电网电压不平衡下并网逆变器功率灵活控制

在不平衡电网电压条件下对光伏并网逆变器功率的有效控制,是高渗透率光伏系统并网运行需解决的重要问题之一。当电网电压不平衡时,有功、无功功率出现不可控的波动,且并网电流不平衡。故在静止坐标系下,提出一种灵活的有功、无功功率控制策略。该方法可通过调节参数,实现有功、无功功率波动的平滑抑制,且可在参数范围的两个极端将有功或无功功率波动消除。通过仿真和实验验证了所提控制策略的有效性。     

光伏并网逆变器在不平衡电网下的控制策略研究

目前,在平衡电网电压下的光伏并网逆变器的控制已较成熟,而在不平衡电网下,光伏并网逆变器的传统控制策略会引起电压不稳定和有功无功功率的二次脉动.通过建立并网逆变器的数学模型,获得dq坐标系下的并网逆变器的动态方程,分析并网逆变器控制策略的关键是其锁相环能够准确提取电网三相电压相位.传统的三相同步锁相环(SRF-PLL)在电网三相电压不平衡时无法准确提取电压的相位,在举例分析国内外几种获取电压准确相位方法的优缺点的基础上,采取了一种基于解耦双同步参考坐标系下的锁相环(DDSRF-PLL)的控制策略,通过dq旋转轴的坐标系和解耦网络,可准确获取三相不平衡电网的电压相位,并采用旋转dq坐标系分离正负序分量,完成独立控制.最后构建电网三相电压平衡和不平衡工况下的光伏并网逆变器的仿真模型,验证了该控制策略的可行性和有效性.     

不平衡及谐波电网下并网逆变器的谐振滑模控制技术

为提高不平衡及谐波电网下并网逆变器的运行性能,通过建立不平衡及谐波电网下并网逆变器的数学模型,以输出电流正弦或输出有功/无功功率平稳为控制目标,提出不平衡及谐波电网下并网逆变器基于谐振滑模的直接功率控制策略。为避免不平衡及谐波电网电压对相位检测精度的影响,所提谐振滑模控制策略在静止坐标系中实现。此外对所提控制策略的稳定性和收敛性进行深入分析。最后通过构建并网逆变器实验系统,对所提控制策略的正确性和可行性进行实验验证。     

光伏电站低电压穿越技术研究

针对大规模光伏电站在电网故障或扰动时突然脱网造成的严重后果,提出了一种光伏电站低电压穿越(Low Voltage Ride Through,LVRT)控制策略,详细分析了基于正负序同步旋转坐标系的并网逆变器控制策略以及LVRT有功电流和无功电流协调控制的实现方法。以单相电压跌落故障为例,进行了仿真和实验研究,结果表明所提出的控制策略不仅能保证并网逆变器在电压跌落期间输出不过流,而且能够向电网发出一定的无功功率以支撑并网点电压的恢复,实现低电压穿越。     

负载不平衡条件下并网逆变器的运行策略研究

针对新能源并网出现的配电网分布式和不平衡负载问题,提出了通过电网统一调度实现并网逆变器对电网的有功功率、无功功率和不平衡负载进行分布式补偿的运行策略。上级电网根据线路的运行数据,对线路上并网逆变器进行调度,完成对电网的有功功率、无功功率和不平衡负载的调节。对不平衡负载的调节会导致逆变器直流侧电压的波动,造成逆变器的输出电流产生谐波等问题。针对该问题,提出基于陷波器的直流侧电压控制策略,抑制输出电流谐波。通过仿真和样机实验验证了该运行策略的可行性。     

不平衡电压下并网逆变器的预测电流控制技术

在推导不平衡电网电压下并网逆变器数学模型和瞬时有功、无功功率数学模型的基础上,提出了实现三相电流平衡、有功功率或无功功率无波动为控制目标的控制策略,讨论了不同控制目标下并网逆变器正序和负序电流指令计算原则。为实现正序和负序电流的准确控制,设计了在两相静止坐标系下的预测电流控制方案,以消除电流采样延时和正负序分解导致系统运行性能下降的影响。最后搭建了3 kVA并网逆变器的试验系统,试验结果验证了理论分析的正确性以及预测电流控制方案的有效性。     

