System and method for detecting faults in medium voltage circuits

Abstract

A fault detection system (800) for a medium voltage circuit comprises a power distribution cable (205), a first transformation center (210) comprising a first transformer and a first grounding conductor (603), a first grounding system comprising a first grounding resistor (RPAT1) connected to the first transformer via the first grounding conductor (603), the first transformation center (210) being connected to a first end of the cable (205), a second transformation center (220) comprising a second transformer and a second grounding conductor (603), a second grounding system comprising a second grounding resistor (RPAT2) connected to the second transformer via the second grounding conductor (603), the second transformation center (220) being connected to a second end of the cable (205).

Description

[0001] Purpose of the Invention

[0002] The object of the present invention is a method and system for detecting faults in an insulated power distribution cable. Background Art

[0003] Currently, there are various methods, devices and systems for detecting and locating faults in power distribution cables.

[0004] Existing methods, devices and systems either involve continuous monitoring of the power lines (phases) of a medium voltage section, or generate excitation signals of different nature, which are applied to the cable or a part thereof after a fault occurs, and from which an effect is generated that can be measured by the equipment and is related to the location of the fault.

[0005] Some known methods involve detecting pressure or sound waves when a current pulse is applied to the cable, arc generation by applying a high voltage directly to the cable, reflection of high-frequency waves, and the like.

[0006] In the case of methods, devices and systems based on continuous monitoring of power lines (phases) in the medium-voltage section, the main drawback of these methods, devices and systems, in addition to the complexity of the monitoring system itself, is the complexity of the insulation when working at high voltage levels. Since the power is distributed in a three-phase system, this means that monitoring must be carried out on each of the three phases that make it up. This means that the monitoring equipment for the medium-voltage network is bulky and complex.

[0007] In the case of methods, devices and systems based on the generation of excitation signals and subsequent processing of the effects produced by the signals, they have the disadvantage of requiring direct access to the cables and disconnection from the power supply network, which hampers operation and prolongs the time required for positioning. Summary of the Invention

[0008] In electrical power distribution systems, insulated cables are commonly used that run underground in most cases, especially in urban areas. Figure 1 As shown, these power distribution cables (101) are made up of an electrical conductor (101) through which the electricity flows and composed of different layers that are coaxial with the main conductor until they reach the cover (103). One of the layers of these cables is a screen or mesh (102), also made of conductive material, which is used as a ground terminal, that is, it is connected to the grounding system.

[0009] It is possible that the internal insulation of the cable has degraded for various reasons and that a conductive path has developed between the conductor (101) and the mesh (102) connected to the ground, usually by arcing, generating a so-called fault current. The grid protection system is responsible for taking action when this situation is detected in the network, de-energizing the network, thereby minimizing the resulting damage and allowing the relevant repairs to be carried out before implementing a network reset.

[0010] From a network operability perspective, the time from the moment a fault occurs and the network loses service until service is restored should be as short as possible. This time consists of three main components: travel time for the repair personnel, fault location time, and fault repair time.

[0011] The main purpose of the fault detection method and system according to the present invention is to reduce the fault location time of a medium voltage cable, thereby improving the service quality index of the network operator.

[0012] The method according to the invention differs from methods based on continuous monitoring of power lines and from methods based on generating an excitation signal after a fault has occurred. In particular, the method according to the invention is based on continuous monitoring of the current flowing through the grounding conductor connecting the metal structure of the substation transformer to the grounding system.

[0013] Thus, in a first aspect, the present invention relates to a method for locating faults in a power distribution cable, the method being applicable to a medium voltage circuit, comprising at least one power distribution cable, a first transformer center consisting of a first transformer and a first grounding conductor, a first grounding system consisting of a first grounding resistor, the first grounding system being connected to the first transformer via the first grounding conductor. The first transformer center is connected to a first end of the cable. The medium voltage circuit further comprises a second transformer center consisting of a second transformer and a second grounding conductor, the second grounding system comprising a second grounding resistor connected to the second transformer via the second grounding conductor. The second transformer center is connected to a second end of the cable.

[0014] The method comprises identifying a fault in the cable by monitoring a current threshold in at least one ground conductor, and in case of identifying a fault, measuring a first current value I flowing through the first ground conductor. med1 , obtain the value of the first resistor, and measure the second current value I passing through the second grounding conductor med2 , obtain the value of the second resistor and calculate the relative distance d1 from the point where the fault occurs on the cable to the center of the first transformation, so that:

[0015]

[0016] The method further includes calculating a distance d2 from the point of the cable where the fault occurs to the second transformation center;

[0017]

[0018] The method also includes determining a location of the fault based on the distances d1 and d2.

