Study On The Operating Characteristics And Safety Control Measures Of HVDC Islanded Operation System

The uneven distribution of China’s energy reserves and demand promotes the rapiddevelopment of long-distance and large-capacity HVDC power transmission. Islandedoperation of large-capacity HVDC system can eliminate the influence of power flowtransferring on the AC system when monopole/bipole Block of the DC system occur, enhancethe stability of long-distance transmission system and increase the capacity of thetransmission corridor. How to guarantee the secure and stable operation of islanded systembecomes a critical problem of power system and DC operation arrangement.Under islanded operation, the HVDC system has no electrical connection with the mainAC system, so its effective inertia constant and short circuit ratio(SCR) are much lower thanthat under HVAC-HVDC parallel operation.Therefore, the interaction between AC and DCsystem under islanded operation is more significant, which leads to a sequence of problemson angle stability, frequency stability, overvoltage, etc. In order to keep the islanded systemsafe and stable, well-designed speed governing and excitation control should be introduced onthe generator side. While on the DC side, additional frequency control and overvoltagecontrol should be introduced.Yunnan-Guangdong ultra high voltage direct current transmission system (UHVDC) isthe first HVDC system which takes islanded operation as its normal operation mode during itsdesign stage in China. Due to more than250km away from its two supporting power plants,the rectifier Chuxiong convertor station is weak, the effective short circuit ratio(ESCR) isvery low and the system’s operating condition is stringent. Therefore, the operation andcontrol of Yunnan-Guangdong UHVDC is much more difficult than any other HVDC systemin the world. Aimed at the possible problems, comprehensive theoretic research was carriedout in the early stage and a series of on-site commissioning experience were organized toverify the effectiveness of the proposed control approaches.During these commissioning experience in the islanded Yunnan-Guangdong UHVDCsystem, some special problems of islanded operation gradually emerged. In frequency control,the main problems include frequency periodic oscillation caused by mismatch of frequencycontrol measures between the generator side and the DC side, adaptability problem of controland protection caused by frequency remaining higher than52Hz for a long time aftermonopole Block, and synchronous condenser operation of generators after bipole Block. Involtage control, the main problems include voltage periodic oscillation caused by high gain ofAVC, power frequency overvoltage after the Block of HVDC, voltage sag caused by valve groups deblocking. In stability control, an oscillation mode exist between Xiaowan generatorsand Jinanqiao generators in the islanded system, the conventional PSS can effectively increasethe damping of the oscillation, but the reverse-regulation of PSS after monopole Block willbring adverse influence on the frequency control and overvoltage control of the islandedsystem.This thesis studies the physical characteristics behind those special phenomenamentioned above. On this basis, this paper proposes control measures to enhance theoperation performance of islanded system. The major works of this paper include:(1)Based on the on-site commissioning experience of islanded operation of theTianshengqiao-Guangdong HVDC and the Yunnan-Guangdong UHVDC, the frequencyregulation strategy is proposed. In order to avoid frequency periodic oscillation, the dead zoneof primary frequency regulators is enlarged and the dead zone of DC frequency limit control(FLC) is reduced, the FLC is preferred in frequency control of islanded system. During loadadjustment of the islanded HVDC system, the generators should be adjusted in advance andHVDC automatically follow them by FLC.(2)Aimed at the problem of frequency sharp rise after monopole Block, threefactors the generators’ input mechanical power, output electromagnetic power, rotationalinertia and their effect on frequency dynamics are analyzed. For the rising process offrequency, a method is proposed to reduce the peak value with the combination of modifyingthe parameters of governor, optimizing the DC overload control and generator tripping. Forthe falling process of frequency, the thesis points out that the PSS will lead to significantdecline on AC voltage and trigger the inverter control to switch to the I mode which will slowdown the recovery of frequency. In this thesis, Wide-area Power System Stabilizer is proposedto solve this problem.(3)After the bipolar tripping under islanded operation, it was observed that some of thegenerators turned into condenser operation. Mechanism of this phenomenon is interpreted byusing the theory of three-stage distribution of power disturbance in power system, and thecontrol strategy to avoid this phenomenon is proposed.(4)The problem of power frequency overvoltage after the Block of HVDC underislanded operation is studied. According to the difference of main factors which affect thelevel of overvoltage on time scale, the dynamic process of overvoltage is divided into threestages. The dynamic process of overvoltage under three different circumstances monopoleBlock, bipole Block and successively bipolar Block is analyzed in detail. A solution schemeusing high side voltage control is proposed to improve the system performance during the second stage of overvoltage.(5) The problem of dynamic stability of sending system under islanded operation isstudied. This thesis points out that the conventional PSS can effectively increase the dampingof the oscillation mode between Xiaowan generators and Jinanqiao generators, but bringadverse influence on the overvoltage control of the islanded system after monopole Block.The reason is studied from the principle of the conventional PSS, and a solution scheme usingwide-area signals based PSS is proposed based on the mechanism of electromechanicaloscillations. The wide-area PSS can increase the damping of the system and avoids theadverse influence on overvoltage control at the same time.The study in this paper is based on the first UHVDC system which considered sendingend islanded operation as its normal operation mode in China.This paper summarizes the keyproblem of islanded operation in frequency contol, overvoltage control and system stability.The conclutions of this paper not only provid the theoretical and technical basis for thearrangement of Yun-Guang HVDC islanded operation scheme, but also have reference valuefor other HVDC system which considered islanded operation as normal operation mode.