dsadas dissertation

Commissioning Of A Doubly Fed Induction Generator

by

John McDonagh

This Report is submitted in partial fulfilment of the requirements of the Honours Degree in Electrical and Electronic Engineering (DT021) of the Dublin Institute of Technology

June, 2nd, 2010

Supervisor: Joseph Kearney

II

DECLARATION

I, the undersigned, declare that this report is entirely my own written work, except where otherwise accredited, and that it has not been submitted for a degree at any other university or institution. Signed: _____________________________ Date: ________________

III

Acknowledgement

The author would like to this opportunity to thank Joseph Kearney, project supervisor,

for presenting the opportunity to carry out project work on his research topic for

advancing in his academic career and also for his assistance and help throughout the

duration of the project Thanks to the laboratory technical officer, Terrance Kelly for his

time and help. Special thanks to Elizabeth Hendrick, John Byrne and Iain Hendrick for

proof reading the following report.

IV

Abstract

Doubly Fed Induction Generators (DFIG) today are the single most used generators in

wind turbine systems, with some globally recognised manufacturers such as Vestas and

GE utilising these generators. Key features of the DFIG include those of being able to

operate below, above and through synchronous speeds. As well as this there are less

electrical losses in the power electronics circuits.

The following documentation covers various elements such as wind turbine systems that

have been used in the past and the present day, including theory of Doubly Fed

Induction Generators. Also covered are explanations of components used throughout

the DFIG including signal conditioning circuits, current transducer, voltage transducers,

incremental shaft encoders and back to back pulse width modulated convertors with full

commissioning reports.

Attached with this report is a log book and design file covering stages of research, design,

implementation and testing of equipment.

V

Table of Contents Introduction....................................................................................................................1

1. Wind Turbine Systems ................................................................................................2

1.1 Fixed Speed Wind Turbines...................................................................................3 1.2 Variable Speed Wind Turbine ................................................................................3

1.3 Variable Speed Wind Turbines with Doubly Fed induction Generator ..................5

2. Doubly Fed Induction Generators for Wind Turbines ................................................6 2.1 Equivalent circuit of the Doubly Fed Induction Generator....................................8

3. Space Vectors Theory................................................................................................10

3.1 Clarke and Park Transformations ........................................................................10 3.2 Mathematical Model of Clarke Transform ...........................................................11

3.3 Mathematical Model of Park Transform ..............................................................11

3.4 Inverse Mathematical Models of Clarke and Park Transforms .............................11 3.5 Space Vector Modulation ....................................................................................12

4. System Hardware and Digital Control .......................................................................14

4.1 Texas Instruments eZdsp TMS320F2812 Microprocessor ...................................15 4.2 Scherbius Drive ...................................................................................................15

4.2.1 Rotor side converter .....................................................................................15

4.2.2 Grid side converter .......................................................................................16 4.3 Current Transducer .............................................................................................17

4.4 Voltage Transducer .............................................................................................18

4.5 Signal Conditioning .............................................................................................20 5. Acquisition of Analogue Variables.............................................................................21

6. Incremental Shaft Encoder........................................................................................23

7. Test Results ...............................................................................................................25

7.1 Commissioning of Current Transducers ..............................................................26 7.2 Commissioning of Voltage Transducers ..............................................................27

7.3 Commissioning of Incremental Shaft Encoder ....................................................27

7.3.1 Comments on Incremental Shaft Encoder ....................................................28 7.4 Commissioning of Rotor Side IGBTs .................................................................28

7.4.1 Comments on Rotor Side IGBTs.................................................................29

7.5 Commissioning of Grid Side IGBTs...................................................................29 7.5.1 Comments on Gide Side IGBTs ..................................................................29

8. Conclusion and recommendations.............................................................................31

Appendix A - Nomenclature .........................................................................................32 Appendix B Current & Voltage Transducer Test Results............................................33

Appendix C Commissioning Checklists......................................................................34

Appendix D Hall Effect .............................................................................................43 Appendix E Voltage and Current Transducer Datasheet ............................................44

Appendix E Wiring Diagram And Cable Schedule .....................................................51

Reference: .....................................................................................................................79

1

Introduction

The Doubly Fed Induction Generators (DFIG) has long been established in wind

turbines (WTs). The large investment of two nations, Germany and USA, saw the

growth of the DFIG in multi-megawatt wind turbines as result of the oil price crisis in

the 1970s [1].

The DFIG uses AC-AC converters (also known as a Scherbius drive) in the rotor circuit.

This configuration has become a standard option for high power applications. The major

advantage that a DFIG has is that the power electronics circuit only has to handle a

fractional amount (20% - 30%) of the total system power, it can also operate at sub-

synchronous speeds. This means that the losses in the power electronic equipment can

be reduced in comparison to power electronic equipment that has to handle total system

power. Thus the power converters need only be rated to handle power on the rotor side

of the machine.

The DFIG test bench described in this project uses back-to-back pulse width modulated

(PWM) converters connected between the rotor and the grid. Both a DC-link and

cycloconverter are used in the rotor circuit. The test bench also incorporates a

functional hardware board. The hardware board used is Texas Instruments DSP

TMS320F2812. The board is used to handle and measure signals such as currents and

voltages. The PWM pattern and control signal to the IGBT drives are generated by the

board.

Chapter 1 Wind turbine systems

2

1. Wind Turbine Systems

Wind turbines (WT) can operate with either fixed speed drives or variable speed drives.

For a fixed speed WT, the generator is directly connected to the grid. Since the speed is

almost fixed to the grid frequency and is not controllable, it is not possible to store the

turbulence of the wind in the form of rotational energy. Thus, for a fixed speed system

the turbulence of the wind will give power variations and therefore affect the power

quality of the grid.

