Ebrahim Vaahedi

Large scale dynamic security screening and ranking using neural networks

This paper reports on the findings of a completed Canadian Electric Association (CEA) funded project exploring the application of neural network to dynamic security contingency screening and ranking. The idea is to use the information on the prevailing operating condition and directly provide contingency screening and ranking using a trained neural network. To train the two neural networks for the large scale systems of BC Hydro and Hydro Quebec, in total 1691 detailed transient stability simulation were conducted, 1158 for BC Hydro system and 533 for the Hydro Quebec system. The simulation program was equipped with the energy margin calculation module (Second Kick) to measure the energy margin in each run. The first set of results showed poor performance for the neural networks in assessing the dynamic security. However a number of corrective measures improved the results significantly. These corrective measures included: (a) the effectiveness of output, (b) the number of outputs, (c) the type of features (static versus dynamic), (d) the number of features, (e) system partitioning and (f) the ratio of training samples to features. The final results obtained using the large scale systems of BC Hydro and Hydro Quebec demonstrates a good potential for neural network in dynamic security assessment contingency screening and ranking.

Dynamic security contingency screening and ranking using neural networks

This paper summarizes BC Hydros experience in applying neural networks to dynamic security contingency screening and ranking. The idea is to use the information on the prevailing operating condition and directly provide contingency screening and ranking using a trained neural network. To train the two neural networks for the large scale systems of BC Hydro and Hydro Quebec, in total 1691 detailed transient stability simulation were conducted, 1158 for BC Hydro system and 533 for the Hydro Quebec system. The simulation program was equipped with the energy margin calculation module (second kick) to measure the energy margin in each run. The first set of results showed poor performance for the neural networks in assessing the dynamic security. However a number of corrective measures improved the results significantly. These corrective measures included: 1) the effectiveness of output; 2) the number of outputs; 3) the type of features (static versus dynamic); 4) the number of features; 5) system partitioning; and 6) the ratio of training samples to features. The final results obtained using the large scale systems of BC Hydro and Hydro Quebec demonstrates a good potential for neural network in dynamic security assessment contingency screening and ranking.

Voltage stability contingency screening and ranking

The objective of contingency screening and ranking function is to shortlist a specified number of critical contingencies from a large list of credible contingencies and rank them according to their severity. This paper summarizes the work conducted as part of the EPRI/BC Hydro on-line voltage stability project in developing a contingency screening and ranking (CS&R) module. The two methods of reactive support index (RSI) and iterative filtering are derived in this paper and tested on the large scale systems of BC Hydro and another major unnamed utility. The results obtained indicate that RSI on its own or in combination with the iterative filtering method can be used for CS&R depending on the acceptable level of misranking. The RSI method is a very fast and powerful CS&R method and is suggested for systems where some misclassification of contingencies can be tolerated. On the other hand, for systems where an exact list of critical contingencies is intended, then the iterative filtering method can be used for screening complemented with another method like RSI for ranking. The latter technique is being integrated in the EPRI/BC Hydro on-line voltage stability tool.

Voltage Stability Monitoring Based on the Concept of Coupled Single-Port Circuit

Summary form only given: This paper reveals that the impedance match (or the Thevenin circuit) based voltage stability monitoring techniques have problems to predict voltage stability limits when applied to multi-load power systems. Power system loads are nonlinear and dynamic. They cannot be simply represented as Thevenin circuit parameters for impedance match analysis. To overcome these difficulties, a new concept called “coupled single-port circuit” is proposed in this paper. The concept decouples a meshed network into individual single generator versus single bus network and, as a result, a modified version of the impedance match theorem can be used. This leads to a real-time voltage stability monitoring scheme without the need to estimate Thevenin parameters. The scheme can estimate voltage stability margin and identify weak areas in a system based on the SCADA and PMU data. Case studies conducted on several test systems have verified the validity of the proposed method.

