After the asynchronous interconnection of Yunnan Power Grid (YPG) and the main grid of China Southern Power Grid (CSG), the system stability characteristics of YPG will change dramatically, and the problem of system frequency will become the main stability problem. In order to ensure the operation security of YPG after asynchronous interconnection, a series of experiments on the asynchronous interconnected system is carried out by CSG to validate the frequency control measures including the primary frequency regulation, the frequency limit controller (FLC) of HVDC, the electric power system stability control system, and the third line of defence. The experiment items from April 7 to April 9 are introduced in this paper. The main problems and cases occurred in the system experiment are analyzed, and simulation and comparison are carried out. The results show that the FLC of HVDC and the electric power system stability control system are able to adapt to the operating requirements of YPG after asynchronous interconnection, while the PID parameters of hydro trubine governors and the dead-zone settings of primary frequency regulation still need to be further optimized and regulated.

The wide area response based transient stability detection and control technology has become a hot research area in power system stability studies. Currently several different technologies have been proposed, but lots of problems still need to be solved before they can be applied to practice. In order to effectively analyze and compare the validity and practicability of these transient stability detection technologies, a wide area measurement system (WAMS) based prototype system is developed for power system real-time transient stability detection and control. Based on real-time measured data from PMU, the system simultaneously implements various response based transient stability detection and control technologies and provides visual presentation of results. The design of framework structure, implementation of function modules and engineering applications of the system are presented. Meanwhile, suggestions on future orientation of wide area response based transient stability control are given in the following aspects including time delay, data acquisition and control schemes generation. Engineering application demonstrates that the prototype system has a good perspective and is an effective tool for analysis and comparison of the wide area response based transient stability detection methods.

Yunnan Power Grid is planned to operate asynchronously with the main grid of China Southern Power Grid (CSG). When the time comes, the anti-interference ability of Yunnan Power Grid will be poor since it has a small system capacity and operates independently. There might be power angel problems and high frequency stability problems after DC system is blocked, and low frequency problems after large generators are tripped. This paper analyzes the stability characteristics of Yunnan Power Grid after asynchronous interconnection with CSG, also analyzes and assesses the effects of five stability control measures groups through simulation and comparison, finally proposes control measures such as generator tripping combined with FLC for high frequency problems, and decreasing DC power through FLC and shedding load for low frequency problems.

Frequency stability will become the main security risk of Yunnan Power Grid as multi-DC sending end with small load and large sending-power after the asynchronous interconnection with CSG. The influences of load model and governor parameters on frequency characteristics are investgated. The principle of coordination and optimization of over-requency generator tripping are proposed. Based on the simulation and analysis of 4 alternative schemes, the over-frequency generator tripping scheme is proposed for Yunnan Power Grid to meet the requirements of asynchronous interconnection.

During the implementation of the asynchronous interconnection of Yunnan Power Grid and the main grid of China Southern Power Grid (CSG), the system stability characteristics will change dramatically. In the implementation periods, power transmission section of Yunnan needs to be disconnected with different 500 kV AC tie lines, moreover, since the times of Luxi Back-to-back HVDC and Yongfu HVDC project being put into operation are uncertain, different asynchronous interconnection interim schemes have influence on the security and stability of the power system. In order to reduce the risk of system operation, this paper proposes interim operation modes for the implementation stage of the asynchronous interconnection projects and analyzes the corrsponding stability characteristics, with a focus on electric power system stability control system adaptability, configuration of the third line of defence, and engineering commissioning of Luxi Back-to-back HVDC and Yongfu HVDC projects, etc, effective suggestions are provided to improve the security and stability level of the power system.

The asynchronous interconnection mode between Yunnan Power Grid and China Southern Power Grid is planned to be put into practice in 2016. However, during the practical experiment of raising DC power in asynchronous mode, the frequency of Yunnan Power Grid oscillates in ultra-low frequency for a long period. In this paper, through detailed analysis of the experiment data, the governor systems of hydropower units are found to be the direct reason of the oscillation, and the mechanism of governor systems offering negative damping during the oscillation is analysed. Furthermore, off-line data simulation is carried out to reproduce the process of this practical experiment, the results prove the correctness of the theory, finally some preliminary recommendations are proposed for further work.