A variable speed generator is controlled by power electronic equipment and this makes it

possible to control the rotor speed. This way the power fluctuations caused by wind

speed variations can be more or less absorbed by changing the rotor speed and power

variations originating from the wind conversion so that the drive train can be reduced.

The power quality issues caused by the wind turbine can therefore be improved

compared to a fixed speed turbine.

The rotational speed of wind turbines is fairly low and as a result must be adjusted to

match the electrical frequency. This can achieved in two ways; (1) with a gearbox or (2)

with the number of pole pairs in the generator. The number of pole pairs defines the

mechanical speed of the generator with respect to the electrical frequency. Similarly the

gearbox adjusts the rotor speed of the turbine to the mechanical speed of the generator.

This section will cover the following wind turbine systems;

1. Fixed speed wind turbines

2. Variable speed wind turbines

3. Variable speed wind turbines with doubly fed induction generator


3

1.1 Fixed Speed Wind Turbines

Fixed speed wind turbines consist of a directly grid connected squirrel cage induction

machine (SCIM), the rotor of which is connected to the turbine shaft through a gearbox.

Whether the turbine is acting as a motor or as a generator the SCIM consumes reactive

power from the grid therefore a capacitor bank is connected at the SCIM terminal for

reactive power compensation. This can be seen in figure 1.

Gearbox SCIM

Transformer

Capacitor Bank

To 50 Hz AC Grid

Figure 1.1 Fixed speed wind turbine with SCIM

There a few variations of this wind turbine type. The variations consist of having two

fixed speeds which is accomplished by having two generators with different ratings and

pole pairs. Alternatively it can be a generator with two windings having different ratings

and pole pairs. These configurations were the most popular wind turbine types

manufactured in the 1990s.

1.2 Variable Speed Wind Turbine

The configuration of the wind turbine in figure 2 is equipped with a Scherbius drive

connected to the stator. This variation of wind turbine can either have an induction or

synchronous generator.


4

Gearbox G

Transformer

To 50 Hz AC Grid

=

=

Power Electronics Converter

Figure 1.2 Variable speed WT with either induction or synchronous generator

The design of the gearbox is to correspond to the rated speed of the generator at

maximum rotor speed. Synchronous generators or permanent magnet synchronous

generators can be designed to have multiple poles which would imply that there is no

need for a gearbox. Figure 3 shows the configuration of this system. The configuration in

both cases takes total system power across the power electronic converters which

increase losses, however, this system is well developed and has robust control techniques

and is commonly used throughout other applications.

Figure 1.3 Gear-less variable speed WT with synchronous generator


5

1.3 Variable Speed Wind Turbines with Doubly Fed induction Generator

The configuration below (figure 4) consists of a WT with a doubly fed induction

generator i.e. the stator is directly connected to the grid and the rotor windings are

connected via slip rings to the Scherbius drive.

Gearbox DFIG

Transformer

=

=

To 50 Hz AC Grid

Figure 1.4 Variable speed WT with DFIG

As the Scherbius drive is connected to the rotor it only has to handle approximately 20

30% of the total system power compared to that of a system which has to deal with total

system power which can be seen in 1.2 Variable Speed Wind Turbines. Also in addition to

this, the costing of the converter becomes lesser as it need only be rated to handle power

on the rotor side of the machine.

The DFIG over the past few years has increased in popularity among wind turbine

systems and is becoming an industry standard for wind turbine generators. To date

manufacturers such as Vestas, Nordex and GE Wind Energy are producing doubly fed

induction machines as generators.

Chapter 2 DFIG For Wind Turbines

6

2. Doubly Fed Induction Generators for Wind Turbines

A variable speed system with a limited variable speed range i.e. 30% of synchronous

speed, the DFIG is an attractive solution. As mentioned in Chapter 1 the reason for this

is that the power electronics converter on the rotor side of the machine only has to

handle approximately 20 30% of the total system power. This means that the losses in

the power electronic equipment are reduced compared to systems which have to deal

with total system power through the power electronic equipment. The stator side of the

DFIG is connected to the grid whilst the rotor is connected to the Scherbius drive

through slip rings. Figure 2.1 shows this configuration.

Figure 2.1 Principle configuration of DFIG

A more detailed image of the DFIG can be seen in figure 2.2 with the back-to-back

converters shown. The back-to-back converters are equipped with two converters

connected back-to-back i.e. a machine side converter and a grid side converter.

Between the two converters, a DC-link capacitor is connected to the system. The DC-

link capacitor is aimed at keeping the voltage variations (ripple) small.


7

Figure 2.2 DFIG with back-to-back converters

As there is a machine side converter, this makes it possible to control the torque

and/or speed of the DFIG and also the power factor at the stator terminals. The main

objective for the grid side converter is to keep the DC-link voltage constant. The

speed-torque characteristics can be seen in figure 2.3. It can also be seen that the

DFIG can operate both as a motor and as a generator. The DFIG is a typical

application for wind turbines and operate with a limited speed range of approximately

30% around synchronous speed depending on the number of poles in the machine.

Figure 2.3 Typical Torque speed Measurement Curve


8

2.1 Equivalent circuit of the Doubly Fed Induction Generator

The equivalent circuit of the doubly fed induction generator can be seen below. The

equivalent circuit is on a per phase basis and is valid for both Y-connected and -

connected configurations.

