تجزیه و تحلیل حساسیت بر اساس خط انتقال برای مدیریت تراکم

Transmission line limits impede power transfers and cause congestion greatly reducing the effectiveness of systems and increasing the cost of power transmissions. Through several methods, congestion can be effectively eliminated either by building a new transmission line or by increasing the capacity of the original line between congested zones. Both methods cause susceptance change and line capacity increase between nodes. Therefore, the sensitivity of a system to variations of its parameters becomes important in terms of operation and planning. In this paper, congestion relief as a function of line susceptances and line capacities is investigated. Mathematical derivations for calculating sensitivities based on line susceptances are given systematically, and numerical studies were performed by using sequential quadratic optimization programming to determine congestion in terms of line susceptances. It is shown that the optimum susceptance range becomes critical when the line parameters are changed dynamically by using thyristor-controlled series-compensators. To relieve congestion in the operation of a network may require tracking this optimal parameter range whenever system states change.

The restructuring of the electric utility industry in many countries has provided open access to their transmission systems [1] and [2]. Generators and loads can access transmission system in a non-discriminatory and equitable manner. Open access to transmission systems is vital for healthy competition in power industry. As the number and magnitude of power transactions increase in open access, power transactions between generators and loads when implemented may sometimes congest the electric power network. Therefore, congestion management has become very important and difficult to deal with in the emerging deregulated electricity markets [3]. In a power market, transactions between producers and consumers are limited by line parameters, when the limitations exist for certain transactions, cost of electrical energy production will increase. Also, because of these limitations, in some regions market power may prevail. Electric markets need to set some rules or methods to manage the transactions to deal with congestion problem. Transmission congestion management may be categorized into short-term and long-term. The short-term transmission congestion management is based on rules and pricing [4], [5], [6] and [7]. The FERC expresses its preferences for market-based plans, and notes that the pricing and expansion program should be compatible with the pricing signals for shorter-term solutions to congestion management so that market participants can choose the least-costly response [8]. Congestion always increases the cost of electrical energy and persistent congestion in similar zones of transmission lines must be removed in the long-term by installing new lines or by utilizing FACTS devices so that competitive energy markets are accomplished through increased transmission capacities. The statistical methods proposed in literature may be used in congestion studies for long-term planning [9]. Grid planning is a long-term approach that requires decision-making tools to determine which part of a network should be developed in the future. Competitive markets expose transmission planners to new uncertainties. These are handled using a decision-analysis approach whose key contribution is quantifying and minimizing risk [10]. Long-term congestion relief between zones can be achieved through the strengthening of transmission lines. The strengthening of a transmission line can be accomplished either by building a new transmission line between the congested zones or by increasing the capability of the original line. There are many difficulties in most developed countries to building new additional lines to increase transmission capacity [11]. Power engineers may prefer the simple approach of constructing new lines to meet unexpected needs and to trade power over large regions. However, the reality is that the construction of new transmission lines is often opposed by local residents, landowners and environmentalists and, therefore, new lines may be difficult to achieve nowadays and it will become more difficult in the future. Therefore, engineers are looking for new ways of increasing transmission capacity without building new lines. Flexible AC Transmission System (FACTS) devices increase the capacity by loading the transmission lines up to their thermal limits and control the power flows by changing their parameters. FACTS from advanced energy management systems allows control of transmission flows without requiring new line construction. A study describing an analytical approach for operation of series-compensators to relieve power flow congestion has been published [12] in which changes in the direction and the amount of compensation have been determined according to sensitivity values derived from a DC power flow model under the assumption that generation dispatch is maintained. In this reference, a computer algorithm is proposed to relieve congestion while utilizing the controlled lines. This paper is organized as follows: the subsequent section is devoted to the thyristor-controlled series capacitor (TCSC) model and brief information about this device is given. In Section 3, congestion relief as a function of line susceptance and line capacity is investigated based on the bid functions of supplies. To achieve this goal, mathematical derivations to calculate sensitivities based on line susceptances are given. Studies based on sensitivity analysis and sequential quadratic programming (SQP) are implemented in Section 4, using an example power system to determine the congestion regions in terms of line parameters and the total cost when line capacities and line susceptances are varied. In Section 5, effects of DC and AC power flow analysis are compared as the line susceptances are changed. Section 6 summarizes the conclusions of the work done in this paper.

A sensitivity analysis and numerical study which should be important as an instrument for planning and operations engineers working within an open market structure has been performed to observe the effects of variations in transmission line susceptances and line capacities. Performing sensitivity analysis during congestion reveals the change in system variables from given susceptance deviations. Subsequently, the most effective susceptance deviations can be used to reach the objective. This study shows that the most sensitive line(s) may not be the congested line when the total system cost is considered. Moreover, there may be an optimum susceptance range, above and below which more congestion will be caused and total cost increased. An optimum susceptance range could be important when the line parameters are changed dynamically through the widespread use of FACTS devices in the future. To operate a network efficiently and relieve congestion may require tracking these optimal parameter ranges as system states change over time. Allowable susceptance deviations for removing congestion, and thus reduce system cost, are slightly smaller when DC power flow analysis is used, leading to more conservative results than using AC power flow. Therefore, in normal conditions, DC power flow analysis should be favored for large systems, where fast decisions are essential, due to its simplicity and computational speed. Because sensitivities change while supplies and demands vary, congestion management over the long-term requires careful statistical study of cost sensitivities compared to line parameters. Such studies will guide planners in selecting which lines to strengthen first to efficiently eliminate long-term congestion.