Harmonic Characteristic Analysis And Harmonic Power Flow Calculation Of The Voltage Source Converter Based On The Harmonic Coupled Admittance Matrix Model

In recent years, with the wide use of nonlinear power electronic devices in power system, the harmonic pollution problems have become increasingly prominent. Three-phase voltage source converter based on uncontrolled rectifier is an important harmonic source in the power grid, which has been widely used in many fields, such as AC-DC-AC frequency converter, electric vehicle, and so on. The harmonics that created by this kind of harmonic source are added in power system and lead to many problems, such as the additional power loss of equipments, the misoperation of relays, resonant overvoltage and so on, affecting the safety and economic operation of power system severely.Currently, there are many researches on harmonic modeling of three-phase voltage source converter based on uncontrolled rectifier. And among the existing models, the harmonic coupled admittance matrix model is the most accurate one. The model can directly show the coupling relationship between each harmonic voltage and each harmonic current at the VSC supply terminals, thus the harmonic model takes into account the effect of interactions between the harmonic source and power system. Based on the proposed matrix model, however, there has been lack of deep research on the harmonic generating characteristics and the harmonic proliferation in the system. With the accurate mathematical model, the research on the harmonic generating characteristics and harmonic power flow of harmonic sources has an important significance for formulating harmonic suppression strategies and reducing harmonic pollution.Based on the harmonic coupled admittance matrix model, deep study of harmonic generating characteristics and harmonic power flow of three-phase voltage source converter containing uncontrolled rectifier has been carried out. The decoupling characteristics and physical meanings of the harmonic coupled admittance matrix model are analyzed. And the harmonic generating characteristics under two different operational modes, current discontinuous mode and continuous mode, are studied for three-phase voltage source converter. Furthermore, the system harmonic power flow and simplified harmonic model of the three-phase voltage source converter are analyzed, and the accuracy and precision of them has been verified, respectively. Main research contents of this paper are listed as follows:(1) Based on harmonic coupled admittance matrix model for nonlinear electronic devices, the fundamental theory, model form, and physical meanings of admittance matrix element were analyzed. By transforming the matrix elements’amplitudes into per-unit, the contribution and coupling relationship of each harmonic voltage and current were analyzed and the influences of harmonic current are depended on the orders of harmonic voltage. Harmonic characteristics of three-phase voltage source converter containing uncontrolled rectifier are studied under both discontinuous and continuous modes. The main similarities can be summarized as follows:the contribution of harmonic voltage towards harmonic current decreases as the harmonic order increases, the self-coupling effect is more significant than the harmonic mutual coupling effect. The main differences can be summarized as follows:the self-coupling effect of each harmonic voltage and current is more significant, and the contribution of high order harmonic voltage to harmonic current is far less than fundamental frequency voltage.(2) An iterative power flow calculation algorithm is proposed based on harmonic coupled admittance matrix model for three-phase voltage source converter containing uncontrolled rectifier. The fundamental frequency power flow and harmonic power flow are calculated separately according to the decoupling effect of the model. By solving the system matrix equations and harmonic source model equations together, the system harmonic results can be obtained. Considering that the firing angle and extinction angle are affected by the variation of harmonic voltage at input port of the harmonic source, meanwhile, the harmonic coupled admittance matrix model should be corrected until the result of the power flow is convergent during the computational process. Apply the proposed algorithm to system with single and multiple converters and compare the error between the results of the power flow obtained by the time-domain simulation and program calculation quantitatively, and the high accuracy of the proposed algorithm can be verified.(3) The’conjugate matrix ignored’simplified model and Norton equivalent circuit simplified model of for three-phase voltage source converter containing uncontrolled rectifier are proposed. The contribution of each harmonic voltage is closely related to admittance matrix elements. Based on the complete harmonic model, the’major elements’ with greater contribution are reserved in simplified models, and the’minor elements’with less contribution are ignored. Simplified models include the main coupling effect of harmonic voltage and current. The harmonic power flow calculation of simplified models for system with harmonic source is analyzed. Through comparing the harmonic power flow and calculation error of the two kinds of simplified models with complete model under different background voltage distortion conditions, the results show that ’conjugate matrix ignored’ simplified model and Norton equivalent circuit simplified model are accurate under the current continuous mode. On the other hand, the calculation error of simplified models increases with the increasing of input voltage distortion extent.