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Note for Flexible AC Transmission System - FACT By Premananda Pany

  • Flexible AC Transmission System - FACT
  • Note
  • Biju Patnaik University of Technology Rourkela Odisha - BPUT
  • Mechanical Engineering
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FLEXIBLE AC TRANSMISSION SYSTEMS MODULE-I (As per BPUT Syllabus) B.Tech (6th Semester) Prepared by Dr. Premananda Pany FACTS concept and General System Considerations: Flow of Power in an AC System FACTS stands for Flexible AC transmission system which integrate power electronics based static controllers to control both active and reactive power and enhance power transfer capability of the AC lines. AC transmission systems incorporating the power electronic-based converters used to enhance controllability and increase power transfer capability. In the Fig.1 shown below Bus 1and Bus 2 represent two AC transmission systems where power is to be transmitted from V1 to V2 through a line of impedance r+jx. Power transmitted is given by P= 𝑉1 𝑉2 𝑋 sin 𝛿 1

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Where δ = phase angle. The power transmitted can be increased or decreased by changing the effective value of ‘x’. In addition it modifies reactive power needed.  Increase or decrease of ‘x’ will change value.  Maximum power that can be transmitted is obtained when δ=900 The FACTS technology is not a single high-power controller but to a certain extent a collection of controllers, which can be applied individually or in combination with others to control one or more of the interrelated system parameters such as i. Voltage ii. Current iii. Series impedance iv. Shunt impedance v. Phase vi. Damping of oscillations at various frequencies below the required system frequency. The FACTS controllers can allow a line to carry power nearer to its thermal rating power. In flexible AC systems, the controllable parameters are 2

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a) Line reactance. b) Phase angle δ when it is not large which controls the active power flow. c) Injecting voltage in series with line and at 90° phase with line current i.e. Injection of reactive power in series. This will control active power flow. d) Injecting voltage in series with line at variable phase angle. This will control both active & reactive power flow. e) Controlling the magnitude of either V1or V2. f) Variation of line reactance with a series controller and regulating the voltage with a shunt controller. This can control both active and reactive power. Flow of Power in A.C Systems: In AC power systems the electrical power at source and load must be balanced. To some extent, the electrical system is self regulating. If generation power is less than load requirement, the voltage and frequency drop. However there is only a few percent of margin for such a self regulation .If voltage propped up with reactive power support, then the load will go up and consequently frequency will drops and system will collapse .If there is inadequate reactive power support, the system voltage can be collapsed. When adequate power generation is available, active power flows from the surplus generation areas to shortcoming areas through all parallel paths available this frequently involves EHV (extra high voltage) and medium voltage lines. Power Flow in Parallel Paths: Consider power flow through two parallel paths from a surplus generation area to a deficit generation area as shown in Fig.2 (a). Without any control, power flow is based on the inverse of various transmission line Impedances. As a result line 3

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impedance may be overloaded and limit the loading on both the paths even though the higher impedance path is not fully loaded. Fig.2(a) AC power flow with parallel paths With HVDC, power flows as ordered by the operator because with HVDC power electronic converters power is electronically controlled. An HVDC line can also help the parallel AC transmission line to maintain stability. However, HVDC is expensive for general use, and is considered when long distances are involved. Fig. 2(b) shows the two parallel lines same to paths, but one of these has HVDC transmission line. Fig. 2(b) Power flow control with HVDC. Fig. 2.(c) and Fig. 2.(d) show one of the transmission lines with different types of series type FACTS Controllers. By controlling impedance as shown in Fig. 2(c) or phase angle as shown in Fig.2(d) or series injection of appropriate voltage a FACTS Controller can control the power flow as required. Maximum power flow is limited to its rated limit under contingency conditions when this line is expected to carry more power due to the loss of a parallel line. 4

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