ES2532677A1 - Method and system of detection of ground faults in dc networks of isolated cables (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Method and detection system for earth faults in isolated direct current networks. The present invention relates to a method and system for selective detection of ground faults in dc networks formed by isolated cables that have their screens connected to ground in at least one of their two ends. The cable that has a ground fault is identified, by measuring and analyzing the currents that circulate through the cable screens. Said defect intensities circulating on the screens can be measured in several ways. This patent tries to solve the problem of detecting a ground fault selectively in electrical networks with cables isolated in direct current, and has the advantage that the measurement of the current in the screens is simpler than in the driver itself, since that the insulation voltage required for the measuring equipment is much lower than if it were measured in the cable. (Machine-translation by Google Translate, not legally binding)

Description

OBJECT OF THE INVENTION

The object of the present invention is to present a new method and system for detecting ground faults in direct current electric power networks with insulated cables. This new method offers a perfect detection of the insulated cable with defect, while the new system when implementing said method and putting it into practice, guarantees the indication of the cable with defect and provides a selectivity

15 total to avoid unwanted or erroneous shots.

BACKGROUND OF THE INVENTION

In electricity supply networks composed of insulated cables in

20 DC, when a ground fault occurs due to deterioration of the insulation of the cable, the value of the defect intensity can have very high values, making its rapid detection and elimination essential, since all the power elements that make up the network of DC power supply are subject to high thermal stresses, causing severe damage.

25 One way to detect, in a non-selective way, the defective cable is to inject a voltage signal of small amplitude and a certain frequency into the supply network. Therefore, the insulation resistance of the network itself is calculated permanently. To know which cable has the defect, go

30 removing from service one by one until said insulation resistance rises to normal values that indicate good insulation. Normally, when an isolation level corresponding to a defect situation is reached, the protection system first provides an alarm signal and, in the event of further isolation decrease, a second trip signal. This technique has come

35 normally used in low voltage power distribution networks.

DESCRIPTION OF THE INVENTION

The present invention makes it possible to selectively detect the insulated cable or cables that are in a ground fault situation in DC power supply networks.

Earth defects represent more than 90% of the total faults that occur in

10 electric power networks, so its detection and precise location is essential. If only voltmetric protections are used that measure the voltages at different points of said distribution networks, this type of defect cannot be found. Therefore, it is necessary to measure other quantities, and in this sense the present invention proposes to measure the intensities that circulate through the cable screens

15 isolates that conform the DC distribution network.

In electrical systems for the supply of direct current electric power, when an earth fault occurs in a cable and its active part is in contact with its screen, intensity circulates both through the corresponding cable screen and through the 20 screens of the rest of the cables without defect. Once the initial moments of the transitory phenomenon of the defect have elapsed, through the screens of the other cables without defect that form said electrical system, no intensity circulates since there is no mutual flow between the active parts of the cables and their corresponding screens, attenuating completely its value. In these conditions and in the cables without defect,

25 the capacitors formed by the capacities between the active parts and the screens, as well as the capacitors formed between the screens and ground, are charged. This situation means that only the intensity of the fault can circulate through the cable screen that has a ground fault.

30 The principle of operation of the new method is based on the measurement and analysis of the intensities that circulate through the screens of the cables in service. The screen or screens through which intensities are circulating permanently for times longer than previously established thresholds, correspond to the defective cables.

This new method discriminates between the intensities that circulate on the screens in the maneuvers of connection of the cables and the intensities that circulate through the screens when a defect has occurred.

The new protection method identifies based on the characteristics of the measured intensities in the cable screens, if these intensities that circulating on the screens correspond to energizing maneuvers of the cables or defect situation. In the case of connection maneuvers, the intensities read have the following characteristics:

1.-They take positive and negative values until their complete attenuation. 2.-Therefore, sign oscillations appear in its derivative with respect to time until its complete attenuation.

In the case of cable defect, the measured intensities have the following features:

1.-No oscillations appear in your sign. 2.-Therefore there are no oscillations in the sign of its derivative with respect to weather.

