The Advantages Of Using DFIG

The Advantages Of Using DFIGIn step-up to its stator, the DFIGs rotor is also connected to the grid using a effect converter (Picture 3). This type is certain common for wind turbines as it offers original advantages comp atomic number 18d to other typesIt tail end scarper like a synchronized writer and at changeable speed, although its stator is directly connected to the grid. The converter put up adequately underwrite the potentiality (phase and magnitude) that is applied to the rotor and as a consequence curtail the magnetic fields frequency or speed. It genuinely forms an AC excitation with a variable frequency, just like the DC excitation affaird in synchronous generators. This exertion gives the DFIG the benefits of a synchronous generator. These benefits include the separate control of re energetic and active billet, or the control of the wind turbines causation occurrenceor. The DFIG wind turbines atomic number 18 better than other designs in scathe of g rid compatibility.It grass operate at variable speed, sub- or super synchronously. The optimum speed can be chosen by poseing the frequency and phase of the electromotive force that is applied to the rotor, and is such that maximum motive is obtained by the wind, in different wind speeds.Since it can operate as synchronous generator, a study advantage of large DFIG wind turbines is that they can contribute to the schemes stability after(prenominal) a rupture occurs. However, this relies heavily on the control options given by the power converter. Moreover, A DFIG wind turbine can generate reactive power even when the mechanical part is non operating and in non delivering active power.Relative to other variable speed generators, DFIGs power converter is rated at get off power (i.e. just about 30% of the wind turbines rated power), since only part of the numerate power delivered by the wind turbine is transferred through the converter. This makes the wind turbine cheaper and lighter.The power mentioned supra, can be each delivered by the rotor (when the turbine rotates with a advanceder than the synchronous speed), or abstracted (when its speed is below the sync speed). This double mode gives the DFIG the ability to operate at speeds below or above even by 50% of the sync speed, although actually lower pas seuls are chosen.Compared to other generators with variable speed, which engross external rotor resistances to allow variable speed, DFIG are more naught efficient, since there is little power dissipated in the converter.DFIG can be accurately controlled due to the fact that the power converter can coiffe both the magnitude and phase of the electromotive force applied to the rotor. This attribute, combined with pitch-control in wind turbines offers the performer more accurate power control, especially in graduate(prenominal) winds where high power output can face exacting damage to the equipment.Finally DFIG shares the benefits of sync hronous and inductive generators regarding its contribution to power systems stability. earmark control can improve the damping of power variations in the system, without compromising potency control.+ emf controlPicture 1Active worry in the dispersion intercommunicate 4, 5, 6The dispersed nature of renewable resources necessitates their connection at the distribution network, which was designed in order to convey power from high to low potential difference. Now, with the introduction of renewable electricity sources, the power can copy any(prenominal) direction, affecting the power and potentiality quality as fountainhead as the security measure of the system. Therefore considerable amounts of money should be pass for their upgrade, if the optimum renewable potential is to be achieved. A nonher, less pricy onslaught is the transition from the networks traditional passive operation to their active management.Under active management approach, new control and communicati on technologies are incorporated into the system and allow the operator to control in substantive time the voltage, the power flows and even the fault take aims.The main options under active management are reason flow managementThe risks regarding the network power flows must(prenominal)(prenominal) be sufficiently eliminated. The possibility of the distributed generators to deliver power above the systems ratings-capacity can importantly threat the systems operation. This is a serious issue, especially in racing shell of a circuit outage. The power flow management protects the systems parts taking into report their nominal capacity. potential hook controlThe voltage at a motor mickle is affected by the sure and reactive power on that bus as well as the R and X values of the line. The voltage change at that bus is approximately equal to. The most probative issue accruing from the introduction of distributed generators at the distribution networks is the voltage rise at th e connected bus. The operator can choose appropriate R and X values but this would require an expensive network upgrade. The control of P and Q is a less expensive option, and this is why it is preferred. The main actions commonly taken areOn- stretch along transformer tapping The operator aptitude reduce/ accession the voltage at the primary substation reducing/increasing all the subsequent voltages. Nevertheless, in case of reducing the voltage, a possible generator disconnection might admit the voltage below its permissible limit.Power factor or/and voltage control see question 4Reactive power earnings Appropriate devices (capacitor banks, STACOM etc) can be connected on critical buses, so that they deliver/absorb reactive power and fix the voltage profile. times curtailment another way to mitigate a voltage rise, would be to curtail real power times from the distributed generators. However, this sets a meaningful bashfulness in the renewable get-up-and-go that can be ex ploited.Fault level management see belowLoad managementReducing or shifty adjustable loads can alleviate the pressure on the networks operation, especially in case of very low renewable generation.Power quality managementThe operator exploits various capabilities at the generators, loads or other special devices to maintain the powers high quality in terms of harmonics, voltage disturbances etc.Power Factor vs Voltage Control 7, 8, 9The dominant allele negative impingement of connecting generators in the distribution network is the distortion of the voltage profile. The simplest, but at the same time the most expensive, solution for this caper is the upgrade of the local grid. However, there are more efficient ways, (borrowed by the operation of the transmission networks) that could allow for larger DG penetration without outstanding distortions at the voltage profile. Two of them, concerning the generators operation, arePower factor control the generator operates at a continu ous power factor. The reactive follows the real power output so that their ratio remains constant. This kind of operation was required by the