The main objective of this thesis was to design a STATCOM when its fundamental reac-tive current is prioritized over acreac-tive harmonic filtering and when it is required to produce nominal fundamental reactive current/power and perform active harmonic filtering at the same time. For such a combined RPC and AHF operation of the studied STATCOM, it was of interest to investigate how adding AHF functionality affects its design in terms of the needed number of SMs, DC link voltage capacity, MV busbar voltage, transformer and coupling inductor reactance. Additionally, how to sum the fundamental and AHF current and Impact on zero sequence current demand due to such a combined operation in unbalance grid operation were also among those interests
Starting with the design case where RPC operation was prioritised over AHF, it was found that producing AHF current affects the STATCOM design in three ways. First was the magnitude of AHF current where the number of needed SMs to produce the required voltage increases with increasing AHF current magnitude. The second was the phase angle references of AHF current. If they are chosen such that the peaks of produced VSC’s fundamental and harmonic voltages are aligned, then the situation of worst peak angle occurs. In this situation, the needed number of SMs to produce the same magni-tude of AHF current increases in comparison to the situation where VSC voltage peaks are either not aligned or in opposite directions. The third effect on STATCOM design was based on the order of AHF current to be produced. Here, simulation results suggested that the number of needed SMs to produce the same magnitude of AHF current in-creases as the order of harmonic of filtering current inin-creases. The highest SM number was needed when same current capacity (RMS) was used for a current of three com-bined harmonics. Alongside, it was also found that increasing the reactance of trans-former and coupling inductor results into increase in the needed number of SMs to pro-duce the same amount of voltage, as with lower reactance values.
Another option for aforesaid prioritisation case (extra AHF when RPC is not needed) is not to utilize the whole current capacity and limit the operation based on the number of submodules needed in RPC mode only, instead not to produce so high AHF current that number of sub-modules should be increased). This brings benefit to a network operator most of the time without over dimensioning the STATCOM and saving significantly in term of additional hardware costs.
In case of designing STATCOM capable to produce nominal reactive power and per-forming active harmonic filtering at the same time, simulation results suggested that op-erating with the maximum current in both operations creates a bottleneck with the overall current limit of studied STATCOM in delta winding. Here, the maximum fundamental reactive current and maximum filtering current can not be achieved at the same time with an economical design (geometrical summation of currents). However, achieving maxi-mum RMS values for both of these currents is possible in a conservative design (arith-metical summation of currents); with a trade-off between significant safety margin of VSC current in delta winding due to high MVAr rating (and higher cost) or optimum utilisation of current capacity (even closer to the limit) with less MVAr rating. Hence, an optimum design current summation principle exists between these two designs which fulfils all the requirements and seems to be quite close to the geometrical summation principle of economical design.
Further, testing the designed STATCOM with unbalanced supply, it was noticed that the maximum demand of zero sequence current occurs when STATCOM was producing fundamental reactive current and negative sequence AHF current in the MCOP region simultaneously. And, here peaks of positive and negative sequence network voltage were aligned along with the worst peak angle reference of AHF current. Furthermore, when STATCOM was producing only reactive power in MIOP region, then the highest demand of zero sequence was recorded when peaks of positive and negative sequence network voltage were aligned.
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APPENDIX A: PARAMETERS OF SIMULATIONS
Appendix A, Table 1. Simulation parameters for case 1
Appendix A, Table 2. Simulation parameters for case 2
APPENDIX B: SIMULATION RESULT SUMMARY
Appendix B, Table 1. Summary of simulation results from case 1 (RPC operation prior-itized over AHF)
Appendix B, Table 2. Summary of simulation results from case 2 (Full RPC support in parallel to AHF operation)