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Flexible AC Transmission Systems (FACTS) Overview and Applications

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  1. Flexible AC Transmission Systems (FACTS) Overview and Applications Claudio CañizaresDepartment of Electrical & Computer EngineeringPower & Energy Systems (www.power.uwaterloo.ca) WISE (www.wise.uwaterloo.ca)

  2. Outline • Compensation. • Thyristor Control: • Thyristor Controlled Reactor–Fixed capacitor (TCR-FC). • Static Var Compensator (SVC). • Thyristor Controlled Series Capacitor (TCSC). • Thyristor Controlled Voltage Regulator (TCVR) and Thyristor Controlled Phase Angle Regulator (TCPAR). • Voltage-Sourced Converters (VSC): • VSC operation. • Shunt Static Synchronous Compensator (STATCOM). • Series Static Synchronous Compensator (SSSC). • Unified Power Flow Controller (UPFC). • Interline Power Flow Controller (IPFC). • Convertible Static Compensator (CSC). • HVDC light. • D-FACTS: • DSTATCOM. • DSMES. • Applications.

  3. Compensation • Generator-system (generator-infinite bus) model:

  4. Motivation • In steady state: • Maximum power that can be transmitted is Pmax= E'V2/X • The operating point is defined by Pm = PG = PL = o

  5. Compensation • Series compensation:

  6. Compensation • Steady state:

  7. Compensation • The system is more “stable” because: • The maximum power that can be transmitted from the generator to the system is increased. • The generator operating angle ois reduced. • These can be associated with an increase in decelerating energy in the system (equal area criterion), i.e. a larger stability region.

  8. Compensation • Shunt Compensation:

  9. Compensation • Steady state:

  10. Compensation • As with series compensation, the system is more “stable”: • The maximum power transfer for the system is larger. • The generator operating angle ois smaller. • The stability region is larger.

  11. Compensation • For a phase shifter (phase-shifting compensation):

  12. Compensation • Hence for  = 10o:

  13. Compensation • As in the case of shunt and series compensation, the system is more stable because: • The maximum power that can be transmitted from the generator to the system is increased. • The generator operating angle ois reduced.

  14. FACTS

  15. TCR-FC • SVC and TCSC controllers are based on the following basic circuit topology: FC + v(t) - TCR

  16. TCR-FC • Each thyristor is “fired” every half cycle. • The firing angle αis “synchronized” with respect to the zero-crossing of the voltage (or current). • As αincreases, the TCR-FC equivalent impedance changes from inductive to capacitive.

  17. SVC

  18. SVC • The controller is connected in shunt through a step-down transformer to reduced the voltage level on the thyristors. • The thyristor firing is synchronized with respect to the bus voltage. • The main objective is to control the bus voltage magnitude. • Filters may be used to reduced harmonics.

  19. SVC • The steady state control characteristics are:

  20. TCSC

  21. TCSC • Somewhat similar to the SVC controller but connected in series with a transmission line. • The thyristor firing is synchronized with respect to the line current. • Filters are usually not used in this case, which lead to stringent limits on the firing angle α. • In steady state, the device controller operates in the capacitive region; the inductive region is only used during transient operation.

  22. TCSC • The controller has a resonant point that must to be avoided, as the controller becomes an open circuit:

  23. TCSC • The typical controls for the TCSC are:

  24. TCSC • The power flow or “slow” control is designed to maintain a constant controller impedance. • The stability or “fast” control is usually designed to reduced system oscillations after contingencies. • The typical use of this type of controller in practice is for the control of inter-area oscillations (e.g. North-South ac interconnection in Brazil). • For simple series compensation, MSC are a much cheaper option; however, these can lead to Sub-synchronous Resonance (SSR) problems.

  25. TCVR & TCPAR • The typical topology is:

  26. TCVR & TCPAR • TCVR and TCPAR are basically ULTC and phase-shifters, respectively, with thyristor switching as opposed to electromechanical switching. • Thus, these controllers have better dynamic response, i.e. smaller time constants, than the corresponding electromechanical-based devices. • Controls are typically discrete, but with certain designs these can be continuous.

