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EP-4298702-B1 - TRANSFORMER OVERCURRENT PROTECTION

EP4298702B1EP 4298702 B1EP4298702 B1EP 4298702B1EP-4298702-B1

Inventors

  • BISHOP, MARTIN
  • WILLIAMS, STEPHEN
  • PEREZ, MARCELO

Dates

Publication Date
20260513
Application Date
20220222

Claims (12)

  1. A method for determining when an electronic interrupting device (14) will open in response to detecting overcurrent, the electronic interrupting device (14) protecting a transformer (12) having a winding in a power distribution network, the method comprising: obtaining a time/current through fault protection curve (54) that is defined by a plurality of time/current points for the transformer (12) that identifies when the transformer (12) may experience thermal or mechanical damage in response to a certain current flow over a certain time in the transformer winding; selecting a time multiplier; the method is characterised in further comprising determining an operating curve (64) for the electronic interrupting device (14) by multiplying the multiplier and a time portion of each of the plurality of time/current points on the through fault protection curve, where the operating curve (64) identifies when the electronic interrupting device (14) will open in response to a certain current flow over a certain time.
  2. The method of claim 1, wherein determining the operating curve (64) also includes providing an overload portion (66) of the operating curve (64) that defines an overload current that if not exceeded for any amount of time the interrupting device (14) will not open, providing a two fundamental power frequency cycle portion (68) of the operating curve (64) that defines a current that if exceeded will cause the interrupting device (14) to open after two fundamental power frequency cycles and providing a one fundamental power frequency cycle portion (70) of the operating curve (64) that defines a current that if exceeded will cause the interrupting device (14) to open after one fundamental power frequency cycle.
  3. A system for determining when an electronic interrupting device (14) will open in response to detecting overcurrent, the electronic interrupting device (14) protecting a transformer (12) having a winding in a power distribution network, the system comprising: means for obtaining a time/current through fault protection curve (54) that is defined by a plurality of time/current points for the transformer (12) that identifies when the transformer (12) may experience thermal or mechanical damage in response to a certain current flow over a certain time in the transformer winding; means for selecting a time multiplier; the system is characterised in that it further comprises means for determining an operating curve (64) for the electronic interrupting device (14) by multiplying the multiplier and a time portion of each of the plurality of time/current points on the through fault protection curve (54), where the operating curve (64) identifies when the interrupting device (14) will open in response to a certain current flow through the transformer windings over a certain time.
  4. The method according to claim 1 or the method of claim 2 or the system of claim 3 wherein the multiplier is less than one.
  5. The method according to claim 1 wherein determining an operating curve (64) includes providing an overload portion (66) of the operating curve (64) that defines an overload current that if not exceeded for any amount of time the electronic interrupting device (14) will not open; or the system according to claim 3 wherein the means for determining an operating curve (64) provides an overload portion (66) of the operating curve (64) that defines an overload current that if not exceeded for any amount of time the electronic interrupting device (14) will not open.
  6. The method according to claim 1 wherein determining an operating curve (64) includes providing a two fundamental power frequency cycle portion (68) of the operating curve (64) that defines a current that if exceeded will cause the electronic interrupting device (14) to open after two fundamental power frequency cycles; or the system according to claim 3 wherein the means for determining an operating curve (64) provides a two fundamental power frequency cycle portion (68) of the operating curve (64) that defines a current that if exceeded will cause the electronic interrupting device (14) to operate after two fundamental power frequency cycles.
  7. The method according to claim 1 wherein determining an operating curve (64) includes providing a one fundamental power frequency cycle portion (70) of the operating curve (64) that defines a current that if exceeded will cause the electronic interrupting device (14) to operate after one fundamental power frequency cycle; or the system according to claim 3 wherein the means for determining an operating curve (64) provides a one fundamental power frequency cycle portion (70) of the operating curve (64) that defines a current that if exceeded will cause the electronic interrupting device (14) to open after one fundamental power frequency cycle.
  8. The method according to claim 1 wherein the transformer (12) is a distribution transformer that converts medium voltage to low voltage to be delivered to low voltage customer loads; or the system according to claim 3 wherein the transformer (12) is a distribution transformer that converts medium voltage to low voltage to be delivered to low voltage customer loads.
  9. The method according to claim 1 wherein the interrupting device (14) is a cut-out mounted interrupting device (14); or the system according to claim 3 wherein the interrupting device (14) is a cut-out mounted interrupting device (14).
  10. The method according to claim 1 wherein the interrupting device (14) is a single phase self-powered magnetically actuated recloser; or the system according to claim 3 wherein the interrupting device (14) is a single phase self-powered magnetically actuated recloser.
  11. The method according to claim 1 wherein the interrupting device (14) includes a vacuum interrupter.
  12. The method according to claim 1 wherein the interrupting device (14) and the transformer (12) are mounted to the same utility pole (16).

Description

BACKGROUND Field This disclosure relates generally to a method for programming the operation of an electronic interrupting device to closely follow a through fault protection curve for a transformer and, more particularly, to a method for selecting an operating curve for an electronic interrupting device that closely follows a through fault protection curve for a distribution transformer by multiplying a defined time of the through fault protection curve by a multiplier in a selected current operating range. Discussion of the Related Art An electrical power distribution network, often referred to as an electrical grid, typically includes power generation plants each having power generators, such as gas turbines, nuclear reactors, coal-fired generators, hydro-electric dams, etc. The power plants provide power at 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 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 three-phase feeders that carry the same current for balanced loading, but are 120° apart in phase. Three-phase and single phase lateral lines are tapped off of the feeders that provide the medium voltage to distribution lines that each include a distribution transformer, where the voltage is stepped down to a low voltage and is provided to loads, such as homes, businesses, etc. Periodically, faults occur in the distribution network as from 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, 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. Traditionally, a fuse is employed as a primary overload protection device for protecting distribution transformers that has a certain rating so that the fuse will operate above a transformer inrush current, but below a transformer through fault protection withstand or damage curve. Primary overload protection is also expected to protect the transformer from damage due to long overloads and secondary faults by removing the transformer from service after a severe overload of significant time duration. Secondary faults producing winding currents that are many times full load magnitude should also result in operation of the protection device before thermal or mechanical damage occurs to the transformer. The time limits for winding current as a multiple of full load current are established by IEEE C57.109, Guide for Liquid-Immersed Transformer Through-Fault-Current Duration. As a general guideline, the fuse time current characteristic (TCC) curve should be below and to the left of the transformer through-fault-duration withstand curve so as to minimize the loss of transformer life due to the mechanical and thermal effects of sustained faults and longtime overload conditions. A primary side protective device used for a pole-type transformer is subject to energizing inrush transient currents and cold-load currents, which should be accounted for, so that the protection is not activated during these events. Typical inrush current magnitudes for distribution transformers are twelve times full load at the 0.1 second point and 25 times full load at the 0.01 second point. Cold-load inrush currents that are above rated full load current for many seconds must be carried by the fuse or other protective device after an outage. Typical cold-load inrush points are two times full load at 100-300 seconds, three times full load at 10 seconds, and six times full load at one second. Some utilities may experience twice full load current for 30 minutes and three times full load for 30 seconds when resistive load predominates. The inrush points and cold-load pickup points form a TCC curve that must fall below and to the left of the protective device's minimum TCC curve. In all applications, the primary protective device for a pole-type distribution transformer should maintain proper coordination with upstream devices. It must be the first device to trip, thus avoiding the operation of upstream devices that result in a large number of affected customers. Primary protection of pole-type distribution transformers must be the fastest in the system, but only tripping when it is necessary. 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 down