Arc Detection and Prevention in a Power Generation System

Abstract

A method for arc detection in a system including a photovoltaic panel and a load connectible to the photovoltaic panel with a DC power line. The method measures power delivered to the load thereby producing a first measurement result of the power delivered to the load. Power produced by the photovoltaic panel is also measured, thereby producing a second measurement result of power produced by the photovoltaic panel. The first measurement result is compared with the second measurement result thereby producing a differential power measurement result. Upon the differential power measurement result being more than a threshold value, an alarm condition may also be set. The second measurement result may be modulated and transmitted over the DC power line.

Claims

1. A system comprising: a first photovoltaic string configured to produce a first string power, wherein the first photovoltaic string comprises a first plurality of photovoltaic panels; a module configured to determine the first string power to produce a measured string power; a load configured to be connected to the first photovoltaic string and receive the first string power from the first photovoltaic string; a controller configured to be connected to the load and configured to: transmit a first signal to the first photovoltaic string, wherein the first signal comprises an instruction to continue power production by the first photovoltaic string; measure a total power delivered to the load; receive the measured string power; compare the total power delivered to the load and the measured string power to produce a power difference; determine that the power difference is greater than a threshold value; and cease transmission of the first signal to the first photovoltaic string to stop power production of the first string power.

2. The system of claim 1, wherein the module is configured to determine the first string power by adding a first panel power and a second panel power.

3. The system of claim 1, further comprising a second photovoltaic string configured to produce a second string power, wherein the second photovoltaic string comprises a second plurality of photovoltaic panels configured to provide the second string power to the load.

4. The system of claim 3, wherein the controller is configured to transmit a plurality of signals, wherein the plurality of signals comprises the first signal and a second signal, wherein the second signal comprises an instruction to continue power production by the second photovoltaic string, wherein the controller is configured to transmit the plurality of signals by power line communications, wherein the first signal has a first frequency corresponding to the first photovoltaic string, and wherein the second signal has a second frequency corresponding to the second photovoltaic string.

5. The system of claim 4, wherein the controller is configured to transmit the second signal to instruct the second photovoltaic string to continue power production of the second string power after transmission of the first signal has been ceased to stop power production of the first string power.

6. The system of claim 5, wherein the controller is configured to transmit the second signal periodically.

7. The system of claim 1, wherein the controller is configured to transmit the first signal to the first photovoltaic string by wireless communications.

8. A method comprising: transmitting, using a controller, a first signal to a first plurality of photovoltaic panels, wherein the first signal comprises an instruction to continue power production by the first plurality of photovoltaic panels; measuring, using the controller, power delivered to a load; receiving, using the controller, a determined string power produced by the first plurality of photovoltaic panels; comparing, using the controller, the power delivered to the load and the determined string power to produce a power difference; determining, using the controller, that the power difference is greater than a threshold value; and ceasing, using the controller, transmission of the first signal to the first plurality of photovoltaic panels to stop power production by the first plurality of photovoltaic panels.

9. The method of claim 8, further comprising determining the determined string power by adding a first panel power and a second panel power.

10. The method of claim 8, further comprising transmitting, using the controller, a second signal, wherein the second signal comprises an instruction to continue power production by a second plurality of photovoltaic panels.

11. The method of claim 10, wherein the controller is configured to transmit the first signal and the second signal by power line communications, wherein the first signal has a first frequency corresponding to the first plurality of photovoltaic panels, and wherein the second signal has a second frequency corresponding to the second plurality of photovoltaic panels.

12. The method of claim 10, further comprising transmitting, using the controller, the second signal to instruct the second plurality of photovoltaic panels to continue power production by the second plurality of photovoltaic panels after ceasing transmission of the first signal to stop power production by the first plurality of photovoltaic panels.

13. The method of claim 8, further comprising transmitting, using the controller, the first signal to the first plurality of photovoltaic panels by wireless communications.

14. An apparatus comprising: a controller configured to: transmit a first signal to a first plurality of photovoltaic panels, wherein the first signal comprises an instruction to continue power production by the first plurality of photovoltaic panels; measure power delivered to a load; receive a determined string power produced by the first plurality of photovoltaic panels; compare the power delivered to the load and the determined string power to produce a power difference; determine that the power difference is greater than a threshold value; and cease transmission of the first signal to the first plurality of photovoltaic panels to stop power production by the first plurality of photovoltaic panels.

