Systems and apparatus for fault detection in DC power sources using AC residual current detection

A fault in a DC power source, such as a battery string or a string of photovoltaic cells, 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 modulated DC bus of an uninterruptible power supply (UPS) system comprising an inverter having an input coupled to the DC bus. The inverter may be configured to generate an AC output voltage and the AC component has a frequency that is a harmonic of a fundamental frequency of the AC output voltage, such as a third harmonic of the fundamental frequency of the AC output voltage.

BACKGROUND

The inventive subject matter relates to power systems and methods and, more particularly, to fault detection in power systems using DC power sources.

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 U.S. patent application Ser. No. 12/779,522, filed May 13, 2010.

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 U.S. Pat. No. 6,593,520, U.S. Pat. No. 6,320,769, U.S. Pat. No. 7,079,406, U.S. Pat. No. 6,856,497, U.S. Pat. No. 7,005,883 and U.S. Pat. No. 6,930,868.

SUMMARY

Some embodiments of the inventive subject matter 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.

DETAILED DESCRIPTION OF EMBODIMENTS

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. In the drawings, like numbers refer to like elements. It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items.

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. 1illustrates apparatus and methods according to some embodiments of the inventive subject matter. First and second DC busses10a,10bare 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 circuit20interfaces a DC power source30to the DC busses10a,10b. The DC power source30may 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 circuit20may include, for example, an intervening conversion circuit (e.g., a DC/DC converter) or direct connection between the DC power source30and the DC busses10a,10b.

As further illustrated, an AC fault identification circuit100is configured to sense a residual (i.e., net) current iRof the DC power source30and to detect a fault of the DC power source30responsive to an AC component of the detected residual current iR. For example, in embodiments described below, the AC fault detection circuit30may 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 busses10a,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. 2illustrates an example application of the inventive subject matter. A DC source, in the form of an electrochemical battery cell or photovoltaic cell string30′, is interfaced to DC busses10a,10busing a half-bridge converter circuit20′. The converter circuit20′ includes first and second switches S1, S2(e.g., transistors) that selectively couple the string30′ to the DC busses10a,10bvia an inductor L. An AC fault detection circuit200is configured to sense a residual current iRof the cell string30′ and to detect a fault in the string30′ responsive to an AC component of the sensed residual current iR.

FIG. 3illustrates implementation of such an arrangement in a power conversion system including an inverter50that is configured to generate an AC output from power delivered from DC busses10a,10b. The system ofFIG. 3includes a half-bridge converter circuit20′ including switches S1, S2and an inductor L1configured to interface a battery or photovoltaic cell string30′ to the DC busses10a,10balong the lines discussed above. The system further includes a neutral coupling circuit40, including switches S3, S4that are configured to selectively couple the DC busses10a,10bto a neutral node N via an inductor L2. The neutral coupling circuit40may be used to generate an AC component in the DC bus voltages vDC+and vDC−.

As explained, for example, in U.S. Pat. No. 7,088,601 to Tracy et al., the disclosure of which is incorporated herein by reference in its entirety, such a neutral coupling circuit may be used to modulate or shift the DC busses10a,10bwith 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., 60 Hz) 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 iRfor purposes of fault detection, but other techniques may be used within the scope of the inventive subject matter.

FIG. 4illustrates a UPS system400according to further embodiments. The UPS system400including a rectifier circuit60that is configured to be coupled to an AC source70(e.g., a three-phase AC utility source) and to generate first and second DC voltages vDC+and vDC−on first and second DC busses10a,10b. The system400also includes a neutral coupling circuit40that is configured to modulate the DC bus voltages vDC+and vDC−with respect to a neutral N, which is shown as grounded. The system400further includes an output inverter50coupled to the DC busses10a,10band configured to generate an AC output, and a converter circuit20′ that interfaces a DC power source, e.g., a battery or photovoltaic cell string30′, to the DC busses10a,10b. An AC fault detection circuit200is configure to detect faults in the battery or photovoltaic cell string30′ 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 circuit200may be a standalone device (or combination of devices) or may be integrated with the battery or photovoltaic cell string30′ or with the UPS system400.

