Voltagestability assessment and control are not considered as any new issue , but they have now attained special attentions to maintain the stability of the transmission networks in order to avoid recurrence of major blackouts as experienced by the particular countries. The powersystem can be classified in the voltagestability region if it can maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance , . In order to be reliable, the powersystem must be stable most of the time. The study on voltagestability can be break down into various approaches, but the estimation
. The probabilistic analysis was firstly introduced into studying powersystem small signal stability by Burchett and Heydt in . A series of work later on [3–6] have fur- ther improved the various aspects of the analytical method of powersystem probabilistic small signal stability. In [7, 8], a method of probabilistic analysis was proposed to directly calculate the probabilistic density function of critical eigen- values of a large scale powersystem from the probabilistic density function of gird connected multiple sources of wind power generation to investigate the impact of stochastic un- certainty of grid-connected wind generation on power sys- tem small-signal stability . Reference  presented a comparative analysis of the performance of three efficient estimation methods when applied to the probabilistic as- sessment of small-disturbance stability of uncertain power systems. In , an analytical approach was proposed to in- volve the effects of correlation of wind farms in probabilis- tic analytical multi-state models of wind farms output generation. Reliability models of wind farms considering wind speed correlation are proposed in .
An estimation of TSA has been used to study the effects of series & shunt FACTS devices on the transient stability. The modeling and optimal tuning of various FACTS devices for a dynamic stability enhancement of multi machine power systems is studied. The efficacy of SVC and TCSC controller in improving voltagestability and total transfer capacity using multi-objective GA was investigated. Modeling and interfacing techniques for SVC and TCSC for a long term dynamic simulation was studied. This paper presents the placement of SVC and TCSC controller at their optimal position in IEEE 3 machine 9 bus powersystem. The paper also investigates the comparison of both controllers in improving transient stability enhancement.
The load flow solution gives the nodal voltages and phase angles and hence the power injection at all the buses and power flows through interconnecting power channels (transmission lines). Load flow solution is essential for designing a new powersystem and for planning extension of the existing one for increased load demand. These analysis require the calculation of numerous load flows under both normal and abnormal (outage of transmission lines, or outage of some generating source) operating conditions. Load flow solution also gives the initial conditions of the system when the transient behavior of the system is to be studied.
Whenever there is a mismatch in power, speed changes. As seen earlier, the governing system senses this speed change and adjusts valve opening which in turn changes power output. This action stops once the power mismatch is made zero. But the speed error remains. What should be the change in power output for a change in speed is decided by the „regulation‟. If 4% change in speed causes 100% change in power output, then the regulation is said to be 4% (or in per unit 0.04). The regulation can be expressed in the form of power – frequency characteristic as shown in Fig.4 (a & b). At 100% load the generation is also 100%, frequency (or speed) is also 100%. When load reduces frequency increases, as generation remains the same. When load reduces by 50%, frequency increases by 2%, in the characteristic shown. When load reduces by 100%, frequency increases by 4%. In other words 4% rise in frequency should reduce power generation by 100 %.
The coordinated voltage control has been done with DGs reactive power control and OLTC operation. The result indicates that involving DGs reactive powers in the voltage control will result in a reduction of number of OLTC operations and the reduction of the voltage level in the distribution system. Further, the results also indicate that from the coordinated voltage control, the losses can be decreased. In recent years, distributed generation, as clean natural energy generation and cogeneration system of high thermal efficiency, has increased due to the problems of global warming and exhaustion of fossil fuels. Many of the distributed generations are set up in the vicinity of the customer, with the advantage that this decreases transmission losses. However, output power generated from natural energy, such as wind power, photovoltaic’s, etc., which is distributed generation, is influenced by meteorological conditions. Therefore, when the distributed generation increases by conventional control techniques, it is expected that the voltage change of each node becomes a problem. Proposed in this Seminar report is the optimal control of distribution voltage with coordination of distributed installations, such as the load ratio control transformer, step voltage regulator (SVR), shunt capacitor, shunt reactor, and static var compensator. In this research, SVR is assumed to be a model with tap changing where the signal is received from a central control unit. Moreover, the communication infrastructure in the supply of a distribution system is assumed to be widespread. The genetic algorithm is used to determine the operation of this control
and TCSC was done. From the study the TCSC has better performance and less cost than SVC. The series controller is most commonly used for the power flow control because by driving the line voltage the series controller can control the power flow in the transmission line. The series controller injects voltage in series to the line so it will improve voltage profile also. The series controller have the capability of bypassing the short circuit current and it can also handle the over loading of the transmission line. Because of the above mentioned capabilities it is concluded that the series controller is much better than the shunt controller. So it is used for the application of congestion management. For relieving the congestion management TCSC and UPFC are commonly used. But TCSC is largely used for the case of congestion management. Because the cost of UPFC is larger than TCSC. As per  the TCSC has the following advantages:
