PERMANENT MAGNET SYNCHRONOUS GENERATOR BASED STANDALONE SYSTEM

Nandini.A, Isha T.BDepartment of electrical and Electronics Engineering

Amrita Vishwa VidyapeethamAmrita Nagar, Ettimadai,

Coimbatore, India

Abstract— Energy crisis faced today, has thrown in new type oftechnological challenges forcing to search new power generation sources, especially for those living in remote and rural locations. In this paper, a standalone power generation system using Permanent Magnet Synchronous Generator (PMSG) is investigated. The system consists of a Permanent Magnet Synchronous Generator (PMSG), a diode rectifier, a buck-boost converter and an IGBT based inverter with a DC link capacitor. A stand alone load is connected to the inverter terminals. A simulation study of a wind turbine driven PMSG is done using SIMULINK in MATLAB. A laboratory model experimental set up of the system is developed in which the wind turbine was emulated by a separately excited dc motor mechanically coupled to the PM machine. The load voltage and frequency was kept constant.

Keywords— permanent magnet synchronous generator (PMSG),standalone, diode rectifier, wind turbine, space vector modulation, constant voltage and frequency.

I. INTRODUCTION

In many countries, there are remote communities where connection with the power grid is too expensive or impractical and diesel generators are often the source of electricity. Under such circumstances, a locally placed small-scale standalone distributed generation system can supply power to the customers. Presently, doubly fed induction generators (DFIGs) are widely used as the generator in a variable speed wind turbine system. In case of DFIG, there is a requirement of the gearbox to match the turbine and rotor speed. The gearbox many times suffers from faults and requires regular maintenance, making the system unreliable. The reliability of the variable speed wind turbine can be improved significantly using a direct drive-based permanent magnet synchronous generator (PMSG). PMSG has received much attention in wind energy applications because of its self-excitation capability, leading to a high power factor and high efficiency operation. In this paper, a control system is developed to control the load voltage in terms of the amplitude and frequency in a stand alone mode.

In a standalone system, the output voltage of the load side converter has to be controlled in terms of amplitude and frequency. Literature related to PMSG based variable speed wind turbine is mostly concentrated on grid connected system. Much attention has not been paid for a standalone system. Many countries are affluent in renewable energy resources; however they are located in

remote areas where power grid is not available. Locl, small scale stand-alone distributed generation system can utilize these renewable energy resources when grid connection is not feasible. In this paper, load voltage in terms of the amplitude and frequency are controlled in a standalone mode. Here wind turbine is used as the prime mover for PM generator. Since the characteristics of a wind turbine matches with that of a separately excited DC motor, it can be used to emulate the wind turbine characteristics. Wind speed variation is obtained by varying the input voltage to a dc motor, at rated field current.

II. PROPOSED STANDALONE SYSTEM

The basic block diagram of the system is shown in Figure 3.1.

Figure 1: Basic block diagram of the standalone system

It consists of the following: Wind turbine Permanent magnet synchronous generator (PMSG), which

is directly driven by the gearless wind turbine. DC link capacitor Three phase diode bridge rectifier and a buck-boost dc-dc

converter A space vector controlled voltage source inverter

A. Permanent Magnet Synchronous Generator

The PMSG consists of a rotor and stator. The rotor has poles lined on the outer wall. The permanent magnets are usually kept in pairs thereby to obtain the required flux. The stator has copper cable wound around it. A cross section of the PMSG is shown in Figure 2. It consists of

A steel spine and shaft.

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A stator containing coils of wire Two magnet rotors

The rotor of the PMSG is usually coupled to a prime mover.Normally the generator will be coupled to the wind turbine. When the rotor starts to rotate the flux linkage with respect to the stator winding changes and hence an emf is induced in the stator winding. This induced emf enables the flow of current.

Figure 2: Cross section of PMSG

There are two PM synchronous machine types, the surface-mounted PM (SPM) and the interior-buried PM (IPM). The magnets of the SPM machine are attached on the surface of the rotor, while the IPM machine is buried inside. Due to the structure, the rotor of the SPM machine does not have saliency. The inductance measured at the motor terminal is constant regardless of the rotor position. For the IPM, however, the reluctance of the magnetic flux path varies according to the rotor position. Because of this, the inductance at the motor terminal varies according to the rotor position. Due to saliency, the control for an IPM machine is more difficult than SPM machine. A high value of overallefficiency can be achieved, while keeping the mechanical structure of the turbine simple.

B. Wind turbine

A wind turbine is a mechanical device that is capable of converting the potential energy present in the wind speed to kinetic energy which is used to generate electric energy. Typical modern wind turbine has one of two basic operating modes: constant or variable speed

The power developed in a wind turbine is given by

Pm= (1/2) ρ A Cp V³

Pm: the power generated by the wind turbine. ρ: the air density (1.225 kg/m³) A: the turbine blade sweep area (m2) Cp: the Aerodynamic Power Coefficient. V: the wind velocity (m/s)

C. OUTPUT VOLTAGE CONTROL OF BUCK-BOOST CONVERTER

Here the input of buck-boost converter varies with the variation in the wind speed. But the output voltage of the converter should bekept constant in order to have a constant voltage across the load. Thus the output voltage of the converter is kept constant despite thevarying input voltage.