During the practical asynchronous interconnection experiment, Yunnan Power Grid experienced an ultra-low frequency oscillation for a long period, which threatened the security and stability of China Southern Power Grid(CSG). In this paper, the effects of different hydro governor parameters and HVDC frequency limitation control (FLC) on the frequency stability are analyzed in a single-generator system and the BPA simulation system. From a large amount of simulation, the measures of optimally adjusting the dead zones and PI parameters of hydro governors are proposed, and the characteristics of different measures are compared. The proposed measures can effectively solve the ultra-low frequency oscillation problem in asynchronous interconnection mode of Yunnan Power Grid, which has been verified by the large and small disturbance experiments on the actual power grid.

After the asynchronous interconnetcion of Yunnan Power Grid (YPG) and the main grid of China Southern Power Grid (CSG), the operation characteristics of power grids will have a great change, and the problem of frequency instability will become one of the major risk of power system operation. Based on the real-time simulation platform of CSG AC & DC hybrid power system, an overall simulation system of YPG and CSG in asynchronous interconnection operation is constructed, and a series of simulations on the frequency responses of YPG and control characteristics of HVDC projects in asynchronous interconnection operation are carried out considering the frequency control measures of primary frequency modulation, frequency limitation controllor of HVDC and special protection system. In comparison between simulation results and site records, the governor and turbine models are optimized to improve the simulation accuracy of the control characteristics of CSG in asynchronous interconnection operation, which provides technological support for the asynchronous interconnetcion project of YPG and CSG.

The response-based transient stability control technology is a hot issue in real-time transient stability analysis of power system. It has such advantages as independent of modle and parameters of system, real-time detctction and less calculation. Transient instability detection method based on angular velocity-rotor angle trajectory is currently a very active branch of response-based real-time transient stability analysis. Firstly, the theory of this method is introduced, then the problems in application of the method to multi-machine power systems are analyzed and discussed.The simulation results on IEEE-39 bus system show that when this method is applied to multi-machine power system, because of incorrect generator-clustering and the time-varying parameter of the equivalent system, this method will produce incorrect stability detection.

The main circuit connection modes and operating voltage characteristics of VSC-HVDC system have brought new technical problems on equipment type selection. Aiming at symmetric monopole connection mode and based on the asynchronous interconnection project between Yunnan power grid and CSG, the key type selection of connected transformer for VSC-HVDC system is systemetically studied, a technical principle is proposed to determine the valve side insulation tolerance level of connected transformer by calculating the maximum load voltage drop based on reactive power controlled mode, the recommended process and methods of insulation parameters configuration are also given, and the relative insulation test method for winding terminal is put forward. Furthermore, considering that the symmetrical bipolar connection mode may be used in VSC-HVDC system with higher voltage level and lager current in the future, the paper proposes a calculation formula to calculate the DC and AC withstand voltage at valve side winding. The research results have important technical guiding significance for the type selection, technical standards formulation and test of the connected transformer.

Aiming at the differences of DC and AC harmonic current characteristics between valve bridge arm reactor for VSC-HVDC system and conventional DC smoothing reactor, based on the Yunnan asynchronous interconnection project, this paper studies the necessity of additional AC temperature rise test for bridge arm reactor, recommends AC temperature rise test methods and compares temperature rise experiments. Results show that firstly, the methods of temperature rise design and test for bridge arm reactor should not simply follow those methods for conventional DC smoothing reactor based on DC current distribution characteristics; secondly, the bridge arm reactors for Yunnan asynchronous interconnection project meet the temperature rise limit value requirements in AC-DC combined current. The research can provide reference for type design and selection of reactors under AC-DC combined large current projects in future.

According to the system characteristics of ±350 kV Luxi back-to-back VSC-HVDC transmission project for asynchronous interconnection between Yunnan Power Grid and China Southern Power Grid, key technical principles are proposed for the design and type selection of the VSC-HVDC valve AC side wall bushing considering the requirements of system equipment adaptability, parameter configuration, typical structure selection, monitoring device and test inspection. The research shows that key influencing factors, such as system topology and grounding bias leading to AC and DC voltage superposition, various operation mode resulting in wide range fluctuation of voltage and current, different installation location affecting the rated current calculation, should be considered during the design of VSC-HVDC wall bushing. Both the dry-capacitor type structrure form and the dry-inflation type structrure form of wall bushing are analyzed, and it is proved that temperature compensation function is not needed in the SF₆ pressure monitoring device of the wall bushing. Research results of this paper have been applied to the design and type selection of the wall bushing of ±350 kV Luxi back-to-back VSC-HVDC project.