Energization of subsea power transformers

Energizing large transformers often cannot be done direct-on-line due to the negative effects of the inrush current. The typical schemes used in the past are energization via a high impedance, or via a tertiary winding connected to an auxiliary AC power source. These schemes require additional equipment resulting in increased foot print and complexity. For offshore and subsea installations, the increase in foot print often greatly exceeds the cost of the additional equipment. This paper presents an alternative solution allowing direct-on-line energization of large transformers, both topsides and subsea. Different solutions for subsea auxiliary AC power supplies not requiring high-voltage circuit-breakers are presented. For offshore and subsea power systems, the circuitbreakers supplying power to the transformers are standard 3.3 kV to 36 kV class devices having a 3-pole operating mechanism. The solution presented allows circuitbreakers to be switched such that the closing of the poles occurs at the point on the voltage waveform where the resulting inrush current is the least. For subsea power distribution systems, the auxiliary power required for the subsea control equipment is provided by a high-voltage DC auxiliary power link from the shore station and the control and communication link is via optical fiber. The solutions presented use standard proven technology and can be integrated within the subsea modules required for supplying power to the loads. This keeps the number of penetrators and subsea connector systems to a minimum. The importance of redundancy and maintenance in obtaining and keeping the required system availability are discussed.