Testing performances of IEC 61850 GOOSE messages
One of the frequent requests for relay protection devices is support for the IEC 61850 standard. As part of the standard special messages are also planned for a quick exchange of information between the IEDs – so called GOOSE (Generic Object-Oriented SubStation Event). These are mainly trip, interlocking, breaker failure and similar signals. Time of transfer of these signals is critical, its delay may cause undesirable blackouts or damage to equipment.
In this paper we explore which software architecture is most appropriate to achieve the required performance. Software for sending / receiving GOOSE messages can be located in real time (RT) or user space of the operating system. We will consider the RT and user space implementations of two different microprocessor architecture – ARM9 and PowerPC.
Performance degradation can occur from 2 reasons:
- Protection function has the highest priority. At least 500 μs during each millisecond GOOSE thread will be deprived of CPU time.
- In the case of pure user-space implementation, the operating system will interrupt GOOSE task in a completely nondeterministic way.
User Space Test
To test the performance of GOOSE messages in user space, the environment is developed based on the ARM7 architecture:
- ARM7 with integrated Ethernet for sending, receiving and time-stamping of messages.
- The PC application for setting parameters and collecting the results.
- Figure 1 Test configuration for user space test
The essence of the test is as follows: ARM7 board launches a series of messages and records the time for each outgoing message. ARM9 and PowerPC boards are set up to immediately respond to received GOOSE messages with identical message and with the same serial number.
ARM7 registers the answer and uses the serial number to match with the original message and calculates the elapsed time.
- Figure 2 Analysis time
On the figure above we can see the analysis of time. A and B are negligible. Due to the nature of the test 2C + D can be accurately measured but we can’t know exactly the amounts of C and D are respectively. But ultimately this is not important from the point of standards. Let’s look at test results. ARM7 board launches a series of GOOSE messages with pause of 100ms. Results are measured and displayed in Excel.
To make it more realistic result overcurrent protection was turned on. Y axis shows the time in milliseconds and the X axis shows GOOSE messages.
- Figure 3 ARM9 100ms (X axis – number of messages, the Y axis the time of transfer)
We see that during 20 seconds response time oscillates around 2 milliseconds. The next step was to involve several protection functions. It is expected that the GOOSE performance will drop.
This is actually happening as we see in the following figure:
- Figure 4 ARM9 100ms, 700μs (X axis – number of messages, the Y axis the time of transfer)
The time now oscillates about 7 ms. Although it is expected that the performance will decline, it is still above expectations. 7 milliseconds is still enough for some applications. These are the results from the ARM9 platform. PowerPC platform has proved to be something better, because it has almost 2 times more processing power. On the next 2 images we see the results.
- Figure 5 PowerPC 100ms (X axis – number of messages, the Y axis the time of transfer)
- Figure 6 PowerPC 100ms, 700μs (X axis – number of messages, the Y axis the time of transfer)
In a small load time oscillates around 0.8 ms and at most about 2.5 ms. The measured times are suitable for a solid range of applications. Unfortunately, these times are only valid if the GOOSE task is only active task. In the case of other tasks – for example, disturbance recorder, event recorder, embedded web server, IEC 61850 MMS server and so on … transfer time become unpredictable and can go up to 80ms, which is of course unacceptable.
Real Time Test
- Figure 7 Test configuration for real-time test
Although the real time GOOSE is something more difficult to implement, it offers some significant advantages as we shall see. Test environment for real-time is significantly different. The network analyzer was used. The program is available as a free download from the Internet (1). The essence of the test is as follows: protection relays is configured to receive GOOSE messages from a laptop computer and to immediately respond with the same value in the dataset. When analyzing a series of messages network analyzer will come to the moment when the relay and laptops are sending an identical value.
The time between the moment when the laptop starts broadcasting and the moment the relay begins to broadcast the same value as the laptop is the required time.
In the following figure we can see the results displayed in the network analyzer.
- Figure 8 Ethereal Network Analyzer
- Figure 9 Goose series with a time of receipt of messages, network addresses and protocol label
Message number 42 is from a laptop, a message 43 from relay protection. If you subtract the time of receipt: 3.757 to 3.753 = 4msec. When measurements are repeated result oscillates around 4ms. The reason for this is that the task for sending and receiving is set to be run every 2 milliseconds.
