DFIG-DVR系统在不平衡电网电压下的运行特性

针对双馈风电机组(DFIG)在电网电压不平衡时,二倍频扰动分量会造成定转子过电流、功率脉动、转矩脉动等一系列电气和机械的问题,提出了新型DFIG-DVR系统,即串联DVR始终维持DFIG定子端电压恒定,从根源上隔离电网不平衡故障的影响,从而在整个故障运行过程中,DFIG仍可以实现转子侧变换器功率解耦控制和网侧变换器维持直流电压恒定的目标。采用PSCAD/EMTDC建立DFIG-DVR系统模型,对比分析了电网电压不平衡时DVR的不投切与投切对DFIG的影响。结果表明,在电网电压不平衡条件下,所提控制方案可以实现DFIG的平衡运行。     

Steady-state performance of DC motors supplied from photovoltaicgenerators with step-up converter

A simple step-up converter circuit consisting of a single power transistor and an inductor is used as an interface between a PV (photovoltaic) generator and a shunt DC motor driving a centrifugal water pump. The step-up converter allows maximum power output from the PV generator to the motor at all insolation levels. Steady-state performance of the motor is vastly improved as its input voltage and current are stabilized by the regenerative action of the converter. The PV generator operates at maximum power regardless of insolation variations. The converter duty ratio can be set at a fixed optimal value which is valid for all insolation levels. This remarkable property makes this device economically attractive since it is easy to build and does not require any insolation-dependent control as compared to other peak-power tracking devices     

基于PMU的发电机参数量测对电网输送能力提高的研究

针对目前难以获得同步发电机实际运行工况参数的问题,利用同步相量测量单元（PMU）能直接测量发电机的功角以及机端电压和电流相角的特点．计算获得发电机实际运行的同步参数xd。将该直接测量的发电机运行参数运用于电力系统仿真计算,可提高电力系统暂态功角稳定水平和输电能力,同时也提高了系统的静态功角稳定水平。通过算例表明了此方法的实用性和方便性。     

Review of the maximum power point tracking algorithms for stand-alone photovoltaic systems

A survey of the algorithms for seeking the maximum power point (MPP) is proposed. As has been shown, there are many ways of distinguishing and grouping methods that seek the MPP from a photovoltaic (PV) generator. However, in this article they are grouped as either direct or nondirect methods. The indirect methods (“quasi seeks”) have the particular feature that the MPP is estimated from the measures of the PV generator's voltage and current PV, the irradiance, or using empiric data, by mathematical expressions of numerical approximations. Therefore, the estimation is carried out for a specific PV generator installed in the system. Thus, they do not obtain the maximum power for any irradiance or temperature and none of them are able to obtain the MPP exactly. Subsequently, they are known as “quasi seeks”.Nevertheless, the direct methods (“true seeking methods”) can also be distinguished. They offer the advantage that they obtain the actual maximum power from the measures of the PV generator's voltage and current PV. In that case, they are suitable for any irradiance and temperature. All algorithms, direct and indirect, can be included in some of the DC/DC converters, Maximum power point trackings (MPPTs), for the stand-alone systems.     

A novel method to optimize photovoltaic generator operation

Photovoltaic (PV) energy is an alternative energy source for the future due to the world's limited energy resources. In this paper, a new technique to improve the performance of a PV array is presented. The study is based on the determination of the optimal configuration of a PV generator for a fixed number of modules. The aim of the study is to extract the maximum power from a PV generator connected in a direct coupling to a load. Different considerations (commutation current alone, commutation voltage alone, commutation current and voltage) are described. The presented method is based on microcontroller utilization. Very simple calculations allowed to decide, in real time, which configuration is appropriate for any load and any work conditions. Theoretical and experimental results are included. The effectiveness of the applied method is assessed by simulations and experiments. The method using commutation current and commutation voltage leads to good results unlike the other techniques. Copyright © 2008 John Wiley & Sons, Ltd.     

Differentiation of multiple maximum power points of partially shaded photovoltaic power generators

Partial shading conditions have a major effect on the electrical characteristics of photovoltaic (PV) power generators. In this paper, the effects of partial shading on maximum power points (MPPs) of a PV power generator have been systematically studied by using Simulink simulation model of a PV power generator composed of 18 series-connected PV modules. It is shown that the local MPPs can be classified into MPPs at low and high voltages based on the MPP operating point of the PV generator. The results also show that based on the MPP current and voltage it is possible to directly know if the MPP at high voltages is a local or a global MPP. The differentiation between local and global MPPs at high voltages is based on the voltage difference between the actual MPP voltage at high voltages and the theoretical MPP voltage under corresponding uniform conditions. This differentiation method was also tested to work correctly by utilizing experimental measurements of the Tampere University of Technology Solar PV Power Station Research Plant. By using this method, it can be identified if the system is operating at a local or a global MPP. This method can further be utilized to develop global MPP tracking algorithms.     

