The mission of the power system is the provision of the electricity required to meet demand at all points in time, in such a way that all customers’ residential, industrial, tertiary and public service are supplied with the needed amount of electricity at the required locations, following their required power consumption profile in different time-frames. Whereas electricity services across India are affected by severe shortcomings at all stages of generation, transmission and distribution. Supply is highly unreliable, outages and voltage fluctuations are frequent. Quiet often peak demand cannot be met. The problem exacerbates when living in rural areas. Customers invariably rank power interruptions high on their lists of problems. Outages, momentary or sustained causing service disruption have significant economic consequences. The high demand of electrical power due to rapid development, industrialisation and rural electrification; the economic, social and political climate in which the electric power supply industry operates does require ensuring the generation of electrical power to be augmented and remain uninterrupted. The service is to make electricity available for use to the end-user customers of the power supply system with continuous, quality qualified electricity at a reasonable rate. Reliability of electricity supply therefore becomes a primary concern.
Reliability in power system
The term reliability has a very wide range of meanings and cannot be associated with a single specific definition. It is therefore necessary to recognise its extreme generality and to use it to indicate, in a general rather than specific sense. In context to reliability of power system, it is the overall probable ability of the electricity system to perform its functions satisfactorily, under specific conditions during a given period. It is an ability of the power system or its components to withstand instability, uncontrolled events, cascading failures, or unanticipated loss of system components. It relates to how the power system should work and commingles the operational security, flexibility and adequacy properties.
Reliability has typically been synonymous with “grid reliability” or “system reliability”. Consumer reliability derives from a series of initiatives over several decades; the continuous improvement in energy efficiency to satisfy the customer demand. From the utility industry perspective, reliability is traditionally defined through metrics describing power availability or outage duration, frequency, and extent. Reliability within the utility industry is to manage the system operates within limits and avoids instabilities or the growth of disturbances.
Standard definitions of reliability predominantly focus on the frequency, duration, and extent of power outages. Nevertheless formal definition of power reliability based on the frequency, duration, and extent of power outages is rather insufficient to ensure system integrity and available electric power in the face of climate change, natural hazards, physical attacks, cyberthreats, and other intentional or accidental damages.
In short the reliability of an electricity supply system reflects its ability to perform its function under stated conditions. The function in this context is to supply electricity as economically as possible with an acceptable degree of service continuity.
The continuity of electricity supply or reliability of power supply depends on the performance of generation, transmission and distribution of electricity. Reliability is looked individually and in combinations of these three areas. These different functional areas offer different degrees of reliability. Still the actual degree of reliability experienced by a customer varies from location to location.
The function of the generation system is to make sure that enough capacity is available to meet the load/demand at any time. The existing power grid systems are under pressure to meet the energy requirement, leading to an imbalance of supply, thereby affecting the consumers with load shedding and unbalanced voltage. For reliable power supply it is extremely essential to maintain a continuous and instantaneous balance between generation and consumption of electricity in power systems. It is done by keeping some margin of generation above the expected demand load, so the system can deal with unexpected mismatches between supply and demand leading to power shortages. Capacity market allocates system reserve efficiently. To achieve uninterrupted and reliable power supply even in remotest parts of the country may be through embedded generation.
Transmission and distribution systems need to be reliable in making sure the electricity generated can be efficiently delivered to the consumers. Reliability criteria for transmission systems has to address both local interruptions of power supply at points in the network as well as widespread interruptions affecting population centers or entire regions. Local and widespread interruptions are typically caused by different types of events and require different evaluation approaches. Additional transmission facilities always increase reliability. Transmission systems must meet performance standards and criteria that ensure an acceptable level of quality of electric service.
The distribution segment is the weakest link between the source of supply and the customer load points. Electrical distribution systems are usually radially exposed to the rates of interruption because of system components of transformers and breakers and switching devices. The major problem encountered in this area is reducing the number of interruptions experienced by customers. Distribution automation, corrective and preventive maintenance of the lines and the establishment of secondary feed lines minimise outage time for reliability of supply of electricity. A distribution system has to be designed and operated in a way that allows for quick restoration of supply in the event of any failure of equipment.
Threats and hazards to the electricity system represent anything that can cause disruption and outages. Vulnerabilities are points of weakness within a system that increase susceptibility to such threats. The physical vulnerabilities and specific risks to the electric power system vary among infrastructure components and by geographic location. In face of hazards the major steps has to be carried for continuity of power supply. Out-of-step protection, automatic load shedding, frequency, overvoltage, under voltage, equipment overload line load and stability disturbance protections can effectively deal the situations.
