Experimental Assessment with Wind Turbine Emulator of Variable-Speed Wind Power Generation System using Boost Chopper Circuit of Permanent Magnet Synchronous Generator

Abstract

This paper presents experimental results and its assessment of a variable-speed wind power generation system (VSWPGS) using permanent magnet synchronous generator (PMSG) and boost chopper circuit (BCC). Experimental results are obtained by a test bench with a wind turbine emulator (WTE). WTE reproduces the behaviors of a windmill by using servo motor drives. The mechanical torque references to drive the servo motor are calculated from the windmill wing profile, wind velocity, and windmill rotational speed. VSWPGS using PMSG and BCC has three speed control modes for the level of wind velocity to control the rotational speed of the wind turbine. The control mode for low wind velocity regulates an armature current of generator with BCC. The control mode for middle wind velocity regulates a DC link voltage with a vector-controlled inverter. The control mode for high wind velocity regulates a pitch angle of the wind turbine with a pitch angle control system. The hybrid of three control modes extends the variable-speed range. BCC simplifies the maintenance of VSWPGS while improving reliability. In addition, VSWPGS using PMSG and BCC saves cost compared with VSWPGS using a PWM converter.

I. INTRODUCTION

Wind power has become one of the most attractive energy resources for electricity production because it is virtually pollution free (if noise is not considered as pollution). Thus, a great deal of research has focused on the development of a new turbine design to reduce the cost of wind power and to make wind turbines economical and efficient. The investigation of wind power system involves high-performance wind turbine emulators (WTEs), particularly for the development of optimal control solutions. Several studies are being conducted on variable-speed wind generator systems that change the windmill rotational frequency in proportion to wind velocity to utilize renewable wind energy effectively by using power generating systems [1]-[6]. The energy conversion efficiency of propeller windmills depends on wind velocity and windmill speed. Therefore, a converter connected to the generator must control the windmill speed in proportion to the wind velocity to maintain the maximum energy conversion efficiency [7].

In recent years, variable-speed wind generator systems using the PWM converter (PWM converter scheme) have been introduced [3]-[6]. PWM converter is costly because many switching devices are used, and its control system is complicated. However, the variable-speed wind generator systems using PWM converters are used for variable-speed large-capacity wind generator systems that require windmill speed control.

The supply of excitation energy with a converter is not required when a permanent magnet synchronous generator (PMSG) is used. Therefore, wind generator systems using PMSG and a diode bridge rectifier to charge the battery are mainly operated at the rated wind velocity because the induced voltage is sufficiently high [8], [9]. These wind generator systems are low-cost converters. However, the windmill speed control is difficult to achieve because the converter cannot control the PMSG torque by itself. Furthermore, the induced voltage necessary for system interconnection cannot be obtained at low wind velocity. To date, a diode bridge rectifier scheme that realizes windmill speed control and system interconnection simultaneously at low wind velocity has not been reported.

This paper describes the experimental verification of a variable-speed wind power generation system (VSWPGS) that uses PMSG and boost chopper circuit (BCC; BCC scheme), with three speed control modes for wind velocity. The diode bridge rectifier and BCC are used as a converter. The speed control mode of low wind velocity controls the windmill speed by controlling the PMSG torque with BCC. BCC can control the generator torque by adjusting the armature current of PMSG. The speed control mode of middle wind velocity controls the windmill speed by controlling the PMSG torque with the system interconnection inverter. The system interconnection inverter can control the PMSG torque by adjusting the DC link voltage. The speed control mode over the rated wind velocity controls the windmill speed by controlling the windmill torque with the pitch angle control system. The pitch angle control system can control the windmill torque by adjusting the pitch angle. The extension of the variable-speed range is realized by using the hybrid control of three speed control modes. Consequently, system interconnection for all speed control modes is achieved.

 

II. VARIABLE-SPEED WIND GENERATOR SYSTEM

A. System Configuration

Fig. 1 shows the variable-speed wind power generation system. The gearless mechanism that directly connects PMSG with the windmill without using a speed-increasing gear is realized by adopting the multipole PMSG as a generator. The windmill has a pitch angle control system. The pitch angle control system is activated beyond the rated wind velocity, and the windmill rotational speed is kept at the rated windmill speed. Under the rated wind velocity, the windmill rotational speed is controlled to maintain maximum energy conversion efficiency. The converter is connected to the PWM inverter for system interconnection through the DC link condenser. The PWM inverter supplies the power system with the active power obtained from the converter side.

Fig. 1.Variable-speed wind power generator system.

