Multi-point multi-wire grounding fault detection and arc extinction cooperation method for power distribution network

Abstract

The invention discloses a power distribution network multi-point multi-line grounding fault detection and arc extinction matching method, which comprises the following steps of: verifying an actual measured value of zero-sequence current of each line and a normal circulation value of the zero-sequence current of each line under a fault voltage on the basis of not being influenced by transition resistance, distribution network structure parameters and fault types; and the independent discrimination of the fault line and the independent extraction of the fault current of the line are synchronously realized. According to the multi-point multi-line fault detection method provided by the invention, the fault line can be accurately identified only by collecting the zero-sequence electrical characteristic information of each line after the fault, and the detection processes of each line are mutually independent and are not interfered by the fault conditions of other fault lines; according to the fault line removal and arc extinction cooperation method provided for different grounding fault situations, the line tripping number and power failure loss are reduced, and the operation and maintenance efficiency of workers and the power supply reliability of multipoint and multi-line grounding faults are improved.

Description

Technical Field

[0001] The invention relates to a measurement and control method of electrical technology, in particular to a multi-point and multi-line grounding fault detection and arc extinguishing coordination method for a distribution network. Background Art

[0002] Most of my country's medium and low voltage distribution networks operate in a low-current grounding mode. When a single-phase grounding fault occurs, the line voltage remains symmetrical and the fault current is weak, so the system is allowed to operate with power for 1 to 2 hours. However, as the system operating environment becomes increasingly complex, long-term operation with faults can easily lead to breakdown at weak insulation points of the line, causing multi-point grounding faults or phase-to-phase short circuits, and arc grounding can easily cause system overvoltage, resulting in insulation flashover and damage to electrical equipment. In addition, due to bad weather or external forces, multi-point grounding faults are also prone to occur between lines, causing problems such as electric shock and cable trench fires. Therefore, in order to ensure timely control of fault risks, it is necessary to collect ground fault currents in real time to track the development trend of the fault, detect all faulty lines and remove them, and ensure safe and stable operation of the power grid.

[0003] In recent years, with the increasing proportion of multi-circuit overhead lines on the same tower and multi-circuit cables in the same trench, the probability of multi-point grounding faults in distribution networks has also increased. Therefore, multi-point fault detection has gradually attracted the attention of researchers. Traditional fault detection methods are mainly based on the steady-state fault characteristics of multi-circuit grounding in low-resistance grounding systems, and multi-line faults are identified by analyzing the relationship between the amplitude and phase of the zero-sequence current of each outgoing line. However, this method requires the step-by-step judgment of the fault line, resulting in a long line selection time and low line selection efficiency. Therefore, quickly and accurately detecting all fault lines in a grounding fault has important engineering value and research significance for ensuring the safe and stable operation of the distribution network.

[0004] The Chinese patent application number is 202410190724.0, the authorization announcement number is CN 118244047 A, and the patent name is: A method, system and storage medium for selecting lines for two-point grounding faults in the same phase of different lines. This patent judges the first grounding fault and performs the first grounding fault line selection by the zero-sequence voltage mutation, and then judges the second grounding fault according to whether there is a zero-sequence current transfer in the system, and then selects the second grounding fault line by referring to the Euclidean distance corresponding to the zero-sequence current signal of the two grounding faults; however, this patent needs to judge the fault line step by step, and it is impossible to select all fault lines at the same time, and the fault line selection time is relatively long. The Chinese patent application number is 202210017013.4, the authorization announcement number is CN 114487701A, and the patent name is: A method and system for selecting lines for two-point grounding faults in the same phase of a medium-voltage distribution network. This patent selects the second fault line by using the method with the largest difference in the zero-sequence current conversion value before and after the first fault line is cut off, but this patent can only detect grounding faults in the same phase of different lines, and cannot cover all types of multi-point grounding faults; Summary of the invention

[0005] Technical problem: The technical problem to be solved by the present invention is to provide a method for detecting and arc extinguishing multi-point and multi-line grounding faults in a distribution network. When a multi-point and multi-line grounding fault occurs in the system, it is only necessary to verify the actual measured values ​​of each line and the normal flow values ​​of each line under the fault zero-sequence voltage to achieve rapid identification of all faulty lines and independent calculation of the fault current of the line, and to perform fault phase identification, transition resistance calculation and arc extinguishing control when the number of grounding points is different.

