12-09-2014, 12:49 PM
VOLTAGE STABILITY ENHANCEMENT BY USING STATCOM
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ABSTRACT
In recent years, power demand has increased substantially while the expansion ofpower generation and transmission has been severely limited due to the limited resources and environmental restrictions. As a consequence, some transmission lines are heavily loaded and the system stability becomes a power transfer-limiting factor. Flexible AC transmission systems (FACTS) controllers have been mainly used for solving various power system steady state control problems and function of power flow control. Among the different variants of facts devices, static compensator are proposed as the most adequate due to they can supply required reactive current even at low values of bus voltage and also for the real power modulation. A more flexible model may be realized by representing the STATCOM as a variable voltage source for which the magnitude and phase angle may be adjusted using suitable algorithm, to satisfy a specified voltage magnitude at the point of connection with AC network.
The STATCOM will be represented by a synchronous voltage source with maximum and minimum voltage magnitude limits and also it is represented as a voltage source for the
full range of operation.
INTRODUCTION
Electric power plays an exceedingly important role in the life of community and in the development of various sectors of economy Infact the modern economy is very dependent on the electricity as a basic input. This in turn has led to increase in the number of power stations and their capacities and consequent increase in the power transmission line that connect the generating station to the load centers. Most if not all of the worlds, electric power systems are widely interconnected. We need these interconnections because, apart from delivery, the purpose of transmission network is to pool power plants and load centers in order to minimize the total power generation capacity and cost. Transmission interconnections enable taking advantage of diversity of loads, availability of sources, and full price in order to supply electricity to the loads at minimum cost with a required reliability. Transmission is often an alternative to the new generation resource. One cannot be sure about what the optimum balance is between ge4neration and transmission unless the system planners use advanced methods of analysis which integrate transmission planning into an integrated value-based transmission/generation planning scenario.
Hence, we need to incorporate some control mechanism in order to increase the power transfer capability and enhance the controllability. On the other hand, as power transfer grow, the power system becomes increasingly more complex to operate and the system can become less secure for riding through the major outages. It may lead to large power flows with inadequate control, excessive reactive power in various parts of the system, large dynamic swings between different parts of the system and bottlenecks, and thus the full potential of transmission interconnections can not be utilized. The power systems of today largely, care mechanically controlled. The problem with mechanical devices is that control cannot be initiated frequently,because these mechanical devices tend to wear out very quickly compared to static devices.
FLOW OF POWER IN AN AC SYSTEM
At present, many transmission facilities confront one or more limiting network parameters plus inability to direct power flow at will. In ac power systems, given the insignificant electrical storage, the electrical generation and load must balance all the times. To some extent, the electrical system is self-regulating. If generation is less than load, the voltage and frequency drop, and there by the load, goes down to equal the generation minus the transmission losses. However, there is only a few percent margin for such a self-regulation. If voltage is propped up with reactive power support, them the load will go up, and consequently frequency will keep dropping, and the system will collapse. Alternatively, if there is inadequate reactive power, the system can have voltage collapse. The basic requirement of power system is to meet the demand that varies continuously. That is, the amount of power divided by the power companies must be
equal to that of consumer’s need. The power transmitted over an AC transmission line is a function of the line impedance, the magnitude of the sending and receiving and voltages and the phase angle voltages between voltages. The compensators have been provided to control any
one of the function variable. Traditional techniques of reactive line compensation and step like voltageadjustment are generally used to alter these parameters to achieve power transmission
control. Fired and mechanically switched shunt and series reactive compensation are employed to modify the natural impedance characteristics of transmission line in order to establish the desired effective impedance between the sending and receiving ends to meet power transfer requirements. Voltage regulating and phase shifting transformers with mechanical tap changing gears are also used to minimize voltage variation and control power flow. These conventional methods provide adequate control under steady state and slowly changing conditions, but are largely ineffective in handling dynamic disturbance. The power systems can be effectively utilized with prudent use of FACTS technology on a selective, as needed basis. FACTS technology opens up new opportunities for controlling power and enhancing the usable capacity of present, as well as new and upgraded lines. These opportunities arise through the ability of FACTS controllers to control the interrelated parameter that govern the operation of transmission systems. These constraints cannot be overcome while maintaining the required system reliability, by mechanical means with lowering the usable transmission capacity. By providing added flexibility, FACTS controllers can enable a line to carry power closer to its thermal ratings.
AC SYSTEM SCENARIO
Flow in AC lines is generally uncontrollable. As a result of the lack of control in AC lines the following disadvantages are present in AC systems:
1. The power flow in AC lines (except short lines of lengths below 150 km) is limited by stability considerations. The expression for power flow in a lossless AC line with voltage magnitude v at sending and receiving end is given by:
Zc and θ denote the characteristic impedance and electrical distance. Note
that peak power transfer capability is The normal power flow in a line is kept much below the peak value. This margin (or reserve) is required to maintain system security under contingency
conditions. The fact implies that the lines may operate normally at power
levels much below their thermal limits.
2. The AC transmission network requires dynamic reactive power control to maintain satisfactory voltage profile under varying load conditions and transient disturbances. The voltage profile of a long line with the two ends maintained at voltage magnitude v for different loading conditions.
3. AC lines while providing synchronizing (restoring) torque for oscillating generator rotors may contribute negative damping torque which results in undamped power oscillations.
4. The increases in load levels are accompanied by higher reactive power consumption in the line reactances. In case of mismatch in the reactive power balance in the system, this can result in voltage instability and collapse. Recent developments involving deregulation and restructuring of Power industry, are aimed at isolating the supply of electrical energy (a product) from the service
involving transmission from generating stations to load centers. This approach is feasible only if the operation of AC transmission lines is made flexible by introducing fast acting high power solid-state controllers using thyristor or GTO valves. This led to the development of FACTS technology.
STATCOM OPERATING PRINCIPLE
The STATCOM generates a balanced 3-phase voltage whose magnitude and phase can be adjusted rapidly by using semiconductor switches. The STATCOM is composed of a voltage-source inverter with a dc capacitor, coupling transformer, and signal generation and control circuit.
The voltage source inverter for the transmission STATCOM operates in multibridge mode to reduce the harmonic level of the output current. Fig. below shows a single-phase equivalent circuit in which the STATCOM is controlled by changing the phase angle between the inverter output voltage and the bus voltage at the common point connection point. The inverter voltage Vi is assumed to be in phase with the ac terminal voltage Vt
MODELLING OF STATCOM
Statcom is ashumt connected reactive power compensation devic that is capable of generating/obsorbing reactive power and in which the output can be varied to ocntrol the specfic parameters of an electric power system. It is in general a solid state switching converter capable of generating independatly controllable reactive power at its output terminal. The statcom is placed in the bus m and is represented by a shunt reactive current source is as shown n fig above
CONCLUSION
In this thesis, various aspects regarding voltage stability have been presented and the importance to maintain voltage profile has been discussed. Various concepts regarding the FACTS technology and the important features of some of the FACTS devices have been presented. The Newton raphson method has been presented to solve the power flow problem in the power system with static synchronous compensator (STATCOM). In this thesis we had discussed about the STATCOM modeling and analysis when connected to a bus and made it to maintain a flat voltage profile of the full range of operation when there is a need. There by the reactive power compensation was successfully done in the particular transmission whenever it is required. The power flow and the voltage profile in various transmission lines along with and without the placement of STATCOM in a specific transmission line is obtained in order to improve the system performance by using the load flow studies.
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Hence our objective to maintain voltage stability have been successfully achieved with the incorporation of Static Synchronous Compensator (STATCOM).