Wireless Communication Systems Supporting IEC 61850 Applications

Authors: Andre Smit and Suraj Chanda, Siemens Industry, Inc., USA, and Terry Fix, Dominion Energy, Richmond, USA

There was always a common theme in that we only trusted copper or fiber as mediums to transmit data between protection devices. Wireless communication systems were perceived not suitable as we could not trust the systems to provide adequate latency, bandwidth, availability and security. The modernization and digitization of the grid however require us now to move outside the substation fence and communicate to field devices.

WiMAX radio installation on recloser: The deployment of more and more distributed generation increases the need to communicate information between field devices and the substation devices (Figure 1).

Wireless communication is really the only cost-effective medium to use. Fiber buildouts will definitely help us in the future, but this is not always feasible or immediately available. The IEC 61850 standard has really made a difference for considering the use of wireless communication systems. The GOOSE message is really ideally suited for use on wireless systems. It is a small data packet that can contain digital and analogue information including important quality information.

In this article we will discuss the use of 2 different wireless technologies that were applied in protection and control schemes over the past 8 years.

WiMAX

WiMAX was the first wireless technology we could really use in 2010 to transmit GOOSE messages between protection field devices.

WiMAX is a point to multi point radio Architecture:  The system consists of a central base station radio, typically mounted on a tower, and remote subscribers. The field protection / automation controllers are connected to the subscriber radios. This architecture forms a layer two Ethernet network over the radio links. This communication system, much like any standard Ethernet network, supports IEC61850 GOOSE messaging (Figure 2).

Unlike Wi-Fi, the WiMAX standard calls for Quality of Service to be implemented in WiMAX radios. This is likely the most important feature for using a wireless communication system to communicate protection and control/automation data between field and substation devices. The quality of service is used to set aside a guaranteed bandwidth and a priority for the transmission of the GOOSE messages. The protection GOOSE messages cannot be impacted by another network traffic. The latency of this channel is the best for each radio link and is predictable.

In Wi-Fi radios tested, all traffic is funneled through the same communication pipe. If you were to increase traffic e.g. download a fault recording file, it could increase the latency of the GOOSE messages. Should Wi-Fi radio systems incorporate quality of service features they can be used for the transmission of GOOSE messages much like WiMAX. The WiMAX provided a consistent latency of 70 ms to get a GOOSE message from one IED to another IED.

Installation - The higher the base stations can be mounted the farther the reach to the subscriber. The minimum height we found was 200 feet to get above the tree lines on the east coast of the US. We could reach 6 to 9 miles from the base station to field devices. We reached distances of up to 15 miles on installations using high gain antennas and base stations mounted at over 300 feet.

On some applications the distance to cover can be too great to reach with a single Base Station to subscriber link. In these instances, we used a hop, back to back subscribers linking to separate base stations. A hop between devices could potentially double the latency and must be considered in the system design.

Although we recommend doing a formal path study we found we could easily use Google Earth to find high ground for towers and create paths to the coordinates of the field equipment. The elevation tool in Google Earth can be used to determine if there are any obstacles in the communication path. We added tree clutter to the highest points to see that we had a good clear communication path from the base station to the subscriber.

Tower assets are very valuable if you consider using WiMAX. We for example used water tower assets where radio towers were available.

We found that it was of utmost importance to first install the base stations on the tower assets and then physically test every link to prove the RF links.

The two signal statistics that must be recorded are the Received Signal Strength Indicator (RSSI) and Carrier to Interference plus Noise Ratio (CINR).  RSSI in dBm is an indication of the power level being received by the antenna. Therefore, the less negative the number (close to zero) the stronger the signal. CINR in dB is an indication of the quality of the signal.  A higher CINR is desirable in order to adequately distinguish the signal from interference from other antennas or environmental factors. The radio can use higher Quadrature amplitude modulation QAM with lesser interference. We strived to get to QAM 32 to 64 on all protection communication links.

Figure 3 was used to determine adequate RSSI. 

If RSSI is - 82dB or more, the link is good. If the RSSI was between -82dB and -86dB the link was tested again with higher gain antenna or height was adjusted or repositioned.

At RSSI - 60dB and higher, the signal is too strong and will cause saturation. The antenna can be adjusted to face away from the base station direction until acceptable signals are measured. In many instances we made use of bucket trucks to physically test the field radio links and establish antenna mounting height and primary switch location.

Testing WiMAX Radio Link: Once we determined the best location for radio link the installation of the field recloser or switch could be finalized (Figure 4).

WiMAX provides an excellent platform for IEC 61850 devices. It is a modern standard supporting Ethernet and it is very easy to establish an Ethernet network between IED’s. Each feeder system can be placed on its own unique VLAN. We could, for example, create on one RF link separate data service tunnels for different VLANs and also a separate service tunnel for DNP3 traffic.

We found that reading the IEC 61850 stack from the field devices to a control center HMI Client was problematic. The size of the packets was sometimes too big “Jumbo Packets” and the radios struggled to keep the Ethernet links up with the client and the field devices. We moved to DNP3 and the radios coped better with the smaller better managed DNP packets.

The small ±300 byte GOOSE messages did not present a problem to the radios as long as the maximum time between GOOSE messages was long enough. The radios definitely could not support 10 or even 100 ms between messages. We could stretch the maximum time significantly as we could not detect any packet loss in the WiMAX radio systems.

In a fault event an avalanche of GOOSE will be created. We found that if the automation system operates at high speed we could locate a fault, isolate it and close a tie switch before there was any sign that the radios had trouble to keep up with the delivery of the GOOSE packets. It is very important to consider the number of GOOSE applications required to perform the protection and control functions. For the most part 3 applications per relay will be supported.

Lastly the time between GOOSE massages must be established through exhaustive testing of the protection and control functions making sure the radios will support the system. These tests are best performed in a laboratory environment. There is no easy set of rules on how the GOOSE applications should be configured.

Network Configuration -

  • VLAN
  • Maximum time
  • Minimum time
  • VLAN priority

A Virtual LAN (VLAN) is any broadcast domain that is partitioned and isolated in a communication network at the data link layer (OSI layer 2). The Layer II VLAN parameter is the key differentiation feature for an IEC 61850 based GOOSE message. It is crucial to have a unique VLAN assigned to each P&C system. This VLAN assignment avoids any duplicates and/or collisions of Layer II GOOSE messages between devices from two different systems.

WiMAX is likely the best wireless communication system for the transmission of protection and control data. The deployment is however quite difficult especially if tower assets are not readily available. The system must be continuously maintained to ensure availability. We used the GOOSE message quality information to monitor and report communication link failures.

WiMAX provides a private layer 2 network with adequate security if security features available are used.

In two instances we did find that WiMAX radios were installed by others that interfered with the RF links. Frequencies were changed to overcome the interference. This could have been avoided by proper registration of all assets with the FCC. The new WiMAX registration management system will improve the registration process and management though service registration providers that will eliminate this phenomenon experienced in an unlicensed spectrum.

BeijingSifang June 2016