Electricity access drive that aims to grid connect all homes is pleasurable contemplation yet just connecting homes with no guarantee of quality supply means a big cost to consumers. They end up paying a minimum charge for the grid supply even without making the best use of it supplies in the most productive hours. Reliability of electricity supply is a customer-oriented to the end user. The user does not consider the origin of the causes of interruptions, disturbance or preventions. In fact he is simply concerned with the quantity of power available to him. The situation has developed to the point of readiness to get the reliability of power supply. Reliability of electric supply is at the heart of energy challenge India is facing today.
The reliability of power supply pertains to the probability of providing customers with continuous service and with a voltage and frequency within prescribed ranges around the nominal values. The basic techniques for reliability evaluation have been categorised in terms of their application to the main functional zones of an electric power system pertaining generation system, composite generation and transmission of bulk power and distribution system. Reliability assessment at hierarchical level I (HL-I) is solely concerned with the generation facilities. At this level, the total system generation including interconnected assistance is examined to determine its ability to meet the total system load demand. Reliability evaluation at hierarchical level II (HL-II) includes both the generation and transmission in an assessment of the integrated ability of the composite system to deliver energy to the bulk supply points. This analysis is termed as composite system or bulk power system reliability evaluation. Overall assessment considering all three functional segments is known as HL-III analysis.
Reliability is first and foremost a function of design, operation and maintenance practices. In order to maintain the operational state of the power systems at the required levels and subsequently to meet the load demand satisfactorily, the power system operations reliability analysis is essentially to be carried out at regular intervals. The daily operation and planning activities of an electric utility therefore requires the prediction of the electrical demand of its customers. In general it requires load forecasts which can be further categorised into short-term, mid-term, and long-term forecasts. The system requires sufficient reserve in the way that no load will lose power if any one line or any one generator fails. Power system has also to react in abnormal conditions and faults comprising protection of the equipment against failures so that electric power system recovers to its normal operating mode. Reliability evaluation is the criterion by which the performance of an electric power system in reacting to component failures can be judged acceptable or unacceptable.
Reliability analysis of a power system can be conducted using either deterministic or probabilistic techniques. In deterministic reliability assessment, the decision is founded on the requirement that each outage event in the contingency set results in system performance that satisfies the chosen performance evaluation criteria. It only considers the specific configurations and ignores the stochastic or probabilistic nature of real power system. The method is simple in concept and application but results are often misleading, as it does not capture the effect of outage likelihood, non-limiting events and failure conditions, violation severity, and uncertainty in operating conditions. These effects influence the evaluation of near future operating conditions. The probabilistic model refers reliability of power system to the probability of supplying the load when the power network is subject to random outages of its equipment. A random variable is evaluated that describes the difference between capacity of generation units and load demand and evaluation is also done in the negative domain. It is known that probabilistic methods constitute powerful tools for effective decision-making for problems affecting reliable power system.
The society is becoming increasingly dependent on electricity supply. The more the societies are developed, the more vulnerable they are to electricity supply interruptions. Remedy is to achieve or produce something better by preventing and mitigating disturbances. Problem areas experiencing remarkable poor reliability of supply need to be identified. Investigating disturbances and incorrect protection operation and its origin track record is an essential for trusted performances. Once the causes of incorrect protection operations are known, it is possible to decide upon actions to be taken to improve the reliability of the protection systems. Improving of electricity supply reliability may be improved by reducing planned or unplanned, momentary or sustained interruption, its frequency and duration by knowing the root causes of faults to prevent faults from occurring. Existing components can be replaced with comparable components that are less prone to failure. To improve component performances overhead networks can be replaced by underground networks or by replacing aged components with new components. Setting up executing inspection and maintenance plans. Establishing permanent network solutions incorporating such options as automatic reclosing, manual and automatic feeder sectionalising, automatic feeder fault locating, remote monitoring and control capabilities and emergency supplies from adjacent feeders as and when required.
Holistic economic development of a country is largely dependent on reliable power supply. The basic principle applies that the higher the level of reliability of supply, the more difficult it is to improve it further in a socio-economic manner. However ideally, electricity supply regulation should firstly decide upon a level of reliability that must be optimal from a socio-economical point of view. Urban, rural and suburban circuits should be classified separately and there should be minimum performance guarantee to ensure that every customer receive a minimum level of service as defined. Reliability metrics and reporting should be continuously refined. There must be rules to decide upon reasonable allowed revenue for each network company corresponding to requisite reliability level and allow utilities to engage in power quality and reliability service contracts for customers. Reliability requirements must be urged through some kind of penalisation.