Zhao Jinli

An improved power system stability criterion with multiple time delays

Since there exist significant time delays in the data of phasor measurement unit (PMU) and wide-area measurement system (WAMS), to evaluate their impact is very important for power system online stability assessment and controller design in the wide-area environment. In this paper, we use Lyapunov stability theory and linear matrix inequality (LMI) method to analyze the impact of time delays on power system stability. An improved delay-dependent stability criterion for power system stability analysis with multiple time delays is presented. Based on Lyapunov-Krasovskii theory, a proper Lyapunov functional is firstly constructed. And, in deduction of its derivative function along the system trajectory, some necessary slack variables are introduced so as to reduce the method conservativeness. Then the derivative function is expressed as a set of linear matrix inequalities, which can be easily solved with LMI toolboxes in Matlab or Scilab. Finally, a typical two-dimension time-delayed system and WSCC 3-generator-9-bus system with two time delays are employed to validate the method effectiveness. It is revealed that the presented method is correct, effective and with less conservativeness. Work of this paper is helpful for power system stability assessment and control with considering time delays under wide-area environment.

Network reconfiguration for distribution system with micro-grids

Nowadays, technologies of distributed generation (DG) and distributed energy resource (DER) are developing rapidly. It has been demonstrated to be more reliable and economical that DGs or DERs are integrated into the traditional power grid based on micro-grid. So, more and more micro-grids will occur in distribution system in the future. In this paper, we mainly concern and discuss the impact of micro-grids on the distribution network reconfiguration. A model suitable for reconfiguration of distribution system with micro-grids is presented in this paper. Once a fault occurs in a distribution system, it can be applied to construct some islands so as to guarantee the power supply for some important customers and to reduce the system power loss at the same time. The problem is decomposed into a capacity subproblem and a reconfiguration sub-problem. The former is used to determine the optimal capacity for each island, and the latter is used to find the optimal reconfiguration with less power loss. They are called iteratively to get the optimal solution. Finally, some typical systems are employed to validate the effectiveness of the presented method.

Voltage stability control based on real power flow tracing

After some disastrous instabilities and blackouts, power system voltage stability has drawn even more attention. When a serious fault occurs, proper control should be quickly adopted to terminate the system deterioration. In this paper, a control method based on power flow tracing is presented to enhance power system voltage stability. Firstly, a branch-admittance-decreasing algorithm is introduced to determine the minimum power cutting after a serious fault. Power flow tracing is then used to allocate the power to load and generator buses so as to get the corresponding load shedding and generation tripping. Comparing with the previous methods based on power flow sensitivity, the presented method can appropriately consider influence of some nonlinearities in power system, such as the discrete tap changing of OLTC, exciter hard-limit. Since power flow tracing is mainly used in the congestion management, allocation of power loss and transmission charges in the deregulation study, the presented method is compatible to the power system operation under deregulation. Finally, New-England system and IEEE 118-bus system are employed to verify the effectiveness of the presented method.

A LMI based approach to power system stability analysis with time delay

Delay margin means the maximum delayed time that power system can sustain without losing its stability. Since, there exists significant time delay in the data of phasor measurement unit (PMU) and wide-area measurement system (WAMS), to evaluate system delay margin is very important for power system online stability assessment and controller design in the wide-area environment. In this paper, we use Lyapunov stability theory and linear matrix inequality (LMI) method to evaluate power system delay margin. Lyapunov-Krasovskii criterion is firstly introduced to construct a suitable Lyapunov functional. Its derivative along the system trajectory is then expressed as a set of LMIs, which can be easily solved with LMI toolbox in Matlab or Scilab to obtain the system delay margin. In order to reduce conservativeness of the method, some slack variables are introduced in the derivation. Comparing with some existing numerical methods, such as eigenvalue based method, root clustering paradigm method, the presented method has a little conservativeness. However, it has much more flexibility and applicability in stability analysis to the time variant, nonlinear, switched and uncertain time delay systems. Finally, single-machine-infinite-bus (SMIB) system and WSCC 3-generator-9-bus system are employed to validate the effectiveness of the method.

Equivalent network for determination of voltage stability limit

In order to improve the efficiency of power system voltage stability analysis a new static equivalent method is presented in this paper including the selection of the correlation area using L/sub 1/ index described in detail. The correlation area selected from the external area is composed of generators, buses, transformers and loads. The proposed equivalent method is a kind of modified Ward-type equivalent. This method can increase the computation velocity and remain the precision of the voltage stability limits. Test results of a real power system have proved that this new equivalent method is promising for voltage stability study of power system.

Implementation of power system security check and its visualization system

Nowadays, with the introduction of competitive electricity markets, large scale interconnected power grids are operated closer to their operation limits, and power system operators often have a dual and conflicting responsibility for system security and for profit maximization. Insuring power grid operating safely and stably under complicated operating conditions has become a rigorous challenge to electric utilities. This paper discusses some online security check technologies, and presents a power system security check software package implemented with advanced visualization technologies. The application of power system security check is based on CIM model so as to facilitate the passing of power system data with other applications.

Data integration algorithm of power system security check based on CIM model

During the application of power system security check, a variety of raw data acquired from different sources, including the information of current operation state, day-ahead forecasting and scheduling etc., should be integrated to generate the cross-sections dataset describing the future operation. The original power system modeling information is exported by SCADA/EMS in CIM format. Furthermore, the problem of translating, accessing and integrating data into the proprietary format for the application of power system security check is challenging, especially the problem of integrating data representing day-ahead power system. This paper explores the problem above and discusses solutions to some challenges in data integration. Finally, the generation of data files in the PSS/E format is implemented.

Fast assessment of regional voltage stability based on WAMS

Voltage stability margin of regional network is not a fixed quantity but closely related to operation condition and operation constrains. This paper proposes an effective method to estimate the real-time voltage stability margin based on some locally synchrophasor measurements. A Radial Basis Functions (RBF) neural network is employed to estimate the regional margin. The validity of the method is illustrated by case studies on the IEEE 30-bus system.

Assessment method of dynamic reactive power valuation for transient voltage stability

Dynamic reactive power sources can make response for reactive power demand rapidly in order to improve power system transient voltage stability. As reactive power is a local product, its value to transient voltage support very much depends on the location in the power system. Meanwhile, the value of reactive power compensation at the same location is nonlinear as well. It has resulted in a great need to quantify the value of reactive power support from dynamic reactive power sources for transient voltage stability. This paper presents a method of equivalent dynamic reactive power valuation for transient voltage stability. This method emphasizes that the valuation of dynamic reactive power sources should be based on their contributions to system transient voltage stability. The effectiveness of the proposed method has been tested on a 10-bus test system.

Visualization tool for transient stability monitoring of bulk power systems based on DSR

Dynamic security region (DSR) gives power system engineers systematic and global information about the feasible operation region. This paper is a result of an investigation into the use of DSR concept and its effective application in bulk power systems. It particularly concerns with developing a software-based tool for the transient stability monitoring in the on-line operation environment. A practical DSR solving method based on massively parallel computer clusters is described, and a new software package, named power system dynamic security region (PSDSR) is developed. Following a brief discussion about the main requirements, the paper gives a summary of the development of PSDSR from both the software architecture and the function implementation. A most distinctive character, the practical application in monitoring the cut-set power flows, with result visualization is emphasized.