Patent Description:
Power conversion systems that are used to serve AC loads often include an inverter that generates an AC output from a DC voltage provided by a power source. For example, uninterruptible power supply (UPS) systems, which are used to provide uninterrupted power in critical applications, commonly use a battery or other DC power source to provide backup power to an inverter in the event of the failure of a primary power source, such as an AC utility source. Converters used to interface photovoltaic panels to AC power distribution systems also commonly include an inverter that operates off of DC power provided by the photovoltaic panels. Some UPS systems may also be designed to provide power to AC loads from photovoltaic panels, as described, for example, in <CIT>.

In many such applications, the DC power source may be operated such that it "floats" with respect to a system ground. However, ground faults may occur in such systems due to environmental contamination, electrolyte leakage, impact damage and/or other events. Such ground faults may pose operational and safety problems. Techniques for detecting and dealing with ground faults in battery and photovoltaic systems are described, for example, in <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>. Attention is also drawn to <CIT>, which relates to an AC Detector for DC ground faults and high capacitance in high-voltage DC power supplies. The ground fault detector comprises a detector circuit for sensing a relatively low electrical impedance, which represents a ground fault current, between a part of a secondary circuit of a high voltage power supply and chassis ground to generate an alternating voltage. A rectifying circuit is operatively connected to the detector circuit for rectifying the alternating voltage. A warning indication network is responsive to the rectified alternating voltage for visually indicating the presence of the ground fault current.

Attention is drawn to <CIT>, which shows an inverter system for a vehicle comprising a housing, a primary stage, a secondary stage and a fault detection circuit. The primary stage is configured to receive a first voltage signal from an energy power source to generate a second voltage signal. The secondary stage is configured to generate a third voltage signal in response to the second voltage signal. At least one of the primary and the secondary stages define at least one resistance point for discharging leakage current responsive to generating the third voltage signal. The fault detection circuit is configured to electrically couple the primary stage and the secondary stage to provide the second voltage signal to the secondary stage and to measure a portion of the third voltage signal to determine whether the leakage current being discharged through the at least one resistance point is within a predetermined current range. Further, <CIT> is related to A ground fault detection method for detecting a ground fault in a direct current circuit comprising a load and a supply line connecting the direct current power supply and the load, wherein an alternating current signal is injected between the supply line and the ground, and the supply line And detecting a ground fault of the direct current circuit from the detected alternating current and alternating voltage.

In accordance with the present invention, a method and a system for monitoring a DC voltage source as set forth in claims <NUM> and <NUM>, respectively, are provided. Further embodiments of the invention are inter alia disclosed in the dependent claims. Some embodiments of the inventive subject matter inter alia provide methods of monitoring a DC power source, such as a string of electrochemical or photovoltaic cells. A fault in the DC power source is identified by detecting a change in an AC component of a residual current of the DC power source. In some embodiments, the DC power source is coupled to at least one DC bus and the methods further include generating an AC voltage on the at least one DC bus. For example, the DC power source may be coupled to a DC bus of an uninterruptible power supply (UPS) system comprising an inverter having an input coupled to the DC bus and the methods may include generating a voltage having an AC component on the DC bus of the UPS system. In some embodiments, the UPS system comprises first and second DC busses and a neutral and generating a voltage having an AC component on the DC bus of the UPS system comprises shifting the first and second DC busses with respect to the neutral. The inverter may be configured to generate an AC output voltage and the AC component has a frequency that is a harmonic (e.g., a third harmonic) of a fundamental frequency of the AC output voltage.

In further embodiments, identifying a fault in the DC power source by detecting a change in an AC component of a residual current of the DC power source may include identifying a first fault in the DC power source by detecting a change in the AC component of a residual current of the DC power source. The methods may further include identifying a second fault in the DC power source by detecting a change in an amplitude of the residual current of the DC power source. Identifying a second fault in the DC power source by detecting a change in an amplitude of the residual current of the DC power source may include detecting a change in RMS or peak value of the residual current.

