Rotor Resistance Control of Wound Rotor Induction Generator (WRIG) using PSCAD/EMTDC

Rotor Resistance Control of Wound Rotor
Induction Generator using PSCAD/EMTDC
Wind And Solar Electrical Systems (EEPE-32)
Anmol Dwivedi - 107115009
Kritesh Patel - 107115047
April 15, 2019

ABSTRACT
The primary objective of the carried out mini-project is to develop a variable slip (Type-2)
wind turbine capable of working over a wide speed range. Models in this report includes
representations of general turbine aerodynamics, the mechanical drive-train, and the electri-
cal characteristics of the induction generator (speciﬁcally Wound-Rotor Induction Genera-
tor) as well as the control systems used. The software used to carry out the simulation is
PSCAD/EMTDC, and the results show the working of the wind turbine over a broad speed
range.
i

TABLE OF CONTENTS
ABSTRACT i
LIST OF TABLES iii
LIST OF FIGURES iv
CHAPTER 1 Introduction 1
1.1 Background and Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Project Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
CHAPTER 2 Modeling of Wind Turbine 2
2.1 Introduction to Wind Turbine Modeling . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Pitch Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Tip Speed Ratio, Power Coefﬁcient (Cp) and Torque Calculation . . . . . . . 5
2.3 Mechanical Drive Train . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
CHAPTER 3 Wound Rotor Induction Generator Implementation 8
3.1 Rotor Resistance Control Concept . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Model Implementation in PSCAD . . . . . . . . . . . . . . . . . . . . . . . 8
CHAPTER 4 Results and Discussion 10
4.1 Case-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Case-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3 Case-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Conclusion 18
Appendices 19
A Variable-Slip (Type 2) Single Turbine Estimated Ratings and Parameters 20
ii

LIST OF TABLES
A.1 Wind turbine Speciﬁcations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
A.2 Induction Generator Speciﬁcations . . . . . . . . . . . . . . . . . . . . . . . . 21
iii

LIST OF FIGURES
2.1 Modern wind turbine schematic [3] . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Block Diagram for Wind Turbine [3] . . . . . . . . . . . . . . . . . . . . . . 3
2.3 Pitch Control System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4 CP function [6] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5 PSCAD Implementation of a Wind Turbine with controls . . . . . . . . . . . . 6
2.6 PSCAD Implementation of drive train . . . . . . . . . . . . . . . . . . . . . . 6
2.7 Overall Mechanical PSCAD model of Wind Turbine . . . . . . . . . . . . . . 7
3.1 Rotor Injection Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Rotor Injection circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1 Considered Power System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Stator and Injected Rotor Currents . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3 Active and Reactive Power Attained . . . . . . . . . . . . . . . . . . . . . . . 11
4.4 Shaft torque applied to induction generator . . . . . . . . . . . . . . . . . . . . 11
4.5 Machine Speed variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
iv

CHAPTER 1 INTRODUCTION
.
1.1 BACKGROUND AND MOTIVATION
Wind power has proved to be the most promising renewable energy source over the past
decades, which can overcome the concern of energy shortage in future because of its en-
vironment friendliness and sufficient availability. With the priority status accorded to it in
many countries, the share of wind power in relation to overall installed capacity has in-
creased significantly and in some countries, the share of wind in relation to the overall in-
stalled capacity is already approaching the 50% mark [1]. As wind turbine technology ma-
tures and wind power penetration levels increase, interconnecting a large-scale wind power
plant (WPP) into the bulk power system has become a more important issue. Wind power
generation capacity in India has significantly increased in recent years. As of 31 December
2018 the total installed wind power capacity was 35.288 GW, the fourth largest installed
wind power capacity in the world [2].
1.2 PROJECT OBJECTIVES
The primary objective of the work described herewith was to develop a variable slip (Type-2)
Wind Turbine Generator using PSCAD/EMTDC simulations. The emphasis is on develop-
ment and validation of a simulation model similar to those for other power system apparatus.
Salient features of the developed models are:
• They are generic models;
• They are three-phase, time-domain models implemented in PSCAD/EMTDC which
can also be used in power system studies;
• They can model fast and slow phenomena: electromagnetic transients (1ms) to system-
wide controls (50s);
• They are scalable and the entire system is developed using per-unit representations.
(Wind Turbine and Power System models);
• They include basic wind turbine aerodynamic and mechanical characteristics;
1

