CONCLUSIONES Y RECOMENDACIONES

With a SLG fault in either onshore converter, Ponshore will drop to some extent if the maximum phase

current is limited below 1.5 pu. Under this condition, if active power generated by wind farms remains,

PWF will be higher than Ponshore, and dc voltage will keep increasing and may cause severe damage.

Nevertheless, regulating ac voltage in PCC points, offshore converters cannot directly control dc terminal

voltage or active power through the widely used dc voltage margin and droop control. As a solution, a

cascaded droop control is proposed, which builds a nearly linear relationship between the offshore

converter side dc terminal voltage and the active power generated by the WF, using the ac voltage

magnitude as an intermediate variable.

Figure 6-10 illustrates the dc voltage control schemes with different MTDC configurations. In Figure

6-10(a), the onshore converter I is in charge of dc voltage regulation, while onshore converter II employs

the dc voltage margin control. When dc voltage is higher or lower than the pre-set levels, the margin

control will be triggered and onshore converter II will take responsibility for dc voltage control until

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adjust their active power references according to the dc terminal voltage variations. By choosing proper

initial operation point and droop constant, the active power generation of WFs can drop autonomously

under SLG fault and keep dc voltage increase within a desirable range. In addition, the active power

sharing between offshore converters can also be adjusted by using different droop curves.

Without ac grid, however, the dc voltage margin and droop control cannot be implemented. Therefore,

in Figure 6-10(b), a cascaded droop control, which includes vdc – vac droop in offshore converter and vac –

P droop in the WF emulators, is adopted. During SLG fault, the magnitude of ac voltage vac_w1 and vac_w2

will drop with dc voltage increase, and according to the droop curves, the lower magnitude of vac_wf1 and

vac_wf2 will induce wind power reduction in both WFs.

In practice, the wind turbines cannot reduce its output power instantaneously due to their inherent

inertia, and dc chopper resistors are used to absorb unbalanced active power until their thermal limit is

reached. Meanwhile, the wind turbine pitch angle is regulated according to the droop curve to reduce the

mechanical power and thus active power generation. A dead-band is also added in the droop curve to keep

the normal operation of offshore converters and WFs uninterrupted, as illustrated in Figure 6-11. Take vdc

– vac droop for example, if dc voltage deviation is smaller than the preset dead-band ΔVdc, the offshore

converter should operate in constant ac voltage control mode, otherwise the droop mechanism is initiated.

To avoid sudden voltage variation during modes change, the real dc voltage is used as the initial point

of vdc – vac droop curve when it exceeds the specified dead-band. Then, the ac voltage reference of

offshore converters will be defined by the droop curve and change with the dc voltage variation. The vac –

P droop operates in a similar manner. Different pre-fault operation points determine distinct droop curves and thus different steady state WF power generation during ac fault conditions.

Ideally, the coordination of vdc – vac and vac – P droop is equivalent to vdc – P droop in Figure 6-10(a),

and vac serves as an intermediate variable. The leakage impedance of the ac transformer, however, leads to

ac voltage magnitude deviation between vac_w1 and vac_wf1, which interferes in the droop control and make

the operation point diverge from the designed values. This effect can be compensated through droop

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(a) With dc voltage margin (onshore II) and dc voltage droop (offshore I and II) control

D C ca bl e 3 DC ca ble 4

(b) With dc voltage margin (onshore II) and cascaded droop (offshore and WF emulator I and II) control

Figure 6-10. Cascaded droop control in four -terminal HVDC platform.

(a) vdc – vac droop in offshore converter (b) vac – P droop in windfarm emulator

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(a) Control diagram of vdc – vac droop with dead-band in offshore converter

(b) Control diagram of vac – P droop with dead-band in windfarm emulator converter

Figure 6-12. Implementation of cascaded droop control.

Figure 6-12 gives the diagram of the cascaded droop control in offshore converter and WF emulator.

Instead of constant Vdref in Figure 6-5 and Pref in Figure 6-6, the ac voltage magnitude and active power

references in Figure 6-12 move on the droop curves until a steady state is arrived. To avoid stability

issues, the control bandwidth of offshore converter should be higher than that of the WF emulator [142].

Otherwise, the offshore converter cannot track the active power change and the system will finally be

instable due to the continuous error accumulation. For the same reason, the references generated by droop

curves should change slower than the corresponding control schemes, e.g., a low pass filter is added in vdc

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