time sharing concept. So
some references [e.g. 5,6] on multi-input converters are
available which are based on the concept of the transformer
flux addition, and converters delivering power individually
and simultaneously. But such an arrangement requires four
switches in each input side converter.
The hybrid energy system that is considered here is
comprised of two wind turbines, one photovoltaic module and
one micro hydro. The battery is considered as storage for the
system. Figure 1 shows the basic block of the proposed hybrid
energy configuration.
The objective of this paper is to propose a multi-input dc-dc
converter and simulate the topology in MATLAB simulink
environment. This paper also describes the proposed control
strategies for hybrid energy system.
Figure 1: Basic block diagram of the system
II. CONVERTER TOPOLOGIES FOR MULTI-SOURCE HYBRID
POWER SYSTEM
A multi input (MI) power converter is a practical example
which can be used for accommodating multiple sources. With
multiple inputs, the energy source is diversified to increase the
reliability and utilization. The basic difference between
parallel operation of the single input converter and multi input
converter is shown in figure 2. A multi-input converter
accommodates all the inputs whereas in parallel operation
separate converters work in parallel.
Figure 2: Basic configuration of parallel operation of
converters and MI converter
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IEEE NECEC November 8, 2007 St. John’s, NL
Though this concept of multiple inputs in one converter is new,
it has gained acceptance in a couple of fields like hybrid
electrical vehicle and renewable energy system.
Generally, the paralleling of power converter modules
offers a number of advantages over a single, high-power,
centralized power MI converter. Paralleling of standardized
converter modules is an approach that is used widely in
distributed power systems. A desirable characteristic of a
parallel supply system is that individual converters share the
load current equally and stably. Parallel modules are usually
not identical due to differences in power level in each power
stage and control parameters.
III. MULTI-PORT HALF BRIDGE CONVERTER
The proposed half bridge converter for multiple input
configurations is same as half bridge converter [figure 3]; but
it has multiple input sides and one output. The configuration is
shown in figure 4. It consists of two input-stage circuits, a
three winding coupled transformer, and a common output-
stage circuit. The number of the input-stage circuit can be
increased to number of input dc sources
Figure 3: Half bridge push-pull converter
Figure 4: Multi-input half bridge converter
requirement while the output-stage circuit remains unchanged.
The operation of multi-input half bridge converters are similar
to the single stage half bridge converter while all the input
sources are working at same duty cycle. In order to explore
more the half bridge multi input converter, simulation has
been carried out in Matlab-Plecs environment. Detailed study
of the simulation is given in the next section.
IV. SIMULATION RESULTS
Half bridge multi-port dc-dc converter is simulated for
different configurations and in different cases. The control
variables in multi-port dc-dc converter are the duty cycle of
each input-stage. The converter can operate either in same
duty cycle for each input stage or in different duty cycle for
each input stage. Case1 is the situation when all the input
stages are operating at same duty cycle but in case2 each input
stage is running at different duty ratio. In the plots (figure 5, 6,
9, 10), the current is in amp and x-axis is in time with units in
seconds.
For simulation study, input voltage of source #1 is considered
to be 48 volts, for second source- input voltage is 24 volts, the
output voltage is 24 volts and the power rating for this
converter is 400 watt.
Case1: Both input-stages of the converter are operating at the
same duty cycle and frequency.
Figure 5 shows the simulated result of case1. The result shows
the currents of each input stage and output circuit. Here both
the input-stage shares power equally. In this case M1 and M3
are turned ON and OFF at the same time where as M2 and M4
are switched ON and OFF at the same time. No feedback is
present in this case.
Figure 5: Simulation results of MI half bridge converter (case1)
Figure 5 is the current wave shape for different stage with gate
signal for M1. Here 48% duty cycle is considered.
Case 2: Both input-stage of the converters are operating at
different duty cycle
In a practical situation both the input-stage cannot run at the
same duty cycle all the time. (say two renewable sources are
producing power at different level. Power output of source 1 is
more than power output of stage 2) In that case MI converter
input-stage 1 will be on for more time than input-stage 2. And
in this case power is feedback to input stage 2 from input-
stage 1. Voltage induced in secondary winding is more than
the voltage induced in primary winding of input-stage 2. As a
result negative power flows will occur in the input-stage 2.
The simulated result is shown in figure 6.
