Philip F. Keebler, EMC Group, Electric Power Research Institute, Knoxville, Tennessee USA; Stephen Berger, TEM Consulting, LLC, Georgetown, Texas USA
Nuclear power plants (NPPs) in the United States have been undergoing upgrades from analog instrumentation and control (I&C) equipment to digital equipment over the past several years. Upgrades have been occurring on the plant floor for systems such as generator controls, turbine supervisory controls, and chiller controls as well as control systems in the plant control room. Plant events involving electromagnetic interference (EMI) continue to occur with existing analog equipment and with some digital equipment. Because of the increased focus on safety and efforts to eliminate plant events, electromagnetic compatibility (EMC) is still a growing concern. The migration from analog I&C equipment to digital I&C equipment warrants the need to investigate the EMC characteristics of changing electromagnetic environments. These characteristics have been identified through Electric Power Research Institute (EPRI) research by conducting long-term emissions measurements before analog I&C systems are removed, and then again after new digital I&C systems were installed and operational. This paper presents the first-of-its-kind analysis of a complete set of radiated emissions measurement data from 100 Hertz to 6 GHz as part of an upgrade inside a control room to replace an analog control system with a digital control system for one operating unit of a nuclear plant in the United States.
Keywords- Digital upgrade, control room, radiated emissions, electromagnetic interference
INTRODUCTION
Electromagnetic characterization of spaces where electrical and electronic equipment must coexist is a necessary function of EMC for reasons discussed below. These spaces include areas inside and outside facilities that serve residential, commercial, industrial, and specialty needs such as healthcare and power plants. Operations of equipment in these spaces create the overall electromagnetic environment (EME).
Diverse Equipment Designs and Design Changes
About the only commonality between electronic equipment in today’s modern world, including digital I&C equipment used to upgrade older analog I&C equipment in existing power plants, is the need for equipment to use AC or DC power to operate. With rapidly changing semiconductor technologies, the growing use of new digital devices, and the proliferation of software development and its embedded use to enhance the I&C functions of NPPs, I&C equipment manufacturers are developing new types of I&C equipment. The need for smaller more efficient equipment with faster processing speeds and increased network connectivity with higher reliability causes an increase in radiated and conducted emissions. Although filtering and shielding technologies are getting better, manufacturers still only use the amount of filtering and shielding needed to pass EMC regulatory tests. Designers are not keeping pace with the new mitigation technologies for controlling emissions. I&C designs are moving faster into digital than EMC mitigation devices are being used to control emissions generated by the digital devices. Regardless of which type of electronic device is brought into a plant, one can rest assured that the plant’s EME will include its emissions characteristics. Moreover, emissions characteristics are more additive than subtractive, resulting in cumulative emissions increases over time as existing power plants continue to install new equipment. Manufacturers are focused on producing equipment designs that meet existing critical US Nuclear Regulatory Commission (NRC) requirements. In some cases where NRC requirements for digital I&C equipment have not yet been developed or are not yet mature, manufacturers are working with plant engineers, the NRC and EPRI to develop new requirements.
Changes in Equipment Shielding Characteristics
Shielding provides a two-way function for EMC performance—helping to protect equipment from external emissions (e.g., from cell phones and walkie-talkie radios) and helping to reduce emissions generated inside equipment (e.g., from power supplies and microprocessors). Shielding manufacturers and users have no formal method of determining the shielding effectiveness of shields smaller than a two-meter cube. Thus, small shields that are used in portable radio devices and digital I&C equipment, for example, may not be performing as manufacturers expect. However, the Institute of Electronic and Electrical Engineers (IEEE) is presently sponsoring a project (IEEE P299.1) to develop a new standard describing new test methods for measuring the shielding effectiveness of shields having dimensions between 0.1 and 2 meters. This standard will be published in 2011.
Changes in Design and Use of Portable Radio Devices
Rapid development of sophisticated devices (e.g., cell phones, wireless headsets, electronic book readers, etc.) has increased. Networks (i.e., the tower) can initiate changes in radio power to ensure connectivity resulting in increased power levels. The increased use of portable radios and radio applications results in the increased difficulty in controlling use.
