A transient current based busbar protection scheme using Wavelet Transforms

Busbar protection is an essential component in power system design, protecting the most important system node for network stability and security. When faults occurs on busbar itself, it takes much time to isolate the bus from source which may cause much damage in the bus system. Faults in power system are classified as internal and external faults. Faults within the zone are termed as internal faults whereas, the faults outside the Zone are called as external faults. Ideally, a relay looking after the protection of a zone should operate only for internal faults. It should restrain from operating for external faults or through faults. In this paper the busbar protection using differential protection scheme has been investigated for internal and external faults. An algorithm has been developed to improve the selectivity of the relay and the same has been tested in IEEE9 bus system for internal and external faults. Separation of De-noised signal from fault signal is made using wavelet transform so that the nature of fault occurs on the system can be identified. In this study Daubechies 4 at level 3 is used to separate original signal and de-noised signal. The entire simulation has been done using MATLAB R2017a.

The growing demand for electricity, integration of renewable energy sources, and recent advances in power electronics have driven the development of HVDC systems. Multi-terminal HVDC (MTDC) grids, enabled by Voltage Source Converters (VSCs), provide increased operational flexibility, including the ability to reverse power flow and independently control both active and reactive power. However, fault propagation in DC grids occurs more rapidly, potentially leading to significant damage within milliseconds. Unlike AC systems, HVDC systems lack natural zero-crossing points, making fault isolation more complex. This paper presents the implementation of a wavelet-based protection algorithm to detect faults in a four-terminal VSC-HVDC grid, modelled in MATLAB and SIMULINK. The study considers several fault scenarios, including two internal DC pole-to-ground faults, an external DC fault in the load branch, and an external AC fault outside the protected area. The discrete wavelet transform, using Symlet decomposition, is applied to classify faults based on the wavelet entropy and sharp voltage and current signal variations. The algorithm processes the decomposition coefficients to differentiate between internal and external faults, triggering appropriate relay actions. Key factors influencing the algorithm’s performance include system complexity, fault location, and threshold settings. The suggested algorithm’s reliability and suitability are demonstrated by the real-time implementation. The results confirmed the precise fault detection, with fault currents aligning with the values in offline models. The internal faults exhibit more entropy than external faults. Results demonstrate the algorithm’s effectiveness in detecting faults rapidly and accurately. These outcomes confirm the algorithm’s suitability for a real-time environment.

A fault protection method is proposed for flexible direct-current (DC) power systems based on the cosine similarity of currents in current-limiting reactors. The typical characteristics of external and internal faults in a flexible DC power system were analyzed. The principles of cosine similarity and the fault current characteristics of current-limiting reactors were used to distinguish between internal and external faults. The ratio of the average positive and negative voltages of the current-limiting reactor was then used to distinguish the fault types and fault line. Finally, a simulation model of a circular flexible DC power system was constructed using MATLAB/Simulink software (2018b). The simulation results show that the proposed protection scheme can quickly and accurately identify the location and type of faults, and has a strong ability to withstand fault resistance and is less affected by distributed capacitance, noise, and communication delay.

The usage of the high-harmonic restraint is one of the most common ways to ensure the non-operation of the power transformer differential protection when a magnetizing current inrush occurs. The application of this method allows not only to lower the minimum operating current of the protection, which increases its sensitivity to the developing inter-winding faults, but also provides inaction of the protection when unbalance currents increase during a transient caused by an external fault. However, there is no accurate information in the literature on possible ranges of variation of content of higher harmonics in the differential current in the cases when external or internal faults occur. This complicates the calculation of settings of protective relays, ensuring their most effective functioning. In this paper authors analyze the content of the higher harmonics in the differential current in the cases of external and internal faults, taking into account the saturation of the measuring current transformers in order to determine the range of variation of the higher harmonics under these conditions. It is shown that in all cases the ranges of variation of higher harmonics overlap which makes it difficult to create an effective differential protection that recognizes emergency and normal operation conditions operation only by harmonic analysis of the operating signal

There is a smoothing reactor and DC filter between the inverter and the direct current line to form a boundary in the HVDC transmission system. Since this boundary presents the stop-band characteristic to the high frequency transient voltage signals, the high-frequency transient voltage signal caused by external faults through boundary will be attenuated and the signals caused by internal faults will be unchanged. A new algorithm based on mathematical morphology is presented for fast discrimination between internal fault and external fault. The method is based on the principle that the waveform caused by internal fault contains much wave peak and wave trough and the waveform caused by external fault changes slightly.

Discrimination of different DC faults near a converter end of a DC section consisting of a filter, a smoothing reactor, and a transmission line is not an easy task. The faults occurring in the AC section can be easily distinguished, but the internal and near-side external faults in the DC section are very similar, and the relay may cause false tripping. This work proposes a method to distinguish external and internal faults occurring in the DC section. The inputs are the voltage signals at the start of the transmission line and the end of the converter filter. The difference in voltage signals is calculated and given to an intelligent controller to detect and discriminate the faults. The intelligent controller is designed using machine learning (ML) and deep learning (DL) techniques for fault detection. The long short-term memory (LSTM-) based relay gives better results than other ML methods. The proposed method can distinguish internal from external faults with 100% accuracy. Another advantage is that a primary relay is suggested that detects faults quickly within a fraction of milliseconds. Nevertheless, another advantage is that a backup relay has been designed in case the primary relay cannot operate. Results show that the LSTM-based protection scheme provides higher sensitivity and reliability under different operation modes than the conventional traveling wave-based relay.

