ABSTARCT:
With the development in the technology and the need for preserving a healthy atmosphere, renewable sources are finding their way in the power generation systems. In the renewable sector, wind energy is the most clean and non-polluting energy adding power to the grid. But harnessing stable energy from wind is not as easy as it sounds. Power generated from wind turbines is unstable due to varying wind speed. This instability in power causes serious damage to grid and subsequently to the load attached to it. Maintaining the quality of power generated by wind turbines is a major challenge faced by the engineers. The instability in the power is due to reactive power and harmonics created by the distortion of electric current. This research article discusses the main factor behind the instability of power and presents the measures to provide consistent and stable power.
KEYWORDS: Wind-farm, DFIG, harmonics.
Background:
Wind energy usage in power generation has paved its way in the modern world. Also with the expansion of wind power capacity, the influence of wind turbines on the power grid has become highly distinguished. This trend has surprisingly spread all over due to wind power generation being renewable, cost effective and also less pollutant.
Wind velocity is a major factor in the power generation by this method. Thus, in geographically befitting areas with a roughly stable wind velocity such generation plants are being installed. However, the inconsistent nature of wind as a source of power is giving a rise to problems of voltage and frequency oscillations and instability in wind energy conversion systems (WECS).
Along with voltage/frequency variations issues, the maintenance of the power quality, the wiring network compatibility, wind power transmissions over long distances, the power system security, proper siting of turbines may pose as a hurdle in the stability of wind power generation.
The transmission of wind power is a barrier because the sites of wind farms or turbines installation are preferred to be country areas, which are far away from urban centers. For this a secured transmission system must be implemented which also retains the quality of power being supplied.
The location of wind turbines is usually suited to be in high wind pressure areas, away from residential areas due to the noise and its potential hazard to wildlife although it doesn’t affect worker’s health as badly as the other sources of power do. However, wind energy is thought of as reliable economically because it can lessen the high rates of utilities.
One major issue arising is of voltage fluctuation. Voltage regulation and the management of the flow of active and reactive power are tasks to be resolved. Because to maintain grid stability, rapid electrical generation and consumption should supposedly remain steady and in balance, this variation can present considerable challenges to integrating massive amounts of wind power in a grid system.
For the enhancement of power quality, the lasted equipment in are with microprocessor based control and power electronic devices, which are more sensitive to power quality than those that were used in past. Doubly fed induction generators (DFIGs) are used for quality power production. To upsurge overall productivity of the system, use of modifiable speed motor, power factor correction give outcome as increase of harmonics level in the system. Deregulations of utilities, distributed generations have increased the power quality problem. On the local level, voltage variations are the main problem associated with wind power. This can be the limiting factor on the amount of wind power which can be installed.
Wind power generator is commonly at the termination of the power grid, so if in case the grid fails, the voltage drop is the core reason on distressing the stable operation of the wind turbines, and the wind turbines are affected by voltage flickering, whereas the deviations will affect both the stability of the power grid and changes from the active and reactive power.
INTROUCTION:
As the topics name implies wind it is obviously related to its generation, transmission & distribution for being most efficient and feasible source of energy ,it is a renewable energy means that type of energy which can restores itself/used again after its consistent and frequent usage.
Our usage of renewable sources contributes from sources of nature like sun, wind, hydel power, coal etc. A lot of progress is made to improve renewable source of energies, and many methods are devised to exploit wind energy.
Basically wind is the movement of air in atmosphere and the heating down of earths masses by un-even sun rays results in its production. Wind is necessary, one can say crucial task to balance the global temperature between natures of the world. Direction and speed clearly define wind.
Wind energy is not the only source of energy. It changes continuously and emits energy randomly. Means its strength depends on the disturbance of the atmospheric pressure from lower to higher areas, like sometimes it thrusts in the air with 50% of its power at less than half of its operational time.
So, we can never be assure of power consistency .Therefore ,we need a system with huge stockpile capabilities like hydro, desalination plant to reduce the economic issues due to change in the resources .
