informe de auditoría entral guacolda 2015 · 2015. 11. 25. · informe de auditoría central...

Informe de Auditoría Central Guacolda - 2015

25 de noviembre de 2015

Informe de Auditoría Central Guacolda - 2015 2

1 Introducción

La operación de la central Guacolda, la oferta de energía proveniente de fuentes renovables no convencionales (solares y eólicas) y las limitaciones del sistema de transmisión troncal han provocado que el despacho en la Zona Norte del Sistema Interconectado Central (SIC) resulte complejo desde el punto de vista de la flexibilidad operacional. Dado lo anterior esta Dirección Técnica, y en vista de las atribuciones dispuestas en el artículo 36 del Decreto Supremo 291 del 2007 (realizar “las auditorías que sean necesarias para el cumplimiento de las funciones que la normativa eléctrica vigente encomienda a esta Dirección”), ha decidido realizar una auditoría sobre algunos de los parámetros de operación de las unidades generadoras de la central Guacolda que pudieran incidir en la flexibilidad de su operación. En apoyo a esta auditoría, la DO contrató los servicios especializados de la empresa STEAG Energy Services GmbH. En el Anexo B de este informe de auditoría se puede encontrar su reporte. El Proceso de Auditoría incluyó la revisión de la operación de las unidades generadoras y el análisis de los documentos solicitados por la DO al Coordinado de la central Guacolda. Esta auditoría contempló además la información recabada en terreno durante la visita a la central realizada entre los días 1 y 4 de septiembre de 2015, junto con entrevistas al personal de operaciones y mantenimiento e inspección de las instalaciones en esa oportunidad.


2 Parámetros Auditados y Resultados

2.1 Lista de Parámetros Auditados

La siguiente lista corresponde a los parámetros auditados y sus respectivas definiciones:

Mínimo Técnico (MW). Potencia activa bruta mínima con la cual una unidad puede operar en

forma permanente y estable inyectando energía al sistema eléctrico en forma continua.

Tiempo de Estabilización (horas). Corresponde al tiempo en que la unidad debe permanecer en

carga estable antes de recibir una orden de:

a) reducción de carga cuando la unidad está aumentando su generación; o,

b) aumento de carga cuando la unidad está disminuyendo su generación.

Tiempo de Partida en Frío (horas). Corresponde al tiempo transcurrido entre la orden de partida

de la unidad y la operación a mínimo técnico. Supone que la unidad se encuentra detenida y a

temperatura ambiente o cercana a ella.

Tiempo de Partida en Caliente (horas). Corresponde al tiempo transcurrido entre la orden de

partida de la unidad y la operación a mínimo técnico. Supone que la unidad se encuentra detenida

a temperatura de operación.

Tiempo Mínimo en Servicio (horas). Corresponde al tiempo mínimo que la unidad debe estar en

servicio antes de dar orden de detención.

Tiempo Mínimo Fuera de Servicio (horas). Corresponde al tiempo mínimo que la unidad debe

estar fuera de servicio antes de dar la orden de partida.

Costo de Partida en Frío (US$). Costo expresado en dólares incurrido por la unidad desde recibida

la orden de partida hasta que la unidad alcanza su mínimo técnico, considerando una partida en

frío.

Número Máximo de Partidas al Año (#/año). Número máximo de partidas en frío que la unidad

puede realizar en un año y sin afectar su vida útil.

Originalmente, la lista incluía también:

Costo de mantener unidades en “banking” (US$/h)

Costo medio para operar entre 0 MW y Mínimo Técnico, en escalones de 10 MW (US$/MWh)

Durante el Proceso de Auditoría se constató que estos últimos dos parámetros no requerían ser establecidos porque a juicio del Consultor no es posible mantener las unidades operando en “banking” ni operar en forma estable entre 0 MW y su Mínimo Técnico. Este antecedente es coincidente con lo informado por el propietario de la central en su comunicación del día 1 de junio de 2015 (Anexo A).


2.2 Valores de los Parámetros Informados por el Propietario Los valores de los parámetros informados por el propietario en comunicaciones de fecha 1 de junio de 2015 y 18 de agosto de 2015 son los siguientes:

U1 U2 U3 U4

a) Mínimo Técnico MW 60 60 75 60

b) Tiempo de Estabilización h 2:00 2:00 2:00 2:00

c) Tiempo de Partida en Frío h 13:00 13:00 10:00 10:00

d) Tiempo de Partida en Caliente h 2:30 2:30 1:10 1:10

e) Tiempo Mínimo en Servicio h 8:00 8:00 4:00 4:00

f) Tiempo Mínimo Fuera de Servicio h 4:00 4:00 4:00 4:00

g) Costo de Partida en Frío US$ 46.770 46.770 41.170 41.170

h) Número Máximo de Partidas al Año #/año 10 10 10 10

En relación al parámetro Tiempo Mínimo Fuera de Servicio, de acuerdo a la información

proporcionada por el Coordinado en su comunicación AES Gener N° 55/2015, de fecha 10 de

noviembre de 2015 (Anexo C), se desprende que el valor originalmente informado está siendo

modificado a un valor nulo, es decir las unidades generadoras de Guacolda no tienen restricciones

para atender una orden de partida inmediatamente después de una detención.

