Apparatus and method for monitoring a hybrid power system

Abstract

An apparatus is configured to monitor a hybrid power system comprising an alternating current (AC) bus and a direct current (DC) bus. The apparatus comprises a signal generator configured to inject a time-varying AC signal into a conductor of the AC bus and / or into a conductor of the DC bus; a sensor configured to detect a response signal from the hybrid power system, the response signal being indicative of a leakage current from the hybrid power system; and a control unit comprising a response signal analysis unit configured to analyse the detected response signal to identify whether the leakage current originates in the AC bus and / or in the DC bus.

Description

[0001] M&C PB167529GB

[0002] 1

[0003] Apparatus and method for monitoring a hybrid power system

[0004] Technical Field

[0005] The present invention relates to an apparatus and method for monitoring a hybrid power system.

[0006] Background

[0007] Hybrid power systems combine both alternating current (AC) and direct current (DC) supplies. For this reason, hybrid power systems are often known as hybrid AC / DC systems. Hybrid AC / DC systems are becoming increasingly prevalent in applications such as renewable energy, electric vehicles, unmanned and robotic vehicles (RVs) including Remotely Operated Vehicles (ROVs), industrial systems, and power generation plants. In hybrid AC / DC systems, ensuring the safety and integrity of electrical insulation is critical for preventing ground faults, which can cause system malfunctions, safety hazards, and costly equipment damage. Ground faults can occur due to insulation degradation or failure, leading to unintentional electrical connections to ground. Monitoring and detecting such faults in hybrid AC / DC systems presents unique challenges due to the differing electrical characteristics of AC and DC supplies.

[0008] Conventional insulation monitoring techniques face significant challenges in accurately measuring leakage currents in hybrid AC / DC systems. The presence of rectifiers in these systems tends to completely block, or partially block and distort, measurement signals, which often leads to inaccurate or invalid results. Additionally, conventional techniques may struggle to differentiate between faults occurring in the AC section from those occurring in the DC section of the network.

[0009] There is a need for improved techniques to monitor insulation resistance and / or detect ground faults in hybrid AC / DC systems. An improved technique should be adaptive to different system voltages and varying electrical loads while maintaining high accuracy and reliability across both AC and DC networks.

[0010] Summary

[0011] According to a first aspect of the present invention there is provided an apparatus for monitoring a hybrid power system comprising an alternating current (AC) bus and a directM&C PB167529GB

[0012] 2

[0013] current (DC) bus, the apparatus comprising: a signal generator configured to inject a time-varying AC signal into a conductor of the AC bus and / or into a conductor of the DC bus; a sensor configured to detect a response signal from the hybrid power system, the response signal being indicative of a leakage current from the hybrid power system; and a control unit comprising a response signal analysis unit configured to analyse the detected response signal to identify whether the leakage current originates in the AC bus and / or in the DC bus.

[0014] The response signal analysis unit may be configured to decompose the detected response signal into a component corresponding to a leakage current in the AC bus and / or a component corresponding to a leakage current in the DC bus.

[0015] The response signal analysis unit may be configured to decompose the detected response signal by applying a transform to the response signal to convert the response signal from the time domain to the frequency domain.

[0016] The response signal analysis unit may be further configured to identify a conductor of a plurality of conductors of the AC bus as a source of the leakage current based on a phase and / or magnitude of the detected leakage current signal.

[0017] The response signal analysis unit may be further configured to determine an insulation impedance of a conductor of the AC bus or an insulation impedance of a conductor of the DC bus based on the voltage of that conductor and the detected leakage current.

[0018] The response signal analysis unit may be further configured to compare the determined insulation impedance of the conductor to a threshold value and, if the determined insulation impedance fails to satisfy an impedance criterion, the control unit may be configured to generate an alert.

[0019] The control unit may be configured to output a signal to isolate a portion of the hybrid power system determined to contain a fault.

[0020] The control unit may be configured to output a signal to switch the hybrid power system to backup power or to shut down the hybrid power system in response to detecting a fault.M&C PB167529GB

[0021] 3

[0022] The time-varying AC signal may have a frequency lower than a line frequency of the AC bus.

