Abstract:
An apparatus selectively generates a disturbance in a three-phase supply voltage provided to a load. The apparatus includes input connections for receiving a first phase voltage, a second phase voltage and a third phase voltage of the three-phase supply voltage. The apparatus includes a voltage disturbance generator for selectively adjusting the amplitudes of the first, second and third phase voltages according to a first test method, a second test method or a third test method. Output connections are provided for connecting to the load to provide the load the first, second and third phase voltages as altered according to the first, second or third test method. In the first test method, a phase-to-phase voltage disturbance is introduced between the first and second phase voltages by altering the amplitude of the first phase voltage against the second phase voltage. In the second test method, a phase-to-phase voltage disturbance is introduced between the first and second phase voltages by altering the amplitudes of the first and second phase voltages in reference to each other. In the third test method, a phase-to-phase voltage disturbance is introduced between the first and second phase voltages by altering the amplitude of the first and second phase voltages in reference to a neutral connection. The apparatus provides for selecting the first, second or third test method without disconnecting any of the first, second and third phase voltages from the input connections, without disconnecting the load from any of the output connections, and without interrupting the load.

Description:
FIELD 
   This invention relates to the field of power disturbance generators. More particularly the invention relates to a voltage sag/swell generator for use in testing of loads, such as automated process equipment in a manufacturing plant. 
   BACKGROUND 
   In most cases, machines and components used in performing industrial processes receive power from public utility companies. Unfortunately, power supplied by utility companies is often subject to transient reductions in voltage level (sags) or increases in voltage level (swells). These sags and swells can have deleterious effects on sensitive industrial processes. 
   Sag/swell generators are devices that are typically placed in the power circuit between a power supply and a load to introduce controlled and repeatable voltage sags or swells. Using these generators, engineers can perform tests to observe the effects of voltage sags and swells on industrial machines and processes. Using information gathered during such tests, the engineers can determine ways to adjust the machines and processes to minimize harmful effects of voltage variations. 
   The usefulness of prior voltage sag/swell generators has been limited because they have not provided test engineers the ability to easily select between different modes of generating a voltage sag or swell using a single generator. For example, if a test engineer wished to do a first test wherein a voltage sag in a three-phase power system is described by a reduction in amplitude of a single phase-to-neutral voltage (simulating a line-to-ground fault on the utility system), and then do a second test wherein the sag is described by a phase-to-phase reduction (simulating two overhead distribution lines coming into contact with one another), those tests would have to be performed using different sag generators. This is inconvenient due to the time and effort involved in setting up two different generators. The cost in providing two sag generators can also be significant. 
   What is needed, therefore, is a sag/swell generator having multiple modes of operation whereby the amplitude and phase relationships of each phase in a three-phase system is selectable by the test personnel. 
   SUMMARY 
   The invention provides a portable voltage disturbance generator combined with a built-in data acquisition system. The invention serves as a diagnostic tool for determining ride-through characteristics of industrial processes and machinery when those processes are subjected to disturbances in power line voltage. With the invention connected in series between the voltage supply and the load, the user can induce voltage disturbances of controlled amplitude and duration, or turn the supply voltage off momentarily, while monitoring voltages, currents, or other signals from within the process. This allows investigators to quickly identify vulnerable process components. The invention allows the investigators to easily choose between three different methods for simulating a voltage disturbance in a three-phase power system. The method selected will depend upon the type of supply voltage disturbance that the investigators wish to simulate. Once the investigators have identified the weak links in process components, it is often possible to apply local ride-through solutions that are much more economical than whole-system power conditioning. Some typical industrial process components to monitor during a voltage disturbance event include DC power supplies, relays, contactors, and load currents. 
   A preferred embodiment of the invention is controlled by a laptop computer running graphical user interface software based on a Windows or equivalent operating system. The invention creates voltage disturbances by switching rapidly between nominal supply voltage and a reduced or increased voltage. The reduced or increased voltage is preferably supplied by variable autotransformers that have been adjusted to the desired percentage of nominal voltage. The transfer from the nominal voltage to the decreased or increased voltage and back to nominal is performed by solid-state switches, such as Insulated Gate Bipolar Transistors (IGBTs), that are synchronized with precise timing signals. IGBTs offer important advantages over other types of switches and provide a very fast, uninterrupted transition between the two voltage levels. 
