Research Of Flexible Arc-suppression Control Strategy In Distribution Network Grounding Fault

With the increasing extension of cable and nonlinear load in the distribution network,there are quantities of active and harmonic components contained in the faulty current under the single line-to-ground(SLG)fault.Since the arc-suppression coil can not tackle these components,the faulty residual current is large and the arc-suppression result deteriorates.The flexible arc-suppression method based on the three-phase cascaded H-bridge converter(FAMCHC)provides a novel scenario for the fully compensation of faulty current.The FAMCHC is connected to the distribution network via connection inductors.When a SLG fault is occurred in the distribution network,the faulty current and faulty phase voltage can be constrained or declined as long as a compensated current is injected to the distribution network by the converter,which makes the arc self-extinguish readily and be less possible to reignite.On the basis of analyzing the SLG faulty features of non-effective earthing system,the FAMCHC theory,which includes the calculation of compensated current,the transient process of flexible arc-suppression and the principle of suppressing the harmonic component of the faulty current is built up.Then the main circuit parameters,modulation and control strategy are researched according to this theory.The design of control strategy consists of DC voltage stability control,DC balancing control and AC current tracking control.To achieve different objective in the normal and grounding fault state,a multiple objective DC voltage stability control strategy is employed.The partial energy exchange method is adopted in the DC voltage balancing control,which forces the submodules having the maximum capacitor voltage or minimum capacitor voltage to exchange partial energy.Regarding the AC current tracking control,the P control,PI control,inertia element plused P control and inertia element plused sliding mode are compared and analyzed.With the help of PSCAD simulation,it is revealed that the proposed DC stability and DC balancing control strategy are effective and the inertia element plused P control have the advantages of good arc-suppression effect,low harmonic content in modulation waveform and robustness.So it is used as the AC current tracking controller in this paper.Furthermore several PSCAD simulations prove that the proposed flexible arc-suppression control strategy still can control the faulty residual current to near zero effectively and function well in the arc-suppression under the circumstances of different grounding resistance,arcing ground or contamination of harmonic source.Finally to promote the economy of FAMCHC,the multi-function application of cascaded H-bridge converter is researched.The converter is used to eliminate the harmonic when the distribution network is in normal,while it plays the roles of arc-suppression and harmonic elimination in the SLG fault simultaneously,which improves the power quality greatly at the same time of arc-suppression.The instantaneous power theory is applied to detect the harmonic and the multi-function scheme of cascaded H-bridge converter is also validated to be feasible by the simulation.