Transformer internal fault reclose block

A fault detection system that that prevents a recloser from reclosing if a fault is determined to be internal to a transformer, where the recloser is configured to perform a reclosing operation in response to detecting overcurrent. The recloser includes a sensor, such as a light sensor, directed towards the transformer and detecting a fault event. If the recloser detects overcurrent, but the sensor does not detect the fault event, it is assumed that the fault is internal to the transformer and the recloser is prevented from reclosing.

BACKGROUND

Field

This disclosure relates generally to a fault detection system that prevents a recloser from reclosing if a fault is determined to be internal to a transformer and, more particularly, to a fault detection system that prevents a recloser from reclosing if a fault is determined to be internal to a transformer, where if the recloser detects overcurrent and does not detect an arc external to the transformer reclosing is prevented.

Discussion of the Related Art

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 to a medium voltage for distribution. The substations provide the medium voltage power to a number of three-phase feeders that carry the same current, but are 120° apart in phase. A number of three-phase and single phase lateral lines are tapped off of the feeders that provide the medium voltage to a number of distribution lines that each include a distribution transformer, where the voltage is stepped down to a low voltage and is provided to a number of loads, such as homes, businesses, etc. Traditionally, a fuse that is an independent electrical device that is not in communication with other components or devices in the network is provided at the location where the distribution line is tapped off of the lateral line, where the fuse creates an open circuit if an element within the fuse heats up above a predetermined temperature.

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 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 by the vacuum. A vapor shield is provided around the contacts to contain the arcing.

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 load 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.

Fault interrupting devices, for example, single phase self-powered magnetically actuated 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. It has become increasingly more popular to replace the traditional fuse with a fault interrupting device at the location where a distribution line is tapped off of a lateral line just before the distribution transformer so as to reduce the number of service calls to replace fuses in response to temporary faults that can be cleared by the fault interrupting device. One of those devices used for this purpose is known as the VacuFuse® resettable interrupter, available from S&C Electric Company, Chicago, Ill., USA.

Reclosers and fault interrupters of this type typically detect the current and/or 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. For example, a recloser may employ a Rogowski coil, well known to those skilled in the art, that is wrapped around the power line and measures current flow on the line by means of the voltage that is induced in the coil being proportional to the rate of change of current flow. 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 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 3-6 cycles or 50 to 100 ms 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, which could cause significant stress on various components in the network.

Some faults are permanent faults that can't be cleared by a reclosing operation, where the fault interrupter remains in an open state until the fault is fixed. One of those types of faults can be an internal fault in a distribution transformer. However, it is also desirable not to perform the reclosing operation to test for the fault if the fault is internal to the transformer because putting fault current on a damaged transformer could cause a catastrophic failure of the transformer, thus creating a safety issue. Currently, these types of reclosers and fault interrupters are not able identify that a fault is internal or external to the transformer.

SUMMARY

The following discussion discloses and describes a fault detection system that that prevents a recloser from reclosing if a fault is determined to be internal to a transformer, where the recloser is configured to perform a reclosing operation in response to detecting overcurrent. The recloser includes a sensor, such as a light sensor, directed towards the transformer and detecting a fault event. If the recloser detects overcurrent, but the sensor does not detect the fault event, it is assumed that the fault is internal to the transformer and the recloser is prevented from reclosing.

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

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the disclosure directed to a fault detection system that prevents a recloser from reclosing if a fault is determined to be internal to a transformer is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses. For example, the discussion below refers to the transformer as being a distribution transformer. However, the fault detection system may be applicable for other types of transformers.

FIG.1is an illustration of a power distribution system10including a distribution transformer12mounted to a utility pole16and a cut-out mounted recloser14mounted to the utility pole16by a mount18and an insulator20. The transformer12includes an outer can22having a lid24typically filled with oil that houses primary and secondary coils (seeFIG.3). The recloser14is intended to represent any reclosing or fault interrupting device of the type discussed above, such as a single phase self-powered magnetically actuated recloser that employs a vacuum interrupter, suitable for performing reclosing operations at a location where a power distribution line36is tapped off of a lateral line (not shown). The recloser14includes a vacuum interrupter30, a magnetic solenoid and all of the necessary electronics, components and sensors for measuring current, operating the vacuum interrupter30, harvesting current, processing signals, etc. including a current sensor50, a controller26and a suitable communications device28for transmitting and receiving signals. If the recloser14performs a reclosing operation and is unable to clear a fault, it will be released from a connector32and pivot on a hinge34to provide a visual indication that is has operated.

The power distribution line36at medium voltage that is tapped off of the lateral line is electrically coupled to one terminal of the recloser14and a power line38at medium voltage is electrically coupled to the other terminal of the recloser14and the primary winding in the transformer12through a bushing40, where the bushing40includes an internal conductor and an outer insulation body. An arrester42is mounted to the transformer12and ground to provide over-voltage protection. The transformer12steps down the medium voltage on the line38to a lower voltage, and, in this example, provides 120 volt power at a terminal44or46and a ground terminal48and 240 volt power between terminals44and46, where the ground terminal48is electrically coupled to a neutral line (not shown) in most applications. The general configuration of the system10is typical, and therefore additional discussion of its structure and function is omitted for brevity.

