Fault location methods are crucial for reducing fault restoration time, and thus improving a network's system average interruption duration index (SAIDI) and customer outage cost. Resonant-earthed systems pose problems for traditional fault location methods, leading to poor accuracy and a need for additional complexity. In this context, methods that detect fault direction (fault-passage indicators, FPI) at multiple points in the network may show advantages over a central distance-estimation method using fault locators (FL) of poor accuracy. This paper includes a comparative study of these two major fault location methods, comparing the reliability benefit from a varied number of FPIs or a central method. The optimal placement of the fault locating devices is found by formulating a mixed-integer linear programming (MILP) optimization approach that minimizes both outage and investment costs and assesses SAIDI. This approach has been tested on an example distribution system. However, to justify the universality of the algorithm, the RBTS reliability test system has also been analysed. The comparison of location methods and placement method of FPIs are useful for reliability centred planning of resonant-earthed distribution systems where fault location is to be used. Results show that a small number of FPIs that give accurate identification of direction may give more cost effective increase in reliability than a distance estimate by FL with typical levels of inaccuracy.

We propose a method for the fault passage indication of earth faults in resonant-earthed networks, based on phase current measurements alone. This is particularly relevant for electricity distribution systems at medium-voltage levels. The method is based on the relative magnitudes of the phasor changes in the phase currents due to the fault. It is tested for various network types and operation configurations by simulating the network in pscad and using the simulated currents as the input for an implementation of the method in matlab. In over-compensated networks, the method shows reliable detection of the fault passage, with good selectivity and sensitivity for both homogeneous and mixed (cable and overhead line) feeders. However, for the less common under-compensated systems, it has limitations that are described further in this study. The method has good potential for being cost effective since it requires only current measurements, from a single location, at a moderate sampling rate.

A major challenge in electric distribution systems is to increase the reliability of the electricity supply in an economical way. At the final medium-voltage level in the system, such as 10 kV or 20 kV, a single feeder may supply tens of medium/low-voltage transformers and hundreds of customers, with many branches of lines and cables where faults can occur. Resonant earthing is often used for these networks in Sweden, enabling earth-fault currents to be kept at low levels such as 10 A. This brings some advantages with safety and with avoidance of supply disruption during transient faults. However, the low fault currents, much less than typical load currents, make it hard to locate faults. In some countries, this type of system could be operated while an earth fault is traced, or a lower-impedance could be switched into the system earthing to increase the fault current to a clearer level. In Sweden, the legal safety requirements in most types of medium voltage networks require prompt disconnection of earth faults, which means that a single earth fault anywhere in an extensive medium voltage network can cause an outage for all customers on its feeder or section until it is located and repaired. Several approaches have been taken by network companies to reduce these outage times and thereby increase supply reliability.  

The focus of this thesis is to develop methods for determining where a fault is, in order to speed up both the repair process and any switching operations that can restore supply to customers outside the faulty section. Such methods include true fault location (FL), which gives an estimated distance to a fault, and fault-passage indication (FPI), which tells whether a fault is detected downstream of a particular point. A comparison of FL and FPI is made, showing that classic central FL needs an accuracy beyond what is currently available, if it is to give a better improvement in supply reliability than a small number of well-placed FPI units. Three algorithms suitable for FPIs are proposed in this thesis that use only current measurements from different parts of the network to identify the faulty section. One of the methods is based on the information from the phase angles of the healthy phases relative to the faulty phase. It shows good results for under-compensated and high load conditions but struggles for feeders with high charging currents. The next method uses incremental phase currents and shows promising results for over-compensated cases but unreliable results for under-compensated cases. The third method uses the zero sequence current magnitude and the location information of the measurements. It shows good results for the homogeneous feeders in both over- or under-compensated cases. A method for estimating the fault distance using multiple measurements from the network is also presented and tested. Its main trouble is its sensitivity to the fault resistance.

Cross-Country faults (CCFs) are characterized by the situation when two phase-to-earth faults are simultaneously active on different phases at different locations in a network. Specially for resonant-earthed systems the current through the earth during a CCF becomes many times higher than during a single phase-to-earth fault. So far, few studies have been carried about these faults on resonant-earthed networks, specially evaluating the performance of distance protection. In this paper, simulations in RSCAD/RTDS® using real data obtained from a resonant-earthed network in Scandinavia are performed and different effects on distance protection are simulated. Four types of CCFs showing different patterns are defined and explained. Phase-to-phase loops of distance protection proved to be quite ineffective to protect against CCF faults since the fault outside the protected line/cable increases the impedance path. Phase-to-earth loops are accurate for low-resistance faults in a conductor with single infeed (Types I and II). However, when the line/cable is fed from both ends, some challenges can appear (Types III and IV). For Type III, the non-faulted Ph-E loop can be measured inside the protection zone due to the high residual current while for Type IV Ph-E loops will have problems to operate at all due to the lack of residual current.

Cross-Country Faults (CCFs) are defined by the occurrence of two Single Phase-to-Ground faults taking place simultaneously in different phases and at different locations of the galvanically connected network. Few studies about these faults in MV systems have been done so far, particularly with real fault data and simulations. In this work, first a mathematical model is derived to understand basic properties of CCFs. Then, simulations in RSCAD/RTDS (R) using real data obtained from an utility in Scandinavia are discussed and validated with two real faults measured in the field for resonant-grounded networks in Sweden and Norway. The mathematical calculations proved to have a good accuracy and showed important properties of CCFs such as the dependency of both faults of each others fault resistance and location. Furthermore, it was observed that such faults can be very different from more common types of faults in the power system. Interesting behaviors can appear particularly when feeders are connected in ring, where an extra current with smaller magnitude and 180 degrees appears on the measurement point, as well as in lines with double infeed where a very large difference is detected depending on the fault location which influences directly both ends of the line.

