风力发电系统中的远程故障管控系统设计【说明书论文开题报告外文翻译】
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译文题目： Fault ride through and voltage regulation for grid connected wind turbine 电网连接风力涡轮机的故障穿越和电压调整 Fault ride through and voltage regulation for grid connected wind turbineabstractHigh penetration of wind generation challenges wind turbine operators to supply reliable power andextract optimum power from the wind.Hence, the fault ridethrough (FRT) capability of wind turbinetogether with the optimum power tracking and regulation of wind turbine output voltage due to fluctuating nature of the wind becomes essential. In this paper, a method is proposed to ensure that the double fed induction generator (DFIG) wind turbine continues to operate during severe grid faults and maintains a constant output voltage, irrespective of the fluctuating wind. The proposed controller also allows the DFIG wind turbine to track optimum power from the wind. Extensive simulation is performed using PSCAD/EMTDC software and results obtained show that the DFIG output voltage fulfills the grid code requirements.The results also show that the proposed method is able to track the optimum power, regulate the DFIG output voltage and perform fault ride through of wind turbine.1. IntroductionThe Earths climate is changing. The average global temperature has risen by 0.6 since the beginning of the 20th century. The effects of recent warming can be seen in an increased incidence of heat-waves, storminess and flooding, the retreat of glaciers and ice sheets, and altered responses in plants and animals. Although climate change is a natural and constant process, it has been highly influenced by the increasing atmospheric concentrations of carbon dioxide and other greenhouse gases. There is also general agreement that average temperatures are likely to rise even faster, particularly in the second half of this century, unless action is taken to limit and reduce greenhouse gas emissions. Hence, most of the countries are moving their electrical power generation to renewable sources such as wind, solar, tidal, hybrid system and so on. Wind energy is clean, abundant in source and has remarkable growth in the last decade. For instance, in the United States, the total installed capacity is 6350MW and it is projected that by 2020, the total installed capacity will touch 100,000 MW 1. There is about 58,982 MW of wind capacity installed worldwide in 2005, which generates nearly 1% of the world electricity especially in some countries such as Denmark, Spain and Germany 2.In present day, the most popular type of wind turbinesinstalled worldwide is the variable speed wind turbines, which has better advantages such as controllability of speed and flexible operation compared with fixed speed wind turbines. Other advantages of using variable speed generators include improved power quality, speed control, reduced mechanical stresses, decoupled control of active and reactive power as well as more power generation than fixed speed generator under the same circumstances 2.Two types of variable speed generator are commonly used. First type is direct-drive synchronous generator which is completely decoupled from the grid by a power electronics converter connected to the stator winding. The grid side of this converter is a voltagesource converter. The generator side can be a voltage-source converter or diode rectifier. The direct-drive generator is excited using an excitation winding or permanent magnets. Another type is double fed induction generator (DFIG) which also uses power electronics. One end of a back-to-back voltage-source converter feeds the three-phase rotor winding and other end connected to stator winding or power grid as shown in Fig.1. Among the two types, DFIG is more favorable due to the following facts.Economical converter cost, because converter rating generallycan be 1040% of total system power, while the speed range is also 33% around the synchronous speedCost effective converter filters and EMI filters since filters are rated for 0.1e0.4 pu of the total system power, and converter harmonics represent asmaller fractionof total systemharmonics.DFIG converter is able to decouple control of active and reactive powerDuring the fault, disconnection of wind generator can take place if the wind generator does not support the voltage dip or sag. Disconnecting a wind generator too quickly could have a negative impact on the power system grid, especially with large wind farms. Another issue is the mechanical power output from wind generator which is directly proportional to the torque of wind turbine. During fluctuating wind speed, the output voltage will also fluctuate. If voltage fluctuation is out of the limit, it introduces negative impact on the power system. From economic point of view, tracking optimum power from wind is economically effective for wind turbine operators. Present day technology is available to ensure that the wind turbine is connected to the grid during fault or network disturbance. Additional circuitry is utilized in order to overcome fault ride through capability or voltage regulation but at the expense of additional cost. However, future improvement is required for more reliable power system and because near future grid codes may require wind turbines to have better ride through capability and voltage regulation features. A novel control strategy for low voltage ride through (LVRT) for wind turbines with DFIG was discussed in ref. 1. The authors designed a controller using HN technique and m-analysis. Authors in ref. 2 investigated the performance of the wind turbines with DFIG during a voltage dip caused by an external short circuit fault. Authors in ref. 3 described a comprehensive time domain model of DFIG using decoupled dq controller, which was proposed to keep generator operating during transient grid faults. Authors in ref. 4 presented wind farm fault ride through capability and the performance of converter protection schemes based on different resistor protection with crowbar and series dynamic resistor. In ref. 5, the authors used a real time digital simulator (RTDS) to introduce a new control strategy for LVRT for DFIG and also used current limiters which are controlled by thyristor switches to counter the effect of fault on DFIG operation. Authors in ref. 6 used passive control to mitigate the voltage sag or swell. Authors in ref. 7 discussed the LVRT of wind farms using STATCOM compared to thyristor controlled static var compensator (SVC). The transient stability margin is proposed as the indicator of LVRT capability. A simplified analytical approach based on torque-slip characteristic is proposed to quantify the effect of the STATCOM and the SVC on the transient stability margin. Authors in ref. 8 used an additional circuitry which is Five-level cascade multilevel inverter based STATCOM to improve the fault ride through control strategy of wind farm as well as to mitigate voltage fluctuation. The improvement of dynamic model of DFIG wind generator and