CHAPTER 2 MODELING OF WIND TURBINE
Induction machines are popular as generating units due to their asynchronous nature, since
maintaining a constant synchronous speed in order to use a synchronous generator is difﬁcult
due to variable nature of wind speed. Power electronic converters may be used to regulate
the real and reactive power output of the turbine. Broadly wind turbines have been classiﬁed
into four basic types:
• Type 1: Fixed-speed wind turbines
• Type 2: Variable-slip wind turbines
• Type 3: Doubly-fed induction generator (DFIG) wind turbines
• Type 4: Full-converter wind turbines
In this report, we implement a Type-2 variable-speed wind turbine model which are de-
signed to operate at a wide range of rotor speeds as they employ blade-pitching. Variable-slip
(VS) turbines control the resistance in the rotor circuit of the machine to allow a wide range
of operating slip (speed) variation. However, power is lost as heat in the rotor resistance.
Figure 2.1: Modern wind turbine schematic [3]
2

2.1 INTRODUCTION TO WIND TURBINE MODELING
The dominant technology for utility-scale applications is the horizontal axis wind turbine.
Typical ratings range from 500 kW to 5 MW. It must be noted that the power output is inher-
ently ﬂuctuating and non-dispatchable. A typical wind turbine consists of a Rotor (consists
of blades and hub), Drive-train (gearbox, couplings, mechanical brake, and electrical gener-
ator), Electrical system (switchgear, transformers, and power electronic converters), Nacelle
and main-frame. Wind turbines are designed to capture the kinetic energy present in wind
and convert it to electrical energy. From a modeling standpoint, a variable-speed wind tur-
bine consists of the following components:
• Turbine rotor and blade assembly;
• Shaft and gearbox unit (Drivetrain and Speed changer);
• Induction generator;
• Control system;
Figure 2.2: Block Diagram for Wind Turbine [3]
The interaction between each of the components listed above determines how much
kinetic energy is extracted from the wind. Figure 2.2 illustrates the interaction between the
wind turbine components. Modeling of the electrical subsystems is fairly straightforward,
as power system modeling software usually includes a built-in induction machine model.
3

However, modeling of the aerodynamics and mechanical drivetrain is more challenging.
The complete model has been implemented in PSCAD/EMTDC for the purposes of this
report. The following sub-sections describe the modeling of the components listed above:
2.1.1 Pitch Controller
Blade pitching prevents high-wind speeds from causing the wind turbine to operate at higher-
than-rated power output. In this strategy, a control system changes the angles of the tips of
the rotor blades or rotates the entire blade to control the angle of attack and to control ex-
tracted power. Pitch-regulated wind turbines can extract more energy from similar wind
regimes than non-pitch controlled machines, but require additional controllers and machin-
ery, and increase complexity and cost.
Figure 2.3: Pitch Control System
This model ensures that for wind speeds lesser than the rated speed, the pitch of the
blades is kept at zero degrees. Above the rated speed, it keeps the pitch at a fixed value. The
variable Pset fixes the percentage of the rated MW capacity that is required to be generated
by the wind farm. It is usually kept fixed at 1 pu. The values of the constants and limits
are very important in this block to ensure that the pitch remains zero at lesser wind speeds.
Therefore, the pitch depends on both the instantaneous wind turbine generator speed as well
as the real power output of the wind turbine. The pitch control model is shown above in
Figure 2.3.
4

