AU2021221479A1

AU2021221479A1 - Detection of earth faults in DC power systems by use of signal injection

Info

Publication number: AU2021221479A1
Application number: AU2021221479A
Authority: AU
Inventors: Daniel JESSEN; Jeremy Whaley
Original assignee: Red Earth Energy Storage Ltd; Think Logic Pty Ltd
Current assignee: Red Earth Energy Storage Ltd; Think Logic Pty Ltd
Priority date: 2020-09-08
Filing date: 2021-08-24
Publication date: 2022-03-24

Abstract

A ground fault monitoring system for detecting ground faults in a DC power network enables detection of low resistance connections to earth. The system includes: a bus system and a ground reference of the DC power network; a signal generator for providing an injected signal; and a signal processor. The signal generator and the signal processor are coupled to the bus system and the ground reference of the DC power network. In use, the signal generator injects the signal into the bus system to define an injected signal, and the signal processor measures the injected signal to determine whether a ground fault exists in the DC power network. 1/3 100 115 120 105 135 135 110 DC Bus + Signal Generator Coupling Signal Conditioner Mechanism DC Bus (Coupling Earth/Ground ---Mechanism 1201 135 125 Analogue and Digital Signal Processing ...... Communications Interface (Controller / Processor) 140 Alarm Interface 130 145 Figure. 1

Description

1/3

100

115 120 105 135 135 110

DC Bus

+ Signal Generator Coupling Signal Conditioner Mechanism DC Bus

(Coupling Earth/Ground ---Mechanism

1201 135 125

Analogue and Digital Signal Processing ...... Communications Interface (Controller / Processor)

140 Alarm Interface 130

145

Figure. 1

TITLE

Detection of earth faults in DC power systems by use of signal injection

FIELD OF THE INVENTION

[0001] The present invention relates generally to the detection of earth faults, and in particular to a system and method for detecting earth faults in DC power systems by use of signal injection.

BACKGROUND

[0002] Earth faults in DC power systems and networks, such as photo voltaic (PV) arrays and battery storage systems, increase the potential for harm or damage to equipment and personnel. Most PV and battery storage installations are required to monitor for an earth fault condition, and take action (such as raise an alarm or prevent operation of equipment) when an earth or ground fault is detected. An earth fault condition typically involves a DC bus having a lower resistance to earth than is expected.

[0003] Current methods for detection of earth faults typically involve momentarily establishing a resistive connection to earth, and monitoring for current flow through the resistance. However, these implementations come with certain limitations and drawbacks.

[0004] For example, using current methods a fault generally can be detected only when sufficient voltage is present on a DC bus. In the case of solar PV, this means that earth faults cannot be detected in very-low and no light scenarios. Further, the fault can only be detected when sufficient fault current is sourced from the DC bus.

[0005] In the event that an earth fault exists, connecting a resistive load to earth causes fault-current to flow through the resistance. In larger systems with multiple DC sources connected to the DC bus, this has the potential to momentarily create a false positive, triggering earth fault alarms on other equipment, if the other equipment detects and interprets the presence of the resistance as a fault condition.

[0006] There is therefore a need for an improved system for detecting earth faults in DC power systems.

OBJECT OF THE INVENTION

[0007] It is an object of the present invention to overcome and/or alleviate one or more of the disadvantages of the prior art or provide the consumer with a useful or commercial choice.

SUMMARY OF THE INVENTION

[0008] In a first aspect, although it need not be the only or the broadest aspect, the invention resides in a ground fault monitoring system for detecting ground faults in a DC power network, the system comprising: a bus system and a ground reference of the DC power network, a signal generator for providing an injected signal, and a signal processor, wherein the signal generator and the signal processor are coupled to the bus system and the ground reference of the DC power network, and wherein, in use, the signal generator injects the signal into the bus system to define an injected signal, and the signal processor measures the injected signal to determine whether a ground fault exists in the DC power network.

[0009] In one aspect, although it need not be the only or the broadest aspect, the injected signal injection varies with time.

[0010] Preferably, the signal generator or signal processor is coupled to the bus system or ground reference by one or more of: capacitive coupling, inductive coupling, resistive coupling, or direct coupling.

[0011] Preferably, the signal processor measures the injected signal voltage before, at, or after a point of coupling to the bus system.

[0012] Preferably, the signal processor measures the injected signal current in a return ground path.

[0013] Preferably, the signal processor measures magnitude and phase of current and voltage of the injected signal.

[0014] Preferably, the signal processor provides analogue and digital signal analysis, the signal processor configured to isolate the injected signal from other signals or noise.

[0015] Preferably, the signal processor is configured to determine absence, presence, or presence and magnitude, of the ground fault.

[0016] Preferably, the ground fault monitoring system comprises an alarm device to indicate presence and/or status of the ground fault.