[0006] Technical solution: To solve the above technical problems, the present invention proposes a method for detecting multi-point and multi-line grounding faults in a distribution network based on line zero-sequence current verification matching, the method comprising the following steps:

[0007] Step a: When the distribution network is operating normally, measure and record the system zero-sequence voltage And the zero sequence current of each line (i=1,2,3…n; n is the total number of lines);

[0008] Step b: monitor the system zero-sequence voltage in real time. If the zero-sequence voltage increment exceeds 3.5% of the nominal phase voltage, it is determined that a ground fault occurs in the system. Otherwise, return to step a.

[0009] Measure and record zero-sequence voltage after system ground fault And the zero sequence current of each line , calculate the normal theoretical value of zero-sequence current of each line under the zero-sequence voltage of the fault :

[0010] Formula 1

[0011] In the formula, For the Zero-sequence admittance of the line , For line Distributed capacitance to ground, For line Conductivity to ground;

[0012] Step d: According to the theoretical value of zero-sequence current of each line under normal condition obtained in step c , respectively, and the actual measured value of the line zero-sequence current Perform numerical matching check. If the normal theoretical value of the line zero-sequence current is equal to the actual measured value, the line is judged to be a normal line. Otherwise, if the two values ​​are not equal, the line is judged to be a fault line, and the zero-sequence current check deviation value of the line is equal to the ground fault current of the fault line. :

[0013] Formula 2

[0014] Step e: Measure the three-phase voltage of the distribution network The amplitude and phase of the fault line ground fault phase and fault type are determined: if there is a ground current of the fault line If the voltage phase angle of a phase in the circuit is equal, the phase is judged to be the fault phase, and the line is a single-phase grounding fault, and step f is executed to calculate the ground fault conductance of the line when the single-phase grounding fault occurs; if the grounding current of the faulty line is equal to If the voltage phase angles of any phases in the circuit are not equal, the phases on both sides of the ground fault current phase are fault phases. At this time, the line is determined to be a two-phase ground fault, and step g is executed to calculate the ground fault conductance of the line under the two-phase ground fault condition;

[0015] Step f: Calculate the ground fault conductance when a single-phase ground fault occurs in the line :

[0016] Formula 3

[0017] In the formula, It is the line grounding fault current when a single-phase grounding fault occurs on the line; It is the power supply potential of the fault phase when a single-phase grounding fault occurs on the line;

[0018] Step g: Calculate the ground fault conductance when a two-phase ground fault occurs in the line :

[0019] Formula 4

[0020] Formula 5

[0021] In the formula, It is the line grounding fault current when a two-phase grounding fault occurs on the line; are the two fault phases when a two-phase grounding fault occurs in the line, and the transition conductances are ;

[0022] Step h: according to different ground fault types of the system, adopt corresponding coordination methods of fault arc extinguishing and fault line removal;

[0023] Furthermore, the specific meaning of the ground fault current of the fault line in step d is as follows:

[0024] The ground fault current of the fault line includes the ground fault current under different fault conditions such as single-phase ground fault, two-phase ground fault and three-phase ground fault:

[0025] Formula 6

[0026] Formula 7

[0027] Formula 8

[0028] In the formula, Respectively represent the ground fault current when the fault line has a single-phase, two-phase, or three-phase ground fault; They are the three-phase power supply potentials when a three-phase grounding fault occurs in the distribution network, and the transition conductances are and ;

[0029] Furthermore, in step h, according to different grounding fault types of the system, the corresponding coordination method of fault arc extinguishing and fault line removal is as follows:

[0030] Step h-1: according to the detection results in step d and step e, the system fault type is divided into a single-point grounding fault, a multi-point same-phase grounding fault and a multi-point different-phase grounding fault; if a single-point grounding fault or a multi-point same-phase grounding fault occurs in the system, step h-2 is executed to perform fault arc extinguishing processing; if a multi-point different-phase grounding fault occurs in the system, step h-3 is executed to perform fault arc extinguishing processing;

[0031] Step h-2: If a single-point grounding fault or a multi-point same-phase grounding fault occurs in the system, there is no need to cut off any fault line. Instead, a compensation current is directly injected according to the system parameters and the fault phase to clamp the fault phase voltage to zero, thus achieving 100% arc extinguishing of the fault.