Further embodiments of the inventive subject matter provide a system for monitoring a DC power source. The system includes a current sensor configured to detect a residual current of the DC power source and a fault detection circuit coupled to the current sensor and configured to detect a change in an AC component of the residual current of the DC power source and to identify a fault in the DC power source responsive thereto. The DC power source may be coupled to at least one DC bus and the system may further include means for generating an AC voltage on the at least one DC bus.

Further embodiments provide a UPS system including a DC bus, a DC bus modulation circuit configured to generate an AC component on the DC bus and an inverter having an input coupled to the DC bus and configured to generate an AC output voltage therefrom. The system further includes a DC power source coupled to the DC bus, a current sensor configured to detect a residual current of the DC power source and a fault detection circuit coupled to the current sensor and configured to detect a change in an AC component of the residual current of the DC power source and to identify a fault in the DC power source responsive thereto. The DC bus may include first and second DC busses and the DC bus modulation circuit may be configured to shift the first and second DC busses with respect to a neutral. The DC bus modulation circuit may include a neutral coupling circuit configured to selectively couple the first and second DC busses to the neutral.

Specific embodiments of the inventive subject matter now will be described with reference to the accompanying drawings. This inventive subject matter may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive subject matter to those skilled in the art. As used herein the term "and/or" includes any and all combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive subject matter. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms "includes," "comprises," "including" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive subject matter belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

As will be appreciated by one of skill in the art, the inventive subject matter may be embodied as systems and methods. Some embodiments of the inventive subject matter may include hardware and/or combinations of hardware and software. Some embodiments of the inventive subject matter include circuitry configured to provide functions described herein, It will be appreciated that such circuitry may include analog circuits, digital circuits, and combinations of analog and digital circuits.

Embodiments of the inventive subject matter are described below with reference to diagrams of systems and methods according to various embodiments of the inventive subject matter. It will be understood that each block of the diagrams, and combinations of blocks in the diagrams, can be implemented by analog and/or digital hardware, and/or computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, ASIC, and/or other programmable data processing apparatus, such that the instructions, which execute via the processor of the computer and/or other programmable data processing apparatus, create means for implementing the functions/acts specified in the diagrams.

<FIG> illustrates apparatus and methods according to some embodiments of the inventive subject matter. First and second DC busses 10a, 10b are at respective first and second voltages vDC+ and vDC- with respect to a neutral node N, which, in the illustrated embodiments, is connected to ground. Each of the bus voltages vDC+ and vDC- includes an AC component with respect to the neutral node N. The AC component may be generated in a number of different ways, as explained in greater detail below. An interface circuit <NUM> interfaces a DC power source <NUM> to the DC busses 10a, 10b. The DC power source <NUM> may include, for example, one or more strings of serially-connected electrochemical battery cells or one or more strings of serially-connected photovoltaic cells (e.g., solar panels). The interface circuit <NUM> may include, for example, an intervening conversion circuit (e.g., a DC/DC converter) or direct connection between the DC power source <NUM> and the DC busses 10a, 10b.

As further illustrated, an AC fault identification circuit <NUM> is configured to sense a residual (i.e., net) current iR of the DC power source <NUM> and to detect a fault of the DC power source <NUM> responsive to an AC component of the detected residual current iR. For example, in embodiments described below, the AC fault detection circuit <NUM> may be configured to detect an AC current component having a frequency associated with a harmonic of an AC voltage generated by an inverter coupled to the DC busses 10a, 10b. Such detection information may be used, for example, to discriminate between low and high impedance faults, and to take corresponding action based on the nature of the fault detected.

