Key Issues Research On Parallel Inverters System

With the development of the Power Electrical technology, the applications of high power devices in power field such as FACTS,HVDC are more important than before. It is usually used that several inverters operate in parallel operation for increasing capability. Dead time influence to improve the output voltage distortion and current limit protection (such as short circuit) for system safety are more and more attention.On the occasion of the large inductor current ripple, the dead-time effect is eliminated at most zero-crossing zone of the inductor current, which is hard to explain using the conventional average dead time analysis. For this problem, a usual analysis metod based on seven situations of inductor current according to zero current clamping phenomenon is proposed. This method breaks inductor current into 7 situations using fundamental value,current ripple and zero current clamping phenomenon. The analyses of dead time effect using in unipolar and bipolar SPWM are obtained.For the inverter with output LC filter, the dead time effect is is not only relative DC voltage, dead time, switching cycle, but also relative modulation method, filter frequency, load current and so on. Thus, an estimating method for dead time effect is proposed for the inverter working in no load, and the dead time effect estimating functions are built for biopolar SPWM and unipolar SPWM. the curves is obtained to visually express the relation between LC filer frequency,switching frequency and dead time effect.The proposed dead-time compensation method is based on adjusting promptly the compensation voltage in accordance with the seven situations of inductor current in theory. However, it is difficulties to determine seven situations around the zero current clamping (ZCC), as the seven situations of inductor current are variable with change of the load current, DC voltage. Thus, an improved inductor current detecting method with high precision is presented, which estimates the inductor current by detecting load current instead of inductor current. Therefore, the proposed dead-time compensation method is based on compensation voltage according to the three zones (positive zone, zero-crossing zone, and negative zone) of estimated inductor current in practice. The simulation and experiment results show the validity of the analysis and the effectiveness of the proposed dead-time compensation.For parallel inverters system without dead time, the active power and reactive power are connected with voltage amplitude, voltage phase. The 3D figure is the flat, which means that it is possible to decouple between active power and reactive power using a constant coefficient matrix. However, there is a no sensitive area once adding the dead time. The change of active power, reactive power is large with change of voltage amplitude and phase without in this area. On the contrary, the power change is not obvious with the varying of voltage amplitude and phase.For analysis of current limit protection technique in the parallel inverters system. Controlling the output voltage in normal operation constantly is named the voltage control mode (VCM), and controlling the output current constantly in abnormal operation conditions (such as short circuit) is named the current control mode (CCM). A hybrid control strategy is proposed. On the one hand, the PID control and the repetitive control are used in VCM to obtain the fast dynamic responses and low harmonic distortions. On the other hand, the state feedback control is used in CCM. Pole assignment has been employed in designing parameter of the PID controller and state feedback, and the repetitive control design process is given.For ensuring reliable operation of high power inverter happening in load shocking (such as short circuit), the switching model between VCM and CCM becomes one of key technologies. However, it is possible to yield large shock between several parallel inverters when switching from CCM to VCM isn't in same time for parallel inverters. Thus, the large circulating current results in power flooding inversely, even more stoping inverters. An improved switching model technique is proposed only according to the synchronizing signals for ensuring the same time from CCM to VCM. Experimental results validate the proposed control strategy using two 400KVA parallel inverters.