Secondary ground fault protected luminous tube transformer

Details Secondary ground fault protected luminous tube transformer Secondary ground fault protected luminous tube transformer

2002-07-01

2004-11-02

Sircus, Brian (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Ground fault protection

C361S043000

Reexamination Certificate

active

06813125

ABSTRACT:

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 frequ

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