Guide for Measurement of Radio Frequency Interference from Hv and Mv Substations

Total Page:16

File Type:pdf, Size:1020Kb

Guide for Measurement of Radio Frequency Interference from Hv and Mv Substations 391 GUIDE FOR MEASUREMENT OF RADIO FREQUENCY INTERFERENCE FROM HV AND MV SUBSTATIONS Joint Working Group C4.202 August 2009 GUIDE FOR MEASUREMENT OF RADIO FREQUENCY INTERFERENCE FROM HV AND MV SUBSTATIONS DISTURBANCE PROPAGATION, CHARACTERISTICS OF DISTURBANCE SOURCES, MEASUREMENT TECHNIQUES, CONVERSION METHODOLOGIES AND LIMITS Joint Working Group CIGRE/CIRED C4.202 L.-E. Juhlin (SE) Convener R. Hubbard (ZA), G. Lucca (IT), H.-G. Öhlin (SE), C. Rajotte (CA), W.H. Siew (GB), D. Thomas (GB), Gy. Varju (HU) Invited Specialists: J. Eriksson Skansen (SE), E. Petersson (SE), P. Stenumgaard (SE) Corresponding members: R.H. Braeunlich (CH), D.O. Campones do Brasil (BR), B. Cramer (US), J. Diesendorf (AU), N.H. Jendal (NO), G.L. Solbiati (IT) Copyright©2009 “Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of use for personal purposes. Are prohibited, except if explicitly agreed by CIGRE, total or partial reproduction of the publication for use other than personal and transfer/selling to a third party. Hence circulation on any intranet or other company network is forbidden”. Disclaimer notice “CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law”. ISBN: 978-2-85873-078-0 TABLE OF CONTENTS 0. Executive summary .......................................................................................................................... 6 0.1. Introduction .............................................................................................................................. 6 0.2. Scope of the guide .................................................................................................................... 7 0.3. Characteristics of the disturbances ........................................................................................... 7 0.4. Theoretical background ........................................................................................................... 7 0.5. Establishment of RFI limits for substations ............................................................................. 8 0.6. Measurement procedure and measurement uncertainties ...................................................... 10 0.7. Conclusions ............................................................................................................................ 11 1. Introduction .................................................................................................................................... 12 1.1. Scope ...................................................................................................................................... 12 1.2. Background ............................................................................................................................ 12 1.3. Purpose of this guide .............................................................................................................. 12 1.4. Description of the phenomena ............................................................................................... 13 1.5. The state of the art.................................................................................................................. 13 1.6. Design and verification procedures ....................................................................................... 14 2. Background noise........................................................................................................................... 15 3. Characteristics of the radio receivers ............................................................................................. 16 3.1. General ................................................................................................................................... 16 3.2. Equipment immunity ............................................................................................................. 16 3.3. Different detectors used ......................................................................................................... 16 3.4. Communication in the frequency band 9 kHz to 150 kHz..................................................... 18 3.4.1. Background .................................................................................................................... 18 3.4.2. Services using this frequency range ............................................................................... 18 3.4.3. Detector principle........................................................................................................... 19 3.5. AM broadcasting .................................................................................................................... 19 3.6. FM broadcasting and TV ....................................................................................................... 19 3.7. Digital communication........................................................................................................... 19 3.8. Amateur radio ........................................................................................................................ 19 3.9. Other radio services ............................................................................................................... 20 3.9.1. General ........................................................................................................................... 20 3.9.2. Aviation radio and radar ................................................................................................ 20 3.9.3. Communication with mobile services ............................................................................ 20 3.9.4. Cellular telephones......................................................................................................... 20 3.9.5. Suitable detectors ........................................................................................................... 20 3.9.6. Conclusive summary ...................................................................................................... 20 4. Characteristics of the disturbance sources ..................................................................................... 21 4.1. General ................................................................................................................................... 21 4.2. Generation mechanism........................................................................................................... 