RU2614187C1 - Determination method of the circuit insulation resistance and insulation resistance of the joined alternating current circuit with insulated neutral - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: measure the average values of the voltage between the positive and negative poles of the three-phase rectifier bridge, assembled on semiconductor diodes according to Larionov's circuit and connected to the phases of AC mains, as well as between the positive and negative poles of the three-phase rectifier bridge and the "ground". At that the voltages grading at the circuit phases is performed, by connecting parallel to the three-phase rectifier bridge poles of two series-connected the first and the second resistors, the common point of which is connected to the "ground". Measure the average value of the current through the wire, connecting the common point of the first and the second resistors with the "ground", measure the average values of the differential currents, flowing through the circuit connections, by means of the sensors of differential currents for measurement of the current average values, after its connecting initially to one of the poles of the third resistor three-phase rectifier bridge, one terminal of which is connected to the common point of the first and second resistors, and then to the other pole of the fourth resistor three-phase rectifier bridge, one terminal of which is connected to the common point of the first and second resistors. The insulation resistance values of the entire circuit as a whole and the insulation resistance values of connections are determined from the corresponding expressions.

EFFECT: extension of functionality by measuring the insulation resistance of connections, reduction of the line-to-earth voltage imbalances value, arising at the circuit insulation resistance and the connections insulation resistance determination.

11 dwg

Description

The invention relates to electrical engineering and can be used to create devices for monitoring and measuring the insulation resistance of AC networks with isolated neutral.

A known method of measuring and monitoring the insulation resistance of isolated from the ground power electric networks of alternating current under operating voltage (RU 2377581 C1, published December 27, 2009), which consists in applying a stabilized constant voltage to the power network by connecting a measuring circuit to a controlled network, consisting of series-connected resistor current sensor Rdt, stabilized constant voltage source Ust and additional resistor Rd with filter capacitor Cf. The additional resistor Rd is selected k-times larger than the value of the resistor current sensor Rdt, then the voltage U1 at the resistor current sensor is measured, after which the equivalent insulation resistance R from the controlled network is calculated by the formula: Riz = Rdt [Ust / U1- (k + 1 )], where Riz is the equivalent insulation resistance of the power network.

A disadvantage of the known analogue is that this method cannot be measured insulation resistance of individual connections.

A known method for monitoring the insulation status of the feeders of a three-phase AC network with isolated neutral (RU 2400764 C1, published September 27, 2010), in which the phase angle between the current and the zero-sequence voltage is determined, the current insulation resistance of each controlled feeder is calculated according

R = 3 * U _0/3 * I ₀ * cosφ * 10 ^-3,

where R is the insulation resistance of the feeder,

3 * I ₀ - zero sequence current in mA,

3 * U ₀ - zero sequence voltage in V,

ϕ * is the phase angle between the current and voltage of the zero sequence.

The disadvantage of this method is the complexity of its implementation, due to the fact that it is necessary to determine not only the current and voltage of the zero sequence, but also the phase angle between them.

The closest analogue to the proposed invention is a method for determining the insulation resistance of the network and the insulation resistance of the AC connections with an isolated neutral (Ivanov E., Dyachkov A. How to correctly measure the insulation resistance of electrical installations, Electrical News, No. 2 (14), 2002, p. 50-52), in which, a three-phase rectifier bridge assembled on semiconductor diodes according to the Larionov circuit is connected to the phases of the AC network, while the voltmeter of the magnetoelectric system is alternately from eryayut three voltages: U _cp - bridge output, U ₁ - bridge between a positive pole and the ground, U ₂ - bridge between the negative pole and the ground. The calculation of the insulation resistance of the network R is performed according to the formula:

R = R _v * (U _cp / (U ₁ + U ₂ ) -1), where R _v is the resistance of the voltmeter, similar to the formula for the method of three samples of the voltmeter in DC networks.

