Zhiguo Hao

Study on security protection of transmission section to prevent cascading tripping and its key technologies

Cascading overload trip due to lack of transmission section protection is an important reason of large-scale blackout. This paper illustrates the meaning of transmission section and points out that the objective of transmission security protection is to maintain its integrality and transmission capacity, and to avoid cascading overload trip. In addition, the implement condition and key technology are discussed, which are online capture of transmission section, real-time prediction of cascading overload and real-time emergency control to avoid cascading trip. This paper has brought forward the primary scheme. Its validity is verified by the calculation result of CEPRI 36 test system and a province system.

A Novel Fast Searching Algorithm for Power System Self-Adaptive Islanding

Self-adaptive system islanding refers to the separation of an interconnected power system into electrically isolated islands automatically based on the identification of unstable mode. A fast searching algorithm for self-adaptive power system islanding, which is called multilevel reduced graph partitioning (MLRGP) algorithm, is presented in this paper. The aim of this algorithm is that the generation load imbalance in each island is as small as possible. The algorithm computes a reasonable islanding strategy of power system G=(V, E) in O(|E|) time. A C++ program is developed based on the algorithm. The verification of the islanding program is proven with simulations on a practical 212-bus power system. It costs only about 20 ms to get a reasonable islanding strategy on an ordinary PC, which satisfies the speed demand of self-adaptive system islanding.

Analyzation of current characteristics on untransposed double-circuit lines

The parameters of transmission lines are usually not completely symmetrical in actual power grid. As to paralleled double-circuit line, this kind of asymmetry can bring current unbalance even under normal condition, and therefore may have a negative impact on the line temperature, losses and even traditional relay protection equipments. In this paper, we first discuss some factors that may affect the parameters of transmission lines. Then we provide a method which can be used to calculate the circulating currents between the paralleled two lines under normal condition. At last, we give a detailed analysis of one double-circuit transmission line in Northwest Power Grid using this method. By analyzing the proportion of current unbalance in different phasing arrangements, we can make some useful conclusions and a few suggestions to limit the current unbalance.

Study on power system self-adaptive islanding

Self-adaptive system islanding is a new research direction in the field of islanding control. The sub problems and framework of self-adaptive islanding were studied in this paper. The research contents are as follow. First, the traditional out-of-step islanding schemes is introduced and simulated. The simulation results show some defects of traditional islanding. These defects indicate the necessary of self-adaptive islanding researching. Second, an instability mode detection scheme based on out-of-step relays and WAMS is studied and presented. Simulations on IEEE 118-bus power system show the proposed method is effective. Third, a novel and fast islanding boundary searching method was presented. The aim of this method was that the generation load imbalance in each island be as small as possible. A C++ program was developed based on the method. Simulations on IEEE 118-bus power system show the proposed method is effective and fast. Finally, based on the research content mentioned above, the framework of islanding control system was designed. The demand of each part of the system was analyzed, and the flowchart of islanding control system is discussed.

The analysis of non-electrical parameter accumulative effect under transformer external fault

Transformer is a key component of power system and its steady and safe running is of much importance to the whole power system. So electrical engineers develop many protection device to detect transformer abnormal state and fault state. Non-electrical parameter relay protection mainly reflects nonelectrical parameter changes between transformer normal state and abnormal state. Pressure relief valve is one these devices and it is used to protect transformer itself, which feels oil pressure increase and act to inject hot oil to surroundings. It is set according to its pressure on the wall of tank, usually half or 0.6 times of its inherent oil pressure. However, in practice, oil pressure valve always act in external short circuit fault. Towards it, there is few research on it. There is no relevant test research to study non-electrical parameter changes during transformer fault, which is still nearly empty. So we designed a transformer non-electrical parameter measure system and used it to measure non-electrical parameter changes during transformer external short circuit fault, and analyze data obtained. Through these results, we figured out something as following, 1. Oil pressure will increase sharply and fluctuate fiercely during external short circuit, which may arrive beyond open pressure 2. The second harmonic component counts most in oil pressure and it will increase sharply when transformer meet an external short circuit fault after an external short circuit impact. By using these conclusions, we can figure out the reason why oil pressure relief valve misact during external short circuit fault and the difference between external short circuit fault and internal tank fault. Thus, we can come up some measures to improve oil pressure relief valve and coordination of every nonelectrical protection components. Moreover, we can use these conclusions to build new fast and reliable transformer relay protection.