[0019] This method uses comparative measurements from sensor devices distributed across each transformer center that is part of the monitored circuit. The advantages of this method are that it detects and locates faults in real time, without intervention, in an unattended manner, and uses compact equipment that does not require the necessary insulation to monitor medium voltage lines, thus significantly reducing its size.

[0020] In a second aspect, the present invention relates to a fault detection system for a medium voltage line, the system comprising several groups of power distribution cables, a first transformation center comprising a first transformer and a first grounding conductor, and a first grounding system comprising a first grounding resistor, the first grounding system being connected to the first transformer via the first grounding conductor. The first transformation center is connected to a first end of the cable. The medium voltage circuit further comprises a second transformation center comprising a second transformer and a second grounding conductor, a second grounding system comprising a second grounding resistor connected to the second transformer via the second grounding conductor. The second transformation center is connected to a second end of the cable. The fault detection system comprises a set of sensors comprising wireless communication means and a processing unit, wherein the first sensor comprises a means for connecting to the first grounding conductor and is configured to detect a cable fault by detecting a threshold current passing through the first grounding conductor. In the event of fault identification, the first sensor is configured to measure a first current value I passing through the first grounding conductor med1 , obtain the value of the first resistor and set I med1 and transmitted to the processing unit. In the system, the second sensor includes a device connected to the second ground conductor and is configured to detect a cable fault by detecting a threshold current through the second ground conductor. In the event of fault identification, the second sensor is configured to measure a second current value I med2 , obtain the value of the second resistor and set I med2 The processing unit stores instructions for calculating the relative distance d1 from the point where the fault occurs on the cable to the first transformation center, such that:

[0021]

[0022] The processing unit also stores instructions to calculate the distance d2 from the point on the cable where the fault occurred to the second transformation center such that:

[0023]

[0024] The location of the fault is determined based on the distances d1 and d2. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] To supplement the ongoing description and to help better understand the characteristics of the method and system for detecting faults in a medium voltage cable according to a preferred example of its practical implementation, a set of drawings is attached as an integral part of said description, in which, for illustrative and non-limiting purposes, the following are represented:

[0026] Figure 1 Showing components of a medium voltage cable.

[0027] Figure 2 A medium voltage circuit is shown in a ring configuration.

[0028] Figure 3 A medium voltage circuit in a ring configuration is shown in a fault condition.

[0029] Figure 4 Circuit showing fault current.

[0030] Figure 5 Shows the transient behavior of the fault current.

[0031] Figure 6 An example of sensors distributed according to the present invention is shown.

[0032] Figure 7 An example of a sensor according to the present invention is shown.

[0033] Figure 8 An example of a fault detection system according to the present invention is shown. DETAILED DESCRIPTION

[0034] Figure 2 A medium voltage circuit (200) is shown with a ring structure having a plurality of cables (205), supplied by an outlet of a transformer substation (201), and having four transformer centers (210, 220, 230, 240) as part of a fault detection system. Each transformer center (210, 220, 230, 240) is associated with a grounding system, represented in the figure below by grounding resistors (RPAT1-4) and grounding conductors (603) of transformers, allowing the metallic elements of the transformers of the transformer centers (210, 220, 230, 240) to be connected to the grounding system represented by (RPAT1-4).

[0035] Specific to one of the cables (205) of the circuit (200) of the present invention, a first transformation center (210) can be seen, which includes a first transformer and a first grounding resistor (RPAT1) as part of a first grounding system connected to the first transformation center (210). The first transformation center (210) is in turn connected to a first end of the cable (205). The grounding conductor (603) of the transformer allows the metal components of the transformation center (210) to be connected to the grounding system represented by the resistor (RPAT1).

[0036] Furthermore, the fault detection system further comprises a second transformer center (220) comprising a second transformer and a second grounding resistor (RPAT2), which is connected to the second transformer via a grounding conductor (603) as part of a second grounding system. The second transformer center (220) is further connected to the second end of the cable (205).

