Abstract
DC short-circuit faults are one of the challenges for modular multilevel converter (MMC) based DC grid. It is vital for proper design of protection system to estimate the fault currents and voltages. The existing calculation methods based on RLC equivalent model of MMC have enough accuracy in estimating the branch currents but suffer from poor accuracy in estimating the node voltages. To better reflect the dynamics of MMC control during the fault, MMC is equivalent to a RLC series circuit in parallel with a variable controlled current source. This model not only considers the discharge of sub-module capacitors but also the AC active power and MMC control. Then, based on the discrete adjoint model of the equivalent MMC model and the RL series equivalent model of DC lines, the fault voltages and currents for the pre-fault and faulted DC grids could be easily obtained. From the aspect of power balance, the importance of AC active power on estimating the fault currents and voltages is discussed then. At last, based on the Zhangbei bipolar DC grid, comparisons are conducted between the simulations on PSCAD, the numerical calculation under the proposed method and the existing methods. The results show that the proposed method and the existing methods are both able to accurately estimate the fault currents within a relative error of 1%. However, compared with the error of the existing methods in calculating the fault voltages, the relative error for the proposed method is limited to less than 5% for the whole DC gird.
Highlights
High-voltage large-capacity flexible DC transmission, with its advantages of controllable trend, easy to absorb new energy, and ability to provide reactive power support, has become an important pillar of the future DC grid [1,2,3,4]
From the perspective of power balance, this paper proposes a highly accurate estimation method of the fault currents and fault voltages for the whole bipolar DC grid
The following conclusions could be drawn from the comparisons between the electromagnetic transient (EMT) simulation and numerical calculation
Summary
High-voltage large-capacity flexible DC transmission, with its advantages of controllable trend, easy to absorb new energy, and ability to provide reactive power support, has become an important pillar of the future DC grid [1,2,3,4]. In [15], a generic calculation method for poleto-pole short-circuit fault currents in DC grids is proposed. To improve the efficiency of the calculation method, MMCs are equivalent to RLC series circuits and the phase-mode conversion method is adopted to simplify the model of the whole DC grid in [20]. A MMC is equivalent to a RLC series circuit in parallel with a variable controlled current source. (1) The influence of the AC active power on estimating the node voltages and branch currents is fully analyzed.
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