[0023] According to a second aspect of the present invention, there is provided a method of monitoring a hybrid power system comprising an alternating current (AC) bus and a direct current (DC) bus, the method comprising injecting, by a signal generator, a time-varying AC signal into a conductor of the AC bus and / or into a conductor of the DC bus; detecting, by a sensor, a response signal from the hybrid power system, the response signal being indicative of a leakage current from the hybrid power system; and analysing, by a response signal analysis unit, the detected response signal to identify whether the leakage current originates in the AC bus and / or in the DC bus.

[0024] Brief Description of the Drawings

[0025] So that the invention can be fully understood, the following drawings are included, in which:

[0026] Figure 1 is a circuit diagram illustrating a hybrid power system according to an embodiment of the invention;

[0027] Figure 2A is a block diagram illustrating a monitoring system according to an embodiment of the invention;

[0028] Figure 2B is a circuit diagram illustrating the monitoring system;

[0029] Figure 3 is a block diagram illustrating a control unit of a monitoring system according to an embodiment of the invention;

[0030] Figure 4 is a flow chart illustrating operations carried out by the monitoring system according to an embodiment of the invention;

[0031] Figure 5 is a chart illustrating a response signal decomposed into components thereof; andM&C PB167529GB

[0032] 4

[0033] Figure 6 is a circuit diagram illustrating potential fault locations within a hybrid power system according to an embodiment of the invention.

[0034] Detailed Description

[0035] Embodiments of the invention provide a comprehensive solution for ground fault and insulation monitoring in hybrid power systems that include both alternating current (AC) and direct current (DC) power supplies from a single point of monitoring. Embodiments of the invention utilise an injected time-varying AC measurement signal to actively measure the complex insulation impedance of the network, providing a reliable and precise method to monitor insulation and to measure ground faults in conductors. By analysing the characteristics of the response signal, the section in which a fault occurs can be identified.

[0036] Figure 1 illustrates a hybrid power system 1 according to an embodiment of the invention. The hybrid power system 1 comprises an alternating current (AC) bus 2 and a direct current (DC) bus 3. The hybrid power system 1 comprises an AC / DC conversion unit 4. The AC / DC conversion unit 4 can act as a rectifier to convert an AC supply to a DC supply. The AC / DC conversion unit 4 can additionally or alternatively act as an inverter to generate an AC supply based on a DC input. As such, the hybrid power system 1 is a hybrid system in the sense that it provides both AC and DC power supplies. Also shown in Figure 1 are a potential ground fault FAC in the AC bus 2 and a potential ground fault FDC in the DC bus 3.

[0037] The hybrid power system 1 comprises a monitoring system 5. In Figure 1 , the monitoring system 5 is connected to the AC bus 2. However, in other embodiments, the monitoring system 5 may be connected to the DC bus 3. In still other embodiments, a first monitoring system 5 may be connected to the AC bus 2 and a second monitoring system 5 may be connected to the DC bus 3.

[0038] The monitoring system 5 is configured to monitor conductors of the hybrid power system 1. The monitoring system 5 may be configured to detect faults such as ground faults or faults arising in insulation of the AC and DC sections of the hybrid power system 1. Referring to Figure 2A, the monitoring system 5 comprises a signal generator 6, a sensor 7 and a control unit 8. The signal generator 6 is configured to inject a time-varying ACM&C PB167529GB

[0039] 5

[0040] signal into the hybrid power system 1. The time-varying AC signal may be a time-varying voltage signal having a known amplitude (e.g., peak-to-peak amplitude) and frequency. The time-varying AC signal may be time-varying in the sense that the amplitude of the AC signal may vary with respect to time. The signal generator 6 may be configured to inject an AC signal by adding a voltage signal to the AC bus 2 and / or to the DC bus 3, so as to produce a resultant voltage that is the sum of the added signal and the bus voltage.

