Patent Description:
The present disclosure relates generally to a system and method for performing a low energy pulse testing operation in a power distribution network to determine if a fault condition is present.

An electrical power distribution network, often referred to as an electrical grid, typically includes a number of power generation plants each having a number of power generators, such as gas turbines, nuclear reactors, coal-fired generators, hydro-electric dams, etc. The power plants provide power at a variety of medium voltages that are then stepped up by transformers to a high voltage AC signal to be connected to high voltage transmission lines that deliver electrical power to a number of substations typically located within a community, where the voltage is stepped down by transformers to a medium voltage for distribution. The substations provide the medium voltage power to a number of three-phase feeders including three single-phase feeder lines that provide medium voltage to various distribution transformers and lateral line connections. A number of three-phase and single-phase lateral lines are tapped off of the feeder that provide the medium voltage to various distribution transformers, where the voltage is stepped down to a low voltage and is provided to a number of loads, such as homes, businesses, etc..

Power distribution networks of the type referred to above typically include a number of switching devices, breakers, reclosers, interrupters, etc. that control the flow of power throughout the network. A vacuum interrupter is a switch that has particular application for many of these types of devices. A vacuum interrupter employs opposing contacts, one fixed and one movable, positioned within a vacuum enclosure. When the interrupter is opened by moving the movable contact away from the fixed contact the arc that is created between the contacts is quickly extinguished as the AC current goes through zero in the vacuum. A vapor shield is typically provided around the contacts to contain the by-products of the arcing. For certain applications, the vacuum interrupter is encapsulated in a solid insulation housing that may have a grounded external surface.

Periodically, faults occur in the distribution network as a result of various things, such as animals touching the lines, lightning strikes, tree branches falling on the lines, vehicle collisions with utility poles, etc. Faults may create a short-circuit that increases the stress on the network, which may cause the current flow from the substation to significantly increase, for example, many times above the normal current, along the fault path. This amount of current causes the electrical lines to significantly heat up and possibly melt, and also could cause mechanical damage to various components in the substation and in the network. These faults are many times transient or intermittent faults as opposed to a persistent or bolted fault, where the thing that caused the fault is removed a short time after the fault occurs, for example, a lightning strike. In such cases, the distribution network will almost immediately begin operating normally after a brief disconnection from the source of power.

Fault interrupters, for example, reclosers that employ vacuum interrupters, are provided on utility poles and in underground circuits along a power line and have a switch to allow or prevent power flow downstream of the recloser. These reclosers detect the current and voltage on the line to monitor current flow and have controls that indicate problems with the network circuit, such as detecting a high current fault event. If such a high fault current is detected the recloser is opened in response thereto, and then after a short delay closed to determine whether the fault is a transient fault. If a high fault current flows when the recloser is closed after opening, it is immediately re-opened. If the fault current is detected a second time, or multiple times, during subsequent opening and closing operations indicating a persistent fault, then the recloser remains open, where the time between detection tests may increase after each test. For a typical reclosing operation for fault detection tests, about <NUM>-<NUM> cycles or <NUM> to <NUM> of fault current pass through the recloser before it is opened, but testing on delayed curves can allow fault current to flow for much longer times.

When a fault is detected, it is desirable that the first fault interrupter upstream from the fault be opened as soon as possible so that the fault is quickly removed from the network so that the loads upstream of that fault interrupter are not disconnected from the power source and service is not interrupted to them. It is further desirable that if the first fault interrupter upstream from the fault does not open for whatever reason, then a next fault interrupter upstream from the fault is opened, and so on. In order to accomplish this, it is necessary that some type of communications or coordination protection scheme be employed in the network so that the desired fault interrupter is opened in response to the fault.

During the traditional reclosing operation discussed above, the vacuum interrupter contacts in the recloser are closed without regard to a desired phase angle. This results in a random closing angle that often creates an asymmetrical fault current, where the current cycle is offset from zero, i.e., has high magnitude peaks in one polarity and lower peaks in the reverse polarity relative to zero. The high magnitude fault current peaks, depending on the length of time they are occurring, causes significant forces and stresses on the components in the network that may reduce their life. For the traditional reclosing operation having current flow times over <NUM>-<NUM> cycles and longer times for delayed curve operation, these forces and stresses can be considerable. When considering the life of a transformer winding, one cause of end of life can be fatigue in the winding, which is the accumulation of high mechanical and thermal stress cycles. Stress is the result of the current in the winding, where higher current results in higher stress. Doubling the stress that can cause fatigue from the asymmetrical fault currents described above can result in a tenfold or more reduction in fatigue life, i.e., the life before fatigue causes cracking. This stress can be reduced by reducing the peak current and by reducing the number of stress cycles.

