low voltage ride through using pmsg for grid stability

LOW VOLTAGE RIDE THROUGH USING PMSG FOR GRID STABILITY by Neha Srivastava (0809120038) Pratibha Gautam (0809120044) Pushyamitra Vyas (0809120045) Sonia Singh (0809120068)

Submitted to the Department of Electrical Engineering in partial fulfillment of requirements for the degree of Bachelor of Technology in Electrical Engineering

Under the Supervision of Mr. Amit Kumar Roy

JSS Academy of Technical Education, Noida G. B. Technical University May, 2012

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CHAPTER 1

INTRODUCTION

1.1 OVERVIEW Human efforts to utilize wind for energy have started in ancient times, when they used to sail the ships and boats. Later wind energy served the mankind by providing the energy for girding mills and water pumps. During its transformation from these crude and heavy devices to today‟s efficient and sophisticated machines, the technology went through various phases of development. In Holland, several decisive improvements were made on wind mills in the 16th century, leading to new types of mill, so called Dutch wind mill. The era of wind electric generators began close to 1900‟s. The first modern wind turbine, specifically designed for electricity generation was constructed in Denmark in 1890. The wind electric generators became commercially available in American market by 1925. The research on wind turbines started during 1950. However during this period electricity generated from wind costs 8-10 times more than that of fossil fuels. But oil crisis in1970‟s forced the world to think about wind power generation and to work for cost efficient measures. Wind power which has been proved as a potential source for generation of electricity with minimal environmental impact, is the fastest growing source for electric power generation and it is expected to remain so in future. Countries like Germany, United States of America, China and India have taken a lead in harnessing this nonpolluting and replenishable source of energy. In terms of economic value the wind energy sector has now become firmly installed as one of the important players in energy markets. Harnessing wind energy for electric power generation is an area of research interest and at present, the emphasis is given to the cost effective utilization of energy resource for

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quality and reliable power supply. There are various techniques to harness this wind energy. The permanent magnet synchronous generator with its lower maintenance demands and high energy yield appeared to be a good solution for such applications. WECs fitted with PMSG does not absorb reactive power from the grid there by giving a higher efficiency. Since permanent magnet is used no additional power supply for the magnetic field excitation is required. As a result there are no field losses thereby improving the thermal characteristics of the machine. So PMSG is preferred for wind energy plants.

1.2 LITERATURE REVIEW Wind power has proved as a potential source for generation of electricity with minimal environmental impact. Countries like Germany, United States of America, China and India have taken a lead in harnessing this renewable energy [1]. India figures fifth among countries utilizing wind power. World wind energy is expected to be 250 GW worldwide by 2012 [2].

Musgrove [3] has presented the summary on wind energy and wind energy conversion systems. An overview of various types of wind turbines is also given. Wind turbine characteristics and aerodynamic forces on the wind turbine blade have been explained. The recent technical advances in wind turbines, power electronics, and machine drives have made wind energy very competitive to fossil fuel power.

Power electronics technology has gone through dynamic evolution in the last four decades from mercury arc converters to modern power semiconductor converters. The advent of solid-state variable-frequency inverters, various converter topologies, advanced PWM techniques and improved control and estimation methods gradually have resulted into high performance AC drives of various types [4].

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Anderson, Richon and Campbell [5] have implemented an aerodynamic moment control by enclosing a pitch able section of the blade in an active control loop, using the external aerodynamic load as feedback variable. Muljadi and Butterfield [6] have discussed the behavior of the wind turbine generator with pitch control capability under turbulent winds. Two methods to adjust the aerodynamic power have been investigated: pitch control and generator load control.

Thiringer and Linders [7] have implemented a variable rotor speed control of a fixedpitch wind Turbine using a special technique, in which operating point of the wind turbine is determined by using the measured rotor speed and power. Xingjia et al. [8] have proposed the active vibration control based on expert PID pitch control strategy. Experts PID controller combine the expert system and the PID controller. Muller et al. [9] have presented a comparison of various generating systems used for wind energy conversion with the help of the power curves.

Whereas Datta and Ranganathan [10] compared the fixed speed and variable speed wind energy conversion system using grid connected induction generator. The comparison has been done on the basis of major hardware components required, operating region and energy output with a defined wind function.

The control strategy of FESS to reduce the frequency variation proposed by Takahashi and Tamura [11] is based on adjustment of output of the main power plant in cooperation with the FESS depending on its energy charge level and direct frequency control. The appropriate dynamic model has been studied for dynamic or transient conditions. The d-q transformation which deals with the transformation of three phase stationary coordinates to d-q rotating coordinate system has been studied [12]. In d-q transformation the variables associated with the stator of synchronous machines are replaced with the variables associated with fictitious winding rotating with the rotor, in other words stator variables are referred to a reference frame fixed in rotor. Brereton [13]

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applies Park transformation to induction machines and transformed the stator variable of induction machine to a frame of reference fixed in rotor. In theory given by Stanley [14] the rotor variables in voltage equations of induction machine are transformed to variables associated with fictitious stationary winding. The speed of this reference frame is zero.

Hammons [15] has proposed a detailed induction generator model and analyzed the transient behavior of induction generator running at a speed close to synchronous speed when directly connected to electric distribution system.

Demoulias and Dokopoulos [16] have used simulation program to investigate the system behavior when wind turbine is disconnected from the grid under various circumstances. The transient behavior of a wind driven induction generator after its disconnection from the power grid has been investigated using experimental arrangement [17]. The cause of the capacitor failures has been identified by finding the relationships between the voltage rise and the characteristics of the wind turbines using digital simulation [18].

Petru and Thiringer [19] have presented wind turbines modeling for power system studies and various parts of a wind turbine model, such as aerodynamic conversion, drive train, and generator representation, are analyzed. Popa and Et.al [20] have proposed a method that allows an induction machine to run a turbine at its maximum power coefficient. The rotational speed is the controlled variable in proposed control strategy.

1.3 OBJECTIVE OF THE WORK The objective of the present work is to demonstrate the low voltage ride through capability of the variable speed driven permanent magnet synchronous generator for wind energy conversion system during fault under Matlab Simulink environment. This

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work will also discusses the interconnection issues of permanent magnet wind generators as per prevailing grid standards during healthy and fault conditions.

1.4 ORGANIZATION OF THE PROJECT REPORT The report has been organized into five chapters. The present chapter summarizes the overview, work carried out by various researchers and the outline of the report. Chapter 2 introduces wind as a source of electric power with the laws governing the amount of power extracted from wind. It also includes the various wind energy conversion schemes and methods of controlling wind turbine. Global status of wind energy has also been indicated. Chapter 3 includes the major grid problems and the corresponding fault ride through requirements. It also summarizes the wind turbine technologies and the grid code requirements. Chapter 4 details the modeling of wind turbine, PMSG and the various block parameters. Chapter 5 discusses the main model along with the simulation results. Finally chapter 6 summarizes the conclusions and scope of future work.

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CHAPTER 2

WIND AS A SOURCE OF ELECTRIC POWER

2.1 INTRODUCTION The history of wind power shows a general evolution from the use of simple, light devices driven by aerodynamic drag forces; to heavy, material-intensive drag devices; to the increased use of light, material-efficient aerodynamic lift devices in the modern era. Wind power has been used for centuries for different purposes such as: pumping water, propelling boats or grinding corn. Wind turbines are used to extract the energy from the wind by transferring the thrusting force of the air passing through the turbine rotor into the rotor blades. There are two primary physical principles by which energy can be extracted from the wind; these are through the creation of either drag or lift force or through a combination of the two. The cross section of rotor wing as shown in Fig. 2.1 will help understand these principles [3]. The wind passes over both surfaces of the airfoil shaped blade. As wind passes more rapidly over the longer (upper) side of the airfoil, it creates a low- pressure area above the airfoil. The pressure differential between top and bottom surfaces creates a thrust force, called aerodynamic lift.In addition to lift force, a drag force perpendicular to the lift force impedes rotor rotation [3]. A prime objective in wind turbine design is for the blade to have a relatively high lift- todrag ratio. This ratio can be varied along the length of the blade to optimize the turbine‟s energy output at various wind speeds. In an aircraft wing, this force causes the airfoil to rise, lifting the aircraft off the ground. Since the blades of a wind turbine are constrained to move in a plane with the hub as its center, the lift force causes rotation about the hub. As a result, the lifting force is converted into

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a mechanical torque and this torque makes the shaft, as part of the turbine rotor, turn. The power in the shaft is used to produce electricity by coupling it with generator.