Changes in Definition, Use, and Management of Electromagnetic (Radio) Spectrum
Increased use of stationary and portable electronic equipment combined with additional radio and television (digital) broadcast towers and wireless services results in more complex spectrum. Increased use of high-speed data communications in NPPs will also impact the spectrum. Changes in the use and management of spectrum will be seen in the future with new rulings by the US Federal Communications Commission (FCC). Changes in other countries may also occur that will affect use of the spectrum and it energy in NPPs abroad. Composite effects of each additive electromagnetic energy source needs to be identified as NPPs continue to change before NPPs reach a plateau where new EMI problems begin to surface.
Use of Spectral Data
It is a reasonable, standardized, and customary practice to collect spectral data from EMEs, especially when industries can report that EMI problems continue to occur, present serious plant operations, and that equipment environments are changing. In the NPP industry, this is a two-fold problem. First, in existing plants, digital upgrade projects continue in growing numbers as plants meet planned needs and identify new needs to replace older analog I&C equipment. For various reasons, some plants plan and request limited-scope surveys at the point-of-installation (POI). Surveys are carried out to gain additional knowledge regarding the EME prior to the upgrade and how the installation of the new equipment affected the EME. Of the surveys that EPRI has carried out between 2001 and 2010, unexpected knowledge regarding the EME was always learned after the survey. Utility customers reported after their review with the NRC regarding their digital upgrade projects that the initiative taken to do the survey and the information gained from doing it were positive steps in helping the plant, the NRC, and the NPP industry to understand more about EMC concerns and help achieve enhanced EMC for digital upgrade projects.
Secondly, utility engineers engaged in the design of advanced NPPs have expressed the importance of having POI surveys carried out prior to actually constructing new advanced plants. One might ask, “How can this be done?” As part of the design process, a pre-operational demonstration is built for the digital I&C equipment planned for use in new plants. Survey activities can be carried out in these areas for each utility planning an advanced plant. Measurements to characterize the low-frequency radiated magnetic fields and low- and high-frequency radiated electric fields can be made. Conducted emissions measurements of low- and high-frequency can also be made. In fact, there is technical benefit to making these measurements in these areas away from the cluttered EMEs of advanced plants after they are built. Data from such measurements will be useful in the development of an emissions analysis database and can be used to compare to the emissions captured during EMC certification of digital I&C equipment, emissions from analog I&C equipment, from historical surveys in existing plants, recent surveys in existing plants and emissions captured when advanced plants are completed as well as emissions captured during an EMI investigation.
ABOUT THE ORIGINAL SURVEY PROJECT FOR DCS UPGRADE
As a part of the digital control system (DCS) upgrade program for Units 1 and 2, a major US nuclear power plant requested that a survey for radiated magnetic and electric fields be conducted in three areas: 1) Control Room – near the system cabinets in the control room where the existing analog control system is to be retrofitted with the new digital control system for Units 1 and 2, 2) the Operator Assist Computer (OAC) Computer Room area for Units 1 and 2, and 3) the Testing and Training Facility (TTF) Facility where the DCS was set up for testing. A survey plan was designed to investigate the radiated EME in each areas. The investigation was carried out by conducting a partial EMC survey measuring the radiated emissions for Unit 1 and 2 for electric fields from 10 kHz to 6 GHz and for magnetic fields from 20 Hz to 100 kHz with the analog control system in place and operational. A full EMC survey would entail measuring both radiated and conducted emissions. If requested as a part of the survey, conducted emissions could have been measured along power and data cables on the existing analog control system. This in situ study on a DCS is the first of its kind. Only the electric field emissions from 10 kHz to 6 GHz are reported in this paper.
Once the DCS was set up for testing and training in the TTF facility, a second visit was made to the site. The same groups of measurements were made but with the DCS mounted only in wooden racks without any metallic system cabinets in place. (These measurements are not provided here.) After the DCS was installed and operational, the next visit was made to the site where emissions measurements (discussed in this paper) were again made in the control room at the same antenna positions. Measurements were also taken with selected system cabinet doors open for comparison but are also not included in this paper. A new automated emissions measurement system, developed by EPRI was used to capture the emissions data and is further described in Section III. B.