不平衡电压下并网逆变器的预测电流控制技术

在推导不平衡电网电压下并网逆变器数学模型和瞬时有功、无功功率数学模型的基础上,提出了实现三相电流平衡、有功功率或无功功率无波动为控制目标的控制策略,讨论了不同控制目标下并网逆变器正序和负序电流指令计算原则。为实现正序和负序电流的准确控制,设计了在两相静止坐标系下的预测电流控制方案,以消除电流采样延时和正负序分解导致系统运行性能下降的影响。最后搭建了3 kVA并网逆变器的试验系统,试验结果验证了理论分析的正确性以及预测电流控制方案的有效性。     

低压网络中并网光伏逆变器调压策略

为解决光伏接入电网引起的并网点电压越限问题,增大光伏在电网中的渗透率,可充分利用并网光伏逆变器的无功调压能力向电网提供无功支撑。为此,从电网电压降落角度研究光伏发电接入电网前后并网点电压的变化,并深入分析了德国电气工程师协会(Verband Deutscher Elektrotechniker)提出的无功功率控制策略,在此基础上,提出一种基于并网点电压幅值与光伏有功出力的Q(U,P)无功功率控制策略。该控制策略继承了德国电气工程师协会提出的策略的优点,在此控制策略下,光伏系统所有逆变器参与电网电压调节,在维持电网电压在要求范围的前提下,无功吸收总量最低,电网有功、无功损耗最小。算例结果验证了该策略的有效性。     

不平衡电网电压下虚拟同步发电机模型预测控制

虚拟同步发电机(Virtual Synchronous Generator, VSG)控制算法,可为电网提供惯性并参与电压及频率调节,但在电网电压出现不平衡时会导致并网电流不平衡及有功、无功功率波动的问题,严重影响了并网电能质量。针对此问题,提出了一种不平衡电网下VSG模型预测控制策略,通过基于快速电压矢量选择的模型预测控制策略来控制所重构的正序及负序电流分量,达到有功功率恒定、无功功率恒定及电流平衡的控制目标。此外还引入负序电流调节系数,进而实现这三个目标的协调控制。     

非理想电网光伏并网逆变器控制策略研究

当电网电压中含有谐波或三相不平衡以及电网基频偏移时,必须考虑并网逆变器在非理想电网下的并网控制策略。为提高并网电能质量,分别从电网电压畸变不平衡和电网基频偏移角度进行了分析,提出并网逆变器在非理想电网下的控制策略,即在保证并网电能质量前提下,通过调节电流控制指令中负序分量的权重,实现并网电流幅值不平衡和瞬时有功波动之间的协调控制。理论分析进一步表明,并网逆变器建立在α,β坐标系下不仅可避免LCL滤波器参数摄动对并网电流的影响,而且可避免锁相环节中复杂的三角函数运算,降低系统运算量,其控制性能要优于并网逆变器在d,q同步旋转坐标系下的控制方案。仿真与实验结果也验证了所提控制策略的可行性和有效性。     

Modeling and enhanced control of DFIG under unbalanced grid voltage conditions

This paper presents a mathematical model of a doubly fed induction generator (DFIG) based on stator voltage orientation (SVO) in the positive and negative synchronous reference frames under unbalanced grid voltage conditions. The oscillations of the DFIG electromagnetic torque and the stator active and reactive powers are fully described during grid voltage unbalance. A new rotor current controller implemented in the positive synchronous reference frame is proposed. The controller consists of a proportional integral (PI) regulator and a harmonic resonant (R) compensator tuned at twice the grid frequency. Thus, the positive and negative sequence components of DFIG rotor currents are directly regulated by the PI + R controller without the need of involving positive and negative sequence decomposition, which indeed improves the dynamic performance of DFIG-based wind power generation system during small steady-state and relatively larger transient network unbalances. The theoretical analysis and the feasibility of the proposed unbalanced control scheme are validated by simulation studies on a 1.5-MW wind-turbine driven DFIG system. Compared with conventional single PI current control design, the proposed control scheme results in significant elimination of either DFIG power or torque oscillation under unbalanced grid voltage conditions.     

