Abstract:Modular multilevel converter (MMC) system with high voltage and capacity has been widely applied among the VSC-HVDC projects in the world.In MMC application,capacitor voltage balancing ranking algorithms became a hot topic.This paper respectively focused on voltage balancing ranking algorithms suited for high-level MMC to summarize the state of the art literatures of this field methodically,and then to explore the cutting-edge research to show the core issues and concerns in the area.The sorting methods are clarified according to the purposes of reducing time complexity or switching frequency and the collective main ideas of them are extracted.This paper also explored the evaluating indicators for MMC ranking algorithms.By simulations,the capacitor voltage balancing effects of three algorithms with different complexities are compared.The switching frequencies of algorithms with maintaining factor are measured.In the end of this paper,the development direction of ranking algorithms in FPGA (field program gate array) for MMC real-time simulation is discussed.

Abstract:Ice covering on the transmission line has great impact on power system security.Form-finding of transmission line is the basis for mechanical analysis of iced transmission line by ANSYS.Current form-finding methods end with the convergence of sag or the horizontal stress,which is hard to obtain in transmission lines with height difference while the maximum sag isn't in the center position.Therefore,this paper presented a form-finding method of transmission lines consulting the lowest point.The method obtained the lowest point by analytic method first,then modified the elastic modulus for coming near the lowest point,and repeated the iteration until the line reaches the lowest point after changing the elastic modulus back to actual value.Simulation results show the effectiveness of the proposed method for different types of transmission lines.

Abstract:To improve the processing ability of asymmetrical eletromechanical side fault as well as the flexibility of electromechanical and electromagnetic transient hybrid simulation,a calculation method of complex asymmetrical fault in the electromechanical and electromagnetic side at the same time is proposed.This method obtains the fault circuit admittance matrix according to the type of asymmetric fault.By modifying the admittance element associated with the faulty node,the fault circuit admittance matrix is superimposed on the normal admittance matrix to obtain the admittance matrix after fault.The network solution is finally achieved by solving the network equation after fault.Based on the linear circuit superposition theorem,the Thevenin equivalent voltage of the interface node is calculated.To reduce the modeling scale of the electromagnetic side,positive sequence equivalent conductance of the interface is used for the conversion of the Thevenin equivalent voltage into Norton equivalent current.At last,through the electromechanical and electromagnetic transient hybrid simulation platform of a IEEE 39 system containing a HVDC line based on PSCAD+C frame,the simulation accuracy and effectiveness of the method is validated.

Abstract:The DC voltage droop control strategy is often used for the voltage source converter based muIti-terminal HVDC which power flow is changed frequently.But the existing DC voltage droop control can not meet the requirements of different operating conditions on the droop parameters.This paper presents a DC voltage droop control strategy based on fuzzy control.By using the fuzzy control theory,the conventional DC voltage droop control strategy is converted to the variable droop control which is changed according to operating conditions.Finally,a four terminal VSC-HVDC is built and simulated in PSCAD/EMTDC which is connected with Matlab.The simulation results demonstrate that the DC voltage fuzzy control strategy can maintain the stability of the system and accelerate system s recovery speed effectively.

Abstract:The large receiving end power grid with characteristics of UHV AC/DC mixed was prone to a cascading failure.In the event of HVDC pole blocking,frequency stability problem may occur.For multiple dc coordination control,pumped storage control,accurate load control and other frequency emergency control methods,this paper conducted frequency emergency control characteristic simulation based on the actual power grid model.The simulations verified the effectiveness of the existing frequency emergency control methods.And specific improvement measures were also put forward.

Abstract:To study the impact of ultra high-voltage direct current (UHVDC) hierarchical connection mode on power system,the calculation methods of hierarchical interaction factor (HIF) are proposed and the causes of the deviation between different methods are analyzed.The impacts of UHVDC hierarchical connection mode on HIF,effective short circuit ratio (ESCR) and transient over-voltage (TOV) are investigated compared with the traditional connection mode.The results indicate that the HIF is smaller,the ESCR is larger and the TOV is smaller under the hierarchical connection mode.The hierarchical connection mode is more superior with the traditional one.

Abstract:In Xizhe HVDC project the PROFIBUS DP communication false alarm occurs frequently,which is triggered by the detection of the heartbeat signal.It is found that the PROFIBUS DP communication system is a multi-link asynchronous system.The time window of the heartbeat signal is changed by the data transfer in the multi-link asynchronous system,and the continuous multi-frame has the same heartbeat signal,which causes the false alarm of PROFIBUS DP.In this paper,the characteristics of heartbeat signal transmission in multi-link asynchronous system are analyzed in detail.The research method can be applied to similar structure commnunication systems.