Figure 2.4 Equivalent circuit of DFIG

Applying Kirchhoffs voltage law to the above circuit the following can be deduced;

)( rmrsssssss IIIjXIjXIRV ++++= eqn. 2.1

)( rmrsmrrrr IIIjXjXI

s

R

s

V++++= eqn. 2.2

)(0 rmrsmrmm IIIjXIR +++= eqn. 2.3 Where the following notation is; Vs stator voltage; Rs stator resistance; Vr rotor voltage; Rr rotor resistance; Is stator current; Rm magnetising resistance; Ir rotor current; Xs stator reactance; Irm magnetising resistance current; Xr rotor reactance; S slip; Xm magnetising reactance; The slip, s, is the ratio of relative speed to the synchronous speed and;

s

rss

= eqn. 2.4

Where s is the synchronous speed and r is the rotor speed. Also the air gap flux, the stator flux and the rotor flux can be defined as;

)( rmrsmm IIIL ++= eqn. 2.5

mssrmrsmsss ILIIILIL +=+++= )( eqn. 2.6

mrrrmrsmrrr ILIIILIL +=+++= )( eqn. 2.7


9

The resistive losses of the induction generator can be obtained by the following formula;

Ploss = )(222

rmmrsss IRIRIR ++ eqn. 2.8

The electro-mechanical torque can be expressed by;

Te = ( ) ( )** 3Im3 rrprmp InIn = eqn 2.9 Where np is the number of pole pairs. The table below shows some typical parameters of induction machines (all values are in per unit)

Table 2.1 Typical parameters of the induction machine in per unit [2]

Small Medium Large Machine Machine Machine

A 4 kW 100 kW 800 kW

Stator and rotor resistance Rs and Rr 0.04 0.01 0.01 Leakage inductance Ls + Lr L 0.2 0.3 0.3 Magnetizing inductance Lm LM 2.0 3.5 4.0

Denoted by Ss and the apparent power on the rotor is given by Sr. The apparent powers

can be found by the following;

*

1

2

1

2* 3333 smsssssss IjILjIRIVS ++== eqn. 2.10

*

1

2* 3333 rmw

rsrrrrr IsjIsLjIRIVS ++== eqn. 2.11

Chapter 3 Space Vector Theory

10

3. Space Vectors Theory

The objective behind space vectors is to describe the induction machine with two phases

as opposed to three phases. The induction machine comprises of been supplied with

three stator currents and in turn forms a rotating flux in the air gap of the machine. The

same rotating flux can be formed with only two phases. This is the main principle behind

space vectors. The transform from three phasor currents to two phases can be

preformed by implementing Clarke and Park transformations. With the use of the Clarke

transform the real (Ids) and imaginary (Iqs) currents can be identified. The Park transform

can then be implemented to realise the transformation of the Ids and Iqs currents from a

stationary frame to a rotating frame thus used to control the relationship between the

stator vector current and the rotor flux vector.

3.1 Clarke and Park Transformations

The Clarke transformation is also known as the transforms which use three phase

currents ia, ib, ic to calculate currents in the in the two phase orthogonal stator frame

which are known as i and i. The two currents i and i are in the fixed coordinate stator

phase are transformed to the isd and isq current components in the rotating d, q frame of

the Park transform.

+

ia

ib

ic

iq

id

i

i

Figure 3.1 Phasor diagram of stator current in the d-q rotating frame and its relationship with the a, b, c stationary frame


11

3.2 Mathematical Model of Clarke Transform

As per 3.1 states, the Clarke transform modifies three-phase currents into a two phases

stator frame. Where the i and i can be found to be;

)(3

1

3

2cba iiii = eqn. 3.1

)(3

2cb iii = eqn. 3.2

)(3

2cbao iiii ++= eqn. 3.3

Where i and i are components in an orthogonal reference frame and io is the electrical

symmetrical component (homopolar) of the system.

For this application and many others the homopolar component can be neglected. In the

absence of the homopolar component the space vector can equate to v = v + jv which

can represent the original three-phase input.

3.3 Mathematical Model of Park Transform

The two phases, and , which are calculated using the Clarke transform are then fed

onto a rotating vector block where it is rotated over an angle of degrees to track the

frame d and q associated with the rotor flux. The rotation over angle can be found by

use of the following;

)sin()cos( iiisd = eqn. 3.4

)cos()sin( iiisq += eqn. 3.5

3.4 Inverse Mathematical Models of Clarke and Park Transforms

As the above transforms are taking a three phase supply current and transforming into a

two phase representation there is an associated transform to reverse the process. The

vector lying in the d, q frame is transformed from the d, q frame to the two phases of the

and frame with a rotation of degrees can be can calculated by use of the following

formulas;

)sin()cos( sqsd iii = eqn. 3.6

)cos()sin( qsd iii += eqn. 3.7


12

From this point with an orthogonal is two-phase , frame the following equations

can be preformed to return to a three-phase system;

iia = eqn. 3.8

iiib2

3

2

1+= eqn. 3.9

iiic2

3

2

1= eqn. 3.10

3.5 Space Vector Modulation

Space Vector Modulation (SVM) is an algorithm which is developed for the control of

pulse width modulation (PWM). SVM refers to the switching scheme of a 3-phase power

converter with IGBTs. The structure of a 3-phase power converter is shown below in

figure 3.1. The 3-phase converter shown has to be controlled so that at no time are both

IGBTs in the one leg of the circuit switched on together. Should both switches be on in

the same leg of the converter it will create a short circuit. To meet the requirement of

operation i.e. when Q1 is on and Q4 is off and vice versa leads to 8 possible

combinations of switching vectors.

Figure 3.2 Three-phase power converter layout

A


13

Vector Q1 Q2 Q3 Q4 Q5 Q6 V12 V23 V31

V0 = {000} Off Off Off On On On 0 0 0 Zero Vector V1 = {100} On Off Off Off On On +Vdc 0 -Vdc Active Vector V2 = {110} On On Off Off Off On 0 +Vdc -Vdc Active Vector V3 = {010} Off On Off On Off On -Vdc +Vdc 0 Active Vector V4 = {011} Off On On On Off Off -Vdc 0 +Vdc Active Vector V5 = {001} Off Off On On On Off 0 -Vdc +Vdc Active Vector V6 = {101} On Off On Off On Off +Vdc -Vdc 0 Active Vector V7 = {111} On On On Off Off Off 0 0 0 Zero Vector

Table 3.1 Switching Vectors To implement the SVM technique a reference signal Vref is to be sampled with frequency

fs (where Ts = 1/fs). The reference signal may be generated from three separate phase

references. The reference vector is then synthesised using a combination of two adjacent

switching vectors and one or both of the zero vectors.

