CONGESTION MANAGEMENT UNDER TRANSMISSION OPEN ACCESS

Abstract

The deregulation of previously monopolistic power sector and open access to transmission grid has resulted into the economic benefits and improvement in quality of services to society. However, the system operators are more concerned about the efficient, secure and reliable day-to-day operation of restructured powersystem. Hence, efficient and robust methodologies are required to be developed by the researchers, which may be trustworthy for the system operator to handle power system operation under deregulated environment. Under the prevailing deregulated environment, although different countries have adopted different models of restructuring of their power systems, but all are having same goal to create a competition friendly environment. For maintaining competitive environment in the field of electricity sector, the production of electric energy has been treated separately as a commercial product and transmission is considered to be the transportation service, which transports energy from suppliers to buyers. When considered as a commodity, electric energy can be traded in a free market, regulated only by the bids of consumers and suppliers. With the advent of deregulation in electricity sector, a number of Generation and Distribution Companies have emerged into the picture and replaced the vertically integrated structure of electric utilities. However, the interconnected transmission system is considered to be a natural monopoly so as to avoid the duplicity, inherent problems of right-of-the-way, environmental issues and huge investment for new infrastructure, and to take the advantage of the interconnected network. The interconnected transmission grid allows the efficient and reliable use of resources and reduces risk of power failures. In order to ensure a healthy competitive environment, an Independent System Operator (ISO) has emerged as a generic operator, which sets rules and protocols for open and non-discriminatory access of transmission services. The open access transmission framework results in more intensive use of the transmission system by the market participants, and in turn may lead to the situations in which transmission network is not able to accommodate all the desired transactions due to violation of some system constraints. This phenomenon is commonly termed as Abstract "congestion". The congestion in transmission system is one of the serious hindrances in front of perfect competition in deregulated environment. Thus, management of transmission congestion is one of the important functions of ISO. Congestion is not a new phenomenon as it was also there in transmission networks ofpre-deregulation periods, but then its management was relatively less challenging, since all the generation together with transmission (and sometimes distribution also) were regulated by a unique government company. After the deregulation, when electric energy is being traded like a commodity in open market, its prices are negotiated depending upon supply and demand. As it is well known fact that electricity can not be stored in bulk, so time-of-use is an important factor, which decides the price of this commodity. In other words, losses and/or transmission network congestion may make delivery of electric energy from the least expensive generator to some different location, most expensive or even impossible. The spot pricing theory, developed by Caramanis, Bohn and Schweppe in 1982, makes use of supply and demand conditions at a particular instant to determine the price at that instant. The optimal spot prices at different locations or locational marginal prices (LMPs) in a power system are determined based on criterion which not only include the cost of power produced but also its delivery at that particular location and at that particular time. So, the LMPs have the capability to provide economic signals regarding the congestion in the system. Specifically, LMPs of real power are the shadow prices associated with the bus real power balance constraints of market dispatch problem. In pool based deregulated electricity markets, ISO develops the day-ahead generation and demand schedule based on the market dispatch problem. For that, ISO takes supply and demand bids from Generation Companies (GenCos) and Distribution Companies/ bulk consumers (DistCos), respectively, and formulates the market dispatch problem as to maximize the social welfare (DistCos benefit minus GenCos cost) subject to operational and security constraints. The supply bids of GenCos and demand bids of DistCos equivalently represent their marginal costs and marginal benefits, respectively. In this market scenario DistCos make payment and GenCos receive payment based on LMPs ofreal power at their respective locations. In deregulated environment, the supply of reactive power comes under the ancillary services required to be procured. However, supply of reactive power by the generators is ii Abstract mandatory upto a certain level. Therefore, GenCos must be compensated for their beyondmandatory supply of reactive power in deregulated environment. In this research work, capability curve of synchronous generator is explored to develop reactive power procurement cost function. The reactive power procurement cost to be paid to a GenCo is represented in terms of lost opportunity to trade real power reserve by its generating unit. The social welfare objective function of market dispatch problem is modified to include cost of reactive power procurement in addition to demand benefit and real power generation cost. This will incorporate an economic signal regarding production and consumption of reactive power within LMPs payable to the suppliers and chargeable from the consumers at different locations. In the power system of pre-deregulation periods, the power was dispatched based on a common criterion of generation cost minimization. In deregulated market dispatch, inclusion of benefit functions (of elastic DistCos) in the objective function of market dispatch problem would guarantee that LMPs charged at the demand buses is less than or equal to DistCos' marginal benefit, earned byselling that power to retail customers. In case ofcongestion in the system, the market dispatch would give signal to reduce the demand by a particular amount at those buses for which marginal benefit is less than