Excitation Synchronous Wind Power Generators With Maximum Power Tracking Scheme

Excitation Synchronous Wind Power Generators

With Maximum Power Tracking Scheme

Tzuen-Lih Chern, Ping-Lung Pan, Yu-Lun Chern, Wei-Ting Chern, Whei-Min Lin, Member, IEEE,

Chih-Chiang Cheng, Jyh-Horng Chou, Senior Member, IEEE, and Long-Chen Chen

Abstract—This paper presents a novel excitation synchronous

wind power generator (ESWPG) with a maximum power tracking

scheme. The excitation synchronous generator and servo motor

rotor speed tracks the grid frequency and phase using the proposed

coaxial configuration and phase tracking technologies. The

genera-tor output can thus be directly connected to the grid network

without an additional power converter. The proposed maximum

power tracking scheme governs the exciter current to achieve stable

voltage, maximum power tracking, and diminishing servo motor

power consumption. The system transient and static responses over

a wide range of input wind power are examined using simulated

software. Experimental results from a laboratory prototype

ESWPG demonstrate the feasibility of the proposed system.

Index Terms—Excitation synchronous generator, maximum

power tracking, servo motor control, wind power.

I. I

T

HE GLOBAL market demand for electrical power

produced by renewable energy has steadily increased,

explaining the increasing competitiveness of wind power

tech-nology. Wind power generators can be divided into induction

and synchronous types [1]

–[8]. The excitation synchronous

generator driven by hydraulic, steam turbine, or diesel engines

has been extensively adopted in large-scale utility power

gener-ation owing to desired features such as high efficiency,

reliabili-ty, and controllable output power. A wind power generator in

grid connection applications, except for doubly fed induction

generators, achieves these features using variable speed constant

frequency technology. However, most excitation synchronous

wind generators cannot be connected directly to the grid, owing

to instabilities in wind power dynamics and unpredictable

prop-erties that influence the generator synchronous speed. The

direct-drive permanent magnet synchronous wind generator (PMSWG)

uses variable speed and power converter technologies to fulfill

the grid connection requirements, which has advantages of being

gearless. Various power transfer technologies are applied for

ac/dc transformation to obtain a constant frequency ac power

[9]–[16]. However, extensive use of power electronic devices in

those systems that will cause unavoidable power losses from the

rectifier’s conducting resistance and high-frequency power

switches, which will increase power consumption. Therefore,

a converterless method for a high-ef

ficiency excitation

synchro-nous wind generator is an important issue, especially for middle

and high output voltage wind power generators.

This paper presents a novel converterless wind power

gener-ator with a control framework that consists of an excitation

synchronous generator, permanent magnet (PM) synchronous

servo motor, signal sensors, and servo control system. The wind

and servo motor powers are integrated with each other and

transmitted to the excitation synchronous generator via a coaxial

configuration. When the wind speed varies, the servo motor

provides a compensatory energy to maintain constant generator

speed. The additional servo motor power is also transformed into

electricity, and output into the load. This means that the motor

power is not wasted. Using a precise phase tracking function

design, the proposed robust integral servo motor control scheme

reduces the output voltage phase shift in the excitation

synchro-nous generator from wind disturbances. According to the servo

motor power magnitude and the generator power, the proposed

maximum power tracking scheme controls the excitation

field

current to ensure that the excitation synchronous generator fully

absorbs the wind power, and converts it into electricity for the

loads. Based on physical theorems, a mathematical model for the

proposed system is established to evaluate how the control

function performs in the designed framework. The detailed

structure and experimental results will be discussed in the

following sections.

II. P

F

S

For simplicity, assume that all energy transmission elements

behave ideally, allowing us to ignore the mechanical power

losses of the wind turbine, the servo motor, and the excitation

synchronous generator. Fig. 1 shows the power

flows of the

proposed system, where

,

, and

denote the torques and

,

, and

are the wind turbine, servo motor, and excitation

synchronous generator speeds, respectively. The total excitation

synchronous generator input power is the product of

and

.

