Method for detecting ground fault and frequency converter system with ground fault detection

Abstract

A method for detecting a ground fault at a frequency converter (3) connected between a first power grid (1) operating at a first frequency (f1) and a second power grid (2) operating at a different second frequency (f2) includes: determining the common-mode voltage (U) of the side of the converter (3) connected to the second power grid (2). 2cm ), determine the common-mode voltage (U 2cm The converter (3) determines the level of the components of the first frequency (f1) and the second frequency (f2) in the converter and determines whether the level is higher than a preset threshold level (L1, L2), thereby indicating that there is a ground fault on the side of the converter (3) connected to the power grid (1, 2) operating at the corresponding frequency (f1, f2).

Description

Technical Field

[0001] This disclosure relates to a method for detecting ground faults near a frequency converter connected between two AC power grids. Furthermore, this disclosure relates to a frequency converter system with ground fault detection. Background Technology

[0002] Ground fault events at the converter terminals or inside the converter can cause serious damage to the converter and other connected electrical equipment due to the resulting high current or voltage. Therefore, accurate detection and rapid protective response to ground fault events are crucial. In particular, determining which side of the converter the ground fault occurs on is essential, allowing specific protective measures to be initiated for the affected side.

[0003] Prior art documents US 7663850 B2 and US 7978446 B2 disclose methods for detecting ground faults by using voltage sensors at the location where a ground fault occurs. For ground faults in converters, the voltage sensors are located on different sides of the converter. Summary of the Invention

[0004] Embodiments of this disclosure relate to an improved method for detecting grounding faults in AC-AC frequency converters.

[0005] According to a first aspect, a method for detecting a ground fault at a frequency converter connected between a first power grid and a second power grid, wherein the first power grid operates at a first frequency and the second power grid operates at a second frequency, wherein the first frequency is different from the second frequency, the method comprising determining a common-mode (CM) voltage on the side of the converter connected to one of the power grids (e.g., the second power grid). The method further comprises the steps of: determining the levels of components of the first and second frequencies in the CM voltage, and determining whether the level of the components is higher than a predefined threshold level, thereby indicating the presence of a ground fault on the side of the converter connected to the power grid operating at the corresponding frequency.

[0006] This method allows for the detection of ground faults on both sides of a converter using only voltage measurements from one side of the converter. Therefore, voltage measurements from the second grid side can also detect ground faults on the first grid side. Thus, a voltage sensor only needs to be installed on one side of the converter. This not only reduces costs but also improves detection reliability by reducing the number of potentially faulty components. Furthermore, less space is required to install the voltage sensor.

[0007] The first and second frequencies can be the base frequencies of the two AC power grids. For example, one grid might operate at 50 Hz, while the other might operate at 16.7 Hz. The power grids can be single-phase or multi-phase. For example, one grid might be three-phase, while the other is single-phase.

[0008] The detectable ground fault is an asymmetrical ground fault, meaning that this type of ground fault does not affect all phases of the power grid, and therefore its effects are not canceled out in the common-mode voltage.

[0009] For example, a ground fault may only exist in one or two phases of a three-phase power grid.

[0010] In the method, determining the level of the first frequency may include performing a first filtering procedure on the CM voltage, and determining the level of the second frequency may include performing a second filtering procedure on the CM voltage. Two different filtering procedures are used to extract the frequency components. The level of the component can then be determined by calculating an average value (e.g., the root mean square (RMS) value of the filtered voltage). Other values, such as the maximum value, may also be calculated. For example, a first RMS value is calculated after applying the first filtering procedure to the CM voltage on the second grid side, and a second RMS value is calculated after applying the second filtering procedure to that voltage.

[0011] A first filtering procedure for the CM voltage may include transmitting a first frequency and attenuating a second frequency. A second filtering procedure may include transmitting a second frequency and attenuating a first frequency. In both filtering procedures, one or more CM frequencies, such as the third harmonic of the fundamental frequency of the first or second grid, may also be attenuated. This is advantageous if CM harmonic injection is used on the first or second grid side in the converter's control algorithm.

