The DC loop resonance is analyzed in this paper in consideration of harmonic transfer characteristic on both sides of AC/DC and distribution parameters of long overhead DC line. Based on the characteristics of harmonic transfer between AC and DC of the equivalent converter of controlled source, the DC side equivalent impedance of the converter is derived by taking the example of single pole earth return mode. What’s more, aiming at the suppressing measures of DC blocking filter, the influences of elements parameters and arrangement modes on DC loop impedance are analyzed. It is proved that the cross layout of blocking filters in neutral bus will have an evident suppression effect on 50 Hz/100 Hz DC loop resonance. It is also recommended that blocking filters should be based on reactor and capacitor, and the variation range of reactor and capacitor parameters should be limited. Detailed DC loop model and DC simulation model on PSCAD/EMTDC are built. By combining impedance frequency characteristic scanning and time domain simulation, the above effects are simulated and verified, which provides reference for the research on DC loop resonance of long-distance UHVDC transmission.

Multiple AC feeders in urban distribution network usually apply closed loop connection and open-loop operation mode, and do not have controllable power support between lines. The flexible interconnection of multi-circuit 10 kV lines by back-to-back multi-terminal converter can transform the open-loop operation mode of traditional distribution network into closed-loop operation mode. Based on balanced load strategy, a DC power real-time modulation technology is proposed for multiple 10 kV lines with flexible interconnection to DC distribution center, which uses active power distribution coefficient of feeder to realize active power deviation control and load balancing among multiple lines. A flexible interconnected AC/DC power distribution system based on DC distribution system with four terminal hybrid module MMC is constructed by using PSCAD/EMTDC simulation software. On the basis of master-slave control, the control strategy of DC distribution center is improved by using power modulation technology based on load balancing strategy. The simulation results show that the proposed DC power modulation strategy can balance the load between the feeders in real time and improve the equipment utilization and operation reliability of the AC distribution network under the condition of reducing the load measurement points of the distribution power system.

On-load tap changer(OLTC) is one of the most important components of converter transformer. Due to the complex operating conditions and increasing capacity of converter transformers, the switching process and type-selection technology of OLTC of converter transformer are different from those of ordinary power transformer. Taking the typical dual-resistance tap changer as an example, this paper introduces its switching principle, switching process simulations are carried on OLTC which is equipped on power transformer and converter transformer, the particularity of OLTC operation condition of the converter transformer is revealed. The analytical formulas of the recovery voltage of main and auxiliary contacts during the switching process of OLTC are derived, and they are compared with the simulation results. A type-selection technology of OLTC combined with the qualitative waveform simulation and the quantitative key parameter calculation is put forward, which provides reference and guiding significance for the design, type-selection and operation maintenance of OLTC of the converter transformer in the actual HVDC projects.

The voltage phase angle calculated by using DFT algorithm in phasor measurement unit(PMU) contains large errors because of the volatility of the actual power grid frequency. For lacking of the other method in real power grid to measure the phase angle difference, it is impossible to verify the error of phase angle difference measured by PMU. The voltage phase angle difference between different measuring points has not been obtained accurately in actual power grid. This paper presents a method to calculate the phase angle difference of voltage at different measuring points using the voltage waveform data uploaded by the fault recorder. The synchronization of the recorded data measured at different substation is realized by using the zero-sequence-current waveform at the time of single-phase earth fault, and the voltage phase angle difference between different measuring points is calculated by the correlation function. The results of theoretical analysis and actual data analysis show that the accurate voltage phase angle difference at different measurement points of power grid is accurately obtained by this method. With the calculation results of this method, it is possible to modify the measurement error of the voltage phase angle difference calculated by PMU. This method is important to analyze the measurement accuracy of voltage phase angle difference between voltage phasors at different nodes of power grid and improve the value of fault recorder data.

UPFC(unified power flow controller)is the most advanced flexible AC transmission system (FACTS) technology, but the conventional cross decoupling control method in the dq rotating coordinate system still exists coupling between the active control and reactive control loop. The couple characteristics between active power and reactive power in voltage d-q rotating frame are analyzed, and a novel control strategy in polar frame is proposed, which adopts active power-phase, reactive-amplitude decoupling control method of the power. The detail simulation model of HVDC transimission system is built, which includes UPFC. Simulation results comparison shows that the proposed strategy effectively reduces the couple between the active and reactive power control loop, enhances the UPFC system power-flow control performance especially under solely active power or reactive power control scenario.

The review of mechanism of low frequency oscillation caused by speed governor and governor additional damping controller is presented. The mechanism of negative damping of hydroelectric speed governor is interpreted in depth. A kind of governor additional damping controller (GPSS, governor power system stabilization) is designed and proposed for hydroelectric speed governor in Yunnan grid. The time-domain simulation for Yunnan grid fully shows the effectiveness of GPSS to suppress very low frequency oscillation. The conflict between operational stability and adjust rapidity of hydroelectric unit can be solved effectively by installing GPSS on hydroelectric speed governor.

The access of a photovoltaic power station with a hydrogen energy storage system (HESS) to the distribution network can increase the proportion of photovoltaic consumption, reduce the distribution network losses, enrich the types of renewable energy, and improve economic efficiency. Therefore, an active-reactive power coordination control strategy considering hydrogen storage energy and reactive power control of grid-connected inverters is proposed. Considering the uncertainty of photovoltaic output, the active-reactive optimization model of the distribution network is constructed which contains the HESS in response to the time-of-use price, the reactive power of the inverter, the tap adjustment of the on-load transformer and the switching of the shunt capacitors. And a new HESS multistage start-stop control strategy is also proposed. Taking the largest PV consumption, the maximum operating revenue of hydrogen storage and distribution network as the optimization goal, the DICOPT solver of commercial planning software GAMS are used to solve the problem. The improved IEEE33 node system is used to analyze the effectiveness of the proposed model and coordinated control strategy.

Remotely operated vehicle (ROV)for submarine cable inspection is a kind of large equipment based on the use of a large construction vessel. It is necessary to design a ROV dedicated for submarine cable inspection so as to carry out routine inspection by operation and maintenance companies. Firstly, based on the actual situation of Hainan interconnection project, the ability of anti-ocean-current for ROV is obtained from the statistics and analysis of hydrology and ocean current data information, the basic parameters such as power, propulsive force and travel speed are calculated, and the positioning method is designed. Secondly, according to the needs of submarine cable inspection, various types of detection instruments to be mounted on ROV are analyzed and determined. Combined with the characteristics of the vessel, the safe operation modes of ROV based on platforms without dynamic positioning are analyzed. Finally, the special design requirements for the ROV used for 500 kV submarine cable detection in Hainan interconnection project are proposed according to operation and maintenance conditions.

With the development of distribution transformers, the new distribution transformers represented by vegetable insulating oil distribution transformers and high overload distribution transformers, have been applied to distribution networks. Compared with normal distribution transformers, the new distribution transformers have obvious advantages in technical performance, which influence the selections of distribution transformers. In this paper, by analyzing the technical and economic characteristics of vegetable insulating oil distribution transformers and high overload distribution transformers, a life cycle cost model and a optimization method are proposed, which can be used to compare the economic benefits of different distribution transformers, and obtain the optimal selections according to the load level during planning period and the least principle of the life cycle cost. Finally, based on the load data of 10 kV feeders, the schemes of distribution transformers under four typical load curves are optimized by this method. The results show that the method proposed in this paper has strong feasibility and can guide the reasonable selections of distribution transformers.