Novel Fault Protection Method for Flexible DC Power Systems

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

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.

Detection and classification of internal faults in bipolar HVDC transmission lines based on K-means data description method

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.

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.

High voltage dc grids are raised high-speed operation and high reliability requirement for relay protection. This article aims at proposing a protection scheme with high operation speed, high sensitivity, and high reliability. First, the symbolic expressions of the line-mode fault component traveling wave in case of the internal and external faults are derived, which consist of the traveling wave propagation term and the dc line boundary term. It is theoretically demonstrated that the index of the traveling wave propagation term in case of internal faults is greater than that for the external faults, and the index is independent of fault impedance. Second, the Levenberg–Marquardt algorithm is introduced to extract the index and an index-based nonunit protection method as well as the protection scheme is proposed. The proposed method is theoretically robust and independent of fault impedance and fault type. In the validation, the fitting results are indicated that the symbolic expressions are correct and the extracted indices are effective to characterize the location of the faults. Numerous fault cases of different fault distances, fault impedances, and fault types have demonstrated that the proposed protection scheme is of high sensitivity, high operation speed, and high reliability.

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.

For the purpose of dc fault identification in multiterminal modular multilevel converter (MMC) high-voltage dc (HVDC) systems, the proposed work addresses the issues of localized protection schemes. The proposed work is focused on selectivity issues, such as differentiating between forward external and internal faults, and the classification of the type of fault contingency, i.e., pole to pole (PTP) or pole to ground (PTG) in the system. The scheme uses an equivalent network of multiterminal MMC-HVDC systems for dc fault identification. Modal transformation is used to analyze line-mode and zero-mode voltages across the current-limiting reactor (CLR) for different possible contingencies in the system. A maloperation region has been defined to address the issue of selectivity of internal HIFs and external low-impedance faults (LIFs). The rate of change of dc voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dU_{\mathrm{ dc}}$ </tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">/dt</i> ) is used as the mitigation technique to differentiate a HIF as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1000 \Omega $ </tex-math></inline-formula> . Furthermore, a sensitivity function is formulated, which gives the variation of decisive parameters i.e., line-mode voltage and zero-mode voltage with respect to change in fault resistance and value of CLR. PSCAD-/EMTDC-based simulations are used to validate the performance of the proposed scheme. The scheme is validated in terms of detection time, robustness to WGN in measurement, security for ac faults, dc circuit breaker (DCCB) operation, and power reversal (load transients).

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

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.

This paper proposes a novel transmission line protection scheme based on frequency-domain model identification method with distributed parameter. Integrated impedance is defined by voltages and currents of two end terminals of transmission line. By analyzing the frequency-domain expressions of integrated impedance in internal and external fault states, the amplitude-frequency characteristics of the integrated impedance are discussed. The integrated impedance in internal fault state shows system impedance characteristic and the integrated impedance in external fault state shows the line distributed capacitive characteristic. The frequency-domain model errors are defined as unbalanced currents respectively in order to identify the internal and external faults. Matrix pencil algorithm is used to calculate the phasors in different frequencies in the proposed scheme. Protection criterions are designed and some tests are performed. The simulation results show that the proposed schemes can detect internal fault quickly and reliably. Furthermore, the influences from capacitive currents and transition resistances can be avoided in the proposed protection scheme. (6 pages)

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.

A transient current based busbar protection scheme using Wavelet Transforms

Voltage source converters (VSCs) are highly vulnerable to DC fault current. Fast and reliable fault location and protection methods are necessities in VSC-based multiterminal DC (VSC-MTDC) systems for its safe and reliable operation. In this paper, a selective transient-voltage based fault protection method is proposed. The protection method first uses fast overcurrent criterion as the starting element. Then based on the boundary characteristic of the supplemental inductors, the transient-voltage difference between the internal fault and the external fault is used to locate the fault line. And the extraction of the transient-voltage is achieved by wavelet transform. The proposed method is based on local information, it can identify the fault line quickly without communication between protection units at different locations. The methods has no auxiliary directional criterion and additional fault pole selection criterion. Besides, this method can be realized with very low sample rates, and it is rather strong against various fault resistances. Corresponding simulations were carried out, verifying its feasibility and validity.

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.