METHOD FOR IMPROVING VOLTAGE STABILITY SECURITY IN A POWER TRANSMISSION SYSTEM

Abstract: A method of locating enhancements for improving voltage stability security of an electric power transmission system having a plurality of buses and a plurality of sources of reactive reserves coupled thereto. The plurality of buses are grouped into a plurality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relationship. A first voltage control area is determined for locating a first enhancement, wherein the first voltage control area exhibits therein a difference in internal reactive loss that exceeds a loss threshold. A second voltage control area is determined for locating a second enhancement, wherein the second voltage control area exhibits therein a difference in reactive reserves that exceeds a reactive reserve threshold. The first and second enhancements may include switchable shunt capacitors, synchronous voltage condensers, static var compensators, or a combination thereof. A third enhancement, which includes series capacitors, parallel lines, or a combination thereof, is located at interfaces between two neighboring voltage control areas determined to exhibit thereacross a difference in either reactive power flow or reactive power loss that exceeds a predetermined threshold.

Description: METHOD FOR IMPROVING VOLTAGE STABILITY SECURITY IN A POWER TRANSMISSION SYSTEM Technical Field This invention relates generally to planning and control of electrical power transmission systems, and more particularly, to a method for improving volt¬ age stability security in electrical power transmis¬ sion systems. Background Art There are a number of potential voltage instability problems which can arise within an elec¬ trical power system. Some of these instability prob¬ lems occur in distribution systems used for distribut¬ ing electrical power to utility customers. Many of the sources of these distribution system voltage sta¬ bility problems have existed for years, and their causes and solutions are well known in the art. Other problems occur in transmission sys¬ tems, which are used for transporting bulk power from generation stations to load centers. These stability problems r« ult from such causes as facility outages, clearing of short circuit faults, and increases in load power or inter-area power transfer in a transmis¬ sion network. Many of these transmission system volt- age instability problems have been encountered only in recent years. These instability problems have oc¬ curred as a result of recent trends toward: locating generation stations distantly from load centers which limits the effectiveness of their voltage controls, requiring utilities to allow power shipment across their transmission system by independent power produc¬ ers or other utilities, and deterring construction of needed transmission networks, to name a few. A slow-spreading, uncontrollable decline in voltage, known as voltage collapse, is a specific type of transmission system voltage instability. Voltage collapse results when generators reach their field current limits which causes a disabling of their exci¬ tation voltage control systems. Voltage collapse has recently caused major blackouts in a number of differ¬ ent countries around the world. In order to reduce the possibility of volt- age collapse in a power system, and more generally, improve the stability of the power system, system planning is performed by many utility companies. First, a mathematical model representative of the basic elements of the power system, and their inter- connection, is constructed. These basic elements include generating stations, transformers, transmis¬ sion lines, and sources of reactive reserves such as synchronous voltage condensers and capacitor banks. Next, various computational techniques for analyzing system stability are performed using a suitably pro¬ grammed computer. Based on this analysis, proposed enhancements are formulated in an ad-hoc manner for improving voltage stability security. The mathemati¬ cal model can be updated based upon these proposed enhancements so that the resulting system stability security can be analyzed. Enhancements which attain predetermined design objectives are then physically implemented in the actual power system. The process of system planning is continual in that it must be regularly performed in light of changing circumstanc¬ es. In mathematical terras, voltage collapse occurs when equilibrium equations associated with the mathematical model of the transmission system do not have unique local solutions. This results either when a local solution does not exist or when multiple solu¬ tions exist. The point at which the equilibrium equa¬ tions no longer have a solution or a unique solution is often associated with some physical or control capability limit of the power system. Current methods for assessing proximity to classic voltage instability are based on some measure of how close a load flow Jacobian is to a singularity condition, since a singular load flow Jacobian implies that there is not a unique solution. These proximity measures include: (i) the smallest eigenvalue approaching zero, (ii) the minimum singular value, (iii) various sensitivity matrices, (iv) the reactive power flow-voltage level (Q-V) curve margin, (v) the real power flow-voltage level (P-V) curve margin, and (vi) eigenvalue approximation measures of load flow Jacobian singularity. The eigenvalue and minimum singular value methods are disadvantageous in their lack i ng an indi¬ cation of the actual locations and cause of voltage instability. Moreover, these methods have been known to produce misleading results with respect to causes of voltage instability as well as the locations and types of enhancements necessary to improve voltage stability security. Furthermore, the computational requirements for the eigenvalue and minimum singular value methods are relatively high. The sensitivity matrix methods have many of the same difficulties as the eigenvalue and singular value methods resulting from being linear incremental measures for a highly-n¬ onlinear discontinuous process. Regardless of the method employed for as- sessing proximity to classic voltage instability, existing methods employed by many utility companies assume that there is only one voltage instability problem. Further, it is assumed that one distributed reactive power loading pattern test detects the one voltage instability problem. It is known that a voltage control area may be defined as an electrically isolated bus group in a power system. Reactive reserves in each voltage con¬ trol area may be distributed via secondary voltage control so that no generator or station would exhaust reserves before all the other generators in the volt¬ age control area. Although this secondary voltage control is effective in preventing classic voltage instability, previously defined voltage control areas are no longer valid whenever the originally existing transmission grid is enhanced so that bus groups are no longer as isolated. A further disadvantage of this approach is that the reactive reserves for controlling each voltage control area are limited to be within the voltage control area. Methods are also known which employ a volt¬ age zone defined as a group of one or more tight¬ ly-coupled generator P-V buses together with the union of the sets of load buses they mutually support. In such methods, the amount of reactive power supply to maintain an acceptable voltage level is controlled. A disadvantage of this approach, however, is that char¬ acterizing a voltage stability margin in terms of voltage does not protect against classic voltage col- lapse. Current engineering methods of locating potential voltage instability problems includes simu¬ lating all single line outage contingencies, and iden¬ tifying those that do not solve as causing voltage instability. However, the lack of a solution is not a guarantee of voltage instability; a lack of a solution can occur because: the load flow Newton-Raphson-based algorithms are not guaranteed to converge from any particular starting solution, but converge only when the starting point is sufficiently close to the solu¬ tion; the load flow convergence is not guaranteed even when the system is close to a solution if the solution is close to a bifurcation; round-off error affects the load flow convergence; and discontinuous changes due to switching of shunt elements, or outages of genera¬ tors or lines can have a dramatic effect on whether the load flow algorithm will converge to a solution. The converged solutions for all single outages only indicates that there are no bifurcations. In order to attempt to prove that the absence of a converged solu¬ tion is caused by voltage instability, substantial manpower and computer processing time are required. In one such method, the absence of a converged solu- tion is determined to be due to voltage collapse if one can add a fictitious generator with infinite reac¬ tive supply at some bus to obtain a converged load flow solution. This method is not foolproof, and furthermore, does not indicate the cause of voltage instability nor indicate where it occurs. Once a voltage instability problem is detected, siting and selecting appropriate enhance- ments to ameliorate the instability problem is per¬ formed. There are several ad-hoc rules of thumb em¬ ployed by electrical utility companies for determining possible sites for enhancements. These rules include: adding parallel lines and/or series compensators at interfaces where the most severe voltage collapse-inducing contingencies occur; adding parallel lines and/or series compensators at lines with large I 2 X losses before and after voltage collapse-inducing contingencies; citing a synchronous voltage condenser at extra-high voltage (EHV) buses near load centers which are on the path of large reactive flows from generation centers; siting synchronous voltage con¬ densers at EHV buses around the system which have the space for installation of these devices; and adding switchable shunt capacitors or static var compensators (SVCs) so that generators do not exhaust reactive reserves (short term or long term) . However, current methods are incapable of identifying all of the many different voltage stabili- ty problems that can occur in a transmission system. A very routine operating change or supposedly insig¬ nificant contingency in a remote region of the system, followed by another contingency, can cause voltage instability. Furthermore, voltage instability may occur in many different sub-regions of the system. Current methods lack diagnostic procedures for identi¬ fying causes of specific voltage stability problems, as well as systematic and intelligent enhancement procedures for preventing voltage instability prob¬ lems. Summary ofthe Invention For the foregoing reasons, the need exists for a method of identifying potential locations of voltage instability problems, and determining correc¬ tive measures to reduce the likelihood of voltage instability. It is thus an object of the present inven- tion to provide an improved method for correcting potential voltage instability problems in an electri¬ cal power transmission system. Another object of the present invention is to provide an intelligent control of an electrical power transmission system in which the reactive re¬ serves used for controlling each voltage control area are not limited to be within the voltage control area. In carrying out the above objects, the pres¬ ent invention provides a method of enhancing voltage stability in a region of an electric power transmis¬ sion system having a plurality of buses and a plurali¬ ty of sources of reactive reserves coupled thereto. The