Information
-
Patent Grant
-
6813125
-
Patent Number
6,813,125
-
Date Filed
Monday, July 1, 200222 years ago
-
Date Issued
Tuesday, November 2, 200419 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Sircus; Brian
- Nguyen; Danny
Agents
- Waddey & Patterson
- Brantley; Larry W.
-
CPC
-
US Classifications
Field of Search
US
- 361 84
- 361 91
- 361 100
- 315 119
- 315 225
-
International Classifications
-
Abstract
A transformer assembly and method for powering a load with a secondary fault protected isolated secondary. The fault fault path is isolated from ground allowing voltage detection of faults and the return terminal is isolated from the midpoint for multiple load connection schemes using the midpoint as a ground connection. A power control system is connected between the primary winding and the input terminal with a ground fault detection circuit connected between the fault path and the ground terminal, where the ground fault detection circuit is operable to detect a fault and activate the power control system to disconnect the source of power from the primary winding in response to detecting the fault. Also disclosed is a high frequency filter adapted to reduce the effects of high frequency transients and a capacitive reactance connected between the input terminal means and the ground terminal. The capacitive reactance is adapted to provide a ground fault path for fault signals. Another improvement teaches the improved performance of an optocoupler using a breakover device for improved bias control.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to transformers for powering luminous loads and more particularly, this invention pertains to secondary ground fault protection for neon tube transformers.
For luminous tube transformers presently utilized in industry, the output voltage from one output terminal to ground cannot exceed 7500V. To provide a design capable of producing output voltages in excess of 7500V, a “mid-point grounded” secondary design is employed in which two secondary coils are used. These coils produce voltages that are 180° out of phase with each other in order to develop a terminal-to-terminal voltage that is twice that measured from any one terminal to ground.
New industry regulations have been developed that require the addition of secondary ground fault protection to such designs. As noted by UL 2161 subsection 20.4 “An isolated output neon supply shall have a current to ground that is 2 milliamps or less when measured in accordance, with the Isolated Output Determination Test, Section 24A.” (revised Mar. 16, 1999). Subsection 24A.1 then notes: “To determine compliance with 20.4, a neon supply is to have any protective circuitry that prevents the supply from operating without an output load connected to it disabled. The neon supply is to be connected to a source of supply adjusted to rated input with no load connected to the output. While energized, the current from each output lead or terminal to ground is to he measured. The maximum current shall not exceed 2 mA rms.” (added Mar. 16, 1999). The intent is to provide a level of protection and to detect i secondary side fault to ground as a measure to reduce any potential fire hazards that may exist as a result of arcing.
As shown in
FIGS. 1 and 2
, mid-point grounded transformer designs
100
,
200
for prior art applications are typically constructed with input terminal means
130
for receiving an input source of power, a primary winding
103
also known as a primary coil
103
with input leads
134
, a core
106
, at least one secondary winding
104
also known as a secondary coil
104
with output leads
136
, high voltage external output terminals
132
, external ground terminal
112
, and chassis
1108
. One endpoint
102
of each secondary coil
104
is electrically common to form a secondary midpoint
110
. This secondary midpoint
110
in turn is connected to the transformer core
106
. The core
106
is then connected to earth ground
114
. The ends
102
,
1202
of the secondary coil
104
and earth ground
114
are also connected to the transformer enclosure
108
,
208
, if the enclosure is conductive, by a ground lead
138
providing a chassis
108
ground connection to earth ground
114
. A ground wiring terminal
112
is provided on the enclosure
108
that provides a direct connection to the secondary midpoint
110
and the earth ground
114
.
The luminous tube loads
116
are operated by the transformer designs
100
,
200
using wiring connections
118
,
218
shown in FIG.
1
and FIG.
2
.
FIG. 1
illustrates a “series” connection
118
of the luminous tube load
116
. A possible problem with this method is that the length of conductor
120
, shown is high voltage potential wiring
122
, required to connect the secondary windings
104
to the tube load
116
may become excessive causing higher than acceptable leakage currents. This problem is overcome by utilizing a parallel wiring connection
218
shown in FIG.
