Studies On The Compensation Mechanism And Control Strategies Of STATCOM Integrated With Distribution Transformer

With the rapid development of power electronics,the number of power electronics devices in modern distribution system increases day by day.In general,this kind of devices has a smaller time constant than conventional ones,which makes it more difficult and complex for reactive power compensation and power quality enhancement.To address the above problems,voltage source converter-based STATCOMs have been widely used nowadays.However,the handle voltage of power switches is always limited.Therefore,the conventional transformer-less STATCOM has complex structure and high cost,which can not meet the requirements of economy and technicality.Thus it can be concluded that reducing the operation voltage to an appropriate level,increasing the connection point voltage of STATCOM without attaching more auxiliary devices and promoting devices integration are important development trends in the future.In order to reduce the voltage stress and composite cost,improve the integration level of the devices and optimize the comprehensive compensation performance,this paper chose the distribution transformer located at the joint of high and low voltage as a critical point for reactive power compensation and power quality control.The compensation mechanism and nonlinear passivity-based control strategy of STATCOM integrated with distribution transformer(DT-STATCOM)are the major topics of this thesis.There are various connection alternatives for STATCOM in distribution system.This paper compared different connection types of STATCOM in detail.From the point of view of voltage stress reduction,four different structures of STATCOM integrated with distribution transformer were given to achieve a flexible connection point voltage for STATCOM.Then,the compensation mechanism was focused.The mathematical model reflecting the special coupling relationship between STATCOM and transformer was built by multi-winding transformer theory.The compensation principle of DT-STATCOM was explained from the point of circuit,magnetic field and power conservation.In order to prevent the windings from overcurrent after current injection,the winding currents distribution after compensation current injected was studied detailedly.The constrained conditions were also derived.Both simulation and experiment results were provided to verify the amplitude and phase variation characteristics after compensation current injected.In view of the realization of DT-STATCOM system,the overall design was completed from primary structure of power circuit and critical control loops design.Firstly,the cascaded multilevel converter-based STATCOM was implemented for compensation current injection,which can satisfy the requirements of flexible expansibility and high level integration.The LCL filter with passive damping was designed to reduce the filter size and attenuate the high frequency switching harmonics.Then,an improved reference currents generation method across elements was proposed considering that the signal detection point and compensation currents injected point were not the same points.The magnitude and phase compensation was applied to overcome the differences of compensation point and detection point.The compensation capacity constrain was also considered.To achieve the dc capacitors voltage balancing control,a three-layer balancing control strategy was presented,as well as the design procedures of the parameters for each control loop.A dynamic weight assignment-based comprehensive compensation algorithm was proposed to meet the requirement of multi-object compensation.Finally,simulation results have verified the expansibility and performance of the proposed structure and control strategy for reactive power compensation.The traditional inner current control loop has poor flexibility and transient response.To enhance the performance of inner loop control,this paper started with the analysis of the energy dissipation of DT-STATCOM.The passivity-based control(PBC)model of DT-STATCOM was established by applying the Euler-Lagrangian equation in mechanical dynamics.An extra virtual damping was attached to improve the speed of transient response.In addition,the passivity-based control law was studied in detail,which revealed that PBC was essentially a compound control.The design method of the injected damping was also proposed.To address the effects of parameters perturbation on the performance of PBC,this paper proposed an adaptive PBC.Both simulation and experiment results have indicated that the proposed DT-STATCOM with PBC has a good transient response characteristic and steady-state accuracy.Developing control system of STATCOM in the traditional way is inefficient.Design and verification can not be accomplished at the same time.Therefore,this paper studied a model-based design procedure for the control system development of DT-STATCOM.The graphical modeling tools and auto code generation were applied to improve the efficiency.Meanwhile,the semi-physical simulation was adopted,which made it possible to verify the algorithm earlier.Furthermore,this paper focused on the ceffects of the simulation step on the accuracy of the PWM signals.Two interpolation methods were proposed to compensate the error of the PWM pulses.The effects of jitter and delay on the accuracy of PWM were mitigated and the confidence of the semi-physical simulation was improved.On the foundation of theoretical analysis and semi-physical simulation,a cascaded multilevel converter-based dynamic simulation test bench was developed to test the performance of DT-STATCOM.Various experiments were carried out,including winding currents distribution,reactive power compensation,selective harmonic elimination,comprehensive compensation of both reactive power and harmonics,and load unbalance compensation.The results have verified the validity and effectiveness of the proposed mechanism analysis and control strategies.They also indicate that DT-STATCOM has well compensation performance and fast dynamic response.