F. Gubina

Local voltage-stability index using tellegen's Theorem

Summary form only given. In the paper, the Tellegens theorem and adjoint networks are used to derive a new, local voltage-stability index. The new approach makes it possible to determine the Thevenins parameters in a different way than adaptive curve-fitting techniques, from two consecutive phasor measurements. The new index was rigorously tested on different test systems. The results were obtained on a static two-bus test system and on the dynamic Belgian- French 32-bus test system that includes full dynamic models of all power-system components crucial to the voltage instability analysis. The results show advantages of the proposed index: it is simple, computationally very fast, and easy to implement in the wide-area monitoring and control center or locally in a numerical relay.

A simple approach to voltage stability assessment in radial networks

Analytical approach to voltage collapse proximity determination is proposed for radial networks. Under corresponding assumptions, a radial network with arbitrary bus loads is transformed into a two bus equivalent. The voltage phasors at the generator bus and at the last load bus of the radial network are transformed to form the voltage phasors of the two bus equivalent. The latter are further used for assessment of voltage collapse proximity. Exact stability limit relations for a two bus system derived from Jacobian matrix can be exploited. Moreover, an analytical expression is derived for calculation of active and reactive power reserve margins for a radial network equivalent. The proposed procedure has been tested for practical examples of radial networks with inductive and capacitive loads.

A new concept of voltage-collapse protection based on local phasors

A new algorithm for protection against voltage collapse is proposed. The algorithm makes use of the magnitudes and angles of the local phasors (i.e., bus voltages and load currents). The change in an apparent power-line flow during a time interval is exploited for computing the voltage-collapse criterion. The criterion is based on the fact that the line losses in the vicinity of the voltage collapse increase faster than the delivery of the apparent power and, at the voltage-collapse point, the losses consume all of the increased power. The selected criterion equals 0 when a voltage collapse occurs. The proposed algorithm could be easily implemented in a numerical relay. The information obtained by the relay can be used at two levels-for the coordinated system-wide control action or for automatic action on the local level. The algorithm is simple and computationally very fast. It was tested on the IEEE 118-bus test system.

Modified topological generation and load distribution factors

In this paper, a modification of the topological generation load distribution factor method of power flow tracing is reported. The modification is needed since the existing method introduces additional fictitious nodes on system lines in order to take into account the transmission losses. Due to the expansion of a system, the existing algorithm based on the augmented matrix equation requires more mathematical effort and memory and longer calculation time to obtain a solution. The new approach successfully avoids the matrix expansion by introducing matrix decoupling, which is its main improvement. The second novelty is an introduction of an equivalent model of a line that unites the nodal reactive power with the power produced by shunt admittances. Thus, the modified method can be also applied to the reactive power flows and transmission-loss allocation.

Determination and allocation of typical load profiles to the eligible consumers

Information concerning the consumers load profile (LP) in a deregulated environment is of initial importance for load balancing purposes and for billing the consumers, which deviated from contracted schedules. Establishment of a system that enables consumers, without the quarter-hourly load measurements, to participate at the retail market, requires the typical load profiles (TLP) of the various types of consumers. Furthermore, simple and straightforward method for the TLP assignment to the particular eligible consumers should be established. In the paper, a methodology for consumers load profiles classification is presented. The measured load profiles (MLP) are classified by hierarchic clustering method and fuzzy logic. Probability neural network is used to assign the TLP to the particular group of consumers. The results demonstrate that the proposed method is efficient for and assigning TLP to the consumers.

A novel concept for voltage collapse protection based on local phasors

Voltage collapse is basically a dynamic phenomenon with rather slow dynamics. Therefore mostly static methods based on the system Jacobian matrix were proposed for its analysis. Because of computational burden these methods are slow and not appropriate for estimating the proximity of power system to voltage collapse in real time. Therefore a new scheme for protection against voltage collapse based on local phasors is proposed. Detection of critical lines instead of critical nodes is used. The difference between apparent power flows on the receiving and the sending end of the line is used for computation of voltage collapse indicator. According to the developed protection scheme, the relay sends a triggering signal to reactive power sources to increase reactive power production. Because of the computational simplicity the proposed protection scheme can be used for on-line system monitoring as well as for off-line analyses.

Cost-based models for the power-reserve pricing of frequency control

One of the tasks of the system operator (SO) is to control the frequency of the system within defined limits. In order to do this, the SO has to provide enough power reserves which can be provided on the basis of bilateral contracts or on the relevant competitive market. In this paper, several methods for the power-reserve pricing of frequency control are presented. These methods are well suited to small power systems, where an insufficient number of potential bidders makes an ancillary-services market difficult to organize. The proposed pricing methods are meant for estimating the annual costs for power-reserves provision when these reserves are provided on the basis of bilateral contracts. The proposed pricing methods are applied to the Slovenian power system.

Ex-ante transmission-service pricing based on load-flow patterns

In this paper, a new pattern-based transmission-service pricing technique is reported. It is based on power-flow tracing techniques that determine the power contributions of participants in line power flows throughout the whole network. Since producers supply a consumer through the transmission paths considering Kirchoffs Laws, it is obvious that the actual paths are not strongly correlated with the contractual paths. This means that instead of financial paths, only the actual power-flow paths should be considered when calculating the transmission-service price. Power-flow data from previous years can be utilized for this purpose. Several power systems and different operating states that realistically represent actual system operating conditions were statistically analyzed. The analysis showed a constant pattern of power-flow shares in lines to consumers, which can be utilized in ex-ante transmission pricing for a network. Since the new approach reflects the characteristics of transaction- and nontransaction-based methods, it combines their benefits too.

Prediction of Power System Security Levels

In the paper, Markov chains in conjunction with Monte Carlo simulation are used to predict the power system security level. The new approach uses a Markov chain for each identified security range to track the development of security level through time. Based on the forecasted and recorded data, the proposed procedure offers a useful tool to the system operator to predict a future power system security level. The method has been tested on the Bosnian power system using the recorded data on security levels during a one-year period. The forecasted results show a striking coincidence with the real security levels. By means of a simple and computationally fast method, the system operator can estimate the probability of a power system blackout. The presented method could be incorporated in the wide-area monitoring system in control centers.

Intelligent coordinative voltage and reactive power control

A novel local voltage control is proposed where each voltage controller of a generating unit is equipped with additional coordinative voltage controller (CVC), which incorporates intelligent control into the power system. The controllers use artificial neural networks (ANN), trained on a large set of power system states. Optimal power flow (OPF) results serve as the optimal target in the ANN training process. All data the controller needs for operation are acquired locally. The proposed ANN voltage control maintains a sub-optimal power system voltage profile and reduces the power system losses. When equipped with ANN controllers, the system responds satisfactorily to changes in the power system topology, thus improving the power system security.