Single-phase high-resistance grounding fault line selection and phase selection method suitable for flexible grounding system

Abstract

The application discloses a single-phase high-resistance grounding fault line selection and phase selection method suitable for a flexible grounding system and belongs to the field of medium-voltage distribution network fault diagnosis. In view of the problems that the existing protection method of the flexible grounding system has low transition resistance endurance and does not consider the influence of power grid parameter imbalance on criterion, under the condition of considering power grid parameter imbalance, the application provides a single-phase high-resistance grounding fault line selection and phase selection method suitable for the flexible grounding system. Firstly, system zero sequence voltages before and after the input of a parallel small resistor are acquired, phase characteristic quantities are calculated, and fault phase selection is realized. Then, the zero sequence measurement admittance and zero sequence measurement resistance of each line are calculated through the zero sequence current variation and the system zero sequence voltage variation of each line, and finally, fault line selection is realized. The method is not affected by power grid parameter imbalance and zero sequence current transformer polarity reverse connection, and can effectively reduce the requirement of protection on the precision of the zero sequence current transformer. In addition, the method can also realize the measurement of the transition resistance and the ground admittance of a single line by using fault information.

Description

Technical Field

[0001] This invention relates to the field of fault diagnosis in medium-voltage power distribution networks, and specifically to a method for selecting the line and phase of a single-phase high-resistivity grounding fault applicable to flexible grounding systems. Background Technology

[0002] In recent years, to address the issues of high overvoltage levels in resonant grounding systems and low power supply reliability in low-resistance grounding systems, my country's power supply departments have proposed flexible grounding systems, which are now being promoted and used in some regions. The initial purpose of flexible grounding systems is to activate arc-suppression coils during normal operation and transient ground faults to extinguish the arc of transient ground faults. If the fault initiation conditions are still met after a short delay, it is determined that a permanent ground fault has occurred in the system, and a parallel small resistor is activated to increase the current in the faulty line, in order to make it meet the operating conditions of zero-sequence overcurrent protection.

[0003] Current flexible grounding systems still use zero-sequence overcurrent protection, commonly employed in low-resistance grounding systems. However, the setting values ​​for zero-sequence overcurrent protection are often set to avoid the maximum ground capacitance current of each line, resulting in generally large setting values. This leads to low reliability and a tendency to fail to operate when facing single-phase high-resistance grounding faults. Furthermore, existing research methods require high accuracy from the instrument transformers and do not consider the impact of unbalanced current on the judgment criteria, posing a risk of protection misjudgment. Therefore, with the widespread adoption of flexible grounding systems, there is an urgent need to conduct research on high-resistance grounding fault protection in flexible grounding systems. Summary of the Invention

[0004] To address the challenges mentioned in the background section, this invention proposes a method for selecting the line and phase of a single-phase high-resistivity grounding fault applicable to flexible grounding systems.

[0005] To achieve the above-mentioned technical objectives and effects, the present invention provides the following technical solution: a method for selecting the line and phase of a single-phase high-resistance grounding fault applicable to flexible grounding systems, comprising the following steps:

[0006] Step 1: Determine the line with the lowest insulation resistance in the system, and use the insulation resistance value of this line as the line selection threshold;

[0007] Step 2: Monitor the system zero-sequence voltage in real time. If the system zero-sequence voltage is greater than 5% of the phase voltage, activate the protection.

[0008] Step 3: Record the system zero-sequence voltage and the zero-sequence current of each line;

[0009] Step 4: If the zero-sequence voltage of the system drops below 5% of the phase voltage after a 1-second delay, return to step 2; otherwise, connect a small parallel resistor and proceed to step 5.

[0010] Step 5: Record the system zero-sequence voltage and the zero-sequence current of each line after the parallel small resistor is connected. If the zero-sequence current of a certain line meets the zero-sequence overcurrent protection action threshold, disconnect the line; otherwise, proceed to step 6.

[0011] Step 6: Based on the data recorded in Steps 2 to 5, calculate the phase characteristic quantity and transition resistance in sequence to determine the faulty phase;

[0012] Step 7: Calculate the zero-sequence measurement admittance and zero-sequence measurement resistance of each line, and cut off the line with the zero-sequence measurement resistance less than the line selection threshold.

[0013] Furthermore, the minimum insulation resistance in step 1 The calculation formula is:

[0014]

[0015] in, The ground susceptance of the k-th line, These are the three-phase-to-ground capacitances of line k, and min indicates taking the minimum value.

[0016] Furthermore, the moment when the parallel small resistor is connected in step 4 is when the system zero-sequence voltage crosses zero.