Generally, the voltage collapse mainly affected by the large distances between generation and load, under load tap changing transformers performance during low voltage conditions, unfavourable load characteristics, and poor coordination between various control and protective systems. In addition, the system may experience uncontrolled over-voltage instability problem at some buses due to the capacitive behaviour of the network and under excitation limiters that preventing generators and synchronous compensators from absorbing excess reactive power in the system. This can arise if the capacitive load of a synchronous machine is too large. Examples of excessive capacitive loads that can initiate self-excitation are open-ended high voltage lines, shunt capacitors,and filter banks from HVDC stations. The phenomena of voltagestability can be classified into small disturbance and large disturbance voltagestability. Small-disturbance voltagestability refers to the system’s ability to maintain steady voltages when subjected to small perturbations such as incremental changes in system load. A criterion for small disturbance voltagestability in that, at a given operating condition for every bus at the system, the bus voltage magnitude increases as the reactive power injection at the same bus increased. A system is voltage-unstable if, for at least one bus in the system, the bus voltage magnitude decreases as the reactive power injection at the same bus increased. Large-disturbance voltagestability refers to the powersystem ability to maintain steady voltages following large system disturbance such as loss of generation, loss of critical lines, system faults, or protection system failures. Investigation of this form of stability requires the examination of the dynamic performance of the system over a time sufficient to capture the interactions of such devices as under load tap changing transformers and generator field current limiters. The voltagestability can be classified in terms of time into short-term
Most of the large powersystem blackouts, which occurred worldwide over the last twenty years, which are caused by heavily stressed system with large amount of real and reactive power demand and low voltage condition. When the voltages at powersystem buses are low, the losses will also to be increased. This study is devoted to develop a technique for improving the voltage and eliminate voltage instability in a powersystem. Application of Flexible AC Transmission System (FACTS) devices are currently pursued very intensively to achieve better control over the transmission lines for manipulating power flows. They can provide direct and flexible control of power transfer and are very helpful in the operation of power network. The powersystem performance and the
In this paper, PV curve has been generated by identifying stable and unstable condition at the buses. Along with PV curve, line stability index method is used which determine the line stability factor shows best optimum location to place the STATCOM. By placing STATCOM at more sensitive bus results in increment in voltage magnitude .
ABSTRACT: The increase in power demand has forced the powersystem to operate closer to its stability limit. Voltage instability and line overloading have become challenging problems due to the strengthening of powersystem by various means. The nature of voltagestability can be analysed by the production, transmission and consumption of reactive power. One of the major causes of voltage instability is the reactive power unbalancing which occurs in stressed condition of powersystem. Flexible AC transmission system (FACTS) devices play an important role in improving the performance of a powersystem, but these devices are very costly and hence need to be placed optimally in powersystem. FACTS device like static var compensator (SVC) and thyristor controlled series compensator (TCSC) can be employed to reduce the flows in heavily loaded lines, resulting in a low system loss and improved stability of network. In this paper, a method based on line stability index, real power performance index and reduction of total system VAR power losses has been proposed to decide the optimal location of TCSC.
Since 1960s, low frequency oscillations have been observed when large power systems are interconnected by relatively weak tie lines. These oscillations may sustain and grow to cause system separation if no adequate damping is available [1-8]. Although PowerSystem Stabilizer's PSS’s provide supplementary feedback stabilizing signals, they suffer a drawback of being liable to cause great variations in the voltage profile and they may even result in leading power factor operation under severe disturbances. In this context, Flexible Alternating Current Transmission System FACTS technology, based on modern powerful disturbances. FACTS technology, based on modern powerful semiconductor devices, enables the transmission system to improve its efficiency by regulating the power flow, enlarging the loading capability, increasing the system security and providing greater flexibility, just to enumerate a few benefits.
ABSTRACT: This paper presents a further significant development to the developed Flexible Line-Commutated Converter (LCC) based High Voltage Direct Current (HVDC) system with controllable capacitors, which can provide AC voltage/reactive power control. The development involves the installations of fixed parallel capacitors at the valve side of converter transformer, which brings the following significant benefits: 1) AC filter banks at the AC side of converter transformer are not needed as a better harmonic filtering performance can be achieved: 2) significant reduction of the HVDC station land requirement (compared with traditional LCC HVDC), as the AC filters together with the switchgear can occupy over 50% of the HVDC station footprint: 3) up to 50% reduction of the required voltage rating and more than 60% reduction of the capacitance of controllable capacitors for commutation failure elimination can be achieved while similar powersystem dynamic performance (AC voltage/reactive power control) compared with that can be demonstrated. Detailed analyses are presented to illustrate the effective commutation process and superb harmonic filtering performance. Selections of the component values are presented. Simulation results in RTDS are presented to verify the effectiveness of commutation failure elimination, powersystem dynamic performance, and harmonic filtering performance and show voltage/current stress of the fixed parallel capacitors.
That is because they allow all other system quantities to be computed such as real and reactive power flows, current flows, voltage drops, power losses, etc … power flow solution is closely associated with voltagestability analysis. It is an essential tool for voltagestability evaluation. Much of the research on voltagestability deals with the power-flow computation method.
