U.S. patent application number 13/604113 was filed with the patent office on 2013-02-21 for high voltage sensing capacitor and indicator device.
This patent application is currently assigned to SMC Electrical Products, Inc.. The applicant listed for this patent is Bill Blankenship, Sam Handshoe. Invention is credited to Bill Blankenship, Sam Handshoe.
Application Number | 20130043891 13/604113 |
Document ID | / |
Family ID | 44059930 |
Filed Date | 2013-02-21 |
United States Patent
Application |
20130043891 |
Kind Code |
A1 |
Handshoe; Sam ; et
al. |
February 21, 2013 |
High Voltage Sensing Capacitor and Indicator Device
Abstract
A high-voltage sensing capacitor as an interface apparatus that
may be used to attach an indicator unit to a high-voltage AC
electrical bus and to provide safety to maintenance personnel. The
high-impedance nature of the sensing capacitor effectively isolates
the indicator unit from the high-voltage source to which it is
connected. The sensing capacitor can be directly mounted between a
high-voltage busbar and an indicator unit to provide visual and/or
audible alerts to maintenance personnel when high voltage
conditions are detected on the busbar. The sensing capacitor is
comprised of a portable, unitary capacitive structure that includes
a molded insulator body encapsulating two electrodes. The
electrodes only partially or incompletely overlap within the
insulator body. The electrode spacing and configuration is
structured to provide a deliberate amount of coupling between the
two electrodes in the presence of an AC electric field.
Inventors: |
Handshoe; Sam; (US) ;
Blankenship; Bill; (US) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Handshoe; Sam
Blankenship; Bill |
|
|
US
US |
|
|
Assignee: |
SMC Electrical Products,
Inc.
|
Family ID: |
44059930 |
Appl. No.: |
13/604113 |
Filed: |
September 5, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12622722 |
Nov 20, 2009 |
8294477 |
|
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13604113 |
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Current U.S.
Class: |
324/686 |
Current CPC
Class: |
G01R 15/16 20130101;
G01R 27/2605 20130101 |
Class at
Publication: |
324/686 |
International
Class: |
G01R 27/26 20060101
G01R027/26 |
Claims
1-24. (canceled)
25. A sensing capacitor comprising: a portable, unitary capacitive
structure that includes; an insulator body of a dielectric
material; an elongated electrode extending a distance into the
insulator body with a first terminal at one end of the insulator
body; and a hollow electrode extending a distance into an opposite
end of the insulator body with a second terminal at an opposite end
of the insulator body, the hollow electrode surrounding a section
of the elongated electrode opposite the first terminal for an
incomplete overlap and a physical separation between the elongated
electrode and the hollow electrode, the insulator body configured
to allow external electrical connections to be made to the first
terminal end and the second terminal.
26. The sensing capacitor of claim 25, wherein the dielectric
material is cycloaliphatic epoxy resin.
27. The sensing device of claim 25, wherein the second terminal is
configured to be attached to a high voltage busbar of an AC power
source.
28. The sensing device of claim 27, wherein the first terminal is
configured to attach an indicator device to the high voltage
busbar.
29. The sensing device of claim 27, wherein the second terminal
comprises: an inner portion extending a distance into the insulator
body connected with the hollow electrode; and an outer portion
threaded to allow mounting of the high voltage sensing capacitor
onto the high voltage busbar.
30. The high voltage sensing capacitor of claim 27, wherein the
second terminal comprises: a threaded inner portion extending a
distance into the insulator body connected with the hollow
electrode to allow mounting of the sensing capacitor onto the high
voltage busbar.
31. The sensing capacitor of claim 25, wherein said insulator body
is substantially cylindrical.
32. The sensing capacitor of claim 31, wherein said insulator body
comprises a plurality of surface undulations on an outermost
surface thereof.
33. The sensing capacitor of claim 32, wherein at least one of the
following is dependent on a range of operating voltages for said
sensing capacitor; a length of said substantially cylindrical
insulator body; and a diameter of said substantially cylindrical
insulator body; and number and shape of said surface
undulations.
34. The sensing capacitor of claim 31, wherein a length and a
diameter of the hollow electrode is dependent on at least one of
the following: a length and a diameter of the insulator body; and a
degree of electrical coupling desired between the elongated and the
hollow electrodes.
35. A high voltage sensing capacitor comprising: an insulator body
of a dielectric material; a hollow cylindrical shaped electrode
extending a distance into one end of an insulator body; a elongated
electrode extending into an opposite end of the insulator body
extending into the hollow cylindrical shaped electrode so the
hollow cylindrical shaped electrode incompletely overlaps the
elongated electrode with a physical separation between the
elongated electrode and the hollow cylindrical shaped electrode
throughout a region of the overlap to provide voltage isolation
between the elongated electrode and the hollow cylindrical shaped
electrode, the insulator body substantially completely
encapsulating the elongated electrode and the hollow cylindrical
shaped electrode; and wherein one of the hollow cylindrical
electrode and the elongated electrode are configured to be
electrically connected directly to a high voltage busbar of an AC
(alternating current) power source and an opposite one of the
hollow cylindrical electrode and the elongated electrode is
configured to attach an indicator unit to the high voltage
busbar.
