1. Maximum power point tracking of micro hydropower system and grid connected control Zhuoyu jiang Advisor:Dr. Adel Nasiri
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.
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.
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.
P（W）
Calculated Power
Power Errors
Error percent
speed(RPM)
rad/s
torque（N/m)
calculated torque
Torque error
-38.74
0.52%
600.00
62.83
103.45
104.07
-0.62
0.69%
36.14
0.48%
650.00
68.07
101.37
100.84
0.53
0.59%
78.46
1.05%
700.00
73.30
98.22
97.15
1.07
1.19%
-11.77
0.16%
750.00
78.54
92.95
93.10
-0.15
0.17%
-34.57
0.46%
800.00
83.78
88.33
88.74
-0.41
10.08
0.13%
850.00
89.01
84.26
84.15
0.11
0.12%
-27.83
0.37%
900.00
94.25
79.05
79.34
-0.30
0.33%
1.70
0.02%
950.00
99.48
74.38
74.37
0.02
48.68
0.65%
104.72
69.71
69.25
0.46
0.51%
13.10
109.96
64.12
64.00
0.12
44.95
0.60%
115.19
59.03
58.64
0.39
0.43%
-5.74
0.08%
120.43
53.14
53.19
-0.05
0.06%
11.00
0.15%
125.66
47.75
47.66
0.09
0.10%
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=-1.23ω ω
Tm=P/ ω=-1.23 ω / 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.
Figure 1. Micro hydropower system
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.
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:
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 , 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
4. Microhydro power Systems. (n.d.). Retrieved March 23, 2017, from
5. Siegfried Heier, "Grid Integration of Wind Energy Conversion Systems," John Wiley & Sons Ltd, 1998, ISBN X
Figure 3. block diagram of PEU system
Figure 10. grid current FFT analysis.
Conclusions: 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.
Figure 4. Simulink schematic of the system