Because electrical consumption and generation must be in equilibrium for grids stability , this alternation can be quite challenging to Incorporating vast amounts of wind power into a grid system. So, this irregular behaviour of wind can be problematic when supplying power in low quantity to the total demanded. Setting of wind turbines can be quite intriguing on behalf of its structure which contributes in its haphazardness to wild life. Transmission is another obstacle because of the distance of wind research centers from the sites where wind is in high intensity.
With the rapid growth of the wind power, the influence of wind turbines on the power grid is rising in recent times. When the grid fails, the voltage drop is the foremost reason on upsetting the steady process of wind turbines. Most wind turbines employ GE typical model at present, control mode by constant reactive power factor. To study individuality of wind turbines component, it is being built single infinite system, which set the export three phase short circuit fault. The wind turbines are not noteworthy difference in the total active power compared to thermal power under the improvement of stern power grid faults, but it is not conductive to grid constancy when reactive power is not sufficient. The different operating characteristics of wind turbines compared with conservative synchronous generators, mainly with respect to inertial response, will cause a major influence on the characteristics of power grid. Variable speed wind turbines, including doubly fed induction motor and direct drive synchronous generator types demand has frequently became important because of their higher competence and consistency. When the amount of wind power capacity in grid increases, the impact of this on system frequency stability is increasingly becoming severe.
FACTORS CAUSING INSTABILITY OF POWER:
As due to nonlinear and delicate in nature of these loads are majorly pretentious by the power worth problematic .There are dual sorts of turbines namely fixed speed and variable speed turbine, in fixed speed turbine variation in haste offer to large voltage changing on network where as in variable speed wind turbine’s situation, where we use power electronic devices, in this can we need to take into attention about harmonic distortion.
A: Voltage Variations
On the native level, voltage variances are the core issue related with wind power. This can be the limiting factor on the quantity of wind power which can be installed. In ordinary operational condition the impact of attaching a wind farm on the gird voltage is directly related to the short circuit power level.
The short circuit power level at a specified point in the electrical system indicates the strength of a system. If the voltage at a distant point is represented by Us which is a constant voltage and the short circuit power level (commonly represented by SSC )is demarcated as Us2 / Zk where Zk represents total impedance between the points concerned. The system voltage at the supposed inestimable busbar and the voltage at the Point of Common Coupling (PCC) are Us and Ug, correspondingly. The output power and reactive power of the generation entity are Pg and Qg, which corresponds to a current Ig. It is clear that the variations in the produced power will cause variations in the voltage at PCC. If the impedance Zk is small then the voltage deviations will be small. Similarly, if Zk is large, then the voltage deviations will be large .
B: Steady-state voltage
Operation of airstream turbines may disturb the voltage in the concomitant network. If essential, the suitable methods should be taken guarantee that the wind turbine installation does not fetch the amount of the voltage outside the compulsory restrictions. It is suggested that load-flow analyses be accompanied to evaluate this effect.
Several wind turbines are fortified with induction generators which ingest reactive power. At no load the reactive power intake is around 35-40% of the rated active power, and rises around 60% at rated power. Reactive power is one of the main reasons of voltage uncertainty in the system due to the associated voltage drops in the transmission lines, reactive current also subsidizes to system losses.
C: Voltage Fluctuation
Rise and fall in the system voltage may be the source of observable light flicker depending on the amount and frequency of the variation. This kind of disorder is called voltage flicker.
The flicker extent is based on the measurements of three sudden phase voltages and currents shadowed by means of “flicker algorithm” to compute the Pst and Plt. where Pst is the short term flicker severity factor and computed over 10 minutes, and the long term flicker severity factor Plt is distinct for two hour periods. The flicker calculations can also be accompanied with simulation procedure.
Turbulences just noticeable are supposed to have a flicker severity factor of Pst = 1 The flicker emanations, Pst and Plt may also be predictable with the coefficient plus factors, cf(Ψk, va ) and kf(Ψk) attained from the measurements, which are generally given by wind turbine manufacturers.
It is suggested that Plt≤0.50 in 10-20 kV networks and Plt≤0.35 in 50-60 kV networks are considered satisfactory. However, unlike utilities may have different flicker emission limits.