plurality of buses are grouped into a plurality of voltage control areas such that each of the buses within each voltage control area has a similar corre¬ sponding reactive power versus voltage relationship. At least one interface between two neighboring voltage control areas which exhibits thereacross a difference in reactive power flow, between a first operating point and a second operating point of the power sys¬ tem, that exceeds a reactive flow threshold is deter¬ mined. At least one enhancement is located at the at least one determined interface. The present invention further provides a method of enhancing voltage stability in a region of an electric power transmission system having a plural¬ ity of buses and a plurality of sources of reactive reserves coupled thereto. The plurality of buses are grouped into a plurality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relationship. At least one interface between two neighboring voltage control areas which exhibits thereacross a difference in reactive power loss, be¬ tween a first operating point and a second operating point of the power system, that exceeds a reactive loss threshold is determined. At least one enhance¬ ment is located at the least one determined interface. The present invention still further provides a method of enhancing voltage stability in a region of an electric power transmission system having a plural¬ ity of buses and a plurality of sources of reactive reserves coupled thereto. The plurality of buses are grouped into a plurality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relationship. At least one voltage control area which exhibits therein a difference in reactive reserves, between a first operating point and a second operating point of the power system, that exceeds a reactive reserve threshold is determined. At least one enhancement is located within the at least one determined voltage control area. The present invention yet still further provides a method of enhancing voltage stability in a region of an electric power transmission system having a plurality of buses and a plurality of sources of reactive reserves coupled thereto. The plurality of buses are grouped into a plurality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relationship. At least one volt¬ age control area which exhibits therein a difference in internal reactive loss, between a first operating point and a second operating point of the power sys- tern, that exceeds a loss threshold is determined. At least one enhancement is then located within the at least one determined voltage control area. The present invention further provides a method of locating enhancements for improving voltage stability of an electrical power transmission system having a plurality of buses and a plurality of sources of reactive reserves coupled thereto. The plurality of buses are grouped into a plurality of voltage con¬ trol areas such that each of the buses within each voltage control area has a similar corresponding reac¬ tive power versus voltage relationship. A first volt¬ age control area is determined for locating a first enhancement, wherein the first voltage control area exhibits therein a difference in internal reactive loss, between a first pair of operating points of the power system, that exceeds a loss threshold. A second voltage control area is determined for locating a second enhancement, wherein the second voltage control area exhibits therein a difference in reactive re¬ serves, between a second pair of operating points of the power system, that exceeds a reactive reserve threshold. These and other objects, features and advan¬ tages will be readily apparent upon consideration of the following description, appended claims, and accom¬ panying drawings. Brief Description Of The Drawings FIGURE 1 is a flow chart of performing a contingency analysis according to the method of the present invention; FIGURE 2 is a flow chart of grouping buses into voltage control areas according to the method of the present invention; FIGURE 3 is a flow chart of determining a reactive reserve basin according to the method of the present invention; FIGURE 4 is a flow chart of performing a single contingency analysis according to the method of the present invention; FIGURE 5 is a flow chart of performing a multiple contingency analysis according to the method of the present invention; FIGURE 6 is a flow chart of determining voltage control areas according to the method of the present invention; FIGURE 7 is a flow chart of performing a contingency selection according to the method of the present invention; FIGURE 8 is a flow chart performing a reac¬ tive reserve basin security assessment according to the method of the present invention; FIGURE 9 is a flow chart demonstrating ro¬ bustness of the reactive reserve basins according to the method of the present invention; FIGURE 10 is a flow chart performing a sta¬ bility security assessment according to the method of the present invention; FIGURE 11 is a flow chart enhancing voltage stability in a region of an electric power transmis¬ sion system according to the method of the present invention; FIGURE 12 is a flow chart of another embodi¬ ment of enhancing voltage stability in a region of an electric power transmission system according to the method of the present invention; FIGURE 13 is a flow chart of a further embodiment of enhancing a region of an electric power transmission system according to the method of the present invention; FIGURE 14 is a flow chart of another embodi¬ ment of enhancing a region of an electric power trans¬ mission system according to the method of the present invention; and FIGURE 15 is a flow chart of system planning for an electric power transmission system according to the method of the present invention. BestModes For Carrying Out The Invention The present invention is first discussed herein generally, with a more detailed description following thereafter. In general, then, the method of the present invention is capable of identifying total¬ ly independent voltage stability problems that affect fairly isolated sections of one or more utilities. A unique voltage stability problem occurs when a Q-V curve computed at any bus in a sufficiently coherent group has the same shape, minimum, and reactive re¬ serve basin. The neighboring voltage control areas with reactive supply devices that exhaust nearly all reactive reserves upon reaching the minimum of the Q-V curve computed in some critical voltage control area is a reactive reserve basin for that critical voltage control area. A global voltage stability problem occurs when the reactive reserves in a large number of volt- age control areas are exhausted. Global reactive reserve basins for different global voltage stability problems do not contain any of the same voltage con¬ trol areas. Each global voltage stability problem is prevented by a unique and non-overlapping set of reac¬ tive supply devices belonging to its reactive reserve basin. For each global stability problem, a large set of local stability problems lie nested therewithin. In turn, each local stability problem has a different reactive reserve basin associated therewith. However, these local reactive reserve basins overlap. As a result, the possibility exists that a generator, swit¬ chable shunt capacitor or SVC belongs to several local reactive reserve basins. When the reactive reserves in a voltage con¬ trol area are exhausted, all reactive reserve basins to which that voltage control area belongs experience a significant step change toward voltage instability. The local reactive reserve basin that exhausts all reactive reserves in all voltage control areas due to contingencies or operating changes is the local reac¬ tive reserve basin that experiences voltage instabili¬ ty, as long as the contingencies or operating changes directly impact the critical voltage control area where the Q-V curve is computed to determine that reactive reserve basin. The exhaustion of all reac¬ tive reserves for all voltage control areas in a local reactive reserve basin produces voltage instability for that critical voltage control area because that critical voltage control area cannot obtain all the reactive supply needed to cope with the contingencies or operating changes. As used herein, a contingency may be any unexpected discrete change in the transmis- sion system due to equipment loss (such as a genera¬ tor, transmission line, or transformer) or a short circuit (typically referred to as a fault contingen¬ cy) . A locally most vulnerable critical voltage control area and reactive reserve basin is one that belongs to almost every local reactive reserve basin also belonging to a global reactive reserve basin. This locally most vulnerable reactive reserve basin has relatively small reserves that exhaust rapidly for Q-V curve stress tests computed for almost every local critical voltage control area which has local reactive reserve basins that are subsets of a global reactive reserve basin. Such locally most vulnerable reactive reserve basins should be the focus of any system en¬ hancements. It should be noted that local voltage sta¬ bility problems are those brought on by contingencies or operating changes and not the global voltage sta- bility problems which would most often only develop out of a spreading local voltage stability problem. Generally, all such local voltage stability problems need be addressed, not just the locally most vulnera¬ ble. This is so because each local stability problem, including the locally most vulnerable, may be brought on by different contingencies or operating changes that cause reduction of, or partially cut off, the reactive reserves associated with the critical voltage control area. More specifically, now, the method of the present invention employs Q-V curve tests for deter- mining a hierarchical control structure which indi¬ cates that voltage instability occurs when a lack of controllability is evident. Performing a multiple contingency analysis is illustrated by the flow chart shown in Figure 1. The multiple contingency analysis is to be performed for a region of a power system having a plurality of buses and a plurality of sources of reactive reserves coupled thereto. In block 100, the plurality of buses are grouped into voltage control areas in dependence upon a corresponding reactive power versus voltage rela¬ tionship for each of the buses. More specifically, each voltage control area is defined as a coherent bus group where adding a reactive load at any bus in the group produces nearly identical Q-V curves in both shape and magnitude. As a result, each voltage con¬ trol area has a unique voltage instability caused by a local incremental reactive supply problem. In block 102, determining a corresponding reactive reserve basin for each of at least one of the voltage control areas is performed. Each reactive reserve basin comprises at least one source of reac¬ tive reserves selected in dependence upon a quantity representative of the reactive reserves exhausted at a predetermined operating point of the power system. The at least one source of reactive reserves contained within the reactive reserve basin form a set of stabi¬ lizing controls for the corresponding voltage control area. Preferably, the predetermined operating point of the power system is the minimum of the Q-V curve. It is also preferred that the tote reserves in a voltage control area be depleted by a certain percent- age and/or below a certain level before the reactive sources in the voltage control area added to a reac¬ tive reserve basin. A single contingency analysis is performed by block 104. More specifically, a quantity represen¬ tative of the reactive reserves