1 Impact of Demand Response on Distribution System Reliability Salman Mohagheghi, Member, IEEE, Fang Yang, Member, IEEE Bamdad Falahati, Student Member, IEEE Abstract—Demand response (DR) is a market driven and aspect of the Smart Grid. Modern distribution automation sometimes semi-emergency action performed at the utility level systems adopt new techniques for fault detection, isolation or at the Demand Response Service Provider (aggregator) with and restoration so that the smallest possible part of the system the objective of reducing the overall demand of the system during is de-energized as a result of a fault while the rest of the peak load hours. If implemented successfully, DR helps postpone customers are supplied through alternative paths and the capacity expansion projects related to the distribution network, and provides a collaborative framework for the alternative resources [3], [4]. In addition, solutions such as liberalized energy market of the Smart Grid. Customers Uninterruptible Power Supplies (UPS), energy storage devices subscribed to the DR program are requested to reduce their and distributed energy resources are some of the alternatives demand or turn off one or more energy consuming appliances in introduced at the distribution level in order to boost the system exchange for financial incentives such as extra payments or reliability for the customers. Although these devices are discounted electricity rates. This would change the concept of efficient in providing short term energy, their role and distribution system reliability as is traditionally known. From one hand, DR could lead to a higher amount of unserved energy; effectiveness is still complementary to that of the grid, and on the other hand, it does not qualify as an unwanted lost load. cannot replace the need for high reliability provided by it. This paper tries to provide a qualitative analysis on the impact of Clearly, higher levels of reliability can be achieved by demand response on distribution system reliability. adding to the redundancy level of the distribution grid through Index Terms—Distribution system, reliability, availability, capacity expansion (CAPX) projects. This is however smart grid, demand response, demand dispatch. restricted in part due to the cost of additional investments on the infrastructure and partly because of environmental I. INTRODUCTION concerns triggered by installing more overhead lines and ISTRIBUTION System Reliability, according to the generation units. The limited capacity of the distribution DIEEE dictionary definition, is defined as the ability of the system jeopardizes its performance especially during the peak distribution system to perform its function under stated hours where all or most of the system capacity is being conditions for a stated period of time without failure [1]. utilized and the grid functions at little or no safety margin. Reliability of the distribution system is specifically important Under such circumstances, a simple overheating of a line or as it is directly associated with the satisfaction level of the transformer, or malfunction of a component can potentially customers. However, in practice, many factors may affect the lead to catastrophic consequences. Reducing the power losses performance of the distribution network. For one, these can help to some extent, as in the United States 5-12% of all networks are often exposed to natural phenomena and may be the power generated is lost, and 60% of this figure is directly impacted by severe weather conditions. Moreover, in many attributable to the distribution system [5]. Advanced cases due to the radial structure instead of a more complicated distribution and feeder automation systems can adopt ways to and more redundant meshed/loop structure, especially in reduce these power losses in the distribution system, for North America, any single outage and failure can potentially instance through voltage and Var optimization (VVO). affect a large number of customers. Components that are However, the amount of losses that can be reduced this way is normally prone to failure are the distribution lines, distribution limited and may not be adequate for relieving the strain from cables, power transformers, service transformers, capacitors the power network. and voltage regulators. In fact, a typical distribution system Active demand is another alternative that can help reshape accounts for 80% of customer reliability problems [2]. the demand profile of the system by partial curtailment of the With the advent of new sensitive electronic devices in the load, or shifting it from peak hours to off-peak hours, thereby grid for the residential, commercial and industrial consumers, reducing the peak demand and relieving the system capacity the quality of supply and continuity of service are becoming during the peak hours. Although, this is not considered as a more important than before. This has made reliability a major permanent solution, in the short term it can help delay the construction of new lines and generation units. The cost S. Mohagheghi and F. Yang are with ABB Corporate Research, Raleigh, savings, in conjunction with the environmental impacts of NC 27606 USA (email: salman.mohagheghi, [email protected]). postponing system expansion plans, has made demand B. Falahati is with the Department of Electrical and Computer response an attractive solution for the utilities worldwide. Engineering, Mississippi State University, Starkville, MS 39762 USA (email: [email protected]). During this work, he was an intern with ABB Corporate What further strengthens the position of DR as one of the Research, Raleigh, NC. 978-1-4577-1002-5/11/$26.00 ©2011 IEEE 2 pillars of the Smart Grid paradigm is the financial benefits it this category are listed below [10], [11]: creates for both the utility and the consumers by reducing the • System Average Interruption Frequency Index (SAIFI) volatility of the electricity market, deferring investments and [int/yr] = total number of customer interruptions in one the reduction in electricity rates. year / total number of customers served. Sometimes it is Regardless of the type of DR program, the consequence of defined as the number of supply interruptions per 100 a DR event triggered by the utility as a semi-emergency action connected customers. could be involuntary reduced consumption for one or more • Customer Average Interruption Frequency Index (CAIFI) customers1. Although this can help alleviate the system [int/yr] = total number of customer interruptions in one capacity and reduce the chances of service unavailability on year / total number of customers affected. This index the large scale, it would still add to the number of shows trends in customers interrupted and helps to show interruptions experienced by the customers, which in the the number of customers affected out of the whole traditional sense of power system reliability, could have been customer base. interpreted as lower reliability indices and poorer service • System Average Interruption Duration Index (SAIDI) performance. This clearly contradicts with the widely [hr/yr] = sum of customer interruptions durations / total accepted notion [6]-[8] that demand response can improve the number of customers served. This is sometimes also reliability of the distribution system. Hence, the traditional referred to as availability index indicating the minutes lost distribution system reliability assessment should be modified per connected customer. to integrate DR event properly so that the resulting reliability • Customer Average Interruption Duration Index (CAIDI) indices can reasonably reflect the impact of DR on the overall [hr/int] = sum of customer interruptions durations / total distribution system reliability. It is therefore imperative that number of customer interruptions. CAIDI gives the the impact of demand response is evaluated in conjunction average outage duration that any given customer would with the reliability of the power system both at the system experience, and can also be viewed as the average level and for the individual customers. The objective of this restoration time. paper is to provide a qualitative assessment on the impact of • Momentary Average Interruption Frequency Index DR on distribution system reliability in the light of the new (MAIFI) [int/yr] = total number of customer interruptions Smart Grid paradigm. less than the defined time / total number of customers served. This index is not as often reported as the other II. POWER SYSTEM RELIABILITY indices. It helps track the momentary power outages Power system reliability can be evaluated in two aspects: caused by transient faults that are otherwise hidden in the adequacy, which is related to the existence of sufficient notion of SAIDI or SAIFI. Of course, the definition of the facilities in the system to satisfy the load demands within the momentary interruption varies from one utility to another. system constraints, and security, which is the ability of the • Average Service Availability Index (ASAI) = customer distribution system to overcome the disturbances occurring hours of available service / customer hours demanded. inside it [9]. Adequacy is related to the static balance between This index has also been referred to as the Index of load and generation (and existence of system facilities to meet Reliability (IOR). the demand), while security is associated with the dynamic • Average System Unavailability Index (ASUI) = customer response of the system to disturbances