   In a preferred embodiment, the system is equipped with sixteen data acquisition channels for monitoring responses of process components during voltage disturbances. Preferably, eight of the sixteen channels are low-voltage channels that are ideal for use with current probes or other sensors, while the other eight are isolated, high-voltage channels provided for direct connection to the equipment under test. 
   According to one preferred embodiment, the invention provides an apparatus for selectively generating a disturbance in a three-phase supply voltage provided to a load. The apparatus includes input connections for receiving a first phase voltage, a second phase voltage and a third phase voltage of the three-phase supply voltage. The apparatus includes a voltage disturbance generator for receiving the first, second and third phase voltages and selectively adjusting the amplitudes of the first, second and third phase voltages according to a first test method, a second test method or a third test method. The apparatus has output connections for connecting to the load to provide to the load the first, second and third phase voltages as altered according to the first, second or third test method. 
   In the first test method, a phase-to-phase voltage disturbance is introduced between the first and second phase voltages by altering the amplitude of the first phase voltage against the second phase voltage. In the second test method, a phase-to-phase voltage disturbance is introduced between the first and second phase voltages by altering equally the amplitudes of the first and second phase voltages with respect to each other. In the third test method, a phase-to-phase voltage disturbance is introduced between the first and second phase voltages by altering the amplitudes of the first and second phase voltages in reference to a neutral connection. Alternatively in the third test method, a phase-to-phase disturbance is introduced between the first and second phase voltages by altering the amplitude of only the first or second phase voltage with respect to a neutral connection. The third test method is versatile because it additionally allows simultaneous three-phase sags in a balanced or unbalanced configuration. 
   In this preferred embodiment, the apparatus provides for selecting the first, second or third test method without disconnecting any of the first, second and third phase voltages from the input connections, without disconnecting the load from any of the output connections, and without interrupting the load. 
   In one embodiment, the invention provides an apparatus for selectively generating a disturbance in a three-phase supply voltage provided to a load during a test. The apparatus of this embodiment includes input connections, a voltage disturbance generator, output connections and a neutral connection. The input connections include first, second and third input connections for connecting to the three-phase supply and receiving first, second and third phase voltages, respectively. The output connections include first, second and third output connections for connecting to the load and providing to the load the first, second and third phase voltages, respectively, as altered according to the test. 
   The voltage disturbance generator receives the first, second and third phase voltages from the input connections and selectively adjusts the amplitudes of the first, second and third phase voltages according to a particular test method. The voltage disturbance generator of this embodiment includes a first transformer and a second transformer. When configured according to the particular test method, the first transformer has a first winding connection connected to the first input connection, a second winding connection connected to the second input connection, a first tap connection connected to the first output connection, a second tap connection connected to the second output connection, and a center tap available for connection to the neutral connection. When configured according to the particular test method, the second transformer provides a third winding connection connected to the third input connection, a fourth winding connection connected to the neutral connection, and a third tap connection connected to the third output connection. 
   Also in a preferred embodiment, the invention provides a voltage disturbance generator for receiving a supply voltage from an input connection and for selectively adjusting the amplitude of the supply voltage to be provided to an output connection. The voltage disturbance generator of this embodiment includes at least one voltage switching network. The voltage switching network comprises a first switching device, a bridge rectifier circuit, a bipolarized protection component, and a unipolarized protection component. The AC terminals of the bridge rectifier circuit are connected between the input connection and the output connection. The first switching device is connected to the DC terminals of the bridge rectifier. The bipolarized protection component is connected between the input connection and the output connection for protecting the first switching device and the bridge rectifier circuit from damage due to transient voltages that naturally occur during rapid switching. The unipolarized protection component is connected across the first switching device for protecting the first switching device from damage due to transient voltages that naturally occur during rapid switching. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