It is significant that in such cases, measurements should be made with a voltmeter of the magnetoelectric system, since only average voltage values are carriers of information about the value of insulation resistance. This method is suitable for use in single-phase and three-phase AC networks, in networks with controlled and uncontrolled rectifiers.

The disadvantage of the closest analogue is that this method cannot be used to measure the insulation resistances of individual connections, and this method also causes significant voltage imbalances between the phases and the ground, which leads to increased voltage on the individual phases relative to the ground, which means to reduce the isolation resource.

The objective of the invention is to provide a method that allows to eliminate the disadvantages of the known method.

The technical result is the expansion of functionality by measuring the insulation resistance of the connections, reducing the voltage imbalance between the phases and the "ground" that occurs when determining the insulation resistance of the network and the insulation resistance of the connections.

The technical result is achieved by the fact that in the method for determining the insulation resistance of an alternating current main with an isolated neutral and the insulation resistances of the connections of an alternating current main with an insulated neutral, the average voltage between the positive and negative poles of a three-phase rectifier bridge assembled on semiconductor diodes according to the Larionov circuit and connected to the phases of the alternating current network, as well as between the positive and negative poles, three-phase about the rectifier bridge and ground, equalize the voltage on the phases of the network by connecting parallel to the poles of the three-phase rectifier bridge two series-connected first and second resistors, the common point of which is connected to the ground, at the same time measure the average current through the wire connecting the common point of the first and a second resistor with ground, measure the average of the differential currents flowing through the network connections, using differential current sensors for measuring average values currents, after connecting first to one of the poles of the three-phase rectifier bridge of the third resistor, one of the terminals of which is connected to the common point of the first and second resistors, and then to the other pole of the three-phase rectifier bridge of the fourth resistor, one of the terminals of which is connected to the common point of the first and second resistors, and the values of the insulation resistance of the entire network as a whole and the insulation resistance of the connections are determined from the expressions:

R _i = | (U ₊ + U _- -U) / (I _{i +} -I _i- ) |;

R = | (U ₊ + U _- -U) / (I ₊ -I _- ) |,

Where

R is the total insulation resistance of the entire network;

R _i is the total insulation resistance of the i-th connection;

U ₊ is the average value of the voltage at the positive pole of the rectifier bridge relative to the "ground" when a third resistor is connected to it;

U _- is the average value of the voltage at the negative pole of the rectifier bridge relative to the "ground" when a fourth resistor is connected to it;

U is the average voltage between the positive and negative poles of the rectifier bridge;

I ₊ is the average value of the current through the wire connecting the common point of the first and second resistors to the ground when connected to the positive pole of the rectifier bridge of the third resistor;

I _- is the average current through the wire connecting the common point of the first and second resistors to the ground, when the fourth resistor is connected to the negative pole of the rectifier bridge;

I _{i +} is the average value of the differential current of the i-th connection caused by the connection of the third resistor to the positive pole of the rectifier bridge;

I _i- is the average value of the differential current of the i-th connection caused by the fourth resistor connected to the negative pole of the rectifier bridge.

The essence of the method is illustrated by the diagrams shown in FIG. 1 and 2 and the waveforms in figures 3-11.

In FIG. 1 shows a power source 1 of an alternating current network with isolated neutral 17, load connections 22, 23, 24, 25 with capacitances and active resistances of their insulation. For example, connection 22 with load 13 is connected to phases a, b and c, connection 23 with load 14 is connected to phases b and c, connection 24 with load 15 is connected to phases a and b, connection 25 with load 16 is connected to phases a and from. In addition, the diagram (Fig. 1) shows resistive elements 5 and 6 connected via keys 7 and 8 to the poles “-” and “+” of rectifier 2, respectively, voltage measuring devices 9, 10 and 12 (voltmeters for measuring average values) , devices 18, 19, 20 and 21 for measuring average values of differential currents (differential current sensors for measuring average values of currents) flowing through connections 22, 23, 24 and 25, resistors 3, 4 connected in series and connected in parallel to the poles of rectifier 2 , milliammeter 11 for measuring average values of currents flowing in the wire connecting the common point of resistors 3, 4, 5 and 6 and "ground".