22 4.2.1. General ........................................................................................................................... 22 4.2.2. HV corona ...................................................................................................................... 22 4.2.3. Sparking ......................................................................................................................... 22 4.2.4. Arcing ............................................................................................................................ 23 4.2.5. Commutations ................................................................................................................ 23 4.2.6. PLC ................................................................................................................................ 27 5. Disturbance propagation ................................................................................................................ 28 5.1. General ................................................................................................................................... 28 5.2. Sources of RFI ....................................................................................................................... 28 5.2.1. Power electronic installations ........................................................................................ 28 5.2.2. Corona from substation equipment ................................................................................ 28 5.2.3. RFI emission due to spread out of high frequency currents .......................................... 28 2 5.3. RFI in the three zones in Figure 5.1 ....................................................................................... 29 5.3.1. Zone E ............................................................................................................................ 29 5.3.2. Zone S ............................................................................................................................ 29 5.3.3. Zone L ............................................................................................................................ 29 5.4. Disturbance reach................................................................................................................... 29 6. Modelling and calculation.............................................................................................................. 30 6.1. Introduction ............................................................................................................................ 30 6.2. Analytical models of common wave shapes .......................................................................... 30 6.2.1. General ........................................................................................................................... 30 6.2.2. The double
Recommended publications
  • Social Determinants of Unmet Hospitalisation
    Nagulapalli, S 2013 Data from “Social determinants of unmet hospitalisation need amongst the poor in Andhra Pradesh, India: A cross-sectional study.” Journal of Open Public Health Data 1(1):e6, DOI: http://dx.doi.org/10.5334/jophd.af DATA PAPER Data from “Social determinants of unmet hospitalisation need amongst the poor in Andhra Pradesh, India: A cross- sectional study.” Srikant Nagulapalli1 2 1 Government of Andhra Pradesh, India 2 Collector and District Magistrate of Nellore, Andhra Pradesh, India The dataset is of a health survey amongst the 21.5 million poor families of the Indian state of Andhra Pradesh conducted during April and May 2013. The dataset captures individual characteristics and household characteristics of the past 365 days. Data was collected by 2022 trained field staff of Aarogyasri Health Care Trust (AHCT) of Government of Andhra Pradesh using a questionnaire mod- elled after that used for the health surveys by National Sample Survey Organisation of India. Keywords: health survey of Andhra Pradesh; Rajiv Aarogyasri; unmet hospitalisation need Funding statement The data is not the result of any funded project. (1) Overview (2) Methods Context Steps a) Design of questionnaire: National Sample Survey Spatial coverage Organisation (NSSO), Ministry of Statistics, Government Description: 23 districts Adilabad, Nizamabad, Karimna- of India conducts yearly consumer expenditure surveys as gar, Warangal, Hyderabad, Rangareddy, Medak, Mahbub- well as focussed surveys on health care. The health survey nagar, Nalgonda, Anantapur, Kurnool, Kadapa, Chittoor, questionnaire and the consumer expenditure question- Nellore, Prakasam, Guntur, Krishna, Khammam, West naire used for the 60th round (2004-05) of NSSO were Godavari, East Godavari, Visakhapatnam, Vizianagaram integrated by suitably abridging the consumer expendi- and Srikakulam of Andhra Pradesh state, India were cov- ture details and used for this survey.
    [Show full text]
  • The BPL Dilemma
    Reprinted with permission from CQ VHF Magazine, Spring 2004 issue. Copyright CQ Communications 2004 The BPL Dilemma Hams claim Broadband over Power Lines will interfere with their on-the-air operations. The utility companies claim not. Read how they are both right . sort of. By Gary Pearce,* KN4AQ still academic. They haven’t encountered Because of the importance of the it yet. I will provide a quick tutorial. Broadband over Power Lines (BPL) The basics of BPL are simple. It is a issue, “FM” columnist Gary Pearce, method of delivering high-speed internet KN4AQ, devotes his space this time to the to homes and small businesses using the investigation of a BPL test site and the local power lines that crisscross neigh- surrounding area. He will be back in the borhoods either overhead or under- next issue of CQ VHF with his regular ground. This is a brilliantly obvious idea column material. —N6CL (“the wires are already there!”) that was delayed because the AC power grid is a really noisy, crappy signal-delivery ince last fall, I’ve been up to my medium for anything above 60 Hz. The eyeballs in BPL—Broadband over march of technology, however, is mak- Power Lines—and its effect on ing it feasible. It is the third method of Samateur radio. If you’re up on current TV doing that, following DSL (Digital culture, you can call it “HF Eye for the FM Subscriber Line) on the phone lines and Guy.” Our area has been “lucky” enough cable TV (nobody’s come up with a cute to host one of the few BPL trials, courtesy name or acronym for broadband over of my local power company, Progress cable TV; they just call it “cable”).