[0017] Preferably, the ground fault monitoring system communicates with a wired or wireless external device, the external device comprising an interface.

[0018] In another aspect, the invention resides in a networked ground fault monitoring system for monitoring ground faults in a plurality of DC power networks, the networked system comprising two or more ground fault monitoring systems.

[0019] In another aspect, the invention resides in a method for monitoring ground faults in a DC power network, the method comprising: coupling a signal generator and a signal processor to a bus system and a ground reference of the DC power network, sending an injected signal into the bus system using the signal generator, measuring the voltage and current of the injected signal using the signal processor, and analysing the voltage and current of the injected signal to determine whether there is a ground fault.

[0020] Preferably, the method further comprises varying the injected signal by: sending multiple signals, or sending multiple signals of one or more frequencies, or sending multiple signals at varying predetermined times.

[0021] Preferably, the method further comprises measuring magnitude and phase of current and voltage of the injected signal.

[0022] Preferably, the method further comprises conducting analogue and digital signal analysis using the signal processor, and isolating the injected signal from other signals or noise.

[0023] Preferably, the method further comprises using the signal processor to determine absence, presence, or presence and magnitude, of the ground fault.

[0024] Preferably, the method further comprises issuing an alarm to indicate presence or status of the ground fault.

[0025] Preferably, the method further comprises communicating with a wired or wireless external device, the external device comprising an interface.

[0026] In another aspect, the invention resides in a method for networked monitoring of ground faults in a plurality of DC power networks, the method comprising: determining the ground faults of two or more DC power networks.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] To assist in understanding the invention and to enable a person skilled in the art to put the invention into practical effect, preferred embodiments of the invention are described below by way of example only with reference to the accompanying drawings, in which:

[0028] FIG. 1 is a schematic diagram of a system for detecting earth faults in DC power systems/networks by use of signal injection, according to some embodiments of the present invention.

[0029] FIG. 2 is a flow chart of a method for detecting earth faults in DC power systems/networks by use of signal injection, according to some embodiments of the present invention.

[0030] FIG. 3 is a diagram of a networked system for detecting earth faults in DC power systems/networks by use of signal injection, according to some embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0031] The present invention relates to an improved system for detecting earth faults in DC power systems by use of signal injection. Elements of the invention are illustrated in concise outline form in the drawings, showing only those specific details that are necessary to understanding the embodiments of the present invention, but so as not to clutter the disclosure with excessive detail that will be obvious to those of ordinary skill in the art in light of the present description.

[0032] In this patent specification, adjectives such as first and second, left and right, above and below, top and bottom, upper and lower, rear, front and side, etc., are used solely to define one element or method step from another element or method step without necessarily requiring a specific relative position or sequence that is described by the adjectives. Words such as "comprises" or "includes" are not used to define an exclusive set of elements or method steps. Rather, such words merely define a minimum set of elements or method steps included in a particular embodiment of the present invention.

[0033] According to one aspect, the present invention is defined as a ground fault monitoring system for detecting ground faults in a DC power network, the system comprising: a bus system and a ground reference of the DC power network, a signal generator for providing a signal, and a signal processor, wherein the signal generator and the signal processor are coupled to the bus system and the ground reference of the DC power network, and wherein, in use, the signal generator injects the signal into the bus system to define an injected signal, and the signal processor measures the injected signal to determine whether a ground fault exists in the DC power network.

[0034] Advantages of some embodiments of the present invention include a system that can detect low resistance connections to earth, even when no voltage is present on a DC bus, such as during low or no light conditions on a PV array. The present invention utilises signal injection to detect an earth fault and does not need to create current flow from the DC bus to earth.

[0035] Further, some embodiments of the present invention allow detection of conditions that have the potential to cause earth faults, rather than only detecting the presence of an existing fault to earth. This may permit determination of a non-binary "state of health" metric for the DC bus, in addition to the presence or absence of a fault. Some embodiments of the present invention can also diagnose and detect faults in complex systems with multiple sources and DC systems.

[0036] Those skilled in the art will appreciate that not all of the above advantages are necessarily included in all embodiments of the present invention.

[0037] FIG. 1 is a schematic diagram of a system 100 for detecting earth faults in DC power systems/networks by use of signal injection, according to some embodiments of the present invention. Both a positive line 105 and a negative line 110 of a DC bus are connected to a signal generator 115 via a coupling mechanism 120. The signal generator 115 is suitable for generating, conditioning, and filtering a signal which is injected into the positive line 105 and negative line 110 of the DC bus.

[0038] The signal generator 115 is also connected to an earth and ground 125 via a coupling mechanism 120, allowing a signal to be injected into the earth and ground 125 return path. The connection through a coupling mechanism 120 can be through one or more of: capacitive coupling, inductive coupling, resistive coupling, or direct coupling. The person skilled in the art will understand that different methods of coupling may be utilised to achieve the same effect and allow signal injection.