[0041] The time-varying AC signal may have a DC offset. The DC offset may be variable (in other words, the DC offset may be time-varying). Providing a variable DC offset to the time-varying AC signal can help better distinguish a leakage current from the DC bus 3 from the induced leakage due to the signal injected by the signal generator 6.

[0042] The time-varying AC signal may be a low-frequency signal. In other words, the injected AC signal may have a frequency significantly less than the line frequency of one or more conductors of the AC bus 2 of the hybrid current distribution network 1. Applying a signal having a lower frequency than the line frequency is advantageous because it can help reduce interference with the line voltage. Applying a relatively low frequency injection signal also provides a low impedance path through an AC supply transformer (if present). As such, it is not necessary to connect the signal generator 6 to each of the conductors individually. Furthermore, using a low injection frequency provides a broad insulation resistance measurement range especially when the overall capacitance within the hybrid current distribution network 1 is high.

[0043] Figure 2B is a circuit diagram illustrating the monitoring system 5 in connection with the AC bus 2 or DC bus 3. The sensor 7 may comprise a circuit configured to sample a signal on a conductor of the AC bus 2 or the DC bus 3. The circuit may include an analogue-to-digital converter (ADC) 70 to convert an analogue voltage signal on the conductor into a digital value representative of the analogue voltage signal. The sensor 7 may be configured to derive a leakage current based on a measured line-to-earth voltage signal and a known shunt impedance. The monitoring system 5 may comprise a protective impedance to avoid drawing an excessive current from the AC bus 2 or DC bus 3.

[0044] As would be understood by the person skilled in the art, a leakage current is a current between a bus and earth due to insulation degradation. As explained in more detailM&C PB167529GB

[0045] 6

[0046] below, the leakage current determined by a sensor 7 based on the measured voltage signal may be processed to determine a location associated with the leakage current. As such, the monitoring system described herein has the advantage that a single sensor 7 may be deployed within the hybrid power system 1 because the signal detected by the sensor 7 can be processed to determine a location of a ground fault within the hybrid power system 1. This is in contrast to prior systems where a sensor would have to be provided for each conductor within a multiple conductor system.

[0047] Referring to Figure 3, the control unit 8 comprises a response signal analysis unit 9 configured to analyse a response signal detected the sensor 7 that is indicative of a leakage current. The control unit 8 comprises a communication interface 10. The response signal analysis unit 9 is configured to determine whether a detected leakage current originates in the AC distribution network 2 or in the DC distribution network 3. The response signal analysis unit 9 may be configured to cause the communication interface 10 to output information regarding an insulation condition of a conductor of the hybrid power system 1. Such output information may include impedance information of a conductor of the hybrid power system 1. The response signal analysis unit 9 may be configured to cause the communication interface 10 to output a signal indicative of a ground fault for a particular conductor if the response signal analysis unit 9 determines that the impedance information of that conductor indicates that a ground fault is present. The control unit 8 comprises one or more processors 11 and one or more memory units 12. The one or more processors 11 may be configured to execute instructions stored on the one or more memory units 12.

[0048] Figure 4 is a flow chart illustrating operations carried out by the monitoring system 5 in use. The signal generator 6 applies a time-varying AC signal to the hybrid power system 1 at step 401. The time-varying AC signal injected into the hybrid power system 1 may have a frequency lower (or higher) than the line frequency. For example, the injected AC signal may have a frequency significantly lower than the frequency of the AC power supply across the AC bus 2. Where the AC power supply across the AC bus 2 may have a frequency of 50 Hertz (Hz), the frequency of the injected AC signal may, for example, be less than 10 Hz.

[0049] At step 402, a response signal is detected by the sensor 7. As explained above, a line-to-earth voltage signal may be measured by the sensor 7. A corresponding leakageM&C PB167529GB

[0050] 7

[0051] current signal may then be derived from the measured line-to-earth voltage signal. The leakage current signal may take the form of the response signal 50 shown in Figure 5.