<FIG> illustrates an example application of the inventive subject matter. A DC source, in the form of an electrochemical battery cell or photovoltaic cell string <NUM>', is interfaced to DC busses 10a, 10b using a half-bridge converter circuit <NUM>'. The converter circuit <NUM>' includes first and second switches S1, S2 (e.g., transistors) that selectively couple the string <NUM>' to the DC busses 10a, 10b via an inductor L. An AC fault detection circuit <NUM> is configured to sense a residual current iR of the cell string <NUM>' and to detect a fault in the string <NUM>' responsive to an AC component of the sensed residual current iR.

<FIG> illustrates implementation of such an arrangement in a power conversion system including an inverter <NUM> that is configured to generate an AC output from power delivered from DC busses 10a, 10b. The system of <FIG> includes a half-bridge converter circuit <NUM>' including switches S1, S2 and an inductor L1 configured to interface a battery or photovoltaic cell string <NUM>' to the DC busses 10a, 10b along the lines discussed above. The system further includes a neutral coupling circuit <NUM>, including switches S3, S4 that are configured to selectively couple the DC busses 10a, 10b to a neutral node N via an inductor L2. The neutral coupling circuit <NUM> may be used to generate an AC component in the DC bus voltages vDC+ and vDC-.

As explained, for example, in <CIT>, such a neutral coupling circuit may be used to modulate or shift the DC busses 10a, 10b with respect to the neutral N to create an AC voltage component in the DC bus voltages vDC+ and vDC- In three-phase UPS applications, this AC component may have a frequency that is a third harmonic of the fundamental frequency (e.g. , <NUM>) of the AC output produced by the inverter of the UPS. It will be understood that this technique represents one way of producing an AC component in the residual current IR for purposes of fault detection, but other techniques may be used within the scope of the inventive subject matter.

<FIG> illustrates a UPS system <NUM> according to further embodiments. The UPS system <NUM> including a rectifier circuit <NUM> that is configured to be coupled to an AC source <NUM> (e.g., a three-phase AC utility source) and to generate first and second DC voltages vDC+ and vDC- on first and second DC busses 10a, 10b. The system <NUM> also includes a neutral coupling circuit <NUM> that is configured to modulate the DC bus voltages vDC+ and vDC- with respect to a neutral N, which is shown as grounded. The system <NUM> further includes an output inverter <NUM> coupled to the DC busses 10a, 10b and configured to generate an AC output, and a converter circuit <NUM>' that interfaces a DC power source, e.g., a battery or photovoltaic cell string <NUM>', to the DC busses 10a, 10b. An AC fault detection circuit <NUM> is configure to detect faults in the battery or photovoltaic cell string <NUM>' responsive to an AC component of a residual current of the battery or photovoltaic cell string using, for example, techniques described above. It will be appreciated that the AC fault detection circuit <NUM> may be a standalone device (or combination of devices) or may be integrated with the battery or photovoltaic cell string <NUM>' or with the UPS system <NUM>.

Some UPS systems employ a scalable modular architecture using UPS modules that provide functionality along the lines of the system <NUM> of <FIG> and that are coupled in parallel to provide power to a load. <FIG> illustrates an example of such a UPS module <NUM>, including a rectifier circuit <NUM>, an inverter circuit <NUM> and a neutral coupling circuit <NUM>. A DC/DC converter circuit <NUM> is used to interface a battery or photovoltaic cell string <NUM>' to the module <NUM>. Faults in the battery or photovoltaic cell string <NUM>' may be detected by an AC fault detection circuit <NUM> along the lines described above. It will be appreciated that the AC fault detection circuit <NUM> may be a standalone device (or combination of devices) or may be integrated with the battery or photovoltaic cell string <NUM>' or with the UPS module <NUM>,.

<FIG> illustrates an example of a fault detection solution for a modular UPS system <NUM> that is configured to interface to both battery and photovoltaic power sources. First and second UPS modules 620a, 620b are coupled in parallel to a load <NUM>. Each of the UPS modules 620a, 620b includes converter circuits <NUM>, <NUM>, <NUM> coupled to a DC bus <NUM>. In the first module 620a, a first converter circuit <NUM> is configured to operate as a rectifier, coupled to an AC source <NUM>. A second converter circuit <NUM> is configured to provide neutral coupling and inverter operations. A third converter circuit <NUM> is configured as a DC/DC converter that interfaces a battery string <NUM> to the DC bus <NUM>.