Figure 3.1 Switching Vectors

Figure 3.2 shows the possible switching vectors for a 3 leg converter using space vector

modulation. Within figure 3.2 is Vref max which is the maximum amplitude of Vref before

non-linear over modulation is reached.

Chapter 4 System hardware and digital control

14

4. System Hardware and Digital Control

Represented below is DFIG test system.

Analogue ConditioningBoard and

DSP Control System

Shaft Encoder

DC Motor

+

-

110V

CTs

VTsCTs

PWM PWM

Crotor Cgrid

TransformerFilter

PC

Supply

Transformer

Rotor

Stator

Ias Ibs Ics

IarIbrIcr

Va

Vb

Vc

Fuses

c

=

=

Vdc

If

Iag Ibg Icg

Figure 4.1 Doubly Fed Induction Machine Test Rig

The DFIG system consists of the following equipment;

Induction machine

DC shunt machine

Scherbius drive in the rotor circuit

LEM current & voltage transducers

Analogue conditioning boards

Optical incremental shaft encoder

A PC which programs Texas Instruments (TI) Digital Signal Processor

(TMS320F2812)


15

4.1 Texas Instruments eZdsp TMS320F2812 Microprocessor

The DFIG is controlled by a digital signal processor, in this case Texas Instruments (TI)

TMS320F2812 is utilised. The choice of this board is preferred for this project as it has

high speed operation and a wide range of functionality. Some of the key features of the

board are listed below (a full range of specifications for the TI TMS320F2818 can be

found in design file);

Operating speed of 150 MIPS

16, 12 bit ADC channels

12 PWM channels

Quadrature encoder pulse (QEP) interface

ADC channels with fast conversion rate of 80ns at 25 MHz clock speed

The inbuilt PWM channels are advantageous for use on the DFIG as 16 PWM pulses

can be generated independently or synchronised. The pulses generated can be

implemented across both converters which is suitable for the control of the rotor and

grid side converters simultaneously. The control algorithms are developed in C using

Code Composer Studio (CCS) which is then loaded directly to TIs TMS320F2812 via

PC connection. The eZdsp F2812 is stand alone PCB and is power with a 5V power

supply.

4.2 Scherbius Drive

The Scherbius drive consists of AC-AC converters in the rotor circuit. These converters

are known as the rotor side and grid side converters and are discussed below.

4.2.1 Rotor side converter

Figure 4.2 provides a detailed image of the rotor side converter. Featured in figure 4.2 are

the supply voltages Vas, Vbs, Vcs which are obtained through LEM voltage transducers

(see section x). These voltages are modulated through an analogue conditioning board

and then connected to the analogue to digital converter (ADC) of Texass Instruments

TMS320F2812 digital signal processor (DSP). Three rotor currents Iar, Ibr, Icr are

measured through LEMs current transducers (see section x), the currents are also

connected to an analogue conditioning board which are then connected TIs

TMS320F2812.


16

Figure 4.2 Rotor side converter

Figure 4.2 shows six insulated gate bipolar transistors (IGBT) labelled Q1 Q6. These six

power transistors are controlled by PWM gating signals a, b, c, a, b and c. These signals

determine the shape of the output voltages supplied to the rotor windings. The PWM

gating signals are the outputs from the 3V to 15V level shifting board. The inputs to the

board are the 3V outputs from TIs TMS320F2812. The 3V PWM output pattern from

TIs TMS320F2812 DSP chip depends on the control algorithms developed in Chapter

3.

4.2.2 Grid side converter

Below shows the configuration of the grid side converters. The voltages obtained

through LEM voltage transducers are modulated through an analogue conditioning

board which is then connected to the ADC input of Texas Instruments TMS320F2812.


17

Figure 4.3 Grid Side Converter

The three grid side currents (Iag, Ibg, Icg) seen in figure 4.3 are measured through LEM

current transducers and the outputs are connected the analogue conditioning board and

also to the ADC input of Texas Instruments TMS320F2812. As in the case of the rotor

side converters, the six power transistors are controlled by PWM gating signals a, b, c, a,

b, and c. These signals determine the power supplied to the DC-link. The gating signals

mentioned are the outputs from the 3V to 15V level shift conditioning board. The 0

3V PWM output pattern is dependant on the control algorithms which are developed in

Chapter 3.

4.3 Current Transducer

Currents absorbed by the DFIG system, necessary to the control algorithms, are

measured by LEM Module LA 55-P. From figure 4.4, the current, Ip, is to be measured.

The magnetic flux created by primary current is balanced through a secondary coil using

a Hall device and associated electronic circuit. The secondary (compensating current) is

an exact representation of the primary current.


18

Figure 4.4 Scheme of current transducer & connection of current transducer LA 55-P

The output signal is the voltage drop on the measuring resistance, Rm, caused by the

secondary current. The LEM current transducer has a maximum value of 50A which

corresponds to 50mA on the secondary coil (turns ratio of 1/1000). To obtain a voltage

of 1.5V on the secondary coil, Ohms law can be used to obtain the value of resistance

where;

=== 3005.0

5.1

I

VRm eqn. 4.1

4.4 Voltage Transducer

Voltage transducer used are LEMs LV25-P, these transducers are used for measuring the

voltage (Vdc) on the DC-link across the converters and also to measure the three phase

voltages from the supply i.e Va, Vb and Vc. All of the mentioned can be seen in figure 4.1.

Below in figure 4.5 the voltage transducer connection is shown. The use of the voltage

transducer is necessary to acquire signals for control algorithms. The principle

characteristics of LEMs LV 25-P can be seen in appendix x.