Figure 3: Control of buck -boost converter in the system

In this work, the range of input voltage selected is 50-120 volts. In this range output voltage is maintained at a constant value of 75 volts. This is done by comparing the output voltage with a reference value and the error is used to change the duty ratio of the switch. Thus each time when the input of buck boost converter changes, corresponding duty ratio of the switch is changed and output of converter is maintained constant. Reference value is selected depending on the output load voltage required. The buck-boost power converter circuit is shown in Figure 3.

D. CONTROL OF INVERTER USING SPACE VECTOR MODULATION

TECHNIQUE

The load side inverter is controlled to keep the frequency at constant value. Being a standalone load, the output voltage is controlled in terms of amplitude and frequency.

The stator windings of a three-phase ac machine (with cylindrical rotor), when fed with a three-phase balanced current produce a resultant flux space-vector that rotates at synchronous speed in the space. The flux vector due to an individual phase winding is oriented along the axis of that particular winding and its magnitude alternates as the current through it is alternating. The magnitude of the resultant flux due to all three windings is, however, fixed at 1.5 times the peak magnitude due to individual phase windings. The resultant flux is commonly known as the synchronously rotating flux vector. The main aim here is to keep the output frequency constant at 50Hz. Amplitude of voltage is maintained constant by buck-boost converter and this DC voltage is the input to inverterwhich is converted to ac of fixed frequency by space vector modulation of the inverter. After passing through the output filterthe sinusoidal voltage will have fixed amplitude and frequency.

III. DETAILS OF PMSG USED:

PMG0.5kW, Number of poles=12, 200V, 1.5A, 500rpmDC Motor ratingsArmature: 1.1kW, 4.7A, 1500rpm, 230/440VField : 0.8A, 230/440V

IV. SIMULATION OF STANDALONE SYSTEM WITH PMSG

A simulation block diagram of this system is shown in Figure 4. Here PMSG is driven by wind turbine. Output terminals of the

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PMG are connected to a rectifier and buck-boost converter and theinverter through DC link capacitance. Output LC filter is used for smoothening purpose. The output of buck boost converter is measured and is compared with reference value .This generates triggering pulses for the switch. Each time wind speed varies, the error signal varies and thus duty ratio of converter switch varies to keep voltage magnitude constant. Space vector code for inverter is written as embedded MATLAB function to keep frequency constant. Thus both the voltage and frequency are maintained constant in the system using simulation. The simulation results yielded a constant voltage and frequency at the load terminals.

Figure 4: Simulation of PMSG based standalone system

A. Simulation Results

Output of the buck-boost converter as per the variations in wind speed is shown in Figure 5, as observed by simulation. Irrespective of wind speed variation, the buck-boost output voltage is found to be constant.

Figure 5: Wind speed variation in steps from 12-18 m/s and corresponding dc link voltage

Output line voltage of 415V, 50Hz obtained at the output terminals of the inverter is shown in Figure 6.

Figure 6: Inverter output voltage

Figure 7: Voltage obtained across the load

Inverter output voltage is filtered using an LC filter and the sinusoidal load terminal voltage is given in Figure 7.

V. EXPERIMENTAL VALIDATION

The hardware required for implementing a standalone system with output voltage and frequency constant is listed and explained. The hardware components used to implement the system are as follows:

1. Buck-boost converter2. DC source3. 555 timer circuit4. RC snubber circuit and heat sink for MOSFET5. One driver along with isolation6. Three phase inverter7. -2.5 V, 5 V, 9V, 15 V and -15V DC power supply

A. Load test on PMSG

A load test is performed to study the basic performance namely voltage regulation on PMSG. A DC motor is used as the prime mover .The field current of the dc motor was kept constant. The test results are given in Table 1 and Table 2 for speeds 500rpm and 700rpm respectively. The voltage regulation curve obtained is given in Figure 9. The experimental setup of the system is seen in Figure 8.

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Figure 8: Load test on PMSG

TABLE I

FOR SPEED=500rpm

LOAD VOLTAGE(VOLTS)

LOAD CURRENT

(AMPERES)

254.8 0

251 0.1

249.5 0.2

245.2 0.3

245.9 0.4

240 0.7

TABLE II

FOR SPEED=700 rpm

LOAD VOLTAGE

LOAD CURRENT

(VOLTS) (AMPERES)

362 0

355 0.3

358 0.4

350 0.5

348 0.6

Figure 9: Regulation curves of the PMSG (a) speed =500 rpm (b) speed=700rpm

The voltage regulation of the PM generator is studied by conducting load test on the generator. A good regulation was observed at speeds 500 rpm and 700 rpm and the curves are shown in Figure 9 (a) and 9 (b).