In order to overcome this problem, reclosers have been developed in the art that use pulse testing technologies where the closing and then opening of the vacuum interrupter contacts is performed in a pulsed manner so that the full fundamental frequency multiple cycle fault current is not applied to the network while the recloser is testing to determine if the fault is still present. Typically these pulses are about one-half of a fundamental frequency current cycle. Additionally, these reclosers close at the appropriate point on the voltage waveform to eliminate the asymmetrical current, which reduces the stresses due to high current in the components.

Pulse closing technologies have been successful in significantly reducing fault current stresses on network equipment during recloser testing. However, the switching devices required to generate these short pulse durations are relatively complicated and expensive. For example, vacuum interrupters employed to generate these pulses often use two magnetic actuators, one to close the contacts and one to quickly open the contacts using the moving mass of the opening actuator to reverse the direction of the closing actuator, well understood by those skilled in the art. Document <CIT> discloses a method according to the preamble of claim <NUM>.

The following discussion discloses and describes a system and method for performing a low energy pulse testing operation in a power distribution network that causes recloser contacts to close and then open in such a way that produces current flow for one fundamental frequency cycle after closing on a voltage waveform that produces symmetrical current. The method includes energizing a magnetic actuator to move the movable contact towards the fixed contact, where AC current conducts across a gap between the movable contact and the fixed contact before the movable contact and the fixed contact make contact. The actuator movement to close the contacts pushes against the bias of at least one opening spring coupled to the movable contact. A compliance spring coupled to the movable contact with some preload is compressed further as the two contacts touch. The method also includes de-energizing the magnetic actuator or reversing the voltage on the magnetic actuator when the movable contact makes contact with the fixed contact so as to allow the bias of the opening and compliance springs to first compress absorbing energy from the motion of the moving mass, then to expand moving the movable contact away from the fixed contact so that the amount of time that the current conducts between the movable contact and the fixed contact is about one fundamental frequency cycle, and where energizing the magnetic actuator occurs at a time at or near a peak of the voltage wave so that the current through the switch assembly is symmetric.

Additional features of the disclosure will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.

The following discussion of the embodiments of the disclosure directed to a system and method for performing a low energy fault pulse testing operation in a power distribution network that causes recloser contacts to close and then open in approximately one fundamental frequency cycle of current flow time and close on a voltage waveform that produces symmetrical fault current is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.

The present disclosure proposes a system and method that replaces the known fault pulse testing process in a power distribution network with a low energy fault pulse testing process, which results in lower cost switching devices. The method includes controlling the position of the switch contacts in a recloser so that they conduct for a short time duration that limits the current conduction time to one fundamental frequency cycle. The method also includes closing the switch contacts at a point on the voltage waveform that results in the flow of symmetrical current instead of asymmetrical current, which is accomplished by recognizing that asymmetrical currents occur from closing the contacts at a certain voltage angle, where the preferred angle for asymmetrical current is at or near the peak of the voltage waveform. The disclosed low energy testing process is distinguished from the conventional recloser random closing operation since the recloser protection functions perform their normal fault detection processes, thereby extending the potential fault duration to several power system cycles or a much longer time.

<FIG> is a schematic type diagram of an electrical power distribution network <NUM> including an electrical substation <NUM> that steps down high voltage power from a high voltage power line (not shown) to medium voltage power, a three-phase feeder <NUM> that receives medium voltage power from the substation <NUM>, and a lateral line <NUM> that receives the medium voltage power from the feeder <NUM>. The medium voltage is stepped down to a low voltage by a number of distribution transformers <NUM> strategically positioned along the lateral line <NUM>, and the low voltage is then provided through a secondary service conductor <NUM> to a number of loads <NUM> represented here as homes. The lateral line <NUM> includes a fuse <NUM> positioned between the feeder <NUM> and the first load <NUM> on the lateral line <NUM> proximate to a tap location where the lateral line <NUM> is connected to the feeder <NUM>. The fuse <NUM> is an independent electrical device that is not in communication with other components or devices in the network <NUM>, where the fuse <NUM> creates an open circuit if an element within the fuse <NUM> heats up above a predetermined temperature as a result of high fault current so as to prevent short-circuit faults on the lateral line <NUM> from affecting other parts of the network <NUM>.