D: Harmonics
Harmonic disorders are a phenomenon associated by means of the disturbance of the fundamental sine wave and are shaped by non-linearity of electrical apparatus. Harmonics causes rise in currents, power losses and probable disparaging over-heating in the instrument. Harmonics may as well increase difficulties in communiqué circuits.
Entire harmonic distortion, or THD can be related to either current harmonics or voltage harmonics and it is distinct as the fraction of total harmonics to the amount at fundamental frequency multiplies by 100%
THDv = (V2^2+V3^2+V4^2+…. + Vn^2) ^. 5 * 100%
V1
THDI = (I2^2+I3^2+I4^2+…. + In^2) ^ .5 * 100%
I1
The Pulse Width Modulation (PWM) switching frequency, with a characteristic switching frequency of a few thousand Hz, alternates the harmonics to upper frequencies where the harmonics can be simply detached by smaller filters. Popular general harmonic standards can be encountered by means of contemporary wind turbines.
MODEL OF THE PROSELYTIZERS
“Danish concept” was used to build many low-power wind turbines, in which simple squirrel-cage induction machine is used to convert wind energy into electrical energy, straight joined to a three-phase power grid. The generator shaft with a fixed-ratio gearbox is attached to the rotor of the wind turbine. Several induction generators use pole-adjustable winding arrangements to allow 26 process at various synchronous pace. The attributes of mechanical subcircuits very much contribute to the performance of fixed-speed wind turbines. A fast and strong variation in electric output is observed, when an air knocks the turbine. A sturdy mechanical design are needed to absorb high mechanical stresses, due to load variations. In this design, costly mechanical constructions are required, especially at high-rated power.
a) Doubly Fed Induction Generator (DFIG) Model:
The DFIG is a wound-rotor induction generator in which stator is straight linked with the grid, but the three phase rotor windings are attached via slip rings to the grid through a partly valued power electronics converter. An induction generator is DFIG when rotor voltage is nonzero. The stator flux transients are ignored in the voltage relations for the demonstration of DFIG models in power system stability studies. The converter rating is kept low, as these wind turbines only deals with slip power. Magnitude and phase angle of the rotor voltage can be modified by using the PWM converter inserted in rotor. The flux transient’s items vanish under steady state conditions.
b) Induction Generator (IG) Model:
The rotor of induction generator is a wound-rotor or a squirrel-cage rotor with a short circuit winding not linking to any other source.
Adjustable Speed Generators (ASG’s)
Present high-power wind turbines are able of modifiable speed action. Advantages of adjustable speed generators (ASGs) over fixed-speed generators (FSGs) are: They are economical and offer simple pitch control; pitch control time constants can be made high by the frequency of the generator. It also decrease pitch control complications and peak power necessities. The pitch angle is frequently fixed at lower wind speed but at high wind speed, pitch angle control is used to bound max output power. Gusts of wind can be immersed and mechanical stresses are shortened using ASG’s. They vigorously atone for torque and power beats caused by reverse pressure of the tower. The torque pulsations are equivalent to the turbine rotor speed times the number of rotor wings. Power quality can be improved and thus removes electrical power deviations. System proficiency is enhanced, turbine speed is controlled as a function of wind speed to enlarge output power. Operation at the maximum power point can be realized over a wide power range. Upto 10% improvement is possible in efficiency of energy. Acoustic noises are diminished.
a) Direct-in-Line ASG System
Variable-frequency AC power is constructed using a synchronous generator. This variable frequency AC power can be transformed into a fixed-frequency AC power using a series connected power converter with ASG. The max output power of direct-in-line systems is 1.5 MW but they have several disadvantages also. They are extravagant, as the power converter has to be rated at 1 p.u. total system power. Design is made challenging and costly as the inverter output filters and EMI filters are rated for 1 p.u. output power. The total system productivity over the complete operational range depends on the converter ability.
