CN106093591B - A system and method for measuring capacitive current in a neutral point ungrounded distribution network - Google Patents

Abstract

The invention relates to a capacitive current measurement technology of a distribution network, in particular to a system and method for measuring a capacitive current of a neutral point ungrounded distribution network. The measurement system includes a bus voltage transformer, an adjustable resistor, a variable frequency voltage source and a voltage measuring instrument; The adjustable resistor is connected in series with the variable frequency voltage source and connected to both ends of the third winding of the bus voltage transformer, and the voltage measuring instrument is also connected to both ends of the third winding of the bus voltage transformer. The method of measuring by using the measurement system solves the problem that the short-circuit impedance of the bus voltage transformer affects the measurement accuracy, and considers the influence of the zero-sequence voltage caused by the unbalanced degree of the power grid on the measurement of the capacitance current by the signal injection method, and improves the capacitance of the distribution network. To ensure the accuracy of current measurement, an accurate Thevenin equivalent circuit model is used to solve for the capacitance of the system to ground. The measurement process is carried out on the third winding side of the transformer, without changing the primary equipment wiring, the measurement method is safe and convenient, and the result is accurate and reliable.

Description

A system and method for measuring capacitive current in a neutral point ungrounded distribution network

technical field

The invention belongs to the technical field of capacitive current measurement of a distribution network, and in particular relates to a system and method for measuring a capacitive current of a neutral point ungrounded distribution network.

Background technique

The early medium and low-voltage distribution networks in my country mostly adopted the neutral point ungrounded mode. When a single-phase ground fault occurs, the grounding arc can be self-extinguished under the condition that the grid capacitive current is not large, and the distribution network can continue to supply power with faults for 1~ 2 hours. With the continuous development of the distribution network of the power system, especially the large number of cable lines in the urban power grid, the capacitive current of the line to the ground continues to increase, resulting in a sharp increase in the single-phase ground fault current of the distribution network line, and it is difficult for the grounding arc to be self-sufficient. extinguished, causing a phase-to-phase short circuit fault. In order to ensure that all kinds of grounding arcs in the power grid can be extinguished reliably, when the system capacitive current is greater than the limit value required by regulations, an arc suppression coil must be installed to compensate for the grounding capacitive current. The accurate measurement of the capacitive current of the distribution network is the prerequisite for reasonable compensation of the arc suppression coil. Whether the arc suppression coil is installed or not and the size of the installation capacity depend on the measured value of the capacitive current.

In order to avoid operation on the primary side of the grid, the widely used measurement method is the signal injection method. When measuring the capacitive current of the neutral point ungrounded distribution network, it is usually chosen to inject different frequency signals at the open triangle end of the transformer, and the ground capacitance of the grid is calculated by measuring the current of the injected signal and the voltage of the open triangle end. Considering that the short-circuit impedance of the voltage transformer cannot be ignored, some methods reduce the error by adding an adjustable inductance. However, considering that there will be a certain zero-sequence voltage in the actual operation of the power grid, the zero-sequence voltage will be reflected in the open delta end of the transformer, which will have a great impact on the voltage measurement and often cause large errors in the measurement results.

Contents of the invention

The purpose of the present invention is to propose a measurement process carried out on the third winding side of the bus voltage transformer without changing the wiring of the primary equipment, and considering the influence of the zero-sequence voltage during the actual operation of the power grid when using the injection signal method to measure the capacitive current Influence of measurement results, a neutral point ungrounded distribution network capacitance current measurement system and method that can improve the accuracy of distribution network capacitance current measurement.

In order to achieve the above object, the technical solution adopted by the present invention is: a neutral point ungrounded distribution network capacitive current measurement system, including a bus voltage transformer, an adjustable resistor, a variable frequency voltage source and a voltage measuring instrument; The adjustable resistor is connected in series with the variable frequency voltage source and connected to both ends of the third winding of the bus voltage transformer, and the voltage measuring instrument is also connected to both ends of the third winding of the bus voltage transformer.

