New protection algorithms for HVDC grids

233 p.Los sistemas HVDC representan una alternativa prometedora para futuras expansiones del sistema eléctrico gracias a las ventajas que presentan en comparación con el transporte convencional en corriente alterna. Además, el interés por desarrollar redes HVDC multiterminales ha crecido en los últimos años, sin embargo, su implementación se ha visto ralentizada debido a la complejidad que presenta la protección ante faltas en estos sistemas. El objetivo principal de esta tesis es proponer un nuevo algoritmo de protección contra faltas, apropiado para dichas redes y capaz de superar las limitaciones presentes en algoritmos existentes. El algoritmo propuesto es un algoritmo de tensión de inductancia basado en el cálculo del ratio entre las medidas de tensión tomadas a ambos lados de la inductancia limitadora y la derivada de dicho ratio. Es capaz de detectar faltas rápidamente y de discriminar de manera selectiva entre faltas dentro y fuera de la zona de protección. También se propone una metodología para la selección del valor umbral necesario para la operación de algoritmos locales. A continuación, sedesarrolla un esquema de protección completo que se compone de protecciones de línea primaria y de respaldo, protección de barra y protección ante fallo del interruptor. Esta última protección es, así mismo, un nuevo algoritmo propuesto en la tesis, que presenta una operación más rápida que algoritmos convencionales de detección de fallo en el interruptor. Finalmente, la operación del esquema de protección propuesto es validada y analizada a través de simulaciones en un modelo de red de cuatro terminales con diferentes escenarios de falta, comparándolo con algoritmos existentes

Protection contre les courts-circuits des réseaux à courant continu de forte puissance

Dans le domaine du transport de l'électricité, les qualités intrinsèques des réseaux alternatifs s'estompent devant la difficulté imposée par le transport de la puissance réactive lorsque les lignes aériennes ou, plus particulièrement, les câbles souterrains ou sous-marins atteignent des longueurs critiques. Dans le cadre des réflexions visant à exploiter au mieux les énergies renouvelables d'origine éolienne off-shore ou hydrolienne, l'hypothèse de la création d'un réseau électrique à haute tension continue pour acheminer ces énergies jusqu'aux centres de consommation est considérée. Ce travail de thèse est en lien avec le projet européen TWENTIES (Transmission system operation with large penetration of Wind and other renewable Electricity sources in Networks by means of innovative Tools and Integrated Energy Solutions, ref 249812), financé dans le cadre du programme FP7 de la Commission Européenne. Ces travaux traitent de la protection des réseaux à courant continu contre les défauts d'isolement dans les câbles et au niveau des jeux de barre. L'étude se concentre sur des réseaux multi-terminaux bouclés et/ou maillés, et propose d'étudier la faisabilité d'un plan de protection comportant un algorithme principal et un secours en cas de défaillance d'un disjoncteur.In the area of power transmission grids, the inherent qualities of alternative current networks fade behind the difficulty imposed by the transmission of the reactive power when overhead lines or, particularly, underground or undersea cables reach critical lengths. As part of thought aimed for operate at best renewable energy resources, namely wind or marine resources, the assumption of the creation of a high voltage direct current power grid to dispatch those energies to the consumption centers is considered. This Ph.D work is linked to the European project TWENTIES (Transmission system operation with large penetration of Wind and other renewable Electricity sources in Networks by means of innovative Tools and Integrated Energy Solutions, ref 249812), funded as a part of the 7th framework program of the European Commission. This work deal with the protection of DC grids against insulation faults occurring in the cables or at a busbar. The study focusses on meshed and/or looped multi-terminal grids, and proposes to study the feasibility of a protection plan including a main protection algorithm and a backup in case of breaker failure.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Transient wavelet energy based protection scheme for inverter-dominated microgrid