15. The apparatus of claim 14, wherein the determined string power is determined by adding a first panel power and a second panel power.

16. The apparatus of claim 14, wherein the controller is further configured to transmit a second signal, wherein the second signal comprises an instruction to continue power production by a second plurality of photovoltaic panels.

17. The apparatus of claim 16, wherein the controller is configured to transmit the first signal and the second signal by power line communications, wherein the first signal has a first frequency corresponding to the first plurality of photovoltaic panels, and wherein the second signal has a second frequency corresponding to the second plurality of photovoltaic panels.

18. The apparatus of claim 16, wherein the controller is configured to transmit the second signal to instruct the second plurality of photovoltaic panels to continue power production by the second plurality of photovoltaic panels after transmission of the first signal has been ceased to stop power production by the first plurality of photovoltaic panels.

19. The apparatus of claim 16, wherein the controller is configured to transmit the second signal periodically.

20. The apparatus of claim 14, wherein the controller is configured to transmit the first signal to the first plurality of photovoltaic panels by wireless communications.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0017] FIG. 1a illustrates an example of circuit showing serial arcing.

[0018] FIG. 1b illustrates the circuit of FIG. 1a showing an example of parallel or shunt arcing.

[0019] FIG. 2 shows a power generation system including an arc detection feature according to an embodiment of the present invention.

[0020] FIG. 3 shows a method according to an embodiment of the present invention.

[0021] FIG. 4 shows a method according to an embodiment of the present invention.

[0022] FIG. 5a shows a power generation circuit according to an embodiment of the present invention.

[0023] FIG. 5b shows a method according to an embodiment of the present invention.

[0024] FIG. 5c shows a method step shown in FIG. 5b, in greater detail, the step measures power of a string according to an embodiment of the present invention.

[0025] FIG. 5d shows a method for serial arc detection, according to another embodiment of the present invention.

DETAILED DESCRIPTION

[0026] Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

[0027] Reference is made to FIG. 1a which shows serial arcing 106 in a circuit 10a according to background art. In FIG. 1a, a direct current (DC) power supply 102 provides power between power lines 104a and 104b. Power line 104b is shown at ground potential. Load 100 connects power line 104b to power line 104a. Serial arcing may occur in any part of circuit 10a in power lines 104a, 104b or internally in load 100 or supply 102 for example. A disconnection or poor connection in power line 104a between point C and point A is shown which causes an instance 106 of serial arcing. Typically, if series arc 106 can be detected, circuit breakers (not shown) located at supply 102 or load 100 can be tripped to prevent continuous serial arcing 106.

[0028] Reference is made to FIG. 1b, which shows parallel or shunt arcing 108 in a circuit 10b according to background art. In circuit 10b, a direct current (DC) power supply 102 provides power between power lines 104a and 104b. Load 100 connects power lines 104a and 104b. Parallel arcing may occur in many parts of circuit 10b, examples may include arcing between the positive of supply 102 and the ground/chassis of supply 102, if power supply cable 104a/b is a two core cable; arcing may occur between the two cores, or between the positive terminal 104a and ground 104b of load 100. Parallel arcing 108 may occur as shown between power line 104b at point D and high potential on power line 104a at point C.

[0029] Arc noise is approximate to white noise, meaning that the power spectral density is nearly equal throughout the frequency spectrum. Additionally, the amplitude of the arc noise signal has very nearly a Gaussian probability density function. The root mean square (RMS) arc noise voltage signal (V.sub.n) is given in equation Eq. 1, as follows:

V.sub.N=√{square root over (4KTBR)}  Eq. 1, [0030] where: [0031] K=Boltzmann's constant=1.38×10.sup.−23 Joules per Kelvin; [0032] T=the temperature in degrees Kelvin; [0033] B=bandwidth in Hertz (Hz) over which the noise voltage (V.sub.N) is measured; and [0034] R=resistance (ohms) of a resistor/circuit/load.

