05-07-2014, 09:57 AM
MODELING OF UPFC
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ABSTRACT
Continuous and fast improvement of power electronics technology has made FACTS (Flexible AC Transmission System) a promising concept for power
system development in the coming decade. By means of appropriate FACTS technology, power flow along the transmission network can be more flexibly controlled, as the name implies. Among a variety of FACTS controllers, UPFC is chosen as the focus of investigation for it embraces all the basic attributes of transmission power flow control. Computation of power flow for UPFC embedded power systems is fundamental need for power system analysis and planning purposes.
In this project a method is proposed to calculate the load flow of power system in which Unified Power Flow Controllers (UPFCs) are embedded. First the load flow equations of power system including the UPFCs are derived and the algorithm is developed based on the Newton Raphson Load Flow (NRLF) technique.
The method inherits the basic properties of the NRLF approach
INTRODUCTION
Continuous and fast development of power system has made FACTS an effective tool for its development. Among various FACTS controllers Unified Power Flow Control [UPFC] is chosen as it embraces all basic attributes of the transmission.
The mathematical model of the UPFC is developed and employed for the load flow control studies. By using UPFC the power flow control becomes more flexible than ever. But it has a drawback which requires pre-specified condition such as the power flow in the transmission line where it is being embedded. As no one has prior knowledge about this the pre-specified power flow and voltage are arbitrary.
This projects aims to present a systematic and efficient method for performing load flow calculation of a generalized power system with multi-machines and multiUPFC’s. Since Newton-Raphson Load Flow [NRLF] method together with the techniques of sparsity and optimal ordering has been proved to be more effective. The load flow equations are similar to that of the NRLF and algorithm for load flow studies of UPFC’s is developed based on the traditional NRLF method. The approach keeps the conventional NRLF method intact, during iteration process, it receives almost four power mismatches and few elements of Jacobian Matrix for each UPFC.
As for each UPFC, it only refers to a few elements of the Jacobian Matrix and hence additional computation burden incurred is very little. The developed algorithm is tested on IEEE 14-bus system indicates the effectiveness and reliability of this algorithm.
FACTS
INTRODUCTION TO FACTS
We need transmission interconnections because, apart from delivery, the purpose of the transmission network is to pool plants and load centers in order to minimize the total power generation capacity and fuel cost. Transmission interconnections enable taking advantage of diversity of loads, availability of sources, and fuel price in order to supply electricity to the loads at minimum cost with a required reliability. In general, if a power delivery system was made up of radial lines from individual local generators without being part of a grid system, many more generation resources would be needed to serve the load with the same reliability, and the cost of electricity would be much higher. With that perspective, transmission is often an alternative to a new generation resource. Less transmission capability means that more generation resources would be required regardless of whether the system is made up of large or small power plants. In fact small distributed generation becomes more economically viable if there is a backbone of a transmission grid. One cannot be really sure about what the optimum balance is between generation and transmission unless the system planners use advanced methods of analysis which integrate transmission planning into an integrated value – based transmission/generation planning scenario
Opportunities for FACTS
What is most interesting for transmission planners is that FACTS technology opens up new opportunities for controlling power and enhancing the usable capacity of present, as well as new and upgraded. The possibility that current through a line can be controlled at a reasonable cost enables a large potential of increasing the capacity of existing lines with larger conductors, and use of one of the FACTS Controllers to enable corresponding power to flow through such lines under normal and contingency conditions. These opportunities arise through the ability of FACTS Controllers to control the interrelated
SERIES CONTROLLERS
The series controller could be a variable impedance, such as capacitor, reactor, etc., or a power electronics based variable source of main frequency, sub synchronous and harmonic frequencies to serve the desired need. In principle, all series controllers inject voltage in series with the line. Even a variable impedance multiplied by the current flow through it, represents an injected series voltage in the line. As long as the voltage is in phase quadrature with the line current, the series controller only supplies (or) consumes variable reactive power
SHUNT CONTROLLERS
As in the case of series controllers, the shunt controllers may be variable impedance, Variable source, or a combination of these. In principle, all shunt controllers inject current into the system at the point of connection. Even a variable shunt impedance connected to the line voltage causes a variable current flow and hence represents injection of current into the line. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only supplies or consumes variable reactive power
COMBINED SERIES-SHUNT CONTROLLERS
This could be a combination of separate shunt and series controllers, which are controlled in a coordinated manner, or a Unified Power Flow Controller with series and shunt elements. In principle, combined shunt and series controllers inject current into the system with the shunt part of the controller and voltage in series in the line with the series part of the controller. However, when the shunt and series controllers are unified, there can be a real power exchange between the series and shunt controllers via the power link
THE UNIFIED POWER FLOW CONTROLLER
The unified power flow controller (UPFC) concept was proposed by Gyugyi in 1991.The UPFC was derived for the real time control and dynamic compensation of ac transmission systems, providing multifunctional flexibility required to solve many of the problems facing the power delivery industry. Within the framework of traditional power transmission concepts, the UPFC us able to control, simultaneously or selectively, all the parameters affecting power flow in the transmission line (i.e.,voltage, impedance, and phase angle), and this unique capability is signified by the adjective “unified” in its name. Alternatively, it can independently control both the real and reactive power flow in the line. The reader should recall that, for all the Controllers discussed in the previous chapters, the control of real power is associated with similar change in reactive power, i.e., increased real power flow also resulted in increased reactive line power.
In order to increase the system reliability and provide flexibility for future system changes, the UPFC installation was required to allow self-sufficient operation of the shunt converter as an independent STATCOM and the series converter as an independent Static Synchronous Series Compensator (SSSC). It is also possible to couple both converters together to provide either shunt only or series only compensation over a doubled control range.
BASIC OPERATING PRINCIPLES
From the conceptual view point, the UPFC is a generalized synchronous voltage source (SVS), represented at the fundamental (power system) frequency by voltage phasor Vpq with controllable magnitude Vpq (0< Vpq < Vpqmax) and angle (0<< 2), in series with the transmission line, as illustrated for the usual elementary two-machine system (or for two independent systems with a transmission link intertie) in Figure 4.1. In this functionally unrestricted operation, which clearly includes voltage and angle regulation, the SVS generally exchanges both reactive and real power with the transmission system. Since, as established previously, a SVS is able to generate only the reactive power exchanged, the real power must be supplied to it, or absorbed from it, by a suitable power supply or sink. In the UPFC arrangement the real power exchanged is provided by one of the end buses (e.g., the sending-end bus), as indicated in Figure 4.1. In the presently used practical implementation, the UPFC consists of two voltage-source converters. These back-to-back converters, labeled “Converter 1” and Converter 2” in the figure 4.1, are operated from a common dc link provided by a dc storage capacitor. As indicated before, this arrangement functions as an ideal ac-to-ac power converter in which the real power can freely flow in either direction between the ac terminals of the two converters, and each converter can independently generate(or absorb) reactive power at its own ac output terminal.
DESCRIPTION OF THE UPFC
The Unified Power Flow Controller is designed to meet the defined system requirements, in particular, to provide fast reactive shunt compensation. In order to increase the system reliability and provide flexibility for future system changes, the UPFC installation was required to allow self-sufficient operation of the shunt converter as an independent STATCOM and the series converter as an independent Static Synchronous Series Compensator (SSSC). It is also possible to couple both converters together to provide either shunt only or series only compensation over a doubled control range.