In the neutral point ungrounded distribution network capacitive current measurement system, the third winding of the bus voltage transformer is connected in an open delta.

In the neutral point ungrounded distribution network capacitive current measurement system, the voltage measuring instrument is an electromagnetic instrument, and is connected to the open delta end of the bus voltage transformer.

A method for measuring the capacitive current of a neutral point ungrounded distribution network, comprising the following steps:

S1. Disconnect the variable frequency voltage source and the adjustable resistance branch, and measure the open circuit voltage U ₀ ;

S2. Set the output of the variable frequency voltage source to zero, adjust the resistance of the adjustable resistor to the maximum, connect the variable frequency voltage source and the branch of the adjustable resistor, adjust the resistance of the adjustable resistor to R ₁ , and record the reading of the voltage measuring instrument U ₀₁ ;

S3. Adjust the resistance value of the adjustable resistor to R ₂ , and record the reading of the voltage measuring instrument U ₀₂ ;

S4. Keep the adjustable resistor resistance value R2 _unchanged , set the output voltage frequency of the variable frequency voltage source to 25Hz, the effective value of the output voltage is E1, and record the reading _of the voltage measuring instrument U1 _;

S5. Keep the adjustable resistance value R2 unchanged, set the output voltage frequency of the variable frequency voltage source to 75Hz, the effective value of the output voltage is _E2 , and record the reading _U2 of the voltage measuring instrument _;

S6. According to the measured data in steps S1, S2, S3, S4, and S5, the capacitance of the grid to ground is calculated, and the capacitance current value of the grid is obtained through calculation.

In the neutral point ungrounded distribution network capacitive current measurement method, the variable frequency voltage source in the step S2 is a variable frequency voltage source with an output voltage range of 0 to 5V and a frequency range of 10Hz to 100Hz; the adjustable resistance It is an adjustable resistor with high precision and readable resistance, and the adjustment range is 0~5Ω.

In the method for measuring the capacitive current of the neutral point ungrounded distribution network, if the open circuit voltage is large in the steps S2 and S3, the resistance value of the adjustable resistor is correspondingly increased, and the adjustable resistance range is 1-3Ω.

In the method for measuring the capacitive current of the neutral point ungrounded distribution network, the output voltage of the variable frequency voltage source in the steps S4 and S5 is set at about 5V, and the principle is not to affect the normal operation of the power grid.

In the described neutral point ungrounded distribution network capacitive current measurement method, solving the grid-to-ground capacitance and the capacitor current value of the grid in the step S6 includes the following steps:

S61. Obtain R and |X ₅₀ |;

According to the steps S1, S2, S3, the following equations are obtained:

Among them, U ₀ is the open circuit voltage; C ₀ is the grid-to-ground capacitance converted to the third winding of the bus voltage transformer; R and L are the leakage resistance and leakage inductance converted to the third winding of the bus voltage transformer; X ₅₀ is the capacitance The reactance of C ₀ and inductance L at power frequency 50Hz;

Solve (1) to get R and |X ₅₀ |;

S62. Calculate U _E1 and U _E2 ;

Obtain according to described step S4, S5:

Among them, U ₀₂ is the effective value of the voltage at the measurement point when the voltage source U ₀ acts alone, U _E1 is the effective value of the output voltage when the output frequency of the variable frequency voltage source is 25Hz, and U _E2 is the effective value of the output voltage when the output frequency of the variable frequency voltage source is 75Hz;

Solution (2) to get U _E1 and U _E2 ;

S63. Calculate |X ₂₅ | and |X ₇₅ |;

According to the principle of partial pressure, we can get:

Among them, X ₂₅ is the reactance of capacitor C ₀ and inductance L at 25Hz, and X ₇₅ is the reactance of capacitor C ₀ and inductance L at 75Hz;

Solution (3) gets |X ₂₅ | and |X ₇₅ |;