2
. in which the length of high voltage wiring
122
is minimized. Longer wiring runs are limited to the grounded conductor
124
. This parallel type of wiring
218
of the luminous tube load
116
is commonly referred to as “Mid-Point Grounded”
218
. More recent nomenclatures may also refer to this as a “Mid Point Wired”
218
tube load.
FIG. 3
of the drawings shows a prior design using a grounded series connected protected circuit
300
. With the addition of Secondary Ground Fault Protection
302
connected between the midpoint
110
and the ground
114
, the fault path
304
now passes through a sensor, shown as Secondary Ground Fault Protection
302
, before connecting to ground
114
. When a series tube connection
118
is employed as shown in
FIG. 3
, a secondary fault is detected by the Secondary Ground Fault Protection
302
by sensing the current flow on the fault path
304
from ground
114
to the coil mid-point
102
.
As shown in
FIG. 4
, if the tubing load
116
is connected using a Mid-Point Wired parallel connection
218
, the luminous tube load
116
current paths
402
are the same as a ground fault current fault path
304
. With this connection, any imbalance between the current flowing from S
2
-to-ground and S
1
-to-ground, will appear as a ground fault. This would result in nuisance tripping of the Fault Protector
302
.
Similarly, as shown in the series connection
118
of
FIG. 5
, if the high voltage transformer to tube load wiring
122
exhibits a significant amount of capacitively coupled leakage current, shown schematically as the capacitor
502
, such current will appear as a ground fault. This too would result in nuisance tripping of the fault protector
302
.
Finally, industry requirements dictate that a ground fault protected transformer either: (a) detect faults while chassis ground
112
is not connected to earth ground
114
; or (b) shutdown transformer operation if no earth ground
114
connection is present.
In field applications, the ability to provide a reliable, low impedance earth ground
114
connection may be limited as a result of remote installation such as rooftop or pole mounted installations. The resultant high-impedance or ‘noisy’ ground connection can result in nuisance tripping of the fault circuit
302
.
As an alternative to such protection, the transformer may utilize an isolated secondary coil design in which the output voltage does not have a measurable fixed reference to ground. A transformer or power supply of isolated design is considered to inherently provide Secondary Ground Fault Protection since there is no tendency for a “floating” voltage to seek ground. Such isolated designs are subject to fault tests in which one output is grounded. In such a fault test, the ungrounded output cannot produce a voltage in excess of 7500V. If the output does produce an output in excess of 7500V, to ground, the addition of secondary ground fault protection circuitry is required. The present invention provides an apparatus and method for providing this protection with series or mid-point wired loads. What is needed, then, is an apparatus for improved detection of fault currents in a luminous tube transformer circuit with educed false tripping. This improvement is provided by the Secondary Ground Fault Protected Neon Transformer described herein.
SUMMARY OF THE INVENTION
The present invention is designed to provide a novel transformer utilizing an isolated secondary winding design and incorporating a secondary ground fault protection circuit to provide the end user with the option of series or mid-point wired tube loads. Such a design has been proven to provide a reduction of nuisance tripping of the fault circuit as a result of capacitive coupling of output wiring, unbalanced luminous tube loads, or lamp arc transients.
The apparatus of the present invention is a transformer assembly for powering a load with a Secondary Ground Fault Protection circuit for an isolated secondary. The fault path is isolated from ground and the return terminal is isolated from the secondary midpoint for series and mid-point load connection schemes, including schemes using the midpoint as a ground connection. As an exemplary use of this isolation, a power control system is connected between the primary winding and the input terminal with the ground fault detection circuit connected in the fault path. The ground fault detection circuit is operable to detect a fault and activate the power control system to disconnect the source of power from the primary winding in response to detecting the fault.
Also disclosed is a high frequency filter adapted to reduce the effects of high frequency transients. A further aspect teaches a capacitive reactance connected between the input terminals and the ground terminal, so that the capacitive reactance an provide a ground fault path for fault signals. Yet a further improvement teaches he improved performance of an optocoupler using a breakover device for improved bias control.
