Keywords

1 Introduction

The chinese distribution network mainly adopts two grounding modes, namely, the ungrounded neutral point and the grounded neutral point through arc suppression coil. For fault line selection in small current grounding network, several methods have been proposed [1,2,3,4,5]. For example, wavelet energy method, correlation analysis method, electric popular wave method, zero sequence admittance method, zero sequence reactive power method, injection method, medium resistance method, etc., use rich transient information components to identify fault lines [6,7,8,9,10]. In practical work, the traditional “road test” method is generally used [11]. Literature [11] uses the steady-state zero sequence current information and makes zero sequence current change by pulling the primary switch, so as to select the fault line according to the change characteristics. However, this method needs to add a sound line outage event, which influences stable power supply. On the basis of literature [11], this paper sets the second temporary grounding point by using the existing substation line side grounding knife switch, and conducts fault line selection by comparing the change characteristics of the steady-state zero sequence current of each line before and after the temporary grounding. This method does not need to use high-frequency transient information, does not need to add additional data acquisition devices, and is simple in design. Only the existing station measurement and control devices can be used to measure the steady-state zero sequence current for calculation and judgment, and the D5000 dispatching technology support system can be used for visual display for dispatching line selection decision. At present, the fault line selecting method based on the principle of in-phase two point grounding has been applied and popularized on the site, which will not increase the power outage events of sound lines and has high power supply reliability.

2 Steady State Characteristic Analysis of Grounding Network

2.1 Ungrounded Neutral System

Fig. 1.
figure 1

Zero sequence current flow network in unground network

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Figure 1 shows the single-phase grounded zero sequence network of the distribution network of the neutral ungrounded network. The zero sequence current generated by the line to ground capacitance shall all flow to the ground fault point. For the fault line, the zero sequence current \(I_{0f}\) flowing into all lines from the grounding point, while the sum of sound lines zero sequence current \(I_{0k}\) is detected at the beginning of the line, as shown in Formula (1) and (2).

$$ I_{0f} { = }\sum_{i = 1}^N {I_{0i} } $$
(1)
$$ I_{0k} { = } - \sum_{i = 1}^N {I_{0i} (i \ne k)} $$
(2)

2.2 Neutral Grounding System Through Arc Suppression Coil

Fig. 2.
figure 2

Zero sequence current flow network in petersen coil grounding system

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When single-phase grounding happens in the neutral point through the arc suppression coil grounding system, as shown in Fig. 2, the switch S is closed, and the zero sequence current generated by the grounding capacitance of the sound line flows to the grounding fault point, which is the same as that of the ungrounded system. However, the inductance current in the arc suppression coil also flows into the grounding fault point, so the current flowing into the fault point \(I_{0f}\) is the difference between the zero sequence capacitance current in all lines and the inductance current in the arc suppression coil. The zero sequence current \(I_{0k}\) detected at the beginning of the fault line is the difference between the zero sequence capacitive current of all sound lines and the inductance current \(I_L\) of the arc suppression coil, as shown in Formula (4).

$$ I_{0f} { = }\sum_{i = 1}^N {I_{0i} } - I_L $$
(3)
$$ I_{0k} { = } - {(}\sum_{i = 1,i \ne k}^N {I_{0i} } - I_L ) $$
(4)

As the over compensation scheme is generally adopted, the compensation degree is 10%–15%, and the zero sequence current value of the fault line is not large. In addition, the zero sequence current is inductive current, which flows from the fault point to the bus, while the zero sequence capacitive current of the sound line flows from the bus to the line. Therefore, for the steady zero-sequence current, the current direction of the sound line is consistent with that of the fault line, and the fault line cannot be judged by the direction of the steady zero-sequence current.

