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Power-line carrier (PLC) systems have proven to be susceptible to interference caused by harmonics produced by loads, such as HVDC converters, DC electric arc furnaces, static var compensators, and motor speed controllers. Previous work in this area has provided information regarding the nature and origin of harmonic frequency interference affecting PLC systems as well as discussion of problem mitigation. However, the information provided in this area is contained within a number of works. Therefore, the purpose of this paper is to summarize previous work in this area with the intent of providing a single and practically useful resource for field engineers and others responsible for the operation and maintenance of PLC systems used for communication over high-voltage transmission lines. Information provided includes a description of the nature and origin of harmonic frequency interference in PLC coupling systems, including field measurements. Susceptibility of different coupling systems to harmonic frequency interference is discussed. Methods for mitigating interference are also provided, including an example of interference mitigation on an actual coupling system on a 230-kV line. Finally, a relationship between harmonic frequency interference and standard harmonic distortion limits is discussed.