36. The high voltage sensing capacitor of claim 35, further
comprising: an input terminal connected to the hollow cylindrical
shaped electrode.
37. The high voltage sensing capacitor of claim 36, wherein the
input terminal is configured to be electrically connected to the
high voltage busbar when a voltage on the high voltage busbar is in
the range of one of the following: approximately 2.3 KVAC to
approximately 27 KVAC three phase; and approximately 2.3 KVAC to
approximately 18 KVAC single phase.
38. The high voltage sensing capacitor of claim 37, wherein the
input terminal comprises: an inner portion extending a distance
into the insulator body connected with the hollow cylindrical
shaped electrode; and an outer portion threaded to allow mounting
of the high voltage sensing capacitor onto the busbar.
39. The high voltage sensing capacitor of claim 38, wherein the
input terminal comprises: a threaded inner portion extending a
distance into the insulator body connected with the hollow
cylindrical shaped electrode to allow mounting of the sensing
capacitor onto the high voltage busbar.
40. The high voltage sensing capacitor of claim 35, wherein the
elongated electrode is a metallic cylindrical rod that includes an
output terminal to electrically connect the high voltage sensing
capacitor to an indicator device.
41. The high voltage sensing capacitor of claim 40, wherein the
output terminal comprises: an inner portion extending a distance
into the insulator body connected with the elongated electrode; and
an outer threaded portion to allow connecting of the sensing
capacitor onto the indicator device.
42. The high voltage sensing capacitor of claim 40, wherein the
output terminal comprises: a threaded inner portion extending into
the insulator body connected with the elongated electrode to allow
connecting of the sensing capacitor onto the indicator device.
43. The high voltage sensing capacitor of claim 36, wherein the
elongated electrode, hollow cylindrical shaped electrode, and the
insulator body are molded together, thereby creating a unitary
capacitive structure.
44. The sensing capacitor of claim 35, wherein a capacitive
coupling between said elongated electrode and hollow cylindrical
shaped electrodes is dependent on an extent of said overlap between
said elongated and hollow cylindrical shaped electrodes.
45. A high voltage sensing system configuration comprising: a
sensing capacitor having an insulator body of a dielectric material
that includes: an elongated solid electrode extending a distance
into the insulator body; a substantially hollow cylindrical shaped
electrode that surrounds only a part of the elongated electrode so
as to partially overlap while leaving a space between said
elongated and the substantially hollow cylindrical shaped electrode
throughout the overlap; an input terminal in electrical contact
with the substantially hollow cylindrical shaped electrode to
electrically to be electrically connected to a high voltage busbar
of an AC power source; an output terminal in electrical contact
with the elongated electrode, the insulator body substantially
completely encapsulating the elongated electrode, the substantially
hollow cylindrical shaped electrode, and the space between the
elongated and the substantially hollow cylindrical shaped
electrodes; and an indicator unit electrically connected to the
output terminal so as to receive capacitive current generated by
the sensing capacitor when the high voltage busbar is electrically
energized and to thereby provide an indication of presence of high
voltage on the busbar of the AC power source.
Description
BACKGROUND
[0001] 1. Field of the Disclosure
[0002] The present disclosure generally relates to the field of
sensing or detection of presence of high voltages on electrical
conductors in AC (alternating current) power distribution systems,
and, more particularly, to a high voltage sensing capacitor with
electrodes having an incomplete or partial overlap and are
substantially completely encapsulated within an insulator body.
[0003] 2. Brief Description of Related Art
[0004] High voltage single and multi-phase AC (alternating current)
power sources are utilized in many industries. The use of high
voltage AC power distribution systems is accompanied by the risk
that electrical maintenance personnel (and others) may
inadvertently come into contact with energized conductors and be
electrocuted or seriously injured. Thus, it is desirable to safely
determine if a particular segment of an electrical bus is
energized. Such determination of existence or magnitude of these
high voltage conditions may prevent human injuries or deaths.
[0005] U.S Pat. No. 5,065,142 ("the '142 patent") discusses a
safety apparatus for indicating a live AC voltage condition in an
insulated electrical conductor having a central conductor wire. A
capacitor is constructed around the insulated conductor so the high
voltage central conductor wire within the insulation forms an inner
electrode of the capacitor and the conductor insulation forms the
principal dielectric of the capacitor. The outer capacitor
electrode is slidably placed onto the insulated conductor. Such
capacitor provides a high impedance circuit through a gas discharge
lamp to ground. The current through the capacitor is sufficient to
cause the discharge tube to glow when the high voltage circuit is
energized, thus providing a visible warning to maintenance
personnel. Each of the three phases of an electrical distribution
network can be independently monitored in this manner.
[0006] The safety apparatus discussed in the '142 patent may not be
suitable to be attached to a busbar in a piece of switchgear.
Hence, a busbar-based voltage sensing device may be desirable for
higher voltage applications.
[0007] Furthermore, it is observed here that some existing
high-voltage interface apparatus are constructed such that the
electrodes and the dielectric material of the capacitive interface
are exposed to air. This exposure to air may create corona and
fringing, which may in turn create a partial discharge condition.
