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Protective relays have been designed with different technologies resulting in electromechanical, solid-state, and numerical devices. Speed and reliability are the two most important characteristics of a protective relay. Other capabilities such as monitoring and recording are supposed to be of secondary priority. Without any doubt, present-day numerical relays provide considerable capabilities and advantages but they have not improved speed of operation in comparison to their solid-state counterparts. With advancements in analog technology which has become more mature recently, the idea of using mixed signals in several applications has started. The new technology using combination of field-programmable analog arrays (FPAAs) and digital technologies seems to be a breakthrough and has the potential to provide better performance than the present-day numerical relays. In this paper, use of FPAAs for designing and implementing protective relays is investigated for the first time in literature. A distance relay design and its hardware implementation using FPAAs is described. Test results are reported. Index Terms--Distance Relay, FPAA, mixed signal processing, power system protection, relay technology.

Cooperative transmissions have been shown to be able to greatly improve system performance by exploring the broadcasting nature of wireless channels and cooperation among users. We focus, in this paper, on leveraging cooperation for resource allocation among users such that the network performance can be improved. Two important questions are answered--who should help whom among the distributively located users, and how many resources the users should use for cooperation to improve the performance. To answer these questions, a power-optimization, subcarrier-allocation, and relay-selection problem is formulated over a multiuser orthogonal frequency-division multiplexing (OFDM) network, which is applicable to systems such as wireless local area networks (WLANs). In the multiuser OFDM network, cooperation among different users is conducted by assigning the subcarriers of the helping users to relay a certain part of the helped users' data, while maintaining the desired rates of both helping users and helped users by means of power control and rate adaption. This way, the bandwidth efficiency of the multiuser OFDM system with cooperation is the same as that of the noncooperative OFDM system. The formulated optimization problem is an assignment problem for subcarrier usage and corresponding bit loading as well as power control. We provide an approximate closed-form solution for a two-user two-subcarrier case. Then, a suboptimal heuristic algorithm for a multiple-user multiple-subcarrier case is proposed and implemented in the base station to solve the formulated NP-hard problem. From the simulation results, the proposed scheme achieves up to 50% overall power saving for the two-user system and 19%-54% overall power saving for the multiuser case with random locations, compared with the current multiuser OFDM system without cooperative diversity. The proposed scheme is also compared to a much more complicated orthogonal frequency-division multiple access (OFDMA) system. Index Terms--Combinatorial mathematics, cooperative systems, power control, relays, resource management.

In this paper, nonlinear stabilizers are introduced for voltage-controlled microelectromechanical system (MEMS) relays. The control constructions follow a Lyapunov approach and are based on a nonlinear dynamic model applicable to the two types of MEMS relays--electrostatic and electromagnetic. Two control schemes are presented with the objectives of avoiding pull-in during the microrelay closing and improving the transient response during the microrelay opening. First, an adaptive state feedback stabilizer is proposed to compensate for parametric uncertainty in all mechanical parameters and selected electrical parameters while ensuring asymptotic regulation of the electrode opening and closing. Next, a model-based observer/stabilizer is proposed to account for the lack of velocity measurements. Simulations demonstrate the performance of the two control schemes in comparison to the typical open-loop operation of the MEMS relay. [DOI: 10.1115/1.3023134]

A new strategy (denoted P-SS) for estimating the correct fault zone in distance relays is introduced. The main difference between the P-SS and conventional setting strategies is that the P-SS does not depend on fixed boundaries for backup zones. It depends on exchanging information between local relays in the same station in addition to a command from the remote end relay. With the proposed strategy, zone-1 covers the entire length of the protected line, zone-2 covers the entire length of any next line irrespective of its length, and zone-3 covers the entire length of any line next to zone-2. The governing rules for the proposed strategy are presented. The strategy is examined against a wide range of setting problems of a real part of a high-voltage network with real relay settings. The results confirm that the P-SS achieves the required accurate, sensitive, and selective relay operation. Index Terms--Distance relays, fault zones, pilot schemes, setting problems.

