JP7480444B1 - Insulation monitoring system and insulation monitoring method - Google Patents

Abstract

[Problem] To provide an insulation monitoring system and insulation monitoring device that stably controls the earth fault current flowing through an earth fault current suppression section. [Solution] The insulation monitoring system includes a current suppression section provided on a class B ground wire of a transformer, a current detection section that detects the leakage current flowing through the class B ground wire, a control section that switches the leakage current flowing through the class B ground wire to an unsuppressed state in which it flows through a changeover switch or a suppressed state in which it flows through a current suppression resistor, a calculation section having a first calculation function that calculates a resistive component leakage current caused by the earth insulation resistance of the earth fault phase in the unsuppressed state, a second calculation function that calculates a resistive component leakage current caused by the earth insulation resistance of the earth fault phase and a capacitive component leakage current caused by the earth capacitance in the suppressed state, and a third calculation function that calculates an assumed resistive component leakage current caused by the earth insulation resistance of the earth fault phase when the suppressed state is switched to the unsuppressed state, and a timing section having a fourth calculation function that measures the time since the suppressed state was switched to. [Selected Figure] Figure 2

Description

This disclosure relates to an insulation monitoring system and an insulation monitoring method.

An insulation monitoring device that monitors ground faults in the low-voltage circuit of a transformer constantly monitors the insulation of the low-voltage circuit of a high-voltage private facility, thereby preventing electric shock, fire, and power outages caused by leakage current, and thereby contributing to improving the level of electrical safety. For example, Patent Document 1 discloses an insulation monitoring system that includes a leakage current prevention device that is installed between one end of the secondary side of the transformer and the ground to prevent leakage current in the low-voltage circuit on the secondary side of the transformer, and a monitoring device that outputs an alarm when the current flowing through the leakage current prevention device exceeds a predetermined value. This leakage current prevention monitoring system connects a direct grounding contact that is normally closed in parallel to a parallel circuit of a current limiting element and a voltage limiting element, and controls the direct grounding contact to open when the monitoring device detects that a ground fault has occurred on the secondary side of the transformer.

Patent No. 5455430

In a system such as that of Patent Document 1, the suppression state of the earth fault current is released by directly closing the ground contact when the earth fault or an event that is a precursor to the earth fault is eliminated. However, depending on the conditions, such as when the threshold for switching the earth fault prevention device to the suppression state or non-suppression state of the earth fault current is close to each other, or when the fluctuation of the earth fault current accompanying the occurrence of an earth fault or an event that is a precursor to the earth fault is relatively large, it is expected that an event will occur in which the suppression state or non-suppression state of the earth fault prevention device will switch unstably between the suppression state and the non-suppression state within a short period of time.

The present disclosure aims to provide an insulation monitoring system and insulation monitoring device that stably controls the ground fault current flowing through a ground fault current suppression unit.

In order to achieve the above-mentioned object, the insulation monitoring system according to the present disclosure includes a current suppression unit provided in a class B ground wire of a transformer and having a changeover switch and a current suppression resistor arranged in parallel, a current detection unit that detects a leakage current flowing in the class B ground wire, a control unit that switches the changeover switch to a non-suppression state in which the leakage current flowing in the class B ground wire flows through the changeover switch or a suppression state in which the leakage current flows through the current suppression resistor, a first calculation function that calculates a resistive component leakage current caused by the earth insulation resistance of the earth fault phase from the leakage current detected by the current detection unit in the non-suppression state, a second calculation function that calculates a resistive component leakage current caused by the earth insulation resistance of the earth fault phase from the leakage current detected by the current detection unit in the suppression state, and A calculation unit having a second calculation function for calculating a capacitive component leakage current caused by the capacitance to earth and the capacitance to earth of the power supply, and a third calculation function for calculating an assumed resistive component leakage current caused by the earth insulation resistance of the earth-fault phase when the suppressed state is switched to the non-suppressed state from the leakage current detected by the current detection unit in the suppressed state, and a timing unit having a fourth calculation function for timing the time since the suppressed state was switched to, and the control unit switches to the non-suppressed state when the calculation result by the second calculation function, the third calculation function, or the fourth calculation function satisfies a predetermined threshold in the suppressed state switched to because the resistive component leakage current calculated by the first calculation function is greater than a predetermined first threshold.

In order to achieve the above-mentioned object, the insulation monitoring method according to the present disclosure includes a current suppression unit provided in a class B ground wire of a transformer and having a changeover switch and a current suppression resistor arranged in parallel, a current detection unit that detects a leakage current flowing in the class B ground wire, a control unit that switches the changeover switch to a non-suppression state in which the leakage current flowing in the class B ground wire flows through the changeover switch or a suppression state in which the leakage current flows through the current suppression resistor, a first calculation function that calculates a resistive component leakage current caused by the earth insulation resistance of the earth fault phase from the leakage current detected by the current detection unit in the non-suppression state, and a capacitance calculation function that calculates a resistive component leakage current caused by the earth insulation resistance of the earth fault phase and a capacitance calculation function that calculates a capacitance due to the earth capacitance from the leakage current detected by the current detection unit in the suppression state. An insulation monitoring method in an insulation monitoring system having a calculation unit having a second calculation function for calculating a component leakage current, a third calculation function for calculating an assumed resistive component leakage current caused by the earth insulation resistance of the earth-fault phase when switching to the non-suppressed state from the leakage current detected by the current detection unit in the suppressed state, and a timing unit having a fourth calculation function for timing the time since switching to the suppressed state, in which the control unit switches to the non-suppressed state when the calculation result by the second calculation function, the third calculation function, or the fourth calculation function satisfies a predetermined threshold in the suppressed state switched to because the resistive component leakage current calculated by the first calculation function is greater than a predetermined first threshold.

This disclosure provides an insulation monitoring system and insulation monitoring device that stably controls the ground fault current flowing through the ground fault current suppression unit.

1 is a configuration diagram of an insulation monitoring system according to a first embodiment. FIG. FIG. 2 is a circuit configuration diagram of the substation side of the insulation monitoring system. FIG. 2 is a configuration diagram of an insulation monitoring device. 1 is a diagram illustrating a first calculation function by which the earth fault current suppression device calculates a current non-suppression state. 13 is a diagram illustrating a second calculation function by which the ground fault current suppression device calculates the current suppression state. FIG. 13 is a diagram illustrating a third calculation function by which the ground fault current suppression device calculates the current suppression state. FIG. 4 is a timing chart showing the relationship between a ground fault current of the insulation monitoring system and a state in which the current is suppressed and a state in which the current is not suppressed by the insulation monitoring device. FIG. 11 is a configuration diagram of an insulation monitoring system according to a second embodiment. FIG. 11 is a configuration diagram of an insulation monitoring device according to a third embodiment. FIG. 4 is a configuration diagram of an insulation monitoring device according to a first modified example. FIG. 11 is a configuration diagram of an insulation monitoring device according to a second modified example. FIG. 11 is a circuit configuration diagram of a substation side of an insulation monitoring system including an insulation monitoring device in a modified example 3. FIG. 13 is a circuit configuration diagram of a substation side of an insulation monitoring system including an insulation monitoring device in a fourth modified example.

