Supervisory Control and Data Acquisition Experiment Using the Advanced Communications Technology Satellite Lessons Learned Authors
C. Emrich, R. Acosta, A. Kalu, D. Kifer, and W. Wilson Publication Number
FSEC-CR-1296-01 Copyright Copyright © Florida Solar Energy Center/University of Central Florida 1679 Clearlake Road, Cocoa, Florida 32922, USA (321) 638-1000 All rights reserved.
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SUPERVISORY CONTROL AND DATA ACQUISITION USING THE ADVANCED COMMUNICATIONS TECHNOLOGY SATELLITE: LESSONS LEARNED C. Emrich, W. Wilson, and G. Ventre Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, FL 32922-5703 Phone: (321) 638-1507, Fax: (321) 638-1010, E-mail:
[email protected] A. Kalu Savannah State University, Savannah, GA 31404 Phone: (912) 356-2282, Fax: (912) 356-2432, E-mail:
[email protected] R. Acosta and D. Kifer NASA Glenn Research Center, Cleveland, OH 44135 Phone: (216) 433-6640, Fax: (216) 433-6371, E-mail:
[email protected]
1. ABSTRACT Satellite Supervisory Control and Data Acquisition (SCADA) of a Photovoltaic (PV)/diesel hybrid system was tested using NASA’s Advanced Communication Technology Satellite (ACTS) and Ultra Small Aperture Terminal (USAT) ground stations. The setup consisted of a custom-designed PV/diesel hybrid system, located at the Florida Solar Energy Center (FSEC), which was controlled and monitored at a “remote” hub via Ka-band satellite link connecting two USATs in a SCADA arrangement. The robustness of the communications link was tested for remote monitoring of the health and performance of a PV/diesel hybrid system, and for investigating load control and battery charging strategies to maximize battery capacity and lifetime, and minimize loss of critical load probability. Baseline hardware performance test results demonstrated that continuous two-second data transfers can be accomplished under clear sky conditions with a failure rate of less than 1%. The delay introduced by the satellite (1/4 second) was transparent to synchronization of the satellite modem as well as to the PV/diesel hybrid and the control computer. End-to-end communications link recovery times were less than 36 seconds for loss of power and less than one second for loss of link. The system recovered by resuming operation without any manual intervention, which is important since the 4-decibel margin is not sufficient to prevent loss of the satellite link during moderate to heavy rain. The overall failure rate for twosecond data transfers during the 100+ hours of the SCADA experiment was 9%. This increased rate was attributed to precipitation along the USAT to ACTS slant path and to satellite tracking problems. Hybrid operations during loss of communications link continued seamlessly, but real-time monitoring was interrupted. For this sub-tropical region, the estimated amount of time that the signal fade due to precipitation will exceed the system margin is about 10%. These results suggest that data rates of 4800 bits per second and a link margin of 4 decibels with a ¼-Watt transmitter are sufficient for end-to-end operation in this SCADA application. The next phase of this project is a SCADA/training experiment. In preparation for this, the USAT ground stations have been upgraded with two one-Watt transmitters to increase system margin, and audio/video equipment has been added at both ends of the communications link. Baseline testing of the software and hardware is nearing completion. Delivery of the SCADA/training program is planned for May 2000. This paper will present results obtained during both the SCADA and SCADA/Training experiments. Lessons learned associated with implementation of these tasks will also be discussed.
