Product category:
Industrial Drives/Controls
News Release from: Siemens Large Drives | Subject: Masterdrive VSDs
Edited by the Engineeringtalk Editorial
Team on 16 July 2002
VSDs improve power station performance
An upgrade programme aimed at improving efficiency at Europe's largest coal-fired power station includes significant investment in drive technology.
Drax power station, situated on the south bank of the River Ouse, near Selby in North Yorkshire, is Europe's largest coal-fired power station The six turbine generator sets, each capable of generating 660MW of electricity, require 38,000 tonnes of coal a day when running at full capacity
This article was originally published on Engineeringtalk on 15 Jul 2002 at 8.00am (UK)
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Originally constructed in the late 1960s, Drax began life with three generators.
These were supplemented by a further three units during the second phase of the site development which was completed in June 1986.
The station is significant not only because of its size but also for the technological innovations which have always played an important part in its design and operation.
Post privatisation, during the station's period of ownership by National Power, the decision was taken to install the UK's first retrofitted flue gas desulphurisation plant (FGD).
This removes up to 90% of the sulphur dioxide from Drax's boiler gases; a significant advance environmentally.
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More recently, further efforts have been made by the station's new owners, AES, to improve the energy efficiency, environmental impact and reliability of the plant's boiler feed systems.
This has resulted in the replacement of the primary air fans motor drives that feed the boilers in the original section of the power station.
The refurbishment, which is likely to continue for the next two years, represents a significant investment in drive technology.
The final choice, for the Siemens Masterdrive range of variable speed drives has now been made by the site's new owners, AES.
However, this was not before careful consideration had been given to other technologies that could have been implemented.
Before deciding to replace all of their existing drive systems, Drax engineers considered their options very carefully.
Any investment of this magnitude has to be based on well proven operating experiences and the station had suffered problems previously when they had experienced prototype variable frequency drives.
Nick Parker, project leader for AES, takes up the story: "The original motor and drive assemblies were installed as part of the boiler feed systems which formed a critical part of the original station's generating capability.
Reliability problems had started to occur with all thirty units, which are used to drive the fans that push coal dust into the boilers.
We knew that something had to be done, but had to consider each scenario carefully".
Each of the generator sets at Drax is fed by a coal-fired boiler.
Each of these is in turn provided with pulverised fuel from 10 mills.
The fuel is blown into the boiler using Primary Air (PA) fans, each of which is driven by a motor.
The first phase of Drax, which housed thirty mills supplying three boilers, was originally equipped with induction regulators fitted to commutator induction motors.
Now almost thirty years old, the reliability of the drive units was causing concern.
Because of the need for on-going maintenance and repair, Drax engineers were finding that it was difficult to achieve even 80% availability of the PA fans.
Parker and his team were faced with five possible options: do nothing; refurbish the existing motors; replace with a vane controlled fixed speed drive; replace with fluid coupled fixed speed drives; or adopt a variable speed drive solution.
The first option - do nothing - wasn't viable since the reliability and availability of the motors was beginning to have an impact on the overall generating capability of the station.
Refurbishing was a short-term solution, but the engineering and maintenance resource required to carry this programme throughout the remaining life of the drives was not available on site.
Maintenance alone was costing over GBP 140,000 a year by this stage.
In addition, each time a rotor and stator rewind becomes necessary, the entire unit has to be stripped and sent off site for repair.
Fixed speed replacements were considered to be inefficient and changing to the fluid coupled units used on the second phase of Drax wasn't a viable solution because of the engineering time involved.
Adopting this method would also have meant that the fans themselves would all need to be replaced.
"We had tried a variable speed drive in the 1980s but it had proved unreliable", says Parker.
"However, there are a number of benefits associated with this type of drive technology and we decided to take another look".
One of the main benefits of variable speed drives over their original installation, according to the Drax engineers, is the fast response capability.
Under the new NETA electricity supply agreement, the market for power has become very unpredictable, with rapid reductions and increases demanded in output.
This doesn't suit fixed speed running.
