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The difference between common current transformer and zero sequence mutual inductor and its usage? 

1. the role of common current transformer Ordinary current transformer can convert the primary current with larger value to the secondary current with smaller value through a certain ratio, which can be used for protection, measurement and other purposes. A current transformer with a ratio of 400/5 can convert the actual 400A current to the current of 5A. Working principle of 2. zero sequence mutual inductor The basic principle of zero sequence current protection is based on Kirchhoff's current law: the algebraic sum of complex currents flowing into any node of the circuit is equal to zero. Under the normal condition of the circuit and electrical equipment, the sum of the vectors of each phase current is equal to zero. Therefore, the secondary side winding of the zero sequence current transformer has no signal output and the executing element does not operate. When grounding fault occurs, the vector and sum of the phase currents are not zero. The fault current causes the flux to be generated in the annular core of the zerosequence current transformer. The secondary induction voltage of the zerosequence current transformer causes the actuator to act, drives the tripping device and switches the power supply network to achieve the purpose of grounding fault protection.
A current transformer can be installed on each of the threephase lines, or threephase conductors can pass through a zerosequence current transformer together, or a zerosequence current transformer can be installed on the neutral line N to detect the sum of the threephase current vectors. Zerosequence current protection can be used to install a current transformer (C.T) on each threephase line, or let threephase conductors pass through a zerosequence C.T together, or install a zerosequence C.T on the neutral line N. These C.T can be used to detect the sum of threephase current vectors, i.e. zerosequence current Io, IA+IB+IC=IO, when the line is connected to a threephase negative. When the load is completely balanced (no grounding fault, and do not consider the leakage current of the line, electrical equipment), IO = 0; when the threephase load on the line is unbalanced, then IO = IN, the zerosequence current is unbalanced IN; when a phase grounding fault occurs, a singlephase grounding fault current Id will inevitably occur, and then detected. The zero sequence current IO=IN+Id is the vector sum of the threephase unbalanced current and the singlephase earth current. 


