Cai Zexiang

Study on protective performance of HVDC transmission line protection with different types of line fault

HVDC line protection system is composed of traveling wave protection as the main protection, under voltage protection and differential protection as the backup protection. The responses of the HVDC line protection criterion are not same at different fault situations, and it also makes the operation characteristics of the line protection quite different. In this paper, based on the EMTDC simulation platform and an actual HVDC power transmission system and its protection system, the changes of voltage and current of the HVDC line are measured, and the influences of fault location, fault resistance and the external disturbance to the HVDC line protection are also analyzed in detail. At the same time, the relation between the rejecting act of the protection and the factors mentioned above are also discussed, and some improvement schemes are also proposed.

Mixed Measurements State Estimation Based on Wide-Area Measurement System and Analysis

This paper presents a new state estimation method in rectangular coordinates to solve the problem of lacking data redundancy of wide-area measurement/monitoring system (WAMS). This method utilizes all the measurements of WAMS and partial equivalent current phasor measurements transformed by flow measurements from SCADA system. The iterative formula is proposed and analyzed based on these mixed measurements. The theoretical analysis and tests show this method has constant Jacobian and gain matrices, and the gain matrix is decoupled as to deal with the real vector and imaginary vector of node voltage phasors. This method has fast convergence and is robust with different R/X ratios. Furthermore, the convergence character of this method is analyzed and tested in the case study. The treatment of power injections at buses is also discussed in this paper

A novel algorithm for HVDC line fault location based on variant travelling wave speed

This paper presents a novel fault location algorithm based on variant travelling wave speed for HVDC (High Voltage Direct Current) transimission line. The algorithm effectively reduces the large fault locating errors, casuused by adopting consitent wave speeds, under off-design conditions. Fistly, it is illustrated how travelling wave speed varies with fault distance, and then how the variant speed effect fault locating accuracy is analysized. At last, the new algorithm is proposed to gurrantee the accuracy at any fault distance. The validity of the algorithm is verified by the relevant simulation results in EMTDC.

Analysis of realtime performances of process-level networks based on IEC61850 SCSM Model

In this paper, real time performances of process level networks based on IEC61850 SCSM (Specific Communication Service Mapping) Model are analyzed. IEC61850 SCSM Model which is different from Ethernet OSI (Open System Interconnect) Model of Ethernet is introduced by IEC61850. Firstly, dominating factors of real time performances of process level networks are studied. Then IEC61850 SCSM Model is amplified in detail by comparing OSI Model and how to build a IEC61850 SCSM Model with OPNET Modeler software is interpreted. The paper architects the modeling of IEC61850 SCSM using OPNET Modeler software, studies the real-time performances of process level networks based on IEC61850 SCSM model, and analyses the real time capacity of adopting different network structures and different transmission strategies in the process level networks based on IEC61850 SCSM model.

Reactive power coordinated control at the gateway between provincial and regional power grids

It is the key problem in automatic voltage control of large-scale power grids to quantitatively assess the reactive power regulation capability of provincial power grid to determine the control range of reactive power between provincial and regional power grids. Based on the concept of layered decoupling, firstly the definition of average coordination power factor limits (ACPFL) is given to evaluate the reactive power limit of provincial power grid, and the optimized mathematical model of ACPFL is solved to obtain the correlation curve between ACPFL and load level transmitted from provincial power grid to subordinate power networks; then according to the differences among loads of 500kV substations, the approximate linearization calculation of limit value of the upstream gateway coordinated power factor is performed, and taking it as the constraint a calculation model and fast estimation method for the limit value of downstream gateway coordinated power factor are established to implement dynamic setting of parameters of AVC system; finally a method to implement the AVC system based reactive power coordinated control at the gateway between provincial and regional power grids is given. Simulation results of applying the proposed method to a certain regional power grid in Guangdong power grid show that the proposed method is favorable for energy-saving and loss-reduction, improving voltage quality, enhancing security margin of voltage and reactive power in provincial power grid and bringing the potential of area substation capacity into play.