       FIG. 1  depicts vector diagrams representing three methods of generating a voltage disturbance according to a preferred embodiment of the invention; 
       FIG. 2  depicts a functional block diagram of a voltage disturbance generator according to a preferred embodiment of the invention; 
       FIG. 3A  depicts an example of a 6-cycle voltage sag to 70% of nominal voltage for one phase of a three-phase supply; 
       FIG. 3B  depicts an example of a 5-cycle voltage sag originating at a phase angle of 90°; 
       FIG. 4  depicts a mode switching network according to a preferred embodiment of the invention; 
       FIG. 5  depicts a relay control network according to a preferred embodiment of the invention; 
       FIG. 6A  depicts a preferred embodiment of the mode switching network set to introduce a voltage disturbance according to test method A between phase A and phase B; 
       FIG. 6B  depicts a preferred embodiment of the mode switching network set to introduce a voltage disturbance according to test method B between phase A and phase B; 
       FIG. 6C  depicts a preferred embodiment of the mode switching network set to introduce a voltage disturbance according to test method A between phase B and phase C; 
       FIG. 6D  depicts a preferred embodiment of the mode switching network set to introduce a voltage disturbance according to test method B between phase B and phase C; 
       FIG. 6E  depicts a preferred embodiment of the mode switching network set to introduce a voltage disturbance according to test method A between phase C and phase A; 
       FIG. 6F  depicts a preferred embodiment of the mode switching network set to introduce a voltage disturbance according to test method B between phase C and phase A; 
       FIG. 6G  depicts a preferred embodiment of the mode switching network set to introduce a voltage disturbance according to test method C for phases A, B and C in a wye configuration; 
       FIG. 7  depicts a voltage switching circuit of a voltage disturbance generator according to a preferred embodiment of the invention; 
       FIG. 8  depicts a test control screen generated by voltage disturbance test software according to a preferred embodiment of the invention; 
       FIG. 9  depicts a data channel configuration screen generated by voltage disturbance test software according to a preferred embodiment of the invention; 
       FIG. 10  depicts a log information screen generated by voltage disturbance test software according to a preferred embodiment of the invention; and 
       FIGS. 11A and 11B  depict a snubber circuit according to a preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   In the field of testing industrial machines and processes undergoing supply voltage disturbances, there are three generally accepted methods for simulating voltage sags or swells in a three-phase power system. Those methods, referred to herein as methods A, B and C, and also as first, second and third methods, are represented by the vector diagrams depicted in  FIG. 1 . It will be appreciated that each method is capable of producing a momentary voltage sag (decrease) and a momentary voltage swell (increase), although only voltage sags are represented in  FIG. 1 . Voltage sags and swells will be referred to herein collectively as voltage disturbances. 
   Generally, method A is often utilized by test personnel to introduce phase-to-phase voltage disturbances. The basic concept of method A involves injecting phase-to-phase voltage disturbances by referencing one phase against another phase. With method A, the voltage disturbance test can be performed with or without a neutral connection since the neutral is not referenced. This is illustrated in  FIG. 1 , which depicts a 60 degree phase shift in vector V cn  and a 50% amplitude reduction in vector V ac  relative to the nominal value. The nominal value for each vector is defined as 1.0 per unit (pu) for the purposes of the examples depicted in  FIG. 1 . For a momentary interruption between the two affected phases, method A induces a total of 120 degrees of phase shift for V cn . 
   Method B creates a phase-to-phase voltage disturbance by introducing an equivalent change in voltage amplitude to two phases rather than to a single phase as in method A. As shown in  FIG. 1 , this balanced change results in equal phase shifts for the affected phase-to-neutral vectors. In the example of  FIG. 1 , there is a 50% voltage sag in the phase-to-phase voltage V ac  relative to the nominal voltage. For a momentary interruption between the two affected phases, method B induces a total of 60 degrees of phase shift each for V an  and V cn . Like method A, method B does not require a neutral connection for phase-to-phase testing. 
   Method C creates a phase-to-phase voltage disturbance by referencing a neutral connection. In the example of  FIG. 1 , a 50% sag in nominal phase-to-phase voltage V ac  is created by reducing the two phase-to-neutral voltages V an  and V cn  by 50% of nominal. As  FIG. 1  indicates, method C induces no phase shift between the phase-to-neutral voltages during momentary disturbances. Method C requires a neutral conductor be used. Method C is versatile in that it allows simultaneous three-phase voltage disturbances in a balanced or unbalanced configuration. As discussed in more detail hereinafter, the balanced or unbalanced configuration may be attained through changing the magnitude of any phase or phases by the appropriate tap selection at the autotransformer. 