This paper presents a practical and effective novel approach to solve the coordination problem caused by the curve intersection of overcurrent relays in subtransmission systems. Based on the IEEE and IEC standard characteristic curve equations of digital overcurrent relays, as well as the simulation curve equation for electromagnetic overcurrent relays, lever setting adjustment equations for a selected point on the characteristic curve is derived. Then, centered on the derived equations, a dedicated software program is developed to compute the corresponding relay lever setting with different tap value in order that, passing through the selected point, another curve with the new lever setting is chosen. Finally, assisting with the graphical capability of the commercial ASPEN OneLiner, the proposed fixed-point coordination curve adjustment procedure can completely eliminate the coordination curve intersections. In addition, an actual case of a subtransmission network in Taiwan Power Company is simulated to validate the feasibility of the technique proposed. Index Terms--Overcurrent relay, protection coordination, subtransmission system, sympathy trip.

The problem of resource allocation to maximize the outage exponent over a fading relay channel using the decode-and-forward protocol with quantized channel state feedback (CSF) is studied. Three different scenarios are considered: relay-to-source, destination-to-relay, and destination-to-source-and-relay CSF. In the relay-to-source CSF scenario, it is found that using merely one bit of CSF to control the source transmit power is sufficient to achieve the multiantenna upper bound in a range of multiplexing gains. In the destination-to-relay CSF scenario, the systems slightly outperform dynamic decode-and-forward (DDF) at high multiplexing gains, even with only one bit of feedback. Finally, in the destination-to-source-and-relay CSF scenario, if the source-relay channel gain is unknown to the feedback quantizer at the destination, the diversity gain only grows linearly in the number of feedback levels, in sharp contrast to an exponential growth for multiantenna channels. In this last scenario, a simple scheme is shown to perform close to the corresponding upper bound. Index Terms--Cooperative communications, diversity methods, diversity-multiplexing tradeoff, fading channels, large-deviation analysis, power control, relay channels.

We consider a wireless cooperative relay system with one source, one relay, and one destination node. The average bit-error probability (BEP) of several coherent or differential relay schemes with binary modulation is derived for Nakagami-m fading channels and verified by computer simulation. Our analytical results provide a set of mathematical tools for the design and analysis of cooperative communication systems in general fading channels. Index Terms--Average bit-error probability (BEP), cooperative diversity, Nakagami fading, wireless relays.

This paper deals with a cross-layer approach for cooperative diversity networks, which use a combination of amplify-and-forward (AF) and decode-and-forward (DF) as a relaying strategy. Based on a well-selected ad hoc configuration, the proposed approach combines the cooperative diversity concept with a simultaneous optimization of physical, network, and multiple access control layers. The considered optimization problem requires an appropriate distribution of three roles among the network nodes, which are the diversity relays (AF concept), the intermediate router (DF and routing), and the destination (scheduling). The proposed role assignment is based on the instantaneous channel conditions between the links and jointly supports performance optimization and a long-term fairness concept. To minimize the required complexity, a partial and quantized channel feedback is also proposed. The proposed cross-layer solution is compared with conventional approaches by computer simulations and theoretical studies, and we show that it achieves an efficient performance--complexity tradeoff. Index Terms--Ad hoc networks, amplify-and-forward (AF), cooperative systems, cross-layer design, relay channels.