[0039] A signal processor 130 which processes analogue and digital signals for analysis communicates with the signal generator 115, coupling mechanisms 120, and measurement points 135 which are able to measure the injected signal, determining a magnitude, phase of current, and voltage of the injected signal. The measurement points 135 may be suitably positioned on the DC bus lines 105, 110 and earth and ground 125 at locations before, at, or after a point of coupling to the bus system. The signal processor 130 is able to therefore measure the current of the injected signal in a return ground path.

[0040] The system 100 may include an external communications interface 140 by which an operator or separate system can operate or interact with the system 100 through an interface. The external communications interface 140 may be a dedicated electronic device or interface, a wired or wirelessly connected personal electronic device, or a wired or wirelessly connected computing device, the computing device being on or off site. The injected signal may be selected to contain a single signal, or a combination of signals. Further variations for each signal may include a choice between signal frequencies, multiple frequencies, frequencies which vary with time, or combinations thereof. The skilled person would understand that the generated signal may depend upon a particular situation and refinement as required.

[0041] By comparing the signal provided by the signal generator 115 against the processed data acquired and at the measurement points 135, the signal processor 130 is able to determine whether a ground fault exists in the DC power network. The system 100 is also able to determine the absence, presence, or presence and magnitude of a ground fault based on the data acquired, and issue any warnings or alarms as necessary. This warning or alarm may be provided through an alarm interface 145, providing an alert or information regarding the presence and/or status of the ground fault. Alternatively, the alarm, alert, or information may be communicated through the communications interface 140.

[0042] FIG. 2 is a flow chart of a method 200 for detecting earth faults in DC power systems/networks by use of signal injection, by using for example the system 100 described above, according to some embodiments of the present invention.

[0043] In the method 200, at step 205 a signal generator 115 and a signal processor 130 are coupled to a DC power system via coupling mechanisms 120. The signal generator 115 and the signal processor 130 may be coupled to a bus system and a ground reference of the DC power system. The coupling mechanism can be through one or more of: capacitive coupling, inductive coupling, resistive coupling, direct coupling, or any other suitable means as known in the art. Measurement points 135 are also suitably positioned on the DC bus lines 105, 110 and earth and ground 125 at locations before, at, or after a point of coupling to the bus system.

[0044] At step 210, a signal is generated by the signal generator 115, providing a signal injection into the DC power system. The injected signal may contain a single signal, or a combination of signals, and each signal may consist of a single frequency, multiple frequencies, a signal of which the frequency varies with time, or any combination of these. The variation signal injection time allows for a non-sinusoidal or non-repetitive signal such as a 'chirp'.

[0045] Optionally, the coupled signal may be injected into one or more of the DC bus conductors, allowing for common-mode and differential-mode signals to be used. This allows different signals to be simultaneously applied to conductors of the positive line 105 and negative line 110, or for the same signal to be simultaneously applied to both.

[0046] At step 215, the magnitude and phase of the voltage and current of the injected signal is measured. The current may be measured at, before, or after the point of coupling, or in the return ground path.

[0047] According to some embodiments, analogue and digital signal analysis and processing is performed by the signal processor 130, assisting in measurement acquisition or to isolate the injected signals from other (externally generated) signals or noise. Certain signals may also be avoided in order to reduce interference with other equipment on the DC bus, including other deployments of the present invention. Some or all of the measured signals may have mathematical algorithms applied. The person skilled in the art will understand that this process may include algorithms such as spectral analysis and impedance calculations, or other methods for the removal or isolation of certain frequencies or groups of frequencies.

[0048] At step 220, the measurements, such as magnitude and phase of current and voltage of the injected signal, are processed and interpreted. The interpretation of the data may use some or all of the signals to determine, at step 225, if a ground fault exists in the DC bus. The data may be used to determine absence, presence, or presence and magnitude, of the ground fault.

[0049] According to some embodiments, further information is also determined, measured, or inferred by the measurements, such as a non-binary earth fault state. As mentioned, the calculated measurements and data can also be utilised to determine a general "state of health", allowing the system to also predict the likelihood of a future fault. The present invention therefore goes beyond simply detecting a "fault present" status in a DC power system.

[0050] At step 230, the results and interpreted measurements are reported. In the event of an earth fault, an alarm may be raised by an alarm interface 145 or communicated to the communications interface 140. The communications interface 140 may be a wired or wireless device operated by personnel, the device comprising an interface. Different alarms or notifications for different states of the DC power system, detectable by the signal processor 130, can also be established according to predetermined status warnings as required. These predetermined status warnings may be established by pre-set thresholds relating to magnitude and phase of current and voltage of the injected signal. The person skilled in the art would understand that these thresholds may be determined and set according to different requirements, and may be adjusted dynamically based on various other parameters specific to the installation.

