B. Brusiłowicz

A new method of voltage stability margin estimation based on local measurements

The paper presents a method of monitoring the local voltage stability of the power system at the receiving node. To estimate the voltage stability margin a simplified equivalent circuit - called the Thevenins equivalent is used. Publication concerns three main issues: creation and updating of the Thevenins circuit model, determination of voltage stability margin and verification of proposed methods. Two methods of Thevenins model parameters calculation are presented: analytical and based on local phasors measurements. To define the Thevenins parameters by using analytical method there is a need to know current configuration of power system. The second method uses only available locally measurements. During the changes of load admittance, actual value of bus voltage, apparent power and derivatives of apparent load power against the node voltage dS/dV are determined. This allows to update continuously parameters of Thevenins circuit and to follow the ongoing changes in the power system. These changes may be caused by normal switching operations or unexpected faults. Thevenins model parameters are used to calculate local voltage stability margin. The paper presents also the performance and accuracy tests of the algorithm used in calculation of Thevenins model parameters and algorithm for determining voltage stability margin.

Estimation, control and prediction of voltage level and stability at receiving node

The paper presents issues of receiving node voltage stability. Voltage level is related to voltage stability. However, this value is not always a good indicator of distance to the stability limit. Discrepancies may occur especially when voltage regulation is performed. Such regulation causes changes of voltage level and voltage stability conditions. Thus, it seems reasonable to observe and predict voltage stability margin. In the paper basic information about the voltage stability considered as a local problem and voltage regulation influence on stability conditions are presented. Method of stability conditions calculation using local information and including of this parameter in the regulation processes are proposed. Stability margin calculation is based on on-line local measurements and the range of change of Thevenin model parameters determined in off-line studies. During simulations two models were used: IEEE 14-bus test system (ATP-EMTP) and node model containing voltage regulation (Matlab).

Comparison of reactive power compensation methods

The paper presents comparison of reactive power compensation methods. At the beginning definitions of power and simplified distortion factor are described. Next, two methods are presented. The first method use equality between apparent and active power of fundamental component and the second one between apparent power of fundamental component and total active power. Impacts of these methods on voltage stability and simplified distortion factor are analyzed and described. The last paragraph concerns conclusions from presented studies.

Analysis of protection criterion values algorithms in case of distorted signals

Nowadays fast increase of nonlinear elements number in the power system is observed. Besides of nonlinear loads there are also new energy sources that require electronic and power electronic equipment. These elements cause distortion of electrical signals and presence of higher harmonics in currents and voltages. Algorithms of protection criterion values are matched to fundamental components of currents and voltages and can cause errors due to these harmonics. In the paper some signals and load models are considered as well as the most influenced criterion values including a few models of a reactive power. As a complement practical model of nonlinear heating system is described and analysed.

Voltage stability estimation based on approximate model and voltage measurement

The paper presents method of voltage stability margin estimation based on approximate model and measurements of the node voltage. The first part contains basic information about Thevenin model and methods of parameters of this model calculation. In second part, method of voltage stability margin estimation using approximate model and measurements of voltage is described. For the simulations, the 14-bus IEEE test model was implemented in ATP-EMTP software. Basing on this model the impact of configuration changes on the variations of Thevenin parameters can be determined. This information and voltage measurements allow to calculating range of voltage stability changes. Additionally, determined Thevenin model was used to calculate variations of stability margin for different types of load.