Studies On Current Differential Protection And Implementation Technologies In Active Distribution Networks

Distributed generations (DGs) have been widely used in European and American countries, since the concept of smart grid is proposed. In China, distributed generations close to load is also encouraged, which promote the integration of DGs with small capacity into distribution networks. It can be predicted that in future distribution networks will change into an active distribution network (ADN) with high penetration integration of DGs. It has the characteristics of multi-branch, multi-source, bidirectional power flow and fault current as well as weak-feeding. In this case, both selectivity and sensitivity of conventional over-current protections will face big challenges or even to be lost. In order to solve the problems caused by DGs, current differential protection is introduced in distribution networks. Attentions are focused on the analysis of adaptive protection criterions, protection scheme, implementation technologies, the development and test of set, et al. The following works are investigated in this paper.(1) Amplitude and phase characteristics of positive sequence fault components (PSFCs) in ADN are analyzed. Based on the establishment of PSFC equivalent circuit with different structures and different types of DGs, PSFCs are calculated. Then the amplitude and phase characteristics of PSFCs with internal and external faults are analyzed, which can be used to analyze the amplitude and phase relationship between terminals of protection section with rotating type distributed generations (RTDGs) and inverter interfaced distributed generations (IIDGs). It can be used to establish current differential protection criterions.(2) Based on the PSFC, an adaptive current differential protection scheme in ADN is proposed. This scheme employs conventional current differential protection scheme and the value of braking current and braking coefficient are set according to whether there are non-measured branches in protection section, Taking this scheme, load transfer and power fluctuation in protection section can be adapted. And it has strong adaptation in the types, capacity and position of DGs. Finally, simulations results verify its sensitivity and feasibility.(3) An inverse time current differential protection scheme in ADN is proposed in this paper. In this scheme, self-starting current of motors is taken into account avoided by operation time delay, and amplitude and phase factors are used to establish current differential protection criterion respectively to solve the contradiction in enough ability for braking saturation current of current transformer and improving sensitivity with internal faults.(4) A novel fault instant calculation method based on inversion transduction is proposed. The error of current fault data self-synchronization algorithm based on fault instant detection is analyzed theoretically. And in some case, the error will be large. Basing the above analysis, an accuracy fault instant recognition algorithm is proposed. The aim of this scheme is to find real fault instant using recording data. Via this way, synchronization error can be reduced essentially.(5) A novel fault detection method based on Hankel matrix and modulus maximum is proposed. In this scheme, attention is focused on characteristic of sudden change of current at fault instant. Then singular value decomposition (SVD) is used to detect singular point as fault instant. This scheme can offer a novel method to realize fault data self-synchronization.(6) Basing on advanced hardware platform, a set is developed taking the current differential protection scheme. Platforms of static simulation system and RTDS are established to test and evaluate the protection scheme, synchronization scheme, communication protocol, operation time and so on. Test results demonstrate that the scheme can operate accurately with different fault situations in ADN. And Generic Object Oriented Substation Event can be applied to distribution network protection, and contrast with Transmission Control Protocol/Internet Protocol, Generic Object Oriented Substation Event has obvious advantages in the transmission speed.