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IEEE NECEC November 8, 2007 St. John’s, NL
The induced voltage on each winding of the coupled
transformer is clamped to the value that is proportional to the
output stage. Figure 6 shows that during off time of the input-
stage 2 power is fed back to input stage 1 as well and as the
Figure 6: Simulation result –currents for case2
direction of the source current and feedback current are same,
so current is adding up in input stage 1.
V. HALF BRIDGE DUAL CONVERTER
As described above, in a half bridge multi-input voltage
source converter, power feedback occurs in different input
stages. To overcome this problem of power feedback we could
add diode in series with each switch which blocks the free
wheeling path for the circuit. Another option is current source
half bridge multi-input converter topology for the system.
Applying the duality principle to the transformer-coupled half-
bridge push-pull voltage source converter, the current source
converter is developed [7] and is presented in figure 7.
Figure 7: Dual of the half bridge converter
The attributes of the current source converter are derived from
the voltage source converter using the dual principles as
follows:
• The half bridge voltage source converter (shown in figure 3)
uses two capacitors to divide the supply voltage into two
equal half. Alternate closures of the switches apply either
+Vin/2 or –Vin/2 across the transformer primary. The dual
converter (shown in figure 7) uses two inductors to divide the
available supply current into two. Alternate switch openings
direct either +I/2 or –I/2 to flow through the primary.
• At least one switch is open at all times in the half bridge
voltage source converter. In its dual, at least one switch is
closed all the time in the current source converter.
• The closure of both switches is destructive for the half
bridge voltage source converter, causing a short circuit of the
voltage source input; in the dual converter opening both
switches is destructive, causing a open circuit of the input
current source.
• The half bridge voltage source converter is buck derived,
where as the dual is boost derived.
• The output rectifier stage in the half bridge voltage source
converter uses a current stiff inductive filter. The dual filter in
the current source converter uses a voltage stiff capacitive
filter.
• While the half-bridge voltage source converter is suited to
higher DC bus voltages, the dual converter is suited to higher
DC bus currents.
• When both switches in the voltage source converter are Off
the converter is in free wheeling stage and when any of the
switches is Off it is in power transferring stage. In the current
source converter, when both the switches are ON, the
converter is in free wheeling stage and when any of the switch
is ON it is in power transferring stage.
For multi-input configuration of the half bridge current source
converter, the output stage remain the same only one or
multiple input stages are added to configuration and is shown
in figure 8.
Figure 8: Multi-input current source half-bridge converter
VI. SIMULATION RESULTS OF DUAL CONVERTER
Case3: Switches are operating at the same duty cycle
As the converter is operating at constant duty cycle and same
for both the source, the load is equally divided between the
sources. The simulation result is given in figure 9.
The source current I1, is shared equally between the current
dividing inductors left L1, and right L1. If M1, is closed, the
current flowing in L1, flows into the transformer primary, out
of the primary winding and then through M1. This primary
current produces a secondary current that is rectified and
injected into the output filter capacitor. If M2 is closed, then
the current in L1, flows into the primary and then through M2
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IEEE NECEC November 8, 2007 St. John’s, NL
When both switches are closed, the dual converter is said to be
Figure 9: Simulation results of CS converter (case 3)
in a boost phase or free wheeling stage. When only one switch
is closed, the converter is said to be in a powering phase as
current is delivered to the output rectifier. The same operation
occurs in the input stage 2. Figure 9 shows the simulation
results.
When both switches of each input stage are ON, high
current flows from each source and no power is transferred.
But when only one switch is ON current reduced to half for
each input stage and power is transferred during this time. If
we add the half of the current of each input stage, we would
come up with the load current, which means both the source
are sharing the load equally. Gate signal for M1 and M3 are
also presented in figure 9.
Case4: Input-stages of the converter are running at different
duty cycle:
In this case, the switches associated with first input stage are
Figure 10: Simulation results of MI CS converter (case4)
operating at 70 % duty cycle whereas the switches in the
second-input stage are operating at 60% duty cycle. When the
first input stage is in the power transferring stage and the
second input-stage is in free wheeling stage, the induced
voltage on each winding will be clamped to the output voltage.
So there is power transfer in between the two input stages. The
simulation results for this case are presented in figure 10.
In this case power is supplied by the input-stage that is
operating at higher power level.