[0032] Step h-3: If a multi-point out-of-phase grounding fault occurs in the system, the lines with more than two grounding points shall be quickly cut off. For the remaining lines that still have grounding faults, the number of faulty lines in phases A, B, and C shall be counted, and recorded as a, b, and c respectively. All faulty lines of the phase with the largest number of grounding fault lines shall be selected and retained, and the faulty lines with a smaller number of grounding phases shall be cut off. That is, only multi-point same-phase grounding faults shall be retained through selective tripping, and then the arc extinguishing device shall be put into use to reliably suppress the arc and improve the safety and reliability of power supply.

[0033] Beneficial effects: The present invention proposes a method for detecting and arc-extinguishing multi-point and multi-line grounding faults in a distribution network. First, the system zero-sequence voltage and the zero-sequence current of each line are recorded when the distribution network is operating normally; then, if a single-point or multi-point and multi-line grounding fault occurs in the system, the system fault zero-sequence voltage and the fault zero-sequence current of each line are recorded, and the normal flow value of the zero-sequence current of each line under the fault zero-sequence voltage is calculated; then, according to the matching result of the normal flow value of the zero-sequence current of each line and the measured value, the fault line is quickly identified and the fault current is independently calculated, and the fault phase identification, transition resistance calculation and arc-extinguishing control are performed when the number of grounding points is different. The method has the following advantages:

[0034] 1. An independent calculation method for line grounding current that can effectively deal with single-point or even multi-point grounding faults in the system and is only caused by line faults itself is proposed, which eliminates the influence of the correctness of fault phase identification on the acquisition of fault current information and ensures the accuracy of grounding current calculation.

[0035] 2. Fault line selection can be achieved based only on the zero-sequence current information of the line itself, which is not affected by the operating status of the distribution network and the type of fault. In addition, multiple fault lines can be detected at the same time, which greatly shortens the fault line selection time and improves the efficiency of fault line selection.

[0036] 3. The arc extinguishing coordination method of arc extinguishing device input and fault line removal under different grounding fault conditions is given, which reduces power outage losses, optimizes operation and maintenance efficiency, and improves the power supply reliability of the small current grounding system when dealing with multi-point grounding faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Flowchart of multi-point and multi-line grounding fault detection method in distribution network

[0038] Figure 2 Simulation system topology diagram

[0039] Figure 3 Comparison of fault current calculated value and actual value waveform

[0040] Figure 4 Waveform of arc extinguishing compensation for multi-point same-phase grounding fault current Specific implementation methods

[0041] In order to make the technical means, creative features, technical solutions, advantages and effects of the present invention more clear, the following is a detailed implementation method and specific operation process in conjunction with the drawings in the embodiments of the present invention. The implementation of the present invention can directly measure the zero-sequence voltage signal of the distribution network busbar, and the actual zero-sequence current signal of the feeder can be directly measured from the zero-sequence current transformer of each feeder. Figure 1 The specific implementation steps are introduced in detail as follows.

[0042] Step 1: When the distribution network is operating normally, measure and record the system zero-sequence voltage And the zero sequence current of each line (i=1,2,3…n; n is the total number of lines), where the generation mechanism of zero-sequence current of each line is:

[0043] Formula 1

[0044] Formula 2

[0045] In the formula, and is the three-phase power supply potential of the distribution network; For the Zero-sequence admittance of the line ( ), ; and They are respectively The three-phase equivalent capacitance of the line to ground, and They are respectively The three-phase equivalent conductance of the line to ground; For line Naturally unbalanced voltage;

[0046] Step 2: Monitor the system zero-sequence voltage in real time. If the zero-sequence voltage increment exceeds 3.5% of the nominal phase voltage, it is determined that a ground fault has occurred in the system. Measure and record the zero-sequence voltage after the system ground fault. , the zero sequence current of the fault line and the non-fault line , They can all be expressed by the following formula, otherwise return to step 1.