depleted in response to each of a plurality of single contingencies is computed. These single contingencies include single line outages and single generator outages. Using the information computed in the single contingency analy¬ sis, a multiple contingency analysis is performed in block 106. The multiple contingencies selected for analysis comprise at least two of the single contin¬ gencies whose corresponding reactive reserve depletion quantity exceeds a predetermined threshold. The mul¬ tiple contingency analysis is performed for at least one reactive reserve basin. In Figure 2, a flow chart illustrates group¬ ing the buses into voltage control areas in accordance with the present invention. Voltage control areas are defined as coherent bus groups where the Q-V curve computed at any bus in that coherent group has virtu¬ ally identical voltage and reactive margin at the Q-V curve minimum. Furthermore, the shape and slope of the Q-V curve computed at any bus in the voltage con¬ trol area should be nearly identical. Based on the above definition, the voltage control areas are deter¬ mined using a coherent group clustering algorithm. An initial value of a control parameter, alpha, for the clustering algorithm is selected in block 120. The coherent group clustering algorithm employed is based on eliminating the weakest connections from each net- work bus until the sum of reactive power-voltage Jaco¬ bian elements for eliminated branches is less than a parameter alpha times the largest diagonal element of the reactive power-voltage Jacobian matrix. The iso- lated bus groups identified for a particular alpha are the coherent bus groups for that alpha value. This step of isolating bus groups in dependence upon the alpha parameter is illustrated by block 122. For smaller values of alpha selected in block 120, the bus groups are continuously split until each bus group comprises a single bus. On the con¬ trary, if alpha is selected to be relatively large in block 120, all buses belong to one bus group. In block 124, a level of coherency within bus groups as well as a concomitant incoherency between bus groups is examined based upon the Q-V curves. In particular, the Q-V curves are examined to determine whether all buses in each bus cluster have substantially the same Q-V curve minimum. If the Q-V curve minima are not substantially the same, then flow of the routine is directed back up to block 120 where a new value of alpha is selected. If the Q-V curve minima are sub¬ stantially the same, then the routine is exited by return block 126. Determining the reactive reserve basin for each of at least one of the voltage control areas is illustrated by the flow chart in Figure 3. In block 140, a set of test voltage control areas are selected. The selected test voltage control areas are those that have large shunt capacitive supply, or an increase in reactive loss or reactive supply as Q-V curves are computed in neighboring test voltage control areas. Line charging, shunt capacitive withdrawal, series I 2 X series reactive loss, increased reactive inductive or capacitive shunts due to under load tap changers, or switchable shunt capacitors or reactors cause the increase in reactive loss or supply in a voltage con¬ trol area. A Q-V curve is computed in each test volt¬ age control area that has satisfied these conditions as Q-V curves were computed in other voltage control areas. Reactive reserve basins are only determined for those test voltage control areas, called critical voltage control areas, with Q-V curves having a large voltage and a small reactive margin at the minimum of the Q-V curve. In practice, the minimum of the Q-V curve can be obtained using a standard Newton-Raphson algorithm. For each critical voltage control area, the voltage control areas which experience a reduction in reserves greater than a predetermined threshold at the Q-V curve minimum is selected in block 142. In prac- tice, the predetermined threshold is measured on a relative scale and is selected to be less than 100%. In one embodiment, the reactive reserve basin includes voltage control areas which experience greater than 75% reduction in reserves in computing the Q-V curve down to the Q-V curve minimum. This logic is aimed at guaranteeing that every reactive reserve basin is robust in the sense that no contingency or operating change that causes voltage instability on the test voltage control area can exhaust all of the reactive supply and voltage control reserve in a voltage con¬ trol area outside those voltage control areas con¬ tained in the reactive reserve basin computed. In the flow chart of Figure 3, the reactive reserve basins are computed only for the selected subset of voltage control areas that are predicted to be vulnerable to voltage instability by having large capacitive supply, experiencing large shunt capacitive supply increases, or experiencing inductive increases as Q-V curves are computed in other test voltage con¬ trol areas having Q-V curve voltage minima greater than a threshold and reactive minima smaller than another threshold. Moreover, the use of reactive reserve quantities provides an accumulative proximity measure that makes voltage stability assessment prac¬ tical because it is an exhaustible resource that al¬ ways correlates well with proximity to voltage insta- bility and is easily computed for a contingency. In such a manner, unique global voltage stability problems can be identified that have large numbers of voltage control areas and are nearly dis¬ joint. Most, if not all, voltage stability problems that ever occur are local. Moreover, a multiplicity of local voltage stability problems are associated with each global voltage stability problem. Indeed, local voltage stability problems may be determined with a local reactive reserve basin that is substanti- ally a subset of some global reactive reserve basin. Identifying critical voltage control areas for each local stability problem and their reactive reserve basins identifies the location of each stability prob¬ lem, what reactive reserves prevent each local stabil- ity problem from occurring, and why each local voltage instability occurs. Still further, the locally most vulnerable reactive reserve basin may be determined that lies within virtually every other local reactive reserve basin according to the Q-V curve with nearly the larg- est voltage maxima and nearly the smallest reactive minima. Thereafter, its reserves are rapidly ex¬ hausted for the Q-V curve computed in the critical voltage control areas associated with the global and all nested local reactive reserve basins. However, despite the fact that the Q-V curve may have the larg¬ est voltage minima and the largest reactive margin, it may not be the most probable local voltage stability problem because there may not be severe contingencies that directly impact its critical voltage control area because it lies in a remote and low voltage part of the system. This leads to contingency selection for each local reactive reserve basin where in some utili¬ ties the same contingencies affect the global and all locals, and yet in other utilities different contin- gencies affect different locals within a global reac¬ tive reserve basin. Performing a single contingency analysis is illustrated by the flow chart in Figure 4. This sin¬ gle contingency analysis is performed for each criti- cal voltage control area and its associated reactive reserve basin. In block 160, a single contingency is simulated. Specific types of single contingencies include single generator outages and single line out¬ ages. The reactive reserves in each reactive reserve basin are computed for the single contingency in block 162. Conditional block 164 examines whether there are more single contingencies to be simulated. If so, flow of the routine is directed back up to block 160 where another single contingency is simulated. If no further contingencies are to be simulated, then the contingencies in each reactive reserve basin are ranked from smallest to largest based upon the reac- tive reserves exhausted by block 166. In block 168, the single line outages which exhaust more than a pre¬ determined percentage of the reserves in each voltage control area are listed. In block 170, the two largest reactive ca- pacity generators in each reactive reserve basin which exhaust more than a predetermined percentage of its reserve for some contingency are selected. These generators are placed on a generators list. The two lists formed in blocks 168 and 170 are used in forming multiple contingencies in a subsequent multiple con¬ tingency analysis. Performing multiple contingency analysis is illustrated by the flow chart in Figure 5. Using the list of single contingencies formed in block 168, a list of double line outages is formed in block 180. Similarly, using the list of generators formed in block 170, a list of double generator outages is formed in block 182. In block 184, a combination of line and generator outages from the lists formed in blocks 168 and 170 are used to form a combination list. The step of performing an analysis of contin¬ gencies based upon the lists produced in blocks 180, 182, and 184, is illustrated by block 186. Software for determining the voltage control areas is illustrated by the flow chart in Figure 6. In block 200, an initialization step is performed wherein a seed bus, a number of branches, and a mini¬ mum voltage level are selected in order to define a region of interest. Next, the Q-V curves are run and reactive reserve basins are determined at all buses in the region of interest in block 202. In block 204, a voting procedure is employed to select alpha where the Q-V curves computed at all buses in each bus cluster has substantially the same Q-V curve minimum and reac¬ tive reserve basin. The parameter alpha decides the size of the coherent bus clusters which form voltage control areas. As alpha decreases, the size of the coherent bus clusters increases through aggregation of coherent bus clusters identified for larger alpha values. This search procedure eliminates the need for a user to make a judgment on where the differences in voltage changes at buses within coherent bus groups increases from very small values, and the voltage change differences between buses in different bus groups for a disturbance suddenly increase to large values as alpha decreases. In the search procedure for alpha, a bounded interval of potential values of alpha is first select¬ ed. The procedure places a disturbance, namely a voltage change at some seed bus, and calculates the changes in voltage and angle at each bus due to the disturbance. The procedure finds bus clusters for ten equally-spaced alpha values in this bounded interval, and then finds the smallest alpha value where the voltage and angle changes within the bus group satisfy the following equations: AV j - A v ≤ Jc- ΔV j Δθ^ - Δθ i ≤ k 2 AQ ± where ΔV is a voltage change, Aθ is an angle change, i and j are indices representing two buses within a bus group, and kj and k 2 are fixed parameters. The results are confirmed as voltage control areas by running Q-V curves at all buses in the volt¬ age control areas to establish if alpha was selected properly such that the minima of the Q-V curves and the reactive reserve basin obtained from the minima of the Q-V curves are identical. If the alpha value was chosen correctly so that the Q-V curve minima and reactive reserve basins computed at every bus in the bus clusters selected are identical, the user has obtained the voltage control areas and proper alpha value for obtaining these voltage control areas. If the alpha value was not correctly selected because the Q-V curve minima and reactive reserve