    [Show full text]
  • ABBREVIATIONS EBU Technical Review
    ABBREVIATIONS EBU Technical Review AbbreviationsLast updated: January 2012 720i 720 lines, interlaced scan ACATS Advisory Committee on Advanced Television 720p/50 High-definition progressively-scanned TV format Systems (USA) of 1280 x 720 pixels at 50 frames per second ACELP (MPEG-4) A Code-Excited Linear Prediction 1080i/25 High-definition interlaced TV format of ACK ACKnowledgement 1920 x 1080 pixels at 25 frames per second, i.e. ACLR Adjacent Channel Leakage Ratio 50 fields (half frames) every second ACM Adaptive Coding and Modulation 1080p/25 High-definition progressively-scanned TV format ACS Adjacent Channel Selectivity of 1920 x 1080 pixels at 25 frames per second ACT Association of Commercial Television in 1080p/50 High-definition progressively-scanned TV format Europe of 1920 x 1080 pixels at 50 frames per second http://www.acte.be 1080p/60 High-definition progressively-scanned TV format ACTS Advanced Communications Technologies and of 1920 x 1080 pixels at 60 frames per second Services AD Analogue-to-Digital AD Anno Domini (after the birth of Jesus of Nazareth) 21CN BT’s 21st Century Network AD Approved Document 2k COFDM transmission mode with around 2000 AD Audio Description carriers ADC Analogue-to-Digital Converter 3DTV 3-Dimension Television ADIP ADress In Pre-groove 3G 3rd Generation mobile communications ADM (ATM) Add/Drop Multiplexer 4G 4th Generation mobile communications ADPCM Adaptive Differential Pulse Code Modulation 3GPP 3rd Generation Partnership Project ADR Automatic Dialogue Replacement 3GPP2 3rd Generation Partnership
    [Show full text]
  • Session 12: Corona Inspection of Substations and Lines
    Session 12: Corona Inspection of Substations and Lines Session 12: Corona Inspection of Substations and Lines Selwyn Braver Managing Director, Martec Asset Solutions Abstract This paper will focus on the aspects of Corona in MV & HV substations and lines. An overview will be provided with implications for power networks and methods of identification, predictive and preventative procedures. The following aspects will be covered; Corona phenomenon - Theoretical aspects, physics, chemistry, implications, concerns and outcome; The concept of UV inspection; Corona cameras – Design and principles; UV spectral range Photoelectric and optics; IR cameras compliment UV inspection; Corona and the electrical grid – Where, when and what; Corona on outdoor HV substations & lines – Predictive and preventive procedures and finally UV inspection methodology – Basic and advanced aspects. Introduction Corona discharge is an electrical discharge bought on by the ionization of a medium (air) surrounding a conductor that is electrically energized. The discharge will occur when the potential gradient of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. Partial breakdown of the air manifests as a corona discharge on high voltage conductors at points with the highest electrical stress. The dielectric strength of the material surrounding the conductor determines the maximum strength of the electric field the surrounding material can tolerate before becoming conductive. Conductors that consist of sharp points, or edges with small radii, are more prone to causing dielectric breakdown. Corona is sometimes seen as a bluish glow around high voltage wires and heard as a sizzling sound along high voltage power lines.
