Zhongqing Li

Research on dynamic simulation of the resonance fault current limiter

The resonance fault current limiter (RFCL), which avoids the disadvantages of the series reactor, is feasible for EHV and UHV grid. To contribute to project application of the RFCL, and study its running characteristic and impact on relay protection, the dynamic simulation lab of State Grid simulation center carries out the research on simulation technique for RFCL. The principle and technical properties of RFCL applied in EHV power transmission line are analyzed. Considering the features of dynamic simulation system in the lab, the approach to choose parameters of simulation model and corresponding structural design are described. This simulation model has been connected into dynamic simulation system and the simulation experiments for inspecting the control and protection function of RFCL are performed. Experimental results show that the performances of the developed simulation model for RFCL can meet the design requirement, and can be applied in dynamic simulation tests and researches.

A novel substation area backup protection for smart substation

This paper proposes a new scheme of substation area backup protection system based on substation area range of information. A configuration scheme of relay protection in smart substation was proposed firstly, the structure of substation area backup protection (SABP) was introduced, the information matrix of SABP which can reflect the substation topology was established, the protection algorithm of SABP was proposed, the fault-tolerance performance of the proposed protection scheme was also analyzed. According to the analysis of fault direction information, fault current differential information, as well as the information matrix, the SABP can carry out fault location and make a reasonable tripping strategy. The result of case studies shows that the proposed protection algorithm can implement backup protection and circuit breaker failure protection for each electrical element inside smart substation, and has good adaptability to disconnection fault of CT.

A method for dead zone fault isolation and fast recovery of error cutting element under the existing 3/2 connection mode

Under the existing 3/2 connection mode, the incompletion between the configuration of the protection and transformer leads to the existence of the dead zone. When the fault occurs in the dead zone, line protection and breaker failure protection are generally matched with each other to achieve the complete isolation of the fault. This approach will make the scope of the circuit breaker to expand, and even leads to unnecessary outage of the normal bus. So it seriously affects the safe and stable operation of the system. In view of the above problems, this paper analyzes the features of the dead zone under different protective CT configuration mode, and provides a method for dead zone fault isolation and fast recovery of error cutting element. By introducing circuit breaker opening and closing state information, the method identify dead zone fault and the breaker failure effectively. At the same time, based on the information sharing in the station area, the circuit breaker can be quickly recovered by using the reclosing function of the circuit breaker, which can significantly improve the safe and stable operation level of the system.

Research on new automatic bus transfer method which adapts to security and stability control system

Traditional automatic bus transfer methods have difficulty in correctly telling whether it is short-circuit fault or incoming line tripping by security and stability control devices which leads to bus voltage losing. It may cause automatic bus transfer device incorrect operation or refusing action, which effects the stable operation of the power system and reliability of power supply. This paper proposes a new automatic bus transfer method to adapt to security and stability control system according to the different characteristics of bus voltage and incoming line current when the incoming line is cut due to security and stability control system or incoming line short-circuit fault. The method can lock automatic bus transfer function when security and stability control system cuts the incoming line, and it can also reliably open automatic bus transfer function when the incoming line is removed due to fault. The correctness of this method is verified by simulation test.

A reliability evaluation model for intelligent substation secondary system based on variation coefficient method

It is important to establish reliability analysis model of secondary system for the intelligent substation. In this paper, a method is proposed to evaluate the reliability of secondary system in smart substation. Firstly, according to the reliability indices of secondary system structures for different smart substations, the evaluation matrix are determined and the matrix can be orthogonalized to makes it non dimensional. Secondly, the mean, standard deviation, coefficient of variation, and weighted vector of the matrix are calculated. Then, the weighted data matrix of ideal sample and negative sample is determined. Finally, according to the correlation degree ranking method, the reliability evaluation of each index is obtained. For practical examples, this method can be used to identify the factors influencing reliability of intelligent substation. The method can be used to select an appropriate network topology, improving the reliability of the design level of substation communication system.

Zero-sequence voltages of UHV untransposed double-circuit lines and their influence on pilot protection

For untransposed double-circuit transmission lines on the same tower, there are zero-sequence voltages exerting on both ends of double-circuit transmission lines under normal operation and external phase-to-phase fault conditions. The amplitude and phase of the zero-sequence voltages are affected by wire parameters, wire orientations, the types of external phase to phase fault and system parameters. The zero sequence voltage may reach threshold value of zero-sequence power direction component, causing zero sequence power direction component maloperation with high zero sequence current. This paper analyzes the cause of zero-sequence voltages and their influence on pilot protection. Theoretical analysis and simulation results prove that system zero sequence impedance has influence on zero sequence voltage and zero sequence current. This paper will has great meaning to fault analysis of double-circuit transmission lines and protection configuration.

