Abstract:
A method and apparatus for automatically selecting and employing different DC link pre-charge levels dependent upon different sources providing power to a power conditioning system wherein the different sources are characterized by different source inductances, the apparatus including a source control that selects a first pre-charge level when a utility supply provides power to the system and selects a relatively lower pre-charge level when a generator supply provides power to the system thereby reducing the possibility of inverter disablement due to DC link voltage dips.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to power conditioning configurations and more specifically to a method and apparatus for modifying a pre-charge voltage on a DC link between a rectifier and an inverter where the rectifier may be fed by more than one source and at least two of the potential sources have disparate characteristic impedances. 
     Power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids designed so as to be extremely efficient in delivering massive amounts of power. To facilitate efficient distribution over long distances power is delivered as low frequency three phase AC current. 
     Despite being distributable efficiently, low frequency AC current is not suitable for end use in consuming facilities. Thus, prior to end use utility grid power has to be converted to a useable form. To this end a typical power “conditioning” configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses (i.e., across a DC link) and an inverter linked to the DC link that converts the DC power back to three phase AC power having an end useable form (e.g., three phase high frequency AC voltage). A controller controls the inverter in a manner calculated to provide the voltage waveforms required by the consuming facility. 
     While power plants are typically the most efficient way to provide massive amounts of power to consuming facilities, most consuming facilities are equipped with a backup power supply, typically a generator, to provide power to the facility when utility grid power is cut off. Generators, like power plants, provide three phase AC power to conditioning configurations. 
     A typical rectifier includes a diode or SCR bridge that, while rectifying AC power, causes a three phase 360 Hertz ripple across the DC bus. To filter out the ripple many conditioning configurations include a pre-charge component, typically a large bulk capacitor, linked between the positive and negative DC buses. Unfortunately, under certain circumstances the bulk capacitor can result in a current in-rush into the inverter that can damage the inverter. The magnitude of the current in-rush is a function of two factors. First, if the bulk capacitor is relatively uncharged the difference between a peak applied voltage and the DC bus voltage can be substantial, the potential difference tending to cause the current in-rush. Second, the in-rush magnitude is also a function of the AC source (e.g., the utility) impedance. Where source impedance is relatively large the in-rush is appreciably tempered and where source impedance is relatively small the in-rush is relatively large. 
     Because of the potentially damaging current in-rush, most conditioning configurations including a DC bus bulk capacitor also include a hardware protection mechanism to ensure that the bulk capacitor charge is always above a safe threshold or “pre-charge” level prior to causing the inverter to draw power from the DC bus. The protection mechanism may be as simple as a DC bus voltage sensor for sensing the bulk capacitor charge, a comparator to compare the bulk capacitor charge to the pre-charge level and a mechanism for causing the inverter controller to disable (i.e., opening inverter switches so the inverter does not draw power) the inverter when the bulk capacitor charge dips below the pre-charge level. Thus, upon start-up, the DC bus capacitor is pre-charged to the pre-charge level prior to power being drawn by the inverter. Similarly, during inverter operation if, for some reason, the capacitor charge dips below the pre-charge level, the inverter stops drawing power until the capacitor charge again exceeds the pre-charge level. 
     A typical impedance for a utility source is approximately a relatively low 1 to 3%. Given such a low impedance, a typical pre-charge level is approximately 85% of full bulk capacitor charge or full charge level (FCL). Thus, when the bulk capacitor charge falls below the 85% FCL, the inverter controller disables the inverter. While some conditioning configurations have an adjustable pre-charge level, each configuration requires that the pre-charge level be set to accommodate a specific source impedance. 
     While a utility source is typically characterized by a low impedance, most generators are characterized by relatively high impedances. For example, many generators have impedances that exceed 20%. Such high impedances often cause excessive distortion in AC supply voltage and current waveforms. Referring to FIG. 2 exemplary voltage and current waveforms V and I, respectively, corresponding to a generator source are illustrated. Waveforms V and I were generated by a generator feeding a six-pulse inverter drive under load. While a generator regulator can increase output to provide a desired RMS, the distorted waveforms can cause DC bus voltage dips. 