    [Show full text]
  • Section 9 Summary of Results 9.1 Introduction 9.2
    SECTION 9 SUMMARY OF RESULTS 9.1 INTRODUCTION Section 9.2 summarizes the results of NTIA’s preliminary investigations (Sections 2 – 5). These investigations helped refine the scope and approach of NTIA’s analyses and established certain technical assumptions. Section 9.3 summarizes the results of NTIA’s Phase 1 analyses of interference risks (Section 6), measurement procedures (Section 7) and techniques for prevention and mitigation of interference (Section 8). Section 9.4 summarizes matters requiring further study. 9.2 PRELIMINARY INVESTIGATIONS 9.2.1 Descriptions of BPL Systems NTIA identified three architectures for access BPL networks (Section 2): (1) BPL systems using different frequencies on medium- and low-voltage power lines for networking within a neighborhood and extensions to users’ premises, respectively; (2) BPL use of only medium voltage lines for networking within a neighborhood, with other technologies being used for network extensions to users’ premises; and (3) BPL use of the same frequencies on medium- and low-voltage power lines for networking in a neighborhood and extensions to users’ premises. Responses of BPL manufacturers and operators to the FCC’s BPL NOI generally indicate that BPL systems will operate at or near the Part 15 field strength limits in order to achieve maximum throughput and distance separation between BPL devices. NTIA addressed simple BPL deployment models in the Phase 1 interference risk analyses (Section 6). Specifically, a single BPL device and associated power lines were considered for cases of potential interference to ground-based radio receivers and several co-frequency BPL devices were assumed to be deployed throughout the area covered by an aircraft receiver antenna.
    [Show full text]
  • Corona Power Loss Ion.Ppt
    Contents 1. Abstract 2. Introduction 3. Mechanism of corona formation 4. Types of corona 4.1. Positive corona 4.2. Negative corona 5. Voltage parameters of corona 5.1. Disruptive critical voltage 5.2. Visual critical voltage 6. Factors affecting corona 7. Waveform of corona current 8. Disadvantages 9. Ways to reduce corona 10. Advantages and Applications Abstract Corona is a phenomenon associated with all energized transmission lines. Under certain conditions, the localized electric field near an energized conductor can be sufficiently concentrated to produce a tiny electric discharge that can ionize air close to the conductors (Electric Power Research Institute (EPRI), 1982). This partial discharge of electrical energy is called corona discharge, or corona. Corona Discharge discharge results from electrical discharge and indicates ionization of oxygen and the formation of ozone in the surrounding air. It can eat through a Polymer insulator until it breaks! It produces extremely corrosive Nitric Acid when in a humid atmosphere! Several factors, including conductor voltage, shape and diameter, and surface irregularities such as scratches, nicks, dust, or water drops can affect a conductor’s electrical surface gradient and its corona performance. Corona is the physical manifestation of energy loss, and can transform discharge energy into very small amounts of sound, radio noise, heat, and chemical reactions of the air components. Because power loss is uneconomical and noise is undesirable, corona on transmission lines has been studied by engineers since the early part of this century. Many excellent references exist on the subject of transmission line corona (e.g., EPRI, 1982). Consequently, corona is well understood by engineers and steps to minimize it are one of the major factors in transmission line design for extra high voltage transmission lines (345 to 765 kilovolts (kV)).
    [Show full text]
  • G3/0'4 18909 I
    TECHNICAL MEMORANDUM (NASA) 91 A MICRO-COMPUTER BASED SYSTEM TO COMPUTE MAGNETIC VARIATION A microcomputer-based implementation of a magnetic variation model for the continental United States is presented. The implementation computes magnetic variation as a function of latitude and longitude for general aviation receivers such as Loran-C. by Rajan Kaul Avionics Engineering Center Department of Electrical and Computer Engineering Ohio University Athens, Ohio 45701 March 1984 Prepared for National Aeronautics and Space Administration Langley Research Center Hampton, Virginia 23665 Grant No. NOR 36-009-017 (NASA-CR-1713376) A MICRO-COMPUTER BASED B84-20506 SYSTEM TO COMPUTE MAGNETIC VARIATION (Ohio Univ,) 26 p HC A03/IF A01 CSCL 17G Unclas G3/0'4 18909 I. INTRODUCTION A Mathematical model of magnetic variation in the continental United States (COT48) has been implemented in the Ohio University Loran-C receiv­ er. The model is based on a least squares fit of a polynomial function. The implementation on the micro-processor based Loran-C receiver is pos­ sible with the help of a math chip, Am9511 manufactured by Advanced Micro Devices, which performs 32 bit floating point mathematical operations. A Peripheral Interface Adapter (M6520) is used to communicate between the 6502 based micro-computer and the 9511 math chip. The implementation pro­ vides magnetic variation data to the pilot as a function of latitude and longitude. This report briefly describes the model and the real time implementation in the receiver. II. THE MATHEMATICAL MODEL The model was developed at the United States Geological Survey (USGS) by Fabiano et al. [11, by performing least squares analysis on more than 34,000 data measurements taken between 1900 and 1974.