The network <NUM> includes a number of reclosers of the type referred to above provided at certain intervals along the feeder <NUM> represented by reclosers <NUM> and <NUM> that receive the medium voltage from the substation <NUM> on the feeder <NUM>. Although only shown as a single line, the feeder <NUM> would include three lines, one for each phase, where a three-phase or three separate reclosers would be provided in each line. A number of utility poles <NUM> are provided along the feeder <NUM> and the lateral line <NUM>, where the recloser <NUM> would be mounted on certain ones of the poles <NUM>. The recloser <NUM> includes a vacuum interrupter switch or other switching device <NUM> for opening and closing the recloser <NUM> to allow or prevent current flow therethrough on the feeder <NUM>, where the switch <NUM> is capable of providing pulses for pulse testing consistent with the discussion herein. The recloser <NUM> also includes sensors <NUM> for measuring the current and voltage of the power signal propagating on the feeder <NUM>, an electronic controller <NUM> for processing the measurement signals and controlling the position of the switch <NUM>, and an optional transceiver <NUM> for transmitting data and messages to a control facility and/or to other reclosers and components in the system <NUM>. The configuration and operation of reclosers and switching devices of this type are well understood by those skilled in the art.

<FIG> is a side, cross-sectional type view of a magnetic actuator switch assembly <NUM> in an open position and <FIG> is a side, cross-sectional type view of the switch assembly <NUM> in a closed position, where the switch assembly <NUM> can be used in the switch <NUM>. The switch assembly <NUM> includes a fixed contact <NUM> and a movable contact <NUM> having a defined mass, for example, <NUM>, that would be positioned in, for example, a vacuum bottle of a vacuum interrupter, where the contacts <NUM> and <NUM> are shown spaced apart from each other across a vacuum gap <NUM> in the open position, and where the distance of the gap <NUM> is determined so as to prevent conduction between the contacts <NUM> and <NUM> in the open position based on the voltages employed in the network <NUM>. The switch assembly <NUM> further includes an actuator <NUM> having an actuator housing <NUM> with a wide housing portion <NUM> and a narrow housing portion <NUM>. An open spring <NUM> is wound around the narrow housing portion <NUM> and is positioned against the wide housing portion <NUM> and a conductive structure <NUM>, where the current path on the power line flows through a current transfer coupling <NUM> coupled to the structure <NUM> and the movable contact <NUM>. A flange <NUM> attached to the movable contact <NUM> opposite from the gap <NUM> is positioned within the narrow housing portion <NUM> and engages a flange <NUM> at an end of the narrow housing portion <NUM> opposite to the wide housing portion <NUM>. A compliance spring <NUM> is positioned in the narrow housing portion <NUM> against the flange <NUM> and a wall <NUM>, where the spring <NUM> has, in one non-limiting embodiment, a <NUM> Newton preload and a <NUM> Newton full deflection. The actuator <NUM> further includes a solenoid <NUM> positioned within the wide housing portion <NUM> and including a core <NUM>, a coil <NUM> wrapped around a center portion <NUM> of the core <NUM> and movable core <NUM> attached to the housing <NUM>, where a gap <NUM> is provided between the movable core <NUM> and the core <NUM> when the contacts <NUM> and <NUM> are open. The core <NUM> is an E-shaped core in this non-limiting embodiment, where other shaped cores may be applicable such as round pot cores. A plunger <NUM> is secured to a fixed member <NUM> by a spring <NUM> and having a coil <NUM> wrapped around it.

To close the contacts <NUM> and <NUM>, the coil <NUM> is energized so that magnetic flux across the gap <NUM> between the magnetic elements draws the movable core <NUM> towards the E-shaped core <NUM>, which moves the housing <NUM> against the bias of the open spring <NUM> and causes the compliance spring <NUM> to push the movable contact <NUM> against the fixed contact <NUM>, where the bias of the springs <NUM> and <NUM> hold the contacts <NUM> and <NUM> together in a tight engagement. The coil <NUM> is de-energized which causes the spring <NUM> to move the plunger <NUM> upwards and hold the actuator <NUM> in the closed position. To open the contacts <NUM> and <NUM>, the coil <NUM> is energized to pull the plunger <NUM> downward against the friction of the moving part of the actuator <NUM> and the bias of the spring <NUM> so that the open spring <NUM> and the compliance spring <NUM> push the housing <NUM> to the left and moves the movable core <NUM> away from the E-shaped core <NUM>. Since it is necessary to quickly open the contacts <NUM> and <NUM> when fault current is detected, the spring forces used to move the contact <NUM> away from the contact <NUM> are relatively high. Additionally, it is desirable to coordinate the opening of the contacts <NUM> and <NUM> with other reclosers so that those reclosers closest to the fault are opened first, which requires a force to be applied to the movable contact <NUM> to hold the contacts <NUM> and <NUM> closed for some period of time when a fault is detected to coordinate with faster reclosers.

