Major Causes Of Voltage Instability

Major Causes Of emf In constancyI. INTRODUCTIONPower governing body stability is be as the characteristics of a exponent establishment of rules to remain in a put in of Equilibrium at normal operating conditions and to restore an acceptable realm of equilibrium after a disturbance. The bureau governing bodys be heavily stress due to the change magnitude institutionalizeing and this fartings to potential drop stability problem. Voltage stability fire as well be called as the effect stability. A position system lacks the capability to transfer an infinite amount of electrical cater to the dilute. The main factor causing potency imbalance is inability of the power system to meet the demands for antiphonal power in the heavily distressed systems to keep desired potentials. Voltage instability in the system chiefly occurs in the form of a progressive decay in electromotive force magnitude at some of the tidy sumes. A possible outcome of potential drop inst ability is loss of bear down in an area, or tripping of transmitting rail sop ups and some other(a) elements by their protective system leading to cascading outages. FACTS devices have been used, twain for steady state power precipitate control and combat-ready stability control. Using controllable components, such as controllable serial optical condensers and level shifters, identify decreases can be adjustmentd in such a substance that thermal limits are not violated and losses are minimized. These also increases stability shore, help in fulfilling contr true(a) requirement, without violating system operating limits.II.VOLTAGE STABILITY epitome subsequence Power FlowContinuation power flow was introduced to solve this characteristic problem. The sequel power flow can be described as a power flow resultant that can maintain the stability of the power system nether normal and disturbances conditions. Therefore the main inclination of Continuation Power Flow is to find the continuity of power flow root for a given load change.It employs the predictor-corrector scheme with an addition of load argument and the technique used is local parameterization. ikon 1 predictor and Corrector illustration in CPFAs shown in Fig.1, it starts from a known solvent and uses a tangent predictor to estimate a subsequent solution corresponding to a different value of the load parameter. This estimate is hence corrected using the same NR technique employed by a conventional power flow. The local parametization provides a operator of identifying apiece depict along the solution path and plays an integral part in avoiding uniqueness in the Jacobian.First let represent the load parameter such that0 tinyWhere =0 corresponds to base load and = critical corresponds to critical load. We incorporate into conventional Newton Raphson load flow equations. Load change in PLi and QLi terms are modified by breaking each term into two components- one corresp onds to original load and other represents load change brought about by a change in load parameter . ThusPLi= PLi0 + (kLi Sbase cos i) (1)QLi= QLi0 + (kLi Sbase sini) (2)Where the following definitions are madePLio, QLio original load at plenty i, active and oxidizable respectively.kLi multiplier to designate the rate of load change at sight i as changesi power factor move of load change at bus iSbase- a given quantity of apparent power which is elect to provide appropriate scaling of In addition, the active power multiplication is modified asPGi = PGio (1+ kGi ) (3)Where PGio is the active generation at bus i in the base case and kGi is a constant used to describe the rate of generation as varies. Now the Jacobian gets modified with the addition of a new element d. The tangent vector is calculated and the predicted solution is determined. With the local parametrization technique corrected solution is obtained.B. incident be1) nonoperational payload MarginContingenci es such as unexpected breeze outages lots contribute to potential difference collapse blackouts. These contingencies generally reduce or as yet eliminate the voltage stability adjustment. To maintain security against voltage collapse, it is coveted to estimate the effect of contingencies on the voltage stability margin. Action can then be taken to increase the margin so that presumable contingencies do not cause blackout.Contingency can be defined as to a condition which involves removal of line, disconnection of generator or transformer. This creates a condition which disturbs the normal state of the system and may lead to voltage instability. A number of methods have been proposed for static voltage stability entrap contingency ranking. However there exists a need of in force(p) method requiring minimum computational time to accurately rank the contingencies establish on static voltage stabilityFigure 2 stable loading marginThe system contingencies have been ranked base d on post contingency VAR requirement using two methods-Static Loading Margin (SLM) and antiphonal Compensation Index (RCI). True ranking of the assorted contingencies have been obtained considering post-contingency static loading margin. The fore around step is to perform continuation power flow by using PSAT software. The static loading margin is the distance amid the base case operating establish and the horn in organize. A contingency having smaller value of the static loading margin can be considered more severe.2) Reactive Compensation IndexReactive compensation index is used to perform voltage stability based contingency ranking by measuring severity of the outages. It is based on the premise that the distance between the normal case (pre contingency) nose caput (max loadability express) and the post-contingency case nose point can be approximated by the bring reactive injection required at the load buses to maintain like voltages. The ranking obtained by reactive c ompensation index is compared with the true ranking.III. FACTS DEVICESThe fictile AC transmitting systems controllers have been established as an effective means in improving the system stability including voltage stability, enhancing loadability and also providing voltage control.TCSCFigure 1 shows the simple diagram of TCSC comprised of a series capacitor bank, shunted by a Thyristor Controlled Reactor (TCR), to provide a smoothly changeable series capacitive reactance. It is a one-port circuit in series with transmission line it uses natural commutation its switching frequency is low it contains undistinguished energy storage and has no DC port. Insertion of a capacitive reactance in series with the lines inherent inductive reactance lowers the total, effective impedance of the line and thus virtually reduces its length. As a result, both angular and voltage stability gets im examined. However, the sub coincidental series resonant frequency is produced that introduces damag ing damping of generator models leading to unstable system. That is the reason for not placing TCSC between lines having generators at both the ends.Figure4 Equivalent