[0037] In normal operation (no faults), the current through the transformer grounding conductor is only a few milliamperes. However, when a fault occurs, a conductive path is created between the main conductor of the cable (205) and its mesh. As a result, some of the power flows through the mesh cable (205) to the ends of the cable (205). Its mesh is connected to the resistors (RPAT1, RPAT2) of the grounding system. These resistors are connected to the two transformation centers (210, 220) located at the ends of the cable (205), resulting in an increase in the current value of the transformer grounding conductor (603) in the transformation centers (210, 220). The impedance that "sees" the current in both directions should determine the amount of current flowing in each direction.

[0038] The current value in each direction shall depend on the fault current value and other parameters such as network configuration, cable length, neutral system, fault type, location of fault occurrence and the value of the grounding resistor of the grounding system at each transformation center.

[0039] like Figure 3 As shown, under fault conditions, a fault current I fault , it will bifurcate in two opposite directions, toward the two transformation centers (210, 220) located at both ends of the cable.

[0040] The calculation method for the location of power distribution cable faults is based on Figure 4 The following electrical equivalents are shown, where the fault current I fault is divided into two currents I1 and I2. In turn, these two currents are divided into two parts, so:

[0041] I1=Imed1+IC1;

[0042] I2=Imed2+IC2

[0043] Imed1 and Imed2 are the currents flowing through the ground conductor and are therefore the currents measured by the distributed sensor devices.

[0044] In order to perform the circuit analysis, it is considered that the values ​​of L1, R1, C1, L2, R2 and C2 are obtained by multiplying the distributed impedance values ​​of the cable (205) (L, R and C, whose values ​​are provided by the cable manufacturer) by the distance value from the fault occurrence point to the first transformation center (210) (d1) or the second transformation center (220) (d2), in this way, the following ratio is obtained:

[0045]

[0046]

[0047] If circuit analysis is performed, the following developments can be obtained:

[0048]

[0049]

[0050] When considering the usual values ​​of R, L and C in medium voltage cables, we find the following orders of magnitude:

[0051] parameter Order of magnitude of values L(H / m) <![CDATA[10 -6 ]]> R(Ω / m) <![CDATA[10 -4 ]]> C(F / m) <![CDATA[10 -10 ]]> Cable length(m) <![CDATA[10 2 -10 3 ]]> Frequency(Hz) <![CDATA[10 1 ]]>

[0052] This means that the The previous expression can be simplified to approximately 1:

[0053]

[0054]

[0055] By putting I med1 Divide by I med2 , we can see that:

[0056]

[0057] If the lengths d1 and d2 are normalized to unity, the total length of the cable that failed will be 1y, so d1 + d2 = 1. Taking this into account, the previous expression becomes:

[0058]

[0059] By solving the previous expression for d1 and taking into account R PATX >>>Ljω+R, which is:

[0060]

[0061] Since L, R, and ω are constants, we can define the term K so that the previous expression becomes:

[0062]

[0063] Therefore, you can solve the problem of d2 as:

[0064]

[0065] K = 1 can be used as an effective constant in most cases. Therefore, it can be seen that starting from measuring the current of each ground conductor (603) of each switching center (210, 220), plus the ground resistor R when each switching center is faulted PAT1 、R PAT2 The relative distances where the fault occurred, d1 and d2, can be determined by using the values ​​of

[0066] Figure 5 shows the fault current I fault (500) behavior, which means that at the moment of fault occurrence, there is an initial moment of high-frequency transient phenomena, followed by a static period characterized by the grid frequency signal.

[0067] For this reason, the impedance of the grounding system, represented in the diagram by (RPAT1-4) at the initial time, where the frequency content of the transient current is high, will have a contribution from a resistive component, but also a significant reactive component (capacitive, inductive, or a combination of both), so it is best to be able to determine the values ​​of R, L, and C for each component that makes up the grounding system in order to describe the entire phenomenon, not just the steady state. One way to obtain these equivalents is by realizing a Bode plot of magnitude and phase, for which it is necessary to measure the impedance value and the phase shift between voltage and current at different frequencies (frequency sweep).

[0068] However, it is envisaged that the positioning method of the present invention may include means suitable for acquiring and processing other data from the network itself, such as data relating to the length or segments of the cable, the insulation type of each segment, the path of the line, junction points, etc., in order to achieve a more accurate network model and improve the positioning and identification of faults.

[0069] Based on the above, the fault detection system (800) includes:

[0070] A set of distributed sensors (600) is provided, each sensor being installed in a ground conductor (603) of each transformer center (210, 220, 230, 240) of a circuit (200) to be monitored via a transformer (602), wherein each distributed sensor (600) is configured to continuously monitor a current Imed in the ground conductor (603) and detect a fault in a cable (205) by a current threshold, and to determine a ground resistor (RPAT1-4) at the moment of occurrence of a fault in the cable (205), using a frequency sweep to determine a resistive part and a reactive part (L and C) to obtain a Bode plot of amplitude and phase.