Objects of the present invention include: 1) a high voltage isolated virtual midpoint return terminal, 2) integration of an isolated secondary transformer with a ground fault detection circuit; 3) integration of an isolated secondary transformer with a ground fault detection circuit while maintaining secondary isolation; 4) use of a capacitive component between line voltage supply and chassis ground to provide alternate ground fault path for fault signals; 5) use of a high frequency filter to reduce erroneous ground fault detection of transient events; 6) use of high impedance between transformer secondary windings and chassis ground to maintain isolation effect; 7) use of diac/breakover component to desensitize optocoupler performance: and 8) use of a transistor to discharge ground fault sensor filter capacitors.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a conventional midpoint grounded transformer with series luminous tube connection.
FIG. 2
is a conventional midpoint grounded transformer with midpoint grounded luminous tube connection.
FIG. 3
is a conventional midpoint grounded transformer with secondary ground fault protection using a series luminous tube connection.
FIG. 4
is a conventional midpoint grounded transformer with secondary ground fault protection using a mid-point grounded luminous tube connection.
FIG. 5
is a conventional midpoint grounded transformer with secondary ground fault protection using a series luminous tube connection having high capacitively coupled leakage current.
FIG. 6
is a schematic diagram of mid-point isolated luminous tube transformer with secondary ground fault protection showing an isolated mid-point return terminal in accordance with the present invention
FIG. 7
is a block diagram of one embodiment of the luminous tube transformer device of FIG.
6
.
FIG. 8
is an electrical schematic of one embodiment of the secondary ground fault protection circuit and power control circuits shown in FIG.
7
.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The design of the secondary ground fault protected neon transformer apparatus
600
, also known as an external luminous tube load powering device
600
, of the present invention is illustrated in
FIGS. 6 and 7
. The design incorporates an isolated construction core-and-coil transformer
602
in conjunction with a high impedance fault sense detection and power control circuit
603
. An additional design feature is the use of a ‘virtual mid-point’ terminal
606
connection.
An isolated transformer
602
with a primary winding
103
, an ungrounded core
603
and at least one isolated secondary winding
604
is used. The degree of isolation of the transformer secondary
604
is evaluated prior to integration with the fault detection circuit
603
. The application of a very well insulated or isolated transformer
602
is very important to the overall function of the completed design. The isolation of the secondary windings
604
insures control over the possible fault paths of any fault currents. Isolation of the secondary windings
604
also reduces capacitively coupled currents by eliminating fixed voltage-to-ground references. Additionally, use of an isolated design secondary
604
topology allows for a fault detection circuit
603
that senses a voltage differential as voltage-to-ground references between the fault path and ground rather than relying on sensing fault currents of some particular range.
The device includes external lamp terminals S
1
and S
2
connected to the device chassis
108
. The inclusion of an external ‘virtual mid-point’ secondary connection also attached to the device chassis
108
, also known as a midpoint terminal
606
, allows the user to have alternatives in the physical wiring of luminous tube loads
116
. In order to eliminate the possibility of end-user misuse of the midpoint terminal
606
by shorting it directly to ground, an isolating impedance
608
is located between the secondary winding
604
and the midpoint terminal
606
. The value of the isolating impedance
608
is several orders of magnitude greater than that used in the isolation circuit
758
of fault detect sense circuit
704
, shown as the parallel resistors R
11
and R
12
in FIG.
8
. In the preferred embodiment, no actual component is used to provide the impedance. A dielectric material or air gap isolates the terminal
606
to be a free floating point.
Previous embodiments of ground fault sensors utilized relatively low impedances in order to maintain low voltage-to-ground differentials between the nonisolated secondary winding mid-point
110
and chassis ground
112
(FIGS.
1
and
2
). The ground fault detection circuit
754
of the present invention (
FIGS. 7 and 8
) utilizes very high impedance components in order to restrict current flow between the isolated secondary winding midpoint terminal
606
and chassis ground
112
. However, in and of itself, this is limited in its ability to establish the desired isolation for all connection schemes.