When faults occur in the microgrids, high frequency transients will be superimposed on the system currents and voltages. The magnitude of those transients will attenuate as it encounters the discontinuity points in the network such as busbars, or any other impedance discontinuity points. This phenomenon can also be quantified by wavelet energy, which provides a useful tool to detect faults and locate the faulted feeder in the microgrid. In this paper, a novel protection scheme based on the transient wavelet energy of the superimposed current extracted by the Maximal Overlap Discrete Wavelet Transform (MODWT) algorithm is developed to detect faults and locate the faulted feeder in microgrids. Compared with existing protection schemes, the proposed protection scheme has the advantage of being largely immune to the changes in system fault level, fault types and positions, microgrid operating status and the control strategies deployed on the inverters, while presenting much lower requirement on the sampling frequency (10 kHz) compared with travelling wave-based methods. Unlike the conventional differential protection, the proposed scheme does not require synchronized measurement or high bandwidth communication channels, and thus, it can be considered as an economical and promising solution for microgrids

Challenges, advances and future directions in protection of hybrid AC/DC microgrids

Hybrid microgrids which consist of AC and DC subgrids interconnected by power electronic interfaces have attracted much attention in recent years. They not only can integrate the main benefits of both AC and DC configurations, but also can reduce the number of converters in connection of Distributed Generation (DG) sources, Energy Storage Systems (ESSs) and loads to AC or DC buses. In this paper, the structure of hybrid microgrids is discussed, and then a broad overview of the available protection devices and approaches for AC and DC subgrids is presented. After description, analysis and classification of the existing schemes, some research directions including communication infrastructures, combined control and protection schemes, and promising devices for the realisation of future hybrid AC/DC microgrids are pointed out

DC busbar protection for HVDC substations incorporating power restoration control based on dyadic sub-band tree structures

In this paper, a new direct current (dc) busbar protection for high voltage dc (HVdc) substations is proposed. The proposed scheme relies on the instantaneous current measurements obtained from the elements (lines and converters) connected to a dc busbar. Such current measurements are analyzed through dyadic sub-band tree structures that are used to extract the specific features, such as polarity, wavelet energy, and wavelet energy ratios. The performance of the scheme is assessed through the transient simulation using the verified PSCAD models. The simulation results revealed that the scheme can: 1) discriminate, effectively and within a very short period of time, between the internal and external faults; 2) detect pole-to-pole and pole-to-ground faults (both solid and highly resistive); 3) switch to healthy busbars (if available) to allow continuous operation; 4) re-energize the converter and restore the power to pre-fault conditions; and 5) remain stable during disturbances and external faults

A microprocessor-based system for protecting busbars

Advancements in digital technology have led to the development of microprocessor-based relays. However, most of these relays use algorithms similar in principle to their electromechanical counterparts. Also, busbar protection using microprocessor-based relays has not received adequate attention unlike other power system components. Few algorithms proposed for protecting busbars lack inherent immunity to current transformer (ct) saturation. They achieve stability by using additional measures, such as, using special circuitry, multiple algorithms and changing the restraint factor, which are not likely to be effective during severe ct saturation. The impact of ct ratio-mismatch is countered by using percentage-bias characteristics that reduces the sensitivity of the relay. This thesis presents a new technique for protecting busbars. The technique uses positive-sequence and negative-sequence models of the power system in a fault-detection algorithm. While phase voltages and currents are used to detect faults, parameters of the power system are not used. Only the arguments of the positive-sequence and negative-sequence impedances computed by the relay are used to make trip decisions. The performance of the technique was investigated for a variety of operating conditions and for several busbar configurations. Data generated by empty simulations of model power systems were used in the investigations. The results verify that the proposed technique is able to distinguish faults in a busbar protection zone from those outside the zone correctly. Additionally, its stability during ct saturation, immunity to ct ratio-mismatch and applicability, without any modifications, to busbars of different configurations have been established. An analysis of the performance of the proposed technique during ct saturation and ratio-mismatch conditions is presented. The effect of various parameters, such as, presence of d.c. offset in the currents, mild and severe saturation of the cts, different sampling frequencies and the impact of the size of data-windows on the estimates of the current phasors have been included. The analysis indicates that the technique is stable during ct saturation and inherently immune to ct ratio-mismatch. The proposed technique was implemented using a general purpose relay hardware. The hardware and software constituents of the prototype, the procedure for testing these relays by using a playback simulator and selected test results are presented in this thesis

