This paper investigates the behavior of power transformers under the occurrence of permanent or intermittent winding insulation faults. For the study of these phenomena, a simple and efficient permeance-based electromagnetic transformer model is proposed, which is based on the simultaneous consideration of magnetic and electric equivalent circuits. To incorporate the internal faults in this model, a suitable equivalent circuit of the faulty winding is described. With the aid of this transformer model, the onload exciting current Park's Vector Approach will be applied for diagnosing the occurrence of permanent and intermittent winding faults. Experimental and simulation tests results are presented in this paper, which demonstrate not only the adequacy of the digital transformer model for winding fault studies, but also the effectiveness of the proposed technique for detecting winding interturn insulation faults in operating three-phase transformers.

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A Permeance-Based Transformer Model and Its Application to Winding Interturn Arcing Fault Studies

Two methods are presented for analyzing the dynamic response of electrical equipment tanks completely filled with oil under fault conditions liable to cause explosions. Both methods simulate the same infinite cylindrical model subjected to an eccentric arc, but differ in their approach. The first is based on an analytical formulation of a thin-walled shell containing an incompressible liquid; the solution is obtained by superimposing orthogonal components and numerically integrating the equations of motion. The other approach is more general and involves the use of the highly specialized software PISCES adapted for arc-induced excitation. The method can be applied, after modifications, to the analysis of partially filled equipment such as distribution transformers. Comparison of the computer results provides a means of mutually validating the two methods and thus indicates that the use of PISCES may be extended to complex models with a more representative tank configuration. >

Computational methods for the analysis of explosions in oil-insulated electrical equipment

Low-impedance faults are difficult to contain from the point of view of cover retention and tank rupture in pole-type distribution transformers. When evaluating a transformer's containment capabilities, one needs to consider two aspects: the overpressure caused by an arc of given parameters and the pressure withstand of the tank. This paper presents two methods to calculate overpressures as a function of the arc and all of the relevant geometrical parameters: tank diameter, air-space height, and the depth of the arc below the oil. Both methods have been validated with experiments. Results of tests to determine the pressure withstand of typical tanks are presented. It was found that an average value of the overpressure withstand capability is approximately 100 kPa. For a given tank geometry and fault parameters, a current-limiting fuse would be required if the methods proposed here predict an overpressure of more than 100 kPa.

Estimating Overpressures in Pole-Type Distribution Transformers, Part II: Prediction Tools

When an electric arc is generated in transformer oil, a strong overpressure may rapidly develop in the tank, especially for low-impedance faults. This paper presents the test method and results obtained in a project aimed at assessing the human and environmental risks associated with this type of fault. Limit energy and pressure values were defined in terms of different safety levels. The best method of limiting the energy and pressure at a safety level in the transformer tank during internal faults of greater than 1.7 kA is to protect the transformer with a current limiting fuse. Other ways to increase the mechanical withstand of transformer tanks under such faults are also discussed.

Estimating Overpressures in Pole-Type Distribution Transformers, Part I: Tank Withstand Evaluation

Changing protection practices for overhead power distribution transformers involves mounting the arrester on the transformer tank and using lightning-surge-tolerant fuses to provide protection. Concerns about protecting transformers against low-magnitude internal winding faults as a result of this practice are addressed in this paper. An analysis of a high-power test involving primary and secondary winding faults is presented. The magnitude, duration, and "signature" of the winding faults are determined and categorized. An analysis of the response of surge-tolerant fuses shows that they often will not detect low-current winding faults before they propagate to the available system fault current. Testing also demonstrated the performance of current-limiters to effectively reduce the available system fault current, thereby minimizing stress to the transformer. The use of current limiters is recommended with surge-tolerant fuses. The pressure rise in the tank for primary winding faults is mainly caused by low-fault current and, when protected with current limiters, the relief device easily vents the gases. The pressure rise for secondary winding faults is more rapid and can exceed the capability of the relief device to limit the overpressure.

Detection of and protection for internal low-current winding faults in overhead distribution transformers

While the problem of disruptive failure of distribution transformers has received much recent study, uncertainty still remains as to the precise phenomena which cause cover blowing or tank distortion. This paper presents some new insights into the mechanisms of cover blowing based on a comprehensive and highly instrumented test program. Unique contributions from this study should help clarify many points which have been the subject of much discussion during the last few years. Besides observing and correlating the effects of various arcs drawn inside several different transformers, some actual failed transformers were tested and the effects of the failures observed and measured, giving a better understanding of what occurs while a transformer is in the process of failing. Special tests using high speed photography have confirmed the existence of failure modes which up to now have been only suspected. In addition, measurements from these tests have verified theoretical studies and so have helped provide a more complete picture of the diverse phenomena involved. Practical solutions to the cover blowing phenomena are discussed.

B.L. Damsky

Papers

Overpressure phenomena in distribution transformers with low impedance faults: Experiment and theory