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Harmonic analysis of wind power

2020-06-30 15:30:26

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The influence of wind turbine grid connection on power quality of power grid causes fluctuation and flicker. Wind power generation is the fastest growing renewable energy in the 21st century. With the increasing capacity of wind farms, the impact on the system is becoming more and more obvious. In the early stage, the unit capacity of wind power was small, and most of them used asynchronous generators with simple structure and convenient grid connection, which were directly connected to the distribution network. However, the wind power site is often sparsely populated in the area, which is at the end of the power supply network, and its ability to withstand impact is very weak. therefore, wind power is likely to bring harmonic pollution, voltage fluctuation and flicker to the distribution network. the randomness of wind power brings a lot of difficulties to the formulation of power generation and operation planning. 


Grid-connected Technology of Wind Turbine. 


The grid-connected condition of the alternator is that the output voltage of the alternator is exactly the same as the grid voltage in amplitude, frequency and phase. With the increase of the single unit capacity of wind turbine, the greater the impact on the power grid when connected to the grid. When the shock is serious, it will not only cause a substantial decrease in the voltage of the power system, but also cause damage to generators and mechanical parts (towers, blades, accelerators, etc.). If the impact time of grid connection lasts too long, it may disintegrate the system or threaten the normal operation of other grid-connected units. 

Therefore, the adoption of reasonable grid-connected technology is a problem that can not be ignored.



Grid-connected Technology of synchronous Wind Turbine. 


In operation, synchronous generator can not only output active power, but also provide reactive power, stable cycle and high power quality, so it has been widely used in power system. However, it is not ideal when it is transplanted to wind turbine. This is because the wind speed varies randomly, the torque acting on the rotor is extremely unstable, and its speed regulation performance is difficult to achieve the required precision of the synchronous generator when connected to the grid. If there is no effective control after grid connection, reactive power oscillation and out-of-step often occur, especially under heavy load. Therefore, synchronous generators are rarely used in wind turbines at home and abroad for a long time. However, with the development of power electronics technology in recent years, the use of frequency conversion device between synchronous generator and power grid can solve these problems technically. Therefore, the scheme of synchronous generator has attracted people's attention. 


Grid-connected Technology of Asynchronous Wind Turbine. 


When the asynchronous wind turbine is put into operation, because the load is adjusted by slip, the speed regulation precision of the unit is not high, and the synchronous equipment and whole-step operation are not needed. as long as the speed is close to the synchronous speed, it can be connected to the grid. 

Obviously, the wind turbine equipped with asynchronous generator not only has a simple control device, but also will not produce oscillation and out-of-step after being connected to the grid, and the operation is very stable. 


However, there are also some special problems in asynchronous wind generator grid connection, such as excessive impulse current directly connected to the grid will cause a substantial drop in voltage, which poses a threat to the safe operation of the system; it does not generate reactive power and needs reactive power compensation; excessive system voltage will make its magnetic circuit saturated, reactive power excitation current greatly increased, stator current overload, power factor greatly reduced. If the frequency of the unstable system increases too much, the asynchronous generator will change from the generating state to the electric state due to the increase of the synchronous speed, and the frequency of the unstable system will decrease too much, and the current of the asynchronous generator will increase sharply and overload and so on. Therefore, we must strictly monitor and take corresponding effective measures to ensure the safe operation of wind turbines. At present, the main grid connection modes of asynchronous generators at home and abroad are direct grid connection, quasi-synchronous grid connection, step-down grid connection, capture quasi-synchronous fast grid connection, soft grid connection and so on.



Influence of Wind Power Grid connection on Power quality. 


With more and more wind turbines connected to the grid, more and more attention has been paid to the impact of wind power generation on power quality. The uncertainty of wind resources and the operation characteristics of wind turbines make the output power of wind turbines fluctuate, which may affect the power quality of power grid, such as voltage deviation, voltage fluctuation, flicker, harmonics and so on. Most wind turbines use soft grid-connected mode, but they still produce large impulse current when starting. When the wind speed exceeds the cut-out wind speed, the wind will automatically withdraw from the rated output state. If the wind turbines act almost at the same time in the whole wind power site, the impact on the distribution network is very obvious. Not only that, the change of wind speed and the tower shadow effect of the fan will lead to the fluctuation of the fan output, and its fluctuation is just within the frequency range that can produce voltage flicker (less than 25H z),). Therefore, the fan will also bring flicker problems to the power grid during normal operation. Voltage fluctuation and flicker is one of the main negative effects of wind power generation on power quality of power grid. 


