REACTIVE POWER MANAGEMENT ONBOARD NAVAL VESSEL

Abstract

When at sea, the Electric Power System (EPS) of the ship operates in an islanded mode however, when she is berthed alongside and connected to shore power, it operates in a grid-connected mode. Therefore, we can appreciate that the ship’s EPS shows some resemblance to the terrestrial Microgrid. Unlike the terrestrial microgrid, the ship’s EPS is complex and weak. It is complex in terms of the operating environment (ambient temperatures, vibration, roll/pitch of ship and ship’s motion) and it is weak in terms of availability of inertia for meeting heavy load changes and non-availability of the infinite bus bar. Although, it may be inferred that the ship's EPS is intrinsically weak yet, the reliable operation of the ship’s EPS holds paramount importance as it is associated with the safety of men and material onboard. In a practical scenario, it is experienced that over the ship’s operational cycle, the active power is equally distributed amongst the generators running in synchronism however, the reactive power distribution is unequal. This uneven distribution of reactive power can cause higher MVAr loading stress on the synchronous machines, generation of circulating currents and if not addressed, may lead to catastrophic failure of either complete ship’s' EPS or sensitive equipment connected to it. To address the subject problem, this work concentrates on designing a controller for a replenishment type vessel (oil tanker), capable of equally distributing reactive load amongst the generators running in synchronism. The design is facilitated by introducing a ‘Reactive Power Correction Feedback’ (RPCF) which modifies the reference value of the Automatic Voltage Regulator (AVR). Based on this concept, the control strategy is implemented in cascade with the AVRs of the individual generators for ensuring smooth sharing of reactive power. A SIMULINK model of a replenishment type vessel is designed with a definite mission statement and the RPCF control strategy is implemented on the same. The control strategy is validated by comparing the test results before and after, deployment of the presented strategy in an existing ship-based EPS.