basins are not identical for buses in a voltage control area, several larger values of alpha that produce smaller bus clus¬ ter groups can be examined until bus clusters which have nearly identical Q-V curve minima and reactive reserve basins are found. Hence, computing voltage control areas in this manner is based on both the level of coherency within bus clusters and the level of incoherency across bus clusters. Alternative embodiments can be formed which explicitly use the definition of voltage control area in order to find alpha. More specifically, an alter¬ native embodiment would search for the value of alpha that is as small as possible, i.e. which produces the largest bus cluster, and yet assures that the Q-V curves computed at every bus in each bus cluster has nearly identical Q-V curve minima and reactive reserve basins. The search for alpha would only concentrate on bus clusters in some region of interest, which are buses above a certain voltage rating and at most three circuit branches from some seed bus. Turning now to Figure 7, a flow chart of a contingency selection program is illustrated. As seen therein, a contingency selection and ranking for con¬ tingencies and operating changes that bring a particu¬ lar test voltage control area and its reactive reserve basin closest to voltage instability is performed. The contingency selection and rankings are performed for each critical voltage control area and associated reactive reserve basin. In block 210, a single line outage contin¬ gency is simulated. The reserves in each reactive reserve basin are computed for that contingency in block 212. In conditional block 214, it is determined whether or not there are any other contingencies to be simulated. If there are further contingencies to be simulated, then flow of the method is returned back to block 210. If there are no additional contingencies to be simulated, then flow of the routine advances to block 216. In block 216, the contingencies are ranked in each reactive reserve basin based upon reactive reserves. In block 218, the line outages that exhaust more than P% of the reserves in each voltage control area are selected and placed in a list. Further, the largest two reactive capacity generators in each reac¬ tive reserve basin that exhausts P% of its reserve for some line outage are also selected. These generators are placed in another list. The list of generators is used to produce a set of severe single and double generator outage contingencies. The list of line outages are used to produce a set of severe single and double line outage contingencies. The list of genera¬ tors and line outages is used to produce a set of combination line outage and loss of generation contin¬ gencies. In block 220, the severe single and multiple contingencies are simulated and ranked based upon the reactive reserve in a reactive reserve basin. The contingency selection routine can be run several times in sequence to obtain all of the information on why particular reactive reserve basins are vulnerable to voltage instability. The initial run would entail taking all single line outages in one or more areas, or in one or more zones or areas where voltage insta¬ bility is to be studied, or in the entire system mod¬ el. In a preferred embodiment, the contingency selection routine would output a report summarizing the effects of the worst five contingencies for each critical reactive reserve basin. The output for each reactive reserve basin has an initial summary of the status in the pre-contingency case, including the bus names and numbers for all buses in each of the reac- tive reserve basin voltage control areas, the reactive supply capacity and reserves for generators, synchro- nous condensers, and switchable shunt capacitors at the bus where the component is located. After the initial status of a reactive re¬ serve basin is provided, the five worst contingencies for that reactive reserve basin are given. Each con¬ tingency is described and the reactive supply reserves at all generators and switchable shunt capacitors in each reactive reserve basin voltage control area are given. The order of voltage control areas in the report of voltage control area reactive supply reserves for a particular reactive reserve basin is based on the sequence of reserve exhaustion during computation of the Q-V curve. The order of voltage control areas aid in indicating the order of exhaus- tion as voltage collapse is approached for any contin¬ gency for that reactive reserve basin. The order of the contingencies presented in the output report for a reactive reserve basin is based on the percentage of pre-contingency reactive reserves exhausted with the contingency causing the largest percentage reduction reported first. The order of the reactive reserve basins presented in the output report is sorted so that the reactive reserve basins that experience the largest percentage exhaustion of reactive supply on generators and switchable shunt capacitors for that reactive reserve basin's worst contingency are report¬ ed first. The contingency selection routine assists the user in determining the reactive reserve basins that experience voltage instability because they would be the first to be reported. If no reactive reserve basin experience voltage instability, the reporting of the reactive reserve basins in the order of the larg¬ est percentage reduction in total reserves gives only a partial indication of the reactive reserve basin with the most severe contingencies. Percentage reduc- tion in total reactive reserves of a reactive reserve basin is an excellent indicator of the worst contin¬ gency in a reactive reserve basin and the most vulner¬ able reactive reserve basin when the system is experi¬ encing or is nearly experiencing voltage instability. The number of voltage control areas in a reactive reserve basin that exhausts reserves and the status of whether or not reactive reserves are exhausted on voltage control areas listed at the end of the list given for that reactive reserve basin are effective indicators in judging proximity to voltage instability when the contingency does not bring a reactive reserve basin close to voltage instability. The reason for utilizing both indicators for voltage collapse proxim¬ ity rather than percentage reactive reserve reduction is that the system experiences a quantum step toward voltage instability after each successive voltage control area experiences reserve exhaustion, and expe¬ rience indicates voltage control areas that exhaust reserves near the Q-V curve minimum for the pre-con- tingency case are near the Q-V curve minimum for most contingencies. An alternative embodiment of the contingency selection routine would further include modifying the set of reactive reserve basin voltage control areas reserve level for contingencies that lie in the path between a reactive reserve basin voltage control area and the test voltage control area. Such contingencies can have a reactive reserve basin that does not con- tain the pre-contingency reserve basin voltage control area that is totally or partially disconnected from the test voltage control area by the line outage con¬ tingency. Contingencies that have a modified reactive reserve basin and the voltage control area that should be deleted from the pre-contingency reactive reserve basin both can be detected by looking for contingen¬ cies where a reactive reserve basin voltage control area experiences little reduction in reserve compared to other severe contingencies. The deletion of these voltage control areas from reactive reserve basins for those contingencies will make the contingency ranking based on reactive reserve basin reactive reserves more accurate without requiring the user to make judgments. In Figure 8, performing a reactive reserve basin security assessment is illustrated by a flow chart. An initialization step is performed in block 230 wherein selected data is retrieved. This data includes base case simulation data, values of alpha, values of a lower voltage limit where attempts to compute a Q-V curve minimum are aborted, and the cri¬ terion used for selecting the reactive reserve basin voltage control areas. In block 232, each critical voltage control area is specified along with its test bus. The lists of single line outage, double line outage, single loss of generation, double loss of generation, and combina¬ tion contingencies are read in block 234. In block 236, the Q-V curves are computed for each contingency specified for the base case for each voltage control area. In conditional block 238, a check for a positive Q-V curve minimum is performed. If a Q-V curve has a positive minimum, then execution of the routine is stopped. If there are no positive Q-V curve minima, then execution of the routine pro- ceeds to block 240. In block 240, a transfer pattern and level are read and a Q-V curve is computed for each contin¬ gency and voltage control area. Conditional block 242 checks whether or not there is a Q-V curve with a positive minimum. If a Q-V curve with a positive minimum exists, then execution of the routine is stopped. Otherwise, the transfer level is increased until a positive Q-V curve minimum is obtained in block 244. If, at block 246, there are additional transfer patterns which need evaluation, then flow of the routine is directed back up to block 240. If no additional transfer patterns need evaluation, then a load pattern and level is read in block 248, and a Q-V curve is computed for each contingency and voltage control area. If there is a Q-V curve with a positive minimum as detected by conditional block 250, then execution of the routine is stopped. Otherwise, the load level is increased until a positive Q-V curve minimum is obtained in block 252. If, at block 254, additional transfer patterns need evaluation, then flow of the routine is directed back up to block 248. If no additional transfer patterns need evaluation, then execution of the routine is completed. Ideally, the computed reactive reserve bas- ins are robust. Robustness implies that the voltage control areas that experience near exhaustion of re¬ serves for all reactive supply and voltage control devices at the Q-V curve collapse point in the pre- contingency case can experience exhaustion of reserves at the Q-V curve collapse point after: any single contingency, transfer, or loading pattern change; or after any combination line outage and loss of reactive resource contingency; or after any combination line outage/loss of reactive resource contingency and any transfer or loading change in any pattern. Demon¬ strating that the reactive reserve basins are robust based on the above definition is illustrated by the flow chart in Figure 9. In block 260, a set of line outage contin¬ gencies, loss of resource contingencies, transfers, real power loading pattern changes, operating changes, and combination line outage/loss of resource contin¬ gencies that are known to exhaust reactive reserves in one or more specified reactive reserve basins as well as test buses in critical voltage control areas for computing the Q-V curves that produce each of these reactive reserve basins are provided as input to the routine. These inputs can be provided from the output of the contingency selection routine. In block 262, the voltage control areas belonging to a specified reactive reserve basin are determined by computing the Q-V curve and its minimum for each single or double contingency or operating change specified. The reactive reserve basins of the Q-V curve computed at a test bus in a critical voltage control area for each single or double contingency or operating change are outputted into a table for that critical voltage control area by block 