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Full-order thermal models are often used by machine designers to analyze the induction machine thermal behavior. The real-time implementation of such models for the purpose of motor thermal protection is difficult because an extensive knowledge on motors and sufficient computational resource are required. This paper demonstrates that full-order thermal models can be systematically reduced via pole-zero cancellation or Hankel singular-values-based model reduction techniques without additional physical assumptions. As a result, a system of a substantially lower dimension, which has nearly the same response characteristics in the frequency band of interest, is obtained. Both the estimated rotor cage temperature, which is extracted from the voltage and current measurements by a sensorless rotor temperature estimator, and the measured stator winding temperature are used to evaluate the performance of low-order thermal models. Given a certain tolerance for the modeling error, a reduced low-order thermal model can be used to characterize the thermal dynamics of a small-to medium-sized line-connected induction machine and to provide proper protection against motor overheating. Index Terms--Hankel singular values (HSVs), induction machine, model reduction, motor thermal protection, pole-zero cancellation, rotor slot harmonics, temperature estimation, thermal model.

A traditional distance protection relay uses a lumped parameter model of the transmission line. For a long 1000-kV UHV line with large distributed capacitance, such a simplified line model is inadequate and can cause malfunction of the relay due to large errors in the impedance measurement. This paper presents a new fine model for a distance relay and the procedure of reliable fault impedance measurement and relay setting. It describes the proposed application of the relay developed to a 645-km 1000-kV UHV transmission line including shunt reactor compensation, which is currently being constructed in China. Tests are carried out on an RTDS (real time dynamic system) to evaluate the relay performance, showing significant improvement compared to a more traditional distance relay. The new distance relay is shown to satisfy the requirement of 1000-kV UHV transmission line protection. Index Terms--Distance relay, protection, transmission line equations, 1000-kV UHV transmission line.

The purpose of this paper is to describe several different methods of protection of tuned capacitor banks and how the relay protection settings for each are determined. Problems with the different schemes are described, and a new protective scheme is proposed. Index Terms--Capacitor, harmonics, protective relays.

The study of power system of relays requires some previous experience in this field. Laboratories focusing on teaching and researching the area of power-system protection have therefore been amply reported. However, these facilities require the actual presence of the students in the laboratories. To overcome this problem, a power-system-relaying remote laboratory has been developed. To implement this laboratory, a testing system of the relay operating characteristic, together with Matlab-based software, was developed. This remote laboratory enables real power-system-relaying experiments through the Internet. In fact, the user can remotely test a real relay disoperation. This system will therefore allow proficient analysis of sensitivities to relay settings and network configurations. Index Terms--Distance learning, remote laboratory, testing system of relay operating characteristic.

Defense systems are needed to prevent catastrophic failures of a power grid due to cascaded events. Cascaded events can be attributed to improper operations of protective relays. The most challenging problem for the design and implementation of a defense system is the performance in accuracy and speed in a real-time environment. Protective devices are normally designed to operate fast in order to isolate the fault(s). This paper proposes a new methodology to distinguish line overloads from actual faults for distance relays. In order to distinguish between line flow transfers from a line outage and an actual fault, the line outage distribution factor (LODF) and generation shift factor (GSF)-based power flow estimation method, and a secure peer-to-peer (P2P) communication structure are adopted. Computer simulations of cascaded events for a six-bus system and the Korean power grid have been performed to establish the feasibility of the proposed scheme. Index Terms--Blackouts, cascaded events, defense system, distance relay, zone 3 relay.

From a direct three-phase circuit analysis, an accurate fault-location algorithm has been obtained for the line-to-line fault as an extension of the author's previous work for line-to-ground fault location. Robustness of the proposed algorithm to load impedance uncertainty is enhanced by the introduction of impedance compensation using voltage and current measurements. Simulation results show a high degree of accuracy and robustness to load uncertainty. Index Terms--Distribution automation, fault location, protective relaying, short-circuit analysis.

Power transformer differential protection has been used for decades on standard three-phase power transformers. However, special industrial transformers, such as 24-pulse converter transformers, could not typically be protected easily with the standard power transformer differential relays. The main reason is the nonstandard phase angle shift of 24-pulse converter transformers. Such 24-pulse converter transformers are often used in industrial and railway applications. This paper will show that it is possible to provide differential protection for such special transformers by using standard numerical transformer differential protection relays and external interposing current transformers (CTs). However, the external interposing CTs can be designed in a standardized way. This approach will significantly simplify the application of differential protection for special industrial transformers. Index Terms--Converter transformers, power transformers, power transformer protection, protective relaying.

This paper presents a comparative study of the performance of distance relays for transmission lines compensated by shunt connected flexible ac transmission system (FACTS) controllers/devices. The objective of this study is to evaluate the performance of various distance protection schemes on transmission lines with shunt-FACTS devices applied for midpoint voltage control. The impact of two types of shunt FACTS devices, static var compensator (SVC) and static synchronous compensator (STATCOM) on the transmission line distance protection schemes is studied for different fault types, fault locations and system conditions. The power system elements and the shunt-FACTS devices with their associated controllers are modeled using RSCAD/RTDS software. The results are based on the performance of commercial distance relays using a real time digital simulator (RTDS). Index Terms--Distance channel aided schemes, distance protection relay, flexible ac transmission system (FACTS), power system protection, real time digital simulator, shunt compensation, static synchronous compensator (STATCOM), static var compensator (SVC).

