Neutral-point-clamped hybrid multilevel converter with DC fault blocking capability for medium-voltage DC transmission

This paper proposes a novel hybrid multilevel converter with DC fault-blocking capability, i.e., the neutral-point clamped hybrid multilevel converter(NHMC).By employing two types of unipolar full-bridge submodules along with director switches, which are composed of seriesconnected insulated-gate bipolar transistors, the NHMC combines the features and advantages of the neutral-point clamped converter and the modular multilevel converter.The basic topology, operating principles, modulation scheme, and energy-balancing scheme of the NHMC are presented. The DC fault-blocking capability of the NHMC is investigated. The number of power electronic devices used by the NHMC is calculated and compared with other multilevel converters, showing that the proposed NHMC can be an economical and feasible option for medium-voltage DC transmission with overhead lines. Simulation results demonstrate the features and operating scheme of the proposed NHMC.

Study on Fault Lateral Sealing Based on SGR Method

Bozhong A Oilfield is located in the northeast of Huanghekou Sag in the Bohai Bay Basin, and is a complex fault block oilfield. The main oil bearing horizon is the lower Minghuazhen Member, belonging to shallow water delta sedimentation. For the rolling exploration and evaluation of adjacent fault blocks, this paper uses the mudstone smear method to analyze and accurately characterize the lithological contact relationship between the two walls of the fault. Based on the establishment of different reservoir docking modes on the two walls of the fault, the sealing parameters are calculated to evaluate the sealing ability of the fault, thereby evaluating its reservoir formation characteristics, and predicting the height of the oil column. The above methods provide geological basis for further rolling expansion and potential tapping in the study area and surrounding areas, and have guiding significance for exploration and rolling potential tapping in similar blocks in the Bohai Sea.

Exploration Strategies for Complex Fault Block Reservoirs in the Subei Basin

The geological background of the Subei basin is that of small relief subsidence, low geothermal gradient, multi-sedimentary hiatuses, intense reconstruction of the basin, frequent magmatic activity, and a unique combination of source rock and reservoir. This geological background resulted in characteristics such as many small fault blocks, multiple oil-bearing formations, scattered oil distribution, mini- to small-sized reservoirs, and difficulties in exploration. Aimed at such characteristics, an effective exploration strategy was adopted, and the oil reserves, production and economic benefits of the Jiangsu oilfield were significantly increased. This exploration strategy included understanding the hydrocarbon generation mechanism of source rocks, progressive evaluation of oil resources, comprehensive research on the faulted systems, the distribution of oil reservoirs and their controlling factors. The techniques used included integration of acquisition, processing and interpretation with 3-D seismics as the core technology, trap description and evaluation, directional drilling and cluster drilling, integration of cuttings logging, gas chromatographic logging and geochemical logging, and integration of early reservoir description and progressive exploration and development. This strategy could be guidance for other complex fault blocks.

The productivity evaluation model and its application for finite conductivity horizontal wells in fault block reservoirs

It is very difficult to evaluate the productivity of horizontal wells in fault block reservoirs due to the influence of fault sealing.On the basis of the method of images and source-sink theory,a semianalytical model coupling reservoir and finite conductivity horizontal wellbore flow dynamics was built,in which the influence of fault sealing was taken into account.The distribution of wellbore flow and radial inflow profiles along the horizontal interval were also obtained.The impact of the distance between the horizontal well and the fault on the well productivity was quantitatively analyzed.Based on this analysis,the optimal distance between the horizontal well and the fault in banded fault block reservoirs could be determined.According to the field application,the relative error calculated by the model proposed in this paper is within ±15%.It is an effective evaluation method for the productivity of horizontal wells in fault block reservoirs.The productivity of the horizontal well increases logarithmically as the distance between the horizontal well and the fault increases.The optimal distance between the horizontal well and the fault is 0.25-0.3 times the horizontal well length.

Distribution pattern of deltaic sand bodies controlled by syn-depositional faults in a rift lacustrine basin

Take the lacustrine delta in the second member of Paleogene Shahejie Formation in block Wang43, Dongying depression, Bohai Bay Basin as an example, the deposition architectural characteristics of lacustrine deltaic sand bodies controlled by syn-depositional faults in complex fault blocks of rift basin are examined from the aspect of the tectonic-deposition response, using cores, well logs and three-dimensional seismic data. The small-scale syn-depositional faults in complex fault blocks are dense and different in dip, the activity along the strike of syn-depositional fault varies in different positions, and all these control the sedimentary process of deltaic sand bodies. Influenced by syn-depositional faults, the deltaic distributary channel is more likely to pass through the position with weak fault activity, and be deflected or limited at the position with strong fault activity. In downthrown side of a single syn-depositional fault or micro-graben areas, sand bodies increase in thickness and planar scale, and sand bodies of multiple stages are likely to stack over each other vertically. In micro-horst areas controlled by syn-depositional faults, the sand bodies decrease in abundance, and appear in intermittent superimposed pattern vertically. This study can provide new research ideas and theoretical basis for exploration and development research in complex fault blocks.