Abstract:Combined with the state of security and stability control management system (SCMS) and the needs of large power grid dispatching control technology,this paper analyzes the deficiencies of existing SCMS,and proposes the technical scheme of a new integration SCMS based on smart grid dispatching control system platform.A software system contains collection-monitoring-alarm-warning-decision-control is designed.To solve the problem of transition between the new SCMS and service SCMS,the paper propose a project application method of the security and stability control device connect two SCMS with different protocols.A control strategy analysis service based on the class definition and operation factor model of hierarchical security and stability control system and control strategy is developed.It will support strategy analysis service for other application modules thus as Dynamic Security Analysis (DSA).Based on the"horizontal isolation"safety protection strategy,SCMS establishes the connection with the work ticket management system and off-line analysis computation system in management information area,which achieves the setting value list closed loop management and support data for transmission section real time limit calculation and control strategy adaptive check.Finally,the application of the system in Anhui power grid is introduced.

Abstract:The on-line dynamic security assessment system based on intelligent scheduling system calculates and analyses power system using real time on-line data,achieves a leap from the off-line analysis to the on-line analysis of the power grid.The main structure,function and calculation process of the dynamic security assessment system of Jiangsu power grid are briefly introduced in this paper,using the safety and stability analysis of Jiangsu power grid encountering tornado and hail disaster in June 23,2016 and after a substation bus fault in November 4,2016 as calculation examples,the results show that the dynamic security assessment system can calculate and analyze the power grid,and give the corresponding auxiliary decision-making measures timely and effectively,enhancing the emergency handling capacity of power dispatchers in emergency situations,and plays an important role in ensuring the safe and stable operation of the power grid.

Abstract:In order to suppress the forced oscillation of the power system,the damping coefficient of the system is improved by setting the parameters of the UPFC main controller.Then,the auxiliary controller is added into the main controller to track the forced oscillation power fluctuation and to output the inverted power wave to suppress the system forced oscillation.It can further improve the system damping coefficient and enhance the suppression ability of the UPFC to the forced oscillation in the system.A model is built in the PSASP simulation platform to verify that the proposed UPFC main controller and the auxiliary controller can effectively suppress the forced oscillation on the tie line between the system regions.Thereby the damage to the system caused by forced oscillation can be minimized.

Abstract:T-type three-level circuit is a novel clamp topology,which achieves connection of the DC side neutral point and the load through the two-way switch.Compared with the traditional Neutral-Point-Clamp (NPC),the circuit uses fewer components,the device loss is uniform,the operation efficiency is high,and therefore it is widely used in new energy power generation such as photovoltaic power generation.Neutal point voltage balance problem is the inherent problem of this type of circuit,which need to be properly resolved.In this paper,the generation mechanism and influencing factors of T-type three-level midpoint potential are analyzed,and then the mathematical model of midpoint potential is established.An analysis method based on modulation wave voltage instruction partition is proposed,and the analytic expression of the zero sequence voltage to the midpoint potential is established.Based on this,the balance control strategy of midpoint potential is established,and the controller is designed.Finally,the correctness of the theoretical analysis is verified by simulation analysis.

Abstract:Considering the phenomenon of asymmetric and special network structure,a distribution network of conventional components and steady/transient model of distributed power supply are established.The intelligent distribution network related algorithm of three-phase flow algorithm,three phase short circuit current and three-phase time-domain simulation algorithm are proposed based on the characteristic of three-phase asymmetric distribution network.Finally,simulation platform of intelligent distribution network is developed,which is applied in Nanjing CBD successfully,verifying its effectiveness and rationality.

Abstract:UHV AC/DC power transmission is of great significance to ensure the power supply reliability of the receiving power grid.In combination with the engineering practice,the coordination and maintenance strategy of transmission equipment for UHV receiving power network is studied in this paper.Take the case of a provincial power grid in central China,the important equipments in this UHV receiving power grid,strongly coupled with UHV transmission power,are found out.According to the quantitative evaluation index of power grid operation risk,maintenance principle and priority of maintenance arrangement for the relevant equipments are put forward.The proposed method can be used for the preparation of the annual maintenance plan for an UHV power grid.

Abstract:As the key equipment in the flexible looped network controller and UPFC,the reliability of the modular multilevel voltage sourced converter directly affects the safe and stable operation of the whole AC system.We established a 3-D model of the flexible looped network controller and Sunan 500 kV UPFC converter using PTC CREO and ANSYS.We calculated the electric field distribution of the converter valve in insulation type test.The results show that the insulation strength of the air gap distance to the ground of converter valve,the air gap between layers of converter valve,the air medium outside of converter valve can meet the requirements of the converter valve in insulation type test,which can ensure safe and reliable operation of the converter valve.

Abstract:A fixed phase displacement exists when electronic instrument transformers sample analog signals,which affects the measurement accuray,and needs to be corrected using phase shift method.This paper proposes a digital phase-shift method based on least-square fitting algorithm.The digital phase shifting recurrence formula is deduced,and the performance of phase shifting algorithm is simulated and analyzed.The results show that the method has a relatively high resolution and precision in phase-shifting,furthermore,it can guarantee the amplitude measurement accuracy.The algorithm is applied to the Rogowski coil based electronic instrument transformers,and the feasibility and validity of the algorithm are verified by accuracy comparison test.

