RU2806402C1 - Method for continuous monitoring of insulation resistance in double current electrical network with isolated neutral - Google Patents

Abstract

FIELD: electrical measuring equipment; relay protection.

SUBSTANCE: invention is intended to improve safety in electrical networks of alternating, direct and double current with an isolated neutral. Essence: the method is based on measuring leakage current from an auxiliary test voltage source in the form of a periodic sequence of pulses. The repetition period of test voltage pulses is set equal to an even number of measured voltage periods of the controlled network T=kT_c, where k=2.4,…. The test voltage is synchronized with the voltage of one of the phases of the controlled network. The test voltage is connected through additional resistors to the DC buses of the controlled network, the leakage current is measured by measuring the voltage drop across the measuring resistance connected in series with the test voltage source. A signal is generated that is delayed with respect to the voltage drop across the measuring resistance by an interval . The delayed signal is subtracted from the voltage drop across the measuring resistance, and the resulting signal is converted by correction. The moving average value of the signal is calculated over an interval equal to the period of the voltage of the controlled network, and the insulation resistance is calculated. The obtained value is compared with the setting. When the calculated insulation resistance is less than the setting, the electrical equipment is turned off. Additionally, the output of the test voltage source is connected through resistors to the poles of the DC circuit. The test voltage is synchronized with the voltage of one of the phases of the controlled network. The moving average of the signal value over the test voltage period is measured and the area of insulation failure is identified.

EFFECT: increasing the accuracy of continuous monitoring of electrical insulation resistance and reliability of protection of an electrical network of double current with an isolated neutral when the frequency of the controlled network changes and expanding functionality by determining the section of the electrical network in which an insulation failure occurred.

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Description

The present invention relates to electrical measuring equipment and relay protection and is intended to improve safety in dual current electrical networks with an isolated neutral.

There are known methods for continuous monitoring of insulation resistance in a double-current electrical network with an isolated neutral, based on measuring the leakage current from an auxiliary test voltage source, in which a test voltage is supplied to the controlled network through a resistor star in the form of a periodic sequence of opposite-polarity pulses, and the leakage current is measured in During the part of the pulse action time corresponding to the capacitance of the controlled network charged to a constant voltage, the insulation resistance is calculated, the obtained value is compared with the permissible value, and when the measured insulation resistance decreases below the permissible value, the electrical network is disconnected (RF Patent No. 2144679. MKI G01R 27/18 , Н02Н 3/16 - Published January 20, 2000. Bulletin No. 2; RF Patent No. 2321008, IPC G01R 27/16, 2006; RF Patent No. 2437109, IPC G01R 27/18, 2011; RF Patent No. 2722468. IPC G01R 27/18; N02N 3/16. RF Patent No. 2725898. IPC G01R 27/18. Publ. 07.07.2020. Bull. No. 19; Publ. 06/01/2020. Bull. No. 16; USSR author's certificate No. 1737363, IPC G01R 27/18, 1992).

In known methods, insulation resistance is measured cyclically using a test voltage source in the form of a periodic sequence of differently polarized pulses of a special shape. Each cycle involves two main stages: charging the network capacitance to a given constant voltage and direct measurement of the leakage current in the established mode for the measuring direct current in the electrical network. Next, based on the measured values of leakage currents at positive and negative voltages, the insulation resistance is calculated, which is compared with the permissible value. If the resistance decreases below the permissible value, the electrical network is disconnected.

To ensure noise-resistant measurement, the time of direct measurement of leakage current is usually taken equal to one or several periods of the controlled network. In autonomous electrical systems, for example, in mining dump trucks with diesel generators, as well as local AC electrical networks, the voltage frequency changes. In this case, the averaging of the signal proportional to the leakage current occurs over a time interval that is not a multiple of the voltage period of the controlled network. As a result, the errors in measuring leakage current and calculating insulation resistance increase, and the reliability of the protection of the electrical network decreases.

In addition, known methods do not allow determining the section of the electrical circuit in which the insulation failure occurred.

Consequently, the disadvantages of the known methods for continuous monitoring of insulation resistance in a double-current electrical network with an isolated neutral are: low accuracy of insulation resistance measurement and reliability of protection when the voltage frequency of the controlled network changes, as well as limited functionality, because they do not allow you to determine the section of the electrical network in which the insulation failure occurred.