Fig 2.1 Cross section of a rotor wing

The basic features that characterize lift and drag are: • Drag is in the direction of airflow. • Lift is perpendicular to the direction of airflow. • Generation of lift always causes a certain amount of drag to be developed. • With a good aerofoil, the lift produced can be thirty times greater than the drag. • Lift devices are generally more efficient than drag device.

2.2 THE IDEAL BRAKING AND BETZ LAW Wind turbine extracts the energy from wind by slowing down the wind by blades. The more kinetic energy a wind turbine pulls out of the wind, the more the wind will be slowed down as it leaves the turbine. If all the energy from the wind is extracted, the air would move away with the speed zero, i.e. the air could not leave the

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turbine. In that case no energy could be extracted. In the other extreme case, the wind could pass though blades without being hindered at all. In this case we would likewise not have extracted any energy from the wind. The wind energy can be converted into useful mechanical energy by applying braking between these two extreme cases. An ideal wind turbine would slow down the wind by 2/3 of its original speed. This can be explained by Betz law [21]. The Betz‟ law suggests that only 59% of the kinetic energy in the wind can be converted to mechanical energy using a wind turbine. This can be proved through the following stepsLet the average wind speed through the rotor area is the undistributed wind speed before the wind turbine (V 1) and the wind speed after the passage through the rotor plane( V2). So the average speed is (V1+V2)/2 The mass of the air streaming through the rotor during one second is m =ρA(V1+V2)/2

(2.1)

where m: Mass per second, 𝜌: Density of air, A: Swept rotor area According to Newton's second law the power extracted from the wind by the rotor is equal to the mass times the drop in the wind speed squared.

P = (1/2)m(V12 –V22 )/2

(2.2)

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Substituting eq. (2.1) into eq. (2.2) the power extracted from the wind can be calculated asP = (1/4)(V12 –V22 )/2 (V1 +V2 )ρA

(2.3)

The total power in the undisturbed wind streaming through exactly the same area F, with no rotor blocking the wind isP0 = (1/2)V13A

(2.4)

The ratio between the power extracted from the wind and the power in the undisturbed wind is : (P/P0) = (1/2) (1 - (V1 /V2)2) (1 + (V1 /V2 ))

Fig 2.2 Plot of (P/P0) as a function of (V2 /V1 )

(2.5)

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Fig. 2.2 shows the plot of P/P0 as a function of (V2 /V1) . It can be clearly observed that the function reaches its maximum for (V2/V1) = 1/3 while the P/P0 = 0.59 which proved that maximum power that can be extracted from the wind is 59% of the total power in the wind.

2.3 BENEFITS OF WIND POWER The growth of the market for wind energy is being driven by a number of factors, that include the context of energy supply and demand, the rising profile of environmental issues, like climate change and the impressive improvements in the technology itself. These factors have combined in many regions of the world to encourage political support for the industry‟s development. The benefits of wind power have been summed up under the following points-

2.3.1 ECONOMIC CONSIDERATION The wind power is the least expensive of other forms of alternative energy and the electricity at the cost of around 5 cents per kWh can be generated. This cost is projected to decline even more as technology improves because most of the cost with wind power is initial investment and very little cost is needed to maintain it [1]. For conventional generation technologies, future price developments are a significant risk factor and if current trends are any indication, they are likely to continue rising into the unforeseeable future. Wind power is commercially attractive, especially when taking into account the price of carbon, which is a factor in a growing number of markets. The wind power industry is revitalizing regional economies, providing quality jobs and expanding tax bases in rural regions struggling to keep their economies moving ahead in the face of the global flight to the cities. This is resulting into regional economic development as experienced in Tamil Nadu and Gujarat

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2.3.2 SECURITY OF SUPPLY As energy demand continues to increase, supplies of the main fossil fuels used in power generation, are becoming more expensive and more difficult to extract. Therefore, major economies of the world are increasingly relying on imported fuel at unpredictable cost, sometimes from regions of the world where conflict and political instability threaten the security of that supply [1]. In contrast to the uncertainties surrounding supplies of conventional fuels, and volatile prices, wind energy is a massive indigenous power source which is permanently available in virtually every country. There are no fuel costs, no geo-political risk and no supply dependence on imported fuels from politically unstable regions. Every KWh generated by wind power has the potential to displace fossil fuel imports, improving both security of supply and the national balance of payments.

2.3.3 ENVIRONMENTAL BENEFITS OF WIND POWER The dependency on electric energy is increasing day by day, consequently the need for more power plants. Today, most of the electricity is generated by burning fossil fuels, which emits the green house gases and other polluting gases like NOx, SOx results into climate change [2]. Climate change is now generally accepted to be the greatest environmental threat facing the world, and keeping our planet‟s temperature at sustainable levels has become one of the major concerns of policy makers. More wind energy generation will improve the environment. Greenhouse gases like COx are produced naturally in our environment through volcanic activity and organic matter decomposition and by burning of fossil fuels to generate electricity. Using wind to generate electricity reduces COx emission. Sulphur dioxide and nitrogen oxide is a byproduct of burning fossil fuels. In the atmosphere it can react with other chemicals to form acidic compounds. These gases are the main components of the „acid rain‟. Acid rain threatens our lakes and forests by raising the level of acidity. At present power sector is the largest single source of emissions accounting for about

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40% of COx and about 25% of overall emissions. This is costing billions in additional health costs and infrastructure damage. Wind power does not emit any climate change inducing carbon dioxide nor other air pollutants. The wind energy not only reduces the emission of green house gases it also has a positive effect on the quality of air we breathe.

2.3.4 FULLY RENEWABLE AND REDUCED HAZARDS Wind energy does not deplete natural resources. Unlike fossil fuels that have a finite supply, Wind is renewed every day as the earth heats and cools. While it is true that wind energy supplies can come and go based on weather patterns, seasons and so forth, the overall renewable side of production is one of the benefits of wind energy production that make it very attractive. Approximately one pound of coal is saved by every kilowatt-hour generated by wind. Production of fossil fuel and nuclear power can pose rather large safety hazards. A reduction of these is another one of the many benefits of wind energy production. The benefits of wind energy are rather extensive. Although it is difficult to fully replace the use of other sources of energy with the wind alone, this type of energy can greatly reduce reliance upon them.

2.4 WIND ENERGY STATISTICS The statistics demonstrating the global status of wind energy and status of wind energy in India are presented. These statistics have been summed with available data.

2.4.1 GLOBAL STATUS OF THE WIND ENERGY Wind energy has continued the worldwide success story as the most dynamically growing energy source again in the year 2012. Since 2008, global wind installations has doubled. The market for new wind turbines showed a 30.98% increase with 239000 MW

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of new capacity in 2011.The global year wise new installed capacity is shown in Fig.2.3. Since 2001 the wind energy has grown up by more than 200000 MW from 24322 MW to 239000 MW in the year 2011. All wind turbines installed by the end of 2011 worldwide is enough to cover 3% of world‟s total electricity demand [1].

2003

2004

93927

2002

74122

2001

59024

47693

50000

39295

100000

31181

150000

120903

196653

200000

159766

250000

24322

INSTALLED CAPACITY (MW)

300000

239000

World Total Installed Capacity

0 2005

2006

2007

2008

2009

2010

2011

YEAR

Fig 2.3 World Total Installed Capacity

Based on accelerated development and further improved policies, a global capacity of more than 1500000 MW is possible by the year 2020. This development, led by the United States, Germany, Spain, China and India took the worldwide total to 194.4GW. This shows an increase of 30.98% compared with the 2010 market [2]. Fig. 2.5 and Fig. 2.6 show the cumulative installed capacity and the yearly world market growth respectively.

50

42.432

60

52.8

14

29.50 3.960

4.611

10

5.707

6.200

6.060

14.550

20

21.150

30

27.981

40

0

Installed Capacity by end of 2010

Capacity installed during 2011

Fig 2.4 Top 10 Installed Capacities In 2011

Still the five leading countries stand for the lion share of the world capacity of wind turbines: China, USA, Germany, Spain and India, together representing a total share of 74 % of the global wind capacity. Again in 2011, China continues to dominate the world wind market, adding 8 GW in only 6 months, the highest number ever within the first half year. Within those 6 months, China accounted for 43 % of the world market for new wind turbines, compared with 50 % in the full year of 2010. By June 2011, China had an overall installed capacity of around 52 GW [24]. Most of the European markets showed stronger growth in 2011 than in the previous year: The top markets in Europe continue to be Germany with a new capacity of 766 MW and reaching a total of 27‟981 MW, Spain (484MW, 21‟150 MW in total), Italy (460 MW, 6‟200 MW total), France (400 MW, 6‟060 MW total), the United Kingdom

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(504 MW, 5‟707 MW) and Portugal (260 MW, 3‟960 MW). Only France and Denmark showed a decrease in their new installed capacity compared to the first half of 2010 and Denmark even dropped out of the list of the top 10 markets, while Portugal became the new number 10.

