Pilot Protection Scheme Research Articles

A new pilot protection scheme for modular multi‐level converter‐based HVDC system based on voltage matching factor

Concerning the incorrect operation of line protection in modular multi-level converter-based multi-terminal DC system, a pilot protection method based on voltage matching factor is proposed. First, the dynamic switching process of flexible DC converter sub-module is analyzed, and the expression of DC voltage of converter station considering the influence of alternating current (AC) system is derived. On this basis, the model of DC line voltage considering the influence of AC injection current is built, and the equivalent DC line voltage is calculated according to the voltage model. Then, by constructing the distortion factor which reflects the difference between the equivalent DC line voltage and actual DC line voltage, the protection criterion for the identification of internal and external faults is established. Finally, simulation tests on real time laboratory (RT-LAB) platform verify that the proposed scheme can correctly distinguish between internal and external faults in different fault cases, with fast operation speed and strong immunity to the fault resistance.

A pilot protection scheme for flexible HVDC transmission lines based on modulus power

Existing protection methods for Flexible HVDC transmission lines easily fail to detect the fault location fast and reliably. Concerning this problem, a modulus power based pilot protection scheme is proposed. By analyzing the dynamic switching process of sub-modules in flexible DC converter station, the expression of converter station DC voltage taking into account the influence of AC system is derived. On this basis, the modulus voltage and modulus current are calculated by phase-modulus transformation, and the modulus power calculation model containing DC components alone is built. Then, the expressions of modulus power in different fault cases are derived, and a protection criterion based on the polarity difference characteristics of modulus power is established. Finally, the RT-LAB is used to build the model of a flexible DC system for simulation verification. The results of simulation tests verify that, the proposed protection scheme can detect the fault line and fault pole fast and accurately, with easy threshold setting and low demand on communication synchronization. Besides, it is unaffected by the fault resistance and the specific boundary conditions of current-limiting reactor, etc.

An AC line pilot protection scheme for AC/DC hybrid system based on composite mode power difference

This paper proposes a new AC line pilot protection scheme for AC/DC hybrid system on the basis of composite mode power difference, which can avoid the incorrect operation of AC line protection resulting from commutation failure. By analyzing the post-fault conducting states of inverter valves, the expressions of injection current from DC system in different operating states of inverter are derived. And combining the fault network of AC system, the models to calculate the 0/1/2-mode power injected from the backside systems of AC line are built. Then, the 0-mode power and 2-mode power are used to construct the composite mode power difference. Considering that the composite mode power varies with system topology, this paper forms the protection criterion according to the composite mode power difference which is related with the difference between system models of the backside systems in internal and external fault cases. Simulations based on RT-LAB platform confirm that, the presented method can correctly distinguish between different types of internal and external faults, and it is highly immune to the fault resistance and commutation failure.

A pilot protection scheme for VSC-MTDC grids based on polarity comparison using a combined morphological technique

A pilot protection scheme for VSC-MTDC grids based on polarity comparison using a combined morphological technique

A pilot line protection for MT-HVDC grids using similarity of traveling waveforms

Protection of dc line faults is a major challenge for the operation of multi-terminal high-voltage dc (MT-HVDC) grids. To avoid blocking converter and system shutdown, ultra-fast protection is required to detect and isolate line fault with the application of hybrid dc circuit breaker. This paper proposes a pilot protection scheme based on improved Hausdorff distance to detect the overhead line fault for MT-HVDC grid. Compared with literature, this protection method owns higher withstanding ability of noise interference, start-up delay and data anomaly. Moreover, the fault segment can be correctly identified as independent of fault location, and protection sensitivity is maintained even under internal ground fault with high impedance of up to 1000 Ω. The issue of dead zone fault recognition is considered and solved in this paper with reasonable sacrifice in the sampling frequency requirement. Based on Zhangbei HVDC grid in China, a four-terminal dc grid model is developed in PSCAD/EMTDC environment for fault simulation, and the data processing is performed in MATLAB platform. The feasibility and effectiveness of this protection scheme are verified by a simulation study that considers different scenarios.

A novel pilot protection scheme for transmission lines based on current distribution histograms and their Bhattacharyya coefficient

The current waveform characteristics at both ends of a transmission line under different disturbances are represented by means of probability distribution histograms. By adopting the Bhattacharyya distance (BD) algorithm to quantify the similarity of current distribution histograms, a novel scheme of pilot protection for transmission lines based on Bhattacharyya coefficient (BC) is put forward. The proposed scheme can correctly identify internal faults, external faults, and faults accompanied with current transformer (CT) saturation of transmission lines. It also shows good performance in the cases of abnormal samples. The validity of the proposed scheme is evaluated by various simulation and dynamic analog experiment tests.