In the second module 620b, a first converter circuit <NUM> is inactive while a second converter <NUM> provides inverter and neutral coupling functions and a third converter circuit <NUM> acts as a DC/DC converter interface for a photovoltaic string <NUM>. Respective fault AC fault detection circuits 600a, 600b are provided for the battery string <NUM> and the photovoltaic string <NUM>. It will be appreciated that the AC fault detection circuits 600a, 600b may operate along the lines described above, and that the AC fault detection circuits 600a, 600b may be standalone devices or may be integrated with each other and/or with the UPS modules 620a, 620b.

<FIG> illustrates an example of a UPS module <NUM> utilized in the Eaton <NUM> UPS, which may be utilized in a configuration along the lines illustrated in <FIG>. The module <NUM> includes converter circuits <NUM>', <NUM>' and <NUM> ' that are configured to be coupled to an AC source, an AC load and a DC source, respectively, via respective contactors K1, K3, K2. The converter circuits <NUM>', <NUM>', <NUM>' are interconnected by DC busses 625a, 625b and a neutral bus N. As illustrated, the converter circuit <NUM>', which may be used to interface, for example, a battery or photovoltaic cell string, includes a common mode filter circuit <NUM>, which is designed to filter out high-frequency noise generated by the converters <NUM>,' <NUM>', <NUM>'. As this common mode filter <NUM> is designed to filter higher-frequency noise, it may not interfere with detection of lower-frequency AC harmonic currents that are used in AC fault detection techniques described herein.

A simulation was performed to evaluate potential performance of fault detection techniques according to some embodiments of the inventive subject matter. The simulation utilized a model of a photovoltaic array as shown in <FIG>, which illustrates a plurality of strings #<NUM>-#k of photovoltaic cells M#<NUM>-#n, coupled in parallel to positive and negative DC busses. Ground faults were simulated as connections of a resistance RFAULT at various points in the photovoltaic array. "Normal" leakage of the photovoltaic panels was modeled as connections to ground via resistors and capacitors RP, CP. The simulations assume that the photovoltaic array is connected to a DC bus that is modulated at a third harmonic (<NUM>) of the fundamental AC output frequency, as described in the UPS examples above.

<FIG> illustrates simulated residual current of the photovoltaic array when no ground fault is present. <FIG> illustrates a simulated low impedance fault in one of the strings near one of the DC busses. As can be seen, the residual current increases dramatically. The frequency spectrum of the residual current is shown in <FIG>, which illustrates that a DC component is dominant. These simulations suggest that such a low impedance fault may be detected by monitoring amplitude (e.g., the RMS (root mean squared) value or peak value) of the residual current of the photovoltaic array.

<FIG> and <FIG> illustrate simulated residual current for a relatively high-impedance fault at locations near the end of a photovoltaic string and near the middle of the photovoltaic string, respectively. For the fault near the end of the string (<FIG>), a change in residual current is detectable from the RMS or peak value of the residual current, suggesting that such a technique may be effective in detecting such a fault. However, for the fault near the middle of the string (<FIG>), the RMS and peak values of the residual current may not significantly change, suggesting that it might be difficult to detect such a fault based solely on the RMS or peak value of the residual current.

However, <FIG> and <FIG>, which illustrate the spectral content of the residual current for the fault conditions of <FIG> and <FIG>, respectively, indicate that a significant <NUM> AC component is present in the residual current (the "<NUM>" frequency reference in the ordinate of <FIG> corresponds to <NUM>) in both fault cases. This suggests that monitoring of an AC component of residual current can be used to discriminate such faults.