In this paper, we deal with an amplify-and-forward (AF) cooperative strategy in slot-based, block-fading environments. In contrast with previous schemes, which assume a constant channel during the cooperative frame (several slots), here, we relax this constraint and assume a classical quasi-static block-fading channel (constant for one slot). This additional degree of freedom modifies the behavior of the conventional nonorthogonal AF (NAF) schemes and generates a new block-fading NAF (BFNAF) protocol, where the source can usefully retransmit the same data during the cooperative slot. This new protocol is interesting at low spectral efficiencies, where diversity against fading is more important. To overcome performance degradation, which characterizes cooperative schemes at low signal-to-noise ratios (SNRs), an adaptive version of the considered schemes is also proposed, where cooperation is activated according to the outage behavior of the direct link. Moreover, an optimal hybrid protocol that allows intelligent switching between noncooperation, NAF, and BFNAF protocols is also proposed. Another issue that is discussed throughout this paper is the optimal power allocation of the investigated schemes. The proposed power-allocation strategy uses as an optimization criterion the well-defined asymptotic expressions of the outage probabilities, which are averaged over the fading statistics. The minimization (and avoidance for the nonhybrid protocols) of the required data feedback makes it suitable for practical ad hoc networks with crucial power constraints. The enhancement of the proposed schemes and the efficiency of the power allocation policy are shown by theoretical analysis and simulation results. Index Terms--Ad hoc networks, amplify-and-forward (AF), cooperative systems, power allocation, relay channels.

In this paper, a new relay algorithm is proposed for under-voltage load shedding (UVLS) based on rate of voltage recovery and predicted time to recover above a preset voltage threshold. Simulation experience in the Southern Electric System has shown that conventional UVLS relay logic is ill-suited to the task of mitigating delayed voltage recovery and fast voltage collapse situations while maintaining compliance with existing NERC Reliability Standards. The proposed scheme continuously monitors both voltage magnitude and rate-of-change of voltage magnitude and uses these values to determine a projected voltage recovery time following a contingency event. If the projected recovery time is such that resultant system conditions are unacceptable (e.g., major load loss, loss of generation, or voltage collapse), the relay will operate to shed load at the distribution feeder level. This unique scheme has been implemented in a commercially-available programmable relay and is presently installed and active in a number of substations in Georgia Power Company. Index Terms--Fast voltage collapse, power systems, power transmission planning, under-voltage load shedding, voltage stability.

This paper presents the design of undervoltage relay settings for an industrial plant with cogeneration units to prevent critical loads from tripping due to voltage sags. The critical loads that are sensitive to voltage sags in the plant have been identified from actual system fault contingencies. The power quality of these loads is enhanced by quickly islanding the subsystem with cogeneration units before tie line tripping with an external Taipower system. According to the transient stability analysis of the rest of the industrial power system near a Taipower network, the critical loads that are nonsensitive to voltage sags are disconnected from external system faults in time to restore the system stability with the execution of proper load shedding. To ensure the normal operation of critical and voltage-sensitive loads by considering the voltage tolerance curves, the proper relay settings for tie line tripping of the internal subsystem has been determined. This way, the critical loads and cogeneration units in the industrial power systems can be survived from system faults to mitigate the production loss of industrial customers. Index Terms--Cogeneration system, power quality, transient stability analysis, voltage tolerance curve.

In this paper, we investigate the outage performance of the full-duplex decode-and-forward relay system in a Rician fading environment. We derive an analytical expression for the outage probability of the highest achievable information rate of the system, which can be evaluated by employing numerical techniques. We obtain an upper bound and a lower bound on the outage probability. These bounds are given in simple closed forms and can be easily evaluated. Furthermore, we study the impact of transmit signaling on the outage performance. Index Terms--Decode-and-forward (DF), information rate, outage probability, relay channels, Rician fading.

This paper proposes a cooperation strategy among rational nodes in a wireless cooperative relaying network as an effort to solve two basic problems, i.e., when to cooperate and how to cooperate. First, a symmetric system model comprising two users and an access point (AP) is presented. In this model, each user plays an equal role and acts as a source as well as a potential relay and has the right to decide the amount of bandwidth it should contribute for cooperation. Second, referring to the cooperative game theory, the above problems are formulated as a two-person bargaining problem. Then, a cooperation bandwidth allocation strategy based on the Nash bargaining solution is proposed, in which if a derived condition is satisfied, users will cooperatively work, and each will share a certain fraction of its bandwidth for relaying; otherwise, they will independently work. Simulation results demonstrate that when cooperation takes place, users benefit from the proposed strategy in terms of utility, and those with longer distance to the AP should spend more bandwidth to cooperate with others. Index Terms--Cooperative diversity, cooperative relay, game theory, Nash bargaining solution (NBS).

