RU2082177C1 - Power mains ground detection method - Google Patents

Abstract

FIELD: electrical engineering; ground detection in DC power mains feeding control devices, warning and protective gear of industrial enterprises, including power stations and substations. SUBSTANCE: method depending on forced shift of mains potentials to ground by ac voltage superposition, variation of rate of shift of mains potentials to ground, measurement of ac components of currents, and ground detection by maximal difference in measured values of current components in different sections of mains involves variation of superposed voltage amplitude simultaneously with its frequency; ratios between extreme values of frequency m and amplitude n of superposed voltage are interrelated by expression n = m², where

; U_max, U_min is extreme amplitude value; ω_max, ω_min are extreme frequency values. EFFECT: improved reliability of ground detection due to elimination of branch capacitance effect on results of ground fault location. 2 dwg

Description

 The invention relates to electrical engineering and can be used to create devices for finding damage in DC electric networks, supplying control, protection and signaling devices at industrial facilities, including power plants and substations.

 There is a method of finding the place of insulation reduction in the electric network, based on the forced displacement of the network potentials in relation to the ground by applying an alternating current voltage, changing the alternating current components in the network sections and determining the location with reduced insulation by the maximum EMF in the secondary winding of clamp meters caused by variable component of currents in network sections. The disadvantage of this method is the limited dynamic range of measuring insulation resistances in networks having branches with a large distributed capacitance relative to the ground, due to the fact that with a large branch capacitance and a constant source frequency, the EMF component in the secondary winding of current-measuring clamps is caused by an alternating current of the voltage application source passing through the branch capacitance, it can be measured or even exceed the EMF component caused by the alternating current of the source, going through branch insulation resistance.

The closest in technical essence is the method based on the forced displacement of the potentials of the network relative to the ground by applying an alternating current voltage, changing the frequency of the displacement of the potentials of the network relative to the ground, measuring the alternating current components from the network sections and determining the location with reduced insulation at the maximum difference EMF in the secondary winding of clamp meters caused by an alternating current component over network sections [1]
The disadvantage of this method is the limited dynamic range of measuring insulation resistances in networks having a branch with a large distributed capacity relative to the ground, due to the fact that a change in the frequency of the bias potentials of the network relative to the ground leads to a quadratic change in the EMF component in the secondary winding of the clamp meter caused by alternating current passing through the branch capacitance, and a linear change in the EMF component caused by alternating current passing through the resistances e branch isolation. In this case, the difference between the two EMF values in the secondary winding of the clamp meter at two different frequencies of the source will substantially depend on the branch capacitance.

 An object of the invention is to increase the reliability of finding a place to reduce insulation in the electrical network.

U_max, U_min экстремальные значения амплитуды
ω_max, ω_min- экстремальные значения частоты.The technical result is achieved by the fact that in the method of finding the place of insulation reduction in the electric network, based on the forced bias of the potentials of the network relative to the ground by applying an alternating current voltage, changing the frequency of the bias of the potentials of the network relative to the ground, measuring the alternating current components and determining the location with with reduced isolation at the maximum difference of the measured values of the current components in the network sections, at the same time as the frequency of the applied voltage changes, its amplitude, and magnitude relations between the extreme values of frequency and amplitude m n superimposed voltage are related by:
n = m ² ,
Where

U _max , U _min extreme values of amplitude
ω _max , ω _min - extreme frequency values.

 In this case, an increase in the frequency of the bias of the network potential with respect to the earth, while the quadratic dependence of the voltage of the alternating current source decreases, the EMF component in the secondary winding of the clamp meters caused by the alternating current passing through the branch capacitance remains unchanged in value for different the frequency of the source and does not affect the magnitude of the difference in the EMF in the secondary winding of clamp meters. This ensures that there is no effect of branch capacitance on the results of finding a place to reduce insulation.

 In FIG. 1 shows a structural electrical diagram of a device that implements the method, FIG. 2 change in voltage and frequency in time at the output of the source 5.

 The device contains the poles of network 1, for example, section 2 of the network with reduced insulation, represented by the load, as well as the capacitance and insulation resistance of each pole with respect to the ground, section 3 with normal insulation, but having a large distributed capacity relative to the ground and presented similarly to section 2, the rest part of the electric network 4, represented by its equivalent load, equivalent capacity and insulation resistance of each pole relative to the ground, source 5 for biasing the potentials of the network with respect to to the ground with a periodically changing frequency and amplitude, clamp meters 6, measuring element 7 and amplifier 8.

 The poles of the network 1 are connected to the wires of sections 2, 3 of the network and to the rest of the electric network 4, the source 5 is connected to ground with one output and the other to one of the poles of the network 1, the clamp meter 6 is magnetically connected to the wires of the network section 2 and electric communication with the measuring element 7 directly through the amplifier 8.

