Photovoltaic inverter and insulation resistance detection method thereof

Abstract

The invention discloses a photovoltaic inverter and an insulation resistance detection method thereof, and the method comprises the steps: judging whether the voltage value of a sampled object tends to be stable or not after the ground voltage of a DC bus is disturbed; if the voltage value of the sampled object does not tend to be stable, increasing disturbance time and continuing to judge whether the voltage value of the sampled object tends to be stable; and when the increased disturbance time reaches a preset time threshold value, if the voltage value of the sampled object still does not tend to be stable, selecting voltage values at a plurality of time points and predicting a steady-state value based on the voltage values at the plurality of time points so as to be used for calculating the insulation resistance. By predicting the voltage steady-state value of the sampled object, the detection time can be further compressed on the basis of improving the detection precision, so that the insulation resistance value can still be accurately calculated in a relatively short time under the condition that the ground capacitance is very large in rainy days, and the problem of false alarm of an insulation resistance fault is avoided.

Description

Technical Field

[0001] This application relates to the field of photovoltaic technology, and in particular to a photovoltaic inverter and a method for detecting its insulation impedance. Background Technology

[0002] Currently, photovoltaic (PV) module input cables are numerous, mostly laid along the module frame or buried underground. These cables are susceptible to stress and damage, or being cut by sharp objects. Furthermore, the cable insulation is prone to aging and breakage, leading to poor insulation to ground, especially in TN systems. In addition, due to substandard cable insulation protection construction, DC-side grounding faults have increased dramatically. When a DC-side grounding occurs during PV inverter operation, since the grid is already grounded, there will be a two-point grounding situation, causing the PV inverter to fail. Therefore, before grid-connected operation of PV inverters, it is necessary to perform DC-side insulation fault impedance testing to identify DC-side insulation faults in advance. This prevents insulation impedance problems from causing the fault to escalate after grid connection, leading to risks such as PV inverter damage, explosion, or fire.

[0003] Existing technology uses the bridge method to disturb the positive and negative DC bus voltage values ​​before the photovoltaic inverter is started and connected to the grid. This is done by using relays to calculate the positive and negative insulation impedance of the photovoltaic system to determine if there are any abnormalities. However, the capacitance to ground of photovoltaic modules increases significantly in rainy weather, potentially reaching thousands of times that of sunny weather in extreme cases. The capacitance to ground of a single photovoltaic module in rainy weather can exceed 100 nF. As the power density of photovoltaic inverters increases, the power of a single photovoltaic inverter becomes higher, and the number of photovoltaic modules connected to the inverter also increases. The capacitance to ground of photovoltaic modules connected to the inverter can be considered as a parallel relationship. Therefore, after connecting the photovoltaic inverter, the bus voltage to ground can reach over 100 uF. This means that after the relays are activated, the bus voltage to ground cannot stabilize quickly, and the stabilization time may reach 80 seconds. However, the closing time of existing relays is generally around 1 second, which means that the bus voltage to ground collected when the relay closes is not the true steady-state voltage. If the non-steady-state voltage is used to calculate the insulation impedance, the result will have a huge error. Ultimately, this means that existing technology is prone to false alarms when performing insulation impedance detection in rainy weather due to inaccurate detection, which will prevent grid connection and thus affect power generation. Summary of the Invention

[0004] This application provides a photovoltaic inverter and its insulation impedance detection method to suppress or even avoid the problem of false insulation impedance faults in rainy weather.

[0005] This application provides a method for detecting insulation impedance, the method comprising:

[0006] After disturbing the DC bus voltage to ground, determine whether the voltage value of the sampled object tends to stabilize;

[0007] If the voltage value of the sampled object does not stabilize, the perturbation time is increased and the determination of whether the voltage value of the sampled object stabilizes is continued.

[0008] If the voltage value of the sampled object still has not stabilized when the increased disturbance time reaches a preset time threshold, the voltage values ​​at multiple time points are selected and the steady-state value is predicted based on the voltage values ​​at these multiple time points for use in calculating the insulation impedance.

[0009] This application also provides a photovoltaic inverter, which includes a controller configured to perform the insulation impedance detection method described above.