An infrastructure relay system that transfers information from a base station to a randomly located mobile station (MS) using fixed-location relay stations (RSs) is studied. Transmission rates are derived for serial, parallel, and hybrid relay transmissions, considering the utilization of RSs as a means to extend the dimensionality of a multiple-input--multiple-output (MIMO) transmission. The derived transmission rate for the serial relay transmission is optimized by determining the optimal locations of the RSs and the number of hops. In addition, selection schemes for determining which RSs to activate to maximize the transmission rate are provided for parallel and hybrid relay topologies. Based on the analytic findings, numerical results are obtained for various deployment scenarios to compare and evaluate different relay topologies and design characteristics of a relay system. Index Terms--Capacity, cooperative, multiple-input--multiple-output (MIMO), relay.

This paper investigates in detail the impacts of distorted current waveforms, produced by certain types of fault current limiters on time-overcurrent protection relays. A thyristor-based solid-state fault current limiter is chosen as representative of such a device for a case study which investigates its effects on two coordinated protection relays. A detailed software model of the current limiter has been developed and implemented on the real-time digital simulator platform, modeling a typical distribution system. Relay models are used to obtain initial results, which are later validated by an actual protective relay connected in a hardware-in-the-loop simulation setup. The results illustrate the increase of relay tripping times due to severe current limitation caused by the fixed firing angle control of the current limiter. It is revealed that different current measurement principles employed by the relays, such as fundamental, peak, or true rms, can lead to miscoordination due to the distorted fault current waveform. It is demonstrated that these undesirable effects can be mitigated by employing appropriate control strategies for the firing angle in the current limiter. Index Terms--Distribution system, fault current limiter, grading margin, protection relay, real-time simulation.

In this paper, a new high-speed directional relay is proposed. The relay is based on the current and voltage signals before and after fault occurrence. The voltage signal is compensated and then used as a reference. The relay is used for the directional comparison protection of extremely high voltage transmission fines. An evaluation of the suggested directional relay is investigated using recorded fault data from the Alberta power transmission system. The real effects of the power system elements, which might have not been completely considered in the mathematical model of the power system, have been included in the recorded real fault data. In this case, the performance of the new directional relay can be verified in a more realistic environment than simulation. Index Terms--Directional relay, field data, reference polarized voltage.

Fault resistance is a critical variable in distance relaying. If not considered due to underreaching phenomenon, it may cause the misoperation of ground distance relays for internal faults. Still, as a consequence of the overreaching phenomenon, the unbalanced nature of loads and asymmetry of lines can affect the distance protection operation efficiency. Mainly due to these aspects, there is low precision in protection zone limits of ground distance relays. In this paper, a new algorithm is proposed to increase the precision of these limits, improving efficiency in the distance protection process. The proposed method is based in phase coordinates and uses a fault resistance estimate to develop the trip decision procedure. The results show that the algorithm is suitable for online applications, and that it has an independent performance from the fault resistance magnitude, the fault location, and the line asymmetry. Index Terms--Fault diagnosis, fault resistance, power system faults, power system protection, protective relaying.

Mutual coupling poses difficult problems witnessed during single phase to earth fault, in transmission line protection. In parallel lines on the same right of way, mutually coupled zero-sequence circuits cause error in the apparent impedance seen by the relay. This causes the distance relay at one end of the faulty line to over-reach while the one at the other end to under-reach. Incorrect sensing of the ground fault due to zero-sequence voltage inversion caused by mutually coupled zero-sequence network may lead to false trip of the neighbouring healthy line. The paper develops characteristic expressions for effective sequence impedances of twin circuit lines and presents a non-iterative microprocessor based algorithm for fault distance and under-reach estimation in real time, for under-reach correction in basic distance relaying scheme. The residual current measurement from the parallel circuit is not required for this purpose. Index Terms--Distance measurement, microprocessor applications, power transmission, protective relaying.