The partial discharge may cause a continual degradation in the
capacitor's dielectric material that may eventually cause a
"punch-through" dielectric failure. Such a failure may be
catastrophic and irreversible. The dielectric failure may not only
render the capacitive interface useless to provide the voltage
sensing functionality, but may also create potentially unsafe and
hazardous short-circuit conditions on high-voltage conductors.
Hence, it also may be desirable to provide a high-voltage sensing
device that can avoid potentially destructive partial discharges
when used under ambient surroundings.
SUMMARY
[0008] A high-voltage sensing capacitor according to one embodiment
of the present disclosure is an interface apparatus that may be
used to attach an indicator unit to a high-voltage AC electrical
bus (single-phase or three-phase). Multiple electrical phases can
be interfaced using a plurality of such sensing capacitors. The
sensing capacitor can be directly mounted to a high-voltage busbar.
The indicator unit may provide visual and/or audible alerts to
maintenance personnel when high voltage conditions are detected on
the busbar by the sensing capacitor. Thus, the sensing capacitor
can be used in conjunction with a suitable indicator unit to
provide safety or transducer functions.
[0009] In one embodiment, the present disclosure relates to a
sensing capacitor that comprises a portable, unitary capacitive
structure, which includes a pair of electrical conductors having an
incomplete overlap and a physical separation therebetween; and an
insulator body of a dielectric material substantially completely
encapsulating the pair of electrical conductors and the physical
separation therebetween, wherein the insulator body is configured
to allow external electrical connections to be made to the pair of
electrical conductors.
[0010] In a further embodiment, the present disclosure relates to a
high voltage sensing configuration that comprises a sensing
capacitor and an indicator unit. The sensing capacitor includes: a
first electrode having an inner portion and an outer portion,
wherein the outer portion is electrically connected to a high
voltage busbar of an AC power source; a second electrode having a
hollow cylindrical shape that surrounds only a part of the inner
portion of the first electrode so as to partially overlap the inner
portion while leaving a space between the first and the second
electrodes throughout a region of the overlap; an output terminal
in electrical contact with the second electrode; and an insulator
body of a dielectric material substantially completely
encapsulating the inner portion of first electrode, the second
electrode, the space between the first and the second electrodes,
and the output terminal. The indicator unit is electrically
connected to the output terminal (of the sensing capacitor) so as
to receive capacitive current generated by the sensing capacitor
when the high voltage busbar is electrically energized and to
thereby provide an indication of presence of high voltage on the
busbar of the AC power source.
[0011] In another embodiment, the present disclosure relates to a
system comprising: a first electrode having a threaded portion and
a non-threaded portion, wherein the threaded portion is configured
to be electrically connected to a high voltage busbar of an AC
power source; a second electrode having a hollow cylindrical shape
that surrounds only a part of the non-threaded portion of the first
electrode so as to incompletely overlap the non-threaded portion
while leaving a space between the first and the second electrodes
throughout a region of the overlap; an output terminal in
electrical contact with the second electrode; and an insulator body
of a dielectric material substantially completely encapsulating the
non-threaded portion of first electrode, the second electrode, the
space between the first and the second electrodes, and the output
terminal.
[0012] Thus, the sensing capacitor according to one embodiment of
the present disclosure is comprised of an insulator body
encapsulating two electrodes. The electrode spacing and
configuration is structured to provide a deliberate amount of
coupling between the two electrodes in the presence of an AC
electric field. The sensing capacitor provides a very high
impedance interface to the high-voltage bus, thereby delivering
only microampere-level currents to an indicator unit to enable the
indicator unit to provide the desired indication of the presence of
high voltages on the electrical bus. The high-impedance nature of
the sensing capacitor effectively isolates the indicator unit from
the high-voltage source to which it is connected.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For the present disclosure to be easily understood and
readily practiced, the present disclosure will now be described for
purposes of illustration and not limitation, in connection with the
following figures, wherein:
[0014] FIG. 1 illustrates a simplified view of an exemplary high
voltage sensing configuration according to one embodiment of the
present disclosure;
[0015] FIGS. 2A through 2C depict exemplary mechanical views of the
sensing capacitor according to one embodiment of the present
disclosure;
[0016] FIG. 3 shows a cross-sectional view depicting exemplary
constructional details of the high-voltage sensing capacitor
according to one embodiment of the present disclosure;
[0017] FIG. 4 is an exemplary circuit layout for providing audible
and visual alerts through the indicator unit when high-voltage is
detected by the sensing capacitor on an electrical bus of one of
the AC phases; and
[0018] FIG. 5 is an exemplary circuit layout similar to that in
FIG. 4, but configured to provide only audible alerts through the
indicator unit.
DETAILED DESCRIPTION
[0019] The accompanying figures and the description that follows
set forth the present disclosure in embodiments of the present
disclosure. It is to be understood that the figures and
descriptions of the present disclosure included herein illustrate
and describe elements that are of particular relevance to the
present disclosure, while eliminating, for the sake of clarity,
other elements found in typical AC power distribution systems. It
is contemplated that persons generally familiar with designs,
maintenance, or operation of AC power distribution systems, will be
able to apply the teachings of the present disclosure in other
contexts by modification of certain details. Accordingly, the
figures and description are not to be taken as restrictive of the
scope of the present disclosure, but are to be understood as broad
and general teachings.