[0047] Formula 3

[0048] In the formula, It is the ground fault current generated by the fault phase voltage acting on the fault resistor; Non-fault lines j Natural unbalanced voltage and zero-sequence admittance;

[0049] Step 3: To eliminate the influence of the natural unbalanced voltage of the line on the zero-sequence current of the line, the zero-sequence current of each line before and after the fault is subtracted to obtain:

[0050] Formula 4

[0051] Step 4: Calculate the normal theoretical value of zero-sequence current of each line under the fault zero-sequence voltage :

[0052] Formula 5

[0053] Step 5: According to the theoretical value of zero-sequence current of each line under normal condition obtained in step 4 , respectively, and the actual measured value of the line zero-sequence current Perform a numerical matching check. If the normal theoretical value of the line zero-sequence current is equal to the actual measured value, the line is considered a normal line. Otherwise, if the two values ​​are not equal, the line is judged to be a faulty line, and the zero-sequence current check deviation value of the line is equal to the ground fault current of the faulty line. :

[0054] Formula 6

[0055] Step 6: The ground fault current of the line includes the ground fault current when a single-phase ground fault, a two-phase ground fault and a three-phase ground fault occur in the line, which are recorded as :

[0056] Formula 7

[0057] Formula 8

[0058] Formula 9

[0059] In the formula, is the fault phase power supply potential when a single-phase grounding fault occurs in the line, and the transition conductance is ; are the power supply potentials of the two fault phases when a two-phase grounding fault occurs in the line, and the transition conductances are When a three-phase grounding fault occurs in the line, the transition conductances of the fault phases A, B, and C are and ;

[0060] Step 7: Measure the three-phase voltage of the distribution network The amplitude and phase of the fault line ground fault phase and fault type are determined: if there is a ground current of the fault line If the voltage phase angle of a phase in the circuit is equal, the phase is judged to be the fault phase, and the line is a single-phase grounding fault, and the process goes to step 8 to calculate the ground fault conductance of the line when the line is a single-phase grounding fault; if the grounding current of the fault line is equal to If the voltage phase angles of any phases are not equal, the phases on both sides of the ground fault current phase are fault phases. At this time, the line is determined to be a two-phase ground fault, and the process goes to step 9 to calculate the ground fault conductance of the line under the two-phase ground fault condition;

[0061] Step 8: Calculate the ground fault conductance when a single-phase ground fault occurs in the line :

[0062] Formula 10

[0063] Step 9: Calculate the ground fault conductance when a two-phase ground fault occurs in the line :

[0064] Formula 11

[0065] Formula 12

[0066] Step 10: According to the detection results in step 5 and step 7, the system fault type is divided into a single-point grounding fault, a multi-point same-phase grounding fault and a multi-point different-phase grounding fault; if a single-point grounding fault or a multi-point same-phase grounding fault occurs in the system, it is not necessary to cut off any fault line, and a compensation current is directly injected according to the system parameters and the fault phase to clamp the fault phase voltage to zero, so as to achieve 100% arc extinguishing of the fault; if a multi-point different-phase grounding fault occurs in the system, step 11 is executed to perform fault arc extinguishing processing;

[0067] Step 11: If the system has multiple out-of-phase grounding faults, the lines with more than two grounding points shall be quickly cut off. For the remaining lines that still have grounding faults, the number of faulty lines in phases A, B, and C shall be counted, and recorded as a, b, and c respectively. All faulty lines of the phase with the largest number of grounding fault lines shall be selected and retained, and the faulty lines with fewer grounding phases shall be cut off. That is, only multiple same-phase grounding faults shall be retained through selective tripping, and then the arc extinguishing device shall be put into use to reliably suppress the arc and improve the safety and reliability of power supply.

[0068] According to the above steps, MATLAB / Simulink simulation is used to verify the multi-point and multi-line grounding fault detection method of the distribution network based on line zero-sequence current verification matching: Figure 2 To simulate the system topology, the simulation system is a 10kV distribution network with five feeders. ) two operation modes are used to conduct simulation experiments, and the simulation values ​​of the system zero-sequence voltage and each line zero-sequence current under normal operation are recorded respectively. The simulation measurement results are shown in Table 1.

[0069] Table 1 Measurement results of zero-sequence voltage and zero-sequence current under normal conditions.

