Assessment Of Power System Reliability

ABSTRACT

Electric power has become increasingly important as a way of transmitting and transforming energy in industrial, military and transportation uses. Electric power systems are also at the heart of alternative energy systems, including wind and solar electric, geothermal and small scale hydroelectric generation.

The importance of power system reliability is demonstrated when our electricity supply is disrupted, whether it decreases the comfort of our free time at home or causes the shutdown of our companies and results in huge economic deficits. The objective of Assessment of Power System Reliability is to contribute to the improvement of power system reliability. It consists of six parts divided into twenty chapters. The first part introduces the important background issues that affect power system reliability. The second part presents the reliability methods that are used for analyses of technical systems and processes. The third part discusses power flow analysis methods, because the dynamic aspect of a power system is an important part of related reliability assessments. The fourth part explores various aspects of the reliability assessment of power systems and their parts. The fifth part covers optimization methods. The sixth part looks at the application of reliability and optimization methods.

 DEFINITION OF BASIC TERMS

The following covers some basic terminology used in this work

Failure

The loss of a function under stated conditions.

Failure mode

The specific manner or way by which a failure occurs in terms of failure of the item (being a part or (sub) system) function under investigation; it may generally describe the way the failure occurs. It shall at least clearly describe a (end) failure state of the item (or function in case of a Functional FMEA) under consideration. It is the result of the failure mechanism (cause of the failure mode). For example; a fully fractured axle, a deformed axle or a fully open or fully closed electrical contact are each a separate failure mode.

Failure cause and/or mechanism

Defects in requirements, design, process, quality control, handling or part application, which are the underlying cause or sequence of causes that initiate a process (mechanism) that leads to a failure mode over a certain time. A failure mode may have more causes. For example; “fatigue or corrosion of a structural beam” or “fretting corrosion in an electrical contact” is a failure mechanism and in itself (likely) not a failure mode. The related failure mode (end state) is a “full fracture of structural beam” or “an open electrical contact”. The initial cause might have been “Improper application of corrosion protection layer (paint)” and /or “(abnormal) vibration input from another (possibly failed) system”.

Failure effect

Immediate consequences of a failure on operation, function or functionality, or status of some item.

Indenture levels (bill of material or functional breakdown)

An identifier for system level and thereby item complexity. Complexity increases as levels are closer to one.

Local effect

The failure effect as it applies to the item under analysis.

Next higher level effect

The failure effect as it applies at the next higher indenture level.

End effect

The failure effect at the highest indenture level or total system.

Detection

The means of detection of the failure mode by maintainer, operator or built in detection system, including estimated dormancy period (if applicable)

Risk Priority Number (RPN)

Severity (of the event) Probability (of the event occurring) * Detection (Probability that the event would not be detected before the user was aware of it)

Severity

The consequences of a failure mode. Severity considers the worst potential consequence of a failure, determined by the degree of injury, property damage, system damage and/or time lost to repair the failure.

Remarks / mitigation / actions

Additional info, including the proposed mitigation or actions used to lower a risk or justify a risk level or scenario.

Risk

Risk is here the combination of probability and severity of the failure incident (scenario) occurring.

Reliability may be defined in the following ways:

• The idea that an item is fit for a purpose with respect to time
• The capacity of a designed, produced, or maintained item to perform as required over time
• The capacity of a population of designed, produced or maintained items to perform as required over specified time
• The resistance to failure of an item over time
• The probability of an item to perform a required function under stated conditions for a specified period of time
• The durability of an object.

CHAPTER ONE

1.0                                  INTRODUCTION

The electric utility is expected to provide continuous and quality of electric service to their customers at a reasonable rate by making economical use of available system and apparatus. To maintain reliable service to customers, the utility has to have adequate redundancy in its system to prevent a component outage becoming a service interruption to the customers, causing loss of goods, services or benefits.

Reliability costs are used for rate reviews and request for rate increases. Therefore in order to calculate the cost of reliability, the cost of an outage must be determined. Technique which can be used to determine the cost of outages in relation to loss of goodwill, loss of production, and damaged goods. The economic analysis of system reliability can also be a very useful planning tool in determining the capital expenditures required to improve service reliability by providing the real value of additional investments into the system. Lastly, expectation indices, which are most often used to express the adequacy of the generation configuration, characterize future system performance in a satisfactory manner.