[0020] In the discussion herein, when any numerical value is
referred, such value is understood to be the most
practically-feasible design approximation taking into account
variances that may be introduced by such mechanical operations as
machining, tooling, drilling, threading, molding, etc. Furthermore,
a range of numerical values is understood to include all values
that constitute the range, unless otherwise indicated. Also,
various numerical values are provided merely as examples for
different embodiments of the present disclosure, without
necessarily restricting availability and application of different
numerical values in the context of present disclosure.
[0021] It is noted at the outset that the terms "coupled,"
"connected", "connecting," "electrically connected," etc., are used
interchangeably herein to generally refer to the condition of being
electrically connected. It is further noted that various figures
(including circuit diagrams, component diagrams, or mechanical
drawings) shown and discussed herein are for illustrative purpose
only, and are not drawn to scale. The terms "ground," "circuit
ground," or other terms of similar import (or the symbolic
representation of "ground" by reference numeral "20" in various
figures herein) are used interchangeably herein to refer to a
common circuit ground potential (which may or may not be zero) as
is known in the art.
[0022] FIG. 1 illustrates a simplified view of an exemplary high
voltage sensing configuration 10 according to one embodiment of the
present disclosure. As noted before, high-voltage single and
multi-phase AC power sources (e.g., switchboards or switchgears,
transformers, power distribution boards, substations, or other
electrical apparatus) are utilized in many industries. The use of
high voltage AC power distribution systems is accompanied by the
risk that electrical maintenance personnel (and others) may
inadvertently come into contact with energized conductors and be
electrocuted or seriously injured. Thus, it is desirable to safely
determine if a particular segment of an electrical bus is
energized. A high-voltage sensing capacitor 12 according to one
embodiment of the present disclosure may be used for this purpose
as an interface apparatus between a high-voltage busbar 14 of a
single-phase or three-phase AC power source and an indicator unit
(also interchangeably referred to hereinbelow as "indicator
device") 16 that may be used to provide visual or audible alerts of
high voltage conditions on the busbar 14. In one embodiment, the
sensing capacitor 12 may be directly mounted or attached to the
high-voltage busbar 14 (e.g., a main busbar in a piece of
switchgear) using a combination of mounting studs, hexnuts, and
lockwashers collectively represented by reference numeral "15" in
FIG. 1. In confined or other restrictive applications, a busbar
extension (not shown) can be mounted to the main busbar to allow
the sensing capacitor 12 to be conveniently mounted.
[0023] As is known in the art, the term "busbar" in electrical
power distribution refers to thick, flat strips or hollow tubes of
copper or aluminum that conduct electricity within a switchboard,
distribution board, substation, or other single-phase or
three-phase AC power source. In the discussion below, the terms
"busbar," "AC bus," "bus," or "electrical bus" may be used
interchangeably and such usage may be evident from the context of
discussion. A busbar may provide both a mechanical and an
electrical connection. Hence, busbars may be connected to each
other and to other electrical apparatus by bolted or clamp
connections. One such connection between the busbar 14 and the
sensing capacitor 12 is illustrated in FIG. 1.
[0024] The sensing capacitor 12 provides a very high impedance
interface to the high-voltage busbar 14, thereby delivering only
microampere-level currents to the indicator device 16 to enable the
indicator device 16 to provide the desired indication of the
presence of high voltages (e.g., in the range of approximately 2.3
KVAC to approximately 27 KVAC three phase, and in the range of
approximately 2.3 KVAC to approximately 18 KVAC single phase) on
the electrical bus/busbar 14. The high-impedance nature of the
sensing capacitor 12 effectively isolates the indicator device 16
from the high-voltage source (here, the busbar 14) to which it is
connected. The output of the sensing capacitor 12 may be routed via
a single conductor 17 to the indicator unit 16. A male pin or lead
18 of the conductor 17 may be inserted into an opening of a
threaded output terminal 34 (not shown in FIG. 1, but illustrated
in FIG. 3) of the sensing capacitor 12, whereas the female pin or
lead (not visible in FIG. 1) of the conductor 17 may be connected
to the indicator unit 16 via a plug and cord grip assembly 19.
Under operating conditions, the current may flow from the
high-voltage busbar 14 through the sensing capacitor 12 into the
indicator device 16 (via conductor 17), and finally into an earth
ground connection 20. In one embodiment, the ground 20 may
represent the circuit or chassis ground of the indicator unit
16.
[0025] It is noted here that for ease of illustration and
discussion, FIG. 1 shows the busbar 14 of only one phase of a
3-phase AC power distribution system. Although not shown here,
multiple electrical phases can be interfaced using a plurality of
sensing capacitors. In other words, a sensing capacitor similar to
the capacitor 12 in FIG. 1 may be similarly mounted to a busbar of
each remaining phase, and the outputs from all such AC
phase-specific sensing capacitors may be provided to the indicator
unit 16. The indicator unit 16 may be equipped to receive multiple
inputs. Thus, in one embodiment, outputs from a plurality of
sensing capacitors (not shown) can be connected to the indicator
device 16 via a single point (e.g., via the plug and cord grip
assembly 19) so as to enable the indicator device 16 to
individually monitor high-voltage conditions on each electrical
bus.