Geological Guided Tomography Inversion Based on Fault Constraint and Its Application

Fault block reservoirs are one of the main types of hydrocarbon reservoirs found in offshore basins,and they are widely distributed within the Mesozoic and Cenozoic basins of the northern South China Sea.Conventional seismic profiles of complex fault areas often contain obvious fragmentation and distortion of seismic events that is corresponding to geological structure under the fault.This phenomenon is known as a fault shadow;it occurs in relation to rapid changes in velocity near the fault that deviate the ray path of waves,and it seriously affects the ability to determine the geological structure and subsequently evaluate the reserves of fault reservoirs.In the current conventional tomography inversion method,the velocity model is over-smoothed,which results in distortion of the reflection layer under the fault.Based on the velocity tomography inversion of imaging gathers method and the concept of regularization,this paper first introduces the anisotropy Gauss regularization operator.A high-resolution tomography inversion method is then developed,and the fault-controlled geological guidance is constrained.This technology is then applied to a complex fault block reservoir basin in the South China Sea,and the results show that it can significantly solve the problem of fault shadow imaging and determine the geological structures in the target area.The newly developed method thus has very good application prospects.

Study on Injection-Production System Adjustment of Fault Block Reservoir Based on Equilibrium Displacement

This paper proposed a method of injection-production system adjustment to solve the problem that the water flooding effect was restricted because of the horizontal and vertical contradictions during the development process of fault block reservoirs. Considering the heterogeneity of reservoir, the Buckley-Leverett water flooding theory was applied to establish the relationship between the recovery and cumulative water injection. In order to achieve the goal of vertically balanced recovery of each section, the calculation method of vertical sectional injection allocation was proposed. The planar triangular seepage unit was assumed and sweep coefficients of different oil-water distribution patterns were characterized using multi-flow tube method. In order to balance and maximize the plane sweep coefficient, the calculation method of plane production system optimization was obtained. Then the injection-production system stereoscopic adjustment method based on equilibrium displacement was proposed with vertical sectional injection allocation and plane production system optimization. This method was applied to injection and production adjustment of BZ oilfield in southern Bohai. The effect of water control and oil increase was obvious. This method can greatly improve the effect of water flooding of offshore fault block reservoirs with the adjustment of injection-production system.

3D numerical simulation of heterogeneous in situ stress field in low-permeability reservoirs

Analysis of the in situ stress orientation and magnitude in the No.4 Structure of Nanpu Sag was performed on the basis of data obtained from borehole breakout and acoustic emission measurements.On the basis of mechanical experiments,logging interpretation,and seismic data,a 3 D geological model and heterogeneous rock mechanics field of the reservoir were constructed.Finite element simulation techniques were then used for the detailed prediction of the 3 D stress field.The results indicated that the maximum horizontal stress orientation in the study area was generally NEE-SWW trending,with significant changes in the in situ stress orientation within and between fault blocks.Along surfaces and profiles,stress magnitudes were discrete and the in situ stress belonged to theⅠa-type.Observed inter-strata differences were characterized as five different types of in situ stress profile.Faults were the most important factor causing large distributional differences in the stress field of reservoirs within the complex fault blocks.The next important influence on the stress field was the reservoir’s rock mechanics parameters,which impacted on the magnitudes of in situ stress magnitudes.This technique provided a theoretical basis for more efficient exploration and development of low-permeability reservoirs within complex fault blocks.