A fault location method for extra-high voltage mixed line based on variation of sequence voltage

For extra-high voltage mixed line, the parameters of cable segment and overhead line segment are different, so the fault location method of uniform line cannot be used. Based on the characteristics of mixed line parameters, a fault location method for extra-high voltage mixed lines based on variation of sequence voltage is proposed. A precision, fast calculation method of voltage along the line is researched by using linear equivalent. Through simulation tests the precision of method is high. In terms of frequency domain transfer equations, sequence of the amplitude attenuation and phase delay along fault lines are calculated. Extra-high mixed line fault types can be divided into two categories: the asymmetric fault and the symmetric fault. For the asymmetric fault, before fault happened, the negative sequence voltage along mixed lines is 0. So the fault negative sequence voltage variation is the current time negative sequence voltage. Negative sequence voltage amplitudes along mixed line are calculated by using two-terminal electric parameters of line and compared for asymmetric fault. The fault location is the point with equal negative sequence voltage amplitudes. For the symmetric fault, the line will not appear negative sequence voltage. There is positive sequence voltage only. But, for three phase fault with transition resistance, the fault point voltage is not the lowest. The calculated voltage amplitudes along the line have two intersections: fault point and false point. The intersections have almost equal amplitude, so using numerical magnitude of point amplitude to false identification is hard. To solve the problem what has mentioned above, this paper uses the positive sequence voltage variation for fault location. Before fault happened, the positive sequence voltage along the line is not 0, so the positive sequence voltage variation is the difference of current and previous positive sequence voltage amplitude. Then, for three-phase symmetry fault, the fault location is the point with equal voltage amplitudes variation of positive sequence. Finally, the simulation is carried out to verify the algorithm. The proposed algorithm is computationally efficient with high precision. It is not affected by transition resistance, and no false identification problem.

Fault component comprehensive impedance based back-up protection algorithm for intelligent substation

A novel back-up protection algorithm for intelligent substation based on fault component comprehensive impedance is proposed in this paper. Calculation analyses of fault component comprehensive impedances of bus-line region, transformer-bus region and combination region are completed. The fault component comprehensive impedance of each zone reflects the positive-sequence impedance of the external system when an internal fault occurs. In a situation of external fault, it reflects positive-sequence magnetizing impedance or line positive-sequence capacitive reactance. In a situation of magnetizing inrush current, the fault component comprehensive impedance of the region which contains transformer reflects line positive-sequence capacitive reactance or decreasing positive-sequence magnetizing impedance. Fault location is implemented accurately according to the magnitude of fault component comprehensive impedance of each zone which in external fault situation is much larger than that in an internal fault situation. Then back up protection sends out trip signals after a time delay on the condition that failure still exists and the main protection of the fault element refuses to act. This algorithm is immune to transformer inrush current and high resistance fault, which has higher sensitivity than substation-area differential backup protection. Simulation tests based on PSCAD simulation model verify the proposed algorithms validity and superiority.

Maloperation prevention study of zero sequence direction protection to broken-line faults of parallel circuits on same tower

A maloperation prevention method of zero sequence direction longitudinal protection was presented in this paper directing at broken-line faults of double-circuit transmission lines on a same tower. Firstly, the maloperation reasons were analyzed in detail. Then, the zero sequence direction amplitude-comparison criterion and the negative sequence direction amplitude-comparison criterion were constructed. The zero sequence criterion is used to detect earth faults sensitively and the negative sequence criterion is used to avoid maloperation of zero sequence criterion resulting from electrical and magnetic coupling during broken-line period. Meanwhile, the zero sequence current threshold and negative sequence current and voltage threshold were set for insuring the reliability of the whole protection scheme. The interior fault logic is that the zero sequence direction criterions of the two sides of lines both reveal forward direction. If that occurs, the zero sequence longitudinal protection acts and the fault line is removed. Otherwise, the protection will not act. Finally, the simulation results demonstrated the availability of the presented protection method.

Identification methods of SV distortion of smart substation relay protection

The identification methods of sample value (SV) distortion are proposed in this paper to avoid protection mal-operation in smart substations. The methods contain abnormity identification using data from same sources, using relevant data, and according to data features. The same source data include the double A/D data of one merging unit (MU), doubled communication networks of one protection, and doubled independence protection system. The relevant data include current data from a same branch, voltage data from a same node, and current differential data from a close district. Data feature identification will set algorithms which can distinguish abnormal SV points whose characteristics are different with normal data. Finally, theory analysis and simulation results demonstrate that the above methods can identify SV distortion exactly so that solving measures can be carried out timely. Thus the reliability of protection system will be enhanced dramatically in smart substation.