Novel pilot protection scheme for line‐commutated converter high voltage direct current transmission lines based on behaviour of characteristic harmonic impedances

Novel pilot protection scheme for line‐commutated converter high voltage direct current transmission lines based on behaviour of characteristic harmonic impedances

A Differential Pilot Protection Scheme for MMC-Based DC Grid Resilient to Communication Failure

Primary single-end line protection strategies of modular multilevel converter (MMC)-HVdc systems are difficult to make a tradeoff between fast detection speed and high reliability. To improve reliability, the pilot protection schemes based on communication and data exchange can be adopted. However, the communication-based schemes suffer from potential communication failure problems, such as data error, data loss, and time synchronization error. To avoid blocking of protection devices during communication failures, a resilience-oriented differential pilot protection method is proposed in this article. To address the problem of synchronization error, a startup element based on the multiresolution morphological gradient (MMG) of traveling wave (TW) is proposed. For the problems of data error and data loss in communication, the sampled data are preprocessed by morphological filtering (MF). Also, the correlation of TWs is used to identify the internal and external faults; the ratio of the morphological gradient of pole voltages is adopted to discriminate the faulty poles. A four-terminal MMC-based dc grid model is built in PSCAD/EMTDC interfaced with the optical fiber-based communication system built in MATLAB/Simulink. The simulation results show that the protection scheme can effectively identify the faults against serious communication problems of 1‰ bit error rate and 5% data loss.

Pilot protection scheme for distribution network considering low voltage ride‐through of inverter‐interfaced distributed generators

According to the latest grid‐connected standard, the failure of IIDG (inverter‐interfaced distributed generator), represented by wind power and photovoltaic power, is characterized by strong nonlinear output. Therefore, the protection scheme based on the traditional fault characteristics is difficult to be applied to the distribution network with high permeability IIDG access. Based on the low voltage traversing operation characteristics of PQ control strategy IIDG, the work analyzed the distribution characteristics of positive sequence fault components in distribution network, and proposed a pilot protection scheme for distribution networks based on the comparison of current amplitudes on both sides of line. This scheme not only reduced the strict requirement of pilot protection for synchronous sampling of communication facilities, but also eliminated the influence of load current. Moreover, it had good practicability and economy without the protection and cooperation with other components. At the same time, in order to avoid the misoperation of amplitude ratio protection criterion for non‐fault line due to the existence of unmeasured load branches, the distribution characteristics of node branch current abrupt changes were used to establish the auxiliary criterion for node‐branch current mutation. It solved the action dead zone of the ratio protection criterion. Finally, the feasibility of the proposed protection principle was verified by DIgSILENT simulation. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Permissive pilot protection adaptive to DC fault interruption for VSC-MTDC

The voltage source converter based multi-terminal high-voltage DC (VSC-MTDC) system is a novel technique that has many advantages over conventional DC systems. However, the widely used half-bridge modular multi-level converter (MMC) is defenseless against DC faults unless the main protection operates fast enough to trip the local DC circuit breaker. For this reason, there is an urgent need to develop high-speed protection schemes apply to the VSC-MTDC. However, many state-of-the-art protection strategies in field applications require the configuration of the fault current limiting reactors at each end of the DC line, which not only increases investment but also has a negative impact on the dynamic response capability of the system. Also, the dilemma between the improving of sensitivity and the ensuring of selectivity of existing protection schemes needs to be further addressed. In this study, a novel permissive pilot protection scheme is proposed based on the energy ratio between the backward traveling wave and the forward traveling wave. The proposed protection scheme requires only a logic signal from the remote end, avoiding the heavy burden caused by the data exchange and synchronization. Simulation results indicate that the proposed protection has an adequate reach to cover the entire length of the DC line, sufficient speed to isolate internal faults before the blocking of the converter, sufficient sensitivity and security for a 500-kV system, and sufficient robustness against disturbances.

A Pilot Protection Scheme for Transmission Lines of Half-Bridge MMC-HVDC Grids Using Cosine Distance Criterion

One of the major challenges in half-bridge modular multilevel converter-based high-voltage dc (MMC-HVDC) grids is the rapid detection and isolation of short-circuit faults before converter blocking and the entire grid shutdown. With the advancement of the hybrid dc circuit breakers (HDCCBs) in the ultra-fast isolation of faults, a considerable part of the problem has been solved. In this paper, relying on the performance of these circuit breakers and using the cosine distance criterion, a new pilot protection scheme is designed and presented for detecting short-circuit faults in transmission lines of half-bridge MMC-HVDC grids. While this scheme requires a comparatively low sampling frequency, its fault detection speed is quite independent of fault resistance. Moreover, this protection has the correct performance for any fault location along the line under protection and is completely stable against all external faults. The results of the evaluations on a 4-bus test grid under different fault conditions confirm the correct, ultra-fast, stable, selective, and reliable performance of the proposed line protection scheme.