Dynamic security constrained optimal power flow/VAr planning

Traditionally security constrained optimal power flow and VAr planning methods consider static security observing voltage profile and flow constraints under normal and post contingency conditions. Ideally, these formulations should be extended to consider dynamic security. This paper reports on a BC Hydro/CEPEL joint effort establishing a dynamic security constrained OPF/VAr planning tool which considers simultaneously static constraints as well as voltage stability constraints. This paper covers the details of formulation and implementation of the tool together with the test results on a large scale North American utility system and a reduced Brazilian system.

General nonlinear modal representation of large scale power systems

In this paper an approach is devised to represent, and study the behavior of, nonlinear dynamic systems using general nonlinear modal representation. This approach is shown to compare favorably to the normal form of vector field technique, the other methodology used for this purpose, in that it is valid under resonance conditions and it does not require nonlinear transformation. By representing the system more accurately and in terms of its modes and their interactions, it would be better suited to be used for understanding and analyzing complex behavior of stressed power system and also in designing various controls for the system. This method can represent a nonlinear system with any order of nonlinearity and provides the solution to the system nonlinear differential equation employing Laplace transformation. The accuracy and robustness of the method has been validated using three systems, two of which model realistic utility power systems under stressed conditions.

Voltage stability load parameter determination from field tests on BC Hydro's system

This paper reports on the findings of an EPRI project exploring the determination of load characteristic for voltage stability analysis from field tests. The results of the extensive load tests performed on the BC Hydro system are summarized. Based on the these results, important factors affecting test design and load parameter estimation are identified and typical load parameters are determined. Finally, a simple and effective load test procedure is proposed for future tests.

A general purpose method for on-line dynamic security assessment

Time domain-based online dynamic security assessment (DSA) systems require a fast and reliable method for simulation termination and margin calculation. B.C. Hydros online DSA uses the second kick method which proved to be an elegant and reliable method for this purpose. This method was later enhanced to improve implementation and computation requirements. The methods presented so far require the mode of disturbance (MOD) information. In this paper, the fast second kick method is expanded to remove the need for the MOD data. The new method has been tested on the large scale systems of B.C. Hydro and Hydro Quebec and the results have been compared to those of the fast second kick method. The results obtained indicate that the new method predicts the stability limits accurately. They also indicate that the new method takes in average 20% longer than the fast second kick method to calculate the stability limit. The implementation requirement for both methods is similar.

Distribution System State Estimation Based on Nonsynchronized Smart Meters

Distribution systems are undergoing many enhancements and developments to enable the future smart grid, and distribution system state estimation (DSSE) provides the control centers with the information necessary for several of its applications and operational functions. However, the quality of DSSE typically suffers from a lack of adequate/accurate measurements. Recently, many electric utilities have started to install fairly accurate smart meters throughout their distribution networks, which create an opportunity to achieve higher quality DSSE. However, the signals provided by smart meters are generally not synchronized and the difference between the measurement times of smart meters can be significant. Therefore, a complete snapshot of the entire distribution system may not be available. This paper proposes a method to deal with the issue of nonsynchronized measurements coming from smart meters based on the credibility of each available measurement and appropriately adjusting the variance of the measurement devices. To illustrate the effectiveness of the proposed method, two IEEE benchmark systems are used. The results show that the proposed method is robust and improves the accuracy of DSSE compared with the traditional DSSE approach.

A Network Decoupling Transform for Phasor Data Based Voltage Stability Analysis and Monitoring

It is well known that a power network can be represented as a multinode, multibranch Thevenin circuit connecting loads to generators. This paper shows that eigen-decomposition can be performed on the Thevenin impedance matrix, creating a set of decoupled single-node, single-branch equivalent circuits. The decoupled circuits can reveal important characteristics of a power system. By applying the transform to calculated or measured voltage phasor data, a technique for tracking the modes of voltage collapse and for identifying areas vulnerable to voltage collapse has been developed. Case studies conducted on multiple power systems have confirmed the effectiveness of the proposed method. In addition to voltage stability applications, the proposed transform presents a new approach for processing and interpreting multilocation phasor data.