Figure 1. Transfer Capabilities and Related Terms
The increasing demand for electricity in the recent decades had resulted in several problems in the normal operation of the power system. The transmission lines are forced to operate near their limits; critical components are going out of service and also the efficiency of transmission lines is reducing. To address all these issues, evaluation of transfer capability of transmission lines is important. Many deterministic and probabilistic methods are available for evaluation of transfer capability of transmission lines. On computing the available transfer capability of power system, the efficiency of the transmission lines can be evaluated and congestion management in the system is also possible. By knowing the efficiency of existing transmission lines, necessary steps can be initiated to improve the same and hence to operate the entire system more reliably and economically. This paper aims at discussing the fundamentals of ATC computation and reviewing various technical papers related to the same.
The concept of competitive industries compared to the regulated one has become prominent in the past few years. Economists and political analysts have promoted the idea that free markets can drive down costs and prices thus reducing inefficiencies in power production. This change in the climate has fostered regulators to initiate reforms to restructure the electricity industry to achieve better service, reliable operation, and competitive rates. In deregulated power systems, transmission networks are subjected to various bilateral service contracts between customers and suppliers. A bilateral transaction can be represented by a source (positive injection) connected to the point of injection and a sink (negative injection) connected to the point of extraction. The source and the sink are assumed to be of the same size and that the other generation and consumption of the system remains unchanged. In the issue of bilateral transactions, Available Transfer Capability (ATC) can be used as an indicator of relative system security and the profits are made in these bilateral power transactions. ATC is a measurement of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses [12]. Transfer capability is the maximum power that can be transferred from one area to another area. In 1996, North American Electric Reliability Council (NERC) defined a framework for Available Transfer Capability (ATC) definition and evaluation. According to the NERC definition, ATC is the transfer capability remaining between two points above and beyond already committed uses. The ATC value between two points is given as:
ATC = TTC - TRM - CBM - ET
Here TTC is Total Transfer Capability, TRM is Transmission Reliability Margin, CBM is Capacity Benefit Margin, and ETC is Existing Transmission Commitment including customer services between the same two points [6]. Figure 1 represents transfer capability and related terms.
Figure 1. Transfer Capabilities and Related Terms
The need for transfer capability computations are listed as follows.
Conventionally, the following methods are used for ATC (Available Transmission Capability) computation.
The basic concept of OPF approach is formulating the TTC calculation as an optimization problem, with equity constraints of power flow, inequality constraints from basic operation and equipment limits, and transient stability security requirements. The objective function, obviously, is the maximum power flow on the specified transmission route. To determine the total transfer capability, the objective is to maximize the power transfer between the two areas subjected to the conditions that there is no voltage or thermal or stability limit violations [2]. This method can be implemented by many optimization approaches, such as interior point approach, neural network, and two-level optimization approach [11].
Continuation power flow method is a comprehensive tool for tracing the steady state behavior of the power system due to parametric variation. The parameters which are varied, include bus real and/or reactive loads, area real and/or reactive loads, and real power generations at generator or P-V buses. Continuation methods are also known as curve tracing or path following which are used to trace solution curves for general non-linear algebraic equations with a parametric variation. These methods have four basic elements:
This is a mathematical way of identifying each solution for quantifying next solution or previous solution.
To find an approximate point for the next solution. Tangent or secant method is used for this purpose.
To correct error in an approximation produced by the predictor before it accumulates.
To adapt the step length for shaping the traced solution curve [2].
This method repeatedly solves power flow equations at a succession of points along the specified load generation increment. The advantage of this approach is its simple implementation and the ease to take security constraints into consideration [2]. This method involves the solution of a base case, which is the initial system conditions, and then increasing the transfer. After each increase, another load flow is solved and the security constraints are tested [1].
TTC is calculated using probabilistic approaches, such as non sequential Monte Carlo and stochastic programming. In these studies, sometimes maximization of power transaction between areas is only emphasized and sometimes commercial aspects. One of the most common approaches for transfer capability calculations is the Continuation Power Flow (CPF). In principle, CPF increases the loading factor in discrete steps and solves the resulting power flow problem at each step. CPF yields solutions at voltage collapse points. However, since CPF ignores the optimal distribution of the generation and the loading together with the system reactive power, it can give conservative transfer capability results [9] .
