Abstract:
A method of and system for monitoring condition of a large capacitor connected across an output of a rectifier circuit in an operating electrical power transmission circuit in order to anticipate capacitor failure and facilitate appropriate corrective action is disclosed. The method includes measuring a ripple voltage on the capacitor and ripple current through the capacitor, determining from a representative signal whether the signal exceeds a predetermined threshold; and sending an output to a controller on a system operator if the signal exceeds the predetermined threshold. The ripple current and ripple voltage measurements may be provided as inputs to a digital to analog converter which produces and sends the representative signal to a microprocessor to generate the output to the controller.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 61/548,674, filed Oct. 18, 2011, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE TECHNOLOGY 
     This disclosure relates generally to high power electrical transmission systems and more particularly to high power inverters and converters employing electrolytic capacitors. 
     BACKGROUND 
     Many converters are often out in the field, exposed to the elements, and at times in a very hot or very cold environment. Because they contain electrolytic capacitors, in particular, large electrolytic capacitors to support some internal high-voltage dc bus, they often end up not lasting very long because the capacitors die prematurely even if the remainder of the circuitry is good. In some cases, the capacitor may not completely die, but it may degrade to a point where its performance is reduced dramatically, so that eventually it shuts off for safety reasons. 
     The same problem occurs in the power supplies of computers and other electronic equipment, wherein the capacitors eventually die out. What is needed is a system and method for monitoring capacitor health in such electronic equipment and for making capacitors easily exchangeable. 
     SUMMARY OF THE DISCLOSURE 
     A method of monitoring condition of a capacitor connected across an output of a rectifier circuit in an operating electrical power transmission circuit in accordance with the present disclosure includes operations of measuring a ripple voltage on the capacitor; measuring a ripple current through the capacitor; determining a representative signal from the ripple voltage and current measurements; determining whether the signal exceeds a predetermined threshold; and sending an output to a controller on a system operator if the signal exceeds the predetermined threshold. This then can facilitate notification that the capacitor being monitored should be replaced soon. In particular, the ripple current and ripple voltage measurements may be provided as inputs to a digital to analog converter. This converter then produces and sends the representative signal to a microprocessor to generate the output to the controller. The measuring of ripple current may be performed by a rectifier and amplifier module to produce the direct current representative signal. The measuring of ripple voltage may be performed by a voltage divider module. A temperature of the capacitor may also be utilized, sending a temperature signal to the digital to analog converter. 
     A power inverter in accordance with the present disclosure may include a box housing such as a conventional enclosure having an electrical circuit therein including a dc to ac converter; and at least one electrolytic capacitor mounted to an exterior of the box. The capacitor is removably connected with the electrical circuit within the box. Preferably, the capacitor or capacitor bank may include a cutout switch for selectively removing the capacitor from electrical connection to the circuitry within the box. Such a power inverter preferably includes a capacitor health monitoring circuit operably connected to the capacitor. This monitoring circuit may be located within the box or on the capacitor enclosure. 
     The monitoring circuit may include a microprocessor programmed to perform operations of: measuring a ripple voltage on the capacitor; measuring a ripple current through the capacitor; determining a representative signal from the ripple voltage and current measurements; determining whether the signal exceeds a predetermined threshold; and sending an output to a controller if the signal exceeds the predetermined threshold. Such an output could be used to schedule maintenance on the capacitor. 
     Preferably the monitoring circuit senses voltage ripple across the capacitor and also senses ripple current through the capacitor. The ripple current and ripple voltage may be analyzed in a microprocessor programmed to compare a signal representative of the ripple current and ripple voltage to a predetermined threshold, and issue an output signal when the ripple current and ripple voltage exceed the predetermined threshold. 
     A health monitoring system for monitoring a condition of a capacitor in a power inverter circuit in accordance with the present disclosure may include a ripple voltage detector connected across the capacitor, a ripple current detector connected in series with the capacitor, and a measuring module connected to the voltage and current detectors operable to produce a signal representative of the ripple voltage and current and send an output to a controller if the signal exceeds a predetermined threshold. The measuring module may includes an analog-to digital-converter connected to a microprocessor which generates the output to the controller. The ripple voltage detector may be a voltage divider module and the ripple current detector may be a rectifier and amplifier module. The outputs of the voltage divider module and rectifier and amplifier module are preferably fed to an analog-to-digital converter, which in turn produces an output fed to the microprocessor. 
     The disclosure includes methods and apparatuses which perform these methods, including data processing systems which perform these methods, and computer readable media containing instructions which when executed on data processing systems cause the systems to perform these methods. Other features will be apparent from the accompanying drawings and from the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements. 
         FIG. 1  is a schematic overview of a power inverter system in accordance with the present disclosure. 
         FIG. 2  is a schematic representation of a capacitor module in the system shown in  FIG. 1 . 
         FIG. 3  is a circuit diagram for sensing capacitance leakage in a capacitor module in accordance with the present disclosure. 