Conclusion
At first glance, real-time and user space implementation operates in a similar timeframe. But there is a substantial difference. GOOSE RT implementation task may share the processor with an arbitrary number of other task such as the disturbance recorder and others. This architecture greatly reduces the ultimate cost of the device and gives the user more functionality. Otherwise the GOOSE software would have to reside on separate hardware.
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AUTHOR OF ARTICLE:
Veljko Milisavljević | ABS Control Systems, Serbia
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MOXA | Video Surveillance in Power Substations
Power substations play a crucial role in delivering electricity to consumers by converting transmission voltage to the lower voltage used in homes and businesses. Since power plants are often located far from the population centers they serve, electricity needs to be transmitted across long distances at a higher voltage.
Power lines deliver electricity from the plant to the power substations where it is converted before being distributed to the local community. It is therefore imperative that power substations are constantly monitored for safety and maintenance as they are often located near or in a populated area. This white paper explains the benefits of real-time video monitoring and IP video technology, as well as factors to consider in deploying an optimal IP video surveillance system for a power substation.
Video Surveillance Benefits
Since power substations are widely distributed and unmanned, remote monitoring is extremely crucial. Real-time video surveillance of power substations offers automatic monitoring and control capabilities in addition to enhancing remote monitoring applications with visual management. These capabilities not only save management costs for manpower, but also realize complete network automation.
Supervisory Control and Data Acquisition (SCADA) systems, which are already deployed in power substations to provide data about the system’s status, can be easily integrated with video surveillance technology. By installing a real-time video monitoring system at power substations, system administrators are able to receive visual data to complement the raw SCADA data. Real-time video monitoring can help ensure normal operations for power equipment, protect against intrusion and tampering by unauthorized personnel, and prevent accidents. For example, intruders, physical obstructions, or smoke indicating a fire can be seen via video so engineers no longer need to visit the site in-person each time to diagnose an anomaly, saving both time and costs.
Remote video surveillance systems can play an important role in monitoring equipment, detecting intruders, and responding to emergency situations. For example, video surveillance can be used to monitor the appearance of the power transformer and relay, fueling and flammable equipment, and the status of the isolation switch. Video surveillance can also monitor the security situation inside and outside the substation by detecting intruders through sound and visual monitoring. In addition, video surveillance can be integrated with the alarm system and RTU (remote terminal unit) over a SCADA system to provide real-time visual information to prevent accidents and assist emergency response personnel in the event of a fire.
Why IP Video?
In the past, video surveillance systems such as CCTV networks relied upon analog video cameras. Due to advances in video digitization and compression technologies, high quality digital video images can now be sent over Ethernet TCP/IP networks. By using such devices, system integrators can easily integrate video surveillance applications into their SCADA system. As a result, Internet Protocol (IP) video technology is the current trend in video surveillance systems. The benefits of IP video surveillance include:
One Network - Using the existing IP network saves cabling costs and increases installation flexibility, especially for widely distributed substations. Ethernet TCP/IP networks can accommodate a variety of I/O monitoring and control devices in addition to transmitting data, video, voice, and even power (PoE) over a single network.
One System - Integration with SCADA or alarm systems (such as fire, intrusion, etc.) increases monitoring efficiency and creates an event-driven video surveillance system. This means the video images can be displayed and recorded and real-time responses can be received when an event or alarm occurs.
Constructing an Optimal IP Video Surveillance System
Given the critical role played by power substations in our daily lives, it is important for the IP video solution to be well-designed to ensure that the video surveillance system works properly. System integrators should consider factors such as applicability, reliability, integration, and user-friendliness in order to construct an optimal IP video surveillance system.
Applicability – System integrators need to consider video requirements such as image viewing, recording, and analysis, as well as interoperability with other systems (such as SCADA, Access Control, etc.) when deploying an IP video surveillance system. They also need to know how many cameras are required for the system and whether IP cameras or video encoders are suitable for the application. Network transmission factors such as bandwidth, multicast, and IGMP requirements, and whether the project requires a single network or separate networks for data and video are also important. Central management concerns, including system resources (PCs, servers, cost, etc.), software requirements (pure video or video integrated with another system), storage capability and database management, and whether or not a decoder is required, should also be considered.