[0070]

[0071] In order to verify the accuracy of the fault line selection of Formula 6 when a single-phase grounding fault occurs in the system, single-phase grounding faults with different fault conditions are randomly set in the system, and the zero-sequence voltage and zero-sequence current of each line after the fault are measured and recorded. The zero-sequence voltage after the single-phase grounding fault occurs in the distribution network is substituted into Formula 5 to obtain the theoretical value of the zero-sequence current under normal conditions of each line, and the fault line selection is realized according to the verification result with the measured value of the zero-sequence current of the line. The line with the normal theoretical value and the actual measured value of the zero-sequence current is equal to the normal line, and vice versa, it is the fault line. For the fault line, the verification deviation value between the theoretical and actual zero-sequence current is the grounding fault current, and the fault phase selection and transition resistance value calculation are completed based on the principle that the fault current is consistent with the phase angle of the fault phase. The single-phase grounding fault identification results of the neutral point ungrounded system and the arc suppression coil grounding system are shown in Table 2.

[0072] Table 2 Identification results of single-phase grounding fault line

[0073]

[0074] In order to verify the accuracy of formula 6 for detecting multi-point and multi-line grounding faults in the system, multi-point and multi-line grounding faults with different fault conditions are randomly set in the system, and the zero-sequence voltage and zero-sequence current of each line after the fault are measured and recorded. Formula 5 is used to calculate the normal flow value of zero-sequence current of each line under the zero-sequence voltage of the fault. The normal theoretical value and the actual measured value of the zero-sequence current of each line are checked. If the two values ​​are equal, the line is a normal line. If the two values ​​are not equal, the line is a fault line, and the calibration deviation signal between the actual measured value and the normal theoretical value of the zero-sequence current of the fault line is equal to the fault current of the line. The fault phase selection and fault resistance calculation are realized according to the principle of the phase angle of the fault phase and the fault current. The detection results of multi-point and multi-line grounding faults in the distribution network are recorded in Table 3.

[0075] Table 3. Detection results of multi-point and multi-line grounding faults in distribution network

[0076]

[0077] It can be seen from Table 2 and Table 3 that the ground fault detection method based on the actual value of the zero-sequence current of the verification line and the normal flow value of the zero-sequence current of the line under the fault zero-sequence voltage can quickly detect the fault line for both single-point ground fault and multi-point ground fault. When a multi-point and multi-line ground fault occurs in the distribution network, the detection method can simultaneously detect all fault lines and obtain the line ground fault current caused only by the fault of the line itself, and the detection process of each line is independent and does not interfere with each other, which greatly shortens the fault line selection time and improves the efficiency of fault line selection.

[0078] To verify that the second equation of formula 6 can independently extract the ground fault current of the fault line, a single-phase ground fault is set in the fault phase C of distribution network line 1, and the ground fault resistance is 1000 ohms. The actual value of the ground fault current is collected and compared with the theoretical calculated value of the ground fault current. The actual value and calculated value of the ground fault current are as follows: Figure 3 As shown in the experimental results, the fault current calculated by the deviation between the actual zero-sequence current value of the fault line and the theoretical zero-sequence current value is almost consistent with the actual fault current waveform, which further verifies that the present invention can realize parallel processing of multi-fault line detection with short time consumption and simultaneously obtain the grounding current information of the fault line.

[0079] In order to verify the accuracy of the arc extinguishing device for all grounding point currents when multiple same-phase grounding faults occur in the distribution network proposed by the present invention, it is possible to set line 1 and line 2 to have a phase A grounding fault with a transition resistance of 1000 ohms and 2000 ohms at t=0.1s. When t=0.2s, the compensation device is put into use to observe the change of grounding currents of the two fault lines. The fault current compensation effect is as follows: Figure 4 As shown. The experimental results show that when the compensation device is put into the neutral point of the system, the ground fault current of each fault line can be accurately compensated to zero, and the compensation accuracy is not affected by the fault line, the location of the fault point and the transition resistance, and the compensation accuracy is high. Therefore, when multiple points of the same-phase grounding fault occur in the system, the compensation device can achieve full compensation of all fault currents without cutting off the fault line.