B. Boost Converter Scheme

Fig. 1(a) shows the VSWPGS that uses the diode bridge rectifier and BCC (BCC scheme). The operation in the motoring mode cannot be obtained, because the diode bridge rectifier limits the current direction of the DC link. However, this condition does not become a problem, because the motoring mode is not used in this VSWPGS. The value of the DC link current depends on the relationship between the induced voltage of PMSG and the DC link voltage. Therefore, the diode bridge rectifier cannot control the generator torque. Therefore, BCC is added to improve its variable-speed range. BCC can control the DC link current regardless of the induced voltage value. In the low wind velocity range, in which the diode bridge rectifier cannot control the generator torque, BCC can control the generator torque by controlling the DC link current.

C. PWM Converter Scheme

Fig. 1(b) shows the VSWPGS that uses the PWM converter (PWM converter scheme). The PWM converter controls the windmill rotational speed by adjusting the generator torque. The PWM inverter maintains the DC link voltage to be the constant voltage necessary for the system interconnection. The PWM converter scheme can perform high-speed and high-precision generator torque control from the cut-in wind velocity to the rated wind velocity, given that the PWM converter scheme can apply the vector control system. Therefore, the frequency characteristics and variable-speed range of the windmill rotational speed control serve as a benchmark. The PWM converter scheme is referred to as a comparison object to evaluate the performance of the BCC scheme.

 

III. CONTROL SYSTEM

A. WTE

The basic structure of the WTE is shown in Fig. 2. The wind turbine model is based on blade element momentum theory (BEMT) [8]. The wind turbine model consists of three-dimensional (3D) table data of wind turbine characteristics and the mechanical model considering the difference between real wind turbine and induction motor (IM). The 3D table data are composed of the torque Q for wind velocity V0 and windmill rotational speed ωr. The 3D data are calculated by applying the wind turbine parameters of Fig. 3 to BEMT. V0 is provided as a condition of emulation. ωr is detected with the rotary encoder (RE).

Fig. 2.WTE.

Fig. 3.Wind turbine blade shape.

cl and cd are obtained via fluid analysis with finite element method for each point and each meeting angle from 0° to 51°. The concrete numeric data are not given because of the space constraints. Fig. 4 shows the calculated 3D data.

Fig. 4.3D table data of output power.

B. Mechanical Model

The difference of mechanical models between real wind turbine and WTE is explained with Fig. 5. The moment of inertia and coefficient of viscosity are different between the wind turbine and IM. The differences affect the transient and steady-state responses. The differences must be compensated because wind velocity, V0, always changes. The difference can be compensated by the following equation:

where Timref is the torque reference of IM [Nm], Tt is the torque of wind turbine [Nm], Jim is the moment of inertia for IM [kgm2], Jt is the moment of inertia for wind turbine [kgm2], Bim is the coefficient of viscosity for IM [Nm/(rad/s)], Bt is the coefficient of viscosity for wind turbine [Nm/(rad/s)], and ωt is the rotational speed of WTE.

Fig. 5.Mechanical model.

Fig. 6 shows the logic for the control of wind turbine and the steady state in which sufficient time has passed under the condition of fixed V0. The timing of changing the control schemes is divided into Modes 1, 2, 3, and 4.

Fig. 6.Logic for the control of wind turbine.

C. Duty Control System for the BCC Scheme [9], [10]

Fig. 7 shows the duty ratio control system for BCC. The change of wind velocity includes the ranges of low wind velocity, middle wind velocity, and high wind velocity. For the low wind velocity, the duty control system adjusts the DC link current by controlling the BCC. The DC link current corresponds to the necessary generator torque to control windmill rotational speed. The DC link voltage is maintained at 35 V, which is the necessary voltage for the system interconnection by the inverter, when the duty control system is switched on. The duty becomes 0, because the DC link voltage is maintained at 35 V by the inverter, when the windmill speed rises and when the line voltage of PMSG exceeds 35 V. The operating range corresponds to a middle wind velocity. Moreover, only the rectification using the reactor and diode bridge rectifier is performed.

Fig. 7.Duty ratio control system for the BCC scheme.

where KP is the proportional gain, and KI is the integral gain. Simulink model is constructed by Equation (2) and the pulse generator block, which estimates the Bd based on Duty.

D. Converter Control System for the PWM Converter Scheme

Fig. 8 shows the converter control system. In the operating range under the rated wind velocity, the speed regulator is used jointly with the torque current feed-forward control. In the operating range over the rated wind velocity, the pitch angle control system controls the windmill rotational speed, and the converter control system conducts the electric power control and torque current feed-forward control. The converter control system is configured by the following equations. The torque current feed-forward control is described as follows:

Fig. 8.Converter control system for the PWM converter scheme.