In short, the detection of the ground fault is done by checking that the The intensity of the default cable screen is greater than an adjustable threshold, threshold that has been previously set by a predefined time, and whose threshold adjusted will depend on the value of the current on the screen.

In addition, the threshold previously set for a certain time, must overcome the current through the insulated cable screen based on an expression mathematics that relates time and current, so that the higher the Minor effect current will be the firing time.

It should also be noted that only one defect in a cable will be considered isolated, if the current on the corresponding screen shows no sign changes during a defined time threshold, or if the derivative with respect to the time of

The current on the screen shows no sign changes during a defined time threshold.

It is also possible to consider the fact that when an intensity circulates on the screen of a cable without a defect due to a defect that occurs in another cable, the blocking of the cable trip without defect occurs because it is the derivative of the current flowing through the corresponding screen of the opposite sign to the derivative of the current of the screen of the cable with defect during a defined time threshold, and the blocking can also occur when the derivative of the intensity with respect to the time in the insulated cable without defect, presents pending positive and negative

Likewise the object of the invention is the system for detecting ground faults in direct current networks of insulated cables, said system comprising a current measurement equipment that circulates through the insulated cable screen to obtain a measurement signal, complementing it with an analyzer device responsible for analyzing the measured signal. said means analyzing device for obtaining the amplitude of the measured signal, as well as comparison means responsible for buying the amplitude of the measured signal with a determined value of acoustic intensity and obtaining an output signal indicative of the existence or not default, with the particularity that the comparison can be made during a predetermined time or establish the comparison with a determined value for a predetermined time, depending on the measured intensity.

The system also incorporates an analyzer means responsible for analyzing the measured signal.

BRIEF DESCRIPTION OF THE FIGURES

A series of drawings that help to better understand the invention and that expressly relate to an embodiment of said invention which is presented as a non-limiting example thereof is described very briefly below.

Figure 1 shows a DC power supply network where the following elements are observed:

(one ):

DC voltage source.

(2):

Main DC bar.

(3):

DC cables

(4):

DC cable screens.

(5):

Measurement of intensity in direct current.

(6):

Defect in a DC cable.

(7):

Default Intensities

(8):

Grounding the cable screens.

(9):

Grounding of the DC voltage source.

Figure 2. - It shows a simplified scheme of the preferred embodiment, where the elements arranged in Figure 1 and also the following appear:

(10): Power cut-off element (11): Trip order of the new protection system (12) to the power cut-off element (10). (12): New protection system.

Figure 3.- Shows the block diagram to compare the polarities, which It consists of the following elements: (13): Indication of defect in the insulated cable.

(14)

Comparator

(fifteen)

Adjusted intensity or threshold value.

(16)

current value circulating on the insulated cable screen. Figure 4.- Shows a block diagram similar to that of Figure 3, but including also: (17): Timer.

Figure 5.- Shows a diagram similar to the diagram of Figure 4 which also It has the following element: (18): OR logic gate.

Figure 6.- Shows a diagram similar to the diagram of Figure 5 and which also It has the following element: (19): Trip time calculator.

5 Figure 7.- Shows a diagram similar to the diagram of Figure 5 and also It has the following elements: (20): Positive value intensity indicator.

(twenty-one)

AND logic gate.

(22)

Negative value intensity indicator.

Figure 8.- Shows a diagram similar to the diagram of Figure 7 without the elements 20 and 21, also including the following elements: (23): Indicator of intensity derivative with respect to the positive value time.

(24) Indicator of intensity derivative with respect to the negative value time.

Figure 9 shows a diagram similar to the diagram of Figures 7 and 8 without the logic gates 21 and now have three logical inputs.

Figures 10 to 16. - They show the logical sequence of firing, according to 20 different alternatives provided.

PREFERRED EMBODIMENT OF THE INVENTION

An embodiment of the new protection system is described below without excluding other possible configurations using the described method 30.

In figure 1, a direct current power supply network is represented, comprising a direct current voltage source (1), a main direct current bar (2), direct current cables (3), cable screens of 35 direct current (4), the measurement of direct current intensity (5), the defect in

a direct current cable (6), the fault currents (7), the grounding of the cable shields (8) and the grounding of the direct voltage source (9).