S64. Calculate the capacitance C ₀ ;

Comparing the sizes of |X ₅₀ |, |X ₂₅ | and |X ₇₅ | obtained in steps S61 and S63;

a. If |X ₇₅ | is the smallest, then there is

b. If |X ₅₀ | is the smallest, then there is

c. If |X ₂₅ | is the smallest, then there is

Wherein, ω ₇₅ , ω ₅₀ and ω ₂₅ are angular frequencies corresponding to frequencies 75Hz, 50Hz and 25Hz, respectively;

Solve (4), (5), (6) to obtain capacitance C ₀ ;

S65. Convert the capacitance C ₀ obtained in step S64 into the ground capacitance C ₀ ′ of the power grid;

The conversion formula is:

Wherein, k ₁₃ is the transformation ratio from the first winding to the third winding of the voltage transformer;

S66. Find the capacitive current of the grid at last:

Among them, I _C is the capacitive current of the grid, is the rated phase voltage of the grid.

In order to solve the problem that the short-circuit impedance of the bus voltage transformer affects the measurement accuracy, and consider the influence of the zero-sequence voltage caused by the unbalanced degree of the power grid on the signal injection method to measure the capacitive current, improve the accuracy of the capacitive current measurement of the distribution network, The proposed neutral point ungrounded distribution network capacitive current measurement system includes a bus voltage transformer, an adjustable resistor, a variable frequency voltage source and a voltage measuring instrument; when the bus voltage transformer is in a normal working state, its third winding is an open triangle Connection method, the variable frequency voltage source and the adjustable resistor are connected in series to the open triangle end of the voltage transformer, and the voltage measuring instrument is an electromagnetic instrument, which is connected to the open triangle end of the transformer for measuring the effective value of the voltage.

At the same time, a method for measuring the capacitive current of the neutral point ungrounded distribution network is proposed, including the following steps:

1) Disconnect the variable frequency voltage source and the adjustable resistance branch, and measure the open circuit voltage;

2) Set the output of the variable frequency voltage source to zero, adjust the resistance value of the adjustable resistor to the maximum, connect the variable frequency voltage source and the branch circuit of the adjustable resistor, adjust the resistance value of the adjustable resistor, and record the reading of the voltage measuring instrument;

3) Change the resistance value of the adjustable resistor and record the reading of the voltage measuring instrument;

4) Keep the resistance value of the adjustable resistor unchanged, set the variable frequency voltage source to output a voltage of 25Hz, and record the readings of the voltage measuring instrument;

5) Keep the resistance value of the adjustable resistor unchanged, set the variable frequency voltage source to output a voltage of 75Hz, and record the readings of the voltage measuring instrument;

In the above step 1), the output voltage range of the selected variable frequency voltage source is 0-5V, and the frequency range is 10Hz-100Hz. The selected adjustable resistor has an adjustment range of 0-5Ω, and the resistance value is readable and has high precision.

In the above steps 2) and 3), it is more appropriate to adjust the resistance value of the adjustable resistor within the range of 1-3Ω, which should not be too small or too large. In order to improve the measurement accuracy, if the open circuit voltage is large, the resistance value can be adjusted accordingly. bigger.

In the above steps 4) and 5), the output voltage of the variable frequency voltage source can be set at about 5V, which will not affect the normal operation of the power grid.

The beneficial effect of the present invention is that under the condition of considering the zero-sequence voltage of the system and the short-circuit impedance of the transformer, an accurate Thevenin equivalent circuit model is used to solve the ground capacitance of the system. In principle, it not only avoids the influence of the short-circuit impedance of the voltage transformer on the measurement results, but also solves the problem of unavoidable power frequency voltage interference when the general signal injection method measures the capacitive current. The measurement process is carried out on the third winding side of the transformer, without changing the primary equipment wiring, and the measurement method is safe and convenient. When using the injection signal method to measure the capacitive current, the problem of the influence of the zero-sequence voltage in the actual operation of the power grid on the measurement results is solved, and the measurement results are accurate and reliable.

