Renewable generation sources have recently been integrated in power systems at an accelerated pace due to their relatively low capital cost, short project delivery time and quick return on investment compared to conventional thermal generators. Large scale wind and solar generation generally incorporate power electronic based converters, produce significant harmonic currents, and when installed at remote buses, introduce new challenges to Transmission Service Operators in regards to harmonic management. If not managed effectively, harmonic levels can lead to gradual degradation of both customer plant and network equipment. Information from practical case studies is a vital step in managing and mitigating harmonic issues. This paper identifies and examines harmonic allocation issues associated with the integration of wind and solar plants in transmission systems through application of a recommended allocation methodology. It also includes recommendations for improvements to harmonic management in transmission systems with increased penetration of power electronic based generation.