[0026] In the embodiment of FIG. 1, the electrodes of the sensing
capacitor 12 are not visible because they are encapsulated within
an insulator body 22 (of dielectric material) having a plurality of
surface undulations 24. Thus, during operation of the capacitor 12,
only the insulator body 22 (and its surface undulations 24) may
remain exposed to ambient surroundings or air, thereby
significantly reducing partial discharge conditions due to corona
and fringing. In one embodiment, the insulator body 22 may be
constructed using a cycloaliphatic epoxy resin as dielectric
material. This resin may provide extremely high dielectric
characteristics with extremely low partial discharge performance.
Thus, degradation of dielectric material and, hence, a catastrophic
"punch-through" dielectric failure can be significantly minimized
using such resin in the sensing capacitor configuration illustrated
in more detail in FIG. 3 and discussed later hereinbelow.
[0027] FIGS. 2A through 2C depict exemplary mechanical views of the
sensing capacitor 12 according to one embodiment of the present
disclosure. The insulator body 22 and surface undulations 24 may
form the outer surface of the capacitor 12 that is exposed to air
or other ambient surroundings as mentioned before. An outer portion
25 of capacitor's input electrode 28 (not visible in FIG. 2A, but
shown in FIG. 3) may remain outside of the insulator body 22 so as
to allow the capacitor 12 to be electrically connected, e.g., to
the busbar 14. In one embodiment, the outer portion 25 is threaded
so as to allow mounting of the capacitor 12 onto the busbar 14 via
studs, hexnuts, and lockwashers 15 as illustrated in FIG. 1. FIG.
2B illustrates a left-hand side view and FIG. 2C illustrates a
right-hand side view of the front view shown in FIG. 2A. The
reference numeral "27" in the view in FIG. 2C relates to the
opening 35 of the output terminal 34 encapsulated within the
insulator body 22 as shown in FIG. 3 and discussed later
hereinbelow. The opening 35 allows the male pin 18 of the conductor
17 to be inserted into the capacitor 12 for electrical
connections.
[0028] FIG. 3 shows a cross-sectional view depicting exemplary
constructional details of the high-voltage sensing capacitor 12
according to one embodiment of the present disclosure. As
illustrated in FIG. 3, the sensing capacitor 12 according to one
embodiment of the present disclosure is comprised of the insulator
body 22 encapsulating two electrodes--a high-voltage input
electrode 28, and an output electrode 30. The electrode spacing and
configuration may be structured to provide a deliberate amount of
coupling between the two electrodes 28, 30 in the presence of an AC
electric field. In the embodiment of FIG. 3, the electrodes 28, 30
are physically separate from each other, but they have a partial
(incomplete) overlap between them within the insulator body 22. As
mentioned before, the sensing capacitor 12 further comprises
creepage surface undulations 24 forming an external part of the
insulator body 22. An output terminal 34 with an opening 35 may
also form part of the capacitor structure and may allow an external
electrical connection to be made to the output electrode 30 as
discussed below.
[0029] As illustrated in the embodiment of FIG. 3, the insulator
body 22 may be substantially cylindrical and, in one embodiment,
may be constructed of cycloaliphatic epoxy resin as dielectric
material for the sensing capacitor 12. In the embodiment of FIG. 3,
there are five surface undulations each having a substantially
annular (sloping ring- or disc-type) shape as can be more clearly
seen from FIGS. 1 and 2A. However, depending on the range of
operating voltages that may be applied to the sensing capacitor 12
in the field, one or more of the length, the diameter, and the
number and shape of the surface undulations 24 may be varied during
construction of the capacitor 12 so as to accommodate the desired
range of operating voltages. The surface undulations 24 provide the
surface creepage distance between the input electrode 28 and the
output terminal 34. In one embodiment, the creepage distance is
12.8 inches for 18 KVAC per IEC (International Electrotechnical
Commission) standards for light dust. Whereas, in another
embodiment, the creepage distance is 20 inches for 25 KVAC per IEC
standards for medium dust.
[0030] The input electrode 28 may consist of the outer portion 25
and an inner portion 26. The inner portion 26 may extend a
prescribed distance into the insulator body 22. In one embodiment,
the outer portion 25 may be V2 -13 threaded to allow mounting of
the sensing capacitor 12 onto the busbar 14 as mentioned before.