2.2 TIP SPEED RATIO, POWER COEFFICIENT (CP) AND TORQUE CALCULA-
TION
The tip-speed ratio or TSR, denoted by , is the ratio of the blade-tip linear speed to the wind
speed. The TSR determines the fraction of available power extracted from the wind by the
wind turbine rotor. The TSR can be calculated as follows:
λ =
ωrotor ·Rrotor
Vwind
(2.1)
The TSR, together with the user-defined blade pitch angle, are used to calculate the rotor
power coefficient, denoted by CP. The rotor power coefficient is a measure of the rotor
efficiency. There is a constant value of which, if maintained for all wind speeds, will result
in an optimal CP curve and optimal power extraction from the wind. Variable-speed wind
turbines are equipped with a pitch-change mechanism to adjust the blade pitch angle and
obtain a better power coefficient profile.
Cp =
Protor
Pwind
(2.2)
The aerodynamic torque developed (in Nm) can then be calculated:
τrotor =
0.5·ρ ·Cp ·πR2 ·Vwind
3
ωrotor
(2.3)
The value of CP can be obtained either with the help of a look-up table or function. In
this report, we implement CP as a standard function shown in fig below:
Figure 2.4: CP function [6]
Overall the implementation block of these aforementioned calculation is summarized in
the block diagram below:
5

Figure 2.5: PSCAD Implementation of a Wind Turbine with controls
2.3 MECHANICAL DRIVE TRAIN
The mechanical block consists of the rotor shaft, generator shaft, and a gearbox. The wind
turbine drivetrain can therefore be modelled as a two-mass system coupled through a gear
train. The quantities on the wind turbine rotor side of the gearbox can be reﬂected to the
generator side. This eliminates the gear ratio and results in a two-mass representation of
the wind turbine. Moreover, the aerodynamic torque from the wind turbine rotor and the
electromechanical torque from the direct-connect induction generator act in opposition to
each other.
Figure 2.6: PSCAD Implementation of drive train
θturbine =
Γturbine −D·(ωturbine −ωgenerator)−K ·(θturbine −θgenerator)
Jturbine
(2.4)
• Jturbine - Moment of Inertia of the Wind Turbine rotor;
• Γturbine - Wind turbine aerodynamic torque;
6

• ωturbine - Wind turbine rotor speed;
• θturbine - Wind turbine rotor speed;
• D, K - equivalent damping and stiffness;
Figure 2.7: Overall Mechanical PSCAD model of Wind Turbine
7

CHAPTER 3 WOUND ROTOR INDUCTION GENERATOR
IMPLEMENTATION
While fixed-speed wind turbines are simple and robust, they have a significant disadvantage:
they cannot optimally extract power from the wind. It would be preferable to have the
generator continue to output rated power at high wind speeds. To achieve this, variable-
speed wind turbines are employed. While largely relying on the same concepts as fixed-
speed wind turbines at lower-than-rated wind speeds, they typically incorporate blade pitch
and output power controls to optimize power extraction at higher-than-rated wind speeds.
These Type-2 turbines use rotor resistance control to achieve output power control.
3.1 ROTOR RESISTANCE CONTROL CONCEPT
A desired value of torque can thus be achieved at many different speeds, by varying the
external rotor resistance. In a wound rotor induction machine, an external resistance may be
inserted into the rotor circuit during starting, and when operating under load, the external
resistance can be shorted out, thus achieving both objectives: high starting torque and low
running losses. The objective of a rotor resistance controller in this report is to seek the
operating point at which power extraction from the wind is maximized, and also prevent the
power extracted from exceeding the machines ratings.
3.2 MODEL IMPLEMENTATION IN PSCAD
In PSCAD/EMTDC, a wound-rotor induction machine model is available. The same ma-
chine parameters are mentioned in Appendices. The reference real and reactive power are
compared with that of the instantaneous real and reactive power respectively and their out-
puts are fed to the PI controller’s. The output of the PI controllers are the reference d-q
current values which are to be transformed to the respective ABC values with the help of
instantaneous slip angle.
8

Figure 3.1: Rotor Injection Control
Figure 3.2: Rotor Injection circuit
9

CHAPTER 4 RESULTS AND DISCUSSION
The overall system considered for simulation is shown in Figure 4.1.
Figure 4.1: Considered Power System
4.1 CASE-1
At base wind speed = 11m/s.
Figure 4.2: Stator and Injected Rotor Currents
10

Figure 4.3: Active and Reactive Power Attained
Figure 4.4: Shaft torque applied to induction generator
11

