Photovoltaic rapid shutdown and arc sensing system

Abstract

The present disclosure provides a system, apparatus and method for providing rapid shutdown for photovoltaic power systems and provides a system, apparatus and method for providing arc sensing for photovoltaic power systems. An AC current can be put on the DC bus to control PV panel shutdown. Local mean decomposition can be used to sense arcing on the DC bus.

Claims

1. A method for providing emergency de-energizing of a photovoltaic panel, the method comprising: injecting AC current over a DC bus of a PV panel array; isolating said AC current from said DC bus; and using said isolated AC current to cause a passive circuitry to allow flow of DC current from said PV panels over said DC bus, wherein an interruption of said AC provides said emergency de-energizing of said DC bus.

2. The method as defined in claim 1, wherein said using said isolated AC current to cause a passive circuitry to allow flow of DC current from said PV panels over said DC bus comprises: receiving said isolated AC current by a rectifier gate signal circuit providing a switch gate signal; and closing at least one normally open switch using said switch gate signal to allow flow of DC current from said PV panels over said DC bus; wherein upon interruption of said AC current said at least one normally open switch opens and stops flow of DC current over said DC bus.

3. The method as defined in claim 2, wherein said isolating said AC current from said DC bus comprises using a transformer to isolate the AC current.

4. The method as defined in claim 1, wherein said injecting AC current over a DC bus of a PV panel array comprises injecting said AC current at a frequency different from a working of an inverter receiving said DC voltage.

5. The method defined in claim 1, further comprising: collecting a raw signal from said DC bus; applying threshold Local Mean Decomposition (LMD) to extract a first set of peaks and valleys from said raw signal; finding a first set of envelopes using said first set of peaks and valleys; applying Windowed Local Mean Decomposition (LMD) to extract a second set of peaks and valleys; finding a second set of envelopes using said second set of peaks and valleys; comparing said first set of envelopes and said second set of envelopes; removing said AC current over said DC bus when said comparison of said first set of envelopes and said second set of envelopes concurrently show an arc.

6. The method as defined in claim 5, further comprising applying a blanking method to said first set of envelopes and said second set of envelopes.

7. The method as defined in claim 5, further comprising applying a leaky bucket method to said first set of envelopes and said second set of envelopes.

8. The method as defined in claim 5, further comprising applying mean subtraction to said first set of envelopes and said second set of envelopes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which:

(2) FIG. 1 is a schematic illustration of a building equipped with rooftop PV panels showing an outside remote shutdown control and an optional arc sensing control at the combiner box and an inside inverter connected to the PV panels by a DC bus with isolation switches at the PV panels;

(3) FIG. 2 is a schematic block diagram of the electrical system involved in the illustration of FIG. 1; and

(4) FIG. 3 is a schematic diagram of the isolation switch involved in the illustrations of FIGS. 1 and 2.

(5) FIG. 4 shows the regions that an arc-fault circuit-interrupter should be capable of detecting or interrupting arcing based on UL699 standard

(6) FIG. 5 is a flowchart of the steps included in the method used for detecting an arc in accordance with one embodiment of the present disclosure

(7) FIG. 6 is a schematic illustration of the Raw Signal Received by the processor for performing Threshold LMD.

(8) FIG. 7 is a schematic illustration of finding peaks and valleys of the raw signal shown in FIG. 6 using Threshold LMD method in accordance with one embodiment of the present disclosure.

(9) FIG. 8 is a schematic illustration of reconstruction the signal using peaks and valleys (shown in FIG. 7) using Threshold LMD method in accordance with one embodiment of the present disclosure.

(10) FIG. 9 is a schematic illustration of finding reconstructed signal (shown in FIG. 8) envelopes in Threshold LMD method in accordance with one embodiment of the present disclosure.

(11) FIG. 10 is a schematic illustration of the Raw Signal Received (also shown in FIG. 6) by the processor for performing Threshold LMD.

(12) FIG. 11 is a schematic illustration finding peaks and valleys within a sliding window at point 1 using Windowed LMD method in accordance with one embodiment of the present disclosure.

(13) FIG. 12 is a schematic illustration finding peaks and valleys within the sliding window shown in FIG. 11 when it is at point 2.

(14) FIG. 13 is a schematic illustration finding peaks and valleys within the sliding window shown in FIG. 11 when it is at point 3.

(15) FIG. 14 is a schematic illustration of reconstruction the signal using peaks and valleys (shown in FIG. 13) using windowed LMD method in accordance with one embodiment of the present disclosure.

(16) FIG. 15 is a schematic illustration of finding the envelopes of the reconstructed signal of FIG. 14 using windowed LMD method in accordance with one embodiment of the present disclosure.

(17) FIG. 16 shows the comparison and overlapping of the envelopes found using threshold LMD and windowed LMD for concurrent conclusion of arc presence.

DETAILED DESCRIPTION

(18) Reference throughout this specification to one embodiment, an embodiment, or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases in one embodiment, in an embodiment, and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

(19) Moreover, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. Reference will now be made in detail to the preferred embodiments of the invention.

