Analysis for Effects of Load Characteristics on Power System Voltage Stability

This paper investigates on-load tap changer OLTC effect on voltage stability. Chaos theory used in this analysis is via the calculation of the maximum power transmission. Resulted chaotic behavior is exhibited by singular induced bifurcation which clearly shows the voltage stability of the power system with OLTC from a singular induced bifurcation point of view. The paper is organized as follows: 1) increasing in the maximum reactive power via increasing the ratio of OLTC and the possibility of voltage collapse decrease under the same load command. 2) Noting that the voltage instability is directly affected by the load characteristics, shown via the effect of OLTC on the voltage instability under different static load models. 3) Studying voltage stability in state space and the effects of the generator excitation current limit, the OLTC and load dynamics on voltage collapse. In addition, a small-disturbance voltage stability region of a power system is constructed. The voltage stability of the system is determined by the dynamic characteristics of both the OLTC and the load.

An innovative technique to evaluate network equivalent for voltage stability assessment in a widespread sub-grid system

Gerilim kararlılığı, güç sistemlerinde incelenmesi gereken önemli bir konudur. Sürekli artan elektrik enerjisi talebi, artan talebi karşılamak için yeni elektrik üretim santrallerinin kurulumu ve güç sistem arızaları gerilim kararlılığını olumsuz yönde etkilemektedir. Günümüzde gerilim kararlılığını iyileştirmek için güç sistemlerinde Esnek Alternatif Akım İletim Sistemleri (FACTS cihazları) kullanılmakta olup, ayrıca enerji talebini karşılamak ve gerilim kararlılığını iyileştirmek için güç sistemlerine Dağıtık Üretim Kaynakları (DGs) entegre edilmektedir. Bu çalışmada, IEEE 30 baralı güç sistemi ile Kuzeybatı Anadolu (KBA) güç sistemi, gerilim kararlılığı bakımından temel (nominal), aşırı yüklenme ve hat kopması durumlarında incelenmiştir. İncelenen güç sistemlerine FACTS cihazlarından Statik Var Kompanzatör (SVC) ve Tristör Kontrollü Seri Kapasitör (TCSC), DGs’lerden ise Yakıt Hücresi (YH), Güneş Enerji Santrali (GES) ve Rüzgâr Enerji Santrali (RES) ayrı ayrı ve birlikte bağlanmıştır. Güç sistemlerinin bara gerilim değerlerinde ve yüklenme parametresinde meydana gelen değişimler analiz edilmiştir.

This paper analyzes the impact of inter-connection of large-scale wind power on the voltage stability of power system. First it presents three different voltage indexes obtained with the power flow equation. Then, it discusses the impacts with different penetration rates and interconnection locations. Simulation with typical power system shows that different indexes vary a lot with the interconnection of wind power and the impacts are different with respect to different locations.

Integrating large-capacity wind power into a power system has a great impact on the voltage stability of a power system. Especially, a fault in such a system can result in the tripping of a large amount of wind power generation or system collapse. This article studies the effect of large-capacity wind power generation on power system voltage stability under a fault condition. The adopted wind power model consists mainly of a wind turbine, drive device, asynchronous generator, and control system. The proposed analysis approach is applied to a practical power system with wind power, which is located in Gansu province in China. Through simulation and analysis, the characteristic and stability principle of an asynchronous wind generator group are recognized. A speed increase in a wind generator group is closely related to voltage drop after fault, which may cause voltage instability if they both impact each other after fault. The detailed results are presented in the article.

The voltage stability problem in long-distance transmission systems can be exacerbated by geomagnetic disturbance (GMD) even in middle and low latitude areas where the effects of GMDs are considered to be mild compared to auroral areas. With the high voltage and the long-distance transmission lines, power system in China has to face the voltage instability risk. To clarify and measure the risk from GMD represented by geoelectric field, method for analysis of relationship between voltage stability of the long-distance transmission system and the size and direction of geoelectric field is provided. On the basis of calculation for geomagnetically induced currents (GICs) of power system and the additional reactive power losses of transformer due to GIC, the model of long-distance transmission line affected by geoelectric field is established. To measure the impact on the voltage stability of power system and the sensitivity of voltage to geoelectric field, the voltage stability index and the set of voltage limit violation nodes are proposed and calculated considering different geoelectric field and different initial operation conditions of power system. By taking the Northwest 750 kV power system in China as an example, voltage stability with geoelectric field magnitude from 1 V/km to 10 V/km, direction (0°) from north to south (180°) is analyzed and the voltage stability index is calculated, and the set of voltage limit violation nodes are summarized as well. The results show that the method is feasible and the index can reflect actually the relationship between the long-distance transmission system voltage stability and the geoelectric field, and the set of the voltage limit violation nodes can indicate which nodes are most susceptible to GMD.

Coordinated voltage control of wind-penetrated power systems via state feedback control

Power system voltage instability is major threats to reliable operation of power system networks. Nowadays maintaining a stable power system is therefore a challenging issue. If we consider economic and environmental concerns, grid tied renewable energy-based generation are growing over the past few years. These kind of generation can have both advantages and disadvantages. The most commonly used renewable energy resource is solar photovoltaic. This paper mainly focuses on voltage stability analysis of power network when penetrated with solar photovoltaic (PV). For the investigation, a comprehensive model of solar PV penetration in IEEE-14 bus test system has been used. Load flow study has been performed to analyse the voltage stability of the system. Critical eigenvalues of Q-V modal matrix is used to examine the effect of PV generator on voltage stability of power system. The effects of scattered and concentrated PV penetration on power network are also analyzed in this paper.

