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Maximum power point tracking of micro hydropower
system and grid connected control
Zhuoyu jiang
Introductions: Hydropower has great potential for development as a sustainable energy resource. However, the development
of hydropower plant is limited because large power plant will cause unpredictable environmental problem. Micro hydropower system
is a flexible and environmental way of utilizing the hydropower sources considering its small sizes. Due to the simplified construction,
micro hydropower system has no control of the water flow which will cause instability of both frequency and magnitude of output
voltage. This can cause stability problem in the grid. In order to connect the micro hydropower system to the grid, a power electronic
system is applied for the frequency and power flow control. The rotation speed of generator doesn’t need to fit with the frequency of
the grid because of power electronic system which means the generator can work in variable speed. Generators working in variable
speed can give larger output power compared to fixed speed generators. Thus, the innovated maximum power point of hydropower
system based on permanent magnetic synchronous generator(PMSG) will be analyzed and applied to control strategies of power
electronic system.
Micro hydropower Introduction: Micro hydropower
systems utilize the energy from flowing water to provide electricity
energy. Although there are several ways to harness the moving water
to produce energy, run-of-the-river systems, which do not require
large storage reservoirs, are often used for micro hydro power
systems
For run-of-the-river micro hydropower systems, a portion of a river's
water is diverted to a water conveyance -- channel, pipeline, or
pressurized pipeline (penstock) -- that delivers it to a turbine or
waterwheel. The moving water rotates the wheel or turbine, which
spins a shaft. The motion of the shaft can be used for mechanical
processes, such as pumping water, or it can be used to power an
alternator or generator to generate electricity. It can be connected to
grid or it can stand alone. you need access to flowing water on your
property. A sufficient quantity of falling water must be available, which
usually, but not always, means that hilly or mountainous sites are
best. Other considerations for a potential micro hydropower site
include its power output, economics, permits, and water rights.
Advisor:Dr. Adel Nasiri
Permanent magnet synchronous generator: The
permanent magnet synchronous generator (PMSG) is the most likely
candidate from among the generator types used in SHPs because
of its potential for a high pole number (a gearbox is not needed
for variable speeds) and a high efficiency under a wide range of
loads. The diode rectifier will convert the AC voltage into DC and then go
through a DC chopper if the voltage is not high enough for the inverter. The
control of inverter is vital for this system because it has to match the
frequency and voltage magnitude of its output to the grid side voltage. Also
the maximum power point tracking will be implemented in the inverter and
boost converter control.
Calculat
ed
Power Error speed(RP
P（W） Power Errors percent
M)
Figure 1.
Micro hydropower system
rad/s
calculat
torque（
ed
Torque Error
N/m)
torque error percent
6500.00 6538.74 -38.74 0.52%
600.00
62.83
103.45
104.07
-0.62
0.69%
6900.00 6863.86 36.14
0.48%
650.00
68.07
101.37
100.84
0.53
0.59%
7200.00 7121.54 78.46
1.05%
700.00
73.30
98.22
97.15
1.07
1.19%
7300.00 7311.77 -11.77 0.16%
750.00
78.54
92.95
93.10
-0.15
0.17%
7400.00 7434.57 -34.57 0.46%
800.00
83.78
88.33
88.74
-0.41
0.46%
7500.00 7489.92 10.08
0.13%
850.00
89.01
84.26
84.15
0.11
0.12%
7450.00 7477.83 -27.83 0.37%
900.00
94.25
79.05
79.34
-0.30
0.33%
7400.00 7398.30 1.70
0.02%
950.00
99.48
74.38
74.37
0.02
0.02%
7300.00 7251.32 48.68
0.65%
1000.00
104.72
69.71
69.25
0.46
0.51%
7050.00 7036.90 13.10
0.17%
1050.00
109.96
64.12
64.00
0.12
0.13%
6800.00 6755.05 44.95
0.60%
1100.00
115.19
59.03
58.64
0.39
0.43%
6400.00 6405.74 -5.74
0.08%
1150.00
120.43
53.14
53.19
-0.05
0.06%
6000.00 5989.00 11.00
0.15%
1200.00
125.66
47.75
47.66
0.09
0.10%
Figure 6. Speed-Power Characteristic curve at different head flow
Variable speed generator system :The electricity supply
system, that is in present electric power used commonly in the whole
world, synchronous generator operating with fixed speed. The
synchronous generator fulfills simultaneously two main requirements.