[0012] For example, bandpass and bandstop filters with characteristic frequencies equal to the first or second frequency and their optional harmonics can be used. If CM harmonic injection is used on one side of the power grid, a bandstop filter with a characteristic frequency equal to the CM frequency of the power grid is used in both filtering procedures.

[0013] The method may further include activating protection measures on the side of the converter where a ground fault is detected. Therefore, depending on the detected ground fault, protection measures may be activated only at the first power grid, only at the second power grid, or at both power grids. Different protection measures may be implemented for the first and second power grids.

[0014] According to another aspect, a frequency converter system with selective ground fault detection includes: a frequency converter connected between a first power grid and a second power grid; a voltage sensor connected to the side of the converter connected to the second power grid for determining a common-mode voltage; and an analyzer for determining the levels of a first frequency and a second frequency component of the common-mode voltage, and determining whether the levels are higher than a preset threshold level. This frequency converter system can be configured to perform the method described above.

[0015] The frequency converter system may include a first protection device and a second protection device, which are used to perform protection measures on a first power grid and a second power grid, respectively. The analyzer is configured to provide a signal to the first protection device and / or the second protection device when a ground fault is detected on the corresponding power grid side. Alternatively, signals may be sent to both the first and second protection devices to perform protection measures when a ground fault is detected only on one power grid side.

[0016] This disclosure includes several aspects and embodiments. Each feature described with respect to one aspect and embodiment is also disclosed herein with respect to other aspects and embodiments, even if the corresponding feature is not explicitly mentioned in the context. Attached Figure Description

[0017] Further features, improvements, and conveniences will become clear from the following description of exemplary embodiments in conjunction with the accompanying drawings. In the drawings, elements that are structurally and / or functionally identical may be indicated by the same reference numerals. It should be understood that the embodiments shown in the drawings are illustrative representations and are not necessarily drawn to scale.

[0018] Figure 1 A schematic diagram of a converter system according to an embodiment is shown.

[0019] Figure 2 A schematic diagram of a method for detecting ground faults according to an embodiment is shown.

[0020] Figures 3A to 3F A graph of the common-mode voltage according to an embodiment is shown to illustrate the filtering procedure.

[0021] Figures 4A to 4D The diagram and method steps for detecting ground faults in a first power grid are shown.

[0022] Figures 5A to 5D The diagram and method steps for detecting ground faults in a second power grid are shown. Detailed Implementation

[0023] Figure 1A first power grid 1 and a second power grid 2 are shown, which are connected by a frequency converter 3 for AC-AC frequency conversion. The converter can have common topologies, such as AC-DC-AC with two-level or three-level NPC (neutral point clamp), as well as indirect MMC (modular multilevel converter) and AC-AC with direct MMC.

[0024] Depending on the application, each of power grids 1 and 2 includes one or more phases. For example, each of power grids 1 and 2 can be a three-phase power grid or a single-phase power grid (e.g., a railway power grid). Power grids 1 and 2 can have the same number of phases or different numbers of phases. As a specific example, the first power grid 1 can be three-phase with a base frequency of 50 Hz. The second power grid 2 can be single-phase with a base frequency of 16.67 Hz.

[0025] Power grids 1 and 2 are AC power grids, wherein, hereinafter, the fundamental frequency of the first power grid 1 is denoted as a first frequency f1, and the fundamental frequency of the second power grid 2 is denoted as a second frequency f2. The first frequency f1 and the second frequency f2 are different from each other. The fundamental frequency of each of power grids 1 and 2 can be a fixed frequency or a time-varying frequency. For time-varying frequencies, the frequencies of power grids 1 and 2 differ from each other at a given operating time. For example, time-varying frequencies can be used in variable machine drive buses. Both power grids are grounded with high impedance. For example, both power grids may operate at a constant frequency, or both power grids may operate at a time-varying frequency, or one power grid may operate at a constant frequency while the other operates at a time-varying frequency.

