OutputandEfficiencyIncreaseonInstalledGasTurbineSetsConsiderationofinvolvedRisksitsMitigationandAssociatedCostsonElectricalPowerEquipment

Contents1. Task ................................................................................................................................32. Summary .........................................................................................................................43. Overview of Electrical Components ................................................................................54. General Solution Finding Process ....................................................................................65. Electrical Components .....................................................................................................7

5.1 Generator ......................................................................................................................75.2 Excitation .....................................................................................................................95.3 Generator Leads / Bushings ..........................................................................................95.4 Current Transformers / Voltage Transformers ...............................................................95.5 Generator Bus bars...................................................................................................... 105.6 Generator Breaker ....................................................................................................... 105.7 Transformer ................................................................................................................ 105.8 Protection Systems ...................................................................................................... 11

6. Conclusion .................................................................................................................... 11

1. Task

It is foreseen to provide an overview of the risks and the associated mitigation costsfor the electrical key components:

For the analyses a 7EA GT in single cycle operation is considered:

a) GT Power Output at ISO-Conditions: 82,3 MWb) Performance at Sea Level and 120F/49C Ambient: 65,8 MW

In order to increase the performance to or above ISO-conditions the installation of sixTurboPHASE Modules is envisaged with a theoretical power output of 83,3 MWwhich represents a theoretical power increase of 26 % to the ambient performancementioned above.

Provided the voltage level of the Generator will remain the same as well as theelectrical functionality within the limits of the power chart, a Stator Current Increaseof about 20% percent must be considered (the typical Generator Terminal VoltageLevels on the targeted GT fleet in Middle East Region are 11 kV, 13,8 kV, 15 kV).

The increased current will bring two main tasks*:

a) Leading Higher Current andb) Handling the Higher Temperatures cause by the higher current

*The generator capability of handling the higher magnetically flux in the core isdefined by its design and assumed to be provided as most Generators handle ISOcondition outputs already in winter time.

2. Summary

Generally the electrical components can be split into two categories:

a) Standard andb) Non-Standard.

The Standard items can be seen as standardized globally by its OEM irrespective ofthe location, the ambient conditions and do have a design capacity according to ISOStandards, i.e. Generator Stators, Generators Rotors, to some extent oil cooledtransformers (who capacity wise are following the Generation Equipment).

The Non-Standard items have been adapted to serve the ambient conditions (Coolers,Heat Exchangers, Pumps and Drives) and handle the electrical power at these ambientconditions (Excitation/ Exciters, Generator Breaker, Bus Bars and Leads, CTs/VTs).

The detailed considerations, which will follow this summary, lead to the below chartin which a risks and associated costs are simplified presented.

COST

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Figure 1: Risk/Cost Overview

3. OverviewofElectricalComponents

The Electrical Power Components on a turbine set which need to be reviewed in caseof an upgrade can generally be summarized as follows (refer to figure 2):

1. Generator2. Excitation / Exciters3. Leads / Bushings4. Current Transformers / Voltage Transformers (CTs / VTs)5. Bus Bars6. Generator Breaker7. Transformer8. Protection Systems

Figure 2: Electrical Components Overview

4. GeneralSolutionFindingProcess

For all above named components individual considerations have to be carried outaccording to below simplified chart.

Figure 3: Solution Finding Process Flow

5. ElectricalComponents

5.1Generator

As most of the generators are designed to operate in ISO conditions (this is easilyconfirmed by consulting the manuals and name plates), their design capability is given,i.e. a 7EA generator is able to handle 82 MW and its related current.

The electrical output of a generator in the above set-up is depending on the temperaturearound its main components, i.e. Stator Bars, Rotor Coils, Iron Core and the capability oftransferring this heat away from it. This temperature is generally named Cold Air / ColdGas / Cool Air temperature and is defining the temperature inside the generator beforetouching the main components (refer to figure 4).

Figure 4: General Cooling Scheme inside Generator

Further, the cold air temperature is the temperature at the downstream side / dischargeside of the Generator Coolers whereas the Hot Air/ Hot Gas temperature is measuredwhen the Air/Gas flow has passed already the main components (refer to Figure 4).Depending on the two main cooling configurations

a) Open Ventilated - OVb) Total Enclosed Water to Air Cooling - TEWAC

the hot air is discharged outside the Generator (a) or into the upstream side of theGenerator coolers (b). It has to be mentioned that Hydrogen Cooled Generators (i.e. 7FHetc.) are operating in the enclosed configuration.

The Cold Air Temperature is hence to be understood to be kept stable irrespective of theambient temperatures which is a function of the cooler capacity which again depends onthe cooling water inlet temperature, the cooling water flow, etc. The cold air flow throughthe coolers is not to be changed as it is defined by the ventilator on both sides of thegenerator rotor (refer to figure 4) and would need expensive reverse engineering.Example:

A 7EA GT operating in simple cycle is coupled to a Brush Generator with below designcapabilities (refer to figure 5). For providing the electrical output equivalent to the GToutput as per ISO conditions of 82 MW, the cold air temperature (cooler dischargetemperature) need to be maintained at around 25C/77F, irrespective of the ambientconditions.

Figure 5: Diagram of a Frame 7 Generator, BRUSH made

In case the cold air temperature is not maintained at the same level, as a result the hot airtemperature increases and can run into alarm and trip level. Changing these temperaturesettings to a higher value is not recommended and normally not considered as a solution.

