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Cathodic protection improves National Grid sustainability

When BAC Corrosion Control Ltd (BAC) began to investigate the possibility that switch mode power (SMP) supply technology could be applied to Impressed Current Cathodic Protection (ICCP) systems, the company turned to REO to supply the SMP units. Here, Tony Gerrard, managing director of BAC, explains how REO helped his company introduce ICCP to the National Grid at over 100 sites around the UK.

In 2010 BAC was approached by the UK’s high pressure gas network operator, National Grid, to supply replacement power supplies across Great Britain’s high pressure natural gas pipeline network. National Grid needed to replace the element of the pipeline network made up of outdated oil-cooled, pole mounted units, which posed problems due to the need monitor and refill the oil reservoir. The remainder of the network is made up of low pole mounted, air cooled transformer rectifiers

BAC and National Grid identified an SMP unit capable of producing 48V at 20A DC as the technology of choice.  Then, during project implementation, it was discovered that the National Grid’s Abriox remote monitoring equipment system could directly switch the SMPS units without the need for an additional relay. 

Cathodic protection ICCP systems limit the corrosion of a metal surface by turning that surface into a cathode in an electrochemical cell. They are in widespread use at thousands of oil and gas pipeline and production facilities, and prevent failure of piping and other facilities which may lead to uncontrolled release of chemical products, resulting in safety hazards to workers and the public, plus environmental contamination.

Cathodic Protection (CP) transformer rectifiers (TR) are used to convert AC current to DC current in order to supply impressed DC current to the structure to be protected. The interior of these rectifiers traditionally contain numerous energised electrical components, posing a slightly greater risk of accessing hazardous voltages.

Over recent decades there have been numerous manufacturers supplying old style transformer rectifiers to the market, many of which are still operating in the oil and gas industry. Often these units are out-dated and can be less safe to work with compared to the newer style SMP units, and are becoming less appealing.

Linear power supplies have been the mainstay of power conversions for the CP industry for many years and often contain steel or iron laminated transformers and chokes, adding weight and bulk to units where neither are needed or wanted.

New SMP technologies

The SMPs supplied by REO bring a range of complex technologies together to form one unit, resulting in advantages over the traditional alternative. An SMP unit works by rectifying the 115V or 230V AC voltage, resulting in a high tension DC voltage. The DC is then switched on and off at high frequency, typically switching at 30-500kHz and creating a crude AC waveform which is passed across a much smaller ferrite transformer.

This voltage is then converted into the DC output voltage by another set of diodes, capacitors and inductors. Control and regulation of the output is achieved by controlling the percentage of time the switching elements are switched on during each cycle. The SMP units BAC uses contain a microcontroller to achieve this, allowing the user to set the digital parameters from a keypad and display. This is an improvement on older potentiometer systems allowing remote changes to be made more easily.

The new units are packaged for simple kiosk installation and are also extremely accurate throughout the full range of rated outputs and provide precise control at lower output.

Moreover, they bring a host of installation and maintenance benefits. For instance, no on-site portable appliance testing (PAT) is required and if failure in the units occur, replacement units can be easily installed with little downtime or inconvenience. The units are usually situated in roadside glassfibre reinforced plastic (GRP) cabinets for easy access.

In addition there are a number of environmental benefits associated with the product, including very low risk of contaminating the surrounding land in the event of an oil leak from an old unit.

“SMPs have many advantages over conventional units,” said Jason Peters, senior design engineer at BAC. “This mainly relates to the much faster switching frequencies used in the design, where the high frequency means that inductive components are reduced in size, dramatically decreasing the overall size and weight of the unit. With increasing electricity prices, the running cost of the unit has become ever more important. The advanced topology means they are very efficient, giving the SMP system a clear edge in the marketplace.”

The design stage

REO has worked with BAC for five years and was involved in the National Grid ICCP project from the start of the design process. “We were involved in the design of the total system and integration of our SMP from the first steps,” explained Steve Hughes, managing director of REO UK. “We wrote bespoke software for instance.”

“In this application it is crucial that the voltage and current be controlled to zero. Similarly there has to be a very low ripple, so effective filtering of the DC output is crucial. In this sense the process requirements are not unlike the demands of the electro-coating stage in car painting. Here, the low ripple of the SMP helps to achieve a very high quality finish without pin holes. In fact, REO SMP units have been used for many other applications, where performance, size, connectivity and ease of servicing are overarching requirements.

“Rather than working at the drawing board, our involvement in the design process was very participative – we attended lots of meetings and adapted our product to suit. The interesting thing for us was that the process was based on chemical, rather than electrical references. I would say that we are now quite expert at cathodic protection!
“There were also some simple adaptations to the product that needed to be made,” offered Hughes. “For instance, our traditional format is for the SMP to be backplate-mounted with a small footprint, but in this instance it needed to be squatter to fit into a panel. Re-formatting the housing was a simple change.

“One unusual requirement that REO had to comply with was the submission of a document on the meantime between failure (MTBF) of the SMPs,” continued REO’s Hughes. “This is typical in the CP industry, where servicing can be a costly exercise due to site locations

“To make things simple, REO provided a fixed price rolling maintenance contract over an eight-year period. It is not standard for the electronics industry, but our focus is on delivering the right package for the customer,” contended Hughes. 

Other representative applications of REO SMP units include precise heat control in semiconductor manufacture, especially in the production of solar cells. The units can also be used to emulate battery performance and to facilitate the testing of motors and other equipment used in automotive applications.

REO offers SMPs with industry standard 4-20 mA inputs, but also a variety of fieldbus options, like profibus, CanBus, EtherCat and EtherNet IP. Single units can be manufactured up to 25kW, with larger power, current and voltage using series or parallel connection of multiple units.

Future developments include a modular system which will lend itself to multi-zonal protection of concrete reinforced structures and the implementation of more control intelligence to allow the units to become a node within a more complex system.

A long process

“One of the really interesting things about this application is the fact that it clearly demonstrates the move towards remote monitoring, field bus integration and GSM communications. For me, this represents one of the fascinating ways cathodic protection is moving forward. The other is the shift towards powering the process using solar or another form of renewable power,” explained Hughes. 

BAC secured the first part of its contract with National Grid in 2010, which involved the manufacture of an initial 151 SMP units at a rate of 16 units per month.

The replacement of the existing National Grid units with the new SMP type is a relatively long process due to the re-connection of the AC power supplies to the units. These are being installed with ‘smart’ meters as opposed to the original un-metered electricity supply tarrifs (UMS), which will in turn reduce overall running costs. However, it is estimated that over 100 units are already in use across the UK.

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