【作者】 胡胜；

【导师】 康勇；

【作者基本信息】 华中科技大学 ， 电气工程， 2012， 博士

【摘要】 双馈风力发电机的变流器容量仅为发电机转差率容量,与直驱型风电系统中的全功率变换器相比,具有体积小、重量轻和损耗低等优点,因此目前双馈感型风电机组在1WM以上的风电机组中占据着主导地位,是国际风电机组研制的主流技术。然而大规模风电的发展,要求并网运行的风电机组能够满足和适应电力系统运行的需要而具备低电压穿越能力和抵御电网电压不平衡影响的能力。本文围绕双馈感应发电机的低电压穿越技术和电网电压不平衡条件下的控制技术进行了较为深入的研究,主要内容如下：电网故障时双馈感应发电机的电磁暂态特性对于分析其在故障期间的的行为和实现低电压穿越控制是非常重要的,目前对称故障下双馈电机转子侧短路的电磁暂态特性的研究比较清楚,但缺少不对称故障转子侧短路情况下的电磁暂态特性的理论分析。本文在对称故障研究的基础上,对其在不对称故障转子侧短路情况下的暂态特性进行了理论分析,首先建立了双馈感应发电机的数学模型,然后利用其空间矢量模型对相关参量的数学表达式进行了理论推导,仿真结果验证了理论分析的正确性。从磁链、电流、功率和转矩的数学表达式分析可知,定子磁链负序分量是引起故障期间定子电流不对称、转子电流畸变、功率和电磁转矩脉动的根本原因。Crowbar保护方案中的Crowbar电阻的选取是实施有效保护的关键。目前其选取的范围是基于特定运行条件下和一系列近似处理之上通过数学推导给出的,因此存在误差,并且该取值范围的推导不是基于极端运行条件得出的,也没有综合考虑其对功率、电磁转矩的影响。针对上述不足,本文提出了一种Crowbar电阻选取的方法,首先考虑故障前的运行条件对转子电流的影响,通过仿真得出了确定Crowbar电阻值的极端运行条件,然后在该条件下,根据Crowbar电阻对转子电流大小的抑制和定子电流大小、功率和电磁转矩抑制的一致性,提出了在Crowbar投入时尽量抑制转子电流大小和兼顾考虑Crowbar电阻压降不能超出转子变换器的最大输出电压的原则确定Crowbar电阻取值的方法。仿真结果验证了采用该方法确定的Crowbar电阻实现故障穿越的有效性。研究表明,由该方法确定Crowbar电阻的取值所构成的保护方案,可以对定转子电流、功率、电磁转矩的振荡达到最大的抑制。电网故障时定子磁链中的故障分量会在转子侧感应很高的过电压,为了防止转子侧过电流,则必须要求转子变换器输出很高的电压与之抗衡,但这会远远超出变换器的电压输出能力；而灭磁控制是通过控制转子电流来抵消定子磁链中故障分量对转子侧的作用的一种低电压穿越方案,但电压深度跌落时为了抵消定子磁链中故障分量的作用,会使灭磁电流超过了变换器通流极限致其损坏。本文针上述方案的不足,提出了转子侧变换器改进的故障穿越控制策略,利用转子变换器的输出电压裕量来构造虚拟阻抗来减小了故障时所需的灭磁电流的大小,在深度跌落时,也可以保证灭磁电流小于转子变换器的最大允许电流；网侧变换器的负载电流前馈的低电压穿越控制策略由于忽略了输入滤波器的损耗,故障时直流链电压的波动较大,为了减小直流电压的波动,本文提出了网侧变换器改进的故障穿越控制策略,考虑输入滤波器的损耗,前馈电流用变换器侧功率来计算。仿真结果验证了改进的机侧和网侧变换器实现低电压穿越的有效性。研究表明,与原控制策略相比,改进的机侧和网侧故障控制策略在电网电压出现严重故障时也能够有效的抑制转子变换器的过电流和直流链电压的过大波动,从而提高了风电机组实现低电压穿越的能力。在不平衡控制中,正负序分量的分解实施控制的前提,可以使用低通滤波器或者陷波器完成,但滤波器分离的速度慢,有较大的延时,从而会影响系统的稳定性；再者要求以高的阶数来保证分离的精度,但是阶数越高,对系统的稳定性能影响越大,因此滤波器的使用增加了确定PI参数的难度。为了克服滤波器法存在的问题,本文采用了一种快速的正负序分量分解的方法,基于该方法提出了改进的机侧和网侧变换器的不平衡控制策略,仿真验证了该快速分解方法和不平衡控制策略的有效性,研究表明基于该算法的改进的机侧和网侧的不平衡控制策略能够有效实现设定的不平衡条件下各种控制目标,从而提高了机组抵御电网电压不平衡的能力。更多还原

