After the occurrence of a DC-side feeder faults on HVDC transmission systems, protection and fault detection systems are anticipated to minimize their onerous effects, by initiating fault-clearing actions such as selective tripping of circuit breakers. Following the successful fault clearance, a subsequent action of significant importance, is the meticulous estimation of its location as a means to accelerate the line restoration, reduce down-time, limit recovery and repair costs, and hence elevate the overall availability and reliability of the transmission system. In order to capture DC-side fault transients for protection and fault location applications, measuring equipment is required to be placed on HVDC installations. This paper focuses primarily on reviewing the available technologies from the perspective of enabling protection, fault location and automation applications in HVDC systems. The review constitutes a mapping of protection and fault location functions, against the available voltage and current measuring technologies, ultimately unlocking insights for selecting measuring equipment based on the desirable characteristics of protection and fault location systems. The review also revealed that the frequency characteristics of each sensing scheme, primarily refers to the bandwidth of the primary sensor, whereas the overall bandwidth of the complete measuring scheme may be further restricted by the secondary converter and corresponding data acquisition system and signal processing electronics. It was also identified that the use of RC voltage dividers has prevailed for voltage measurements for HVDC applications, due to their superior advantages. The choice of a suitable device for current measurement, depends mainly on the fault detection method used and the frequency range it operates. In particular, the review revealed that fault detection and protection methods are mainly concentrated in a frequency spectrum ranging from a few kHz to 100 kHz, while fault location methods require measurements with a frequency range from 100 kHz up to 2 MHz.

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Voltage and Current Measuring Technologies for High Voltage Direct Current Supergrids: A Technology Review Identifying the Options for Protection, Fault Location and Automation Applications

In this paper, a novel relaying scheme is proposed for bipolar line commutated converter (LCC) high voltage direct current (HVDC) transmission lines that detects the fault, identifies the pole of fault and estimates the location of the fault. The scheme uses features extracted from rectifier end DC current and voltage signals. Long short term memory (LSTM), a deep learning method has been designed as classifier as well as predictor for carrying out different relaying tasks. Three modules have been designed namely LSTM-FD (LSTM module for fault detection (FD)), LSTM-FI (LSTM module for fault pole identification (FI)) and LSTM-FL (LSTM module for fault location estimation (FL)). The voltage and current signals are obtained from measuring units. Then with a moving window of one cycle, the RMS of signals is calculated. The current and voltage features are obtained in the time domain. The voltage and current features obtained are used as input to fault detection and fault pole identification module. For fault location estimation, half cycle samples of RMS current and voltage after the fault have been taken. From half cycle sample, maximum value of current and minimum value of voltage has been obtained. This single value of current and voltage are used as input feature to the fault location module. All the relaying modules have been tested varying fault types, locations, resistances, smoothing reactors, noisy signals, etc. Sensitivity and reliability of the proposed relaying scheme is 100% with all the tested fault cases. Error in location estimation is within 1% for all the tested fault cases. Advantage of the method is that it does not require communication link. Another advantage of the proposed method is that it can work with low sampling frequency. Additionally, reliability and sensitivity of proposed method is very high, hence can be used as an alternative to travelling wave based relaying schemes.

/pdf/a-novel-relaying-scheme-using-long-short-term-memory-for-5aynvcv6rr.pdf

A Novel Relaying Scheme Using Long Short Term Memory for Bipolar High Voltage Direct Current Transmission Lines

High voltage direct current (HVdc) transmission systems play an essential role in our modern power grids, not only providing bulk power transmission and asynchronous ac connection, but also enabling renewable energy integration into power grids. Renewable energy integration through HVdc transmission systems leads to the creation of multiterminal HVdc (MT-HVdc) grids. Recently, the idea of electrifying remote communities by HVdc transmission systems has emerged, and multi-terminal small-scale HVdc grids have been investigated in such applications. Despite numerous applications and significant potential of HVdc transmission systems, various technical and economic challenges in utilizing these systems still persist. In this article, an extensive literature review is conducted on HVdc transmission systems, particularly focusing on their role in renewable energy integration, and their potential application in remote communities. Future research directions are also recommended.

HVDC Transmission and its Potential Application in Remote Communities: Current Practice and Future Trend

High voltage direct current (HVdc) transmission systems play an essential role in our modern power grids, not only providing bulk power transmission and asynchronous ac connection, but also enabling renewable energy integration into power grids. Renewable energy integration through HVdc transmission systems leads to the creation of multiterminal HVdc (MT-HVdc) grids. Recently, the idea of electrifying remote communities by HVdc transmission systems has emerged, and multi-terminal small-scale HVdc grids have been investigated in such applications. Despite numerous applications and significant potential of HVdc transmission systems, various technical and economic challenges in utilizing these systems still persist. In this article, an extensive literature review is conducted on HVdc transmission systems, particularly focusing on their role in renewable energy integration, and their potential application in remote communities. Future research directions are also recommended. 

Existing converter models are normally reduced as simplified electrical components, whereas it overlooks the impacts of fast dynamics of converter current control loops. Therefore, the dc fault current calculation with the traditional simplified converter models becomes invalid when the simulation time is longer. To overcome this problem, this article incorporates the control effects of converters during the transient process and proposes three dc fault analysis models of classical three converter topologies for the high-voltage dc (HVdc) application including line communicated converter (LCC), two-level voltage-source converter (VSC), and modular multilevel converter (MMC). For an LCC-based rectifier, the nonlinear part of original model is first linearized with the least square method due to the large dc-link voltage deviation during dc fault, and the proportional integral (PI) current regulator with the reduced linearized model can be equivalent as an RC circuit for dc fault analysis. However, for two-level VSC or MMC, the dc-link voltage deviation does not drop that much with a large dc-link capacitor discharging process during transient dynamics. As a result, the original model is linearized relying on the small-signal method based on prefault operation points, and the dynamics of fast inner current loop with the linearized model can be represented as an RL circuit for dc fault current calculation. for dc fault analysis The proposed dc fault analysis models of three converters as equivalent RLC circuits are well testified under different fault resistances and dc reactors, control parameters, and operating conditions. Moreover, dc fault simulations of a hybrid HVdc system have verified the effectiveness of the proposed converter models.