In [4], a novel method was proposed to select the uncertainty faults affected ATC greatly, then the calculation time can be saved and efficiency can be improved without effect the compute accuracy. Different generator output adjustment mode is also considered when calculating ATC. When getting the possible states based on the contingency selection, to each specific state the repeated linear iteration based on sensitivity analysis is used to calculate ATC.
In [8], the determination of the Available Transfer Capability (ATC) is formulated as a problem of finding the solution of a system of equations using the point wise maximum function, which collapses all operating constraints of the system into one equation. The resulting equations are semi smooth. The semi smooth equations are solved by the use of a smoothing function. Two solution algorithms are presented. One is a smoothing Newton method in which the smoothing parameter is treated as an independent variable. Another one is a smoothing decoupled Newton method, which incorporates the inherent weak-coupling characteristics of power systems into the algorithm and is suitable for solving large scale problems.
In [13], a problem based on transient thermal circuit equation and modified thermal limits are studied, which involves mathematic model of Optimal Power Flow (OPF) about Available Transfer Capability (ATC). The weakest lines are found applying modal analysis, then it is estimated whether the transmitting capacity is conditioned by the temperature variation of the lines by using constraints of temperature substitute for the thermal limits of the weakest lines. The new model is constructed in order to overcome conservatism of judgment by current capacity, and the latent transmitting capacity will be fully excavated.
The research paper [14] mainly focuses on the evaluation of the impact of FACTS control on Available Transfer Capability (ATC) enhancement. Technical merits of FACTS technology on ATC boosting are analyzed. An optimal power-flow-based ATC enhancement model is formulated to achieve the maximum power transfer of the specified interface with FACTS control. For better studying the capability of FACTS control, a power injection model of FACTS devices, which enables simulating the control of any FACTS devices, is employed.
In [5], a method has been described for determining the ATC between any two locations in a transmission system (single-area or multi-area) under a given set of system operating conditions. The method also provides ATCs for selected transmission paths between the two locations in the system and identifies the most limiting facilities in determining the network's ATC. In addition, the method can be used to compute multiple ATCs between more than one pair of locations.
Three currently used methods of TTC determination are presented and compared in [10]. Besides these methods, transfer-based security constrained OPF (TSCOPF) method is proposed in this paper as a replacement of conventional SCOPF method, for use in the deregulation environment. Both TRM and CBM, which account for reliability of the system, are seldom mentioned in the papers associated with ATC. This paper presents a probabilistic method to assess TRM, proposes rules and a procedure to allocate CBM and two methods of incorporating CBM into ATC. A modified IEEE RTS is utilized to demonstrate the proposed methods and the results show that the values of ATC are quite different when margins are taken into account and the methods of incorporating ATC affect the ATC value significantly.
The paper [3] presents a detailed formulation and implementation of a fast program for Available Transfer Capability (ATC) calculations. The formulation is based on the linear incremental power flow to account for the line flow thermal loading effects. An efficient linear ATC implementation enhances speed in processing a large number of contingencies to determine ATC for each specified transfer.
The work in [7], have described about the ACPTDF based approach for multi transaction cases using power transfer sensitivity and Jacobean matrix. The results have been determined for intact cases taking multi-transaction (simultaneous) as well as single transaction cases. This paper presents the application of the Static Var Compensator (SVC) to enhance the transfer capability of a power system incorporating the reactive power flows in ATC calculations. By redistributing the power flow, the ATC is improved. Studies on a sample IEEE 24-bus RTS power system model are presented to illustrate the effectiveness of SVC device to improve available transfer capacity as well as voltage profile.
This paper presents the importance of Available Transfer Capability (ATC) and methods available for ATC computation by reviewing the existing literature. The different methods for evaluation of ATC have considered various constraints and their own merits and demerits. Few methods are computationally complicated and few may result in reduced accuracy to evaluate ATC. Other than these, many computationally intelligent techniques may also be implemented to evaluate ATC. The computation of ATC will definitely enhance the efficiency of existing transmission lines and hence to operate entire power system more reliably and economically.