[0035] Reference is now made to FIG. 2, which shows a power generation system 201 including an arc detection feature according to an embodiment. A photovoltaic panel 200 is preferable connected to an input of a module 202. Multiple panels 200 and multiple modules 202 may be connected together to form a serial string. The serial string may be formed by connecting the outputs of modules 202 in series. Multiple serial strings may be connected in parallel across a load 250. Load 250 may be, for example, a direct current (DC) to alternating current (AC) inverter or DC-to-DC converter. An electronic module 202 may be included to measures the voltage and/or current produced by a panel 200. Module 202 may be capable of indicating the power output of a panel 200. Attached to load 250 may be a controller 204. Controller 204 may be operatively attached to modules 202 via power line communications over DC power lines connecting load 250 to the serial strings and/or by a wireless connection. Controller 204 may be configured to measure via sensor 206, the power received by load 250. Each panel 200 has a chassis, which may be connected to ground. An instance of serial arcing 106 may occur between two panels 200. An instance of parallel arcing 108 may be shown between the positive terminal of a panel 200 and ground of the panel 200.

[0036] Reference is now made to FIG. 3, which shows a method 301 according to one embodiment. Method 301 may be configured to detect serial and/or parallel arcing. Central controller 204 may be configured to measure one or more parameters such as the power received by load 250 (step 300). Module 202 may be variously configured such as to measure the power of one or more panels 200 (step 302). Module 202 may be variously configured. In one embodiment, it transmits a datum representing the power measured of the one or more panels 200 via wireless or power line communications to controller 204. Controller 204 calculates the difference between power generated at panel(s) 200 and the power received at load 250 (step 304). In this example, if the difference calculated in step 304 shows that the power generated at panel(s) 200 may be greater than the power received at load 250 (step 306) according to a predefined criteria, an alarm condition of potential arcing may be set (step 308). Otherwise, in this example, the arc detection continues with step 300.

[0037] Reference is now made to FIG. 4, which shows an illustrative method 401. Method 401 is a method, which may be utilized for detecting serial and/or parallel arcing. In a method according to this example, central controller 204 measures (step 400) the root mean square (RMS) noise voltage of load 250. Module 202 may then measure (step 402) the root mean square (RMS) noise voltage of one or more panels 200. Module 202 may be configured to transmit a datum representing the RMS noise voltage measured of panel(s) 200 via wireless or power line communications to controller 204.

[0038] One or more controllers may be configured to compare the noise voltage at panel(s) 200 with the noise voltage at the load 250 by, for example, calculating the difference between noise voltage measured at panel 200 and the noise voltage measured at load 250 (step 404). In this example, if the difference calculated in step 404 shows that noise voltage measured at panel(s) 200 may be greater than the noise voltage measured at load 250 (step 406) according to one or more predefined criteria, an alarm condition of potential arcing may be set (step 408).

[0039] Further to this example, the comparison (step 404) also may involve comparisons of previously stored RMS noise voltage levels of panel§ 200 and/or load 250 in a memory of controller 204 at various times, for example, the time immediately after installation of power generation system 201. The previously stored RMS noise voltage levels of both panel§ 200 and load 250 are, in this example, in the form of a look-up-table stored in the memory of controller 204. The look-up-table has RMS noise voltage levels of both panel(s) 200 and load 250 at various times of the day, day of the week or time of year for example, which can be compared to presently measured RMS noise voltage levels of both panel(s) 200 and load 250.

[0040] In this exemplary example, if the comparison of the measured load 250 RMS noise voltage datum with the measured panel(s) 200 RMS noise voltage datum may be over a certain threshold (step 406) of RMS noise voltage difference an alarm condition of potential arcing may be set (step 408) otherwise arc detection continues with step 400.

[0041] Reference is now made to FIG. 5a which shows a power generation circuit 501a according to an embodiment of the present invention. Power generation circuits 501a have outputs of panels 200 connected to the input of modules 202. The outputs of panels 200 may be configured to provide a DC power input (P.sub.IN) to modules 202. Modules 202 may include direct current (DC-to-DC) switching power converters such as a buck circuit, a boost circuit, a buck-boost circuit, configurable buck-or-boost circuits, a cascaded buck and boost circuit with configurable bypasses to disable the buck or boost stages, or any other DC-DC converter circuit. The output voltage of modules 202 may be labeled as V.sub.i.