ANISOTROPIC FEATURE OF QIANGTANG MASSIF TEXTURE

Qiangtang Massif is located in the hinterland of Qinghai—Tibet plateau, which belong to the mid\|east section of Tethys Tectonic Domain.1 Features of the whole texture and structure of Qiangtang massif By synthetic analysis of gravity,magnetic field,MT,seismic surveying,etc. Geophysical data, the massif, lied in the tectonic setting and geodynamic setting mingled by the south,north tectonic belts, have the features of massif,basin and tectonic belt three forming an organic whole,multi\|degree coupling in plane and section with division of region in south\|north trend,division of block\|fault in east\|west trend,division of sphere\|layer in vertical direction. (1) Belting in south\|north trend: Qiangtang massif could be divided into four units from north to south, that is north edge doming zone, west part doming area,Qiangtang Basin and south edge doming zone. Qiangtang Basin also can be divided into four tectonic units—north Qiangtang down\|warping region, middle downing zone, south Qiangtang down\|warping and east part slope region. The near east\|west trend tectonic zones are well developed. There is aero\|magnetic anomaly distributed in belting with east\|west trend but also concentrated in section. Gravity anomaly is high in the south\|west part and low in the northeast part. Inter\|crust low resistance layer alternately distributed with high and low belting of sou th\|north trend in plane.

Enhancing HVRT Capability of DFIG-based Wind Farms Using Cooperative Rotor-side SMES Considering the Blocking Fault of LCC-HVDC System

Line-commutated converter based high voltage direct current(LCC-HVDC)has been applied for transferring bulk power from wind farms to load centers through long distances in many countries.When the blocking fault of the LCC-HVDC system occurs,the surplus reactive power accumulated at the sending end will lead to an overvoltage,thus causing the discon・nection of the wind turbines.To maintain the reliable connection of wind turbines and ensure the stability of the power system,this paper introduces a superconducting magnetic energy storage(SMES)unit to connect in parallel with the rotor side of the doubly fed induction generator(DFIG).By controlling the energy side converter and rotor side converter to inject demagnetizing current and reactive current into the rotor,the proposed scheme can effectively stabilize key parameters of the wind turbine and provide desirable reactive power support,showing a favorable high voltage ride through(HVRT)performance.Several cases based on PSCAD/EMTDC&MATLAB/Simuiink co-simulation together with economic analysis are conducted to demonstrate the feasibility and superiority of the proposed scheme on enhancing HVRT capability of DFIG-based wind farms.

GPS time-series and its response to M_S=8.1 Kunlunshan earthquake

In this paper, observation data in 25 GPS reference stations of China have been analyzed by calculating GPS position coordinate time-series with GIPSY. Result shows there is an obvious trend variation in such time-series. The trend variations of time series along the longitude and latitude coordinate reflect the motion of each position in the global-plate, in which the trend variation in the vertical direction reveals some large-scale construction information or reflects the local movement around the positions. The analysis also shows that such time-series have a variation cycle of nearly 1.02 a, but the reason still remains to be further studied. At the end of this paper, response of the time-series to MS=8.1 Kunlunshan earthquake was analyzed, and the seismogenic process of MS=8.1 Kunlunshan earthquake, according to the time proceeding and the feature of anomaly, was divided into 3 phases-changes in blocks with forces, strain accumulation, quick accumulation and slow release of energy. At the initial stage of seismogenic process of MS=8.1 earthquake and at the imminent earthquake, coseismic process as well as during the post earthquake recovery, anomaly in vertical direction is always in a majority. The anomalous movement in vertical direction at the initial stage resulted in a blocking between faults, while at the middle stage of seismogenic process, the differential movement between blocks are in a majority, which is the major reason causing energy accumulating at the blocking stage of faults.

Reverse-blocking modular multilevel converter for battery energy storage systems

Energy storage systems with multilevel converters play an important role in modern electric power systems with large-scale renewable energy integration.This paper proposes a reverse-blocking modular multilevel converter for a battery energy storage system(RB-MMCBESS). Besides integrating distributed low-voltage batteries to medium or high voltage grids, with the inherited advantages of traditional MMCs, the RB-MMC-BESS also provides improved DC fault handling capability. This paper analyzes such a new converter configuration and its operating principles. Control algorithms are developed for AC side power control and the balancing of battery state of charge. The blocking mechanism to manage a DC pole-topole fault analyzed in depth. Comprehensive simulation results validate both the feasibility of the RB-MMC-BESS topology and the effectiveness of the control and fault handling strategies.

Relationship between Landslides and Active Normal Faulting in the Epicentral Area of the AD 1556 M～8.5 Huaxian Earthquake,SE Weihe Graben(Central China)

In this paper, we focus on the characteristics of the landslides developed in the epicentral area of AD 1556 M^8.5 Huaxian Earthquake, and discuss their relations to the active normal faults in the SE Weihe Graben, Central China. The results from analyzing high-resolution remote-sensing imagery and digital elevation models（DEMs）, in combination with field survey, demonstrate that：（i） the landslides observed in the study area range from small-scale debris/rock falls to large-scale rock avalanches;（ii） the landslides are mostly developed upon steep slopes of ≥30°; and（iii） the step-like normal-fault scarps along the range-fronts of the Huashan Mountains as well as the thick loess sediments in the Weinan area may facilitate the occurrence of large landslides. The results presented in this study would be helpful to assess the potential landslide hazards in densely-populated areas affected by active normal faulting.

