Abstract: A distance relay apparatus includes a directional relay element which performs computation to detect a fault, which occurs in the forward direction from an installing point of the relay apparatus, based on a voltage and a current which are received from an object to be protected, a zone-1 distance relay element which performs computation to detect a fault within a predetermined zone, a fault detecting relay element which performs computation to detect a fault within a zone that is narrower than that of the zone-1 distance relay element in terms of data time length which is shorter than that used for the computation of the zone-1 distance relay element, and a logic element which outputs a relay signal in accordance with a detecting operation of the zone-1 distance relay element, the fault detecting relay element, and the directional relay element.

Abstract: In a digital protection relay with a time sync function, the sampling timing of which is specified based on a reference timing transmitted from a time signal generator to a time sync unit, with a determination value. The time sync unit includes a reception circuit that receives a discrimination code and time data transmitted from the time signal generator, a code discrimination circuit that discriminates the reference timing on condition that the received discrimination code coincides with a desired code, a time calculation circuit that calculates the sampling timing on the basis of the discriminated reference timing and the time data, and a sampling sync circuit that specifies the sampling timing of a digital quantity of electricity on the basis of the calculated sampling timing.

Abstract: A protective relay having an embedded web server to allow communication with a remote device having a standard web browser package. The relay can receive and transmit HTML files according to the HTTP protocol over a communications network. The relay can receive commands from the remote device, and can generate and return requested data to the remote device.

Abstract: A composite power quality indication generated from user weighted statistical contributions of selected system parameters provides an overall indication of power quality in an electric power distribution system. The user defined weightings are maintained over time by continually updating sensitivities that normalize the component contributions to the composite power quality indication. The current composite power quality indication is displayed in relation to its long-term mean and to dynamic thresholds determined as multiples of the standard deviation of the long-term mean.

Abstract: A digital directional relay converts a voltage value and a current value of a power system into digital voltage data and digital current data by periodic sampling and determines a direction of a fault using the digital voltage data and the digital current data. In the digital directional relay, a variation in unbalanced component voltage and a variation in unbalanced component current are computed using both voltage data at a reference point in time and voltage data at a point in time elapsed from the reference point in time by a given sampling interval and using both current data at reference time and current data at time elapsed from the reference time by a given sampling interval, respectively. A relationship in phase between these variations is obtained to determine whether a forward fault or a reverse fault occurs.

Abstract: Disclosed is a method for detecting a fault by using harmonics and a state transition diagram. When the fault occurs on transmission lines, the fault is detected by using harmonics and the state transition diagram. Two logic signals are outputted by using apparent impedance of a steady state characteristic and a sum of harmonics components of a transient state characteristic. Such two logic signals are used as input parameters of the state transition diagram so that the state of an electric power system is presumed based on a state transition of the electric power system. It is possible to prevent a distance relay from being malfunctioned under an increase of load and unstable voltage. The state of the electric power system is presumed by using harmonics and the state transition diagram, and the fault on the transmission line is reliably detected.

Abstract: The present invention relates to a method for calculating the distance to fault in a section of a power transmission network, which section is arranged with line terminals at both ends, each terminal including impedance relays (AA, BB), including the steps of, at the occurence of a fault; measuring the apparent impedances by each relay, measuring the relaying currents, determining the type of fault, checking whether the fault involves a fault resistance or not, and, if so, solving a quadratic equation for complex numbers, resolving the quadratic equation for the real and imaginary components, obtaining two quadratic equations for a fault distance in which the coefficients are real numbers, combining the two equations and obtaining a distance to fault, or, if not so, solving a quadratic equation for real numbers, obtaining two solutions for the distance to fault d1, d2 and comparing the solutions as 0<(d1 or d2)<1 pu, where pu is the length of transmission lines between the line terminals.

Abstract: When a disturbance (for example, lightning strike) occurs on a high-voltage overhead transmission line, the line must be checked for any possible damage. If the distance-to-fault is known, line crews can be quickly dispatched for any necessary repair. The present invention is a fault location method and system that uses filtered, time-synchronized positive- or negative-sequence voltage and current measurements from both ends of the overhead transmission line to determine the exact distance to the fault with respect to either end.

Abstract: In a protective relay system according to an aspect of the present invention, a direction of a fault is correctly determined during system oscillation using one of a negative-phase-sequence relay and a zero-phase-sequence relay. A circuit breaker is tripped only when an internal fault is determined at both terminals to be protected using a carrier signal. Thus, even though a fault occurs during system oscillation, the circuit breaker can be tripped against the internal fault.

