The so-called ‘War of the currents’ was being fought over a century ago. Thomas Edison was a devoted supporter of direct current (DC) power transmission while Nikola Tesla believed alternating current (AC) was the way to go. At that time, it seemed like AC had won the battle. Primarily because long-distance transmission became possible after the invention of the transformer.
AC is still the main form of power transmission today, but DC is again gaining traction. Most modern equipment uses DC in one way or another, for example, LED lights, computers, cell phones, and frequency converters. Utilizing DC instead of AC might improve system efficiency and system footprint by 10–20%. There is a significant potential for energy saving especially when combining a DC grid, energy storage, and/or variable speed generators.
The main benefits of using DC are:
- Less AC to DC and DC to AC conversion
- Fewer filters and transformers
- No reactive current
- No synchronization is needed
- In a drives system, regenerative braking energy (energy coming back from the motor) can be reused without any conversion
However, one of the major challenges facing the DC grid is how to ensure selectivity and protection in case of a short circuit.
As with any grid, at any point on a DC grid, there is a risk that a faulty circuit will occur. To prevent accidents or injuries, sophisticated selective short-circuit protection devices are needed. Danfoss Drives has therefore developed the new VACON® NXP DCGuard™, a short-circuit current suppressor based on current interruption by switching of IGBT transistors. VACON® NXP DCGuard™ detects and cuts off any faulty DC currents and isolates the defective part of the system in microseconds, leaving the healthy part unaffected by the fault. Hence full selectivity between DC grids.
VACON® NXP DCGuard™ is based on a standard VACON® NXP inverter and new application software. The complete product range 3–4140 A and 465–1100 VDC is already Type-approved by DNV-GL.
With the introduction of VACON® NXP DCGuard™, it means Danfoss Drives has all the components needed to build a DC grid. More and more often, we are seeing how DC grids can be used, for example, to save energy and make use of renewable energy. With global regulations, such as the Paris Agreement, placing stricter demands on the energy efficiency of vessels, and the IMO’s stipulation for a 50% reduction in emissions by 2050, one of the main industries that can benefit from a DC grid is the marine industry. There is also a common requirement in the marine industry that failure in one part of the system shall not affect the whole system, making VACON® NXP DCGuard™ a vital addition to the vessel’s electrical grid.
Proven experience
Already in 2009, Danfoss delivered one of the first DC grid systems for a river cruiser. Today, more than 50 of these vessels are in operation. In 2016, the first VACON® NXP DCGuard™ was piloted in a river-crossing ferry in Amsterdam. By including all Danfoss Drives hybridization products in the same DC grid, energy efficiency and system selectivity were taken to a whole new level. And, in 2018, the first DNV-GL and ABS-approved VACON® NXP DCGuard™ will be in operation in a sea-going vessel on one of Norway’s busiest ferry routes.
Author: Håvard Wolden, Product Manager, Applications, Danfoss Drives