Solution:

As the cooling systems and especially the balance of plant with its different configurationsmay differ from site to site, the suggestion is to build a database based on current and yetto gained site set-up know how. This database provides a standardized approach for eachplant by adapting different measures such as (Fin Fan Coolers, New Pumps, Pre-Coolers,etc). As an input base to such database a simple questionnaire should be used givinginformation about respective parameters (water temperature, water flow, coolernameplate, pump nameplate, etc). The questionnaire can be used by sales force whenvisiting side or by sending to the customer. As per my experience as long as the customersees the generator is operating within in the manual provided charts and diagrams, he isassured the Generator is safe. In case of design and reverse engineering inside theGenerator, the whole picture changes and the OEM is requested to approve, which shouldbe avoided.

5.2Excitation

In order to provide a higher power output, the generator current has to be increased. Byramping up the rotor current (also known as field current or excitation current) the rotorfield will be more intense which leads to a higher generator current.

The increase of the excitation current is proportional to the power increase. The Excitercomponents anyhow are considered to be designed to handle the output as GT Output perISO conditions and its related current.

Solution:

The nominal Field Current requested for the respective generator output is provided withthe manual and datasheets in the customer documentation. These values have to bechecked with the nameplates and data sheets of the electrical components. In the rare caseof the excitation will not be able to handle the output, several components have to bechanged (entire exciter, Thyristors). A questionnaire can be provided by requesting allthese information and a database be maintained.

5.3GeneratorLeads/Bushings

The Generator Leads or Bushings are the links between the Generator Stator and the busbars and are made of flexible design (copper braids or laminated links). Depending on theGenerator type these links are cooled by the same air flow already described in thegenerator section.

Solution:

The Generator Leads are designed to handle the Generator current at its designed value,which is considered to be the GT output at ISO conditions. The links are subject to aircooling flow inside the generator; hence the same logic applies here as described on thegenerator section. Anyhow, the leads can be easily replaced by ones with bigger cross-section in case required.

5.4CurrentTransformers/VoltageTransformers

The current and voltage transformers are installed at several places on the electricalperiphery and measure the electrical currents and voltages of different levels. The highcurrent or high voltage (i.e. 5,000 A AC and 13,800 V AC) of the generator leads aretransformed to a lower value (i.e. 1 or 5A AC and 100 120 V AC) and these valuestransferred to the systems such as protection, synchronization, excitation, TGC andmetering cubicle for governing and controlling a safe operation.

Solution:

Same as the generator leads, the CTs and VTs are designed to handle the Generatorcurrent at its designed value, which is considered to be the GT output at ISO conditions.Their real capability can be easily confirmed by consulting the nameplates and datasheets. In the rare case the CTs are not dimensioned sufficiently, they can be replaced onthe generator as well as on the transformer side.

5.5GeneratorBusbars

The Generator bus bars are leading the electrical power from the Generator to theTransformer. The bus bars are massive copper bars which are enclosed by a pipe-likehousing on its entire length. The bus bar housing is in some cases slightly overpressurized in order to keep humidity and dust away and outside.

Solution:

The bus bars are designed to handle the Generator current at its designed value, which isconsidered to be the GT output at ISO conditions. The capability of the bars can beconfirmed by manuals or nameplates. The capability of handling higher current is definedby either the cross-section of the bar or by the ventilation of the bars in case the cross-section is not sufficient. Considering an adaption, ventilation would be the moreeconomical solution.

5.6GeneratorBreaker

In some of the power stations a generator breaker is installed between the generator andtransformer. The other set-up is to connect the GT-set to the grid by only the breaker onthe HV-side of the transformer without any disconnection between the generator andtransformer. The set-up without Generator breaker is mainly found power stationsoperating in simple cycle, where only one generator is connected to the transformer.

Solution:

The generator breaker is designed to handle the generator load which is considered to bethe GT output at ISO conditions. The capability of the breaker can be confirmed bymanuals or nameplates. The generator breaker is an enclosed system and need to bereplaced in rare case it is not be able to handle the load.

5.7Transformer

Transformers in power stations are normally considered as oil cooled and are designed toat least the current maximum output the GT delivers at ISO conditions.

As already mentioned in the generator section, the oil running through the inside of thetransformer has to have a certain temperature to handle the losses represented as heat.The oil is in forced to run through radiator banks (heat exchangers) which are installedoutside the transformer housing where a set of ventilators is established in order toenforce the cooling through the heat exchangers.

Solution:

The capability of a transformer is to be confirmed by name plates or datasheets availableto the customer. As the temperature inside the transformer (enclosed system) isdepending of the convection of the installed radiators / coolers, there are certain measuresthat can be adapted when transformer reaches its capacity limits, including forcing theconvections by ventilators of different size, increasing the number of coolers / radiatorsor using different cooler types, such as water to oil coolers.

5.8ProtectionSystems

The protections relays are governing and controlling the electrical operation of thegenerator and transformer and using the signals given by the CTs and VTs. As the VTsand CTs secondary side current is predefined and will not exceed the maximum limits(i.e. 5 A AC or 120 V AC), the panel and electrical cabling are not to be considered riskyneither are the inputs for the protection relays.

Solution:

Generally, the protection settings are to be set according to the designed value of thegenerator. It can easily be checked by consulting the commissioning or service reports andcan be changed in case required.

6. Conclusion

Provided the components that lead current are sufficiently designed for handling thecurrent which is driven at ISO conditions, the main task is to compensate the heatincrease which is caused by the higher ambient temperature.

The risk, perceived by the customer, can be minimized by operating the generator per theOEM guidelines and increasing the heat exchanger capability as required. This willrequire site specific application engineering.