264. This table is used to confirm that contingencies or operat- ing changes do not exhaust reserves on voltage control areas where all reactive supply and voltage control reserves are not nearly or completely exhausted when a Q-V curve is computed for the pre-contingency case at a test bus in a critical voltage control area. Performing an intelligent voltage stability security assessment is illustrated by the flow chart in Figure 10. The procedure involves determining, at block 270, the voltage control areas, i.e. the bus clusters where the Q-V curves computed at any bus have the same shape and the same curve minimum, and the same reactive reserve basin. These bus clusters are found based on coherency, in other words, the same voltage and angle changes are exhibited at all buses in the voltage control area due to any disturbance. Alternatively, the bus clusters are found based on controllability, observability, or modal properties. Next, the subset of all of the reactive supply resources within voltage control areas that exhaust all of their reactive supply at the minimum of the Q-V curve computed at any bus in the test voltage control area is determined at block 272. The minimum of the Q-V curve can generally be obtained using a normal Newton-Raphson algorithm using a standard pro- cedure that will obtain the minimum when the direct application of the Newton-Raphson algorithm would stop obtaining solutions short of the minimum. A second condition for buses to belong to a voltage control area is that the Q-V curve computed at each bus in a test voltage control area exhausts the same reactive supply resources in the same set of voltage control areas at the Q-V curve minimum. The subset of reactive supply resources in a system ex¬ hausted at the Q-V curve minimum is called the reac¬ tive reserve basin for that voltage control area. The slope of the Q-V curve decreases discontinuously each time all of the reactive supply reserves in one of the voltage control areas in the reactive reserve basin is exhausted. The reactive supply from a reactive re¬ serve basin voltage control area to the test voltage control area is maintained as long as one of the volt¬ age controls associated with reactive supply devices in a voltage control area is active and holds the voltage in that voltage control area. The discontinuity in the slope of the Q-V curve occurs not only due to loss of reactive supply from the reactive reserve basin voltage control area, but occurs due to the increased rate of increase in reactive losses with voltage decline that accompanies loss of all voltage control in a voltage control area. The reactive reserve basins are computed for only selected subsets of voltage control areas that are predicted to be vulnerable to voltage instability. The voltage control areas that can experience voltage collapse are predicted by determining those that have large shunt capacitive supply or experience large reactive network loss change for Q-V curves computed to determine the reactive reserve basin for a neigh¬ boring voltage control area. A further step entails determining, at block 274, those reactive reserve basins and their associat¬ ed test voltage control areas that are most vulnerable to single or multiple contingencies. The five worst contingencies, which either cause voltage collapse by exhausting all reactive reserves in the reactive re¬ serve basin or bring the reactive reserve basin clos¬ est to voltage instability by exhausting the largest percentages of the reactive reserves in that reactive reserve basin, are also found at block 276. A file of single worst line outage contin¬ gencies that exhaust P% or more of the reactive re¬ serves in any reactive reserve basin is produced at block 280. Further, a list of worst generator outage contingencies is also produced, at block 280, by iden¬ tifying the two largest capacity generators from each reactive reserve basin where one or more line outage contingencies exhaust P% or more of the reactive re- serve basin reserves. These two contingency lists are used to produce, at block 282, a list of all single line outages, all single generator outages, all double line outages, all double generator outages, and combi¬ nation line and generator outages. Also, a list of test voltage control areas where P% or more of the reactive reserves were exhausted by single line outag¬ es is produced. These files are used to compute Q-V curve minima and reactive reserve basin voltage control areas with reactive reserves for every contingency in the lists for each reactive reserve basin test voltage control area specified. Although the number of con¬ tingencies in the lists is preferably limited to the projected ten worst contingencies, a user may be al- lowed to run all of the other contingencies. In block 284, a security assessment for single and multiple contingencies with different tran¬ sfer and loading patterns is performed. Transfer limits are determined for each anticipated transfer pattern (specified by a group of generators with in¬ creasing generation in some percentage of the total transfer level and a group of generators with decreas¬ ing generation in some percentage of the total trans¬ fer level) . The transfer level is increased in incre- ents and Q-V curves are computed for all reactive reserve basin critical voltage control areas and all single and multiple contingencies. If all Q-V curves for all single and multiple contingencies in every critical voltage control area have negative Q-V reac- tive minima (implying voltage stability) the total transfer level is incremented again and all Q-V curves are recomputed. This process is repeated until one Q- V curve has Q-V curve positive minima (implying volt¬ age instability) . The total transfer level limit for the transfer pattern is thus determined. A transfer pattern level limit is computed for each anticipated transfer pattern and the reactive reserve basin where the Q-V curve is positive for one or more single or multiple contingencies is noted. The same process is repeated for loading patterns to find those reactive reserve basins that have positive Q-V curve minima for one or more contin¬ gencies. The reactive reserve basins that constrain each transfer (or loading pattern) and the contingen- cies that cause the voltage instability for that tran¬ sfer (or loading pattern) are used as the basis of designing enhancements that prevent voltage instabili¬ ty in that reactive reserve basin for those contingen- cies and a desired level of transfer (possibly larger than the current transfer limit) . It should be noted that the general planning design criterion for voltage instability only requires that a power system survive a worst combination generator and line outage and does not require that a system survive a double line outage contingency. If the load flow will not solve for some contingency, transfer pattern and level, or loading pattern and level, reactive reserves are increased in all generators in each global reactive reserve basin, one at a time. If the addition of reactive reserves in some global reactive reserve basin allows a Q-V curve load flow solution to be computed, then the contingency, transfer pattern and level, and loading pattern and level would cause a voltage instability in that global reactive reserve basin. This feature allows one to determine whether a contingency, or transfer or loading pattern causes a voltage instabil- ity in some other global reactive reserve basin than the one being studied. If one has performed the above assessment of transfer limits for each anticipated transfer pattern and loading limits for each anticipated loading pat- tern, one can determine the transfer pattern limits that need to be increased and the desired level, as well as the loading pattern limits that need to be increased and their desired levels. For each transfer (or loading) pattern where the design criterion is not satisfied out to the desired limit, one knows the local reactive reserve basin or basins and the contin¬ gencies that cause voltage instability in that reac- tive reserve basin or basins. This reactive reserve basin transfer pattern and contingencies are used to design the enhancements for that reactive reserve basin. The enhancements to be made for a particular combination of single and multiple contingencies, transfer pattern and level changes, and loading pat¬ tern and level changes identified via the security assessment require determining interfaces between voltage control areas, boundaries of voltage control areas, and the internal quantities within the control areas that suggest the causes of the voltage stability problem observed. One cause of voltage instability is "black hole" effect on any element which occurs when reactive power flows into the series inductive element of any component model from both terminals. This "black hole" can draw large (up to 11 p.u. MVAR) reactive power flows from all over the system and, more impor- tantly, choke off reactive flow through the element. "Black holes" develop as real and reactive flows in¬ crease, and are most severe when the direction of the real and reactive flow and the increase through the element are the same. "Black holes" also develop as voltages decline on both terminals. "Black holes" have only been detected on boundaries of voltage control areas or on interfaces between voltage control areas. Because "black holes" can by themselves produce voltage instability, one must search for them for each security assessment determined severe contingency, transfer pattern and level change, and loading pattern and level change. One must also search for "black holes" in combination with a Q-V curve stress test in some critical voltage control area affected by the contingencies or operat- ing changes. Series capacitive compensations of lines or addition of a parallel line can eliminate a "black hole" on several interfaces and boundaries, not just the interface and boundary with the worst "black hole" problem evidenced by the I 2 X losses observed on that branch. Another cause of voltage instability is voltage control areas with large net shunt capacitive supply to the rest of the system that experience sig¬ nificant shunt capacitive supply withdrawal as voltage declines. Conventional engineering wisdom has consid¬ ered voltage control areas that have large capacitive supply to be least vulnerable to voltage instability since they have large capacitive supply when voltage instability is known to be due to lack of reactive supply to a bus, voltage control area, or region. The truth is, however, that if a voltage control area experiences a large shunt capacitive withdrawal due to inadequate control of voltage, reac¬ tive power will flow toward that voltage control area from all over the system. A voltage control area with large shunt inductive increase will likewise be vul¬ nerable to voltage instability if sufficient reactive power cannot be imported to meet the shunt capacitive supply withdrawal or shunt inductive reactive load increase. A voltage control area can have above 4.0 p.u. net capacitive reactive supply if it is the ter¬ minus of several long high voltage lines with large line charging components, it has switchable shunt capacitors or fixed capacitors, or is on the low volt¬ age side of an under load tap changing transformer where tap setting increases to raise voltage. Trans- formers are almost always on voltage control area interfaces so one bus is in one voltage control area and the other bus is in another voltage control area. Tap changers can thus make a voltage control area, where tap settings are intended to raise voltage, have large shunt capacitive reactive supply and the other voltage control area