Alouette aluminium producer increases efficiency with new control system

Reprint from Aluminium 2006/3

Reliability, efficiency, availabilityand continuity are an aluminiumproducer’s most important needs.“Running of the plant withoutpower interruption: That’s whatwe expect from our systems toprovide,” says Ms. Line Bérubé,Vice President Corporate Affairsat Alouette. Alouette Phase II inCanada, part of the biggest alu-minium plant in North America,has increased its efficiency usingABB’s new electric power controlsystem AC 800PEC. The result:increased flexibility as well as ef-ficiency.

Alouette phase II

Alouette, an aluminium producerin Sept-Îles, Quebec, Canada, cur-rently has a capacity from its twopotlines of 550,000 tonnes peryear of primary metal. The three-year Phase II expansion was com-pleted in 2005, ahead of schedule.The 161 kV Gas Insulated Switch-gear (GIS) system is fed by HydroQuebec. The potline nominal cur-rent is 350 kA at 1320 VDC.

As shown in fig. 2, ABB deliv-ered the GIS, high voltage cables,five regulating transformers, fiverectifier transformers, five 89 kArectifier cubicles, 1320 VDC incl.

heat exchanger, five 48 MVar ofharmonic filters and the wholerectifier control system with AC 800PEC. All products and in-stallations meet CSA (CanadianStandard Association) standards.The AC 800PEC had to be integrat-ed without any problems in orderto quality for CSA approval.

Technical description

Redundancies

Five rectifiers, for N-1 operation,provide the potline current. Each12-pulse rectifier also has N-1 re-

Fig. 1: Alouette aluminium producer in Sept-Îles, Quebec

Alouette aluminium producer increases efficiency with new control systemR. Stadler, A. Ackeret and J. Ackeret, Turgi

Fig. 2: Alouette Phase II - single line diagram

dundancy per diode branch and anominal current of 89 kA. In emer-gency situations the rectifier canoperate without any controller upto 105 kA. All the required protec-tion is assured by the hardwiredbackup trip, even during emergen-cy operations.

Flexibility, integration into different networks and systems

Two independent ethernet com-munication networks are installedas shown in fig. 3. One is dedicat-ed to the data transfer between themaster and the units. This linkcontains time critical data. Theother one takes care of plant oper-ation via HMI. The rectifier unitsand the master communicate viaethernet MMS to the connectivityserver.

The CEX (Communication Ex-tention Bus) plug-in enables theAC 800PEC to connect to the mostused protocols worldwide, such as Modbus RTU, Profibus DP V2etc.

In Alouette phase II this flexibil-ity is used for the connectionbetween the AC 800PEC of themaster and the potline protectionsystem via Modbus RTU.

Via the PEC-INT board, which ispart of the AC 800PEC family, it iseasy to integrate existing PSR IIcontrollers or to integrate an

AC 800PEC into an existing PSR IInetwork.

A PowerLink connects the AC 800PEC Combi I/O boards,which are used for fast signal pro-cessing. Up to eight (distributed)Combi I/O’s are connectable onone Controller, with the option ofredundant FibreOpticLinks.

Signal isolation

To ensure the rectifier system op-erates reliably, the signal isolation

is built up over several stages. Thefirst stage is the isolation provid-ed by the device: the Combi I/O,binary inputs provide a high iso-lation level. The second stage isthat all Combi I/O devices are con-nected via a fibre optical link tothe AC 800PEC, which adds an ex-tra isolation level. Other benefitsof the fibre connection are its im-munity to electromagnetic distur-bances and the option to integratea remote I/O concept.

Digital signal processing

The Combi I/O meets the needs ofaluminium smelters. One of thegoals is to provide fast I/O han-dling for protection and control ofthe rectifier system.

Each analogue input can be pro-cessed through individual digitalfilters. Several digital filters can beselected and cascaded. This allowsthe optimal signal processing forevery kind of signal. It is very easyto filter the AC or DC componentsof a signal. Provisions are alsomade to calculate the RMS or av-erage values of a signal, for exam-ple of the line current. This set-uphas improved the stability and theresponse time of the current con-troller at Alouette.