[0017] Furthermore, the zero-sequence overcurrent protection action threshold in step 5... for:

[0018]

[0019] Among them, K rel The reliability coefficient ranges from 1.3 to 1.5. = Let be the ground capacitance current of the k-th line, and max represents the maximum value.

[0020] Furthermore, the calculation method for the phase characteristic quantity in step 6 is as follows:

[0021]

[0022] Among them, R N For parallel small resistor values, These are the zero-sequence voltages of the system before and after the parallel small resistor is connected. These are the A-phase power supply potential and the three-phase asymmetry vector sum, respectively.

[0023] Furthermore, the method for determining the faulty phase in step 6 is as follows: if the phase of the phase characteristic quantity is between (-70°, 50°), it is determined to be a phase A fault; if the phase of the phase characteristic quantity is between (50°, 170°), it is determined to be a phase B fault; and if the phase of the phase characteristic quantity is between (170°, -70°), it is determined to be a phase C fault.

[0024] Furthermore, the transition resistance in step 6 The calculation formula is:

[0025]

[0026]

[0027] in, For ground admittance, =1 / , L is the power frequency angular frequency. p Here, j represents the inductance of the arc suppression coil. This is the total admittance of the system's three phases to ground.

[0028] Furthermore, the zero-sequence measurement admittance of any line k in step 7 is:

[0029]

[0030] in, Let be the zero-sequence current change of the k-th line. This represents the zero-sequence voltage change in the system.

[0031] Furthermore, the zero-sequence measured resistance of any line k is:

[0032]

[0033] Where Real represents the real part of the imaginary number.

[0034] Furthermore, in step 7, the zero-sequence measurement admittance of the improved line is the same as the line's ground admittance:

[0035]

[0036] in, These are the ground admittance and zero-sequence measurement admittance of the sound line i, respectively. To improve the zero-sequence current change of line i.

[0037] Furthermore, in step 7, the zero-sequence measurement admittance of the faulty line is composed of the line's admittance to ground and its grounding admittance.

[0038]

[0039] in, These are the ground admittance and zero-sequence measurement admittance of the faulty line j, respectively. Let be the change in zero-sequence current of faulty line j.

[0040] Furthermore, the zero-sequence measurement resistance is less than The line is considered a faulty line and is disconnected.

[0041] This invention discloses a method for selecting the faulty line and phase in a single-phase high-resistance grounding fault applicable to flexible grounding systems. By collecting the zero-sequence voltage and zero-sequence current of the system before and after the connection of a parallel small resistor, the faulty line and faulty line can be identified. Its principle is clear, easy to implement, and highly practical. It is not affected by the reverse polarity of the zero-sequence current transformer or the imbalance of power grid parameters. Furthermore, while selecting the line and phase, it can also measure the ground admittance and transition resistance of the healthy line, which has high engineering application value. Attached Figure Description

[0042] Figure 1 This is an operation flowchart for a single-phase high-resistance grounding fault line and phase selection method applicable to flexible grounding systems.

[0043] Figure 2 It is a phase relationship diagram between the fault phase and the phase characteristic quantities.

[0044] Figure 3 This is a simulation topology diagram of a flexible grounding system.

[0045] Figure 4 This is a waveform diagram of the zero-sequence current of each line when a 3000Ω high-resistance ground fault occurs on line 4. Detailed Implementation

[0046] The invention will now be described in more detail with reference to the accompanying drawings and simulation examples.

[0047] A method for selecting the line and phase of a single-phase high-resistivity grounding fault applicable to flexible grounding systems, such as... Figure 1 As shown, the operation steps are as follows:

[0048] Step 1: Determine the line with the lowest insulation resistance in the system, and use the insulation resistance value of this line as the line selection threshold;

[0049] Step 2: Monitor the system zero-sequence voltage in real time. If the system zero-sequence voltage is greater than 5% of the phase voltage, activate the protection.

[0050] Step 3: Record the system zero-sequence voltage and the zero-sequence current of each line;

[0051] Step 4: If the zero-sequence voltage of the system drops below 5% of the phase voltage after a 1-second delay, return to step 2; otherwise, connect a small parallel resistor and proceed to step 5.

[0052] Step 5: Record the system zero-sequence voltage and the zero-sequence current of each line after the parallel small resistor is connected. If the zero-sequence current of a certain line meets the zero-sequence overcurrent protection action threshold, disconnect the line; otherwise, proceed to step 6.

