RU2585930C1 - Method of measurement of insulation resistance in electric networks - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: method of measuring insulation resistance of electrical networks, which consists in the fact that a controlled network connected DC power source of constant values produce the charge capacity of the network to a predetermined voltage, shut off the constant current source of constant values that connect the source of measuring DC voltage setpoint and calculates an estimate of the steady current value i_1set by extrapolation, to produce the measurement of the current i₁, i₂, i₃ at three different points in time t₁, t₂, t₃, and t₃/t₂= t₂/t₁, calculates an estimate of the final value of current i_1set, using the values of current i₁ i₂, i₃. Then connect to the network controlled by a constant current source of constant values of opposite polarity. Produce the charge capacity of the network to a predetermined voltage of opposite polarity. Unplug the DC power source of constant values. Plug-in power measuring DC voltage setpoint of opposite polarity. Then calculate the estimate of the final value of current i_2set by extrapolation, to produce the measurement of the current i₄, i₅, i₆ at three different points in time t₄, t₅, t₆, using the values of current i₄, i₅, i₆, calculates an estimate of the final value of current i_2set and treated with the test.

EFFECT: insulation resistance measurement of electrical networks under operating voltage and de-energized and isolated from the "ground".

1 cl, 3 dwg

Description

The invention relates to electrical measurements, namely to measurements of the insulation resistance of electrical networks that are under operating voltage or de-energized and isolated from the “earth”.

A known method of measuring insulation resistance [Ivanov EA, Kuznetsov S.E. Methods of monitoring the insulation of ship electrical systems. Tutorial. - St. Petersburg: "Elmore", 1999. S. 53-54], which can be used in AC and double current networks. The essence of the method is as follows. A three-phase rectifier bridge assembled according to the Larionov circuit is connected to the phases of the AC network. Then, three average voltage values are measured in turn: at the output of the bridge, between the positive pole of the bridge and the ground, between the negative pole and the ground. Then, the insulation resistance is calculated by the formula.

The main disadvantage of this method is the low speed due to the need to measure the average voltage values, since it is the average voltage values that are carriers of information about the value of insulation resistance. In addition, this method is unsuitable for de-energized networks.

The closest in technical essence to the proposed invention (prototype) is a method of measuring the insulation resistance of electrical networks [USSR Author's Certificate No. 1737363, class. G01R 27/18, 1992]. This method is as follows. The network capacitors are charged with respect to the ground by a constant current of constant value up to the set voltage value, the constant current source is turned off, the set constant voltage measuring source is connected and the leakage current is measured, then the measurement cycle is repeated with the voltage polarity changing on the network capacitors and the measurement results are processed . This method can significantly reduce the measurement time due to the acceleration of the charge of the network capacitors from a constant current source.

The main disadvantage of this method is that it does not take into account the absorption current that flows through the insulation after connecting the measuring DC voltage source. Current measurement (in both half-cycles) is performed immediately after connecting the measuring DC voltage source. Theoretically, this is correct, since immediately after connecting the measuring DC voltage source, the charge current of the network capacitors is zero, while the current flows only through the insulation resistance of the network. In fact, due to the imperfection of the dielectrics, immediately after connecting the measuring DC voltage source, the absorption current continues to flow through the network capacitors, which changes in time and drops to zero in some time. This leads to the fact that in the presence of large capacities in the network, the measured current (in both half-cycles) has an unacceptably large error, therefore, the result of measuring the insulation resistance will have an unacceptably large error. Therefore, the functionality is limited - the device cannot work if there are large capacities in the controlled network, since at the same time it will have an unacceptably large error in measuring the insulation resistance.

The objective of the proposed method is the expansion of functionality. The technical result consists in reducing the measurement error while maintaining high speed in the presence of large capacities in the controlled network.

The task is achieved due to the fact that a constant current source of constant value is connected to a controlled network, a network capacity is charged to a predetermined voltage, a constant current source of DC voltage is turned off, a measuring constant voltage source of a given value is connected, then a constant current source of constant current is connected to a controlled network values of opposite polarity, charge the network capacitance to a given voltage of opposite polarity, turn off the source nick DC constant current value, connect the source of measuring dc voltage predetermined opposite polarity values and treated with the measurement results, wherein after the connection of a power measuring direct voltage calculating the estimate (estimate) a steady current value i _1yst via extrapolation for that produce measurement current i _1, i ₂ , i ₃ at three different times t ₁ , t ₂ , t ₃ , and t ₃ / t ₂ = t ₂ / t ₁ , using the values of current i ₁ , i ₂ , i ₃ calculate the estimate (forecast) of the steady-state value current i _1yst according to the formula, after connecting the measuring DC voltage source of a given value of opposite polarity, the estimate (forecast) of the steady-state current i _{2yst is} also calculated using extrapolation to measure current i ₄ , i ₅ , i ₆ at three different times t ₄ , t ₅ , t ₆ , and t ₆ / t ₅ = t ₅ / t ₄ , using the values of current i ₄ , i ₅ , i ₆ , calculate the estimate (forecast) of the steady-state value of current i _2yst according to the formula.

