Zhu Yiying

Studying on Measures of Restraining DC Current through Transformer Neutrals

Monopole ground return mode may cause DC current through the transformer neutrals which are connected to the ground and make the transformers DC biases. With the increase of HVDC transmission power, some transformers through which there will be more DC current going may be saturated, which brings some problems such as acutely shaking, increased noises and overheating when power system is in the normal operation. Not only the safety of transformer but also the normal operation of power system may be impacted in this case. There are three main measures to restrain DC current through transformer neutrals. They are: (1) putting a resistor in series between transformer neutral and ground, (2) putting a capacitor in series on AC transmission line and (3) putting a capacitor in series between transformer neutral and ground. Combining with the real instance of the transformers at Lingao and Dayawan nuclear power stations, these three measures to restrain the DC current caused by monopole ground return mode of HVDC transmission system are studied by building a module of power system with ground grid according to the DC current values measured at the transformer neutrals of Lingao and Dayawan nuclear power stations. The results of study show that the optimum measure to restrain DC current through transformer neutrals at Lingao and Dayawan nuclear power stations is to put a capacitor in series between transformer neutral and ground. This measure is studied further. Some main technology parameters are put forward according to the results of study

Study on Sharing Earth Electrode of UHVDC

This paper analyzes the different operation modes of the UHVDC system sharing earth electrode from the aspect of power system stable operation. Two UHVDC system rectifiers and inverters sharing earth electrode were studied respectively by electromagnetic transient analysis software EMTP-RV based on the main parameters of Xiang jiaba to Nan hui plusmn800 kV UHVDC transmission system. The influence on the normal operation of UHVDC system sharing earth electrode is simulated and analyzed. The merits and demerits of sharing earth electrode are summarized in detail. The feasible suggestions for the corresponding control and protection of UHVDC system sharing earth electrode are put forward.

Unit commitment problems using GA

In this paper, unit commitment (UC) using the genetic algorithm (GA) with different implementation techniques, such as different sampling space methods, different selection schemes, different fitness value scaling methods and different crossover/mutation rates, is developed and implemented in 10 units system and 110 units system. This paper thoroughly studies the effects on convergent time, convergent generation number and convergent value of GA based on different implementation techniques, which indicates that the algorithm is effective in a certain degree, and reveals that different implementation techniques have different effects on convergent time, convergent generation number, and convergent value of GA-based UC. A good foundation is settled for further practicable study of GA-based UC.

Security and stability study of planning UHVAC/DC hybrid transmission system

This paper presents the security and stability study of planning UHVAC/DC hybrid transmission system using digital and analog hybrid real-time simulation. The building of digital electromagnetic transient and physical simulation models of planning UHVAC/DC power grid was introduced, then detailed simulation of UHV power backbone was presented. The study include as following: Single Fault, Critical failure of ac system, pole\bipolar emergency outage of dc system. Simulation results show that: Ac power system failure will lead power fluctuation at UHVAC transmission channels, but the system will restore stable operation after ac fault was cleared. When dc system occurs polar emergency outage, planning UHV system was stable, but for bipolar emergency outage, it need trip off generators at sending system to ensure the system stability. 2015 UHV power backbone meet the requirements of Guidelines for Power System Stability, this network structure is feasible.

Real-time simulation and parameter optimization for SVC control tragedy of XiZang grid

Since Qingzang HVDC system put into operation in 2011, higher requirements for voltage stability in Xizang grid must be taken into account. It plans to install several SVC devices at three 220 kV stations which include Duodi, Naiqiong and QuGe, in order to improve the voltage quality of Xizang Grid and the transmission capacity of QingZang HVDC system. To optimize and validate the control tragedy of these SVCs before their installation, a real-time digital/analog hybrid simulation platform is setup by HYPERSIM. This platform is composed of a digital model which includes detail 220 kV and 110 kV network of XiZang grid, an analog HVDC controller for Qingzang HVDC system, and all the analog SVC controllers. It includes two types of SVC devices for the real project, one is TCR+SC, and another is TSC. Based on the simulation platform, the SVC control tragedies of these two types are validated respectively. The simulation results show: through adding SVC devices at the 220 kV stations, the Xizang grid has better voltage recover characteristic after short-circuit fault. It can draw a conclusion that the voltage control tragedy used in this real project has favorable control characteristic for the voltage stability of Xizang Grid.

Power connection technology for full-digital real-time simulation and analog simulator

The study of UHVAC/UHVDC hybrid power grid demands various means in every stage, such as in planning, designing, building and operating. Full-digital real-time simulator has the merits such as very large simulation scale and very easy modified model. And analog HVDC simulator has the merits such as reliable and precision simulation results. To bring the merits of these two simulators into play, a power connection technology for combining them is developed, which is developed by distributed parameters transmission line model. Using this power connection technology, a hybrid simulation model for a simple AC/DC hybrid power grid is set up. Results of load flow, start-up and fault test show that this power connection technology is precision, stable and practical, thus it suits to realtime simulation for parallel operation of AC/DC hybrid power grid.

Real-time simulation on UHVDC coordinated control for transmitting large scale wind power

UHVDC link is one of the appropriate solutions to transmit large-scale renewable energy resources. An advanced real-time digital/analogue hybrid simulation platform is applied, which can accurately simulate both large-scale wind farms with digital models and UHVDC link with physical models. The UHVDC coordinated control for transmitting large scale wind power is researched in depth. According to the characteristics of the active power adjustments and equipment operating characteristics of UHVDC link, a novel type of active power control strategy is proposed to track the fluctuation of wind power. Combined with dynamic characteristics of power grid stability recovery, a reactive power coordinated control strategy of UHVDC link is put forward to improve the fault recovery and enhance the system stability when the wind farms are tripped out from power grid. The proposed coordinated control strategies are testified and verified by real-time digital/analogue hybrid simulation test. The study results provide an indispensable technical support to the application of large scale wind power transmission via UHVDC link.

Study on digital-analog hybrid model of synchronous generator with direct drive wind turbine

A digital-analog hybrid model of synchronous generator with direct drive wind turbine is formed based on realtime digital simulator HYPERSIM and Voltage Source Converter analog model. It combines with the merits of digital simulator and analog simulator. The dynamic behavior and responding character of the model is be studied by simulating steady and transient tests including network faults. The technical problems of real-time simulating the dynamic character of IGBT in wind turbine exactly are solved. This model provides an indispensable technical measure to study the strategy and parameters of the real wind turbine controllers. It can also be used to study the dynamic characteristic of the wind turbine connected to grid. Basic guidelines for the preparation of a technical work for the IEEE Power & Energy Society are presented.