[0052] At step 403, the response signal is then analysed by the response signal analysis unit 9 to determine impedance information of a conductor in the hybrid power system 1. In some embodiments, the response signal analysis unit 9 is configured to decompose the detected response signal into one or more components. In some embodiments, the response signal analysis unit 9 applies a Fourier transform to the response signal. Applying the Fourier transform converts the response signal from the time domain into the frequency domain allowing individual frequency components of the response signal to be identified and isolated from other frequency components. An inverse Fourier transform may then be applied to each identified frequency component to transform that component back into the time domain for further analysis. As an alternative to a Fourier transform, a wavelet transform may be applied. Alternatively, targeted bandpass filtering may be performed. Rather than transforming the signal back to the time domain, the phase and magnitude in the frequency domain may be analysed using a Goertzel algorithm.

[0053] Referring to Figure 5, a response signal 50 is detected by one or more of the sensors 7-1 7-n. The response signal 50 may be analysed by the response signal analysis unit 9 by applying a Fourier transform to transform the response signal into the frequency domain. The application of a Fourier transform to the response signal allows various frequency components in the response signal to be identified and isolated from each other in the frequency domain. The response signal 50 is decomposed into a response component 51 corresponding to the injected variable AC signal itself, an AC response component 53 and a DC response component 52.

[0054] As mentioned above, the injected AC signal may have a frequency significantly less than the line frequency of the AC supply in the AC bus 2. Furthermore, the AC / DC conversion unit 4 comprises a full-wave rectifier so that the rectified signal in the DC bus 3 has a frequency twice that of the line frequency of the AC supply in the AC bus 2. For example, if the line frequency in the AC bus is 50 Hz then the line frequency in the DC bus 3 will be 100 Hz. Thus, the response component 51 corresponding to the injected AC signal, the AC response component 53 and the DC response component 52 may be separated from each other in the frequency domain.M&C PB167529GB

[0055] 8

[0056] Once the components of the response signal have been separated, an inverse Fourier transform may be performed on each of the separated components to transform each component back into the time domain. The response component 51 corresponding to the injected time-varying AC signal itself, an AC component 53 and a DC component 52 shown in Figure 5 have been transformed back into the time domain.

[0057] If a leakage current in the AC section of the hybrid power system 1 is present, then this may be observed as a sinusoidal waveform at the system’s line frequency. In a three-phase AC system, the phase of the leakage current in the AC section may also be analysed to identify whether the fault is located on the L1 , L2 or L3 line. Once the Fourier transform has been applied to the response signal 50, the response signal corresponding to the leakage current in the AC bus 2 of the hybrid power system 1 may be separated from response component 51 corresponding to the injected AC signal itself. This separated component may then be Fourier transformed back into the time domain as AC response component 53. After transforming the signal into the time domain, the phase of the AC component 53 may be analysed to determine which conductor L1 , L2 or L3 in a three-phase AC bus 2 has a ground fault. Alternatively, as mentioned above, the phase and magnitude in the frequency domain may be analysed using Goertzel algorithm processing. A fault on the neutral wire could be identified as the amplitude of the AC and DC line leakage current would be close to zero, while the leakage current induced by the measurement signal would be large.

[0058] Furthermore, the magnitude of the leakage current in the AC section of the hybrid power system 1 can be used to determine how many of the AC conductors have a ground fault. The magnitude of the leakage current can be used to determine if there is a fault to earth on one, two or three of the AC conductors (L1, L2, L3) to ground. Specifically, the magnitude of the AC line leakage current component is related to the vector sum of the line-to-earth voltages of the AC conductors. If the leakage from each conductor is balanced, the overall observed leakage current is close to zero. However, the presence of one or more faults alters this balance. A single fault will reduce the line-to-earth voltage on that conductor and increase the measured leakage. Faults on two of the AC conductors would produce a mid-level leakage magnitude, while equal faults on all three conductors would again cancel out the vector sum of the line to earth voltages and the leakage current would be small.M&C PB167529GB

[0059] 9

[0060] In practice the magnitude might first be used to determine how many conductors have faults, and then the phase could be used to determine which conductor(s) the fault(s) are present on.