【Abstract】 As the converter only manages the rotor power, it can be rated at slip rated power, which can achieve advantages of small size, light weight, low cost, and small losses compared to direct-drive wind power systems with a full scale power converter, nowadays, doubly fed induction generator (DFIG) based wind power system is dominant above 1WM wind power system and it is an essential aspect of wind power technology. With the development of large scale wind power, the capability of low voltage ride through(LVRT) and resisting grid voltage unbalance for grid connected wind power system, Transient behavior of DFIG, LVRT and unbalanced control technology are studied in this paper. The primary contents and original contributions of this dissertation are as follows:The transient characteristic of DFIG under grid faults is very important for analysis of its behavior during grid and LVRT control, now the transient characteristic under symmetrical grid fault with rotor side terminal shortcircuit is clear, but the theoretical analysis of electromagnetic under asymmetrical grid faults is lack,based on the study of symmetrical grid fault, the theoretical analysis of electromagnetic under asymmetrical grid faults are studied in this paper,The mathematical models under different reference frame of DFIG are established, based on its space vector model, the transient behaviors of DFIG are derived and analyzed, the simulation results verify the theoretical derivation. It concludes from expressions of flux, current, power and torque that the negative component is the root cause of active power, reactive power and torque pulsation.Crowbar resistor selection is crucial for carring out effective protection with traditional crowbar protection scheme.Its range can be gotten with mathematical derivation based on a series of approximations and specified running condition,therefore, it has errors and its specific value is not given, in addition, the derivation of crowbar range is not based on the extreme running condition and it not also considered its influence on power and torque.In this paper, a selection method of crowbar resistor is proposed, firstly, considering influence of the running conditions on rotor current, the extreme running conditions are confirmed with simulations, then, under such condition, according to consistence that limiting rotor current is simultaneously limiting power and torque oscillation, the value of crowbar resistor is decided by considering the use of crowbar limits the rotor current at the most, its decision also considers the voltage drop cross crowbar resistor is lower than the maximum value of output voltage of rotor side coverter, the simulative results validated effectiveness of LVRT with crowbar resistor selecte with such method, It concludes that the current of stator and rotor, the power and the torque can be restrained at most after optimization.The DC and negative component of stator flux can induce very large voltage at rotor terminal, so in order to avoid overcurrent at rotor side, rotor side converter must output a very large voltage to balance with it, but the large voltage can exceed the maximum output voltage of rotor side converter much more; demagnitizing control strategy is based on the control of machine side convter itself, its effectiveness is limited by the severity of grid fault and the leakage resistance of DFIG, the magnetizing current under deep dip may exceed the limit of rotor sid converter current, which can damage the converter, in light of such defect, an improved LVRT control strategy for rotor side converter is proposed, the margin of the output voltage of rotor side converter is used to introduce virtual resistance, which can decrease the required demagnitzing current during grid faults; load current forward control strategy is employed for grid side converter during grid faults, because the power loss of input filter is ignored in this strategy, which leads to a large flucuation of DC link voltage during grid faults, in this paper, an improved control strategy of grid side convter is proposed in which the power loss of input filter is not ignored and the computation of the forward current is with convter side power instead of grid side input power, the simulative results validated the effectiveness of rotor sided control strategy and grid side control stragtey, even in the case of severe grid faults, the improved control strategies can still effectively decrease overcurrents on rotor side converter and large fluctuation of DC link,hence, the LVRT capability is improved.Positive and negative component extraction is the pre-conditions of unbalance control, which can be employed with low pass filter or noch filter, however, the filter is an independent part in control system, the decomposed speed is slow and it leads to a large delay,which can influence the stablity,in addition, in order to gurantee the decomposed accuracy, the filter order must be high, but the higher filter order, the worse the system stablity, so the use of filter is also increase the difficulty of PI parameter selection of controller, to vercome the shorcomings of filter, a fast positive and negative extrcation method is adopted, based on it, an improved machine side strategy and an improved grid side control stragtey are proposed, the simulation validated the effectiveness of the fast extraction method and the improved unbalanced control stragtegy, It concludes that the improved machine side and rotor side unbalanced control strategies can effectively realize the specified control targets.更多还原