This can enable more sophisticated system monitoring and control. For example, as illustrated in <FIG>, a power system, such as a UPS system, that is interfaced to a DC source such as a battery or photovoltaic cell string may monitor residual current (block <NUM>). If the RMS or peak value of the residual current exceeds a first threshold indicative of a relatively low impedance fault, the system may trip a breaker or take other immediate action to protect equipment and/or personnel (block <NUM>). If the RMS or peak current is not above the threshold, the system may further determine whether an AC component (e.g., a third harmonic <NUM> component along the lines described above) is greater than a second threshold indicative of a relatively high impedance fault. In response to detecting such a fault, the system may take less dramatic, but still valuable actions, such as logging the fault to identify it for future maintenance actions (block <NUM>) that prevent a more damaging fault in the future. The thresholds may be based, for example, on impedance measurements of the DC source, and may be adaptively modified. It will be further appreciated that it may be possible to further discriminate among faults using such component analyses such that, for example, the location of the fault may be estimated.

It will be appreciated that AC fault detection circuitry described above with reference to <FIG> may implement operations along the lines described with reference to <FIG>. For example, the fault detection circuitry may comprise digital or analog circuitry configured to decompose a residual current signal into frequency components and to analyze these components for various artifacts indicative of faults. This circuitry may also be configured to perform additional functions related to fault detection, such as data logging, alarm generation, activation of protection components (e.g., circuit breakers) and the like.

Although use of a neutral coupling function in a UPS system to provide AC excitation for fault detection purposes is described above, it will be appreciated that other techniques may be used to provide similar excitation. For example, <FIG> illustrates a modification of the configuration of <FIG> (like reference numerals refer to like components) wherein a DC power source <NUM> is connected to DC busses 10a, 10b via a common mode inductor assembly <NUM>. The common mode inductor assembly <NUM> includes an auxiliary winding that is coupled to an AC voltage generator circuit <NUM> that provides AC excitation of the busses of the DC power source <NUM>. <FIG> illustrates another modification of the configuration of <FIG>, wherein a DC power source <NUM> is connected to DC busses 10a, 10b via a common mode inductor assembly <NUM> and AC excitation is provided by an AC voltage generator circuit <NUM> capacitively coupled to the DC power source <NUM>.

It will also be appreciated that any of a number of techniques may be used to detect components of residual current according to embodiments of the inventive subject matter. For example, residual current measurements may be resolved into frequency components using digital techniques. <FIG> illustrates an example wherein a residual current sensor <NUM>, e.g., a current transformer, Hall Effect sensor or the like, is used to sense residual current in conductors <NUM> of a DC power source. An analog to digial converter (ADC) <NUM> converts the current sense signal to digital samples that are provided to a processor <NUM>, e.g., a microprocessor or microcontroller. The processor <NUM> may be programmed to implement a spectrum analyzer <NUM> that determines frequency components of the sensed residual current and, for example, analyzes these components to detect ground fault conditions as described above. The processor <NUM> may also be programmed to perform other functions, such as communication of information regarding detected faults to a supervisory controller in a UPS or other system control component.

It will be appreciated that fault detection may use analog circuitry that performs similar functions. For example, as illustrated in <FIG>, a residual current sensor <NUM> may be coupled to a tuned circuit <NUM> that is configured to receive a particular frequency component of the sensed residual current, e.g., an AC power frequency harmonic component, and a detector circuit <NUM> (e.g., a comparator circuit or similar circuitry) that is configured to compare a magnitude or other measure of the received component to a particular threshold to detect fault conditions.

Claim 1:
A method of monitoring a DC voltage source (<NUM>), the method comprising:
selectively coupling a neutral (N) to first and second DC busses (10a, 10b) that are coupled to the DC voltage source (<NUM>) to generate a voltage having an AC component on the first and second DC busses (10a, 10b);
detecting a residual current associated with the generated voltage at a point between the DC voltage source (<NUM>) and the first and second DC busses (10a, 10b) connected thereto; and
identifying a fault in the DC voltage source (<NUM>) by detecting a change in an AC component of the detected residual current.