74.1

59

47.6

31.1

23.9

17.4

13.6

10.2

50

7.6

100

39.4

120.3

150

93.8

200

194.4

158.7

250

6.1

Global Cumulative Capacity (GW)

Global Wind Power Cumulative Capacity

0

Year

Fig 2.5 Global Wind Power Cumulative Capacity

The cumulative global wind power capacity has increased by almost 100% since 1996 to the year 2010. A remarkable increase of about 32% has been noticed during the year 2009. With this continuous increase in wind power capacity worldwide, wind has proved to be one of the most promising energy resource for electric power generation.

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NEW INSTALLED CAPACITY 2010-2011 (MW) 25.5 21.682 18.405 16

Ist Half 2010

2nd Half 2010

Ist Half 2011

2nd Half 2011*

Fig 2.6 Annual Growth in World Wind Market

2.4.2 WIND ENERGY IN INDIA India had a record year for new wind energy installations in 2010, with 2,139 MW of new capacity added to reach a total of 13,065 MW at the end of the year. Renewable energy is now 10.9% of installed capacity, contributing about 4.13% to the electricity generation mix, and wind power accounts for 70% of this installed capacity. Currently the wind power potential estimated by the Centre for Wind Energy Technology (CWET) is 49.1 GW, but the estimations of various industry associations and the World Institute for Sustainable Energy (WISE) and wind power producers are more optimistic, citing a potential in the range of 65- 100 GW [25]. Historically, actual power generation capacity additions in the conventional power sector in India been fallen significantly short of government targets. For the renewable energy sector, the opposite has been true, and it has shown a tendency towards exceeding the targets set in the five-year plans. This is largely due to the booming wind power sector. Given that renewable energy was about 2% of the energy mix in 1995, this growth is a

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significant achievement even in comparison with most developed countries. This was mainly spurred by a range of regulatory and policy support measures for renewable energy development that were introduced through legislation and market based instruments over the past decade. The states with highest wind power concentration are Tamil Nadu, Maharashtra, Gujarat, Rajasthan, Karnataka, Madhya Pradesh and Andhra Pradesh.The Indian Ministry of New and Renewable Energy (MNRE) is working to create an attractive environment for the export, purchase, wheeling and banking of electricity generated by wind power projects. State Electricity Regulatory Commissions (SERC) are also promoting wind energy, through preferential tariffs and a minimum obligation on distribution companies to source a certain share of electricity from renewable energy. Ten states have set up renewable purchase obligations, requiring utilities to source up to 10% of their power from renewable. India has a solid domestic manufacturing base, including global player Suzlon, which accounts for over half of the market. In addition, other international companies have set up production facilities in India, including Vestas, Repower, Siemens, LM Glasfiber and Enercon.

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Table 2.1 Total Installed Capacity in India

Year

2000

2001

2002

2003

2004

2005

2006

2007

2008

MW

220

1,456

1,702

2,125

3,000

4,430

6,270

7,845

9,655

2009

2010

10,926 13,065

Table 2.2 State wise wind power installed capacity in India S.No.

State

Capacity in MW

1

Tamil Nadu

6160

2

Maharashtra

2358

3

Gujarat

2284

4

Karnataka

1765

5

Rajasthan

1643

6

Madhya Pradesh

276

7

Andhra Pradesh

198

8

Kerala

35

9

Others

4

Total

14723

19

Fig 2.7 State wise percentage shares in wind energy production

In India maximum wind power can be extracted from Tamil Nadu with 43% of the total wind energy production. Maharashtra and Gujarat occupying the second and third position with 19% and 15% wind energy production respectively.

2.5 DIFFICULTIES WITH WIND POWER Despite of many benefits of wind power, there are some difficulties also in converting the wind energy into electric power. Various difficulties with wind power are also discussed here with under the following headings.

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2.5.1 VARIABILITY OF WIND The main difficulty with wind energy is that wind is intermittent. Wind speeds vary with time of day, time of year, height above ground, and location on the earth‟s surface. Near the earth‟s surface, winds are usually greater during the middle of the day and decrease at night. Wind speeds also vary strongly with time of year. In the southern Great Plains, the winds are strongest in the spring and weakest in the summer (July and August). Utilities here are summer peaking, and hence need the most power when winds are the lowest and the least power when winds are highest. Due to this variability in wind , the power output will be zero (or very small) for perhaps 10% of the time, rated for perhaps another 10% of the time, and at some intermediate value the remaining 80% of the time. For a fixed speed system the turbulence of the wind will result in variation in power output. However, output fluctuations of wind generators cause network frequency variations in power systems [17]. This can decrease the power quality and then cause are striction of wind farm installation, especially in an isolated power system. This means that some sort of storage is necessary to mitigate these power quality problems and to compensate variation in power. Following are the most commonly used storage systems. 1. Flywheel storage. . 2. Compressed air energy storage. 3. Superconducting magnetic energy storage. 4. Battery storage.

2.5.2 NOISE The noise from the wind turbines is incredible. It sounded like airplanes or helicopters. New turbines may have quieter bearings and gears, but the huge magnetized generators cannot avoid producing a low-frequency hum and the problem of 100-foot rotor blades chopping through the air at over 100 mph also is insurmountable. Every time each rotor

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passes the tower, the compression of air produces a deep resonating thump. Only a gravelly "swishing" may be heard directly beneath the turbine, but farther away the resulting sound of several towers together has been described to be as loud as a motorcycle, like aircraft continually passing overhead. In mountainous areas the sound echoes over larger distances.

2.6 PITCH AND STALL CONTROL OF WIND TURBINE An effective control system should be designed to control the output power of wind turbine to maximize their output. In case of stronger winds it becomes necessary to waste part of the excess energy of the wind in order to avoid damaging the wind turbine [5]. The wind turbines are designed to yield maximum output at wind speeds of 12-15 metres per second because the probability of this speed of wind is very high. It is not economical to design turbines that maximize their output at stronger winds because such strong winds are rare.

2.6.1 PITCH CONTROL Pitch control is a method of controlling the speed of a wind turbine by varying the orientation or pitch of the blades [6]. On a pitch controlled wind turbine, the turbine's electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it sends a corrective signal to the blade pitch mechanism which immediately pitches (turns) the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again. In designing a pitch controlled wind turbine, it should be ensured that the rotor blades pitch varies exactly by the required amount. On a pitch controlled wind turbine, the computer will generally pitch the blades a few degrees every time the wind changes in order to keep the rotor blades at the optimum angle in order to maximize output for all wind speeds.

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2.6.2 STALL CONTROL Wind turbine stalling works by increasing the angle at which the relative wind strikes the blades (angle of attack), and it reduces the induced drag (drag associated with lift). Stalling increases automatically when the winds speed up. The rotor blade has been aerodynamically designed to ensure that the moment wind speed becomes too high [7]. stalling creates turbulence on the side of the rotor blade not facing the wind. This stall prevents the lifting force of the rotor blade from acting on the rotor. A rotor blade for a stall controlled wind turbine is twisted slightly along its longitudinal axis. This is partly 12 done in order to ensure that the rotor blade stalls gradually rather than abruptly when the wind speed reaches its critical value. The basic advantage of stall control is that one avoids moving parts in the rotor itself, and a complex control system. On the other hand, stall control represents a very complex aerodynamic design problem, and related design challenges in the structural dynamics of the whole wind turbine, e.g. to avoid stallinduced vibrations. Nearly two thirds of the wind turbines currently being installed in the world are stall controlled.

2.7 WIND ENERGY CONVERSION SCHEMES Several kinds of generator technologies have been developed and are in use today [4]. Broadly the generator may be working in stand alone or synchronized with grid. A short overview of grid connected wind generator topologies employing squirrel cage induction generator is presented. These topologies can be broadly classified on the basis of use of power electronic circuitry as1. Direct grid connected operation. 2. Grid operation employing power electronics.