Deformation simulation and analysis of power transformer windings

Power transformers are critical in power systems and the stable operation of power transformers is a vital guarantee of system stability. However, with the development of capacity and voltage level, the external short-circuit fault threats the stable and secure operation of power transformers more seriously than ever before. Transformer windings experience complicated dynamic deformation transient process during external short-circuit faults, and many issues still remain unknown in the field of winding deformation simulation and analysis. In view of this, this paper deals with the simulation and analysis of the transformer winding deformation characteristics. At first, the mathematical model of winding deformation concerning magnetic field and solid mechanic field is presented and demonstrated. Furthermore, a 3D geometric simulation model is established based on the actual size parameters of a 500 kV single-phase two-winding transformer. Based on this, the muti-physic-field coupling technique based on the finite-element method (FEM) has been employed to calculate the dynamic deformation characteristics of transformer winding structures during the transient process. The simulation result indicate that ideal intact windings can withstand the most severe impact from external short-circuits without plastic deformation. Even so, relatively heavy radial deformation will appear in the middle position near return yokes. Consequently, the winding structure in the middle-height position near the ferromagnetic circuit should be checked and strengthened to prevent windings from plastic deformation and radial instability.

The analysis of frequency spectrum of oil pressure signal in transformer external fault

When testing transformer is under an external short circuit fault, its wall will get rapid oil pressure fluctuation. This signal changes in a certain law, and it reflects some non-electrical parameter feature of the situation of transformer external short circuit fault. Using the method of experiment testing, this article analyze a 110kV transformers oil pressure signal feature, and get some conclusion about its harmonic distribution.

Novel power transformer differential protection scheme based on improved short-window algorithm

This paper presents a new approach to improve the performance of the power transformer differential relay. The proposed approach enhance its anti-CT saturation performance during serious faults based on short data window improved Pronys algorithm, and is capable of acting rapidly when external fault converts to internal fault. The improved Pronys algorithm can obtain power frequency phasors of the fault current in a short time with unsaturated segment of the CTs distorted current. The reduced order method and optional sampling rate calculation method are employed to improve the efficiency of the improved Pronys algorithm. Moreover, a discrimination region is added into the traditional dual-slope differential characteristic. When the differential current trajectory enters into this discrimination region, an improved equivalent instantaneous inductance of transformer method is proposed to distinguish inrush current from internal fault and reliably block the protection during the transformer inrush current. A 242/66kV power transformer is simulated applying PSCAD/EMTDC for evaluating the performance of the proposed transformer differential protection. Extensive simulation studies show that the proposed approach results in a more sensitive and faster differential protection scheme.

Axial stability analysis and simulation of power transformer windings

The stable operation of power transformers is important guarantee of power system stability. Nevertheless, accidents of winding instability occurs intermittently, which exerts huge harm to the secure operation of transformers and even power systems. Moreover, the axial instability is a major type of winding failure. When the natural frequency of windings approaches to the frequency of axial electromagnetic forces, winding structures will resonate intensively, causing the axial instability to occur. Therefore, the axial resonance of windings should be avoided. This paper presents the axial stability analysis and simulation of transformer windings to understand the resonance mechanism. At first, the theoretical model is put forward to demonstrate the axial vibration of transformer windings. Moreover, a 3-D modal analysis simulation model of winding structures are established and solved by means of finite element method (FEM). Furthermore, the natural frequency under different values of pre-stress has been calculated and the relationship between the axial natural frequency and pre-stress has been analyzed. Based on this, the first three axial vibration modes of windings are extracted and analyzed. The simulation results indicate that, natural frequencies will continue to decline as the pre-stress decreases over time, and thus maintaining the pre-stress in a relatively high level will be an effective axial vibration-proof measure contributing to avoiding the axial resonance. The simulation results and the method employed in this paper can provide reference for the transformer design, manufacture, operation and maintenance.

Residual flux estimation method for three-phase transformers without zero-sequence circuit on the switch-off side

Part of the magnetic flux will remain in the transformer core after it is switched off, referred to as the residual flux. When an unload transformer being energized, inrush current may flow in the voltage jump side, causing relay malfunction and many other problems. This paper presents a method to estimate the residual flux for a three-phase transformer without zero-sequence circuit on the switch-off side. The estimating method is based on the analysis of the switching-off transient process and mechanical characteristics of the vacuum circuit breaker. With this method, we could choose appropriate voltage angles to close the transformer for avoiding inrush current. The method has been tested in our laboratory. The results shows that inrush current has been restrained to a lower level after estimating the residual flux.