Marine AC UPS for critical offshore installations

Equipment installed in ships or offshore installations typically operate under tough conditions, and any possible equipment failures can have serious cost impact. Despite this, products not really suited for the applications are often chosen based on the initial cost rather than looking at the total cost of ownership.

One example is the use of a modular marine AC UPS versus the use of a modified commercial standard UPS for critical applications on-board ships and offshore installations. The same principle does of course apply to any other demanding industrial application.

This is particularly true for a smaller UPS, up to 20 kW, as this is typically the power range where the purchase price has a tendency to weigh even more heavily in the purchase process than the total cost of ownership.

Traditional UPS

Typically, a traditional UPS is a monoblock design. This means that it has a fixed rectifier and inverter capacity and a battery with sufficient energy storage to maintain the load for as long as it will take to ensure that the alternative energy supply is up and running or to correct the fault that caused the power outage.

The traditional UPS is available in various types and can run in different modes. Typically for marine applications it is required that the UPS is running in online mode, meaning that the power is always fed from the inverter, with the bypass ready to take over should the inverter fail.

In a modular UPS, it is easy to meet future power demands as modules can easily be added or removed.

Modular UPS

For critical industrial applications operating on DC, modular systems have been around for decades, pro-viding redundant and very reliable power systems. For AC UPSs this has not been so - and particularly not for lower power ratings due to system complexity and synchronization issues. Recently technology has been made available, enabling modular AC UPS with a very high reliability and performance.

A modular UPS is equipped with as many slots for power modules as the maximum expected power requirement will require. The system will then be equipped with the required number to support the current load plus one (or more) redundant module(s) to ensure that the system will continue to operate if a module fails.

Modular UPSs are also available in many different configurations with complete parallel-able UPS modules with its own rectifier, inverter and battery or modular rectifiers and inverters connected to a com-mon battery. Recently, bidirectional converters combining the rectifier, the inverter and the static bypass in one box has further reduced the complexity, cost and size of the sys-tem while improving the overall reliability and efficiency.

In a modular UPS, it is easy to meet future power demands as modules can easily be added or removed. This means, in the event of a failure, a non-skilled person can easily replace a module.

Advantages of a Modular UPS

When all electric installations are completed, it is often not the best environment on board some vessels. Dust, humidity, metal chips from cutting, grinding or welding or other elements may be present, which are not healthy for electron-ics.

In a modular system, the cabinet or frame with all external connections, bolting etc. can be completely installed early in the process, which protects the electronics during the construction phase. All critical part can be plugged in after construction is completed, when power is needed for the connected equipment.

Scalability

In a modular UPS it is easy to upgrade the system to higher power capacity or additional redundancy by simply adding additional modules. It does, how-ever, require that there are additional module slots available and that the upstream fuses and cables are correctly dimensioned.

Better scalability gives the opportunity to exactly configure the system according to the current requirements as well as making it simple to connect additional equipment to the modular UPS in the future. For the traditional UPS, one would have to either over dimension the UPS initially or add a complete UPS to manage increasing loads.

If there is a high probability that the system will require additional capacity during its lifetime, it is also sensible to allocate space for additional batteries in the system design.

Serviceability

In a modular UPS, designed for redundancy, a potential failure in the electronics will not have any operational impact. A module failure will initiate an alarm and any-one - with or without any special training - can replace the faulty module in a matter of seconds.

For a traditional UPS, this typically means that the whole unit needs to be serviced by a certified technician or the whole UPS needs to be replaced. On board a ship or an offshore installation, this can be extremely costly.

Rectifier/Inverter (or Rectiverter) modules typically have a very high mean time between failure (MTBF), so the requirement for spares is limited.

Ingar Sørensen, Global Director Marine & Offshore, is convinced that we need to focus on the Total Cost of Ownership rather than initial purchasing price

Design Life

In a modular UPS for marine applications it is easy to choose components with higher quality specifications and longer design life. Power modules are typically designed for industrial applications with a long expected lifetime and high MTBF.

Batteries are also available in various "Design lifetimes". This reflects thickness of copper plates, strength of electrolyte and other parameters that affect both life-time and cost. In a traditional UPS, it is typical with batteries with a design lifetime of 3 to 5 years, which means that the batteries must be replaced 2 to 3 times during the expected product life-time.

In a modular UPS, longer design-life batteries are normally chosen, and the batteries should last the expected lifetime of the product.

A modular system is also typically has more advanced battery management, which contributes to additional practical battery life-time.

Designed for Marine

Equipment designed for use on-board ships is designed to meet electrical requirements as well as vibration and other elements affecting the product lifetime. In a modular UPS, these requirements are typically met on a lower level (component/building-block level) rather than on system level.

This has an impact on the overall system quality and it simplifies the ship certification process.

Bidirectional Rectifier/Inverter

Although not widely available, Eltek has launched a bidirectional power converter (the Rectiverter) that combines the function of rectifier and inverter in one module. In addition it also includes the functionality of a static bypass switch in a traditional AC UPS.

The Rectiverter is a 3-port (AC in, AC out and DC in/out) device where the DC port is bidirectional. This means that the module in normal operation will provide both AC and DC output. In the case of a loss of AC input, the unit will immediately switch to pure inverter mode drawing power into the DC power and feeding the AC out-put without interruption.

The result leads to a significantly reduced total cost of ownership and a system that meets the reliability and uptime expectations for the maritime industry.

Such products simplify the complexity of modular systems significantly. Reliability is improved, cost and size reduced and the total cost of ownership is minimized.

Total Cost is Key

When deciding on power solution for critical loads on board ships, it is important to look at total cost of ownership. There are many alter-natives that seem attractive based on the initial purchase price, but it is the final accumulated bill that really counts.

A modular UPS may have a slightly higher initial cost, but the advantages listed below, by far out-weighs this:

  • Scalability - build as your load grows or add redundancy
  • Serviceability - module replacement in seconds when spares are available
  • Design life - with high-quality components the system will remain operational much longer
  • Installation can be completed without any sensitive electronics in place (modules can be pulled out and stored in a proper place when it is less favourable environment during installation)
  • Reliability - besides the cost element, all of the above obviously impact the reliability of the supply of power to critical functions on-board

The result leads to a significantly reduced total cost of ownership and a system that meets the reliability and uptime expectations for the maritime industry.


This article was first published in "Scandinavian Oil - Gas Magazine",  Issue 3, 2016

About the author: Ingar Sørensen is the Global Director Marine & Offshore for Eltek. He has nearly 30 years of experience at Eltek, having gotten his start in the company as a system design and project manager for marine and offshore solutions. In 2001, Mr Sørensen was named manager for the Nordic business unit, and in 2010 he became responsible for the development of a range of standard marine and offshore solutions with DNV and ABS approvals.

Ingar Sørensen Mar 21, 2018
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