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2.7.1 DIRECT GRID CONNECTION Induction generator is the prevalent choice for fixed speed wind energy systems because of its simplicity, robustness and relative low cost. This concept consists of a rotor connected to a squirrel cage induction generator through a gear box is shown in Fig. 2.9 The gear box is needed because the optimal rotor and generator speed ranges are different. This concept is known as Danish concept and many low power wind turbines build to date has been constructed according to this Danish concept [22]. The inductive circuit of squirrel cage induction generator consumes reactive power so to improve the power factor of whole system capacitors are often added to supply magnetizing current. The generator is directly grid coupled; therefore, speed variations are very small. since the speed variations are very small the turbine is normally considered to operate at constant speed. Often the fixed speed wind turbine systems are provided with two fixed speeds. This is accomplished by using generators with adjustable pole winding 13 configuration. This will increase the aerodynamic capture as well as reduces the magnetizing losses of generator at low wind speeds. The performance of fixed-speed wind turbines very much depends on the characteristics of mechanical sub circuits e.g. pitch control time constants, main breaker maximum switching rate, etc. The response time of some of these mechanical circuits may be in the range of tens of milliseconds. As a result, each time a gust of wind hits the turbine, a fast and strong variation of electrical output power can be observed. When induction machines are operated using vector control techniques, fast dynamic response and accurate torque control are obtained.

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Fig 2.8 Direct grid connected wind energy conversion scheme

2.7.2 GRID OPERATION EMPLOYING POWER ELECTRONICS In this scheme permanent magnet synchronous generator is connected to grid through power electronics converter-inverter set. Schema of this scheme is given in Fig. 2.10 Rectifier changes variable frequency output of permanent magnet synchronous generator to constant dc and this dc is boosted up by dc-dc converter and then converted to constant frequency ac by firing angle control of converter inverter set [22-23]. This ac output from converter is supplied to the grid.

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Fig 2.9 Variable speed system using permanent magnet generator

2.8 CONCLUSION With the pace of development the demand of energy is growing rapidly. It has become impossible to meet this growing need of energy with conventional methods of generation because of depleting fossil fuel reserves and increasing crude oil prices. To sustain this pace of development it is necessary to search and develop non conventional sources of energy. Wind energy is a very good alternative to this problem. There is large potential of wind energy in India and in rest of the world. Wind energy is cheaper than other renewable sources of energy like solar energy. After the payback period it is totally free of cost for rest of its operating period. Wind energy also gets intention because of the rising concern towards climate changes as wind is a clean source of energy. Wind energy also have some negative impacts and all of these negative aspects will only become worse if even a small part of the industry's plans for hundreds of thousands of towers becomes reality however, the negative impacts must of course be weighed against the benefits and these are negligible.

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CHAPTER 3

GRID CODES AND FAULT RIDE THROUGH REQUIREMENTS

3.1 INTRODUCTION Increasing wind power penetration levels to the power systems of many regions and countries has led to the elaboration of specific technical requirements for the connection of wind farms with the grid which is generally known as the grid codes issued by the Transmission System Operators (TSOs). These requirements typically refer to large wind farms, connected to the transmission system, rather than smaller stations connected to the distribution network [26]. The new grid codes stipulate that wind farms should contribute to power system control (frequency and also voltage), much as the conventional power stations, and focus on wind farm behavior in case of abnormal operating conditions of the network (such as in case of voltage dips due to network faults). Grid code requirements are the major driver for the development of WT technology and several relevant publications are already available.

The technical requirements within grid codes vary between electricity systems. The typical requirements for generators can be grouped as follows: • Tolerance - the range of conditions on the electricity system for which wind farms must continue to operate; • Control of reactive power - often this includes requirements to contribute to voltage control on the network; • Control of active power - often this includes requirements to contribute to frequency control on the network;

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• Protective devices; and • Power quality. It is important to note that these requirements are often specified at the Point of Common Coupling (PCC) between the wind farm and the electricity network. In this case, the requirements are placed at wind farm level, and wind turbines may be adapted to meet these requirements. It is also possible for some requirements to be met by providing additional equipment, as for example FACTS devices.

3.2 MAJOR GRID PROBLEMS Substantial wind penetration exists today and will only increase over time. The impacts on the transmission network are viewed not as an obstacle to development, but rather as obstacles that must be overcome [27]. High penetration of intermittent wind power greater than 20 percent of generation meeting load and may affect the network in the following ways and are detailed as follows:

3.2.1. POOR GRID STABILITY For economic exploitation of wind energy, a reliable grid is as important as availability of strong winds. The loss of generation for want of stable grid can be 10% to 20% and this deficiency may perhaps be the main reasons for low actual energy output of WECs compared to the predicted output in known windy areas with adequate wind data.

3.2.2. LOW FREQUENCY OPERATION Low frequency operation affects the output of WECs in two ways. Many WECs do not get cut-in, when the frequency is less than 48 Hz (for standard frequency of 50 Hz) through wind conditions are favorable, with consequent loss in output. This deficiency apart, the output of WECs at low frequency operation is considerably reduced, due to

28

reduced speed of the rotor. The loss in output could be about 5 to 10% on the account of low frequency operation.

3.2.3. IMPACT OF LOW POWER FACTOR WECs fitted with induction generators need reactive power for magnetizing. Normally in conventional energy systems, generators apart from supplying active power will be supplying a reactive power. But in case of WECs fitted with induction generators, instead of supplying reactive power they absorb reactive power from the grid, which undoubtedly is a strain on the system. Suitable reactive power compensation may be required to reduce the reactive power burden on the grid. Lower the power factor, higher is the reactive power absorption.

3.2.4. POWER FLOW It is to be ensured that the interconnecting transmission or distribution lines will not be over-loaded. This type of analysis is needed to ensure that the introduction of additional generation will not overload the lines and other electrical equipment. Both active and reactive power requirements should be investigated for an ideal power flow.

3.2.5. SHORT CIRCUIT It is required to determine the impact of additional generation sources to the short circuit current ratings of existing electrical equipment on the network to avoid any damage due to short circuit in the existing network.

3.2.6. POWER QUALITY Fluctuations in the wind power may have direct impact on the quality of power supply. As a result, large voltage fluctuations may result in voltage variations outside the regulation limits, as well as violations on flicker and other power quality standards. Therefore it is necessary to maintain the quality of power supplied to the consumers.

29

In order to mitigate the above mentioned grid problems in WECs several solutions are provided by wind turbine manufacturers. However, problems such as power quality, contribution to short circuit level and protection system settings are not the key issues for large wind power stations connected to the transmission system. So main emphasis is placed on the requirements that have been introduced in the last years and concern active and reactive power regulation, voltage regulation and wind farm behavior during grid disturbances.

3.3

WIND

TURBINE

TECHNOLOGIES

FOR

GRID

REQUIREMENTS Here a brief presentation is made of available technologies for modern, commercial wind turbines, in terms of their electrical system configuration, as far as their response to grid disturbances and compliance to grid code requirements is concerned. In this Section a brief review is presented of wind turbine technology aspects, associated with grid code compliance. Wind turbines are generally divided in two main technological categories:

3.3.1. CONSTANT SPEED WIND TURBINES Constant speed wind turbines, which are equipped with squirrel cage induction generators are directly connected to the grid. The rotational speed of the rotor is practically fixed, since they operate at a slip around 1%. Since the induction machine absorbs reactive power from the grid, connection of compensating capacitor banks at the wind turbine (or wind farm) terminals is necessary. Their aerodynamic control is based on stall, active stall or pitch control. A variation of this scheme utilizes a wound rotor induction generator and electronically controlled external resistors to the rotor terminals, permitting a very variation of speed (typically up to 10% above synchronous).

30

3.3.2 VARIABLE SPEED WIND TURBINES Variable speed wind turbines, consists of rotors where the speed varies significantly, according to the prevailing wind conditions. Two major types are available: The first utilizes a Doubly-Fed Induction Generator (DFIG) and a rotor converter cascade of reduced rating, while the second employs a synchronous or induction generator, the stator of which is interfaced to the grid via a full-power converter. In case of DFIGs the generator‟s stator is directly connected to the grid while the rotor is connected through a cascade of two voltage source converters (rectifier-inverter, connected back-to-back). Wind turbines with full converter use either a synchronous or an asynchronous generator, whose stator is connected to the grid via an AC/DC/AC converter cascade. In this case, the converter handles the total generator power to the grid and therefore no size economies are possible [27]. The latest grid codes require that wind farms must remain in operation during severe grid disturbances, ensure fast restoration of active power to the prefault levels, as soon as the fault is cleared, and in certain cases produce reactive current in order to support grid voltage during disturbances. Depending on their type and technology, wind turbines can fulfill these requirements to different degrees, as explained in the following. Starting with constant speed wind turbines, their low voltage behavior is dominated by the presence of the grid-connected induction generator. In the event of a voltage dip, the generator torque reduces considerably (roughly by the square of its terminal voltage) resulting in the acceleration of the rotor, which may result in rotor instability, unless the voltage is restored fast or the accelerating mechanical torque is rapidly reduced. Further, operation of the machine at increased slip values results in increased reactive power absorption, particularly after fault clearance and partial restoration of the system voltage. This effectively prevents fast voltage recovery and may affect other neighboring generators, whose terminal voltage remains depressed. Since the dynamic behavior of the induction generator itself cannot be improved, measures that can be taken in order to

31

enhance the fault ride-through capabilities of constant speed wind turbines are the following: 

Improvement in the response of the wind turbine aerodynamic control system, in order to perform fast limitation of the accelerating mechanical torque, to prevent rotor over speed. Physical limitations of the blades and the pitch regulation mechanism impose a limit on the effectiveness of such an approach.



