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Long-Duration Voltage Variations

Long-Duration Voltage Variations. X. R. I. Load. V 1. V 2. V 1. jX I. V 2. RI. I. At given pf at full load, nominal V 2. Voltage Regulation. Definition: Voltage regulation (at point x) is the percent voltage rise caused by unloading a power system (at point x)

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Long-Duration Voltage Variations

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  1. Long-Duration Voltage Variations

  2. X R I Load V1 V2 V1 jX I V2 RI I At given pf at full load, nominal V2

  3. Voltage Regulation • Definition: Voltage regulation (at point x) is the percent voltage rise caused by unloading a power system (at point x) • Assumption 1: The original power factor at point x is given • Assumption 2: The original voltage is the nominal value at point x, or a given value if not nominal; the source voltage is fixed • Assumption 3: Original system is at full load, or a given value if not full load

  4. V2,NL = V1 V2FL At no load, V2 normally rises to equal V1 Last equality assumes full-load voltage is nominal or rated value for the system

  5. The system inductive reactance usually causes voltage drops under normal loading • If the load pf is leading or if very long transmission lines at EHV (345 kV and up, the line charging current may be very large), then regulation may be negative

  6. Root Cause • Most long-duration voltage variations are caused by too much impedance (Zth) in the power delivery system • The power system is too weak for the load • voltage drops to a low value under heavy loads (lagging pf) • voltage rises to a high value under light loads (more leading or less lagging pf)

  7. Solutions to Improve Voltage Regulation • Add shunt capacitors to increase the load power factor (not leading however) tending to decrease the load kVA by decreasing the load kVAr • Add static var compensation or other dynamic reactive power compensation (same reason as shunt capacitor addition, but better control) • Add series capacitors to lines to cancel part of the jXI voltage drop (long transmission lines and (rarely) short lines with impact loads)

  8. Solutions to Improve Voltage Regulation • Add voltage regulators to boost V under heavy load and buck voltage under light load • Increase the size of conductors to reduce Z

  9. … Loads Step voltage regulators Raise Lower Source side Load side

  10. V sensing and gate control … … Source side Load side Electronic tap-switching voltage regulator

  11. … Loads voltage regulator set at 105% without line-drop compensation V(x) voltage profile for light load 126 V 120 V x voltage profile for heavy load 114 V

  12. … Loads voltage regulator set at 100% with line-drop compensation V(x) voltage profile for light load 126 V 120 V x voltage profile for heavy load 114 V

  13. V(x) 126 V x 120 V 114 V

  14. Voltage profile after load rejection V(x) 126 V x 120 V 114 V Needs rapid runback controls

  15. Flicker Sources of flicker -Load change -Induction motor starting -Variable power generation Observable flicker is dependent on the following: -Size (VA) of potential flicker-producing source -System impedance (stiffness of utility) -Frequency of resulting voltage fluctuations

  16. Example of Flicker 4.5[MVA]/0.5[MVA] 0.6[MVA]/0.1[MVA] 3.3[kV]/6.6[kV] 6.6[kV]/3.3[kV] 1.16+j0.599 [Ω] D D DG HG Y Y 4.5[MW]/0.5[MW] 0.6[MW]/0.1[MW] 1.53+j0.790 [Ω] 0.378+j0.195 [Ω] 1.5[MVA] HG 3.3[kV]/6.6[kV] 6.0[MVA]/2.0[MVA] 0.1[MW] D Y DG 0.23[kV]/6.6[kV] D 1.5[MW] Load 6[MW]/2[MW] 1.16+j0.600 [Ω] PL, QL 0.6[MVA] 0.6[MW] VG SMES 0.48[kV]/6.6[kV] PWG Unit Ps, Qs D WG Y 0.305 MVAR C Power system model at Ulleung Island of South Korea.

  17. System Responses 12 10 Wind speed [m/s] 8 6 0 10 20 30 40 50 60 Time [sec] Wind speed data. Responses of active power and system frequency.

  18. System Responses with SMES Responses of active power and system frequency with SMES .

  19. Flicker Mitigation Techniques -Adding series reactor Source side Load 3rd 5th 7th capacitors configured as harmonic filters Thyristor-controlled reactor One type of static var compensator

  20. Source side Load capacitors are gated fully on in sequence Thyristor-switched capacitor Another type of static var compensator

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