The Polish electricity supplier, PSE manages a network of more than 1,700 high-voltage substations across the country. These distribute electricity to 35 million people and many industrial operations – so reliability and high availability are very important. PSE has long used communications systems based on the IEC61850 substation automation design standard in both new and modernised substation systems, and back in 2006 its systems integrator, Elkomtech installed a WindEx SCADA system, which supports surveillance centres by allowing remote access to many different substation control systems.
Key elements of this monitoring system are Ethernet switches, and over the last two years, Elkomtech has ordered around 100 Westermo units through Tekniska, the latter’s Polish agent. For new developments, Elkomtech has chosen Westermo’s R208 switch, a device that is said to deliver the highest time stamping accuracy on the market.
Meeting PSE’s additional requirement for high reliability, Elkomtech also equips these switches with a redundant power supply. Two redundant inputs are provided. The power supply draws power from the input that has the highest potential difference when compared to the alternate supply. This allows the use of a 48V source as primary supply with a 24V battery as back up.
Latency jitter
In industrial automation, Ethernet networks have always been limited by the latency jitter of data packet transmission. Depending on network load, data packet size and the number of switches between the server and the client, data transmission inside an Ethernet switch can cause time latency jitter of several milliseconds. A packet that enters the switch may even be transmitted after one that arrives later, due to switch QoS (Quality of Service) techniques.
Furthermore, a network with several off-the-shelf switches will accumulate time synchronisation errors for each switch on the network path between the time server and time client. The precise data analysis needed for high- or medium-voltage substation automation then becomes impossible.
Westermo’s R208 switch allows precise time-stamping, providing a full, detailed history of data events. Thus sophisticated or critical industrial processes – whether or not they are in real time – can be managed using the off-the-shelf SNTP Ethernet protocol. The R208 allows industrial Ethernet networks to be installed in a redundant ring configuration, eliminating network failure caused by breaks in the fibre or copper infrastructure. The reconfiguration time is just 30ms.
Elkomtech’s R&D department has been working on new data monitoring techniques like ‘synchrophasor’ acquisition and disturbance recording, where an overall time resolution of less than ten microseconds is required. Westermo’s one microsecond time synchronisation resolution thus proved very useful for this project.
The monitoring network
The WindEx SCADA system enables continuous monitoring of high voltage equipment, detecting danger points, alarms and abnormalities. There are six control centres equipped with 12 real-time servers. Six of these, interconnected via a wide area network (WAN), ensure full redundancy, while the other six servers have additional functions. This creates a coherent system with an integrated database but distributed functions.
Every control centre has dedicated operator terminals, and all terminals in the WindEx system have a main connection to the local network and backup to other nodes. In the event of WAN failure every server can work alone, supporting its substation control systems through local terminals. All remote substation control systems have two connections to the WindEx system, through servers in two different control centres. If the main connection breaks, the system automatically switches to the backup connection.
WindEx receives information from the substation control systems. In the highest voltage stations, Elkomtech uses its own system, Ex-LHV, which enables full monitoring and remote access to recorders and other substation automation devices. Ex-LHV is a system with a distributed structure, comprising field drivers, substation computers, HMI workstations and web servers. For information interchange, the system uses the PN EN 60870-5-104 protocol.
A substation local area network (LAN) places demands on the physical and functional reliability of any hardware on the network, and this includes the Ethernet switches, which must guarantee redundant connection, fast reconfiguration and full diagnostics of LAN operation, as well as an ability to work in tough environments (temperature extremes and strong electromagnetic fields). Substation LANs must allow high accessibility, and this can only be achieved with hardware that offers very favourable meantime between failure and by good design.
An Ethernet LAN for small to medium sized substations consists of a few Westermo R200 switches working in a redundant fibre optic ring. Large substations use multi-ring topology. Thanks to redundant servers and LAN with redundant ring topology, potential failure of one module does not break the whole system. Specifically for IEC61850, critical GOOSE (Generic Object Oriented Substation Event) messages can be sent reliably using the multicast and prioritisation functionality within Westermo’s switches. This essential feature allows new SSiN (Control and Managing System) elements to be installed more easily.