## 1 Introduction
On the other hand, SGs with the help of wireless monitoring sensors and proper fault diagnostics, can remarkably reduce power losses and long-term maintenance budget and does not require expensive communication cables, thus enhancing the system's reliability and efficiency [13]. Efficient energy utilization and network lifetime are considered as the main design parameters in the previous research conducted for WSN [8]. Additionally, the feasibility of mobility usage in WSN as demonstrated in [14] can improve the network lifetime by replacing the traditional static wireless sensor nodes with mobile wireless sensor nodes. Similarly, if the center node (or sink) is mobile, it needs more computation power in comparison to other sensor nodes [14]. The mobility and more computation power will drain out the battery of that sink node after a few rounds of sensing and aggregation, and its energy should be replenished in a timely manner. The mobility of a sink node can be controlled or randomly planned depending upon the particular scenario of the WSN and in this way, many traditional problems like hot-spot problem [14] can be avoided. With reference to [15], researchers have proposed a working-sleeping cycle strategy in which free nodes go to sleep to save their battery power and have a proper node scheduling scheme for efficient data transmission. These node scheduling schemes were categorized as synchronous and asynchronous working-sleeping cycle, which means that the network lifetime can be prolonged by changing the node scheduling schemes in accordance with the scenarios. Although it can affect the link stability between sensor nodes, it also creates an opportunistic node connection to exist between nodes due to asynchronous working-sleeping scheduling. According to [16; 17; 18; 19; 20], Opportunistic Routing (OR) is a paradigm for wireless networks which benefit from broadcast characteristics of a wireless medium by selecting multiple nodes as candidate forwarders, to improve network performance. In [18; 19; 20] a set of nodes are selected as potential forwarders and the nodes in the selected set forward the packet according to some criteria after they receive the packet. This group of nodes in OR is called a candidate set (CS). The performance of OR depends on several key factors such as OR metric, candidate selection algorithm and candidate coordination method. [PERSON] et al. in [20] discussed the basic function of OR by highlighting its ability to overhear the transmitted packet and to coordinate among relaying nodes. In OR, by using a dynamic relay node to forward the packet, the transmission reliability and network throughput can be increased. Thus, the term \"Opportunistic Routing\" can be defined as the routing scheme in which the next best forwarder is dynamically selected with respect to OR metric, candidate selection algorithm and candidate coordinate method as in [18; 19; 20]. The reasons why opportunistic node connections exist in WSNs can be summarized as follows:  
* WSNs are often deployed in harsh environments (smart grids NAN in our case), where wireless signals are susceptible to interference, thus causing link instability [21; 22], further leading to opportunistic node connections;
* The sink node mobility usually leads to intermittent links in the network, resulting in opportunistic node connections [21];
* Due to the limited energy of the nodes, the sensor nodes adopt an asynchronous working-sleeping cycle strategy to save energy and accordingly, the adjacent nodes may not be able to communicate with each other continuously as in [22], thus bringing about opportunistic node connections. By utilizing asynchronous working-sleeping cycle strategy, multiple nodes in WSNs can apply the concept of OR by overhearing their working neighbor's transmission.