VII. PROBLEM IDENTIFICATION
Half-bridge multi-port dc/dc converter could be a good option
for accommodating four energy sources. But this topology has
disadvantage of having feedback from other sources. If all
sources are operating at same duty cycle, this converter can be
applied in hybrid power system. But in practical all the input-
stages of the converter will not operate at a same duty cycle.
From simulation and also with practical implementation, it is
observed the feedback exist in different input-stage. Using
series diode with MOSFET cannot mitigate the problem of
feedback; it blocks the freewheeling path of the inductor. Dual
of the half bridge converter is also studied and simulated for
this application. But this topology required large inductor.
Power transfer stage is less than 50% of the total cycle.
Energy stored in inductor is not properly discharged and
require snubber circuits and this topology does not transfer the
energy efficiently. In order to remove energy stored in
inductor we need to have some energy recovery circuit which
makes this topology very complicated [7].
The simple way to overcome these problems is to use
individual boost or buck-boost converter with each source.
This is parallel operation of the converters.
VIII. PARALLEL OPERATION OF DC-DC CONVERTERS
As indicated in section II single input dc-dc converter may be
used in a parallel arrangement to connect sources at different
voltage levels to a common dc bus. In the hybrid power
system under consideration, the micro-hydro and photovoltaic
in system produce nominal output voltage of 48V. However
this voltage may vary from 44V to 55V. The wind energy
conversion system produces a nominal voltage of 24 V. In
order to connect the different level of voltage to a common dc
bus with a constant voltage of 48V, buck-boost and boost
converters are required. Figure 11 shows a block diagram
representing the hybrid power system containing multiple dc-
dc converters.
If we look at the basic block diagram of the total system, we
can see the total system can be subdivided in to five main
blocks; i.e.
• Input Sources
• Load Side
• Power electronics interface circuitry
• Controller and
• Sensors
Input sources: The input sources consist of renewable energy
sources. For our system the input sources are wind turbine,
photovoltaic and micro hydro. Though in our system two wind
turbines are considered but for simplicity only one block of
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IEEE NECEC November 8, 2007 St. John’s, NL
wind turbine is shown in the figure 11. The input sources are
connected to load through power electronics circuit.
Load side: The load side represent consumer load. But it also
include dump load and battery bank. The dump load is nothing
but pure resistive load. In some applications it is heating load.
The dump load is connected to dc busbar through a solid state
relay. The relay is controlled by the supervisory controller. If
Figure 11: Basic block diagram of the hybrid power system
the generation is more than customer demand, supervisory
controller generate control signal to relay to connect the dump
load to the system. Battery is considered here as load side
component, though it supply power for short duration but it
also consumes power for charging. The battery bank voltage
maintain the constant voltage of the DC busbar and also it is a
control variable. Depending on this voltage, supervisory
controller decides whether the battery needs charging or not. If
the voltage is below a threshold voltage, it means batteries
need to be charged and supervisory controller generate signal
accordingly.
Power electronic interface circuitry: Power electronics
interfacing circuitries are like bridges between input stages to
the output stage. There should be isolation from source side to
load side, power electronics devices established this isolation.
If the sources are directly connected to load due to variation in
source voltage, load side voltage will vary and also different
voltage rating sources cannot connect directly to load. As
previously mentioned two different voltage rating sources are
connected to common DC bus through power converters. As
described in the figure wind turbines are connected to DC bus
through boost converters where as micro-hydro and
photovoltaic are connected to DC bus bar voltage through
buck-boost converter.
Controller: the controllers in the system can be further
subdivided in to supervisory controller and device level
controller. The supervisory controller supervises overall
system performance and generates signal according. It also
generates reference signal for device level controller. These
reference signals are associated with maximum power point
tracking algorithm with the device level controller. The
supervisory controller takes decision whether which system
should put on or should turn off. It produce signal for dump
load relay. The device level controllers are solely responsible
for generating duty ratios for converters. Here the supervisory
controller and device level controllers are micro controller
based which is very easy to operate and suitable for stand-
alone application.
Sensors: In order to operate successfully, the controller needs
to sense some signals. Depending on the control algorithm,
number of input signal are measured and conditioned. In the
system all the source input current, load current and load side
voltage are control variable. They are sensed by proper
transducers. As output of the current transducers are not pure
dc, so low pass filter is used to block the high frequency
ripples. Also the current sensor output is very low in
magnitude so amplifier stage is used to amplify the signal.