3 Analysis of the Principle of Same Phase Two Point Grounding

3.1 Analysis on the Principle of Two Point Grounding in Ungrounded Neutral Point Network

In the neutral point ungrounded system, line m1 and m2 occur and phase A is grounded meanwhile. The zero sequence current generated by the grounding capacitance of a sound line will be shunted at two grounding points, and the distribution coefficient is related to the zero-sequence impedance of the line between the two grounding points and the bus.

$$ I_{0m1} = - \left| {\frac{{Z_{0m2f} }}{{Z_{0m1f} + Z_{0m2f} }}} \right| \cdot \sum_{i = 1}^N {I_{0i} } (i \ne m1,i \ne m2) $$
(5)
$$ I_{0m2} = - \left| {\frac{{Z_{m1f} }}{{Z_{m1f} + Z_{m2f} }}} \right| \cdot \sum_{i = 1}^N {I_{0i} } (i \ne m1,i \ne m2) $$
(6)

where \(Z_{0m1f}\) and \(Z_{0m2f}\) are respectively the zero-sequence impedance from the grounding point in the fault line m1 and m2 to the bus. When the grounding point of fault line m1 is close to the bus, \(Z_{0m1f} \ll Z_{0m2f}\) and \(\left| {I_{0m1} } \right| \gg \left| {I_{0m2} } \right|\) are inferred.

When only line m2 of the system is grounded at one point:

$$ I_{0m2}^{\prime} = - \sum_{i = 1}^N {I_{0i} } (i \ne m2) $$
(7)

By comparing Formula (6) and Formula (7), \(\left| {I_{0m2}^{\prime} } \right| \gg \left| {I_{0m2} } \right|\). It can be confirmed that the steady zero-sequence current in the fault line in case of one point grounding is far greater than that in case of two point grounding of different lines.

Consider the case that two point grounding in the same phase occurs at different times. When the ungrounded neutral point ung network is grounded at the first point of the line, the zero-sequence capacitive current of the grounding point flows into all sound lines. When any other sound line is grounded at the same phase, a part of the zero-sequence capacitive current of the previous grounding point will be diverted to the second grounding fault point. If the second grounding point is too close to the bus, most of the zero-sequence capacitive current will be distributed, and the steady zero sequence current detected by the line where the first grounding point is located will have a significantly reduced change. The zero sequence capacitive current detected at the beginning of any other sound line is still generated by the capacitance to ground of lines, and the value is basically unchanged.

According to the above rule analysis, when the network is grounded at a single grounding point, the switch line side of a sound line is selected to set the second grounding point of the same phase (the grounding knife switch that can be operated remotely can be used for grounding). The line where the first grounding point is located can be identified by detecting the sudden change of the steady zero-sequence current which is detected at the beginning of each line before and after the operation in the second grounding point.

3.2 Analysis of Two Point Grounding Principle of Neutral Grounding System Through Arc Suppression Coil

In arc suppression coil grounding network, when m1 and m2 different lines are grounded at two points in the same phase, the zero-sequence capacitive current of all sound lines and the inductance current from the arc suppression coil are shunted at two grounding points, and the shunting formulas are (8) and (9).

(8)
$$ I_{0m2} = \left| {\frac{{Z_{0m1f} }}{{Z_{0m1f} + Z_{0m2f} }}} \right|(P\sum_{i = 1}^N {I_{0i} } + I_{0m1} + I_{0m2} ) $$
(9)

where P is the compensation degree in arc suppression coil. When \(Z_{0m1f} \ll Z_{0m2f}\), \(\left| {I_{0m1} } \right| \gg \left| {I_{0m2} } \right|\).

If only line m2 is grounded at one point: \(I_{0m2}^{\prime} = (P\sum_{i = 1}^N {I_{0i} } + I_{0m2} )\).

Therefore, when the arc suppression coil grounding network is grounded at one point (line m2 is grounded), the switch line side of a sound line is selected to set a grounding knife switch for grounding (line m1 is grounded, and the grounding phase is the same as line m2), forming a two-phase grounding state in the same phase, and \(\left| {I_{0m1} } \right| \gg \left| {I_{0m2} } \right|\), \(I_{0m2}^{\prime} = I_{0m1} + I_{0m2}\).\(\left| {I_{0m2}^{\prime} } \right| \gg \left| {I_{0m2} } \right|\) is reasoned out. The grounding line in case of one point grounding can be judged by the sudden change of steady zero-sequence current of all lines before and after setting the second grounding point, and the steady zero-sequence current of the sound line is basically unchanged.