Mechanism analysis of voltage fluctuation and flicker. 


The fundamental cause of voltage fluctuation and flicker caused by wind power generation lies in the fluctuation of the output power of grid-connected wind turbines. 


At present, there are three representative flicker measuring instruments in the world, namely, Japanese flicker meter, British ERA arc furnace flicker meter and flicker meter recommended by IEC and UIE. However, the lighting voltage in China is 220V, which is close to the flicker test with 230V lighting voltage and 60W incandescent lamp recommended by IEC/UIE, and it is more scientific and accurate to evaluate the severity of flicker by using short-time flicker and long-time flicker in IEC standard. 


Mathematical analysis of fluctuations and flickers: 


1.Relative voltage fluctuation. 


The difference between the two extremes of the root mean square of voltage Umax and Umin Δ U is often expressed as a percentage of the rated voltage UN, that is,

The voltage fluctuation value is a waveform which takes the voltage root mean square or the envelope of the peak value as the time function. In the analysis, the power frequency voltage U is abstractly regarded as the carrier, and the fluctuation voltage Δ U is regarded as the amplitude modulation wave.


2. Flicker detection rate F (%). 


According to the experimental conditions recommended by IEC, the power frequency voltage of AM wave with different waveforms, frequencies and amplitudes is used as the carrier to supply light to the power frequency 230V and 60W incandescent lamp. According to the statistics of the flicker perception experiment of the observer, the ratio of the number of people with obvious perception and intolerability to the total number of observers can be obtained, that is,


                    

                                 

In the formula, A: the number of people who are not aware of it, B: the number of people who are slightly aware, C: the number of people who are obviously aware, and D: the number of people who are unbearable. 


If the ratio is more than 50%, it means that more than half of the experimental observers have obvious or unbearable visual reflection. If F (%) is greater than 50% as the flicker limit, then the corresponding voltage change value is the allowable value of voltage fluctuation under the experimental conditions. 


Voltage fluctuation and flicker suppression

At present, most of the compensation devices used to improve power quality have the function of restraining voltage fluctuation and flicker, such as static var compensator (SVG) and active power filter (APF).. 


Static Var compensator (SVG). 


Voltage flicker is a special reflection of voltage fluctuation. the severity of flicker is related to the voltage change caused by load change. in high-voltage or medium-voltage distribution network, voltage fluctuation is mainly related to the change of reactive load and the short-circuit capacity of power network. When the short-circuit capacity of the power network is fixed, the voltage flicker is mainly caused by the drastic change of reactive load. Therefore, the most commonly used method to restrain voltage flicker is to install static var compensation device. However, because some types of SVC also produce low-order harmonic currents, they must be used in parallel with passive filters. In actual operation, some harmonics may be seriously amplified due to the harmonic resonance of the system.Therefore, in the compensation, it is required to use the reactive power impulse current and harmonic current compensator with short response time and can directly compensate the load. 


Active power filter (APF). 


In order to restrain the voltage flicker, the reactive current must be compensated in real time with the change of load in the case of sharp fluctuation of load current. In recent years, the active filter composed of power transistor (GTR), turn-off thyristor (GTO) and pulse width modulation (PWM) technology can compensate the load current in real time. The working principle of the active power filter is completely different from that of the traditional SVC. It uses power electronic devices that can be turned off and is controlled based on the instantaneous reactive power theory based on the coordinate transformation principle. Its working principle is to use the power electronic controller to provide the necessary distortion current to the load instead of the system power supply, so as to ensure that the system only needs to provide sinusoidal fundamental current to the load. 


Compared with ordinary SVC, active power filter has the following advantages: fast response time, high compensation rate for voltage fluctuation and flicker, reduced compensation capacity, stable operation without harmonic amplification and resonance, strong control, control of voltage fluctuation, flicker and voltage stability, as well as effective filtering of high-order harmonics and compensation of power factor.

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