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

Abstract

A method of detecting a ground fault at a frequency converter (3) connected between a first electrical network (1) operating at a first frequency (f1) and a second electrical network (2) operating at a second, different frequency (f2) comprises determining a common mode voltage (U2 cm) at a side of the converter (3) connected to the second electrical network (2), determining levels of components of the first frequency (f1) and the second frequency (f2) in the common mode voltage (U2 cm), and determining whether the level is higher than a predetermined threshold level (L1, L2) in order to indicate the presence of a ground fault on the side of the converter (3) connected to the electrical network (1, 2) operating at the respective frequency (f1, f2).

Description

Technical Field

[0001] The present disclosure relates to a method for detecting a ground fault in the vicinity of a frequency converter connected between two AC grids. Furthermore, the present disclosure relates to a frequency converter system with ground fault detection. Background Art

[0002] Ground fault events at the converter terminals or inside the converter can cause severe damage to the converter and other connected electrical equipment due to the high current or voltage generated. Therefore, accurate detection and fast protection reaction to ground fault events are critical. In particular, it is critical to determine on which side of the converter the ground fault occurred. This allows specific protection measures to be initiated for the affected side.

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

[0004] Embodiments of the present disclosure relate to an improved method of detecting a ground fault in an AC-AC frequency converter.

[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 at a 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 levels of components of the first frequency and the second frequency in the CM voltage, and determining whether the levels of the components are above a predefined threshold level, thereby indicating that a ground fault exists at the side of the converter connected to the power grid operating at the corresponding frequency.

[0006] The method allows to detect ground faults on both sides of the converter by using only the voltage measurement on one side of the converter. Thus, the voltage measurement on the second grid side also enables to detect ground faults on the first grid side. Therefore, it is only necessary to provide a voltage sensor on one side of the converter. This not only reduces the cost, but also improves the reliability of the detection, since there are fewer components that may fail. In addition, the space required to install the voltage sensor is also smaller.

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

[0008] Detectable ground faults are asymmetrical ground faults, ie such ground faults do not affect all phases of the network and therefore their effects do not cancel out in the common mode voltage.

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

[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, such as a root mean square (RMS) value of the filtered voltage. Other values, such as a maximum value, can also be calculated. For example, a first RMS value is calculated after applying a first filtering procedure to the CM voltage on the second grid side, and a second RMS value is calculated after applying a second filtering procedure to the voltage.

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

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

[0013] The method may further comprise initiating a protective measure for the side of the converter where the ground fault is detected. Thus, depending on the detected ground fault, the protective measure may be initiated only at the first grid, only at the second grid, or at both grids. Different protective measures may be performed for the first grid and the second grid.

[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 a side of the converter connected to the second power grid, for determining a common mode voltage; and an analyzer, for determining levels of components of a first frequency and a second frequency in the common mode voltage, and determining whether the levels are higher than a preset threshold level. The frequency converter system can be configured to perform the method as 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 the first power grid and the second power grid respectively, wherein 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. It is also possible to send a signal to the first protection device and the second protection device to perform the protection measure when a ground fault is detected only on one power grid side.

[0016] The present disclosure comprises several aspects and embodiments. Each feature described with respect to one of the aspects and embodiments is herein also disclosed with respect to the other aspects and embodiments, even if the corresponding feature is not explicitly mentioned in the context. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0018] Figure 1 shows a schematic diagram of a converter system according to an embodiment,

[0019] Figure 2 A schematic diagram showing a method for detecting a ground fault according to an embodiment is shown,

[0020] FIG. 3A to FIG. 3F A diagram showing a common mode voltage according to an embodiment for illustrating the filtering procedure,

[0021] 4A to 4D A diagram and method steps are shown for detecting a ground fault in a first electrical network,

[0022] FIG. 5A to FIG. 5D A diagram and method steps are shown for detecting a ground fault in a second electrical network. DETAILED DESCRIPTION

[0023] Figure 1A first grid 1 and a second grid 2 are shown, which are connected via 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 Clamped), as well as indirect MMC (Modular Multilevel Converter) and AC-AC with direct MMC.

[0024] Depending on the application, each of the power grids 1, 2 includes one phase or more than one phase. For example, each of the power grids 1, 2 can be a three-phase power grid or a single-phase power grid (e.g., a railway power grid). The power grids 1, 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] The power grids 1 and 2 are AC power grids, wherein, hereinafter, the fundamental frequency of the first power grid 1 is represented as the first frequency f1, and the fundamental frequency of the second power grid 2 is represented as the second frequency f2. The first frequency f1 and the second frequency f2 are different from each other. The fundamental frequency of each of the power grids 1 and 2 may be a fixed frequency or a time-varying frequency. For the time-varying frequency, the frequencies of the power grids 1 and 2 are different from each other at a specific operating time. For example, the time-varying frequency can be used in a variable machine drive bus. Both power grids are grounded with high impedance. For example, both power grids operate at a constant frequency, or both power grids operate at a time-varying frequency, or one of the power grids operates at a constant frequency and the other power grid operates at a time-varying frequency.