         FIG. 4  is a circuit diagram of the internal capacitor module in accordance with the present disclosure. 
         FIG. 5  is a process flow diagram for the system in accordance with the present disclosure. 
         FIG. 6  is a perspective view of an exemplary inverter box in accordance with the present disclosure. 
         FIG. 7  is a graph of capacitor life expectancies for a long life aluminum capacitor. 
         FIG. 8  illustrates a capacitor having a cutout switch for selectively removing the capacitor for electrical connection according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description and drawings are illustrative and are not to be construed as limiting. Numerous specific details are described to provide a thorough understanding. However, in certain instances, well known or conventional details are not described in order to avoid obscuring the description. References to one or an embodiment in the present disclosure are not necessarily references to the same embodiment; and, such references mean at least one. 
       FIG. 1  shows an overview of a power inverter  100  in accordance with the present disclosure. An initial circuitry  102 , typically referred to as a chopper, takes in the DC power from the source  101  and converts the power to radio frequency (RF) power. The size and weight and cost of transformers typically shrinks in accordance with T=1/square of the frequency. Thus, the higher the frequency, the smaller the transformer. However, this is counterbalanced by switching losses, that increase with frequency, and so, for each system there is an optimal point or range of transformer physical configurations in which to operate. The RF power is typically fed, from the chopper circuit  102 , to a transformer  103  which is used to increase the voltage (in some applications the transformer may be unnecessary). Then the RF power is then typically fed to a rectifier module  104  that rectifies the high voltage RF to produce high voltage DC. Capacitor  105  supports the high voltage DC bus coming out of the rectifier module  104 , typically with a large capacitor or even a bank of capacitors, because, depending on the output frequency of the final inverter stage module  106 , the current demand on the bus may exceed the supply fed by module  104 . 
     For purposes of this document, capacitor and capacitor bank shall be used interchangeably. Typically, inverter module  106  takes the high voltage DC and creates a one-, two-, or three-phase supply output  107 , indicated as P 1 , P 2 , and P 3 , respectively, to feed the power transmission grid or a local AC supply. Because the grid typically runs at much lower frequency than the RF chopper  102 , the capacity of capacitor  105  must be very large, large enough to hold the differences in current flow due to the waveforms and frequencies typically used in an AC grid. 
       FIG. 2  shows a schematic overview of an exemplary capacitor module  200 , according to the system and method disclosed herein. The capacitor or capacitor bank  105  is, in this example, connected to a ripple current sensor  201 . In some cases sensor  201  is an inductive transformer, although in other cases it could be a Rogowski transformer or coil, which is basically a toroid of wire used to measure an alternating current through a cable encircled by the toroid. 
     Alternatively a very low-resistance resistor, typically in the low milliohms may be used in series with the capacitor  105 , just enough to measure the ripple current. Some specialized low ohm measuring resistors that may be utilized have four leads, to allow a more accurate measurement. The large capacitor  105  is connected to the inverter stage circuit  106 . 
     The inverter circuit  106  is shown in  FIG. 2  in greater detail than in  FIG. 1 . The capacitor controller or supervisory controller circuit  202  is shown by example in more detail in  FIG. 4 . The controller circuit  202  has a conventional built-in test circuit for the capacitor  105 , because, preferably, large capacitors must be tested for every line cycle (in this case, typically twice the line frequency, so, 100, Hz or 120, Hz respectively). Also present are connection  203  to measure the ripple current and connection  204  for measuring the voltage on the dc bus from rectifier module  104  described above, as well as the amplitude and frequency of any ripple voltage on the bus. Connection  205  could be a wireless connection or other network connection to another processor, etc., to send maintenance reports and notifications reflecting the measurements of the ripple current. 
       FIG. 3  shows an exemplary circuit  300  for sensing capacitor leakage, according to the system and method disclosed herein. Capacitors, such as capacitors  301   a - n , (of which two are shown, for exemplary purposes), are placed in banks, in series and sometimes in parallel, or both. Often some high-resistance resistors  302   a - n , are used across the capacitors  301   a - n , to align the voltages. Such resistors are also part of a discharge circuit that is typically required for safety reasons to dissipate the charge on the capacitors  301   a - n , when they are electrically removed from service. 
     As one of the capacitors  301   a - n , starts to dry out, its capacity reduces, and accordingly the voltage distribution, as measured by elements  303  and  304 , typically voltage dividers, can be used to monitor any such changes and could connect to controller  202  as well. These elements  303  and  304  may be used to measure a drift off the voltage balance, indicating the dry-out of one or more of the capacitors, resulting in reduced performance and eventual failure. 