[0010] The photovoltaic inverter and its insulation impedance detection method provided in this application can further reduce the detection time while improving the detection accuracy by predicting the steady-state voltage value of the sampled object. This allows the insulation impedance value to be accurately calculated in a short time even when the capacitance to ground is large in rainy weather, thereby avoiding the problem of false alarms of insulation impedance faults. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of a photovoltaic system provided in an embodiment of this application;

[0012] Figure 2 This is a schematic diagram of an insulation resistance detection circuit provided in an embodiment of this application;

[0013] Figure 3 This is a schematic diagram illustrating the change in the voltage of the photovoltaic array to ground capacitor under a clear sky after being disturbed, as provided in an embodiment of this application.

[0014] Figure 4 This is a schematic diagram illustrating the change in the voltage of the photovoltaic array to ground capacitor after disturbance during rainy weather, provided in an embodiment of this application.

[0015] Figure 5 This is a schematic diagram of the insulation impedance detection method provided in the embodiments of this application.

[0016] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0017] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer and more understandable, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit the scope of this application.

[0018] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0019] Figure 1 This is a schematic diagram of a photovoltaic system provided in an embodiment of this application.

[0020] like Figure 1 As shown, the photovoltaic system includes a photovoltaic array PV, a PV positive electrode-to-ground capacitance C1, a PV negative electrode-to-ground capacitance C2, and a PV positive electrode-to-ground impedance R. PV+ The impedance R of the negative electrode of the photovoltaic array to ground PV- DC to DC converter, insulation resistance detection circuit, DC bus capacitors C3 and C4, DC to AC converter, voltage sampling circuit, controller.

[0021] In some examples, the DC-DC converter, insulation resistance detection circuit, DC bus capacitors C3 and C4, DC-AC converter, voltage sampling circuit, and controller can be integrated into the same cabinet, which can be called a photovoltaic inverter.

[0022] In some examples, the photovoltaic array PV is the DC input of the photovoltaic inverter; the DC-DC converter can convert DC to DC, either by step-up or step-down; the DC-AC converter can convert DC to AC; the insulation impedance detection circuit can disturb the voltage of the positive bus (i.e., positive DC bus BUS+) to ground and / or the voltage of the negative bus (i.e., negative DC bus BUS-) to ground; the voltage sampling circuit can sample at least one of the voltage of the positive bus to ground, the voltage of the negative bus to ground, the bus voltage sampling, the voltage of the positive input (i.e., PV positive) to ground, and the voltage of the negative input (i.e., PV negative) to ground.

[0023] In some examples, the DC-to-DC converter and the DC-to-AC converter are connected via the positive DC bus BUS+ and the negative DC bus BUS-, and the insulation resistance detection circuit is located between the positive DC bus BUS+ and the negative DC bus BUS-.

[0024] In some examples, DC-DC converters include two-level or three-level boost circuits to achieve MPPT (maximum power point tracking) tracking and boost functionality. Two-level boost circuits include single-boost or buck-boost circuits; three-level boost circuits include dual-boost or flying capacitor boost circuits.

[0025] In some examples, the DC / AC converter includes single-phase inverter circuits and three-phase inverter circuits. Single-phase inverter circuits include, but are not limited to, Heric circuits, H5 circuits, H6 circuits, and single-phase H-bridge circuits; three-phase inverter circuits include, but are not limited to, type I three-level three-phase bridge circuits, ANPC three-level three-phase bridge circuits, type T three-level three-phase bridge circuits, and three-phase full-bridge circuits.

[0026] In some examples, the controller can perform the steps of the insulation resistance detection method, as detailed below.

[0027] In some examples, the controller controls the operation of switches in the insulation resistance detection circuit to change the impedance values ​​of the positive and / or negative bus terminals to ground, thereby disturbing the voltage of the positive and negative bus terminals to ground, and thus calculating the impedance R of the photovoltaic array PV positive terminal to ground. PV+ and the impedance R of the negative electrode of the photovoltaic array to ground PV- .

[0028] In some examples, the controller can provide control signals to the voltage sampling circuit and the insulation impedance detection circuit, receive the voltage value uploaded by the voltage sampling circuit, determine whether the voltage is approaching a steady state, predict the steady-state voltage value, and calculate the insulation impedance.