Description of drawings

Fig. 1 is a system schematic diagram of a specific embodiment of the present invention;

Fig. 2 is the measurement schematic diagram of a specific embodiment of the present invention;

Among them, 1-bus voltage transformer, 2-adjustable resistor, 3-variable frequency voltage source, 4-voltage measuring instrument, A-mark of the opening end of the third winding, B-mark of the opening end of the third winding.

Detailed ways

Embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

Fig. 1 is the schematic diagram of the capacitive current measuring system of neutral point ungrounded distribution network of the embodiment of the present invention, comprises bus voltage transformer 1, adjustable resistance 2, variable frequency voltage source 3 and voltage measuring instrument 4; Said adjustable resistance 2 After being connected in series with the variable frequency voltage source 3, it is connected to both ends of the third winding of the bus voltage transformer 1, and the voltage measuring instrument 4 is also connected to both ends of the third winding of the bus voltage transformer 1.

In the above-mentioned capacitive current measurement system of the neutral point ungrounded distribution network, the third winding of the bus voltage transformer 1 is connected in an open delta. The voltage measuring instrument 4 is an electromagnetic instrument, and is connected to the open triangle end of the bus voltage transformer 1 .

During specific implementation, the neutral point ungrounded distribution network capacitive current measurement system includes a bus voltage transformer 1 , an adjustable resistor 2 , a variable frequency voltage source 3 and a voltage measuring instrument 4 . The busbar voltage transformer 1 is in normal working condition, its third winding is open delta connection, the opening ends are marked as A, B, after the adjustable resistor 2 is connected in series with the variable frequency voltage source 3, it is connected to A, B of the voltage transformer At both ends, the voltage measuring instrument 4 is an electromagnetic instrument, which is connected to both ends of A and B to measure the effective value of the voltage at the open triangle end.

As shown in Fig. 2, it is the method of measuring the capacitive current of the neutral point ungrounded distribution network using the above-mentioned measurement system. Due to the influence of the unbalanced degree of the power grid, a zero-sequence voltage will be generated at the open triangle end of the voltage transformer, U ₀ That is, the opening voltage at the open triangle end of the voltage transformer. Looking from both ends of A and B to the grid side, its Thevenin equivalent circuit is shown in part I of Figure 2, and its open circuit voltage is U ₀ ; C ₀ is the grid-to-ground capacitance converted to the third winding of the voltage transformer; R and L are the leakage resistance and leakage inductance converted from the voltage transformer to the third winding. In addition, R' is an adjustable resistor; E is a variable frequency voltage source.

The above measurement method specifically includes the following steps:

① Disconnect the variable frequency voltage source and the adjustable resistance branch, and measure the open circuit voltage U ₀ ;

②Set the output of the variable frequency voltage source to zero, adjust the resistance value of the adjustable resistor to the maximum, connect the variable frequency voltage source and the branch circuit of the adjustable resistor, adjust the resistance value of the adjustable resistor to R ₁ , and record the reading U ₀₁ of the voltage measuring instrument;

③ Adjust the resistance value of the adjustable resistor to R ₂ , and record the reading of the voltage measuring instrument U ₀₂ ;

④Keep the resistance value of the adjustable resistor R ₂ unchanged, set the output voltage frequency of the variable frequency voltage source to 25Hz, the effective value of the output voltage is E ₁ , and record the reading of the voltage measuring instrument U ₁ ;

_⑤Keep the resistance value of the adjustable resistor R2 unchanged, set the output voltage frequency of the variable frequency voltage source to 75Hz, the effective value of the output voltage is _E2 , and record the reading U2 of the voltage measuring instrument _;

According to the above steps ①, ②, ③, the following equations can be obtained:

Among them, X ₅₀ is the reactance of capacitor C ₀ and inductance L at the power frequency of 50Hz, and R and |X ₅₀ | can be obtained from this equation group.