MEASUREMENT METHODS FOR COLLECTING RADIATED EMISSIONS DATA
The NRC NUREG 1.180 (Rev. 1) 2003 and the EPRI TR-102323 (Rev. 3) 2004 Documents
The document, “Guidelines for Evaluating Electromagnetic and Radio-Frequency Interference in Safety-Related Instrumentation and Control Systems”, U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide, NUREG 1.180 (October 2003) Rev. 1 was developed and published by the NRC. The purpose of this document is “to provide guidance to licensees and applicants on additional methods acceptable to the NRC staff for complying with the NRC’s regulations on design, installation, and testing practices for addressing the effects of electromagnetic and radio-frequency interference (EMI/RFI) and power surges on safety-related instrumentation and control (I&C) systems.” This guidance document focuses heavily on acceptable test methods to measure emissions generated by safety-related I&C equipment and to determine its immunity to man-made emissions and disturbances.
The survey presented in this article was not conducted to provide any guidance as to where the system cabinets or the DCS in the cabinets should be located in the control room as that information was already pre-determined by the customer as part of their upgrade program for the plant’s control system. This survey was conducted to determine if any of the POI areas (without and with the DCS installed) have emissions characteristics that violate specific emissions envelopes currently in use by the NPP industry. These include the bounded envelope for plant emissions limits defined in the EPRI TR-102323-2004 (Rev 3) guidance document, “Guidelines for Electromagnetic Interference Testing in Power Plants” and the susceptibility line at 140 dBμV/m (10 V/m) defined in the NUREG 1.180 (Rev 1). These limits lines are included in the radiated emissions graphs presented later in this article for reference.
The NUREG 1.180 was also carefully reviewed along with the appropriate emissions measurement procedures included in MIL-STD-461E and the IEEE 473-1985 (R1991), “IEEE Recommended Practice for an Electromagnetic Site Survey (10 kHz to 10 GHz).” In addition, the research, data, and recommendations developed in published in EPRI TR-102323 were also carefully reviewed before this survey was carried out. Before the survey was conducted, two applicable survey methods—one based on MIL-STD-461E and the other based on IEEE 473—were reviewed. (For a comparative discussion on these methods, please see the article, “Measuring and managing electromagnetic interference: selecting the right antenna for your E3 program” which appeared in ITEM’s EMC Directory and Design Guide 2006, pp. 36-51.)
In an effort to closely characterize the location area of interest in the Control Room of this major US nuclear power plant, the following EMC measurement equipment was used: two 461E antennae—one broadband discone antenna with a frequency range 100 Hz to 1 GHz for radiated electric field measurements and one large loop magnetic field antenna with a frequency range 20 Hz to 5 MHz for radiated magnetic field measurements, one mini directional antenna with a frequency range 1 GHz to 6 GHz for radiated electric fields above 1 GHz, and two measurement methods were employed. The use of a single broadband discone antenna was applied with the use of an automated emissions measurement system as a more appropriate technique to improve the measurement process for high-frequency radiated electric fields. The IEEE 473 method was also attractive given the use of an automated emissions measurement system discussed below in the next section of this paper.
Emissions Measurement and Data Storage System Used
The traditional measurement system used for conducting surveys in the past has been the spectrum analyzer with minimal on-board data storage. Although spectrum analyzers have continued to develop over the years to provide for hundreds of on-board functions necessary for radio and EMC engineering and spectral analysis, little has been done regarding their ability to program long-term scans for surveys and to provide for large amounts of data storage. Limitations associated with the use of a traditional spectrum analyzer inclu
- Inability to program long-term cycling emissions tests across multiple frequency ranges
- Difficulty in capturing enough sweeps to properly represent the needed characteristics of an EME without having to dedicate a large number of man hours at the site
- Difficulty in capturing emissions sweeps associated with transients produced by the operation of devices such as relays, solenoids, valves, etc.