[0061] As such, the monitoring system 5 can identify a fault occurring in any of positions 1 to 4 shown in Figure 6 which schematically represent a ground fault on each of the four conductors in the AC bus 2, any of which is detectable by the monitoring system 5. Furthermore, the amplitude of the leakage current may be determined from the AC component 53.

[0062] If a leakage current in the DC section of the hybrid power system 1 is present then this may be observed as a DC offset with a rectified sinusoidal ripple superimposed upon the previously identified components. Once the Fourier transform has been applied to the response signal 50, the signal corresponding to a leakage current in the DC section of the hybrid power system 1 may be separated from response component 51 corresponding to the injected signal itself. This separated component may then be transformed back into the time domain as DC component 52. The value of the leakage current may be determined from the DC response component 52. Alternatively, the value may be obtained in the frequency domain.

[0063] In the example shown in Figure 5, the DC component 52 corresponding to a DC-bus leakage current is shown as a positive signal indicating that a fault is present on the DC+ line of the DC bus 3. Similarly, a fault present on the DC- line of the DC bus 3 would be represented as a negative signal. As such, a ground fault in the DC bus 3 may be identified as occurring on the DC+ line or on the DC- line by identifying whether the DC response component 52 is a positive signal or a negative signal respectively. As such, the monitoring system 5 can identify a fault occurring either position 5 or 6 in Figure 6, which represent schematically a ground fault on each of the two conductors in the DC bus 3.

[0064] If no leakage current is detected in the AC bus 2 of the hybrid power system 1 then the AC response component 53 will be zero. Likewise, if no leakage current is detected in the DC bus 3 of the hybrid power system 1 then the DC response component 52 will be zero.M&C PB167529GB

[0065] 10

[0066] The value of a leakage current for a particular conductor once determined as described above may then be used to determine the complex impedance for the given conductor using Ohm’s law.

[0067] The determined impedance for a conductor may be compared to a stored threshold value. If the determined impedance of a conductor fails to satisfy an impedance criterion associated with the threshold (e.g. if the determined impedance is less than the impedance threshold value) then a signal may be generated by the signal analysis unit 9 and output by the communication interface 10 of the control unit 8 of the monitoring system 5 indicating that a ground fault has occurred. For example, an alert signal may be generated by the analysis unit 9 to signal to devices connected to the hybrid power system 1 that a fault has occurred.

[0068] Additionally or alternatively, when a ground fault or insulation breakdown is detected, the monitoring system 5 may be configured to automatically isolate the affected portion of the hybrid power system 1. For example, the AC or DC supply may be disconnected from a load. Alternatively, an affected portion of the hybrid power system 1 may be switched to backup power. Alternatively, the hybrid power system 1 may be shut down.

[0069] The monitoring system 5 may also communicate, via the communication interface 10, with external control systems to log the fault event and to initiate maintenance procedures.

[0070] The hybrid power system 1 described above comprises a monitoring system 5 connected to the AC bus 2. Advantageously, by injecting a low-frequency AC signal into the hybrid current distribution network 1 , the response of conductors in both the AC bus 2 and the DC bus 3 can be monitored simultaneously. Furthermore, the injection of an AC signal (rather than a DC injection signal) allows the response of the DC+ and DC- minus conductors to be monitored simultaneously. This compares advantageously with systems using a DC injection signal which require the polarity of the injected DC signal to be inverted to monitor both DC+ and DC- conductors.

[0071] The monitoring system 5 can be easily integrated into existing hybrid power infrastructures or deployed as a standalone monitoring solution. A single monitoringM&C PB167529GB

[0072] 11

[0073] system 5 may be connected into the hybrid power system 1 at an easily accessible location, such as the power supply.

[0074] The monitoring system 5 is configured to work across a range of voltages and frequencies, making it suitable for low-voltage, medium-voltage, and high-voltage applications in both AC (single and 3-phase) and DC configurations.

[0075] While various embodiments of the invention have been described, the scope of the invention is defined by the appended claims.