it is connected to have large shunt inductive reactive loss. Both voltage control areas connected by tap changers that are at high volt¬ age ratings can be vulnerable if the tap settings change or hit limits. Changing a tap changer setting causes large increases in inductive shunt loss in the voltage control area with reactive supply or access to reactive supply. Reaching tap setting limits can allow the shunt capacitive supply in the other voltage control areas where voltage was being increased to withdraw as voltage declines due to tap changer set¬ tings being at limits. A search is needed to find voltage control areas with large shunt capacitive reactive supply withdrawal or large shunt inductive reactive loss increase due to security assessment determined severe contingencies, transfer pattern and level changes, and loading pattern and level changes, possibly in combi¬ nation with a Q-V curve stress test in critical volt- age control areas affected by the contingencies or operating changes. Still another cause of voltage instability is exhaustion of all reactive reserves in all voltage control areas in some local or global reactive reserve basin. These reactive reserves include reserve on synchronous generators, synchronous condensers, static var compensators, and switchable shunt capacitors. A search is finally needed to identify reactive reserve basin voltage control areas where all reactive re¬ serves are nearly exhausted due to computation of a Q- V curve in its critical voltage control area. Exhaus¬ tion of reactive reserves in each voltage control area causes a cutoff in reactive supply rate from that voltage control area due to voltage drop in the criti¬ cal voltage control area where the Q-V curve is com- puted. Exhaustion of reactive reserves in a voltage control area also causes a dramatic increase in reac¬ tive losses due to black hole effects, shunt capaci¬ tive withdrawal, and shunt inductive increase due to the loss of voltage control on voltage control devices in the reactive reserve basin voltage control area. Plots of changes in total series I 2 X losses on all elements in voltage control area boundaries and in specifically identified voltage control area bound¬ aries and interfaces for Q-V curves computed in criti- cal voltage control areas are needed. These plots show the relative effectiveness of each enhancement in solving these black hole problems and their magni¬ tudes. Plots of total shunt capacitive withdrawal and shunt inductive loss increase in all voltage control areas and the specifically identified voltage control areas are also needed. These plots show the relative effectiveness of each enhancement in solving these shunt capacitive supply withdrawal and shunt inductive increase problems. These above plots, along with plots of reac¬ tive reserves in each voltage control area in a reac- tive reserve basin, show the disastrous effects that only occur instantly when all reactive reserves ex¬ haust in any reactive reserve basin voltage control area as a Q-V curve stress test is applied to a criti¬ cal voltage control area. These plots need to be made for Q-V curves computed for all critical voltage con¬ trol areas affected by the security assessment identi¬ fied severe single and multiple contingencies, trans¬ fer patterns and levels, and loading patterns and levels. Perhaps the most important aspect of the design of enhancements is noting the sequence of how I 2 X losses develop on voltage control boundaries and interfaces, shunt capacitive withdrawal or inductive shunt increase occur in voltage control areas, and exhaustion of reserves occur in reactive reserve basin voltage control areas as a Q-V curve is computed in the critical voltage control area. This sequence appears to occur for line outages, generator outages, load patterns and transfer patterns that affect this critical voltage control area. Understanding this sequence can allow much improved coordinated and in¬ telligent design that in turn provides much improved robust protection against voltage instability at much lower financial investment. After performing such a voltage stability security assessment, suitable enhancements are select¬ ed and designed to remedy any potential voltage insta¬ bility problems. Four ways of preventing loss of voltage stability for a local or global reactive re¬ serve basin are as follows. Synchronous voltage condensers can be locat¬ ed in the critical voltage control area of the reac- tive reserve basin. For the purpose of this discus¬ sion, this enhancement is referred to as a Type I enhancement. This is the conventional logic used by utilities since it locates reactive supply where the Q-V curve is most sensitive to added reactive load or reactive loss. Though it is generally a good techni¬ cal option for radial voltage instability, it is not a good technical option for classic voltage instability. Locating a synchronous voltage condenser in the test voltage control area does not attack the causes of classic voltage instability, namely the lack of reac¬ tive reserves in a reactive reserve basin and reactive losses that choke off reactive supply from non-reac¬ tive reserve basin generation. A Type II enhancement entails an addition of lines or compensation to lines that increases the reactive supply rate from one or more non-reactive reserve basin voltage control areas to the global test voltage control area where the Q-V curve is computed. The rate of reactive supply from a voltage control area is monitored by the plots of reserves in these voltage control areas. The addition of lines or se¬ ries compensation of lines reduces the I 2 X losses on voltage control area boundaries and reactive losses within voltage control areas. Type II enhancements are generally the last of the four enhancement options to be assessed because these enhancements are the most expensive and would only be the enhancement of choice if the IX losses on lines was the major design flaw in the system. In accordance with the foregoing description of a Type II enhancement, an embodiment of enhancing voltage stability in a region of an electric power transmission system is illustrated by the flow chart in Figure 11. The electric power transmission system which is to be enhanced comprises a plurality of buses and a plurality of sources of reactive reserves cou- pled thereto. In block 290, the plurality of buses are grouped into a plurality of voltage control areas. As a result, each voltage control area comprises buses which have similar corresponding reactive power versus voltage relationships. Based upon the grouping per- formed in block 290, block 292 determines at least one interface between two neighboring voltage control areas which exhibits a large difference in reactive power flow between two operating conditions of the power system. More specifically, a difference between the reactive power flow at a first operating point of the power system and the reactive power flow at a second operating point of the power system is compared to a reactive flow threshold. The first operating point of the power sys- tern corresponds to a predetermined stressed or unstressed pre-contingency case. The second operating point of the power system corresponds to a predeter¬ mined collapse point, a predetermined post-contingency case, or a predetermined critical point for the pre- contingency case. For an interface which exhibits a difference in reactive power flow that exceeds the reactive flow threshold, at least one Type II enhance- ment is located at the interface. This step is repre¬ sentatively performed by block 294 in the flow chart. The Type II enhancement includes one or more series capacitors, one or more parallel lines, or a combina- tion of both. An alternative Type II enhancement of a region of an electric power transmission system is illustrated by the flow chart in Figure 12. In a similar manner as the embodiment of Figure 11, the buses of the electric power transmission system are grouped into a plurality of voltage control areas in block 300. In block 302, a step of determining at least one interface between two neighboring voltage control areas which exhibits thereacross a difference in reactive power loss between two operating condi¬ tions of the power system that exceeds a reactive loss threshold is performed. The two operating conditions of the power system can be formed from a pre-contin¬ gency operating point and a post-contingency operating point as with the embodiment of Figure 11. In block 304, at least one Type II enhancement is located at the at least one interface determined in block 302. A Type III enhancement is based on siting synchronous voltage condensers to add reactive reserves to all critical local and global reactive reserve basins that are vulnerable to voltage insta¬ bility for combination line outage/loss of resource contingencies, loading pattern and level change, and transfer pattern and level change. The SVC sites are often electrically close to the locally most vulnerab¬ le reactive reserve basin that belongs to sets of local reactive reserve basins that contain similar voltage control area subsets of the global reactive reserve basin voltage control areas and are the first to exhaust reactive reserves on computing the Q-V curve for the global test voltage control area from plots of voltage control area reactive reserves. A preferred enhancement may be two or more SVCs rather than one SVC of the same total reactive capacity so that reactive reserves are distributed among reactive reserve basin voltage control areas so that they exhaust reserves as close to the voltage minimum as possible for the Q-V curve computed in the global or local critical voltage control area for each severe global or local contingency. The severe con¬ tingencies should exhaust all or most of the reserves of the local reactive reserve basin. When reactive reserves are exhausted simultaneously, the reactive losses that build up rapidly after a voltage control area exhausts reserves and accelerates reserve exhaus¬ tion in the other reactive reserve basin voltage con- trol area is eliminated. Thus, the reactive reserves are distributed so they primarily serve contingency- induced loss of reactive supply, namely reactive sup¬ ply on outage of reactive source contingencies and line charging on line outage contingencies, and are not so totally consumed serving contingency-induced voltage decline and reactive losses that accelerate the voltage instability. A second, and more primary, purpose of lo¬ cating Type III enhancements is to prevent shunt capacitive withdrawal that could still be large even after synchronous voltage condensers are located to distribute reactive reserves as described above. Additional SVCs or synchronous generators may need to be located in the center of a set of non-reactive reserve basin voltage control areas with significant shunt capacitive withdrawal in the reactive reserve basin subregion. These SVCs or synchronous generation would hold voltage in non reactive reserve basin volt¬ age control areas, thus preventing this shunt capaci¬ tive withdrawal. These SVCs not only prevent shunt capacitive withdrawal but also should add another voltage control area to the global reactive reserve basin. In accordance with the foregoing description of a Type III enhancement, an embodiment of enhancing a region of an electrical power transmission system is illustrated by the flow chart in Figure 13. In block 310, a step of grouping the buses of the power system into a plurality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relationship is performed. This step of grouping can include the step of examining whether each of the buses within each voltage control area has a