Theoretical and conditional monitoring of a small three-bladed vertical-axis micro-hydro turbine

Theoretical and conditional monitoring of a small three-bladed vertical-axis micro-hydro turbine

Energy Conversion and Management 86 (2014) 727–734 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 86 (2014) 727–734

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage:

Theoretical and conditional monitoring of a small three-bladed vertical-axis micro-hydro turbine Sy-Ruen Huang c, Yen-Huai Ma e,⇑, Chia-Fu Chen d, Kazuichi Seki b, Toshiyuki Aso a a

THK Co., Ltd., Nishi-Gotanda, Shinagawa-ku, Tokyo, Japan Research Institute of Science and Technology, Tokai University, Tokai, Japan c Department of Electrical Engineering, Feng Chia University, Taichung, Taiwan d Department of Electronic Engineering, Feng Chia University, Taichung, Taiwan e Ph. D. Program of Electrical and Communications Engineering, Feng Chia University, Taichung, Taiwan b

a r t i c l e

i n f o

Article history: Received 22 December 2013 Accepted 27 May 2014

Keywords: Condition monitoring system Micro-hydro generator Three-phase permanent magnet symmetric generator

a b s t r a c t This paper presents a novel 3-kW three-bladed vertical-axis micro-hydro turbine (VAMHT) system. The experimental results reveal that using this type of turbine in water is distinct from using it in wind. The micro-hydro turbine system uses a three-phase permanent magnet symmetric generator that transforms mechanical energy into electrical energy. The output voltage and frequency of the generator depend on water flow speed, and voltage steady equipment is used to maintain the maximum output power of the DC bus. According to the maximum power point tracking of the micro-hydro turbine system, the condition monitoring of the novel micro-hydro turbine requires no water flow meter. Furthermore, the construction and installation of the new micro-hydro turbine is simple, economical, and stable. This system combines a micro-hydro generator and electrical state-monitoring system, which can measure the speed, output power, DC-bus voltage, and all electrical characteristics of the micro-hydro turbine system. The results of comparing turbine between wind and water show that the speed ranges of water flow is narrower than that of wind, and the status transformation from cut-into stable power generation is short. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Because of recent problems with the depletion of fossil fuels and global warming, renewable energy is being rapidly developed. Hydroelectric power is clean renewable energy that can be developed large-scale using current technology. In addition, the cost of hydroelectric generation is clearly lower than that of commonly used thermal, nuclear, solar, and wind power [1–5]. Almost all developed countries are considering hydroelectric power to reduce the threat of carbon emissions. For the electrical power industry, hydro energy is one of the most favorable regulatory powers that can easily control and adjust the gates of dams. Traditional steam thermal power must burn coal completely to generate sufficient steam for starting a power supply. Only petroleum and natural gas are comparable with hydroelectric power in regulation performance. In China the 12–5 construction projects, hydropower occupies an important place. ⇑ Corresponding author. Tel.: +886 9 28484353. E-mail address: [email protected] (Y.-H. Ma). 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.

To China, the industry and the economy is about to take off, and also its hydroelectric power occupies 15 percent of all power supply. Hoping that it continues development, could reduce 40–50% carbon emissions in 2020 [6]. In Taiwan, the total capacity of hydroelectric power installation is approximately 925,000 kW, and the annual generating capacity is as much as 47 million kW per hour. Therefore, based on the water supply and ecological and environmental features, low hydroelectric power is necessary. Micro-hydro technology is recent and is still at the developmental stages. This paper presents a vertical-axis micro-hydro turbine (VAMHT) system that exploits the structure of wind turbine systems to absorb the energy of water. These patterns of hydropower are rarely discussed in relevant literature. Using micro-hydro turbines enables obtaining energy from water without intercepting the water. Generally, the structures of hydropower generation must intercept water to obtain energy from water such as at dams and with run-of-the-river hydroelectricity and pumped-storage hydroelectricity. However, these types of hydroelectricity require large and complex designs that are expensive and cannot exist