Protection of physically compact multiterminal DC power systems

The use of DC for primary power distribution has the potential to bring significant design, cost and efficiency benefits to microgrid, shipboard and aircraft applications. The integration of active converter technologies within these networks is a key enabler for these benefits to be realised, however their influence on an electrical network's fault response can lead to exceptionally demanding protection requirements. This represents a significant barrier to more widespread adoption of DC power distribution. The principle challenge within the field is to develop protection solutions which do not significantly detract from the advantages which DC networks offer. This objective leads the thesis to not only consider how the protection challenges may be overcome but also how this can be achieved in a manner which can benefit the overall design of a system, inclusive of various system design objectives. The thesis proposes that this objective can be achieved through the operation of network protection within the initial transient period following the occurrence of a fault. In seeking to achieve this aim, the work presented within this thesis makes a number of contributions. The thesis categorises converter type based on the components which influence their fault response and then presents an analysis of the natural fault response of compact multiterminal DC power distribution networks containing these converters. Key factors such as the peak magnitudes and formation times of fault current profiles are determined and quantified as a function of network parameters, enabling protection system operating requirements to be established. Secondary fault effects such as voltage transients are also identified and quantified to illustrate the impact of suboptimal protection system operation. The capabilities of different protection methods and technologies for achieving the proposed operating requirements are then analysed. Significant conclusions are: solid state breaking technologies are essential to achieving operating targets and severe limitations exist with the application of protection methods available within literature for this application. To overcome these shortfalls, novel fault detection approaches are proposed and analysed. These approaches enable fault detection time targets to be met as well as aid with the effective integration of future circuit breaking technologies

A new approach to the fault location problem: using the fault's transient intermediate frequency response

The fault location problem has been tackled mainly through impedance-based techniques, the travelling wave principle and more recently by machine learning algorithms. These techniques require both current and voltage measurements. In the case of impedance-based methods they can provide multiples solutions. In the case of the travelling wave approach it usually requires high sampling and synchronized frequency measurements together with sophisticated identification algorithms. Machine learning techniques require training data and re-tuning for different grid topologies. In this work we propose a new fault location method based on the fault's transient intermediate frequency response of the system immediately after a fault occurs. The transient response immediately after the occurrence of a fault is characterized by the travelling wave phenomenon together with intermediate frequencies of oscillation in the range of 5 to 500 kHz. These intermediate frequencies of oscillations are associated with the natural response of the cable/line system to the fault event. Their frequencies of oscillation are dependent on the faulted section and the fault location within that section. The proposed fault location methodology aims to leverage on that dependency, by firstly identifying these intermediate frequencies for different fault location scenarios for a given network. This process is performed offline using a linear time invariant (LTI) representation of the network. To compute this LTI representation, as part of this work an impedance representation in the modal domain is established for cable/line sections, which is able to capture the frequency-dependence and distributed nature of its electrical parameters. The offline methodology identifies these intermediate frequencies for different fault location scenarios, and then proceeds to fit the fault location dependence of each intermediate frequency using a polynomial regression. An online methodology is also proposed to perform the fault location in real time by solving the polynomial regressions computed during the offline methodology using measurements of the intermediate frequencies present in the frequency spectrum of transient signals. The fault location is thus solved by using voltage or current measurements of the fault’s transient response at different locations in the network, together with simple signal processing techniques such as the Fast Fourier Transform. The full method is tested with an EMT simulation in PSCAD, using the detailed frequency dependent model for underground cables, together with realistic load models in a low voltage distribution network test system.Open Acces

DC Line Protection for Multi-Terminal High Voltage DC (HVDC) Transmission Systems