Of the known methods, the closest in terms of the achieved result to the proposed technical solution is the method of continuous monitoring of insulation resistance in an electrical network of double current with an isolated neutral, in which the voltage period of the controlled network T _c is measured, a test voltage is generated in the form of a periodic sequence of pulses of the form

where U ₁ , U ₂ are constant voltages, U ₁ ≥U ₂ ; τ - time interval, T - repetition period of test voltage pulses, ; the repetition period of the test voltage pulses is set equal to an even number of periods of the voltage of the controlled network T=kT _c , where k=2.4,..., the voltage drop across the measuring resistance connected in series with the test voltage source is measured, a delay with respect to the voltage drop across the measuring voltage is formed resistance T per interval signal, subtract the delayed signal from the voltage drop across the measuring resistance, convert the resulting signal u _and (t) by correction into a signal

where ε is a small time interval, ΔT≤ε<τ; ΔT - maximum increment value of the test voltage period, calculate the moving average value signal u _to (t) over an interval equal to the period T _from the voltage of the controlled network, the insulation resistance is calculated using the formula

where r _t is the internal resistance of the source; r ₀ is the resistance of the measuring resistor, compare the obtained value with the setting R ₀ and when r _from ≤R ₀ , turn off the electrical equipment, while the output of the test voltage source is connected through a resistor star to the AC buses of the controlled network (RF Patent No. 2732790. IPC G01R 27 /18(2020.05); N02N 3/00(2020.05). Publ. 09.22.2020. Bulletin No. 27).

The time for direct measurement of leakage current is usually taken equal to one or several periods of the voltage of the controlled network in order to ensure noise-resistant measurement. In autonomous electrical systems, for example, in mining dump trucks with diesel generators, as well as local AC electrical networks, the voltage frequency changes. In this case, the averaging of the signal proportional to the leakage current occurs over a time interval that is not a multiple of the voltage period of the controlled network. As a result, the errors in measuring leakage current and calculating insulation resistance increase and the reliability of protection of the electrical network decreases.

Consequently, the disadvantages of the known method for continuous monitoring of insulation resistance in a double-current electrical network with an isolated neutral are low accuracy of insulation resistance measurement and reliability of protection when the voltage frequency of the controlled network changes, as well as limited functionality, because they do not allow you to determine the section of the electrical network in which the insulation failure occurred.

The purpose of the present invention is to increase the accuracy of continuous monitoring of electrical insulation resistance and the reliability of protection of an electrical network of double current with an isolated neutral when the frequency of the controlled network changes and to expand functionality by determining the section of the electrical network in which an insulation failure occurred.

This goal is achieved by the fact that in the known method of continuous monitoring of insulation resistance in an electrical network of double current with an isolated neutral, in which the voltage period of the controlled network T _c is measured, a test voltage is generated in the form of a periodic sequence of pulses of the form

where U ₁ , U ₂ - constant voltages, U ₁ ≥U ₂ , τ - time interval, T - repetition period of test voltage pulses, the repetition period of the test voltage pulses is set equal to an even number of periods of the voltage of the controlled network T=kT _c , where k=2, 4, ..., the voltage drop across the measuring resistance connected in series with the test voltage source is measured, a delayed one with respect to the voltage drop across the measuring resistance is formed resistance T per interval signal, subtract the delayed signal from the voltage drop across the measuring resistance, convert the resulting signal u _and (t) by correction into a signal

where ε is a small time interval, ΔT≤ε<τ; ΔT - maximum value of the test voltage period increment, u _to (t), calculate the moving average value signal u _to (t) over an interval equal to the period T _from the voltage of the controlled network, the insulation resistance is calculated using the formula

where r _t is the internal resistance of the source; r ₀ - resistance of the measuring resistor, compare the obtained value with the setting R ₀ and when r _from ≤R ₀ , turn off the electrical equipment, additionally connect the output of the test voltage source through resistors to the poles of the DC circuit, the test voltage is synchronized with the voltage of one of the phases of the controlled network, measure a moving average during the test voltage period the signal value and identify the area of insulation failure: when - AC circuit; at - DC bus with positive potential; at - DC bus with negative potential.