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individually. Therefore, this paper presents a vertical micro-hydro turbine structure, of which the micro-hydropower feasibility was explored in existing canals or irrigation channels. The ease of future installations and removals in flood or maintenance conditions was considered. A micro-hydro electrical condition-monitoring system was constructed to monitor the micro-hydro turbine rotational speed, output power, and DC (Direct Current)-bus voltage. The electrical characteristics of the micro-hydro turbine were detected and recorded by the condition-monitoring system for machine diagnostics and power-generation analysis. Establishing a data monitoring system can be observed in the future or predict the effects of different variables on the system, also proposed on research paper. These study using neural network (ANN) Feed Forward Neural Networks (FFNs) to predict the water level of the lagoon. After a long observation that lagoon water level would change with the effects of different variables, then collected the training data hourly, and with its features build a model of neural network. During to the change of water reservoir, the hydroelectric water flow rate would change. If the water level can be estimated, its micro hydro power generation can be estimated [7–9]. However, the vertical wind turbine using in the hydropower that the characteristic of the turbine is different. This paper describes the speed ranges and status transformation of the turbine. In this study the hydraulic turbine must withstand strong steam flow. Due to the force of the water is 1000 times greater than the wind but the speed is slower than wind. Thus the design of the micro hydraulic turbine must be stronger than wind turbine. With those constraint, the bearing, structure, blades should be make an appropriate redesign as well as the generator. As an independent energy source, micro-hydro turbines have an internal high-magnetic flux density that is widely applied by a permanent magnet synchronous generator. Such turbines have no gear drive system, and their advantages are a small power generation system; the development system capacity ranges up to several hundred kilowatts [10]. In considering this trend, obtaining the optimal topology for the conversion system in the micro-hydro turbine involves connecting the synchronous generator with simple three-phase rectifier diodes and DC–DC maximum powertracking circuits. The three-phase rectifier diode is more economical than the three-phase IGBT (Insulated Gate Bipolar Transistor) converter [11–14]. 2. Micro-turbine concept The energy conversion processes for water and wind are similar and entail two procedures. At the first stage, kinetic energy is converted to mechanical energy by a rotating blade. At the second stage, the mechanical energy is converted to electric energy by a generator, as shown in Fig. 1. Many mechatronic energy conversion thesis had been proposed, but it is relative complex system and cost more. In Fig. 1, the system

configuration is relative stable and easier to construct. The structure and operation diagrams of the small three-bladed VAMHT are show in Fig. 2. In hydro-electric power, rotating blades absorb energy from water. Energy is exchanged between the machines and the potential of water. The rotating blades are connected to an electrical generator, and the electrical generator produces electrical power through the rotation of the blades. Hydropower uses the potential energy of water and converts it into mechanical energy, and then produces electrical energy from the generator by using rotating blades. Three terms are used to describe the various hydroelectric types: Conventional (e.g., dams) hydroelectric power is generated from the potential energy of dammed water, which powers the water turbine and generator. Hydro turbines and generators are installed in dams or along rivers. Run-of-the-river (ROR) hydroelectricity power systems are built at middle and lower reaches, to draw or divert water into turbines. Pumped-storage hydroelectricity (PSH) involves a reservoir sluice in which electricity peaks during the day. Pumping water into the height place stored for supplying high power consumption of the power generation. These types of hydroelectricity require large and complex designs [15–17]. Simple construction and installation, which were the perceived benefits of using micro-hydro turbines in this study, allow micro-hydro turbines to be installed in any type of canal or irrigation channel without affecting the water flow. Micro-hydro turbines can exist individually in electrical power supplies. Figs. 3 and 4 depict the construction of the micro-hydro turbine. Hydropower can be calculated by using an equation. Water flows from top to bottom. Because falling water creates kinetic energy, higher water from a river or dam is diverted to a lower area, and the water flow pressure causes turbines to rotate, which is how water energy is produced. The density of water is considerably greater than that of wind, and the flow speed produces the kinetic energy of water. The kinetic energy of a unit area is calculated using the mass and area of water in unit time with proportional available power (in water).

PW ¼

1 1 qAV  V 2 ¼ qAV 3 2 2


where q = water density (1225 kgm3). V = water speed (ms1). A = water turbine rotor swept area (m2). The sum of the available power of water can be approximated using the hydraulic power generated by a hydraulic turbine, as follows:

1 P ¼ K qAV 3 2


K ¼ C p gg gb


gg = efficiency of generator. gb = efficiency of bearing. Cp ¼

Tt  x 0:5  A  V 3


Cp = Power Coefficient of performance. This water turbine torque is given by:

Ts ¼

Fig. 1. Schematic diagrams of the small three-bladed VAMHT.



Ts = torque of water turbine. P = power of water turbine. xs = rotor speed of water turbine.


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Fig. 2. Operational diagrams of the small three-bladed VAMHT.

Fig. 3. Structural drawing of the small three-bladed VAMHT.

