FACTS-based reactive power compensation of wind energy conversion system Seminar

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FACTS-based reactive power compensation of wind energy conversion system

ABSTRACT
Voltage control and reactive power compensation in a distribution network with embedded wind energy conversion system (WECS) represent main concern of this paper. The WECS is of a fixed speed/constant frequency type that is equipped with an induction generator driven by an unregulated wind turbine. The problem is viewed from short term (10 seconds) and mid-term (10 minutes) time domain responses of the system to different wind speed changes. Being disturbed by a variable wind speed, the WECS injects variable active and reactive power into the distribution network exposing nearby consumers to excessive voltage changes. In the FACTS-based solution approach, the Unified Power Flow Controller (UPFC) is used at the point of the WECS network connection to help solve technical issues related to voltage support and series reactive power flow control.

INTRODUCTION
Recently, alternative solutions treating distributed generation of electrical energy have appeared as a consequence of strong ecological concerns with regard to almost all major industrial branches. Moreover, initiatives of potential investors come along with liberalization of electrical energy market. It results with an --additional impact to a need for conducting a new kind of technical analysis. Grid integration aspects of renewable sources have become increasingly important as incentives come in large numbers. From distribution network viewpoint, connection of small power plants with dispersed generation of electricity calls for urgent attention. In case of increased power ratings, dispersed power plants could be integrated in a transmission network. Dispersed generation of electricity is often a subject of polarized discussions. At one side, experienced engineers motivated by wide knowledge of complex power system operation are concerned regarding fundamental realization of massive introduction of unregulated and uncontrollable Generators into a distribution network. At the other side, enthusiastic proponents of renewable sources believe that such generating units are a necessity in operation should domestic and international requirements for reduction of CO2 emission be fulfilled. Moreover, they are convinced that renewable decrease dependence on dominant energy fuels (gas, oil, coal…) in times of large international crisis. Increased penetration of renewable such as wind energy creates an uncontrollable component in electric power system. Based on weather forecasts it is possible to predict a mean wind speed in short-term time period, but not dynamic changes as well, smaller or larger, which take place around a base speed. Dynamic changes of wind speed make amount of power injected to a network highly variable. Depending on intensity and rate of changes, difficulties with frequency and voltage regulation could appear making a direct impact to quality level of delivered electrical energy. Conditions of economic justification set project requirements for wind power plant installations in areas with high wind utilization. Such areas are often located in rural zones with relatively Weak electrical networks. In order to establish a balance between polarized attitudes, it is necessary to provide answers concerning technical, economic, and security aspects related to grid integration of wind power plants. From that viewpoint, the objective of this paper is set as to create a countermeasure Without a countermeasure, it is possible that at some locations only a small number of wind turbines could be connected due to weak voltage conditions and increased losses in the nearby network. That would not only leave assessed wind potential unused, but also it could also prohibit installation of larger number of wind turbines jeopardizing the economics of the whole project. In an attempt to overcome negative dynamic impacts caused by wind speed changes, the voltage regulation and reactive power compensation problem is approached here not only from a conventional aspect, but from a FACTS based one as well. Wind power plant induction generator is viewed as a consumer of reactive power. Its reactive power consumption depends on active power production. Conventionally, shunt capacitor banks are connected at the generator terminals to compensate its reactive power consumption. In some schemes, shunt capacitor banks could be automatically switched on/off by using feedback signal from generator reactive power. The capacitor switching is triggered through an algorithm if a generator reactive power is outside an allowed dead-band for a specified time period. Further on, continuous voltage control and reactive power compensation at the point of the WECS network connection is provided by using FACTS-based device. Among FACTS devices, the Unified Power Flow Controller (UPFC) is chosen due to its versatile regulating capabilities. The UPFC consists of shunt and series branches, which could be interchangeably used. Being located at the point of the WECS connection to the distribution network, it is made possible to simultaneously control the WECS bus voltage magnitude and/or series reactive power flow that WECS exchanges with the network. This countermeasure is expected to contribute in making assessed wind site viable for connecting larger number of wind turbines.

ABOUT FACTS
With the rapid development of power electronics, Flexible AC Transmission Systems (FACTS) devices have been proposed and implemented in power systems. FACTS devices can be utilized to control power flow and enhance system stability. Particularly with the deregulation of the electricity market, there is an increasing interest in using FACTS devices in the operation and control of power systems with new loading and power flow conditions. A better utilization of the existing power systems to increase their capacities and controllability by installing FACTS devices becomes imperative. Due to the present situation, there are two main aspects that should be considered in using FACTS devices: The first aspect is the flexible power system operation according to the power flow control capability of FACTS devices. The other aspect is the improvement of transient and steady-state stability of power systems. FACTS devices are the right equipment to meet these challenges.

Definition of FACTS
According to IEEE, FACTS, which is the abbreviation of Flexible AC Transmission Systems, is defined as follows:
Alternating current transmission systems incorporating power electronics based and other static controllers to enhance controllability and power transfer capability.

FACTS categories and their functions
FACTS categories
In general, FACTS devices can be divided into four categories
 Series facts devices
Series FACTS devices 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 (or a combination) to serve the desired need. In principle, all series FACTS devices inject voltage in series with the transmission line.
 Shunt facts devices
Shunt FACTS devices may be variable impedance, variable source, or a combination of these. They inject current into the system at the point of connection.
 Combined series –series facts device:
Combined series-series FACTS device is a combination of separate series FACTS devices, which are controlled in a coordinated manner.
 Combined series –shunt facts devices:
Combined series-shunt FACTS device is a combination of separate shunt and series devices, which are controlled in a coordinated manner or one device with series and shunt elements.

Introduction of FACTS devices:
Typical FACTS devices and their functions
In this four typical FACTS devices are considered in detail: TCSC (Thyristor Controlled Series Capacitor), TCPST (Thyristor Controlled Phase Shifting Transformer), UPFC (Unified Power Flow Controller) and SVC (Static Var Compensator).