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
Chapter 1 Wind turbine systems
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.
Chapter 1 Wind turbine systems
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
Chapter 1 Wind turbine systems
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.
Chapter 2 DFIG For Wind Turbines
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
Chapter 2 DFIG For Wind Turbines
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
Chapter 2 DFIG For Wind Turbines
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
Chapter 3 Space Vector Theory
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
Chapter 3 Space Vector Theory
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
Chapter 3 Space Vector Theory
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)
Chapter 4 System hardware and digital control
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.
Chapter 4 System hardware and digital control
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.
Chapter 4 System hardware and digital control
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.
Chapter 4 System hardware and digital control
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.
Chapter 4 System hardware and digital control
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.
Chapter 4 System hardware and digital control
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.
Chapter 7 Testing Results
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.
Chapter 7 Testing Results
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.
Input current Input to PCB Input to TI (Output PCB) Programme ADC Clarke
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.
Chapter 7 Testing Results
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.
Chapter 7 Testing Results
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
Chapter 7 Testing Results
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
Input current Input to PCB Input to TI (Output PCB) Programme ADC Clarke
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
Final Year Project DT021/4
Electrical & Electronic Engineering
Commissioning checklist
Doubly Fed Induction Generator
Commissioning of Rotor Side CTs
Location: Room 5, DIT, Kevin St
Signed: _____________________
38
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:
39
Final Year Project DT021/4
Electrical & Electronic Engineering
Commissioning checklist
Doubly Fed Induction Generator
Commissioning of IGBTs Rotor Side
Location: Room 5, DIT, Kevin St
Signed: _____________________
40
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:
41
Final Year Project DT021/4
Electrical & Electronic Engineering
Commissioning checklist
Doubly Fed Induction Generator
Commissioning of IGBTs Grid Side
Location: Room 5, DIT, Kevin St
Signed: _____________________
42
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 input PWM signals
Full clean set of approved drawings available. Comments:
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
45
46
47
48
49
50
51
Appendix F Wiring Diagram And Cable Schedule
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
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