3. Structure and performance of micro-hydroelectric turbine system In the experiment, a three-bladed VAMHT generator was installed in a channel of the Shimen Dam in Taoyuan, Taiwan at a water level 3 m above the ground. The average temperature and average water flow rate were 34 °C and 1.8 m/s, respectively. The rotational area of the rotor was 1 m, the blade angle was 7°, and 9-kW-rated power in the permanent magnet generator generated DC output power through a three-phase bridge rectifier. A brake resistor protects the turbine in emergencies. Fig. 5 shows an electrical block diagram of the small, three-bladed VAMHT. The micro-hydro turbine generator is mounted in an experimental cabin laboratory, which is equipped to collect the measured data daily. A water flow meter was installed in the experimental canal 10 m from the water turbine. In addition, a tachometer was set up to measure the rotational speed of the turbine blade driven by the force of the water. In Table 3, the blade, steam flow and the generator parameter is listed. By the formula the energy generated is proportional to cubic of the steam flow speed. But in contract the faster steam flow, the higher intensity of strength is needed. Energy conversion efficiency and starting torque is differ from various type of blade and the low speed permanent magnetic genera-

Fig. 4. Structural picture of the small three-bladed VAMHT.

tor is appropriate choice for the high density flow and slow speed characteristic. The direct drive generator perform higher efficiency and reduce the gearbox loss. The measurement data are used to calculate the interactions among the flow rate, blade, and generator, as well as the power generated by the water energy. The measurement data of the equipment are recorded on a memory card and analyzed using a computer. Table 4 shows the small, three-bladed VAMHT power generation parameters and measurements of the DC power output (average DC voltage is 150 V). Fig. 5 shows the system configuration combined with a permanent magnet synchronous generator, three-phase diode bridge rectifier, brake resistors, and fixed voltage load. When the turbine is rotated by the force of the water flow, the permanent magnet synchronous generator produces power through the three-phase diode bridge rectifier, and converts the AC (Alternating Current) voltage to a DC voltage output supply.


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Fig. 5. Electrical block diagram of the small three-bladed VAMHT.

Table 1 Micro-hydro turbine installation condition. Location Average water density Canal width Canal depth Water flow

4. Micro-hydro machine condition monitoring device Taoyuan, Taiwan 1127 kg m3 3.1 m 3m 1.5 m/s–3 m/s (minimum 1.0 m/s)

Table 2 Micro-hydro turbine power generation specifications. Number of blade Rated power output Maximum power output Water flow speed (m/s) Power output (kw) Yearly total power (kWh/y) Working hours: 7000 h

3 or 5 3 kW 9 kW 1 0.3 2300

2 2.7 19,000

3 9 64,000

Table 3 Micro-hydro turbine specification. Rated power Cut-in water speed Cut-out water speed Rated water speed Rotor diameter Pitch setting Blade profile Generator type Generator rate voltage Generator rate current Generator rate rpm Generator power

The interface of the condition-monitoring system of the microhydro turbine has four parts, as shown in Fig. 6: measurement of the electrical characteristics, measurement of the hydraulic characteristics of the canals or irrigation channels, data collection and monitoring system, and the data transmission network interface. The purpose of the measurement system is to determine the conversion efficiency among the water flow, turbines, and converters. Data analysis shows the various results of the system, and can be used to diagnose problems in the water turbine. Table 2 lists the micro-hydro turbine power generation specifications. Regulations limit the distributed generation (DG) to maintain the safety and quality of the electricity supply. The main switching element includes a main circuit breaker, which is used to connect or disconnect the generator from the grid. Measuring transformers, power transformers, and current sensors manage and analyze the output voltage and current conversion. Grounding is implemented according to the requirements specified by the grounding standard. 5. Experimental results

1.5 0.8 m/s 1.8 m/s 1.6 m/s 1.5 m/s–3 m/s (minimum 1.0 m/s) Fixed NACA0020 PMSG 220 V 10A 150 rpm 2.2 kw

The voltage load is maintained at a constant value, and the energy generated by the synchronous generator is maintained at a fixed rate to reduce any ripple voltage, which is generated by a floating water flow condition. DC-bus capacitors are used to reduce the voltage ripple and provide a self-sustaining VSC (Voltage Source Converter) DC bus.