This paper presents a simple and practical method which enables to prevent malfunction of protection relay due to differential current caused by one end CT saturation in case of external fault. This method uses difference of magnitude(instantaneous value) between the both end current just before the occurrence of differential current without a separate method to CT staturation detection. One end CT saturation is simulated by current transformer model using type-96 component and the presented method is verified by using EMTP MODELS with respect to internal and external fault with one end CT staturation. The presented method distinguished rightly bewteen external and internal fault with one end CT saturation. This information can be used to prevent malfunction of current differential protection relay in case of external fault. And this method is not affected by sampling rate and has no calculation burden, so it will be applicable to differential current protection relay with ease.

In this study, a hyperbolic S-transform-based method is proposed to discriminate most important transient fault currents of transformers. First, the proposed method discriminates external faults from other disturbances. Then, the S-transform is applied to differential currents. An index is suggested by using absolute deviations of the S-matrix of differential currents to discriminate internal incipient faults in addition to inrush currents and internal faults. The relay issues an alarm signal in the case of the incipient fault but restrains during the magnetising inrush current. If an internal fault is recognised, the relay will issue a trip signal. To study the robustness of the suggested method, a program is developed in the MATLAB environment. The inputs of this program are differential current signals, derived from a system modelled by EMTP software. In order to simulate the internal incipient fault along with internal turn to turn and turn to earth faults, the transformer is modelled as 8×8 RL matrices, derived by using subroutine BCTRAN in EMTP. Also, differential currents are contaminated by noise and it is shown that the suggested method is not affected by noise and it can discriminate incipient faults, inrush currents, internal faults and external faults.

A new differential protection scheme based on Synchro-squeezing transform applied to AC Micro-Grid

Modern microprocessor based relays used for busbar and transformer protection, typically implement a low impedance percentage differential principle using dual slope characteristics. These relays may sometimes misoperate in response to the CT saturation during close-in external faults, and the subsidence current present after clearing external faults. A primary reason for such misoperations is the fact that the traditional percentage differential principle relies exclusively on current magnitudes rather than directionality for tripping decisions. This paper describes two novel principles that distinguish between external and internal faults so as to enhance the security of differential protection; directional phase angle comparison and rate-of-change of differential current. The paper also discusses mathematical models used for each principle and their interaction during operation as well as their interaction with the traditional dual slope method. In addition, the paper compares the performance of a protective device using the traditional dual slope method vs. one with the new principles. Test cases are presented to illustrate the effectiveness of the new principles in enhancing the security for external faults.

The paper presents a methodology on how to discriminate: internal and external faults; for a transmission line protected by differential protection scheme, during the presence of charging current. Transmission line with high voltage levels (400-765kV) peculiarly has this disadvantage. Various plots for a Distributed Transmission line Model with Source at both the ends” with internal and external faults were plotted and analyzed. Through the comparison of current samples at zero crossings for remote end and local end buses, after that implementation of proposed DSA (Difference Squared and Averaged) theory, were able to distinguish whether the fault is internal or external. The techniques better compared to conventional techniques since threshold taken under consideration; obtained through experimentation on various system models for varying degree of parameters via certified thresholding method. And the uniqueness lies with the fact that it provides a provision to distinguish transient disturbances from internal fault.

The busbar protection relays typically implements low impedance percentage differential principle using dual slope characteristics. These relays may sometimes misoperate in response to the CT saturation during close-in external faults, and the subsidence current present after clearing external faults. A primary reason for such misoperations is the fact that the traditional percentage differential principle relies exclusively on current magnitudes rather than directionality for tripping decisions. This paper evaluates the performance of an enhanced bus differential relay which uses different principles that distinguish between external and internal faults so as to enhance the security of differential protection. Extensive testing has been carried out to evaluate the effectiveness of the new principles in enhancing the security for external faults.

Conventional second harmonic current restriction approach is an adopted method which has been implemented to transformer differential protection schemes. Because of unreliable nature of this approach in discrimination between inrush current, internal faults and external faults and due to the saturation of current transformers, we present new features of the differential protection scheme. This helps to determine the feasibility and applicability of the proposed approach in differential protective scheme for power transformer. The Current Transformer Saturation (CTS) phenomenon issue and its revelation is one of the most significant problems in power system protection particularly in the differential protection. In this paper, the saturation phenomenon and its effects on the differential protection are analyzed. Prevalent algorithms in digital signal processing which are implemented in power system cannot correctly recognize this phenomenon. Mal-operation of differential protection relay takes place when saturation in CT is appeared. This paper presents a Third-Difference Algorithm (TDA) for CTS recognition. In order to make comparison between the presented approach and prevalent digital protective algorithm, an identical case study is simulated. Simulation results also show that the TDA is both satisfactory and consistent with expectation. Keywords: Current transformer, Saturation phenomenon, Third-difference Algorithm, Internal and external faults.

This paper presents an on-line digital computer technique used for differential protection of a synchronous generator. It provides fault detection and tripping well within half a cycle and is also capable of discriminating between internal and external faults. In the latter case, sufficient restraint signals are generated to overcome maloperation. Further, the scheme is highly reliable even with CT saturation due to heavy through faults on the generating unit. On-line tests using a laboratory synchronous generator show the results for different types of internal and external faults with varying fault incidence inception.

Several disturbances involving power transformers commonly occur in the Electric Power System. External faults near the transformer, internal faults, as well as transformer energizing and overexcitation are the main events evaluated in the protection of the power transformers. All of the aforementioned events present transients, which can be properly analyzed by the wavelet transform. The Maximal Overlap Discrete Wavelet Transform (MODWT) can be used to detect transients in some power system disturbances. This paper presents an algorithm based on MODWT for detection and identification of external faults near or far the transformer, internal faults, and transformer energization by using differential currents in the wavelet domain. A system of 230 kV was modeled using ATP/EMTP software. The results reveal the advantages of using this type of modeling, since It provides reliability and speed in detection of these events.