Claims

M&C PB167529GB12CLAIMS:

1. An apparatus for monitoring a hybrid power system comprising an alternating current (AC) bus and a direct current (DC) bus, the apparatus comprising:a signal generator configured to inject a time-varying AC signal into a conductor of the AC bus and / or into a conductor of the DC bus;a sensor configured to detect a response signal from the hybrid power system, the response signal being indicative of a leakage current from the hybrid power system; anda control unit comprising a response signal analysis unit configured to analyse the detected response signal to identify whether the leakage current originates in the AC bus and / or in the DC bus.

2. The apparatus of claim 1 , wherein the response signal analysis unit is configured to decompose the detected response signal into a component corresponding to a leakage current in the AC bus and / or a component corresponding to a leakage current in the DC bus.

3. The apparatus of claim 2, wherein the response signal analysis unit is configured to decompose the detected response signal by applying a transform to the response signal to convert the response signal from the time domain to the frequency domain.

4. The apparatus of any preceding claim, wherein the response signal analysis unit is further configured to identify a conductor of a plurality of conductors of the AC bus as a source of the leakage current based on a phase and / or magnitude of the detected leakage current signal.

5. The apparatus of any preceding claim, wherein the response signal analysis unit is further configured to determine an insulation impedance of a conductor of the AC bus or an insulation impedance of a conductor of the DC bus based on the voltage of that conductor and the detected leakage current.

6. The apparatus of claim 5, wherein the response signal analysis unit is further configured to compare the determined insulation impedance of the conductor to aM&C PB167529GB13threshold value and, if the determined insulation impedance fails to satisfy an impedance criterion, the control unit is configured to generate an alert.

7. The apparatus of any preceding claim, wherein the control unit is configured to output a signal to isolate a portion of the hybrid power system determined to contain a fault.

8. The apparatus of any preceding claim, wherein the control unit is configured to output a signal to switch the hybrid power system to backup power or to shut down the hybrid power system in response to detecting a fault.

9. The apparatus of any preceding claim, wherein the time-varying AC signal has a frequency lower than a line frequency of the AC bus.

10. A method of monitoring a hybrid power system comprising an alternating current (AC) bus and a direct current (DC) bus, the method comprising:injecting, by a signal generator, a time-varying AC signal into a conductor of the AC bus and / or into a conductor of the DC bus;detecting, by a sensor, a response signal from the hybrid power system, the response signal being indicative of a leakage current from the hybrid power system; and analysing, by a response signal analysis unit, the detected response signal to identify whether the leakage current originates in the AC bus and / or in the DC bus.

11. The method of claim 10, wherein analysing the detected response signal comprises decomposing the detected response signal into a component corresponding to a leakage current in the AC bus and / or a component corresponding to a leakage current in the DC bus.

12. The method of claim 11, wherein decomposing the detected response signal comprises applying a transform to the response signal to convert the response signal from the time domain to the frequency domain.

13. The method of any of claims 10 to 12, wherein analysing the detected response signal comprises identifying a conductor of a plurality of conductors of the AC bus as aM&C PB167529GB14source of the leakage current based on a phase and / or magnitude of the detected leakage current signal.

14. The method of any of claims 10 to 13, wherein analysing the detected response signal comprises determining an insulation impedance of a conductor of the AC bus or an insulation impedance of a conductor of the DC bus based on the voltage of that conductor and the detected leakage current.

15. The method of claim 14, further comprising:comparing the determined insulation impedance of the conductor to a threshold value; andif the determined insulation impedance fails to satisfy an impedance criterion, generating an alert.

16. The method of any of claims 10 to 15, further comprising outputting a signal to isolate a portion of the hybrid power system determined to contain a fault.

17. The method of any of claims 10 to 16, further comprising outputting a signal to switch the hybrid power system to backup power or to shut down the hybrid power system in response to detecting a fault.

18. The method of any of claims 10 to 17, wherein the time-varying AC signal has a frequency lower than a line frequency of the AC bus.

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