Fig. 3: Alouette Phase II - ethernet communication network

Fig. 4: Control builder

Control Builder

The Control Builder programmingtool is a modern and efficient software tool, based on the IEC61131-3 standard. As required bythe standard, the tool allows the AC 800PEC to be programmed indifferent programming languages.These programming languages arenot dependent on a controller andcan therefore be read and pro-grammed by anybody knowingthe standard languages. This is anadvantage, as the maintenancestaff does not need a great deal oftraining. In general, of the fivepossible languages, only functionblock diagram and structured textare used, as can be seen in fig. 4.These two are the most efficient toprogram, are the easiest to readand are the most commonly used.

Efficiency

The AC 800PEC controller familyis designed for the harsh environ-ment of rectifier applications suchas substations of aluminiumsmelters. The extensive operatingtemperature range (-40°C to 70°C),rugged protection circuits, andlack of electrical connectionsbetween the controller and field allincrease the efficiency of thesystem.

AC 800PEC Combi I/O features

The combi I/O is specially devel-oped to meet the needs of alumin-ium smelters. In addition to stan-dard I/O features, the focus hasbeen set on fast I/O handling, asoftware independent trip circuit,device supervision including ahardwired trip relay in case of adevice error, a redundant powersupply, and a redundant fibre op-tical communication link to theAC 800PEC. The combi I/O pro-vides 32 binary inputs, 16 binaryoutputs, 12 fast analogue inputsand 4 fast analogue outputs.

The hardwired trip relays pro-vide a controller and a communi-cation link with an independentalarm circuit to provide a warningif any of the following failures oc-cur:

� power,� communication link,� I/O module.

In addition to the internal boardsupervision mentioned above, thecombi I/O has an integrated, inde-pendent software trip circuit withrelay output. This allows an inde-pendent trip circuit to operatewithout a controller. This so-called backup trip circuit can beused for the following purposes:

1. Run the hardwired backup tripcircuit with a controller in op-eration. This can operate twoindependent breaker coils,thereby increasing the reliabil-ity.

2. Run the rectifier in emergencyoperation mode without the

controller in operation, but stillproviding all necessary protec-tion for the rectifier equipment.

The combi I/O’s 32 optically iso-lated binary inputs include arugged protection circuit allowingsafe operation even in a tough in-dustrial environment. Each inputcan be selected individually toparticipate in the backup trip cir-cuit by means of a dipswitch. Allselected inputs are AND wired andforce the trip relay to drop if oneof the inputs drops. This solutionminimizes the source of errors incomparison to the standard prac-tice of doubling contacts via aninterposing relay. This on-boardsolution eliminates errors due todefective wiring, the breaking of awire during operation, the failureof a relay contact or of a relay.

The combi I/O also provides a re-dundant 24 V power supply. Thispower supply generates all inter-nal supplies and provides EMCprotection.

AC 800PEC CT module features

Like the combi I/O the CT (currenttransformer) module is speciallydeveloped for the needs of alumin-ium smelters, and converts the 1Aor 5A CT current signal to an elec-tronic level. The circuit providesgalvanic isolation and decoupling.Another feature that has been builton board, consists of a replica ofthe rectifier circuit to provide a DCimage of the AC currents. This sig-nal can be used for indication dur-ing operation without the control-ler in operation.

Emergency mode

It is vital for aluminium smeltersto ensure a safe and reliable pow-er supply to the potline. On therare occasion of a total breakdownof the control system, the rectifi-ers can be operated independentlyof the control system. If this hap-pens, it is important to ensure fullprotection of the rectifier systemand easy handling for the opera-tors. The emergency mode is builtas follows:

Fig. 5: AC 800PEC with the optical linkconnection to the Combi I/O

The AC 800PEC combines slowand fast tasks on one controller.The slow tasks (typically >10ms)are programmed in the ControlBuilder. The fast tasks are pre-defined and have to be parameter-ized via Control Builder. The fasttasks contain the current control,the protection and the signal pro-cessing.

