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Flexible Alternating Transmission Systems
- 1. Flexible Alternating Current Transmission System (FACTS) Presented by G.MADHURI, Assistant Professor(C), Department of EEE, JNTUHCEM. madhurigundenm@gmail.com
- 2. Static Shunt Compensation: Objectives of shunt compensation. Midpoint voltage regulation. Voltage instability prevention. Improvement of transient stability. Power oscillation damping. Methods of controllable var generation Variable impedance type static var generators. switching converter type var generators. Hybrid var generators.
- 3. Reactive compensation makes transmission line more compatible with load demand. • The main objective of shunt compensation is To increase power transmitted Increase system stability Reduce losses Voltage regulation Dynamic voltage control Damp power oscillations If the power system is lightly loaded fixed or mechanically switched shunt reactors is used to maintain voltage level. If the power system is heavily loaded fixed or mechanically switched shunt capacitors is used. OBJECTIVES OF SHUNT COMPENSATION
- 4. • Var compensation is used for voltage regulation at the midpoint (or some intermediate) to segment the transmission line. • At the end of the (radial) line to prevent voltage instability, as well as for dynamic voltage control to increase transient stability and "damp power oscillations. • Examples of shunt compensation is STATCOM and SVC. OBJECTIVES OF SHUNT COMPENSATION
- 5. Midpoint Voltage Regulation for Line Segmentation Uncompensated Case: Let us consider a simplified model of the uncompensated system. Power = Sr=Vr Ir* =Vr [ Vs∟δ − Vr∟0˚ 𝒋𝑿 ]* Vr∟0˚ Vs∟ δ X I Vs∟δ Vr∟0˚ Vx δ I
- 6. Sr=Vr Ir* = Vr [ Vs∟δ − Vr∟0˚ 𝑗𝑋 ]* = Vr [ Vs∟δ 𝑗𝑋 − Vr∟0˚ 𝑗𝑋 ]* = Vr [ Vs∟(δ−90) 𝑋 − Vr∟90˚ 𝑋 ]* = Vr [ Vs∟(90−δ) 𝑋 − Vr∟90˚ 𝑋 ] = VrVs∟(90−δ) 𝑋 − Vr2∟90˚ 𝑋 Pr = VrVsCos(90−δ) 𝑋 − Vr2Cos90˚ 𝑋 = VrVsSinδ 𝑋 ------------------------------------------1 Qr = VrVsSin(90−δ) 𝑋 − Vr2Sin90˚ 𝑋 = VrVsCosδ) 𝑋 − Vr2 𝑋 Qr = Vr 𝑋 (VsCosδ − Vr) −−−−−−−−−−−−−−−−−−−−2 If Vr = Vs = V then P=(V2/X) Sinδ = Pmax sinδ Qr = V2 𝑿 (Cosδ − 1)
- 7. If the line is naturally loaded then the voltage profile would be flat i.e the voltage magnitude would be equal at all points along the line and no compensation is required. The voltage profile of the uncompensated transmission line is a maximum at the line ends and minimum at the midpoint, Vm. Application of line compensation means approximating a flat voltage profile. Vs∟δ Vr∟0˚ Vm X/2 X/2 But compensation is distributed along the line. So , the next best approach is to provide compensation at the midpoint, which will divide the line into two equal sections.
- 8. Consider the simple two-machine (two-bus) transmission model in which an ideal var compensator represented by a sinusoidal ac voltage source with an amplitude identical to that of the sending and receiving-end voltages, is shunt connected at the midpoint of the transmission line. |VS|=|VR|=|VM|=V Mid point compensation ∟ δ 𝟐 ∟ δ ∟ 𝟎
- 9. The midpoint compensator in effect segments the transmission line into two independent parts: The first segment, with an impedance of X/2, carries power from the sending end to the midpoint, and the second segment, also with an impedance of X/2, carries power from the midpoint to the receiving end. The midpoint VAR compensator exchanges only reactive power with the transmission line in this process. The compensator does not consume any real power (P=0) since the compensator voltage VM, and its current IM are in quadrature. For the lossless system assumed, the real power is the same at each terminal (sending end, midpoint, and receiving end) of the line.
- 10. Sr=Vr Ir* = Vs∟δ [ Vs∟δ − Vm∟δ/𝟐˚ 𝒋𝑿/𝟐 ]* = Vs∟δ [ Vs∟δ 𝒋𝑿/𝟐 − V̊m∟δ/𝟐 𝒋𝑿/𝟐 ]* = Vs∟δ [ Vs∟(δ−𝟗𝟎) 𝑿/𝟐 − Vm∟(δ 𝟐 −𝟗𝟎) 𝑿/𝟐 ]* = Vs∟δ [ Vs∟(90−δ) 𝑿/𝟐 − Vm∟(90−δ 𝟐 ) 𝑿 𝟐 ] = Vs 2∟(90−δ+δ) 𝑿/𝟐 − VmVs∟(90−δ 𝟐 +δ) 𝑿 𝟐 = Vs 2∟(𝟗𝟎) 𝑿/𝟐 − VmVs∟(90+δ 𝟐 ) 𝑿 𝟐 Pr = Vs 2cos(𝟗𝟎) 𝑿/𝟐 − VmVscos(90+δ 𝟐 ) 𝑿 𝟐 =− VmVs(−𝐬𝐢𝐧 δ 𝟐 ) 𝑿 𝟐 = 2VmVs𝒔𝒊𝒏 δ 𝟐 𝑿 Qr = Vs 2sin(𝟗𝟎) 𝑿/𝟐 − VmVssin(90+δ 𝟐 ) 𝑿 𝟐 = 𝟐Vs 2 𝑿 − 2VmVscosδ 𝟐 𝑿 = 2Vs 𝑿 (Vs − Vmcos δ 𝟐 ).