The structure of the three-bladed VAMHT system includes small, vertical micro-turbine blades, bearings, and a generator, as shown in Fig. 3. The actual architecture and installation of the micro-turbine are shown in Figs. 4 and 7. Table 2 lists the microhydro turbine specifications. Fig. 7 shows an entity graph of a micro-hydro turbine installed in its water intake canal, without a water supply. The water turbine installation process requires the water level to be lowered to ensure safety; complete installation restores the water level and fully covers the blades, thereby obtaining full power output. Eq. (1) shows that the water passing through the area of the blade is proportional to the output power. Therefore, if the water level does not completely cover the blades, then the water turbine cannot reach maximum power. Furthermore, the upper and lower arms, which are subjected to different levels of torque caused by low water levels, may experience blade fractures. Table 1 lists the installation conditions of the three-bladed VAMHT system.

Table 4 The measurements data of water-turbine output from March 22 to April 17, 2012. Date

Daily power (Wh)

Accumulation power (Wh)

Average current (A)

Average voltage (V)

Average power (W)

Average rotational speed (rpm)

2012/3/22 2012/3/23 2012/3/24 2012/3/25 2012/3/26 2012/3/27 2012/3/28 2012/3/29

36,800 37,600 36,500 36,600 36,500 36,300 36,000 35,800

49,300 86,900 123,400 160,000 196,500 232,800 268,800 304,600

9.83 9.89 10.53 10.12 10.32 10.49 9.88 10.16

157.80 153.80 155.20 150.90 152.00 160.90 156.60 160.10

1551.17 1521.08 1633.88 1527.10 1567.98 1688.64 1547.34 1626.30

63 64 64 64 63 63 65 65

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Fig. 6. VAMHT hydroelectric power generation condition monitoring system architecture.

Fig. 7. Micro-hydro turbine installed in its water intake canal, without water supply.

the recorded data. The daily power of the water turbine is stable compared to those of solar energy and other types of DG; the average generated energy and rotational speed is 1.5 kW and 63 rpm, respectively. Table 4 shows the monitoring measurements of water-turbine output from March 22 to April 17. This paper presents a micro-hydroelectric generator, which is installed in channels and rivers, and demonstrates the stable power generation of large dams. Irrigation channels supply abundant and continuous water; thus, power generation is steady. Fig. 8 shows a threebladed VAMHT system daily-generation capacity histogram. The experimental results reported in relevant literature indicate that vertical wind turbine speeds range between 0 and 20 ms1 [18–22]. Air density is a thousand times smaller than water density, which speed ranges scale of wind can produce energy by velocity, but for water, that will be a devastating disaster. Because water density is large, the micro-hydro turbine is easy to cut in into the operation mode. The cut-in water flow is approximately

5.1. Power curve analysis The micro-turbine, through a variety of parameters, analyzes and evaluates the effectiveness and performance of its power generation using the condition-monitoring system. In this study, water flow determines the main factors of the micro-turbine power output. Analyzing the amount of electricity generated by the microturbine includes daily generating capacity, DC-output voltage and current, three-phase voltage and current in the generator, and the transient of the water turbine. The experimental period was from March 22, 2012 to April 17, 2012. The monitoring system measurement data show that the average daily power generation was 36 kWh, the average current was 10 A, and the average voltage was controlled by a constant voltage device at 160 V. The average power can be calculated using

Fig. 8. Three-bladed VAMHT system daily-generation capacity histogram.


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Fig. 9. Summary of power and power coefficient as a function of water flow curve. Fig. 11. Small three-bladed vertical-axis micro-turbine startup transient generator rotational speed, DC current (Idc), and DC voltage (Vdc).

0.7 ms1. However, the short scale of water flow speed also creates an immediate energy conversion limit. The water flow ranges between 0 and 1.8 ms1. Regarding the experimental results reported in this paper, the water flow was not processed by gravitational force or any pressure, but was a natural water flow. Fig. 9 summarizes power and the power coefficient as a function of a water flow curve. 5.2. The transient and steady of vertical-axis micro-hydro turbine system When the water level drops below the upper arm blade, the turbine blade can become damaged; consequently, a dynamic braking resistor is installed to protect the turbine blade. The brake resistor stops the micro-hydro turbine. The dynamic braking resistor releases the micro-hydro turbine when the water level completely covers the upper arm blade again. Fig. 10 shows the transientphase line current (Ia, Ib, and Ic) and phase voltage (VAB) of the blades. A three-phase current is generated by the induction voltage and sent to the system. The transition time of the micro-turbine is 0.5 s