Compared to the closest similar solution, the proposed technical solution has the following new features (operations):

- connect the output of the test voltage source through resistors to the poles of the DC circuit;

- the test voltage is synchronized with the voltage of one of the phases of the controlled network;

- measure the moving average during the test voltage period, the value of the signal u(t);

- identify the area of insulation failure: when - AC circuit; at - DC bus with positive potential; at - DC bus with negative potential.

Consequently, the claimed technical solution meets the “novelty” requirement.

When implementing the proposed invention, the accuracy of monitoring the insulation resistance and the reliability of the protection of electrical equipment are increased when the frequency of the AC power supply network changes. An increase in measurement accuracy is achieved by compensating for interference during the algebraic summation of signals proportional to the leakage current, but shifted in time, and by averaging the summation result over a sliding interval equal to the voltage period of the controlled network. In this case, the delay interval and the averaging interval are adjusted in proportion to the period T _from the voltage of the controlled network, and the test voltage is synchronized with the voltage of one of the phases of the controlled network. Thanks to this, a high level of interference compensation is achieved by averaging the signal over a sliding interval strictly equal to the voltage period of the controlled network. Based on the moving average measurement result During the test voltage period, the value of the signal u(t) determines the area of insulation failure: when - AC circuit; at - bus with positive potential of the direct current section; at - bus with negative DC potential.

Consequently, the proposed method improves the accuracy of monitoring the electrical insulation resistance and the reliability of protection of a double-current electrical network with an isolated neutral when the voltage of the controlled network changes, and also makes it possible to identify a section of the electrical network with damaged insulation.

Consequently, the claimed technical solution meets the requirement of “positive effect”.

For each distinctive feature, a search was carried out for well-known technical solutions in the field of measuring technology and relay protection.

Operations:

- the test voltage is synchronized with the voltage of one of the phases of the controlled network;

- measure the moving average during the test voltage period, the value of the signal u(t);

- identify the area of insulation failure: when - AC circuit; at - bus with positive potential of the direct current section; at - a bus with a negative DC potential has not been found in technical solutions for a similar purpose.

Operation:

1. Connect the output of the test voltage source through resistors to the poles of the DC circuit known in devices for monitoring insulation resistance in DC electrical networks (Olszowiec R. Insulation Measurement and Supervision in Live AC and DC Unearthed Systems. Lecture Notes in Electrical Engineering, Springer, 2014 , pp. 99 - 108. doi: 10.1007/978-3-642-29755-7). In devices for monitoring insulation resistance in dual current electrical networks, the connection of the test voltage source to the DC buses through additional resistors was not detected.

Thus, these features ensure that the claimed technical solution meets the “significant differences” requirement.

The essence of the proposed invention is illustrated by drawings. In fig. 1 shows a simplified schematic diagram of a dual current electrical network, explaining the method of monitoring insulation resistance and protective shutdown in a three-phase electrical network with rectifier B. FIG. Figure 2 shows oscillograms of the test voltage and diagrams of signals generated during data processing in microcontroller 14.

In fig. 1 is indicated: 1 - source of three-phase alternating voltage E _c ; 2, 4 and 6 - insulation resistance of phases A, B and C of the controlled network, respectively r _A , r _B , r _C ; 3, 5 and 7 - capacitances of phases A, B and C of the controlled network, respectively C _A , C _B , C _C ; 8 - voltage sensor of the controlled network; 9 - rectifier; 10 - test voltage source u _t (t); 11 - measuring resistor with resistance r ₀ ; 12 and 13 - additional resistors, resistance of additional resistors r _t1 =r _t2 =r _t ; 14 - amplifier; 17 - microcontroller; 15 and 18 - insulation resistance of the DC network (for feeders connected to the positive and negative poles of the thyristor rectifier TV), respectively r _p1 , r _p2 ; 16 and 19 - DC network capacity; C _p1 , C _p2 , 20 - display unit; 21 - executive relay IR; 22 - complex resistance of the rectifier load Z _n .

