Control Principle And Application Of Cascaded H-bridge Multi-level STATCOM

As a novel static var compensator, STATCOM (static synchronouscompensator), is very important for the security of the power system.It has becomeresearch focus at home and abroad at present. It is important for realizing largecapacity and high voltage reactive power compensation by using cascadedH-bridge multilevel topology. Cascaded H-bridge multilevel STATCOM iscombined by many H-bridge module connected in series.In this way, high voltageis reached, the capacity and switching frequence are increased Modeling andcontrol of cascaded H-bridge STATCOM, DC side capacitor voltage betweenclusters, zero sequence voltage injection method and unbalanced loadcompensation are detailed analyzed in this paper. The detailed contents are asfollows:STATCOM should be modeled and controlled exactly, in order to compensateVar properly.Cascaded H-bridge STATCOM is a strong nonlinear system, becauseof DC-side capacitor voltage fluctuation cause output voltage distortion.According to the modeling of Cascaded H-Bridge STATCOM, a reduced orderHamiltonian model is proposed based on the average model that avoids thesolution difficulty of partial differential equation during the design ofinterconnection and damping assignment passivity-based control (IDA-PBC).Thus open system interconnection and damping assignment passivity-basedcontrol are realized based on the reduced order Hamiltonian model. The dynamiccharacteristic and stability has all been improved by energy shaping and dampinjecting.DC-side capacitor voltage unbalance of Cascaded H-Bridge STATCOM is themost important issue to be dealt with, and the needed DC side balance abilities arevarious in different situations. Carrier phase exchanging is adopted due to theabove problems, and this control method is fit for lower unbalancing situation.Since a more fast control capacity is needed under large fluctuation and fault,according to the impacts on DC-side capacitor voltage by current analysis andswitching states of cascaded H-Bridge STATCOM, a novel DC-side capacitorvoltage control strategy based on voltage redundant states is presented in thispaper.DC side capacitor voltage unbalance between clusters and its reasons areanalyzed. According to the DC side capacitor unbalance of each cluster caused bysystem unbalanced voltage, a zero sequence voltage injecting control method ofcascaded STATCOM based on DC side energy is proposed in this paper. Then, the control ranges in unbalanced system is proposed, where large equipment capacityis needed.Three-phase unbalanced load and balancing method is analyzed in this paper.The novel calculation methods of both reactive power compensation admittanceand the compensation current of STATCOM based on balance components areproposed. Any neutral non-grounded three-phase load can be decomposed intobalanced load and a single line load. The load compensation admittancecalculation can be divided into three parts, the power factor compensation for thebalance load and the single line load together with the balance compensation forthe single line resistive load. Then, the ideal Steinmetz compensation networkalgorithm is amended under the condition of asymmetric voltage. Above all, anoptimization method of reference current of cascaded H-Bridge STATCOM isproposed in this paper, which makes cascaded H-Bridge STATCOM a better use inunbalanced system. Simulation and experimental results verified the advantages ofsimple algorithm and easy realization of the proposed method.Hardware and software are designed for cascaded H-Bridge STATCOM at last,including the choice of DC-side capacitor and steady voltage-balancing circuit.The hardware design of control system based on DSP and FPGA was finished. Asimplified1-D space vector modulation algorithm is presented based on irregularand asymmetrical sampling technique, which could even the switching frequencyof the device. Finally the testing results of field operation are given, and theexperimental results are analyzed and verified.