For its part in the diagram of Figure 2, in addition to the components mentioned and that

5 incorporates the diagram of figure 1, a power cutting element (10), a device order (11) of the protection system (12) and the protection system (12) itself are included.

The preferred embodiment of this invention contemplates its implementation in relays

10 multifunction programmable through logic schemes. A principle provision of the new protection method is shown in Figure 2. Some ways to measure the intensities that circulate through the screens of the insulated cables, can use Hall effect sensors, or measure voltages in shunt resistors connected at the end of the screen between it and earth, etc. The new protection system (12)

15 has the necessary elements to determine if the cable has a ground fault or not. The different logical schemes that are implemented depending on how to detect the ground fault of the insulated cable are indicated in the following figures:

• Figure 3: a comparator (14) is available to compare the

20 fault intensities (16) and adjustment (15). If the default intensity is greater than the adjustment, the comparator indicates that there is a defect with a logical "one" at its output (13).

• Figure 4: the same elements are available as in figure 3 with an additional timer (17). If the default intensity is greater than

25 setting for a time set in the timer, the comparator indicates that there is a defect with a logical ~ one "on its output (13).

• Figure 5: The same elements are available as in Figure 4 with an additional OR logic gate (18). If the default intensity is always higher than the setting and positive, or always higher than the setting and

30 negative, during a time set in the timer (17), a logical one will be present at one of the two inputs of the OR logic gate and it is indicated that there is a defect with a logical "one" at its output (13).

• Figure 6: a comparator (14) is available to compare the

default intensities (16) and adjustment (15). If the intensity read in 35 the screen is higher than the setting, the comparator (14) sends a

start signal to the trip time calculator (19) depending on

the intensity read and adjust. If the calculated time expires,

it is indicated that there is a defect with a logical "one" in its output (13).

•

Figure 7: a comparator (14) is available to compare the

5

default intensities (16) and adjustment (15). While the

Read intensity on the screen is higher than the setting, the

comparator keeps the timer started (17).

Simultaneously, if the value of the signal or intensity (16) read in the

screen is always positive or always negative, there will be two "ones"

10

logic at the inputs of the corresponding logic gate ANO (21).

In this situation there will be a logical "one" in either of the two

logical inputs of the OR gate (18) And it will be indicated that there is a defect in

the exit (13).

In said figure 7, the intensity indicators (20) and (22) are shown

fifteen

of positive and negative value, respectively.

•

Figure 8: a comparator (14) is available to compare the

default intensities (16) and adjustment (15). While the

intensity read on the screen is higher than the setting, the

twenty

comparator (14) keeps the timer (17) started.

Simultaneously, if the derivative with respect to the time of the signal read in

the screen is always positive or always negative, according to what

indicating the indicators (23 or 24), there will be two logical "ones" in

the corresponding logic gate inputs ANO (21). In this

25

situation will have a logical ~ one "in either of the two entries

logic of the OR gate (18) And it will be indicated that there is a defect in the output

(13).

•

Figure 9: a comparator (14) is available to compare the

default intensities (16) and adjustment (15). While the

30

intensity read on the screen is higher than the setting, the

comparator (14) keeps the timer (17) started.

Simultaneously, if the derivative with respect to the time of the signal read in

the screen is always positive or always negative, as indicated by the

indicators (23 or 24), there will be three logical "ones" in the inputs of

35

the corresponding logic gate ANO (21). In this situation you will have

a logical "one" in either of the two logic inputs of the door

OR (18) And it will be indicated that there is a defect in the output (13). In that figure

9, the intensity (20) and (22) indicators of positive and negative value respectively are also included.

Figures 10, 11, 12, 13, 14, 15 and 16 indicate, for better understanding, the logical sequences of the following claims:

o Figure 10: the logical firing sequence according to claim 1 is indicated.

o Figure 11: the logical firing sequence according to claim 2 is indicated.

o Figure 12: the logical firing sequence according to claim 3 is indicated.

