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EVALUATION OF A WAVEGUIDE SYSTEM FOR IEC61850 COMMUNICATION

Krister Landernäs Jimmy Kjellsson ABB Corporate Research – Sweden ABB Corporate Research - Sweden krister.landernas@se.abb.com jimmy.kjellsson@se.abb.com

ABSTRACT

In this paper, we study the use of a waveguide for IEC61850 communication using 802.11a radio. In particular the communication latency is analyzed. The latency of the system will depend on the traffic load and, thus, several test cases with different load are analyzed. In the paper we also describe a pilot installation commissioned at LyondellBasell Industries. The installed switchgear equipment consists of six panels and uses a waveguide for IEC61850 communication.

INTRODUCTION

Traditionally, the backbone in industrial automation networks consists of a number of industrial type switches interconnected using optical fiber. The overhead in system cost from the switches and fiber can be quite substantial in a typical substation installation. To circumvent this it has been proposed to use radio communication in so called wavgeuides to replace optical fiber communication in substation bays [1], [2]. There are several benefits with this approach. The waveguide provides a galvanic isolation with a rugged construction, whereas fibre is sensitive to bends and dents. Another advantage is simplified installation since the waveguides can be integrated in the cubicles. During commissioning the cubicles are then placed next to each other without the need for connectors or additional wiring. The IEC 61850 standard defines communication networks and systems in substations [3]. It includes a broad range of services and tools for monitoring, control, and protective relaying. The standard can be mapped to different protocols such as Generic Object Oriented Substation Event (GOOSE) and Manufacturing Message Specification (MMS). The architecture studied in this paper uses the IEC 61850-8-1 GOOSE messaging. GOOSE messages are used to transmit breaker trip commands, convey lockout commands, breaker failure initiation, and re-closing initiation. GOOSE messaging is an unconfirmed service and, thus, the publisher has no way of knowing if the subscriber has received the message. Because of this, the publisher must continuously transmit messages to the network. The actual rate of messages being published is adaptive and application dependent. An example of GOOSE transmissions is shown

in Figure 1.

Figure 1: GOOSE Transmission Pattern

It is important to note that GOOSE transmission patterns are not specified in the standard and they are user defined.

WAVEGUIDE OVERVIEW

In general, a waveguide is a structure that guides waves, e.g., light or sound. The waveguide studied in this paper is a hollow metal conductor for transmitting electromagnetic waves, see Figure 2. The shielding of the waveguide provides a protected RF environment which makes it well suited for harsh radio conditions.

Figure 2: Waveguide setup

The dimensions of the waveguide are dependent of, and thus, matched to the frequency of the transmitted signal. The waveguide will act as a high-pass filter with a cut-off frequency related to its dimensions. This implies that in order to transmit a signal with low attenuation the wavelength of the signal must not exceed a certain wavelength limit. For a given height, and in the case of a rectangular waveguide, dimensioning of the waveguide can be made according to:

x⋅=≤ 2limλλ , where � is the wavelength of the signal, �lim is the limit

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wavelength and x is the width of the waveguide.

HARDWARE SETUP

In the experimental setup a 1.5 meter waveguide is connected to one WLAN access point and eight WLAN clients. The access point is a Routerboard 133 with a R52 wireless card from Mikrotik running RouterOS 2.9.46 [4], [5]. The clients are Routerboards 112 with R52 running the same OS [6]. Each WLAN client is connected to a processor board which generates GOOSE traffic. The processor board triggers the client to send a GOOSE message according to a pre-set pattern. The setup is shown in Figure 3.

Figure 3: Waveguide Setup

WLAN SETUP

Typically, 802.11a equipment is optimized for usage in a LAN setup with typical operating range of 2-100m distance between access point and clients. It is also designed to handle rather high levels of RF interference originating both from other 802.11a systems and other radio equipment operating in the same frequency band. When using 802.11a in waveguides we create an environment with fundamentally different conditions. In the waveguide the distance between transmitter and receiver is usually rather short and there is virtually no interference originating from other radio equipment operating in the same frequency band. It is, therefore, important to keep the RF output power on the correct level. Since the channel losses are very small it is very easy to saturate the receivers and hence severely impair the system overall performance. Our experiments show that using -55 dBm as a lower limit and -35 dBm as an upper limit in the waveguide creates a good operating environment for the radio equipment. In addition to limiting the output power, the number of retransmissions must also be reduced in order to optimize waveguide performance. WLAN equipment typically has a high number of retries (the default value for the Routerboard cards is 15). The burst like loading conditions created in an IEC61850 installation communicating GOOSE

messages will create a high level of contention during a rather short period of time. This in combination with the rapid increase of the back-off time will occasionally create long latencies. Since GOOSE messages are repeated according to a specific scheme by the nodes generating them, there is already a mechanism handling lost packages in the IEC61850 system. GOOSE messages are of multicast type and will hence not be retransmitted on the route from access point to nodes [7], and the retransmission scheme makes sure that data eventually reaches each node in the system anyway. It is recommended to set the maximum numbers of hardware retransmission to a significantly lower number than 15 in order to optimize the system for GOOSE/Waveguide operation. Using four hardware retransmissions will make sure that the contention window does not grow too much, and takes advantage of the, in this case more effective GOOSE retransmission scheme.