[Embodiment 1]
Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings.

Figure 1 is a configuration diagram of an insulation monitoring system 1. The insulation monitoring system 1 includes a substation 2 (such as a cubicle) and an output device 3. The substation 2 includes a transformer 13, an earth fault current suppression device 4 connected to a secondary circuit 12 (a circuit to be monitored) on the low voltage side of the transformer 13, and an insulation monitoring device 5 that constantly monitors the earth insulation of an electrical device (not shown) that is a load to which power is supplied by the secondary circuit 12. The transformer 13 is connected to a primary circuit 11 on the high voltage side and a secondary circuit 12 on the low voltage side. The insulation monitoring device 5 and the output device 3 are connected so as to be able to communicate with each other by wire or wirelessly through a network.

FIG. 2 is a circuit diagram of the substation 2 side of the insulation monitoring system 1. The insulation monitoring device 5 has a function of measuring the leakage current I ₀ of the secondary electric circuit 12 to be monitored, detecting monitoring information such as abnormal leakage, and notifying the measurement result, the detected monitoring information, or the abnormality judgment result to the external output device 3. The insulation monitoring device 5 can measure the leakage current I ₀ in a live line state without temporarily interrupting the secondary electric circuit 12. The leakage current I ₀ includes a resistive component leakage current Ir caused by the earth insulation resistance R2 that is directly related to the insulation resistance, and a capacitive component leakage current Ic caused by the earth capacitance. The insulation monitoring system 1 of this embodiment employs an I ₀ r measurement method that can measure the resistive component leakage current Ir and the capacitive component leakage current Ic according to the wiring method of the secondary electric circuit 12 by measuring the leakage current I ₀ using the insulation monitoring device 5. In FIG. 2, the primary electric circuit 11 is illustrated as a single-phase two-wire AC circuit including a first phase 11a and a second phase 11b. Additionally, the secondary power circuit 12 is illustrated as a single-phase, two-wire AC circuit including a first phase 12a and a second phase 12b. The first phase 12a of the secondary power circuit 12 is a ground phase.

The earth fault current suppression device 4 is provided on the B-class ground wire 121 of the transformer 13. Specifically, the B-class ground wire 121 is connected to the first phase 12a of the secondary circuit 12 shown in FIG. 2. The earth fault current suppression device 4 also functions as a current suppression unit. The earth fault current suppression device 4 has a changeover switch SW, a current suppression resistor R1, and a gas discharge tube GDT (Gas Discharge Tubes) connected in parallel. The changeover switch SW and the current suppression resistor R1 function as a current control unit 41.

The changeover switch SW has a function of switching between a suppressed state and a non-suppressed state of the current of the current control unit 41. The changeover switch SW is controlled by the insulation monitoring device 5. The current suppression resistor R1 has a function of suppressing the ground fault current (leakage current I ₀ ) flowing through the B-class ground wire 121 when a ground fault or the like occurs. The current suppression resistor R1 is a fixed resistor set to a resistance value of, for example, several kΩ. The gas-filled discharge tube GDT has a function of suppressing a voltage rise between the first phase 12a of the secondary circuit 12 and the ground point when a contact fault or the like occurs in the transformer 13 during high resistance grounding. The DC discharge start voltage of the gas-filled discharge tube GDT is set to a threshold value of, for example, several hundred volts.

Next, the insulation monitoring device 5 will be described. As shown in Fig. 3, the insulation monitoring device 5 includes a control unit 51, a reference voltage acquisition unit 52, a current detection unit 53, a calculation unit 54, a timer unit 55, a communication unit 56, and a storage unit 57. The control unit 51 has a function of switching the changeover switch SW to a non-suppression state in which the leakage current _I0 flowing through the class B ground wire 121 flows via the changeover switch SW, or a suppression state in which the leakage current I0 flows via a current suppression resistor R1.

The reference voltage acquisition unit 52 has a function of measuring the reference voltage _Vref of the secondary current circuit 12. In this embodiment, the voltage (line voltage) between the first phase 12a and the second phase 12b is detected as the reference voltage _Vref . The configuration of the measurement terminal and wiring for measuring the reference voltage _Vref is omitted in the figure. The reference voltage acquisition unit 52 may also calculate the effective value RMS( _Vref ) of the reference voltage _Vref , the frequency F( _Vref ), and the phase difference θ( _Vref ) of the leakage current _I0 with respect to the reference voltage _Vref . The phase difference θ of the leakage current _I0 is obtained, for example, by the zero cross method. The frequency F( _Vref ) of the reference voltage _Vref is also called the reference frequency. The current detection unit 53 is connected to a zero-phase current transformer 58 (so-called ZCT) and detects the leakage current _I0 flowing through the B-class ground wire 121.

The calculation unit 54 has a first calculation function, a second calculation function, and a third calculation function. The first calculation function is a function for calculating a resistive component leakage current Ir caused by the earth insulation resistance R2 of the earth-fault phase (the second phase 12b in FIG. 4) from the leakage current _I0 detected by the current detection unit 53 in the non-suppressed state. The second calculation function is a function for calculating a resistive component leakage current Ir caused by the earth insulation resistance R2 of the earth-fault phase and a capacitive component leakage current Ic (Ic2) caused by the earth capacitance C2 from the leakage current _I0 detected by the current detection unit 53 in the suppressed state. The third calculation function is a function for calculating an assumed resistive component leakage current Ir' caused by the earth insulation resistance R2 of the earth-fault phase when the leakage current _I0 detected by the current detection unit 53 in the suppressed state is switched to the non-suppressed state. The first calculation function, the second calculation function, and the third calculation function will be described in detail later.

The timer unit 55 has a function of timing an elapsed time. The timer unit 55 in this embodiment has, for example, a fourth calculation function of timing the time that has elapsed since the ground fault current suppression device 4 was switched to a state in which the leakage current _I0 was suppressed. The communication unit 56 has a function of the insulation monitoring device 5 transmitting and receiving information to and from an external device. The insulation monitoring device 5 communicates with the output device 3 shown in Fig. 1 and the like via the communication unit 56 in a wired or wireless manner.