The inner portion 26 may be non-threaded and may remain
substantially encapsulated within the insulator body 22 as shown in
the exemplary illustration in FIG. 3. It is noted here that the
inner and outer portions 26, 25 are identified in FIG. 3 for ease
of discussion only, and such identification is not meant to
indicate or imply that the input electrode 28 is composed of two
individual, separate, or disjoint "components" 25, 26. In other
words, the reference numerals "25" and "26" are used merely to
identify two different sections of the same, single
structure--i.e., the input electrode 28. That is, in terms of a
physical structure, each of the three reference numerals "25,"
"26," and "28" relates to only one physical structure--the input
electrode. In one embodiment, the input electrode 28 is a
partially-threaded, solid, metallic cylindrical rod having a
pre-determined length. The partial-threading thus divides the
electrode 28 into the inner non-threaded portion 26 and the outer
threaded portion 25. Different geometries for the input electrode
28 may be used at lower voltages. Also, the length of the input
electrode 28 may change if the overall design of the sensing
capacitor 12 is lengthened. The input electrode 28 may be made of
copper, brass, bronze, or other suitable metal or metal alloy.
[0031] In one embodiment, the output electrode 30 may be of a
hollow, cylindrical shape that extends a prescribed distance into
the insulator body 22 and surrounds only a portion of the input
electrode 28 (i.e., a part of the inner portion 26) so as to
provide an incomplete or partial overlap with the input electrode
28 as shown by way of example in FIG. 3. The placement of the
electrodes 28, 30 within the insulator body 22 is such as to leave
a physical separation therebetween. As illustrated in FIG. 3, the
region of overlap between the electrodes 28, 30 may be indicated by
the reference numeral "32." The shape of the overlap region 32 may
depend on the geometry of the electrodes 28, 30. In the embodiment
of FIG. 3, the overlap region 32 may be hollow cylindrical or
annular in shape. In one embodiment, the output electrode 30 is a
copper mesh cylinder. However, in an alternative embodiment, the
output electrode 30 may be a hollow, solid cylinder of copper,
brass, bronze, or other suitable metal or metal alloy. Additional
geometries (e.g., a hollow rectangular or square tube) for the
output electrode 30 also may be contemplated.
[0032] Although the length and diameter of the output electrode 30
may be predetermined for a given capacitor configuration, in
alternative configurations these length and diameter can be varied
depending on the length and diameter of the substantially
cylindrical insulator body 22 and/or the degree of electrical
coupling desired between the electrodes 28, 30 in view of the
incomplete overlap 32 therebetween. The capacitive coupling between
the input and output electrodes 28, 30 may be dependent on the
extent of overlap 32 between the electrodes 28, 30.
[0033] It is observed here that the incomplete overlap 32 between
the electrodes 28, 30 may be used to provide desired voltage
isolation between two capacitor surfaces or electrodes. For
example, in one embodiment, the input electrode 28 may be bolted
directly onto a high voltage bus assembly having a voltage of
approximately 18 KVAC (single phase) or 27 KVAC (three phase),
whereas the output electrode or capacitor surface may be connected
directly to a low voltage display alarm unit 16 operating at a
voltage in the range of approximately 5-100 volts AC. Therefore,
the high voltage isolation may be necessary to insure proper safe
isolation of the high and low voltages during operation of the
sensing capacitor 12 in conjunction with the indicator unit 16.
[0034] Referring again to FIG. 3, in one embodiment, the output
electrode 30 may be electrically connected to the output terminal
34 having an opening 35 to allow an external electrical connection
to be made to the output electrode 30 (e.g., via insertion of the
male pin 18 of the conductor 17 as mentioned hereinbefore). In one
embodiment, the output terminal 34 may be a solid copper insert,
threaded terminal having a 1/4-20 threaded opening 35 to connect a
single conductor to the indicator device 16. In the embodiment of
FIG. 3, the output terminal 34 is a solid copper insert having a
diametrical shape. However, in other embodiments, the geometry of
the output terminal 34 may vary depending, for example, on the
conductor 17 to be used for electrical connection or on the shape
and/or size of the output electrode 30. The output terminal 34 may
be made of copper, brass, bronze, or other suitable metal or metal
alloy that allows it to be electrically connected to the output
electrode 30 through soldering, brazing, or welding.
[0035] It is observed that the insulator body 22 may substantially
completely encapsulate the electrodes 28, 30 (including the space
in the region of overlap 32 between the electrodes) and the output
terminal 34, except at either ends of the insulator body 22 where
electrode connection points are provided as illustrated in the
exemplary embodiment of FIG. 3. In one embodiment, the insulator
body 22, the electrodes 28, 30, and the output terminal 34 are all
molded together to create a portable, unitary capacitive structure
(i.e., the sensing capacitor 12). A homogenous capacitor assembly
may be created by performing the molding in a single-stage vacuum
de-aeration process using a fixed mold. Hence, a number of
different configurations of the sensing capacitor 12 may be molded
using a given mold to provide for capacitors to sense different
ranges of AC voltages. In the de-aeration process, the
cycloaliphatic epoxy resin may be mixed with hardener and filler
compounds and degassed under high vacuum. The mix may be then
injected into the mold (containing the electrodes 28,30, and the
output terminal 34) and the capacitor assembly thus may be formed
under heat and pressure. Potentially destructive, partial-discharge
creating air voids may be removed from the epoxy resin during
curing of the molded capacitor 12. The sensing capacitor 12 molded
in this manner may provide optimum high-voltage performance. In one
embodiment, a particle discharge (PD) test may be applied to the
molded capacitor 12 by applying a very high voltage (approximately
18 kV) for one (1) minute between the threaded portion 25 and the
output terminal 34 (via the opening 35). The capacitor 12 may
"pass" the test if the PD level remains lower than 10 pC during the
entire minute of testing.