Abstract: The apparatus detects an out-of-step condition following the clearing of an external multi-phase fault in a protective relay. The apparatus includes a circuit which detects the presence of an external multi-phase fault on the power line and a timing circuit for readying an out-of-step logic detection circuit in the protective relay if the external fault remains for a pre-selected period of time. An out-of-step condition is declared and a signal blocking operation of selected distance elements in the protective relay is produced if the positive sequence impedance on the line remains inside an outer protection zone boundary in the impedance plane for a selected period of time, wherein the out-of-step blocking signal continues as long as the positive sequence impedance remains inside the second boundary.

Abstract: A line current differential protective relay and method for protecting transmission lines with power transformers tapped between the substations without any measurements taken at the tap point. The protection method uses distance supervision by an overcharging instantaneous impedance zone to prevent malfunction of the relay during external faults at the distribution busbar of the tapped transformer. The protection method uses modified current differential principle that compares the zero sequence compensated differential and restraining signals to provide relay stability on external ground faults. The protection method combines both the traditional and modified current differential principles to provide phase-selective operation.

Abstract: The invention describes a method, system, device, and computer-readable medium having computer-executable instructions for classifying faults on an electrical power line. In particular, the invention permits the classification of crossover faults using a local measurement technique. The inventive method includes providing a first electrical power transmission line and a second electrical power transmission line, and monitoring the first electrical power transmission line to identify a crossover fault occurring between the first and the second electrical power transmission lines.

Abstract: The invention describes a method, system, device, and computer-readable medium having computer-executable instructions for classifying faults on an electrical power line. In particular, the invention permits the classification of crossover faults, using a local measurement technique. The inventive method includes providing a first electrical power transmission line and a second electrical power transmission line, and monitoring the first electrical power transmission line to identify a crossover fault occurring between the first and the second electrical power transmission lines.

Abstract: The invention describes a method, system, device, and computer-readable medium having computer-executable instructions for classifying faults on an electrical power line. In particular, the invention permits the classification of crossover faults, using a local measurement technique. The inventive method includes providing a first electrical power transmission line and a second electrical power transmission line, and monitoring the first electrical power transmission line to identify a crossover fault occurring between the first and the second electrical power transmission lines.

Abstract: A distance relay for protection of a transmission line, which is capable of estimating apparent impedances with respect to faults on the transmission line involving fault resistances, more particularly faults at local and remote ends of a protection zone of the transmission line, and compensating for the magnitudes and phases of the estimated apparent impedances, so as, to minimize a reactance effect resulting in a mal-operation or non-operation or the distance relay. If a fault occurs on the transmission line, the distance relay determines whether the amount of zero-sequence current resulting from overcurrent exceeds a predetermined threshold value, so as to determine whether the occurred fault is an earth fault or a short fault.

Abstract: The invention is directed to a method for producing a fault signal indicating a short-circuit between a first phase conductor and a second phase conductor of two different, parallel-running, three-phase power transmission lines. The method produces a fault signal indicating a short-circuit which has occurred between a first phase conductor and a second phase conductor of a power transmission arrangement.

Abstract: A safety switch includes a shell having an open top and a closed bottom, with a hollow button pivotally connected to the shell at the open top, and with a first blade, a second blade and a grounding prong extending out of the shell through the closed bottom. A metal piece is housed in the shell and electrically and pivotally connected to an inner end of the second blade. The button has an indicator light received therein. The button is formed with a first tubular portion for receiving a movable pin and a first spring which urges the pin to abut the metal piece. The button is further formed with a second tubular portion for receiving a second spring which is compressed between the grounding prong and the button. Additionally, the indicator light has two terminals electrically connected with the respective spring.

Abstract: The invention relates to a method for generating an error signal which characterizes a fault current in an electrical conductor (130) provided with two conductor ends (120, 123, 126, 129) and comprising charge measuring devices (100, 103, 106, 109) which are connected to each other by data lines (112). At least one device is attached to each end of the conductor. In the inventive method, charge measuring values are determined using charge measuring devices. The measured charge values take into account the direction of the charge flow a total measured charge value is formed by addition. Said error signal is generated when the total measured charge value exceeds a certain threshold value. According to the invention, the measured charge values are determined synchronously and determined repeatedly at regular intervals in such a way that the time interval between each sequential determination is smaller than the predefined measuring period.

