JP2607623B2 - Light ground fault occurrence cable identification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial application field The present invention relates to a case where a light ground fault continues to occur in a high voltage system, which of the cables under monitoring is responsible for the occurrence or monitoring. The present invention relates to a method of determining a light ground fault occurrence cable, which determines whether there is a cause other than the cable below.

(B) Prior art and problems to be solved by the present invention In a non-grounded high-voltage system, if a minor ground fault that does not result in a shutdown accident occurs continuously, what is important for the facility manager is Quickly know if there is a ground fault cause, that is, which of the feeder cables (mains cable) constitutes the cause, or if the cause is in a location other than the feeder cable, and prepare for a possible power failure. Take precautionary measures and take steps to minimize damage.

In the case described above, each feeder cable is generally provided with a ZCT (zero-phase current transformer) and a direction-selective ground fault relay. For this reason, which feeder cable and subsequent circuits have a ground fault cause can be understood by closely examining all the direction-selective ground fault relays. However, this approach is not practical because the direction-selective ground fault relay cannot be easily investigated before the contacts operate. Even if the contact of the direction-selective ground fault relay operates, it cannot be determined whether the feeder cable is the cause or the cause is downstream of the feeder cable.

FIG. 3 and FIG.
This will be described with reference to the drawings. In FIG. 3, feeder cables 2, 2 'are connected to the high-voltage bus 1, and a part thereof is shown. Shielded terminal ground wires 3, 3 'are connected to the feeder cables 2, 2', and zero-phase currents flowing through the feeder cables are detected by ZCTs 4, 4 'and supplied to direction-selective ground fault relays 5, 5', respectively. You. On the other hand, a grounding type instrument transformer 6 is connected to the high-voltage bus 1, and a ground fault voltage relay 7 is connected to the transformer 6 to detect the occurrence of a ground fault voltage to detect a direction-selective ground fault relay. 7 is supplied with a direction standard voltage. The ground fault location of the feeder cable 2 is 8, and its ground fault resistance is Rg. Further, when the high-voltage bus 1 ′ after passing through the feeder cable 2 ′ has a ground fault cause, a ground fault location is set to 8 ′, and the ground fault resistance is set to Rg ′.

Now, it is assumed that the occurrence of the ground fault that does not lead to the interruption is reported by the operation of the grounding type instrument transformer 6 and the ground fault voltage relay 7, and this is maintained. This may be caused by one of the feeder cables (2 in FIG. 3) or at a location other than the feeder cable (1 'after passing through 2' in FIG. 3).
Sometimes it is. In this case, conventionally, the clamp-type ammeter 9 was carried around and inserted into the shielded terminal ground wires 3, 3 'of each feeder cable to measure the ground current, and it was tried to determine the magnitude of the ground current.

FIG. 4 shows the current I measured well by the clamp ammeter 9.
FIG. 9 is a circuit diagram for analyzing (current flowing in 3) and I ′ (current flowing in 3 ′). In FIG. 4, symbols C and C 'denote the total capacitance of the feeder cables 2 and 2' for the three phases, and symbols Co and Co 'denote C and C', respectively, from the total capacitance of the high voltage system for the three phases to the ground. It is the remaining capacitance that was subtracted. Symbol E represents the voltage of the high-voltage system line voltage. Ig and Ig 'are ground fault currents flowing through ground fault resistances Rg and Rg'.

As shown in FIG. 4 (A), the ground current I flowing through the shield terminal ground line 3 is I = Ig−Ic, and C in the current returning from the ground (e) to the high voltage system neutral point (N) is Current sharing
The current obtained by subtracting Ic from the ground fault current Ig will be measured.
Ico is a current flowing through the residual capacitance Co, the magnitude is equal to I, and the direction is reversed.

The current I 'flowing through the shield terminal ground line 3' is shown in FIG. 4 (B).
As shown in FIG. 5, I '=-Ic', and the ground fault current Ig 'is not measured. Only the current Ic' shared by C 'in the current returning from the ground (e) to the neutral point (N) is obtained. Will measure. I c
o 'is a current flowing through the remaining capacitance Co and is equal to Ig'-Ic'.