19

Figure 4.5 Voltage transducer LV 25-P connections

The supply phase voltage considered for use is 220V rms knowing that the supply

voltage is required to determine the series resistance (R1) as well as the measuring

resistance (Rm) on the secondary side of the voltage transducer. In accordance with the

datasheet (see appendix x), R1 is to be calculated so that the voltage measured

corresponds to a peak current of 14mA. With this the calculation of R1 is found to be;

p

rms

rms RI

VR =1 eqn. 4.2

Where Rp is primary resistance denoted by the manufacturer and is 205. Therefore the

value of the series resistance R1 is;

=

=

kR 973.21250

1014

220231

The resistance, R1, will tend to generate heat therefore the resistance needs to be of an

adequate wattage to dissipate the heat levels. The resistance chosen was 22.476k which

corresponds to 7 watts. Therefore the actual primary current (I1) is;

mAI 68.1325010476.22

220231

=+

= eqn. 4.3

Having the primary series resistance and current, the measuring resistance can be

calculated where the desired output voltage is in the range of 1.5V thus;

=

=

=

85.43

1000

2500)1068.13(

5.1

3rms

mkI

VR eqn. 4.4

Where;

kN is the conversion ratio of the voltage transducer (see appendix x)

As the value of resistance for Rm was not available the closest size to the calculated

resistance is 47 which is used in the actual circuit.


20

4.5 Signal Conditioning

Signals that are sent to Texas Instruments TMS320F2812 chip are to be in the range of

0-3V. The 0-3V signals are sent to the analogue to digital converter (ADC) of the

processor, should a voltage of greater than 3V be applied to the ADC input of the

processor the TMS320F2812 chip may malfunction. As mentioned in section 4.2 the

desired output from the current transducer is 1.5V, to have the signal from the current

transducer in the range of 0-3V the signal must be level shifted. To achieve the required

voltage range, a level shifting circuit board is implemented, thus the output from both

the current and voltage transducers are applied to the board. This ensures that an output

of 0-3V will be applied to the ADC input of the processor. The level shifting circuit

includes a low pass filter which eliminates high frequency noise to ensure clean signals

are been applied to the ADC of the processor. The level shifting circuit can be seen in

appendix x.

Chapter 5 Acquisition of analogue variables

21

5. Acquisition of Analogue Variables

Discussed in Chapter 4 were the analogue signal outputs for the different components.

The signals mentioned are modulated through the signal conditioning board and sent to

the TI eZdsp board. In the case of the current and voltage transducers the analogue

conditioning board modulates the output voltage of 1.5V to a positive signal in the

range of 0 3V which is a desirable for the input to the TI F2812 ADC. As there are

numerous inputs required there are two associated circuit boards to accommodate the

signals i.e.

Rotor side currents x 3

Grid side currents x 3

Grid side voltages x 3

DC link voltage x 1

Speed control input (discussed in Chapter 6) x 1

Thus there are a total of eleven inputs with each board accommodating six of the

transducer outputs. Below in figure 5.1 can be seen the connection pin layout of the

eZdsp F2812 and accompanied tables 5.1 and 5.2 are the input/output connections of

the ADC connections.

Figure 5.1 TI F2812 Connector positions [3]

Chapter 5 Acquisition of analogue variables

22

P5 Pin Number P5 Signal Measurement

1 ADCINB0 Grid side Va

2 ADCINB1 Grid side Vb

3 ADCINB2 Grid side Vc

4 ADCINB3 Speed Control Input

5 ADCINB4

6 ADCINB5

7 ADCINB6

8 ADCINB7

9 ADCREFM

10 ADCREFP

Table 5.1 Input ADC Connections for F2812 [4]

P9 Pin Number P9 signal P9 Pin Number P9 Signal Measurement

1 GND 2 ADCINA0 Rotor side Ia

3 GND 4 ADCINA1 Rotor side Ib

5 GND 6 ADCINA2 Rotor side Ic

7 GND 8 ADCINA3 Grid side Ia

9 GND 10 ADCINA4 Grid side Ib

11 GND 12 ADCINA5 Grid side Ic

13 GND 14 ADCINA6 Vdc

15 GND 16 ADCINA7

17 GND 18 VREFLO

19 GND 20

Table 5.2 Input ADC Connections for F2812 [4]

Chapter 6 Incremental shaft encoder

23

6. Incremental Shaft Encoder

An encoder is classed as a feedback device where the device is used to measure rotary

motion of the induction machine. The incremental encoder consists of a perspex grating,

a light source and a detector. The grating itself has uniformly spaced windows which act

as shutter for the detector. The shutter provides a strobe light effect for the detector. By

counting the number of cycles, the incremental shaft encoder is able to establish how far

the machine has moved from an initial position. For position feedback to occur the

encoder must have a predefined home position.

With the incremental shaft encoder there is a once per revolution pulse, this gives the

encoder an absolute starting position. The incremental shaft encoder in this case has two

light sources/detectors which are arranged so that direction of travel can be sensed. The

light source/detectors are arranged to be one quarter out of phase. This is effectively

providing four state changes for each line on the encoder, thus increasing the resolution

by a factor of 4.

The disk of the incremental encoder is patterned with a single line track around its

periphery. A second track is added to the encoder to generate a signal that occurs once

per revolution to indicate absolute position. To obtain the direction, the lines on the disk

are read out by two different elements that look at the disk pattern with a mechanical

shift of one quarter the pitch of a line pair between them. As the disk rotates, two photo

elements generate signals that are shifted 90 out of phase from each other []. These two

signals are the quadrature A and B (QEPA & QEPB) signals. These signals produce

1000 pulses per revolution of the machine.

As mentioned above the light source/detectors are arranged to be one quarter out of

phase which is providing four state changes, therefore the actual count per revolution is

4000 pulses. Using a quadrature counter the rising and falling edges of the pulses can be

counted.

