# PD-Free Design of Insulation Systems: An Application to Laminated Busbars
## 6 Discussion  
Since good validation of the PD inception field estimates was achieved under both AC and DC supply, it can be assumed that the discharge model would hold for both AC and DC voltage. It can also be speculated that the model would fit any type of voltage waveform, since it is based on discharge physics [18] and thus on field rather than voltage waveform. This implies that the PDIV variation with the voltage waveform would be due to electric field profile modifications. The impact of such results is that the three-leg approach can be applied broadly to the design of insulation systems of LBB (and not only), according to the PD-free design paradigm. Here, we focus on surface discharges, but the same approach works also for internal cavities (structural and defects), for which \(k_{s}=1\) in (4), \(l\) is the cavity height and the model parameters are given in Table 1. For this purpose, it is fundamental that PD measurement analytics can provide the indication of the type of PD source, namely surface or internal. Figure 7: (**a**) Global PD pattern at 12.1 kV. (**b**) PD pulse. (**c**) Separation (PCA) map with clustering. (**d**) PD subpatterns. (**e**) Identification of surface PD at % likelihood. Notably, surface insulation design is generally covered by standards on creepage and clearance, e.g., [29], that are based on the overall surface discharge (flashover) criterion. We think that significant innovation from this point of view is brought by the three-leg, PD-free approach, since the design criterion proposed here is to run a PD-free insulation system for all the specified life. Indeed, as mentioned above, for organic materials, the inception of PD causes faster degradation, locally, and premature breakdown. There is another striking aspect that would support turning insulation design from a creepage to a PD-free concept. This is emphasized by Figure 8, where the test assembly for the investigation of the surface roughness effect on the PDIV is used at various electrode distances. The electric field profiles reported in Figure 8b suggest that the surface discharge inception field would not change significantly, increasing the distance between high voltage and ground layers, and, in fact, the PDIV increases about 15% when the distance between the electrodes increases 50% (e.g., it varies from 10 to 15 mm as in the experiments). This will always be the case (with different numbers, of course) when the electric field distribution is far from being uniform, i.e., \(k_{s}\ll 1\). As an example, if the inception electric field was calculated using Equation (4), the parameters for the surface discharge in Table 1, and \(k_{s}=1\) (i.e., assuming uniform electric field as with the creepage concept), the estimated inception electric field would have been 0.82 kV/mm for both AC and DC supply. This value is about half of the inception field estimated for AC and DC at 25 \({}^{\circ}\)C (see Figures 2 and 5), leading to an erroneous estimation of the PDIV. Consequently, the creepage concept, holding for a quasi-uniform field, may no longer be valid. This can occur, for example, in power electronic boards when increasing voltage and reducing insulation dimensions, e.g., for multi-conductor LBBs and for printed circuit boards, PCBs. Increasing the distance between the high voltage and ground contacts may not be able to raise the inception field value for surface PD above the maximum field magnitude in insulation, and then remedy action must be based on reducing the electric field divergence (e.g., by profiling proper metallic parts). As a last observation, we may notice that the presented methodology is quite general, and it could be employed to a variety of situations. For example, in case of aged insulation, the DC electrical conductivity changes (increases) with respect to an unaged sample [30].