Abstract:PSD-CCP and PSASP are two commonly used software for short-circuit current calculation at present in domestic,however,the calculation results differs under different calculation conditions,affecting power grid planning and decision-making.With consideration of three standards for short-circuit current calculation,differences between them are analyzed,then some factors such as line and transformer resistances,line capacitors,reactive power compensation and static load affecting short-circuit current calculation results are analyzed theoretically combined with the equivalent circuit.Finally,six different short-circuit current calculation methods are adopted based on PSD-SCCP and PSASP using Jiangsu power grid planning architecture in some year during 2016-2020 as an example,and recommendations are put forward through comparing the calculation results,in order to provide reasonable reference and ideas for short-circuit current calculation in power grid planning and the early stage of project design.

Abstract:Ultrasonic testing technology for transformer partial discharge as an effective insulation diagnosis technology,has been widely used in UHV substation.By reference to intensive care technology,the transformer intensive monitoring system based on ultrasonic method is developed and applied in the transformer equipment with hidden trouble or emergency shutdown.Based on the characteristic data of partial discharge ultrasonic signal,this system combines real-time data of on-line or off-line characteristic data,such as DGA and SCADA operation load.Data fusion method is adopted to detect and locate the partial discharge source in real time.The field application shows that the system can detect the internal discharge fault of the transformer,providing effective support for the diagnosis and maintenance strategy of transformers.

Abstract:A 2N+1 level modulation method of modular multilevel converter (MMC) is studied in this paper.The N+1 level modulation is analyzed.The realization principle of 2N+1 level modulation method is deduced on the basis of fully understanding the working principle of MMC,and the specific implementation process is given according to the situation of odd and even.Simulation results show that comparing with the N+1 level modulation mode,2N+1 level modulation mode can achieve 2N+1 level output with more waveform levels and low harmonic content,but the cost is the increase of bridge arm circulation and inductance drop.The feasibility and validity of the 2N+1 level modulation method are verified.

Abstract:Under the effect of high current,the conductivity of the cable bracket will change the magnetic field distribution around the cable,affecting the operation and leading to changes in temperature.Conventional electricity standards and regulations has not given a clear explanation of the selection cable support.In this paper,we take NanXu 220 kV substation as an example,the effects of different current carrying capacity and cable materials on the temperature rise of cable are studied by using two-dimensional electromagnetic-fluid-temperature field coupling finite element analysis method.Analysis showed that the temperature rise of cable support due to eddy currents of different materials will not endanger the safety of human and equipment,whether uses a non-magnetic material depends on eddy currents and economy.This article breaks through the limitations of traditional cable temperature calculation,taking into account the impact of cable support,given the theory and practical application foundation when selecting the material,it has important significance in improving the construction s economy and the operational reliability.

Abstract:Precise fault location is an important precondition of quick power supply restoration after transmission line fault occurs.In generally,concerning one-phase to ground fault,it is the transient resistance that causes that the impedance from where the relay is installed to where the fault occurs is caculated incorrectly.On such scenario,a double-ended fault location algorithm is preferred in order to acquire rather precise fault location result.Based on analyzing the source of additive impedance,this essay presents that under some special working conditions whereas double-ended fault location becomes invalid,single-ended fault calculation based on solving triangle calculation gives precise results.And this method can also be applied to hybrid parameter transmission lines which have both overhead lines and cables.The simulation and field data verified that the method has high precision.

Abstract:As a new type of D-FACTS equipment,DSSC can be utilized to eliminate the transmission network congestion.In order to study the cost allocation of DSSC after installed,a cost allocation method based on electrical dissection method is proposed.Responsibility factors of generators and loads are determined quantitatively by the results of electrical dissection,and the cost of the DSSC is distributed to each generator and load according to the congestion responsibility factor respectively.Finally,the simulation results based on the IEEE 39-bus system show that the proposed method can recover the DSSC equipment cost from the generator and load effectively and fairly.

Abstract:Excitation transformer as an important component is connerted to the enclosure type isolated-phase bus in the main circuit and branch circuit of generator,in order to reduce the ground fault,phases fault and eliminate steel structure fever.However,the reversed phase-sequence of an excitation transformer and the isolated-phase bus was found in the construction of a power plant,this paper analyses the influences of excitation change current phase,excitation transformer differential protection,excitation regulator,generator rotor earth protection caused by this situation,and proposes two solutions.Considering the convenient of excitation system and relay setting calculation,the second scheme become the first choice.The simulation results meet the requirements of the excitation transformer differential protection,rotor earth protection and the excitation regulator,avoiding re-purchase and install the isolated-phase bus or replace the excitation transformer.

Abstract:This paper introduces the mode and tripping condition of 110 kV automatic bus transfer equipment in detail,and puts emphasis on analysis of automatic bus transfer equipment in charging/discharging condition and action process.It also elaborates the cause of malfunctions of automatic bus transfer equipment and the serious consequences of the improper action to power grid and equipment,with putting forward the corresponding solutions and measures,and provides a reference for dispatching operators to deal with similar situations in future work.