Power electronic based frequency converters such as solar, wind and batteries, which produce harmonics, are becoming integral parts of modern power systems. Transmission and Distribution Network Operators find it increasingly difficult to manage harmonics in their network. In particular, harmonic allocation to large loads in transmission systems present many challenges for Transmission Network Operators. One of the challenges is that existing methodologies are unclear and often lead to misinterpretation and inconsistent application of harmonic guidelines and standards and ultimately fail to achieve fair and equitable harmonic allocation. Another challenge is that harmonic amplification, damping and absorption capability of transmission systems can be heavily affected by the complex relationship among synchronous machines, transmission lines and reactive network elements such as transformers, reactors and capacitors, under different network scenarios. This paper evaluates the practical application of different harmonic allocation methodologies and recommends the most practical method that allows maximum allocations to loads in transmission systems while ensuring compliance with planning levels.