Under the rated wind velocity, the rotational speed is controlled by regulating the torque current of PMSG. The torque current reference is calculated by the following equation of the PI controller:

Over the rated wind velocity, the windmill rotational speed is controlled by regulating the wind turbine torque. The wind turbine torque is regulated by the pitch angle control system as follows:

Moreover, the output power is kept at the rated value by regulating the torque current of PMSG. The torque current reference is calculated by the following equation of the PI controller:

The stator voltage references are calculated by the following equations:

E. Inverter Control System for the BCC Scheme

Fig. 9 shows the inverter control system for the BCC scheme. The windmill speed control is performed by using the duty control system, when the induced voltage of PMSG is under 35 V. Then, the inverter control system maintains the DC link voltage at 35 V. The duty control system cannot control the windmill rotational speed when the DC link voltage exceeds 35 V. This operating range corresponds to the ranges of middle wind velocity and high wind velocity. In a middle wind velocity under the rated wind velocity, the windmill speed control is performed with the inverter control system. Concretely, the inverter control system adjusts the electric torque of PMSG by adjusting the DC link voltage, and this system controls the windmill rotational speed. In high wind velocity, in which the wind velocity exceeds the rated wind velocity, the windmill rotational speed control is performed with the pitch angle control system. Moreover, the inverter control system performs the electric power control. The inverter control system is configured by the following equations of PI speed controller:

Fig. 9.Inverter control system for the BCC scheme.

Over the rated wind velocity, the rotational speed is controlled by regulating the wind turbine torque. The wind turbine torque is regulated by the pitch angle control system as follows:

When the pitch angle control is activated, the DC link voltage reference is calculated by the following equation of PI power controller:

The DC link voltage is controlled by regulating the active current. The DC link voltage reference is calculated by subtracting outputs of PI speed and power controllers from constant DC link voltage reference:

The reactive current reference is constant at 0 to keep the power factor at 1. The output voltage references are calculated by using the following equations:

F. Inverter Control System for the PWM Converter Scheme

Fig. 10 shows the inverter control system of the PWM converter scheme. The PWM converter scheme performs the speed control and electric power control of PMSG on the converter side. In the PWM converter scheme, the DC link voltage Vdc is controlled at the constant of 35 V to perform speed control and electric power control of PMSG, whereas the system interconnection is conducted in the operating ranges of wind velocity. The d axis current reference isdref of Equation (12) is estimated by using the constant DC link voltage reference Vdcref of 35 V. Moreover, isqref is set at 0 to maintain the power factor of a power system at 1. The current controller is composed of Equations (13) and (14).

Fig. 10.Inverter control system for the PWM converter scheme.

G. Pitch angle control system

Fig. 11 shows the block diagram of the pitch angle control system. The model of the pitch angle drive system describes the response of the displacement servo system and the hydraulic cylinder. This model is described as the first-order lag element, because the hydraulic cylinder has integral characteristics and because it becomes the response of the closed-loop system that comprises the servo gain. The time constant of the first-order lag element is the time constant of the pitch angle control system adopted for the wind generation system.

Fig. 11.Pitch angle control system.

Table I shows the generator and power system parameters. The parameters of PMSG, power system, and transformer are common. Table II shows the control parameters for the BCC scheme. The control parameters of the boost converter and duty control system are designed with the simulations by using the implemented Simulink models. The control parameters of the vector control system applied in the inverter side are designed with the conventional method that considers the arrangement of pole and zero based on the control theory. Table III shows the control parameters of the PWM converter scheme. The control parameters of the vector control system applied in the converter and inverter sides are designed with the conventional method that considers the arrangement of pole and zero.

TABLE IGENERATOR, POWER SYSTEM, AND TRANSFORMER PARAMETERS

TABLE IICONTROL PARAMETERS OF BOOST CONVERTER

TABLE IIICONTROL PARAMETERS OF PWM CONVERTER

 

IV. EXPERIMENTAL SYSTEM

Fig. 12 shows the experimental system of the BCC and PWM converter schemes. The digital signal processing (DSP) unit calculates the wind turbine torque reference from the wind velocity data and rotational speed of the IM. The wind velocity data are provided by the stored data in a PC. The rotational speed of IM is estimated in the DSP by using the signals from the RE. The torque of IM is controlled by the off-the-shelf inverter. The IM is operated based on the wind turbine torque reference sent from a DSP through the D/A board (DS2102). After the WTE is activated, the PMSG starts to rotate, and the converter control system starts to control the windmill rotational speed for the maximum power point tracking. The duty and PWM signals that control the duty and INV are sent from a DSP to the short-circuit prevention circuit through a digital IO board (DS4002). The PWM signals are sent to the CNV and INV through the optical circuit.

Fig. 12.Experimental system.

Fig. 13 shows the change of the wind velocity V0 [m/s] and includes each mode shown in Fig. 6. The change of wind velocity includes the ranges of low wind velocity at 5 m/s, which corresponds to Mode 1, wind velocity at 6 m/s, which corresponds to the switching point between Modes 2 and 3, middle wind velocity at 7 m/s, which corresponds to Mode 2 and Mode 3, and high wind velocity at 11 m/s, which corresponds to Mode 4. High wind velocity denotes the wind velocity that exceeds the rated value. All ranges include the sinusoidal change of amplitude: 0.5 [m/s] and 0.2 [Hz]. The BCC and PWM converter schemes are tested for the same change of the wind velocity, and the control performances of both schemes are discussed.

