### Algorithms for Wireless PD Localization of PD Sources  
The PD pulse phenomenon is random in nature. The quality of HV systems and cables can be assessed by measurement and diagnosis of PD. Various methods have been deployed in the past for the detection and localization of different PD types [5]. Lateration techniques used for PD localization are based on distance. PD source location based on spatially-separated sensors has been explored in the past by using various techniques including radiofrequency (RF) antenna array, time of arrival (TOA), time difference of arrival (TDOA), direction of arrival (DOA), use of SDR USRP N200 (from Ettus research, Santa Clara, CA, USA) and RTL-SDR (from Noolec, New York, NY, USA) etc. [5; 8; 10; 11; 12; 13]. Another interesting PD location mechanism proposed in [14] is based on advanced auscultatory technique uses the amplitude of the received signal to estimate the source location. In recent years, the radiometric RF detection of PD has gained significant popularity due to advancements in the field of communication engineering. The cost of hardware at HF, VHF and UHF operating bands has reduced significantly in recent times, which make it affordable to detect PD in these frequency bands [9; 15; 16; 17]. All these methods are classified as range-based methods, i.e., they form matrices bearing location information and subsequently they estimate the position of the source based on the information held in location matrices.  
Continuous monitoring of PD band-limited signal phenomena requires a real-time location system. Real-time location methods implemented in the past for mobile device positioning can be classified into later, angulation and pattern recognition [12; 18]. PD localization accuracy is limited by the fact that PD pulse is time-limited, i.e., it has a certain rise-time. Owing to measurement systems limitations and propagation effects, the received RF signal will be a band-limited signal. This brings uncertainty in the time-of-flight of PD pulse and hence will cause inaccuracy in location measurement [19].  
The RF antenna array method is based on the radiometric location of a PD source. A wideband RF interference is generated by PD, which can be intercepted by using the radio receiver. The work in [20] has used a 4-antenna array for three-dimensional localization of PD sources. The antenna array with direct sampling can measure the time of arrival of the wave to a nanosecond accuracy [21].  
In TOA, both transmission and receiver synchronizations are necessary [22; 23]. In TDOA only receiver synchronization is necessary [24]. In both these schemes, a small inaccuracy can lead to significant location errors [25; 26]. For multipath interference, it may lead to inaccuracies and hence may sometimes hardly be applicable in practice. In the RSS method, however, there is no synchronization between nodes is required, because the technique work on the received energy rather than the time bound PD pulse and this, therefore, enhances scalability. In all the three schemes above scalability remains the biggest constraint due to the synchronization requirement. However, on the other handside, the scalability also improves accuracy [27; 28]. The RSS method is an energy detection method and is again based on a lateration approach like TOA and TDOA. Generally, and based on literature, PD localization systems can be summarized as shown in Figure 1.