IX. CONTROL ALGORITHM FOR THE HYBRID POWER SYSTEM
The main challenge is the control of the hybrid energy system.
So far no literature has been found about supervisory and
modular controller approach to real time system. Few
literatures are available on supervisory controller but they are
basically on simulation level. Practical implementation of
control algorithm for more than one source is available but
most of that work on current sharing method or individual
level control. One report [8] is available from Riso research
institute, where they worked on simulation studies of
supervisory and modular controller. Though supervisory
controller meet the generation and load demand and modular
controller control the power flow from each source. The
source or loads are assigned with priority number where they
could be easily interchangeable and different source are given
different priority number to generate power. As mentioned in
[8] they developed program in Matlab and did only simulation
of different cases. Also in there system they were are talking
about a system comprised of diesel only, wind-diesel or wind
only. So this will be first work on hybrid supervisory
controller for stand alone wind-photovoltaic and micro hydro
system. Also most of the control systems are PC based, so it
requires external ac power to run the PC itself. Micro
controller based controller overcome this problem for stand
alone system where the necessary small power can be supplied
from a battery.
The control structure can be sub-divided into two distinct
levels:
1. System level control
2. Device level control
The system-level controller or supervisory controller manages
the power flows to the controllable elements of the system to
help, as far as possible, balance supply and demand. In this
level all the inputs are sensed through sensors and generated
signals for device level controller. The device level controller
works on each power-electronics interface. This control action
does not need to be particularly fast but is required to ensure
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IEEE NECEC November 8, 2007 St. John’s, NL
that each individual device functions correctly, producing the
correct current, voltage and waveform.
X. SYSTEM LEVEL CONTROL
The challenge for a supervisory control system is to control
the power flows to maintain the DC voltage on the bus bar
within the operational limits of the power-electronic
converters.
The supervisory controller for hybrid energy system is a plant-
wide control system that synchronizes and supervises system
operation while distributing the operation control with local
controllers and regulators. The distributed control is realized
by allowing the system components use their own regulators
and control systems to safeguard operation. For instance, a
wind turbine has a pitch controller to control its active power
and a voltage regulator to control the voltage.
The supervisory controller is responsible for selecting the best
mode of operation for the system taking into account the
application-specific operating goals, the system design and
constraints. The supervisory controller determines set points
for different components at different modes of operation and
sends them to their individual local controllers.
The controller is separated into two parts: hardware and
software. The hardware is characterized by the type of
microcontroller and its features. The interfaces with the plant
to be controlled (process interface) and the operators (man-
machine interface) are important parts of the hardware and
influence the application programs (software) related to the
I/O operations of each interface. The software is application
programs, which are the algorithms of the control system.
In our system controllable components are two wind turbines,
one photovoltaic module, one micro hydro, one battery bank
and one dump load. Micro hydro is used as base generation
and no device level control is applicable to this source. If it
has sufficient flow to produce power, it will keep producing
power and no maximum point tracker is used in this stage.
For wind and photovoltaic module maximum power point
tracker is used and they are discussed under device level
controller.
If generation exceeds the demand, the excess power has to
utilize or generation needs to cut down. In the system, any
excess variation in generation is dumped through dump load.
The dump load is nothing but pure resistive load. This load is
controlled by relay signal from supervisory controller.
The battery bank in the system keep the DC bus voltage in a
constant value, in our system it is 48V. A drop in busbar
voltage below a certain threshold limit means that the battery
banks need to be charged.
If we consider only generation and demand there may exist
only three condition-Generation equal to demand, generation
greater than demand or generation less than demand.
Depending on these three conditions our supervisory
controller worked in three modes. The following section
described these three modes in detail.
Mode 1: Load is equal to generation: It corresponds to period
of sufficient wind or solar or both to satisfy the total demand.
As micro hydro is used as base generation, so the wind and
solar energy sources have to track the total demand while the
battery bank is inactive. If this is the starting condition, device
level controller will search more maximum power point
tracking. And if the generation increased the increased power
will charge the battery depending on the charge level.
Mode 2: Generation is more than load. The supervisory
controller stops the renewable energy sources for maximum
generation. In this mode, the battery bank is not requested to
supply power to load. On the contrary, under this mode, the
battery bank demands the maintenance current or recharge
current and becomes part of total demand. The dump load is
turned on in this mode.
