Discussion on online monitoring of low voltage cable insulation in mines

my country's coal mine underground low-voltage power grid adopts the transformer neutral point insulation operation mode, and the power transmission mainly relies on cables. Due to the harsh power supply environment, single-phase leakage or single-phase grounding faults often occur in cable lines, which not only cause personal electric shock, but also may cause gas and coal dust explosions, and even cause electrical detonators to detonate prematurely. Therefore, studying the online monitoring technology of cable insulation parameters is of great significance to improving the safety and reliability of power supply.

1 Principle of online monitoring of insulation parameters
of low-voltage cables in mines Insulation monitoring devices based on zero-sequence voltage and leakage protection devices based on power direction have long been used in coal mines. The former cannot reflect the change after the three-phase insulation of the power grid decreases symmetrically; the latter only sends a trip signal after the cable leaks, and cannot accurately predict the insulation level of the power grid before a single-phase grounding fault occurs. In view of its shortcomings, this paper adopts an online monitoring method of cable insulation parameters based on additional low-frequency power supply detection. This method can not only realize online monitoring of the insulation parameters of each branch cable to the ground, but also realize selective protection of the power grid.
The basic principle of the additional low-frequency power supply method is to add a low-frequency power supply signal to the three-phase AC power grid. The low-frequency power supply enters the power grid through the three-phase reactor, and then enters the ground through the ground capacitance and insulation resistance of the power grid to form a low-frequency current loop. By processing and calculating each low-frequency current signal, the insulation resistance of each branch cable can be obtained, thereby realizing online monitoring.
For low-frequency signals, the reactance introduced by the three-phase reactor and line impedance is extremely small and can be ignored compared with the insulation impedance of the low-voltage power grid. Therefore, the equivalent circuit shown in Figure 1 can be obtained.

The insulation parameters of any branch can be calculated using the following formula:

Formula (1) can be equivalent to the following two formulas:

From formula (2) and formula (3), we can get:

When the grid insulation parameters are symmetrical:

In the formula: U is the low-frequency voltage value; ω is the angular frequency; θ is the phase angle; Ii is the low-frequency current value of the i-th branch; Ri is the total insulation resistance value of the i-th branch; Ci is the total capacitance of the i-th branch to ground; RiA, RiB, RiC are the insulation resistance values ​​of phase A, phase B, and phase C of the i-th branch respectively.

2 Establishment and implementation of system model
This paper uses Matlab software for simulation. Matlab has been quite mature in the application of power systems. In the Simulink environment, select the modules required for system simulation in the power system simulation module library and build a simulation model, as shown in Figure 2.

An ideal three-phase voltage source is used as the power supply of the line, with a line voltage of 0.4 kV and a frequency of 50 Hz. The low-frequency power signal is set to a voltage amplitude of 20 V and a frequency of 10 Hz, and a π-type equivalent circuit is used. The positive sequence parameters of the line are: R1=0.20 Ω, L1=0.40 mH, C1=0.1μF per kilometer. The zero sequence parameters are: R0=0.23Ω, L0=1.72 mH, C0=0.08μF per kilometer. The simulation model contains a total of 3 cable lines.

3 Simulation Analysis
According to the selected module and set simulation parameters, the simulation is performed to obtain the waveform diagram in Figure 3.

As can be seen from Figure 3, on the fault branch, the 10 Hz low-frequency current detected by its zero-sequence current transformer is much larger than that on the branch with intact insulation resistance. Compared with the low-frequency current on the fault branch, the leakage current of the non-fault branch can be ignored. In this way, it is easy to distinguish between the fault branch and the non-fault branch, so that the fault branch can be selectively cut off.
When the insulation parameters are changed, simulation is performed. According to the measured low-frequency current value and its phase angle, the insulation parameter values ​​under various insulation conditions can be obtained by substituting the insulation parameter formula.
From the simulation results, it can be seen that during the operation of the cable, the fault branch can be detected by collecting and analyzing the additional low-frequency signal. Regardless of whether the three-phase insulation of the cable is symmetrical to the ground, this method can reflect its changes. It can not only make an accurate prediction of the insulation level of the power grid before a single-phase grounding fault occurs, but also selectively predict the fault branch with a decreased insulation level. This selection method is simple and easy, and has nothing to do with the neutral point grounding method of the transformer, so that this insulation monitoring method can be applied to the protection system of various power grids and is more versatile. It overcomes the limitations of the leakage protection device based on power direction and the insulation monitoring method based on zero-sequence voltage, and can realize fault line selection in case of fault.
Through simulation and statistics, it can be seen that the additional low-frequency power supply method has the following problems that need to be paid attention to.
3.1 The accuracy of insulation resistance detection decreases with the increase of branch-to-ground capacitance
In order to obtain the relationship between ground resistance measurement accuracy and capacitance, two fault branch ground resistances of 100 kΩ and 10 kΩ are set, and the fault branch-to-ground capacitance is changed from 0 to 5 μF under various ground resistances.