[0071] Furthermore, each distributed sensor (600) is configured to store data internally so that it can be transmitted remotely to a processing unit (818).

[0072] Each distributed sensor (600) includes a wireless transmission device (819) for remotely transmitting data acquired by each sensor device (600) to a remote processing unit (18). Each distributed sensor (600) includes a synchronization system (821) so that data from different distributed sensors have a single time basis when being processed.

[0073] The system (800) includes a processing unit (818) where data is stored and grouped and where analysis can be performed on the received data in real time to obtain distance values ​​d1 and d2.

[0074] The system (800) includes alarm and display means, preferably an alarm system, which can indicate the fault condition and the approximate location of the fault to the user, and the distance values ​​d1 and d2 are obtained from the measurements recorded by the sensor device (600) and their analysis.

[0075] Figure 6 An example of a distributed sensor (600) according to the present invention is shown. The sensor (600) comprises a transformer (602) through which a ground conductor (603) of a transformer at a power substation passes. The electronics of the sensor device (600) are contained in an enclosed housing (604).

[0076] Figure 7 The figure shows a converter element (602) connected to a sensor (600). The converter element (602) consists of two toroidal magnetic cores (605) and (606) through which a ground conductor (603) passes. The magnetic core (605) induces an electromotive force (emf) in the conductor to be monitored via an injection loop set (608), while the other toroid (606) records the current value generated by the emf in the conductor via a measurement loop set (609), which depends on the impedance of the conductor (603). In order to be able to determine the induced voltage in the conductor (603), an additional loop (607) is wound around the measurement core (606).

[0077] Figure 8 The various parts of the system (800) are shown. In particular, Figure 8 Shown are a distributed sensor (600), a transducer (602) as an interface element between the distributed sensor (600) and a ground cable (603), and a processing unit (818) of the system (800).

[0078] like Figure 8 As shown, the distributed sensor (600) includes:

[0079] - a continuous current monitoring system (810) connected to the set of measuring circuits (609), which detects fault conditions by subsequent comparison with programmable thresholds (811).

[0080] - A programmable amplification system (817) allowing automatic range adjustment and connected to the output of the filtering system (815). The programmable gain amplifier (817) and the programmable threshold (811) allow it to be adjusted locally and remotely and allow optimization of fault detection and location in environments with different electrical noise levels and with different expected values ​​of fault currents.

[0081] - a circular buffer (812) connected to a continuous current monitoring system (810) and a programmable threshold (811), configured to, when a fault condition is detected, record at least 10 ms before and 300 ms after the instant the fault occurs, and send a set of recorded data to a storage system (813).

[0082] - A multi-frequency current injection system (814) connected to the injection loop group (608) in order to be able to perform the frequency sweeps necessary for characterizing the reactive equivalents (L and C) of the ground resistor measurements (RPAT1-4). It is preferable to use an amplitude modulation system for this multi-frequency injection system, which will simplify post-processing, measurements and subsequent interpretation.

[0083] - A filtering system (815), connected to the measurement circuit group (609), in order to reduce the noise of the environment to an acceptable level. Preferably, the filtering system comprises an amplitude demodulation system that simplifies the filtering process.

[0084] - A system for measuring the phase shift (816) between the signal from the set of measurement loops (609) and the additional loop (607) of the converter (602).

[0085] - A storage system (813) for collecting data that should be sent to a remote central processing unit (818) for interpreting the data collected by each distributed sensor.

[0086] - Wireless transmission means (819) for bidirectional connection between each sensor device (600) and a remote central processing unit (818).

[0087] - A backup power system (820) to prevent data loss in the event of a power supply failure.

[0088] - a synchronization system (821) that operates with a common time base for all sensor devices distributed across the locations, and

[0089] - A microcontroller (822) for the integration, control and synchronization of all necessary elements, as well as the sequencing of processes and communications.

