Sub-Station Reliability

Introduction

Sub-stations are essential and critical segments of an electric power system and their reliability can have significant impact on overall power system reliability. Sub-stations are mainly composed of three basic components: circuit breakers, bus sections and transformers. Evaluation of sub-stations reliability therefore consists of assessing how adequately these basic elements are able to perform their intended functions. Modern sub-station electrical layouts are receiving considerable attention with regard to reliability.

The immediate consequence of a poor sub-station design is the failure to contain simple disturbances. These often lead to multiple component outages and place undue stress on the power grid. Providing redundancies in the transmission system is an obviously expensive solution. More direct and less costly answers can be found by designing sub-stations for better reliability. The reliability of the sub-station is assessed by checking the effects of faults and outage of each component within the sub-station.

Economic aspects to be considered:

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  Investment cost.

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  Operation and maintenance cost.

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  Reliability/Interruption cost.

Technical aspects to be considered:

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  Flexibility.

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  Safety.

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  Automation level.

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  Security.

The sub-station is a portion of electric power system that forms a link between bulk transmission system and the generating system. Hence a sub-station is essential and critical segment of an electric power system. Its reliability can have significant impact on overall power system reliability. Sub-stations have more complex switching arrangements than the other networks of the power system.

To evaluate the sub-station reliability, decoupling it from the rest of the system will give improved results. This would allow greater detail to be given to the sub-station reliability while keeping the complexity of the entire system evaluation down.

Most utilities have a variety of sub-station ages in their system. Some sub-station may be very new with automation incorporated into their design and the latest technology. Other sub-stations may be relatively old without automated switching and mechanical relays still in use. These sub-stations require crews to be dispatched for switching when failure occurs. Overall network reliability depends on this variety in sub-station designs for any given utility.

The traditional way to plan and operate a power system is the deterministic n-1 criterion. In this method, the power system shall be operated in such a way that after any single contingency the system remains stable and a new operating point without overloading and voltage violations can be reached. Probabilities of different faults are not traditionally taken into account; instead all faults that may limit the transmission capacity are treated equally. This traditional method can lead to conservative utilization of the grid. However, the most efficient contributions to the system reliability enhancement can be found by using the probabilistic rather than deterministic methods. The power systems are usually large, complex and in many ways non-linear. The post-fault phenomena in a power system are dynamic in nature and dependent on the grid connection and load flow situations in different parts of the grid. Thus, the security analysis of a power system is a difficult task. The effects of an unreliable power system transmission can be widespread and affect vast number of people.

Sub-Station Reliability Aspects

Reliability evaluation is an essential part of planning, design and operation of electric power systems. Measures of reliability are often referred to as reliability indices which indicate how well the system will perform its basic functions of supplying electrical energy to its customers. It provides information to assist the power system planners in design, operation and expansion of new facilities.