[0029] Figure 2 This is a schematic diagram of an insulation resistance detection circuit provided in an embodiment of this application.

[0030] like Figure 2 As shown, in the insulation resistance detection circuit, K1, K2, and K3 are switches (including but not limited to relays) that connect and disconnect the two contacts. R1, R2, R3, and R4 are resistors. Resistors R1, R2, R3, and R4 are connected in series between the positive DC bus BUS+ and the negative DC bus BUS-. The connection point between resistors R2 and R3 is connected to the protective ground or ground wire (PE in the figure) through switch K3. That is, one end of switch K3 is connected to the connection point between resistors R2 and R3, and the other end of switch K3 is connected to the protective ground or ground wire. Switch K1 is connected in parallel with resistor R2, and switch K2 is connected in parallel with resistor R3.

[0031] A typical implementation method involves recording the positive DC bus voltage V to ground when switch K1 is closed, switch K2 is open, and switch K3 is closed. BUS+_PE and ground to negative DC bus BUS-voltage V PE_BUS- When switch K1 is open, switch K2 is closed, and switch K3 is closed, record the positive DC bus voltage (V) to ground at this time. * BUS+_PE and ground to negative DC bus BUS-voltage V * PE_BUS- .

[0032] The controller can calculate the impedance R of the positive DC bus BUS+ to ground according to the following formula 1. BUS+_PE and negative DC bus BUS-to-ground impedance R BUS-_PE In some application scenarios, R PV+ Completely equivalent to R BUS+_PE R PV- Completely equivalent to R BUS-_PE In some application scenarios, R can be approximated. PV+ Equivalent to R BUS+_PE R PV- Equivalent to R BUS-_PE Among them, R1, R2, R3, R4, and V BUS+_PE V PE_BUS- V * BUS+_PE V * PE_BUS- All of these are known quantities.

[0033] (1)

[0034] As can be seen from equation (1), this kind of disturbance of the positive DC bus BUS+ to ground voltage V BUS+_PE and ground to negative DC bus BUS-voltage V PE_BUS- Then, the impedance R of the positive DC bus BUS+ to ground is solved by setting up equations. BUS+_PE and negative DC bus BUS-to-ground impedance R BUS-_PE Finally, the impedance R of the positive electrode of the photovoltaic array to ground was obtained. PV+ The impedance R of the negative electrode of the photovoltaic array to ground PV- The detection accuracy of this method is affected by V. BUS+_PE V PE_BUS- V * BUS+_PE V * PE_BUS- The accuracy of these values ​​is directly affected; they must be steady-state values ​​after the disturbance ends, not process quantities. Otherwise, there will be a huge error, causing false alarms or missed alarms.

[0035] It is understandable that insulation resistance detection circuits are not limited to Figure 2 The situation.

[0036] Depend on Figure 1 It can be seen that there is a capacitance C1 between the positive terminal of the photovoltaic array and ground, and a capacitance C2 between the negative terminal of the photovoltaic array and ground. These two capacitances vary with weather conditions, being smaller on sunny days and larger on rainy days. When the voltages of the positive DC bus BUS+ and the negative DC bus BUS- to ground are disturbed, they will be redistributed, potentially resulting in a change in voltage. BUS+_PE Increase, V PE_BUS-Lower or V BUS+_PE Reduce, V PE_BUS- The voltage increases, but due to the presence of the capacitor, it is equivalent to an RC charging and discharging network. There is a slow charging or discharging process across the capacitor; when the capacitor is small, the charging and discharging process is faster, and the voltage across the capacitor will reach steady state more quickly. For example... Figure 3 As shown, it may reach steady state in about 100ms. With a larger capacitor, the charging and discharging process is slower, and the voltage across the capacitor will reach steady state more slowly. Figure 4 As shown, it may take about 80 seconds to reach a steady state.

