Chenguang Yan

Research on integration of transformer protection and winding deformation detecting

In this paper, the integrated conception of transformer protection principle based on equivalent equation and winding deformation on-line detecting has been presented. The transformer protection principle based on equivalent equation has been researched. Both of the equivalent equation protection criterion based on complete or fault component and the threshold on-line setting method are described in this paper. The on-line method uses the variation of the leakage inductance to monitor the transformer winding deformation. According to the parameter identification of winding leakage inductance, this paper puts forward a comprehensive and systematic integrated method of transformer protection principle and winding deformation on-line detecting. Using the transformer dynamic test data to inspect and verify, the conclusion can be brought that the winding parameter identification is accurate as well as be able to meet the requirements of transformer protection and online monitoring. The protection algorithm is theoretically clear and it can correctly and rapidly recognize the various transformer internal short circuits with distinct features.

Simulation and analysis of power transformer internal arcing faults overpressure characteristics

The high-energy arcing faults in power transformer can cause overpressure which would break the tank open, lead to oil-spill and even catching on fire. Facing to the great hazard of transformer overpressure during internal arcing faults duration, this paper has investigated the physical process of tank internal overpressure and revealed the relationship between the electrical faults and overpressure characteristics from the energy conversion perspective. Due to the fact that the arcing faults tests inside power transformers are extraordinarily difficult, destructive and costly, the theoretical model has been proposed and numerical simulation approaches has been employed. At first, the transformer internal arcing fault energy model, the fault pressure source and the overpressure wave propagation have been mathematically modeled considering the whole phenomenon as an acoustic problem. Then, according to the real full-scale 240 MVA/220 kV oil-immersed power transformer structure parameters, a transformer simulator has been set up. Thirdly, by employing 3D FEM, the transient overpressure characteristics such as pressure rise curves and effective pressure indexes at different positions have been explored. The simulation results illuminate that the chief effect to overpressure inside tank is the arc energy, the following is the positions where arcing faults happen. Additionally, both the theoretical model and the simulation method proposed in this paper can be extended to simulate the overpressure phenomenon during arcing faults in other types of oil-immersed transformers and reactors instead of costly and unpractical field tests.

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.

A novel digital non-electrical protection for power transformer based on internal pressure characteristics

Power transformers play a very critical role in electrical energy transmission. Abnormal performance of transformers would seriously affect the reliability and stability of power system. Non-electrical gas relays are usually used as a main protection of power transformers. Indeed, due to the inherent defects, the gas relay misjudgments happen every year across the world. Given this background, this paper employs a theoretical model and its simulation method to study the relationship between electrical faults and non-electrical pressure variations inside the tank. Based on the essence difference of the pressure characteristics among the internal faults, external faults and normal running condition, a novel transformer protection principle is proposed. Numerical test has indicated that the proposed protection is able to reliably clear the transformer internal faults, even the single-turn fault or other weak faults.

Oil flow surge test for 110kV power transformer during external short circuit faults

Large oil-immersed power transformers always have the gas relay equipped between the main tank and conservator. However, due to the inherent defects, the gas relay malfunction caused by external faults happens occasionally all over the world, which jeopardizes the safe operation of bulk power systems. In order to explore the oil flow characteristics inside the pipe during external short circuit faults, we built the transient oil flow measurement system on the 40 MVA/110 kV power transformer, and carried out 34 external short circuit fault test campaigns. The test results confirmed that the deformation and vibration of transformer windings caused by the short circuit current would lead to the oil flow surge through the pipe. The surge appears with a long time delay up to hundreds of milliseconds due to the liquid inertia. The test also shows that both the fault severity and location would affect the oil flow surge.

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.

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.