COST-BENEFIT ANALYSIS OF VOLTAGE SAG MITIGATION STRATEGIES

Abstract

Power Quality (PQ) disturbances can have significant economic consequences for various types of customer facilities. Moreover, the recent trend towards deregulated electricity market, industry automation and widespread use of electronics goods has put electric utilities under severe stress to improve the quality of supply to customer. Among various types of PQ disturbances e.g. interruptions, voltage sags, voltage swells, overvoltages, under-voltages, transients, voltage unbalance, voltage flicker and, harmonics; voltage sags are the most frequent. Voltage sag is a momentary (i.e., 0.5-30 cycles) reduction in the supply RMS voltage (on one, two, or all three phases) between 10 % and 90 % of the nominal voltage. Voltage sags are usually caused by network faults. However, voltage sags can also be caused by switching-on of a large load e.g. starting of large induction motor-load on a weak utility system. The equipment used in modern industrial processes e.g. programmable logic controllers (PLCs), adjustable speed drives (ASDs), personal computers (PCs) and motor contactors are highly sensitive to voltage sags and therefore, due to the increased use of such equipments in modern industrial processes, voltage sags have become one of the central PQ concerns both for utilities and customers. Consequently, the ability of modern industrial process equipments to ride-through voltage sags and to keep equipment operational is becoming more and more important as never before; any down time can be directly co-related to loss of production, revenue, and profits. Production loss in manufacturing units, ranging up to millions of dollars is attributed at times to a single disruption of the process whereas the costs to commercial customers e.g. banks, data centers, customer service centers etc. can be just as high if not higher. These cost include loss of productivity, labour cost for clean-up and subsequent start-ups (e.g. lubrication and thermal cycling), damaged product, reduced product quality, delays in delivery, reduced customer satisfaction, losing customers due to delay in delivery and ageing of machine due to process interruption etc. This has resulted in an urge to improve the ridethrough capability of equipments against voltage sags by equipment manufacturers and customers, and to improve the PQ as a whole by the utilities. Customers with sensitive loads may soon demand from their distribution companies, detailed data showing the Abstract number of voltage sags that should be expected at various locations in the network. Given the probability of voltage sags of different types, magnitude and shape, customer may also want to know from equipment manufacturer how many of these voltage sags will cause their ASDs, PLCs and PCs to trip or fail. Combining these two types of data would indeed allow calculating the expected cost of failures due to voltage sags. This information could then be used to negotiate appropriate connection rates with the distribution companies. It could also guide the choice of location for new plants, selection of manufacturing equipments or suggest the purchase of mitigation devices. Even though grid operators put mucheffort in preventing the occurrences of faults leading to voltage sags, it is impossible to eliminate voltage sags entirely. Still, the general PQ level of the grid can be enhanced by the utility adopting different network topologies; thereby improving the voltage sag performances at various load points of interest where highly sensitive industrial processes might be running. However, modifying the grid topology is not the only solution. The additional way for utility and the customers to reduce process interruptions beyond the general security level of the grid is to individually invest in mitigation devices. In these cases, installing mitigation devices between the grid and the vulnerable processes is the only possibility that can be applied. The goal of mitigation strategies is the reduction in process interruptions, making the total process disruption cost lower. It is, however, not straight forward to choose among various commercial solutions and needs the proper tool for cost-benefit analysis of various solutions available. To determine whether there are cost-effective mitigation methods to avoid or limit the damage, requires detailed information on several aspects, such as an estimation of the expected number of voltage sags at the load points of interest, an overview of possible solutions and a correct economic decision-making criterion are described in this work. Before making any investment for improving the PQ, it becomes mandatory for the utility engineers to evaluate the economic impacts of PQ on the customers being served by the existing system network. The evaluation needs an understanding of the expected voltage sag performance from the supply system. The best choice will then depend upon the damage costs claimed by customers due to poor PQ and the total operating cost of various solutions to improve the PQ performance of the utility supply system. The evaluation requires an understanding of the costs ofPQ disturbances to the customers. 11 Abstract The economic assessment of losses due to voltage sags needs the information about (i) number and characteristics of voltage sags expected at a specific location over a period of interest (ii) information about the sensitivity of various types of equipment connected (iii) number of such equipment participating in an industrial /commercial process along with their inter-connections and finally, (iv) information about the average cost (claimed by different types of customers served atthat point) attributed to a single trip ofthe process due to voltage sags The analysis is performed to identify the vulnerable areas, load points and customer facilities reporting higher losses and therefore, used to enhance the PQ specifically, at these points requiring utmost attention as well as overall PQ of the supply system. Statistical sags characterization of power networks is essential for regulatory purposes as well as for selecting mitigation methods. Statistics on voltage sags may be obtained by means of i) monitoring of the power supply, and /or ii) stochastic assessment of voltage sags. Monitoring is expensive and requires long monitoring periods to get reasonable accuracy. Stochastic assessment is a simulation method that relates the historical fault data with deterministic data regarding the residual voltages during the occurrence of faults. The methods used for voltage sag assessment are - the method of critical distances and the method offault positions. The method of critical distances is applicable strictly to radial networks only. Whereas the method of fault positions is suitable to assess the voltage sag performance of meshed power systems. This method is based on considering a number of fault positions spread throughout the power system. But the drawback of this method is that a large number of fault positions are required