**3. Discussion**

#### *3.1. Model Error Analysis*

According to the above calculation results, this section further discusses the causes of errors. Figure 8 is obtained from the calculation results. It can be seen from Figure 8 that the error of the

dynamic pollution model based on atmospheric parameters is different. The *re* of T1–T5 are relatively small, with its value varying from 7.5% to 14%. However, the *re* of T6–T7 are relatively larger, with its value standing at about 20%.

According to the differences between the different *re*, the characteristics of meteorological parameters are further investigated. It was found that, there are five days of rainfall in the sixth stage; while in the seventh stage, there are eight days of rainfall, and the rainfalls are heavy. Therefore, it can be concluded that rainfall mainly causes the errors in the dynamic accumulation model [27].

#### *3.2. Insulator Structure Coe*ffi*cients*

Now in the power system, the actual insulator's NSDD is usually determined by measuring the NSDD of the reference insulator XP-160. However, even under the same pollution condition, the pollution degrees of insulators with different structures varies. Therefore, insulator structure coefficients *c*1 and *c*2 are further calculated. Table 3 and Figure 9 can be obtained.


**Table 3.** Insulator structure coefficients.

**Figure 9.** Insulator structure coefficients.

As shown in Figure 9, the value of *c*1 (*c*2) witnessed fluctuation, and the change was evident with its figure varying between 0.439 and 0.699 (0.553 and 0.753). In addition, rainfall had a significant influence on the insulators' structure coefficients, since *c*1 and *c*2 increased to a certain extent during the rainfall period. It can be discovered by further calculation that insulator structure coefficient varies due to the change of wind speed, particle size and other factors, with no obvious changing patterns. Hence, this indicates that insulator structure coefficients are dynamic.

Therefore, the original method for obtaining the pollution level of transmission line insulators only by reference insulator is unreasonable. Insulator structure coefficients should be taken into consideration to ge<sup>t</sup> the pollution degree. However, it is time consuming and labor consuming to obtain insulator structure coefficients from natural experiments. Therefore, the insulator structure pollution coefficients can be calculated according to the flow chart based on the proposed model in this paper. And the calculated results are show in Table 4:


**Table 4.** Calculated insulator structure coefficients.

It can be seen that the calculated error of insulator structure coefficients is acceptable, with the maximum value at 19.2% (*re*1) and 22.7% (*re*2), and the average value at 9.0% (*re*1) and 13.5% (*re*2), which verifies the feasibility of the model.