The memory unit 57 may store the control program of the insulation monitoring device 5, the measured values acquired or calculated by the reference voltage acquisition unit 52 and the current detection unit 53, the various calculation results calculated by the calculation unit 54, judgment conditions such as thresholds required to judge the calculation results by the first calculation function to the fourth calculation function (including the first threshold to the fourth threshold, and the cutoff threshold in the second embodiment described below), and various data required for the operation of other control programs. The memory unit 57 stores flag information indicating whether the earth fault current suppression device 4 is in a current suppression state or a non-suppression state. The control unit 51 can, for example, determine whether the state is suppressed or non-suppressed by referring to this flag information.

The insulation monitoring system 1 shown in FIG. 2 shows a case where the changeover switch SW is in a closed state. That is, the earth fault current suppression device 4 is in a non-suppressed state where the leakage current _I0 flowing through the B-class ground wire 121 flows through the changeover switch SW. In the insulation monitoring system 1 of FIG. 2, since no earth fault occurs in the secondary electric circuit 12, the resistive component leakage current Ir due to the ground insulation resistance R2 of the live second phase 12b does not flow, and the capacitive component leakage current Ic due to the ground capacitance C2 flows. In addition, since the changeover switch SW is in a closed state, the first phase 12a is grounded, and the ground capacitance C1 of the first phase 12a can be ignored. Therefore, it can be said that the leakage current _I0 flowing through the B-class ground wire 121 in FIG. 2 is equal to the capacitive component leakage current Ic. Note that, in this embodiment, the ground capacitances of the phases of the secondary electric circuit 12 are approximately the same known value. The capacitance to earth C1 and the capacitance to earth C2 may be values given in advance by design values or the like, or may be values calculated by the calculation unit 54 in a non-suppressed state of the current, for example, C2=Ic/ _jωVref (ω is the angular frequency of the reference voltage _Vref ). The calculation unit 54 can also calculate the capacitance to earth C1 by C1=C2.

Next, the first calculation function, the second calculation function, and the third calculation function will be described in detail. First, the first calculation function is a function for calculating a judged value as to whether or not a ground fault has occurred or an event that is a sign of a ground fault has occurred when the ground fault current suppression device 4 is in a non-current suppression state as shown in FIG. 4. Specifically, the control unit 51 acquires a measured value of the leakage current _I0 detected by the current detection unit 53 using the zero-phase current transformer 58. When the ground fault current suppression device 4 is in a non-current suppression state, no ground fault has occurred in the secondary electric circuit 12, so that _I0 (leakage current) = Ic (capacitive component leakage current) = Ic2 (capacitive component leakage current flowing through the earth capacitance C2). The calculation unit 54 calculates the resistive component leakage current Ir of the second phase 12b as follows:
Ir= _I0 ·cosθ (1)
[θ is the phase difference of the leakage current _I0 with respect to the reference voltage _Vref .]
It is calculated as follows.
In the first calculation function, the resistive component leakage current Ir is calculated as the ground fault current Im. The control unit 51 compares the resistive component leakage current Ir (ground fault current Im), which is the calculation result calculated in the first calculation function, with a predetermined first threshold value to determine whether to switch the current from a non-suppression state to a suppression state.

The second calculation function is a function for calculating a judged value as to whether or not a ground fault or an event that is a precursor of the ground fault has been eliminated when the ground fault current suppression device 4 is in a current suppression state, as shown in Fig. 5. Specifically, the control unit 51 acquires a measured value of the leakage current _I0 detected by the current detection unit 53 through the zero-phase current transformer 58. When the ground fault current suppression device 4 is in a current suppression state, the control unit 51 determines that a ground capacitance C1 and a ground capacitance C2 are generated in the first phase 12a and the second phase 12b, respectively, because a ground fault has occurred in the secondary electric circuit 12, and causes the calculation unit 54 to calculate a resistive component leakage current Ir, a capacitive component leakage current Ic2 of the ground phase (second phase 12b), and a capacitive component leakage current Ic1 of the ground phase (first phase 12a).

First, the calculation unit 54 calculates the voltage to ground V1 of the first phase 12a from the leakage current _I0 detected by the current detection unit 53 and the known value of the current suppression resistor R1 using equation (2).
V1 = _I0 · R1 ... (2)
The calculation unit 54 also obtains the capacitance component leakage current Ic1 of the earth capacitance C1 by the formula (3).
Ic1 = V1 j ω C1
= _I0 R1 j ω C1 ... (3)
Since the reference voltage _Vref , which is the line voltage of the second phase 12b relative to the first phase 12a, is known, the calculation unit 54 calculates the capacitive component leakage current Ic2 of the earth capacitance C2 using equation (4).
Ic2=( _Vref - _I0R1 )jωC2 (4)
Therefore, the resistive component leakage current Ir flowing through the ground insulation resistance R2 can be calculated by the following equation (5) using Ic1 and Ic2 calculated by the equations (3) and (4).
Ir=( _I0 +Ic1)-Ic2 (5)
The calculation unit 54 uses the resistance component leakage current Ir calculated by the formula (5) to obtain
R2=( _Vref -R1· _I0 )/Ir (6)
This makes it possible to obtain the current earth insulation resistance R2 when the earth fault current suppression device 4 is in the suppressed state.

In the second calculation function, since the combined current of the leakage current _I0 and the capacitive leakage current Ic1 is equal to the combined current of the resistive leakage current Ir and the capacitive leakage current Ic2, the combined current of the leakage current _I0 and the capacitive leakage current Ic1 is calculated as being equal to the earth fault current Im for the second phase 12b. Therefore, the calculations of the above formulas (4) to (6) can be omitted from the processing contents of the second calculation function. The control unit 51 compares the combined current of the resistive leakage current Ir and the capacitive leakage current Ic2, which is the calculation result obtained by the second calculation function, with a predetermined second threshold value to determine whether or not to switch to the non-suppression state of the current.

The third calculation function is a function for calculating a judged value as to whether or not a ground fault or an event that is a precursor to a ground fault has been resolved when the ground fault current suppression device 4 is in a current suppression state, as shown in Fig. 6. Specifically, when the ground fault current suppression device 4 is in a current suppression state, the control unit 51 causes the calculation unit 54 to calculate an assumed resistive component leakage current Ir' that is assumed to flow through the ground insulation resistance R2 when the ground fault current suppression device 4 is switched from a current suppression state to a non-suppression state (when the changeover switch SW is switched to a closed state as shown by the dashed line).