[0026] Ground fault events at the converter terminals or inside converter 3 can cause serious damage to converter 3 and other connected electrical equipment. For example, a single-phase ground fault can gradually lead to faults in other phases, resulting in high current or high voltage. Therefore, accurate detection and rapid protective response to ground fault events are crucial. Ideally, the system should be shut down before greater damage occurs. In particular, it is essential to distinguish which side of converter 3 the ground fault occurs on. This helps operators quickly identify and clear the fault, and allows for different protective measures to be taken for ground faults occurring on different sides of converter 3.

[0027] For this purpose, a voltage sensor 4 is installed on the side of converter 3 that is connected to one of the power grids 1 and 2. In the illustrated embodiment, the voltage sensor 4 is installed at the converter terminal connected to power grid 2. For the disclosed method, it is sufficient to install the voltage sensor 4 on only one power grid side, so that only the voltage on that side needs to be measured.

[0028] Analyzer 5 receives and analyzes the measured voltage. Frequency analyzer 5 can be an integral part of voltage sensor 4, or it can be separate from voltage sensor 4. Frequency analyzer 5 is configured to detect the presence of a ground fault and determine the grid side where the ground fault occurred. Therefore, ground faults are selectively detected.

[0029] As shown in the figure, other components may also exist in the frequency converter system 6. Typically, for isolation purposes, a transformer is connected between converter 3 and power grids 1 and 2. The transformer can also be considered part of converter 3. Alternatively, a transformer may not be present.

[0030] Figure 2 A method for analyzing measured voltages to perform selective ground fault detection is illustrated. In the first method step A, the common-mode (CM) voltage is calculated based on the voltage measured for grid 2. The CM voltage can be calculated directly from sensor 4 or analyzer 5. The voltage on the three phases a, b, and c of grid 2 is U. 2a U 2b U 2c A three-phase system with common-mode voltage U 2cm Calculated as

[0031]

[0032] For a two-phase system, the common-mode voltage is...

[0033]

[0034] During normal operation, the CM voltage of the second power grid 2 should contain only a small number of CM frequency components, such as DC components and 3rd and 6th harmonic components.

[0035] In the case of an asymmetrical ground fault (i.e., the ground fault does not exist in all phases of either power grid 1 or 2), the frequency component of the CM voltage in the second power grid 2 is:

[0036] - If a high-impedance single-phase-to-ground fault occurs on side 1 of the first power grid: the significant level of the fundamental frequency f1 of the first power grid (see...) Figure 3C , Figure 3D );

[0037] - If a high-impedance single-phase-to-ground fault occurs on side 2 of the first power grid: the significant level of the fundamental frequency f2 of the second power grid 2 (see...) Figure 3E , Figure 3F ).

[0038] In a further method step B, the filtering procedure extracts the aforementioned frequency components from the CM voltage.

[0039] The first filtering procedure 7 is used to detect ground faults on the first power grid side 1. The calculated CM voltage can be filtered by a second-order IIR (Infinite Impulse Response) filter.

[0040] Filtering program 7 may include the following filters:

[0041] - The characteristic frequency fc is equal to the fundamental frequency f1 of the first power grid 1 (i.e., f c =f1) bandpass filter (BPf1),

[0042] - Characteristic frequency f c Equal to the fundamental frequency f2 of the second power grid 2 (i.e., f c =f2) band-stop filter (BSf2); thus preventing false detection when a ground fault occurs on the second power grid 2,

[0043] -A band-stop filter (BSf) whose characteristic frequency fc is equal to the common-mode frequency of either the first power grid 1 or the second power grid 2. x_cm This is useful when CM harmonic injection is used on one side of the power grid 1 or 2 in the control algorithm of converter 3.

[0044] The second filtering program 8 can be used for ground fault detection on the second power grid side 2.

[0045] Filtering subroutine 8 may include the following filters:

[0046] - Characteristic frequency f c Equal to the fundamental frequency f2 of the second power grid 2 (i.e., f c =f2) bandpass filter (BP),

[0047] - Characteristic frequency f c Equal to the fundamental frequency f1 of the first power grid 1 (i.e., f c =f1) band-stop filter (BSf1); thus preventing false detection when a ground fault occurs on the first power grid 1,

[0048] - Characteristic frequency f c A band-stop filter (BSf) equal to the common-mode frequency of either the first power grid 1 or the second power grid 2. x_cm This is useful when CM harmonic injection is used on one side of the power grid 1 or 2 in the control algorithm of converter 3.