    [Show full text]
  • Report ITU-R SM.2451-0 (06/2019)
    Report ITU-R SM.2451-0 (06/2019) Assessment of impact of wireless power transmission for electric vehicle charging on radiocommunication services SM Series Spectrum management ii Rep. ITU-R SM.2451-0 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio- frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regional Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Resolution ITU- R 1. Forms to be used for the submission of patent statements and licensing declarations by patent holders are available from http://www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series of ITU-R Reports (Also available online at http://www.itu.int/publ/R-REP/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodetermination, amateur and related satellite services P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management Note: This ITU-R Report was approved in English by the Study Group under the procedure detailed in Resolution ITU-R 1.
    [Show full text]
  • Broadband Over Power Lines a Foundation for the Utility of the Future
    Broadband over Power Lines a Foundation for the Utility of the Future Jay M. Bradbury Duke Energy Regulatory Manager – Powerline Communications October 26, 2006 LSU Center for Energy Studies – Energy Summit 2006 Topics • BPL Explored a Primer • The Duke Experience • BPL as an Enabler of the Utility of the Future 1 BPL Explored A Primer 2 BPL Defined • Access Broadband Over Power Line (Access BPL). A carrier current system installed and operated on an electric utility service as an unintentional radiator that sends radio frequency energy on frequencies between 1.705 MHz and 80 MHz over medium voltage lines or low voltage lines to provide broadband communications and is located on the supply side of the utility service’s points of interconnection with customer premises. Access BPL does not include power line carrier systems as defined in Section 15.3(t) of this part or In-House BPL systems as defined in Section 15.3(gg) of this part. (FCC) – Electric utility companies can use Access BPL systems to monitor, and thereby more effectively manage, their electric power distribution operations. – Access BPL systems can also deliver high speed Internet and other broadband services to homes and businesses. • In-House Broadband Over Power line (In-House BPL). A carrier current system, operating as an unintentional radiator, that sends radio frequency energy to provide broadband communications on frequencies between 1.705 MHz and 80 MHz over low- voltage electric power lines that are not owned, operated or controlled by an electric service provider. The electric power lines may be aerial (overhead), underground, or inside walls, floors or ceilings of user premises.
    [Show full text]
  • COUNTY PURCHASING AGENT Fort Bend County, Texas
    COUNTY PURCHASING AGENT Fort Bend County, Texas Gilbert D. Jalomo, Jr., CPPB (281) 341-8640 County Purchasing Agent Fax (281) 341-8645 December 14, 2015 TO: All Prospective Bidders RE: Addendum No. 1 – Fort Bend County Bid 16-041 - Construction of Traffic Signalization to Serve the Intersection of South Mason Road at Delta Lake Drive Addendum 1: See attached is addendum 1. Vendors are to use the Addendum 1 document while preparing their bid response. ****************************************************************************** Immediately upon your receipt of this addendum, please fill out the following information and fax this page to the Fort Bend County Purchasing Department at (281) 341-8645. _______________________________________________________________________ Company Name _______________________________________________________________________ Signature of person receiving addendum Date If you have any questions please contact this office. Sincerely, Debbie Kaminski, CPPB Assistant Purchasing Agent 301 Jackson, Suite 201 ∙ Richmond, TX 77469 December 11, 2015 Ms. Debbie Kaminski, CPBB Assistant County Purchasing Agent Fort Bend County – Purchasing Department 301 Jackson Street, Suite 201 – Travis Annex Richmond, TX 77469 RE: Plans for Construction of Traffic Signalization to Serve the Intersection of S. Mason Road @ Delta Lake Drive – Addendum #1 Dear Ms. Kaminski, Enclosed please find documents pertaining to Addendum No. 1 for the construction of Traffic Signalization to Serve the Intersection of S. Mason Road @ Delta Lake Drive. BID DOCUMENTS: 1. Remove Specification 16730 – ITS Controller Cabinet Assembly and replace with Specification TS2-1, P44 Cabinet with 16 Position Back panel, attached. 2. Remove Specification 16731 – Model 2070 Controller Unit and replace with Specification TS2-1, P44 Cabinet with 16 Position Back panel, attached. 3. Add Special Specification 8775 – 5.8 Ghz Wireless Ethernet Radio, attached.
