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.

Large-scale smart charging stations can effectively satisfy and control the charging demands of tremendous plug-in electric vehicles (PEVs). But, simultaneously, their penetrations inevitably induce new challenges to the operation of power systems. In this chapter, damping torque analysis (DTA) was employed to examine the effects of the integration of smart charging station on the dynamic stability of the transmission system. A single-machine infinite-bus power system with a smart charging station that denoted the equivalent of several ones was used for analysis. The results obtained from DTA reveal that in view of the damping ratio, the optimal charging capacity is better to be considered in the design of the smart charging station. Under the proposed charging capacity, the power system can achieve the best maintained dynamic stability, and the damping ratio can reach the crest value. Phase compensation method was utilized to design the stabilizer via the active and reactive power regulators of the smart charging station respectively. With the help of the stabilizers, damping of the system oscillation under certain operating conditions can be significantly improved, and the power oscillation in the tie-line can be suppressed more quickly.

The rising integration of renewable energy sources has resulted in a diminished capacity for voltage support within the system, which is characterized by low inertia and a reduced short circuit ratio (SCR). In order to improve grid strength and enhance the capacity for renewable energy integration, an initial analysis was conducted on the grid support capabilities of grid-forming (GFM) stations, followed by an investigation into how grid strength influences the dominant operational modes of GFM converters. Subsequently, leveraging the definition of the multi-infeed short circuit ratio, a calculation method for the SCR, applicable to new energy base stations featuring GFM substations, is developed. Additionally, a strategic approach to optimal location selection and sizing of these substations aimed at enhancing the SCR within new energy grids is proposed, with the model being solved through genetic algorithms. Finally, the effectiveness of the proposed method is verified based on the IEEE39-node system and a real new energy station. The results show that the system strength is greatly improved after the optimized configuration of the GFM equipment, and the maximum tolerable space of 90% new energy stations reaches 95% of the theoretical maximum tolerable space of each new energy station.