In the above-mentioned conventional method, although there is a phase difference of 180 degrees between I and I ', it cannot be measured and only its magnitude relationship can be known. Generally, I is greater than I '. However, if the three-phase capacitances C and C 'of the feeder cable occupy a relatively large ratio compared to the remaining capacitance, which cable determines the cause of the ground fault based on the magnitude of the ground current alone? Or, the cable within the measured range does not constitute the cause of the ground fault, and it is difficult to determine that the ground fault has occurred at another place. Furthermore, the actual work by the above method has the disadvantage that it takes a lot of manpower and time since the cubicle can be approached only after the back cover of the cubicle is removed and the cable terminal is exposed, so that the shield terminal ground can be approached. However, there has been a problem that the investigation and work effort have been wasted because of natural resolution.

The present invention provides a method for monitoring whether or not a cable that constitutes a ground fault is present in a group of cables that are provided in advance as facilities to be monitored in a very short time when a light ground fault occurs in a high-voltage system in a very short time. It is an object of the present invention to provide a method of determining a light ground fault occurrence cable, which can easily and accurately determine which cable is.

(C) Means for Solving the Problems According to the present invention, the grounding of the shielded terminals of a group of cables to be monitored is sequentially floated from the ground for each cable and superimposed on a grounding circuit having a specific capacitance. And a step of measuring an AC voltage generated at both ends of the capacitance each time the superimposed injection is performed. In the group of cables, it is determined that there is no cable constituting a ground fault cause. Judge that it is configured.

(D) Operation When the ground of the shield terminal of the group of cables is sequentially floated from the ground for each cable and superimposed on a grounding circuit having a specific capacitance, if a light ground fault does not occur, Although the AC voltage generated at both ends of the capacitance sequentially increases, a measurement characteristic is obtained in which the AC voltage is reduced as the cable after the occurrence of the light ground fault is applied and the AC cable is repeatedly applied thereafter. The cable which constitutes the cause of the ground fault is specified by this characteristic.

(E) Embodiment FIG. 1 shows an embodiment of the method of the present invention. In FIG. 1, the same parts as those in FIG. 3 are denoted by the same reference numerals. The high-voltage bus 1, ZCT4, 4 'and direction-selective ground fault relays 5, 5' are omitted because they are not particularly relevant to the present invention. The terminal grounding lines 3, 3 'are connected to the receiving terminals of the cable grounding panel 10 without being directly dropped to the ground.

The cable grounding panel 10 includes feeder cable ground changeover switches 11, 11 'having a fixed position electrically connected to the terminal grounding lines 3, 3', a specific capacitance 12, a security arrester 13, an AC voltmeter. 14. Equipped with the receiving terminal, ground terminal, and complete wiring. The cable grounding panel 10 can be provided not only in one set but also in other places suitable for collecting other groups of cables connected to the same high-voltage system. If these circuits are assembled after the ground fault occurrence report is obtained, it is not possible to make it in time.

The ground changeover switches 11, 11 'have measurement positions a, a' and normal positions b, b ', and the normal positions b, b' are connected to a ground terminal. Further, the ground changeover switch has a wrap mechanism in which the movable piece does not float at the intermediate position when switching from the normal position to the measurement position. The specific capacitance 12, the arrester 13, and the AC voltmeter 14 are connected in parallel. One connection point is connected to the measurement positions a and a ', and the other connection point is connected to a ground terminal. The AC voltmeter 14 is preferably an automatic range switching digital multimeter having an input resistance of about 10 MΩ.

The reason why 12 is referred to as a specific capacitance is that there is a limit on the capacitance value determined for each high-voltage system. When the capacitance value Cp is 100% ground fault, the high voltage system is connected to the high voltage system through the total capacitance for the three phases to the ground (only the group of cables connected to the cable grounding board 10 is not the three phase capacitance). The impedance must allow the ground fault current returning to the neutral point to pass safely, ie with a voltage drop that does not reach its maximum allowable operating voltage value. For example, the maximum allowable operating voltage
Assuming that the voltage is 240 V and the ground fault current at the time of 100% ground fault occurrence is 5 A, the impedance must be 48 Ω or less, that is, Cp should be 66 μF or more at 50 Hz. Actually, if Cp is set to 80 μF,
The corresponding maximum ground fault current is 6.03 A. The maximum value of the total capacitance of the high-voltage system for the three phases to the ground is 6.6 KV and 5.04 μF at 50 Hz.

The following table shows the hypothetical 6.6K used to illustrate the invention.
It is a total capacitance configuration for three phases of V, 60Hz system.