Some UPS systems employ a scalable modular architecture using UPS modules that provide functionality along the lines of the system400ofFIG. 4and that are coupled in parallel to provide power to a load.FIG. 5illustrates an example of such a UPS module500, including a rectifier circuit510, an inverter circuit520and a neutral coupling circuit540. A DC/DC converter circuit530is used to interface a battery or photovoltaic cell string30′ to the module500. Faults in the battery or photovoltaic cell string30′ may be detected by an AC fault detection circuit200along the lines described above. It will be appreciated that the AC fault detection circuit200may be a standalone device (or combination of devices) or may be integrated with the battery or photovoltaic cell string30′ or with the UPS module500.

FIG. 6illustrates an example of a fault detection solution for a modular UPS system600that is configured to interface to both battery and photovoltaic power sources. First and second UPS modules620a,620bare coupled in parallel to a load80. Each of the UPS modules620a,620bincludes converter circuits622,624,626coupled to a DC bus625. In the first module620a, a first converter circuit622is configured to operate as a rectifier, coupled to an AC source70. A second converter circuit624is configured to provide neutral coupling and inverter operations. A third converter circuit626is configured as a DC/DC converter that interfaces a battery string630to the DC bus625.

In the second module620b, a first converter circuit622is inactive while a second converter624provides inverter and neutral coupling functions and a third converter circuit626acts as a DC/DC converter interface for a photovoltaic string640. Respective fault AC fault detection circuits600a,600bare provided for the battery string630and the photovoltaic string640. It will be appreciated that the AC fault detection circuits600a,600bmay operate along the lines described above, and that the AC fault detection circuits600a,600bmay be standalone devices or may be integrated with each other and/or with the UPS modules620a,620b.

FIG. 7illustrates an example of a UPS module700utilized in the Eaton 9395 UPS, which may be utilized in a configuration along the lines illustrated inFIG. 6. The module700includes converter circuits622′,624′ and626′ 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 circuits622′,624′,626′ are interconnected by DC busses625a,625band a neutral bus N. As illustrated, the converter circuit626′, which may be used to interface, for example, a battery or photovoltaic cell string, includes a common mode filter circuit628, which is designed to filter out high-frequency noise generated by the converters622,′624′,626′. As this common mode filter628is 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 inFIG. 8, which illustrates a plurality of strings #1-#k of photovoltaic cells M#1-#n, coupled in parallel to positive and negative DC busses. Ground faults were simulated as connections of a resistance RFAULTat 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 (180 Hz) of the fundamental AC output frequency, as described in the UPS examples above.

FIG. 9illustrates simulated residual current of the photovoltaic array when no ground fault is present.FIG. 10illustrates 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 inFIG. 11, 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.

FIGS. 12 and 13illustrate 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. 12), 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. 13), 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,FIGS. 14 and 15, which illustrate the spectral content of the residual current for the fault conditions ofFIGS. 12 and 13, respectively, indicate that a significant 180 Hz AC component is present in the residual current (the “0” frequency reference in the ordinate ofFIG. 14corresponds to 180 Hz) 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 inFIG. 16, 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 (block1610). 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 (block1620). 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 180 Hz 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 (block1630) 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 toFIGS. 1-6may implement operations along the lines described with reference toFIG. 16. 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. 17illustrates a modification of the configuration ofFIG. 1(like reference numerals refer to like components) wherein a DC power source30is connected to DC busses10a,10bvia a common mode inductor assembly1710. The common mode inductor assembly1710includes an auxiliary winding that is coupled to an AC voltage generator circuit1720that provides AC excitation of the busses of the DC power source30.FIG. 18illustrates another modification of the configuration ofFIG. 1, wherein a DC power source30is connected to DC busses10a,10bvia a common mode inductor assembly1810and AC excitation is provided by an AC voltage generator circuit1820capacitively coupled to the DC power source30.

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. 19illustrates an example wherein a residual current sensor1910, e.g., a current transformer, Hall Effect sensor or the like, is used to sense residual current in conductors90of a DC power source. An analog to digital converter (ADC)1920converts the current sense signal to digital samples that are provided to a processor1930, e.g., a microprocessor or microcontroller. The processor1930may be programmed to implement a spectrum analyzer1932that determines frequency components of the sensed residual current and, for example, analyzes these components to detect ground fault conditions as described above. The processor1930may 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 inFIG. 20, a residual current sensor2010may be coupled to a tuned circuit2020that is configured to receive a particular frequency component of the sensed residual current, e.g., an AC power frequency harmonic component, and a detector circuit2030(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.

In the drawings and specification, there have been disclosed exemplary embodiments of the inventive subject matter. Although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the inventive subject matter being defined by the following claims.