Research On Dynamic Reactive Power Compensation Device And Control Strategy With Low-order Harmonic Suppression

At present,China’s power demand is developing rapidly.A large number of reactive and non-linear loads are connected to the grid,which leads to serious power quality problems.Power quality management has become an important research topic.In practical applications,a large amount of load injects a large amount of harmonics into the power grid while absorbing reactive power,causing pollution to the power grid.In this context,the dynamic reactive power compensation device and control strategy with low-order harmonic suppression are studied.For two-level grid-connected converters with LCL filtering,the system should consider the stability of LCL control and also the influence of non-ideal conditions,such as grid distortion and imbalance and load asymmetry.The modulation strategy of the two-level converter has a great influence on its differential-mode and common-mode characteristics,and the differences between various modulation methods still need to be further explored.In addition,when the compensation capacity is increased,the design of the output filter inductor is critical to the improvement of the system power density.The power loss of the inductor,as a important performance index,should be design properly.In view of the above problems of control,modulation and inductor loss,this paper mainly completed the following work:The stable control method of the grid-connected converter with LCL filtering and the power fluctuations of the static var generator under reactive power and harmonic compensation are analyzed firstly.Under the condition of grid imbalance and load asymmetry,the grid-side current feedback with capacitor current proportional feedforward are used to achieve stable operation of LCL.Inner current loop based on reduced-order generalized integrator and the outer voltage loop based on adaptive PI are adopted to compensate the reactive power and harmonic current.For the modulation methods of two-level converters,this paper takes a study from the aspects of common-mode voltage(CMV)and differential-mode current.Based on the superposition theorem,the single-phase equivalent model of the converter is analyzed.And a current ripple evaluation method based on the current-ripple-double –envelope(DCRE)is proposed.The ripple and high-frequency harmonic characteristics of each modulation method are analyzed.In addition,according to the impulse equivalent principle,the distribution of the low-frequency CMV of each modulation method is explored in a three-dimensional vector space,and a modulation method with globally minimum low-frequency CMV is obtained.For the iron loss of filter inductor,a core loss estimation method based on the DCRE is proposed.This method can quickly estimate the loss value of the core according to the double envelope and core characteristics without any experiment.Adopting the DCRE method,the influence of pulse width modulation(PWM)schemes,dc voltage,and the amplitude of output current,can be easily evaluated,which provides a satisfactory guidance to the design of the inductor.