When the selected grounding operation line is exactly the grounded line, the zero-sequence current which is detected at the beginning of the line has no obvious change, and the zero-sequence current of other sound lines has no obvious change. When the bus is grounded, the steady zero-sequence current of all lines is generated from capacitive current to ground. If the second point of the same phase is grounded on a line at this time, the grounding point will be shunted with the bus grounding point, and the zero-sequence current which is detected at the beginning of the line will change significantly, while the zero sequence current of other lines will remain unchanged.

4 Fault Line Selection Process

Regardless of the ungrounded neutral point network or the arc suppression coil grounded network, the steady-state zero sequence current values of each line are obviously different due to the different length and type of distribution network lines in each substation. In order to more clearly distinguish the sudden change characteristics of steady zero-sequence current, the line where zero sequence current is the largest is set as the selected operating line of the second grounding point, and the ratio between the change amount of steady zero-sequence current of each line after the setting of the second temporary grounding point and the zero-sequence current before the setting of the second grounding point is used as the basis for judging the current change.

When single-phase grounding occurs, the steady zero-sequence current of each line is recorded. Set the line side grounding knife switch of the same phase on the selected line to ground, and calculate the sudden change percentage of the steady zero-sequence current of each line. When the current sudden change of the selected grounding operation line is the largest, bus grounding is judged. If both the selected grounding operation line and some other line have large current inrush, the latter is the grounding line. If there is no obvious change in the steady zero-sequence current of all lines, the selected grounding operation line is the grounding line. After identifying the grounding fault line, first disconnect the second temporary grounding point, then pull the grounding line, and the grounding fault disappears.

5 Example of Field Equipment Line Selection

D5000 smart grid dispatching support system transmits current, voltage and other data information collected by various telemetry devices in the substation to the power dispatching control center. It is unnecessary to set additional hardware equipment separately to use the steady zero-sequence current information for fault line selection. It is only necessary to use the existing collection device in the station to transmit the zero sequence current data to the D5000 system, and display the data processing results visually for the dispatcher to make decision on fault line selection. The existing acquisition device can make the zero sequence current accurate to 0.01 A, which is sufficient to complete the line selection discrimination.

Fig. 3.
figure 3

Connecting mode of one transformer substation

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Taking a 110 kV substation as an example, the 10 kVI section bus is operated with a set of arc suppression coils, and the wiring mode is shown in Fig. 3. A single-phase grounding event occurred in the station in June 2021, and the fault phase is A. After receiving the bus grounding alarm signal, the dispatching master station grounds the remote remote control A phase grounding knife switch on the pre selected grounding operation line 576. The zero-sequence current information of each line is shown in Table 1. Current 1 and current 2 respectively represent the steady zero-sequence current value of the line before and after closing the A-phase grounding switch of line 576. This table is displayed to the dispatcher in the D5000 system.

Table 1. Zero sequence current variation in lines
Full size table

In the data in Table 1, the sudden change percentage of steady zero-sequence current of line 579 and line 576 under grounding operation is −92% and 378% respectively. The sudden change in steady zero-sequence current of the two lines is significant, and the change of zero sequence current of other lines is very small. From this, it can be confirmed that line 579 is a grounding line. After the dispatcher remotely pulled the A phase grounding knife switch of line 576, pulled the line 579 switch, the grounding phenomenon disappeared, and the fault line selection was correct.

Up to now, the fault line selection method has been popularized and tested in several substations. In a total of twelve 10 kV grounding events, the fault line selection is correct, there is no power failure of the sound line, and the line selection and wiring time is within 10 min. This method not only reduces the times of grounding test, avoids power failure of sound lines, but also greatly reduces the line selecting time.

6 Conclusion

This paper presents a fault line selecting method based on the principle of in-phase two point grounding. This method only uses the existing measurement and control devices in the substation to get the steady zero-sequence current for line selection judgment. The fault identification information features prominently, and no additional hardware equipment is required. It can be visualized through the D5000 dispatching support system, which is convenient for the dispatcher to make decisions. This method can correctly judge in the ungrounded network or in arc suppression coil network, and has wide application scope and strong popularization.