Mode 3: Generation is less than demand. This mode occurs
when generation falls below demand. In this mode battery
bank is requested to supply power to the load and charge of
the battery is monitored constantly. If it falls below a certain
safety limit the supervisory controller has to cut off some load
and the dump load is always turned off in this mode..
The basic flow chart of the supervisory controller is given
below-
Figure 12: Supervisory controller flow chart
The sensors will sense the voltage and currents of each stage
and will help to determine the generation and demand at
regular intervals. If the load is equal to generation, the
controller for each source will search for MPPT algorithm. If
already all the renewable energy sources are at maximum
power point, no extra energy can be generated from the
sources. On the other hand if the sources are not operating at
MPPT, the sources will run at MPPT and the extra energy will
be used to charge the battery bank. If the load and generation
are not equal, there may be two situations; either load is
greater than generation or the load is less than generation. If
the load demand is higher than the generation, the batteries
need to discharge power to meet the load demand. The battery
charge must be monitored at regular intervals, if the battery
charge falls below a certain limit, some load will have to be
turned off. If generation is more than demand, the extra power
will charge the battery bank. A fully charged battery, however,
will require that the extra energy be diverted to the dump load.
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IEEE NECEC November 8, 2007 St. John’s, NL
XI. DEVICE LEVEL CONTROL
The voltage of the main bus bar to which the various DC
devices are connected has been selected as 48V DC. Wind
turbines output voltages are 24 volts whereas PV arrays and
micro hydro output voltage is 48V. However, due to
variations in the output voltage of the sources DC-DC
converters are required for all the sources. The solar PV arrays
and micro hydro sources require buck boost converters,
whereas the wind turbine sources require boost converters.
These converters are controlled by device level controllers.
The device level controllers for the solar and the wind are
discussed in separate section.
XII. PHOTOVOLTAIC SOURCE
Characteristic of the Photovoltaic Array
All the photovoltaic modules considered in the paper are
crystalline silicon- which was used in 94% of all 1200 MW of
produced solar cell modules in 2004 [9]. Such modules have
current/voltage (IV) characteristics as shown in Figure 13 [10].
The current is mainly affected by changes in solar irradiance
(W/m
2
) and, to a lesser extent, by other variables such as
temperature. But open circuit voltage is mainly affected by
temperature and it has negative effect. It means with increase
in temperature, open circuit voltage of the pv modules
decrease.
Figure 13: Typical IV characteristic of a crystalline solar PV
module
Modules are built from a number of cells in order to supply
higher voltage and current. Modules are then connected into
an array, with series and parallel connections used to give
increases in voltage and current. If the panels are wired in
higher voltage series strings, they will have a lower current for
the same power level and hence lower resistive losses and
thinner, less expensive, wires.
Maximum Power Point Tracking (MPPT)
Various methods of maximum power point tracking have been
considered in photovoltaic applications. They can be mainly
categorized into three broad groups [10].
• Look Up assignment table method
• Computational method
• Perturbation and observation method
In this research the perturbation and observation method is
Figure 14: Solar array characteristic curve [11]
proposed in a software program with a self tuning function.
The program automatically adjusts the array reference voltage
and voltage step size. The problem- incapable of tracking
maximum power in fast changing environment, with the
method is wished to overcome by increasing sampling
frequency.
Solar array characteristic curve under a given insolation is
given in figure 14. The internal impedance is low on the right
side of the curve and high on the left side and maximum
power point is located at the knee of the curve. According to
maximum power transfer theory, the power delivered to the
load is maximized when the load impedance is equal to the
source internal impedance. So if the system is required to
operate near maximum power zone, the impedance seen from
the converter side needs to be match with the internal
impedance of solar array.
Figure 15: Flowchart of the MPPT control
As the traditional dc/dc converter has negative impedance
characteristic, the solar array is required to operate on the right
side of the curve to perform the tracking process. The control
flowchart in figure 15 illustrates the details of decision
processes. If the given perturbation leads to an increase in
array power, the next perturbation is made in the same
direction. In this way the maximum power tracker seeks the
maximum power point continuously.