ANALYSIS OF GOOSE PERFORMANCE

Taking the information above into account we can conclude that there are basically two main reasons for delays created for the transmitted data, in our case the GOOSE messages, namely:

• Retransmissions due to collisions in the shared media (air)

• Lost data or delays due to stack overflow or long queues

As can be seen there are two things that can happen to a data telegram in an Ethernet based communication system, it can be delayed or it can be lost. There are no guarantees whatsoever that a transmitted package actually will reach the receiver. This can be dealt with using acknowledge of received data (e.g. TCP/IP) or simply by retransmitting the data several times (as in the GOOSE case). In 802.11 data is actually retransmitted on MAC-level, should it be lost due to collisions in the air but N.B. this is only true for unicast-massages not for broadcast-messages. So if we derive the possible reasons for delayed and lost GOOSE-packages further we can find the following cases (refer Figure 4 for definitions):

1. Message delayed in path 1 due to busy media 2. Messages delayed in path 1 due to collision and

hence contention 3. Message delayed in AP due to queue of data 4. Message delayed in AP due to busy media 5. Message lost in path 2 due to collision or

interference(no retransmission at MAC level due to the message being of multicast-type)

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Figure 4: WLAN Traffic Flow

Theoretically, the usage of VLAN prioritization will only increase performance in case 3. Case 5 creates the worst delays since it is not until the next GOOSE retransmission that the message has a possibility to reach other nodes on the 802.11 network again. It can be seen from the information above that the GOOSE retransmission scheme is crucial to performance in a Waveguide installation based upon 802.11. In order to investigate the performance impact due to the different cases mentioned above the injected interfering traffic should have Virtual LAN (VLAN) prioritization which can be altered.

RESULTS

The experimental results for the waveguide are based on the settings given in Table 1. PARAMETER SETTING DESCRIPTION Number of nodes 8 Number of client

nodes Delay between transmissions

2 ms Delay time between each transmission

Number of repeats 10000 Number of test runs Delay between tests

5 s Time between each test run

Table 1: Test Case Settings

Only GOOSE messages were sent in the first test case. The result from this test is shown in Figure 5. The average latency is 4.1ms and only about 0.06% of the GOOSE messages take more than 10ms to reach their target.

Figure 5: Latency plot for Waveguide communication

The difference in the second test case is that interfering traffic is introduced. The level of interference is about 0.2 Mbit/s UDP broadcast IP traffic, consisting of a 512 byte packet transmitted every 20ms on top of the GOOSE traffic. VLAN priority tagging is used to give priority to the GOOSE messages (This is in accordance with the IEC61850 standard). The VLAN level for the GOOSE messages is seven and for the interfering traffic the corresponding level is four.

Figure 6: Latency plot for Waveguide communication including UDP traffic

The results from Figure 6 show that the impact from the interfering traffic is rather small. The average latency of 4.2ms can be compared to the average latency of 4.1ms in the case without interfering traffic. For comparison a test using seven nodes connected to an

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industrial Ethernet switch instead of the waveguide has been performed. In this test case the same amount of interfering traffic was introduced as in the previous test. The switch is a T208F0 from Ontime and the reason for using only seven nodes is that the switch only has eight channels and one has to be used for interfering traffic. The results shown in Figure 7 shows that the average latency is 1.4ms with a spread of less than 9ms.

Figure 7: Latency plot for switched Ethernet with interfering traffic

PILOT INSTALLATION

A pilot installation using Waveguide technology was commissioned in March 2008 at LyondellBasell Industries. The installed switchgear equipment consisting of six panels uses a Waveguide for communicating Supervisory Control and Data Acquisition (SCADA) information. Part of the pilot installation is shown In Figure 8. The access point (top left of the photo) is connected to the waveguide at the bottom of the picture.

Figure 8: Waveguide pilot installation

CONCLUSIONS AND FUTURE WORK

In this paper, the performance of a waveguide is studied. The increased average latency in the waveguide system, compared to a switched Ethernet system, is in the order of a few milliseconds. Tests running eight nodes in the same segment of the waveguide show that the average latency for a GOOSE message to reach another node in the system is less than 5ms. Current work is focused on optimizing the performance when running both GOOSE and SCADA traffic in the Waveguide. WLAN was considered in this paper; however other communication solutions, such as Ultra Wide Band (UWB), are also studied. REFERENCES [1] K. Scherrer, B. Deck, and A. Reimüller ``Data Pipeline'', ABB Review, Vol. 4, pp. 26--29, 2006. [2] D. Ebbinghaus and A. Reimüller ``IEC61850:"Ubertragungstechnik zur

Kommunikation in MS-Schaltanlagen'', etz, Vol. S1, pp. 68--72, 2006. [3] IEC 61850-8-1:2004-05 Communication networks

and systems in substations – Part 8-1: Specific Communication Service Mapping (SCSM) – Mappings to MMS (ISO 9506-1 and ISO 9506-2) and to ISO/IEC 8802-3. Genf/ Schweiz: Bureau Central de la Commission Electrotechnique Internationale (ISBN 2-8318-7425-4).

[4] Mikrotik RouterOS v2.9 Reference Manual, Rev.

3.40, 2007. [5] RouterBOARD 133 series User´s Manual, Rev.\ B,

2007. [6] RouterBOARD 111/112 series User´s Manual, Rev.\

A, 2006. [7] S. Min-Te, H. Lifei, A. Arora, and L. Ten-Hwang ``Reliable MAC layer multicast in IEEE 802.11

wireless networks'', Int conf on Parallel Processing 2002}, pp. 527--536, 2002.

[8] Dujovne Diego, Turletti Thierry “Multicast in 802.11

WLANs: An Experimental Study”. INRIA Research Report 5947

[9] Sun Min-Te et.al “Reliable MAC Layer Multicast in

IEEE 802.11 Wireless Networks”, Ohio State University 2002