JPS6243142B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a facility for a live-line insulation monitoring system that measures the insulation resistance of the entire high-voltage distribution line or individual power cables connected to it under live-line conditions and monitors the progress of deterioration. In particular, the present invention relates to a method for grounding a plurality of grounding transformers installed at different locations on a high-voltage distribution line.

FIG. 1 is a diagram showing a conventional grounding method for a grounding transformer. 1 is a power transformer that steps down from extra high voltage to high voltage, 2 is a main distribution line connected to the secondary side of the transformer 1, and 2', 2'' is a plurality of sets branched from the main line 2 (here, Two sets of branch distribution lines are shown using cables together with the main line 2. 3 is a grounding transformer connected to the main line 2 near the transformer 1; 3', 3''; are grounding transformers connected near the branch parts of the branch distribution lines 2', 2'', respectively, and these grounding transformers 3, 3', 3'' have a star connection on their primary side and a star connection on their secondary side, respectively. is used in open delta connection. 4, 4', 4'' are large capacity capacitors inserted and connected between the primary side neutral point of the grounding transformer 3, 3', 3'' and the earth, and 5, 5', 5'' are respectively An electromagnetic contactor (hereinafter referred to as a switch) connected in parallel to the capacitors 4, 4', and 4'' so as to short-circuit or open both ends thereof, and is normally closed to short-circuit between the neutral point and the ground. The state is maintained, but only when measuring the insulation resistance of the distribution line or the cable connected thereto under live wires, it is electrically energized (or deenergized) from the operating power supply device 8 via the control cable 7 and released. is configured to be 6,
6', 6'' are arresters connected in parallel to the capacitors 4, 4', 4'' and switches 5, 5', 5'' as safety devices. 9 is the DC power source for measurement or the cable insulation resistance side. It is a constant ammeter, one end of which is connected to the primary side neutral point of the grounding transformer 3, and the other end of which is grounded.The reference numeral 9 contains a wave device and other components, but these are omitted here.

The measurement method will be explained. In order to measure the insulation resistance of the entire distribution line or each cable connected to it, the distribution line must be grounded at the neutral point with low impedance for AC, but isolated from the ground for DC. There is a need to.
Therefore, first, the operation power supply device 8 is activated and an electric signal is sent to the switch 5, via the control cable 7.
5', 5'' and operate these switches to change the short-circuit state between the primary side neutral point of each of the grounding transformers 3, 3', 3" and the earth to an open state. As a result, the capacitors 4, 4', 4'' are automatically inserted between the primary side neutral points of the grounding transformers 3, 3', 3'' and the earth, as expected. state is achieved. Then, if 9 is a DC power source for measurement, if it is operated, the DC voltage for measuring the insulation resistance of the entire distribution line or the insulation resistance of each cable connected to it will be superimposed on the AC voltage on the distribution line. 9
If it is an ammeter for measuring cable insulation resistance, it can measure the insulation resistance of each cable by sequentially applying a separate measurement DC power source between the shield of each cable to be measured and the ground. be.

When the measurement is completed, the DC power supply for measurement is deenergized, and the switches 5, 5', 5'' are returned to the closed circuit state by operation from the operation power supply device 8 via the control cable 7, and the grounding Return to the normal state in which the primary neutral points of the transformers 3, 3', and 3'' are short-circuited to the ground.

The conventional method of grounding multiple connecting transformers has the following drawbacks.
In other words, the individual locations of multiple installed grounding transformers are often far apart from each other. For example, grounding transformer 3 is located in the receiving substation, 3" is located in the first electrical room, and 3" is located far apart from each other. They are located in the second electrical room, and the distance between them is several hundred meters or more.Also, by the time we start monitoring the insulation under live wires of distribution lines, the lines are old and have deteriorated to a certain extent. After many years of operation, we had to install a new control cable to remotely control the opening/closing mechanism between the neutral point of the grounding transformer and the earth. Compared to laying a distribution line at the beginning of construction, it is extremely difficult to select a route and carry out construction work, and the cost is enormous, making it uneconomical.If this cost is too high, there is no route for laying the distribution line, or if the construction is impossible. In this case, it becomes impossible to carry out insulation monitoring under live wires.

It is an object of the present invention to provide a remote control mechanism only for a specific grounding transformer that is close to the measurement location and for which control wiring can be easily installed among the multiple grounding transformers installed on the distribution line. An object of the present invention is to provide a grounding method for a grounding transformer that enables insulation monitoring of a high-voltage distribution line under a live line by laying a control cable that is connected to the ground. Specifically, the primary neutral point of a specific grounding transformer is always directly grounded and floated above the ground only during measurement, but other grounding transformers are grounded directly at all times and during measurement. The primary neutral point is always connected to the ground through a capacitor so that it is floating directly above the ground, and a resistor for vibration suppression is inserted in series with this.

The present invention will be described in detail below with reference to the drawings.