Distributed space-time coding is a cooperative transmission scheme for wireless relay networks. With this scheme, antennas of the distributive relays work as transmit antennas of the sender and generate a space-time code at the receiver. It achieves the maximum diversity. Although the scheme needs no channel information at relays, it does require full channel information, both the channels from the transmitter to relays and the channels from relays to the receiver, at the receiver. In this paper, we propose a differential transmission scheme, which requires channel information at neither relays nor the receiver, for wireless relay networks. As distributed space-time coding can be seen as the counterpart of space-time coding in the network setting, this scheme is the counterpart of differential space-time coding. Compared to coherent distributed space-time coding, the differential scheme is 3dB worse. In addition, we show that Alamouti, square real orthogonal, and Sp(2) codes can be used differentially in networks with corresponding numbers of relays. We also propose distributed differential space-time codes that work for networks with any number of relays using circulant matrices. Index Terms--Wireless relay networks, cooperative diversity, differential transmission, space-time block coding.

In Italy, the increase of railway traffic and of the power demand by the trains, especially the high-speed trains, has caused a continuous increase of the currents through the electric traction lines rated at 3 kVdc. As a consequence, in some cases, the load currents may approach the value of the fault currents for faults located far from the supply stations. The detection of the faults in such a situation has become difficult for the electromechanical protections at present in operation in Italy in the lines above. In addition, transient phenomena related to the train absorption, frequently cause untimely trippings of the protections above. For such a reason, in the years 1999-2000, a field test campaign was performed in the 3-kVdc lines Campoleone-Cisterna and Farneta-Montallese in Italy, in order to investigate the phenomena that lead to the untimely trippings above. On the basis of the field test campaign, it was found that such an inconvenience is related to the inner characteristics of the electromechanical protections in use. A digital protection, operating on new protective algorithms, was thus set up and thereafter put in operation in the years 2003-2004. This paper presents the basic principles of the new protection above and the results of the first two years of operation. Index Terms--Digital relay protection, electric railways, fault protection.

Microrelays with liquid metal wetted contacts have been demonstrated using bidirectional electrothermal electromagnetic actuators. These relays were fabricated with the MetalMUMPs foundry process, which has a 20-[micro]m-thick nickel structural layer. The operating voltage is under 0.5 V. The measured breakdown voltage and OFF-State resistance are greater than 200 V and 100 M[OMEGA], respectively, and the gold-to-gold contact resistance is around 0.3 [OMEGA]. When the contacts are wetted with liquid gallium alloy (melting point at -20 [degrees]C), the measured contact resistance can be as low as 0.015 [OMEGA] As such, these bidirectional relays could have potential applications in high-power switching systems with low contact resistance using liquid metal wetted contacts. [1706] Index Terms--Bidirectional actuators, electromagnetic, electrothermal, liquid metal, MetalMUMPs, microrelays.

This paper proposes a method for determining the coordination of the rate of change of frequency (ROCOF) and under/overfrequency relays for distributed generation protection considering islanding detection and frequency-tripping requirements. The method is based on the concept of application region, which defines a region in the trigger time versus active power imbalance space where frequency-based relays can be adjusted to satisfy the anti-islanding and frequency-tripping requirements simultaneously. This method can be used to optimally determine the instantaneous and time-delay settings of different frequency-based relays in a unified manner. The application region can be also employed to evaluate how much an existing frequency-based protection scheme violates one of the above-mentioned requirements. In addition, it can be used to set different combinations of frequency-based relays in order to make the distributed generator frequency-based protection system as efficient as possible, since it respects the region that satisfies both requirements. Index Terms--Distributed generation, dynamic simulation, frequency relays, islanding detection, rate of change of frequency relays, synchronous generators.

