Real time evaluation of wavelet transform for fast and efficient HVDC grid non-unit protection

This paper presents a real-time evaluation of a Wavelet Transform (WT) for HVDC grid non-unit protection. Due to its time and frequency localisation capability, WT can successfully extract the necessary information present in the voltage transients following a DC fault. This capability is exploited to achieve fast and selective HVDC grid protection. A Digital Signal Processor (DSP) is employed to execute real-time Stationary Wavelet Transform (SWT) on voltage signals using discrete convolution to efficiently compute the WT coefficients. Hardware-in-the loop (HIL) simulation is performed to test a WT-based hardware module using a Digital Real-Time Simulator (DRTS), in which a meshed HVDC grid is modelled. The closed-loop interaction enables the hardware device to emulate a protection relay that can generate trip commands for the HVDC breakers integrated within the HVDC grid model. The real-time simulations demonstrate the technical feasibility, speed and robust performance of the SWT implementation

A novel fault let-through energy based fault location for LVDC distribution networks

Low Voltage Direct Current (LVDC) distribution systems have recently been considered as an alternative approach to electrical system infrastructure as they provide the additional flexibility and controllability required to facilitate the integration of more low carbon technologies (LCTs). However, DC protection systems and, more specifically high accuracy DC fault location, have been recognised as a key challenge to facilitating post-fault network maintenance. Most of the existing fault location techniques rely on current derivative or communications-based methods that are either very sensitive to noise, or require a high level of data synchronisation. Fault energy has been recognized as a reliable indicator of more accurate fault location estimations. Therefore, this paper develops a mathematical model for describing fault energy during the transient period of DC faults. The method subsequently proposes a new fault let-through energy based DC fault location working strategy to facilitate post-fault network maintenance. The proposed method does not require data synchronisation regardless of the voltage, current, and the size of the converters connected to the LVDC feeder. The capabilities of the proposed fault location strategy are validated against different faults applied on an LVDC test network in PSCAD/EMTDC and shown to be more reliable and accurate than existing methods

Fault location in DC microgrids based on a multiple capacitive earthing scheme

This paper presents a new method for locating faults along feeders in a DC microgrid using a multiple capacitive earthing scheme. During fault conditions, capacitors within the earthing scheme are charging by transient currents that correlate to the fault distance and resistance. Therefore, by assessing the response of the capacitive earthing scheme during the fault, the distance to fault is estimated. The proposed method uti- lizes instantaneous current and voltage measurements (obtained from the feeder terminals and earthing capacitors) applied to an analytical mathematical model of the faulted feeder. The proposed method has been found to accurately estimate the fault position along the faulted feeder and systematic evaluation has been carried out to further scrutinize its performance under different loading scenarios and highly-resistive faults. Addition- ally, the performance and practical feasibility of the proposed method has been experimentally validated by developing a low- voltage laboratory prototype and testing it under a series of test conditions

Review and evaluation of the state of the art of DC fault detection for HVDC grids

This paper reviews the state of the art of DC fault discrimination and detection methods of HVDC grids, and summarises the underlying principles and the characteristics of each method. To minimize HVDC grid disturbance and power transfer interruption due to DC faults, it is critically important to have protection schemes that can detect, discriminate and isolate DC faults at high speeds with full selectivity. On this basis, this paper lists the advantages and disadvantages of the most promising fault detection methods, with the aim of articulating the future directions of HVDC protection systems. From the qualitative comparison of relative merits, the initial recommendations on HVDC grid protection are presented. Moreover, a comprehensive quantitative assessments of different fault detection methods discussed above are carried out on a generic 4-terminal meshed HVDC grid, which is modelled in PSCAD environment. The presented simulation results identify that the voltage derivative and wavelet transform are the most promising methods for DC fault detection and discrimination

Investigation of the impact of interoperability of voltage source converters on HVDC grid fault behaviour

