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&#39; 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.