In protection relaying schemes, the digital filter unit plays the essential roles to calculate the accurate phasor. However, while the fault current contains plentiful decaying dc component, the over-reach of distance protection will cause sever problem. This work develops an adaptive mimic phasor estimator to remove the decaying dc oscillation between voltage and current and obtains the accurate apparent impedance. First, a discrete Fourier transform-based mimic phasor estimator is developed. Then, an adaptive scheme is proposed to obtain the decaying time constant. Unlike the fixed decaying dc time constant used in a digital mimic filter, the proposed algorithm adopts the transmission-line parameters information hiding in the voltage and current measurements to adaptively approximate the decaying dc time constant to the accurate value. Thus, the estimation error in the mimic filter due to the time constant mismatch can be eliminated. Both full-cycle and half-cycle versions are developed in this work. Simulations results illustrate the effectiveness of this new algorithm for distance relaying applications. Index Terms--Adaptive mimic phasor estimator, decaying dc component, discrete Fourier transform, distance relaying, phasor.

Determining optimally designed structures is important for diverse fields of science and engineering. Here we describe a procedure for calculating the optimal design of a switch and apply the method to a bistable microelectromechanical system relay switch. The approach focuses on characterizing the unstable transition state connecting the two stable equilibria to control the force displacements. Small modifications in component shape lead to a substantial improvement in device operation. Fabrication of the optimized devices confirms the predictions.

This paper describes the design, implementation and testing of an adaptive distance relay. The relay uses a fault detector to determine the inception of a fault and then uses data windows of appropriate length for estimating phasors and seen impedances. Hardware and software of the proposed adaptive relay are also described in this paper. The relay was tested using a model power system and a real-time playback simulator. Performance of the relay was compared with a fixed data window distance relay. Some results are reported in this paper. These results indicate that the adaptive distance relay provides faster tripping in comparison to the fixed data window distance relay. Index Terms--Adaptive relaying, distance relays, line protection, microprocessor-based relays.

It has been recently shown that the fast Fourier transform of a sequence which slides over a time-limited rectangular window can be carried out in a non-recursive manner by means of O(N) computations. The same idea is applied in this paper to the digital relaying field, where only certain isolated harmonics, rather than the full spectrum, are needed, leading to O(log(sub 2)N) computations per harmonic. The particular case N = 16 has been used for comparison purposes, showing that the new technique is less expensive than other non-recursive algorithms appeared in the literature. Keywords: Harmonic computation, spectral analysis, discrete Fourier Transform, digital relaying.

A methodology is presented for the consideration of definite-time backup relays in the optimal coordination of directional overcurrent relays using linear programming. It is shown that the influence of considering second-zone distance relays and breaker failure relays impose important requirements for the determination of the time dial settings of directional overcurrent relays. The paper introduces a revised formulation of the optimization problem. Results are presented for the application of the methodology on a power system with 2 generators, 9 buses, 2 transformers and 7 transmission lines. Keywords: Computer Aided Relay Coordination, Optimization Techniques, Power Systems Protection

A new method for the detection of the type of a fault in generator circuits and transmission systems is introduced. Already within a quarter of a cycle after fault inception the method can distinguish between the various fault types. Fitting the parameters of a set of simple equations to voltage and current measurements immediately before and after a fault identifies the fault type. The procedure includes a new method of phasor computation and takes less than 1 ms computation time. As a variant of this method neural networks are employed. Verification using EMTP modeling proved satisfactory operation of both methods even when the current signals were superimposed with heavy noise. Fast decisions for single pole tripping and a crucial basis for algorithms for synchronous switching under fault conditions are provided. Keywords: Fault location, Parameter estimation, Neural network applications

A lateral switching relay structure has been developed which provides a double gold contact with as low as 70-m[Omega] measured contact resistance, 0.45-A current-carrying ability at MEMS compatible force levels, TTL compatible actuation, and air gap isolation when open. The die area used for the relay mechanism itself (distinct from the actuation) is approximately 75 [[micro]meter] by 100 [[micro]meter] and was designed to allow fabrication of the relays in the MCNC MUMP's dual polysilicon foundry process with no assembly. Design analysis shows that substantial characterization is needed to design optimal microrelays. Temperature softening and failure modes have been characterized by current voltage techniques. Polysilicon vernier structures were used to develop force/current/conductance curves. Relays using thermal actuators have been built. Index Terms - Contact physics, microrelay, plastic deformation, relay, tribology.