(20) Rapid shutdown was first introduced in the 2014 National Electrical Code (NEC) with the intent of providing a simple method for fire fighters to de-energize solar system DC conductors easily to ensure a safe condition on the roof of a building during a fire. This is because on a standard string inverter solar system, when the inverter is switched off, the DC wiring from the solar system remains live when the sun is shining.

(21) In the 2017 NEC, rapid shutdown was expanded with different requirements based on how close the PV system conductors are to the PV array boundary, which is now defined as the area 1 foot (305 mm) from the array in all directions.

(22) PV circuits located outside the boundary or more than 3 feet (1 m) from the point of entry inside a building shall be limited to not more than 30 volts within 30 seconds of rapid shutdown initiation.

(23) For PV circuits located inside the array boundary, one of the following three options must be used beginning with an effective date of 1 Jan. 2019:

(24) The PV array shall be listed or field labeled as a rapid shutdown PV array. Such a PV array shall be installed and used in accordance with the instructions included with the rapid shutdown PV array listing or field labeling. PV conductors located inside the boundary or not more than 3 feet (1 m) from the point of penetration of the surface of the building shall be limited to not more than 80 volts within 30 seconds of rapid shutdown initiation. PV arrays with no exposed wiring methods, no exposed conductive parts, and installed more than 8 feet from exposed grounded conductive parts or ground shall not be required to comply with rule 2 above.

(25) In one example, the rapid shutdown system can be applied to PV rooftop installations in a building as schematically illustrated in FIG. 1. In some configurations, an inverter 10 can be located inside the building for converting DC power from a power bus 12 of the PV panels 16. The PV panels 16 are installed as a string configuration 18 on a rooftop exposed to the sun. It will be appreciated that when DC storage of PV energy is desired, the unit 10 can comprise a battery charger.

(26) In one embodiment of the present disclosure, a remote shutdown controller 20 is provided, and as illustrated it may be located on an outside wall of the building. A combiner box 14, may be the location where the PV Strings 18 are connected.

(27) It will be appreciated that in some examples the actuation of the remote shutdown 20 can be under the control of the inverter 10 as well. FIG. 1 also illustrates schematically that isolation switches 30 are provided in the connection between panels 16 as part of the power bus 12. The isolation switches may be provided separately from panels 16 or they may be integrated into panels 16. An isolation switch may be provided with each panel 16, or they may be distributed within the array of panels 16, keeping in mind that the objective of the isolation switches 30 is to maintain the voltage present on the DC bus 12 below a given threshold when a shutdown is required.

(28) As illustrated in FIG. 2, the rapid shutdown system 20 that can an include arc sensing shutdown features can be built around a power supply 24 and multiple isolation devices 30 (for example, one per PV panel 16). The power supply 24 may or may not be coupled through an AC injector transformer 26.

(29) The AC injector transformer can have three windings. A first winding of the transformer may be part of the resonance tank that the power supply 24 uses to generate alternative current. A second winding of the transformer may pass the full PV DC current while injecting the power on the array's DC line 12. A third winding having turns in the opposite direction than the direction of the turn of the first winding are connected to the arc sensing circuit 23. In this way, the injected AC signal is not seen by circuit 23 that will only see noise appearing on bus 12.

(30) The arc sensing circuit is a circuit tuned to identify the signal characteristics caused by arcing and in response to such signal characteristics to output a control signal to the controller 22 to actuate the shutdown, for example by actuating the isolation switches 30.

(31) The power supply 24 may have an operating frequency chosen to optimize the transmission across the PV array. From simulation and testing as well as a literature review, PV panels 16 appear to have a particular impedance. The system disclosed takes advantage of the PV's impedance and the PV's quality factor to minimize the injected power losses at the PV cell. From lab experiments, it has been noted that a frequency between 200 kHz and 300 kHz offers a high-quality factor (low energy dissipation) and a linear (predictable) impedance while being below the AM frequency band. It has been found that at 250 kHz, only 20 Vrms and 40 mA of alternative current is enough to supply all the isolation devices across a PV array with cable length of 80 meters. Thus, the experimental system demonstrated approximately a power consumption of 10 mW per meter of cable length in the PV array (a power consumption less than 20 mW per meter of cable length in the PV array is desirable). Using a frequency below 200 kHz increases power consumption, and therefore is it best not to use a frequency below 100 kHz. Using a frequency above 300 kHz is possible, however, above 440 kHz is too close to the AM band at 540 kHz to be suitable.

