In high voltage direct current (HVDC) converters, the stations are line commutated. This implies that the initial current of the valve can only be delayed in reference a zero value of the converter bus voltage in AC form. Consequently, for better control of voltage, the converter bus is connected to a reactive power source.
Reactive power sources are used to vary capacitors in static systems. The response of the reactive power system is dictated by voltage control in dynamic conditions.
When operating unstable AC systems, problems tend to arise because of unstable voltage and overvoltage surges. A better coordination of reactive power sources is required to simplify the control of the firing angles. As a result, this feature of the reactive power converter is increasingly being applied in modern converters using HVDC.
Reactive Power Control in Steady State
The equations expressing reactive power as a function of active power are given in terms of unit quantities.
Base converter voltage is given by −
$$V_{db}=3sqrt{frac{2}{pi }}times V_{L}$$
Where VL = Line to line voltage (on winding side)
Base DC Current (Idb ) = Rated DC Current (Idr)
Base DC Power (Pdc) = nb × Vdb × Idb, where nb = number of bridges in series
BaseBase AC voltage (Vb) = (Va)
Base AC Power = Base DC Power
$$sqrt{frac{18}{pi }}times V_{a}times I_{db}times n_{b}$$
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