## 1 Introduction  
In order to guarantee the safe and stable operation of high voltage transmission lines, differential protection is adopted as the main protection for the benefits of its phase-selection function and immunity to power swings and operation modes [1]. It trips instantaneously for faults in the protected zone. Figure 1 shows the functional setup of the current differential protection. At each terminal, the phase sequence network converts the three-phase currents from the line current transformers into a single-phase voltage, which is applied to the relay circuits via the saturating transformer. Acting as an impedance matching device between the relay circuit and the data acquisition circuit, the primary winding is connected to the restraint and operating circuits [2]. The local and remote current values are sampled to calculate the differential current. When a fault occurs in the protected zone, the differential current value will exceed the operation threshold and the protection will operate to clear the fault. A stabilization technique named percentage restraint is commonly used to conquer the errors from different ratios, saturation of the current transformer (CT) channel delay measurement, and finite sampling frequency [3]. According to the principle of differential protection, a remote quantity containing the current information needs to be transferred to the local end for the criterion. Common to all the current differential protection is the need for a fast and reliable communication channel. A 64 kbps communication interface is commonly used for differential protection [4]. The communication channel can be twisted pair cables, power line carriers, and microwaves. Nowadays, advantages of optical fiber are becoming more prominent as pilot communication media in the virtue of its significance and particularity of Optical-fiber Composite Overhead Ground Wires (OPGWs). The dedicated fiber connection is typically deploying a single mode fiber, 1310 nm or 1550 nm LED or laser depending on fiber distance. The laser option can typically be applied for up to 100 km [4]. However, OPGWs are vulnerable to various forms of natural disasters and malicious physical events, such as lightning strikes and icing [5]. When optical fibers disconnect, single end information-based backup protection, for instance distance protection, will be accelerated [6]. However, it cannot guarantee whole-line quick-action. Therefore, the development of reconstructed communication channels (RCCs) is urgently needed, which should give priority to maintaining the speed and reliability of differential protection. To achieve this goal, there are two thoughts in general, wherein one is to utilize remaining sound communication channels, and the other is to build dedicated channels when there is no sound communication channels in a wide area. For the former thought, the use of self-healing ring architectures and PRP (Parallel Redundancy Protocol) has been widely used in industrial applications. Synchronous optical networks (SONETs) and synchronous digital hierarchy (SDH) allow the development of network topologies that are able to achieve communication reconstruction. They can survive the disasters by reconfiguring and maintaining alternate routing. Protection switching in a ring topology may be unidirectional [7]. The unidirectional switch means that only the faulted path is switched while the non-faulted path follows the original route. It may introduce permanent, unequal propagation delays that are not acceptable by protection standards [4]. In such cases, 2 M leased lines without self-healing rings are introduced [8], but problems such as sensitive data security and high cost of public telecommunication remain to be solved. The latter thought has drawn the attention of power companies and attracted research interests. The very promising evolution from conventional networks to smart grids integrating several communication technologies [9; 10; 11] may come as a solution.