The projected global energy shortage and concerns about greenhouse emissions have led to the significant developments in offshore wind farm projects around the globe. It is also envisaged that in the near future, a number of existing onshore converter stations and offshore stations will be interconnected to form a Multi-terminal (MT) HVDC systems, whereas protection issues remains a major challenge. This is largely due to the low inductance in DC network compared to AC interconnection which usually results in a sudden collapse in the DC voltage and rapid rise in the fault current thus reaching damaging levels in few milliseconds. Therefore faults in MT-HVDC system must be detected and cleared quickly before it reaches a damaging level; typically 4 – 6ms (including circuit breaker opening time) following the inception of the fault. For this reason, transient based protection techniques are ideal candidates if the protection scheme must be reliable and dependable. Transient based protection algorithms utilises the higher frequency components of the fault generated signal to detect a fault, therefore making it possible to detect the fault while the fault current is still rising and well before the steady state. The traditional protection algorithms developed for conventional high voltage AC (HVAC) systems such as distance protection are steady state based and as such not suitable for the protection of MT-HVDC systems. Another major issue is selectivity as only the faulty section must be isolated in the event of a fault. This constitutes a major challenge considering the anticipated lengths of the cables. Traditional protection techniques developed for two-terminal HVDC systems are also not suitable for MT-HVDC since it will de-energise the entire network and other sub-grids connected to the main network. DC line protection devices which will operate at a sufficient speed and which will isolate only the faulty section in the event of a fault are therefore required to avoid a total system failure during short circuit. It is anticipated that it will be achieved by the use of HVDC breakers, whereas the implementation and realisation of such circuit breakers still remain a major issue considering speed, complexity, losses and cost. However, two major vendors have proposed prototypes and hopefully these will be commercially available in the near future. The key issue still remains the development of a fast DC line fault detection algorithm; and it is on these premise that this research was undertaken. The work reported in this thesis is a novel time domain protection technique for application to HVDC grids. The protection principle developed utilises the “power” and “energy” accompanying the associated travelling wave following the occurrence of a fault to distinguish between internal and external fault. Generally, either the “power” or “energy” can provide full discrimination between internal and external faults. For an internal fault, the associated forward and backward travelling wave power; or the forward and backward wave energy must exceed a pre-determined setting otherwise the fault is regarded as external. This characteristic differences is largely due to the DC inductor located at the boundaries which provides attenuation for the high frequency transient resulting from an external fault, hence making the power and energy for an internal fault to be significantly larger than that for external fault. The ratio between the forward and backward travelling wave power; or between the forward and backward travelling wave energy provides directional discrimination. For a forward directional fault (FDF) with respect to a local relay, this ratio must be less than unity. However, the ratio is greater than unity for reverse directional faults (RDF). The resulting wave shape of the “travelling wave power” (TWP) components also led to the formulation of a novel protection algorithm utilising the wave shape concavity. For an internal fault, the second derivative of the resulting polynomial formed by the TWP must be negative, thereby indicating a “concave-upwards” parabola. However, for an external fault, the second derivative of the resulting polynomial formed by the TWP components must be positive indicating a “concave-downwards” parabola. The developed and proposed protection techniques and principles were validated against a full scale Modular Multi-level Converter (MMC) – based HVDC grid, and thereafter the protection algorithm was implemented in MATLAB. Wider cases of fault scenarios were considered including long distance remote internal fault and a 500Ω high resistance remote internal fault. In all cases, both the pole-pole (P-P) and pole-ground (P-G) faults were investigated. The simulation results presented shows the suitability of the protection technique as the discrimination between internal and external faults was made within 1ms following the application of the fault. Following this, the protection algorithm was implemented on both a low-cost experimental platform utilising an Arduino UNO ATmega328 Microcontroller and on a Compact RIO FPGA-based experimental platform utilising LAB-View. The experimental results obtained were consistent with those obtained by simulations. An advantage of the proposed technique is that it is non-unit based and as such no communication delays are incurred. Furthermore, as it is time domain - based, it does not require complex mathematical computation and burden / DSP techniques; hence can easily be implemented since it will require less hardware resources which ultimately will result in minimal cost