Impedance measurement of grounding systems

Abstract

A procedure for measuring the impedance of a grounding system comprising a loop, the procedure comprising generating, for each frequency fi of a set of frequencies F of a sweep applied to the grounding system loop, a modulation signal S having a fixed frequency fm signal (2) Generate a carrier signal S with a frequency fi carrier (1), and obtain the frequency components with frequencies fm and fi and based on S signal (2) and S carrier (1) The amplitude modulated signal S modulated (3).

Description

[0001] Purpose of the Invention

[0002] The objects of the present invention include a method and an apparatus for measuring ground resistance. Background Art

[0003] Traditional methods for measuring ground resistance are primarily based on driving auxiliary electrodes into the ground. More recently, methods and devices have been developed that exploit the phenomenon of induction to inject current into a loop that is part of the grounding system, thereby obtaining a resistance measurement for that loop.

[0004] Normally, the resistance of a grounding closed loop is composed of the sum of the conductor resistance and the grounding resistance.

[0005] Whatever the procedure used, the general technique involves injecting into the ground either direct current "DC" or alternating current "AC". In the case of AC, some installations have more than one injection frequency fi, usually close to each other.

[0006] Currently, the two most common configurations for measuring ground resistance are:

[0007] By inserting an electrode in physical ground, circulating AC or DC current between 2 points and reading the potential at the same point or other points.

[0008] By using an inductive transformer, an electromotive force (EMF) is introduced in the loop that is proportional to the resistance of said loop. If this loop is closed by physical grounding, the measurement value provided by the measurement will depend on the grounding resistance.

[0009] The latest technical document related to the measurement of ground resistance is CN1133878, which mentions a measurement method in which current injection is generated with a stake using a 62% method, in which only amplitude modulation is used as a functional element of the electronic equipment, in particular the control power elements (IGBTs).

[0010] US4283794A relates to a method and apparatus for obtaining radio frequency impedance information of an electrical network, such as an antenna, by using a current converter for introducing radio frequency current into the network and a voltage converter for applying radio frequency voltage across the network. Summary of the Invention

[0011] In the proposed procedure, a frequency sweep is intended to be performed in order to obtain the Bode plot of the phase and amplitude of the grounding system loop to be monitored, for which it is necessary to inject a set of frequencies, ranging from a few Hz to at least tens or hundreds of kHz, into the loop to be monitored, as part of the grounding system. Due to the complexity of the reading and the need for adequate filtering of the signal coming from the ground, a method and a device for its realization are proposed, characterized in that the current injected into the ground consists of an amplitude modulated wave (AM), whose modulation signal S signalIt is low frequency and remains at a fixed frequency fm during the entire frequency sweep process, while the carrier signal S carrier It varies according to the frequency range F required for the sweep.

[0012] The procedure and apparatus to which this invention relates are based on measurements using an inductance transformer. In this type of apparatus:

[0013] On the one hand, alternating current (AC) is induced in the loop of the monitored system, which is part of the grounding system; and

[0014] On the other hand, the electromotive force induced in said loop is measured and depends on the resistance of the loop and therefore also on the ground resistance.

[0015] In commercial devices, current is typically introduced at frequencies of several hundred hertz, down to a few thousand hertz, to determine the impedance value at the injection frequency (impedance and resistance only coincide at DC). This method cannot introduce DC current into the circuit. Therefore, with this method, the measured value is the impedance at the device's injection frequency, meaning that different devices and different injection frequencies may provide different results.

[0016] The purpose of the procedure and device described in the present invention is to describe the circuit to be monitored, which is particularly interesting in those devices where various functional elements are connected to the grounding system and are therefore part of the circuit, such as the shielding of medium voltage insulated cables, distribution towers, etc.

[0017] Thus, in a first aspect, the invention relates to a method for obtaining the impedance of a grounding system including a loop, the method comprising a frequency sweep characterized by a set of frequencies F, typically consisting of a few tens of Hz to a few hundred kHz, preferably F=

[0018] [100Hz-500kHz], applied to the grounding system loop to generate a modulation signal, where the modulation signal S signal , with a fixed frequency fm, for example several hundred Hz, preferably fm = 220 Hz, generating a carrier signal S with a varying frequency fi carrier , and corresponds to each frequency in the frequency set F of the frequency sweep, so that fi∈F, we get signal and S carrier The amplitude modulated signal S modulated , and S modulated The signal is applied to the return path of the ground system.

[0019] In addition, the method further includes measuring an electromotive force induced on a loop of the grounding system, wherein the electromotive force is proportional to an impedance value of the grounding system at each frequency fi in a frequency set F.