In order to retain the isolation benefits of the transformer assembly, any connection between the secondary windings
604
and chassis ground
112
should be of high impedance.
As shown in
FIGS. 7 and 8
, a lo-pass high frequency filter
706
(comprising R
11
, R
12
, C
6
, and C
5
) is preferably added to the secondary ground fault sense circuit
754
that serves two functions. First, the low-pass filter
706
serves to aid in reducing is any high frequency transients that could trigger the opto device
708
. These transients are commonly present during each half cycle of luminous tube load
116
operation, such as during the re-strike of the arc. They may also be present in the initial startup of the luminous tube load
116
, depending upon the initial phase angle of the input voltage waveform when power is first applied to the transformer
602
. Additionally, the capacitors C
5
and C
6
(
FIG. 8
) in the lo-pass filter
706
serve as charge storage elements during a fault condition. If sufficient energy, due to fault currents, is developed, the resultant voltage across the capacitors C
5
and C
6
will be sufficient to drive the breakover voltage device
709
, also known as diac D
3
, and allow current flow to the opto device
708
.
As shown in
FIG. 7 and 8
, the output
710
of ground fault detection circuit
754
is electrically coupled to the input
712
of a relay control circuit
752
using an isolation circuit
708
. In a preferred embodiment, the isolation circuit
708
uses an optocoupler device U
1
because of the high dielectric rating between the ground fault detection circuit output
710
and relay control circuit input
712
. This assures isolation between the primary
103
and isolated secondary windings
604
and/or between the isolated secondary windings
604
and ground
114
. However, a limitation of the optocoupler device U
1
exists in the manufacturers' ability to provide a device with a given current ‘trip level’ range. As a result of not having a predictable and reliable minimum current level to work with, use of a conventional opto device U
1
can result in inconsistent activation levels, causing nuisance tripping as a result of system ‘noise’ or by not tripping at desired minimum fault current levels. The inclusion of a diac
709
(
FIG. 8
) as a reliable device with known breakover voltage characteristics in series with the output
710
helps in preventing ‘noise’ activated faults. The use of the isolated transformer
602
results in a voltage to ground sense circuit
754
that relies on a voltage levels such that the concern of minimum fault current levels no longer exists. If a ground fault occurs external to the transformer enclosure
108
, a fixed voltage, reference condition is developed. This voltage is sufficient to drive the breakover device
709
into conduction and allows the opto device U
1
to conduct, creating a fault signal. The breakover device
709
can be embodied in a variety of devices such as the bilateral trigger diac used in the preferred embodiment.
Any circuit design that performs transformer output shutdown based upon the absence of a very low impedance chassis
108
ground to earth ground
114
connection would likely create field performance problems. This is largely due to the difficulty associated with obtaining a quality earth ground
114
connection in a remote installation of the transformer itself. The present design uses a capacitive reactance
714
(
FIG. 8
) connected between the input voltage grounded conductor (LW
1
B on
FIG. 8
) to chassis ground
114
(LW
2
A) as a “Y-cap”, with the added benefit of providing a conductive path to earth ground from chassis ground in the event that a quality chassis ground connection is not available.
The following detailed discussion of the circuit overview of
FIGS. 6
,
7
, and
8
provides construction details for this preferred embodiment.
FIG. 7
is a block diagram of the ground fault protection circuit and power control circuits, further showing connections to the transformer and device terminals. The input terminals
130
are connected through a power disconnect relay K
1
to the primary winding
103
. The operation of the power disconnect relay K
1
is enhanced with a relay snubber
750
and is controlled by the relay control circuit
752
. The relay control circuit
752
is connected to the ground fault detection circuit
754
through an isolation circuit
708
to maintain primary to secondary isolation. The isolation circuit
708
is connected to a consistent bias breakover detection device
709
which detects the secondary faults and triggers the relay control circuit
752
. The consistent bias breakover detection device
709
is connected to the secondary winding
604
through the low pass filter
706
and the secondary isolation circuit
758
. The low pass filter
706
is a capacitive type of filter which may need to be discharged through the connected filter discharge circuit
756
when a non-fault charge occurs on the low pass filter such as a charge caused by normal leakage currents or lamp rectification. The secondary isolation circuit
758
provides a circuit bias that ensures isolation during load operation. The secondary isolation circuit
758
is also connected to the midpoint terminal
606
through an isolating impedance
608
to allow for the possibility for a grounded midpoint terminal
606
. The secondary isolation circuit
758
is connected to the isolated secondary winding
604
to monitor the operation of the secondary windings
604
for ground faults. A detailed electrical schematic with component parameters is provided in FIG.