� A key switch can be turned tobypass the controller.

� Main Breaker and DC Isolatorscan be operated by means of acontrol switch or manually.

� Regulating Transformer OLTCcan be operated by means of apush button or manually.

� Saturable reactor current canbe adjusted by means of poten-tiometers.

� All necessary indications areprovided via hardwired cir-cuits: OLTCposition; Saturablereactor currents; DC currents ofstar and delta system, rectifierready; breaker and isolator po-sitions.

� All devices independent of con-trollers like Buchholz, Temper-ature, over current, fuse moni-toring etc. are in operation dueto hardwired backup trip cir-cuit.

The special features described inthe previous chapters of the com-bi I/O and CT module ensure safe,reliable and easy to use operationsduring emergencies.

Trip and alarm handling

All binary inputs provide the op-tion of recording events SOE (se-quence of events). This means thata changing signal at a binary in-put channel can be identified to-gether with the time stamp whenthe change occurred. These eventsare transmitted via Ethernet/MMSto the rectifier common OPC con-nectivity server. As all AC 800PECinternal clocks are synchronized,an exact event list can be generat-ed out of the OPC connectivityserver data. This has the big ad-vantage that if an incident occurs,

the exact sequence of eventsis available for fast and effi-cient fault tracing.

Potline current control

For a given number of pots,potline load is consideredfairly stable. However, as allpots are not connected onthe first day, the voltage in-creases progressively duringstart-up. After all pots havebeen connected, mainte-nance work will periodicallytake pots out of operation, there-by changing the potline voltage.Even with a consistent number ofpots in operation, the potline volt-age is disturbed due to a phenom-enon called “anode effect”. Thisaffects the stability of the current.For these reasons the rectifier volt-age/current needs to adapt to everchanging potline conditions.

As shown in fig. 6, the potlinecurrent during an anode effect re-mains stable. This was achieved bytwo cascaded current controlloops. The inner one was imple-mented on the AC 800PEC of theunit, and maintains the current atsetpoint given by the AC 800PECof the master. The superimposedcurrent controller on the mastermaintains the potline current atthe given potline setpoint. Theunit measures its AC current in theCT of the rectifier transformer andcalculates the DC current. Thismethod is very reliable and has anaccuracy rate of more than Cl 0.5at the operating point.

A LEM helps to carry out thepotline DC current measurement.The signal is read by a third partysystem and sent over Modbus RTUto the master controller. The mas-

ter controller verifies continuous-ly the values of the rectifier unitsand the LEM. If the LEM unit fails,it switches back to the values sentfrom the rectifier units. Thatmeans that no overshoot of thepotline current occurs even if thefeedback signal to the master PIcontroller is missing. All transfersof setpoints are made seamlessly.The result: the potline works con-tinuously and is fully reliable.

Fig. 6: Typical anode effect, red channel: potline current, blue channel: potline voltage

Authors

Räto Stadler, electrical engineer, headof Base Metal Engineering, High PowerRectifiers, ABB Switzerland Ltd., Turgi,Switzerland.

Alexander Ackeret, electrical engineer,responsible for engineering and com-missioning of the Alouette phase IIproject, and deputy of head of BaseMetal Engineering, High Power Rectifi-ers, ABB Switzerland Ltd., Turgi, Swit-zerland.

Jenny Ackeret, Marketing & Communi-cation, High Power Rectifiers, ABBSwitzerland Ltd., Turgi, Switzerland.

ABB Switzerland LtdHigh Power RectifiersAustrasseCH-5300 TurgiSwitzerland

Web www.abb.com/rectifiersEmail hpr@ch.abb.comTel: +41 - 58 - 589 39 39Fax: +41 - 58 - 589 30 00 3B

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