First, the calculation unit 54 performs calculations such as those from equation (2) to equation (6) described in the second calculation function to calculate the current earth insulation resistance R2. The calculation unit 54 calculates the assumed resistive component leakage current Ir' that flows through the earth insulation resistance when the changeover switch SW is switched to the closed state as shown by the dashed line, using the current earth insulation resistance R2 and the reference voltage _Vref, as follows: Ir' = _Vref /R2 (7)
It is calculated as follows.

In the third calculation function, the assumed resistive component leakage current Ir' of the earth insulation resistance R2 calculated by equation (7) is calculated as the earth fault current Im. The control unit 51 compares the assumed resistive component leakage current Ir', which is the calculation result calculated by the third calculation function, with a third threshold value determined in advance, and determines whether to switch the current to a suppressed or non-suppressed state.

The calculation unit 54 can execute the calculation of the earth fault current Im using the first calculation function, the second calculation function, and the third calculation function at a predetermined period (or sampling frequency). The calculation period of the earth fault current Im is set to, for example, several minutes or several hours. Furthermore, the calculation period of the earth fault current Im may be set differently in some or all of the first calculation function, the second calculation function, and the third calculation function.

Next, the operation of the insulation monitoring system 1 will be described. FIG. 7 is a timing chart showing the relationship between the earth fault current of the insulation monitoring system 1 and the current suppression state and non-suppression state by the insulation monitoring device 5. In this timing chart, the horizontal axis shows time and the vertical axis shows the earth fault current Im. Below, judgment examples 1 to 8 will be described as methods for determining whether the control unit 51 switches the earth fault current suppression device 4 to a current suppression state or a non-suppression state, but any combination of each judgment example may be used.

(Judgment Example 1)
First, judgment example 1 will be described.

In FIG. 7, during the period from timing t0 to timing t1 when the earth fault current Im is 0 [mA], the earth fault current suppression device 4 controls the changeover switch SW to the closed state as the initial state, and is set to the non-suppression state. In addition, the control unit 51 causes the calculation unit 54 to calculate the earth fault current Im using the first calculation function from timing t0 to timing t1. Timing t1 is, for example, the timing when a ground fault accident occurs. When the control unit 51 detects that the earth fault current Im satisfies the first threshold value (for example, a current value of 50 [mA] or more continues for 60 [seconds]) as a result of the calculation using the first calculation function, it switches the changeover switch SW to the open state at timing t2, and switches the earth fault current suppression device 4 to the current suppression state. The period D1 from timing t0 to timing t2 in FIG. 7 indicates the period from the initial state to switching the changeover switch SW to the open state. In addition, the period D3 from timing t1 to timing t2 indicates the period from timing t1 when the earth fault accident occurs to switching the changeover switch SW to the open state.

The period D2 from timing t2 to timing t3 indicates a period of the current suppression state in which the earth fault current suppression device 4 switches the changeover switch SW to the open state. During period D2, the control unit 51 determines whether or not to switch the calculation unit 54 to the non-suppression state by the second calculation function, the third calculation function, the fourth calculation function, or other processing. For example, when the control unit 51 detects that the earth fault current Im calculated as a result of the second calculation function satisfies a predetermined second threshold, it switches the changeover switch SW to the closed state at timing t3 to switch the earth fault current suppression device 4 to the current non-suppression state. Whether or not the predetermined second threshold is satisfied is determined, for example, by whether or not the earth fault current Im is a current value equal to or less than 50 [mA].

In addition, in the period D2, the judgment condition for switching to the non-suppression state of the current may be set so that a delay time (or waiting time) is provided between when the earth fault current suppression device 4 switches to the current suppression state and when it returns to the non-suppression state. Specifically, the judgment condition for switching to the non-suppression state of the current is set stricter than the judgment condition for switching to the suppression state of the current. For example, the earth fault current Im calculated by the second calculation function includes the capacitive component leakage current Ic and the resistive component leakage current Ir, while the earth fault current Im calculated by the first calculation function includes the resistive component leakage current Ir and does not include the capacitive component leakage current Ic. Therefore, in this case, even if the thresholds of the earth fault current Im calculated by the first calculation function and the earth fault current Im calculated by the second calculation function are the same, the judgment condition for switching to the non-suppression state of the current is set stricter as a threshold for evaluating the resistive component leakage current Ir than the judgment condition for switching to the suppression state of the current.

During a period D1 after timing t3, the ground fault current suppression device 4 is in a state where the current is not suppressed, and the control unit 51 monitors the leakage current I ₀ (ground fault current Im) using the first calculation function in the same manner as from timing t0 to timing t2.

The control unit 51 may perform output processing such as a warning when the ground fault current Im meets a predetermined threshold during period D2. The threshold for determining whether or not to perform output processing such as a warning is set to, for example, a value greater than the first threshold. The output processing is, for example, the control unit 51 transmitting a warning instruction to the external output device 3 (see FIG. 1) via the communication unit 56. The output processing can be processing that can be perceived by humans, and any means can be used, such as display by a display device provided in the output device 3, light emission by a light-emitting device, sound emission by a speaker, notification to a terminal connected to the output device 3, etc.

For example, the insulation monitoring device 5 switches the changeover switch SW to the open state when the calculation of the first calculation function in period D1 indicates that the earth fault current Im is 50 mA or more. At this time, the output device 3 outputs a message that the changeover switch SW has been switched to the open state through output processing. The output device 3 also performs output processing to output a first alert when the earth fault current Im transitions from a state of less than 50 mA to 50 mA or more in period D2. The output device 3 also performs output processing to output a second alert when the earth fault current Im transitions from a state of 50 mA or more and less than 200 mA to 200 mA or more in period D2. The second alert is set to a stronger alarm level than the first alert.

(Judgment Example 2)
Next, another determination example in the current suppression state in period D2 will be described. In determination example 2, when the current suppression state in period D2 is in the current suppression state, the control unit 51 switches to the non-suppression state when the ground fault current Im calculated by the third calculation function is less than or equal to a third threshold value (e.g., 50 mA).

(Judgment Example 3)
In addition, in judgment example 3 in the current suppression state of period D2, the control unit 51 switches to the non-suppression state when the earth fault current Im, which is the calculation result by the second calculation function and the third calculation function, is less than a predetermined second threshold value (e.g., 50 mA) and less than a predetermined third threshold value (e.g., 50 mA), respectively, in the suppression state.