The power

flow equation can thus be defined as

1949-3029 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. Manuscript received August 17, 2013; revised December 29, 2013 and April

14, 2014; accepted May 25, 2014. Date of publication June 30, 2014; date of current version September 16, 2014. This work was supported in part by the National Science Council of Taiwan under Grant NSC 102-3113-P-110-005.

T.-L. Chern, P.-L. Pan, Y.-L. Chern, W.-T. Chern, W.-M. Lin, and C.-C. Cheng are with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan (e-mail: [email protected]).

J.-H. Chou is with the Graduate Institute of Electrical Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan (e-mail: [email protected]).

L.-J. Chen is with the Department of Mechanical and Electromechanical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan (e-mail: [email protected]).

Color versions of one or more of thefigures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TSTE.2014.2327130

Fig. 2 shows the corresponding coaxial con

figuration. The

wind generator rotor shaft input-end receives rotating torques

from the speed increasing gear box. The tail-end of the generator

rotor shaft is coupled with a servo motor. The input energy of the

excitation synchronous generator is the sum of the wind power

and servo motor powers. The speed and rotating direction for the

wind turbine output, servo motor, and excitation synchronous

generator is the same, i.e., the system speeds satisfy

. This arrangement can reduce the power

trans-mission losses.

III. C

P

P

W

P

G

S

Fig. 3 depicts the control framework of the proposed system.

The control system design concepts maintain power

flow balance

between the input and the output and, simultaneously, force the

generator frequency to synchronize with the utility grid. When

the system complies with these conditions, the generator output

can be connected to the utility grid network, subsequently

reaching the high efficiency and maximum power tracking

objectives. The control signals, including the generator voltage,

current, grid phase, motor encoder, and output power, are sensed

and transferred to the microprocessor control unit (MCU). The

servo motor controller plays an important role in output power

and grid voltage phase tracking. A situation in which the

controller detects a power increase from the servo motor implies

decreasing wind speeds. At this moment, the system regulates the

exciter current to reduce the excitation generator output power. A

chain reaction subsequently occurs in which the servo motor

power returns to a balanced level. During the energy balance

periods, the servo motor consumes only a slight amount of

energy to stabilize the shaft speed. Once (1) is satis

fied, both

the maximum power and the constant speed can be obtained by

the designed control scheme.

Fig. 4 schematically depicts the servo motor and maximum

power tracking control (MPTC) loops which are designed to

stabilize the speed, frequency, and output power of the excitation

synchronous generator under wind disturbances. The wind

turbine provides mechanical torque to rotate the generator shaft

via the speed-increasing gear box. As the generator shaft speeds

reach the rated speed, the generator magnetic

field is excited. The

MPTC then controls the output voltage reaching grid voltage.

Moreover, the generator output waveform is designed in phase

with the grid using the servo motor control track grid sine

waveform. Owing to the difficulty in precisely estimating the

wind speed, the proposed MPTC scheme measures the motor

output power as the reference signals to determine the generator

output power. The excitation synchronous generator output

frequency, voltage-phase, and output power are fed back into

the control scheme. The phase/frequency synchronization

strat-egy in Fig. 4 compares the grid voltage-phase and frequency with

the generator’s feedback signals, and produces the position

command

with pulse-type signals to the servo motor driver.

The MPTC also adjusts the excitation

field current

based on

the wind power and motor power inputs, where

denotes the

servo motor rotor mechanical rotor angular displacement

de-tected by an encoder. Due to the coaxial configuration, detecting

the relative position of the rotor allows us to determine the

generator voltage phase during the wind power generator system

Fig. 1. Powerflow block diagram.

Fig. 2. Proposed coaxial construction configuration.

Fig. 3. Proposed wind power system framework.

Fig. 4. Proposed wind power generator system.

waveform in phase with the grid. Consequently, the generator

fully absorbs the wind power and grid connection requirement

can be ensured. To observe the system feasibility, the transient

responses for wind power, generator power, motor power, and

generator shaft speed were measured as shown in Fig. 14(a) and

(b). The system is assumed to have a stable point at a wind power

of 2500 W in the time period beginning in Fig. 14(a) and (b).

However, the measured generator power output is only 2000 W.