   The voltage disturbance generator of the present invention offers test personnel the option of testing a load using any of methods A, B or C simply by selecting an appropriate switch setting. A test setup incorporating a preferred embodiment of the invention is depicted in  FIG. 2 . The test setup  10  receives a three-phase input supply voltage  12  (phase A i , phase B i , and phase C i ) such as from the electric utility grid or from a motor/generator set. The supply voltage  12  is connected to an input port  14  of a voltage disturbance generator  24 . The input port  14  includes first, second and third input connections  14   a – 14   c  and a neutral connection  14   d.  An output port  16  of the generator  24  provides a three-phase output voltage (phase A o  phase B o  and phase C o ) to the load under test  18 . The output port  16  includes first, second and third output connections  16   a – 16   c  and a neutral connection  16   d.    
   In the preferred embodiment of the invention, the voltage disturbance generator  24  operates under the control of test software running on a control computer  22 . Within the voltage disturbance generator  24  is an autotransformer network  26 , a voltage switching network  27  and a mode switching network  28 . As described in more detail below, the type, magnitude and duration of the voltage disturbance created by the voltage disturbance generator  24  are determined by tap settings within the autotransformer network  26 , positions of manual mode selection switches SW 1  and SW 2  on the control panel of the generator  24  and control signals provided by the control computer  22 . A preferred embodiment of the invention includes a data acquisition board  30  and controller  32  for acquiring various test signals  19  associated with the load under test  18  during a voltage disturbance test. 
     FIG. 3A  depicts an example of a 6-cycle voltage sag to 70% of nominal voltage for one phase of the three-phase supply. Herein, voltage sags are described as a percentage of nominal voltage. For example, if nominal voltage is 120V, and the RMS voltage during the sag is 84V, then the event is described as “a sag to 70%”. As described in more detail hereinafter, using the voltage disturbance generator  24  of the present invention, test personnel have control over the point-on-wave of disturbance events. For example,  FIG. 3B  shows a 5-cycle voltage sag originating at 90°. As  FIG. 3B  indicates, the transfer time from nominal voltage to the sag voltage is nearly instantaneous. 
   The computer  22  used to control the voltage disturbance generator  24  is preferably a PC-based Pentium laptop computer with a Windows operating system or equivalent. Preferably, the computer  22  has a universal serial bus (USB) port for connection to the data acquisition board  30  of the generator  24 . In the preferred embodiment of the invention, the controller  32  is responsible for both acquiring data and controlling the voltage switching network  27  of the voltage disturbance generator  24 . 
   The computer  22  preferably performs at least the following functions which are described in more detail hereinafter: controls the disturbance duration; controls the phase angle at which the disturbance is applied; triggers disturbance events; displays waveform data acquired on selected data acquisition channels during the disturbance event; automatically detects a “trip” during a disturbance event; saves/recalls waveforms for further analysis; and keeps a log of disturbance test activity. 
   A preferred embodiment of the mode switching network  28  and the autotransformer network  26  is depicted in  FIG. 4 . The mode switching network  28  comprises a network of relays K 1 –K 14  for selecting relationships between the input voltages (A i , B i , C i ,) and the output voltages (A o , B o , C o ,) based on tap positions in autotransformers T 1  and T 2 . Transformer T 1  includes winding connections T 1   H1  and T 1   H2 , tap connections T 1   X1  and T 1   X2  and a center tap T 1   C . Transformer T 2  includes winding connections T 2   H1  and T 2   H2  and a tap connection T 2   X . 
   The selection and activation of the relays K 1 –K 14  are controlled by the relay control network  42 , a preferred embodiment of which is depicted in  FIG. 5 . As shown in  FIG. 5 , the relay control network  42  includes a mode selection switch SW 1  for selecting between test methods A, B and C. In the preferred embodiment depicted in  FIG. 5 , position  1  of switch SW 1  selects test method A, position  2  selects method B and position  3  selects method C. When the mode selection switch SW 1  is in positions  1  or  2 , a phase selection switch SW 2  is used to select the two phases between which a voltage disturbance is to be introduced. For example, when switch SW 2  is in position  1 , the disturbance is generated between phases A and B, when switch SW 2  is in position  2 , the disturbance is generated between phases B and C, and when switch SW 2  is in position  3 , the disturbance is generated between phases C and A. The combination of settings of switches SW 1  and SW 2  cause activation (closure) of the relays K 1 –K 14  according to the schedule listed in Table I. In the preferred embodiment, the switches SW 1  and SW 2  are manual switches operated by knobs disposed on the front panel of the housing in which the voltage disturbance generator  24  is contained. However, it should be appreciated that the switches SW 1  and SW 2  could be relays or other electromagnetically-controlled switches activated by the controller  32  in conjunction with computer-generated commands. 
   
