网压非对称条件下双馈风电系统的PS-VS及NS-VR综合控制分析

当前诸多文献研究了电网电压对称跌落状态下双馈风力发电系统的穿越控制策略,但是电网发生非对称跌落的概率约占网压跌落事件数的80％。该文提出一种电网电压非对称条件下的双馈系统优化动态综合控制策略。分别建立了正序电压支撑（positive sequence voltage support,PS．vs）及负序电压抑SO（negative sequence voltage restrain,NS-VR）的控制模型,并且在变流器的输出电压中将正序电压控制分量与负序电压控制分量给予合成。此控制策略实现了电网电压不对称故障条件下风电场电网公共接入点电压质量的显著提升。论文研究了根据电网电压跌落幅度对PS．VS子功能与NS—VR子功能的容量分配算法。通过Matlab／Simulink对PS．VS控制及NS．VR控制的综合控制仿真结果表明,该控制策略实现了风电场电网电压非对称状态下双馈风力发电系统并网条件的明显提升。     

TRANSIENT STABILITY SIMULATION OF UNBALANCED LARGE SCALE POWER SYSTEMS

ABSTRACT

Conventional transient stability analysis is performed on the basis of a balanced system representation. However, a typical power system may be unbalanced due to a variety of reasons, and the computations of sequence quantities due to these unbalances is necessary in order to facilitate the application of countermeasures. This paper presents an algorithm for the simulation of large scale power system dynamics under unbalanced operating conditions. This algorithm finds potential application in the simulation of systems with abnormal loads such as arc furnace and traction, and also in the realistic simulation of unbalanced faults in bulk power transmission systems.     

A New Affine Arithmetic-Based Optimal Network Reconfiguration to Minimize Losses in a Distribution System Considering Uncertainty Using Binary Particle Swarm Optimization

Abstract

In the present work, Binary Particle Swarm Optimization (BPSO) based optimal re-configuration for balanced and unbalanced radial distribution networks using Affine Arithmetic (AA), with uncertainty in generation and load, is proposed to minimize the system losses. An expression for three phase real affine power loss is derived with partial deviations of real power loss in lines with respect to power injections in other buses and also with respect to power injections in other phases in case of unbalanced distribution systems. The major contribution of the present work is the application of AA based optimal network reconfiguration, to both balanced and unbalanced radial distribution networks with uncertainty. The proposed method is tested on IEEE 16, 33, 85 and 119 bus balanced distribution systems and an unbalanced 123 bus system with Distributed Generation (DG) connected at some buses. The optimal loss intervals obtained by the proposed method are compared with that obtained by Interval Arithmetic (IA) and Monte Carlo (MC) simulations based methods. The simulation results show that proposed AA based analysis gives an optimal reconfiguration, for both balanced and unbalanced radial distribution systems with uncertainty as compared to existing IA based method.     

Generalized Mathematical Modeling of N-Phase Power Systems

ABSTRACT

The analysis of the polyphase power systems is normally carried out by assuming a perfectly balanced system. However, power system is generally unbalanced because of various reasons. Therefore, it is desirable to analyse the power system taking imbalances into account. Hence, it is necessary to develop mathematical formulations of various components of the power systems in phasor co-ordinates. In this paper, detailed mathematical models for general n-phase components have been developed. These models will be useful for conducting the steady state as well as the transient analysis of the multiphase systems in phasor co-ordinates. Finally, the paper concludes with the discussion on the findings based upon these models.     

A Multi-objective Optimization for Sizing and Placement of Voltage-controlled Distributed Generation Using Supervised Big Bang–Big Crunch Method

Abstract—This article presents an efficient multi-objective optimization approach based on the supervised big bang–big crunch method for optimal planning of dispatchable distributed generator. The proposed approach aims to enhance the system performance indices by optimal sizing and placement of distributed generators connected to balanced/unbalanced distribution networks. The distributed generation units in the proposed algorithms are modeled as a voltage-controlled node with the flexibility to be converted to a constant power node in the case of reactive power limit violation. The proposed algorithm is implemented in the MATLAB (The MathWorks, Natick, Massachusetts, USA) environment, and the simulation studies are performed on IEEE 69-bus and IEEE 123-node distribution test systems. Validation of the proposed method is done by comparing the results with published results obtained from other competing methods, and the consequent discussions prove the effectiveness of the proposed approach.     