- 11. ∟ δ 𝟐 ∟ δ ∟ 𝟎 |VS|=|VR|=|VM|=V If Vm = Vs = V then P= 𝟐V2 𝑿 Sin( δ 𝟐 ) = 2Pmax sin δ 𝟐 Qr = 2V2 𝑿 (1−cos δ 𝟐 ) (or) Vs = Vm + X 𝟐 Ism VR = Vm − X 𝟐 ImR Cos( δ 𝟒 ) = Vm Vs𝒎 = Vm V𝒎𝒓 sin( δ 𝟒 ) = X 𝟒 Ism Vs = X 𝟒 ImR V𝑹
- 12. |VS|=|VR|=|VM|=V
- 13. It is observed that the midpoint shunt compensation can significantly increase the transmittable power(doubling its maximum value) at the expense of a rapidly increasing reactive power demand on the midpoint compensator. With long transmission lines a single midpoint compensator may not be adequate to prop up the line voltage and several shunt compensator connected at intervals down the line may be needed.
- 14. NOTE: The midpoint of the transmission line is the best location for the compensator. This is because the voltage sag along the uncompensated transmission line is the largest at the midpoint. Also, the compensation at the midpoint breaks the transmission line into two equal segments for each of which the maximum transmittable power is the same. The concept of transmission line segmentation can be expanded to use of multiple compensators, located at equal segments of the transmission line as illustrated for 4 line segments
- 15. jX/4 Inr Vs Vl Vm Vn Vr Isl Ilm Inr Imn jX/4 Isl jX/4 Ilm jX/4 Imn Vs = Vl + X 𝟒 Isl → 𝑽l = Vs − X 𝟒 Isl Vm = Vl − X 𝟒 Ilm Vn = Vm − X 𝟒 Imn Vr = Vn − X 𝟒 Inr As no of segments increased then active power doubles for each segmentation and angle(δ) decreases by half for each segmentation. Here for 4 line segments: power becomes P= 4V2 𝑿 Sin( δ 𝟒 ) = 4Pmax sin δ 𝟒 .
- 16. End of Line Voltage Support to prevent Voltage Instability A simple radial system with feeder line reactance X and load impedance Z. The nose-point at each plot given for a specific power factor represents the voltage instability corresponding to that system condition. The normalized terminal voltage VR versus power P plot at various different load power factors, ranging from 0.8 lag to 0.9 lead. It should be noted that voltage stability limit decreases with inductive loads and increases with capacitive loads.
- 19. The shunt compensation can effectively increase the voltage stability by supplying reactive load neglecting terminal voltage. NOTE: 1. For a radial line , the end of the line, where the largest voltage variation is experienced, is the best location for the compensator. 2. Reactive shunt compensation is often used too regulate voltage support for the load when capacity of sending –end system becomes impaired.
- 20. Improvement of Transient Stability
- 21. ∟ δ 𝟐 ∟ δ ∟ 𝟎
- 22. POWER OSCILLATION DAMPING: In the case of an under-damped power system, any minor disturbance can cause the machine angle to oscillate around its steady-state value at the natural frequency of the total electromechanical system. The angle oscillation results in a corresponding power oscillation around the steady- state power transmitted. The lack of sufficient damping can be a major problem in some power systems and, in some cases, it may be the limiting factor for the transmittable power. When the rotationally oscillating generator accelerates and angle δ increases (dδ/dt>0), the electric power transmitted must be increased to compensate for the excess mechanical input power.
- 23. Conversely, when the generator decelerates and angle δ decreases (dδ/dt<0), the electric power must be decreased to balance the insufficient mechanical input power.
- 24. SUMMARY OF COMPENSATOR REQUIREMENTS: The compensator must stay in synchronous operation with the AC system at the compensated bus under all operating conditions including major disturbances. Should the bus voltage be lost temporarily due to nearby faults, the compensator must be able to recapture synchronism immediately at fault clearing. The compensator must be able to regulate the bus voltage for voltage support and improved transient stability, or control it for power oscillation damping and transient stability enhancement, on a priority basis as system conditions may require. For a transmission line connecting two systems, the best location for Var compensation is in the middle, whereas for a radial feed to a load the best location is at the load end.