4. AHF EFFECTS ON THE STATCOM DESIGN
4.3 Case 1: RPC operation prioritized over AHF
4.3.2 AHF current’s phase angle impact on design
In combine RPC and AHF operation, VSC’s output current is comprised of fundamental current and harmonic components of filtering current. The magnitude reference of fun-damental reactive current is computed through equation 3.7, mentioned in chapter 3.3.1.
And, its phase angle references are either leading or lagging the VSC voltage by 90°, depending upon the operating point (inductive or capacitive). In case of harmonic filtering current, its magnitude and phase angle references are computed through the required filtering harmonic voltage at the PCC. But in this case 1, since RPC is prioritized, this harmonic current is limited to the capacity that is left after RPC operation.
In all simulations, references to fundamental and harmonic filtering current have been supplied manually to work out with an effective design in the worst conditions possible.
Magnitude reference of reactive current was supplied manually through q-component (𝐼q,ref) and phase angle references while adding/subtracting 90° in detected phase an-gles of PCC voltage. Further, magnitude reference of harmonic filtering current was also supplied manually through 𝐼h,ref current component and phase angles of detected PCC secondary side fundamental voltage (MV busbar voltage) were used as its phase angle references. In last, to produce filtering current of different harmonics (e.g. 3rd, 5th or 7th), manual frequency references (e.g. 150, 200, 250 Hz) were also supplied to the harmonic controller.
Different magnitude of harmonic filtering current affects the STATCOM design differently and that has been shown in the next chapter of this thesis. However, it was of interest to know what phase angle reference values cause the worst-case design. Therefore, ini-tially, a sample magnitude reference case was decided; fundamental reactive current in
maximum capacitive operating point was prioritized such that 10% of VSC current ca-pacity was available to generate harmonic filtering current. Using the abovementioned magnitude references (e.g. 0.10 p,u for 𝐼h,ref) of VSC’s currents, simulation results with different phase angle reference (𝜑ih,ref) of AHF current were carried out, to see their impact on overall design.
Figure 19 represents the VSC voltage results where supplied AHF current’s magnitude reference was at 0.10 p.u and its phase angle references were shifted by +60° w.r.t phases of MV bus bar voltage. The frequency of generated AHF current was 150 Hz (3rd harmonic, positive sequence in nature) and the number of SMs to produce required VSC voltage was 35. Choosing such phase angle references of AHF current make the pro-duced VSC’s fundamental and 3rd harmonic voltage peaks to be aligned, as shown in Figure 19(b). Peak alignment of produced VSC’s fundamental and harmonic voltage fur-ther results into the relatively higher peak of net VSC valve voltage, in comparison to other phase angle references of AHF current.
Figure 19(a) shows when produced VSC’s fundamental and harmonic voltage peaks are aligned, then peak of overall VSC valve (𝑉VSC_valve) voltage is at 67.14 kV. This value of 𝑉VSC_valve is turned out to be at its highest in comparison to other peak values simulated with different phase references of AHF current. Moreover, the available DC link voltage capacity (𝑉DC_max− 𝑉VSCrefpeak = 69.18 − 66.91 = 2.27 kV) in this situation is also at its lowest.
Figure 19. Simulated (a) DC link, VSC valve and MV busbar voltages in MCOP gener-ated with 0.90 p.u of reactive current and 0.10 p.u of AHF current magnitude reference (AHF current phase references shifted with +60°), (b) VSC overall valve, fundamental and harmonic voltage.
Figure 20 represents the VSC voltage results where all parameters, as explained in above scenario were kept the same (e.g. AHF current magnitude reference, harmonic
(a) (b)
type, number of SMs, etc.) except the phase angle references. Here, phase angle refer-ences of AHF current were shifted by +240° w.r.t phases of MV bus bar voltage. Choos-ing such phase angle references of AHF current makes the produced VSC’s fundamental and 3rd harmonic voltage peaks to be opposite to each other, as shown in Figure 20(b).
Since peaks of these voltages are in opposite direction, therefore, the peak of overall VSC valve voltage also got decreased significantly in comparison to its value in Figure 20(b).
Figure 20. Simulated (a) DC link, VSC valve and MV busbar voltages in MCOP gener-ated with 0.90 p.u of reactive current and 0.10 p.u of AHF current magnitude reference (AHF current phase references shifted with +240°), (b) VSC overall valve, fundamental and harmonic voltage.
Figure 20(a) shows when produced VSC’s fundamental and harmonic voltage peaks are in the opposite direction, then the peak of overall VSC valve (𝑉VSC_valve) voltage is at 62.05 kV. Lower 𝑉VSC_valve peak also result into increasing the available DC link voltage capacity (𝑉DC_max− 𝑉VSCrefpeak = 67.93 − 60.88 = 7.05 kV) which supposedly can be uti-lised to decrease the number of required SMs from 35 to 34.
Consequently, the phase angle of AHF current affects the effective STATCOM design in a way that if peaks of produced VSC’s fundamental and harmonic voltages are aligned, then higher number of SMs is required, in comparison to the situation when peaks are not aligned. But if peaks are opposite to each other, then there is a probability at least with low order harmonic filtering current that less SMs might be needed to produce the same VSC voltage. Moreover, because needed AHF current phase varies depending on operating point and is not known in design phase, the worst-case approach needs to be taken to be able to operate in all operating points.
(a) (b)