     In many cases, when the DC bus voltage dips below the pre-charge level, the dip causes pre-charge disablement of the inverter while the bus voltage is re-charged to the pre-charge level. Because pre-charge levels are set for efficient and safe operation assuming a utility source (e.g., 85% of full bulk capacitor charge), and generators often cause relatively large DC bus voltage distortion, switching from a utility source to a generator source often results in the DC bus voltage dropping below the specified pre-charge level and hence causes the controller to disable the inverter during pre-charge sessions. 
     One solution to the voltage dip problem caused by switching from a utility to a generator source is to provide a much larger generator (e.g., 3 to 4 times larger) to lower the source impedance to a level more similar to the utility source impedance. While adopted by several members of the industry this solution is relatively expensive and therefore is not suitable for all applications. 
     Another solution that can be adopted when the pre-charge level is manually adjustable is to adjust the pre-charge level manually to accommodate the generator&#39;s higher characteristic impedance. To optimally manually adjust the pre-charge level as a function of source and system characteristics a system user has to be knowledgeable about many system characteristics and inductive/electrical phenomenon. For example, to optimally tune the user has to be familiar with an expected peak source voltage, source impedance, inverter switch characteristics such as current handling capabilities and so on. In other words, the user has to be relatively highly skilled to effect optimum pre-charge tuning. While effective, manually adjustable systems are burdensome in most cases for several reasons. First, generators are primarily used as backup power sources and therefore are only sporadically employed. For this reason often no user at a consuming facility is familiar with the pre-charge level adjustment procedure and the source/system characteristics described above. Second, even when generators are employed the periods of employment are often limited as utility sources are typically only cut off for relatively short times. Thus, even if the pre-charge level could be manually adjusted at a consuming facility when a generator is employed, the manual adjustment task would likely be for naught when the utility source is back on line. In fact, in such a case, when the utility is back on line the pre-charge level would again have to be adjusted back to the level corresponding to the utility impedance. 
     Other solutions such as halting power consumption when a utility is down or simply allowing dips and pre-charge sessions when a generator source is employed are unacceptable in many applications where relatively constant power and inverter operation is required. 
     In addition to consuming facilities that are primarily powered by utility sources, some remote facilities are powered primarily by dedicated generator sources. In these cases the conditioning configurations that facilitate manual pre-charge level adjustment are advantageous as the pre-charge level can be set to accommodate specific generator characteristic impedance. Unfortunately, even in this case, the task of selecting the pre-charge level is burdensome and increases the time and costs associated with commissioning the conditioning configuration. 
     Therefore, a need exists for a low cost system that facilitates switching of power sources that feed conditioning configurations where the sources are characterized by different impedances without causing unnecessary inverter disablement. In addition, it would be advantageous to have a system that automatically selects a suitable pre-charge limit as a function of the source linked to the conditioning configuration. 
     BRIEF SUMMARY OF THE INVENTION 
     The present inventors have recognized that while the high impedance associated with a generator often causes excessive DC link distortion, the high impedance also impedes current in-rush into an inverter when the difference between a peak applied voltage and a DC bus voltage is large. For this reason, where a generator source is employed relatively larger DC voltage dips can be tolerated without risking in-rush current magnitudes that will damage an inverter. Thus, when a generator is employed instead of a utility source, the present invention automatically selects a lower pre-charge level for the DC link than the pre-charge level employed when a utility source provides power to the DC link. For example, when a utility source is initially linked to a conditioning configuration including a DC bus bulk capacitor and the utility source has a characteristic impedance of 1 to 3% a first suitable pre-charge level of 85% of full charge may be employed. Upon switching from the utility source to a generator source where the generator source has a characteristic source impedance of approximately 20%, a second suitable pre-charge level of between 15 and 50% is employed. 
