Gerd H. Kjølle

RELRAD-an analytical approach for distribution system reliability assessment

The use of reliability analysis within the framework of distribution system planning is addressed. A specific model for reliability calculations in radially operated distribution systems is presented. Due to increased outage costs in terms of specific costs of nondelivered power and energy (NDP and NDE), an increased part of system operating costs is related to reliability aspects. Customers outage costs, based on the willingness-to-pay principle, are given the same principal treatment as power system electrical losses. This approach is becoming generally accepted by the power companies in Norway. The reliability model is based on an analytical approach, connecting component failures to load point outages. The description is based on statistical distributions. >

Vulnerability analysis of the Nordic power system

A comprehensive methodology is developed for the analysis of power system vulnerability with respect to energy shortage, capacity shortage, and power system failures. The basic method is risk analysis, where risk is defined in terms of the probability of events and their consequences. The objective is to identify medium- and high-risk situations that require corrective actions. The method is applied to the Nordic power system. The results show that the system is in a medium-risk state, implying the need to consider various measures. With respect to energy shortage, the probability of extremely high prices over a sustained period of time is once every ten years for todays system and similar in 2010. With respect to capacity shortage, the vulnerability is expected to deteriorate toward 2010. With respect to blackouts, the risk is related to large blackouts in southern Scandinavia, involving many consumers. There is concern with regard to the future effect of changed maintenance routines and reductions in qualified technical personnel.

Risk analysis of critical infrastructures emphasizing electricity supply and interdependencies

Failures in critical infrastructures can cause major damage to society. Wide-area interruptions (blackouts) in the electricity supply system have severe impacts on societal critical functions and other critical infrastructures, but there is no agreed-upon framework on how to analyze and predict the reliability of electricity supply. Thus, there is a need for an approach to cross-sector risk analyses, which facilitates risk analysis of outages in the electricity supply system and enables investigation of cascading failures and consequences in other infrastructures. This paper presents such an approach, which includes contingency analysis (power flow) and reliability analysis of power systems, as well as use of a cascade diagram for investigating interdependencies. A case study was carried out together with the Emergency Preparedness Group in the city of Oslo, Norway and the network company Hafslund Nett. The case study results highlight the need for cross-sector analyses by showing that the total estimated societal costs are substantially higher when cascading effects and consequences to other infrastructures are taken into account compared to only considering the costs of electricity interruptions as seen by the network company. The approach is a promising starting point for cross-sector risk analysis of electricity supply interruptions and consequences for dependent infrastructures.

Adequate interruption cost assessment in a quality based regulation regime

Balancing cost-effectiveness and quality of service is one of the most important tasks for network companies in the deregulated environment. It is widely recognized that interruption costs is a relevant expression for the inconvenience customers feel when service is interrupted and that adequate assessment of such costs is crucial when they are used quantitatively in cost-benefit analyses. In this paper we demonstrate by a distribution system example that typical time variations of component failure rate, repair time, load and specific interruption costs can have a significant impact on the estimated annual interruption cost for delivery points in the system. This observation is possible by a model that can handle such time variations including correlation between the parameters. An interesting application of this model is to quantify the consequence of for example moving planned maintenance to periods when the load and the interruption cost is low.

Reliability and interruption cost prediction using time-dependent failure rates and interruption costs

The main idea presented and discussed in this paper is a model reproducing a time-dependent component failure rate pattern similar to the observed pattern recorded in failure statistics. This pattern includes all types of failures, caused by the weather or by technical and human aspects. Failure causes and mechanisms are not modelled explicitly and the observed pattern is assumed to be representative for the analysis period ahead. Being able to predict and time-tag component failures, the time-dependent variables of load, repair time and customer-specific interruption costs can be adequately combined to calculate annual reliability indices and interruption costs. This fact also permits us to apply an analytical model which will produce expectation values comparable with average values in a Monte Carlo simulation.

Integrated approach for security of electricity supply analysis

The methods and tools needed for security of supply assessment range from long term market models for predicting future generation and load patterns to more detailed network simulation models for analyzing contingencies and reliability of supply for delivery points. This paper describes an integrated approach for security of electricity supply analysis, by the integration of power system reliability analysis with the power market analysis. An integrated methodology will enable the long-term security of supply assessment and provide a more consistent input to the various analyses, as well as a better information exchange and interaction between the different methods and tools involved. The integrated methodology is under development in an ongoing research project focusing the power system reliability analysis, making it possible to take the risk of extraordinary events into account.

Protection System Faults -- a Comparative Review of Fault Statistics

Correct operation of the protection system is of vital importance for the reliability and security of supply. Information of protection system faults is therefore important for different purposes such as quality of supply regulation and as input data to security and reliability analysis. This paper provides a comparative review of the Norwegian and Finnish fault statistics for line and transformer protection systems at the voltage levels 132-400 kV for the period 1999-2004. Unwanted operation is the major fault type, particularly for transformer protection. Human causes dominate by roughly 50%, technical faults count for 20-30% while there are large portions of faults where the cause is not identified. In Norway 20% of the total energy not supplied at these voltage levels are caused by the protection. The corresponding portion in Finland is only 4%

Probabilistic Risk-Based Security Assessment of Power Systems Considering Incumbent Threats and Uncertainties

In-depth security analyses of power systems (PSs) require to consider the vulnerabilities to natural and human-related threats, which may cause multiple dependent contingencies. On the other hand, such events often lead to high impact on the system, so that decision-making aimed to enhance security may become difficult. Introducing the uncertainty, the risk associated to each contingency can be evaluated, thus allowing to perform effective contingency ranking. This paper describes an in-depth security assessment methodology, based on an “extended” definition of risk (including threats, vulnerability, contingency, and impact) aimed to perform the risk assessment of the integrated power and Information and Communication Technology (ICT) systems. The results of the application to test cases and realistic PSs show the added value of the proposed approach with respect to conventional security analyses in dealing with uncertainty of threats, vulnerabilities, and system response.

Quality of supply in the deregulated Norwegian power system

Since 1995 the Norwegian regulator has required annual reporting of interruption data by the network companies. Main results from the reported data in 1995-1999 are presented in the paper. The statistics shows that the annual number of interruptions per delivery point has been relatively constant during this five-year period. The annual energy not supplied (ENS) shows a decreasing tendency due to a quite evident reduction in ENS for notified interruptions. The interruption statistics are utilized for many purposes by the regulator, the network companies and as a basis for research. Some examples of utilization of the statistics are presented in the paper.

Dependability and security-based failure considerations in protection system reliability studies

The reliability of protection systems has a considerable effect on the reliability of supply, and hence appropriate protection system reliability models must be incorporated in power system reliability studies. These studies assume increasing prominence, especially in the wake of influx of smart grid technologies, making it imperative to handle the accompanying failure dependencies in detail. This paper presents the results of one such related investigation carried out to incorporate the impact of transmission line failure modes on account of various protection system response scenarios on supply reliability indices. In addition to the basic frequency and duration indices, indices such as annual power interrupted, annual energy not supplied, and annual interruption costs are computed, built on a minimal cut set based approach. The approach presented circumvents the need for complex Markov models to include protection system reliability considerations. An illustrative case study is employed to draw attention to the impact of identified comprehensive failure scenarios in protection and control equipment on power system reliability. Comparisons are made with an existing simplified method, and also with a case where protection system reliability is assumed to be perfect. The results bring forward the emphasis to be placed on initiatives to include the study of impact of reliability of protection systems on reliability of supply.