The method is as follows. Measure the average voltage between the positive and negative poles of the rectifier bridge with a voltmeter 12. Measure the average voltage between the ground and the positive pole of the rectifier bridge with voltmeters 10, and also measure the average current in the wire connecting the common point of the resistors 3, 4, 5 6 and ground, with a milliammeter 11, measure the average values of differential currents using differential current sensors to measure average values (for example, differential current sensors of the DD type produced by NPP EKRA LLC), flowing through the network connections after connecting the resistor 6 to the positive pole of the rectifier bridge. Measure the average voltage between the ground and the negative bridge of the rectifier bridge using a voltmeter 9, and also measure the average current through a milliammeter 11, measure the average values of differential currents using differential current sensors to measure the average values of currents flowing through the network connections after connecting to the negative pole ryamitelya resistor 5. The insulation resistance value of each of the connections and the network as a whole is determined from the expression:

R _i = | (U ₁₊ + U _2- -U) / (I _{i +} -I _i- ) |; R = | (U ₁₊ + U _2- -U) / (I ₀₊ -I _0- ) |,

Where

R is the total insulation resistance of the entire network;

R _i is the total insulation resistance of the i-th connection;

U ₁₊ is the average value of the voltage at the positive pole of the rectifier bridge relative to the "ground" when a third resistor is connected to it;

U _2- is the average value of the voltage at the negative pole of the rectifier bridge relative to the "ground" when a fourth resistor is connected to it;

U is the average voltage between the positive and negative poles of the rectifier bridge;

I ₀₊ is the average value of the current through the wire connecting the common point of the first and second resistors to the ground when connected to the positive pole of the rectifier bridge of the third resistor;

I _0- is the average current through the wire connecting the common point of the first and second resistors to the ground, when the fourth resistor is connected to the negative pole of the rectifier bridge;

I _{i +} is the average value of the differential current of the i-th connection caused by the connection of the third resistor to the positive pole of the rectifier bridge;

I _i- is the average value of the differential current of the i-th connection caused by the fourth resistor connected to the negative pole of the rectifier bridge.

,

эквивалентное сопротивление изоляции сети 17 переменного тока соответственно при отрицательном и положительном напряжении фаз сети относительно «земли»,

и

эквивалентное сопротивление изоляции первого присоединения соответственно при отрицательном и положительном напряжении фаз сети относительно «земли»,

и

эквивалентное сопротивление изоляции второго присоединения соответственно при отрицательном и положительном напряжении фаз сети относительно «земли»,

и

эквивалентное сопротивление изоляции третьего присоединения соответственно при отрицательном и положительном напряжении фаз сети относительно «земли»,

и

эквивалентное сопротивление изоляции четвертого присоединения соответственно при отрицательном и положительном напряжении фаз сети относительно «земли», R₃ и R₄ сопротивления резисторов, подключаемых с помощью ключей 7 и 8 к выводам выпрямителя соответственно «-» и «+», R₁ и R₂ - сопротивления резисторов 3 и 4.In FIG. 2 shows a simplified equivalent circuit of an alternating current main with isolated neutral for calculating the insulation resistances shown in FIG. 1, where E is the emf at the output of the rectifier,

,

equivalent insulation resistance of the AC network 17, respectively, with a negative and positive voltage of the phases of the network relative to the "ground",

and

equivalent insulation resistance of the first connection, respectively, with a negative and positive voltage of the phases of the network relative to the "ground",

and

equivalent insulation resistance of the second connection, respectively, with negative and positive voltage of the phases of the network relative to the "ground",

and

equivalent insulation resistance of the third connection, respectively, with a negative and positive voltage of the phases of the network relative to the "ground",

and

equivalent insulation resistance of the fourth connection, respectively, with a negative and positive voltage of the network phases relative to the "ground", R ₃ and R _{4 of the} resistance of the resistors connected with the keys 7 and 8 to the terminals of the rectifier, respectively, "-" and "+", R ₁ and R ₂ - resistance of resistors 3 and 4.