marginal price, which will ultimately lead to reduction in system congestion. In other words, we can say that inclusion ofconsumer benefit functions will assure system congestion to be managed at an acceptable level. In this research work, the effect of incorporation of consumers' benefit bids on managing system congestion at acceptable level is demonstrated with the help of decomposition of LMPs of real and reactive power into energy/reference, loss and congestion components. The negative slope bidding of DistCos is able to control the market price and congestion, but it is much likely that DistCos have to reduce their demands during peak hours. On the other hand, the price responsive demand shifting bidding of DistCos, which has been recently introduced in the literature, is able to make up the loss ofload from peak periods to off-peak periods. In this research work, a day-ahead market dispatch methodology based on price responsive demand shifting bidding of DistCos under power system network constraints, has been proposed. The proposed methodology has the advantages that i) the system congestion and LMPs can be controlled, and ii) the in Abstract consumers are able to recover their energy consumptions, which have been lost during peak hours, in day-ahead markets. Broadly, the congestion management approaches can be classified into two categories 1) preventive type, and 2) corrective type. While preventive congestion management takes care of line limits during the market dispatch, the corrective approach performs adjustments in the pre-specified schedule for the management of congestion. In the corrective congestion management approaches, it is customary to represent the line flows in terms of load flow jacobian based sensitivity matrix, which would reduce the computational efforts and time. However the sensitivity matrix developed from load flow jacobian depends upon selection of slack bus, which may lead to biased and unfair operation and allocation of congestion management cost in deregulated environment. Therefore, a more realistic approach is required, which can comply with the prevailing deregulated environment. In this research work, the use of bus impedance matrix is explored to determine the sensitivities of line flows in congested transmission lines as a function of bus injections. Acongestion management optimization problem is formulated as to minimize the cost involved in re-dispatching the generators based on incremental and decremental bids subject to line flows (as determined from bus impedance matrix based sensitivities) congestion constraints and bounds on rescheduled power generations. In contrast to conventional load flow jacobian matrix based sensitivities, the bus impedance matrix based sensitivities are independent of the selection of slack bus; therefore it guarantees a fair rescheduling of generators and allocation of congestion cost in deregulated environment. In the pool based electricity markets, the market clearing price is decided by the scheduling of thermal GenCos, and hydro GenCos are considered to be price takers, which can maximize their revenue by bidding their energy at a price close to but smaller than forecasted "System Marginal Price" of each hour. In general, the hydro GenCos can perform self-scheduling for selling their energy produced in day-ahead market to maximize their own profit. This is somewhat justified on the ground that consideration of hydro GenCos as price makers would create complexities due to the involvement of inter temporal stochastic constraints. On the other hand the time span ofcongestion management process is one hour and congestion re-dispatch is considered hour by hour, so the bids and IV Abstract constraints corresponding to hydro GenCos can easily be incorporated in congestion relieving re-dispatch. In this research work, a congestion management re-dispatch methodology is proposed considering the combined operation of hydro and thermal GenCos in a pool based energy market on hourly basis. The proposed congestion management problem is formulated as mixed binary nonlinear programming problem to minimize the cost of re-dispatching the hydro and thermal GenCos to alleviate congestion subject to operational, line overloading and water availability constraints. A piecewiselinearized unit performance curve is used in this formulation (by making use of binary variables), which takes into account its non-concave nature. The increased competition in electricity markets due to deregulated environment has resulted into the unprecedented use of transmission systems, which often leads to the congestion in transmission networks. The alleviation of transmission congestion is generally being carried out by re-dispatching the available generation capacity, on shortterm basis, usually through an objective of congestion management service cost minimization. Since the congestion is a frequent phenomenon in deregulated environment, the cost of its management becomes too high, which puts extra burden on the market participants. Hence, it would be more economical to invest in transmission network reinforcement, which otherwise is also needed to enhance the reliability of the supply. The Federal Energy Regulatory Commission of USA has stressed upon the "economic planning studies" to reduce congestion and to integrate new resources and loads, through its order no. 890, which has been issued to assist the transmission providers in their development of planning process. In recent developments, the electricity market oriented and congestiondriven approaches are being used in transmission expansion planning. A literature survey of transmission expansion planning has been performed for transmission expansion planning in the deregulated environment. As a general trend, LMP based approaches have been used for transmission expansion planning in the deregulated environment of electricity markets. However, while developing LMPs from social welfare maximization, dc load flow based models have been used, which do not include Q-V sub-problem and pricing signal for reactive power procurement. These approximations would lead to the inaccuracies in the final results and outcome of transmission expansion planning may not present a real life picture to market participants. A pricing signal for reactive power procurement needs to be included in the formulation in order to provide