8
.
As shown in
FIGS. 6 and 7
, Relay K
1
, shown in three parts as coil K
1
:A, contact K
1
:B, and contact K
1
:C, is utilized to control power delivered to the transformer primary
103
via secondary ground fault circuit output connections LW
1
A and LW
1
C. The relay control circuitry
752
operates from a 120v 60 hz source via secondary ground fault circuit connections LW
1
B and LW
1
D. These are supplied power by end user connections to terminals P
1
and P
2
. The intent of the design is to have the common or neutral power connection made to terminal P
1
/LW
1
B. The line or hot connection should be made to terminal P
2
/LW
1
D. Series connected resistors R
7
, R
8
, R
9
, and R
10
are used to lower the effective resistance of the relay coil shown as K
1
:A. Normally closed relay contact K
1
:B allows power to be supplied to the transformer primary
103
. Normally open relay contact K
1
:C is used to latch the relay K
1
to an on state in the event of a fault signal. The ON state of the relay K
1
opens contact K
1
:B and disconnects power to the transformer primary
103
. Components R
5
and C
3
serve as a snubber
750
for the relay contact K
1
:B. Component RV
1
is utilized to suppress line transients that may damage the relay control circuit. Components R
2
, R
3
, C
1
, R
6
, Q
1
, R
4
, and C
2
constitute the triac switching relay control circuit
752
. Introduction of a ground fault condition activates the optocoupler U
1
which is used to sense a fault signal on pins
1
and
2
. Upon sensing fault current flow, the optically isolated output triac T
1
of the optocoupler U
1
allows current flow from pin
6
to
4
. This presents a voltage to pin
2
of triac Q
1
thereby energizing relay K
1
. As previously mentioned, this latches the relay K
1
ON via contact K
1
:C and breaks power to the transformer primary
103
via contact K
1
:B. Component C
4
is a high impedance “Y” cap connected between terminals LW
1
B and LW
2
A. LW
2
A is connected to chassis ground
112
. The benefit of the C
4
component in the circuit is to provide an alternate path to ground in the event that chassis ground is not connected to a reliable earth ground.
Components R
11
, R
12
, D
1
, D
2
, Q
2
, C
5
, C
6
, D
3
, R
13
, and U
1
constitute the round fault detection circuit
754
. The ground fault detection circuit
754
is connected to the transformer secondaries
604
via LW
2
B and LW
2
C. The value of components R
11
and R
12
in the secondary isolation circuit
758
are calculated to insure that the transformer secondaries
604
still have a high degree of isolation with respect to ground
114
under lamp load
116
conditions. In the event that a ground fault occurs in the S
1
-lamp-S
2
current path, a fixed voltage to ground (VFAULT) will be developed at LW
2
B/LW
2
C due to the isolated construction of the transformer. VFAULT is used to drive a fault current signal through the R
11
/ /R
12
-D
2
-D
3
-R
13
-U
1
path back to chassis ground
112
. The presence of a true VFAULT is sufficient to cause the diac
709
to conduct and allow a fault current to flow through the optocoupler U
1
input pins
1
and
2
. The calculated value of R
11
and R
12
is significant because too large a value will not pass enough signal to cause
709
to conduct, and too low a value permits nuisance tripping of the circuit due to normal lamp arc transients.
In order to minimize the presence of normal operating noise signals, components CS, C
6
, R
11
, and R
12
serve as a low pass filter
706
to filter out the transient voltage spikes associated with normal neon tube operation. These transients are characterized by high amplitude, short duration pulses that are effectively filtered out by the low pass filter
706
.