[0049] For power grids with variable frequencies, frequency adaptive filters, such as adaptive second-order IIR filters, can be used.

[0050] Gain compensation can be added to the above cascaded filters to set the total gain at the bandpass frequency to 1 and compensate for the attenuation caused by the filter itself.

[0051] In a further step C, characteristic values ​​of the amplitudes of the first and second frequency components are calculated based on the processed signal. For example, this value can be an RMS value, an instantaneous value, a maximum value, or an average value. The RMS value of signal S is calculated as follows:

[0052]

[0053] Next, determine whether the RMS value is equal to or higher than the preset threshold levels L1 and L2. An RMS value that is equal to or higher than the corresponding threshold levels L1 and L2 at a specific time or over a certain duration indicates that a ground fault error has occurred in the corresponding power grid.

[0054] Depending on the identified ground fault on one of the power grids, protective measures can be activated in step D. To do this, depending on the detected ground fault, a signal is sent to one or more ground fault protection devices 9, 10 to activate protective measures on the first power grid 1 or the second power grid 2. The analyzer 5 and one or more protection devices 9, 10 can also be integrated into the same device.

[0055] Figures 3A to 3F It shows Figure 1 The CMVU of the converter system 6 shown during normal operation and during ground faults 2cm A graph of frequency components.

[0056] The converter system 6 may include high-impedance grounding transformers on both sides. The converter 3 connects to a three-phase 50Hz system (first grid 1) and a single-phase 16.7Hz system (second grid 2). These grids are defined in reverse order. The voltage sensor 4 is located only at the converter terminals connected to the second grid 2. On the first grid 1 side, the converter modulates the injected third harmonic, i.e., 150Hz.

[0057] Figure 3A The figure shows the CM voltage U calculated for the second power grid 2 during normal operation. 2cm It is approximately zero during the monitoring time t.

[0058] Figure 3B It shows the normal operation period Figure 3A The signal's frequency domain is near zero at all frequencies.

[0059] Figure 3C and Figure 3D The CM voltage U is shown during a single-phase ground fault (1ph-G) on side 1 of the first power grid. 2cm The main frequency components are 50Hz and 150Hz.

[0060] Figure 3E and Figure 3FThe CM voltage U is shown during a single-phase ground fault (1ph-G) on side 2 of the second power grid. 2cm The main frequency component is 16.67 Hz.

[0061] Figures 4A to 4D It shows Figure 1 The CM voltage U of the converter system 6 shown is during normal operation and during ground faults occurring only on the first power grid 1. 2cm A graph of its frequency components.

[0062] Figure 4A The diagram shows the voltages on the three phases during a single-phase ground fault (1ph-G) on the first power grid side 1 at time t = 0.3s. The fundamental frequency f1 of the first power grid 1 is 50Hz.

[0063] Figure 4B It shows that in such Figure 4A The CM voltage U shown is when a single-phase ground fault occurs on side 1 of the first power grid. 2cm .

[0064] Figure 4C The CMVU after processing according to the first filtering procedure 7 for detecting ground faults on the first power grid 1 side is shown. 2cm The RMS value. The processed voltage is expressed as U. 2CM，F1，RMS Already applied. Figure 2 The first filtering procedure 7 described in step B has been performed, and the RMS value described in step C has been calculated.

[0065] The applied filtering path for ground faults on the first power grid 1 includes a 50Hz bandpass filter, a 16.7Hz bandstop filter, and a 150Hz bandstop filter.

[0066] The calculated RMS value U 2CM，F1，RMS Approaching 1. The predefined threshold level L1 for the first frequency component is 0.8. These figures are given in per-unit notation. The reference value can be the peak voltage relative to ground of the grid rated voltage. Due to the calculated RMS value U 2CM，F1，RMS The voltage level is higher than this level, therefore a ground fault is detected on the first power grid 1.