Abstract: In a power line current differential protection system, all three phase current values (IA, IB and IC) are obtained from both the local end and the remote end of a power transmission line. The magnitude of the ratio of the remote current values to the local current values are calculated. Also, the angle difference between the local and the remote current values for each phase are calculated. Comparison elements then compare the ratio and angle values against preselected values which establish a restrain region in the current ratio plane. Current values which result in the ratio being within the region do not result in a tripping signal for the circuit breaker on the power transmission line, while current values which result in a ratio outside of the region result in a tripping of the circuit breaker. Similar circuitry is used for negative sequence current quantities, with the negative sequence preselected values being set substantially lower to produce a more sensitive response to possible faults in the line.

Abstract: The system includes an enable signal applied to an output AND element. A comparator compares the voltage ratio of selected measured and calculated phase-to-phase or single phase voltages against a threshold value. An AND gate is responsive to a high signal from the comparator and indications of a three pole open condition, a single pole open condition for selected poles, and a loss of potential condition. A high output from the AND gate is applied to a timer, which produces a high output for a short period of time, which controls the output element. The output element provides a blocking signal for the protective elements under particular conditions and a permissive signal under other conditions.

Abstract: A digital distance relay for deciding whether a fault point is within a stipulated operation region by obtaining an impedance to the fault point. The relay comprises: means for deciding whether the impedance is within a specified region by applying a value proportional to the relative phase; and means for deciding the fault point is within the stipulated operation region if: (a) when the load current indicates a sending direction, the positive-phase resistance and reactance are within a stipulated region based on stipulated positive-phase resistance and reactance, and the impedance is decided within the specified region; or (b) when the load current indicates a receiving direction, the positive-phase resistance and reactance are within a stipulated region based on stipulated positive-phase resistance and reactance, or the impedance is decided within the specified region.

Abstract: The present invention relates to a method for location of a fault which has occurred on one of a plurality of lines in a power distribution network where the location is performed with the measured values of the common supply voltage of the plurality of lines and the currents of the plurality of lines after the occurrence of a fault; calculating the equivalent positive-sequence impedance Z1kf and zero-sequence impedance ZZ0kf of the network in a pre-fault steady state for all M nodes based on knowledge of the configuration and topology of the network, and obtaining, via a superordinate protection system, which of plurality of lines has become faulty and which type of fault has occurred.

Abstract: A protective relay, system, and relay operation method which allows logic functions to be distributed amongst multiple protective relays associated with a power distribution system. The protective relays include communications ports for exchanging messages over a peer-to-peer communications network, and include programmable logic. Each protective relay receives logic operands contained in the messages from remote protective relays, executes logic equations using the received operands, provides protective control if needed, and outputs operands to other protective relays.

Abstract: A short-circuit distance relay has a short-circuit distance relay element that calculates a fault impedance from voltage and current which are introduced from a power system and that compares the fault impedance with a set impedance and uses the fault impedance comparison result within its protection zone. It calculates the amplitudes of the phase-to-phase differences of these voltage amplitudes and a sensitivity constant Vk.

Abstract: A method of operating a protective gear is described, in particular a remote protective gear for an electric power grid. In this method, a voltage signal is derived from the line voltage, and a current signal is derived from the line current. To guarantee reliable detection of a fault in the measurement circuit of protective gear, a voltage drop is gated with a current surge. A monitoring device provided for this purpose gates a first detection signal derived from the voltage signal for the voltage drop with a second detection signal derived from the current signal for the current surge. The monitoring function is suitable for both single-phase and multi-phase networks and permits a reliable differentiation between a network fault and a failure of the measuring circuit.

Abstract: A method and apparatus for automatically controlling gain switching of a monitor. To automatically control gain switching, an electronic circuit splits an input channel into a plurality of gain channels. The plurality of gain channels are simultaneously sampled. Thereafter, the processor accesses a present buffer of a first gain channel. The present buffer contains a predetermined amount of samples which represent the input channel. Next, the processor determines whether at least one of the samples contained in the present buffer is saturated. If at least one of the samples is saturated, the processor selects an alternate gain channel. Otherwise, if none of the samples is saturated, the processor determines whether a previous buffer of the first gain channel is saturated. If none of the samples from the previous buffer are saturated, the DSP selects the first gain channel. Otherwise, the DSP selects the alternate gain channel.