Fig. 13.Change of wind velocity.

Figs. 14 and 15 show the experimental results of the BCC scheme. The system activates at 5 m/s, which exceeds the cut-in wind velocity of 3 m/s, and the power generation started after approximately 9 seconds, in which windmill speed ωg reaches the windmill rotational speed reference ωgref. In the wind velocity of 5 m/s, which corresponds to Mode 1, the speed control is performed by the duty control system. Fig. 14 illustrates that windmill speed ωg and duty can confirm that the speed control is well performed. Fig. 15 illustrates that ωg, duty, and DC link voltage Vdc can confirm that the speed control is performed while the speed control system is smoothly switched at the wind velocity of 6 m/s, which corresponds to the switching point of Modes 2 and 3. At the wind velocity of 7 m/s, which corresponds to Mode 3, the speed control is performed by adjusting Vdc with the inverter control system. ωg and Vdc can confirm that the speed control is well performed. In wind velocity of 11 m/s, which corresponds to Mode 4, the speed control is performed by pitch angle control system, and the electric power control is conducted by the inverter control system. Fig. 15 illustrates that ωg, Vdc, and pitch angle θ can confirm that ωg is controlled at the rated speed and that the generated power Ps is controlled at the rated power of 300 W.

Fig. 14.Experimental results of the BCC scheme.

Fig. 15.Experimental results of the BCC scheme.

Figs. 16 and 17 show the simulation results of the PWM converter scheme. The system activates at the wind velocity of 5 m/s, which exceeds the cut-in wind velocity of 3 m/s. Moreover, the power generation started after approximately 11 seconds, in which ωg reaches in ωgref as well as the BCC scheme. The system is operated under the rated wind velocity, which corresponds to Modes 1 to 3 until about 115 seconds. Thus, ωg follows ωgref. However, the response of the PWM converter scheme has been retarded because the increase of ωgref corresponds to the increase of wind velocity given that the restriction of torque current reference iaqref has been established so that PMSG may not conduct the motoring. The operating range, which exceeds the rated wind velocity and corresponds to Mode 4, starts after 115 seconds. The pitch angle control system starts around 118 seconds. Then, the speed controller of converter control system stops simultaneously, and the maximum electric power tracking control is activated. The sinusoidal change of wind velocity is superimposed in this operating range. ωg is maintained at the rated speed-by-speed control by pitch angle control system and the electric power control by converter control system. Moreover, the generated output Ps is maintained at the rated power of 300 W.

Fig. 16.Experimental result of the PWM converter scheme.

Fig. 17.Experimental result of the PWM converter scheme.

Table IV shows the integrated powers of each mode of the BCC and PWM converter schemes. Integrated Ps of the BCC scheme decreases at 0 seconds to 55 seconds, which corresponds to Mode 1, compared with integrated Ps of the PWM converter scheme, because the tracking characteristic of ωg for ωgref is inferior. The BCC scheme uses the hybrid control of the duty control system and speed control of the inverter side at 55 seconds to 80 seconds, which corresponds to the switching point of Modes 1 and 2. In this operating range, integrated Ps of the BCC scheme decreases, because the tracking characteristic of the speed reference is inferior to the PWM converter scheme. The BCC scheme charges the DC link condenser at 80 seconds to 105 seconds, which corresponds to Mode 3. Therefore, integrated Ps decreases. In this period, speed control is performed by charging the DC link condenser, given that the wind velocity increases. Integrated Ps of the BCC scheme and the PWM converter scheme have a significant difference, given that the DC link capacitor of the BCC scheme discharges in the operating range of 105 seconds to 130 seconds, which exceeds the rated wind velocity and corresponds to Mode 4. At this operating range, the pitch angle control is activated, and the efficiency of the converter of the BCC scheme changes because the operating circuit changes for the operating modes.

TABLE IVINTEGRATED POWER OF EACH MODE

 

V. CONCLUSIONS

This paper proposed the VSWPGS using the BCC scheme. An implementation process that considers all systems for the BCC scheme was proposed to verify the validity. In addition, the implementation process of the VSWPGS using the PWM converter scheme was described as the benchmark comparison. The experimental systems of both schemes were built to evaluate the performance of the BCC scheme. The BCC scheme enabled windmill rotational speed control from the cut-in wind velocity to the overrated wind velocity. Therefore, the BCC scheme obtained a performance similar to that of the PWM converter scheme. The developed WTE can be used to analyze various mechanical and electrical characteristics of a grid-tied PMSG wind power system. Moreover, it can be used to educate students or engineers to understand the operation of grid-tied PMSG wind power systems.