DC Fault Analysis Models of Three Converter Topologies Considering Control Effects

To overcome shortages in the traditional current differential protection for HVDC transmission lines, a pilot protection based on transient energy ratio is proposed. The fault identification criterion is put forward based on the impedance-frequency characteristic of the line boundary, and the fault analysis which shows that under internal faults, with regard to either terminal of the dc line, the transient energy within a specific frequency band on the line side of the boundary is much greater than that on the valve side of the boundary, whereas after rectifier-terminal (inverter-terminal) external faults, the transient energy on the line side of the rectifier-terminal (inverter-terminal) boundary is much less than that on the valve side of the boundary. Moreover, the lightning disturbance identification criterion is implemented based on the magnitude ratio of fault components of high- and low-frequency band currents. Fault-pole selection is constructed by using the fault components of pole voltages. Simulation results demonstrate that the proposed scheme, with low computational complexity, can distinguish internal faults from external ones and protect the entire line reliably. It can also identify high-impedance faults and select the faulted pole correctly. Besides, it is not subject to lightning interferences and the dc line distributed capacitor.

A Pilot Protection for HVDC Transmission Lines Based on Transient Energy Ratio of DC Filter Link

The common way to isolate the line fault in line-commutated converter-based high-voltage direct current (LCC-HVDC) transmission grids is regulating the trigger angle of converters on both terminals of the faulted line, which usually depends entirely on the conventional controller and goes with large current peak and lengthy overcurrent duration. In this study, a trigger-angle setting approach with adaptability is, therefore, proposed to provide a theoretical basis for the generation of fault isolation strategies under different scenarios. First, equivalent circuits of solid pole-to-ground line faults at rectifier and inverter terminals are established, and time-domain expressions of the fault and converter currents are obtained. Secondly, based on the analysis of current characteristics, the non-linear programming method is utilised to obtain the optimal trigger-angle sequence after the fault, which can be applied in the following fault isolation. Finally, the proposed approach is verified via power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) simulation. The results show that it can effectively reduce the current peak and the overcurrent duration of the converter, which is of positive significance for maintaining the power system security and stability.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-gtd.2018.6806

State-space analysis based pole-to-ground line fault isolation strategy for LCC–HVDC systems

The invention discloses a protection method for a HVDC transmission line. The method includes the steps of 1, determining a region inner and outer fault identification criterion; 2, determining a structural locking criterion; 3, determining a failure start criterion; 4, determining a fault pole selection criterion; and 5, determining a protection scheme.

Protection method for HVDC transmission line

The invention discloses a single-pole fault isolation method of a high-voltage direct-current transmission line. The single-pole fault isolation method of a high-voltage direct-current transmission line includes the following steps: 1) the characteristics of the DC current and the fault current of a rectifier and an inverter after the first and tail end metallic faults are selected to determine the objective function and constrained conditions of a fault isolation strategy; 2) the target of the fault isolation strategy is that the average value of the fault current is reduced to 0 after the first and the tail end metal faults; and the constrained conditions are that the inverter is not overcurrent and the DC current of the inverter is not less than 0 after the first end fault and the faultcurrent is reduced and the independent variables of the target and the constrains are equivalent electromotive force of the rectifier and the inverter, and the equivalent electromotive force of the inverter is a fixed value within single 20ms and is obtained following the 20ms period one by one; 3) through the target and the constrained conditions of the first and tail end metallic faults, an electromotive force sequence is obtained, and a firing angle sequence of the rectifier and the inverter is obtained, wherein the firing angle of the rectifier and the firing angle of the inverter are controlled by the control system of the high-voltage direct-current transmission system respectively to be equal to the above obtained firing angle, thus realizing fault isolation.

Single-pole fault isolation method of high-voltage direct-current transmission line

Pole-to-ground faults in hybrid AC/DC power grids may result in DC-bias voltage on AC side of the MMC. If the transformer is YNd11 connected, or Dyn11 connected with small grounding resistance, the DC current caused by the fault will pass the transformer and lead to DC magnetic bias, which would further produce the magnetizing inrush current and the malfunction of differential current protection of transformers. In this paper, with respect to YNd11 connected transformers and Dyn11 connected transformers with different grounding resistance, the impact of pole-to-ground faults on the DC magnetic bias of transformers was analyzed. Influencing factors were subsequently concluded. Secondly, as an example of hybrid AC/DC systems, the DC-type PV grid-connection system model was established in PSCAD/EMTDC to verify the theoretical analysis. Finally, countermeasures, as well as the identification of single-pole fault and single-phase fault were given after analyzing the protection failures caused by DC magnetic bias.

Cheng Zhang

Papers

A Pilot Protection for HVDC Transmission Lines Based on Transient Energy Ratio of DC Filter Link

State-space analysis based pole-to-ground line fault isolation strategy for LCC–HVDC systems

Protection method for HVDC transmission line

Single-pole fault isolation method of high-voltage direct-current transmission line

Impact of Pole-to-ground Faults on Transformer DC Magnetic Bias and Differential Current Protection in Hybrid AC/DC Power Grids