- Lack of proper data storage space on board the analyzer to store data from sweeps
- Inability to record sweeps in real-time and play them back on the screen if a review of emissions data is needed
- Difficulty associated with conducting mathematical operations on a limited set of emissions data to determine characteristics associated with a long-term recording of sweeps to support emissions analysis
To address the limitations listed above and several others, EPRI developed an automated emissions measurement system. This system utilizes a custom written program supporting a series of algorithms placed on a laptop computer that is interfaced to a spectrum analyzer through the IEEE 488 buss. Once activated, the computer program takes over the operation of the analyzer, allowing the EMI investigator to program exactly how the survey should be carried out. A total survey time of a few minutes up to a week can be selected. Once the survey emissions tests are simply programmed into the computer, the investigator clicks the “Start” button, closes up the access panel, locks the cabinet door, and walks away. The programmability and flexibility of this system allows the EMI investigator to set up emissions tests using a customer graphical user interface and determine when those tests would start and stop. The EMI investigator can also specify how much time would be spent on a specific frequency band and if emissions above a certain amplitude should be ignored among other custom settings. The system program also contains a data analysis package, which allows the investigator to conduct statistical analyses on the data, capture any trace or set of traces, and replace any range of traces or the whole data record upon command. Histogram analyses can also be carried out on the recorded data.
The system was built specifically for conducting surveys in critical areas where the location of emissions sources is unknown, where sources of transient emissions may be present and could cause severe malfunction of critical electronic equipment, where increasing the statistical confidence of the data would further improve the validity of the survey data, and where antenna size could possibly place constraints on the survey process thus limiting the amount of data collected. This system has already been used in other critical facilities including hospitals and communications facilities, and to date collected emissions data for more than ten digital upgrade projects in NPPs. Data gathered during this survey process furthered the understanding of the EMC for the DCS project at this major US nuclear power plant.
This automated system continues to be used to conduct POI surveys in NPPs and other types of facilities where surveys are needed or where EMI problems persist. One of the primary benefits of using this system is the permanent data record of emissions traces that the system creates when a survey or set of emissions measurements is carried out. This type of emissions record keeping will be beneficial when EPRI develops an on-line emissions database. Such a database can provide researchers and customers with access to historical and recent emissions data. Data from past surveys may even be converted to digital data which can be uploaded to the database.
MEASUREMENT DATA
Antenna Positions
Figure 1 illustrates the location of the three antenna positions near the system cabinets that now contain the new digital control system. These same cabinets previously contained the analog control system. All three antennae were used at these positions during the emissions measurements.
Figure 1. Location of antenna positions adjacent to system cabinets for plant control system.
High-Frequency Radiated Emissions Data – Electric Fields: 10 kHz – 1 GHz
1) Antenna Position 1
Figure 2 illustrates the final radiated emissions trace (i.e., the maxima of each measurement point in this frequency band occurring among several thousand traces during the collection of data at this antenna position) for electric fields taken at Antenna Position 1 from 10 kHz to 1 GHz adjacent to one of the system cabinets for the plant control system. Figure 1 contains the data for both the analog control system (green trace) and the digital control system (blue trace). From the trace, one can see that a few characteristics of the analog control are that it peaks at 1.34 MHz at 99.2 dBμV/m and at the high-frequency end at 928 MHz at 76.6 dBμV/m.
Figure 2. Radiated electric filed spectra, 10 kHz to 1 GHz, antenna position 1 in control room (Unit 1).
The blue trace from digital control system has a similar signature starting from 10 kHz but lower amplitude and does not contain the 1.34 MHz peak. From 2.31 to 3.51 MHz, the radiated energy from the DCS is higher than that of the analog control system (ACS). From 6.71 MHz out to 1 GHz, the radiated energy from the DCS is just about always higher than that of the ACS. There are two distinctive peaks that are present on the DCS trace, which are not present on the ACS trace. These are at 468 MHz (71.6 dBμV/m) and 826 MHz (94.5 dBμV/m). One of the peaks at the higher frequency area at 928 MHz peaked at 113.5 dBμV/m, which is 36.9 dBμV/m higher when the DCS system was installed.