controllers for network unbalance gird fault ride through capability using PIeR current regulators is investigated in ref. 9. The improvement includes control of the grid- and rotor side converters (GSC and RSC, respectively) during voltage unbalance. Authors in ref. 10 discussed the enhancement of the fault ride through capability, which was achieved by inserting a series-connected voltage-source converter during the fault.In this paper, an improved controller which enables fault ride through, voltage regulation and optimum power tracking from the wind is proposed. The results show compliance with the Malaysian standard and the E.ON Germany standard.2. Grid requirementsThe number of wind farms or turbines installation has increased around the world in the last decade. The increasing number of wind farms connected to the grid has encouraged the power system operators to establish grid code requirements. These requirements impel the wind farms to contribute better quality and continuity of power supply. Grid requirements aremainly focused in the following aspects: voltage ride through, reactive power exchange, voltage control and power quality. However, the demanded requirements vary from one country to another and the behavior of the wind farm also has varying influence in the stability of power system. The general grid code requirement for Malaysia is shown in Fig. 2 11.The main requirement of typical grid codes, is summarized below.2.1. Voltage fluctuation rangeThe voltage fluctuation range for pre-fault or pre-disturbance in most of the utilities is generally 5% but it also depends on the voltage level 11.2.2. Continuous voltage operating rangeThe wind turbines are expected to operate within typical grid voltage variations. In most utilities, the continuous voltage range is from 0.9 to 1.1 pu 12.2.3. Reactive power capabilityThe wind farms are required to control their reactive power output. The power factor range is typically between 0.9 (lag) and 0.9 (lead), and may depend upon the region. The turbines can be required to regulate their terminal voltage 12.2.4. Fault ride through (FRT)During the occurrence of voltage sag, the turbines are required to remain connected for a specific amount of time before being allowed to disconnect. This requirement is to ensure that there is no loss of generation for normally cleared faults. Disconnecting a wind generator too quickly could have a serious negative impact on the grid, particularly dealing with large wind farms. In addition, some utilities require that the wind turbines help support the grid during faults.Fig. 3(a) shows different standard curves for different countries in the world. The E.ON standard is a popular one and is used as the reference in this paper. The E.ON standard demands the continuous connection of generator with the grid by operating above the curve, as shown in Fig. 3(b). Disconnection can take place if the voltage falls below the curve 13.As shown in Fig. 3(b), if the three-phase short circuit of faultrelated symmetrical voltage sag is above the limit line 1, it must not lead to instability of power system. On the other hand, there must be no disconnection of the wind turbines from the system. In the shaded area and above the limit line 2, the following are applied;(i) Plant should experience the fault without disconnection from the grid. If a generating plant cannot fulfill this requirement due to the gird connection concept, it is permitted to shift the limit line while at the same time reducing the resynchronization time and ensuring a minimum reactive power in feed during the fault。(ii) If the individual generator becomes unstable or the generator protection responds, a brief disconnection of the generating plant from the grid is allowed. At the start of disconnection, resynchronization of the plant must take place within 2 s at the latest 2.3. Methodology for DFIG wind turbineThe traditional vector control method with two PWM voltage fed current regulated inverters that are connected back to back in the rotor circuit is used in this paper. DFIG allows power output from the stator winding as well as the rotor winding. By using DFIG, it is possible to get a good power factor even though the machine speed is quite different from synchronous speed. Such machines can therefore operate without the need for excessive shunt compensation 14.Fig. 4 shows a simple layout of the proposed control method of DFIG. The stator of the DFIG is connected to the three-phase grid and the rotor is fed via the back to back IGBT voltage-source inverters with a common DC bus. The grid side converter (GSC) controls the power flow between the DC bus and the AC side, and allows the system to be operated in sub-synchronous (undersynchronous) and super-synchronous (over-synchronous) speed. The active power is generated by considering the wind speed and wind turbine characteristics while reactive power control is set with respect to the utility demand. The proper rotor excitation is provided by the rotor side converter (RSC).FromFig. 4, the DFIG control systemis fed with the reactive power setting , refQrotor speed , reference speed , stator voltage and current , rotor side and nrefsVsIgrid side currents, grid reactive power and bus voltage Vdc. The DFIG Qdcsystemcontrol outputs are the switching signals to the rotor- andgrid-side converters. The blades pitch control is also used in case of higher rotor speeds.To achieve the objective of this paper, the control ability of the DFIG variable-speed wind turbine also involves the turbine control for preventing over-speeding and the control of the power converter during faults. Many authors have described the modeling of wind turbines with DFIG in refs. 3,1419.3.1. Pitch angle control system strategyThe mechanical power generated by wind turbine follows the equation below:Therefore, the mechanical power ( ) depends on the wind speed ( ) and the mechPwVpower coefficient ( ), which is a function of the tip speed ratio ( ) and the pitch Cp angle ( ). ( ) is the air density and ( ) is the air sweeping area of turbine blade. 2RThe l is defined as the relation between the tip speed and the wind speed of the wind turbine:The relation between and l for different pitch angles of is shown in Fig. 5. As Cpshown in Fig. 5, function has the highest value of where the pitch 4.0maxCpangle . In order to capture the maximum power from wind, should equal to 0 zero. For minimum power, should equal 25 for 2 MW wind turbine 15. A pitch angle control is implemented to limit the generator speed during grid disturbances and in normal operation under high wind speeds. Its aerodynamic rate of change also helps in voltage regulation and optimum power tracking.3.2. Fault ride through (FRT) strategy3.2.1. Rotor side converter (RSC)From the mathematical model of DFIG 21, the active power and reactive power generated are:where, is the magnitude of the stator phase voltage. It is obvious that active power Vsand reactive power
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