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

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

[0028] The analyzer 5 receives and analyzes the measured voltage. The frequency analyzer 5 may be an integral part of the voltage sensor 4 or may be separate from the voltage sensor 4. The frequency analyzer 5 is configured to detect the presence of a ground fault and determine the grid side where the ground fault occurs. Thus, the ground fault is selectively detected.

[0029] As shown in the figure, there may be other components in the frequency converter system 6. Usually, for isolation considerations, a transformer is connected between the converter 3 and the power grids 1 and 2. The transformer may also be referred to as a part of the converter 3. The transformer may also not exist.

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

[0031]

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

[0033]

[0034] In normal operation, the CM voltage of the second power grid 2 should contain only a small amount of CM frequency components, such as DC components and third-order, sixth-order, and other harmonic components.

[0035] In the case of an asymmetrical ground fault (ie, the ground fault does not exist in all phases of one of the grids 1, 2), the frequency component in the CM voltage of the second grid 2 is:

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

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

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

[0039] The first filtering procedure 7 is used to detect a ground fault on the side of the first grid 1. The calculated CM voltage may be filtered by a 2nd order IIR (Infinite Impulse Response) filter.

[0040] The filter 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 (ie, f c =f1) of the bandpass filter (BPf1),

[0042] - characteristic frequency f c is equal to the fundamental frequency f2 of the second power grid 2 (ie, f c = f2) of the band-stop filter (BSf2); thereby preventing false detection when a ground fault occurs on the second power grid 2,

[0043] - a band-stop filter (BSf) having a characteristic frequency fc equal to the common mode frequency of 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 grids 1 , 2 in the control algorithm of the converter 3 .

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

[0045] The filtering subroutine 8 may include the following filters:

[0046] - characteristic frequency f c is equal to the fundamental frequency f2 of the second power grid 2 (ie, f c =f2) of the bandpass filter (BP),

[0047] - characteristic frequency f c is equal to the fundamental frequency f1 of the first power grid 1 (ie, f c = f1) of the band-stop filter (BSf1); thereby preventing false detection when a ground fault occurs on the first grid 1,

[0048] - characteristic frequency f c A band-stop filter (BSf) equal to the common mode frequency of 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 grids 1 , 2 in the control algorithm of the converter 3 .

[0049] For power grids with variable frequency, frequency adaptive filters such as adaptive 2nd order IIR filters can be used.

[0050] Gain compensation can be added to the above cascaded filters to set the overall gain to unity at the frequencies after the bandpass and compensate for the attenuation caused by the filters themselves.

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

[0052]

[0053] Afterwards, it is determined whether the RMS value is equal to or higher than a preset threshold level L1, L2. An RMS value equal to or higher than the corresponding threshold level L1, L2 at a specific time or for a duration indicates that a ground fault error occurs in the corresponding power grid.

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

[0055] FIG. 3A to FIG. 3F Shows Figure 1 The CM voltage U of the converter system 6 during normal operation and during a ground fault is shown. 2cm and frequency components.

[0056] The converter system 6 may include high impedance grounded transformers on both sides. The converter 3 connects a three-phase 50 Hz system (first grid 1) and a single-phase 16.7 Hz system (second grid 2). The definitions of these grids are the same in reverse. The voltage sensor 4 is only located at the converter terminals connected to the second grid 2. On the first grid 1 side, the converter modulation injects the 3rd harmonic, i.e. 150 Hz.

[0057] Figure 3A It is shown that during normal operation, the CM voltage U calculated for the second grid 2 2cm During the monitoring time t it is approximately zero.

[0058] Figure 3B Shows the normal operation period Figure 3A The frequency domain of the signal is also around zero at all frequencies.

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

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

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

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

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

[0064] Figure 4C shows the CM voltage U after processing according to the first filtering procedure 7 for detecting a ground fault on the first grid 1 side. 2cm The processed voltage is represented by U 2CM，F1，RMS . 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 filter path used for ground faults on the first power grid 1 comprises a 50 Hz band pass filter, a 16.7 Hz band stop filter and a 150 Hz band stop filter.