(32) As illustrated in FIG. 3, the isolation device 30 is built around a coil coupler or transformer 32 that picks up the supplied alternative current on DC bus 12 and uses a rectifier circuit 34 to supply the gates of switching devices 36. While the AC power provided to bus 12 is extracted using a transformer coil 32, it will be appreciated that equivalent capacitor coupling can be used to extract the required power. Two normally-open switches are illustrated, and the use of two switches 36 provides redundancy in case one switch would fail and also reduces a current load through each one of the switches 36. It will be appreciated that more than two switches or a single switch 36 could be used. A bandpass filter 32, for example an ac coupling capacitor makes sure that the alternative current can always flow no matter the state of the high voltage switches. The filter 38 can be tuned to allow the AC current at the chosen supply frequency to pass, for example, between 200 kHz and 300 kHz. When the picked-up voltage is above a given threshold it energizes the semiconductor switches 36 that will let the PV energy flow to the load. As soon as the isolation device loses it supply, the rectifier circuit 34 stops supplying the gate signal and the switches 36 reverts to an open state making the PV array inert and the whole system safe to touch. Circuit 34 preferably contains capacitors and the time for the gate signals to open switches 36 can be a number of seconds.

(33) In some embodiments, the arc sensor may include an oscilloscope with large memory, high sampling rate and high bandwidth probe. The oscilloscope receives real arc signal and uses Local Mean Decomposition (LMD) and Empirical Mode Decomposition (EMD) to detect an arc within the system. An arc typically generating a wide frequency spectrum, but the arc signal may also contain some harmonics of the natural resonance frequency of the system. The method therefore seeks to find a beat at resonance frequency in order to detect the arc.

(34) In some examples of the method, in order to detect the beat at resonance frequency of the system, the raw signal is passed through a Thresholded LMD and in parallel through a Windowed LMD. The threshold LMD allows us to prevent false triggering on small noise which may be a part of the harmonics associated with the natural resonance frequency of the system. In contrast large noise would pass through this filter as dur to the nature of the arc, a large noise is a part of the signature that we want to detect. The Windowed LMD prevent errors in detecting arcs by preventing the arc detection system from being triggered by very fast events as typically fast transient events are not the beat that we are looking for. Upon completing the Windowed LMDs, the output signal of the filtering would be used to reconstruct the signal from the peaks found and to find the envelopes of the signal. After finding both envelope of the signal, they are compared and if the corroborate then we have detected an arc. The system then needs to blank and use a leaky integrator to make sure it does not trigger too fast.

(35) FIG. 4 shows the regions that an arc-fault circuit-interrupter should be capable of detecting or interrupting arcing based on UL699 standard. Region A as illustrated: For all tests, disrupt arcing event in less than 2.5 seconds, and limit energy not to exceed 200 J; Region B: For all tests, disrupt arcing event in less than 2.5 seconds, and limit energy not to exceed 750 J; and Region C: For any test, arcing time equal to or greater than 2.5 seconds, or energy greater than 750 J, the device is considered non-compliant with the standard.

(36) It will be appreciated by those skilled in the art that in this application envelope of a signal refers to the contour of a waveform; threshold LMD is a signal processing method based on the LMD that uses a predefined deadband, hysteresis or a threshold to search for the signal of interest; and windowed LMD is a signal processing method based on the LMD that uses a sliding window to search for the signal of interest.

(37) Referring to FIG. 5, a flowchart for sensing an arc using the raw signal in a photovoltaic system is illustrated. S51 is the receiving Raw Signal from a sensor connected to a DC bus of the photovoltaic system collecting raw signal from the DC bus. It will be appreciated that the sensor does not have to be connected and can be in proximity of DC bus. For example, the sensor can benefit from galvanic isolation but still provide voltage or current signal. In S52 the method includes applying threshold Local Mean Decomposition (LMD) to extract a first set of peaks and valleys from the raw signal. S54 includes finding wave envelopes using the first set of peaks and valleys. At S56 the method may include applying mean subtraction method to the signal. S53 includes applying Windowed Local Mean Decomposition (LMD) to extract the peaks and valleys of the signal followed by finding envelopes using these peaks and valleys at S55. At S57 the method may include applying mean subtraction method to the envelopes. Then at S60 the two envelopes found in steps S55 and S54 are compared and the system may send an output signal indicating arcing if the envelopes concurrently shown an arc in the system. The method may also include applying Blanking and/or Leaky Bucket at S62.

(38) FIGS. 6 to 15 are illustrations showing the signal in each step of the process as described in FIG. 5. FIG. 6 is a schematic illustration of the Raw Signal Received by the processor from our sensor. FIG. 7 shows finding of peaks and valleys of the signal received from the sensor using Threshold LMD method. FIG. 8 shows the reconstruction the signal using peaks and valleys shown in FIG. 7 as done in Threshold LMD. In FIG. 9 the reconstructed signal is used to find envelopes (contours of the waveform).

(39) FIG. 10 is a schematic illustration of the Raw Signal Received windowed LMD. FIGS. 11 to 13 illustrate finding of peaks and valleys within a sliding window at points 1 to 2 as done in the Windowed LMD method. FIG. 14 is a schematic illustration of reconstruction of the signal using peaks and valleys as done in windowed LMD method followed by the finding of the envelopes of the reconstructed signal in FIG. 15. Lastly, FIG. 16 shows the comparison and overlapping of the envelopes found using threshold LMD and windowed LMD during which a concurrent conclusion of arc presence can result in a conclusion that the arc is happening.