Claims

1. A method for detecting and arc-extinguishing multi-point and multi-line grounding faults in a distribution network, characterized in that: The steps include: Step a: When the distribution network is operating normally, measure and record the system zero-sequence voltage And the zero sequence current of each line (i=1,2,3…n; n is the total number of lines); Step b: monitor the system zero-sequence voltage in real time. If the zero-sequence voltage increment exceeds 3.5% of the nominal phase voltage, it is determined that a ground fault occurs in the system. Otherwise, return to step a. Step c: Measure and record the zero-sequence voltage after the system ground fault And the zero sequence current of each line , calculate the normal theoretical value of zero-sequence current of each line under the zero-sequence voltage of the fault : Formula 1 In the formula, For the Zero-sequence admittance of the line ( ), For Line Distributed capacitance to ground, For Line Conductivity to ground; Step d: According to the theoretical value of zero-sequence current of each line under normal condition obtained in step c , respectively, and the actual measured value of the line zero-sequence current Perform a numerical matching check. If the normal theoretical value of the line zero-sequence current is equal to the actual measured value, the line is judged to be a normal line. On the contrary, if the two values ​​are not equal, the line is judged to be a fault line, and the zero-sequence current verification deviation value of the line is equal to the ground fault current of the fault line. : Formula 2 Step e: Measure the three-phase voltage of the distribution network The amplitude and phase of the fault line ground fault phase and fault type are determined: if there is a ground current of the fault line If the voltage phase angle of a phase in the circuit is equal, the phase is judged to be the fault phase, and the line is a single-phase grounding fault, and step f is executed to calculate the ground fault conductance of the line when the single-phase grounding fault occurs; if the grounding current of the fault line is equal to If the voltage phase angles of any phases in the circuit are not equal, the phases on both sides of the ground fault current phase are fault phases. At this time, the line is determined to be a two-phase ground fault, and step g is executed to calculate the ground fault conductance of the line under the two-phase ground fault condition; Step f: Calculate the ground fault conductance when a single-phase ground fault occurs in the line : Formula 3 In the formula, It is the line grounding fault current when a single-phase grounding fault occurs on the line; It is the power supply potential of the fault phase when a single-phase grounding fault occurs on the line; Step g: Calculate the ground fault conductance when a two-phase ground fault occurs in the line : Formula 4 Formula 5 In the formula, It is the line grounding fault current when a two-phase grounding fault occurs on the line; are the power supply potentials of the two fault phases when a two-phase grounding fault occurs in the line, and the transition conductances are ; Step h: According to different ground fault types of the system, adopt corresponding coordination methods of fault arc extinguishing and fault line removal.

2. A method for detecting and arc-extinguishing multi-point and multi-line grounding faults in a distribution network according to claim 1, characterized in that: The specific meaning of the ground fault current of the fault line in step d is as follows: The ground fault current of the fault line includes the ground fault current under different fault conditions such as single-phase ground fault, two-phase ground fault and three-phase ground fault: Formula 6 Formula 7 Formula 8 In the formula, Respectively represent the ground fault current when the fault line has a single-phase, two-phase, or three-phase ground fault; They are the three-phase power supply potentials when a three-phase grounding fault occurs in the distribution network, and the transition conductances are and .

3. A method for detecting and arc-extinguishing multi-point and multi-line grounding faults in a distribution network according to claim 1, characterized in that: In step h, according to different ground fault types of the system, the corresponding coordination method of fault arc extinguishing and fault line removal is as follows: Step h-1: according to the detection results in step d and step e, the system fault type is divided into a single-point grounding fault, a multi-point same-phase grounding fault and a multi-point different-phase grounding fault; if a single-point grounding fault or a multi-point same-phase grounding fault occurs in the system, step h-2 is executed to perform fault arc extinguishing processing; If a multi-point out-of-phase grounding fault occurs in the system, execute step h-3 to perform fault arc extinguishing processing; Step h-2: If a single-point grounding fault or a multi-point same-phase grounding fault occurs in the system, there is no need to cut off any fault line. Instead, a compensation current is directly injected according to the system parameters and the fault phase to clamp the fault phase voltage to zero, thus achieving 100% arc extinguishing of the fault. Step h-3: If a multi-point out-of-phase grounding fault occurs in the system, the lines with more than two grounding points shall be quickly cut off. For the remaining lines that still have grounding faults, the number of faulty lines in phases A, B, and C shall be counted, and recorded as a, b, and c respectively. All faulty lines of the phase with the largest number of grounding fault lines shall be selected and retained, and the faulty lines with a smaller number of grounding phases shall be cut off. That is, only multi-point same-phase grounding faults shall be retained through selective tripping, and then the arc extinguishing device shall be put into use to reliably suppress the arc and improve the safety and reliability of power supply.

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