     Thus, one object of the invention is to provide a conditioning configuration wherein unnecessary inverter disablement is substantially reduced. To this end, by providing two different pre-charge levels, a first relatively high level corresponding to a first source having a first characteristic impedance and a second relatively low level corresponding to a second source having a second characteristic impedance that is greater than the first characteristic impedance, when the sources are switched, in-rush current is maintained at safe levels while the number of pre-charge sessions is kept to a minimum. 
     One other object to achieve the aforementioned object relatively inexpensively. To this end the present invention can be implemented essentially using existing hardware and simply adding software instructions to be performed by a controller processor. 
     Another object is to provide a system where the pre-charge level is automatically set as a function of the source. To this end, one embodiment of the inventive system includes a controller input that is linkable to a utility source, the utility providing a signal at the input when the utility is providing power. When a signal is received at the input the controller sets the pre-charge level at a level suitable for a typical utility source (e.g., 85% of the normal DC link level). When no signal is provided at the input the controller sets the pre-charge level at a level suitable for a relatively high impedance generator (e.g., 60% of the normal DC link level). 
     Yet another object of the invention is to provide a system where a second pre-charge level for a specific generator can be optimally tuned once by a skilled system operator and thereafter, when the generator is linked either manually or automatically to provide power to the DC bus, the system uses the optimally tuned pre-charge level. It is contemplated that in the case of generators that provide redundant back-up power to utility sources, typically a single generator having specific characteristics (e.g., a specific known source inductance) will be employed and only rarely will the generator be replaced with another generator having different characteristics. In these cases, by storing optimally tuned pre-charge levels for both the utility and generator sources, the task of returning the pre-charge levels upon source switching is eliminated. 
     One more object is to, according to one embodiment of the invention, provide “hard-coded” first and second pre-charge levels corresponding to “worst case” sources of different types. For example, given all commercially available and mass produced generators there will be an expected lowest inductance generator. For instance, the generator having the expected lowest inductance may be characterized by a 10% inductance characteristic as opposed to a more typical 15-70% inductance characteristic. In the case of a 10% inductance generator, the pre-charge level deemed suitable to protect an inverter may be 75% of the normal DC link level. By hard-coding the utility source pre-charge level to 85% and the worst case generator&#39;s pre-charge level to 75%, a generic system to accommodate all generators is provided. 
     One further object is to restrict pre-charge level modifications by unskilled system users. Where pre-charge level is changeable by unskilled users, an unskilled user may set the pre-charge level below an inverter protecting level. for example, a user unfamiliar with a specific generator may believe that a maximum applied generator voltage is much lower than an actual maximum voltage and therefore may set the pre-charge level lower than suitable for protection purposes. By either hard-coding pre-charge levels for expected worst case source inductance or enabling optimal tuning by only skilled system users, suitable pre-charge levels can essentially be assured. 
    
    
     These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to the claims herein for interpreting the scope of the invention. 
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a schematic view the inventive system in the context of an exemplary motor control system according to the present invention; 
     FIG. 2 is a graph illustrating voltage and current waveforms from a high impedance generator feeding a 6-pulse drive under load; 
     FIG. 3 is a graph illustrating DC bus voltage and inverter output with a pre-charge level set to 19%; 
     FIG. 4 is a graph similar to FIG. 3 albeit with the pre-charge level set to 50%; and 
     FIG. 5 is a flow chart illustrating a method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein like reference characters correspond to similar components throughout the several views and, more specifically, referring to FIG. 1, the present invention will be described in the context in the exemplary motor control system  10 . System  10  includes three-phase generator source  12 , a three-phase utility source  14 , a source selector  16 , a rectifier  18 , a capacitor bank  20 , a DC link voltage sensor  22 , an inverter drive  24 , a three-phase load  26 , a source control  28 , an interface  30 , a utility voltage sensor  32  and an inverter controller  34 . 
