January 2020

34 n VARIABLE-SPEED DRIVES January 2020 www.drivesncontrols.com Huge offshore crane uses two DC bus drive systems S hanghai Zhenhua Heavy Industry (known as ZPMC) describes itself as the world’s largest manufacturer of heavy- duty equipment. The Chinese company, founded in 1992, has three main product areas: massive port container cranes (where it holds around 70% of the global market); steel structures and bridges; and offshore projects, such as huge floating cranes, pipe-laying vessels and other platforms. One of its most impressive recent projects has been the design and construction of a large derrick pipe- laying barge that uses the world’s most advanced pipe-laying technology and can lay two pipes simultaneously. The vessel, intended to plug a gap in China’s technological capabilities, has been built for offshore oil exploitation and subsea pipe- laying projects. It has a high utilisation efficiency due to its dual pipe-laying capabilities and a huge on-board crane that can lift weights of 3,000 tonnes while stationary, or 2,000 tonnes while slewing through a full 360-degree circle. Anchoring winches can be used to move the vessel short distances, and it can be used for hoisting and subsea pipe-laying operations in areas with a water depth of 300 metres. The motors that drive the 3,000-tonne crane have a combined power rating of 7.1MW. The crane has two main hooks, two auxiliary hooks, two beams, six slewing gears, two rigging hooks, two cargo control winches and one hook control winch. The AC drive system for the full- circle-swinging crane was specially developed by Vacon for the application, based on a marine design. It consists of two 12-pulse common DC bus systems, which are supplied through a quasi 24-pulse AC drive system. The total harmonic distortion for the system is less than 5%, which means that it meets the requirements of marine classification societies. The vessel also has four Vacon NX rectifiers and 17 NXP inverters that are used to control actuators on- board the crane. A further two NX braking systems are used to dissipate braking energy. The modular drive products for the common DC bus system operate in a closed-loop vector control mode and deliver accurate speed and torque control, with a speed differential of less than 0.01% and torque precision of better than 2%. The system can be controlled at full torque at any speed, including zero speed. Vacon has also supplied AC drives to control similar cranes on two other 3,000- tonne derrick pipe-laying barges. n A massive crane with a 3,000-tonne lifting capacity on-board a Chinese pipe-laying vessel is being controlled by two 12-pulse common DC bus systems. Danfoss drives have found an unusual application in the Czech Republic where they are being used to provide DC-to-AC and AC-to- DC power conversion for four EV charging stations in the village of Žulová, near the Polish border. Because of its location in a remote mountainous area, the village has a weak connection to the power grid. This posed a challenge for the team at Energon Advanced Energetics which designed the charging stations. They developed a hybrid system that uses a 54kW solar panel array to back up the grid, and a 109kWh-capacity battery to ensure that power is available at night and in poor weather. Output from a Danfoss NXA active front-end inverter is fed via an LCL filter to a power transformer that supplies the charging stations. A small UPS provides a “black start” function, allowing the system to be started from a powered-down condition, even if no power is available from the grid. In these circumstances, the inverter provides a magnetising current for the transformer, thus stabilising the system and ensuring a smooth start-up. There are two main operating modes: on- grid, when power is available from the grid; and off-grid, when it is available only from the batteries and solar panels. Transitions between the two modes are smooth, and are invisible to the loads. When operating in on-grid mode, the system can draw power simultaneously from the grid, batteries and solar panels – effectively a peak-shaving arrangement that minimises the load on the weak grid and reduces the need to buy energy from the grid operator. When working off-grid, the system operates as an “island” independent from the grid, drawing power only from the batteries and solar panels. The inverter then operates in a microgrid mode, supporting frequency grouping and providing local control over frequency and voltage. It also ensures reliable synchronisation when power from the grid becomes available again. The hybrid system results in low running costs because, except at night and in dull weather, the charging stations operate mainly with “free” power from the solar panels. Even if the grid supply fails, the charging stations can continue to operate, using power from the solar panels and the batteries. The 3,000-tonne capacity crane on board the Chinese pipe-laying vessel is powered using two 12-pulse common DC bus systems