Figure 4.5: Machine Speed variation
The graphs of Case-2 and Case-3 will be compared to the aforementioned graphs in order
to draw conclusions from the study. Since the speed is less than rated speed, the machine
speed and output power will be dependent on the wind speed. In this case, the output power
is 4.4 MW which is less than rated power since the machine is not operating at its rated
speed. Increasing the speed further would result in increase in output real power and settling
speed of the machine.
4.2 CASE-2
Increasing the speed of wind to rated values = 12m/s
12

13

It is clear that the speed of the machine has increased as a result of the increase in wind
speed which means that the resultant electromagnetic torque has increased with slip. The
output real power is 5.3 MW which is almost equal to the rated machine MVA. With any
more increase in wind speed, the controllers will swing into action and operate as per their
respective references in order to limit the output real power and speed of the machine.
14

4.3 CASE-3
At rated wind speed = 15m/s, set Real Power (P) = 5MW and set Reactive Power (Q) = 0
MW. These measurements would assist the pitch controller to adjust its pitch angle accord-
ingly in order to limit the output real power and speed of the machine. At this speed, which
is above the rated speed of the machine, the control system should adjust the pitch angle
such that the Power Output and hence the speed of the machine remains constant and hence,
shall not exceed the rating of the machine.
15

16

Once the speed of the machine is increased beyond the rated speed of the machine, the
speed of the machine and the rated power of the machine is maintained constant at rated
values.
17

CONCLUSION
• With any increase in wind speed above the rated value, the output power is regulated
by the PI controller (at the rated power) by adjusting the pitching angle. By changing
the pitching angle the output mechanical/shaft torque can be adjusted in order to keep
the output power of the machine within limits.
• As the wind speed increases, slip increases and hence, the corresponding rotor re-
sistance shall increase (decrease in injected current) in order to compensate for the
increase in slip in order to achieve the same torque at different speeds of the machine.
• This method is not widely used due to the high losses in rotor resistance.
Some of the various short comings faced during the simulation study are as follows:
• Ideal current sources are used in order to inject currents i.e there is no actual resis-
tance change in the rotor circuit. The current is being injected which is considered
equivalent to changing rotor resistance.
• This simulation study does not implement MPPT for speeds below rated speed of the
machine and hence, is not the most efﬁcient implementation.
18

VARIABLE-SLIP (TYPE 2) SINGLE TURBINE ESTIMATED
RATINGS AND PARAMETERS
Table A.1: Wind turbine Speciﬁcations
Variable Value
Regulation Method Pitch Control
Maximum Pitch 30o
Cut-in Speed 4m/s
Cut-out Speed 20m/s
Base Wind Speed 11m/s
Rated Wind Speed 12m/s
Nominal Tip Speed Ratio 8.1
Nominal Cp 0.48
Shaft Spring Constant 1.11
Shaft Mutual Damping 1.5
Kp(generator) 5
Ki(generator) 5
Kp(turbine) 15
Ki(generator) 0.1
20

Table A.2: Induction Generator Speciﬁcations
Variable Value
Rated MVA 5.5 MVA
Rated Voltage 0.69kV
Number of Poles 6
System Frequency 60Hz
Stator to Rotor Turns Ratio 0.3
Rated Wind Speed 12m/s
Stator Resistance 0.0054 pu
Wound Rotor Resistance 0.006 pu
Magnetizing inductance 4.5 pu
21

REFERENCES
[1] Wind Energy Potential in India An Overview
[2] https://en.wikipedia.org/wiki/WindpowerinIndiaTamilNadu
[3] Dynamic Models for Wind Turbines and Wind Power Plants - NREL Laboratories
[4] Active and Reactive Power Control of a Variable Speed Pumped Storage System
[5] Modeling of GE Wind Turbine-Generators for Grid Studies
[6] Type 3 Wind Turbine Model - PSCAD Simulation Document
[7] Energy Resources Technology - NPTEL Document by Electrical Engineering, Prof.
S. Banerjee, IIT Kharagpur
All block diagrams/ schematics taken from reference number [3]
22

Rotor Resistance Control of Wound Rotor Induction Generator (WRIG) using PSCAD/EMTDC

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