**5. Comparative Study of Experimental Data**

The DC-IM device with the unbalanced electric bridge method is created, and the schematic overview of application is shown in Figure 8. The MCU performs the switch *Q*<sup>1</sup> and *Q*2, sample the voltage values of *v*<sup>1</sup> and *v*<sup>2</sup> stored in memory, then calculate the *Rf*<sup>1</sup> and *Rf*2, and output the result to the computer. The experiment table is shown in Figure 9. It includes the display interface, DC-IM device, voltage regulating device, insulation resistance selection switch, and GC selection switch. The DC-IM controller uses a PIC18F4580 single-chip microcomputer. The monitoring period is 0.2 s; that is, the switch action occurs every 0.1 s, so the sampling time should satisfy (*t*<sup>1</sup> + 2∆*t*) < 0.1 s. The positive grounding resistance is *Rf*<sup>1</sup> = 1000 kΩ, the negative grounding resistance is *Rf2* = 300 kΩ, the first sampling time *t*<sup>1</sup> = 0.01 s, the bridge resistance is *R<sup>a</sup>* = 1000 kΩ, and *R<sup>b</sup>* = 200 kΩ. The GC value is *C*<sup>Y</sup> = *C*<sup>1</sup> = *C*2. The grounding current waveform corresponding to different GCs is shown in Figure 10. The waveform of *C*<sup>Y</sup> = 0.1 µF can be stabilized in a half cycle. The larger the value of *C*<sup>Y</sup> is, the closer the waveform is to the triangle wave. Therefore, the traditional unbalanced bridge sampling method is used when *C*<sup>Y</sup> < 0.1 µF, and the three-point climbing algorithm is used when C<sup>Y</sup> > 0.1 µF.

The calculation results are compared by changing the different parameters, and relative error (RE%) is determined as

$$\text{RE\%} = \lfloor \text{measured value} - \text{actual value} \rfloor \text{/measured value}.\tag{20}$$

Different parameters are applied in the proposed method. (1) DC voltage *v*dc = 800 V, GC value *C*<sup>Y</sup> = 0.1 µF, and the sampling time interval ∆*t* is changed; the results are shown in Figure 11. The larger the sampling time interval is, the higher accuracy is. (2) DC voltage *v*dc = 800 V, sampling time interval ∆*t* = 0.04 s, and the value of *C*<sup>Y</sup> is changed; the results are shown in Figure 12. The smaller *C*<sup>Y</sup> is, the higher accuracy is. (3) Sampling interval ∆*t* = 0.04 s, *C*<sup>Y</sup> = 0.1 µF, and DC voltage *v*dc is changed; the results are shown in Figure 13. The larger the DC voltage is, the higher accuracy is. When the DC voltage drops to below 200 V, the measurement accuracy is greatly reduced. (4) Under the premise that the parallel value of bridge resistance *R<sup>a</sup>* || *R<sup>b</sup>* is constant and the difference between *R<sup>a</sup>* and *R<sup>b</sup>* is changed; the results are shown in Table 2. The larger the difference between *R<sup>a</sup>* and *R<sup>b</sup>* is, the higher accuracy is. Time interval ∆*t* increases, *C*<sup>Y</sup> decreases, DC voltage *v*dc increases, and the difference between *R<sup>a</sup>* and *R<sup>b</sup>* increases. These factors make the sampled voltage difference larger, which will reduce the error of the final results.

The proposed method is compared with the traditional method to verify the availability and superiority of the former. The monitoring time and relative error of the two methods are shown in Tables 3–5. The relative error is the larger one between *Rf*<sup>1</sup> and *Rf*2. Table 3 is the data at *v*dc = 800 V, *Rf*<sup>1</sup> = 1000 kΩ, *Rf*<sup>2</sup> = 300 kΩ; Table 4 is the data at *v*dc = 400 V, *Rf*<sup>1</sup> = 1000 kΩ, *Rf*<sup>2</sup> = 300 kΩ; Table 5 is the data at *v*dc = 800 V, *Rf*<sup>1</sup> = 100 kΩ, *Rf*<sup>2</sup> = 100 kΩ. The traditional method needs to increase the monitoring time with a large value of GC because it should have a stable sample value after charging the GC. The proposed method has a fixed monitoring time due to the fixed three-point sampling. When GC is large and the proposed method is applied, the error is similar to using the traditional method. When GC is small, such as 10 nF in the table, and the traditional unbalanced bridge calculation method is applied automatically, the calculation results of two methods are almost the same. Overall, the experimental results are consistent with the theoretical conclusion.

**Figure 8.** The schematic overview of application.

**Figure 9.** Display of the experiment table.

**Figure 10.** *Cont*.

**Figure 10.** Waveform of grounding current with different values of *CY*. (**a**) *C<sup>Y</sup>* = 0.1 µF. (**b**) *C<sup>Y</sup>* = 0.2 µ. (**c**) *C<sup>Y</sup>* = 0.4 µF.

**Figure 11.** RE% with different sampling intervals.

**Figure 12.** RE% with different values of CY.

**Figure 13.** RE% with different DC voltages.


**Table 2.** Data results with different bridge resistors.

**Table 3.** Comparison of monitoring data in *v*dc = 800 V, *Rf*<sup>1</sup> = 1000 kΩ, *Rf*<sup>2</sup> = 300 kΩ.


**Table 4.** Comparison of monitoring data in *v*dc = 400 V, *Rf*<sup>1</sup> = 1000 kΩ, *Rf*<sup>2</sup> = 300 kΩ.


**Table 5.** Comparison of monitoring data in *v*dc = 800 V, *Rf*<sup>1</sup> = 100 kΩ, *Rf*<sup>2</sup> = 100 kΩ.