A stand-alone digital relay has been designed and implemented. The relay can be modified easily to accommodate the protection of different types of transformers by changing the software only. Presented in this paper are the real-time experimental results of five selected algorithms implemented using this digital relay. These algorithms are: discrete Fourier transform, Walsh function, rectangular transform, finite impulse response and least-square. based on the experimental results, the discrete Fourier transform (DFT) algorithm is found to be the best algorithm in terms of accuracy and speed for digital implementation of the differential relay for power transformers. Keywords: Digital relay, power transformer, differential protection, testing algorithms.

Micromechanical switches have been fabricated in electroplated nickel using a four-level surface micromachining process. The simplest devices are configured with three terminals, a source, a drain, and a gate and are 30 [[micro]meter] wide, 1 [[micro]meter] thick, and 65 [[micro]meter] long. A voltage applied between the gate and source closes the switch, connecting the source to the drain. Devices switch more than [10.sup.9] cycles before failure and exhibit long lifetimes hot switching currents up to 5 mA. The initial contact resistance is less than 50 m[Omega]. The breakdown (stand-off) voltage between the source and the drain is greater than 100 V and the off-current is less than 20 fA at 100 V. [218] Index Terms - Contacts, nickel surface micromachining, relays.

A design analysis of a telecentric microchannel relay system developed for use with a smart-pixel-based photonic backplane is presented. The interconnect uses a clustered-window geometry in which optoelectronic device windows are grouped together about the axis of each microchannel. A Gaussian-beam propagation model is used to analyze the trade-off between window size, window density, transistor count per smart pixel, and lenslet f-number for three cases of window clustering. The results of this analysis show that, with this approach, a window density of 4000 windows/[cm.sup.2] is obtained for a window size of 30 [[micro]meter] and a device plane separation of 25 mm. In addition, an optical power model is developed to determine the nominal power requirements of a 32 x 32 smart-pixel array as a function of window size. The power requirements are obtained assuming a complementary metal-oxide semiconductor inverter-amplifier and dual-rail multiple-quantum-well self-electro-optic-effect devices as the receiver stage of the smart pixel.

Design, development, and implementation of a computer-aided design (CAD) tool for developing digital controllers and relays is presented in this paper. The development includes modeling A/D converters, software, and the necessary logic. Procedures for modeling A/D converters and simulating the software are described. Application of the CAD tool is demonstrated by evaluating and comparing three different designs of a digital overcurrent relay. The designs are evaluated to determine both the speed and accuracy of the relays. The effects of the size of the A/D converters, the word-size used in computations, and the effects of truncation/rounding are included in the simulation. The paper also describes the algorithm used in the relay design.

Directional relays use voltage as the polarizing quantity. When three-phase faults occur close to the relay bus, the available voltage becomes nearly zero and this creates a problem in estimation of the fault direction. The capacitor coupling voltage transformer subsidence transients add to this problem. The memory voltage used as the polarizing quantity at these situations is a compromise. This paper highlights these issues and proposes a simple solution using the power-flow direction in addition to other information. The performance of the technique is evaluated through simulation in Power System Computer Aided Design. Index Terms--Distal relay, directional relay, fault, phasor estimation, subsidence transient.

This paper reviews the past and present uses of relay models. It discusses the various types of models, what information is required to build such models and the model validation process. Examples of present and possible future use of software models are given. Index Terms--Relay models, relays, simulation, validation.