This paper proposes a novel approach to locate earth-faults in resonant-earthed distribution systems. It uses the fundamental-frequency current measurements to determine the direction of the fault current and thereby to locate the faulted section. It sets the current-angle of the faulty phase as the reference for measuring the angles of the remaining two phase-currents. These three phasor quantities are then processed to determine the direction of the fault from the measurement point. The proposed method requires an adequate resistive current from the neutral for successfully determining the faulted section. The validity of the method has been tested by PSCAD simulations for a small-scale overhead distribution system.

Reliability indices of a distribution system can be improved by reducing failure rate and restoration time. A resonant-earthed distribution system has a low failure rate because numerous transient faults become self-extinguishing. However, in such networks, it can be difficult and time-consuming to locate nontransient faults resulting in aggravating the restoration time. This paper analyzes how different fault location methods affect the restoration time and SAIDI. Two major fault location methods are modeled for the calculation of the reliability indices and then applied to a radial feeder of a medium-voltage distribution system. The results show that SAIDI varies depending on the applied fault location method and its accuracy. The influence of fault location methods on labour costs is also discussed.

Series capacitors are used in transmission lines for enhancing power transmission limit. However, they complicate the line’s protection due to impedance change of the line, voltage inversion, current inversion and sub-synchronous oscillation. Distance and differential protections are used in different arrangements in transmission line protection. Often they are used together as main and backup protection. In this paper, the fault tree method is used to compare the reliability of three common transmission line protection schemes. The schemes considered here are distance (main)-distance (backup)(Z; Z), differential (main)-distance (backup) (delta;Z) and differential (main)-differential (backup) (delta;delta). Fault trees are used to calculate the reliability of protection schemes in terms of both unavailability and failure rate. The analyses show that, for series compensated lines, using distance protection reduces protection system reliability. Differential protection performs best in terms of reliability despite depending entirely on communication.

Line differential protection is popular for its good selectivity and simplicity as long as there is a dependable communication system between the two ends of the line. However, the sensitivity needs to be compromised when traditional line differential scheme is applied for UHV-AC lines because of the large charging current. This paper presents a study of the impact of UHV transmission line characteristics on line differential protection and a proposed solution based on compensation of the phase shift that exists between the sending and receiving end currents.

Secondary frequency control plays a vital role inpower systems and has therefore been the focus of muchresearch. Recent focus is being targeted towards developingdistributed solutions. This paper proposes a fast converging,distributed solution for secondary frequency control on the basisof the Alternating Direction Method of Multipliers (ADMM).For economic benefits the proposed solution integrates the useof rapid start units. Rapid start units are fast response and highramp offline units, which can provide the reserve power in caseof need. The proposed control scheme is a distributed solution tocombine secondary frequency control, economic dispatch and theRS start up process. Finally, the developed algorithm is tested fora power system model on a real-time co-simulation platform. Theresults show a fast converging algorithm that provides secondaryfrequency control compliant with ENTSO-E requirements, andthe economical benefits of the inclusion of a rapid start unit.

Fault location methods are crucial for reducing fault restoration time, and thus improving a network's system average interruption duration index (SAIDI) and customer outage cost. Resonant-earthed systems pose problems for traditional fault location methods, leading to poor accuracy and a need for additional complexity. In this context, methods that detect fault direction (fault-passage indicators, FPI) at multiple points in the network may show advantages over a central distance-estimation method using fault locators (FL) of poor accuracy. This paper includes a comparative study of these two major fault location methods, comparing the reliability benefit from a varied number of FPIs or a central method. The optimal placement of the fault locating devices is found by formulating a mixed-integer linear programming (MILP) optimization approach that minimizes both outage and investment costs and assesses SAIDI. This approach has been tested on an example distribution system. However, to justify the universality of the algorithm, the RBTS reliability test system has also been analysed. The comparison of location methods and placement method of FPIs are useful for reliability centred planning of resonant-earthed distribution systems where fault location is to be used. Results show that a small number of FPIs that give accurate identification of direction may give more cost effective increase in reliability than a distance estimate by FL with typical levels of inaccuracy.

We propose a current-based method for fault passage indication of earth faults in resonant-earthed networks. This type of network is commonly found in electricity distribution systems at medium-voltage levels. The proposed method is based on the magnitude of the changes in the phase currents due to the fault, and can therefore be implemented using just current sensors. It is implemented in MATLAB and tested on data from simulations in PSCAD for various network types and operation configurations. In over-compensated networks the method shows reliable detection of the fault passage, with good selectivity and sensitivity for both homogeneous and mixed (cable and overhead line) feeders. However, for under-compensated systems it has limitations that are described further in this study. The method has good potential for being cost-effective since it requires only current measurements, from a single location, at a moderate sampling rate.

Fault location methods help to reduce the restoration time and thus improve the SAIDI (System average interruption duration index) of the network. Besides that, restoration time has a direct impact on customer outage cost. Traditional fault location methods struggle to perform adequately and need additional features for a resonant-earthed system. This paper assists in the reliability centred planning of such a system with the focus on fault location methods. Two major fault location methods are modelled for the study. The optimal placement of the fault locating devices is found by formulating a MILP optimization approach that minimizes both outage and investment cost and asses SAIDI. Moreover, a comparative study among the fault location methods is done to find the best case for an actual resonant-earthed distribution system.