Supply of reactive power through static compensation devices at the wind turbine or wind farm terminals, such as SVCs or STATCOMs. These devices would provide high amounts of reactive power during faults, to effectively support the terminal voltage and therefore limit the magnitude of the voltage dip experienced by the wind turbines. Nevertheless, FACTS are complicated and costly devices, while there is an obvious limitation to the voltage correction they can achieve, particularly in the event of nearby system faults.

Variable speed wind turbines, on the other hand, present the distinct advantages of direct generator torque and reactive current control and the possibility to endure large rotor speed variations without stability implications. For this reason, grid disturbances affect much less their operation and they are capable of meeting stringent requirements. In case of voltage disturbances, rotor over speed becomes an issue of much smaller significance, since a limited increase of speed is possible (e.g. 10-15% above rated), the rotor inertia acting as an energy buffer for the surplus accelerating power, until the pitch regulation becomes effective. In case of severe voltage dips, an energy surplus may occur in the electrical part, potentially leading to DC over voltages which can be dealt via proper redesign of the converter controllers and the increase of the local energy storage capacity (e.g. capacitor size) or even by providing a means of local power dissipation. However, even with variable speed wind turbines there still exist LVRT issues affecting their response. In the case of DFIG wind turbines, the direct connection of the generator stator to the grid inevitably results in severe transients in case of large grid disturbances. Hence, the stator contributes a high initial short circuit current, while large currents and

32

voltages are also induced in the rotor windings, as a consequence of the fundamental flux linkage dynamics of the generator .Furthermore, the depressed terminal voltage converter, leading to an increase of the dc bus capacitor voltage. To protect the power converters from over voltages and over currents, DFIGs are always equipped with a device known as a crowbar, that short circuit the rotor terminals as soon as such situations are detected. Once the crowbar is activated, the DFIG behaves like a conventional induction machine, i.e. control is lost over the generator [27]. Notably, crowbar activation is possible not only at the instant of a voltage depression, but also in case of abrupt voltage recovery, after clearance of a fault. Conceptually, two crowbar options are available: 1. The passive crowbar, utilizing a diode rectifier or a pair of antiparallel thyristors to short the rotor terminals. The disadvantage of this option is the lack of control on the deactivation of the crowbar, leading to sustained operation with short-circuited rotor. 2. The active crowbar that uses IGBT switches to short the rotor. This enhances considerably the operation of the device, with a faster elimination of the rotor transients (typically within 100 ms) and therefore faster regain of control. After deactivation of the crowbar, full controllability over the wind turbine behavior is resumed. Hence, although voltage dips inevitably cause torque and power transients in the DFIG wind turbine,which excite the rotor crowbar protection for a limited time interval, the various implementations of the active crowbar can improve the stability of the wind turbine and its response to sudden voltage changes.

3.4 GRID CODE REQUIREMENTS One of these new connection requirements regarding wind energy is fault ride-through capability. In the past, wind generators were not allowed to remain connected to the utility when voltage at the PCC fell below 85 %, forcing their disconnection even when

33

the fault happened far from the wind farm. That is the reason from Turbine to Wind Farms - Technical Requirements and Spin-Off Products why, in grids with significant wind energy penetration, the voltage dip and the subsequent wind farm disconnections would create an important stability problem. Therefore, it is important to check the compliance with Grid Codes. The Compliance with Grid Codes can be checked by means of in-field test or by simulation of validated models [26]. The present Section includes the requirements encountered in the majority of grid codes concerning wind farm interconnection. These include fault ride-through, system voltage and frequency limits, active power regulation and frequency control, as well as reactive power/power factor/voltage regulation [27].

3.4.1. FAULT RIDE THROUGH REQUIREMENTS

The large increase in the installed wind capacity in transmission systems necessitates that wind generation remains in operation in the event of network disturbances. For this reason, grid codes issued during the last years invariably demand that wind farms (especially those connected to HV grids) must withstand voltage dips to a certain percentage of the nominal voltage (down to 0% in some cases) and for a specified duration. Such requirements are known as Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT) and they are described by a voltage vs. time characteristic, denoting the minimum required immunity of the wind power station. The FRT requirements also include fast active and reactive power restoration to the prefault values, after the system voltage returns to normal operation levels. Some codes impose increased reactive power generation by the wind turbines during the disturbance, in order to provide voltage support, a requirement that resembles the behavior of conventional synchronous generators in over-excited operation. The requirements depend on the

34

specific characteristics of each power system and the protection employed and they deviate significantly from each other.

The required fault behavior of a wind farm can be summarized into 4 requirements: 1. For system faults that last up to 140 ms, the wind farm has to remain connected to the network. For supergrid voltage dips of duration greater than 140 ms, the wind farm has to remain connected to the system for any dip-duration on or above the heavy black line of fig. 3.1 2. During system faults and voltage sags, a wind farm has to supply maximum reactive current to the Grid System without exceeding the transient rating of the plant. 3. For system faults that last up to 140 ms, upon the restoration of voltage to 90% of nominal, a wind farm has to supply active power to at least 90% of its pre-fault value within 0.5 sec. For voltage dips of duration greater than 140 ms, a wind farm has to supply active power to at least 90% of its pre-fault value within 1 sec of restoration of voltage to 90% of nominal. 4. During voltage dips lasting more than 140 ms, the active power output of a wind farm has to be retained at least in proportion to the retained balanced super grid voltage [28]. It should be noted that in cases where less than 5% of the turbines are running, or under very high wind speed conditions where more than 50% of the turbines have been shut down, a wind farm is permitted to trip.

35

Fig 3.1 Ride Through Capability of Wind Farm for Super Grid Voltage Dip

3.4.1.1. REQUIREMENTS FOR REACTIVE CURRENT SUPPLY DURING VOLTAGE DIPS Some grid codes prescribe that wind farms should support the grid by generating reactive power during a network fault, to support and faster restore the grid voltage. E.ON [26] requires wind farms to support grid voltage with additional reactive current during a voltage dip, or increased reactive power consumption in the event of a voltage swell as shown in figure 3.2. The voltage control must take place within 20 ms after fault recognition by providing additional reactive current on the low voltage side of the wind turbine transformer amounting to at least 2 % of the rated current for each percent of the voltage dip. A reactive power output of at least 100 % of the rated current must be possible if necessary. The above applies outside a ±10% dead band around nominal voltage. According to the Spanish grid code, wind power plants are required to stop

36

drawing reactive power within 100 ms of a voltage drop and to be able to inject reactive power within 150 ms of grid recovery as shown in Fig.3.2.

Fig 3.2 Reactive output current during voltage disturbances, according to theGerman and Spanish grid codes

3.4.2. ACTIVE POWER AND FREQUENCY CONTROL

These requirements refer to the ability of wind farms to regulate their power output to a defined level (active power curtailment), either by disconnecting turbines or by pitch control action. In addition wind farms are required to provide frequency response that is to regulate their active output power according to the frequency deviations. The grid

37

codes of the following countries demand that wind farms have the ability of active power curtailment: • Germany, with a ramp rate 10% of grid connection capacity per minute. • Ireland, with a ramp rate 1-30 MW per minute. • Nordic Grid Code, with a ramp rate 10% of rated power per minute. • Denmark, with a ramp rate 10-100% of rated power per minute.

3.4.3. VOLTAGE AND FREQUENCY OPERATING RANGE

Wind farms must be capable of operating continuously within the voltage and frequency variation limits encountered in normal operation of the system. Further, they should remain in operation in case of voltage and frequency excursions outside the normal operation limits, for a limited time and in some cases at reduced output power capability. It is obvious that the most extreme frequency limits are 47 Hz and 54 Hz. In countries like Ireland, characterized by an isolated power system with weak interconnections, frequency limits are expectedly wider. It is remarkable that New Zealand‟s grid code prescribes a frequency range of 45-55 Hz. The strictest continuous operation limits for frequency appear in the British code (47.5-52 Hz) and for voltage in the Danish code (90-106% nominal voltage).