Components CS and C
6
also serve as charge storage devices for fault signals occurring during one-half of a 60 hz cycle. If an excessive amount of charge is developed, a discharge will occur through the filter discharge circuit
756
using path D
3
-R
13
-U
1
. To guard against any unintentional triggering as a result of charge being developed over several cycles, components for the controlled discharge switch including transistor Q
2
, and charge detection circuit D
1
, and D
2
were added as a discharge circuit to discharge these unwanted charges on C
5
and C
6
.
Thus, although there have been described particular embodiments of the present invention of a new and useful secondary ground fault protected luminous tube transformer, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.
Claims
- 1. A device for powering an external luminous tube load comprising:an device chassis; an external ground terminal electrically connected to define a chassis ground; input terminal means operable to receive a source of power; a transformer mounted to the chassis, the transformer having a core, a primary winding electrically connected to the input terminal means and at least one secondary winding, the secondary winding having at least two electrical endpoints, the to transformer core and secondary endpoints being electrically isolated from the chassis ground; at least two high voltage external output terminals electrically connectable to the luminous tube load, the high voltage external output terminals electrically connected to the secondary endpoints; and an external midpoint terminal electrically adapted to provide a midpoint wiring location for the external luminous tube load, the external midpoint terminal electrically isolated from the chassis ground and the secondary winding.
- 2. The device of claim 1 further comprising:a ground fault detection circuit electrically connected between the secondary winding and the chassis ground.
- 3. The device of claim 2, the ground fault detection circuit comprising:power control system electrically connected between the primary winding and the input terminal means, and a fault detection circuit electrically connected between the secondary winding and the chassis ground and operable to detect a fault and activate the power control system to disconnect the source of power from the primary winding in response to detecting the fault.
- 4. The device of claim 3, the ground fault detection circuit including high impedance components connected between the secondary winding and chassis ground.
- 5. The device of claim 2, the ground fault detection circuit operable to detect a fault as a voltage differential between the fault path and the earth ground.
- 6. The device of claim 2, the ground fault detection circuit adapted to maintain the isolation between the secondary winding and chassis ground to control fault current fault paths, the ground fault detection circuit operable to detect a voltage differential between the fault path and the earth ground and control the power control system to disconnect the source of power from the primary winding in response to detecting the voltage differential.
- 7. The device of claim 4, the ground fault detection circuit including a high frequency filter adapted to reduce the effects of high frequency transients between the secondary winding and chassis ground.
- 8. The device of claim 7, the high frequency filter including a chargeable element, the device further comprising:a controlled discharge switch electrically connected to the high frequency filter, the controlled discharge switch adapted to controllably discharge unwanted charges collected in the high frequency filter.
- 9. The transformer apparatus of claim 8, the controlled discharge switch comprising:a transistor controlled by a charge detection circuit.
- 10. The transformer apparatus of claim 1, further comprising:a capacitive reactance connected between the input terminal means and the ground terminal, the capacitive reactance adapted to provide a ground cult path for fault signals.
- 11. A predictable operation coupling apparatus having a consistent operating bias and adapted to isolate an input signal from an output signal, the apparatus comprising:an optocoupler adapted to provide electrical isolation between a coupler input and a coupler output; and a breakover component including an breakover input and a breakover output, the breakover component adapted to receive the input signal at the breakover input and provide a consistent operating bias for controlling the breakover output, the breakover output having a minimum on-signal output higher than the minimum consistent on-signal input signal necessary for operation of the optocoupler, the breakover output connected to the input signal of the optocoupler such that the breakover component and optocoupler are adapted to provide a predictable bias for operation of the optocoupler.
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Name |
Date |
Kind |
3731142 |
Spira et al. |
May 1973 |
A |
5680286 |
Pacholok |
Oct 1997 |
A |
5751523 |
Ballard et al. |
May 1998 |
A |
6111732 |
Beland |
Aug 2000 |
A |