[0066] The calculated RMS value U 2CM，F1，RMS is close to 1. The predefined threshold level L1 of the first frequency component is 0.8. These figures are given in per unit. The reference value can be the peak value of the rated relative voltage of the power grid. Since the calculated RMS value U 2CM，F1，RMS Above this level, the presence of a ground fault on the first electrical network 1 is therefore detected.

[0067] Figure 4D The CM voltage U is shown after being processed according to the second filtering procedure 8 for detecting a ground fault on the second grid 2 side. 2cm The processed voltage is represented by U 2CM，F2，RMS . 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 filter path used for ground faults on the second grid 2 comprises a 16.7 Hz bandpass filter, a 50 Hz bandstop filter and a 150 Hz 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. Due to the calculated RMS value U 2CM，F2，RMS Below this level, it is therefore detected that no ground fault is present on the second electrical network 2 .

[0070] Therefore, it is detected that the ground fault exists only on the first grid 1 , and corresponding protective measures are triggered for the first grid 1 but not for the second grid 2 .

[0071] FIG. 5A to FIG. 5D Shows Figure 1 The CM voltage U 2 of the converter system 6 is shown during normal operation and only during a ground fault on the second grid 2 . 2cm And a plot of the extracted frequency components.

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

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

[0074] Figure 5C shows the CM voltage U after processing according to the first filtering procedure 7 for detecting a ground fault on the first grid 1 side. 2cm The RMS value of Figure 4C The same filtering procedure and RMS value calculation as described in .

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

[0076] Figure 5D Shows that according to Figure 4D Description of the processed CM voltage U 2cm The calculated RMS value U 2CM，F2，RMS Above the predefined level L2=0.8.

[0077] Therefore, it is detected that the ground fault is present only on the second electrical network 2 and corresponding protective measures are triggered.

[0078] In some embodiments, the predefined voltage levels L1 , L2 for detecting a ground fault of the first power grid 1 and the second power grid 2 may be different from each other.

[0079] In summary, the disclosed method and converter system 6 allow selective detection of a ground fault on the first grid 1 side by using only voltage measurements on the converter 3 side. 4A to 4D ) and a ground fault on the second grid 2 side ( FIG. 5A to FIG. 5D ).

[0080] Reference numerals

[0081] 1. First Grid

[0082] 2 Second Grid

[0083] 3 Converter

[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 The voltage of the three phases on the second AC grid

[0094] U 2cm Common mode voltage on the second AC mains

[0095] U 2CM，F2，RMS RMS value of filtered common mode voltage, filter for grid 2 fault

[0096] U 2CM，F1，RMS RMS value of filtered common mode voltage, filter for grid 1 fault

[0097] BPf1 is a bandpass filter with characteristic frequency f1

[0098] BPf2 is a bandpass filter with characteristic frequency f2

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

[0100] BSf2 is a band-stop filter with characteristic frequency f2.

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

[0102] RMS rms value

[0103] f c Characteristic frequency of the filter

[0104] A, B, C, D method steps

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), Determining a common mode voltage (U) at a side of the converter (3) connected to one of the power grids (1, 2) 2cm ), determine the common mode voltage (U 2cm ) and determining whether the levels are higher than preset threshold levels (L1, L2), thereby indicating that a ground fault exists on one side of the converter (3) connected to the grid (1, 2) operating at the corresponding frequency (f1, f2).

2. The method according to claim 1, in, Determining the level of the component of the first frequency (f1) includes determining the common mode voltage (U 2cm ) applies a first filtering procedure (7), and wherein determining the level of the component of the second frequency (f2) comprises filtering 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 comprises calculating an average value or a 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) comprises using the characteristic frequency (f c ) is a bandpass filter (BPf1) of the first frequency (f1) and a characteristic frequency (f c ) is a band stop filter (BSf2) for the second frequency (f2), and the second filtering procedure (8) comprises using the characteristic frequency (f c ) is equal to the second frequency (f2) of the band pass filter (BPf2) and has a characteristic frequency of the first frequency (f1) of the band stop filter (BSf1).

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 a characteristic frequency (f c ) is equal to a harmonic of the first frequency (f1) and / or the second frequency (f2) of the band-stop filter (BSf 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 overall gain of the transmitted frequencies to unity.

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 according to any one of the preceding claims, The method comprises initiating protective measures on one side of the converter (3) indicating the presence of the ground fault or on both sides of the converter (3).

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

12. A method as claimed in any one 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); a voltage sensor (4) for determining a 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 an analyzer (5), the analyzer is used to determine the levels of the components of the first frequency (f1) and the second frequency (f2) in the common mode voltage, and determine whether the levels are higher than a preset threshold level (L1, L2).

14. The frequency converter system (6) according to claim 13, in, The voltage sensor is installed on only 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 to provide a signal to both protection devices (9, 10).

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