3.4.4

REACTIVE

POWER

CONTROL

AND

VOLTAGE

REGULATION Reactive power control is important for wind farms, because not all wind turbine technologies have the same capabilities, while wind farms are often installed in remote areas and therefore reactive power has to be transported over long distances resulting in power losses. Recent grid codes demand from wind farms to provide reactive output regulation, often in response to power system voltage variations, much as the

38

conventional power plants. The reactive power control requirements are related to the characteristics of each network, since the influence of the reactive power injection to the voltage level is dependent on the network short-circuit capacity and impedance. Some codes prescribe that the TSO may define a set-point value for voltage or power factor or reactive power at the wind farm‟s connection point.

39

CHAPTER-4

WIND DRIVEN PMSG SYSTEM MODEL This chapter describes the modeling of various blocks used in the main simulink model such as wind turbine, permanent magnet synchronous generator (pmsg). It also lists the various block parameters.

4.1 WIND TURBINE MODEL 4.1.1 DESCRIPTION

Fig 4.1 Wind turbine model

The model is based on the steady-state power characteristics of the turbine. The stiffness of the drive train is infinite and the friction factor and the inertia of the turbine must be combined with those of the generator coupled to the turbine. The output power of the turbine is given by the following equation.

40

𝜌𝐴

Pm = Cp(ƛ,β). 2 V3wind

(4.1)

Where Pm : Mechanical output of the turbine (W) Cp : Performance coefficient of the turbine ρ : Air density (kg/m3) A : Turbine swept area (m2) Vwind : Wind speed (m/s) λ : Tip speed ratio of the rotor blade tip speed to wind speed β : Blade pitch angle (deg) Eqn.4.1 can be normalized. In the per unit (pu) system the eqn. can be expressed as, Pm_pu = KpCp_puV3wind_pu

(4.2)

Where: Pm_pu : Power in pu of nominal power of nominal power for particular values of ρ and A Cp_pu : Performance coefficient in pu of the maximum value of Cp Vwind_pu : Wind speed in pu of the base wind speed. The base wind speed is the mean Value of the expected wind speed in m/s. Kp : power gain for Cp_pu = 1 and Vwind_pu – 1pu,Kp is less than or equal to 1 A generic equation is used to model cp(λ,β). This equation, based on the modeling turbine characteristics is:

41

𝐶2

Cp(λ,β) = 𝐶1 (

𝜆𝑖

- 𝐶3 β - 𝐶4 )𝑒

−𝐶 5 𝜆𝑖

+ C6λ

(4.3)

With 1 𝜆𝑖

1

0.035

= 𝜆+0.08𝛽 – 1 + 𝛽 3

(4.4)

The coefficients c1 to c6 are: c1 = 0.5176, c2 = 116, c3 = 0.4, c4 = 5, c5 = 21 and c6 =0.0068. The cp-λ characteristics, for different values of the pitch angle β, are illustrated below. The maximum value of cp (cpmax = 0.48) is achieved for β = 0 degree and for λ = 8.1. This particular value of λ is defined as the nominal value (λ_nom).

Fig 4.2 CP-𝝀 characteristics for different values of pitch angle,β

42

The Simulink model of the turbine is illustrated in the Fig 4.3. The three inputs are the generator speed (ωr_pu) in pu of the nominal speed of the generator, the pitch angle in degrees and the wind speed in m/s. The tip speed ratio λ in pu of λ _nom is obtained by the division of the rational speed in pu of the base rotational speed (defined below) and the wind speed in pu of the base wind speed. The output is the torque applied to the generator shaft.

wind speed pu 3

-K-

u(1)^3

Wind speed (m/s) 1/wind_base

Pm_pu Pm_pu

wind_speed^3

Product

Avoid division by zero

-Kpu->pu

cp_pu

1 Generator speed (pu)

-K-

-Kpu->pu

lambda cp

Product

beta

lambda_nom

cp(lambda,beta)

-K-

cp_pu

1/cp_nom

2 Pitch angle (deg) -1

1 Tm (pu)

Avoid division by zero

Fig 4.3 Simulink Model of the Turbine

43

4.1.2 PARAMETER DESCRIPTION 

Nominal mechanical output power - The nominal output power in watts (W).



Base power of the electrical generator - The nominal power of the electrical generator coupled to the wind turbine, in VA. This parameter is used to compute the output torque in pu of the nominal torque of the generator.



Base wind speed - The base value of the wind speed, in m/s, used in the per unit system. The base wind speed is the mean value of the expected wind speed. This base wind speed produces a mechanical power which is usually lower than the turbine nominal power.



Maximum power at base wind speed - The maximum power at base wind speed in pu of the nominal mechanical power. This parameter is the power gain kp already defined.



Base rotational speed - The rotational speed at maximum power for the base wind speed. The base rotational speed is in pu of the base generator speed.



Pitch angle beta to display wind turbine power characteristics - The pitch angle beta, in degrees, used to display the power characteristics. Beta must be greater than or equal to zero.



Display wind turbine power characteristics - If this parameter is checked, the turbine power characteristics are displayed for different wind speeds and for the specified pitch angle beta.

44

4.2 PMSG (Permanent Magnet Synchronous Generator) MODEL 4.2.1 DESCRIPTION

Fig 4.4 PMSG model

The Permanent Magnet Synchronous Machine block operates in either generator or motor mode. The mode of operation is dictated by the sign of the mechanical torque (positive for motor mode, negative for generator mode). The electrical and mechanical parts of the machine are each represented by a second-order state-space model. The sinusoidal model assumes that the flux established by the permanent magnets in the stator is sinusoidal, which implies that the electromotive forces are sinusoidal. For the trapezoidal machine, the model assumes that the winding distribution and flux established by the permanent magnets produce three trapezoidal back EMF waveforms.

The block implements the following systems 

Sinusoidal Model Electrical System



Trapezoidal Model Electrical System

45

Trapezoidal Model Electrical System: These equations are expressed in the phase reference frame (abc frame). Note that the phase inductance Ls is assumed constant and does not vary with the rotor position. 𝑑𝑖 𝑎 𝑑𝑡 𝑑𝑖 𝑏

1

= 3𝐿 [2vab + vbc – 3Rsia + λpωr(-2φʹa + φʹb + φʹc)] 𝑠

1

[-vab +vbc -3Rsib + λpωr(φʹa - 2φʹb + φʹc)]

𝑑𝑡

=

𝑑𝑖 𝑐

= -( 𝑑𝑡𝑎 +

𝑑𝑡

3𝐿𝑠 𝑑𝑖

𝑑𝑖 𝑏 𝑑𝑡

)

Te = pλ[φʹa.ia + φʹb.ib + φʹc.ic] where the electromotive force Φ' is represented by the given graph. The graph shows the relative variation of electromotive force with the electrical angle, θ electrical. Electromotive force, φ‫׳‬

θelectrical Fig4.6 Variation of electromotive force,φ‫ ׳‬with θelectrical

46

Ls : Inductance of the stator windings R : Resistance of the stator windings ia, ib, ic : a, b and c phase currents Φa', Φb', Φc' : a, b and c phase electromotive forces vab, vbc : ab and bc phase to phase voltages ωr : Angular velocity of the rotor λ :Amplitude of the flux induced by the permanent magnets of the rotor in the stator phases p : Number of pole pairs Te : Electromagnetic torque

Mechanical System (for both models) 1

d𝜔/𝑑𝑡= 𝐽 (Te - F𝜔r – Tm) where J : Combined inertia of rotor and load F : Combined viscous friction of rotor and load Θ : Rotor angular position Ts : Shaft mechanical torque

47

Mechanical input It allows us to select either the torque applied to the shaft or the rotor speed as the Simulink signal applied to the block's input. If we select Torque,Tm it specifies a torque input, in N.m., and changes labeling of the block's input to Tm. The machine speed is determined by the machine Inertia J and by the difference between the applied mechanical torque, Tm and the internal electromagnetic torque Te. The sign convention for the mechanical torque is the following: when the speed is positive, a positive torque signal indicates motor mode and a negative signal indicates generator mode. Speed w is selecetd to specify a speed input, in rad/s, and change labeling of the block's input to w. The machine speed is imposed and the mechanical part of the model (Inertia J) is ignored. 4.2.2 BLOCK PARAMETERS 

Stator phase resistance - The stator phase resistance Rs (Ω).