     Selector  16  defines six inputs, first, second and third inputs linked to generator supply lines  36 ,  38  and  40  and fourth, fifth and sixth inputs linked to utility supply lines  42 ,  44  and  46 , respectively. Selector  16  also includes three outputs linked via first, second and third output lines  48 ,  50  and  52  to three inputs of rectifier  18 . In addition to the inputs and outputs, selector  16  includes three single pole double throw switches  54 ,  56  and  58 . Switch  54  links output  48  to either first input line  36  or fourth input line  42 , switch  56  links output line  50  to either second input line  38  or fifth input line  44  and switch  58  links output line  52  to either third input line  40  or sixth input line  46  such that each of switches  54 ,  56  and  58  is capable of linking a corresponding output line to either a generator input line or a utility input line. Switches  54 ,  56  and  58  are synchronized such all of the switches together select either generator input lines  36 ,  38  and  40  or utility input lines  42 ,  44  and  46 . 
     A control bus  60  links source control  28  to each of switches  54 ,  56  and  58  so that control  28  controls the positions of each of switches  54 ,  56 , and  58 . Thus, selector  16  is used to select either three phase AC input from generator  12  or three-phase AC input from utility  14 , providing the selected AC input voltages to rectifier  18  on output lines  48 ,  50  and  52 . 
     Rectifier  18  receives the three-phase input voltages on lines  48 ,  50  and  52  and, as well known in the power conditioning industry, converts the three-phase voltages to a DC potential which is provided across positive and negative DC buses  62 ,  64 , respectively. The DC buses  62  and  64  together are referred to herein as a DC link. A large bulk capacitor bank  20  is linked between buses  62  and  64  and is provided to reduce ripple on the DC link. 
     The DC link provides a DC voltage to inverter drive  24 . Also as well known in the power conditioning industry, inverter drive  24  includes a plurality of switches which are configured and controlled so as to convert the DC voltage at the inverter inputs to three-phase AC output voltages on supply lines  66 ,  68  and  70 . Each of lines  66 ,  68  and  70  is separately linked to one of three phase windings (not illustrated) of three-phase load  26 . Operation of exemplary three-phase loads  26  such as motors are well known in the controls art and therefore will not be explained herein detail. 
     Inverter controller  34  is linked via a control bus  72  to the switches of inverter drive  24  to independently control the states (e.g., open, closed) of the drive switches in a manner calculated to provide desired voltage on output lines  66 ,  68  and  70 . When the inverter switches are closed power is drawn from the DC link (i.e., buses  62  and  64 ) and delivered to load  26  thereby tending to reduce the energy on the DC link. When inverter switches are opened power is not drawn from the DC link and the energy provided by rectifier  18  increases the energy stored across the DC link. 
     Sensor  22  is linked to buses  62  and  64  and senses the voltage difference across the DC link. A signal indicative of voltage difference is provided to inverter controller  34  via a sensor line  74 . 
     Referring still to FIG. 1 source sensor  32  senses the presence of a utility voltage on line  46  and provides a signal indicating the presence of utility voltage via a sensor line  76  to source control  28 . 
     Source control  28  performs several functions according to the present invention. First, based on the presence of a signal on sensor line  76 , source control  28  control switches  54 ,  56  and  58  to select either generator supply lines  36 ,  38  and  40  or utility supply lines  42 ,  44  and  46  to be linked to output lines  48 ,  50  and  52 . In this regard, specifically, when a signal is not received via sensor line  76 , source control  28  sends signals to switches  54 ,  56  and  58  via bus  60  causing switches  54 ,  56  and  58  to link generator input lines  36 ,  38  and  40  to output lines  48 ,  50  and  52 , respectively. To this end, switches  54 ,  56  and  58  may be normally closed to generator source lines  36 ,  38  and  40  so that, when there is no signal on line  76 , source control  28  does not provide signals to switches  54 ,  56  and  58  thereby allowing those switches to remain in their normally closed states. In this case, the absence of signals on bus  60  would in effect be signals indicating that the normally closed states of switches  54 ,  56  and  58  should be maintained. When a signal is provided on line  76  indicating that utility source  14  is linked to selector  54  and that voltage is being provided on line  46 , source control  28  controls switches  54 ,  56  and  58  to close those switches to connect utility supply lines  42 ,  44  and  46  to output lines  48 ,  50  and  52 , respectively. 