substan¬ tially similar voltage value and reactive margin at the minimum of the corresponding reactive power versus voltage relationship. In block 312, a step of determining a volt¬ age control area which exhibits a difference in inter¬ nal reactive loss, between two operating conditions of the power system, which exceeds a reactive loss thres- hold is performed. The two operating conditions can comprise a first operating point based upon a stressed or unstressed pre-contingency case, and a second oper- ating point based upon a collapse point, a post-con¬ tingency case, or a critical point for the pre-contin¬ gency case. At these determined voltage control ar¬ eas, at least one Type III enhancement is located. This step is representatively performed by block 314 in the flow chart. The at least one Type III enhance¬ ment can comprise one or more synchronous voltage condensers, one or more synchronous generators, or a combination of both. Adding reactive reserves to voltage control areas in a reactive reserve basin is referred to as a Type IV enhancement. Type IV enhancements are often inexpensive because switchable shunt capacitors can be switched in when contingencies occur. These enhance- ents are beneficial when the transmission system is not heavily clogged after line outage contingencies and heavy loads and transfers, and when there is lit¬ tle shunt capacitive withdrawal when reactive reserves in a global voltage control area are properly distrib- uted so that reactive reserves in every reactive reserve basin voltage control area are exhausted si¬ multaneously. Type IV enhancements can be employed to distribute reactive reserves in reactive reserve basin voltage control areas so they all exhaust simul¬ taneously near the voltage minimum after severe global contingencies. In accordance with the aforementioned description of Type IV enhancements, an embodiment of enhancing a region of an electric power transmission system is illustrated by the flow chart in Figure 14. In block 320, the buses of the power system are grouped into voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relationship. In block 322, at least one voltage control area which exhibits a difference in reactive reserves, between two operating conditions of the power system, that exceeds a reactive reserve thresh¬ old is determined. In block 324, at least one Type IV enhancement is located at the at least one voltage control area determined in the previous step. The at least one Type IV enhancement comprises one or more switchable shunt capacitors, one or more synchronous voltage condensers, or a combination thereof. Type IV enhancements should be the first enhancement type investigated in most utilities since it maximizes the current investment in transmission and reactive reserve basin resources, namely SVCs, synchronous condensers, and synchronous generation. Switchable shunt capacitors would be switched into every reactive reserve basin voltage control area when a major contingency occurs that could exhaust these reserves in voltage control areas of the global or local reactive reserve basin affected by the contin- gency. The switchable shunt capacitors would be dis¬ tributed in a reactive reserve basin so that each global or local reactive reserve basin voltage control area exhausts reserves simultaneously for all severe combination line outage and loss of generation contin- gencies. Although it may be difficult to distribute reactive reserves in the global or local reactive reserve basin voltage control areas so that the reac¬ tive reserves exhaust nearly simultaneously for all transfer and loading patterns after every severe glob¬ al contingency, preferably it should be attempted to the greatest possible extent. Progressive, rather than simultaneous, exhaustion of reactive reserves in reactive reserve basin voltage control areas prevents the progressive buildup of shunt capacitive withdrawal in voltage control areas and I 2 X losses on voltage con- trol area boundaries that are brought on by voltage decline and choke off reactive supply from non-reac¬ tive reserve basin voltage control areas. If reactive reserves cannot be exhausted in a voltage control area at voltages above .90 per unit (pu) after contingencies, changes in loading patterns or levels, or changes in transfer patterns or levels, then the shunt capacitors added at buses in that volt¬ age control area works effectively. The shunt capaci¬ tive reactive power added by switching in the Type IV capacitors would act as a constant power reactive supply because voltage would not change. However, if reactive reserves can be exhausted in a reactive re¬ serve basin voltage control area at voltages above .90 pu by plausible changes in transfer pattern and level, plausible changes in loading pattern and level, or other contingencies that would not switch in the Type IV shunt capacitors, then switching in capacitors in voltage control areas with no reactive reserves after contingencies may not achieve significant beneficial effect. Furthermore, it may make the system even more vulnerable to voltage instability for a subsequent contingency. It has been observed that additional reactive losses on the order of 3-10 times the amount of shunt capacitive withdrawal for a voltage control area without reactive reserves and voltage control can be caused. Constant power reactive supply, i.e. Type III enhancement, are employed when significant capaci¬ tive withdrawal occurs even when switchable shunt capacitors are switched in after certain contingen¬ cies, and the switchable shunt capacitors have been added to cause near simultaneous reactive reserve exhaustion in reactive reserve basin voltage control areas for Q-V curve calculation for each of these contingencies. Type III enhancements are also employed when some or all reactive reserve basin voltage control areas exhaust reserves due to: contingencies other than those where Type IV switchable shunt capacitors are switched in, plausible changes in transfer pattern and level, or plausible changes in loading pattern and level. In these cases, SVCs, synchronous condensers, or additional synchronous generation reactive reserves must be added as constant power sources so that Type IV enhancements behave as constant power sources. Type III enhancements can be located in reactive reserve basin voltage control areas just as Type IV switchable shunt capacitors are. Type III enhancements can also be located outside existing reactive reserve basin voltage control areas. This would add another reactive reserve basin voltage con¬ trol area, and distribute reactive reserves to other reactive reserve basin voltage control areas if prop¬ erly sited. If properly located in a nonreactive reserve basin voltage control area, Type III enhance¬ ments should be operated at a proper voltage set point and sized with sufficient reactive capacity so that it exhausts at or near the voltage at the minimum of the Q-V curve so that it does not allow internal voltage control area reactive losses to develop in these non- reactive reserve basin voltage control areas. Locat¬ ing two or more SVCs can provide greater flexibility in achieving objectives of enhancement design. Turning now to Figure 15, a system planning flow chart is presented which attempts to solve for a particular voltage stability problem with the least costly enhancement type. Further, the design is aug¬ mented with a more costly enhancement type if the causal factor addressed in designing the less costly enhancement type was not the only causal factor. More specifically, enhancements are investigated in the order of Type IV, Type III and Type II, which is the order of the cost of the hardware required to imple- ment each type. As the use of a more costly enhance¬ ment type is addressed, the design procedure further addresses design of less costly enhancements as sup¬ plements to cure affects of causal factors which may still remain, and are not specifically cured by the more costly enhancement type. The procedure attempts to optimize the design of each enhancement type in combination with lessor costly enhancement types to increase the effectiveness and minimize the cost of the design. Referring to Figure 15, a step of performing the voltage stability security assessment is indicated by block 330. Within this broad step, such steps as grouping the buses of the electric power system into voltage control areas, determining corresponding reac¬ tive reserve basins, performing contingency analyses, are performed. Furthermore, the insecure global and their associated local reactive reserve basins can be determined after running Q-V curves in their critical voltage control areas. The worst single and multiple contingencies for the global and its associated local reactive reserve basins can be determined based upon the contingency analysis. In block 332, Type IV enhancements are de¬ signed in reactive reserve basin voltage control areas so that reactive reserves in each voltage control area exhaust nearly simultaneously. More specifically, the Type IV enhancements attempt to distribute reactive reserves in voltage control areas so that all voltage control areas exhaust reserves substantially simulta¬ neously at or near the minimum of the Q-V curve for the more severe contingencies, plausible transfer pattern and level changes, and all plausible loading pattern and level changes. If the exhaustion of reserves in reactive reserve basin voltage control areas can be prevented for transfer pattern and level changes, loading pattern and level changes, and less severe contingencies, and if exhaustion of reserves on all reactive reserve basin voltage control areas can be achieved at the minimum of the Q-V curve for the more severe contingencies in operating changes, then only Type IV enhancements are implemented. If the Type IV enhancements fail to achieve either of these two objectives, then Type III enhance¬ ments are implemented outside the reactive reserve basin voltage control areas to better achieve these objectives through the use of reducing internal volt¬ age control area reactive losses. This, in turn, makes it more difficult to exhaust the reserves in all reactive reserve basin voltage control areas at or near the minimum of the Q-V curve for all worst con¬ tingencies, and to prevent exhaustion of reserves in reactive reserve basin voltage control areas for other contingencies, plausible transfer pattern and level changes, and plausible loading pattern and level changes. The Type III enhancements are implemented to prevent internal voltage control area reactive losses, to maintain reactive reserves on all voltage control areas down to the Q-V curve minimum for all severe contingencies, and to maintain reactive reserves of the reactive reserve basin voltage control areas for all severe contingencies, plausible transfer pattern and levels, and plausible loading pattern and level changes. This step is indicated by block 334. After designing the Type IV and Type III enhancements, a step of designing Type II enhancements is performed by block 336. Type II enhancements should be implemented if series I 2 X losses on voltage control area boundaries are large after severe global or local contingencies. Adding series line compensa¬ tion or adding parallel lines to voltage control area interfaces with large I 2 X losses are generally the preferred Type II enhancements. Type II enhancements on a path to a non-reactive reserve basin voltage control area may add that non-reactive reserve basin voltage control area to the reactive reserve basin and thereby eliminate large I 2 X losses as well as utilizing existing non-reactive reserve basin reactive supply as part of the reactive reserve