 The reliability standards for electricity supply in a developing country, like Nigeria, have to be determined on past engineering principles and practice. Because of the high demand of electrical power due to rapid development, industrialization and rural electrification; the economic, social and political climate in which the electric power supply industry now operates should be critically viewed to ensure that the production of electrical power should be augmented and remain uninterrupted. This paper presents an economic framework that can be used to optimize electric power system reliability. Finally the cost models are investigated to take into account the economic analysis of system reliability, which can be periodically updated to improve overall reliability of electric power system.

1.1                           OBJECTIVE OF THE PROJECT

The objectives of assessing power system reliability are:

1. To apply engineering knowledge and specialist techniques to prevent or to reduce the likelihood or frequency of power system failures in Umuahia.
2. To identify and correct the causes of failures that does occur despite the efforts to prevent them.
3. To determine ways of coping with failures that does occur, if their causes have not been corrected.
4. To apply methods for estimating the likely reliability of new designs, and for analyzing reliability data.

1.2                                           PURPOSE OF THE PROJECT

The reason for the priority emphasis is that it is by far the most effective way of working, in terms of minimizing costs and generating reliable power supply. The primary skills that are required, therefore, are the ability to understand and anticipate the possible causes of failures, and knowledge of how to prevent them. It is also necessary to have knowledge of the methods that can be used for analyzing designs and data.

1.3                                SCOPE OF THE PROJECT

Reliability engineering for “complex systems” requires a different, more elaborate systems approach than for non-complex systems. Reliability involves:

• System availability and mission readiness analysis and related reliability and maintenance requirement allocation
• Functional System Failure analysis and derived requirements specification
• Inherent (system) Design Reliability Analysis and derived requirements specification for both Hardware and Software design
• System Diagnostics design
• Fault tolerant systems (e.g. by redundancy)
• Predictive and Preventive maintenance (e.g. Reliability-Centered Maintenance)
• Human Factors / Human Interaction / Human Errors
• Manufacturing- and Assembly-induced failures (effect on the detected “0-hour Quality” and Reliability)
• Maintenance-induced failures
• Transport-induced failures
• Storage-induced failures
• Use (load) studies, component stress analysis, and derived requirements specification
• Failure / reliability testing (and derived requirements)
• Field failure monitoring and corrective actions
• Spare parts stocking (Availability control)
• Technical documentation, caution and warning analysis
• Data and information acquisition/organization (Creation of a general reliability development Hazard Log and FRACAS system)

Effective reliability engineering requires understanding of the basics of failure mechanisms for which experience, broad engineering skills and good knowledge from many different special fields of engineering.

   1.4                     SIGNIFICANCE OF THE PROJECT

This systems is designed to play the biggest role in providing such high availability levels. They are meshed networks (grids) that deliver large amounts of electric power at high voltages. Because transmission systems are networks, the loss of any one segment, such as a transmission line, transformer or generator, usually causes only a minor disturbance to the system. The network allows the power to take different paths from the generation source to the load, and it usually can take a second or even third contingency before disastrous results occur. Typical causes of transmission outages include lightning strikes, transformer failures, line splice failures, switching surges, wind toppled towers, lines in contact with trees or vegetation, and insulator flash overs due to animals or contamination buildup.

While distribution systems are pretty reliable, they do not enjoy the high availability rate that transmission systems do. Just like with transmission systems, distribution system design plays a significant role in system performance. Distribution systems are usually, but not always, “radial” systems, which means power flows from the supply point, usually a substation connected to the transmission system, downstream to customers distributed along the line. Unlike the transmission system, little if any redundancy exists on a distribution feeder: It is a series of segments and components, resembling a chain, and like a chain, it is only as strong as its weakest link.

If any link fails, all customers downstream of that link are out of service until that link is restored. Equipment failure, trees, lightning, wind, birds and car-pole accidents are some of the most likely causes of distribution outages. Most Nigeria distribution systems are designed with strategically placed sensationalizing switches, so that loads can be switched between feeders to restore service to customers while repairs are being made to faulted line sections.