[0042] The outputs of modules 202 and module 202a may be connected in series to form a serial string 520. Two strings 520 may be shown connected in parallel. In one string 520, a situation is shown of an arc voltage (V.sub.A) which may be occurring serially in string 520. Load 250 may be a DC to AC inverter. Attached to load 250 may be a central controller 204. Controller 204 optionally measures the voltage (V.sub.T) across load 250 as well as the current of load 250 via current sensor 206. Current sensor 206 may be attached to controller 204 and coupled to the power line connection of load 250.

[0043] Depending on the solar radiation on panels 200, in a first case, some modules 202 may operate to convert power on the inputs to give fixed output voltages (V.sub.i) and the output power of a module 202 that may be dependent on the current flowing in string 520. The current flowing in string 520 may be related to the level of irradiation of panels 200, e.g., the more irradiation, the more current in string 520, and the output power of a module 202 is more.

[0044] In a second case, modules 202 may be operating to convert powers on the input to be the same powers on the output; so for example if 200 watts is on the input of a module 202, module 202 may endeavor to have 200 watts on the output. However, because modules 202 may be connected serially in a string 520, the current flowing in string 520 may be the same by virtue of Kirchhoff's law. The current flowing in string 520 being the same means that the output voltage (V.sub.i) of a module should vary in order to establish that the power on the output of a module 202 may be the same as the power on the input of a module 202. Therefore, in this example, as string 520 current increases, the output voltage (V.sub.i) of modules 202 decreases or as string 520 current decreases, the output voltage (V.sub.i) of modules 202 increases to a maximum value. When the output voltage (V.sub.i) of modules 202 increases to the maximum value, the second case may be similar to the first case in that the output voltage (V.sub.i) may be now effectively fixed.

[0045] Modules 202 in string 520 may have a master/slave relationship with one of modules 202a configured as master and other modules 202 configured as slaves.

[0046] Since current may be the same throughout string 520 in this example, master module may be configured to measure current of string 520. Modules 202 optionally measure their output voltage V.sub.i so that the total string power may be determined. Output voltages of slave modules 202, in this example, may be measured and communicated by wireless or over power line communications, for instance to master unit 202a so that a single telemetry from module 202a to controller 204 may be sufficient to communicate the output power of the string. Master module 202a in string 520 may be variously configured, such as to communicate with the other slave modules 202 for control of slave modules 202. Master module 202a, in this example, may be configured to receive a ‘keep alive’ signal from controller 204, which may be conveyed to slave modules 202. The optional ‘keep alive’ signal sent from controller 204 communicated by wireless or over power line communications, may be present or absent. The presence of the ‘keep alive’ signal may cause the continued operation of modules 202 and/or via master module 202a. The absence of the ‘keep alive’ signal may cause the ceasing of operation of modules 202 and/or via master module 202a (i.e. current ceases to flow in string 520). Multiple ‘keep alive’ signals each having different frequencies corresponding to each string 520 may be used so that a specific string 520 may be stopped from producing power where there may be a case of arcing whilst other strings 520 continue to produce power.

[0047] Reference is now also made to FIG. 5b which shows a method 503 according to an embodiment of the present invention. In step 500, one or more strings 520 power may be measured. In step 502, the load 250 power may be measured using central controller 204 and sensor 206. The measured load power and the measured string powers may be compared in step 504. Steps 500, 502 and 504 may be represented mathematically by Equation Eq. 2 (assuming one string 520) with reference, in this example, to FIG. 5a, as follows:

V.sub.TI.sub.L=ΣP.sub.IN−V.sub.A[I.sub.L]I.sub.L+ΣV.sub.iI.sub.L  Eq. 2, [0048] where: [0049] V.sub.A[I.sub.L]=the arc voltage as a function of current I.sub.L; [0050] V.sub.T I.sub.L=the power of load 250; [0051] Σ P.sub.IN=the power output of modules 202 when modules 202 may be operating such that the output voltage (V.sub.i) of a module varies in order to establish that the power on the output of a module 202 may be the same as the power on the input of a module 202 (P.sub.IN); and [0052] Σ V.sub.i I.sub.L=the power output of modules 202 with fixed voltage outputs (V.sub.i) and/or power output of modules 202 (with variable output voltage V.sub.i) when string 520 current decreases sufficiently such that the output voltage (V.sub.i) of modules 202 increases to a maximum output voltage level value. In all cases, the maximum output voltage level value (V.sub.i) and fixed voltage outputs (V.sub.i) may be pre-configured to be the same in power generation circuit 501a.

