In this study, a new algorithm for discrimination between internal faults and inrush current conditions in power transformers is presented. First, the proposed algorithm removes the pre-fault conditions of the fault currents. Then, those currents are processed using the discrete wavelet transform to obtain the wavelet matrix. The correlation matrix is computed from wavelet matrix to know the eigenvalues associated with each frequency scale and to reduce the analysed information. This algorithm, which the authors are going to call wavelet correlation modes, is used to identify inrush and internal fault currents in power transformer. To validate the proposed algorithm, simulations are carried out in power systems computer-aided design software, and the obtained signals are analysed in order to evaluate its performance and sensitivity.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-gtd.2015.1147

Algorithm for transformer differential protection based on wavelet correlation modes

Sympathetic interaction between transformers is a quite normal phenomenon in power systems. For the purpose of preventing transformer differential protection relays from malfunction, this paper proposes a morphological method for the identification of sympathetic inrush, which is the first time when mathematical morphology is applied in this field. Since the waveform of differential current is symmetrical in an internal fault case while asymmetrical in a sympathetic inrush case, the proposed method uses a morphological operator to extract the peaks and valleys of the differential current to distinguish sympathetic inrush. Considering the possible current-transformer (CT) saturation conditions, this paper combines a morphological gradient with a weighted mathematical morphological operator to improve the effectiveness of the proposed method. The proposed method is evaluated on data collected from simulation cases established in PSCAD/EMTDC and from laboratory experiments, respectively. Identification results have verified that by comparing with the traditional second harmonic restrain method, the proposed method can distinguish sympathetic inrush from an internal fault current more accurately and more effectively, even when the CT is fully saturated.

An Identification Method Based on Mathematical Morphology for Sympathetic Inrush

This study introduces the concept of decision tree (DT) classification, a new approach to power transformer differential protection The proposed technique is based on processing the differential current The suggested method detects winding insulation failures and distinguishes them from magnetising inrush and sympathetic inrush conditions with classification accuracies of 100% for simulations and 95% for real-time studies The internal faults can be accurately recognised from inrush current conditions in a few sampling cycle after the occurrence of a disturbance Another advantage of the proposed method is that the fault detection algorithm does not depend on the selection of thresholds Performance analysis of the DT is achieved by the simulation of different faults and switching conditions on a power transformer in power system computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) Furthermore, the proposed method is also tested in laboratory environment The accuracy of DT is also compared with support vector machine Both experimental and simulation results are presented and discussed

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-epa.2013.0407

Power transformer protection based on decision tree approach

Sympathetic inrush is a magnetizing inrush phenomenon between two or more transformers when one of them is switched on under no load condition. This phenomenon has caused transformer differential protection maloperation in several cases due to the failure of the second harmonic restraint algorithm. Sympathetic inrush is very different in both the mechanism and characteristics from general inrush generated by a single transformer. Hence, a special identification method is required. The physical mechanism of sympathetic inrush between transformers is thoroughly studied to understand its characteristics, especially the alternate saturation feature. A substation-area information-based identification method is proposed to identify sympathetic inrush of the in-service transformer (IT). The alternate opposite saturation characteristic is used in the method. Both locking and unlocking criteria are designed for current differential protection to prevent the IT protection from maloperation. The proposed method is verified to be effective under different system conditions by simulations conducted in PSCAD and RTDS.

Transformer Sympathetic Inrush Characteristics and Identification Based on Substation-Area Information

Current transformer (CT) saturation can cause the maloperation of transformer differential protection under external faults. To avoid this problem, a method based on the histogram of the normalized rotated current (NRC) is proposed. NRC is obtained by a symmetric waveform transformation of a part differential current. An index based on the histogram is defined to evaluate the NRC frequency distribution. The external faults with different degrees of CT saturation, internal faults, and transferring faults can be clearly distinguished by the proposed method. Moreover, the detection time is invariably less than a half cycle after the obvious increase of the differential current. Simulation results validated the proposed method.

Histogram-Based Method to Avoid Maloperation of Transformer Differential Protection Due to Current-Transformer Saturation Under External Faults

In substations with an inner bridge connection, the two main transformers are generally connected in parallel. The differential protection of the transformer, under normal operating conditions, may maloperate when the other transformer is switching on through the bridge breaker after scheduled maintenance. This paper analyzes the factors that may lead to the maloperation of transformer differential protection and concludes that local transient saturation of the current transformer (CT) caused by the decaying dc component in the magnetizing inrush is the main cause of the maloperation of the transformer differential protection. By detecting the time difference between the instant of sudden changes occurring in the transformer wye-side current and the instant of the differential current increasing, a new algorithm based on the time differential method is proposed in this paper, which can prevent the maloperation of transformer differential protection in substations with an inner bridge connection. On the basis of detailed analysis and simulation work, the new algorithm is verified to be applicable under the conditions of sympathetic inrush and CT saturation.

J. Gu

Papers

A New Algorithm to Avoid Maloperation of Transformer Differential Protection in Substations With an Inner Bridge Connection