The voltage sensor of the controlled network 8, connected by input to phases A and B of the network, generates a signal proportional to the voltage of the controlled network. This signal is supplied to the input of microcontroller 17. The voltage diagram of the controlled network is shown in Fig. 2a. The microcontroller 17 measures the voltage period of the controlled network T _c and generates a control signal for the test voltage source 10. The shape of the test signal is shown in Fig. 2b. The test voltage u _t (t) is a sequence of multi-polar pulses of a special shape. The repetition period of the test voltage pulses is set by the microcontroller 17 equal to the even number of periods of the measured voltage of the controlled network T=kT _c , where k=2.4,…. The test voltage is synchronized with the voltage of the controlled network, i.e. the moment the test voltage begins coincides with the moment the voltage of the controlled network passes through 0. With this formation of the test voltage, the measurement results are invariant with respect to changes in the frequency of the controlled network.

The voltage from source 10 through additional resistors 12 and 13 enters the DC circuit of the controlled electrical network. The current flowing in the circuit: “test signal source” u _t (t) - additional resistors 12 and 13 - insulation resistance - ground, is controlled by the voltage drop across the measuring resistor 11 (r ₀ ). The voltage from the measuring resistor 11 through the amplifier 14 is supplied to the input of the microcontroller 17. The insulation resistance value is calculated depending on the measured voltage drop across the measuring resistor 11 and the known test voltage. The executive relay IR 21, the control input of which is connected to the output of the microcontroller MK 17, is intended to disconnect the protected section of the network. The display unit 20 displays the insulation resistance value and the section of the network in which the insulation resistance has decreased.

In the time interval 0<t≤τ, the test voltage is equal to u _t (t)=U ₁ and provides an accelerated process of transition of the electrical system to a steady state, namely, a forced charge of capacitors in AC and DC circuits. At time t=τ, the voltage on the capacitance reaches the value u _e (τ)≈U ₂ . In the time interval test voltage source 10 generates voltage u _t (t)=U ₂ .

At 0<t≤τ, i.e. with accelerated charging of the capacitance, the current flowing through the measuring resistor is equal to

Where - time constant of the charging circuit;

U _p - voltage of the DC network section;

- component of the leakage current caused by the phase voltages of the controlled network.

During the time interval the current flowing through the measuring resistor 11 is determined by the expression

When u(t)≈U ₂ the expression for the current through the measuring resistor in the interval takes the form

where ΔU is the difference between the voltages of the charged container and the test voltage U ₂ at time t=τ;

- component of the current flowing through the measuring resistor and caused by the transient process when switching the test voltage,

Voltage is a monotonically decreasing function.

At the capacity is charged at an accelerated rate. The current flowing through the measuring resistor 11 is equal to

During the time interval the current flowing through the measuring resistor 11 is determined by the expression

At expression for the current through the measuring resistor 11 in the interval takes the form

where ΔU' is the difference between the voltage of the charged capacitor and the test voltage -U ₂ at a time .

The voltage drop across the measuring resistor 11 is equal to

u(t)=r ₀ i _and (t).

An oscillogram of the voltage drop u(t) across the measuring resistor is shown in Fig. 2c. The voltage u(t) through the amplifier 14 is supplied to the input of the controller 17. Further, in mathematical expressions, for simplification, the transmission coefficient of the amplifier 8 is taken equal to 1. Data processing is performed in the microcontroller 17. A signal is being generated by delaying the signal u(t) by half the test voltage period. The oscillogram of the signal u _з (t) is shown in Fig. 2g. Next, the difference is calculated in microcontroller 17

u _and (t)=u(t)-u _з (t).

Signal oscillogram shown in Fig. 2d. The signal u _and (t) are corrected according to the equation

The oscillogram of the corrected signal u _to (t) is shown in Fig. 2e. The correction is performed to exclude from the signal processing procedure the components corresponding to the charge of the network capacitance, taking into account changes in the period of the test pulses. To do this, when - ε<t≤τ+ε the signal u _to (t) is assigned a constant value u _and (-ε, recorded at the end of the second half-cycle of the previous measurement cycle. When signal u _to (t) is assigned a constant value . Value 6 is selected from the relation ΔT≤ε<τ, where ΔT is the maximum change in the period of the test voltage in relation to k periods of the controlled network voltage.