Reliability - Since video monitoring is used to ensure safety and security in remote and disperse locations, reliability is a key factor in designing an optimal IP video surveillance system. Factors for reliability include surge protection and fiber transmission to reduce electromagnetic interference. Redundancy, high MTBF (meantime between failures) and IP protection are also important factors to consider for optimal reliability.
Integration - System integrators should consider integrating video surveillance into the central management system, as well as other systems, including SCADA/HMI, remote monitoring, and access control. This not only reduces cabling and network installation costs, but also makes central management and control easier to handle for system administrators. Interoperation with other devices for event-driven video monitoring is another benefit. For example, the system can begin recording video once a card reader or sensor is activated.
User-friendliness - IP video involves new applications and technologies that power system administrators need to learn. For this reason, it is recommended that system integrators choose ready-to-use hardware and software solutions to reduce the time needed to set up an IP video surveillance system. Not only does this simplify the system integrator’s task, but it will also be easier for system administrators to learn and use.
Video Surveillance in Power Substations
VPort Solutions
Moxa’s VPort industrial video networking solutions include video encoders and decoders, IP cameras, and IP video surveillance software designed for mission-critical video surveillance applications. Since most mission-critical application environments are demanding, the rugged design features of Moxa’s VPort solutions are particularly suitable for these kinds of applications.
Video servers – Digital video images require large data files, so video compression (reducing the quantity of data used to represent video images) is required for transmission and storage. Video servers include encoders and decoders. Encoders are used to convert analog video images from cameras into an easy to transfer digital format such as MJPEG or MPEG4. Decoders are used to convert images from compressed formats (MJPEG or MPEG4) back into analog for use with legacy monitors or displays.
IP cameras – IP cameras bypass the need for video encoders because the images are automatically encoded into a digital format (MJPEG or MPEG4) by the camera itself, and are easily transferred via Ethernet/Internet. Moxa’s VPort series of IP camera offers a wide operating temperature range of -40 to 50°C without the need for a fan or heater, IP66-rating for rain and dust protection, one camera lens for both day and night use, up to 30 frames per second at 720 x 480 resolution, and direct-wired power input and PoE (power over Ethernet) for power redundancy. Moxa’s industrial-grade video servers offer 12/24 VDC or 24 VAC redundant power inputs, DIN-Rail mounting and panel mounting accessories, IP30 protection, -40 to 75°C operating temperature range for T models, and RJ45 or fiber optic Ethernet ports.
Software – Moxa’s SoftDVR Surveillance Software, which includes SoftDVR Lite (4-ch) and SoftDVR Pro (16-ch), is designed for IP-based video surveillance systems. The client/server-based network infrastructure makes it easy to build a user-friendly video surveillance system. SoftDVR offers multi-screen viewing, event-driven recording, easy to use search and playback, data storage to network hard disks, scheduling feature for recording and alarm activation, and remote access by web browser.
SDK – Most video surveillance systems require customized video management functions, or must be integrated with other applications such as SCADA, access control systems, and fire alarms. For this reason, a user-friendly SDK (software development kit) is a good tool to have available for building customized video management systems. Moxa’s VPort SDK, which includes CGI Commands, ActiveX, and a C library, is available free of charge to system integrators and third-party software developers. Learning to use the Moxa VPort SDK is easy, and detailed documentation and sample code is provided for quick reference.
Summary
Transmitting video, voice, and data simultaneously over Ethernet/Internet is becoming a standard feature due to the ever-increasing popularity of IP networks. Versatile and advanced video digitizing and compression technologies, such as MJPEG and MPEG4, are also making it possible to migrate analog CCTV surveillance systems to IP-based platforms.
Power substations play an instrumental role in delivering electricity from power plants to end-users, so managing and ensuring the safety and security of these installations through an optimal video surveillance system is imperative. Since power substations are unmanned and widely distributed installations, video surveillance grants system administrators visual management capabilities in addition to data management provided by existing central control systems that only provide raw quantitative data. The versatility of IP video technology and its ability to be integrated with existing central control systems make it an attractive option for remote video monitoring in power substation applications.
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SOURCE: MOXA, Harry Hsiao, Product Manager at Moxa; www.moxa.com; harry.hsiao@moxa.com
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