[0067] Figure 4D The CMVU after processing according to the second filtering procedure 8 for detecting ground faults on the second side of the second power grid is shown. 2cm The RMS value. The processed voltage is expressed as U. 2CM，F2，RMS Already applied. Figure 2 The second filtering procedure 8 described in step B has been performed, and the RMS value described in step C has been calculated.

[0068] The applied filtering path for ground faults on the second power grid 2 includes a 16.7Hz bandpass filter, a 50Hz bandstop filter, and a 150Hz bandstop filter.

[0069] The calculated RMS value U 2CM，F2，RMS After a small peak due to filter overshoot or filter surge, it approaches zero. The predefined voltage level L2 for a fault on the second grid 2 is also 0.8. This is due to the calculated RMS value U... 2CM，F2，RMS The voltage level is below this level, therefore no ground fault is detected on the second power grid 2.

[0070] Therefore, the ground fault is detected to exist only on the first power grid 1, and the corresponding protection measures for the first power grid 1, but not the second power grid 2, are triggered.

[0071] Figures 5A to 5D It shows Figure 1 The CMVU of the converter system 6 shown is measured during normal operation and during ground faults occurring only on the second power grid 2. 2cm And a graph of the extracted frequency components.

[0072] Figure 5A The diagram shows the voltages on two phases during a single-phase ground fault (1ph-G) on side 2 of the second power grid at time t = 0.3s. The fundamental frequency f2 of the second power grid 2 is 16.7Hz.

[0073] Figure 5B It shows that in such Figure 5A The CM voltage U shown is during a single-phase ground fault on side 2 of the second power grid. 2cm .

[0074] Figure 5C The CMVU after processing according to the first filtering procedure 7 for detecting ground faults on the first power grid 1 side is shown. 2cm The RMS value. Already applied to... Figure 4C The same filtering procedure and RMS value calculation are described in [the document / document].

[0075] The calculated RMS value U 2CM，F1，RMS After a brief peak, it approaches 0. This is due to the calculated RMS value U. 2CM，F1，RMS The level is below the predefined level L1 = 0.8, therefore no ground fault is detected on the first power grid 1.

[0076] Figure 5D It shows according to Figure 4D Description of the processed CM voltage U 2cm The RMS value. The calculated RMS value U 2CM，F2，RMS It is higher than the predefined level L2 = 0.8.

[0077] Therefore, the ground fault was detected to exist only on the second power grid 2 and the corresponding protection measures were triggered.

[0078] In some embodiments, the predefined voltage levels L1 and L2 used to detect grounding faults in the first power grid 1 and the second power grid 2 can be different from each other.

[0079] In summary, the disclosed method and converter system 6 allow for the selective detection of ground faults on the first power grid 1 side by using only voltage measurements on the converter 3 side. Figures 4A to 4D ) and ground faults on the second side of the power grid ( Figures 5A to 5D ).

[0080] Figure Labels

[0081] 1 First Power Grid

[0082] 2 Second Power Grid

[0083] 3 Converters

[0084] 4. Voltage sensor

[0085] 5. Frequency Analyzer

[0086] 6. Converter System

[0087] 7 First Filtering Procedure

[0088] 8. Second Filtering Procedure

[0089] 9. Protective devices

[0090] 10. Protective devices

[0091] f1 First frequency

[0092] f2 First frequency

[0093] U 2a U 2b U 2c Voltage of the three phases on the second AC power grid

[0094] U 2cm Common-mode voltage on the second AC power grid

[0095] U 2CM，F2，RMS The RMS value of the filtered common-mode voltage, used for filters in power grid 2 faults.

[0096] U 2CM，F1，RMS The RMS value of the filtered common-mode voltage, used for filters in power grid 1 faults.

[0097] BPf1 bandpass filter with characteristic frequency f1

[0098] A bandpass filter with characteristic frequency f2.

