MMC performance evaluation concerning different dc breakers and converters

Abstract

Renewable energy resources are the most successful, promising, and economical mode of energy generation. Moreover, wind energy is also one of the victorious candidates in generating renewable energy. Currently, High Voltage Direct Current (HVDC) systems are designed to integrate the energy harvested in the Offshore Wind Farms (OWF) to the grid system. The HVDC system uses AC-DC converters, and then with the help of DC cables, the power is transmitted from the offshore to the onshore area, where it again gets converted back to AC. The converters used here are called high voltage converters, where these power electronic components have the ability to sustain high voltages and current. Further, in this thesis, Modular Multilevel Converter (MMC) is used for the conversion of AC to DC and vice versa. The MMC is built using stacked Insulated Gate Bipolar Transistors (IGBTs), where these power electronic components are current sensitive in nature. Thereby, if there is a fault, and if this fault current flows through them, then these components get damaged permanently. Moreover, if the offshore MMC goes down, the OWF system has to undergo a restart. Therefore, the DC Circuit Breaker (DCCB) is used in order to clear the DC fault in an HVDC system. Meanwhile, designing a DCCB for the HVDC system is not easy, one major reason is that there are no natural zero crossings in a DC fault. Also, the fault must be cleared very quickly because DC faults have a very high rate of rising in fault currents. Earlier, the MMC protects itself by using the blocking algorithm. Here, the switching devices turn off temporarily until the fault is cleared. When they are turned off, the fault current flows through the freewheeling diodes, as diodes have the capability to withstand high currents. Indeed, if the MMC is blocked, the purpose of DCCB is not satisfied. Therefore, in this thesis, the performance of the MMC is examined with the presence of DCCBs for a 525 KV system. Besides, there are two types of DCCBs used, in order to evaluate the more suitable breaker for the system. Further, with the initial performance, the fault behavior of MMC is analyzed and illustrated. Consequently, the DC inductance, converter inductance, and arm inductance were modified in order to improve the performance of the system. On the other hand, there are two types of MMC, namely Half Bridge MMC (HBMMC) and Full Bridge MMC (FB MMC). Generally, the HB MMC are called fault feeding converters, whereas the FB MMC are called fault blocking converters. This is due to the topology of the FB MMC, where it completely blocks the fault current by itself. Therefore, the performance of the HB MMC with a DCCB is compared with the FB MMC with blocking protection for a 320 KV network.