Industrial Ethernet-based communication is becoming increasingly important for many applications, for example, in power supply. Even the briefest interruption in communication can lead to great economic loss, especially in this sector. This calls for networks that have no reconfiguration time at all in case of an error.
Today, communication without any interruption at all is possible, thanks to a new redundancy method, Parallel Redundancy Protocol (PRP), described in the international standard IEC 62439-3. As the name suggests, the data are sent in parallel over redundant paths. If one communication channel is interrupted, the second frame reaches its target without interruption, running in parallel on the redundant path.
To use this procedure, the stations must meet certain technical requirements. If a user wishes to transmit the same information on redundant paths, the data must be sent twice, and the recipient must recognize that a frame was received twice. For this to work, the Industrial Ethernet frames need an amendment. This amendment includes all the information necessary to send both frames using two different networks and for the recipient to recognize the frames as duplicates. The connected terminal devices also need amendments because the two frames must be fed into redundant networks accordingly. This means that the terminal devices require two integrated interfaces, or an upstream device must be used.
A redundant communications scheme for a protective relay which allows high-speed, reliable, and fault tolerant communications between peer protective devices in a power distribution network. Communications over a select-able primary communication channel are interrupted and switched to a secondary communication channel, which can operate according to a different communication protocol, when a fault is detected on the primary communication channel. The fault detection and switching is performed transparently to the main protective relay processor.
Communication using PRP networks
During the set-up of these two parallel networks, components are used that – depending on the application – meet the necessary requirements of the environment and support the forwarding of PRP frames. These networks, through which each duplicate frame is transmitted with its amendment, are physically separated and can have different sizes and structures. The different frame runtimes that result do not negatively affect the redundancy concept. Due to these different runtimes, the frames usually do not reach the recipient simultaneously. The recipient receives the first frame, saves it in an internal table, and forwards it either within the device itself or – in the case of an upstream connection box – to the addressed terminal device. Once the second frame arrives, the table is consulted to see whether the corresponding frame is already present. If it is, the second frame is discarded and the frame stored in the table is deleted. If the second frame does not reach the recipient due to a network interruption in one of the two networks, the first frame is deleted from the table after a specified period.
This new redundancy mechanism allows the implementation of communications networks with no reconfiguration times in case of an error. Through the parallel networks, two equivalent transmission paths are created, and disabled parallel paths do not first need to be enabled in case of an error as with other procedures.
Simple frame duplication for devices with network interfaces
In the area of switchgear automation, Siemens already offers protection devices that are equipped with two PRP-capable network interfaces. The applications communicating with these switching devices usually run on servers. These servers must also be connected to the parallel network structures. Simatic NET offers a software package for these server applications. Softnet-IE RNA (Redundant Network Access) allows the user to duplicate frames in the server and send both frames using two network cards integrated in the server, such as Simatic NET CP 1612 A2. A corresponding upstream device is available for all devices in which it is not possible to integrate two interfaces. Scalance X204RNA allows for the connection of non-PRP-capable terminal devices to redundant network structures. The bottom line is that the new, standardized PRP procedure is suitable for all applications in which the high availability of the Industrial Ethernet-based communications network is critical and even the shortest of network reconfiguration times – such as those encountered in the case of ring redundancy – may lead to great economic loss.
Further redundancy methods
Support of other procedures for reconfiguration of data networks, for example, Rapid Spanning Tree Protocol (RSTP, according to IEEE 802.1D), Media Redundancy Protocol (MRP, according to IEC 62439), Media Redundancy for planned duplication (MRPD, according to IEC 61158), High-availability Seamless Redundancy Protocol (HSR, according to IEC 62439-3)
Additional solutions for the energy and transportation market through the new product line Ruggedcom
Advantages of PRP communication
Very high plant availability due to parallel data transmission using separate network structures
No frame delays during reconfiguration of one of the two network structures
High flexibility during network set-up, as the networks can be realized as line, tree, star, or ring structures
Rapid commissioning without mandatory configuration.
SUMMARY OF THE INVENTION:
The present invention solves the above-mentioned problems, and achieves additional advantages, by providing for a redundant communications scheme for a networked control device in a power distribution system. According to exemplary embodiments of the invention, a digital protective relay is provided with a redundant communications circuit which can communicate relay information with peer devices over a network using a primary ethernet communication channel. The communications circuit is capable of detecting the presence of a fault or failure on the primary communication channel, and of switching the communication from the primary channel to a secondary channel. The circuit performs the detection and switching in a manner which is transparent to the main relay processing circuitry. Preferably, the primary communication channel type can be selected by a user without reprogramming the relay. Further, the communications circuit is industrially hardened to withstand operating conditions associated with electric utility substations, which can include a temperature range of approximately -40 communications circuit advantageously provides multiple fiber communications ports on a single card.