Research on a Multiport Parallel Type Hybrid Circuit Breaker for HVDC Grids:Modeling and Design

This paper proposes a star type multiport hybrid circuit breaker(Star-HCB)topology for protection of multiterminal DC transmission.Reliability and stability of high voltage DC(HVDC)grids are determined by their capabilities to withstand DC-side faults.In order to maintain reliability of HVDC grids,both ends of each line should be equipped with hybrid circuit breakers(HCB).This method will increase expenditure of the HVDC,especially the meshed topology.The n-port Star-HCB consists of ultra-fast mechanical disconnectors,load current switch and only one transferring branch which is formed by improved half-bridge sub-module.Compared with existing traditional hybrid circuit breakers and other multiport hybrid circuit breakers,the proposed topology can realize the same short-circuit blocking goal using fewer components.Detailed mathematical transient process calculation and timedomain simulation of the proposed Star-HCB are given to verify its superiority.

Techno-economic Analysis of Contingency Reserve Allocation Scheme for Combined UHV DC and AC Receiving-End Power System

In a UHV DC and AC combined system the UHVDC’s blocking fault becomes the most serious load balance contingency of the receiving end power system due to its large capacity.The amount of contingency reserve thus is largely increased.This paper proposes three distinct schemes for allocating contingency reserve to cover the power shortage caused by UHVDC’s blocking fault,and compares the economic cost of these schemes.An operation simulation method using a unit commitment technique is proposed for evaluating the operation costs associated with increasing contingency reserve.A real case study for a Jiangsu Province power system in China is presented to demonstrate that the best scheme is to share the contingency reserve with neighborhood power systems through the transmission capacity of UHVAC lines.

Reservoir formation conditions and key exploration & development technoloiges in the Yingdong Oilfield in western Qaidam Basin

Located in east part of Yingxiongling structural belt in the Qaidam Basin,the Yingdong oilfield has a extremely complicated ground condition.Due to no significant discovery,this oilfield was considered to have no favorable geologic conditions for formation of oil or gas reservoir.In the past few years,with continuous improvement in the mountain 3D seismic surveys and logging data interpretation,some breakthroughs were obtained in 2010,and the Yingdong oilfield,the largest-scale reserves of a single reservoir with highest organic matter abundance,most favorable physical property and optimal development efficiencies in the Qaidam Basin,had been discovered,the production capacity was up to 0.55×10^(6) t.Through detailed analyses of the Yingdong oilfield,some studies,such as hydrocarbon accumulation conditions and technical challenges,are carried out,and following conclusions can be achieved.The Yingxiongling area is located in Mangya hydrocarbon-generation sag in the west part of the Qaidam Basin,its oil sources are rich;the Neogene Xiayoushashan Formation and Shangyoushashan Formation are dominated by wide and gentle delta frontdshore-shallow lacustrine sediments with interbeds of sandstone and mudstone,the sandbodies are widely distributed with favorable physical condition,and the mudstone is the key caprock,combined with high-quality Paleogene hydrocarbon source rocks,a complete source-reservoir-cap assemblage can be formed.Large-scale detachment faults of the Yingdong area connect high-quality Paleogene hydrocarbon source rocks with middle-shallow buried structural traps,thus,reservoirs formed in the early stage are modified,and at the same time,hydrocarbons formed in the later stage continue to migrate and accumulate;in this way,the deep and shallow faults form a relay-style hydrocarbon transport system,and hydrocarbons are accumulated in the shallow structural traps in the later stage;in this area,the middle-shallow faults have good lateral plugging performance which is favorable for preservation of oil and gas.For complex landforms and reservoir features in the Yingdong area,the integral 3D seismic acquisition,processing and interpretation technology is developed for complex mountain areas to provide a reliable foundation for hydrocarbon exploration.For some problems in the Yingdong oilfield like long oil/gas-bearing intervals,great diffi-culty in identification of fluids,the development mode of multiple oil/gas/water systems in the long intervals is established,and the geologic modeling technology with constraint of multiple conditions on complex fault blocks is also developed.Thus,hydrocarbon accumulation mechanism in the Yingdong oilfield is clear,and some complex key technology of engineering are well solved,providing necessary geologic theories and technical supports for high-efficiency development and rapid production construction in the Yingdong oilfield.