(Judgment Example 4)
In addition, in judgment example 4 in the current suppression state of period D2, the control unit 51 switches to the non-suppression state when the elapsed time obtained by the fourth calculation function in the suppression state is equal to or greater than a predetermined fourth threshold value (e.g., 1 hour).

(Judgment Example 5)
In addition, in judgment example 5 in the suppression state of the current during period D2, the control unit 51 switches to the non-suppression state when the elapsed time obtained by the fourth calculation function in the suppression state is equal to or greater than a predetermined fourth threshold value (e.g., 1 hour) and the calculation result by the second calculation function is less than or equal to a predetermined second threshold value (e.g., 50 mA).

In addition, in judgment example 5, when the elapsed time acquired by the fourth calculation function is less than the fourth threshold value, the control unit 51 executes the second calculation function, or the second calculation function and the third calculation function, at a predetermined sampling period.

(Judgment Example 6)
In judgment example 6 in the suppression state of the current during period D2, the control unit 51 switches to the non-suppression state when the elapsed time obtained by the fourth calculation function in the suppression state is equal to or greater than a predetermined threshold (e.g., 1 hour or more) and the calculation result by the third calculation function is equal to or less than a predetermined threshold (e.g., 50 mA).

In addition, in judgment example 6, when the elapsed time acquired by the fourth calculation function is less than the fourth threshold, the control unit 51 executes the third calculation function, or the second calculation function and the third calculation function, at a predetermined sampling period. In other words, the control unit 51 can be configured to execute one or both of the second calculation function and the third calculation function at a predetermined sampling period when the elapsed time acquired by the fourth calculation function is less than the fourth threshold in judgment example 5 or judgment example 6.

(Judgment Example 7)
In judgment example 7 in the suppression state of the current during period D2, the control unit 51 switches to the non-suppression state when the elapsed time obtained by the fourth calculation function in the suppression state is equal to or greater than a predetermined threshold value, and the calculation results by the second and third calculation functions are equal to or less than a predetermined second threshold value and a predetermined third threshold value, respectively.

In the above, in the first embodiment, the insulation monitoring system 1 is provided in the class B ground wire 121 of the transformer 13, and includes a current suppression unit (such as the ground fault current suppression device 4) having a changeover switch SW and a current suppression resistor R1 arranged in parallel, a current detection unit 53 that detects the leakage current _I0 flowing in the class B ground wire 121, a control unit 51 that switches the changeover switch SW to a non-suppression state in which the leakage current _I0 flowing in the class B ground wire 121 flows through the changeover switch SW or a suppression state in which the leakage current I0 flows through the current suppression resistor R1, and a calculation unit 54 or a timing unit 55 that has first to fourth calculation functions. Here, the first calculation function is a function that calculates a resistance component leakage current Ir caused by the earth insulation resistance R2 of the ground fault phase from the leakage current _I0 detected by the current detection unit 53 in the non-suppression state. The second calculation function is a function for calculating a resistive component leakage current Ir caused by the earth insulation resistance R2 of the earth-fault phase and a capacitive component leakage current Ic caused by the earth capacitance C2 from the leakage current _I0 detected by the current detection unit 53 in the suppressed state. The third calculation function is a function for calculating an assumed resistive component leakage current Ir _{' caused by the earth insulation resistance R2 of the earth-fault phase when the suppressed state is switched to the non-suppressed state from the leakage current I0} detected by the current detection unit 53 in the suppressed state. And the fourth calculation function is a function for measuring the time since the state was switched to the suppressed state.

Furthermore, as a calculation performed in the suppression state of the earth fault current suppression device 4, the control unit 51 has been described as being configured to switch to the non-suppression state when the calculation result of the second, third or fourth calculation function satisfies a predetermined threshold (second, third or fourth threshold) in the suppression state switched to because the resistance component leakage current Ir calculated by the first calculation function is greater than a predetermined first threshold.

The earth fault current suppression device 4 executes the judgment conditions for switching to the current suppression state using the calculation result of the first calculation function, and executes the judgment conditions for switching to the current non-suppression state using a calculation function (second calculation function to fourth calculation function) different from the first calculation function. By making the judgment conditions for switching to the current non-suppression state stricter than the judgment conditions for switching to the current suppression state (for example, judgment examples 1 to 3 and judgment examples 5 to 7), or by using the elapsed time, which is the calculation result of the fourth calculation function, as the condition for switching to the non-suppression state (for example, judgment examples 4 to 7), it is possible to provide a delay time (or waiting time) between when the earth fault current suppression device 4 switches to the current suppression state and when it returns to the non-suppression state. This makes it possible to prevent the earth fault current suppression device 4 from returning to the current non-suppression state even though the state in which the earth fault is eliminated is not stable, or to prevent the occurrence of a phenomenon in which the suppression state and the non-suppression state are repeated in a short period of time. Therefore, it is possible to configure an insulation monitoring system and an insulation monitoring device that stably control the earth fault current flowing through the earth fault current suppression unit.

[Embodiment 2]
Next, an insulation monitoring system 1A of the second embodiment will be described. Fig. 8 is a configuration diagram of the insulation monitoring system 1A. The insulation monitoring system 1A has a substation 2A, which is a monitored system, instead of the substation 2, and has an output device 3A, which is a monitoring system connected to the monitored system so as to be able to communicate with it via a network, instead of the output device 3. The substation 2A also has an insulation monitoring device 5A instead of the insulation monitoring device 5 of the first embodiment. In the description of the insulation monitoring system 1A, configurations different from those of the insulation monitoring system 1 of the first embodiment will be described, and other configurations are similar to those of the insulation monitoring system 1, so that the description will be omitted or simplified by assigning the same reference numerals, etc.

The output device 3A includes a control unit 31, a calculation unit 32, a timing unit 33, a communication unit 34, a memory unit 35, and an output unit 36. The control unit 31, the calculation unit 32, and the timing unit 33 have the same functions as the control unit 51, the calculation unit 54, and the timing unit 55 of the insulation monitoring device 5 in FIG. 3, respectively. The calculation unit 32 and the calculation unit 54 have a first calculation function, a second calculation function, and a third calculation function. Furthermore, the timing unit 33 has a fourth calculation function.

The control unit 31 uses the calculation results of any one of the first, second, third, and fourth calculation functions according to judgment examples 1 to 7 described in embodiment 1, and compares them with a predetermined first threshold value to determine whether to switch to a suppressed or non-suppressed state of the current. The control unit 31 has a control function to switch the earth fault current suppression device 4 to a non-suppressed or suppressed state via a network using the communication unit 34 and the communication unit 56 provided in the insulation monitoring device 5A.