[0020] In a second aspect, the invention relates to a device for measuring the impedance of a grounding system, including a loop, according to the preceding claims, wherein the device comprises a modulated signal S signal Generator at frequency fm, multiple carrier signals S at frequency fi carrier The generator has a frequency fi within the frequency range of the frequency sweep F, and the amplitude modulator is configured to obtain multiple modulation signals S modulated , the inductor is configured to introduce multiple modulation signals S in the loop modulated The related electromotive force, current sensor, is configured to measure in the loop with multiple modulated signals S modulated Related signal set S out , demodulator, the frequency fm of the group signal S out demodulates, filters, tunes to frequency fm and is configured based on S signal Get the demodulated signal S demodulated , and based on S modulated Device for calculating the loop impedance Z(fi) of the grounding system. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to supplement the description and better assist in understanding the characteristics of the impedance measurement procedure of the grounding system, a set of figures is attached as an integral part of the description, according to a preferred embodiment of its actual implementation, in which, for illustrative and non-limiting purposes, the following are represented:

[0022] Figure 1 An RLC circuit is shown.

[0023] Figure 2 The three signals participating in the amplitude modulation are shown.

[0024] Figure 3 An example of the effect of the modulation index on the AM modulation process is shown.

[0025] Figure 4 A device for measuring the impedance of a grounding system according to the invention is shown. DETAILED DESCRIPTION

[0026] Figure 1 The circuit (100) shown refers to the system characteristics that determine its resistive (R), inductive (L) and capacitive (C) components, which can be equivalently represented as RLC in a simplified and compact manner.

[0027] As we all know, Figure 1 The transfer function of the circuit (100) is given by the following expression, where s is the Laplace operator.

[0028]

[0029] Knowing the shape of the system transfer function, performing a frequency sweep to obtain the Bode plot makes it easy to obtain the asymptotic response, and thus the values ​​of R, L, and C, which allow a complete description of the system being evaluated.

[0030] In a grounding loop, where there are different interconnected elements, it is important to determine the purely resistive part (R) and separate it from the reactive part (L and C), since most normative provisions refer to this parameter R, rather than the impedance Z that takes into account the contributions of the three components (Z = f(R,L,C)), depending on the frequency used in the method. To obtain the RLC equivalent of the monitored loop, the procedure involves injecting a set of frequencies, from a few hertz to several hundred kilohertz, also known as a frequency sweep.

[0031] Performing a frequency sweep means injecting several frequencies and subsequently reading these same frequencies. For this purpose, it is necessary to intersperse filters to eliminate all those frequencies that do not constitute useful information. Given a set of frequencies of interest, this set of filters should be tuned to each frequency of interest, which means that the number of tuned filters is as high as the number of frequencies included in the sweep. Another possible solution is to use configurable or programmable filters, which is also quite complex.

[0032] Advantageously, in order to simplify the filtering process, the procedure of the present invention comprises the use of Figure 2 The amplitude modulation (AM) technique shown, generally, involves a carrier S in the high frequency fi frequency range carrier (1), and the modulation wave S of the lower frequency fm signal (1). The result of amplitude modulation is the modulated signal S modulated (3), where the carrier S in the range of F carrier (1) Frequency fi and modulation wave S signal The fixed frequency fm of (1) is reflected.

[0033] Therefore, in the measurement method according to the invention, the modulator Ssignal(1) is introduced with a fixed frequency fm, the carrier S carrier The frequency fi of (1) changes frequently (from a few Hz to hundreds of kHz) within the sweep frequency range of frequency fi∈F.

[0034] The parameter to be considered in this process is the modulation index (m), which is defined as the carrier signal (S carrier (1)) and the amplitude ratio of the modulating signal (Ssigna(1)):

[0035]

[0036] The modulation index m is the modulating signal S modulated Morphological indicators such as Figure 3As shown in carrier (1) and S signal (1) The modulation signal S with three different amplitude relationships signal (1) are the same, and thus have different modulation indices m = (0.3, 0.5 and 0.7), respectively.

[0037] The measurement procedure according to the invention provides a set of values ​​Z, Z(fi), based on the calculation of the modulation index m, the graphical representation of which corresponds to the amplitude of the Bode plot of the loop impedance at each frequency fi that is part of the sweep of frequencies F.

[0038] In addition, the program also provides a set of offset values Its graphical representation corresponds to the Bode plot of loop impedance.

[0039] A Bode plot is generated based on the impedance and offset measurements taken at each frequency of the sweep being placed into a table (stored in the microcontroller's memory). Subsequently, a graphical representation of each impedance and offset value (Y-axis) at each frequency (X-axis) yields the magnitude and phase of the Bode plot.

[0040] The main advantage of the method according to the invention is its immunity to noise, which allows the filtering during the reading process to be fixed and highly selective using a very simple and known procedure, without having to use multiple filters or configurable filters, which means greater reliability and less processing time.

[0041] Another advantage is that the frequency range can be extended, since filtering tuned to the fixed frequency of the modulator can be used, without having to add or modify the components used to filter and demodulate the signal.

[0042] Figure 4 A measuring device (400) according to the invention is shown. In particular, the measuring device 400 is used in a loop as part of a grounding system. The measuring device (400) comprises a modulated signal S at a fixed frequency fm. signal (1) A generator (12) of (1). A microcontroller (13) comprising a carrier signal S with various frequencies within the range F carrier (1) Generator, used to sweep the frequency to characterize the impedance of the grounding system.