不平衡电网电压下基于FPNSC算法的逆变器并网输出电流峰值控制策略

针对电网电压不平衡下电压负序引起的逆变器输出功率波动、电流峰值上升、谐波增大等问题,提出了一种基于灵活正负序控制(FPNSC)算法的逆变器并网输出电流峰值控制策略,在降低了并网电流畸变率的同时,将输出电流约束在允许范围内,保障了有功传输与动态无功支撑。通过引入正负序控制因子,FPNSC算法使输出正负序有功与无功的控制更加灵活。文中给出了不同控制目标下正、负序控制因子的选取及参考电流的计算方法;根据不平衡电网电压下逆变器控制目标,提出了基于FPNSC算法的电流峰值限制方法。仿真与实验验证了所提方法的正确性与有效性,该方法可推广应用于分布式并网发电系统中。     

电网电压不平衡下MMC的无源控制策略

提出了电网电压不平衡条件下模块化多电平换流器(MMC)的无源控制方法,其能很好地解决电网电压不平衡条件下MMC存在的交流侧三相电流不对称、有功功率出现二次脉动、无功功率出现二次脉动的问题。根据MMC的拓扑结构,建立MMC在电网电压不平衡条件下的数学模型,分析MMC在电网电压不平衡条件下的内部特性,并在此基础上设计电网电压不平衡条件下的环流控制策略。基于存储函数的无源控制理论,针对上述3个不同的控制目标,制定相应的非线性无源控制策略。     

不平衡电压下双馈变速风电机组的控制策略

提出了不平衡电网电压下双馈发电机的控制策略,并建立了双馈发电机在正、反旋转坐标系下的数学模型。在此基础上推导和分析了电网电压不平衡条件下双馈发电机输出的瞬时有功、无功功率的组成。提出了4种可供选择的不平衡电压控制方案,并给出了不同控制目标下转子的正、负序电流目标值的计算原则。通过MATLAB/SIMULINK仿真验证了控制方案的有效性。     

电网不对称故障下直驱风电机组低电压穿越技术

分析了直驱风电变流器在电网不对称情况下的表现,提出一种适用于该情况的网侧变流器控制策略,并将其与直流侧Crowbar电路配合,实现电网不对称故障下的低电压穿越.直驱风电机组与电网间的功率交换完全通过变流器实现,电网不对称故障引起的有功功率波动在变流器上表现为直流侧电压大幅波动.控制策略以稳定输出有功功率为目标,基于同步旋转坐标系和正负序分解得出控制模型,并综合考虑电流安全限值、系统复杂程度等问题,给出额定功率附近运行时发生不对称故障的穿越方案.使用MATLAB建立仿真模型,给出不对称故障下的仿真结果,证明方案可行性.     

An Enhanced LVRT Scheme for DFIG-based WECSs under Both Balanced and Unbalanced Grid Voltage Sags

Due to the latest grid codes, wind energy conversion systems (WECSs) are required to remain connected to grid under grid voltage sags and supply reactive power into the grid. So, this paper proposes an enhanced scheme to improve low-voltage ride through (LVRT) capability of doubly fed induction generator (DFIG)-based WECSs under both balanced and unbalanced grid voltage sags. The proposed scheme is composed of active and passive LVRT compensators. The active compensator is performed by controlling the rotor- and grid-side converters of the DFIG to decrease the stator flux oscillations and inject reactive power into the grid. The passive compensator is based on a three-phase stator damping resistor (SDR) located in series with the stator windings. The proposed scheme decreases the negative effects of grid voltage sags in the DFIG system including the rotor over-currents, electromagnetic torque oscillations, and DC-link over-voltage and also injects reactive power into grid to support the grid voltage. So, the LVRT capability of DFIG is enhanced and new grid code requirements are addressed. Simulation results on a 1.5-MW DFIG-based WECS using MATLAB/Simulink demonstrate the effectiveness of the proposed LVRT scheme under both balanced and unbalanced grid voltage sags.     