Stator inductances  Sinusoidal model: The d-axis and q-axis stator inductances Ld (H) and Lq (H).  Trapezoidal model: The stator phase inductance Ls (H).



Specify a machine constant - It lets us select the machine constant that we wish to specify for block parameterization:  Flux linkage established by magnets  Voltage Constant  Torque Constant

48

Once a constant is selected, we can enter its value in the appropriate parameter field, while the other two parameters become inaccessible and are only shown for information. 

Flux linkage established by magnets - The constant flux λ (Wb) induced in the stator windings by the magnets.



Voltage Constant - The peak line to line voltage per 1000 rpm. This voltage represents the peak open circuit voltage when the machine is driven as a generator at 1000 rpm.



Torque Constant - The torque per ampere constant. This constant assumes that the machine is driven by an inverter which provides a perfect synchronization between the current and the Back-EMF.  Sinusoidal model: A sine wave current is assumed.  Trapezoidal model: A square ware current is assumed.



Back EMF flat top area - The width of the flat top for a half period of the electromotive force Φ' (degrees) (only for trapezoidal machine).



Inertia, friction factor and pole pairs - The combined machine and load inertia coefficient J (kg.m2), combined viscous friction coefficient F (N.m.s), and pole pairs p.



Initial conditions - Specifies the mechanical speed (rad/s), mechanical angle Θ m (degrees), and instantaneous stator current (A): [wm, Θm, ia, ib]

Note that since the stator is wye-connected, the current ic is given by ic = -ia-ib.

49

4.3 RECTIFIER [UNIVERSAL BRIDGE MODEL] 4.3.1 DESCRIPTION

Fig 4.8 Rectifier Model

The Universal Bridge block implements a universal three-phase power converter that consists of up to six power switches connected in a bridge configuration. The type of power switches and converters configuration are selectable from the dialog box. The Universal Bridge block allows simulation of converters using both naturally commutated or line-commutated power electronic devices (diodes or thyristors) and forced-commutated devices (GTO, IGBT, MOSFET). The Universal Bridge block is the basic block for building two-level voltage-sourced converters (VSC).

4.3.2 BLOCK PARAMETERS 

Number of bridge arms – It is set to 1 or 2 to get a single-phase converter (two or four switching devices) or to 3 to get a three-phase converter connected in Graetz bridge configuration (six switching devices).

50



Snubber resistance Rs - The snubber resistance, in ohms (Ω). The Snubber resistance Rs parameter is set to inf to eliminate the snubbers from the model.



Snubber capacitance Cs - The snubber capacitance, in farads (F). The Snubber capacitance Cs parameter is set to 0 to eliminate the snubbers, or to inf to get a resistive snubber. In order to avoid numerical oscillations when the system is discretized, Rs and Cs snubber values for diode and thyristor bridges are specified. For forced-commutated devices (GTO, IGBT, or MOSFET), the bridge operates satisfactorily with purely resistive snubbers as long as firing pulses are sent to switching devices. If firing pulses to forced-commutated devices are blocked, only antiparallel diodes operate, and the bridge operates as a diode rectifier. In this condition appropriate values of Rs and Cs must also be used.

When the system is discretized, the following formulas are used to compute approximate values of Rs and Cs: 𝑇

Rs > 2.𝐶𝑠 𝑠

Cs < 1000

𝑃𝑛 2𝜋𝑓 𝑉𝑛2

Where Pn = Nominal power of single or three phase converter (VA) Vn = Nominal line-to line AC voltage (Vrms) f = Fundamental frequency (Hz) Ts = Sample time (sec)

These Rs and Cs values are derived from the following two criteria:  The snubber leakage current at fundamental frequency is less than 0.1% of nominal current when power electronic devices are not conducting.

51

 The RC time constant of snubbers is higher than two times the sample time Ts.  These Rs and Cs values that guarantee numerical stability of the discretized bridge can be different from actual values used in a physical circuit. 

Power electronic device -The type of power electronic device to be use in the bridge is selected.  When Switching-function based VSC is selected, a switching-function voltage source converter type equivalent model is used, where switches are replaced by two voltage sources on the AC side and a current source on the DC side. This model uses the same firing pulses as for other power electronic devices and it correctly represents harmonics normally generated by the bridge.  When Average-model based VSC is selected, an average-model type of voltage source converter is used to represent the power-electronic switches. Unlike the other power electronic devices, this model uses the reference signals (uref) representing the average voltages generated at the ABC terminals of the bridge. This model does not represent harmonics. It can be used with larger sample times while preserving the average voltage dynamics.



Ron - Internal resistance of the selected device, in ohms (Ω).



Lon - Internal inductance, in henries (H), for the diode or the thyristor device. When the bridge is discretized, the Lon parameter must be set to zero.



Forward voltage Vf - This parameter is available only when the selected Power electronic device is Diodes or Thyristors. Forward voltage, in volts (V), across the device when it is conducting.

52



Forward voltages [Device Vf, Diode Vfd] - This parameter is available when the selected Power electronic device is GTO/Diodes or IGBT/Diodes. Forward voltages, in volts (V), of the forced-commutated devices (GTO, MOSFET, or IGBT) and of the antiparallel diodes.



[Tf(s) Tt(s)] - Fall time Tf and tail time Tt, in seconds (s), for the GTO or the IGBT devices.

4.4 IGBT 4.4.1 DESCRIPTION

Fig 4.10 IGBT Model

The IGBT block implements a semiconductor device controllable by the gate signal. The IGBT is simulated as a series combination of a resistor Ron, inductor Lon, and a DC voltage source Vf in series with a switch controlled by a logical signal (g > 0 or g = 0) The IGBT turns on when the collector-emitter voltage is positive and greater than Vf and a positive signal is applied at the gate input (g > 0). It turns off when the collectoremitter voltage is positive and a 0 signal is applied at the gate input (g = 0).

53

The IGBT device is in the off state when the collector-emitter voltage is negative. Note that many commercial IGBTs do not have the reverse blocking capability. Therefore, they are usually used with an antiparallel diode. The IGBT block contains a series Rs-Cs Snubber circuit, which is connected in parallel with the IGBT device (between terminals C and E).

4.4.2 BLOCK PARAMETERS 

Resistance Ron - The internal resistance Ron, in ohms (Ω).



Inductance Lon - The internal inductance Lon, in henries (H). The Inductance Lon parameter cannot be set to 0.



Forward voltage Vf - The forward voltage of the IGBT device, in volts (V).



Current 10% fall time - The current fall time Tf, in seconds (s). This parameter is not modeled when the Enable use of ideal switching devices parameter of the Powergui block is selected.



Current tail time - The current tail time Tt, in seconds (s). This parameter is not modeled when the Enable use of ideal switching devices parameter of the Powergui block is selected.



Initial current Ic -An initial current flowing in the IGBT can declared and is usually set to 0 in order to start the simulation with the device blocked. If the Initial Current IC parameter is set to a value greater than 0, the steady-state calculation considers the initial status of the IGBT as closed. Initializing all states of a power electronic converter is a complex task. Therefore, this option is useful only with simple circuits.



Snubber resistance Rs - The snubber resistance, in ohms (Ω).The Snubber resistance Rs parameter is set to inf to eliminate the snubber from the model.

54



Snubber capacitance Cs - The snubber capacitance in farads (F). The Snubber capacitance Cs parameter is set to 0 to eliminate the snubber, or to inf to get a resistive snubber.



Show measurement port - If selected, it adds a Simulink output to the block returning the diode IGBT current and voltage

4.5 LINEAR TRANSFORMER 4.5.1 DESCRIPTION

Fig 4.14 Linear transformer Model The Linear Transformer block model shown consists of three coupled windings wound on the same core.

Fig 4.15 Linear Transformer Block model

55

The model takes into account the winding resistances (R1 R2 R3) and the leakage inductance (L1 L2 L3), as well as the magnetizing characteristics of the core, which is modeled by a linear (Rm Lm) branch. The Per Unit Conversion In order to comply with industry, the block allows to specify the resistance and inductance of the windings in per unit (pu). The values are based on the transformer rated power Pn, in VA, nominal frequency fn, in Hz, and nominal voltage Vn, in Vrms, of the corresponding winding. For each winding, the per unit resistance and inductance are defined as 𝑅(𝛺)

R (pu) = 𝑅

𝑏𝑎𝑠𝑒

𝐿(𝐻)

L (pu) = 𝐿

𝑏𝑎𝑠𝑒

The base impedance, base resistance, base reactance, and base inductance used for each winding are 𝑉2

Zbase = Rbase = Xbase= 𝑃𝑛

𝑛

Lbase =

𝑋 𝑏𝑎𝑠𝑒 2𝜋𝑓𝑛

For the magnetization resistance Rm and inductance Lm, the pu values are based on the transformer rated power and on the nominal voltage of winding 1. 4.5.2 BLOCK PARAMETERS 

Units - Specify the units used to enter the parameters of the Linear Transformer block. Select pu to use per unit. Select SI to use SI units. Changing the Units parameter from pu to SI, or from SI to pu, will automatically convert the parameters displayed in the mask of the block. The per unit conversion is based

56

on the transformer rated power Pn in VA, nominal frequency fn in Hz, and nominal voltage Vn, in Vrms, of the windings. 