    [Show full text]
  • BPL) SYSTEMS to FEDERAL GOVERNMENT RADIOCOMMUNICATIONS at 1.7 - 80 Mhz
    NTIA Report 04-413 POTENTIAL INTERFERENCE FROM BROADBAND OVER POWER LINE (BPL) SYSTEMS TO FEDERAL GOVERNMENT RADIOCOMMUNICATIONS AT 1.7 - 80 MHz Phase 1 Study VOLUME I technical report U.S. DEPARTMENT OF COMMERCE National Telecommunications and Information Administration NTIA Report 04-413 POTENTIAL INTERFERENCE FROM BROADBAND OVER POWER LINE (BPL) SYSTEMS TO FEDERAL GOVERNMENT RADIOCOMMUNICATIONS AT 1.7 - 80 MHz Phase 1 Study VOLUME I U.S. Department of Commerce Donald Evans, Secretary Michael D Gallagher, Acting Assistant Secretary For Communications and Information APRIL 2004 ACKNOWLEDGEMENTS The measurements and studies underlying this report were performed by NTIA’s Spectrum Engineering and Analysis Division (SEAD) and Institute for Telecommunications Science (ITS). The following NTIA personnel contributed substantially to this report and the underlying studies: Brent Bedford (ITS lead) ITS Gary Patrick SEAD Ernesto Cerezo SEAD Alakananda Paul (tech. lead) SEAD Nick DeMinco ITS James Richards (admin. lead) SEAD Ed Drocella SEAD Robert Sole SEAD Phil Gawthrop SEAD Thomas Sullivan SEAD Randall Hoffman ITS Clement Townsend SEAD Gerald Hurt SEAD Cecilia Tucker SEAD Bernard Joiner SEAD Cou-Way Wang SEAD Yeh Lo ITS Jonathan Williams SEAD Norman Maisel SEAD Robert Wilson SEAD NTIA is grateful for the cooperation provided by the BPL parties whose systems were subject to measurement. NTIA also greatly appreciates the equipment, special radio network access and radio operator support provided by the Department of Homeland Security for NTIA’s testing of BPL interference. Finally, NTIA would like to thank the Interdepartment Radio Advisory Committee (IRAC) for its review of this report and the inputs it provided. iii PREFACE This Report contains two Volumes.
    [Show full text]
  • Boeung Prek Lapouv Management Plan January 2014 – December 2018 Part 1: Description and Evaluation
    Boeung Prek Lapouv management plan January 2014 – December 2018 Part 1: Description and Evaluation Contents Acronyms and abbreviations used ................................................................................................................ 4 1. Summary ................................................................................................................................................... 5 2. Legislation and Policy ................................................................................................................................ 8 2.1. Prime Ministerial Decree (Sub Decree) .............................................................................................. 8 2.2. Fish Sanctuary .................................................................................................................................... 8 2.3. Inundated Forest Protection Zone ..................................................................................................... 8 2.4. Zones & Boundaries ........................................................................................................................... 8 2.5. Other relevant legislation and policies ............................................................................................ 10 3. Description of Boeung Prek Lapouv ........................................................................................................ 11 3.1. Brief historical timeline ...................................................................................................................
    [Show full text]