 The definition of the site with reduced insulation is as follows. Connect the source 5 to one of the poles of the network 1. Using current-measuring clamps 6 and a measuring element 7, showing the difference between the two values of the component of the alternating current, take alternate measurements on the network sections. The damaged area is determined by the largest value of the difference in currents estimated by the measuring element 7. Next, the location of the insulation damage is determined, moving from the source 5 deep into the damaged area.

Отношение максимальной угловой частоты ω_max к минимальной угловой частоте ω_min принято равным:

Значения ЭДС на вторичной обмотке токоизмерительных клещей при максимальном напряжении и минимальной частоте источника для неповрежденного участка e_1н и поврежденного участка e_1п определяются по известным из электротехники формулам:

где M- взаимная индуктивность между первичной и вторичной обмоткой токоизмерительных клещей, R_н и C_н сопротивления изоляции и емкость относительно земли соответственно неповрежденного участка сети, R_п и C_н сопротивление изоляции и емкость относительно земли соответственно поврежденного участка сети.In FIG. 2 shows the amplitude-frequency-modulated voltage fluctuation at the output of the source 5. The ratio of the maximum voltage U _max to the minimum voltage U _{min is} taken to be:

The ratio of the maximum angular frequency ω _max to the minimum angular frequency ω _{min is} taken to be:

The values of the EMF on the secondary winding of the clamp meter at maximum voltage and minimum source frequency for the undamaged section e _1n and damaged section e _{1n are} determined by the formulas known from electrical engineering:

where M is the mutual inductance between the primary and secondary windings of the current clamp, R _n and C _n insulation resistance and capacitance relative to the ground, respectively, of the undamaged network section, R _p and C _n insulation resistance and capacitance relative to the ground, respectively, of the damaged network section.

Используя формулы (3) (6), определим разность между двумя экстремальными значениями ЭДС на поврежденном участке Δe_п и неповрежденном участке Δe_н, используя принятые соотношения (1) и (2):

Определим, как отличаются разности между двумя экстремальными значениями ЭДС на поврежденном и неповрежденном участках сети:

Известно, что наибольшие трудности встречаются при поиске поврежденного участка, если неповрежденный участок имеет большую емкость относительно земли, т. е. при C_н>>C_п. В этом случае первое слагаемое равенства (9) будет положительной величиной, так как сопротивление изоляции поврежденного участка относительно земли (R_п) всегда больше, чем у неповрежденного участка R_н и

. Второе же слагаемое равенства (9) будет отрицательной величиной, так как (C_п-C)_н<0
Для повышения надежности поиски поврежденного участка сети желательно отстроиться от влияния на результаты измерений емкостей относительно земли участков сети. Это можно сделать, приняв следующее соотношение
n=m².The values of the emf on the secondary winding current clamp at the minimum voltage and the minimum frequency source undamaged portion _2N and e _2n e damaged portion defined by the formulas:

Using formulas (3) (6), we determine the difference between two extreme values of the EMF in the damaged section Δe _p and the undamaged section Δe _n , using the accepted relations (1) and (2):

Let us determine how the differences between the two extreme values of the EMF in the damaged and undamaged sections of the network differ:

It is known that the greatest difficulties are encountered when searching for a damaged area if the undamaged area has a large capacity relative to the ground, i.e., when C _n >> C _p . In this case, the first term of equality (9) will be a positive value, since the insulation resistance of the damaged area relative to the ground (R _p ) is always greater than that of the undamaged area R _n and

. The second term of equality (9) will be a negative quantity, since (C _n -C) _n <0
In order to increase reliability, it is advisable to look for a damaged network section from influence on the results of capacitance measurements relative to the ground of network sections. This can be done by adopting the following relation
n = m ² .

In this case (nm ² ) = 0 and the second term of equality (9) will take zero value. With the accepted ratios (1), (2), and (10), equality (9) will always be a positive value, and the value of the branch capacitance will not affect the results of finding the place of insulation reduction, thereby increasing the reliability of finding the place of insulation reduction in the electric network .

Claims (1)

The method of finding the place of insulation reduction in the electric network, based on the forced bias of the potentials of the network relative to the ground by applying an alternating current voltage, changing the frequency of the bias of the potentials of the network relative to the ground, measuring the alternating current components and determining the location with reduced insulation by the maximum difference of the measured values components of the current over the network sections, characterized in that at the same time as the frequency of the applied voltage changes, its amplitude is also changed, and other relations between the extreme values of the frequency m and the amplitude n of the applied voltage are related by
nm ² ,
where n U _max / U _min ;
m = ω _max / ω _min ;
U _max , U _min extreme values of the amplitude;
ω _max , ω _min extreme frequency values.

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