Abstract: This device for protecting a line (10, 12) of a network of supply lines comprises two protective relays (18, 20; 22, 24) furnished with means for detecting faults and a circuit breaker adapted so as to disconnect the line from the network of lines in the presence of a fault, each relay being adapted so as to monitor a first zone (Z1) of the line corresponding to a zone of certain detection of a fault (10, 12) and at least one second zone (Z2) juxtaposed with the first zone and corresponding to a zone of uncertain detection of a fault. At least one of the relays comprises means for detecting the opening of the circuit breaker associated with the other relay in response to a detection, by the latter, of a fault in the first zone in such a way as to cause the opening of the circuit breaker associated with the said at least one relay (18, 20; 22, 24).

Abstract: A protective relay that derives and processes power system data from multiple user-configurable data source points. Data from multiple source points can be combined in the relay as a single source of data useful to perform a wide variety of monitoring and protection tasks. Multiple digital signal processing modules can be used to provide additional processing resources, including the ability to provide protection and revenue-class metering in a single device.

Abstract: A method of protecting a section of a line is disclosed.

Abstract: A digital distance relay for deciding whether a fault point is within a stipulated operation region by obtaining an impedance to the fault point. The relay comprises: means for deciding whether the impedance is within a specified region by applying a value proportional to the relative phase; and means for deciding the fault point is within the stipulated operation region if: (a) when the load current indicates a sending direction, the positive-phase resistance and reactance are within a stipulated region based on stipulated positive-phase resistance and reactance, and the impedance is decided within the specified region; or (b) when the load current indicates a receiving direction, the positive-phase resistance and reactance are within a stipulated region based on stipulated positive-phase resistance and reactance, or the impedance is decided within the specified region.

Abstract: A reach-measurement method is used in connection with a series-compensated line of a power system. The series-compensated line includes an installed series capacitance having a bus side and a line side, and a non-linear protection device parallel to the installed series capacitance. The series-compensated line has a line current, a bus side voltage, and a line side voltage. The series capacitance and the non-linear protection device have a capacitance voltage thereacross equal to the bus side voltage minus the line side voltage. In the method, a number (n) of line current samples are measured, where such samples are representative of values of a line current waveform at successive instants of time on the series-compensated line. Capacitance voltage values are computed based on the measured line current samples in accordance with an equation which takes into account the non-linear protection device parallel to the installed series capacitance.

Abstract: A combination exercise gripper and calculator includes an electronic calculator, with a keypad, a power source, a visual display means, such as a liquid crystal display, and a calculating computing means, which is responsive to manual input from sequential depression of selected keys of the keypad for determining mathematical calculations. A switch is used to calculate these mathematical inputs. The manual griping means, including a pair of depressible handles, which are forcibly separated by a compressible spring wherein compression of the handles towards each other compresses this spring to provide a resistance against the compression. The switch includes a physical condition sensor, communicating with the calculating means. The switch is responsive to the calculating means upon compression of the handles. A counter is used to count the number of compressions the user has made by squeezing the handles.

Abstract: When directional overcurrent protection relays are included at the load end of a set of parallel feeders fed from an AC power source, a minimum fault-current threshold is specified based on the potential of a phase-to-phase fault at the source to cause a relay to operate misleadingly. This threshold is, for plain (transformerless) feeders, approximately 50% and, for transformer feeders, approximately 85% (nominally 86.7%) of rated pre-fault load current. These thresholds can, however, be ignored if the relay outputs are processed in a 2-out-of-3 circuit, so that a relay on one phase will only operate its associated circuit breaker if a relay on another phase has also operated, a condition which has been found not to obtain in the case of a source fault.

Abstract: An improved reactance characteristic for an overall ground distance quadrilateral characteristic embodies a particular phase selector, possibly among a plurality of phase selectors and variables, that eliminates undesirable generation of a tripping signal other than only for a selected phase to ground fault. This phase selector is I1FZ1, where Z1 is the positive sequence replica impedance and I1F is the fault component of the positive sequence current component, referenced to the proper phase. Preferably, the pre-fault load current is removed from this quantity. Additionally, an improved restricted phase directional function is implemented, further improving the overall performance of the quadrilateral characteristic relay.