[0043] Furthermore, the measuring device (400) comprises an amplitude modulator (11) to adjust the amplitude of the signal based on S signal (2) and S carrier (1) Obtain the amplitude modulated signal S at frequency fi modulated (3) Optionally, the power amplifier (10), and the inductor (9), are configured to induce an electromotive force (emf) in the ring conductor of the grounding system.

[0044] Furthermore, the measuring device (400) comprises a current sensor (15) for measuring the current introduced in the circuit, identified as S out The electromotive force is given by S modulated (3) Combining with the noise and subjected to the typical changes in the impedance of the loop through which it propagates, the amplitude of the demodulator (16) is tuned to the modulation signal S signal The filter (17) of the frequency fm, the device (20) for obtaining the impedance Z(fi), in particular the device for measuring the demodulated signal S modulated Offset The measuring device (400) comprises means (18), preferably a phase-locked loop "PLL", and sweeps each frequency F, which can complement the Bode plot obtained by measuring the modulation index. In addition, the measuring device (400) comprises means (19) for calculating the modulation index m:

[0045]

[0046] Advantageously, the measuring device (400) can perform a frequency sweep by injecting a number of frequencies corresponding to the sweep of the frequency F through an amplitude modulated signal and then, after demodulation, the modulation signal S signal (2) reads the demodulated signal at the frequency fm, thus including tuning to the modulation signal S signal A single filter (17) for the frequency fm of (2) is sufficient.

Claims

1. A method for measuring the impedance of a grounding system comprising a loop, characterized in that The method comprises: For each frequency fi in the set of frequencies F of the frequency sweep applied to the loop of the grounding system: Generate a modulated signal S with a fixed frequency fm signal (2), fm=220Hz; Generate the carrier signal S of the frequency fi carrier (1); Based on the modulation signal S signal (2) and the carrier signal S carrier (1) Obtaining an amplitude modulated signal S having the fixed frequency fm and the frequency fi frequency component modulated (3); The amplitude modulated signal S modulated (3) introducing the loop of the grounding system; and Based on the amplitude modulated signal S in the loop of the grounding system modulated (3) Measurement signal S out ; Conditioning the signal S out , eliminate the carrier signal S carrier (1); The signal S is tuned with a fixed frequency fm out Filtering is performed to obtain the modulated signal S signal (2) The demodulated signal S at the fixed frequency fm demodulated ; Based on the demodulated signal S demodulated Obtaining the impedance Z(fi) of the grounding system; Wherein, based on the modulation signal S demodulated Obtaining the impedance of the grounding system includes: demodulated Calculate the modulation index where the modulation index m is defined as the ratio of the amplitudes, Based on the respective modulation signal S of each frequency fi demodulated obtaining phase measurements; and Based on Z(fi), and the modulation index m of each of the frequencies fi, and obtain the Bode diagrams of the frequency set F respectively.

2. A device (400) configured to measure the impedance of a grounding system according to the method of claim 1, the device (400) comprising a loop, characterized in that The device (400) comprises: Generator (12), configured to generate a modulation signal S of a fixed frequency fm signal (2); A generator (13) configured to generate a plurality of carrier signals Scarrier (1) having frequencies fi included in a swept frequency range F; Amplitude modulator, configured to obtain a plurality of amplitude modulated signals S modulated (3); The inductor (9) is configured to introduce a current into the loop that is related to the plurality of amplitude modulated signals S modulated (3) associated electromotive force; A current sensor (15) is configured to measure the current in the loop relative to the plurality of amplitude modulated signals S modulated (3) Related signal S out A collection of The demodulator (16) modulates the signal S at the fixed frequency fm. out The set of is demodulated; filter (17), tuned to the fixed frequency fm and configured to be based on the modulation signal S signal (2) Obtaining the demodulated signal S demodulated ; Based on the modulation signal S demodulated means for calculating said impedance Z(fi) of said grounding system; Obtain the demodulated signal S of each frequency fi demodulated Phase measurement value of device (18); Calculate the modulation index for each frequency fi device (19); wherein the modulation index is defined as the ratio of the amplitudes; and Based on Z(fi), and the modulation index m of each of the frequencies fi, and a device for respectively obtaining the Bode diagram of the frequency set F.

3. The device (400) configured to measure the impedance of a grounding system according to claim 2, characterized in that The multiple carrier signals S carrier The generator (13) of (1) includes a microcontroller.

4. The device (400) configured to measure the impedance of a grounding system according to claim 3, characterized in that The microcontroller includes a memory configured to store the Z(fi) and the Z(fi) of each frequency in the frequency set F of the frequency sweep.

5. The device (400) configured to measure the impedance of a grounding system according to any one of claims 2 to 4, characterized in that: The device further comprises: is configured to amplify a plurality of said amplitude modulated signals S modulated (3) power amplifier (10).

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