A New Atp Add-on for Modeling Internal Faults in Power Transformers

The more sophisticated a protection algorithm, the more information on both steady-state and transient behavior of a protected element required for proper designing, setting and testing of a given relay. In the case of a power transformer, the most important phenomena to be modeled from the standpoint of protective relaying are: magnetizing inrush including initial, recovery and sympathetic inrush as well as out-of-step synchronization of a connected generator, stationary overexcitation of the core due to short-term steady-state overvoltage and/or frequency reduction, internal short-circuits including turn-to-turn, interwinding and earth faults, external faults combined with transformer and Current Transformers (CTs) ratio mismatch, on-load tap changer operation, and saturation of the CTs, double contingency events such as switching-in a faulted transformer or internal fault occurring in course of an external fault, etc. Since field recordings of transformers abnormal conditions, especially for internal faults, are seldom available, the information needed for investigation of protective systems may be achieved exclusively by means of digital simulation. The main directions in the computer modeling for analysis of transformers are classified as follows: Self and mutual inductances. The approach is commonly used in transient short-circuit calculations since adopted as the transformer model in EMTP-ATP. The method uses accurate formulae for calculation of self and mutual inductances between the windings. The presence of an iron core, however, makes the values of inductances close one to another which results in ill-conditioned equations. This problem has been efficiently solved by subtracting the common flux when computing the inductances. Leakage inductances. This model represents adequately the leakage inductances of a transformer but shows difficulties in representing properly the iron core. Principle of duality. This approach deals accurately with the iron core, but the leakage inductances, in turn, cannot be modeled properly. Measurements. This family of methods focuses on representing a transformer as a terminal equivalent in a wide spectrum of frequencies. Methods of this group match the parameters of an assumed structure of a model with the experimental frequency response of a tested transformer. Weak basis for generalization is the major drawback of those methods. Electro-magnetic fields. This group of methods use three-dimensional finite element algorithms for analysis of electro-magnetic fields in a power transformer. The approach is very accurate and complete but rather design oriented due to a very heavy computational burden. Most of the above models have been primarily developed as global models they give a terminal equivalent of the device. In order to simulate disturbances such as internal faults, one needs a model with inner nodes in its windings rather than very accurate representation of the core or emulation of the frequency response of a transformer. This paper presents a model of a power transformer with an internal fault as well as the software implementation of this model. The approach taken in this paper is based on the BCTRAN procedure of ATP. The terminal equivalent of a transformer delivered by BCTRAN is rearranged using the custom-built software to incorporate model of an internal fault. The resulting BCTRAN-like data file is then processed by ATP for simulation. First, a terminal equivalent of a transformer is presented. Second, the modifications are described that yield the internal fault model. Third, the developed ATP add-on software is presented. The numerical example is given to illustrate the simulation method.