Parallel Operation Analysis and Optimization of Cables in Urban DC Distribution System

With the rapid development of national energy conservation and emission reduction policies, the wide connection to the distribution network with photovoltaic, energy storage and electric vehicles, and the increasing number of power electronic equipment made the DC distribution system an important development direction. In the future DC distribution system, as the line load continues to increase, more and more single-core cables work in parallel with multiple cables of the same pole., and how to evenly distribute the parallel circuit current has always been a key issue restricting the parallel operation of the cable in same-pole. Starting from the operation control of grid connected AC/DC converter under unbalanced grid voltage, this paper gives theoretical derivation that the busbar current of DC system always has double frequency harmonics. Then, referring to the three-phase AC system, based on the impedance parameter calculation method of“conductor-earth”, the calculation method of AC impedance parameters of multi-circuit cable lines in symmetrical bipolar and monopole DC system is deduced. On the basis of the theoretical analysis, conclusion is put forward, which is, in symmetrical monopole DC system, the current sharing is the best when the mutual impedance between the positive and negative pole cables is equal in each circuit. Finally, supported by a symmetrical bipolar DC power distribution demonstration project in Shenzhen, this paper optimizes the DC bus layout scheme with two cables running in parallel through the finite element calculation. It also verifies derivation of harmonic current sharing in the symmetrical monopole DC system proposed in this paper.