不对称故障条件下并网光伏逆变器峰值电流抑制策略

不对称故障在电网实际运行过程中时有发生,其导致的输出电流峰值较大的问题严重影响了光伏发电系统的可靠运行。首先对不对称故障条件下并网光伏逆变器输出电流峰值过大的机理进行了分析,进而提出了一种考虑输出电流峰值的参考电流算法,在已有参考电流算法中加入多个调节参数,能够对输出电流峰值和功率波动进行调节。在所提出改进参考电流算法基础上,提出了一种有功无功协调控制策略,有效解决了不对称故障条件下并网光伏逆变器的过流问题,拓展了光伏系统的有功输出能力。通过在PSCAD/EMTDC中建立仿真模型验证了所提出控制策略的正确性。     

不平衡电网下并网逆变器功率模型预测控制

当电网电压不平衡时,并网逆变器侧有功/无功功率波动、并网电流谐波显著增加。为解决该问题,提出一种新型的模型预测功率直接控制策略MPDPC(model predictive direct power control)。在该控制策略中,采用电网电流、电网电压及其1/4周期延迟信号和电容电压差搭建功率模型;然后选择功率模型预测控制中最优的开关状态,有效地消除了逆变器输出有功功率2倍频脉动,电流谐波,实现给定有功功率和无功功率的精确跟踪。仿真和实验结果表明,该方法能消除电流谐波,实现三相电流基本平衡。     

A flexible power control strategy for rotor-side converter of DFIG under unbalanced grid voltage sags

Doubly fed induction generator (DFIG) is widely applied in variable-speed wind energy conversion system. The disconnection of a substantial amount of DFIG may arouse the instability problem of power system, thus wind power generators have to remain connected during short-time grid faults. As a result, the voltage sag will lead to overcurrent and overvoltage in the rotor winding of a DFIG, moreover the unbalanced voltage sags will also cause serious fluctuations in its electromagnetic torque and output power. This paper studies the relationship of the stator instantaneous powers with the three-phase stator voltage and rotor current of a DFIG under unbalanced grid voltages. A generalized formula of current reference for the rotor-side converter of DFIG is constructed by introducing continuous adjustment coefficients. Meanwhile, the analytical equations of rotor peak current, stator active and reactive power fluctuations are derived to characterize the operating performance of DFIG. The impacts of adjustment coefficients on DFIG control performance and the feasible region of coefficients restrained by the rotor current constraint are discussed. In consideration of the rotor current limit, the flexible power control strategy for DFIG in unity power factor mode (UPFM) and reactive power supporting mode (RPSM) is presented. The correctness of proposed method is verified by simulation and experiment tests of single-DFIG and multi-DFIG systems.     

Operation and control of a grid-connected DFIG-based wind turbine with series grid-side converter during network unbalance

This paper proposes a control scheme of a grid-connected doubly-fed induction generator (DFIG) wind turbine with series grid-side converter (SGSC) to improve the control and operation performance of DFIG system during network unbalance. The behaviors of DFIG system with SGSC under unbalanced grid voltage conditions are described. The SGSC is controlled to inject voltage in series to balance the stator voltage. Therefore, the adverse effects of voltage unbalance upon the DFIG such as large stator and rotor current unbalances, electromagnetic torque and power pulsations are removed and the conventional vector control strategy for the rotor-side converter (RSC) remains in full force under unbalanced conditions. Meanwhile, three selective control targets for the parallel grid-side converter (PGSC), such as eliminating the oscillations in total active or reactive power, or no negative-sequence current injected to the grid are identified and compared. Besides, the proportional resonant controllers in the stationary reference frame are designed for both the SGSC and PGSC to further improve the dynamic performance of the whole system. Finally, the ratings and losses of the SGSC and the injected transformer are discussed and the effectiveness of the proposed control scheme is verified by the simulation results of a 2 MW DFIG-based wind turbine with SGSC under steady state and small transient grid voltage unbalance.