circuit of TCSCSSSCStatic Synchronous Series Compensator (SSSC) is a voltage sourced converter based series FACTS device that provides capacitive or inductive compensation independent of line current. The SSSC is a synchronized voltage type compensator which is analogous to an ideal electromagnetic generator that produces a good deal of alternating voltages at the desired fundamental frequency with controllable bounteousness and phase angle. The operating principle is based on conventional series capacitive compensation which is used as a means of trim back the line impedance, which in turn increases voltage, current and transmitted power crosswise given physical line. The SSSC offers fast control and it is inherently neutral to sub-synchronous resonance.Figure 5 Equivalent circuit of SSSCIV. SENSITIVITY ANALYSISF or optimum localization of series FACTS devices, linear sensitivity of loading factor () with respect to line reactance has been computed using MATLAB coding technique. The calculation of the index is done such that transformers and lines machine-accessible between generators at both ends are excluded. The line having the most detrimental value of the /Xij sensitivity factor for the critical contingency cases has been identified for optimal series FACTS devices musical arrangement.Sensitivities are calculated under severe outage conditions at a stressed point near to maximum loadability point i.e. the determine of voltage, angle and power factor are taken at the critical point as obtained from Continuation Power Flow results. The optimal location of TCSC posture has been considered in a line producing maximum in each line A criterion for the optimal placement of TCSC, in this work, has been that it should not be placed in aq line connecting two generator buses.V. alive(p) ANA LYSISDynamic voltage stability is analyzed by monitoring the Eigen set of the linearized system as a power system is progressively loaded. When the parameter varies, the equilibrium points of the dynamic system also vary then, and so do the Eigen values of the corresponding state matrix ASYS. The equilibrium points are asymptotically stable if all the Eigen values have negative real parts. The point where a complex conjugate pair of Eigen values reaches the conceptional bloc with respect to changes in is known as Hopf Bifurcation point.Power system oscillations are associated with a pair of complex Eigen values of equlibria crossing the imaginary axis of rotation of the complex plane, from the left half plane to the right half plane, when the system undergoes sudden changes. If this particular dynamic problem is studied using lingering changes it can be viewed as Hopf bifurcation problem. Thus by predicting these types of bifurcations well in advance, a possible dynamic inst ability problem may be avoided.Figure 6 Hopf Bifurcation pointVI. RESULT AND DISCUSSIONSTo implement the optimal placement two case studies were taken of IEEE 14 bus system and 39 bus New England system. The software packages used in the system analysis are MATLAB, and PSAT (power system analysis toolbox)A.STATIC ANALYSISContinuation power flow for intact system was performed and SLM was found toState of the systemDivergence pointWeakest BusIntact2.885With TCSC3.104With SSSC3.204Table 2 Comparison of loadability of the 14 bus systemLine outagesSLMTrue RankingRCIRanking1-2*10.30812-30.559320.279821-50.559640.129235-60.6550830.127147-90.706650.08155Table 1 Contingency based ranking for SLM and RCI for 14 bus systemFig7 PSAT simulink model of IEEE14 bus Test systemFor the placement of series FACTS devices sensitivity index for unhomogeneous cases is calculated and it is inferred that line 1-5 is the optimum location as the index is having most negative value for it. Newton Raphson loa d flow was performed on the 14 bus system and the values of P load were incremented in travel of 0.2 percentage of loading with NR load flow being performed again on the modified system. The NR diverged at 2.88 times of loading and system suffered voltage collapse. The weakest bus observed to be is the 5m bus followed by 4m bus. With TCSC placement, the NR diverges at 3.1times loading epoch it does the same for 3.2 when SSSC is incorporated. Thus proving that compensation provided by series FACTS devices enhances the capacity of the system to bear stress in the form of increased load. The advancement in the voltage profile for the 5th bus is better with SSSC as compared to TCSC.B. DYNAMIC ANALYSISThe test case considered for the dynamic analysis is IEEE 14 bus system. The approach to study the stability is Hopfield Bifurcation as already mentioned.For the analysis of dynamic stability, dynamic model of 14 bus system was made which includes synchronous generators and AVR connected at the PV buses apart from the other static components. After obtaining the Hopf bifurcation of the system for the weakest bus the optimal location of the FACTS devices is determined and they are placed accordingly to provide stability to the system.The power flow is performed for the intact system and the Eigen values are found to be negative with PL = 0.076 p.u. at bus 5. When the PL is increased to 2.419 times of the initial load, two Eigen values cross the imaginary axis leading to Hopf bifurcation and instability. To render stable conditions, TCSC is optimally placed in line 1-5 based on the sensitivity index already calculated and it is found that the load can be increased till 5.58 times of the actual load before reaching the Hopf bifurcation point. The comparison of the stable and unstable system due to increased loading effect is depicted by the Figures 9 and 10.The placement of SSSC is not possible as it produced negative compensation in the system which can be related to the presence 3 synchronous condensers in the IEEE 14 bus system. This is inferred from the fact that SSSC, also acting as a VAR generator at times, adds to the reactive power generated from the other dynamic components present in the system.VII. CONCLUSIONStatic voltage instability in the system may occur due to deficit of reactive power. The reactive power requirement of the system may increase under severe contingencies. Therefore, contingency ranking based on Static voltage stability criterion, can be obtained based on the extra reactive support requirement from existing sources.Dynamic voltage instability, on the other hand has been attributed to Hopf bifurcation when one pair of Eigen values of the systems state matrix reaches imaginary axis, following change in the system parameters such as load.FACTS devices prove to be an effective remedy in enhancing system voltage stability. however due to high cost of FACTS controllers their placement should be such as to improve both s tatic and dynamic voltage stability. The comparison between the placement of TCSC and SSSC has been shown for both the static and dynamic analysis. SSSC is found to be ruff suited for the static stability enhancement whereas the dynamic stability improvement incorporates only TCSC.