Accurate and fast convergence method for parameter estimation of PV generators based on three main points of the I–V curve

Currently there are different methods to estimate the parameters necessary to model a PV cell or generator. The simplest ones are based on reducing the number of parameters, disregarding some of them or assigning an initial value. These methods use simple equations, often obtained empirically, and very easy calculations. Usually, the results are simple approximations, for which reason they must be considered unreliable.On the contrary, methods that give good results are often characterized by their complexity. Generally they need a large amount of data, which in some cases require statistical processing, in addition to equations and laborious calculations.The purpose of this paper is to present a low complexity calculation procedure that allows estimating the ideality factor and the loss resistances of any PV cell or generator. The proposed method is based on the five-parameter model and as input data it only requires the open circuit voltage, the short circuit current and the maximum power point voltage and current. The results obtained with it allow simulating any PV module of crystalline silicon with great accuracy.     

Apparatus for supplying an isolated DC load from a variable-speedself-excited induction generator

This paper describes the design and laboratory testing of novel generation apparatus for supplying an isolated DC load from a self-excited induction generator operable at variable speed. The variable-speed generating apparatus consists of a self-excited induction machine, a controlled Graetz bridge rectifier, a voltage-boost power converter, and a control system. The induction generator supplies the rectifier. The voltage-boost power converter interfaces the variable output voltage of the rectifier to the fixed DC voltage required for the load. The rectifier is operated at levels of average DC current and voltage which control machine voltage to the rated AC voltage and which also draw the necessary power to supply the DC load. Performance is enhanced with respect to earlier apparatus in that both the DC voltage supplied to the load and the AC voltage on the machine are simultaneously controlled to fixed reference levels over broad operating ranges of load and speed     

Voltage-frequency control of a self-excited induction generator

A new strategy for controlling voltage and frequency of a self excited induction generator (SEIG) is presented. The SEIG operates in the linear region of the core magnetizing curve, so that efficiency and performance are upgraded. An external excitation circuit, comprising permanently connected capacitors and electronically switched inductances is used. The external circuit allows to compensate for the generator reactive demand. A detailed analysis is performed, showing some salient aspects related to the connection of the external excitation circuit on the control performance. Asynchronous switching is used, but some important considerations must be taken into account related to the instantaneous phase angle between stator voltage and external inductor current at the switching instant, if good transient response is desired. Sliding mode controllers are proposed, showing good dynamic response and robust behavior upon changes in load and generator parameters. Computer simulations are used to demonstrate the validity of the proposed scheme     

Control of a switched reluctance generator for variable-speed wind energy applications

This paper presents a novel control system for the operation of a switched reluctance generator (SRG) driven by a variable speed wind turbine. The SRG is controlled to drive a wind energy conversion system (WECS) to the point of maximum aerodynamic efficiency using closed loop control of the power output. In the medium and low speed range, the SRG phase current is regulated using pulsewidth-modulation (PWM) control of the magnetizing voltage. For high speeds the generator is controlled using a single pulse mode. In order to interface the SRG to the grid (or ac load) a voltage-source PWM inverter is used. A 2.5-kW experimental prototype has been constructed. Wind turbine characteristics are emulated using a cage induction machine drive. The performance of the system has been tested over the whole speed range using wind profiles and power impacts. Experimental results are presented confirming the system performance.     

基于磁流体发电机的波浪能最大功率追踪策略研究

为了提高某给定海试样机的波能转换效率,提出一种组合控制策略：采用爬山法确定最优参考电流,模型预测控制算法决定MOSFET开关管的关断跟随最优参考电流。当波高降低时,如果直接采用爬山法确定最优参考电流,扰动方向会判断错误,因此加入平均功率减小阈值的判断。仿真结果表明,改进后的爬山法无论波高上升还是下降都能正确判定扰动方向,能够随波高的任意变化自适应调整Boost电路的等效负载,使磁流体波浪发电机得以输出最大功率。最后,搭建硬件电路,利用直流电压源模拟磁流体波浪发电机,通过实验验证控制策略的有效性。     

Single-phase operation of a three-phase induction generator using a novel line current injection method

By using two capacitances and a current injection transformer, a three-phase induction generator can operate with good phase balance and line power factor while delivering power to a single-phase power grid. This paper presents a systematic analysis on this novel induction generator configuration. The solution of the system's inspection equations using the method of symmetrical components enables the steady-state generator performance at any speed to be computed. The conditions for achieving perfect phase balance are deduced from the phasor diagram. It is shown that the capacitances that result in perfect phase balance depend on the generator admittance, power factor angle, as well as the turns-ratio of the current injection transformer. Where possible, the computed results are verified by experiments conducted on a 2-kW induction machine. An experimental investigation on the system waveforms and harmonics is also carried out.     