A novel pilot protection scheme for HVDC lines based on waveform matching

The traditional current differential protections for HVDC transmission lines are easily affected by distributed capacitive current. Although some improved differential principles have been presented in literatures, they still risk maloperation caused by synchronization error, noise or parameter errors. To overcome the problems above, this paper proposes a novel pilot protection scheme based on waveform matching. After the electrical quantities on both sides are low-pass filtered, the HVDC line is simplified to two PI models from the midpoint, which can eliminate the effect of distributed capacitive current and has simple computation as well as satisfying precision. The midpoint currents, which can be calculated based on the simplified DC line model with the information of either side, will be matched under external faults while have opposite trend under internal faults. The improved Pearson correlation coefficient (IPCC) is proposed to measure the similarity between the waveforms of the calculated midpoint currents and establish the protection criterion. A ± 800 kV HVDC test system is modeled in PSCAD/EMTDC. Extensive simulations of various fault situations indicate that the new protection has absolute selectivity, great reliability and strong fault resistance withstand ability. Sensitivity analysis and comparisons with other pilot protections further demonstrate that the proposed protection stands out for high accuracy and strong robustness to many non-ideal factors.

A Pilot Protection Scheme of DC Lines for Multi-Terminal HVDC Grid

Multi-terminal HVDC grid has extremely high demand for protection speed of DC lines, consequently, the sampling time window of the main protection is very short, which affects its selectivity and reliability. To make up for the insufficiency of the non-unit main protection, this paper proposes a pilot protection scheme based on the voltage polarities of current-limiting reactors at the ends of DC lines. Analysis shows that the voltage polarities of terminal reactors at the two ends of the DC lines are both positive when internal fault occurs, while they are opposite for external faults. Based on the aforementioned characteristic difference, a pilot protection criterion is built to discriminate between internal and external faults. To avoid mal-operation of the protection on healthy pole generated by the coupling effect, the polarity mutation of reactor voltage on healthy pole due to coupling effect is analyzed when single-pole-to-ground fault occurs. Based on the analysis, the fault voltage of current-limiting reactor is integrated, and its polarity can be identified based on the integration value. At last, a four-terminal modular multilevel converter (MMC)-based HVDC grid model is built in PSCAD/EMTDC, and the effectiveness of the protection scheme is verified by extensive simulations.

Pearson correlation coefficient of current derivatives based pilot protection scheme for long-distance LCC-HVDC transmission lines

This paper proposes a pilot protection scheme for long-distance LCC-HVDC transmission lines based on Pearson correlation coefficient of current derivatives. According to the impedance frequency characteristics of converter and DC filter, and the superposition principle, the correlation characteristics between line current derivative (LCD) and DC filter current derivative (DFCD) are analysed for internal and external faults. For internal fault, there is completely negative correlation between line current derivative and corresponding DC filter current derivative at both rectifier side and inverter side; for external fault, the correlation of current derivatives (CDs) at the faulty side is positive, yet completely contrary at the other side. The Pearson correlation coefficient (PCC) is utilized to detect the correlation characteristics of current derivatives. And based on these characteristics, a novel protection scheme for long-distance HVDC transmission lines can be constructed to detect internal and external faults. Comprehensive simulation results show that the proposed protection scheme can quickly identify internal and external faults with high reliability. Furthermore, it is insensitive to fault resistance, fault location and measurement noise, and does not require data synchronization.

A New Two-Port Network Model-Based Pilot Protection for AC Transmission Lines

Conventional pilot protections used for EHV/UHV ac transmission lines require data synchronization between the two terminals of transmission line, and their performances are affected by the line capacitive charging current and load conditions. To tackle these problems, this paper proposes a new pilot protection scheme based on two-port network model of ac transmission lines. The new protection scheme calculates the two-port network parameters from the online measurements via the sequence components method, and then compares the solved parameters with their setting values to distinguish the internal and external faults. It does not require the data synchronization between two terminals of the transmission line. Moreover, its performance is not affected by the line capacitive charging current and load conditions. The applicability of the proposed scheme for series compensated lines is also analyzed. Effectiveness and accuracy of the proposed protection scheme have been verified by simulation tests.