In the interval the signal u _to (t) is equal to

If the period of the test voltage T is equal to an even integer number of periods of the voltage of the controlled network T _c , the components of the leakage current due to the phase voltages of the controlled network are

Taking into account (6) and (7), expression (5) takes the form

Similarly in the interval the signal u _to (t) is equal to

Taking into account expressions (8) and (9), as well as the monotonically decreasing nature of the function signal values u _to (t) in the intervals 0<t≤τ and assuming are equal respectively:

Thus, due to the operations of delaying the signal, proportional to the leakage current, by half the period of the test voltage, and subtracting the delayed signal from the original one, with strict correspondence of the period of the test voltage to the period of the voltage of the controlled network, firstly, the invariance of the measurement result with respect to changes in the frequency of the controlled network and DC network voltage, and, secondly, compensation in the measuring signal for the leakage current component caused by the voltage of the controlled network.

When averaging the adjusted signal u _to (t) over a sliding interval equal to the voltage period of the controlled network, a signal is generated

Solving equation (10) for r _from gives the formula for calculating the insulation resistance

Oscillogram of the averaged signal shown in Fig. 2g.

The procedure for calculating the insulation resistance value in accordance with formula (11) is performed by microcontroller 17. The microcontroller continuously generates a signal proportional to the insulation resistance averaged over the voltage period of the controlled network. In this case, the delay in determining the fact of a decrease in insulation resistance and, consequently, the activation of protection does not exceed

The microcontroller 17 calculates the average voltage value across the measuring resistor 11 over the period of the test signal. In general, the average voltage value is

Taking into account relations (1) - (4), expression (12) takes the form

Leakage current component caused by the phase voltages of the controlled network, is a periodic function with a frequency that is a multiple of the test voltage frequency. Hence,

In expression .

In the event of a violation of the insulation resistance in the alternating current circuit, the direct voltage source does not enter the leakage current circuit. Hence, .

If the insulation resistance in the DC circuit is violated, the average voltage value across the measuring resistor 11 over the period of the test signal in accordance with equation (13) is equal to

From equation (14) it follows that when the insulation resistance of the positive pole decreases because U _p >0, and when the insulation resistance of the negative pole decreases because U _p <0.

The algorithm for generating an emergency shutdown signal contains:

- calculation of the value of equivalent insulation resistance r _from ;

- comparison of r _from with the setting R ₀ (for example, 10 kOhm);

- generation of a shutdown signal for executive relay 21.

Thus, the proposed method for monitoring insulation resistance and protective shutdown of the electrical network provides increased accuracy of insulation resistance measurement, reliability of protection and expanded functionality due to:

- regulation of the test voltage period in proportion to the voltage period of the controlled network;

- compensation of interference during algebraic summation of signals proportional to the leakage current, but shifted in time;

- averaging the summation result over a sliding interval equal to the voltage period of the controlled network;

- continuous calculation of the insulation resistance value in the microcontroller;

- identifying the section of the electrical network in which the insulation failure occurred.

In order to confirm the positive effect when using the proposed technical solution, the device was simulated using MATLAB-Simulink. Parameters of the simulated system: E _c =220 V; U ₁ =75 V U ₂ =75 V (for the purpose of simplicity, U ₁ =U ₂ is assumed); C _A =C _B =C _C =0.1 µF; C ₊ =C _- =0.1 µF; r _t =10 kOhm; r ₀ =5 Ohm. The test voltage period is 0.08 s; τ=0.012 s.

In fig. Figure 3 shows oscillograms of the test voltage u _t (t), the corrected signal u _k (t) and the current average voltage value at the measuring resistance u (t) with a decrease in the insulation resistance of the AC circuit at time t = 0.08 with a jump of 1 MΩ up to 20 kOhm.

In fig. Figure 4 shows oscillograms of the test voltage u _t (t), the corrected signal u _to (t) and the current average value of the voltage at the measuring resistance u (t) with a decrease in the insulation resistance of the bus with a positive potential of the direct current circuit of the alternating current circuit at time t = 0 .08 with a jump from 1 Mohm to 20 kOhm.