  27. VSC • A typical six pulse VSC with GTO switches (IGBTs are used for “low” voltage applications):

  28. VSC • To reduce harmonics, multi-pulse converters and filters are used. • For example, for a 12-pulse VSC:

  29. VSC

  30. VSC

  31. VSC • Pulse-width modulation (PWM) control techniques may also be used (“popular” in low voltage level applications). • Beside the control advantages, this technique eliminates certain lower harmonics, although it creates high level harmonics. • For example, for a 6-pulse VSC:

  32. VSC Fire valves when carrier and modulation signals cross CARRIER: MODULATION:

  33. VSC • This leads to: • Changing the modulation ratio, i.e. the magnitude of the modulation signal, results in changes of the ac voltage magnitudes. • Shifting the modulation signal leads to phase shifts on the ac voltages.

  34. STATCOM

  35. STATCOM • It is basically a VSC controlling the bus voltage. • The phase-locked loop (PLL) is needed to reduce problems with spurious zero voltage crossings associated with the high harmonic content of the signals for this controller, especially with PWM controls.

  36. STATCOM • Two types of controls can be implemented: • Phase control in a multi-pulse VSC: By controlling the phase angle of the voltage, the capacitor can be charged (< ) controller absorbs P) or discharged (> ) controller delivers P), thus controlling the voltage output Vi.

  37. STATCOM • PWM control in a 6-pulse VSC: the voltage output Vi can be controlled through the modulation ratio m independently of its phase angle , which in turn controls Vdc.

  38. STATCOM • The typical steady state control strategy is: • The current limits are due to the valve current limitations.

  39. STATCOM • This device is typically model using a voltage source, neglecting dc voltage dynamics and losses; this is a rough approximation. • There are several applications of this type of converter, but most of them at distribution voltage levels (using IGBT technology). • Additional controls may be added to effectively damp system oscillations (the same applies to SVC).

  40. STATCOM • Compared to an SVC: • The STATCOM occupies significantly less space. • There is more control flexibility (e.g. PWM, more reactive support at the limits). • Costs are higher due to the cost of switching devices, i.e. installation costs: • MSC  10 USD/kvar • SVC 50-60 USD/kvar (100 Mvar) 35-40 USD/kvar (200 Mvar) • STATCOM  1.2-1.3 SVC

  41. SSSC

  42. SSSC • Similar to the STATCOM but connected in series and synchronized with respect to the line current. • A phase angle control charges and discharges of the capacitor, thus controlling the output voltage Vi. • PWM controls can be decoupled or coupled:

  43. SSSC • Decoupled PWM controls:

  44. SSSC • Coupled PWM controls (better overall performance):

  45. UPFC

  46. UPFC • This controller is basically the STATCOM and SSSC combined, with independent controls, especially for PWM: • The STATCOM controls the sending-end voltage Vkand dc voltage Vdc. • The SSSC controls the power on the line Pland Ql. • There is a “demo” UPFC controller in Ohio (AEP-EPRI venture).

  47. IPFC • A combination of 2 SSSCs connected independently to 2 lines is referred to as an Interline Power Flow Controller (IPFC): • In this case the power on both lines can be controlled independently.

  48. CSC • A combination of 2 SSSC and 2 STATCOMS connected to 2 independent lines is referred to as a Convertible Static Compensator (CSC). • In this case the control possibilities are many, as it can work as a STATCOM, SSSC, UPFC and IPFC. • The CSC has been implemented in NY to relief congestion (NYPA-EPRI venture) [E. Uzunovic et al, “NYPA convertible static compensator (CSC) application phase I: STATCOM,” Proc. Trans. & Dist. Conf. and Expo., vol. 2, 2001, pp. 1139-1143]:

  49. CSC

  50. HVDC Light • Based on VSCs as opposed to the current sourced converters (CSCs) used in classical HVDC: • IGBTs (Insulated-gate bipolar transistors have a FET gate and a BJT switch) instead of GTOs are used as switches; have lower losses, higher frequency switching capacity, are cheaper, but have less reverse voltage blocking capacity. • These switches allow using PWM controls, which yield greater control flexibility.