     
       
             
             
             
             
             
             
           
         
             
               TABLE I 
             
             
                 
             
             
                 
                 
                 
                 
                 
               Schematic 
             
             
               SW1 
               SW2 
               Test 
                 
                 
               Depicted 
             
             
               Position 
               Position 
               Method 
               Phases 
               Relays Closed 
               in 
             
             
                 
             
           
           
             
               1 
               1 
               A 
               A–B 
               K2, K3, K4, K5, K14 
               FIG. 6A 
             
             
               1 
               2 
               A 
               B–C 
               K1, K7, K8, K10, K14 
               FIG. 6C 
             
             
               1 
               3 
               A 
               C–A 
               K1, K2, K4, K10, K13 
               FIG. 6E 
             
             
               2 
               1 
               B 
               A–B 
               K2, K3, K5, K6, K14 
               FIG. 6B 
             
             
               2 
               2 
               B 
               B–C 
               K1, K7, K8, K10, K13 
               FIG. 6D 
             
             
               2 
               3 
               B 
               C–A 
               K2, K3, K4, K10, K13 
               FIG. 6F 
             
             
               3 
               n/a 
               C 
               A–N, 
               K2, K3, K5, K6, K9, 
               FIG. 6G 
             
             
                 
                 
                 
               B–N, 
               K11, K12 
             
             
                 
                 
                 
               C–N 
             
             
                 
             
           
        
       
     
   
   As indicated in Table I,  FIGS. 6A–6G  depict schematic diagrams of the circuits formed within the mode switching network  28  based on the corresponding relay closures listed in Table I. 
   As depicted in  FIG. 7 , each voltage switching network  27   a–c  comprises a set of relays K 18 , K 16  and K 17 , and switches SW 3  and SW 4 , and SW 5  that switch in sequence to create voltage disturbances. Although  FIG. 7  depicts the voltage switching network  27   a  for phase A, it should be appreciated that the same switching scheme is preferably implemented in voltage switching networks  27   b  and  27   c  for phases B and C, respectively. The configuration depicted in  FIG. 7  corresponds to a situation wherein switches SW 1  and SW 2  are both in position  1  (test method A for a voltage disturbance between phases A and B). 
   In the preferred embodiment of the invention, switches SW 3  and SW 4  are insulated gate bipolar transistors (IGBTs) and switch SW 5  is a silicon controlled rectifier (SCR). The purpose of the relay K 18  is to apply or remove the input voltage A i  to the remainder of the switching network  27   a  and the autotransformer T 1  either by software control or by manual load start/stop buttons. Preferably, the voltage disturbance generator  24  prohibits closure of relay K 18  and application of a voltage to the load  18  until test control software is running on the computer  22 . Exiting the software preferably causes relay K 18  to open. 
   In the preferred embodiment, a bypass relay K 16  remains closed until test personnel initiate a voltage disturbance using the test software. Upon the initiation of a disturbance test, relay K 16  opens, thereby allowing the switches SW 3 , SW 4  and SW 5  to open and close in proper sequence and create a voltage disturbance. When the disturbance is complete, relay K 16  closes again. In this manner, the relay K 16  provides a bypass for the switches SW 3  and SW 5  so that they do not generate heat by continuously carrying load current. This eliminates the need for large heat sinks. 
   The relay K 17  is a safety relay that disconnects the transformer&#39;s variable output voltage from the circuit when the transformer T 1  is not needed. Preferably, the relay K 17  operates simultaneously with relay K 16 , but in the opposite state. 
   In the preferred embodiment, the switches SW 3  and SW 4  can carry and switch load current up to about 200A. However, certain loads have high inrush currents when power is applied. The same inrush can also occur immediately after voltage disturbances. As mentioned above, switches SW 3  and SW 4  are preferably IGBT switches. Because IGBT switches generally cannot handle large pulse currents, the switch SW 5  provided in parallel with the switch SW 3  is preferably an SCR, which can handle inrush currents up to 2000A peak. As an additional protection feature, the preferred embodiment includes an over-current protection circuit to protect switches SW 3  and SW 4  from excessive inrush currents. In case of excessive inrush current, the switches SW 3  and SW 4  will open and the test personnel will be alerted of the condition. 
   As will be appreciated by one skilled in the art, a uni-polar switching device that conducts in only one direction (such as an IGBT) can be configured to switch alternating current (AC) in two directions with the use of a bridge rectifier circuit  66  as depicted in  FIG. 11A . To avoid unnecessarily complicating  FIG. 7 , a bridge rectifier circuit is not depicted therein. However, it will be appreciated that in those embodiments wherein switches SW 3  and SW 4  are IGBT devices, a bridge rectifier circuit may be provided between points P 1  and P 2  for switch SW 3  and between points P 3  and P 4  for switch SW 4 . In a preferred embodiment, the bridge rectifier  66  depicted in  FIG. 11A  comprises a SanRex DF200AA160. 
   In one preferred embodiment, the voltage switching networks  27   a–c  include snubber circuit assemblies to protect the IGBT switches SW 3  and SW 4  and the bridge rectifier circuits from damage due to transient voltages. As shown in  FIG. 11A , each snubber circuit assembly  64  preferably includes a bipolarized protection component  64   a  and a unipolarized protection component  64   b.  In the preferred embodiment, the bipolarized protection component  64   a  comprises a metal oxide varistor (MOV). Also in a preferred embodiment, the unipolarized protection component  64   b  comprises the snubber circuit depicted in  FIG. 11B . Information regarding the components of the embodiment depicted in  FIG. 11B  is listed in Table II. It will be appreciated that the component information listed in Table II is provided merely as one example of an embodiment of the snubber circuit  64 , and the invention is not limited to the particular component values or part numbers listed in Table II. 
   
