| The output characteristic of inverter-interfaced distributed generation(IIDG)is totally decided by control strategy.When it's connected into distribution network,the current flow and short-circuit current will change.What's more,the protection setting and cooperation will be affected.Especially when IIDG connects into the network as a teed line,the reliability and selectivity of longitudinal differential protection will be affected.So it's necessary to analyze the fault characteristic of double-power supply system.Based on it proposed the improved scheme of longitudinal differential protection.In this paper,introduced the method about modeling of IIDG,which is represented by photovoltaic power system.Based on this,summarized the output characteristic of IIDG.Later,put forward the calculation method of composite sequence circuit of unbalanced fault through transition resistance.Then analyzed the relationship between the fault position,fault type,transition resistance,capacity of IIDG,angular phase difference of the system potential and the differential current.Furthermore,proposed the method to calculate the allowed penetration level of IIDG under the influence of longitudinal differential protection.When the capacity of IIDG exceeded the allowed penetration level,based on the information from potential transformers on high voltage bus,a protection scheme was proposed,which used the difference between the positive-sequence compensating voltages as an assistant criterion.By using the phase difference between positive-sequence compensating voltage and positive-sequence differential current,the fault dead zone can be eliminated.At last,a 110 kV high voltage distribution network with IIDG case was built in PSCAD.The validity of the calculation of maximum penetration capacity of IIDG was verified.Compared with the traditional longitudinal differential protection,the reliability and effectiveness of the improved scheme was also verified. |
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