Two limit lines are placed on the plot as well. One is the 140 dBμV/m limit line (red line)—a susceptibility limit line defined in NUREG 1.180 (Rev 1) and also in EPRI TR-102323. The second limit line (yellow line) is the highest composite plant emissions envelope limit, originally defined in EPRI TR-102323 (Rev 1) in 1997. While there is more than an 8 dB safety margin between the peak of either trace and the 140 dBμV/m limit line, one will notice that the emissions from the DCS equipment at 928 MHz are near the allowable plant emissions limit line.
Two other limits are also placed on the graph of Figure 2. These are equipment emissions limit lines. One is the limit line defined in NURED 1.180. The other is also an equipment emissions limit line defined in EPRI TR-102323 (Rev. 3). Although these limit lines are intended to determine if the emissions from a single piece of equipment or system are too high, the emissions from both the ACS and the DCS equipment do exceed these limit lines.
2) Antenna Position 2
Figure 3 illustrates the final radiated emissions trace for electric fields taken at Antenna Position 2 from 10 kHz to 1 GHz adjacent to one of the system cabinets for the plant control system. This trace contains the data for both the analog control system (red trace) and the digital control system (blue trace). From Figure 2, one can see that a few characteristics of the analog control are that it peaks at 1.04 MHz at 99.3 dBμV/m and again at 4.55 MHz at 88.5 dBμV/m. Again, these two traces cross the NUREG 1.180 and EPRI TR-102323 equipment emissions limit lines for high frequency radiated emissions.
Figure 3. Radiated electric field spectra, 10 kHz to 1 GHz, antenna position 2 in control room (Unit 1).
While the radiated emissions in the region between 139 kHz and 2.61 MHz have dropped as a result of converting the plant control system from analog to digital, there are other regions (e.g., A, B, and C) that have increased in amplitude. 132 These three example areas have experienced amplitude increases ranging from a few dB to as much as high as over 40 dB. With the nature of radiated emissions being cumulative with increasing digital devices in areas such as Control Rooms, areas such as A, B, and C will experience significant growth in amplitude more closely approaching the plant emissions limit line (yellow line) defined by EPRI TR-102323.
As additional digital control equipment is installed in the Control Room, these emissions levels will grow. Moreover, with the new advanced nuclear plants presently under design (some under early construction), I&C engineers can expect new concerns regarding higher emissions levels and new EMI problems as digital I&C controls are brought on line. This is an area that deserves careful consideration in efforts to lower the risk of allowing an EMI problem to occur in the fleet of advanced nuclear plants built and placed on the grid over the next ten years. Efforts put into place to gather emissions data for new digital I&C equipment slated for use in the new plants will provide much needed emissions guidance and aid in the prevention of future EMI problems.
High-Frequency Radiated Emissions – Electric Fields: 1 – 6 GHz
1) Antenna Position 1
Figure 4 illustrates the radiated emissions trace for electric fields taken at Antenna Position 1 from 1 to 6 GHz adjacent to one of the system cabinets for the plant control system. This trace contains the data for both the analog control system (red and blue traces) and the digital control system (purple trace only).
Figure 4. Radiated electric field spectra, 1 – 6 GHz, antenna position 1 in control room (Unit 1).
From red and blue (upper) traces, one can see that there are no significant peaks associated with the analog control system. However, with the digital control system there are peaks at 1.35 GHz at 49.9 dBμV/m, 1.88 GHz at 50.5 dBμV/m, 1.92 GHz at 53.4 dBμV/m, 2.41 GHz at 76.3, 2.46 GHz at 54.4 dBμV/m, and 5.82 GHz at 60.6 dBμV/m. Some of these spectral components are higher at Antenna Position 1 than at Antenna Position 2.
From increases in the usage of digital equipment in other mission-critical environments where surveys have been carried out, it is reasonable to predict that the above components will experience a growth in amplitude in addition to the development of new components with faster processors (and using more switch-mode power supplies) as more digital I&C systems are installed in the control room and other areas supporting the control room. The control rooms of the new advanced plants are, of course, no exception. They will also experience higher levels of radiated emissions in this frequency range and also extending up to 10 GHz.