A Harmonic Based Pilot Protection Scheme for VSC-MTDC Grids with PWM Converters

This paper presents a selective harmonic-based pilot protection scheme for detecting faults happened in the DC transmission section of VSC-MTDC grids with pulse width modulation (PWM) voltage source converters (VSCs). When a DC fault occurs in VSC-MTDC grids with PWM converters, first carrier frequency harmonic (FCFH) currents will be generated by all VSCs through the grid. FCFH currents have different flowing directions depending on the characteristics and the location of the fault. According to the characteristics of the existing FCFH in the fault currents, a selective pilot protection algorithm is designed for VSC-MTDC grids. Considering the internal and external DC transmission faults for specific zones, and the circulating flow of FCFH current in the DC link capacitors, the relays cannot detect FCFH currents for external faults, while for the internal faults, FCFH currents are clearly detected. To design the selective protection algorithm, Hilbert-Huang transform (HHT) is used to detect the instantaneous frequency and the instantaneous amplitude of the high frequency intrinsic mode function (IMF)s, which are extracted from the fault current waves. Multiple faults with different characteristics are applied to CIGRE DCS-2 VSC-MTDC grid with two-level and three-level VSCs modeled in PSCAD, and the HHT-based selective protection scheme is designed in MATLAB. According to the results, the proposed algorithm can truly discriminate between internal and external faults.

A Pilot Protection Scheme for Transmission Lines in VSC-HVDC Grid Based on Similarity Measure of Traveling Waves

The voltage-source converter (VSC)-based high-voltage direct-current (HVDC) grid is an effective means for large-scale integration of renewable energies. This paper proposes a pilot protection scheme as the backup protection for transmission lines in the VSC-HVDC grid. The traveling wave (TW) propagating processes and waveform characteristics at both ends of the transmission line are analyzed under different fault conditions. Based on the different characteristics between internal and external fault conditions, the protection is formulated utilizing similarity measure of TW waveforms. A four-terminal VSC-HVDC grid is modeled on PSCAD/EMTDC platform to test the performance of the proposed protection. Various simulation results demonstrate that the proposed protection is able to accurately discriminate internal faults from external faults and identify the faulty pole. The protection performance analysis shows that the protection is insensitive to the fault resistance, the protection sampling rate, and the inductance value of the reactors at the end of lines. The protection algorithm is simple with small calculation amount and does not require data synchronism. Furthermore, the proposed protection has superiority in the fault resistance withstand ability compared with other backup protections.

A novel pilot protection scheme for LCC-HVDC transmission lines based on smoothing-reactor voltage

This paper proposes a novel pilot protection scheme for LCC-HVDC transmission lines based on smoothing-reactor voltage (SRV). The proposed scheme is derived taking into consideration transient capacitance current and fault resistance. The polarity characteristics of SRVs are theoretically analyzed under internal and external faults in the fault transient period. And for internal fault, the polarities of SRVs at both sides are positive; when an external fault occurs, the polarity of SRV is negative at faulted side and positive at the other side. Based on these characteristics, a new back-up protection scheme is proposed. Moreover, a fault pole selection criterion is proposed. A ±800kV LCC-UHVDC test system is established to verify the accuracy and validity of the proposed protection scheme under different fault conditions. Comprehensive simulations show that the proposed method can identify internal and external faults reliably, with high sensitivity and selectivity, and its performance is inspiring under different fault locations and fault resistances. Furthermore, the proposed protection needs a lower sampling frequency and does not require data synchronization, making it valuable for applications in HVDC transmission system.

Improved travelling wave protection for extra‐high‐voltage/ultra‐high‐voltage transmission lines

Travelling wave protection has some very attractive advantages such as fast operation speed, immunity to load condition, immunity to power swing, immunity to charging current, and immunity to series compensation. Consequently, travelling wave protection is deemed very suitable for extra-high-voltage/ultra-high-voltage transmission line protection applications. However, its practical application has some challenging issues, especially in regard to security and reliability. For example, harmonics may cause wrong fault direction detection for the classic travelling wave pilot protection, and can even lead to mal-operation in some cases. How to improve the security and reliability under such practical operation conditions is, therefore, a key topic in the research of travelling wave protection. A novel travelling wave pilot protection solution will be discussed, which is based on some new methods such as 4-polarity comparison, a time synchronised pilot protection scheme, and high-frequency unblocking. The analysis and simulation results show that the proposed solution has much better security and reliability than the classic travelling wave pilot protection.

Pilot protection scheme for transmission line of a hybrid HVDC system based on polarity characteristics of current and voltage fault components

Pilot protection scheme for transmission line of a hybrid HVDC system based on polarity characteristics of current and voltage fault components