In fig. Figure 5 shows oscillograms of the test voltage u _t (t), the corrected signal u _k (t) and the current average voltage value at the measuring resistance when the insulation resistance of the bus with a negative potential of the DC circuit of the AC circuit decreases at time t=0.08 with a jump from 1 MΩ to 20 kΩ.

The transition time for the signal u _to (t) with a decrease in insulation resistance is 0.04 s, i.e. half the test voltage period.

If the insulation in the AC circuit is damaged, the value If the insulation of a bus with a positive potential of the DC circuit is violated, the value . If the insulation of a bus with a negative potential of the DC circuit is violated, the value .

Thus, by identifying a network section with reduced insulation resistance, the functionality of the method for monitoring insulation resistance in a dual current electrical network is expanded.

Consequently, the use in the proposed method of continuous monitoring of insulation resistance in an electrical network of a double current with an isolated neutral, in which the period of the voltage of the controlled network T _c is measured, generates a test voltage in the form of a periodic sequence of pulses of the form

where U ₁ , U ₂ are constant voltages, U ₁ ≥U ₂ ; τ - time interval, T - repetition period of test voltage pulses, ; the repetition period of the test voltage pulses is set equal to an even number of periods of the voltage of the controlled network T=kT _c , where k=2.4,..., the voltage drop across the measuring resistance connected in series with the test voltage source is measured, a delay with respect to the voltage drop across the measuring voltage is formed resistance per interval signal, subtract the delayed signal from the voltage drop across the measuring resistance, convert the resulting signal u _and (t) by correction into a signal

where ε is a small time interval, ΔT≤ε<τ; ΔT - maximum value of the test voltage period increment, u _to (t), calculate the moving average value signal u _to (t) over an interval equal to the period T _from the voltage of the controlled network, the insulation resistance is calculated using the formula

where r _t is the internal resistance of the source; r ₀ - resistance of the measuring resistor, compare the obtained value with the setting R ₀ and when r _from ≤R ₀ , turn off the electrical equipment, additionally connect the output of the test voltage source through resistors to the poles of the DC circuit, synchronize the test voltage with the voltage of one of the phases of the controlled network, measuring the moving average u(t) over the period of the test voltage signal value and identification of the area of insulation failure by signs: when - AC circuit; at - bus with positive potential of the direct current section; at - a bus with a negative potential of direct current, increases the accuracy of continuous monitoring of electrical insulation resistance and the reliability of protection of an electrical network of double current with an isolated neutral when the frequency of the controlled network changes and expands the functionality by determining the section of the electrical network in which an insulation failure occurred.

The use of the proposed technical solution in electrical systems for various purposes will improve the reliability and safety of electrical equipment.

Claims (7)

A method for continuous monitoring of insulation resistance in a double current electrical network with an isolated neutral, in which the voltage period of the controlled network T _c is measured, a test voltage is generated in the form of a periodic sequence of pulses of the form where U ₁ , U ₂ are constant voltages, U ₁ ≥U ₂ ; τ - time interval, T - repetition period of test voltage pulses, ; the repetition period of the test voltage pulses is set equal to an even number of periods T _from the voltage of the controlled network T=kT _c , where k=2, 4, ..., the voltage drop u(t) is measured across the measuring resistance connected in series with the test voltage source, a delayed one is formed in relation to the voltage drop across the measuring resistance per interval signal, subtract the delayed signal from the voltage drop across the measuring resistance, convert the resulting signal u _and (t) by correction into a signal where ε is a small time interval, ΔT≤ε<τ; ΔT - maximum value of the test voltage period increment, u _to (t), calculate the moving average value signal u _to (t) over an interval equal to the period T _from the voltage of the controlled network, the insulation resistance is calculated using the formula where r _t is the internal resistance of the source; r ₀ is the resistance of the measuring resistor, compare the obtained value with the setting R ₀ and when r _from ≤R ₀ , turn off the electrical equipment, characterized in that they additionally connect the output of the test voltage source through resistors to the poles of the DC circuit, the test voltage is synchronized with the voltage of one of phases of the controlled network, measure the moving average during the test voltage period, the value of the signal u(t) and the area of insulation failure is identified: when - AC circuit; at - DC bus with positive potential; at - DC bus with negative potential.

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