[0053] The comparison between string power of string 520 and of the power (V.sub.T×I.sub.L) delivered to load 250 may be achieved by subtracting the sum of the string 520 power (ΣP.sub.IN+ΣV.sub.i I.sub.L) from the power delivered to load 250 (V.sub.T×I.sub.L) to produce a difference. If the difference may be less than a pre-defined threshold (step 506), the measurement of power available to string 520 (step 500) and load 250 (step 502) continues. In decision block 506, if the difference may be greater than the previously defined threshold, then an alarm condition may be set and a series arc condition may be occurring. A situation of series arcing typically causes the transmission of a ‘keep alive’ signal to modules 202 from controller 204 to discontinue, which causes modules 202 to shut down. Modules 202 shutting down may be a preferred way to stop series arcing in string 520.

[0054] Reference is now made to FIG. 5c which shows method step 500 (shown in FIG. 5b) in greater detail to measure power of string 502 according to an embodiment of the present invention. Central controller 204 may send instructions (step 550) via power line communications to master module 202a. Master module 202a may measure the string 502 current as well as voltage on the output of master module 202a and/or voltage and current on the input of master module 202a to give output power and input power of module 202a respectively. Master module may instruct (step 552) slave modules 202 in string 502 to measure the output voltage and string 502 current and/or the input voltage and current of modules 202 to give output power and input power of modules 202 respectively. Slave modules 202 may then be configured to transmit (step 554) to master module 202a the input and output powers measured in step 552. Master module 202a receives (step 556) the transmitted power measurements made in step 554. Master module 202a then adds up the received power measurements along with the power measurement made by master module 202a (step 558) according to equation Eq. 2. According to equation Eq. 2; Σ P.sub.IN=the power output of modules 202 when modules 202 may be operating such that the output voltage (V.sub.i) of a module varies in order to establish that the power on the output of a module 202 may be the same as the power on the input of a module 202 (P.sub.IN); ΣV.sub.i I.sub.L=the power output of modules 202 with fixed voltage outputs (V.sub.i) and/or power output of modules 202 (with variable output voltage V.sub.i) when a string 520 current decreases sufficiently such that the output voltage (V.sub.i) of modules 202 increases to a maximum output voltage level value. In all cases, the maximum output voltage level value (V.sub.i) and fixed voltage outputs (V.sub.i) may be pre-configured to be the same in power generation circuit 501a. The added up power measurements in step 558 may be then transmitted by master module 202a to central controller 204 (step 560).

[0055] Reference is now made to FIG. 5d, which shows a method 505 for serial arc detection, according to another embodiment of the present invention. First differential power result 508 occurs in circuit 501a, with load current I.sub.L now labeled as current I.sub.1 and with voltage V.sub.T across load 250 (as shown in FIG. 5a). First differential power result 508 may be produced with reference to FIG. 5a and equation Eq. 3 (below) as a result of performing method 503 (shown in FIG. 5c). Eq. 3 is as follows:

V.sub.TI.sub.1=ΣP.sub.IN−V.sub.A[I.sub.1]I.sub.1+ΣV.sub.iI.sub.1  Eq. 3, [0056] where: [0057] V.sub.A[I.sub.1]=the arc voltage as a function of current I.sub.1; [0058] V.sub.T I.sub.1=the power of load 250; [0059] Σ P.sub.IN=the power output of modules 202 when modules 202 may be operating such that the output voltage (V.sub.i) of a module varies in order to establish that the power on the output of a module 202 may be the same as the power on the input of a module 202 (P.sub.IN); and [0060] ΣV.sub.i I.sub.L=the power output of modules 202 with fixed voltage outputs (V.sub.i) and/or power output of modules 202 (with variable output voltage V.sub.i) when string 520 current decreases sufficiently such that the output voltage (V.sub.i) of modules 202 increases to a maximum output voltage level value. In all cases, the maximum output voltage level value (V.sub.i) and fixed voltage outputs (V.sub.i) may be pre-configured to be the same in power generation circuit 501a.

