RU2551657C1 - Method of monitoring of zero wire break and break of connection of substation earthing loop with transformer neutral point - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: according to the method if at disappearance of current in a zero wire the phase voltage system became asymmetrical when phase voltages differ by an absolute value and change both towards decrease, approaching to a zero value, and towards increase, approaching to a linear value, depending on connection or disconnection of single-phase consumers, then the conclusion is made on the zero wire break and break of connection of the substation earthing loop with the transformer neutral point.

EFFECT: expansion of the method functionality by obtaining the information on break of the zero wire and break of connection of the substation earthing contour with the transformer neutral point.

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Description

The invention relates to the automation of electrical networks and is intended to control the breakage of the neutral wire and the disconnection of the ground circuit of the substation with the neutral point of the transformer.

A known method for automatically monitoring the parameters of the neutral wire of 0.4 kV overhead lines, which consists in measuring the difference in current in the neutral wire at the beginning of the line and the current in the neutral wire for the first re-earthing switch. And if this difference exceeds the minimum acceptable value for normal operation, then turn off the monitored line [patent RU 2356151 C1, cl. H02J 5/10, publ. 05/20/2009, bull. No. 14].

The disadvantage of this method is the impossibility of using it to control the breakage of the neutral wire and the disconnection of the ground circuit of the substation with the neutral point of the transformer.

The objective of the invention is to expand the functionality of the method by obtaining information about a broken wire and a broken connection of the ground loop of the substation with the neutral point of the transformer.

According to the proposed method, if, when the current disappears in the neutral wire, the phase voltage system becomes asymmetric, when the phase voltages are different in absolute value and change both towards decreasing, approaching the zero value, and increasing, approaching the linear value, depending on switching on or off single-phase consumers, they conclude that the neutral wire is broken and the connection between the ground circuit of the substation and the neutral point of the transformer is broken.

The essence of the invention is illustrated by drawings, where:

- figure 1 presents a structural diagram containing elements for implementing the method;

- figure 2 is a diagram of the signals at the outputs of the elements shown in figure 1.

The circuit (see figure 1) contains: power transformer 1; 2, 3 and 4 - variable resistance of different phases of the line; 5, 6 and 7 - linear wires of the line; 8 - zero wire line; 9, 10 and 11 - repeated grounding of the neutral wire; 12 - break point of the neutral wire; 13 - break point of connection of the ground loop of the substation with the neutral point of connection of the secondary windings of the transformer; 14 - ground circuit of the substation; 15, 16 and 17 - zero currents that flow through repeated grounding 9, 10 and 11 in the event of a break in the neutral wire, for example, at point 12 (see figure 1); element NO 18; 19 - current sensor (DT); 20, 21 and 22 - voltage sensors (DN); 23 - element And; 24 - recording device (RU).

The signal diagrams at the outputs of the elements shown in figure 1, have the form (see figure 2): 25 - at the output of the element 18; 26 - at the output of element 19; 27 - at the output of element 20; 28 - at the output of element 21; 29 - at the output of element 22; 30 - at the output of element 23; 31 - in RU 24.

Figure 2 also shows: t ₁ is the instant of breakage of the connection of the ground loop to the neutral point of connection of the secondary windings of the transformer and the breakage of the neutral wire at unbalanced load; t ₂ is the point in time when the symmetrical load mode has started in the line; t ₃ - the moment of time when the mode of asymmetric load began in the line.

The method is as follows.

In the normal operation of the substation, when there are no breaks at points 12 and 13 (Fig. 1) in both symmetrical and asymmetric loads, the phase voltages between wires 5 and 8, 6 and 8, 7 and 8 are equal in absolute value and form a three-phase symmetrical stress system. Moreover, in the symmetric load mode, the current in the neutral wire is zero, and in the asymmetric load mode, the current in the neutral wire is determined by the vector sum of the phase currents and is not equal to zero. In Fig.2, the symmetric load mode is observed during the time from t ₂ to t ₃ . In the remaining time intervals from t ₀ to t ₂ and from t ₃ onwards, an asymmetric load mode is observed. Due to the fact that at the first input of the And 23 element only a signal from DT 18 is received, and at the second, third and fourth inputs of this element, due to the fact that the three-phase voltage system is symmetrical, no signals are received, so the circuit is in control mode .

When the neutral wire breaks, as well as the connection breaks between the substation ground loop and the neutral connection point of the secondary windings of the transformer (see Fig. 1, points 12 and 13) with an asymmetric load, the vector sum of the phase currents is not zero, and if only the neutral wire breaks at point 12, the zero current 15, 16 and 17 could pass to the transformer 1 through repeated grounding of the neutral wire 9, 10 and 11, then in this case there is no way to pass the zero current, so with DT 18 the signal will disappear (Fig. 2, diagram. 25) and a signal will appear on you the course of the element NOT 19 (Fig. 2, Diagram 26), which will go to the first input of the And 23 element. However, the condition of the first Kirchhoff law - the vector sum of the currents in the node is zero - must be satisfied. Therefore, the voltage drops at different phases will be distributed in proportion to the resistance of these phases, and since phase resistances are different for asymmetric loads, the voltage drops at the phases will be different - an asymmetry of phase voltages appears (voltage imbalance). This will lead to the appearance of output signals from DNs 20, 21 and 22 at time t ₁ (Fig. 2, diagram 27, 28 and 29), when the ground connection of the substation to the neutral point is added to the open circuit at point 12 connection of the secondary windings of the transformer at point 13. These signals will go to the second, third and fourth inputs of the element And 23, it is triggered (figure 2, diagram 30). The signal of this element will go to RU 24, and it will display information about a broken wire and a broken connection between the ground circuit of the substation and the neutral point of the transformer (Fig. 2, Diagram 31).

Thus, the proposed method allows to obtain information about the broken wire and the broken connection of the ground loop of the substation with the neutral point of the transformer.

Claims (1)

A method for monitoring the interruption of the neutral wire and the disconnection of the ground circuit of the substation with the neutral point of the transformer, which consists in measuring the current in the neutral wire, characterized in that if the phase voltage system becomes asymmetric when the current disappears in the neutral wire, when the phase voltages are different in absolute value and change both in the direction of decreasing, approaching the zero value, and in the direction of increasing, approaching the linear value, depending on whether or not single-phase consumes oil, then they conclude that the neutral wire is broken and the connection of the ground circuit of the substation to the neutral point of the transformer is broken.

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