The insulation monitoring device 5A may be configured without the calculation unit 54 and the timer unit 55 (see FIG. 3).

In the insulation monitoring system 1A of embodiment 2, the monitored system and the monitoring system (in FIG. 8, the substation 2A and the output device 3A) can be installed in remote locations, so that the monitored system, that is, the substation 2A, can be easily configured.

[Embodiment 3]
Next, a third embodiment will be described. Fig. 9 is a configuration diagram of an insulation monitoring device 6 applicable to the insulation monitoring system 1. The insulation monitoring device 6 includes an earth fault current suppression unit 4B functioning as a current suppression unit, and an insulation monitoring unit 5B. The earth fault current suppression unit 4B has a similar configuration to the earth fault current suppression device 4. Moreover, the insulation monitoring unit 5B has a similar configuration to the insulation monitoring device 5 shown in Fig. 3.

In this way, the insulation monitoring device 6 has the insulation monitoring unit 5B and the ground fault current suppression unit 4B integrated into one device, so that it can be configured to be compact overall while still having the functions of the ground fault current suppression device 4 and the insulation monitoring device 5 in embodiment 1 (including judgment examples 1 to 7).

(Variation 1)
Next, a first modified example of the third embodiment will be described. FIG. 10 is a configuration diagram of an insulation monitoring device 6B applicable to the insulation monitoring system 1. The ground fault current suppression unit 4B (current suppression unit) has a current suppression resistor R1, a changeover switch SW arranged in parallel with the current suppression resistor R1, and a gas-filled discharge tube GDT. The insulation monitoring device 6B further includes a current suppression resistor R3 (second current suppression resistor) arranged in series with the current suppression resistor R1 and the changeover switch SW and having a lower resistance than the current suppression resistor R1 on the ground point side. In FIG. 10, the current suppression resistor R3 is arranged in series between the ground fault current suppression unit 4B and the ground point. The current suppression resistor R3 is set to several Ω to several tens of Ω. Therefore, the insulation monitoring device 6B can set the ground resistance by the current suppression resistor R3 even in a non-suppressed state in which the changeover switch SW is closed. The insulation monitoring device 6B can suppress the leakage current I ₀ from flowing excessively even when the resistance value of the B-class ground wire 121 is low. Therefore, the insulation monitoring device 6 and the ground fault current suppression unit 4B can be protected from overcurrent.

The current suppression resistor R3 may be provided inside the ground fault current suppression unit 4B or outside the insulation monitoring device 6D.

(Variation 2)
Next, a second modified example of the third embodiment will be described. Fig. 11 is a configuration diagram of an insulation monitoring device 6C applicable to the insulation monitoring system 1. In the description of the insulation monitoring device 6C, the same components as those of the insulation monitoring device 6 are denoted by the same reference numerals, and the description thereof will be omitted or simplified. The earth fault current suppression unit 4C provided in the insulation monitoring device 6C has a current suppression resistor R4 and a gas-filled discharge tube GDT. The current suppression resistor R4 is composed of a variable current limiting element. The resistance value of the current suppression resistor R4 can be changed, for example, in a range from 0 Ω to several kΩ by control by the insulation monitoring unit 5B.

The insulation monitoring device 6C can be made compact overall because it does not require a changeover switch SW.

(Variation 3)
Next, a third modification of the third embodiment will be described. Fig. 12 is a diagram showing an insulation monitoring system 1D using an insulation monitoring device 6D according to the third modification. In the description of the insulation monitoring system 1D, the same components as those of the insulation monitoring system 1 and the like described above will be denoted by the same reference numerals, and the description thereof will be omitted or simplified.

The substation 2D includes a ground fault current suppression unit 4B, an insulation monitoring unit 5D, and circuit breakers 7 and 8 provided on the secondary electric circuit 12 side. In the example shown in the figure, the circuit breaker 7 is arranged as a main circuit breaker, and one or more circuit breakers 7 are arranged as molded-case circuit breakers. The insulation monitoring unit 5D has a similar configuration to the insulation monitoring unit 5B shown in Fig. 9, but the current detection unit 53 has a function of detecting a leakage current _I0 of the circuit breaker 7 connected to the secondary electric circuit 12 by a zero-phase current transformer 58. In other words, the current detection unit 53 detects a ground fault current of the secondary electric circuit of each circuit breaker 8 provided on the secondary electric circuit 12, which is a low-voltage circuit of the transformer 13.

The control unit 51 of the insulation monitoring unit 5D has a function of interrupting one or both of the electric circuits of the circuit breaker 7 and the circuit breaker 8 when the calculation result by the second calculation function, the third calculation function, or the fourth calculation function described in the first embodiment satisfies a predetermined interruption threshold in the current suppression state. Whether or not the interruption threshold is satisfied can be set, for example, as a case where the ground fault current Im is equal to or greater than a predetermined threshold even though the ground fault current suppression unit 4B is in the current suppression state (the state in period D2 in FIG. 7).

The insulation monitoring system 1D has a function of suppressing the ground fault current using the ground fault current suppression unit 4B and a function of disconnecting the secondary circuit 12 using the circuit breaker 8 (or a function of disconnecting the secondary circuit 12 using the circuit breaker 7), so it can improve the safety of the entire system, including the secondary circuit 12 and the electrical equipment on the load side.

(Variation 4)
Next, a fourth modification of the third embodiment will be described. Fig. 13 is a diagram showing an insulation monitoring system 1E using an insulation monitoring device 6E according to the fourth modification. In the description of the insulation monitoring system 1E, the same components as those of the insulation monitoring system 1 and the like described above will be denoted by the same reference numerals, and the description thereof will be omitted or simplified.

The insulation monitoring system 1E is connected to the secondary electric circuits 12 of multiple transformers 13a to 13d (13). The insulation monitoring system 1E is applied to, for example, a single-phase two-wire secondary electric circuit 12 similar to the insulation monitoring system 1 of embodiment 1. The first phase 12a (not shown in FIG. 13), which is the ground phase of each transformer 13, is connected to a ground point via a common class B ground wire 121. The insulation monitoring device 6E is provided on this common class B ground wire 121. Note that the class B ground wire 121 is not limited to a configuration in which it is connected to the secondary electric circuits 12 of four transformers 13, but may be a configuration in which it is connected to the secondary electric circuits 12 of two or more multiple transformers 13.