Power System Fault Calculation under Special Circumstances and Corresponding Effect on Protective Relaying

This paper presents the power system fault calculation model for multi-circuit lines with mutual zero-sequence impedance and describes the procedure of the common and integrated node zero-sequence admittance matrix. It makes the complex fault calculation be able to extend faulted location into the inside of coupling lines. The transformation for 4-circuit coupling lines from sequence-component model to phase-component model is depicted so as to improve utilizing the multiple-coupling-line model in EMTP. The precision of fault calculation is verified by the results derived from EMTP package. Aiming at the typical operating conditions on the actual 4-circuit lines with 6-pair mutual impedance, the characteristics related to the measured impedance of earth-fault protective relaying are analyzed through the use of the fault calculation results. Adjusting the distance relays zero-sequence current compensation factor and adopting automatic inter-tripping technique can help to reduce mutual coupling effects

Influence of inter-line faults on distance protection in parallel lines

The conventional configuration pattern of relaying protection was challenged by parallel transmission lines inter-line faults, which produced an apparent effect on the distance protection. It was absolutely essential to study parallel transmission lines inter-line faults and their influence on distance protection. The analytical analysis method of parallel transmission lines inter-line faults for relaying protection was presented in this paper, which could accurately analyze the influence tendency and boundary of inter-line faults on the main distance element. So as to extract related fault characteristic by using current branch coefficient to study the influence, fault port equivalent impedance was solved by injecting current on the zero sequence mutual inductance decoupled model, and the analysis of different types inter-line faults were dealt with, combining the way of zero sequence current controlling voltage source with the fault boundary conditions. The research result indicated that the distance protection would not exceed the first part of distance value when the inter-line grounding faults occurred, and the inter-phase measure impedance calculated with twice current of fault phase would reflect the actual measure impedance more accurately when the inter-line faults occurred without grounding. With the simple calculation and the clear physical conception, the proposed analytical analysis method also was fit to study the influence of inter-line faults on the phase element and other protection elements, and could be applied to the calculation and analysis of multi-circuit parallel lines inter-line faults. Considering the influence of mutual inductance intensity, difference between both sides power source, fault location and other related factors, it could definitely and accurately analyze protection range with the analytical expression, which provided the theoretical basis for the setting and disposition of relaying protection. Finally, the analysis result was verified by the example of Guangdong power grid.

Influence on zero-sequence components of autotransformer neutral grounding via small reactance in one 500kV substation

Transformers neutral grounding accessing small reactance is an effective measure to restrain short circuit current in 500kV power grid. But accessing small reactance will bring changes to the zero-sequence parameters of system. The relay protection, especially the main transformer protection and transmission line protection nearby will be affected and act incorrectly when unsymmetrical earth fault occurs in the transmission line. An equivalent system model based on the Guangdong power grids actual parameter and autotransformer in 500kV substation is built, the effects on power systems zero-sequence parameters as only one autotransformer neutral point grounded via small reactance is analyzed Meanwhile, a quantitative calculation is accomplished based on the relative variation of zero-sequence components from three aspects: fault position, resistance value and transmission line length. Furthermore, the effects on protective relaying and coordination between the adjacent lines are analyzed according to curves drawn by Matlab. Zero-sequence current protection of autotransformer neutral should be reset to meet sensitivity.

Research on networking strategy and optimization simulation of process level network in smart substation based on OPNET

To verify the rationality of networking and the real-time reliability of network transmission for the process level in smart substation, this paper proposes a method of modeling and simulating for smart substation based on OPNET, which models the key network equipments after analyzing the simulation requirement of process level network in smart substation. And several networking modes and optimization strategies for packet transmission are researched. Then the simulation example of Qinyun Substation is used to prove the rationality and effectiveness of the networking strategy, combined with static multicast and different star networks for SV and GOOSE packets, in the practical engineering application. It is certified that the networking strategy meets the applied requirement about the network flow and transmission delay of process level network in smart substation at the moment.

Operation characteristic analysis of same-tower transmission lines and its influence on relay protection

The wide application of same-tower transmission lines has brought new challenges to fault analysis and relay protection operation in power system. In this paper, the diversity of construction form of same-tower transmission lines is introduced. And its operation characteristics, such as strong magnetic coupling and easy occurrence of inter-line fault, are analyzed. Then the influence of same-tower transmission lines on relay protection is studied. The influence matrix of distance protection, zero sequence protection and fault phase selection components is summarized. Finally, from the point of view of protection setting and configuration, power system operation and management, and protection device, several solutions to minimize the influence of same-tower transmission lines are proposed.