As mentioned above, this disclosure describes a low energy fault pulse testing process that causes about one fundamental frequency cycle of symmetrical current during each pulse test. Process algorithms control the switch assembly <NUM> to provide point-on-wave closing of the contacts <NUM> and <NUM> to obtain the short duration symmetric current flow that includes controlling the relative position of the vacuum interrupter contacts <NUM> and <NUM> to provide the desired time to start and end current conduction across the contacts <NUM> and <NUM> by controlling the time when the actuator <NUM> begins moving to close the contacts <NUM> and <NUM> and controlling when the coil <NUM> is turned off. <FIG> are graphs to help illustrate the control of the switch assembly <NUM> in this manner.

<FIG> is a graph with time on the horizontal axis and magnitude on the vertical axis showing a relationship between symmetrical and asymmetrical current relative to a voltage waveform. Graph line <NUM> is the measured voltage at the switch assembly <NUM> during the fault and shows the zero cross-overs of the voltage signal. Graph line <NUM> shows the measured fault current at the switch assembly <NUM> if the contacts <NUM> and <NUM> begin conduction at a zero cross-over of the voltage, and shows the current being offset or asymmetrical where the positive peaks are much higher in absolute magnitude than the negative peaks, which leads to significant stress on the electrical components in the network <NUM> that are subjected to the fault current because of the high current magnitude. Graph line <NUM> shows the measured fault current if the contacts <NUM> and <NUM> begin conducting at a voltage angle <NUM>°, and shows the current being symmetrical where the positive and negative peak values are at the same magnitude relative to zero. Thus, although the peak-to-peak current is the same for symmetrical and asymmetrical currents, the absolute magnitude of the peaks is less for symmetrical current, which reduces the stress on the electrical components in the network <NUM> that are subjected to the fault current.

<FIG> is a graph with time on the horizontal axis and the position of the actuator housing <NUM> on the vertical axis, where graph line <NUM> shows the position of the actuator housing <NUM> from when it starts moving to close the contacts <NUM> and <NUM> until it returns to the contact open position. The zero position is the position of the actuator housing <NUM> where the movable core <NUM> is touching the E-shaped core <NUM>, as shown in <FIG>. When the coil <NUM> is energized and the actuator housing <NUM> moves to close the contacts <NUM> and <NUM>, initially compressing the springs <NUM> as the movable contact <NUM> is moved towards the fixed contact <NUM>, the gap <NUM> will be reduced. When the position of the actuator housing <NUM> is at about <NUM> shown by point <NUM> at about time <NUM>, the gap <NUM> is about <NUM>, and conduction between the contacts <NUM> and <NUM> will begin across the gap <NUM>, known as a pre-strike. As the actuator housing <NUM> continues to move, the contacts <NUM> and <NUM> will engage at actuator position of about <NUM> shown by point <NUM> at about time <NUM>, and the current to the coil <NUM> will be shut off. The momentum of the actuator housing <NUM> will continue moving the actuator housing <NUM> until the position of the actuator housing <NUM> is about <NUM> at point <NUM> at about time <NUM>, shown as a gap <NUM> between the flanges <NUM> and <NUM> in <FIG>, when the compression forces of the springs <NUM> and <NUM> are holding the contacts <NUM> and <NUM> closed and the actuator housing <NUM> will reverse its direction. At point <NUM>, about time <NUM>, the contacts <NUM> and <NUM> will separate, but there still is conduction across the gap <NUM> until the gap length is about <NUM>, which occurs at point <NUM> at about time <NUM>. One fundamental frequency cycle at <NUM> is about <NUM> for <NUM>, so the time from the beginning of the conduction at time <NUM> until the end of conduction at time <NUM> is <NUM>, which is about three-fourths of the cycle, where the current stops at the next zero current crossing. Thus, the current conduction time through the contacts <NUM> band <NUM> is one fundamental frequency cycle at <NUM>. These times can be adjusted for systems that operate at other frequencies.