The first requirement is a conversion of mechanical into electrical power,
while the second requirement is production of high quality electrical
power. The high quality electrical power means stable amplitude of ac
voltage, instantaneous voltage values that corresponds to sine function
and stable strictly standardized, frequency. The power variation is
caused only by driving torque since the speed is bond to the frequency
of grid. This improvement of power is limited by the mechanic
characteristics of turbine. Any increase of speed results in size and mass
reduction of prime movers and generators. Increase of produced
frequency also brings generator size and weight reductions. However,
conventional directly grid connected synchronous generator can not run
in variable speeds condition. Thus we need power electronics unit(PEU)
to decouple the grid frequency and generator speed.
Figure 5. Speed-Power Characteristic curve from Dr. Amano
Maximum power point estimation of the system:
based on figure 5 , there is a maximum power output for the micro hydro
power system. For example, at a certain head flow( can be consider as
fixed head flow), the relationship between the output power and speed is
shown in figure 5. as we can see that the system gives maximum power
output at around 890 rpm and the power will decrease as the turbine speed
decrease. The output power and speed relation are given by :
P=-1.23ω2+223.1ω-2623.2
Tm=P/ ω=-1.23 ω+223.1-2623.2/ ω
From other researches about the micro power system (with PMSG) we can
find some similar speed-power characteristic curve. Under a certain torque,
the speed of generator can not be infinitely increase. However, at present
there isn’t any analytical expression of the maximum power point. All of
these curve are based on turbine characteristic simulation.
Thus, a algorithm for the speed control is necessary at a dynamic torque
input in order to extract more power from the micro hydro turbine. The
inverter control in PEU need to be coupled with the speed-power
characteristic curve.
Simulation results: The simulation is based on the curve in figure 5.
The speed of PMSG was narrowed from 890 RPM and deliver 7KW power to
the grid. In the simulation results, the output value is at rated value but the
generator speed is higher than the idealized speed which indicates the error of
the equation of the turbine parameters.
The generator speed can follow the MPPT algorithm reference speed well to
produce more power to the grid. The current harmonic distortion was shown n
figure 10.
Figure 7: generator speed vs reference speed
Figure 2. variable speed generator operation range
Power electronic unit: the power
electronics unit has two main functions. The
first one is to convert generator electricity of
constant frequency synchronized to the grid.
The second function is to implement variable
speed operation by regulating the power of the
generator in accordance with the actual
hydrological conditions. Moreover, power
electronic unit can improve the power quality
by minimizing the production of harmonics,
thus improving the ride-through generation
capacity and controlling the reactive power
and voltage regulation the system connection
point. PEU in wind power system has the
similar application in order to adapt to the
power grid. This can bring a similar question to
the micro hydro power system: the maximum
power point.
Figure 8. grid voltage and current
Figure 9. output active and reactive power of grid side
Reference:
Figure 3. block diagram of PEU system
Figure 10. grid current FFT analysis.
Conclusions:
Figure 4. Simulink schematic of the system
For such a certain model of micro
hydro turbine it is necessary to control the generator
speed within a certain range for higher efficiency. With
the MPPT control of the generator the system can
extract the maximum power at 7KW and deliver to grid.
The total harmonic distortion s less than 5% which
meet the IEEE criteria.
1. M. Chinchilla, S. Arnaltes and J. C. Burgos, "Control of permanent-magnet
generators applied to variable-speed wind-energy systems connected to the
grid," in IEEE Transactions on Energy Conversion, vol. 21, no. 1, pp. 130135, March 2006.
2. Teodorescu, R., Liserre, M., & Rodriguez, P. (2011). Grid converters for
photovoltaic and wind power systems. Chichester: Wiley.
3.Indra, A. (2012, October 4). Industrial Fibre Optics In Wind Energy
Applications. Retrieved March 08, 2017, from
http://www.displayplus.net/news/articleView.html?idxno=39636
4. Microhydro power Systems. (n.d.). Retrieved March 23, 2017, from
https://energy.gov/energysaver/microhydropower-systems
5. Siegfried Heier, "Grid Integration of Wind Energy Conversion Systems,"
John Wiley & Sons Ltd, 1998, ISBN 0-471-97143-X