2. INTRODUCTION The lack of electrical energy in the rural communities of developing countries is well known, as is the economic unfeasibility of providing much needed energy to these regions via electric grids. Renewable energy (RE) can provide an economic advantage over conventional forms in meeting some of these energy needs. The use of Supervisory Control and Data Acquisition, and distance education via satellite could enable experts at remote locations to provide technical assistance to local personnel while they acquire a measure of proficiency with a newly installed RE system through hands-on training programs using the same communications link. Upon full mastery of the technology, indigenous personnel could employ similar SCADA/training arrangements with their subsequent constellation of RE systems. The portability of the Ultra Small Aperture Terminal and the versatility of NASA’s Advanced Communications Technology Satellite, as well as the advantages of Ka band satellites, provide an opportunity for testing strategies to meet the energy challenges of rural communities in developing countries. Likewise, Central Florida’s subtropical climate provides an ideal environment for assessing the impact of frequent rain events with high rain rates on Ka-band signal strength. 3. EXPERIMENT DESCRIPTION The setup for the SCADA experiment consisted of a custom designed PV/diesel hybrid system located at the Florida Solar Energy Center and a Ka-band communications link using two 0.6 meter antenna USATs and ACTS in a SCADA arrangement. A satellite communications link was established between the control personal computer (PC) and the PV/diesel hybrid. To achieve this link the signal was converted from asynchronous RS232 to synchronous RS449 and back by a series of black boxes. The purpose of this conversion was to allow the computer and the hybrid, with their asynchronous RS232 input/output standard, to interface with the satellite modems, which require synchronous RS449. Figure 1 shows a schematic of the setup for the SCADA experiment. Actually, only one USAT had a transmitter (which transmitted at ¼ Watt for both the control PC and the hybrid). The other USAT was a receive-only unit. Continuous two-second data transfers were performed to determine data transfer failure rates. Key system parameters of the hybrid system were monitored and controlled in real time via satellite from a separate location at FSEC using custom-designed software on the control PC. The SCADA experiment was designed to test the robustness of the communications link for remote monitoring of the health and performance of a PV/diesel hybrid system, and for investigating load control and battery charging strategies to maximize battery capacity and lifetime, and minimize loss of critical load probability. For the SCADA/training experiment, the USAT ground stations located at the Florida Solar Energy Center were connected to satellite modems tuned to different frequencies, establishing two channels for full duplex Ka-band communications. A short training program on operation and maintenance of the system will be delivered while simultaneously monitoring and controlling the hybrid using the same satellite communications link. The trainees will include faculty and students from Savannah State University, and staff from FSEC. An interactive internet link will be used to allow participation in the training session by an audience not connected by satellite. Figure 2 includes a schematic of the setup for the SCADA/training experiment. The internet link is not shown in this figure, but will be achieved using a software program such as Net Meeting. Preliminary testing of this setup is in progress and the actual demonstration experiment is planned for May 2000. The training program is expected to last between 1 to 1 1/2 hours. Since the training will be interactive,
the format will be kept flexible and rather informal. The objective is to simulate the type of hands-on training that might take place in a real-life scenario, with the experts in Florida and the trainees with their hybrid system in remote village of some developing country.
LOCAL USAT
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Figure 1 SCADA experiment setup
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Figure 2 SCADA/training setup
PV/Diesel Hybrid
3. RESULTS Results for the SCADA experiment suggest the delay introduced by the satellite (1/4 second) is transparent to synchronization of the satellite modem as well as to the PV-hybrid and control PC. Data rates of 4800 bits per second and a link margin of 4 decibels with a ¼Watt transmitter proved sufficient for end-to-end operation in this application. Under clear sky conditions during hardware checkout, two-second data transfers failed less than 1% of the time. However, the failure rate for the actual SCADA experiment was 9%. Two-second data transfers failed at rate of 8.8% on days when local precipitation (measured at ground station via tipping bucket) occurred during data collection period. Curiously, the error rate was 9.9% for days when there was no measurable rainfall at the ground station site during the data collection period. These were most likely due to satellite tracking problems.
Data Transfer Statistics (9/8/99) 1.3 .10 1.25 .104 4
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Figure 3 Unsuccessful data transfers due to rainfade Data transfer failures that were not consecutive were due to the internal hybrid controller giving priority to routine collection and archival functions, which are automatic. These were predictable and accounted for less than 1% of the failures. Episodes of two or more consecutive unsuccessful data transfers were analyzed to determine the etiology. To identify failures due to rainfade, failure times were compared with time-of-tip data from a tipping bucket rain gage located at the USAT site. Figure 3 shows an example of an increased number of data transfer failures due to loss of communications link, which correlated well with the timing of a local rain event recorded by the rain gage.