to get the required accuracy for voltage sag assessment. In addition, analytical approaches contrary to the method of fault positions do not require the assignation of fault positions. In these methods, results are based on the solution of analytical expressions, which are valid for any location of faults in the power system. After the assessment of voltage sag performance, a methodology is discussed for probabilistic assessment of financial loss due to voltage sags, which is applicable to both assessment of individual customer losses and assessment of total network losses. It takes into account, all the uncertainty associated with the voltage sag calculation, sensitivity of in Abstract customer's equipment to voltage sags, the interconnection of the equipment within the industrial process, the customer types and the location of the process in the network, in a probabilistic manner. After the assessment of the associated economic losses, without making any investment, the power system network can be reconfigured by the supplier as to obtain the optimal topology resulting in the minimization of financial losses with the help of existing network switching devices. However, to further reduce the financial losses due to voltage sags, mitigation devices such as Flexible AC Transmission Systems (FACTS) devices have to be incorporated into the system at suitable locations. In this thesis, various methods of voltage sag assessment are described e.g. method of critical distance, method of fault position and the analytical method. Specially, Voltage sags produced by balanced and unbalanced short-circuits are analyzed by means of a new analytical method. The main feature of this approach is that contrary to the method of fault positions, it does not require a pre-assignment of discrete fault positions along the lines. After determining the remaining voltages at network buses with the help of the proposed analytical method, the historical fault rates of variouspower networkequipments have been used to calculate the expected frequencies of voltage sags of different voltage magnitudes and durations. The sag durations are assigned with the help of typical operating times of protection devices placed at different levels of power system network. These sags are compared with the sensitivities of various equipments participating in industrial /commercial processes running at the selected buses of the existing network which provides the information about the number of process trips due to expected voltage sags at these locations. This information is used in the assessment of financial losses due to voltage sags at individual buses as well as for the whole network. After the assessment of financial losses for the existing power network, efforts are made for the minimization of financial loss with reconfiguration of network topology. The developed methodology starts with a selected number of switches, which generate various topologies. First of all, load flows are performed to ascertain the feasibility of these topologies and then for each feasible topology, voltage sag assessment is again performed at different buses in the network and to calculate financial losses incurred by voltage sags at buses of interest. These topologies are then ranked in the increasing order of total number of voltage sags, process trips and financial losses. It has been observed that depending on the criteria of rankings, the optimal topologies are not identical. IV Abstract For further reduction of financial losses due to voltage sags, suitable locations for mitigation devices can be selected for each individual optimal topology based upon the maximum number of voltage sags reported, maximum number of process trips or the maximum financial losses reported at different buses of interest. Under conventional vertically integrated electricity market, if the utility is interested in improving the voltage sag performance of the system network, the best location for installing a mitigation device would be the bus reporting maximum number of voltage sags for the optimal topology chosen on the basis of total number of voltage sags. However, as the enhancement of overall power quality of the system needs investment, the best location from the utility point of view is the bus having maximum financial losses reported for the optimal topology chosen on the basis of total financial losses. However, for the optimal topology chosen on the basis of financial losses it might be possible that at the location having maximum process trips reportedmay not be having maximumfinancial losses reported but the frequent process trips may irritate the customers connected at such load points and the utility might face the risk of losing valuable customers to other competing utilities, specially, in modern deregulated power scenario. Therefore these customers would ask the utility to place mitigation devices at their supply points and the utility may charge more for the improved power quality or the utility may advice the customers to have mitigation devices installed themselves and later price negotiation can be made for new tariffs. Therefore, in this thesis two criteria for placement of mitigation (FACTS) devices are proposed - (i) based upon utility criteria and (ii) based upon customer criteria. The proposed work describes the static modeling of FACTS devices for minimization of financial loss due to voltage sags. A new approach for voltage sag assessment using system impedance matrix that incorporates FACTS devices is presented. The detailed mathematical models of two types of FACTS devices - D-STATCOMand SVC using sequence network and system impedance are described. This simplifies the process of voltage sag performance analysis in large systems with FACTS devices and enables huge amount of voltage sag data to be obtained in an efficient way which is then used for financial loss assessment due to voltage sags. The best solution is determined using economic analysis which includes the costs of the voltage sag problem (cost to customers for disruptions caused by the voltage sags) and the cost of the mitigation alternatives. In this work, author has developed a complete methodology to gather this information and carry out these estimates.

This work focuses on the best mitigation device, which minimizes the total annual costs i.e. (PQ costs + solution costs). The PQ cost of each alternative is evaluated in terms of lowering the financial losses due to voltage sags whereas the solution costs include capital cost as well as the running cost. Therefore, the cost-benefit analysis of two abovementioned FACTS devices is performed to choose the best mitigation device. Four types of economic analysis methods, such as the payback period, net present value (NPV), internal rate of return method (IRR) and profitability index (PI) are used. The developed methodology has been applied to the two Indian systems namely, Uttarakhand transmission system and Haridwar district distribution system. In addition, a real case study of cement industry is also presented.