       FIG. 4  shows an exemplary layout of the inside of controller circuit  202  shown in  FIG. 2  in more detail, according to the system and method disclosed herein. Input connections  204  (voltage ripple sensing) and  203  (current ripple sensing) are conditioned by reconditioning circuits such as a voltage divider module  402  and rectifier and amplifier module  401  respectively. Their outputs are then fed, as direct current, into an analog-to-digital converter (DAC)  403 . The DAC  403  delivers the digitized results into microprocessor  404 . Microprocessor  404  is an embedded microprocessor that contains memory, program storage, communications, etc. An output signal line  205  extends from the microprocessor  404  to communicate with the rest of the system  100 . 
     Exemplary circuit  202  also contains a temperature sensor  405  that is also connected to the DAC  403 . Some microprocessors made for industrial applications may have some or all the elements described here built in, some may even include the temperature sensor  405  and output to or incorporate a wireless communication network. For the purposes of describing the system and method disclosed herein, all these variations are included. 
       FIG. 5  shows an exemplary process  500  that may be executed by a programmable computer or by code in the microprocessor  404  for implementation of the system, according to one aspect of the system and method disclosed herein. Control flow begins at operation  501 . In operation  501  the system measures the local ambient temperature, typically expected to be about the same as the capacitor temperature. Control then transfers to operation  502 . 
     In operation  502 , the bus voltage is measured, as well as the amplitude and frequency of any ripple voltage on the bus at connection point  204 . In some cases, more than one voltage may be measured, such as, for example, if there is a series of capacitors, as described above in the discussion of  FIG. 3 . Control then transfers to operation  503 . 
     In operation  503 , the ripple current at point  203  is measured. Control then transfers to operation  504  In operation  504  the ripple voltage and ripple current data is saved into a storage unit  505 , which could be, for example, an EPROM, a flash, or other suitable nonvolatile memory, either in or attached to the microprocessor (not shown). Control then transfers to operation  506 . 
     In operation  506  the system loads a data raster with past data and the current measured data, and control transfers to operation  507 . 
     In operation  507  the system executes a trend analysis on the loaded current and past measured data. Control then transfers to query operation  508  where the query is made whether the trend analysis projection is at or above a predetermined threshold or below the predetermined threshold. 
     If the trend analysis results in projections exceeding certain predetermined threshold values, control transfers to operation  509  where the system sends a message to a power master management unit coupled to it via communication link  205  (not shown). Such a message typically may include a unit ID, a time and date stamp, the type and location of the trouble threshold, and expected time to initial failure. This message preferably will permit maintenance personnel to schedule exchange of the capacitor (bank) in a timely, non-disruptive manner. If, in operation  508 , the predetermined values are not exceeded, and are not trending toward exceeding those predetermined values, control returns to operation  501  and the process begins again. 
     The sending of a message to the power master management unit by the system in accordance with the present disclosure is further discussed in our U.S. Pat. No. 7,602,080,, the disclosure of which is incorporated herein by reference in its entirety. 
       FIG. 6  is a simplified schematic perspective view of an exemplary inverter box  600 , according to the system and method disclosed herein. Box  600  typically has a bottom plate  601  where cables, such as the feeding cable  602  enter and grid cable  603 , come out. The internal circuitry within the box  600  is described above with reference to  FIGS. 1-5 . On the front  604  of the box  600  are two capacitor bank units  605   a , and  605   b , with handles  606   a , and  606   b , and securing latches  607   a , and  607   b . This proposed configuration is similar to the configuration used in high-power fuse blocks that support arc-free or arc-secured removal while in operation without requiring the operator to touch any conductors. Such configurations have conductor blades that are broad enough for low-impedance connection to the rest of the circuitry, thus enabling the system to continue operation during the removal and replacement of the capacitor or capacitor bank or block. 
     With more than one block or capacitor bank units, capacitors can be swapped without having to shut down the transmission system. In cases where only a single block is used, a shut-off button or switch (not shown) may be placed next to the block, so that the operator can switch the block off and then wait for an indicator light (not shown) such as, for example, a green light, to indicate that it is now safe to remove the block without danger of discharge. 
     This interchangeable capacitor bank approach in accordance with the present disclosure has a huge advantage over the use of a conventional aluminum capacitor bank because of the size and weight burden that aluminum capacitors place on the typically used circuit board inside a converter or inverter, and also because they are vibration-sensitive and need to be ruggedized for applications such as, for example, automotive, airplane, and marine uses, etc. 
       FIG. 7  is a graph that shows a diagram of the life expectancy of a long-life aluminum capacitor in a high ripple current use application, such as in inverters and converters utilized in high power transmission applications. For most applications, the operating temperature of inverters is such that capacitors run between 55° C. and 65° C. Therefore, having more than one capacitor or capacitor bank available, and monitored, at all times achieves an overall lower actual time to failure, and increases the chance of the system operators being notified before a catastrophic system failure occurs. 
     In the foregoing specification, the disclosure has been described with reference to specific exemplary embodiments thereof. It is clear that many modifications and variations of the system and method disclosed herein may be made by one skilled in the art without departing from the spirit of the novel art of this disclosure. These modifications and variations do not depart from its broader spirit and scope as set forth in the following claims.