The principles of the above steps ④ and ⑤ are as follows:

Under the combined action of two voltage sources U ₀ and E ₁ or E ₂ with different frequencies, the reading U ₁ or U ₂ of the voltage measuring instrument has the following relationship with the effective value of the voltage at the measuring point when the two power sources act separately:

Among them, U ₀₂ is the effective value of the voltage at the measurement point when the voltage source U ₀ acts alone, U _E1 is the effective value of the output voltage when the output frequency of the variable frequency voltage source is 25Hz, and U _E2 is the effective value of the output voltage when the output frequency of the variable frequency voltage source is 75Hz. U _E1 and U _E2 can be obtained according to the above formula.

According to the principle of partial pressure, the following equations can be obtained:

In the above formula, X ₂₅ is the reactance of capacitor C ₀ and inductance L at 25Hz, and X ₇₅ is the reactance of capacitor C ₀ and inductance L at 75Hz. By substituting data into the equations, |X ₂₅ | and |X ₇₅ | can be obtained.

⑥ According to the obtained |X ₅₀ |, |X ₂₅ | and |X ₇₅ |, calculate the ground capacitance C ₀ of the power grid, the method is as follows:

Judging the size of |X ₅₀ |, |X ₂₅ | and |X ₇₅ |,

a. If |X ₇₅ | is the smallest, then there is

b. If |X ₅₀ | is the smallest, then there is

c. If |X ₂₅ | is the smallest, then there is

In the above formula, ω ₇₅ , ω ₅₀ and ω ₂₅ are the angular frequencies corresponding to the frequencies 75Hz, 50Hz and 25Hz respectively, and C ₀ can be obtained by solving the equations.

⑦Finally, convert the obtained C ₀ into the ground capacitance C ₀ ′ of the power grid, and the conversion formula is Where k ₁₃ is the transformation ratio from the first winding to the third winding of the voltage transformer.

grid capacitive current in, is the rated phase voltage of the grid.

In order to verify the effectiveness of the specific implementation of the present invention, three groups of different capacitance values are set in Matlab/Simulink to carry out simulation analysis respectively, the first group is 3 μ F, 3.05 μ F, 3.1 μ F; the second group is 10 μ F, 10.3 μ F, 10.5 μF; the third group is 30.1μF, 30.5μF, 30.8μF.

For different combinations of system capacitances to ground, simulations were carried out, and the data obtained are shown in Table 1.

Table 1. Simulation measurement data and calculation results under different ground capacitance combinations

In the above simulation results, the measurement errors under different ground capacitance combinations are all within 2%, which verifies that the method in this embodiment is not only effective, but also has high accuracy.

In this specific implementation manner, an accurate Thevenin equivalent circuit model is used to solve the ground capacitance of the system. In principle, it not only avoids the influence of the short-circuit impedance of the voltage transformer on the measurement results, but also solves the problem of unavoidable power frequency voltage interference when the general signal injection method measures the capacitive current.

It should be understood that the parts not described in detail in this specification belong to the prior art.

Although the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, those of ordinary skill in the art should understand that these are only examples, and various variations or modifications can be made to these embodiments without departing from the principles and principles of the present invention. substance. The scope of the invention is limited only by the appended claims.

Claims (1)

1. A capacitance and current measuring method for a distribution network with a neutral point ungrounded comprises a bus voltage transformer, an adjustable resistor, a variable frequency voltage source and a voltage measuring instrument; the adjustable resistor is connected with a variable frequency voltage source in series and then is connected to two ends of a third winding of the bus voltage transformer, and the voltage measuring instrument is also connected to two ends of the third winding of the bus voltage transformer; the third winding of the bus voltage transformer is in open delta connection; the voltage measuring instrument adopts an electromagnetic instrument and is connected to the opening triangular end of the bus voltage transformer;

the method is characterized by comprising the following steps:

s1, disconnecting a variable frequency voltage source and an adjustable resistance branch, and measuring open-circuit voltage U₀；