b) Doubly Fed Induction Generator (DFIG) ASG System
New advancement pursue to escape most drawbacks of direct-in-line converter based ASGs. An alternate ASG idea that comprises of a doubly fed induction generator (DFIG) with a four-quadrant ac-to-ac converter based on insulated gate bipolar transistors (IGBTs) connected to the rotor windings. DFIG provides some advantages when compared to the direct-in-line systems: Inverter rating required is 25% of total system power and the speed range of the ASG is ±33% about the synchronous speed, so the cost of inverter is decreased. The inverter filters and EMI filters needed are of 0.25 p.u. total system power, so the cost of these filters are also decreased. A smaller part of total system harmonics is represented by an inverter harmonics. The losses of generator and IGBT inverters are presented individually. Approximately 2-3% efficiency improvement can be obtained. The DFIG system principally works similar to a synchronous generator, so power-factor control can be employed at lower cost. The converter has to deliver only excitation energy.
WIND FARM OPERATIONS:
Several investigations have been made in order to solve these difficulties. In this part we will discussed some possible explanations which are mandatory in order to solve these problems.
A. Frequency and Power Regulator
The generation of the real power by the wind turbine can be reduced but it is problematic to increase the output power because of the limitations in the wind speed. However some spinning reserve is stored when the turbine is working in inferior power then the existing power this means that the power generation is reduced and therefore proceeds also reduced.
The response may be present in the large scale energy storage devices. For this persistence some devices which responds fast is wanted for this work. From the point of view of system operator a system hot backup distribution amount the generation units may be more cost effective to look for the difficulty if it is possible.
B. Reactive Power Compensation
There are a number of wind turbines which holds induction generators in them. Due to which reactive power is more consumed and when there is no load the reactive power usage is about 35% to 40% of the rated active power. And this value is increased up to 60% of the rated power. The problem of voltage immovability in the network due to deviations in the voltage is caused by the reactive power and the reactive current also play its role in the system loses.
To overwhelm this difficulty capacitor banks may be connected in order to compensate the reactive power consumed by the induction generator. To avoid the power loses and to increase the steadiness of the voltage the reactive power can be controlled by self-commutated power electronic system in the wind turbine. These wind turbines by controlling the reactive power can control the voltage deviations and can have a proper power factor of 1.00. And for the large scale wind farm, certain devices are used known as central reactive power compensation device that is SVC or STATCOM which gives a stable reactive power regulation.
C: Stability Support
The accumulation of the large scale wind farm creates a difficulty it has some impacts on the system constancy and transient behavior. Power system faults in the network linked with system constancy such as short circuit, tripping of transmission lines and loss of production capability. The active and reactive power is disturbed due to these failures and the flow of power becomes unstable. However the capacity of the operating generators is satisfactory. The disturbance of active and reactive power in the network will cause the voltage instability. Because of this brown out (low voltage) can occur and it may convert into the complete loss of power (blackout).
Many difficulties in the power system have been solved by the relay protection of transmission system. It can be done either by disconnection or by disconnection and fast reclosure. In the above cases there is a short period of low or no voltage but after some time the voltage returns to normal and this whole situation is controlled by a nearby wind farm. In the starting days of the development of wind energy there were very little amount of wind turbines that are associated to the grid so at that time when a fault occurs in the system which cause the voltage to drop at wind turbine was separated to the grid until the fault is removed and the voltage returns to normal because of the low penetration power of wind in the early days the constancy of power system is not effect by the disconnection of wind turbine or wind farm to the grid. But with the increase in the wind penetration power the role of power generated by wind energy is very effective. If the wind farm is suddenly disconnected from the grid at its maximum generation thin the system will lost the ability for further generation unless the power plants which are operating have enough spinning reserve to overcome the loss in short period of time. There is a huge deviation in the frequency and in the voltage and it may cause the complete loss of power. To solve these problems there is a need of new generation wind turbines that are able to counter this problem of disturbance and disconnection of wind farm from the grid.
To keep the system in steady condition it is very necessary to ensure that the wind turbines are working correctly and there is no error in the system and performing their operations normally. In different types of wind turbine technology this could have different focuses such as the permanence of system voltage and these technologies includes the reactive power compensation devices known as interface power electronics i.e. SVC, STATCOM that keeps the generator at specific speed by varying the power.
