Design and implementation of single-phase grounding line selection device for small current grounding system

In China's 6~66 kV medium and low voltage power supply grid, the neutral point is generally ungrounded, grounded through arc suppression coils or high resistance grounding. These grounding methods do not need to cut off the power supply immediately when a single-phase grounding fault occurs, because the fault voltage is still symmetrical, which improves the reliability of power supply. However, when a fault occurs, the phase voltage of the non-fault phase rises to the line voltage amplitude. Long-term operation with the fault may cause insulation breakdown of the transmission line or power accidents such as phase-to-phase short circuit. Therefore, this type of grounding fault needs to be resolved within a certain period of time. Generally, it is stipulated that the equipment can operate for 2 hours under this type of fault [1]. When
a single-phase grounding fault occurs in a distribution system with an ungrounded neutral point, grounded through arc suppression coils or high resistance grounding, a relatively small zero-sequence current will be generated at the fault point, the symmetry of the three-phase voltage is not destroyed, and the equipment operates normally. Therefore, this type of fault is very hidden, which brings certain difficulties to detection. Based on the relationship between the current and voltage changes before and after the fault, this paper designs a real-time, reliable and easy-to-operate real-time small current detection system.
1 Analysis of small current line selection schemes
At present, the methods for small current line selection are mainly divided into two types: line selection schemes based on transient components and line selection schemes based on steady-state components [2]. Among them, the line selection schemes based on transient components mainly include wavelet transform method, first half-wave method and maximum fundamental wave method. The line selection schemes based on steady-state components mainly include zero-sequence current amplitude ratio method, group amplitude ratio and phase ratio method. In addition, there are line selection methods such as injection fundamental wave method and S injection method [3].
Although the line selection method based on transient components has strong real-time judgment for instantaneous grounding of lines, especially intermittent arc grounding faults, this line selection method has major theoretical defects and there is no experience in using it [4]. Therefore, this system does not adopt the transient component line selection scheme. The injection signal method requires the injection of the required frequency signal into the power grid, which not only increases the accuracy requirements of the sensor, but also affects the power quality. Therefore, this line selection system does not adopt it. In order to develop a reliable low-current line selection system, and considering that the transient component detection is effective for some instantaneous grounding such as arc grounding, the system can return to normal after the arc disappears, so the line selection scheme adopted in this paper is mainly based on the steady-state component scheme.
When a fault occurs in a low-current grounding system, the most obvious change is that the phase voltage in the circuit increases to the line voltage amplitude, so this system first detects the voltage value of the transmission line. After a low-current grounding fault occurs, the zero-sequence current value generated by the circuit becomes larger, and the system can further detect the zero-sequence current of the circuit to improve the reliability of detection. For a system with a neutral point grounded through an arc suppression coil, in the overcompensation mode, the steady-state zero-sequence current of the fault line is the excess inductance current generated during overcompensation, and its direction is the same as that of the non-fault line. Therefore, in this case, the detection method for zero-sequence current may fail. The zero-sequence compensation admittance method [5] is used in the system for the final judgment.


The judgment of the compensated zero-sequence admittance in this system is based on the deviation between the measured zero-sequence admittance and the set zero-sequence admittance for monitoring. For the line where the voltage of the transmission line increases, the zero-sequence current generated in the line increases significantly, or the compensation admittance deviation increases, it can be determined to be a faulty line. The use of the zero-sequence compensation admittance method to monitor the transmission line not only increases the reliability of the detection system, but also does not cause much change in the hardware. The use of the zero-sequence compensation admittance method in conjunction with the zero-sequence voltage and zero-sequence current method to monitor the small current grounding circuit is theoretically reasonable and easy to implement in reality. The solution is feasible.
3 Software and hardware system design and implementation
3.1 Design of the hardware system
The hardware structure diagram of the system is shown in Figure 1. The hardware system mainly includes sampling, filtering and signal conditioning of zero-sequence current and zero-sequence voltage, touch screen input and output, clock, power supply and central processing unit S3C2440. The main functions of the small current monitoring system are:

(1) Real-time detection of the zero-sequence voltage of the line. When the zero-sequence voltage of the circuit is greater than the set value, the zero-sequence current and zero-sequence admittance detection tasks are started. If the system finds a fault, the alarm program is started.
(2) The touch screen is the most important human-computer interaction channel of the system. The setting of various threshold value parameters in the system and the output information inside the system are mainly realized through the touch screen.
(3) For the system's external clock, an independent lithium battery is used to ensure the accuracy of the clock signal. After the system calibrates the clock once, it does not need to be calibrated again.
(4) Real-time performance of the system. The circuit information detected by the system in real time will be displayed on the monitor. If a problem occurs, the fault information can also be displayed correctly and clearly.
3.2 Design of the software system
The main task flow of the small current monitoring system is shown in Figure 2.

The main tasks of the software system include zero-sequence voltage monitoring task, zero-sequence current monitoring task, compensation admittance monitoring task, time task, key task and display task. The main process is as follows:
(1) In order to improve the real-time performance of the system, the system first monitors the zero-sequence voltage of the transmission line in real time. When the measured value of the zero-sequence voltage exceeds the set threshold value, the zero-sequence current monitoring task and the compensation admittance monitoring task are started.
(2) When the detection value of the zero-sequence monitoring task exceeds the set current or the compensation admittance changes, the system will start the alarm program.
(3) The system clock is powered by a battery to ensure the normal operation of the clock and increase the reliability of the system.
(4) The human-computer interaction of the system is mainly realized by a touch screen. The touch screen is responsible for displaying the information of the small current grounding system and setting the internal parameters of the system.
For the zero-sequence compensation admittance method, the system judges by detecting the phase angle of the zero-sequence current and the zero-sequence voltage. When the line is operating normally, the zero-sequence current leads the zero-sequence voltage by 90°; when the line fails, the zero-sequence current lags the zero-sequence voltage by 90°. According to the direction relationship between the zero-sequence current and the zero-sequence voltage, it is judged whether the line is faulty.
When a line has a ground fault, the zero-sequence current of the faulty line has the opposite sign to the zero-sequence current of the remaining lines. When the zero-sequence admittance produces a difference, the system will obtain the signal of the phase angle transmitter to obtain the offset angle of the zero-sequence admittance, and determine whether a small current ground fault has occurred by comparing the measured offset angle with the set offset angle. In order to improve the accuracy of line selection, the system samples the phase angle transmitter signal twice in succession. If the comparison results of the two sampled signals are consistent with the set values, the system will output the diagnosis results; if the sampling results are inconsistent, the phase angle transmitter signal will be read for the third time and the results of the last two times will be compared. If the number of readings of the phase angle transmitter signal in a continuous comparison is equal to 5 times, it is considered that the line has a fault, and the fault information is output and the fault alarm device is activated.
The data information processing unit of this system uses the ARM processor S3C2440, and the processor core adopts the RISC architecture, which reduces the development difficulty. At the same time, a 5-level pipeline is used to further increase the execution speed of the system, and the maximum instruction execution speed can reach 500 MIPS. In the selection of embedded systems, the requirements of real-time performance are fully considered, and the μC/OS-II operating system with the best real-time performance is adopted, which also improves the real-time performance of the system in software.
In view of the current situation that small current grounding faults are difficult to detect and determine, this paper designs a line selection method based on zero-sequence current and zero-sequence voltage, and proposes and utilizes the line selection criterion based on the conductor-to-ground capacitance. The factors affecting the correctness of line selection are analyzed and solutions are proposed. The use of an embedded operating system with excellent real-time and reliability greatly improves the real-time and accuracy of line selection. In a power supply network with small current grounding, this system can accurately detect grounding faults in the line.
References
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