[0037] To achieve rapid detection of insulation impedance, existing technology samples the positive DC bus voltage V to ground approximately 1 second after the disturbance. BUS+_PE and ground to negative DC bus BUS-voltage V PE_BUS- Used for calculations. On sunny days, given the magnitude of the photovoltaic module's capacitance to ground, 1 second is sufficient for voltage stabilization. Therefore, using the sampled steady-state value for calculation allows for a relatively accurate determination of insulation impedance. However, on cloudy or rainy days, the capacitance to ground of the photovoltaic module increases dramatically, potentially reaching approximately 1000 times that of sunny days. Therefore, after a disturbance, it may take 80 seconds to reach a steady state. Clearly, 1 second is far from sufficient. The sampled voltage is only a process quantity; substituting this value into the formula will produce an incorrect result, significantly different from the true value. This explains why many photovoltaic inverters in photovoltaic power plants are prone to falsely reporting insulation impedance faults on rainy days, when it's not a real fault but simply due to inaccurate detection. However, if a blanket increase to 80 seconds or more is applied, while it may allow for steady-state sampling on rainy days, it's unreasonable for sunny days, as stabilization takes very little time. An 80-second delay would drastically increase the insulation impedance detection time, which is unacceptable to customers.

[0038] Based on this, such as Figure 5 As shown in the figure, this application provides an insulation resistance detection method, which includes the following steps:

[0039] S11. After disturbing the DC bus voltage to ground, determine whether the voltage value of the sampled object tends to stabilize.

[0040] It should be noted that the voltage value of the sampled object can be at least one of the following: the voltage between the positive terminal of the photovoltaic array and ground, the voltage between ground and the negative terminal of the photovoltaic array, the voltage between the positive terminal of the busbar and ground, and the voltage between ground and the negative terminal of the busbar.

[0041] S12. If the voltage value of the sampled object does not tend to stabilize, the perturbation time is increased and the determination of whether the voltage value of the sampled object tends to stabilize is continued.

[0042] S13. When the increased disturbance time reaches a preset time threshold, if the voltage value of the sampled object still has not stabilized, select the voltage values ​​at multiple time points and predict the steady-state value based on the voltage values ​​at these multiple time points for use in calculating the insulation impedance.

[0043] It should be noted that the selection of the preset time threshold depends on the requirements for detection time. If a short detection time is desired, the smaller the value, the better; if a long detection time is desired, the larger the value, the better.

[0044] In some examples, determining whether the voltage value of the sampled object tends to stabilize includes:

[0045] The voltage values ​​of the sampled object at different time points are subtracted, and the absolute value of the difference is compared with a preset threshold value to determine whether the voltage value of the sampled object tends to be stable.

[0046] If the absolute value of the difference is lower than the preset threshold, it is determined that the voltage value of the sampled object tends to be stable; if the absolute value of the difference is higher than the preset threshold, it is determined that the voltage value of the sampled object has not tended to be stable.

[0047] It is understandable that the above method can still be used for judgment in steps S12 and S13. The difference is that the preset threshold values ​​in steps S11, S12, and S13 can be the same or different. The selection of the threshold value depends on the accuracy requirement. If the accuracy requirement is high, the smaller the value, the better. If the accuracy requirement is low, the value can be appropriately large.

[0048] In some examples, after determining whether the voltage value of the sampled object tends to stabilize after disturbing the DC bus-to-ground voltage, the method further includes:

[0049] If the voltage value of the sampled object tends to stabilize, the disturbance ends and the voltage value of the sampled object sampled at the last moment is used for the calculation of insulation impedance.

[0050] In some examples, after increasing the perturbation time and continuing to determine whether the voltage value of the sampled object tends to stabilize, the method further includes:

[0051] If the voltage value of the sampled object tends to stabilize before the increased disturbance time reaches the preset time threshold, the disturbance ends and the voltage value of the sampled object sampled at the last moment is used for the calculation of insulation impedance.

[0052] In some examples, selecting voltage values ​​at multiple time points and predicting the steady-state value based on these voltage values ​​includes:

[0053] Select voltage values ​​at at least three time points, and predict the steady-state value based on a preset curve showing the voltage changing over time.

[0054] The preset curve of voltage changing with time is determined by the following equation:

[0055] Where V(t) represents the voltage value at time t, k1, k2, and b are unknowns, and b is the steady-state value. It should be noted that variations of the equation based on the given equation are also within the scope of this application.

[0056] It should be noted that in the same criterion mentioned above, "higher than" and "lower than" can be either "greater than or equal to" or "less than", or "greater than" or "less than or equal to".