15 or Figure 13: the logical firing sequence according to claim 4 is indicated.

o Figure 14: the logical firing sequence according to claim 5 is indicated.

o Figure 15: the logical firing sequence is indicated according to claim 6.

o Figure 16: the logical firing sequence according to claim 7 is indicated.

Claims (3)

1a ._ Method for detecting ground faults in DC networks of insulated cables, where the earthing of the insulated cables is carried out by means of screens, characterized in that the measurement of the intensity that circulates through the screen of the Corresponding insulated cable, checking that the current flowing through the screen of the defective cable is greater than an adjustable threshold. 2a ._ Method for detecting ground defects in DC networks of insulated cables, according to claim 1, characterized in that the adjustable threshold that must be exceeded by the intensity or current measured in the insulated cable screen, corresponds to a threshold adjustable for a previously defined time. 3a._ Ground fault detection method in DC networks of insulated cables, according to claim 1a, characterized in that the adjustable threshold that must be exceeded by the intensity or current measured in the insulated cable screen, corresponds to a adjustable threshold for a previously defined time, said adjustable threshold depending on the value of the current flowing through the insulated cable screen. 4a._ Method for detecting ground defects in DC networks of insulated cables, according to any of the preceding claims, characterized in that a defect in an insulated cable is only considered if the current on its screen does not show sign changes during a defined time threshold. 5 <1._ Method for detecting ground faults in DC networks of insulated cables, according to previous claims, characterized in that only a defect in an insulated cable is considered, if the derivative with respect to the time of the current by its Screen does not show sign changes during a defined time threshold. 68._ Method for detecting ground faults in DC networks of insulated cables, according to previous claims, characterized in that when the current circulates through the screen of a cable without defect due to a defect occurring in another cable, it is blocked the trip of the cable without defect being the derivative of the current flowing through the screen of opposite sign to the derivative of the current of the default cable screen for a defined time threshold. 7a._ Ground fault detection method in DC networks 5 isolated, according to previous claims, characterized in that when circulating intensity by the screen of an insulated cable without defect, due to a defect that occurs on another cable, it blocks the trip of the cable without defect when presenting the derivative of the intensity with respect to time in the insulated cable without defect positive slopes or negative aa._ Ground fault detection system in DC networks insulated, where the grounding of the insulated cables is carried out by means of screens characterized in that it comprises: 15 - A measuring device (5) of the current flowing through the screen (4) of the insulated cable (3) that obtains a measurement signal (16) at the output;

-

An analyzer device responsible for analyzing the measurement signal (16), according to claim la and having:

•

Means for obtaining the amplitude of the measured signal (16);

•

Comparison means (14) responsible for comparing the amplitude of the measured signal (16) with a determined value of adjustment intensity (15) and obtaining a signal of adjustment intensity (15), and obtaining a signal of

25 output (13) indicative of the existence or not of defect. ga._ Ground fault detection system in DC networks isolated, according to claim 8, characterized in that it also incorporates means (14) to compare the amplitude of the measured signal (16) with a given value (15) 30 for a predetermined time according to claim 2, and obtaining an output signal (13) indicative of the existence or not of defect 10.-Earth fault detection system in DC networks of insulated cables, according to claims 8 and g, characterized in that it incorporates 35 also means for comparing the amplitude of the measured signal (16) with a determined value (15) for a predetermined time depending on the intensity measured according to claim 3, and obtaining an output signal (13) indicative of the existence or not of defect. 5. 11-Earth fault detection system in DC networks of insulated cables according to claims 8 to 10, characterized in that it also incorporates an analyzer in charge of analyzing the measured signal (16) according to claim 4. 12. 12. Earth fault detection system in DC networks of insulated cables, according to claims 8 to 11, characterized in that it incorporates an analyzer responsible for analyzing the measured signal (16) according to claim 5. 13. 13. Earth fault detection system in DC networks of insulated cables according to claims 8 to 11, characterized in that it incorporates an analyzer responsible for analyzing the measured signal (16) according to claim 6. 14. System for detecting ground defects in DC networks of insulated cables according to claims 8 to 13, characterized in that it incorporates an analyzer responsible for analyzing the measured signal (16) according to claim 7.