As discussed above, the recloser14measures current and is able to perform a reclosing operation in response to detecting overcurrent from, for example, a fault on the line38or within the transformer12. However, as mentioned above, known reclosers are unable to determine whether a fault is internal or external to the transformer12, where it would be desirable for the recloser14to not perform the reclosing operation if the fault was inside of the transformer12because of safety concerns.

In order to address this issue, this disclosure proposes fitting the distribution transformer12with an internal fault detector (IFD), which is a known spring actuated, single operation device that is triggered by overpressure that is offered by the IFD Corporation of Vancouver BC, Canada. The IFD is mounted inside of the transformer12and pushes a mechanical indicator out if there is a sudden increase in pressure associated with an internal fault. A short-range radio, such as Zigbee or Bluetooth radio, is coupled to the IFD that is turned on by a switch that is actuated by the motion of the mechanical indicator in the IFD. When the IFD detects overpressure, it turns on the radio, which transmits a signal to the communications device28inside the cutout mounted recloser14. When the recloser14receives the signal, it does not perform the reclosing operation and as such would trip and remove the faulted transformer.

The radio includes a power supply that would be powered by the low voltage transformer output and as such power would always be available before the fault occurs when the transformer12is energized. The power supply can be design to work over a voltage range of 20 V peak to 200 V peak for use on a 120 V transformer output. This range would allow the power supply to stay energized even when the transformer12has some degree of internal fault that depresses the output voltage. Each winding of the transformer12can be a source that makes the input power redundant at least from the secondary winding perspective. The power supply would have energy storage in the form of capacitors (film, electrolytic, or super capacitors) so that the radio can be powered for a sufficient time once the recloser14opens to clear the fault. It is during this open interval that the radio communications needs to transmit the reclosing blocking signal. For such a short duration, the radio signal can be strong and simple ensuring the transmittal and reception of the blocking signal.

FIG.2is a depiction of a transformer60modified in this manner that can replace the transformer12, where like elements are identified by the same reference number. Particularly, the transformer60includes an IFD66mounted to and extending through and into the can22. The IFD66includes a sensor68that monitors pressure within the can22and if the pressure exceeds a predetermined pressure, which may occur as a result of a fault in the transformer60, will cause a mechanical indicator70to extend. A radio72is also mounted to the can22and is coupled to the IFD66that transmits the blocking signal to the recloser14.

FIG.3is a schematic diagram of a fault detection system80including the IFD66and the radio72separated from the transformer60. The system80also includes a primary coil82and a secondary coil84that would be housed within the can22, where the radio72is powered by a power supply86that receives power from the secondary coil84. A power storage unit88, such as a bank of capacitors, stores power to operate the radio72when the recloser14is open and power is not being provided to the transformer60. The radio72includes a switch90that is mechanically coupled to the indicator70so that when the indicator70extends in response to the detection of high pressure, the switch90will close, which causes a transmitter92to transmit a signal that is received by the communications device28in the recloser14. The device28sends a message to the controller26and the controller26prevents the recloser14from reclosing.

The embodiment discussed above employed a technique for determining that a fault is internal to the transformer60. In another embodiment, a technique is employed for determining that the fault is external to the transformer12, such as by detecting an arc from the top of the bushing40to the can22or along the line38, where if it is determined that the fault is not external to the transformer12it is assumed that it is internal to the transformer12. An electric arc on the outside of the transformer12across the bushing40, from the arrester42, or from the drop lead to ground, will emit a strong flash of light, an over-pressure wave and/or high-frequency radiation. By providing a simple sensing element in connection with the recloser14, a fault can be identified that is outside of the transformer12. The intensity of the arc-flash light is quite high making sensing of the light practical even in outdoor sunlight. The recloser14is generally mounted in close proximity to the transformer12enhancing the ability to detect the arc flash. Increased sensing reliability to differentiate over high sunlight conditions can be obtained by looking for a sudden change in the light intensity over the average ambient lighting, which can be adjusted as the ambient light changes.

As a non-limiting example, a simple light sensor52that is part of the recloser14and is directed towards the transformer12could detect a bright or sudden flash of light in combination with the detection of overcurrent, where a lens54can be provided to expand the field of view of the sensor52. The light sensor52is a known technology and can be powered, monitored and analyzed by the controller36inside of the recloser14that is responding to the fault overcurrent. If overcurrent trips the recloser14and a flash of light is detected by the sensor52, then the fault is outside of the transformer12and the controller26allows reclosing. If overcurrent trips the recloser14and a flash of light is not detected by the sensor52, then the fault is assumed to be inside of the transformer12and the controller26does not allow reclosing.

Other sensors could be utilized as a replacement or supplemental for the light sensor52. An antenna could sense higher frequency electromagnetic radiation that would be high for external arcs and low for internal arcs. Also, a pressure sensor could detect the overpressure associated with the external arc. It is noted that the sensor52is intended to represent any of these various types of sensors and combination of sensors suitable for the purposes discussed herein.