FIG. 2 is a diagram showing a method of grounding the grounding transformer of the present invention. 1 is a power transformer that steps down from extra high voltage to high voltage, 2 is a main distribution line connected to the secondary side of the transformer 1, and 2', 2'' is a plurality of sets branched from the main line 2 (here, These branch distribution lines (shown in two sets) are mostly cables, similar to the main line 2. 3 is a grounding line connected to the main line 2 near the transformer 1. The transformer in this position is usually designated as a special grounding transformer. Between the neutral point on the primary side of this transformer 3 and the earth, there is an electromagnetic contactor 5 as a remote operation switching mechanism, and a control cable 7 is connected to the ground. It is operated from the operating power supply device 8 through the power supply unit 8.Here, only one specific grounding transformer is shown, but there may be more than one.For example, if the bus bar is a double bus bar, one grounding transformer is provided for each bus bar. When the transformers are connected, a total of two transformers may serve as special grounding transformers. Between the primary side neutral point of transformer 3, which represents the special grounding transformers, and the earth, Furthermore, a large-capacity capacitor 4 and a safety arrester 6 are inserted. 9 is a DC power supply for measurement or a rated ammeter on the cable insulation resistance side, and one end thereof is connected to the primary side neutral point of the special grounding transformer 3. and the other end is grounded.This 9 includes a wave device and other components inside, but they are omitted here.If there is not one specific grounding transformer but multiple, the primary side neutral of each Individual electromagnetic contactors may be inserted between the point and the ground, or one electromagnetic contactor may be inserted after sharing the primary neutral point of each. In the former case, 9 can be connected between the primary side neutral point of any one of the specified grounding transformers currently connected to the distribution line and the earth.The opening/closing operation of the electromagnetic contactor when measuring insulation resistance is performed using the specified grounding. Regarding the transformer, the method is exactly the same as the conventional method.

Next, 3', 3'' are grounding transformers connected near the branch parts of the branch distribution lines 2', 2'',
This is a representative grounding transformer other than the specific grounding transformer mentioned above. A safety arrester 6 is installed between the primary neutral point of these transformers and the earth.
6'', as shown in Figure 2, a circuit is inserted in which large capacity capacitors 4', 4'' and resistors 10', 10'' are connected in series.Here, the rating of the grounding transformer is If the ratings are the same for 3' and 3'', the ratings for large capacity capacitors 4' and 4'' and the resistors 10' and 10'' may be the same, but if they are different, the same constants must be used for each. . The capacitance value of a large-capacity capacitor is preferably at least 100Ω or less, which is defined as the third type grounding resistance value, as its own impedance, so it is usually 40μF (impedance at 50Hz 79.6Ω). Take the above. Normally, the zero-sequence current that flows through this capacitor is several mA, and even in the event of a single-line ground fault, it is usually around several hundred mA, so the voltage drop due to the impedance of the capacitor is small and the voltage drop on the secondary side of the grounding transformer is small. There is almost no effect on the ground fault detection function. Rather, due to the presence of capacitance, it is excited by the inrush zero-sequence current at the time of an accident, causing series resonance with the zero-sequence circuit effective inductance of the grounding transformer, generating abnormal voltage, which causes the grounding There is a greater concern that the insulation of power transformers and capacitors will be destroyed.
In order to suppress the occurrence of such vibrations, the method of the present invention constantly inserts a resistor in series with the capacitor.

In FIG. 2, the resistors 10' and 10'' are placed on the side near the neutral point, and the capacitors 4' and 4'' are placed on the ground side, but the placement may be reversed. A current-limiting resistor is originally installed at the open end of the secondary delta connection of the grounding transformer, and under normal conditions, the value converted to the primary zero-phase circuit is the value between the neutral point and the earth. However, in the very early stages of an accident, the excitation characteristics of the core cannot follow the rapidly changing phenomenon over time, so only the DC resistance value of the primary winding of the grounding transformer is It simply exists as a vibration suppressing resistance, and since this value is generally low, a sufficient vibration suppressing effect cannot be expected. Therefore, a fixed resistor is always inserted between the neutral point and the ground, and its value is such that since the fixed resistor is always inserted, it obstructs the zero-sequence current detection function, which is the original responsibility of the grounding transformer. To avoid this, the converted value of the secondary current limiting resistor to the primary side should be
The maximum value is 10%. Let's now consider the resistance value to be inserted on the primary side of a grounding transformer with the following rating.

A Primary rated voltage 3300V B Secondary 110/3V C Voltage that appears at the open end of delta connection in case of complete ground fault 110V D Secondary current limiting resistance 50Ω E Secondary current in case of complete ground fault 2.2A F Primary current 0.127A g Primary side converted resistance value 50Ω x (2.2/0.127) ² = 15000Ω Therefore, the maximum resistance value that can be inserted between the primary side neutral point and the earth is 1500Ω in this case. The actual resistance value to be inserted is the value obtained by subtracting the 3-phase parallel DC resistance value of the primary winding from this value, but it is 1500Ω including that value.
is inserted, and the capacitance value of the capacitor connected in series with it is 40μF.

