| Capacitive voltage transformers(CVT)are mainly used to provide reliable voltage signals for power system metering,protection and control devices,and has high accuracy under power-frequency.However,due to the existence of energy storage components and nonlinear components inside the CVT,the transient response error of the CVT is large,and it is difficult to meet the measurement requirements under the high amplitude and wideband transient voltage of the power grid.Therefore,it is necessary to establish an accurate CVT electromagnetic transient physical model to realize the CVT response simulation under complex transient conditions,and to provide a reliable data source for power system energy measurement,relay protection,and other related research.At the same time,it provides a basic model for calculating the original signal of the power system through the output of the secondary side of the CVT.The normal operation of CVT depends on the resonance of the capacitive divider and the compensation reactance.Once the frequency changes or the core is saturated,the CVT will exhibit complex electromagnetic characteristics such as frequency characteristics and core nonlinearity.However,the existing CVT models with frequency characterization capabilities are mostly linear models,and it is difficult to take into account the influence of the intermediate transformer core components on its voltage transmission characteristics,resulting in a sharp increase in the response error of the model in a specific frequency band or when the intermediate transformer core is saturated.Therefore,it cannot completely and accurately simulate the electromagnetic transient response of the CVT under wide frequency band and wide amplitude excitation.Therefore,the purpose of this thesis is to construct a CVT wideband nonlinear model which can simultaneously simulate frequency characteristics and nonlinearity.To describe the frequency characteristics,the CVT port scattering parameters are discretized and characterized in the equation of state,and then the CVT wideband admittance sub model is established through Norton’s equivalent to realize the accurate representation of the wideband voltage transmission characteristics of the CVT.Aiming at the nonlinear characteristics representation,an electromagnetic dual model of the intermediate transformer that can characterize the difference in core saturation is derived through the principle of electromagnetic duality with considering the physical structure of the intermediate transformer.And then a CVT low frequency nonlinear sub model considering the deep saturation characteristics of the intermediate transformer core is established.And a CVT wideband nonlinear coupling model is established through the mutual admittance difference and parallel expansion.Finally,parameter identification and model construction of a typical 35 k V CVT are carried out,and low-frequency inrush current,voltage transfer characteristic test and lightning impulse voltage test are designed to verify its nonlinearity and frequency characteristic representation capabilities.The main research work and important achievements of the thesis are as follows:(1)The development of the CVT wideband admittance model that represents frequency characteristics.By equating the CVT to a two-port network,the CVT wideband admittance can be calculated by the scattering parameter method.Then the frequency domain expression of the CVT wideband admittance parameters is obtained by the vector fitting method.And the continuous state equation of wideband admittance is derived and discretized by the reverse Laplace transform and the central difference method.Then the equivalent circuit of wideband admittance model is developed by Type-94 component.Finally,the CVT wideband admittance sub model is constructed based on the Π structure to realize the accurate characterization of CVT frequency characteristics.(2)The development of the CVT electromagnetic dual model considering core saturation characteristics.The electromagnetic duality model topology of the intermediate transformer is derived based on the duality principle.And a direct measurement method of CVT deep saturation inductance is proposed to realize the accurate identification of the spatiotemporal nonlinearity of the core.Finally,CVT wideband nonlinear model,which can realize the electromagnetic transient characteristics of the CVT under complex conditions,is established through the mutual admittance and parallel expansion of the CVT low frequency nonlinear sub model and the wideband admittance sub model.(3)The accuracy and effectiveness of the 35 k V CVT wideband nonlinear model are verified.The parameter identification of the model are carried out on a 35 k V CVT.Then the CVT wideband nonlinear model is established in ATP-EMTP;low-frequency inrush current,voltage transfer characteristics and lightning impulse voltage tests are carried out to validate the representations of the frequency characteristics and nonlinearity.Finally it is further verified that the proposed transformer model has the ability to simulate complex electromagnetic transients by comparing with the typical CVT electromagnetic transient model.The results show that the CVT wideband nonlinear model proposed in this thesis has the ability to accurately simulate the complex electromagnetic transient response of the CVT under the excitation of the wideband and wide amplitude excitation,and can realize the accurate representation of the nonlinearity and frequency characteristics of the CVT at the same time.It can also provide the basic data and model support for reliable operation and condition monitoring of a CVT-based system. |
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