[0057] The following example illustrates the insulation resistance testing process in detail:

[0058] First, the DC bus voltage to ground is disturbed. The voltage sampled at disturbance time t0 is V0, and the voltage sampled at disturbance time t1 is V1. The absolute value of V1-V0 is then compared with the preset threshold value ΔV. th1 Compare the threshold values ​​ΔV. th1 Depending on the required accuracy, a smaller value is better if high accuracy is required, while a larger value can be used if low accuracy is required. The voltage sample value at each time point can be the instantaneous value at that moment, the average value over a short delay, or a filtered value. The following statements regarding threshold values ​​and voltage sample values ​​at each time point are similar.

[0059] If |V1-V0|≤ΔV th1 If the disturbance ends, the voltage value sampled at the last moment will be used to calculate the insulation impedance.

[0060] If |V1-V0|>ΔV th1 Then, the disturbance time is increased to t2, and the voltage V2 at time t2 is sampled. The absolute value of V2-V1 is compared with the set threshold value ΔV. th2 Comparisons are made. Where ΔV th1 With ΔV th2 They can be equal.

[0061] If |V2-V1|≤ΔV th2 If the disturbance ends, the voltage value sampled at the last moment will be used to calculate the insulation impedance.

[0062] If |V2-V1|>ΔV th2 Then the perturbation ends, and the three points (t0,V0), (t1,V1), and (t2,V2) are substituted into the equation. In the equation, the specific expression is the curve of voltage changing with time. The steady-state value can be predicted from this curve, where b is the predicted final steady-state value. This steady-state value is then used to calculate the insulation impedance.

[0063] It should be noted that the above example uses t2 as the upper limit of delay, but in reality it can be t. n That is, the upper limit of delay t n The inner can contain (t0,V0), (t1,V1), (t2,V2), ..., (t n V n There are n points in total. After entering the prediction stage, at least 3 points are selected from the n points and substituted into the equation. In this process, the steady-state value is ultimately predicted and used to calculate the insulation impedance.

[0064] Taking a sunny day as an example, with a relatively small capacitor, the voltage might stabilize in about 100ms. If this technical solution is used, sampling voltage V0 at 0s and voltage V1 at 0.5s, if the voltage changes significantly after a disturbance, the absolute value of V1-V0 will definitely be greater than ΔV. th1 Then the disturbance time will be increased to 1 second, and the voltage V2 will be sampled for 1 second. Since it stabilizes after 0.1 seconds, the absolute value of the difference between V2 and V1 will be less than ΔV. th2 Afterwards, the disturbance will end, and the voltage value sampled at 1 second will be used to calculate the insulation impedance. It can be seen that compared to existing technologies, this scheme does not increase the detection time in clear weather.

[0065] Taking rainy weather as an example, the capacitor has a large capacitance, so it may take about 80 seconds to stabilize. Assuming the upper limit of the delay time is set to 4 seconds, if this technical solution is used, sampling voltage V0 at time 0 seconds and voltage V1 at time 0.5 seconds, the voltage is still changing. Therefore, the absolute value of V1 - V0 is greater than ΔV. th1 The perturbation time is then increased to 1 second, and the voltage is sampled as V2. Since the voltage may not yet be stable, the absolute value of V2-V0 is greater than ΔV. th2 The perturbation time was then increased to 2 seconds, and the voltage was sampled as V3. Since the voltage may not yet be stable, the absolute value of V3-V2 is greater than ΔV. th3 The perturbation time was then increased to 4 seconds, and the voltage was sampled as V4. Since the voltage may not have stabilized yet, the absolute value of V4-V3 is greater than ΔV. th4 However, since the delay limit of 4 seconds has been reached, the disturbance will end and the prediction phase will begin. We can choose to substitute the three points (0,V0), (2,V1), and (4,V2) into the equation. In the calculation, the values ​​of K1, K2, and b are shown in Equation 2 below:

[0066] , , (2)

[0067] Where b is the predicted final steady-state value, which is used to calculate the insulation impedance. It can be seen that compared with existing technologies, this scheme can use the collected or predicted steady-state value to calculate the insulation impedance value in rainy weather, thereby effectively improving the accuracy of insulation impedance detection. Furthermore, the existence of the prediction scheme does not significantly extend the detection time.