     In addition to providing control signals on bus  60 , source control  28  also provides a signal via a line  80  to inverter controller  84  indicating which of utility  14  and generator  12  is linked to output lines  48 ,  50  and  52 . 
     Inverter controller  34  also performs a plurality of functions. As indicated above, controller  34  controls the switches (not illustrated) of inverter drive  24  so as to provide three-phase AC voltages on lines  66 ,  68  and  70 . In addition, controller  34  performs the protection function of disabling the inverter switches whenever the DC link voltage level drops below a pre-charge level corresponding to a pre-charge level deemed adequate to eliminate the possibility of an on-rush current into inverter drive  24 . With respect to utility source  14 , the predetermined pre-charge level in the exemplary system  10  will be assumed to be 85% of a normal DC link voltage level. Thus, when utility source  14  is linked to lines  48 ,  50  and  52  to provide voltage to rectifier  18 , if the DC link voltage across buses  62 , and  64  drops by 15%, inverter controller  34  disables inverter drive switches to allow the DC link voltage across capacitor  20  to recharge up to the desired DC link pre-charge level. 
     Where generator source  12  is linked to output lines  48 ,  50  and  52  to provide power to rectifier  18 , inverter controller  34  is programmed employ a second relatively lower pre-charge level to protect inverter drive  24  switches. In one embodiment of the invention, the relatively lower pre-charge level corresponding to generator  12  is hard-coded into controller  34  and is selected to be a pre-charge level which will accommodate essentially all generators  12  independent of generator inductance corresponding to a specific generator type. In this case, the generator pre-charge level is selected so as to accommodate the lowest expected inductance corresponding to a generator  12 . Thus, where the lowest expected generator inductance is 10%, the hard-coded pre-charge level corresponding to generator  12  may be selected to be a relatively high 75% of the normal DC link voltage. 
     In the alternative, the generator pre-charge level may be adjustable via interface  30  within a range corresponding to expected generator inductance levels. As illustrated in FIG. 1, an exemplary range of selectable generator pre-charge levels may be between 15% and 50%. 
     Referring now to FIGS. 1 and 5, in FIG. 5, an exemplary method  100  according to the present invention is illustrated. Method  100  is performed by source control  28  and inverter controller  34  and is generic in the sense that it is independent of whether or not the pre-charge level corresponding to generator  12  is hard-coded or manually adjustable, initially it is assumed that each of the generator pre-charge level PCL 1  and the utility charge level PCL 2  have been set. 
     For the purpose of this explanation it will be assumed that the pre-charge level PCL 1  is 65% of the normal DC link voltage level. It will also be assumed that the utility suitable pre-charge level PCL 2  is set to 85% of the normal DC link voltage level. In addition, this example it will be assumed that switches  54 ,  56  and  58  are normally closed such that lines  36 ,  38  and  40  are linked to lines  48 ,  50  and  52  and therefore, source control  28  “selects” generator  12  for linking to rectifier  18  by providing no signals on bus  60 . 
     At process block  104  sensor  32  senses voltage on utility supply line  46  and, if a voltage is present on line  46 , sensor  32  generates a signal V u  on line  76  which is provided to source control  28 . At decision block  106 , source control  28  determines whether or not a voltage present signal V u  has been received. If no voltage presence signal V u  has been received, control passes to block  102  where control  28  provides  no  signal on line  80 . When no signal is provided on line  80  controller  34  sets the pre-charge level PCL to PCL 1  corresponding to the generator pre-charge level. In addition, at block  102  control  28  controls link selector  16  to select generator  12  as the source linked to output lines  48 ,  50  and  52 . At block  106 , if a signal V u  is provided on line  76  indicating the presence of source  14  control passes to block  108  where control  28  provides a signal to controller  34  via line  80 . When controller  34  receives a signal on line  80  controller  34  sets the pre-charge level PCL equal to the utility suitable charge level PCL 2 . In addition, at block  108  control  28  controls selector  16  to link utility  14  to output lines  48 ,  50  and  52 . 
