As new energy power generation technology continuously advances,
and the capacity of distributed generation (DG) integrated to the distribution
network ever increases, traditional distribution network that
operates open-loop in a radiating pattern has gradually become an
active distribution network with multiple terminals of power supply.
This development has resulted in a dramatic change in power flow
distribution and has made it difficult to determine the operation mode
and short circuit capacity of distribution network [1–5]. Traditional
over-current protection based on fixed setting value cannot accurately
reflect the fault condition of distribution network with massive DG
integration, thus the risk of mal-operation or operation-rejection is high
[6–10]. Therefore, in order to ensure the safe and stable operation of
distribution network, it is of great importance to study current protection
schemes applicable to distribution network with DG integration.
Currently, several experts and scholars have conducted in-depth
research on distribution network current protection. Ref. [11,12] calculates
the setting values of current fast-tripping protection according
the maximum capacity and minimum capacity of DG, which could
prevent the protection from refusing to operate. However, when the
capacity of DG changes dramatically, the protection might mal-operate.
Ref. [13] calculates the setting value of the operation current online
according to the current operation mode and the fault type, so that it is
adaptable to the variation of system operation mode after DG is integrated.
However, the fault transient model of DG is simplified to a
certain degree. In Ref. [14], the setting formula of adaptive current fasttripping
protection is derived based on the detailed model of full-inverter
DG. However, the effect of other types of DG on the fault is not
considered. Besides, there is scarcely any research on the problem of
delayed operation-rejection of protection caused by voltage drop in
medium-/low-voltage distribution network [15].
In view of the above problems, considering the uncertain type and
capacity of DG integrated to the distribution network, an adaptive directional
current protection scheme based on fault steady-state component
is proposed in this paper. First, by analyzing the fault transient
characteristics and fault equalization methods of different types of DGs,
the expression of system short circuit current is derived, and the relationship
between three phase components of the short circuit current
is revealed. On this basis, according to the fault boundary conditions,
the fault steady-state components are derived. And then, combined with
the measured voltage and measured current at the relaying point, the
equivalent voltage and equivalent impedance at the backside of protection
are calculated. Thus the adaptive directional current protection
criteria for different fault types are formed. Simulation results verify the
correctness and effectiveness of the proposed scheme.
مشاوره و انجام پایان نامه رشته برق
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