Nominal power and frequency - The nominal power rating Pn in volt-amperes (VA) and frequency fn, in hertz (Hz), of the transformer. Note that the nominal parameters have no impact on the transformer model when the Units parameter is set to SI.



Winding 1 parameters - The nominal voltage V, in volts RMS, resistance, in pu or ohms, and leakage inductance, in pu or henries. The pu values are based on the nominal power Pn and on V1. Set the winding resistances and inductances to 0 to implement an ideal winding.



Winding 2 parameters - The nominal voltage V2 in volts RMS, resistance, in pu or ohms, and leakage inductance, in pu or henries. The pu values are based on the nominal power Pn and on V2. Set the winding resistances and inductances to 0 to implement an ideal winding.



Three windings transformer - If selected, implements a linear transformer with three windings; otherwise, it implements a two-windings transformer.



Winding 3 parameters - The Winding 3 parameters parameter is not available if the Three windings transformer parameter is not selected. The nominal voltage in volts RMS (Vrms), resistance, in pu or ohms, and leakage inductance in pu or henries. The pu values are based on the nominal power Pn and on V3. Set the winding resistances and inductances to 0 to implement an ideal winding.



Magnetization resistance and inductance - The resistance and inductance simulating the core active and reactive losses. When selected, the pu values are based on the nominal power Pn and on V1. For example, to specify 0.2% of active and reactive core losses, at nominal voltage, use Rm = 500 pu and Lm =

57

500 pu. Rm must have a finite value when the inductance of winding 1 is greater that zero. 

Measurements - Winding voltages is selected to measure the voltage across the winding terminals of the Linear Transformer block. Winding currents is selected to measure the current flowing through the windings of the Linear Transformer block. Magnetization current is selected to measure the magnetization current of the Linear Transformer block. All voltages and currents is to measure the winding voltages and currents plus the magnetization current. Multimeter block can be placed in the model to display the selected measurements during the simulation.

. 4.6 IGBT/DIODE [UNIVERSAL BRIDGE] 4.6.1 DESCRIPTION

Fig 4.16 Model of Universal bridge

58

The Universal Bridge block implements a universal three-phase power converter that consists of up to six power switches connected in a bridge configuration. The type of power switches and converters configuration are selectable from the dialog box. The Universal Bridge block allows simulation of converters using both naturally commutated or line-commutated power electronic devices (diodes or thyristors) and forced-commutated devices (GTO, IGBT, MOSFET). The Universal Bridge block is the basic block for building two-level voltage-sourced converters (VSC).

REFERENCE MODEL

Fig 4.18 Reference Model for Universal Bridge

59

The model compares three Voltage Source Converter (VSC) models using the Universal Bridge block. Each model is controlled in open loop with a SPWM pulse generator (switching frequency=1080 Hz ; modulation index m=0.85) Model 1 is a detailed model using 6 IGBTs and 6 antiparallel diodes.[29]

4.7 FUNCTION BLOCK PARAMETERS 4.7.1 WIND TURBINE Nominal mechanical output power (W)

= 1.5e6

Base power of the electrical generator (VA)

= 1.5e6/0.9

Base wind speed (m/s)

= 12

Maximum power at base wind speed (pu of nominal mechanical power)

= 0.73

Base rotational speed (p.u. of base generator speed)

= 1.2

Pitch angle beta to display wind-turbine power characteristics

= 0

(beta >=0) (deg)

4.7.2 PERMANENT MAGNET SYNCHRONOUS GENERATOR CONFIGURATION: Back EMF Waveform

=

Trapezoidal

Mechanical input

=

Torque, Tm

Stator phase resistance Rs (ohm)

=

18.7

60

Stator phase inductance Ls (H)

=

8.5e-3

Flux Linkage established by Magnets

=

0.1717

Voltage Constant (V_peak L-L / krpm)

=

71.9215

Torque Constant (N.m / A_peak)

=

0.6868

Torque Constant (N.m / A_peak)

=

120

Inertia, friction factor, pole pairs [ J(kg.m^2) F(N.m.s) p()]

=

[2.26e-005 005 2]

Initial conditions [ wm(rad/s) thetam(deg) ia,ib(A) ]

=

[0,0, 0,0]

4.7.3 THREE PHASE CIRCUIT BREAKER Initial status of Breakers

=

Closed

Transition times (s)

=

5/60

Breakers resistance Ron (ohms)

=

0.001

Snubbers resistance Rp (Ohms)

=

1e6

Snubbers capacitance Cp (Farad)

=

Inf

Number of Bridge arms

=

3

Snubber resistance Rs(ohms)

=

.1

Snubber capacitance Cs (F)

=

.001

4.7.4 UNIVERSAL BRIDGE

1.349e-

61

Power Electronic device

=

Diodes

RON(Ohms)

=

1e-3

LON(Ohms)

=

0

Forward voltage Vf(V)

=

.8

Measurements

=

Device currents

4.7.5 SERIES RLC BRANCH Table 4.1 Parameters of Series RLC Branch Branch Type

Resistance(Ohms)

Inductance(H)

Series RLC Branch

RL

1

1e-3

Series RLC Branch1

L

0

1

Series RLC Branch2

L

0

1e-3

62

4.7.6 PULSE GENERATOR Table 4.2 Parameters of Pulse Generator Pulse Type

Time (t)

Pulse

Time

Use

Generator

based

simulation

Amplitude (V)

Period (sec)

Pulse Width

Phase

( %of

Delay

period)

(sec)

1

100e-6

50

0

1

51e-6

50

0

time Pulse

Time

Use

Generator1

based

simulation time

4.7.7 LINEAR TRANSFORMER Units

SI

Nominal power and frequency [Pn(VA) fn(Hz)]

=

[ 250e6 60 ]

Winding 1 parameters [V1(Vrms) R1(ohm) L1(H)]

=

[7.35e+005

4.3218

0.45856] Winding 2 parameters [V2(Vrms) R2(ohm) L2(H)]

=

[7.35e+005

4.7938

0.084225] Winding 3 parameters [V3(Vrms) R3(ohm) L3(H)]

=

[3.15e+0005 0.084225]

0.7938

63

Magnetization resistance and inductance [Rm(ohm) =

[1.0805e+006 2866]

Lm(H)]

4.7.8 IGBT Table 4.3 Parameters of IGBT IGBT

IGBT 1

0.01

0.01

Inductance Lon (H)

0

0

Forward voltage Vf (V)

1

1

Current 10% fall time Tf (s)

1e-6

1e-6

Current tail time Tt (s)

1e-6

1e-6

0

0

10000

10000

Inf

Inf

Resistance Ron (Ohms)

Initial current Ic (A) Snubber

resistance

Rs

(Ohms) Snubber capacitance Cs (F)

64

4.7.9 DIODES Table 4.4 Parameters of Diodes

Resistance Ron (Ohms) Inductance Lon (H) Forward voltage Vf (V) Initial current Ic (A) Snubber resistance Rs (Ohms) Snubber capacitance Cs (F)

DIODE 1

DIODE 2

DIODE 3

DIODE 4

0.001

0.001

1

1

0

0

1

1

0.8

0.8

0.8

0.8

0

0

0

0

500

500

500

500

250e-9

250e-9

250e-9

250e-9

4.7.10 UNIVERSAL BRIDGE 1(IGBT/DIODES) Number of Bridge Arms

= 3

Snubber resistance Rs (Ohms)

= 10000

Snubber capacitance Cs (F)

= inf

Power Electronic Device

= IGBT/Diodes

Ron (Ohms)

= 1e-4

Forward voltages [ Device Vf(V) , Diode Vfd(V)]

= [1 1 ]

[ Tf(s) , Tt(s) ]

= [ 1e-6 , 2e-6 ]

65

4.7.11 DISCRETE PWM GENERATOR

Generator Mode

=

Double 3-arm bridge(12 pulse)

Carrier frequency(Hz)

=

5000

Sample time

=

51.8e-6

Modulation index (0