Abstract: A bus-bar protection relay equipment includes a data-acquisition means, a check zone protection means, a fault bus-bar judging means, and a tripping bus-bar judging means. A data-acquisition means transforms analog current signal from the current transformer connected to the bus-bar with two or more dependency regions into digital data at every predetermined cycle. The check zone protection means computes amount Id (vector sum) of operation, and the amount Ir (scalar sum) of inhibition using the digital data transformed by this data-acquisition means. The check zone protection means transforms analog current from the current transformer of a line segment into digital data “n” multiplied with. The amount Id of operation and the amount Ir of inhibition are computed. An actuating signal is outputted when predetermined relationship is satisfied by using the amounts Id and Ir.

Abstract: As a variation of the non-orthogonal filter, a phasor estimate is computed by using an N-point window. An aspect of the sub-window cosine filter is to repeat the basic cosine filter for only selected points of the window. In the end, a least-squares fit is used to obtain an estimate for the phasors components. Previous cosine techniques use a data window whose length is greater than 1 cycle while the present invention requires only 1 cycle.

Abstract: A high sample rate cosine filter eliminates DC components by summing them such that they sum to zero. A non-orthogonal cosine filter is also provided. When the cosine filter is applied for N=4 samples per cycle, the samples are separated by 90 degrees. However, at higher sampling rates, it is not necessary to wait for 90 degrees to estimate the phasor value. Non-orthogonal components are used to estimate the phasor value. The time delay associated with the cosine filter is reduced in the process.

Abstract: A protective relay receives current and voltage inputs, digitizes them and reduces the effects of high and low frequency noise. In the case of a distance relay, incorporated in the relay is a signal generator which generates "operate" and "restraint" sinusoidal signals. In the case of a current or voltage relay, operate is a digitized version of current (voltage) and restraint is a pre-specified constant. In all cases, a magnitude comparator, implemented using short (two sample) data window algorithms, compares the magnitude of the operate and restraint signals and outputs a trip condition signal. The trip condition output is limited to a finite number of values by a limiter system and sent to an adaptive trip counter system. The adaptive trip counter system employs trip counter processing, which determines whether a trip signal will be initiated, immediately or after a period of delay.

Abstract: Impedance protection method for detecting short-circuits on a monitored section of an electrical power supply line in which impedance values formed by current and voltage during a short-circuit in order to extract a tripping signal are checked to see whether they lie within a specific tripping characteristic. A relatively small first tripping characteristic in relation to the monitored section is used, and impedance values first determined are compared to this first tripping characteristic to see whether they lie inside or outside this tripping characteristic; the tripping signal is generated if the impedance values lie within the characteristic and, if not, the system switches to a maximum tripping characteristic corresponding to the monitored section. Impedance values following the impedance values first determined are checked to see whether they lie within the maximum tripping characteristic and, if they do, the tripping signal is generated.

Abstract: The system impedance ratio (SIR) is determined for the original zone 1 distance element reach of a protective relay. The reduction in the original zone 1 reach is then determined, using the SIR value and a known table of reduced values. When a fault is recognized the minimum reach to just detect the fault is determined. A trip signal is provided without any delay if the minimum reach is less than or equal to the reduced reach. The trip is delayed for a selected time if the minimum reach is greater than the reduced reach but not greater than the original reach.

Abstract: An adaptive distance relaying system provides improved performance for parallel circuit distance protection. The system utilizes the parallel circuit's current, when available, in conjunction with measurements of voltage and current on the protected line to compensate for the zero sequence current mutual coupling effect. The sequence current ratio (zero or negative sequence) is used to avoid incorrect compensation for relays on the healthy circuit. If the parallel circuit current is not available and the line operating status is, the best zero sequence current compensation factors are selected accordingly as a next level adaptation. If both the parallel circuit current and line operating status are unavailable, a fallback scheme that offers better results than classical distance protection schemes is employed.

Abstract: A data switch for transferring data from either actual line readings or from stored memory to the input of a relay tester. The device uses a digital switch set between the A/D converter and the CPU of an electronic relay. The switch permits technicians and engineers to either pass data to the CPU from the system through the A/D converter or, by switching, feed the CPU data from the RAM, which has been loaded with the archived data. In this way, the relay system software can be examined and tested using actual event data under ordinary conditions as desired, with relay settings changed between tests to verify the efficacy of the changed settings to perform as desired for the recorded conditions. The digital switch can be either a hard-wired element, or can be a soft switch, operated through software instructions.

Abstract: Both fault location and fault resistance of a fault are calculated by the present method and system. The method and system takes into account the effects of fault resistance and load flow, thereby calculating fault resistance by taking into consideration the current flowing through the distribution network as well as the effect of fault impedance. A direct method calculates fault location and fault resistance directly while an iterative fashion method utilizes simpler calculations in an iterative fashion which first assumes that the phase angle of the current distribution factor D.sub.s is zero, calculates an estimate of fault location utilizing this assumption, and then iteratively calculates a new value of the phase angle .beta..sub.s of the current distribution factor D.sub.s and fault location m until a sufficiently accurate determination of fault location is ascertained. Fault resistance is then calculated based upon the calculated fault location.

Abstract: A protective relaying system includes a local protective relay operatively coupled to a protected line segment at a first location, and a remote protective relay operatively coupled to the protected line segment at a second location. The remote protective relay generates a signal (PILOT RECEIVE) to indicate the detection of a fault. This signal is communicated to the local protective relay and employed therein to supervise single-pole trip operations.

Abstract: A digital decaying current offset correction method and apparatus, in one aspect, separates the requirements of detecting fault existence from the requirements of detecting fault location. Once the decaying offset removal routine is initiated, current and voltage are sampled and, for each current and voltage sample, current and voltage phasors are generated. The current phasor values, which are not offset corrected, may be used to determine whether to trip a breaker. Once the breaker is tripped, the decaying offsets are removed from the current phasors. The offset corrected current phasors are then used to locate the fault.

Abstract: A quadrilateral characteristic relay system for determining a fault associated with a multiple-phase electric power transmission system is disclosed. The system dynamically adapts one line of the quadrilateral trip region to account for the interaction of load current and fault resistance. As a result of the adaptation, the system exhibits better overreach and underreach performance characteristics. The system employs impedance and voltage plane quadrilateral characteristics using negative sequence current as a proxy for fault current. In the voltage plane, the fault determination is made through a series of comparisons, avoiding computationally inefficient mathematical divisions.

Abstract: A microprocessor-based multifunction overcurrent relay for protecting power systems has a plurality of overcurrent elements implemented in a software program. Sufficient overcurrent elements are provided to accomplish a selected one of three independent applications, including feeder protection, motor protection, or breaker failure protection. The relay includes elements for selecting one application by the user from the three possible applications and produces output signals when an overcurrent condition is determined.

Abstract: At least two different time constants are used to produce the polarizing memory reference voltage which in turn is used in a protective relay for power transmission lines. For most conditions, a relative short time constant of 1.75 cycles is used, while under other specific conditions, determined automatically, a longer time constant of 15.75 cycles is used.

Abstract: A process for identifying the type of fault detected by a protective relay includes measuring pre-fault and post-fault samples of current waveforms on the phase-A, phase-B and phase-C conductors of a transmission line. Next, increment currents DELIA1, DELIB1, and DELIC1 are calculated, whereDELIA1=(I.sub.ar -I.sub.apr).sup.2 +(I.sub.ai -I.sub.api).sup.2DELIB1=(I.sub.br -I.sub.bpr).sup.2 +(I.sub.bi -I.sub.bpi).sup.2DELIC1=(I.sub.cr -I.sub.cpr).sup.2 +(I.sub.ci -I.sub.cpi).sup.2.The increment currents are then employed to identify a two-phase to ground fault. In calculating the increment currents, I.sub.ar represents a post-fault value of the real part of the phase-A current, I.sub.apr represents a pre-fault value of the real part of the phase-A current, I.sub.ai represents a post-fault value of the imaginary part of the phase-A current, and I.sub.api represents a pre-fault value of the real part of the phase-A current. The phase-B and phase-C values are denoted accordingly.

Abstract: An out-of-step blocking circuit is established with two additional zones in the impedance plane, with the circuit determining the time that the positive sequence impedance takes to move between the two zones. The distance elements for selected zones of protection for the relay are blocked if the impedance moves between the two zones at a rate less than a first threshold. However, a trip signal results if the rate of change is greater than the first threshold but less than a second threshold, indicating an unstable swing. Further, an inner blinder zone is established in the impedance plane within the additional two zones of protection. If the positive sequence impedance entering that zone does not move from that zone within a certain time during an otherwise out-of-step condition, the blocking signals are terminated.