Also, the stopping and starting experienced on two shift operation, burns out the commutators on the old units.
The soft start technology employed on variable speed drives overcomes this problem.
"A variable speed solution was, on the face of it, our favoured route", explains Parker.
"We were happy that the technology had developed beyond our original experiences.
It is now a proven solution.
We also knew that Siemens had built a large installed base for this type of unit in Europe".
Cost was naturally a major consideration.
For the Drax team, this extends well beyond the cost of installation and encompasses the whole life costing of the entire system.
Variable speed drives are extremely energy efficient and the fast response to frequency changes, coupled with the low maintenance costs, helped to convince AES that they needed to look in this direction.
However, all variable speed drives exhibit these characteristics, so choosing the supplier for what will eventually be an order worth well in excess of GBP 2 million, had to be considered carefully.
For Parker, the next challenge, having selected the technology that was to replace their ageing motors and drives, was the ease with which a retrofit could be carried out.
Installing a new drive system in an existing plant, which wasn't designed to accommodate this type of technology, without interrupting power generation and utilising as much of the existing infrastructure as possible, was to be the biggest challenge of all.
Initially, the Drax engineers wanted to keep as many options open as possible, regarding the siting of the drives.
Because of the hostile environment near the fans, where heat and dust are a problem, it was thought that an installation inside an off plant switchroom would be favoured.
However, this meant replacing all the existing 3.3kV motor cabling.
A pilot system was ordered, which was initially due to be trialled for 12 months before the installation of the remaining systems began.
Siemens Automation and Drives, based in Manchester, set about developing an IP54 rated solution which retained all the existing wiring.
Special cabinets were designed to house water cooling units, which ensure that the variable speed drives can operate in IP54 rated housings.
Siemens had to build these to fit onto the existing concrete plinths adjacent to the fans, which meant designing a 1200 by 1200mm cabinet to hold all the necessary controls and dual redundant cooling pumps.
The existing 3.3kV cabling was retained, but careful consideration had to be given to the harmonic effects generated by 30 drives in close proximity.
"The incoming supply had to be stepped down from 3.3kV to the 500V required for the motors.
By phase shifting the drives in pairs it was possible to cost effectively reduce any harmonic effects", explains Mick Jones, of Siemens Automation and Drives.
"One of the main reasons that we chose Siemens as a supplier was their willingness to modify the standard motor, drive and control specifications to suit the existing installation", says Parker.
"Having decided to site the drives on the boiler house floor, we needed to ensure that they were rated to IP54 and fitted within the confines of the existing plinths, built nearly thirty years ago.
"This meant developing the special control cabinet as well as ensuring that all of the components; water-cooling plant, rectifier and control equipment would operate within the physical confines of the cabinet.
The original plan was to install one Siemens drive unit and test its reliability, across a variety of seasonal load conditions, for 12 months.
However, the benefits in terms of reduced maintenance scheduling quickly became apparent.
"The faster response time available from the Siemens variable speed drive, and the higher operating efficiency of the PA fan, were not really major factors influencing the purchase" explains Parker.
"However, we immediately began to see a marked difference in boiler plant management and control.
The operation of the PA fans had become a real issue internally with plant operators and maintenance fitters.
Almost as soon as the first Siemens variable speed drive became operational, the benefits became apparent".
Encouraged by the early results, and convinced that significant cost savings can be made in terms of reduced maintenance and downtime, AES has accelerated the original replacement schedule.
The remaining 29 PA fans will now be upgraded, in a roll out scheduled by Siemens Automation and Drives to be completed within the next two years.
An important factor influencing this decision was the way in which the Large Drives department, at Siemens Automation and Drives, handled the installation of the initial unit and provided support subsequently.
"There is no doubt in my mind that variable speed drive technology will deliver significant benefits at Drax", concludes Parker.
"Improved energy efficiency, faster response, soft starting and the flexibility which we can now apply to plant control are all bonuses for us.
The real issues of reducing down time and maintenance costs have been solved, and solved convincingly".
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