The impact of large steel mill loads on power generating units

Large steel mills can be potentially harsh loads on an electric power system, producing harmonic currents and unbalanced system loading. Since these large loads are typically supplied from the EHV power system grid, it must be recognized that not only will the system of the host utility be impacted, but neighboring utility systems could be impacted as well. Existing IEEE Standard 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, does not fully address the interharmonic problems generated by steel mill loads. This paper identifies the limits of existing unit protection schemes to detect the potentially damaging impacts which large mill loads can have on nearby power generating units. Preoperational studies are needed to assess the impact of the mill on the power system, followed by field tests to verify the study results. Potential mitigation schemes to protect generating units are identified. Issues are discussed from both a utility and mill perspective. Additionally, the paper provides information needed by both the utility protection engineer and the mill's operators during initial planning discussions prior to operation.