[0036] As mentioned before, electrical coupling between the
electrodes 28, 30 in the molded capacitor 12 is accomplished
through the capacitive action derived from the electrodes 28, 30
and the dielectric material 22. The electrodes 28, 30 comprise the
"plates" of the capacitor 12, whereas the cycloaliphatic epoxy
resin (of the insulator body 22) provides the dielectric of the
capacitor 12. In one embodiment, the dielectric (e.g., the
cycloaliphatic epoxy resin) of the insulator body 22 may have a
nominal dielectric constant of between 3 and 4. In the presence of
an AC electric field, the effective capacitance of the sensing
capacitor 12 creates a high impedance element that allows
microampere level current to flow into the indicator device 16.
Hence, effective isolation is provided between the high voltage
busbar 14 and the circuitry in the indicator device 16. By varying
the geometry of the electrodes 28, 30 (and, hence, the geometry of
the insulator body 22), the amount of effective capacitance of the
capacitor 12 can be adjusted as per the voltage-sensing
requirement.
[0037] In one embodiment, the sensing capacitor 12 may provide for
impedance in excess of 100 mega-ohms (Me) for a nominal range of
input voltages (e.g., in the range of approximately 2.3 KVAC to
approximately 27 KVAC three phase, and in the range of
approximately 2.3 KVAC to approximately 18 KVAC single phase) on
the busbar 14. For example, with reference to FIG. 3, in one
embodiment, the length of the input electrode 28 may remain fixed
at 6.5 inches (with 0.5 inch diameter for the electrode rod 28 and
1.25 inches long threaded outer portion 25) and the diameter of the
cylindrical output electrode 30 may remain fixed at 1.73.+-.0.03
inches, whereas the lengths of the output electrode 30 and the
overlap 32 may be varied to obtain a range of capacitance and
impedance values. For example, average impedance of 153.+-.2
M.OMEGA. and average capacitance of 17.+-.0.3 pF may be obtained
when the output electrode 30 is 4.5 inches long with 2.5 inches
(lengthwise) of overlap 32 between the input and output electrodes.
In another embodiment, average impedance of 145.+-.2 M.OMEGA. and
average capacitance of 18.+-.0.3 pF may be obtained when the output
electrode 30 is 4.25 inches long with 2.25 inches (lengthwise) of
overlap 32 between the input and output electrodes. In a still
further embodiment, average impedance of 131.+-.3.5 M.OMEGA. and
average capacitance of 19.3.+-.0.3 pF may be obtained when the
output electrode 30 is 4 inches long with 2 inches (lengthwise) of
overlap 32 between the input and output electrodes. Additional
impedance and capacitance value may be obtained by appropriately
varying the lengths of the output electrode 30 and the overlap 32
between the electrodes. It is noted here that, like the dimensions
of the input electrode rod 28, the dimensions of other parts in the
sensing capacitor 12 also may remain fixed while accommodating
different geometries of the output electrode 30. For example, again
with reference to FIG. 3, in one embodiment, the cylindrical
insulator body 22 may be 7.5 inches long with 3.1 inch outermost
diameter (which diameter may decrease to 2.2 inches when measured
at the trough of the surface undulations 24), the output terminal
34 may be 1.125 inches long with 0.5 inch outer diameter, and the
opening 35 may be 0.65 inch deep lengthwise inside the output
terminal 34 (with 0.5 inch thread depth) and may have 0.201 inch
diameter. All of these component geometries may be accommodated
within an overall (end-to-end) length of 9 inches for the sensing
capacitor 12.
[0038] It is observed here that the sensing capacitor 12 is
preferably mounted directly onto a high voltage busbar or bus
bracket in a vertical position as illustrated, for example, in FIG.
1. The 1/2-13 threaded portion 25 of the input electrode 28 may be
used along with 1/2-13 mounting hardware 15 to directly mount the
capacitor 12 onto the busbar 14. However, when the capacitor 12 is
mounted in a horizontal or other non-vertical position, the
non-conductive insulating material of the insulator body 22 may be
used as support so as not to reduce the electrical creepage
distance from the high voltage input electrode 28 to ground or to
the end of the low-voltage output terminal 34. Due to the high
voltages involved during operation of the sensing capacitor 12, it
is desirable to have a capacitor design in which adequate creepage
distance is maintained along with usage of proper dielectric
material (with no air voids) for safe operation of the sensing
capacitor 12.
[0039] FIG. 4 is an exemplary circuit layout for providing audible
and visual alerts through the indicator unit 16 when high-voltage
is detected by the sensing capacitor 12 on an electrical bus of one
of the AC phases. And, FIG. 5 is an exemplary circuit layout
similar to that in FIG. 4, but configured to provide only audible
alerts through the indicator unit 16. Referring now to FIG. 4, a
full-wave rectifier circuit 40 (comprising of diodes 42a through
42d) in the indicator unit 16 may be connected directly between the
incoming conductor 17 (associated with the sensing capacitor 12 for
an AC phase) and ground 20 as shown. The rectifier terminals 40a
and 40b may be directly connected respectively to the corresponding
conductor 17 and to ground 20. In the circuit of FIG. 4, the
alternating current picked up by the electrode 30 of the
high-voltage sensing capacitor 12 across insulation 22 is fully
rectified by the diode bridge rectifier 40, the rectified charge
being stored in a corresponding capacitor 44 that is connected
across rectifier output terminals 40c and 40d.