用于海上风电场直流输电的新型变换器

针对永磁直驱风力发电机变流技术的特点,并根据柔性直流输电系统的电压和功率控制要求,提出了一种新型的直流变换器。该变流器采用三电平拓扑结构、内环电流峰值控制,以及适用于三电平Boost变换器电流峰值控制的双梯形波补偿控制方法,经Matlab／Simulink仿真,研究结果证明该变流器具有功率开关电压应力小、电抗器电流脉动小,以及运行可靠、动态响应性能好等优点,适用于海上风电柔性直流输电等大功率、高电压场合。     

Simulations and experiments on a 12 kW direct driven PM synchronous generator for wind power

A direct driven permanent magnet (PM) synchronous generator has been designed and constructed and results from the first experimental tests are presented. The generator has been designed using the finite element method (FEM) and dynamic simulations have been performed to study the generator. The simulations are performed by using an electromagnetic model, which is described by a combined field and circuit equation model and is solved in a finite element environment. The stator winding of the generator consists of circular cables and the rotor has surface mounted, arched PMs. A complete experimental setup has been constructed consisting of a motor, a frequency converter, a gearbox and electrical loads. Oscilloscopes are used to measure the voltage and the current for each phase. Measurements have been performed for both full load and no load at rated speed. The harmonic content of the voltage is analyzed and compared to results from simulations. Furthermore, the generated electric power has been calculated from knowing the voltage and current and is compared to the simulated power. The agreement between experimental results and results from simulations based on finite element calculations is very high, especially considering harmonics. Several sources of error are suggested that could cause the small differences between the simulated results and the measured data for the constructed generator.     

Real time estimation of photovoltaic modules characteristics and its application to maximum power point operation

In this paper, an approximate curve fitting method for photovoltaic modules is presented. The operation is based on solving a simple solar cell electrical model by a microcontroller in real time. Only four voltage and current coordinates are needed to obtain the solar module parameters and set its operation at maximum power in any conditions of illumination and temperature. Despite its simplicity, this method is suitable for low cost real time applications, as control loop reference generator in photovoltaic maximum power point circuits. The theory that supports the estimator together with simulations and experimental results are presented.     

小容量同步发电机组谐波特性分析

相对于大电网,小容量系统惯性小、承受扰动的能力弱,因此小容量同步发电机组受谐波影响较大。为揭示谐波的作用机理,从注入谐波电流后电机内部的电磁关系变化人手,描述了谐波电流形成的旋转磁场与基波磁场叠加后的形态。进而分析了谐波电流对发电机输出电压的影响,并运用瞬时功率理论推导了谐波转矩的解析式,指出了发电机组转轴机械振动的原因。进行了柴油发电机组带非线性负载的实验,观察了机组在谐波影响下的运行状态,实验结果与理论分析一致。     

低压配电网不平衡和谐波问题的综合治理

针对低压配电网三相电流不平衡及谐波而导致三相电压不平衡、线损和变压器损耗增加、中性点电压偏移等问题,提出了基于晶闸管投切电容器和有源滤波器的综合治理方法,给出了利用晶闸管投切电容器进行“粗补”、有源滤波器进行“微补”的小容量综合治理装置及其拓扑结构和控制方法,并提出采用“变P法”和“限无功法”对有源滤波器进行保护。实例仿真结果表明,该综合治理方案行之有效。     

Performance of self-excited single-phase induction generators withshunt, short-shunt and long-shunt excitation connections

The influence of three capacitor excitation topologies (shunt, short-shunt and long-shunt) on the steady-state and dynamic performance of a single-phase, self-excited induction generator is explored in this paper. Attention is focused on the influence of the different capacitor connections on the generator overloaded and output power capabilities. The generator voltage with shunt excitation connection collapses when overloaded, while with either the long or short-shunt excitation connection the generator is able to sustain the load at a lower operating voltage and larger load current     

State space formulation and transient stability of the doubleoutput asynchronous generator

The double output asynchronous generator state space formulation by using the small displacement procedure valid for both grid connected and for autonomous systems is deduced. The study of the dynamics shows that the transient stability is influenced by disturbances of speed of load and of excitation frequency for any RMS value and power factor of the excitation voltage. Because of its linearity, the small displacement model in state space form can be used for control design purposes. From the matrices in generalized form, the transfer functions needed for the design of frequency and current controllers are easily computed