AC neutral neutral bias voltage U _cm , i.e. the voltage of the point O (Fig. 1) relative to the "earth" the greater, the greater the difference between the insulation resistances of the phases relative to the "earth". Moreover, the higher the neutral bias voltage, the higher the voltage differences at the phases of the network relative to the "ground". Mathematical and natural studies have shown that the voltage distortion between the poles of the rectifier bridge and the "ground" ΔU is greater, the greater the bias voltage of the neutral of the AC network

where ΔU = U ₊ -U _- , U ₊ is the voltage at the “+” terminal of the rectifier relative to the ground, U _- is the voltage at the “-” terminal of the rectifier relative to the ground.

Before closing the keys K1 and K2:

voltage at the “+” terminal of the rectifier relative to the “ground”

The voltage at the “-” terminal of the rectifier relative to the “ground”

U _- = U ₊ -E

As can be seen from expressions 1 and 2, the voltage distortion between the poles of the rectifier bridge and the "ground" is less, the smaller the ratio

R ₁ // R _siz- // R _1iz- // R _2iz- // R _3iz- // R _4iz- and R ₂ // R _{siz +} // R _{1iz +} // R _{2iz +} // R _{3iz +} // R _{4 of +}

Thus, when the insulation resistance of the phases of the network relative to the ground deteriorates, the voltages on the phases of the network are balanced against the “ground”, achieving a decrease in the neutral offset. To do this, include in the circuit parallel to the poles of the rectifier bridge two series-connected identical resistors, the common point of which is connected to the "ground". Resistors 3, 4 are selected by a resistance many times smaller than the insulation resistance, for example, a resistance of 100 ... 1000 kOhm.

After closing the key K1, the voltage U ₁₊ at the terminal "+" of the rectifier bridge relative to the "ground"

where R _{Σ from +} is the equivalent insulation resistance of the network and connections at a positive phase voltage relative to the "ground" R _{Σ from +} = R s _{+ +} // R _{1 from +} // R _{2 from +} // R _{3 from +} // R _{4 from +} , R _{Σ from -} - equivalent resistance of the network and connections with a negative phase voltage relative to the "ground" R _Σiz- = R _siz- // R _1iz- // R _2iz- // R _3iz- // R _4iz- .

Expression (3) can be written in terms of conductivity G

U ₁₊ = E * (1 / (G ₂ + G ₄ + G _{Σ from +} ) / (1 / (G ₂ + G ₄ + G _{Σ from +} ) +1 (G ₁ + G _{Σ from-} )),

Where

G ₂ = 1 / R ₂ , G ₄ = 1 / R ₄ , G _{Σ from +} = 1 / R _{Σ from +} , G ₁ = 1 / R ₁ , G _{Σ from} - = 1 / R _{Σ from -}

After closing the key K1, the current I ₀₊ through the milliammeter

After closing the key K2, the voltage U _2- at the “-” terminal of the rectifier bridge relative to the “ground”

After closing the key K2, the current I _0- through the milliammeter:

Using expressions 1-6, you can get:

G _{Σ from +} + G _{Σ from-} = 1 / (R _{Σ from +} // R _{Σ from-} ) = (I ₀₊ -I _0- ) / (Е-U _2- -U ₁₊ )

Thus, the total insulation resistance of the entire network:

R _{Σ from} = R _{Σ from +} // R _{Σ from-} = | (U ₁₊ + U _2- -U) / (I ₀₊ -I _0- ) |,

Where

U ₁₊ is the average value of the voltage at the positive pole of the rectifier bridge relative to the "ground" when a third resistor is connected to it;

U _2- is the average value of the voltage at the negative pole of the rectifier bridge relative to the "ground" when a fourth resistor is connected to it;

U is the average voltage between the positive and negative poles of the rectifier bridge;

I ₀₊ - the average value of the current through the wire connecting the common point of the resistors R ₁ and R ₂ and the "ground" when connected to the positive pole of the rectifier of the third resistor;

I _0- is the average current through the wire connecting the common point of the resistors R ₁ and R ₂ and the ground when connected to the negative pole of the rectifier of the fourth resistor.