Future HVDC grids are expected to incorporate different power converters from multiple vendors in the same system. Even if a complete converter station is developed by a single manufacturer, it might be challenging to integrate this terminal into a DC grid that comprises of several converter stations built by other vendors. Moreover, the different fault response exhibited by each converter technology complicates the design of HVDC protection systems. Therefore, this study investigates the fault response of a multivendor HVDC grid. An illustrative 4-terminal meshed HVDC grid, which is modelled in PSCAD environment, is used to perform studies of interoperability of different converter topologies on a common HVDC network. The investigation of the fault behaviour of such a multivendor HVDC network highlights the main impediments that need to be tackled and a set of actions that needs to be done at a converter level in order to mitigate the impact of DC faults on the HVDC system. Moreover, the key parameters that need to be taken into account when designing a protection scheme for a multivendor HVDC grid are identified

Frequency domain analysis of HVDC grid non-unit protection

Owing to the fact that travelling wave propagation in an HVDC grid is a heavily frequency dependent phenomenon, frequency domain analysis has been identified as a useful means of assessing the capabilities and limitations of non-unit protection. This exploits the fact that the main difference between an internal and external fault is the presence of the series inductor in the fault path. The inductor acts as a high impedance element for high frequency components and consequently the transient voltage frequency response in each fault case is recognisably different. On this basis, the investigation in the frequency domain can reveal significant information for protection design purposes. Towards this aim, the transient voltage at the relay location is meticulously represented in the frequency domain by taking into account the transmission medium length and geometry, the number of other attached feeders to the same bus, the converter parameters, the inductive termination, the fault resistance, and the travelling wave behaviour of DC faults. By performing a sensitivity analysis on these parameters, a deeper understanding of their impact on HVDC non-unit protection is obtained. In addition, the factors that can be adjusted to extend the reach of the protection systems are revealed and a generic approach for analytically calculating the protection threshold is developed

DC fault management strategy for continuous operation of HVDC grids based on customized hybrid MMC

Successful deployment of High-Voltage Direct Current (HVDC) grids necessitates effective DC fault handling strategies, which can minimize the severe consequences caused by DC faults on the AC and DC side of the HVDC grids. Therefore, this paper investigates the enhanced DC fault performance of the Customized Hybrid Modular Multilevel Converter (CH-MMC), in which a limited number of full-bridge sub-modules (FB-SMs) is added into the arms of the conventional MMC in an effort to significantly extend the timespan between fault inception and fault clearance, thus allowing the use of relatively slow and cheaper DC circuit breakers. Based on this converter, a dedicated DC fault handling strategy for CH-MMC based HVDC grids is proposed, which aims to improve the fault resiliency and security of HVDC grids for pole-to-pole faults. Moreover, the proposed DC fault management strategy guarantees the continuous operation of the grid during pole-to-ground DC faults, including full reactive power provision from the converter stations. The performance of the strategy is demonstrated using comprehensive electromagnetic transient (EMT) simulation studies conducted on an illustrative four-terminal meshed HVDC grid, which consider a range of scenarios with different fault current limiting inductors and DC circuit breaker operation times

Review of DC series arc fault testing methods and capability assessment of test platforms for more-electric aircraft

In the new era of increasingly electric aircraft, the need for reliable and safe electrical systems is more important than ever. In addition, the wide scale adoption of DC distribution is considered a key enabling technology for more effcient aircraft operation. In this context, arc fault detection devices have become a topic of interest for the aviation industry with ongoing research to characterize the impact and adequately protect against severe DC series arc faults. Although DC arc faults have been widely investigated for utility applications (such as solar photo-voltaic systems), direct adoption of current practices for validating arc detection devices is not straightforward due to the distinct aircraft operating environment. This paper provides a first of its kind, landscaping exercise of published series arc fault testing based on factors associated with aircraft applications which have the potential to influence the arc characteristics. In addition, an appraisal and associated gap analysis of published arc test platforms is undertaken in order to assess their suitability to support in-depth testing of the impact and mitigation of series arcs within future aircraft DC electrical systems and identify future testing needs in particular to better facilitate a comprehensive performance validation of new arc fault detection devices

Voltage and current measuring technologies for high voltage direct current supergrids : a technology review identifying the options for protection, fault location and automation applications

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