The generator output power is 500 W less than the wind power,

which is mainly due to the power consumptions of mechanical

friction and moment of inertia at running speed of 1800 rpm, and

the partial power loss comes from energy conversion efficiency

of the generator. According to Fig. 14(a) and (b), regardless of

whether the wind is step or sine wave change cases, the proposed

MPTC scheme converts the wind energy into electricity. A slight

amount of power in the servo motor can maintain the generator

shaft speed constantly and achieve excellent power quality.

X. C

This paper presented an excitation synchronous wind power

generator with MPTC scheme. In the proposed framework, the

servo motor provides controllable power to regulate the rotor

speed and voltage phase under wind disturbance. Using a phase

tracking control strategy, the proposed system can achieve

smaller voltage phase deviations in the excitation synchronous

generator. In addition, the maximum output power tracking

scheme governs the input and output powers to achieve high

performance. The excitation synchronous generator and control

function models were designed from the physical perspective to

examine the presented functions in the proposed framework.

Experimental results demonstrate that the proposed wind power

generator system achieves high performance power generation

with salient power quality.

R

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Tzuen-Lih Chern was born in Kaohsiung, Taiwan, in 1958. He received the M.S. and Ph.D. degrees in industrial control and flight control systems from the Institute of Electronics, National Chiao-Tung University, Hsinchu, Taiwan, in 1985 and 1992, respectively.

From February 1992 to January 1998, he was an Associate Professor with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, where he has been a Professor since February 1998. His research interests include industrial control, wind power, motor drives, and power electronics.

Ping-Lung Pan was born in Ping-Tung, Taiwan on March 28, 1959. He received the B.S. and M.S. degrees in electronic engineering from the I.-SHOU University, Kaohsiung, Taiwan, in 2000 and 2003, respectively. Currently, he is pursuing the Ph.D. degree in control systems and wind power system design at the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan.

Yu-Lun Chern was born in Kaohsiung, Taiwan, in 1988. He received the M.S. degree in motor drive from the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, in 2013.

From 2014, he was a TSMC Technology Engineer. His research interests include industrial control, motor drives, and power electronic.

Wei-Ting Chern was born in Taipei, Taiwan, in 1986. He received the M.S. degree in wind power from the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, in 2013.

His research interests include wind power and power electronic.

Whei-Min Lin (M’87) was born in 1954. He received the B.S.E.E. degree from the National Chao-Tung University, Hsin-Chu, Taiwan, the M.S.E.E. degree from the University of Connecticut, Storrs, CT, USA, and the Ph.D.E.E. degree from the University of Texas, Arlington, TX, USA, in 1985.

Currently, he is a Professor with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan. His research interests include geographic information system (GIS), distribution system, supervisory control and data acquisition (SCADA), and automatic control system.

Chih-Chiang Cheng was born in Taipei, Taiwan, on February 27, 1957. He received the B.S. degree in electrical engineering from Chung Yuan Christian University, Chung-Li, Taiwan, in 1981, the M.S. and Ph.D. degrees in electrical engineering from the Uni-versity of Texas at Arlington, Arlington, TX, USA, in 1983 and 1991, respectively, and the Engineer degree in electrical engineering from the University of South-ern California, Los Angeles, CA, USA, in 1985.

Currently, he is a Professor with the Department of Electrical Engineering, National Sun Yat-Sen Univer-sity, Kaohsiung, Taiwan. His research interests include system and control theory, with emphasis on design of nonlinear control systems such as variable structure control, backstepping control, adaptive control, and fuzzy control.

Jyh-Horng Chou (SM’04) received the B.S. and M.S. degrees in engineering science from National Cheng-Kung University, Tainan, Taiwan, in 1981 and 1983, respectively, and the Ph.D. degree in mecha-tronic engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 1988.

Currently, he is the Chair Professor with the Department of Electrical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan, as well as the Distinguished Professor with the Institute of Electrical Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan. His research and teaching interests include intelligent systems and control, computational intelligence and methods, automation technology, robust control, and quality engineering.

Long-Jeng Chen received the Ph.D. degree from the Department of Mechanical Engineering, University of Iowa, Iowa City, IA, USA, in 1990.

He is an Associate Professor with the Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan. His research interests include power genera-tion by water or wind turbine and developments of PEM fuel cells.