The control of the switch assembly <NUM> as being described includes driving the movable contact <NUM> into the fixed contact <NUM> and then immediately turning off the current applied to the coil <NUM>, or reversing the voltage on the coil <NUM> to drive the coil current to zero, so that the closing force between the contacts <NUM> and <NUM> is only provided by the momentum of the actuator <NUM> and the mass of the movable contact <NUM> against the forces provided by the open spring <NUM> and the compliance spring <NUM>, which bounces the movable contact <NUM> off of the fixed contact <NUM>. In other words, the velocity of the actuator <NUM> after the coil <NUM> is turned off, or otherwise has zero coil current, compresses the compliance spring <NUM> until the stored energy in the moving mass of the actuator <NUM> and the movable contact <NUM> is transferred to the compliance spring <NUM>, where the compliance spring <NUM> will push the contacts <NUM> and <NUM> back open again. Thus, by intentionally bouncing the contacts <NUM> and <NUM> for the low energy test in this manner, the potential fault current duration is limited to one fundamental frequency current cycle as described. Specifically, the control of the switch assembly <NUM> is capable of opening the vacuum interrupter contacts <NUM> and <NUM> fast enough to be able to interrupt current flow within <NUM> of the first conduction between the contacts <NUM> and <NUM>, which gives a total conduction time from closing to opening the contacts <NUM> and <NUM> less than <NUM> as measured from a <NUM> open pre-strike contact gap to a <NUM> open contact gap. This results in one fundamental frequency current cycle of symmetrical fault current, which is twice the time that the present single pulse would allow as known pulse testing a faulted circuit results in a half-cycle of fault current. These times can be adjusted for systems that operate at other frequencies.

Thus, the switch assembly <NUM> is designed so that the moving mass of the actuator <NUM> and the movable contact <NUM>, the size and length of the compliance spring <NUM>, the amount of current applied to the coil <NUM>, the time that the current is removed from the coil <NUM> during the closing operation, etc. cause the contacts <NUM> and <NUM> to conduct for about one fundamental frequency cycle of fault current. Since the control of the switch assembly <NUM> allows control of the position of the movable contact <NUM> relative to time, the timing of the conduction between the contacts <NUM> and <NUM> can be controlled so that current begins flowing at a <NUM>° angle of the voltage waveform so that symmetrical currents flow instead of asymmetrical currents when the contacts <NUM> and <NUM> are closed, which reduces forces on the network components. Further, traditional vacuum interrupters can be employed instead of more complex and specialized vacuum interrupters required for the known pulse testing.

Another embodiment includes closing the vacuum interrupter contacts <NUM> and <NUM> at the voltage peak that drives transformers away from saturation, thereby keeping the transformers from saturating, where when a transformer is de-energized there is often a non-zero remnant flux in the transformer core. Energization in-rush current results from saturation of the transformer core when it is energized at a voltage point where the new flux drives the transformer into saturation. This can be reduced/eliminated by keeping a flux model of transformers in some form of memory, closing the vacuum interrupter contacts <NUM> and <NUM> so that the flux will move toward zero rather than away from zero. If the residual flux is the result of positive voltage the vacuum interrupter contacts <NUM> and <NUM> close when the voltage is negative moving the flux toward zero and minimizing the peak flux that drives the transformer core into saturation. The flux at peak voltage is zero because it lags the voltage by <NUM>°. If this is the case, the point-on-wave can be adjusted to correct the residual flux by closing slightly earlier than the peak of the voltage waveform.

Many of the examples used in this discussion were derived for power systems operating at <NUM> fundamental frequency. It is noted that similar techniques can be applied to adjust the mechanism to operate at other fundamental frequencies.

Claim 1:
A method for operating a magnetically actuated switch assembly (<NUM>) to perform a low energy test pulse, the switch assembly (<NUM>) including a fixed contact (<NUM>) and a movable contact (<NUM>), the method comprising:
energizing a magnetic actuator (<NUM>) to move the moveable contact (<NUM>) against the bias of at least one spring (<NUM>, <NUM>) coupled to the movable contact (<NUM>) towards the fixed contact (<NUM>) to make contact therebetween, characterised by the method being such that AC current conducts across a gap (<NUM>) formed between the movable contact (<NUM>) and the fixed contact (<NUM>) as the moveable contact (<NUM>) is moved toward the fixed contact (<NUM>) and before the movable contact (<NUM>) and the fixed contact (<NUM>) make contact; and
reversing the magnetic actuator (<NUM>) to move the movable contact (<NUM>) away from the fixed contact (<NUM>) at least under the bias of the at least one spring (<NUM>, <NUM>) so that the amount of time that the current conducts between the movable contact (<NUM>) and the fixed contact (<NUM>) is about one fundamental frequency cycle, and preferably, about one-half of one fundamental frequency cycle of the current, wherein reversing the magnetic actuator (<NUM>) occurs at a time so that when the movable contact (<NUM>) and the fixed contact (<NUM>) begin conducting an applied voltage on the switch assembly (<NUM>) is at or near a peak of the voltage wave so that the current is symmetric.