from standstill to rotation. The induction voltage state transition is also 0.5 s after the sudden increase into the AC steady state in the micro-turbine. The described micro-hydroelectric monitoring system starts the transient voltage and current in DC-power generation. The micro-turbine system determines the stability of the speed and power generation before transferring power to the output. Starting the dynamic brake resistor and steadily operating the power transmission process requires 42 s. Fig. 11 shows the small, three-bladed vertical-axis micro-turbine startup transient generator rotational speed, DC current (Idc), and DC voltage (Vdc). The micro-turbine ceases operation under several conditions, such as when the turbine is stopped by the dynamic braking resistor switch for repairs; when the blade is broken or damaged by an unknown object; when the turbine stops because the blade loses water force; and when the water level is too low. The insufficient force of the water propelling the micro-hydro turbine can cause the turbine to stop rotating. In addition, a low water level can stop the turbine for safety and protection reasons. Fig. 12 shows that the turbine output waveform of the turbine stopped because of

Fig. 10. Transient-phase line current (Ia, Ib, and Ic) and phase voltage (VAB) of the blades.

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Fig. 12. Small three-bladed vertical-axis micro-turbine shut down transient generator rotational speed, DC current (Idc), and DC voltage (Vdc).

insufficient force from a low water level. Fig. 12 shows that when the rotational speed of the micro-turbine is less than 30 rpm, the water turbine operation mode is changed to a ‘‘cease’’ status. In the block diagram of the system shown in Fig. 5, the threephase rectifier bridge converts the power from AC to DC. Fig. 13 shows the high-frequency switching voltage of the convertor converting the DC output. The waveform diagram of the three verticalaxis blades for the small micro-turbine Vdc and Idc is also shown. In the upper graph of Fig. 13, the Idc value becomes apparent after high-frequency switching; the lower diagram of Fig. 13 shows the output DC constant voltage variable. Micro-hydro turbine operation is long-term and provides stable and reliable energy, which is the goal of the renewable energy field. The corresponding relationship between rotational speed and output power is also shown. The turbine channel regularly provides water downstream for public irrigation from the upstream dam; thus, stable average water can produce a continual power supply. Figs. 14 and 15 show plots of rotor speed, power, versus water flow. The large scatter in the data is due in part to the water at the test site during the experimental period.

Fig. 14. Micro-hydro turbine average power (kWh) and water flow (m/s).

Fig. 15. Micro-hydro turbine rotor speed (rpm) and water flow (m/s).

Fig. 13. Waveform diagram of the three vertical-axis blades for the small micro-turbine Vdc and Idc.



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6. Conclusion


This study derived experimental results from developing and practically demonstrating a vertical-axis water micro-hydro turbine. Micro-hydro turbine condition-monitoring techniques do not require a water flow sensor, thereby enabling simple, reliable, and economical construction and installation. The monitoring system can be adjusted to the maximum power output, and a dynamic braking function is used to prevent injury when water overflows or a low water level is detected, to reduce damage to the micro-turbine blades and ensure the safety of users. That exploits structure of wind turbine system to absorb energy of water. The advantage of using the micro-hydro turbine is that it obtains energy from water without intercepting the water. Simple construction and installation are also the merits of the microhydro turbine, thus allowing the micro-hydro turbine to be installed in any type of canal or irrigation channel without affecting the water flow. Water density is a thousand times larger than air density. Therefore, the micro-hydro turbine is easy to cut in into the operation mode. The cut-in water flow is approximately 0.7 ms1. The water flow ranges between 0 and 2 ms1. The short scale of water flow speed also creates an immediate energy conversion limit. The experimental results reveal that using this type of turbine in water is distinct from using it in wind. The experimental results also show that the speed ranges of the water flow are narrower than those of wind, and the status transformation from cut-into stable power generation is short. Developing a stable and long-term energy generation system is crucial in the field of energy research. This paper presents a 3-kw three-bladed VAMHT power generation system including the characteristics of power generation and a condition-monitoring measurement and management system. The experimental data show the capabilities of having stable and long-term power generation that exhibits the various characteristics of this type of wind turbine structure. The efficiency of this study is constrained by steam flow speed and crossover section area. And the vibration is due to the steam flow is not forcing the blade equally. In the aspect of commercial operation, the generation efficiency, the stability of the duck of the structure, regional development should be a researchable issue. In the future, the optimization of the distance of the configuration of generator connection will be discussed.