     
       
             
             
             
           
         
             
                 
               TABLE II 
             
             
                 
                 
             
             
                 
               Component 
               Description 
             
             
                 
                 
             
           
           
             
                 
               SW3, SW4 
               IGBT-Powerex CM400HA-24H 
             
             
                 
               SW5 
               SGR-Powerex W4DC162PB 
             
             
                 
               64a 
               MOV-Harris 575LA40 
             
             
                 
               D1 
               RURU100120 (1200 V, 100 A)-has input 
             
             
                 
                 
               port D1 i  and output port D1 o  (see FIG. 11B) 
             
             
                 
               C1, C2 
               150 μf, 450 Vdc 
             
             
                 
               R1, R2 
               100 kΩ, ¼ W 
             
             
                 
               R3 
               20 Ω, 5 W 
             
             
                 
                 
             
           
        
       
     
   
   In the preferred embodiment of the invention, the transformers T 1  and T 2  are multi-tapped autotransformers. Test personnel can control the magnitude of the voltage disturbance for each phase by adjusting the position of an external jumper in the variable tap of the transformers T 1  and T 2 . In an alternative embodiment, the magnitude of the voltage disturbance may be adjusted by automated tap selection control of the controller  32  using additional contactors for the autotransformers T 1  and T 2  and computer-generated commands. The transformers T 1  and T 2  are preferably designed for voltage disturbances within the range of 0% to 125% of nominal voltage, with the taps provided in nominal voltage increments such as in 5% or 10% steps. In a preferred embodiment, the transformer T 1  is rated for 600V (480V×125%) and T 2  is rated for 350V (277V×125%). 
   As shown in  FIG. 2 , the preferred embodiment of the invention includes multi-function three-phase power meters  40   a–c  to measure characteristics of the output voltages A o , B o  and C o  for each phase. The power meters  40   a–c  are preferably configured to measure the voltage resulting from the autotransformer tap settings so that test personnel have a measurement of the anticipated disturbance voltage before the disturbance event is initiated. The meters  40   a–c  also preferably measure load current, real power, and apparent power so that the load can be characterized before sag testing begins. 
   In the preferred embodiment of the invention, the voltage disturbance generator  24  is controlled by a software program running in a windows-based user interface on the computer  22 . An example of a main control screen  44  generated by the software on the computer  22  is depicted in  FIG. 8 . This screen  44  is preferably used by test personnel to initiate and control voltage disturbances. In the example of  FIG. 8 , the control program is set up for a twelve-cycle voltage sag triggered at 0 degrees with respect to the control power voltage. As shown in  FIG. 8 , the duration of the voltage disturbance is preferably controlled using a horizontal scroll bar  46 . In a preferred embodiment of the invention, the duration of the voltage disturbance ranges from one quarter cycle up to three seconds in increments of one quarter cycle. In the example of  FIG. 8 , a set of “hot buttons”  48  are available below the scroll bar  46  to allow a quick jump to the selected disturbance duration. While these default durations are chosen because they are commonly used in disturbance testing, they may be changed simply by right-clicking on the button and typing a new value. 
   In the preferred embodiment of the invention, the phase angle (or point-on-wave) at which a disturbance begins is controllable between 0 and 359 degrees using a scroll bar  50  labeled “Point on Wave,” or by typing a value directly into the numerical display box  52 . To use this feature of the control program, an appropriate phase reference should be selected. For example, if the user is set up for three-phase tests and wishes to begin a voltage disturbance at 90 degrees, then it will be necessary to specify which phase will be at 90 degrees at the initiation of the disturbance. The other two phases will be +120 and −120 degrees out of phase at that moment. To synchronize properly, the input voltage selected as the phase reference generally must have a valid and stable signal. In the preferred embodiment of the invention, the control program automatically checks the stability of the selected input voltage and provides a message to the user if the program is unable to synchronize with the selected input voltage. 