[0068] The above method extends the voltage disturbance time and continuously samples the voltage. The absolute value of the difference between the sampled values ​​within a specified time period is compared with a pre-set threshold to determine if the voltage has reached a steady state. If a steady state is reached, the last sampled value is used as the steady-state value to calculate the insulation impedance, thereby effectively improving the accuracy of insulation impedance detection and suppressing or even avoiding false alarms of insulation impedance faults in rainy weather. Furthermore, to shorten the detection time, i.e., the voltage disturbance time, this scheme adds a method for predicting the steady-state voltage value based on the delay. This further compresses the detection time while improving accuracy, allowing for accurate calculation of the insulation impedance value even in rainy weather with high ground capacitance, thus avoiding false alarms of insulation impedance faults.

[0069] The preferred embodiments of this application have been described above with reference to the accompanying drawings, but this does not limit the scope of the claims. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and spirit of this application shall be within the scope of the claims.

Claims

1. A method for detecting insulation resistance, characterized in that, The insulation impedance detection method includes: After disturbing the DC bus voltage to ground, determine whether the voltage value of the sampled object tends to stabilize; If the voltage value of the sampled object does not stabilize, the perturbation time is increased and the determination of whether the voltage value of the sampled object stabilizes is continued. If the voltage value of the sampled object still has not stabilized when the increased disturbance time reaches a preset time threshold, the voltage values ​​at multiple time points are selected and the steady-state value is predicted based on the voltage values ​​at these multiple time points for use in calculating the insulation impedance.

2. The insulation resistance detection method as described in claim 1, characterized in that, The determination of whether the voltage value of the sampled object tends to stabilize includes: The voltage values ​​of the sampled object at different time points are subtracted, and the absolute value of the difference is compared with a preset threshold value to determine whether the voltage value of the sampled object tends to be stable.

3. The insulation resistance detection method as described in claim 2, characterized in that, The insulation impedance detection method further includes: If the absolute value of the difference is lower than the preset threshold value, then it is determined that the voltage value of the sampled object tends to be stable; If the absolute value of the difference is higher than the preset threshold, it is determined that the voltage value of the sampled object has not stabilized.

4. The insulation resistance detection method as described in claim 1, characterized in that, After determining whether the voltage value of the sampled object tends to stabilize after disturbing the DC bus voltage to ground, the process further includes: If the voltage value of the sampled object tends to stabilize, the disturbance ends and the voltage value of the sampled object sampled at the last moment is used for the calculation of insulation impedance.

5. The insulation resistance detection method as described in claim 1, characterized in that, After increasing the perturbation time and continuing to determine whether the voltage value of the sampled object tends to stabilize, the method further includes: If the voltage value of the sampled object tends to stabilize before the increased disturbance time reaches the preset time threshold, the disturbance ends and the voltage value of the sampled object sampled at the last moment is used for the calculation of insulation impedance.

6. The insulation resistance detection method as described in claim 1, characterized in that, The step of selecting voltage values ​​at multiple time points and predicting steady-state values ​​based on these voltage values ​​includes: Select voltage values ​​at at least three time points, and predict the steady-state value based on a preset curve showing the voltage changing over time.

7. The insulation resistance detection method as described in claim 6, characterized in that, The preset curve of voltage changing with time is determined by the following equation: Where V(t) represents the voltage value at time t, k1, k2, and b are unknowns, and b is the steady-state value.

8. A photovoltaic inverter, characterized in that, The photovoltaic inverter includes a controller configured to perform the insulation impedance detection method according to any one of claims 1-7.

9. The photovoltaic inverter as described in claim 8, characterized in that, The photovoltaic inverter also includes a DC-to-DC converter and a DC-to-AC converter connected by a positive DC bus and a negative DC bus, an insulation impedance detection circuit disposed between the positive DC bus and the negative DC bus, and a voltage sampling circuit for sampling voltage.

10. The photovoltaic inverter as described in claim 9, characterized in that, The insulation impedance detection circuit includes a first resistor, a second resistor, a third resistor, and a fourth resistor connected in series between the positive DC bus and the negative DC bus, a first switch connected in parallel with the second resistor, a second switch connected in parallel with the third resistor, and a third switch with one end connected to the connection point between the second resistor and the third resistor and the other end connected to the protective ground or ground wire.

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