The Research On Grounding Type And Short Circuit Protection Of Civil Low Voltage Direct Current Systems

As a power supply mode with energy efficiency and environmental protection,low voltage direct current (LVDC) is showing strong application potential inconventional field such as civilian buildings. The security of LVDC system became thekey obstacle when to promote it in conventional field. The safety of low voltagedistribution system is translated into appropriate grounding practices, and protection ofequipment or human body from electrical faults. This paper outspreads the study infocusing on the two topics of LVDC system grounding issues and its short circuitprotection. Study results provide theoretical basis for further promotion of LVDCsystem.The choice of low voltage power system grounding types depends on the electricshock protection performance and supply electricity continuity, especially the formerfactor. The safety voltage limit is the foundation to study electric shock protectionperformance of low voltage power system. However, effect of electric shock for AC andDC are different in nature. This paper compares effect of electric shock for each currenttype refer to international standard IEC60479-1, and the safety threshold curve for DCis given. Safety voltage limit when human body through DC current in differentdirections also has been analyzed. Considering the phenomenon that DC current mayinclude some high frequency DC ripple inevitably at present, frequency weighting effectto electric shock effect for human body and influence by the DC ripple has beendiscussed.Based on the analysis of DC safety voltage, electric shock protection performanceof two power system supplied by ideal dc source and single-phase bridge rectifier hasbeen simply discussed. What’s more, by using the DC bipolar distribution system basedon DC micro-girds and voltage balancer as the research subject, this paper makes theanalysis on grounding fault parameters, and validates it with the aid of Matlab/Simulinksoftware. On this basis, this paper discusses the power supply continuity, protectionproperty against electric shock and requirement of protection for each grounding type.The result indicates that, in comparison with the traditional AC220V distribution system,the low voltage DC distribution system has a significant advantage in terms of electricalsecurity. In addition, the power supply continuity and the electrical security of the DCTT system equipped with RCD can be improved simultaneously, and which is more suitable for this bipolar distribution system with voltage balancer than IT or TN system.The allowed range values of the grounding resistors and technical requirements aregiven to ensure the electrical security.As to the short fault and protection cooperation of LVDC system, this paperproposes two simplified short fault models on account of external characteristics andswitch function, respectively. Then, this paper focus on the setting up of a small dcpower network based on DC converter in the lab. Fault parameters have been measuredfor line short and load short. The changes of short current in LVDC system and theimpact of short fault to LVDC power system are both presented. In the end, this papertests the protection cooperation situations between DC circuit breaker, DC source andDC loads.