[0061] The impedance of load 250 may be adjusted (step 510) optionally under control of central controller 204. Typically, if load 250 is an inverter, controller 204 adjusts the input impedance of load 250 by variation of a control parameter of the inverter. A change in the input impedance of load 250 causes the voltage across the input of load 250 to change by virtue of Ohm's law. The voltage (V.sub.T) as shown in circuit 501a across load 250 may be therefore made to vary an amount ΔV as a result of the input impedance of load 250 being adjusted. The voltage across load 250 may be now V.sub.T+ΔV and the load 250 current (I.sub.L) may be now I.sub.2.

[0062] A second differential power result 522 may be now produced as a result of performing again method 503 (shown in FIG. 5c) on the adjusted input impedance of load 250 performed in step 510. Second differential power result 522 may be represented mathematically by equation Eq. 4, as follows:

(V.sub.T+ΔV)I.sub.2=ΣP.sub.IN−V.sub.A[I.sub.2]I.sub.2+ΣV.sub.iI.sub.2  Eq. 4, [0063] where: [0064] V.sub.A [I.sub.2]=the arc voltage as a function of current I.sub.2; [0065] (V.sub.T+ΔV)I.sub.2=the power delivered to load 250; [0066] ΣP.sub.IN=the power output of modules 202 when modules 202 may be operating such that the output voltage (V.sub.i) of a module varies in order to establish that the power on the output of a module 202 may be the same as the power on the input of a module 202 (P.sub.IN); and [0067] ΣV.sub.i I.sub.L=the power output of modules 202 with fixed voltage outputs (V.sub.i) and/or power output of modules 202 (with variable output voltage V.sub.i) when string 520 current decreases sufficiently such that the output voltage (V.sub.i) of modules 202 increases to a maximum output voltage level value. In all cases, the maximum output voltage level value (V.sub.i) and fixed voltage outputs (V.sub.i) may be pre-configured to be the same in power generation circuit 501a.

[0068] The first differential power result 508 may be compared with the second differential power result 522 (step 524), for example, using controller 204 to subtract the first differential power result 508 from the second differential power result 522 to produce a difference. The difference may be expressed by equation Eq. 5, which may be as a result of subtracting equation Eq. 3 from equation Eq. 4, as follows:

V.sub.TI.sub.1−(V.sub.T+ΔV)I.sub.2=V.sub.A[I.sub.2]I.sub.2−V.sub.A[I.sub.1]I.sub.1+ΣV.sub.i(I.sub.1-I.sub.2)  Eq. 5

[0069] The summed output power (P.sub.IN) of each module 202 for circuit 501a may be thus eliminated.

[0070] Equation Eq. 5 may be re-arranged by controller 204 by performing a modulo operator function on equation Eq. 5 to obtain an arc coefficient α as shown in equation Eq. 6.

[00001]$\begin{array}{cc}\frac{V_T{I}_1-\left(V_T+\Delta V\right)I_2}{\left(I_1-I_2\right)}=\alpha +.Math.V_i& \mathrm{Eq}.6\end{array}$ [0071] where the arc coefficient α is shown in Eq. 7

[00002]$\begin{array}{cc}\alpha =\frac{V_A\left[I_2\right]I_2-V_A\left[I_1\right]I_1}{\left(I_1-I_2\right)}& \mathrm{Eq}.7\end{array}$

[0072] Controller 204, for example, may be configured to calculate coefficient α according to the above formula and measurements. A non-zero value of arc coefficient α shown in equation Eq. 7 causes an alarm condition to be set (step 528) otherwise another first differential power result 508 may be produced (step 503). A situation of series arcing typically causes the ‘keep alive’ signal to be removed by controller 204, causing modules 202 to shut down. Modules 202 shutting down may be a preferred way to stop series arcing in string 520.

[0073] The definite articles “a”, “an” is used herein, such as “an arc voltage and/or arc current”, “a load” have the meaning of “one or more” that is “one or more arc voltages and/or arc currents” or “one or more loads”.

[0074] While the embodiments of aspects of the invention has been described with respect to a limited number of examples, it will be appreciated that many variations, modifications and other applications of the invention may be made.