基于功率原理的新型发电机不对称故障保护

基于两相坐标系统,对发电机内部不对称故障进行研究发现,瞬时功率中不平衡功率增加而平衡功率下降,解释了该现象的物理原因,提出了瞬时功率不对称度定义,其大小为不平衡功率与平衡功率的比值,且该值能灵敏反映发电机内部不对称故障.以瞬时功率不对称度为主要判据,结合发电机外部故障时不平衡功率相位会出现反相突变的特点提出了一种新型发电机不对称故障保护.动模试验结果表明,该保护能切除少匝数不对称短路故障,对大匝数故障、异相短路故障和振荡中故障均有很高的灵敏度和可靠性;而且接线简单,易与其他类型保护集成互补使用.     

ANALYTICAL APPROACH TO THE OPERATION OF A SYNCHRONOUS MACHINE ASSOCIATED WITH A THYRISTOR BRIDGE

ABSTRACT

A synchronous machine associated with a thyristor bridge operates normally under unbalanced conditions which are described and studied in the present paper. Natural cut-off thyristors is used in the present system where the gates of thyristors are controlled by the rotor position. It is assumed that the machine has salient poles and damper windings. Both motor and generator operations are contemplated.

An analytical approach has been used, and the exact shape of the currents has been considered. The commutation problem is studied in detail, taking into consideration the transient effects of the armature circuits and the limited value of the smoothing reactor. The different expressions are given in terms of the classical machine constants which can be directly measured or furnished by the manufacturer. Comparison between theoretical and practical results are given.     

电网不平衡下虚拟同步发电机功率-电流协调控制策略

虚拟同步发电机(Virtual Synchronous Generator, VSG)技术通过模拟同步发电机运行机理,在理想电网条件下能够使分布式电源友好接入电网。然而,在不平衡电网电压下,则会出现输出功率振荡、电流不平衡等问题。为克服这些问题,提出了一种基于静止坐标系下虚拟同步发电机功率-电流协调控制策略。该控制策略根据瞬时功率理论计算负序电流参考值,建立了恒定有功、无功及电流平衡3个控制目标的统一解析表达式,实现功率-电流的协调控制,提高了系统运行性能。最后仿真结果验证了所提控制策略的有效性。     

Bilinear Control for Three-Phase Microgrids: A Proportional-Integral Passivity-Based Design

Abstract

A bilinear PI control based on passivity theory for the adequate integration of distributed energy resources (DERs) in ac microgrids is presented in this paper. DERs are integrated into the grid by voltage source converters (VSC), the most common and suitable technology for this type of application. The proposed control guarantees asymptotically stable operation for the dynamical system under closed-loop operating scenarios via Hamiltonian and Lyapunov formulations. ZP load models and π-model of the transmission lines are considered in the stability analysis of the microgrid. Conventional PI control is also implemented for comparative purposes. Simulation results in Matlab/Simulink demonstrate the effectiveness and stability of the proposed control’s performance in a radial microgrid composed of a photovoltaic generator, a supercapacitor energy storage (SCES) system and unbalanced loads.     

The effects of unbalanced networks on synchronous and asynchronous machine transient stability

Untransposed lines and feeders, together with single-phase loads, impose permanent unbalanced conditions on the rest of the power system. Unbalanced phase voltages at the terminals of a machine can be represented as balanced sets of positive-, negative- and zero-sequence voltages. The negative-sequence voltage gives rise to negative-sequence current which creates excessive heating, particularly in the rotor circuits. Much attention has been paid to the heating aspect, and protection provided against this, by limiting the levels of negative-sequence current; however, power system stability studies, in particular for multimachine systems, have until now ignored the imbalance and its effects upon the dynamic response of the rotor speed of each machine. This paper reconsiders the transient stability of a single synchronous generator connected to an infinite bus and evaluates the significance of the error incurred by representing the unbalanced line and shunt load by an equivalent balanced system. The paper also reconsiders the transient stability of a multimachine system containing a synchronous generator and an induction motor when subjected to different types and degrees of imbalance and finds that significant errors are incurred under certain conditions.     