When a remotely sited wind farm is connected to the utility power system through a distribution line, the overcurrent relay at the common coupling point needs a directional feature. This paper presents a method for estimating the direction of fault in such radial distribution systems using phase change in current. The difference in phase angle between the positive-sequence component of the current during fault and prefault conditions has been found to be a good indicator of the fault direction in a three-phase system. A rule base formed for the purpose decides the location of fault with respect to the relay in a distribution system. Such a strategy reduces the cost of the voltage sensor and/or connection for a protection scheme which is of relevance in emerging distributed-generation systems. The algorithm has been tested through simulation for different radial distribution systems. Index Terms--Digital relay, directional relay, distributed generation, feeder protection, overcurrent relay, phasor estimation.

This paper presents a new logic of operation for time overcurrent relays. With the application of the proposed technique, the sensitivity of time overcurrent relays is increased. In order to obtain this, the relay operates with a dynamic pickup setting, such as load current. Additionally, fast operation of overcurrent relays is obtained. The functional structure is described and the logic of overcurrent relay is evaluated under different operative states. We describe the functional structure of the proposed relay and its implementation in laboratory tests. Index Terms--Overcurrent relay, pickup current, sensitivity.

A high voltage thyristor valve is the basic component of an HVDC transmission system. Development of a 500kV valve for next generation HVDC transmission systems is described. First, the power loss of the valve is analyzed to decide a reasonable wafer size for the light triggered thyristor. From these results, a six inch diameter wafer size is selected. The light triggered thyristor, with ratings of 8kV and 3.5kA, is developed using the six inch wafer. The designing of the valve employing the thyristor and test results with the prototype valve prove that a 500kV valve can be realized by the design method.

This paper describes testing of the new numerical relaying concept presented in reference [1]. The new technique is applied to distance relaying to offer improved reliability and security of the relay, and improve the speed of relay operation. To validate the algorithm's performance, the scheme was analyzed under a variety of test conditions and compared with conventional relay concepts. This paper presents the results of the testing and shows that the new relaying scheme operated with a high level of security and dependability for all of the test cases. The tests show improvements that the new scheme can offer in comparison to the conventional schemes.

Recently, several papers have discussed the protection of parallel transmission lines connected to common buses. The main purpose of this paper is to discuss problems associated with parallel line relaying schemes, which depend upon the theory of transverse differential protection relaying, and to present a new advanced technique to overcome these problems. The suggested technique relies on current incremental signals in corresponding phases instead of current magnitude. This method eliminates many of the associated problems typically encountered in this area. The possibility of applying the suggested technique on parallel three-terminal lines was investigated and satisfactory results were recorded. The voltage signal and the communication channel have been excluded from this study.

A new distance relay is developed in this paper. The relay is based on two operation criteria - a current criterion and a voltage criterion. Each uses the fault component as operating quantity, and the relay reacts only on the reactance of the transmission line. So, it is a fault component reactance relay. During external fault, two criteria are complementary each other, i.e. at least one of them will not be satisfied. So no maloperation will happen. During internal fault both criteria will be satisfied even if the fault resistance is high. KEYWORDS: Distance relay, fault component, reactance relay

This paper presents the design, simulation, implementation and performance evaluation of a computationally efficient and accurate digital distance relaying algorithm. Published historical data were used in the first phase for validation purpose. Sample results illustrating highly accurate fault impedance estimates for various conditions are reported. The second phase uses voltage and current signals generated by Alternative Transients Program (ATP) and a sample power system for various first-zone, second-zone and third-zone faults. Results of these studies confirming stability and computational efficiency of the algorithm are presented and discussed. In the third phase, a prototype of the relay was developed and tested using real-time fault data generated from physical models of the transmission lines. Oscillographs for these conditions were recorded. Results of these tests indicating high speed relay operation are also discussed. The performance evaluation studies reported in this paper conclusively demonstrate that the new algorithm provides fast and accurate fault impedance estimates for the three-zone protection of transmission lines. Key words: performance evaluation, digital distance relay

This paper presents a digital relay scheme for the primary protection of power transformers. A multi-criteria algorithm is developed based on fuzzy sets for the decision making part of the scheme. The cost of wrong decision-making and the amount of information inflow are used along with several standard criteria to improve the reliability of the protection. It is shown that the proposed scheme, if properly tuned, can enhance the sensitivity and selectivity of the digital relay and mitigate problems associated with conventional relay schemes. Key words: Digital Transformer Protection, Decision Making, Fuzzy Sets.

To facilitate protection system studies on the space station electric power system, there is a need to develop a model that can accurately and conveniently simulate both the power system and the protection system. Models for two major protective devices, the remote bus isolater and the remote power controller, are described. These models have been installed in a power system model resembling one channel of the space station power system. The usefulness of these models in protection system studies is demonstrated.

The design of a new microprocessor based multifunctional relay for use on interconnected 33 kV circuits is described. The relay uses sequence component principles and rented low grade VF communication. It was needed urgently because rented metallic communication circuits, which are in widespread use, are being withdrawn in the UK. The relay development is unusual because of its measurement methods and because it can derive polarising voltage from existing 11 kV VTs. The time scales required were such that development, field trials and installation phases overlapped. The field experience is described.

Use of graph theory simplifies representation of complex bus arrangements in power system stations. This paper presents a new method, based upon graph theory, for selecting bus protection zones in microprocessor-based relays. We use a typical bus arrangement to illustrate the graphical representation of station arrangements, graph operations, and associated matrix operations. We also describe an implementation of the zone selection method and use two examples to demonstrate the advantages of the method. Using the status of switching devices in the station, the zone selection method provides the relay with real-time bus arrangement information. The bus relay uses this information to assign input currents to a differential protection zone and to select which breakers to trip for a bus fault or breaker failure. Index Terms--Busbars, graph theory, microprocessor-based bus relays, protection zone selection, protective relaying.