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IEEE NECEC November 8, 2007 St. John’s, NL
XIII. WIND ENERGY CONVERSION SYSTEM
Characteristic of the Wind Generator
A wind turbine extracts energy from moving air by slowing
the wind down, and transferring this harvested energy into a
spinning shaft, which usually turns a generator to produce
electricity. The power in the wind that is available for harvest
depends on both the wind speed and the area that is swept by
the turbine blades [12-14]. It can be formulated by the
following equation:
23
1
2
t
PR
ρπ
=V
(1)
Where, =Output power in watts;
t
P
ρ
=Air density in kg/m3;
R=Radius of the swept area of wind turbine in m; V=wind
speed in m/s.
The first concept that this formula shows is that when the
wind speed doubles, the power available increases by a factor
of eight. The only way to increase the available power for low
or constant wind speeds is by sweeping a larger area with the
blades.
In 1919 Betz calculated that there is a limit to how much
power a turbine blade can extract from the wind. He
determined that 59.2% of absolute maximum energy can be
extracted from the available power [14]. Beyond the Betz
Limit of 59.26% energy extraction, more and more air tends to
go around the turbine rather than through it, with air pooling
up in front. There are additional losses after the Betz limit;
small wind turbine blades are never fully efficient, even when
running at desired speed; no generator is 100% efficient in
converting the energy in a rotating shaft into electricity; there
are friction losses from bearings, and from any gearing that is
involved in the power conversion. And there are magnetic
drag and electrical resistance losses in the alternator or
generator. Taking these factors into consideration the output
of a wind turbine can be expressed as [15]
13
()
2
PC AV
tp
λρ
= (2)
Where, =Output power in watts; C
P
t
p
=Power co-efficient
(non-dimensional);
λ
=Tip speed ratio (non-dimensional);
ρ
=Air density in Kg/m3; A=Frontal area of wind turbine in
m2; V=wind speed in m/s.
And the term λ is the tip-speed ratio, defined as
R
V
λ
Ω
= (3)
Here Ω is the rotational speed (in rad/sec) of the wind
generator’s rotor.
Maximum Power Point Tracking
The wind generator power curves for various wind speeds are
shown in figure 16. It is observed that for each wind speed
there exists a specific point in the wind generator output
power versus rotating-speed characteristic where the output
power is maximized. The control of the wind generator load
results in a variable-speed operation, such that maximum
power is extracted continuously from the wind.
Figure 16: Power curves at various wind speeds
A commonly used wind generator control system [16] is based
on the optimal power versus the rotating-speed characteristic,
which is usually stored in a microcontroller memory. The
wind generator rotating speed is measured; then, the optimal
output power is calculated and compared to the actual output
power. The resulting error is used to control a power interface.
A control system based on wind-speed measurements has also
been proposed in literature [17]. The wind speed is measured,
and the required rotor speed for maximum power generation is
computed. The rotor speed is also measured and compared to
the calculated optimal rotor speed, while the resulting error is
used to control a power converter.
In permanent-magnet (PM) wind energy conversion systems,
the output current and voltage are proportional to the
electromagnetic torque and rotor speed, respectively. In [1],
Figure 17: Basic flow chart of the MPPT control for Wind
energy conversion system
the rotor speed is calculated according to the measured output
voltage, while the optimal output current is calculated using an
approximation of the current versus the rotational-speed
optimal characteristic. The error resulting from the
comparison of the calculated and the actual current is used to
control a dc/dc converter.
In this research, the MPPT process is based on monitoring
wind generator output power using measurement of output
8
IEEE NECEC November 8, 2007 St. John’s, NL
voltage and current and directly adjusting the dc/dc converter
duty cycle according to successive output power values. Thus
this method neither need to measure wind generator rotational
speed or wind speed. But to measure the reference point it
applies the concept of reference [18].
The flow chart in figure 17 describes the detail of decision
process. Proposed modular controllers and supervisory
controller are still under development at the energy system lab,
Memorial University of Newfoundland. System performance
and experimental test results will be presented in future
publications.
XIV. CONCLUSIONS
This paper proposed a multi-input dc-dc converter topology
for hybrid energy system and investigates its operation
through simulation. The simulation studies show that the
topology is not suitable for multi source hybrid energy system.
So the alternate solution –parallel operation of the converter is
described in this paper. This paper also investigates control
strategies and proposed supervisory and modular controller
approach for a hybrid power system. The basic flow charts for
supervisory and the device level controllers are also presented
in this paper.
ACKNOWLEDGMENT
The Authors would like to thank the National Science and
Engineering Research Council (NSERC) Canada for providing
financial support for this research.
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