[0053] Step 6: Based on the data recorded in Steps 2 to 5, calculate the phase characteristic quantity and transition resistance in sequence to determine the faulty phase;

[0054] Step 7: Calculate the zero-sequence measurement admittance and zero-sequence measurement resistance of each line, and cut off the line with the zero-sequence measurement resistance less than the line selection threshold.

[0055] Furthermore, the minimum insulation resistance in step 1 The calculation formula is:

[0056]

[0057] in, The ground susceptance of the k-th line, These are the three-phase-to-ground capacitances of line k, and min indicates taking the minimum value.

[0058] Furthermore, the moment when the parallel small resistor is connected in step 4 is when the system zero-sequence voltage crosses zero.

[0059] Furthermore, the zero-sequence overcurrent protection action threshold in step 5... for:

[0060]

[0061] Among them, K rel The reliability coefficient ranges from 1.3 to 1.5. = Let be the ground capacitance current of the k-th line, and max represents the maximum value.

[0062] Furthermore, the calculation method for the phase characteristic quantity in step 5 is as follows:

[0063]

[0064] Among them, R N For parallel small resistor values, These are the zero-sequence voltages of the system before and after the parallel small resistor is connected. These are the A-phase power supply potential and the three-phase asymmetry vector sum, respectively.

[0065] Furthermore, the method for determining the faulty phase in step 6 is as follows: if the phase of the phase characteristic quantity is between (-70°, 50°), it is determined to be a phase A fault; if the phase of the phase characteristic quantity is between (50°, 170°), it is determined to be a phase B fault; and if the phase of the phase characteristic quantity is between (170°, -70°), it is determined to be a phase C fault.

[0066] Furthermore, the formula for calculating the transition resistance in step 6 is as follows:

[0067]

[0068]

[0069] in, For ground admittance, =1 / , Where L is the power frequency angular frequency, Lp is the inductance of the arc suppression coil, and j is the imaginary part indicator. This is the total admittance of the system's three phases to ground.

[0070] Furthermore, the zero-sequence measurement admittance of any line k in step 7 is:

[0071]

[0072] in, Let be the zero-sequence current change of the k-th line. This represents the zero-sequence voltage change in the system.

[0073] Furthermore, the zero-sequence measured resistance of any circuit is:

[0074]

[0075] Where Real represents the real part of the imaginary number.

[0076] Furthermore, in step 7, the zero-sequence measurement admittance of the improved line is the same as the line's ground admittance:

[0077]

[0078] in, These are the ground admittance and zero-sequence measurement admittance of the sound line i, respectively. To improve the zero-sequence current change of line i.

[0079] Furthermore, in step 7, the zero-sequence measurement admittance of the faulty line is composed of the line's admittance to ground and its ground conductance.

[0080]

[0081] in, These are the ground admittance and zero-sequence measurement admittance of the faulty line j, respectively. Let be the change in zero-sequence current of faulty line j.

[0082] Furthermore, the zero-sequence measurement resistance is less than The line is considered a faulty line and is disconnected.

[0083] Simulation verification

[0084] A simulation model was built using PSCAD / EMTDC to verify the practical value of the proposed method for selecting the line and phase of a single-phase high-resistivity grounding fault in flexible grounding systems. The specific structure and line length of the simulation model are marked on [the diagram]. Figure 3 In this simulation model, there are a total of 4 different types of lines, R N For parallel small resistors, R N The resistance is 10Ω, S is a switch that controls the connection or disconnection of the parallel small resistor, and L... p For the arc suppression coil, L p Size is 0.5208H, T Z G is a grounding transformer, and G is an infinite power source.

[0085] Table 1 Line Parameters A relative capacitance to ground 1.7901 2.8395 1.2503 0.1300 B relative to ground capacitance 1.8051 2.8294 1.2363 0.1280 C relative to ground capacitance 1.8051 2.8198 1.2440 0.1250 Phase A insulation resistance 101094 61899 166524 1362567 B-phase insulation resistance 101094 61899 166524 1362567 C-phase insulation resistance 101094 61899 166524 1362567

[0086] Calculate the zero-sequence overcurrent protection operating threshold and line selection threshold separately:

[0087] =1.3×5774×(2.8395+2.8294+2.8198)×314×10 -6 ≈20A,

[0088] .