Chapter 6 Incremental shaft encoder

24

Figure 6.1 Optical encoder disk with output signals [4]

The incremental shaft encoder used in this project is manufactured by Hengstler and is a

type RI 32 shaft encoder (see design file for data sheet). The output signals from the

shaft encoder are sent to the TI F2812 interface board which in turn determines the

position, the direction of rotation and the rotational speed of the machine.

The induction machine used for this project has two pole pairs which from induction

machine theory the synchronous speed of the machine can be found to be;

P

f s

)(120= eqn. 6.1

Where;

f is the system frequency i.e. 50 Hz

P is the number of poles

Therefore the synchronous speed equates to 1500 rpm, also this corresponds to 25

rev/sec. The TI F2812 will receive a signal frequency of;

1000 pulses/rev x 25 rev/sec = 25 kHz

Also with each pulse the rotor has advanced by 0.09 which can be found by the

following mathematics.

360 per rev 4000 pulses = 0.09

The Hengstler shaft encoder is to be powered by a +15V, with this the output from the

shaft encoder will be square wave pulses with an amplitude of +15V. The output signals

from the shaft encoder are applied a QEP conditioning board which in turn steps down

the voltage to +3V thus in turn is an acceptable signal to be applied to the TI

TMS320F2812.

Chapter 7 Testing Results

25

7. Test Results

The commissioning of a system comprises of a number of checks on the equipment.

This includes physical, electrical and operational checks. Commissioning involves

ensuring all wiring is secure, correct power supplies are being applied to the equipment

and ensuring correct functioning of all the equipment. It also performs operational tests

to prove that safety devices such as circuit breakers and MCBs trip under fault

conditions to protect the remaining equipment from electrical power surges.

Commissioning checklists are created to take note of all aspects to (1) ensure nothing has

been overlooked during this stage and (2) to note any discrepancies for future reference.

Results and comments on each checklist are stored for future reference. Once the

equipment is commissioned to a satisfactory standard the equipment can be handed over

to the operator and put into full time service.

Fingerprinting of equipment takes place to monitor the efficiency of the equipment

over time. Tests are performed on the equipment and results are obtained which are

stored. The test is performed again over a defined period of time and the results are

compared. Any discrepancy in results shows how the equipment has deteriorated over

time and is a good aide in monitoring the life span of the equipment.

The scope of the project entailed to correctly interface and calibrate current and voltage

signals to the TI TMS320F2812 DSP chip, calibrate an incremental shaft encoder and

both the grid side and rotor side IGBTs. Each section of the commissioning was

recorded through commissioning checklists as can be seen in appendix B. Discussed

below are the components tested with comments noted on each.


26

7.1 Commissioning of Current Transducers

The intent on commissioning the current transducers is to ensure that both the input

current to the ADC cards and to the TI F2812 are in the correct level as intended. Also

obtaining clean voltages signals in the range of 0-3V were essential to testing the current

transducers. To perform the testing of the current transducers a three-phase variac and a

load resistance bank were required. As current measurement was to be obtained the

supply from the three-phase variac was incremented in steps one amp. With this

increment it information could be noted. Below the results of the testing can be seen.

Input current Input to PCB Input to TI (Output PCB) Programme ADC Clarke

1 A 0.3V 1.5V320mV 1.5x10-4 0.0849

2 A 0.6V 1.5V 600mV 2.7x10-4 0.15

3A 0.9V 1.5V 88mV 4.04x10-4 0.202

4A 1.16V 1.5V1.16V 5.5x10-4 0.27

5A 1.46V 1.5V1.52V 6.7x10-4 0.328

6A 1.74V 1.5V1.74V 7.9x10-4 0.394

7A 2.0V 1.5V2.0V 8.9x10-4 0.459

8A 2.24V 1.5V2.24V 0.00104 0.52

Table 7.1 Current Transducer results

In the above results it can be seen that input voltage to the TI F2812 was in the range of

0-3V which was intended as per the design and implementing of the signal conditioning

board. Also in the above table the column Clarke can be seen. The Clarke column is a

digital representation of the current on a per unit scale which is representing the input

voltage to the TI F2812. Increasing the current from 1A 8A the per unit values

increase with respect to increasing voltage level. The maximum value of voltage to be

obtained is to be 3.3V which on a per unit scale would be 1 per unit. The current

transducers were operating in the range as expected and are fit for operation on this

project.


27

7.2 Commissioning of Voltage Transducers

The voltage transducers were commissioned with the same intent as that of the current

transducers. The intent being that the voltage transducers are able to supply clean voltage

signals to the ADC cards and to the TI TMS320F2812. To ensure that the voltage

transducers were operating correctly a three-phase variac supplied power across each of

the phases with current be acquired through ammeters with the aide of a load resistance

bank. In the following table are the obtained results.


1 A 340mv 1.5V360mV 1.89x10-4 0.0876

2 A 680mV 1.5V 680mV 3.4x10-4 0.156

3A 960mV 1.5V 920mV 4.3x10-4 0.218

4A 1.24V 1.5V1.24V 5.5x10-4 0.277

5A 1.54V 1.5V1.54V 7.3x10-4 0.302

6A 1.80V 1.5V1.80V 8.5x10-4 0.433

7A 2.14V 1.5V2.14V 9.8x10-4 0.487

8A 2.40V 1.5V2.40V 0.00107 0.538

Table 7.2 Voltage Transducer Results

In the above results it can be seen that input voltage to the TI F2812 was in the range of

0-3V which was intended as per the design and implementing of the signal conditioning

board. As with the current transducer testing the Clarke column represents a digital

output which can be visualised on screen. The per unit value are coherent with the range

of the voltage levels obtained and the voltage transducers for this project are operating in

the expected regions are fit for operation.

