Introduction

Conventional line differential relays utilize methods of synchronization to ensure that samples compared by relays at all terminals of a power transmission line are taken at the same time instant. The techniques employed are generally based on the premise that propagation delays in the communication link are equal for both send and receive paths. However, this premise is not necessarily valid for SDH/SONET, (Synchronous Digital Hierarchy/ Synchronous Optical Network) communication networks which can exhibit different propagation delays for the send and receive paths. The consequence is that conventional methods of achieving sampling timing synchronization cannot be applied. Line current differential relays that utilize GPS time information can accomplish sampling timing synchronization independent of the characteristics of the communication medium.

For line current differential relays using GPS synchronization the need for the relay to have a comprehensive GPS back-up system is an indispensable requirement from utilities. The back-up system must be able to maintain the performance of the protection in the unlikely event of a failure within the GPS system which may occur over a protracted period. It must also maintain sampling synchronization during periods of GPS signal interruption and accommodate path switching within the SDH/SONET network. The back-up mode function is also required to cover problems that may occur with integral components of the protection system such as the GPS receiver or antenna.

Current sampling timing synchronization lies at the heart of the security of current differential protection. Synchronization design needs to be rigorously proven in conjunction with the telecommunication network. It is of vital importance that any new product undergoes extensive type testing and with such a novel technique it is imperative to undergo field testing and trials. The first commercial products of this type were installed in 2001 and the article includes some observations on the service experience to date.

Impact of SDH/SONET Communication Networks on Line Current Differential Relays

The rapid growth of digital networks and the convergence of telephone and high-speed data networks have enforced the development of new standards. Proposals in ITU-T, (formerly CCITT) for a Broadband Integrated Services Digit a l Network (B-ISDN) were an enabler for a new synchronous multiplexing standard that would better support switched broadband services. The new standard first appeared as SONET in the United States.

Initially, the objective of the SONET standard was to establish a North American standard that would permit inter-working of equipment from multiple vendors. Also at that time, a different standard for digital hierarchies was in use in Japan. Subsequently, the ITU-T was approached with the objective of developing a world-wide standard. Despite the historical differences between the North American, European and Japanese digital hierarchies, this goal was achieved with the adoption of the SDH standard (1988), the international equivalent of SONET. Figure 1 shows these different standards in Europe, North America & Japan and SDH hierarchies.

In synchronous networks, all multiplexing functions operate synchronously using clocks derived from a common source. SDH/ SONET are expected to be in common use for the foreseeable future as the multiplexing structure has been designed to carry not only current services but also emerging ones using ATM (Asynchronous Transfer Mode) and/or IP framing structures for example.

SDH/SONET allows the development of network topologies that are able to achieve 'network protection', that is, they are able to survive failures in the network by reconfiguring and maintaining services by alternate means. One of the methods that can be employed to provide network protection is by the use of self-healing ring architectures. Protection switching in a ring topology can be either 'unidirectional' or 'bi-directional'. The description unidirectional means that only the faulted path is switched whilst the non-faulted path follows the original route.

With bi-directional switching both the go and return path are switched to follow the opposite direction along the ring. The important point to note is that bi-directional switching will maintain equal signal propagation delays for the go and return path (any differences will be transient), whilst unidirectional switching may introduce permanent, unequal propagation delays that can cause severe difficulties for current differential relays. The creation of unequal delays is illustrated in Figure 2, Figure 3, and Figure 4.

Comparison of measured quantities from differential protection relays must be based on pairs of samples that were taken at the same sampling instant. As samples are transferred to the remote end of the protected circuit for comparison, the delay that is introduced by the telecommunications link has to be compensated by the protection relay that performs the comparison. However, if the sampling clocks are synchronized at each terminal (see later), the process is simplified to one of comparing referenced current samples which are derived from synchronous clocks. With the introduction of SDH/SONET communication systems it is essential that the comparison of the respective samples does not depend on equal signal propagation delay times for the send and receive paths nor on stringent limitations for signal propagation time variations.

Figure 5 shows that the occurrence of a different propagation delay time between the send and receive paths in the communication network will result in a sampling synchronization error for a conventional current differential relay.