distribution network operator (DNO) in the condition of the fit and forget rationale, with which all the low voltage networks were initially designed. The major advantage of this method is that it does not disrupt the operation of other normal devices, such as OLTC. However, it may cede a negative impact on the voltage at the connected bus, since it tends to contribute to the voltage variation which is influenced by the changes in power flows. As a consequence, this approach limits the pith generation capacity that can be connected on the network.Voltage control the generator adjusts its reactive power output so that the voltage at the connection bus remains within the allowable limits. This adjustment is done according to the real power output and the voltage drop or rise occurring at the bus. Obviously, the generators power factor is no t constant anymore, although an upper and a lower limit would normally apply. The DNO are not very keen on allowing generators to operate in that way, since this operation could good affect the operation of other voltage regulation devices, such as OLTC transformers. Moreover, if a small generator tries to correct a high voltage change, it might need to set its reactive power output at a critical high or low level. This sort of hard operation close to the thermal or over true limits entails a significant increase in the livelihood cost, or even worse, it can activate the protections and cause a sudden trip.A combination of the two approaches described above is considered the best solution regarding the optimum operation of a distributed generator. The main liking is that the generator operates with a constant power factor until the point where the voltage at the connected bus reaches the upper or lower allowable limit. At this point, the power factor control is deactivated, lett ing the generator to adjust its reactive power output and fix the voltage. Again, the power factor must be kept within its the allowable limits. When the voltage returns within its limits, the power factor control is reactivated.The consequences of new DG capacity on network fault levels and protection 4The connection of distributed generation, using either synchronous or inductive generators, causes an increase in the systems fault levels, owing to the additional generation that could feed a potential fault current. This is especially the case at the very edges and weak parts of the distribution network, where the renewable generators are usually connected. The new fault levels might exceed the rating of the existing protections. such(prenominal) an implication could cause significant damage to the protection equipment and set the systems security at high risk or ultimately incur cater interruptions. But most importantly could set personnels life in to prankish danger. Therefore, every time a new distributed generator is connected, the fault levels must be re-examined and wherever is deemed necessary, protections must be upgraded. In some cases it might be necessary to upgrade the respective part of the network (reduce the R and X values new lines or/and transformers), which would usually require significant amount of money.Nonetheless, in the context of the active management operation of the distribution network, the operator can take some alternative measuresAdvanced converter technology the use of advanced power inverters, makes a generators contribution to the fault current much lower. The more advanced a converter, the higher its cost. cyberspace reconfiguration changing the topology of the network could change the fault level at some buses. This operation is already available in most of the networks for maintenance purposes.Is Limiter a device that can instantly increase the systems resistivity (lower fault level) in case of an incident, but it need s replacement after each use.Sequential switching the contribution at the fault current by one or a team of distributed generators can be keep apartd in case of a fault in a different section.The need for active power balancing in networks with high penetration of renewable energy resourcesIn order for the frequency of a system to be maintained, the total real power generation must always be equal to the total real power load and losses. A distortion of this balance could cause a frequency leaving beyond the allowable limits and hence, damage the equipment and the loads. Therefore, it is of major importance that the system operator must always retain the active power balance.Most of the customary renewable energy resources, such as wind or solar, have an intermittent behaviour and their output relies on the weather/climate conditions and hence, cannot be controlled. Although there were significant advances in the weather forecasting, errors are placid present. Moreover, any sudd en incidents that could trip a big part of the total generation (not only renewable) could also contribute to a real power unbalance. Adding to this the fact that the demand can also vary, the large penetration of renewable resources increases easily the risk for the systems stability.The main techniques that a system operator can use in order to maintain the power balance are remembering Large (compressed air, hydro-pump storage, flywheels etc) or smaller (EV etc) storage facilities can contribute to the real power balancing. They can store energy when the renewable generation exceeds the total load and deliver it back to the network when the total generation is not sufficient to meet the demand. Some storage technologies are better than others in terms of their performance, however all of them are quite capable of right away adjusting their output to support the systems frequency. Nonetheless, most of the storage technologies are still very expensive. Pump storage, a cheaper opt ion, is naturally constrained, while the massive introduction of EV is not feasible in the foreseeable future.Demand Side Management The active power equation has two sides. Available for adjusting or shifting demand could adequately contribute to the systems stability when the renewable sources are lower than expected, if appropriate incentives are given to the consumers.Dispatchable units The unbalances risk that might emerge by the large penetration of renewable energy resources could be offset by large and fast, in terms of start-up and output, generating units, such as hydro or hired gun plants. However, hydro plants are naturally constrained and gas power plants are still causing carbon emissions, albeit fewer than the coal generators.Interconnections Interconnections between large electricity grids could significantly reduce the risk associated with the large penetration of renewable energy sources. Each system could either absorb or deliver real power from its counterpart d epending on its total generation and demand. DC interconnections are more common, since they can transfer more real power and isolate the two systems in terms of frequency control, although they are more expensive than conventional AC interconnections.