The current flowing through a milliammeter is equal to the sum of the differential currents flowing through all the connections.

The expressions for the equivalent insulation resistance of each connection R _i will _be obtained by the example of the first connection for the circuit (Fig. 2) based on the values for the differential currents of the connections, as well as the expressions for the voltage at the poles of the rectifier when the keys K1 and K2 are closed:

R _iiz = R _iiz- // R _{iiz +} ,

where R _{i -} and R _{i + +} - equivalent insulation resistance of the first connection, respectively, with a negative and positive voltage of the phases of the network relative to the "ground".

After closing the key K1, the currents flowing through the insulation of the wires of the first connection (Fig. 1) are equal to the currents flowing through the insulation resistance of the first connection connecting the rectifier bridge “+” and “-” buses and “ground”:

I ₁₊ = U ₁₊ * G _{1 of +} , I _1- = U _1- * G _{1 of-} ,

where U ₁₊ , U _1- is the voltage at the terminals of the rectifier bridge "+" and "-" relative to the "ground", respectively,

G _{1 of +} , G _{1 of-} - the insulation conductivity of the first connection, respectively, with a positive and negative voltage of the phases of the network relative to the "ground".

The differential current of the first connection after closing the key K1:

Where

G _1iz - the total conductivity of the insulation of the first connection,

G _{1 of} = G _{1 of +} + G _{1 of}

After closing the key K2: the currents flowing through the insulation of the wires of the first connection (Fig. 1) are equal to the currents flowing through the insulation resistance of the first connection connecting the rectifier bridge “+” and “-” busbars and “earth”:

,

,

U ₂₊ , U _1- - voltage on the bus "+" and "-" relative to the "ground", respectively, after closing the key K2.

The differential current of the first connection after closing the key K2:

Based on the expressions (7, 8) we obtain the expression for the total (equivalent) insulation resistance of the first connection:

.

.

In general, for i-connection, the total insulation resistance

R _i = | (U ₁₊ + U _2- -U) / (I _{i +} -I _i- ) |,

R _i is the total insulation resistance of the i-th connection;

U ₁₊ is the average value of the voltage at the positive pole of the rectifier bridge relative to the "ground" when a third resistor is connected to it;

U _2- is the average value of the voltage at the negative pole of the rectifier bridge relative to the "ground" when a fourth resistor is connected to it;

U is the average voltage between the positive and negative poles of the rectifier bridge;

I _{i +} is the average value of the differential current of the i-th connection caused by the connection of the third resistor to the positive pole of the rectifier bridge;

I _i- is the average value of the differential current of the i-th connection caused by the fourth resistor connected to the negative pole of the rectifier bridge.

The figure 3 shows the voltage waveform at the positive terminal of the rectifier bridge relative to the "ground" 1 and the voltage waveform at the negative terminal of the rectifier bridge relative to the "ground" 2 with open keys 7 and 8 and good insulation. The figure 4 shows the voltage waveform at the positive terminal of the rectifier bridge relative to the "ground" 1 and the voltage waveform at the negative terminal of the rectifier bridge relative to the "ground" 2 with the key 7 closed and good insulation. The figure 5 shows the voltage waveform at the positive terminal of the rectifier bridge relative to the "ground" 1 and the voltage waveform at the negative terminal of the rectifier bridge relative to the "earth" 2 with the key 8 closed with good insulation.