CONTROLLING OF POWER:
A lot research has been conducted in answering the challenges of controlling the wind farms and maintaining the stability if the power produced. With DFIG based wind turbines use of power electronic converters and are thus able to regulate their own reactive power, so as to operate at a given power factor, or to control grid voltage. The rotor-side converter is controlled by a two stage controller, one is the fast current controller and the other is the slower-power controller. Potential restriction can also be accomplished by rehabilitation of the reactive potential restrictor by an e.m.f manipulator specifying the d-axis current reference. Up to now, this quality of the DFIG based wind power house is mainly used to keep the dynamo reactive power indifferent.
The stability can also be attained with the help of adjustable speed generator as they improve power quality as with adjustable speed generators torque pulsations can be scaled down due to the flexibility of the wind power system. This cancels electrical power fluctuations i.e. less flicker. They also upgrade systems ability as turbines momentum is adjusted as a function of wind speed to exaggerate yielded power. Operation at the ultimate power point can be attained over a wide power domain as an exercise of turbine speed and air speed. As a result, energy ability enhancement is up to 10% is possible & with Doubly Fed Induction plus Adjustable Speed Generators power-factor control can be enforced at lower amount, because the Doubly Fed Induction Generator System (four-quadrant proselytizer and induction apparatus) basically operates identically to a synchronous dynamo. The alternator has to provide only excitation intensity. In addition, compared to silicon-controlled rectifier (SCR) established Kramer drives, the DFIG with a four-quadrant alternator in the rotor circuit allows decoupled supervision of active and reactive potential of the generator.
The development of various control methods for mitigating output fluctuations is proceeding; these techniques offer controlled smoothing effects to compensate for wind power output fluctuations. Some studies have analysed the effects of control techniques using the equipment in a wind power system, such as the pitch control of wind turbine blades, or the control of wind-energy converters.
As far as the periodicity and power stability is concerned the real power propagation of a wind turbine can be modulated but it may be problematic to boost the power gain since the input power is bounded by the wind pace. However, some rotational reserve may be conserved if the wind turbine worked at a minor power level than the accessible power level which means degradation in propagation, and hence curtailed yields. In order to maintain the frequency for periodic fluctuations with a cycle of a few seconds, the frequency is controlled by the grid inertia, which is stored as rotational kinetic energy in the turbines of wind farms, and the self-regulating characteristics of the load.
Some of the wind turbines have induction generators, whereas induction generators consume reactive power. At no hindrance (loafing), the reactive power utilization is approximately 35-40% of the estimated active power, and rises to around 60% at rated power. On the other hand most system faults are resolved by the relay stability of the conveyance system either by disconnection or by detachment and quick re-closures. In all the positions the output is a small period with little or no voltage persuaded by a duration when the e.m.f returns. When a fault somewhere in the lines caused the voltage at the wind turbine to drop, the wind turbine was simply isolated from the grid and was reconnected when the fault was cleared and the e.m.f brought back to normal. But if the whole wind farm is abruptly disconnected at full propagation, the systems will loss production capability. Except the remaining operating power plants have sufficient “rotatory reserve”, to recover the damage within very limited time. There could have been many focuses in several types of wind turbines, and may include for backing the systems e.m.f with reactive power indemnity devices.
However, as wind power infiltration in power systems is growing, it will perhaps be desirable for wind power systems to give e.m.f control. The proselytizer can adjust either the e.m.f or the power factor, but the ultimate possible reactive power output is defined by the alternator ratings.
Conclusion:
This paper concludes that since wind being a renewable source of energy and having a great potential if properly controlled can be used to overcome the energy deficiency not only in Pakistan but all over the world. As discussed in the article regarding the causes and factors of instability in power, the wind power systems are designed to overcome such problem like that of the DFIG, DFIG plus the ASG structure are designed to ensure a better quality power generation as well as to get maximum output over a wide range of wind velocity. To get control over the frequency, flickers, harmonics several power electronics converter are equipped to increase the stability of power.
This paper also discussed that the integration of large scale wind power into power systems present many new challenges. The impact of wind power on power quality, the gird requirements for integration of wind turbines, and discusses the potential operation and control methods to meet the challenges.