Impact of Drivetrain on Wind Farm VAR Control - Upwind

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UPWIND 4.1.6 Wind Farm VAR Control with WTG Reactive Power Control Implementation The wind farm VAR control performed in the previous subsection using the WTG voltage control does not take into account the generator reactive power capability, which could be exceeded in order to bring the voltage back to the required level after any disturbance. To overcome this limitation, a reactive power control loop with reactive power limits can be implemented using a PI controller as shown in. Figure 4-8. The excitation-generator dynamic is approximated by a first order system with a time constant Tv. The values <strong>of</strong> the WTG reactive power limits (Qmax,Qmin) and terminal voltage limits (VWTG_max,VWTG_min) were assumed as in the DFIG machine. The reactive power limits are considered fixed based on a power factor <strong>of</strong> 0.9. The limits could be adapted to the machine operating point. By considering the machine power voltage (PV) curve, the reactive power capability can be extracted and compared to the reference reactive power required for VAR control (Qref). If the reference value is within the machine capability, it is used to determine the WTG reference voltage (VWTG_ref). If Qref is beyond the machine capability at the operating point, Qref is limited to the machine reactive power capability. The parameter Fn represents the portion <strong>of</strong> WTGs online. As explained with the DFIG control, this parameter adjusts the loop gain to maintain the same wind farm VAR time constant with different numbers <strong>of</strong> WTGs online. Vreg_ ref Qref + - Qgen 1 1 + sT r K qi s + - Vreg VWTG_min VWTG_max K qp ( 1 + s) K qi 1 F n VWTG_ref + - VWTG K i s K p Deliverable D5.9.1 42 sT v + Qmax + 1 + sT c 1 VWTG Qmin 1 X total 1 X grid Figure 4-8. Wind farm VAR control block diagram for synchronous machine WTG. 4.1.7 Reactive Power Control Loop The WTG voltage control loop with a synchronous machine is not as fast as that <strong>of</strong> a DFIG with power electronic grid interface. This suggests a modification to the design <strong>of</strong> the reactive power controller to be <strong>of</strong> proportional-integral (PI) type rather than <strong>of</strong> an integral type controller as that with the DFIG machine. Qref Vreg
UPWIND Given knowledge <strong>of</strong> the time constant <strong>of</strong> the WTG voltage control loop (Tv) and setting the desired time constant <strong>of</strong> the reactive power control loop (Tq), the PI controller parameters can be determined such that Kqp/Kqi cancels Tv, and Kqi can be set based on Tq as follows: K qp = TV (5.2) K qi X total K qi = (5.3) Tq It is clear that the system SCR has an impact on the controller since it affects the total system reactance as observed from the equivalent WTG terminal (Xtotal). Table 4-5 lists the WTG reactive power controller parameters. Xtotal was calculated with all WTGs online. The range <strong>of</strong> variation <strong>of</strong> Xtotal depends on the percentage <strong>of</strong> WTGs online. From the previous investigation <strong>of</strong> the static excitation performance, the WTG voltage control time constant was set to 1 second. The reactive power control loop time constant was set to 0.5 seconds. The response <strong>of</strong> the test system to a step change <strong>of</strong> 0.3 pu <strong>of</strong> the aggregate WTG reference reactive power (Qref) for different system SCRs is depicted in Figure 4-9. The full power wind farm was considered with all WTGs online (324 MW). The figure shows the voltage <strong>of</strong> the WTG (o marks) and the POI voltage (x marks). The WTG voltage limits were not applied to investigate the response <strong>of</strong> the reactive power control, which was acceptable according to the desired time constant. Table 4-5 WTG reactive power control loop parameters with static excitation system SCR Tq (sec) Kqi Kqp 20 0.5 0.5 0.5 5 0.5 0.82 0.82 3 0.5 1.08 1.08 Deliverable D5.9.1 43
Page 1 and 2: Project UpWind Contract No.: 019945
Page 3 and 4: UPWIND Contents Executive Summary .
Page 5 and 6: UPWIND Figure 4-21. System response
Page 7 and 8: UPWIND Glossary DFIG Doubly Fed Ind
Page 9 and 10: UPWIND conventional synchronous mac
Page 11 and 12: UPWIND dynamic model basis as in th
Page 13 and 14: UPWIND available wind power is less
Page 15 and 16: UPWIND Table 2-2. Wind farm system
Page 17 and 18: UPWIND 3. Wind Farm VAR Control for
Page 19 and 20: UPWIND 3.1.3 Wind Farm VAR Control
Page 21 and 22: UPWIND SCR:20 SCR:5 y-axis (min) y-
Page 23 and 24: UPWIND SCR:20 SCR:5 SCR:3 Figure 3-
Page 27 and 28: UPWIND +/- 10% of
Page 29 and 30: UPWIND Scenario Case SCR MVA base W
Page 31 and 32: UPWIND Table 4-1. Parameters <stron
Page 33 and 34: UPWIND Magnitude (dB) Phase (deg) 2
Page 37 and 38: UPWIND 20 3 0.2 1.65 3.3 5 3 0.49 0
Page 39 and 40: UPWIND 4.1.5 Results Summary and Co
Page 41: UPWIND Table 4-4. Summary results <
Page 45 and 46: UPWIND reactance (Xgrid). The ratio
Page 49 and 50: UPWIND Table 4-7. Summary results <
Page 51 and 52: UPWIND Magnitude (dB) Phase (deg) 6
Page 55 and 56: UPWIND Vreg_ref + - K K i p ( 1+ s)
Page 57 and 58: UPWIND • Scenario 3: With half po
Page 59 and 60: UPWIND 4.2.4 Wind Farm VAR Control
Page 63 and 64: UPWIND 4.2.5 Test Cases and Results
Page 65 and 66: UPWIND 5. Summary Wind farms can be

Impact of Drivetrain on Wind Farm VAR Control - Upwind

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