Research And Design On The Single-phase Power Electronics Load

With the development of economy, all kinds of power sources have been applied in every respect of production and people's daily life, their qualities are very important, so the stern tests for them are essential. A great number of components , such as resistances, inductances and capacitors which are used in traditional tests, not only waste a lot of test energy, but also take a lot of time for workers to fix and modulate the test circuit. The paper presents a novel equipment named PEL(Power Electronics Load) to test power source, thus the current values, power factor, creast factor and other parameters can be flexibly set to fulfill various tests, and the test energy can be fed back to the grid with little loss , high power factor and low current THD(Totle Harmonic Distortion).In view of the phenomenon that people sometimes confuse the equipment with another apparatus which are used to test the circuit board, the thesis gives the definition of PEL, which is a kind of equipment constructed by the power electronics components and controlled by the control theory of power electronic. Besides, PEL is used to test various power sources with little power loss by recycling the test power.The thesis presents the control strategy and electric circuit to test the DC power source or three-phase AC power source, and analyzes how PEL simulate all kinds of linear or nonlinear loads. The methods used to set the current reference in CCM (Constant Current Mode), CRM (Constant Resistance Mode) and CPM (Constant Power Mode) are introduced, effective ways for calculating and selecting the main circuit elements such as ac inductance and dc capacitor are discussed.The mathematic model is constructed from the circuits, P(Proportion) or PI (Proportion and Integral) control is firstly applied to simplify the design process. Jury test in control theory is used to analyze the stability of the whole system; dynamic parameter method in control theory is used to discuss the steady-state errors of the current loop. The effect and parameters selection of the low-pass filter in the current sampling circuits are discussed.Traditional PI controller can not track the current reference very closely and the band width of the close system is limited, especially in medium or high frequency band, the PI controller will cause great steady state error. For one thing, the improved repetitive controller with P and repetitive controller in parallel connection are proposed. The system with enough band width tracks the nonlinear current reference with nearly zero steady state error; for another thing, it ensures the quality of the recycling current of the grid connection converter. The design process of the controller from P controller, cascade compensation link S₁(z), cascade low-pass filter S₂(z), phase compensation link z^K, to the repetitive gain Kr, is presented.Spectral analysis of DC link voltage ripple in various work modes is provided to design the filters in the DC voltage control loop. The design and realization processes of digital notch filter, low -pass filter and mean filter are presented, and their characteristics are analyzed and compared. The mean filter, due to its best quality of eliminating harmonics, is applied in the controller; the thesis also indicates that the mean filter will deteriorate the dynamic characteristics of the whole system, other control methods should be added in. The effects of the filter's location in the controller are also discussed. Finally the engineering design method of dc voltage control loop is presented.According to the instantaneous power and active power balance, the small signal model of the whole system is constructed. The feedforward control of the input active current reference, ac voltage can eliminate the bad effects of the fluctuations caused by the ac voltage or power flow; it optimizes the steady-state and dynamic state characteristics of the whole system.A 10kWA power electronics load prototype is constructed to testify the whole design and analysis of the thesis. The multifunction equipment under comprehensive protection operates well and stably, which fully demonstrates the fact that PEL can be widely applied in various power sources' tests.