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A new bushing insulation monitoring system based on decentralized measurements of the bushing-tap current is presented. The development of this decentralized system is driven by the need to reduce the implementation cost of bushing monitoring based on the comparison of specimens connected in parallel on the same electrical phase. GPS is used for time synchronization of the decentralized measurements. The phase and amplitude measurement accuracy specifications are respectively 0.01° and 0.1%. These specifications are required to allow monitoring of the insulation degradation as well as variations of the bushing capacitance and power/dissipation factor introduced by the fluctuation of the internal insulation temperature and ambient conditions (e.g. rain). Simulations of the new decentralized measurement system were performed to determine the detailed specifications. The system will be validated by comparing it with on-line measurements performed on nine 735/230-kV transmission transformers using a standard galvanic measurement system. These transformers are located in a substation with HVDC converters, which represents a challenging noisy environment for such accurate measurements.