The insulation monitoring system 1E configured in this manner can suppress ground fault currents originating from any of the multiple secondary circuits 12. In addition, because the insulation monitoring device 6E can be connected to multiple secondary circuits 12, the overall system configuration can be made smaller.

The above describes an insulation monitoring device that includes a current suppression unit that is provided in a class B ground wire of a transformer and has a current suppression resistor; a current detection unit that detects the leakage current flowing in the class B ground wire; a control unit that changes the resistance value of the class B ground wire to switch between a non-suppressed state and a suppressed state of the leakage current; and a calculation unit that has a first calculation function that calculates a resistive component leakage current caused by the earth insulation resistance of the earth-fault phase from the leakage current detected by the current detection unit in the non-suppressed state, a second calculation function that calculates a resistive component leakage current caused by the earth insulation resistance of the earth-fault phase and a capacitive component leakage current caused by the earth capacitance from the leakage current detected by the current detection unit in the suppressed state, and a third calculation function that calculates an assumed resistive component leakage current caused by the earth insulation resistance of the earth-fault phase when the state is switched to the non-suppressed state from the leakage current detected by the current detection unit in the suppressed state. The control unit is also configured to switch to the suppressed state when the resistance component leakage current calculated by the first calculation function is greater than a predetermined first threshold, and to switch to the non-suppressed state when the calculation result by the second calculation function or the third calculation function satisfies a predetermined threshold.

With this configuration, the insulation monitoring devices 6, 6B to 6E can be configured to be compact overall while still having a ground fault current suppression function and an insulation monitoring function. Furthermore, the insulation monitoring devices 6D, 6E can simultaneously monitor the measurement points of multiple electric circuits. Therefore, it is possible to configure the insulation monitoring devices 6, 6B to 6E so that the entire system can be easily configured even when connected to a large number of electric circuits to be monitored.

Although the first and second embodiments of the present disclosure have been described above, each embodiment can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are considered to be within the scope of the invention and its equivalents as described in the claims, as well as within the scope and spirit of the invention.

For example, in the first to third embodiments, the primary circuit 11 and the secondary circuit 12 are described as being of a single-phase two-wire system, but they may be of a single-phase three-wire system, a three-phase three-wire system, or a three-phase four-wire system.

In addition, the insulation monitoring device 5 and insulation monitoring units 5B and 5D have been described as having a communication unit 56 built in, but they may also be provided as part of a function of a communication device (e.g., a gateway).

In addition, in the first embodiment, the capacitance to earth of each phase of the secondary current circuit 12 is described as being approximately the same (C1 = C2) and a known value, but the capacitance to earth C1, C2 of each phase may be unbalanced (C1 ≠ C2).

The above-described technology of the present disclosure is exemplified as follows.
[1]
A current suppression unit provided in a class B ground line of the transformer and having a changeover switch and a current suppression resistor arranged in parallel;
a current detection unit that detects a leakage current flowing in the class B ground line;
a control unit that switches the changeover switch to a non-suppression state in which a leakage current flowing through the class B ground line flows through the changeover switch or a suppression state in which a leakage current flowing through the current suppression resistor;
a calculation unit having a first calculation function of calculating a resistive component leakage current caused by the earth insulation resistance of the earth-fault phase from the leakage current detected by the current detection unit in the non-suppressed state, a second calculation function of calculating a resistive component leakage current caused by the earth insulation resistance of the earth-fault phase and a capacitive component leakage current caused by the earth capacitance from the leakage current detected by the current detection unit in the suppressed state, and a third calculation function of calculating an assumed resistive component leakage current caused by the earth insulation resistance of the earth-fault phase when switched to the non-suppressed state from the leakage current detected by the current detection unit in the suppressed state;
A timer unit having a fourth calculation function that measures the time since the suppression state is switched to;
Equipped with
The control unit switches the suppression state, which is switched to when the resistance component leakage current calculated by the first calculation function is greater than a predetermined first threshold, to the non-suppression state when a calculation result by the second calculation function, the third calculation function, or the fourth calculation function satisfies a predetermined threshold.
Insulation monitoring system.
[2]
The control unit switches to the non-suppression state when the calculation result by the second calculation function is equal to or less than a predetermined second threshold value in the suppression state.
[3]
The control unit switches to the non-suppression state when, in the suppression state, the calculation results by the second calculation function and the third calculation function are below a predetermined second threshold and a third threshold, respectively.
[4]
The control unit switches to the non-suppression state when the elapsed time obtained by the fourth calculation function in the suppression state is equal to or greater than a predetermined fourth threshold value.
[5]
The control unit switches to the non-suppression state when, in the suppression state, the elapsed time obtained by the fourth calculation function is equal to or greater than a predetermined fourth threshold and the calculation result by the second calculation function is equal to or less than a predetermined second threshold.
[6]
The control unit switches to the non-suppression state when, in the suppression state, the elapsed time obtained by the fourth calculation function is equal to or greater than a predetermined threshold and the calculation result by the third calculation function is equal to or less than a predetermined threshold.
[7]
An insulation monitoring system as described in [6], wherein when the elapsed time obtained by the fourth calculation function is less than a fourth threshold, one or both of the second calculation function and the third calculation function are executed at a predetermined sampling period.
[8]
The control unit switches to the non-suppression state when, in the suppression state, the elapsed time obtained by the fourth calculation function is equal to or greater than a predetermined threshold, and the calculation results by the second calculation function and the third calculation function are equal to or less than a predetermined second threshold and a predetermined third threshold, respectively.
[9]
A monitored system including the current detection unit and the current suppression unit;
a monitoring system including the control unit, the calculation unit, and the timing unit, the monitoring system being communicatively connected to the monitored system via a network;
The insulation monitoring system according to any one of claims 1 to 8, comprising:
[10]
A current suppression unit provided in a class B ground line of the transformer and having a changeover switch and a current suppression resistor arranged in parallel;
a current detection unit that detects a leakage current flowing in the class B ground line;
a control unit that switches the changeover switch to a non-suppression state in which a leakage current flowing through the class B ground line flows through the changeover switch or a suppression state in which a leakage current flowing through the current suppression resistor;
a calculation unit having a first calculation function of calculating a resistive component leakage current caused by earth insulation resistance of the earth-fault phase from the leakage current detected by the current detection unit in the non-suppressed state, a second calculation function of calculating a resistive component leakage current caused by earth insulation resistance of the earth-fault phase and a capacitive component leakage current caused by earth capacitance from the leakage current detected by the current detection unit in the suppressed state, and a third calculation function of calculating an assumed resistive component leakage current caused by earth insulation resistance of the earth-fault phase when switched to the non-suppressed state from the leakage current detected by the current detection unit in the suppressed state;
A timer unit having a fourth calculation function that measures the time since the suppression state is switched to;
An insulation monitoring method in an insulation monitoring system comprising:
The control unit switches to the non-suppression state when a calculation result by the second calculation function, the third calculation function, or the fourth calculation function satisfies a predetermined threshold in the suppression state switched to when the resistance component leakage current calculated by the first calculation function is greater than a predetermined first threshold.
Insulation monitoring methods.