incentives to the generator companies for their reactive power supply in deregulated environment. In this context, the present research work proposes a method for transmission expansion planning considering the interconnection of prospective transmission lines (taken care by binary variables) and takes into account the requisite incentives to generator companies for their supply of reactive power. The transmission expansion planning problem is formulated as to minimize annualized investment cost of prospective transmission lines and maximize social welfare, subject to operational and security constraints. The social welfare function is modified to include the reactive power procurement cost function. The proposed problem decides the transmission expansion plan based on a number of potential scenarios of market conditions. The construction of new transmission lines raises many issues such as ecological problems, right-of-way and huge construction cost. Fortunately, the advent of power electronics based Flexible Alternating Current Transmission Systems (FACTS) technology has achieved a sufficient maturity and FACTS devices can be incorporated to capture the unutilized potential of transmission systems. In this research work, the optimal allocation of Thyristor Controlled Series Compensators (TCSCs) is explored for congestion management and social welfare maximization while minimizing the usage cost of TCSCs. The existing approaches for determining the location of TCSC placement are based on single scenario and perhaps neither of the existing approaches for TCSC placement takes into account cost of reactive power procurement in deregulated environment. In this research work, the social welfare function is modified to include reactive power procurement cost to be paid to the generator companies. The proposed technique decides the optimal allocation of TCSCs in deregulated power systems on the basis of a number of potential scenarios of market conditions. Specifically mentioning, the major contributions of this research workencapsulate the following points. 1. The capability curve of synchronous generators is explored to develop the reactive power cost function of GenCos. The social welfare objective function of market dispatch problem is modified to include cost of reactive power VI Abstract procurement. This will incorporate an economic signal regarding production and consumption of reactive power within LMPs payable to the suppliers and chargeable from the consumers at different locations. 2. The inclusion of consumer benefit functions (of elastic consumers) in market dispatch problem of deregulated environment, assure that system congestion to be managed at an acceptable level. The effect of incorporation of consumers' benefit bids on managing system congestion at acceptable level is demonstrated with the help of decomposition of LMPs of real and reactive power into energy/reference, loss and congestion components. 3. The currently practiced negative slope bidding by the DistCos are limited to individual periods and may lead to their loss of load during peak hours. In this research work the price responsive and demand shifting bidding of DistCos, which is able to make up the loss of load from peak hours to off-peak hours, has been explored for controlling the congestion and LMP spikes in day-ahead market. 4. In the corrective congestion management approaches, line flows are represented in terms of load flow jacobian based sensitivity matrix. However the sensitivity matrix developed from load flow jacobian depends upon selection of slack bus, which may lead to biased and unfair operation, and allocation of congestion management cost in deregulated environment. A congestion management optimization problem is formulated for re-dispatching the generators for congestion management cost minimization subject to line flow and power balance constraints. The use of bus impedance matrix is explored to represent the line flows in terms of bus power injections, which is independent of the selection of slack bus. Therefore it guarantees a fair rescheduling of generators and allocation of congestion cost in deregulated environment. 5. A novel congestion management re-dispatch methodology is proposed considering the combined operation of hydro and thermal GenCos in a pool based energy market on hourly basis. The proposed congestion management problem is formulated as mixed binary nonlinear programming problem to minimize the cost of re-dispatching the hydro and thermal GenCos to alleviate

congestion subject to operational, line overloading and water availability constraints. A piecewise-linearized unit performance curve is used in this formulation (by making use of binary variables), which takes into account its non-concave nature. 6. A transmission expansion planning problem is formulated in deregulated environment as to minimize annualized investment cost of prospective transmission lines and maximize social welfare, subject to operational and security constraints. The social welfare function is modified to include the reactive power procurement cost function as in point 1. The proposed transmission expansion planning problem takes into account a number of potential scenarios of market conditionson the basis of one annum and is solved with a mixed integer nonlinear programming (MINLP) approach. 7. An algorithm has been devised for the solution of transmission expansion planning problem as discussed in point 6. The proposed algorithm selects those prospective lines for transmission expansion, which were having large difference in LMPs at their end buses and meet the requirement of minimum pre set increment in social welfare. Moreover, it selects the transmission expansion plan on the basis of a number of potential scenarios of one annum. 8. The placement of TCSCs can be utilized for congestion management in deregulated power systems. The optimal allocation of TCSCs in deregulated power systems is determined by an optimization problem which maximizes social welfare and minimizes usage cost of TCSCs, subject to transmission congestion and operational constraints. As already discussed, the social welfare function is modified to include reactive power procurement cost to be paid to the generators. The proposed MINLP technique for optimal allocation of TCSCs takes into account a number of potential scenarios of market conditions.