FIG. 3 is a diagram showing a grounding transformer in which a resistor is inserted on the primary side. R is the secondary current limiting resistance of 50
Ω, Ro is the primary side insertion resistance of 1500Ω, C is the resistance Ro
The capacitor in series with the capacitor is 40μF. The maximum inductance Lmax that exists in this circuit and does not cause an oscillating state due to the resistance Ro is Lmax=Ro ² C/4 = 1500 ² ×40 × 10 ⁻⁶ /4 = 22.5 (H).

The zero-phase circuit residual inductance of the grounding transformer is not zero, but actually has a considerable inductance value. However, even if there is a large inductance as described above, the insertion of 1500Ω will not cause a vibration state, so as a practical matter, vibration phenomena can be suppressed in most cases.

Next, the inductance value is actually 1/10 of the above value.
Let's also consider the case where there is only 2.25H. Of course, 1500Ω is too high in this case, so no vibration occurs. In the early stages of a complete ground fault, etc., when it is not expected to convert the secondary current limiting resistor into the primary side, 1758V will be applied to both ends of the resistor, and 1758V will be applied to both ends of the capacitor.
A voltage drop of 93V is possible, but if the secondary current limiting resistor is transferred to the primary side, the voltage will be shared by the resistor at 173V and the capacitor at 9V. At this time, the voltage across the secondary current limiting resistor is calculated at 50Hz.
It is 100V, a drop of about 10%.

As described above, in the method according to the present invention, other grounding transformers other than the specified grounding transformer do not have a remote control opening/closing mechanism, and there is a capacitor and a resistor between the primary neutral point of the transformer and the earth. are always inserted to form a series circuit, so in the event of a system ground fault, a zero-sequence current will always flow through the capacitor and the resistor, causing a voltage drop across both ends, but the occurrence of vibration is suppressed by the inserted resistor. Therefore, the insulation of the capacitor and the grounding transformer is not threatened, and the ground fault detection function on the secondary side of the grounding transformer is not significantly affected. Needless to say, installation of control cables for the remotely operated opening/closing mechanism can be avoided. On the other hand, since the primary neutral point of a special grounding transformer is always directly grounded, there is a high probability that a system ground fault will occur in that state. The possibility of a ground fault occurring during measurement is extremely small considering the ratio of non-measuring time to measuring time, so there is no particular need to include a vibration suppression resistor to guard against that slight possibility, but of course it is safe. It is okay to insert it for the sake of.
It is necessary to wire the control cable 7 from the operation power supply device 8 to the specific grounding transformer, but since these are located very close to each other, it is economical to wire the control cable 7 here. There is no problem with gender or anything else.

An advantage of the present invention is that it eliminates the need for a short-circuit opening/closing mechanism between the primary side neutral point of a grounding transformer installed far away and the earth, so there is no need to install control cables. The cost of laying control cables, which is a big barrier when adopting an insulation monitoring system even after many years, is significantly reduced and economical, and the primary neutral point and the earth are always grounded using a capacitor. A resistor with a value of 10% or less of the value converted to the primary side of the secondary current limiting resistance of the grounding transformer, which is floating in direct current, is inserted in series with the capacitor, without impairing the earth fault detection function. It may prevent electrical vibrations from occurring during ground faults, eliminating the risk of insulation breakdown in grounding transformers and capacitors.

[Brief explanation of the drawing]

Fig. 1 shows the grounding method of a conventional grounding transformer, Fig. 2 shows the grounding method of the grounding transformer of the present invention, and Fig. 3 shows a grounding transformer with a resistor inserted on the primary side. FIG. 1...Power transformer, 2...Distribution line main line,
2', 2''...Branch distribution line, 3, 3', 3''...Grounding transformer, 4, 4', 4''...Condenser, 5,
5', 5''...Magnetic contactor, 6, 6', 6''...Arrester, 7...Control cable, 8...Operation power supply device, 9...DC power supply for measurement or ammeter for measuring cable insulation resistance , 10, 10', 10''...Resistance, R
...Secondary side current limiting resistance, Ro...Primary side insertion resistance, C...Condenser.

Claims (1)

[Scope of Claims] 1. In a method for grounding a grounding transformer when insulation is monitored under live lines in a high-voltage distribution line where multiple grounding transformers are installed, only a specific grounding transformer is grounded. A remote control switching mechanism is installed between the primary neutral point and the earth, and a series circuit of a capacitor and a resistor is always inserted between the primary neutral point and the earth for other grounding transformers. A grounding method for a grounding transformer characterized by: 2. In the grounding method described in claim 1, the value of the resistance is equal to the value obtained by converting the value of the secondary current limiting resistance of the grounding transformer to the primary zero-phase circuit.
A grounding method for a grounding transformer characterized by a grounding ratio of 10% or less.