PLC control for vital electrical generation

GE Fanuc PLCs provide control for vital electrical generation back-up solutions.

Broadcrown Projects, a major bespoke solutions provider in the field of electrical power back-up and peak-lopping, is increasingly turning to GE Fanuc PLCs to provide control for its electrical generator sets.

Broadcrown Projects is a sister company to Broadcrown Exports which is a leading global manufacturer of a wide range of standard electrical generating sets in the 22-4500kVA band.

To meet the increasing demand for failure-free electricity supply for such diverse industries as telecommunications, the water supply industry, food refrigeration and banking systems, Broadcrown Projects is involved in the design and manufacture of ever more complex and powerful generating sets.

Each installation will have its own specific requirements of power output and sophistication of switching in the event of mains failure.

The company has also developed CHP (Combined Heat and Power) solutions for large installations such as hospital, university, district heating schemes, theatre complexes and process industries.

Other designs capitalise on the natural production of methane in land-fill and sludge treatment works.

In place of the standard gasoline fuelled engine, a gas-burning engine is used in the generator set.

The electricity generated is used to power equipment used in other parts of the installation, while any excess may be exported to the National Grid.

In sludge treatment applications heat from the engine exhaust and from the engine cooling system can then be recycled to the digesters in the plant, increasing the release of methane still further.

As a basic requirement, the engine needs to be monitored for oil pressure and temperature and coolant level and temperature.

Transducer measurements may act as warnings to an operator, or to provide automatic stoppage in potentially catastrophic conditions.

These measurements are fed to an adjacent control panel or cabinet where they can be displayed as simple analogue or digital meter readings and warning lights.

However, for the more advanced bespoke systems, far more sophisticated control is enabled by channelling the measurements to a PLC.

This also provides far greater flexibility as later changes to the configuration can be handled easily by simply amending the PLC program.

Far greater benefits can accrue from the use of a PLC.

For example, it can communicate directly with a building management system so that gradual switching of the load onto the generator across a complete site can be carefully controlled.

A PLC can also provide signals to a SCADA system.

This enables not only centralised on-site control, but also the possibility to control the system remotely.

This can be particularly useful for remote installations, which may be vital for the pumping of water, or as part of a telecommunications infrastructure.

Signals may indicate the need for maintenance or parts replacement, while some adjustments may be possible from the control site.

Broadcrown, in fact, provides a service to some of its customers to monitor the status of their installations from its main central site at Hixon, Staffordshire.

While Broadcrown purchases the engine and cooling system, the generator, PLC, control cabinet and other electrical components, it designs and fabricates the rest of the system and assembles the complete unit at Hixon.

This construction includes the welded base-frame on which the engine and alternator are assembled; the wiring of the control cabinet or panel; the containerisation if located outside; and any purpose-designed fuel and lubrication oil tanks the particular site may require.

Broadcrown may also design and fabricate bespoke exhaust systems and remote coolant header tanks and radiators.

Remote radiators are commonly used if the unit is installed in an enclosed space, for example in the basement of a building.

The complete systems are normally fully tested at Hixon to minimise onsite testing and commissioning.

In terms of the number of bespoke units that Broadcrown Projects sells each year, the major proportion are sold for use solely in the event of mains failure.

UPS systems may provide temporary back-up with a gradual or rapid transfer of load to the generator set in the event of a prolonged outage.

Different degrees of sophistication are employed for the change over depending on the application.

A vital part of the control is achieved using a PLC.

Broadcrown typically uses GE Fanuc 90O-30 PLCs offering from 80 I/O to 256 I/O.

Becoming increasingly more popular, others are designed as peak-loppers, to be used regularly to avoid high electricity tariffs at times of peak demand, as well as to provide an alternative means of supplying electricity during a mains failure.

A typical peak-lopping generator is rated at 2MW.

Its prime duty would be its availability at all times as a stand-by to the normal mains supply, but would be run during set periods of peak demand on the Grid.

These periods are scheduled in advance based on the supplying electricity board's charging structure.

The generating set will automatically cut-in using a sophisticated soft load transfer, and the user will thus avoid expensive peak tariff charges.

The Triad periods, historically fixed during the winter, are peaks in the demand that the regional supply companies use to set their annual charges, and are therefore particular periods when the peak-loppers will be used to minimise the user's supply costs.

Of vital importance in any electrical generating set when paralleling with the National Grid supply is the need to match the set with the mains i.e maintain the 50Hz frequency and correct output voltage under any load conditions.

Also, there is the ability to synchronise the sine waveform to that of the Grid when exporting excess electricity.

According to Steve Horner, Chief Electrical Engineer at Broadcrown, "This involves a complex combination of feedback and control, based on a detailed experience of the characteristics of the diesel engines and alternators employed and how they respond to different loadings." Each engine has its own governor system which controls the fuel injection based on feedback from the speed of its flywheel, and each alternator has its own Auto-Voltage Regulator (AVR).

However, the generator set will have different load demands on different occasions and these systems are not enough to maintain the steady 50Hz and fixed voltage output to sufficient accuracy.

The turbo-lag interval between the loading of an engine and its response is a typical example of the need for further sophistication.

To overcome problems such as this and maintain the correct voltages and frequency, a special synchroniser unit is installed in the adjacent control cabinet.

The synchroniser also monitors the phase angle of the mains supply and ensures that the generator set matches it before it exports electricity to the Grid.

Broadcrown Projects has recently installed a complete 11kV turnkey project for the South Staffordshire Water installation at Hampton Loade in Worcestershire.

In order to ensure the continued availability of water pumping from the river Severn, and the subsequent water treatment to make it potable, the company ordered three 2MW generator sets from Broadcrown.

Broadcrown was responsible for the design and installation of the complete installation including the building that houses the generators, the noise baffling enclosures for each unit, the fuel and lubricating oil tanks, and the connection to the 11kV sub-station.

An integral element of the project is the system's ability to provide peak-lopping to avoid excessive mains electricity charges.

As this typically involves using the units on a daily basis, system readiness is maintained by keeping the engine coolant and lubricating oil close to their operating temperatures.

Data from the transducers on each generator set is captured by local GE Fanuc Genius I/O units on the SCADA system.

Data is fed back to a 90O-30 PLC in the corresponding control unit in the control room.

The PC in the control room communicates with these PLCs via the Genius Bus which also links with the control system in the electrical sub-station.

"We are now starting to utilise HMI panels to replace the traditional analogue / digital metering and indicator lamps on the control panels.

This provides greater flexibility for amending and upgrading the system once installed.

Re-programming of the PLC to provide additional information on a display is far quicker and more efficient than physically amending and re-wiring the control cabinet.

We have found the GE Fanuc PLCs to be very reliable in operation, easy to program, and competitive on price," says Horner.

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