Through actual measurement, the relationship between the influence of ground capacitance on the measurement accuracy of various ground resistances was obtained (Figure 4). As can be seen from Figure 4, the detection accuracy of ground resistance decreases with the increase of branch ground capacitance, and for the same capacitance, the error increases with the increase of ground resistance. This is because the higher the insulation resistance, the smaller the measured signal.
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3.2 The influence of the increase of grid-to-ground capacitance on the fault branch location
As shown in Table 2, Ii is the fault branch leakage current, and Ik is the maximum non-fault branch leakage current. The grounding resistance is 1 kΩ, the low-frequency voltage amplitude is 20 V, and the frequency is 10 Hz.

When the cable-to-ground capacitance changes, the leakage current of the non-fault branch gradually approaches the leakage current of the fault branch. Since the leakage current in the cable is basically capacitive current and the resistive current can be ignored, it is impossible to distinguish between the fault branch and the non-fault branch.
3.3 Selection of injection frequency
The selection of injection frequency directly affects the effect of the additional low-frequency power supply method applied to the grid insulation fault location. As shown in Table 3, the test power supply frequency is changed. When the frequency increases, the low-frequency current in the measurement loop continues to increase, mainly due to the influence of capacitive current, because I=ωCU increases with frequency, and the capacitive current increases accordingly. The larger the injection frequency, the smaller the difference between the leakage current of the fault branch and the non-fault branch, making it difficult to achieve fault line selection.

To this end, the frequency of the injection signal should be determined according to the following principles:
(1) The injection frequency should be as low as possible to minimize the impact of the grid-to-ground capacitance on the detection accuracy. At the same time, the injection frequency below 50 Hz power frequency will not conflict with the normal working frequencies of the power grid.
(2) The injected sine wave frequency is stable, the waveform distortion coefficient is small, and the signal is easy to extract.
(3) Ensure the measurement accuracy of the sensor for weak currents.
Taking the above factors into consideration, 10 Hz can be selected as the injection frequency. In this way, the power frequency is a whole harmonic of the injection frequency. When using the full-cycle Fourier algorithm for calculation, the influence of the 50 Hz power frequency load signal and other high-order harmonics can be effectively eliminated.

4 Measurement error analysis
Since amplitude and phase errors may occur during the transmission and extraction of current and voltage in actual systems, they may have an adverse effect on the calculation results. According to formula (4), the relative error of insulation resistance measurement and the voltage and current modulus measurement errors satisfy the following relationship:

From formula (6), it can be seen that the insulation resistance measurement error is proportional to the voltage amplitude measurement error. When the voltage amplitude error increases or decreases, the calculation result of the Ri value will increase or decrease accordingly. From formula (7), it can be seen that the influence of the current amplitude is the opposite. When the current amplitude error increases or decreases, the calculation result of the Ri value will decrease or increase accordingly. From the above error analysis, it can be seen that in actual engineering applications, the measurement accuracy of insulation resistance is mainly affected by the amplitude error of current and voltage sensors, which can be used as a reference for sensor design and selection.

5 Conclusion
Theoretical analysis and simulation calculations show that it is completely feasible to use the additional low-frequency power supply method to conduct online insulation monitoring of the underground low-voltage power grid. Through it, the insulation level of the power grid can be observed at any time, and it has good engineering application prospects.