     FIG. 8  also depicts multiple windows for instant display of waveforms captured during disturbances. The user may click on any of the image windows to display a larger image and to change the display characteristics, such as to include scaled axes, zoom in, measure and overlay multiple waveforms. These images may also be copied, pasted, saved and printed. 
     FIG. 9  depicts a data channel configuration screen  54  that displays the low voltage and high voltage data acquisition channels. By clicking on “Acquire New Data”, the software controls the data acquisition board  30  in taking a “snapshot” of the low and high voltage signals applied to the appropriate inputs. After acquisition of the data, the software in the preferred embodiment determines whether each input signal is AC or DC and then calculates a nominal value for each channel. Generally, this is done first when the equipment under test is operating normally so that the acquired signals are representative of a nominal operating condition. Preferably, the nominal value is stored for comparison to disturbance data as it is acquired. 
   In the preferred embodiment of the invention, low voltage and high voltage channels are available for a wide range of signal measuring requirements. Generally, the low voltage channels are intended for use with current probes or other transducers having output in the range of tens of millivolts to ±10 volts maximum. These low voltage channels preferably have a gain that is software selectable, and the displayed data is automatically adjusted so that the magnitude of the signal is shown relative to the input range. In many applications, the low voltage channels are used to measure current. Preferably, channels  1 – 3  are internally connected to measure load current on phases A, B, and C respectively. In the preferred embodiment, the high voltage channels ( 9 – 16 ) have an internal circuit that divides the measured voltage by a fixed ratio, and software scales the data by the same ratio in order to display a properly scaled measurement. 
   In the preferred embodiment, statistical information regarding the data acquisition appears at the bottom of the main control window  44 , as shown in  FIG. 8 . For example, the points/cycle box indicates the data sample rate. Preferably, at least two sampling rates are available to the user: 10 kHz or 5 kHz. 
   Preferably, the invention provides an on-line log that simplifies note-taking during tests by keeping a running record of disturbance test activity. In the preferred embodiment, key disturbance parameters are recorded along with data such as time, date, trip indication, a comment field, and a filename where waveforms are stored on the computer  22 . When the recording log function is enabled, all waveforms are automatically stored using a numeric filename indexing system.  FIG. 10  depicts an example of a display screen showing an interface that prompts a user for input to be stored in the log. In the preferred embodiment, the log file is saved as text and can be read by spreadsheet or word processing software. Waveform files are preferably stored in the same directory as the log file. 
   In one preferred embodiment, the invention also functions as an inrush tester for measuring inrush current when a load is started at any phase point on a supply voltage wave. In inrush test mode, the voltage disturbance is an extended interruption in the voltage, such as for one minute or longer. Power is returned at the end of the extended interruption at a particular point on the waveform. Meanwhile, the data acquisition system is preferably delayed so that data collection begins immediately prior to the power-on event. Usually, inrush current is the main parameter of interest in the data collection. However, the preferred embodiment allows the user to collect data on as many as 16 channels during the test. Preferably, an external current probe is used for this test to improve measurement accuracy. 
   The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as is suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.