In the high voltage system shown in the above table, feeder cables No.1 to No.1
The ten lines up to 0 (total capacitance 2.43 μF) are connected to the cable grounding board 10 and are monitored, and the remaining capacitance 2.295 μF of other cables and the like is not monitored. The total capacitance of the system to ground for three phases is 4.725μF,
On the other hand, as the specific capacitance in the cable grounding board 10,
80 μF is prepared. The maximum allowable operating voltage value of this capacitance is 240 V. However, in this virtual system, no matter where a 100% ground fault occurs, the voltage drop that occurs at a specific capacitance is 24 V
Does not reach 0V.

Now, it is assumed that the No.X cable is a specific cable, a light ground fault has occurred with a total capacitance of 0.225 μF for three phases, and a ground fault resistance Rg = 5 KΩ inside the cable, and this continues. In this case, the ground fault voltage Vo generates 425V (11.2%),
The ground fault current Ig flows 0.757A. Normally, a light ground fault through the insulated body of the cable will recover instantaneously if it is caused by a water tree, and in order for such a light ground fault to continue, the cause must be capacitance. Or a cleave accident at a location where the leakage distance is long, such as a connection or terminal, but the search for the cause is excluded here.

FIG. 2 illustrates a calculation example by simulation in the case where the method of the present invention is applied. In the case where the No. X cable happens to be replaced with any of the feeder cables No. 1 to No. 10, or If it is in a residual cable, or if the accident has not occurred in the cable but has occurred elsewhere (but the ground fault resistance
Rg 'is equal to Rg and 5KΩ). In the calculation, a specific capacitance of 80μF for simplicity
The change in the ground fault current due to the insertion of the current is ignored.

Curve A in Fig. 2 shows the case where there is no No. X cable between feeder cables No. 1 to No. 10 and there is a ground fault at the remaining cable or at a place other than the cable. Is shown. Measurement starts from feeder cable No. 1.
The ground changeover switches 11 and 11 'up to No. 10 are sequentially switched from the normal positions b and b' to the measurement positions a and a ', and the AC voltage at both ends of the specific capacitance measured by the AC voltmeter 14 each time. Is shown. In the curve A, the AC voltage only increases as the feeder cable is superimposed. When the upward trend as shown by the curve A is observed, it is determined that there is no cable constituting the ground fault cause in the group of cables. Here, for example, feeder cable No.2
No.2 even if the ground fault resistance Rg 'exists after passing through
No abnormal phenomena occur when thrown. However, the last No.1
A cable of 0 shall leave doubt just in case.

Curve B in Fig. 2 is No.X at the position of feeder cable No.1.
The transition of the AC voltage measurement when the cable is replaced is shown. In this case, a large voltage is observed only by switching the ground changeover switch 11 to the measurement position a, and thereafter, as the feeder cable is turned on, the AC voltage only decreases. In this case, it is determined that feeder cable No. 1 is the cable that is the cause of the ground fault.

Curve C in Fig. 2 is No.X at the position of feeder cable No.3.
In this case, the AC voltage increases until feeder cable No. 3 is superimposed. N
The voltage jump when o.3 is turned on is large, but this is not a criterion but a reference. If the AC voltage only decreases as the feeder cables after No. 4 are repeatedly superimposed, it is determined that feeder cable No. 3 is the cable constituting the cause of the ground fault. Curve D in FIG.
Indicates the measurement when the feeder cable No. 5 was replaced, and the curve E indicates the measurement when the feeder cable No. 8 was replaced with the No. X cable #. In each case, the AC voltage increases until the No.X cable is turned on, and when the No.X cable is turned on, a jump can be seen to a certain degree, but this is only a reference, not a judgment standard. The cable with the highest voltage value obtained by inserting the feeder cable only after it is confirmed that the AC voltage has dropped by superimposing the feeder cable thereafter, that is, the curve D
Then, it is determined that feeder cable No. 5 and feeder cable No. 8 on curve E are cables constituting the ground fault cause. However, in the case of the curve E, since the jump is not clear, it is necessary to question the next feeder cable No. 9.

