In this final paper, we analyzed instrument transformers, which are crucial for power systems due to their reliability and durability, especially traditional instrument transformers with ferromagnetic cores, which are simple, long-lasting, and cost-effective. We examined the fundamentals of current and voltage instrument transformers and identified the key parameters important for their selection. We also described new technologies in this field and concluded that, although modern technologies offer more advanced alternatives, traditional transformers still play a significant role in power systems, slowing the faster adoption of innovations. With the increasing complexity of systems and the need for faster and more precise measurements, new technologies such as optical fibers and sensors provide advantages like better compatibility, lower costs, and enhanced safety. Unconventional instrument transformers have the potential to take a leading role in the future, but their widespread acceptance depends on ongoing collaboration in further development and standardization, particularly in addressing compatibility issues with existing systems.