Aiming at the influence of the connection of pumped storage power station to power grid on voltage stability, the impedance modulus margin defined by non-linear equivalent analysis method is used as the evaluation index of system voltage stability, and a method for evaluating power system voltage stability considering pumped storage power station is proposed. Considering the peak-cutting function of pumped storage power station, the calculation method of general impedance modulus margin of load nodes is derived. The general impedance modulus margin of load nodes is calculated by changing the output and access position of pumped storage power station, and its influence on the voltage stability of power system is analyzed. Coordinating the access position and output of pumped storage power station to improve the voltage stability of the system. Finally, through the simulation calculation on the standard node system, the feasibility of the project is verified.

The rapid growing in electrical load, and increasing the dependency on renewable energy recourses; highly affect the Jordanian power system stability and operation. This paper presents a static voltage stability analysis for the northern part of the Jordanian grid. The main purpose of this study is finding out the domain of the system from facing voltage instability conditions. In order to analyze voltage stability of the study system, static analysis techniques such as P-V curve and Q-V curve methods are applied. The DIgSILENT PowerFactory® program is used in this study to obtain the results.

Enhancing voltage stability and LVRT capability of a wind-integrated power system using a fuzzy-based SVC

Because of energy and environmental policies in the last decade, a lot of photovoltaic generations will be installed into power systems in Japan in the future. In this situation, it is of great concern that disconnection of PV due to voltage drop caused by a system fault has an impact on power system stability. To avoid its disconnection, Fault Ride Through (FRT) requirement is applied for PVs in Japan. In this paper, transient stability and voltage stability of a multi-machine power system with a large penetration of PV considering several fault ride through requirements and dynamic load characteristics are discussed. The simulation results show that t he fault ride through requirements for PVs don't necessarily improve the power system stability.

In February 1998 a new and unique form of power generator, the Powerformer TM , was introduced into the marketplace. The Powerformer can be operated at transmission system level voltage without the need for a step up transformer and consequently has unique system support capabilities. The aim of the research work, presented in this thesis, was to investigate the unique aspects of the Powerformer from a power system perspective, in order to facilitate optimal power system operation and security.The main focus of the research undertaken by the author of this thesis was to investigate the impact of Powerformer on the relative voltage stability of a power system. The driving force behind the author's desire to look at the impact of the Powerformer on power system voltage stability and security was the realisation that voltage stability issues are becoming increasingly important. The focus for today's modem power transmission systems is undergoing a radical shift from a purely publicly provided service to a financially driven commodity. This ultimately means that power systems are increasingly under pressure to be utilised to their fullest capacity without the need to install new, expensive and sometimes socially unpopular system equipment and facilities. The problem with this situation is that a system's stability can be affected greatly by loading conditions. When a system is heavily loaded the voltage stability limit, rather than the transient angle stability limit constrains its operation. It is for this reason that voltage stability concerns in particular are increasingly being discussed in the literature and becoming an issue for power system operators. A number of different static and dynamic system analysis techniques have been reviewed and compared in this thesis, with the results of these comparisons presented and discussed in this thesis. Amongst the methods discussed in this thesis, methods for use in voltage stability analysis and assessment have been proposed and presented by the author of this thesis. The most suitable techniques are being utilised to determine how the Powerformer can aid in safe system operation under a number of different system contingencies in a number of different test systems, including the 600+ bus Queensland Transmission System. Particular emphasis has been placed in this research on long-term system dynamics. Two major aspects of the Powerformer that differ it from conventional are the lack of a step up transformer and the ability of the Powerformer stator to be overloaded for longer periods than conventional power generators. The lack of a step up transformer means that power that would normally be lost can be supplied to the system and that the high voltage bus, normally on the opposite side of the step up transformer, is directly controlled. The overload capability of the Powerformer means that it can supply reactive power support to the power system for a longer period than a conventional generator. Investigations undertaken into the Powerformer have shown it to have a potentially beneficial ability to allow greater system loading capability and therefore system security margins and a potentially beneficial impact on the time to voltage collapse following a system contingency.

The subject of the paper is the influence of enhanced generator excitation control on voltage and angular stability of a power system. A simple, six busbar power system model is considered which has two distinct oscillatory modes that are of frequencies relevant to the UK network. Dynamic performance of the system is assessed through time response analysis for both small and large disturbances. The recently developed generator transfer function regulator (GTFR) is used as a part of a generator excitation control system. The performance of generators with enhanced excitation control is compared with that of a conventional scheme. It is demonstrated that the GTFR based excitation system exhibits superior control performance, particularly as limiting conditions are approached. The proposed scheme also extends the range of stable operation of the system.

The author considers a nonlinear system at a quasistatically varying, stable equilibrium encountering a linear constraint on the system state and studies the consequent change in system stability. The author formulates the problem of a nonlinear vector field encountering a linear constraint and notes that the stability of the constrained system is usually determined by the stability of the constrained linearization of the system. Therefore he studies the stability of a linear system encountering a linear constraint and gives conditions on the constraint guaranteeing stability or a certain change in the degree of stability when the constraint is encountered. The objective is to study the stability of the constrained system directly from the stability of the system and the constraint without explicitly determining the constrained system. This allows analytic study of the process of encountering a constraint. An application to improving the voltage stability of an electric power system near voltage collapse by selectively locking tap-changing transformers is briefly discussed. >