Claims

1. A method for locating a fault in an electric power distribution cable, characterized in that The method is applicable to a medium voltage circuit comprising at least one power distribution cable (205), a first transformation center (210) comprising a first transformer and a first grounding conductor (603), a first grounding system comprising a first grounding resistor (RPAT1) connected to the first transformer via the first grounding conductor (603), the first transformation center (210) being connected to a first end of the cable (205); and A second transformation center (220) comprising a second transformer and a second grounding conductor (603), comprising a second grounding system connected to a second grounding resistor (RPAT2) of the second transformer via the second grounding conductor (603), the second transformation center (220) being connected to a second end of the cable (205), the method being characterized by comprising: - identifying a fault in the cable (205) by monitoring a current threshold value passing in at least one of the grounding conductors (603), and in case of fault identification: - measuring a first current value I passing through the first ground conductor (603) med1 ; - obtaining the value of the first grounding resistor (RPAT1); - measuring a second current value I passing through the second grounding conductor (603) med2 ; - obtaining the value of the second ground resistor (RPAT2); - Calculating the relative distance d1 from the point on the cable (205) where the fault occurs to the first transformation center (210), such that: - calculating the distance d2 from the point on the cable (205) where the fault occurs to the second transformation center (210); - Identifying the location of the fault based on the distances d1 and d2.

2. The method for locating a fault in a power distribution cable according to claim 1, characterized in that Obtaining the value of the first grounding resistor (RPAT1) includes obtaining a Bode magnitude diagram and a phase diagram by frequency scanning to determine the resistance part and the reactance part of the first grounding resistor.

3. The method for locating a fault in a power distribution cable according to claim 1, wherein: Obtaining the value of the second ground resistor (RPAT2) includes obtaining a Bode magnitude diagram and a phase diagram by frequency sweeping to determine the resistance portion and the reactance portion of the second ground resistor.

4. Method for locating faults in power distribution cables according to the preceding claims, characterized in that Further included notifying a user of the status of the fault and the location of the fault.

5. A fault detection system (800) for a medium voltage circuit, characterized in that: A first grounding system comprising a set of power distribution cables (205), a first transformation center (210) comprising a first transformer and a first grounding conductor (603), a first grounding resistor (RPAT1) connected to the first transformer via the first grounding conductor (603), the first transformation center (210) being connected to a first end of the cable (205); and A second transformation center (220) comprising a second transformer and a second grounding conductor (603), comprising a second grounding system comprising a second grounding resistor (RPAT2) connected to the second transformer via the second grounding conductor (603), the second transformation center (220) being connected to a second end of the cable (205), the fault detection system (800) comprising: - a set of sensors (600) including wireless communication means, and - a processing unit (818), The first sensor (600) comprises a device (602) connected to the first ground conductor (603) and is configured to: - Detecting a cable fault (205) by detecting a threshold current through said first ground conductor (603), wherein, in case of fault identification: - measuring a first current value I passing through the first ground conductor (603) med1 ; - obtaining the value of the first grounding resistor (RPAT1); and -Will I med1 and the value of the first grounding resistor is transmitted to the processing unit (818), The second sensor (600) comprises a device (602) connected to the second ground conductor (603) and is configured to: - Detecting a cable fault (205) by detecting a threshold current through the second ground conductor (603), wherein, in case of fault identification: - measuring a second current value I passing through the second grounding conductor (603) med2 ; - obtaining the value of the second ground resistor (RPAT2); and -Will I med2 and transmitting the value of the second grounding resistor to the processing unit (818); The processing unit (818) stores instructions for: - Calculating the relative distance d1 from the point on the cable (205) where the fault occurs to the first transformation center (210), such that: - calculating the distance d2 from the point on the cable (205) where the fault occurs to the second transformation center (210); - Identifying the location of the cable (205) fault based on the distances d1 and d2.

6. The fault detection system (800) according to claim 5, characterized in that The device connected to the first ground conductor and the second ground conductor (603) comprises a transformer (602).

7. The fault detection system (800) according to claim 5 or 6, characterized in that: A warning and display device is included that is configured to notify a user of the status of the fault and the location of the fault.

8. The fault detection system (800) according to claim 7, characterized in that The sensor (600) includes a current injection system (814) at a plurality of frequencies, the current injection system being configured to perform a frequency sweep to identify resistive and reactive portions of the first and second ground resistors.

9. The fault detection system (800) according to claim 8, characterized in that The sensor (600) comprises means for acquiring a Bode plot, in particular an amplitude measurement system comprising a programmable amplifier (817) and a phase shift measurement system (816) at a plurality of frequencies.

10. The fault detection system (800) according to claim 9, characterized in that The sensor (600) includes a programmable gain amplifier (817) and a programmable threshold (811).

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