Curve F in FIG. 2 is No. 10 at the position of feeder cable No. 10.
When the X cable is replaced, in this case, the highest voltage value is obtained when feeder cable No. 9 is turned on, and the AC voltage decreases when No. 10 is turned on. In this case, it is easy to determine that feeder cable No. 9 is the cable that constitutes the cause of the ground fault, but no jump is seen when No. 9 is inserted, so doubts are made to the next feeder cable No. 10. You need to call. Curves E and F are phenomena that occur when the remaining capacitance (including the remainder of the feeder cable) happens to be close to the superimposed capacitance. In this case, reverse the order of measurement and re-measure from feeder cable No. 10 to No. 1, or start again by measuring suspicious cable first to clarify the tendency and make the final decision easier. Can be done.

When there are a plurality of cable grounding boards, the same measurement is performed on each cable grounding board. Then, it is determined whether or not there is a cable constituting the ground fault cause in the cables of each group connected to each cable grounding panel, and if so, which cable is.

The reason why the characteristic shown in FIG. 2 appears is that, as shown in FIGS. 4A and 4B, the current flowing through the shielded terminal ground wire of the cable constituting the ground fault cause is from (N). If the power flow direction toward (e) is positive, it can be expressed as positive, but in the case of other cables, since it returns from (e) to (N), it will be expressed as negative and the phase will differ by 180 degrees. Therefore, according to the present invention, the ground fault occurrence cable can be easily determined by observing the observation result including the influence of the phase while measuring the absolute value of the voltage.

Although the above embodiment has been described with reference to the setting of a ground fault due to a ground fault resistance, the present invention is not limited to this and can be applied to an apparent one-phase capacitance grounding fault based on capacitance imbalance. In the case of cables, capacitance imbalance can occur due to a disconnection of the shielded copper tape or a single-phase open phase due to fuse blowing, etc., but also in this case, the cable that constitutes the ground fault cause and other cables This is because the current flowing through the shield terminal ground wire has a phase difference of 180 degrees. However, the phase relationship between the high-voltage system line voltage E and the ground fault voltage Vo is different.
Here, how much capacitance unbalance is caused by ground fault resistance Rg
Examining whether or not a ground fault voltage of 425 V corresponding to = 5 KΩ occurs in the above-described virtual high-voltage system, it corresponds to a case where 0.474 μF is missing in one phase. As another example, assuming that one phase of No.X cable is completely open, Vo is 61.5V due to 0.075μF missing, ground fault current of No.X cable is 104, 3mA, and 80μF is inserted into this Even if the ground current does not change, the terminal voltage is 3.46V.

(F) Effect The present invention relates to a high-voltage system in which a sustained light ground fault has occurred, which one of the cables which has been installed and monitored in advance in a short time and with a small number of humans, or which monitors the cause. None of the cables listed below constitute the cause, and it can be easily and accurately determined whether the cause exists elsewhere. For this reason, important information and guidelines can be obtained for taking necessary precautionary measures before a serious accident occurs, which has great benefits in predictive maintenance of electrical equipment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram for implementing the method of the present invention;
FIG. 2 is a diagram showing a characteristic of a voltage value appearing in a specific capacitance when feeder cables are sequentially superimposed by the method of FIG. 1, FIG. 3 is a circuit configuration diagram showing a conventional determination method, and FIG. FIG. 3 is an equivalent circuit diagram for analyzing a ground current flowing through a shield terminal ground line. 1, 1 '... high-voltage bus, 2, 2' ... feeder cable, 3, 3 '... shielded terminal ground wire, 8, 8' ... ground fault location, 10 ... cable grounding board, 11, 11 ' … Ground switch, 12… Specific capacitance, 14… AC voltmeter.

Claims (1)

(57) [Claims] When a light ground fault continues to occur in a high voltage system, the cause of the occurrence of the light ground fault is a group of devices connected to the high voltage system to be monitored and whose shield terminals are grounded. A method for determining whether or not a cable is in a cable, and if the cable is in the group of cables, which cable is in the cable, wherein the grounding of the shielded terminal of the group of cables is sequentially floated from the ground for each cable. Superimposing on a ground circuit having a specific capacitance; and measuring an AC voltage generated at both ends of the capacitance each time the superimposing is performed. If the AC voltage rises as the power is applied more than once, it is determined that there is no cable that constitutes the ground fault in the group of cables, and the subsequent cable input will be delayed after a specific cable input. A method for determining a light ground fault cable, comprising determining that the cable constitutes a ground fault cause when the AC voltage decreases as the power is applied.