Voltage instability in PowerSystem Network (PSN) occurs when a disturbance on the network causes a gradual and uncontrollable decline in voltage. Contingencies such as line or generator outage due to faults, sudden increase in load, external factors, or improper operation of voltage control devices are the causes of voltage instability . Voltage instability can also surface where there is an incongruity between supply and demand of reactive power, that is, inability of the system to meet the reactive power requirements. If measures are not taken to check this voltage instability, it will leads to a decrease in systemvoltage and consequently voltage collapse resulting in a partial or total system blackout. This jeopardizes the essential service of delivering uninterrupted and reliable power supply to consumer , .
The computation time of these two methods for dif- ferent loading conditions are tabulated in Tables 13 and 14. From these tables, it is very clear that the computa- tion time of the proposed method is slightly higher than method-I .The various results obtained by the two meth- ods show that both the methods are quite effective. But, in the proposed method both resistance and reactance are taken into account hence this method is more accurate and yields more computation time. Figure 1 and Figure 2 show the improved voltage profile of the proposed algo- rithm for standard 6 bus ward-hale test system and IEEE-14 bus test system The figure shows that the algo- rithm is capable of obtaining a faster convergence for the three unit thermal system in a very few generations and the solution is consistent.
Abstract - The growth of industry manufacturers and populace, electric power quality turns out to be increasingly vital. The developing amount of power electronics-based equipment has profoundly affected the nature of electric power supply. Presently a day, customers require great power supply for their sensitive loads. Voltage flicker has in this way been an essential power quality worry for supply utilities, regulatory agencies and clients [1-2]. Erratic variety in reactive power requests prompt a fluctuating voltage drops over the impedance of a dispersion framework which brings about voltage change at the point of common coupling (PCC). Traditionally, for essentially inductive supply framework, power quality can be enhanced by utilizing receptive power control techniques. These undesirable power quality issues can be alleviated by interfacing controlled devices either in arrangement or shunt to the load. A couple of such devices are dynamic voltage restorer (DVR) and Distribution Static Compensator (DSTATCOM). Both these gadgets require voltage source converters to palatable operation. Numerous topologies have been proposed in later past for voltage source converters in many published literatures.
effectively. Wear and tear in the mechanical components and slow response were the major problems. As a result, it was needed for the alternative technology made of solid state electronic devices with fast response characteristics. The requirement was further fuelled by worldwide restructuring of electric utilities, increasing environmental and efficiency regulations and difficulty in getting permit and right of way for the construction of overhead power transmission lines. This, together with the invention of semiconductor thyristor switch, opened the door for the development of FACTS controllers . The use of FACTS devices in a powersystem can potentially overcome limitations of the present mechanically controlled transmission systems. FACTS controllers are capable of controlling the network condition in a very fast manner and this feature of FACTS can be exploited to improve the voltagestability, and steady state and transient stabilities of a complex powersystem . In this study, a 5-bus, 2-machine powersystem has been considered for describing the impact of STATCOM in enhancing the voltage level and transient stability of the system. The simulation has been done using the MATLAB/SIMULINK and PowerSystem Analysis Toolbox (by F. Milano)
The Unified Power Flow Controller (UPFC) is a typical FACTS (Flexible AC Transmission Systems) device that is the most sophisticated and complex power electronic equipment and has emerged for the control and optimization of power flow and also to regulate the voltage in electrical power transmission system. This paper propose the real, reactive power and voltage control through a transmission line by placing UPFC at the sending end using computer simulation. The L index is the y is concerned with the ability of a under normal conditions and after being s also become more complicated due dealt with performance analysis of e collapse point and enhancement of
Static voltage instability is actually related to the reactive power imbalance. The reactive power support which the bus receives from the system, can limit loadability of that bus and hence the entire system. If the reactive power support reaches below the limit, the system will approach to maximum loading point or voltage collapse point due to high real and reactive power losses [9-11]. Hence, the reactive power supports should be local and adequate in order to avoid problem associated with its transmission , especially in a stressful condition. This phenomenon can be seen from the plot of the voltage at receiving end versus power transferred. The plots are popularly referred to as P-V curve or “Nose” curve. Figure 1 shows a typical P-V curve of a synchronous generator. As the power transfer increases, the voltage at the receiving end decreases. Eventually, the critical (nose) point, the point at which system reactive power is depleted, is reached where any further increase in active power transfer will lead to very rapid decrease in voltage magnitude [13-14]. Before reaching to critical point, the large voltage drop due to heavy reactive power losses can be observed. The maximum load that can be increased prior to the point at which the system reactive power depleted is called static voltagestability margin or loading margin (LM) of the system. The only way to save the system from voltage collapse is to reduce the reactive power losses in the transmission system or to add additional reactive power prior to reaching the point of voltage collapse [15-16]. This has to be carried out in planning with several system-wide studies.