     Next, after each of blocks  102  and  108  control passes to block  110  where sensor  22  senses the DC link voltage V DC  and provides a signal to inverter controller  34 . At decision block  112 , inverter controller  34  compares the sensed DC link voltage V DC  to the set pre-charge level PCL. At block  106 , assuming that there is no voltage on utility line  46 , the pre-charge level PCL is equal to the generator pre-charge level of 65% of the normal DC link voltage. If the sensed DC link voltage is greater than 65% of the normal DC link voltage level at decision block  112 , control passes to block  116  where controller  34  continues to enable inverter  24 . In the alternative, if the sensed DC link voltage V DC  is less than  65 % of the normal DC link voltage control passes to block  114  and controller  34  disables inverter  24 . The loop from block  112  through block  114  continues so that inverter  24  remains disabled until the sensed DC link voltage V DC  is greater than the 65% pre-charge level at which point control passes to block  116  where controller  34  enables inverter  24 . 
     Although not illustrated, it should be appreciated that the loop including blocks  112  and  144  may be hysteretic in the sense that, after inverter  24  has been disabled at block  114 , the inverter  24  may not be re-enabled until a DC link voltage greater than the pre-charge level (e.g., greater than 65% in the present example) has been achieved so as to limit the disabling and enabling cycles that occur. Hysteretic loops are well known in the controls art and therefore will not be explained here in more detail. 
     Referring still to FIGS. 1 and 5 and specifically to again to block  106 , as indicated above, if a voltage presence signal V u  is received via line  76  indicating that utility source  14  is linked to selector  16  and that a voltage is being provided on line  46 , controller  34  sets the pre-charge level PCL to the pre-charge level PCL 2  corresponding to utility source  14 . In the present example, it has been assumed that the utility source pre-charge level PCL 2  is 85% and therefore, at block  108 , the pre-charge level PCL is set equal to 85% of the normal DC link voltage. 
     Continuing, at block  110 , sensor  22  again senses the DC link voltage V DC  and provides a signal indicative thereof to controller  34  via line  74 . Again, at block  112 , controller  34  compares the sensed DC link voltage V DC  to the pre-charge level PCL. In this case, because utility source  14  is linked to rectifier, the pre-charge level is 85% of the normal DC link voltage and therefore, when the sensed DC link voltage V DC  drops below 85% of a normal DC link voltage, at block  114  inverter controller  34  disables inverter  24 . Similarly, when the DC link voltage V DC  is greater than the pre-charge level (i.e., in this case, greater than 85% of the normal DC link voltage), control passes to block  116  where inverter controller  34  enables inverter  24 . As above, the loop including blocks  112  and  114  may be hysteretic. 
     After inverter  24  has been enabled at block  116 , control again passes up to block  104  where process  100  continues. 
     Referring now to FIG. 3, an exemplary DC link voltage V DC  and an exemplary PWM voltage pulse waveform are illustrated where the source used to provide power to rectifier  18  (see FIG. 1) was a generator and the pre-charge level was set to 81% of the normal DC link voltage. In FIG. 3, the DC bus voltage V DC  is shown on a scale of 100 volts per division and the PWM waveform PWM is shown on a scale of 500 volts per division. It can be see that at a time T 1  controller  34  disables inverter  24 . In-rush current into the inverter was not a problem given the pre-charge level and the generator inductance. 
     Referring also to FIG. 4, FIG. 4 is similar to FIG. 3 in that it shows a DC link voltage waveform V DC  and a PWM voltage waveform PWM. Scales for each of the waveforms in FIG. 4 are identical to the scales to the waveforms in FIG.  3 . The difference between FIGS. 4 and 3 is that the pre-charge level in FIG. 4 was set to 50% of the normal DC link level and, as seen, inverter  24  is disabled at a time T 2  after the DC link voltage V DC  is dropped by 50%. Again, given the pre-charge level and the generator inductance no in-rush current problems were observed. 
     It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. 
     To apprise the public of the scope of this invention, the following claims are made: