Ground protection apparatus for electrical equipment with y-connected windings

Claim 2

Apparatus for detecting grounds at or near the neutral point between Y-connected windings of alternating voltage electrical equipment, the apparatus comprising a resistive bridge circuit, one arm of which comprises a load resistor connected between the neutral point of the windings and ground, a bias voltage source connected to one diagonal of the bridge circuit, the frequency of the bias voltage source differing from the line frequency of the electrical equipment, a potentiometer connected to the second diagonal of the bridge circuit in which an alternating current voltage is induced by either the line frequency or, when a ground occurs at or near the neutral point between the windings, by the bias voltage source, time measuring means connected to the potentiometer to detect the time interval between the zero crossings of the a-c voltage signal induced in the potentiometer, the time measuring means emitting an output signal indicating an occurrence of a ground in the windings if the voltage signal in the potentiometer exceeds a predetermined minimum value during the measured time interval between two successive zero crossings of the voltage in the potentiometer or if the voltage at the end of the time interval between two successive zero crossings is more than a predetermined minimum value.

Claim 3

Ground detection apparatus as set forth in claim 1 in which the time measuring means comprises rectifier means connected to the potentiometer to produce a posiTive rectified output voltage signal, a threshold circuit connected to the output of the rectifier to respond at a predetermined positive voltage output from the rectifier, a clock connected to the output of the threshold circuit, a counter the counting input of which is connected to the output of the clock, counter control means connected between the potentiometer and the counter to control the direction of the counting in the counter by the polarity of the voltage in the potentiometer and the time of the counting by the duration of the voltage cycle of the potentiometer, and evaluating means connected to the output of the counter to emit a signal if a predetermined maximum count is exceeded by the counter or a count above a predetermined minimum is present at the end of the voltage cycle in the potentiometer, both of which signals indicate the occurrence of a ground in the windings.

Claim 4

Ground detection apparatus as set forth in claim 2 in which the counter has at least two outputs and produces an output signal at the first output indicating a ground when the count of the counter exceeds a first predetermined voltage level which is above that voltage reached in the counter when the potentiometer voltage is caused by the line voltage, the first output signal thereby indicating the occurrence of a ground, the counter producing an output signal at the second output when the count of the counter exceeds a second predetermined voltage level, an OR circuit, one input of which is connected to the first output of the counter, the OR circuit thereby emitting a signal indicating a ground when a signal is transmitted to it by the first output of the counter, an AND circuit having a first input connected to the second output of the counter and a second input from a control means comprising a circuit connected between the potentiometer and the AND circuit which emits a signal to the AND circuit after the completion of the voltage cycle in the potentiometer, the output of the AND circuit being connected to the second input of the OR circuit and emitting a signal thereto indicating the occurrence of a ground when the output of the counter is above the second predetermined voltage level after the completion of a counting cycle.

Claim 5

Ground detection apparatus as set forth in claim 3 wherein the control means for the AND circuit comprises a half-wave rectifier circuit producing a positive output during the positive voltage cycle in the potentiometer, a flipflop circuit connected to the output of the rectifier circuit to conduct during alternate voltage cycles in the potentiometer, the output of the flipflop connected to the second input of the AND circuit.

Claim 6

Ground detection apparatus as set forth in claim 4 in which a utilization device is connected to the output of the OR circuit to indicate the occurrence of a ground in the windings.

Claim 7

Ground detection apparatus as set forth in claim 5 in which the utilization device comprises a ground detection relay to indicate the occurrence of a ground in the windings.

Claim 8

Apparatus for detecting grounds at or near the neutral point between Y-connected windings of alternating voltage electrical equipment, the apparatus comprising a voltage transformer having a primary winding connected between the neutral point and ground, and a secondary winding, a load resistor connected across the secondary winding of the voltage transformer and forming an arm of a resistive bridge, a voltage bias source connected to one diagonal of the bridge and having a frequency less than the line frequency of the electrical equipment, a potentiometer connected to the second diagonal of the bridge in which alternating voltages occur proportional to the line frequency or, when a ground occurs in the windings, proportional to the bias voltage, rectifier means connected to the potentiometer to produce a positive rectified output voltage signal when an alternating voltage occurs in the potentiometer, the output of said rEctifier means being fed to a threshold stage adapted to respond to a predetermined positive voltage output from the rectifier means, a clock connected to the output of the threshold stage to produce an output signal triggered by the threshold stage, a counter having a counting input connected to the output of the clock, a reset input, positive and negative directional inputs and two counting outputs, a second rectifier means connected to the potentiometer to produce a positive and negative rectified voltage output signal during the positive and negative voltage cycles of the voltage in the potentiometer, the second rectifier means connected to the positive and negative directional inputs of the counter to trigger the counter to positively count during positive cycles of the potentiometer voltage and negatively count during negative cycles of the potentiometer voltages, a flipflop circuit connected to the positive rectified output of the rectifier means to conduct on alternate cycles of the potentiometer voltage, the output of the flipflop circuit connected to a delay device, the output of which is connected to the reset input of the counter and resets the counter after the completion of a cycle of the potentiometer voltage, the first counting output of the counter connected directly to one input of an OR circuit and adapted to emit an output signal to the OR circuit when a first predetermined count has been exceeded in the counter during a counting cycle, the signal thereby indicating the occurrence of a ground in the windings, the second counting output of the counter connected as an input to an AND circuit and adapted to emit an output signal from the counter when a second predetermined count has been exceeded by the counter, the AND circuit having a second input connected to the output of the flipflop circuit at which a signal occurs on alternate cycles of the potentiometer voltage, the output of the AND circuit connected as the second input to the OR circuit, the AND circuit thereby emitting a signal to the OR circuit indicating the occurrence of a ground when the second predetermined count of the counter has been exceeded and the flipflop circuit has emitted a signal indicating the completion of the counting cycle.

Claim 9

Apparatus for detecting grounds at or near the neutral point between Y-connected windings of alternating voltage electrical equipment, the apparatus comprising a voltage transformer having a primary winding connected between the neutral point and ground, and a secondary winding, a load resistor connected across the secondary winding of the voltage transformer and forming an arm of a resistive bridge, a voltage bias source connected to one diagonal of the bridge and having a frequency less than the line frequency of the electrical equipment, a potentiometer connected to the second diagonal of the bridge in which alternating voltages occur proportional to the line frequency or, when a ground occurs in the windings, proportional to the bias voltage, rectifier means connected to the potentiometer to produce a positive rectified output voltage signal when an alternating voltage occurs in the potentiometer, the output of said rEctifier means being fed to a threshold stage adapted to respond to a predetermined positive voltage output from the rectifier means, a clock connected to the output of the threshold stage to produce an output signal triggered by the threshold stage, a counter having a counting input connected to the output of the clock, a reset input, positive and negative directional inputs and two counting outputs, a second rectifier means connected to the potentiometer to produce a positive and negative rectified voltage output signal during the positive and negative voltage cycles of the voltage in the potentiometer, the second rectifier means connected to the positive and negative directional inputs of the counter to trigger the counter to positively count during positive cycles of the potentiometer voltage and negatively count during negative cycles of the potentiometer voltages, a flipflop circuit connected to the positive rectified output of the rectifier means to conduct on alternate cycles of the potentiometer voltage, the output of the flipflop circuit connected to a delay device, the output of which is connected to the reset input of the counter and resets the counter after the completion of a cycle of the potentiometer voltage, the first counting output of the counter connected directly to one input of an OR circuit and adapted to emit an output signal to the OR circuit when a first predetermined count has been exceeded in the counter during a counting cycle, the signal thereby indicating the occurrence of a ground in the windings, the second counting output of the counter connected as an input to an AND circuit and adapted to emit an output signal from the counter when a second predetermined count has been exceeded by the counter, the AND circuit having a second input connected to the output of the flipflop circuit at which a signal occurs on alternate cycles of the potentiometer voltage, the output of the AND circuit connected as the second input to the OR circuit, the AND circuit thereby emitting a signal to the OR circuit indicating the occurrence of a ground when the second predetermined count of the counter has been exceeded and the flipflop circuit has emitted a signal indicating the completion of the counting cycle.