The figure 6 shows the voltage waveform at the positive terminal of the rectifier bridge relative to the "ground" 1 and the voltage waveform at the negative terminal of the rectifier bridge relative to the "ground" 2 with open keys 7 and 8 and poor insulation of phase A. The figure 7 shows the voltage waveform at the positive terminal rectifier bridge relative to the "ground" 1 and the voltage waveform at the negative terminal of the rectifier bridge relative to the "ground" 2 with the key 7 closed and poor insulation of phase A. In the figures e 8 shows the voltage waveform at the positive terminal of the rectifier bridge relative to the "ground" 1 and the voltage waveform at the negative terminal of the rectifier bridge relative to the "ground" 2 with the closed key 8 with poor insulation of phase A.

Figure 9 shows the waveforms of current 1 through a milliammeter and the waveform of the differential current 2 through the connection with open keys 7 and 8 with poor insulation of phase A. Figure 10 shows the waveforms of the current 1 through the milliammeter and the waveform of differential current 2 through the connection when the switch 7 is closed with bad isolation of phase A. Figure 11 shows the waveforms of current 1 through a milliammeter and the waveform of differential current 2 through connection with a closed key 8 with poor insulation of phase A.

It can be seen that the average values of voltages and currents with a closed key 7 and with a closed key 8 are different.

A device that implements the proposed method for determining the insulation resistance of an AC network with an isolated neutral and the insulation resistance of the AC connections with an isolated neutral is the EKRA-SKI terminal, a rectifier bridge assembled on semiconductor diodes according to the Larionov circuit and sensors as differential current sensors type DDT-25 manufactured by NPP EKRA, successfully tested in the conditions of the testing ground of LLC NPP EKRA.

Claims (12)

A method for determining the insulation resistance of an alternating current mains with an isolated neutral and the insulation resistances of the connections of an alternating current mains with an isolated neutral, in which average voltage values are measured between the positive and negative poles of a three-phase rectifier bridge assembled on semiconductor diodes according to the Larionov circuit and connected to the phases of the alternating current network , as well as between the positive and negative poles of a three-phase rectifier bridge and "ground", characterized in that equalize the voltages on the phases of the network by connecting parallel to the poles of the three-phase rectifier bridge two series-connected first and second resistors, the common point of which is connected to the ground, at the same time measure the average current through the wire connecting the common point of the first and second resistors to the ground, measure the average values of the differential currents flowing through the network connections, using differential current sensors to measure the average values of the currents after connecting first and to one of the poles of the three-phase rectifier bridge of the third resistor, one of the terminals of which is connected to the common point of the first and second resistors, and then to the other pole of the three-phase rectifier bridge of the fourth resistor, one of the terminals of which is connected to the common point of the first and second resistors, and the values of the insulation resistance of the entire network as a whole and the insulation resistance of the connections are determined from the expressions: R _i = ⎥ (U ₁₊ + U _2- -U) / (I _{i +} -I _i- ), ⎢ R = ⎥ (U ₁₊ + U _2- -U) / (I ₀₊ -I _0- ) ⎢ Where R is the total insulation resistance of the entire network, R _i is the total insulation resistance of the i-th connection, U ₁₊ is the average value of the voltage at the positive pole of a three-phase rectifier bridge relative to the "ground" when a third resistor is connected to it, U _2- is the average value of the voltage at the negative pole of a three-phase rectifier bridge relative to the "ground" when a fourth resistor is connected to it, U is the average voltage between the positive and negative poles of a three-phase rectifier bridge, I ₀₊ is the average value of the current through the wire connecting the common point of the first and second resistors to the ground when connected to the positive pole of the three-phase rectifier bridge of the third resistor, I _0- is the average current through the wire connecting the common point of the first and second resistors to ground when connected to the negative pole of the three-phase rectifier bridge of the fourth resistor, I _{i +} - the average value of the differential current of the i-th connection, caused by the connection of the third resistor to the positive pole of the three-phase rectifier bridge, I _i- is the average value of the differential current of the i-th connection caused by the connection of the fourth resistor to the negative pole of the three-phase rectifier bridge.

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