7.3 Commissioning of Incremental Shaft Encoder

The objective of commissioning the incremental shaft encoder was to see how accurate

the readings obtained by the TI TMS320F2812 interpreted the speed of the DFIG and

provide digital displays on the operating PC along with providing direction of rotation.

To perform this test, an incremental build of level 2 of the main code in Code Composer

Studio (CCS) was preformed. Also the induction machine (machine set O) was arranged

to run light i.e. no load attached along with a tachometer to obtain actual machine

speeds.


28

7.3.1 Comments on Incremental Shaft Encoder

With machine set O running light, the tachometer was used to measure the rotational

speed of the machine. Below are listed results of the test.

Tachometer reading (rpm) Display readout on PC (Per unit)

1489 0.997

1480 0.994

1460 0.993

Table 1 Incremental Shaft Encoder Test Results

Below in figure 7.1 is shown a typical of the digital readout as observed from on screen

displays. The readout measurement is set on a per unit scale which is specified in the

coding of the program where 1 per unit represents a synchronous speed of 1500 rpm. It

can be noted that there are some differences in the actual speed to the obtained from the

PC. For example, per unit reading of 0.997 equates to 1495 rpm, however, in comparison

the actual reading was 1489 rpm. This indicates to error within the system, although very

small error which equates to 0.05% error. This subtle error has be noted and accepted as

a tolerable error. Thus, concluding that the incremental shaft encoder is operating as

expected and to satisfactory limitations.

Figure 7.1 Digital readout of From Shaft Encoder

7.4 Commissioning of Rotor Side IGBTs

Commissioning of the rotor side IGBTs entailed testing to ensure that the IGBTs were

firing at the correct frequency as set out in CCS, in this case 5 KHz. Ensuring that power

flow across the convertor did not create any unbalances in voltage, current or frequency

had to be noted. For testing of the rotor side IGBTs build level 1 rotor side had to be

initialised in the main code of Code Composer Studio (CCS). Along with initialising the

code to operate and provide PWM output signals from the TI TMS320F2812, a variable

DC supply, load resistance bank and ammeters were required.


29

7.4.1 Comments on Rotor Side IGBTs

Upon initialising the coding, the output from the TI TMS320F2812 was checked to

ensure the correct frequency of the PWM output was 5kHz. Establishing that there was a

5kHz output, power from the variable DC was applied to the IGBTs. With an

oscilloscope each leg of the convertor was checked for correct frequencies and also

noting that a balanced three-phase current was shared across the load resistance bank.

The rotor side convertor was established to operating in the correct manner and was

satisfactory to expected conditions.

7.5 Commissioning of Grid Side IGBTs

The objective of testing the grid side IGBTs is to ensure power flow across the

convertor does not create any unbalances in current, voltage and frequency. To test the

IGBTs build level 1 (See design file for full coding) of the main code of the TI

TMS320F2812 was initialised in Code Composer Studio (CCS). Build level one initialises

elements which have been developed in various chapters of this report including inverse

Clarke and Park transforms, space vector modulation of IGBTs and PWM. To have

power into the circuit and for measurement of currents, a three-phase variac, ammeters

and a load resistance bank were amongst the equipment to perform the test.

7.5.1 Comments on Gide Side IGBTs

With CCS initialised to operate the grid side IGBTs, the set point level for PWM output

was to be 5 kHz. It was found that at the output PWM signal was approximately 30 kHz

and the same for the frequency level across the IGBTs. Also with power flowing across

convertor there was a current unbalance on the system. By use of an oscilloscope each

leg of the IGBTs were investigated. Through investigation it found that one leg of the

IGBTs was misfiring. The grid side IGBTs presented two problems at this stage. (1)

Incorrect frequencies and (2) miss firing. To overcome the higher frequencies the coding

in CCS was investigated. It was found that CCS was calling the rotor side IGBT variables

which were not collaborating with the main coding. Creation of correct variable names to

be called from the main coding overcame the issue of incorrect frequencies. To confirm

the correct frequencies, by use of an oscilloscope the output PWM signal from the TI

TMS320F2812 and was confirmed to providing a 5 kHz signal as intended. Having the

correct PWM output the IGBTs could have power supplied to them and check for

unbalances. With power supplied across the IGBTs, it was observed that there was still a


30

current unbalance across one of the phases. It was found that leg 2 (see appendix x for

image) was still misfiring and was generating high frequency noise. The problem with the

misfiring of the IGBTs is hoped to not be terminal and can be resolved with further

investigation, however, should the IGBT be un-reparable a new unit will have to be

ordered.

Chapter 8 Conclusion and recommendations

31

8. Conclusion and recommendations

The objectives set out at the beginning of the project were to interface and calibrate

current and voltage signals obtained from the induction generator, to supply the correct

3.3V clean voltage signals to the Texas Instruments TI TMS320F2812 DSP chip and to

calibrate an incremental shaft encoder and interface to the TI DSP chip.

The above objects have all been successfully met and commissioning checklists have

been compiled to prove elements within the system are fit for their intended purpose and

also for future reference if a fault ever occurs within the system it can be traced back to

any comments which may have been made on the commissioning checklist.

Outside of these objectives work also been carried out on numbering cables in the DFIG

panel. Also a full schematic diagram (see appendix E) and a cable schedule of the system

was designed using EPlan software design package which is widely used throughout

industry.

Recommendations for future work would include finalising the project and fully

commissioning the TI TMS320F2812 to provide closed loop control over the system as

well having fully functional ADC cards. Also as this type of generator is widely used

throughout industry, creating a laboratory assignment for students to investigate

performance and analyse the system would be advantageous to develop an appreciation

for a generator that is widely used in industry.

Should a student under take work on the DFIG as project work in the future, the student

could investigate and analyse the system to be coherent with IEC standards and Irish

Grid code for harmonics, fault ride through, flicker and reactive power capability.