Reference Signs List 1, 1A, 1D, 1E Insulation monitoring system 2, 2A, 2D, 2F Transformation device 3, 3A Output device 4 Earth fault current suppression device 4B, 4C Earth fault current suppression unit 5, 5A Insulation monitoring device 5B, 5D Insulation monitoring unit 6, 6B to 6E Insulation monitoring device 7 Circuit breaker 8 Circuit breaker 11 Primary current circuit 11a First phase 11b Second phase 12 Secondary current circuit 12a First phase 12b Second phase 13, 13a to 13d Transformer 31 Control unit 32 Calculation unit 33 Time measurement unit 34 Communication unit 35 Memory unit 36 Output unit 41 Current control unit 51 Control unit 52 Reference voltage acquisition unit 53 Current detection unit 54 Calculation unit 55 Time measurement unit 56 Communication unit 57 Memory unit 58 Zero-phase current transformer 121 Class B earth wire C1 Earth capacitance C2 Earth capacitance D1 to D3 Period F Frequency GDT Gas-filled discharge tube _I0 leakage current Ic, Ic1, Ic2 Capacitive component leakage current Im Earth fault current Ir Resistive component leakage current Ir' Assumed resistive component leakage current R1 Current suppression resistor R2 Earth insulation resistor R3 Current suppression resistor R4 Current suppression resistor SW Changeover switch V1 Voltage to earth V _ref reference voltage t0 Timing t1 Timing t2 Timing t3 Timing θ Phase difference

Claims (10)

A current suppression unit provided in a class B ground line of the transformer and having a changeover switch and a current suppression resistor arranged in parallel;
a current detection unit that detects a leakage current flowing in the class B ground line;
a control unit that switches the changeover switch to a non-suppression state in which a leakage current flowing through the class B ground line flows through the changeover switch or a suppression state in which a leakage current flowing through the current suppression resistor;
a calculation unit having a first calculation function of calculating a resistive component leakage current caused by earth insulation resistance of the earth-fault phase from the leakage current detected by the current detection unit in the non-suppressed state, a second calculation function of calculating a resistive component leakage current caused by earth insulation resistance of the earth-fault phase and a capacitive component leakage current caused by earth capacitance from the leakage current detected by the current detection unit in the suppressed state, and a third calculation function of calculating an assumed resistive component leakage current caused by earth insulation resistance of the earth-fault phase when switched to the non-suppressed state from the leakage current detected by the current detection unit in the suppressed state;
A timer unit having a fourth calculation function that measures the time since the suppression state is switched to;
Equipped with
The control unit switches the suppression state, which is switched to when the resistance component leakage current calculated by the first calculation function is greater than a predetermined first threshold, to the non-suppression state when a calculation result by the second calculation function, the third calculation function, or the fourth calculation function satisfies a predetermined threshold.
Insulation monitoring system. The insulation monitoring system according to claim 1, wherein the control unit switches to the non-suppression state when the calculation result by the second calculation function is equal to or less than a predetermined second threshold value in the suppression state. The insulation monitoring system according to claim 1, wherein the control unit switches to the non-suppression state when the calculation results of the second calculation function and the third calculation function are equal to or less than a second threshold and a third threshold, respectively, that are determined in advance in the suppression state. The insulation monitoring system of claim 1, wherein the control unit switches to the non-suppression state when the elapsed time acquired by the fourth calculation function in the suppression state is equal to or greater than a fourth predetermined threshold value. The insulation monitoring system of claim 1, wherein the control unit switches to the non-suppression state when the elapsed time acquired by the fourth calculation function is equal to or greater than a predetermined fourth threshold and the calculation result by the second calculation function is equal to or less than a predetermined second threshold in the suppression state. The insulation monitoring system of claim 1, wherein the control unit switches to the non-suppression state when the elapsed time acquired by the fourth calculation function is equal to or greater than a predetermined threshold and the calculation result by the third calculation function is equal to or less than a predetermined threshold in the suppression state. The insulation monitoring system of claim 6, wherein when the elapsed time acquired by the fourth calculation function is less than a fourth threshold, one or both of the second calculation function and the third calculation function are executed at a predetermined sampling period. The insulation monitoring system of claim 1, wherein the control unit switches to the non-suppression state when the elapsed time acquired by the fourth calculation function is equal to or greater than a predetermined threshold and the calculation results by the second calculation function and the third calculation function are equal to or less than a predetermined second threshold and a predetermined third threshold, respectively. A monitored system including the current detection unit and the current suppression unit;
a monitoring system including the control unit, the calculation unit, and the timing unit, the monitoring system being communicatively connected to the monitored system via a network;
The insulation monitoring system according to any one of claims 1 to 8, comprising: A current suppression unit provided in a class B ground line of the transformer and having a changeover switch and a current suppression resistor arranged in parallel;
a current detection unit that detects a leakage current flowing in the class B ground line;
a control unit that switches the changeover switch to a non-suppression state in which a leakage current flowing through the class B ground line flows through the changeover switch or a suppression state in which a leakage current flowing through the current suppression resistor;
a calculation unit having a first calculation function of calculating a resistive component leakage current caused by earth insulation resistance of the earth-fault phase from the leakage current detected by the current detection unit in the non-suppressed state, a second calculation function of calculating a resistive component leakage current caused by earth insulation resistance of the earth-fault phase and a capacitive component leakage current caused by earth capacitance from the leakage current detected by the current detection unit in the suppressed state, and a third calculation function of calculating an assumed resistive component leakage current caused by earth insulation resistance of the earth-fault phase when switched to the non-suppressed state from the leakage current detected by the current detection unit in the suppressed state;
A timer unit having a fourth calculation function that measures the time since the suppression state is switched to;
An insulation monitoring method in an insulation monitoring system comprising:
The control unit switches to the non-suppression state when a calculation result by the second calculation function, the third calculation function, or the fourth calculation function satisfies a predetermined threshold in the suppression state switched to when the resistance component leakage current calculated by the first calculation function is greater than a predetermined first threshold.
Insulation monitoring methods.

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