Generalized modeling and unbalanced fault simulation of saturated self-excited induction generators

The paper presents a generalized unified electromechanical current and flux models of self-excited induction generators. A k-factor saturation method is used to account for magnetic saturation in the previous model. A neutral connection through impedance is included for equivalent Park models of wye connected induction generators. Numerical modeling of self-excited induction generator unbalanced transients according to machine grounding systems is proposed. To illustrate application examples, transient performances during unbalanced short-circuit tests of single-cage, double-cage and deep-bar rotor type induction generators are simulated and discussed.     

基于动态电压补偿的VSG电流平衡及峰值电流控制

虚拟同步发电机(VSG)能够实现新能源机组友好并网。然而,传统的VSG技术主要适用于电网电压平衡工况,这使得VSG在电压不平衡情况下面临输出电流不平衡及过流等问题,为此提出一种基于动态电压补偿的VSG平衡电流控制架构及方法。通过负序电流抑制和峰值电流抑制策略,分别生成对应的补偿电压,使得VSG在电压不平衡时仍能输出平衡电流,且可抑制电网电压跌落瞬间的暂态冲击,以及确保稳态运行时的电流不超过安全阈值。同时,所提控制方法并未改变VSG等效为电压源的属性,保留了VSG的电压支撑能力。不同工况下的仿真测试结果验证了所提出控制方法的正确性和有效性。     

DIGITAL SIMULATION OF A RECTIFIER LOADED SYNCHRONOUS MACHINE

ABSTRACT

The digital computer simulation of a rectifier loaded synchronous generator is described in this paper. The digital model of the synchronous generator with either a controlled or uncontrolled rectifier bridge deals efficiently with the complex commutation conditions when the source impedance is significant. From the equations developed in this paper, the performance of synchronous generators with or without damper windings, connected to controlled or uncontrolled bridge-rectifier loads, can be predicted from given loading data, field current, load current, generator speed and bridge delay angle. The approach described in this paper is valid for any bridge topology, and analysis of abnormal fault conditions presents no difficulties once the appropriate transformation matrix is available. The numerical analysis includes calculation of the performance of a synchronous generator with rectifier bridge loads for a variety of operating conditions.     

Distribution feeder line models

Starting with a modified version of Carson's equations, this paper develops distribution feeder models that can be used in the power flow and short circuit analysis of unbalanced three-phase distribution feeders. A simple line section is used to demonstrate the differences in voltage drops that occur between the various models for load conditions varying from balanced to very unbalanced     

Modeling of unbalanced three‐phase driving‐point impedance with application to control of grid‐connected power converters

The dq transformation is widely used in the analysis and control of three‐phase symmetrical and balanced systems. The transformation is the real counterpart of the complex transformations derived from the symmetrical component theory. The widespread distributed generation and dynamically connected unbalanced loads in a three‐phase system inherently create unbalanced voltages to the point of common coupling. The unbalanced voltages will always be transformed as coupled positive‐sequence and negative‐sequence components with double‐frequency ripples that can be removed by some filtering algorithms in the dq frame. However, a technique for modeling unbalanced three‐phase impedance between voltages and currents of same sequences or of opposite sequences is still missing. We propose an effective method for modeling unbalanced three‐phase impedance using a decoupled zero‐sequence impedance and two interacting positive‐sequence and negative‐sequence balanced impedances in the dq frame. The proposed method can decompose a system with unbalanced resistance, inductance, or capacitance into a combination of independent reciprocal bases (IRB). Each IRB basis belongs to one of the positive‐sequence, negative‐sequence, or zero‐sequence system components to facilitate further analysis. The effectiveness of this approach is verified with a case study of an unbalanced load and another case study of an unbalanced voltage compensator in a microgrid application. Copyright © 2015 John Wiley & Sons, Ltd.     

Characteristics of three-phase induction motors under unbalanced rotor impedances

An analysis is developed for three-phase induction motors with balanced or unbalanced rotor impedances. The validity of this analysis is verified by comparing its results, under balanced conditions and in the steady state, with those obtained using conventional methods. The method is also verified experimentally for a 7.3 kW, 220 V, four-pole, 50-cycle slip-ring induction motor with balanced or unbalanced rotor impedances. The analysis is then used to study the characteristics of three-phase induction motors under unbalanced rotor impedances.     