2) Antenna Position 2
Figure 5 illustrates the radiated emissions trace for electric fields taken at Antenna Position 2 from 1 to 6 GHz adjacent to one of the system cabinets for the plant control system. This trace contains the data for both the analog control system (green trace) and the digital control system (blue trace). From the trace, one can see that there are no significant peaks associated with the analog control system. However, with the digital control system there are peaks at 1.17 GHz at 48.8 dBμV/m, 1.92 GHz at 48.9 dBμV/m, 2.42 GHz at 57.7 and 61.2 dBμV/m, and 5.82 GHz at 50.9 dBμV/m. Only data from Antenna Position 1 and 2 are included in this paper. Emissions data at Antenna Position 3 was similar to that of Antenna Position 1 and 2.
Figure 5. Radiated electric field spectra, 1 – 6 GHz, antenna position 2 in control room (Unit 1).
STANDARDS DEVELOPMENT
Presently, the nuclear power plant industry relies on the EPRI guidance document (TR-102323 (Rev. 3)) and the NUREG 1.180 to plan and conduct EMC qualifications testing for I&C equipment (analog and digital). EPRI is leading the effort in developing new standards for the NPP industry with the first standards project focusing on immunity testing of I&C equipment. An update on this standards development effort will also be presented at the conference as part of this presentation.
CONCLUSION
Project engineers responsible for digital I&C upgrades at the this major US nuclear power plant took the right step in having the two areas of concern—Control Room (near cabinets in Unit 1 where DCS was installed), OAC Computer Room, involving the completion of the digital control system (DCS) project—survey for radiated emissions. The DCS equipment is primarily digital (instead of analog) and its radiated emissions signatures were different than its analog counterparts. It is well known in the EMC industry that EMC surveys provide valuable insight as to the electromagnetic conditions of an environment in question, especially one as critical as a Control Room in a nuclear power plant. An analysis of the emissions and immunity test results and witness immunity testing of these proposed digital systems should be conducted prior to installation. Due to the critical nature of he DCS, simple proof of acceptable EMC compliance for this equipment should not be accepted as complete with regards to EMC. Further consideration of electromagnetic compatibility combined with a well-designed EMC installation practice and the results of this survey will further help to ensure that these systems are not interrupted by emissions from the electromagnetic environment in question. Digital I&C equipment slated for use in the advanced plants will also benefit from pre-op surveys in areas where some of the EME conditions can be controlled. These components can be integrated into an Electromagnetic Environmental Effects (E3) program should this power plant elect to establish such a program to maintain EMC throughout the plant. Further information regarding this type of program can be provided upon request.
REFERENCES
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Philip F. Keebler manages the Lighting and Electromagnetic Compatibility (EMC) Group at EPRI where EMC site surveys are conducted, end-use devices are tested for EMC, EMC audits are conducted and EMI solutions are identified. Keebler has conducted System Compatibility Research on personal computers, lighting, medical equipment, and Internet data center equipment. The lighting tasks were associated with characterizing electronic fluorescent and magnetic HID ballasts, electronic fluorescent and HID ballast interference, electronic fluorescent and HID ballast failures, and electronic fluorescent and HID lamp failures. Keebler has drafted test protocols and performance criteria for SCRP tasks relating to PQ and EMC. He served as editor developing a new EMC standard for power line filters, IEEE 1560.
Stephen Berger is president of TEM Consulting, an engineering services and consulting firm dealing in regulatory compliance, wireless, voting equipment and EMC. Berger was the convener and founding chair of IEEE SCC 41, Dynamic Spectrum Access Networks and immediate past chair of the IEEE EMC Society Standards Development Committee. He is a past president of the International Association of Radio and Telecommunications Engineers (iNARTE), a professional certification agency. Currently he works with ANSI ACLASS as a lab assessor and on issues of conformity assessment. Before forming TEM Consulting, Berger was a project manager at Siemens Information and Communication Mobile, in Austin, Texas, where he is responsible for standards and regulatory management. He has provided leadership in the development of engineering standards for 30 years, including five which have been adopted and incorporated into federal regulations by the FCC.