S2, setting the output of the variable frequency voltage source to zero, adjusting the resistance value of the adjustable resistor to the maximum, connecting the variable frequency voltage source and the adjustable resistor branch, and adjusting the resistance value of the adjustable resistor to R₁Record the reading U of the voltage measuring instrument₀₁；

S3, adjusting the resistance value of the adjustable resistor to R₂Record the reading U of the voltage measuring instrument₀₂；

S4, keeping the resistance value of the adjustable resistor to be R₂The frequency of the output voltage of the variable frequency voltage source is set to be 25Hz and the effective value of the output voltage is E without change₁Record the reading U of the voltage measuring instrument₁；

S5, keeping the resistance value of the adjustable resistor to be R₂The frequency of the output voltage of the variable frequency voltage source is set to be 75Hz and the effective value of the output voltage is E without change₂Record the reading U of the voltage measuring instrument₂；

S6, solving the ground capacitance of the power grid according to the data measured in the steps S1, S2, S3, S4 and S5, and calculating to obtain the capacitance current value of the power grid;

the variable frequency voltage source in the step S2 is a variable frequency voltage source with an output voltage range of 0-5V and a frequency range of 10Hz-100 Hz; the adjustable resistor is readable with a high-precision resistance value, and the adjusting range is 0-5 omega;

if the open-circuit voltage is large in the steps S2 and S3, the resistance value of the adjustable resistor is correspondingly increased, and the adjustment range of the adjustable resistor is 1-3 omega;

in the steps S4 and S5, the output voltage of the variable frequency voltage source is set to be about 5V, and the normal operation of the power grid is not influenced on the principle;

the step S6 of solving the capacitance to ground and the capacitance current value of the power grid includes the following steps:

s61, obtaining R and | X₅₀|；

According to the steps S1, S2, S3, the following equation set is obtained:

wherein, U₀Is an open circuit voltage; c₀Converting the capacitance to the ground of the power grid of the third winding of the bus voltage transformer; r and L are leakage resistance and leakage inductance converted from the bus voltage transformer to a third winding; x₅₀Is a capacitor C₀And the reactance of the leakage inductance L under the power frequency of 50 Hz;

solving (1) to obtain R and | X₅₀|；

S62, calculating U_E1And U_E2；

According to the steps S4 and S5:

wherein, U_E1The effective value of the output voltage is U when the output frequency of the variable frequency voltage source is 25Hz_E2Outputting a voltage effective value when the output frequency of the variable frequency voltage source is 75 Hz;

obtaining U by resolving (2)_E1And U_E2；

S63, obtaining | X₂₅I and I X₇₅|；

From the principle of partial pressure, it is possible to obtain:

wherein, X₂₅Is a capacitor C₀And reactance of leakage inductance L at 25Hz, X₇₅Is a capacitor C₀And the reactance of the leakage inductance L at 75 Hz;

solving (3) to obtain | X₂₅I and I X₇₅|；

S64, calculating the capacitance C₀；

Comparing | X obtained in the steps S61 and S63₅₀|、|X₂₅I and I X₇₅The size of |;

a. if | X₇₅If | is minimum, then there is

b. If | X₅₀If | is minimum, then there is

c. If | X₂₅If | is minimum, then there is

Wherein, ω is₇₅、ω₅₀And ω₂₅Angular frequencies corresponding to frequencies 75Hz, 50Hz and 25Hz, respectively;

the capacitance C is obtained by solving (4), (5) and (6)₀；

S65, the capacitor C obtained in the step S64₀Conversion into ground capacitance C 'of power grid'₀；

The conversion formula is as follows:

wherein k is₁₃The transformation ratio from the first winding to the third winding of the bus voltage transformer is shown;

s66, finally, solving the capacitance current of the power grid:

wherein, I_CIs the capacitive current of the power grid,and operating phase voltage for the power grid.