basin. If such enhancements reduce the I 2 X losses and add a reactive reserve basin voltage control area or areas, the design of Type III and Type IV enhance¬ ments should be repeated since the Type II enhance¬ ments may only partially solve the required objec¬ tives, and make Type III and/or Type IV enhancements necessary. For example, if Type III and Type IV enhancements were designed in a first iteration, and I 2 X losses on voltage control area boundaries are large, then Type II enhancements are needed and Type III and Type IV enhancements need to be redesigned. The Type III and Type IV enhancements need to be redesigned as supplements for distributing and increasing reactive reserves in the voltage control areas of the reactive reserve basin, and for reducing internal reactive losses in non-reactive reserve basin voltage control areas. If the I 2 X losses on voltage control area boundaries are small, then there is no need for Type II enhancements, and the Type III and Type IV enhancements are all that are needed to be implemented. The previously described embodiments of the present invention have many advantages. One advantage is that the intelligent control method is capable of curing the locally most vulnerable voltage instability problems, the local voltage instability problems, and the global voltage instability problem. Another ad- vantage is that the present invention identifies a global stability problem and each associated local voltage stability problem that have reactive reserve basins that contain subsets of the voltage control areas that belong to the global reactive reserve ba¬ sin. The loss of stability for each such problem is caused by a lack of sufficient reactive supply to its critical voltage control area. The reactive reserve basin is the critical voltage control that maintains voltage and thereby prevents the reactive losses that consume and choke off reactive supply from outside, as well as inside, the respective reactive reserve basin from reaching the critical voltage control area. Loss of local or global voltage stability occurs by ex¬ hausting the reactive reserve basin reserves that remove the reactive supply to the critical voltage control area and disable the critical voltage controls that allow the reactive losses to develop that choke off remaining reactive supply to the critical voltage control area. A global voltage stability problem generally has many individual local voltage stability problems associated therewith. Each local voltage stability problem can occur due to different contin¬ gencies or in some cases due to the same severe con¬ tingencies. The advantages still further include detect¬ ing each critical voltage control area, its reactive reserve basin, the severe single and multiple contin¬ gencies that cause voltage instability in several local reactive reserve basins and may even cause a global voltage instability. The various embodiments of the present invention can selectively design en- hancements for one or several local reactive reserve basins that are affected by voltage instability sepa¬ rately. For example, one could design enhancements for geographically northern, central, and southern local voltage stability problems that are associated with a global voltage stability problem if they are found affected by different single and multiple con¬ tingency plausible changes in loading pattern and level, and plausible changes in transfer pattern and level. A preferred embodiment makes the method of the present invention robust by siting and selecting en¬ hancements such that reserves are added to all reac¬ tive reserve basins so that, to the greatest extent possible, all single or multiple contingencies, plau¬ sible changes in transfer pattern and level, and plau¬ sible changes in loading pattern and level could not cause local voltage stability problems but would only cause global voltage stability problems because all the reactive reserves in the global reactive reserve basin effectively protect every local critical voltage control area. While this objective is never attain¬ able with finite financial resources, the present inve _ion provides a method showing what may be at- tainable with such finite financial resources. Another advantage of the present invention is that the proximity to voltage instability is great¬ ly improved as compared to enhancements selected by the ad-hoc procedures. A further advantage is that the cost of the enhancements is greatly reduced in comparison to the cost associated with the ad-hoc procedures. Moreover, the loading and transfer levels which can be sustained without voltage instability are greatly increased in comparison to the ad-hoc enhance- ments. While the best modes for carrying out the invention have been described in detail, those famil¬ iar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the follow¬ ing claims.

Claims: What Is Claimed Is: 1. A method of enhancing voltage stability in a region of an electric power transmission system having a plurality of buses and a plurality of sources of reactive reserves coupled thereto, the method com¬ prising: grouping the plurality of buses into a plu¬ rality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relation¬ ship; determining at least one interface between two neighboring voltage control areas which exhibits thereacross a difference in reactive power flow, be- tween a first operating point and a second operating point of the power system, that exceeds a reactive flow threshold; and locating at least one enhancement at the at least one interface. 2. The method of claim 1 wherein grouping a plurality of buses comprises: determining whether each of the buses within each voltage control area has a substantially similar reactive margin at the minimum of the corresponding reactive power versus voltage relationship; determining whether each of the buses within each voltage control area has a substantially similar voltage at the minimum of the corresponding reactive power versus voltage relationship; and determining whether each of the buses within each voltage control area has a substantially similar reactive reserve basin at the minimum of the corre¬ sponding reactive power versus voltage relationship. 3. The method of claim 1 wherein the at least one enhancement comprises a series capacitor. 4. The method of claim 1 wherein the at least one enhancement comprises a parallel line. 5. The method of claim 1 wherein the first operating point is a pre-contingency operating point, and the second operating point is a post-contingency operating point. 6. A method of enhancing voltage stability in a region of an electric power transmission system having a plurality of buses and a plurality of sources of reactive reserves coupled thereto, the method com- prising: grouping the plurality of buses into a plu¬ rality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relation- ship; determining at least one interface between two neighboring voltage control areas which exhibits thereacross a difference in reactive power loss, be¬ tween a first operating point and a second operating point of the power system, that exceeds a reactive loss threshold; and locating at least one enhancement at the at least one interface. 7. A method of enhancing voltage stability in a region of an electric power transmission system having a plurality of buses and a plurality of sources of reactive reserves coupled thereto, the method com- prising: grouping the plurality of buses into a plu¬ rality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relation- ship; determining at least one voltage control area which exhibits therein a difference in reactive reserves, between a first operating point and a second operating point of the power system, that exceeds a reactive reserve threshold; and locating at least one enhancement within the at least one voltage control area. 8. The method of claim 7 wherein the at least one enhancement comprises a switchable shunt capacitor. 9. The method of claim 7 wherein the at least one enhancement comprises a synchronous voltage condenser. 10. The method of claim 7 wherein the at least one enhancement comprises a static var compensa¬ tor. 11. A method of enhancing voltage stability in a region of an electric power transmission system having a plurality of buses and a plurality of sources of reactive reserves coupled thereto, the method com¬ prising: grouping the plurality of buses into a plu¬ rality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relation¬ ship; determining at least one voltage control area which exhibits therein a difference in internal reactive loss, between a first operating point and a second operating point of the power system, that ex¬ ceeds a loss threshold; and locating at least one enhancement within the at least one voltage control area. 12. The method of claim 11 wherein the at least one enhancement comprises a synchronous genera¬ tor. 13. The method of claim 11 wherein the at least one enhancement comprises a synchronous voltage condenser. 14. The method of claim 11 wherein the at least one enhancement comprises a static var compensa¬ tor. 15. A method of locating enhancements for improving voltage stability in a region of an electric power transmission system having a plurality of buses and a plurality of sources of reactive reserves cou¬ pled thereto, the method comprising: grouping the plurality of buses into a plu- rality of voltage control areas such that each of the buses within each voltage control area has a similar corresponding reactive power versus voltage relation¬ ship; determining a first voltage control area for locating a first enhancement, wherein the first volt¬ age control area exhibits therein a difference in internal reactive loss, between a first pair of oper¬ ating points of the power system, that exceeds a loss threshold; and determining a second voltage control area for locating a second enhancement, wherein the second voltage control area exhibits therein a difference in internal reactive reserves, between a second pair of operating points of the power system, that exceeds a reactive reserve threshold. 16. The method of claim 15 wherein the first enhancement comprises a synchronous generator. 17. The method of claim 15 wherein the first enhancement comprises a synchronous voltage con- denser. 18. The method of claim 15 wherein the first enhancement comprises a static var compensator. 19. The method of claim 15 wherein the second enhancement comprise a switchable shunt capaci- tor. 20. The method of claim 15 wherein the second enhancement comprises a synchronous voltage condenser. 21. The method of claim 15 wherein the second enhancement comprises a static var compensator. 22. The method of claim 15 further compris¬ ing determining an interface between two neighboring voltage control areas for locating a third enhance¬ ment, wherein the interface exhibits thereacross a difference in reactive power loss, between a third pair of operating points of the power system, that exceeds a reactive loss threshold. 23. The method of claim 22 wherein the third enhancement comprises a series capacitor. 24. The method of claim 22 wherein the third enhancement comprises a parallel line. 25. The method of claim 15 further compris- ing determining an interface between two neighboring voltage control areas for locating a third enhance¬ ment, wherein the interface exhibits thereacross a difference in reactive power flow, between a third pair of operating points of the power system, that exceeds a reactive flow threshold.

Inventors: SCHLUETER ROBERT ANTHONY, SCHLUETER, ROBERT, ANTHONY
Applicants: UNIV MICHIGAN, MICHIGAN STATE UNIVERSITY
Application year: 1995
Main classification: H02J3
IPC codes: H02J3