[1] Chacra FA, Bastard P, Fleury G, Clavreul R. Impact of energy storage costs on economical performance in a distribution substation. IEEE Trans Power System 2005;20:684–91. [2] Yuan XH, Wang YY, Xie J, Qi XW, Nie H, Su AJ. Optimal self-scheduling of hydro producer in the electricity market. Energy Convers Manage 2010;51:2523–30. [3] Perez-Diaz Juan I, Wilhelmi Jose R, Arevalo Luis A. Optimal short-term operation schedule of a hydropower plant in a competitive electricity market. Energy Convers Manage 2010;51:2955–66. [4] Shafie-khah M, Parsa Moghaddam M, Sheikh-El-Eslami MK, Babaei Ali. Price forecasting of day-ahead electricity markets using a hybrid forecast method. Energy Convers Manage 2011;52:2165–9. [5] Santolin A, Cavazzini G, Pavesi G, Ardizzon G, Rossetti A. Techno-economical method for the capacity sizing of a small hydropower plant. Energy Convers Manage 2011;52:2533–41. [6] Cheng CT, Shen JJ, Wu XY, Chau KW, et al. Operation challenges for fastgrowing China’s hydropower systems and respondence to energy saving and emission reduction. Renew Sustain Energy Rev 2012:2386–93. [7] Taormina R, Chau KW, Sethi R. Artificial neural network simulation of hourly groundwater levels in a coastal aquifer system of the Venice Lagoon. Eng Appl Artif Intell 2012;25:1670–6. [8] Wu CL, Chau KW, Li YS. Predicting monthly streamflow using data-driven models coupled with data-preprocessing techniques. Water Resour Res 2009;45:W08432. [9] Cheng CT, Shen JJ, Y X, Wu, Chau KW. Short-term hydroscheduling with discrepant objectives using multi-step progressive optimality algorithm. J Am Water 2012:464–79. [10] Sun YT, Gao QF. The design and operation of 700 MW class totally air-cooled hydroelectric generators. In: presented at the IEEE EUROCON. Conf.; 2009. [11] Song SH, Kang SI, Hahm NK. Implementation and control of grid connected AC–DC–AC power converter for variable speed wind energy conversion system. In: presented at the Applied Power Electronics. Conf; 2003. [12] Thiyagarajah K, Ranganathan VT, Iyengar BSR. A high switching frequency IGBT PWM rectifier/inverter system for AC motor drives operating from single phase supply. In: presented at the Power Electronics Specialists, Conf; 1990. [13] Ray Prakash K, Mohanty Soumya R, Kishor Nand. Proportional-integral controller based small-signal analysis of hybrid distributed generation systems. Energy Convers Manage 2011;52:1943–54. [14] Shojaeefard MH, Mirzaei A, Babaei A. Shape optimization of draft tubes for Agnew microhydro turbines. Energy Convers Manage 2014;79:681–9. [15] Hammons T, Naidoo P, Musaba L. Run of river bulk hydroelectric generation from the Congo river without a conventional Dam. Nat Resour 2011;2(1):18–21. [16] Margeta J, Glasnovic Z. Role of water-energy storage in PV-PSH power plant development. J Energy Eng June 2011;137:187–97. [17] Deane JP, Gallachóir BPÓ, McKeogh EJ. Techno-economic review of existing and new pumped hydro energy storage plant. Renew Sustain Energy Rev May 2010;14(4):1293–302. [18] Huskey A, Bowen A, Jager D. Wind turbine generator system power performance test report for the Gaia-Wind 11-kW wind turbine. In: presented at the Technical Report NREL/TP-500-46151; Dec 2009. [19] Curtis A, Gevorgian V. Wind turbine generator system power quality test report for the Gaia Wind 11-kW wind turbine. In: presented at the Technical Report NREL/TP-5000-51477; July 2011. [20] Miller A, Muljadi E, Zinger DS. A variable speed wind turbine power control. IEEE Trans Energy Conversion Jun. 1997;12(2):181–6. [21] Shanka PN. Development of vertical axis wind turbines. Acad Proc Eng Sci 2011;2(1):49–66. [22] Teodorescu R, Blaabjerg F. Flexible control of small wind turbines with grid failure detection operating in stand-alone and grid-connected mode. IEEE Trans Power Electronics September 2004;19:1323–32.

Acknowledgments The authors would like to thank Feng Chia University (Contract No. 94GB66) and the National Science Council of the Republic of China, Taiwan (Contract No. NSC 101-2632-E-035-001-MY3) for financially supporting this research.