Research Of Electronic Power Transformer Protection System

The reliability and security of power system is highly related to power transformer which is a key component to connect nodes and exchange power in power system. Electronic Power Transformer (EPT) is a brand new class of transformer that the magnitude and phase angle of voltages in both the primary side and the secondary side of EPT can be controlled in real-time through power electronic converters to achieve flexible regulation of the current and power.Due to the fact that an EPT consists of power electronic converters, the reliability of EPT which is always of special significance for the electric power system is dependent on those power electronic converters and power switches. Hence the fault characteristics, fault detection and protection of those power electronic converters and power switches are of great importance for EPT, whose failure could result in damage to the whole power system and loss to end user. However, seldom research has been done about EPT protection system, let alone its device protection and component protection. Hence this dissertation mainly discusses the protection system for a distribution electronics power transformer.Chapter1introduces the state of the art of EPT, and then gives the topology and operational principles of the EPT. Chapter2focus on the hardware design for the protection system of EPT, including measuring system, controlling system and protection execution system as well as a monitering carbinet. Chapter3is to design a software system for the protection hardware, which divides the protection system into3parts according to the way EPT operates. The detailed fault protections are verified and further discussed with experimental results in chapter4.Chapter5derives some conclusions about EPT fault characteristics, since the waveforms of relevant components heavily depend on the location of the faulty switch, it is very easy to locate the exact position of the faulty switch. Finally, the fault characteristics peculiar to D-EPT are analyzed, and further discussed with simulation on the Saber platform, as well as a fault location diagnosis algorithm.Chapter6discusses a new problem caused by voltage sag and short interruption for input voltage of EPT, and finally comes up with steps to prevent the over current to the IGBT of EPT which is caused by the faults mentioned above by analysis and simulations.Chapter7shows possibilities that EPT can apply hybrid cascaded converter with staircase control scheme, and the total harmonic distortion (THD) and switching losses of the proposed hybrid cascaded converter among staircase, PWM and partial PWM control are compared with analysis and Saber simulation. In the end, a laboratory prototype of a15-level IGBT-based cascaded H-bridge inverter is designed and implemented to output a low distortion, near-sinusoid stepped-voltage, with its total harmonics distortion (THD) less than5%.