B. Buck- boost converter design

Buck boost converter is one of the basic part of the PMSG based standalone system. Main purpose of this system is to maintain the voltage constant. In order to design the converter switch and heat

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sink required are to be selected. The specifications of converter is given below,DC input voltage - 50-100VOutput voltage - 75 VSwitching frequency – 20 kHz

C. Pulse generation using analog circuit for closed loop operation

Closed loop operation of the converter is possible by sensing the dc link voltage using a dc isolation amplifier. The output of this amplifier ranges from 1-5 V. So in this case, 1V at the sensor output is considered to be 25V at the dc link. Thus to keep 75 volts at the dc link, 3V is considered to be the sensor output. This is kept as the reference input for hardware implementation. The hardware developed to keep this reference value at the dc link, is shown in Figure 10.

Figure 10: Circuit diagram of hardware developed to keep dc link at 75volts

D. Three phase voltage source inverter

Here a three phase inverter, SEMIKRON make is used. The controller used is dsPIC30f4011. The details of the card and program developed are described in the subsequent sections. In SEMIKRON make 3Ф inverter used, there are IGBT modules SKM75GB128D, chopper modules SKM100GAL123D, diode bridge MD8TU 100/16, IGBT drivers SKHI 22 AR, heat sink MDP3/250mm,fan and 80˚C thermal strip..Specifications: Maximum input AC voltage up to 3*415V 3Ф Maximum output current up to 30A Maximum switching frequency up to 20kHz PWM Maximum ambient temperature = 40˚C

The SEMIKRON inverter needs a minimum of 13V at the gate of IGBT to drive it. Since the pulses coming from dsPIC is of 5V, it should be amplified before connecting to the gate terminals. So a transistor amplifier circuit is used and the device used is 2N2222A transistor.

The software development is one of the main parts of the project work. Here the controller used is dsPIC30F4011; a 16 bit microcontroller. It is used to generate triggering pulses for the inverter. It keeps the frequency constant at 50Hz by using Space Vector Modulation of the Inverter which is one of the objectives of the project. The coding for the digital signal controller is done in MPLAB IDE v8.10 from Microchip Technology Inc. The whole programming is done in C platform.

For this MPLAB has a language tool suite add-on called MPLAB C30 compiler. The complete standard C library is provided with the MPLAB C compiler for dsPIC. The code is converted to hex file format after compiling and the dsPIC30F4011 DSC is programmed using LabProg IC programmer.

E. Results

1) Buck- boost converter

The developed buck boost converter is tested with dc input. Here pulse is generated by 555 timer for off line testing of the converter.Duty cycle is varied from 60% to 40% for an input voltage variation from 50 volts to 120 volts. A constant dc output of 75 volts is obtained. Pulses for two duty cycles are shown in Figure 11 and Figure 12. Output dc voltage obtained is shown in Figure 13.

Figure 11: Pulse with duty cycle of 40%

Figure 12: Pulse with duty cycle of 60%

Figure13: DC Output voltage of buck- boost converter

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2) Inverter testing

In order to maintain the frequency constant, code for inverter using space vector modulation is written and the output of inverter is obtained as shown in Figure 14.

Figure 14: Inverter output voltage

V. CONCLUSIONS

A PMSG based laboratory model standalone system was designed,developed, fabricated and tested successfully.A complete MATLAB/SIMULINK simulation is done for the analysis of standalone system with PMSG driven by wind turbine. Using the simulation for a wind speed range of 12m/s to 18m/s DC link voltage is maintained constant, also the inverter frequency.Hence, the voltage and frequency across the load is made 415 volts, 50Hz even if the wind speed varies.For hardware implementation buck-boost converter, inverter, controller circuit, three phase inverter are designed and tested. The controller selected is dsPIC30f4011. For implementation, the entiresystem is divided into smaller modules and each module is tested independently and verified. A program is written for keeping the frequency constant.

VI. FUTURESCOPE

From the hardware implementation it is observed that dsPIC30f4011 requires a lot of instruction cycles for performing complex operations. This leads to slow dynamic response. So advanced microcontrollers can be used for this purpose.This is a standalone system. This can be connected to the grid and methods can be implemented to maintain constant voltage and frequency in such a system. This system can be extended to perform maximum power point tracking at different wind speeds.

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International Power Electronics and Motion Control Conference, 2010[7] T. Tafticht , K. Agbossou and A. Chériti , in the paper ”DC Bus Control of Variable Speed Wind Turbine using a Buck-Boost Converter”[8] Mahmoud M. Hussein, Tomonobu Senjyu, Mohamed Orabi, Mohamed A. A. Wahab, and Mohamed M. Hamada, “Control of a Variable Speed Stand Alone Wind Energy Supply System”, 2012 IEEE International conference on Power and Energy , 2-5 December 2012, Kota Kinabalu Sabah, Malaysia[9]Ned Mohan, Tore M.Undeland,William P. Robbins, “Power Electronics Converters, Applications and Design”,3rd ed., Wiley India,2003[10]Microchip dsPIC30F family reference manual

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