Provision of Balanced Voltages Under Severe Unbalanced Load Currents in a Microgrid Using a 3D-SVM Technique

Abstract

The current source inverter (CSI) is more suitable for use in photovoltaic applications than the voltage source inverter (VSI), but this structure has two main constrains: inability to feed unbalanced load and high common mode voltage (CMV). Despite the merits, use of CSI is limited due to these constrains. A three-phase CSI with a neutral-leg is presented in this paper, compared to the conventional three-phase CSI to overcome mentioned limitations, in which load unbalance under a PV system is dealt in a microgrid. An innovative three-dimensional space vector modulation (3D-SVM) is suggested for the four-leg CSI inverter. This novel 3D-SVM technique is very simple compared to the existing methods and computational complexity is too low. Operating principles, equivalent circuit and SVM is discussed for the four-leg inverter. Then, a balancing technique is applied to the four-leg inverter using the developed 3D-SVM, where simulations validate the capability of the inverter in supplying balanced load voltages; 4-leg inverter generates balanced sinusoidal voltages even under severe unbalanced load current.     

不平衡负载下三相离网逆变器的序阻抗建模及特性分析

孤岛微电网中三相负载不平衡是引起微电网电压不对称的主要原因。为了清晰直观地分析和改善电压不对称问题,从序阻抗角度,采用一种谐波线性化方法,建立了不平衡负载下三相离网逆变器采用无正负序分离电压控制模式的正负序阻抗模型,在不同PI控制参数、不同的运行工况下对三相离网逆变器的正负序阻抗特性进行了对比分析。分析表明PI控制参数对正负序阻抗特性影响较大,运行工况产生的影响较小。然后在三种不同运行工况下,对单序dq控制模式和无正负序分离电压控制模式进行了仿真对比分析。得到的序阻抗特性曲线和输出波形表明:采用单序dq电压控制模式时离网逆变器的输出电压幅值不平衡度高,而采用无正负序分离电压控制模式时,由于三相离网逆变器的负序等效输出阻抗在基频下较小,不平衡负载引起的负序电流流过阻抗产生的压降也较小,使逆变器的输出电压幅值基本保持平衡。最后,仿真验证了无正负序分离电压控制模式下的序阻抗模型的有效性。     

Mathematical modeling and analysis of brushless doubly fed reluctance generator under unbalanced grid voltage condition

The brushless doubly fed reluctance generator (BDFRG) can be an attractive choice for wind power application where mostly located at remote areas with unbalanced grid voltages, however there is not any study on the BDFRG under this condition up to now. In this paper, a mathematical model for the BDFRG under unbalanced grid voltage condition has been developed. Its equivalent circuit in dq reference frames has been extracted, and the torque and power equations have been derived, all based on the positive and negative sequence components. Also, a real-time separation method has been proposed to separate the BDFRG positive and negative sequences. The developed model and equations express the basis of the BDFRG operation under unbalanced grid voltage condition. The proposed model is simulated in MATLAB/Simulink software and the accuracy of the proposed model and equations has been validated by comparison with simulation results of an existing BDFRG model under balanced condition, which has already been experimentally verified in the literature.     

Ground-Fault Protection of an Emergency or Stand-by Power System with Multiple Neutral-to-Ground Connections

SYNPOSIS

In the operation of an engine-generator set, when used for emergency power, it is required that the neutral conductor be connected to the nearest available effectively grounded electrode. Therefore, any three-phase, four-wire system with properly grounded engine generator sets will have multiple neutral-to-ground connections. One connection will be at the service entrance with the other connection at each engine-generator set.

When an electrical system has multiple neutral-to-ground connections, there may be problems in obtaining proper sensing of ground fault currents as well as nuisance tripping with unbalanced loads. These problems persist with the system in either its normal or emergency supply modes of operation or both. This paper discusses some of these problems and the methods of solution. A new and special ground-fault sensing scheme which can overcome these problems for such a multiple neutral-to-ground system is also presented.