[0040] A neon discharge bulb 46 also may be connected across the
rectifier output terminals 40c-40d, in parallel with the
corresponding capacitor 44. The neon discharge bulb 46 will not
conduct until the voltage across the bulb reaches approximately 80
volts, when an avalanche discharge occurs and the capacitor 44
discharges instantly, creating a bright flash of light. The
flashing continues at a frequency determined by the voltage on the
busbar 14, the geometry of various components (e.g., the electrodes
28, 30) of the sensing capacitor 12 (and, hence, the amount of
voltage isolation provided thereby), and other parameters as long
as the high voltage busbar 14 is energized. The rate of flashing
also may be determined, among other factors, by the phase to ground
voltage of the AC power supply (e.g., the busbar 14 in FIG. 1), by
the capacitance of the storage capacitor 44, and by the geometry of
the sensing capacitor 12. At very high voltages, the flashing
frequency may increase to a point where the effect on an operator's
eye may be one long continuous flash. It may be however desirable
to have the flashing frequency at a rate low enough for the
operator's eye to sense that the neon bulb 46 flashes on and
off.
[0041] In addition to the AC phase-specific visual alert provided
by the flashing neon bulb 46, a sound-generating piezo electric
device 48 may be connected in series with the neon bulb 46 as
illustrated by way of the dotted circle in FIG. 4. The sound
generator 48 may produce a sharp "chirp" each time the bulb 46
flashes. This option may thus provide both visible and audible
alert signals. In another embodiment, where only an audible signal
would be required, the circuit configuration illustrated in FIG. 5
may be used instead of that in FIG. 4 for a single AC phase. An
avalanche diode 50, or any other suitable device for conducting
electrical current in an avalanche mode in response to a
predetermined voltage, may be substituted for the neon bulb 46 (in
FIG. 4), in series with the piezo electric sound generator 48 as
illustrated in FIG. 5.
[0042] It is reiterated here that the circuit configurations in
FIGS. 4 and 5 are for only one of the AC phases. Three such circuit
configurations--one per AC phase--may be used per installation of
the indicator device 16 to monitor voltage conditions in all three
electrical phases. Such additional configurations may provide
protective redundancy for maximum safety. In case a circuit
associated with one of the neon bulbs or avalanche diodes is
defective, there are two other circuits for back-up. Furthermore,
apart from the safety advantages, the visual and/or audible alerts
can also save time by indicating the loss of high voltage to the
equipment being monitored.
[0043] The foregoing describes a high-voltage sensing capacitor as
an interface apparatus that may be used to attach an indicator unit
to a high-voltage AC electrical bus and to provide safety to
maintenance personnel. The high-impedance nature of the sensing
capacitor effectively isolates the indicator unit from the
high-voltage source to which it is connected. Multiple electrical
phases can be interfaced using a plurality of such sensing
capacitors. The sensing capacitor can be directly mounted to a
high-voltage busbar. The indicator unit may provide visual and/or
audible alerts to maintenance personnel when high voltage
conditions are detected on the busbar by the sensing capacitor. The
sensing capacitor is comprised of a portable, unitary capacitive
structure that includes a molded insulator body encapsulating two
electrodes. The electrodes of the capacitor only partially or
incompletely overlap within the insulator body. The electrode
spacing and configuration is structured to provide a deliberate
amount of coupling between the two electrodes in the presence of an
AC electric field. The sensing capacitor provides a very high
impedance interface to the high-voltage bus, thereby delivering
only microampere-level currents to an indicator unit to enable the
indicator unit to provide the desired indication of the presence of
high voltages on the electrical bus.
[0044] A high voltage sensing configuration including the sensing
capacitor and indicator unit according to one embodiment of the
present disclosure may be used, for example, to sense high voltage
conditions in single phase and three phase AC systems applications
because the configuration can respond to phase to ground voltages
on each of the three (3) AC phases. The sensing capacitor and
indicator unit combination is "self-contained" because it does not
require separate power supply or batteries and does not need any
transformers, switches or fuses. Furthermore, the combination of
the sensing capacitor and indicator unit can be readily
incorporated into new electrical equipment and also can be easily
retrofitted to existing equipment. The sensing capacitor and
indicator unit can be applied in circuit locations where
alternative indicators would be impractical or too costly. Also,
the sensing capacitor and indicator unit combination according to
one embodiment of the present disclosure requires low maintenance
because it does not require any adjustment or calibration, as can
be evident from the discussion hereinabove.
[0045] While the disclosure has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope of the
embodiments. Thus, it is intended that the present disclosure cover
the modifications and variations of this disclosure provided they
come within the scope of the appended claims and their
equivalents.
* * * * *