[0089] Table 2. Determination of Faulty Phase Line 4 2000Ω A -0.275 A Line 4 2000Ω B 120.267 B Line 4 2000Ω C 240.412 C

[0090] Table 3 Calculation of Transition Resistance 150 154 2.667 500 508 1.600 1000 1017 1.700 3000 3007 0.233 4500 4496 -0.089

[0091] Table 4 shows the zero-sequence measured resistance of each line when ground faults of different resistance values ​​occur. Line 1 150 149 20508 54306 447009 Line 1 Line 1 500 492 20226 55408 459779 Line 1 Line 1 1000 971 21929 53668 461674 Line 1 Line 1 3000 2751 20262 52597 412490 Line 1 Line 1 4500 3978 21029 53170 423085 Line 1

[0092] From the data in Tables 2 to 4 above, it can be seen that the method proposed in this invention can not only accurately identify the faulty phase and faulty line using fault information, but also calculate the transition resistance with small errors, meeting the needs of real-world working conditions.

[0093] Figure 3 and Figure 4 The figures show the waveform changes of the zero-sequence current and system zero-sequence voltage of Line 4 and Line 1 during a certain time period when a 3000Ω single-phase high-resistance ground fault occurs in phase A of Line 4 at 0.5s. A small parallel resistor is connected to the system at 1.5s. Figure 4 Before the small parallel resistor was connected, the zero-sequence voltage of the system was much greater than the 5% phase voltage of 288.7V, but Figure 3 Since the zero-sequence current of each line is less than the zero-sequence overcurrent protection threshold, the process proceeds to calculate the zero-sequence measurement resistance of each line. The line with a zero-sequence measurement resistance of 7499Ω is identified as a faulty line.

[0094] The above description and embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for selecting the line and phase of a single-phase high-resistivity grounding fault applicable to flexible grounding systems, characterized in that, Includes the following steps: Step 1: Determine the line with the lowest insulation resistance in the system, and use the insulation resistance value of this line as the line selection threshold; Step 2: Monitor the system zero-sequence voltage in real time. If the system zero-sequence voltage is greater than 5% of the phase voltage, activate the protection. Step 3: Record the system zero-sequence voltage and the zero-sequence current of each line; Step 4: If the zero-sequence voltage of the system drops below 5% of the phase voltage after a 1-second delay, return to step 2; otherwise, connect a small parallel resistor and proceed to step 5. Step 5: Record the system zero-sequence voltage and the zero-sequence current of each line after the parallel small resistor is connected. If the zero-sequence current of a certain line meets the zero-sequence overcurrent protection action threshold, disconnect the line; otherwise, proceed to step 6. Step 6: Calculate the phase characteristic quantities sequentially based on the data recorded in Steps 2 to 5. The phase characteristic quantity, in conjunction with the transition resistance, is calculated according to the following formula: in, For parallel small resistors, These are the zero-sequence voltages of the system before and after the parallel small resistor is connected. These are the phase A power supply potential and the three-phase asymmetry vector sum, respectively. The faulty phase is determined based on the phase characteristic quantity: if the phase is between (-70°, 50°), it is determined to be phase A fault; if the phase is between (50°, 170°), it is determined to be phase B fault; and if the phase is between (170°, -70°), it is determined to be phase C fault. Step 7: Calculate the zero-sequence measurement admittance and zero-sequence measurement resistance of each line, and cut off the line with the zero-sequence measurement resistance less than the line selection threshold.

2. The method for selecting the line and phase of a single-phase high-resistivity grounding fault applicable to a flexible grounding system as described in claim 1, characterized in that, The minimum insulation resistance in step 1 The calculation formula is: in, The ground susceptance of the k-th line, These are the three-phase-to-ground capacitances of line k, and min indicates taking the minimum value.

3. The method for selecting the line and phase of a single-phase high-resistivity grounding fault applicable to a flexible grounding system as described in claim 1, characterized in that, Transition resistance in step 6 for: in, For ground admittance, =1 / , Where L is the power frequency angular frequency, Lp is the inductance of the arc suppression coil, and j is the imaginary part indicator. This is the total admittance of the system's three phases to ground.

4. The method for selecting the line and phase of a single-phase high-resistivity grounding fault applicable to a flexible grounding system as described in claim 1, characterized in that, The zero-sequence measurement admittance of any line k in step 7 for: in, Let be the zero-sequence current change of the k-th line. This represents the zero-sequence voltage change in the system. Zero-sequence measured resistance of any line k for: Where Real represents the real part of the imaginary number; The zero-sequence measurement admittance of the line is the same as the line's ground admittance: in, These are the ground admittance and zero-sequence measurement admittance of the sound line i, respectively. To improve the zero-sequence current variation of line i; The zero-sequence test admittance of a faulty line is composed of the line's admittance to ground and its ground conductance. in, These are the ground admittance and zero-sequence measurement admittance of the faulty line j, respectively. The change in zero-sequence current of faulty line j; The zero-sequence measurement resistance is less than The line is considered a faulty line and is disconnected.

Images