In FIG. 1. shows a simplified diagram of a device that implements the method in terms of impact on a controlled network, FIG. 2. and FIG. 3 shows time diagrams of currents explaining this measurement method.

The essence of the proposed method is as follows.

Two half-cycles of measurement are necessary in order to eliminate the influence of the interfering constant component. The fast charge of the network capacities is achieved due to the linear law of voltage change, not exponential. After charging the network capacitance to voltage U ₁ and connecting the voltage source, the absorption current due to the imperfect dielectric continues to flow through the network capacitance. The absorption current changes over time and drops to zero over a long time. However, there is no way to measure the leakage current through the insulation resistance separately from the absorption current of the containers. It is possible to measure the sum of the leakage current through the insulation resistance and absorption currents of the network capacitors. If you use the current value obtained immediately after connecting the voltage source, the result of measuring the insulation resistance will have an unacceptably large error. It is impractical to wait for the time required for the end of the absorption current, since the absorption current can drop to zero in a very long time.

In the proposed method, extrapolation is performed, that is, the estimation (forecast) of the steady-state current value is calculated without waiting for the absorption current to end. Let's consider how this is done in more detail.

When measuring large values of insulation resistance (1-10 MΩ), leakage currents are very small and, as a rule, amount to units or tens of microamps. As a rule, a current containing 3 components is considered: a charge current, an absorption current, and a leakage current. In this case, the charge current is created by a constant current source. After the charge, a voltage source is connected, while only the absorption current and leakage current flow in the measuring circuit, FIG. 2.

With a constant measuring voltage, the change in the absorption current in time obeys a power law, the total current with the leakage current will have the form [1]

. Получены соответствующие значения тока i₁, i₂, i₃ фиг. 3. По этим значениям составляем систему уравненийTo achieve high performance it is necessary as soon as possible to determine the steady-state current value i _yst , without waiting for the end of the absorption current. Let the measurements taken at times t ₁ , t ₂ , t ₃ , and $$\frac{t_3}{t_2}=\frac{t_2}{t_{\mathrm{one}}}$$

. The corresponding current values i ₁ , i ₂ , i _{3 of} FIG. 3. Based on these values, we compose a system of equations

There are three unknowns in this system of equations - i _mouth , a, n and three equations. Therefore, there is only one solution. We transform the system of equations

, можно записатьAs $$\frac{t_3}{t_2}=\frac{t_2}{t_{\mathrm{one}}}$$

can be written

After the conversion, we get the formula for extrapolation

Extrapolation is carried out both in the first half-cycle, the value of i _{1st is calculated} , and in the second half-cycle, the value of i _{2st is calculated} . Then the insulation resistance is calculated by the formula

Due to the use of extrapolation, there is no need to wait for the end of the absorption current, and high performance is maintained even if large capacities exist in the controlled network. In comparison with the prototype method, the error decreases, since in the prototype method the measured current values will have an error due to the absorption current, and in the proposed method, due to extrapolation, the obtained current values i _1st and i _2st are equal only to the leakage current through the insulation resistance . Thus, the expansion of functionality is achieved, the application of the method allows to reduce the measurement error while maintaining high speed in the presence of large capacities in the controlled network.

Literature

1. The effect of absorption current on the process of measuring insulation resistance. Lachin V.I., Solomentsev K.Yu., Nguyen K.U. Izv. universities. North - Kavk. region. Tech. Sciences. - 2013. - No. 6. - S. 32-35.

Claims (1)

A method of measuring the insulation resistance of electrical networks, which consists in connecting a constant current source of constant value to a controlled network, charging the network capacity to a predetermined voltage, disconnecting a constant current source of DC voltage, connecting a measuring constant voltage source of a predetermined value, then connecting to a controlled network a constant current source of a constant value of opposite polarity, they charge the network capacitance to a given voltage against bying polarity disconnect the constant current source of constant value, connect the source of measuring dc voltage predetermined opposite polarity values and treated with the measurement results, characterized in that the measured current i _1, i _2, i ₃ after connection of a power measuring DC voltage at three different instants t time ₁ , t ₂ , t ₃ , and t ₃ / t ₂ = t ₂ / t ₁ , calculate the estimate of the steady-state value of current i _1ust , using the values of current i ₁ , i ₂ , i ₃ , using extrapolation according to the formula, measure the current i ₄ , i ₅ , i ₆ after connecting the measuring DC voltage source of a given value of opposite polarity at three different time instants t ₄ , t ₅ , t ₆ , and t ₆ / t ₅ = t ₅ / t ₄ , and the estimate of the steady-state current value i ₂ current i ₄ , i ₅ , i ₆ , using extrapolation according to the formula.

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