To meet the ultra-fast, selective, and reliable requirements of the DC fault protection, a protection scheme for modular multilevel converter based HVDC grids is proposed in this paper. The protection scheme is consisted of main protection, directional pilot protection, and faulty pole identification. The new fault directional criterion is achieved by employing the surge arriving time difference of line-mode backward traveling wave and line-mode forward traveling wave. This criterion is valid even when the fault current limiting reactors(FCLRs) are installed at the converter side of DC bus. The main protection without communication and complex computation relies on co-operation of the derivative of line-mode forward traveling wave and the directional criterion. The directional pilot protection by comparing the double end directional criteria can be used as a backup protection. The accumulation of zero-mode component is adopted to discriminate the faulty pole. Simulations have demonstrated that the proposed scheme is capable of identifying various types of faults with different fault resistances at different positions. The robustness analysis shows that the protection scheme presented is not sensitive to the FCLR size, noise environment, sampling frequency, and the operation of hybrid DC circuit breakers.

Accurately detecting and locating the fault point of the DC line is significant for eliminating the fault and restoring the power supply of the flexible DC distribution network as soon as possible. Firstly, a direction pilot protection scheme for a complex DC distribution network is proposed based on the integral of the current superposition to identify the fault direction. Then, an online time-domain fault location method based on the least square (LS) method to solve the overdetermined equations is proposed by analyzing the fault loop circuit after the DC line fault. The proposed location scheme utilizes fault data at both ends of the line to eliminate the theoretical impact of fault resistance, and the calculation of the current difference method is discussed to reduce the location error of whether the fault current-limiting reactor (CLR) exists. Finally, various simulations by PSCAD/EMTDC V4.5 demonstrate that the proposed scheme has high protection reliability and location results after different fault positions and resistances. The proposed scheme has low requirements for the sampling rate, fault data length, and implementation costs, which can meet practical application requirements.

Integrating battery energy storage systems (BESS) into a coal-fired generator can enhance power systems’ secondary frequency regulation capability. To this end, this paper proposes a policy to coordinate the BESS and the coal-fired generator to meet the automatic generation control (AGC) requirements and subsequently investigates the optimal BESS capacity to maximize the net profit gained from the frequency regulation. Firstly, probability characteristics of AGC instruction duration period, interval period, regulation rate, and regulation direction are investigated according to the real AGC datalog. The AGC regulation direction would change randomly. Directly switching the BESS between charging and discharging to match the requirement would significantly deplete the lifetime of the battery. Therefore, this paper divides the BESS into two groups, which will be controlled stay in charging and discharging states to respectively respond to the AGC requirement of “up” and “down”. To obtain a cost-effective BESS investment, this paper develops a new sizing method, which optimizes the BESS capacity by simulating the operation of the hybrid coal-fired generator and BESS power station (HCGBPS) over an example day. Finally, the case studies justified the developed sizing method could make satisfying BESS investment decisions to ensure a maximum net profit in operation.