32

Appendix A - Nomenclature

Abbreviations

ADC Analogue to Digital Converter CCS Code Composer Studio CT Current Transducer DAC Digital to Analogue Converter DFIG Doubly Fed Induction Generator DSP Digital Signal Processor Hz Hertz IGBT Insulated Gate Bipolar Transistor KHz Kilo-Hertz PC Personal Computer PWM Pulse Width Modulation SCIM Squirrel Cage Induction Machine SVM Space Vector Modulation TI Texas Instruments VT Voltage Transducer WT Wind Turbine WTG Wind Turbine Generator Symbols G Generator S slip SG Synchronous Generator Subscripts Irm Magnetising resistance current Ir Rotor current Is Stator current Rm Magnetising resistance Rr Rotor resistance Ra Stator resistance Vr Rotor voltage Vs Stator voltage Xm Magnetising reactance Xr Rotor reactance Xs Stator reactance

33

Appendix B Current & Voltage Transducer Test Results

Results for Channel 2 Current Transducer


1 A 0.36V 1.5V320mV 1.5x10-4 0.0847

2 A 0.66V 1.5V 600mV 2.7x10-4 0.143

3A 0.9V 1.5V 900mV 4.0x10-4 0.193

4A 1.15V 1.5V1.2V 5.2x10-4 0.259

5A 1.48V 1.5V1.50V 6.4x10-4 0.32

6A 1.7V 1.5V1.68V 7.6x10-4 0.389

7A 2.0V 1.5V2.0V 8.5x10-4 0.444

8A 2.24V 1.5V2.24V 0.0010 0.513

Results for Channel 3 Current Transducer Input current Input to PCB Input to TI (Output PCB) Programme ADC Clarke

1 A 0.380mV 1.5V320mV 1.8x10-4 0.0833

2 A 0.640mV 1.5V 600mV 3.14x10-4 0.142

3A 0.9V 1.5V 900mV 4.3x10-4 0.217

4A 1.16V 1.5V1.16V 5.5x10-4 0.265

5A 1.44V 1.5V1.5V 6.4x10-4 0.328

6A 1.72V 1.5V1.72V 7.6x10-4 0.397

7A 1.96V 1.5V1.98V 8.5x10-4 0.415

8A 2.24V 1.5V2.24V 0.00101 0.514

Results for Channel 5 Voltage Transducer Input current Input to PCB Input to TI (Output PCB) Programme ADC Clarke

1 A 0.360mV 1.5V340mV 1.8x10-4 0.0849

2 A 0.6V 1.5V 640mV 3.1x10-4 0.15

3A 0.940mV 1.5V 940mV 4.3x10-4 0.207

4A 1.24V 1.5V1.24V 5.5x10-4 0.28

5A 1.52V 1.5V1.52V 6.7x10-4 0.343

6A 1.84V 1.5V1.84V 8.2x10-4 0.404

7A 2.1V 1.5V2.1V 9.5x10-4 0.485

8A 2.4V 1.5V2.4V 0.00107 0.546

34

Appendix C Commissioning Checklists

Please ote that the below are strictly for outlining purposes only. Within the printed

dissertations contain the written content.

35

Final Year Project DT021/4

Electrical & Electronic Engineering

Commissioning checklist

Doubly Fed Induction Generator

Commissioning of Grid Side CTs

Location: Room 5, DIT, Kevin St

Signed: _____________________

36

Physical Inspection of Equipment Yes No

Check for physical damage which could have occurred Check layout complies with electrical drawings

Check all labelling is correct Check that panel door closes securely

Check all wiring is secure

Electrical function and integrity check Yes No

Check three phase supply Check circuit breakers are in the operation position

Check input current Check output current

Check ratio of instrument transformers

Full clean set of approved drawings available. Comments:

37





Commissioning of Rotor Side CTs


Signed: _____________________

38










39





Commissioning of IGBTs Rotor Side


Signed: _____________________

40










41





Commissioning of IGBTs Grid Side


Signed: _____________________

42








Check input PWM signals


43

Appendix D Hall Effect

The Hall Effect is a production of voltage difference across an electrical conductor. The

Hall Effect is due the nature of the current in a conductor where the current consists of

many small charges. Moving charges experience the Lorentz force when a magnetic field

is present. Without a magnetic field present the charges follow an approximately straight

line. With a magnetic field present perpendicular to the charge, the charges path is seen

to be curved which in turn it can be noted that moving charges accumulate on side of the

material. Thus with the accumulation of charges on one face of a material this leaves an

equal and opposite charge exposed on the other face. This results in a asymmetric

distribution of charge across the Hall element which is perpendicular to both the straight

path of charges and the applied magnetic field. This separation of charges establishes an

electric field which opposes passage of further charges. This produces a steady electrical

potential which is existent as long as charge is flowing.

44

Appendix E Voltage and Current Transducer Datasheet

51

Appendix F Wiring Diagram And Cable Schedule

79

Reference:

[1] D. Ehlert, H. Wrede, Wind Turbines With Doubly-Fed Induction Generator Systems With Improved Performance Due To Grid Requirements, IEEE 2007 [2] T. Thiringer and J. Luomi, Comparison of reduced-order dynamic models of induction machines, IEEE Trans. Power Syst., vol. 16, no. 1, pp. 119126, Feb. 2001. [3] eZdspTM F2812, Technical Reference, page 2-5 [4] Joseph Kearney, Project report on DFIG [5]TMS320x280x, 2801x, 2804x Enhanced Quadrature Encoder Pulse (eQEP) module, Reference guide, page 9 Thomas Ackermann, Wind Power In Power Systems, John Wiley & Sons Ltd, 2005. Bhag S. Guru, Huseyin R. Hiziroglu, Electric Machinery and Transformers, First Edition, 1988

dsadas dissertation

Documents

fed induction generators

wind turbine systems

used generators

variable speed wind

fixed speed wind turbines

following report

project thanks

project supervisor