31-08-2012, 11:15 AM
POWER SYSTEM OPERATION AND CONTROL USING FACTS DEVICES
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ABSTRACT
In recent years, power demand has increased substantially while the expansion of power generation and transmission has been severely limited due to 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. However, recent studies reveal that FACTS controllers could be employed to enhance power system stability in addition to their main function of power flow control. The literature shows an increasing interest in this subject for the last two decades, where the enhancement of system stability using FACTS controllers has been extensively investigated. This paper presents a comprehensive review on the research and developments in the power system stability enhancement using FACTS damping controllers. Several technical issues related to FACTS installations have been highlighted and performance comparison of different FACTS
controllers has been discussed.
INTRODUCTION:
According to IEEE definition it is defind as “: Alternating current transmission systems incorporating power-electronic based and other static controllers to enhance controllability and increase power transfer capability. The need for more efficient and fast responding electrical systems has given rise to innovative technologies in transmission using solid-state devices. These are called FACTS devices which enhance stability and increase line loadings closer to thermal limits. The development of power semiconductor devices with turn-off capability(GTO, MCT) opens up new perspectives in the development of FACTS devices. FACTS devices are the key to produce electrical energy economically and environmental friendly in future.
STATIC SYNCHRONOUS COMPENSATOR (STATCOM):
STATCOM is the voltage-source converter, which converts a DC input voltage into AC output voltage in order to compensate the active and reactive needed by the system. STATCOM has several advantages of being small/compact, high response speed and no harmonic pollution. Shunt controllers is also variable impedance, variable source, or a combination of these. All shunt controllers inject current into the system at the point of connection. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well. Static Synchronous Compensator (STATCOM) is one such controller.
UNIFIED POWER FLOW CONTROLLER (UPFC):
Two inverters, namely shunt inverter and series inverter which operate via a common dc link with a dc storage capacitor, allow UPFC to independently control active and reactive power flows on the line as well as the bus voltage.
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 shunt part of the controller 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.
APPLICATION IN DEREGULATED ENVIRONMENT
Apart from its traditional application for voltage control, power flow control and enhancing steady state and dynamic limits, FACTS controllers are finding new applications in the present deregulated environment. One of the applications is in controlling the “parallel flow” or “loop flow”. Loop flow results in involuntary reduction in transmission capacity that may belong to some other utility and hence foreclose beneficial transactions through that line. Utilities can also make use of FACTS controllers in their tie lines, either to shield it from the neighbouring effects, such as wheeling transactions or to participate in such transaction. FACTS devices can also be implemented to ensure the economy in operation by placing it in a suitable line such that least cost generators
can be dispatched more. It can also be used to reduce the losses in the system. Yet, another application is to use FACTS to relieve the congestion in the system. FACTS devices can be strategically placed such that congestion cost is reduced, curtailment is decreased and price volatility due to congestion is minimized.
CONCLUSION
Today, FACTS devices are individually controlled. But according to a new EPRI report, inventive strategies incorporating system-wide control logic could further increase power transfer capability, stability and reliability of transmission systems. Controllers would be able to maximize available transfer capacity which maintaining dynamic stability and security, which could help accommodate even more electricity transactions. The all solid-state implementation of power-flow controllers will result in a significant reduction in equipment size and installation labour, dramatic improvements in operating flexibility and performance, and a progressive reduction in capital cost that is fuelled by advances in power semiconductor technology. Furthermore, the uniform, all solid-state approach is expected to reduce manufacturing cost and lead-time by
allowing the use of standard, prefabricated power inverter modules in different applications.