| With the rapid development of power industry and increasingly strictrequirement of reactive power, the traditional reactive power compensator has beendifficult to meet the requirements of some special occasions. Static SynchronousCompensator (STATCOM) based on cascaded multilevel inverter technology is a newtype of reactive power compensator, it cascade through the full bridge or half bridgemodule and features with the characteristics such as compensating large capacity ofreactive power quickly in high voltage distribution network, overcoming the poorvoltage withstanding performance of single power electronic device, easy expansion,low switching frequency of device, low generation of current harmonics,etc. In recentyears, domestic and overseas scholars made a lot of previous research in cascademultilevel STATCOM, the cascade multilevel STATCOM has also begun to put intoapplication.This article revolved around cascade multilevel STATCOM, to be specific, it’smodulation method, voltage stability of dc side, load balancing compensation methodof triangular cascade multilevel STATCOM, negative sequence compensation controlmethod of star cascade multilevel STATCOM, the negative sequence compensationcharacteristics of the modular multilevel STATCOM, and other key technologies wasstudied in turn, to further improve harmonic characteristics and stability of cascademultilevel STATCOM, and expand its use function. The research emphasis andachievements mainly reflect in the following respects.(1) This paper expounds the working principle of the cascade multilevelSTATCOM and put forward the suitable single carrier modulation method formultilevel STATCOM. This improved modulation strategy, which uses a singletriangular carrier to achieve multi-level modulation, has advantages of both goodharmonic characteristics of carrier phase disposition modulation and switch balanceddistribution by carrier phase shifted modulation. As well as, the combination ofsingle-carrier modulation strategy and the voltage and current double loop control isapplied to the cascade multilevel STATCOM. Simulation compared effects of thecascade multilevel STATCOM using single carrier modulation and carrier phase shiftmodulation control, the results show that the output voltage and currentcharacteristics of the single carrier modulation is better than that of the carrier phaseshifting. (2) The effects of filter in dc side voltage feedback loop on dc side voltagestability of cascade multilevel STATCOM was studied. According to principle ofenergy balance, multilevel STATCOM was equivalent to an integrity component;considering the filter in dc side voltage feedback loop, a closed-loop model based onPI control was established. What is more, the stability of cascade multilevelSTATCOM’s dc voltage control loop was analyzed by using Roth-Hurwitz criterion,the results show that the time constant of PI regulator integral greater than of filter isa necessary and sufficient condition for the robust stability of the system. Finally, theproposed analysis was verified by experimental prototype.(3) To compensate imbalance load by using cascade multilevel STATCOM withdelta configuration, this paper presents a rapid triangle compensator instructioncurrent extraction method based on the instantaneous power theory. Through thegeometric analysis of compensation circuit phasor diagram and combined with theinstantaneous reactive power theory, the dq/transformation matrix is deduced,which can be applied to the negative sequence compensation system and thecomprehensive compensation system. The negative sequence active currentcomponent and the negative sequence reactive current component in a two-phasestationary coordinates is obtained, by transform instantaneous value of three-phasecircuits through the abc/dq transformation matrix and low pass filtering. And thenthrough the dq/transformation matrix, the negative sequence compensation phasecurrent command can be achieved for cascade multilevel STATCOM with deltaconfiguration. In order to further simplify the extraction of comprehensivecompensation instruction current, computing the zero sequence circulating of phasecurrent and derived the zero sequence current of cascade multilevel STATCOM withdelta configuration. Proposed the zero sequence circulating current computationmethod based on instantaneous power theory and simplified comprehensivecompensation instruction current extraction method. All parameters involved in theproposed instruction current extraction are instantaneous value which avoid thephasor recognition when compute the compensated admittance based on the principleof Steinmetz. Moreover, the speed of response is better than the existing methods.Finally, simulation model is constructed in PSIM to verify the effectiveness and thesuperiority of the proposed method.(4) Set up a split-sequence model based on cascade multilevel STATCOM withstar configuration, expounds the working principle of compensator to compensatereactive power and negative sequence: adjust the positive and negative sequence component of output voltage of the compensator can control the output current;choosing the appropriate voltage zero sequence component can orthogonalize theoutput voltage and current orthogonal, so the dc side voltage stability can bemaintained. Neutral point trajectory is obtained according to mathematical deductionof the zero sequence component with the phasor diagram. The neutral pointcoordinates show that the zero sequence components is influenced by ratio betweenreactive component and negative sequence component of the output current. Theoutput voltage and negative sequence compensator ability are often limited inpractical application, so an improved voltage current double closed loop controlstrategy is proposed in this paper to suppress the negative sequence components in theinstruction current. Add zero sequence component into the modulation signal in thedouble closed loop control, and the necessity of the improvement is verified by thesimulation.(5) This paper elaborates the working principle of modular multilevelSTATCOM to compensate reactive power and negative sequence current, comparedthe characteristic with cascade multilevel STATCOM compensation. Comparison andanalysis the cascade multilevel STATCOM with delta configuration and modularmultilevel STATCOM in negative sequence compensation performance, the resultsdemonstrates that: modular multilevel STATCOM possess less current stress whenapplied to the negative sequence compensation and less installed capacity than thecascade multilevel STATCOM with delta configuration. The conclusion providestheory foundation for rational topology selection in the negative sequencecompensation engineering application. |
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