| With the development of economy and industrial control technology, the high voltage and high-power converters are widely applied in the application fields. At present, in the application of oilfield oil-pumping exploitation, compared to the widely-used two level converter, the multilevel converter has many advantages, such as small switch stress, high dc-link voltage grade, low harmonic content of output voltage and so on. Among them, the diode neutral point clamped(NPC) three-level converter is one of the most widely used structures, which has the lower requirement of power device.Thus, it is more and more widely researched and applied in the field of frequency conversion control.For the neutral-point voltage deviating problem in diode neutral point clamped(NPC)three-level converter, the paper analyzes the mechanism of neutral-point voltage deviation,explains the modulation principle of SVPWM and carries out two methods by utilizing the positive and negative small vectors for capacitors neutral-point voltage balancing.Nevertheless, this strategy will cause the low frequency oscillation of neutral-point voltage in heavy current and higher modulation index.Concerning the low frequency oscillation of neutral-point voltage, an improved method of virtual space vector pulse width modulation(VSVPWM) is introduced, which recommends small vectors to the virtual space vectors modulation strategy for balancing the neutral point voltage. Nevertheless, owing to the fact that the balance of neutral point can not be implemented by smalls vectors in some vector areas, some scholars put forward the mixed modulation strategies. This paper proposes a varied virtual space vectors pulse width modulation(VVSVPWM), which modifies the neutral-point current of virtual middle vectors and distributes the action time of small vectors to balance the neutral point voltage.The strategy above solves the low frequency oscillation of neutral-point voltage caused by SVPWM and the problem that the neutral point voltage can’t be controlled in some vector areas by virtual space vector modulation.The exact inductance parameters need to be known for the conventional current decoupling controlling in two-level rectifier. The inductance parameters are difficult to measure in engineering. It is hard to achieve an ideal dynamic controlling performance withthe conventional decoupling method, because of the changed inductance by saturation. The paper comes up with a current decoupling strategy without inductance parameters in three level rectifier, and the voltage outer loop is implemented using traditional PI. Meanwhile, to improve the anti-load performance of system, a current feed-forward control by the current observer is proposed for three level rectifier, which improves the dynamic, static performance and robustness of rectifier. The feasibility of the proposed strategy is verified through the simulation and experiment results.The direct power control for PWM rectifier is a common control scheme. The varied switching frequency brings some difficulties to the PWM control of three level rectifier, for example, the difficulty on balancing the capacitance neutral point voltage. A sliding mode direct power control with fixed switching frequency for three level rectifier is studied in the paper. The mathematical model of sliding-mode direct power control is deduced under the two-phase stationary coordinate. The proposed strategy does not need the phase detection for the synchronous rotating coordinate transformation and is easy to be realized. Finally,the simulation of the proposed direct power sliding mode control is implemented in the paper and the results verify the effectiveness of the proposed strategy. |