[0051] FIG. 3 is a diagram of a networked system 300 for detecting earth faults in DC power systems/networks by use of signal injection, according to some embodiments of the present invention.

[0052] In a further embodiment, as shown in FIG. 3, the invention resides in ground fault monitoring system and/or method for monitoring ground faults in a plurality of DC power networks, each with their own ground fault monitoring system. The networked system 300 thereby comprises two or more separate ground fault monitoring systems, each system similar to the system 100 for example, and each coupled to a DC power system and associated device. Each instance of the separate ground fault monitoring systems can communicate with a primary control device 305 which is a central monitoring device of the entire networked system 300. The primary control device 305 is suitable for comparing and monitoring the results and status of each separate ground fault monitoring system.

[0053] The person skilled in the art will understand that the networked system 300 as shown in FIG. 3 can extend to any quantity of 'n' devices per bus over any quantity of 'm' buses, with the entire networked system 300 controlled by one or more primary control device(s) 305. Optionally, the networked system 300 can also be scaled up to define additional tiers of management and control hierarchy, such that a separate control device may manage a plurality of primary control devices 305, the primary control devices

305 each managing a plurality of networked 'n' devices with their own ground fault monitoring system. The person skilled in the art will further understand that the primary control device 305 or separate control device may comprise its own interface means and be connected to devices in the network via wired or wireless means.

[0054] Those skilled in the art will appreciate that various components of embodiments of the present invention can be made of various materials and as various integrated or non-integrated designs.

[0055] The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. Numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. Accordingly, this patent specification is intended to embrace all alternatives, modifications and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above described invention.

Claims

1. A ground fault monitoring system for detecting ground faults in a DC power network, the system comprising:

a bus system and a ground reference of the DC power network; a signal generator for providing an injected signal; and a signal processor, wherein the signal generator and the signal processor are coupled to the bus system and the ground reference of the DC power network, and wherein, in use, the signal generator injects the signal into the bus system to define an injected signal, and the signal processor measures the injected signal to determine whether a ground fault exists in the DC power network.

2. The ground fault monitoring system of claim 1, wherein the injected signal contains one or more signals consisting of one or more frequencies.

3. The ground fault monitoring system of any preceding claim, wherein the injected signal injection varies with time.

4. The ground fault monitoring system of any preceding claim, wherein the signal generator or signal processor is coupled to the bus system or ground reference by one or more of: capacitive coupling, inductive coupling, resistive coupling, or direct coupling.

5. The ground fault monitoring system of any preceding claim, wherein the signal processor measures the injected signal before, at, or after a point of coupling to the bus system.

6. The ground fault monitoring system of any preceding claim, wherein the signal processor measures the injected signal in a return ground path.

7. The ground fault monitoring system of any preceding claim, wherein the signal processor measures magnitude and phase of current and voltage of the injected signal.

8. The ground fault monitoring system of any preceding claim, wherein the signal processor provides analogue and digital signal analysis, the signal processor configured to isolate the injected signal from other signals or noise.

9. The ground fault monitoring system of any preceding claim, wherein the signal processor is configured to determine absence, presence, or presence and magnitude, of the ground fault.

10.The ground fault monitoring system of any preceding claim, wherein the ground fault monitoring system comprises an alarm device to indicate presence and/or status of the ground fault.

11.The ground fault monitoring system of any preceding claim, wherein the ground fault monitoring system communicates with a wired or wireless external device, the external device comprising an interface.

12.A networked ground fault monitoring system for monitoring ground faults in a plurality of DC power networks, the networked system comprising two or more of the ground fault monitoring system as claimed in any preceding claim.

13.A method for monitoring ground faults in a DC power network, the method comprising: coupling a signal generator and a signal processor to a bus system and a ground reference of the DC power network, sending an injected signal into the bus system using the signal generator, measuring the injected signal using the signal processor, and analysing the injected signal to determine whether there is a ground fault.

14.The method of claim 13, further comprising: varying the injected signal by: sending multiple signals, or sending multiple signals of one or more frequencies, or sending multiple signals at varying predetermined times.

15.The method of claims 13 or 14, further comprising: measuring magnitude and phase of current and voltage of the injected signal.

16. The method of any one of claims 13 to 15, further comprising: conducting analogue and digital signal analysis using the signal processor, and isolating the injected signal from other signals or noise.

17. The method of any one of claims 13 to 16, further comprising: using the signal processor to determine absence, presence, or presence and magnitude, of the ground fault.

18. The method of any one of claims 13 to 17, further comprising: issuing an alarm to indicate presence or status of the ground fault.

19. The method of any one of claims 13 to 18, further comprising: communicating with a wired or wireless external device, the external device comprising an interface.

20.A method for networked monitoring of ground faults in a plurality of DC power networks, the method comprising: determining the ground faults of two or more DC power networks each using the method of any one of claims 13 to 19.