22-05-2014, 12:58 PM
POWER FLOW CONTROL BY UPFC
POWER FLOW CONTROL.docx (Size: 377.46 KB / Downloads: 12)
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.
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.
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 parameters that govern the operation of transmission systems including series impedance, shunt impedance, current, voltage, phase angle, and the damping of oscillations at various frequencies below the rated frequency. These constraints cannot be overcome, while maintaining the required system reliability, by mechanical means without lowering the useable transmission capacity. By providing added flexibility, FACTS Controllers can enable a line to carry power closer to its thermal rating. Mechanical switching needs to be supplemented by rapid-response power electronics. It must be emphasized that FACTS is an enabling technology, and not a one-on-one substitute for mechanical switches. The FACTS technology is not a single high-power Controller, but rather a collection of Controllers, which can be applied individually or in coordination with others to control one or more of the interrelated system parameters mentioned above. A well-chosen FACTS Controller can overcome the specific limitations of a designated transmission line or a corridor. Because all FACTS Controllers represent applications of the same basic technology, their production can eventually take advantage of technologies of scale. Just as the transistor is the basic element for a whole variety of microelectronic chips and circuits, the thyristor or high-power transistor is the basic element for a variety of high-power electronic Controllers. FACTS technology also lends itself to extending usable transmission limits in a step-by-step manner with incremental investment as and when required. A planner could foresee a progressive scenario of mechanical switching means and enabling FACTS Controllers such that the transmission lines will involve a combination of mechanical and FACTS Controllers to achieve the objective in an appropriate, staged investment scenario. It is also worth pointing out that, in the implementation of FACTS technology, we are dealing with a base technology, proven through HVDC and high-power industrial drives. Nevertheless, as power semiconductor devices continue to improve, particularly the devices with turn-off capability, and as FACTS Controller concepts advance, the cost of FACTS Controllers will continue to decrease. Large-scale use of FACTS technology is an assured scenario.
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-SERIES CONTROLLERS:
This could be a combination of separate series controllers, which are controlled in a coordinated manner, is a multilane transmission system or it could be a united controller in which series controllers provide independent series reactive compensation for each line but also transfer real power among the line via the power link. The real power transfer capability of the unified series – series controller, referred to as interline power flow controller, makes it possible to balance both the real and reactive power flow in the lines and there by maximize the utilization of the transmission system. Note that the term “Unified” here means that the dc terminals of all controller converters are all connected together for real power transfer.