The highest measured failure rate was 21.7% during a 6.5-hour data collection period. The satellite link was lost for a 1.331-hour period. While most outages correlated well with measured local rainfall, this one occurred two hours prior to the onset of rain in the area, as shown in Figure 4. The cause of this outage may have been due to precipitation along the USAT to ACTS slant path far enough away to escape detection by the local rain gage. However, similar outages occurred on days when no precipitation was present along the slant path, as shown in Figure 5. These outages were most likely due to satellite tracking errors. Data Transfer Statistics (10/4/99) 1.3 .104 1.25 .104 1.2 .104 1.15 .10 4 1.1 .10 4
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Figure 4 Unsuccessful data transfer precede rain event The SCADA experiment was conducted on a noninterferring basis with an antenna wetting experiment, which used the same equipment. Satellite time for the SCADA experiment was usually during business hours Monday through Friday, but not necessarily the same time each day. Satellite time was dedicated to the antenna wetting experiment the rest of the time. Due to ACTS’ inclined orbit, tracking equipment and software were used to maximize signal strength during the SCADA experiment. However, the antennae were parked for the antenna wetting experiment. This caused problems because the tracking program automatically updated a tracking table based on peak signal strength. This tracking table was then used to find the satellite. Because of the intermittent scheduling of the SCADA experiment, the tracking table was never completely updated causing the trackers to lose the satellite from time to time. Tracking errors most likely were the cause of failed data transfer rates above 1% on days with no measurable precipitation. On days with rain, a threshold rain rate (above which
loss of satellite link was certain) could not be established since this varied widely from one data collection period to the next. Data requests during rain events demonstrated that precipitation along the USAT to ACTS slant path does not affect the operation of the PV/diesel hybrid but may result in temporary loss of communications link and consequently failed data transfers. The hybrid also continued routine operation during loss of link due to tracking problems, as shown in Figure 6.
Data Transfer Statistics (9/29/99) 1.8 .10
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Figure 5 Loss of communications link with no rain Due to the Central Florida location and the time of year data was collected (summer), lightning and hurricanes also presented challenges during this experiment. Protection was added after several components sustained damage from lightning. In addition, the USATs had to be dismantled and removed from their rooftop locations on several occasions to prevent damage from approaching hurricanes (Dennis, Floyd, and Irene). Control strategies to maximize battery capacity and minimize loss of load were also investigated during this experiment. The results of this investigation will be deferred for a photovoltaic/renewable energy focused audience. As of the writing of this paper, preliminary baseline hardware and software tests of the SCADA/training arrangement were nearing completion. Prior to upgrade, rates of ¼ T1 (384 Kbps) with two channels for full duplex were achieved using a ¼-Watt transmitter. However, the system margin with this setup was less than 4 decibels. Upgrading the ground stations with two one-Watt transmitters provided
a system margin of slightly greater than 10 decibels. The training program consists of a short introduction, an overview slide presentation, a real time video on location showing the different features and compartments of hybrid, a demonstration of the monitoring and control software, and an opportunity for students at remote locations to control the hybrid through software sharing. Preliminary testing showed that audio and video quality of this program was sufficient at these data rates with present equipment.
Data Transfer Statistics (9/29/99) 60 55 50 45 40 35
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Figure 6 Interrupted monitoring due to loss of link
4. CONCLUSIONS The PV/diesel hybrid is designed to operate autonomously for several weeks before maintenance is required. As a result, hybrid performance is seamless to loss of communications link under normal operation, making a low system margin totally acceptable for this application. However, unanticipated problems such as failure of the diesel engine can lead to battery discharge and automatic load shedding. In a real-life scenario this incident could result in total loss of power to a village. So in the absence of local trained personnel, SCADA via satellite is necessary to detect such problems and prevent untoward situations by using control strategies such as load shifting. For a sub-tropical region such as Central Florida, the estimated percentage of time that the signal fade due to rain will exceed the 4-decibel margin is about 10%. The overall data transfer failure rate for the SCADA experiment was 9%, including rainfade and satellite tracking problems. Even frequent losses of the communications link for extended periods (the maximum being slightly longer than 82 consecutive minutes) were not a problem in this particular application due of the forward-thinking design of the hybrid system. Adding audio/video equipment to this setup provides for interactive, ongoing, and opportunistic
training, as well as an ideal method for trouble-shooting and problem solving. While 4800 bits per second is sufficient for SCADA, ¼ T1 (384 Kbps) was necessary for quality audio/video delivery. If training sessions were very informal and short in duration perhaps slower data rates would be tolerable. Similarly, a system margin of 4 decibels might be adequate if frequent link losses could be tolerated; however, upgrading the ground stations from ¼-Watt to 1-Watt transmitters would increase the system margin (in this case by greater than 6 decibels), thereby improving the robustness of the communications link. Future plans include modifications to the setup and hybrid design based on lessons learned. After testing of these modifications, implementation and evaluation of this type of arrangement in a remote village in a developing country would be the next logical step. Just as frequent brief but intense rain events, hurricanes, and lightning provided lessons for this project, the climate of the targeted location would need to be factored into the system design to minimize unanticipated challenges.