[0099] BSf1 band-stop filter with characteristic frequency f1

[0100] BSf2 bandstop filter with characteristic frequency f2

[0101] BSf x_cm Band-stop filter with characteristic frequency of CM frequency of power grid 1 or 2

[0102] RMS rms value

[0103] f c Filter characteristic frequency

[0104] Methods and steps A, B, C, and D

Claims

1. A method for detecting a ground fault at a frequency converter (3) connected between a first power grid (1) and a second power grid (2), wherein, The first power grid (1) operates at a first frequency (f1), and the second power grid (2) operates at a different second frequency (f2). Determine the common-mode voltage (U) at the side of the converter (3) connected to one of the power grids (1, 2). 2cm ), determine at the common-mode voltage (U 2cm The converter (3) determines the levels of the components of the first frequency (f1) and the second frequency (f2) in the converter and determines whether the levels are higher than preset threshold levels (L1, L2), thereby indicating that there is a ground fault on the side of the converter (3) connected to the power grid (1, 2) operating at the corresponding frequencies (f1, f2).

2. The method according to claim 1, in, Determining the level of the component of the first frequency (f1) includes determining the level of the common-mode voltage (U). 2cm The first filtering procedure (7) is applied, and wherein determining the level of the component of the second frequency (f2) includes adjusting the common-mode voltage (U). 2cm Apply the second filtering procedure (8).

3. The method according to claim 2, in, Determining the level of the component involves calculating the average or maximum value after applying the first filtering procedure (7) and after applying the second filtering procedure (8).

4. The method according to any one of claims 2 and 3, in, In the first filtering procedure (7), the first frequency (f1) is transmitted and the second frequency (f2) is attenuated, and in the second filtering procedure (8), the second frequency (f2) is transmitted and the first frequency (f1) is attenuated.

5. The method according to any one of claims 2 to 4, in, The first filtering procedure (7) includes using the characteristic frequency (f c ) is the bandpass filter (BPf1) at the first frequency (f1) and the characteristic frequency (f) c The second filtering procedure (8) includes using a band-stop filter (BSf2) at the second frequency (f2), and the second filtering procedure (8) includes using a characteristic frequency (f c The bandpass filter (BPf2) is equal to the second frequency (f2) and the bandstop filter (BSf1) has the characteristic frequency of the first frequency (f1).

6. The method according to any one of claims 2 or 5, in, The first filtering procedure (7) and the second filtering procedure (8) include using the characteristic frequency (f) c A band-stop filter (BSf) equal to the harmonics of the first frequency (f1) and / or the second frequency (f2). x_cm ).

7. The method according to any one of claims 2 to 6, in, Gain compensation is performed after the first filtering procedure (7) and the second filtering procedure (8) to set the total gain of the transmitted frequency to 1.

8. The method according to any one of the preceding claims, The first threshold level (L1) is different from the second threshold level (L2).

9. The method according to claims 1 to 7, in, The first threshold level (L1) is equal to the second threshold level (L2).

10. The method as described in any of the preceding claims, This includes activating protective measures on the side of the converter (3) where the grounding fault is indicated or on both sides of the converter (3).

11. The method as described in any of the preceding claims, in, At least one of the first power grid (1) and the second power grid (2) operates at a time-varying frequency.

12. The method as described in any of the preceding claims, in, At least one of the first power grid (1) and the second power grid (2) operates at a fixed frequency.

13. A frequency converter system (6) with selective ground fault detection, the frequency converter system comprising: A frequency converter (3) connected between a first power grid (1) configured to operate at a first frequency (f1) and a second power grid (2) configured to operate at a second frequency (f2); and a voltage sensor (4) for determining the common-mode voltage (U). 2cm The voltage sensor (4) is connected between the frequency converter (3) and one of the power grids (1, 2); and the analyzer (5) is used to determine the level of the components of the first frequency (f1) and the second frequency (f2) in the common-mode voltage, and to determine whether the level is higher than a preset threshold level (L1, L2).

14. The frequency converter system (6) according to claim 13, in, The voltage sensor is installed only on one side of the converter (3).

15. The frequency converter system (6) according to any one of claims 13 or 14, comprising a first protection device (9) and a second protection device (10), the protection devices being used to perform protection measures on the first power grid (1) and the second power grid (2) respectively, wherein, The analyzer (5) is configured to provide a signal to the first protection device (9) and / or the second protection device (10) when a ground fault is detected on the corresponding grid side, or is configured to provide a signal to both protection devices (9, 10).

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