2.3 Valores de los Parámetros Observados en el Proceso de Auditoría

Habida cuenta de todos los antecedentes recabados en el proceso de auditoría, se puede concluir que

los parámetros auditados tienen los valores observados que se muestran en la siguiente tabla. Cabe

tener presente que por valores observados se debe entender: valor justificado por el fabricante,

valores constatados con la operación real de las unidades o valores esperables para este tipo de

unidades, según se explica más adelante para cada caso:

U1 U2 U3 U4

a) Mínimo Técnico MW 60 60 75 60

b) Tiempo de Estabilización h 2:00 2:00 2:00 2:00

c) Tiempo de Partida en Frío h 15:03 13:51 11:33 13:09

d) Tiempo de Partida en Caliente h 1:00 7:36 2:06 5:06

e) Tiempo Mínimo en Servicio h 2:00 2:00 2:00 2:00

f) Tiempo Mínimo Fuera de Servicio h 0:00 0:00 0:00 0:00

g) Costo de Partida en Frío US$ 49.435 49.352 62.831 45.171

h) Número Máximo de Partidas al Año #/año 10 10 10 10


Al respecto, cabe realizar los siguientes comentarios:

(a) Mínimo Técnico

Los valores de Mínimo Técnico de cada una de las unidades que se pueden deducir de las

observaciones realizadas durante la auditoría son consistentes con los valores informados por el

propietario a la DO.

Dichos valores observados fueron deducidos a través de evidencia documental presentada por el

propietario y a través de los registros de operación del Sistema de Control Distribuido (DCS por su

sigla en inglés).

En relación con la evidencia documental, el fabricante del equipo ha informado al propietario de la

central que efectivamente el mínimo técnico de las unidades es el 40% de la capacidad máxima

(MCR: Maximum Continous Rate). En el caso particular de la unidad 3 aparece una restricción

adicional en la caldera debido al uso de petcoke, lo que causa que el mínimo técnico de esta

unidad sea un 50% del MCR. Los registros del DCS proporcionados por el propietario son

consistentes al respecto.

Si bien la información entregada por Guacolda está debidamente respalda con los informes del

fabricante de las unidades, en opinión del Consultor, basada en su experiencia en instalaciones de

similar capacidad y configuración, los mínimos técnicos de diseño para la turbina están

generalmente en el rango de 10% a 20% de MCR.

(b) Tiempo de Estabilización

Los valores de Tiempo de Estabilización de cada una de las unidades han sido estimados de las

observaciones realizadas durante la auditoría y son consistentes con los valores informados por

el propietario a la DO. En el caso de la unidad 2, esta consistencia supone que efectivamente se

concrete el cambio de material de los tubos del sobrecalentador 4.

En este caso los valores observados fueron deducidos del análisis de los datos del DCS entregados

por el propietario para un escenario de variación de carga en cada unidad. En dichos escenarios, a

partir de los registros efectivamente se aprecian oscilaciones en algunas de las variables,

principalmente de temperatura y presión ante un cambio en su punto de operación.


El Consultor que apoyó a la DO en esta auditoría ha señalado que, de acuerdo a su experiencia en

instalaciones similares, el tiempo de estabilización debería ser próximo a los 15 minutos para

cambios de carga bajo el 75% de MCR. Para cambios de carga sobre este porcentaje no debería

requerirse un tiempo de estabilización.

A juicio del Consultor, la causa más probable de la diferencia entre el Tiempo de Estabilización

observado y el estándar es la sintonización de los lazos de control para operar con subidas y

bajadas de carga, dado que dicha sintonización estaría con ajustes originales para operar a carga

base. Por otra parte, no se observó que la expansión térmica de la turbina fuese actualmente una

limitante para la estabilización de las unidades.

(c) Tiempo de Partida en Frío

Los Tiempos de Partida en Frío informados a la DO por el propietario son consistentes con los

tiempos observados en el Proceso de Auditoría. Las diferencias entre ambos conjuntos de

valores pueden atribuirse a las particularidades de cada caso.

Los valores de Tiempo de Partida en Frío de cada una de las unidades fueron estimados a partir

del análisis de los datos del DCS entregados por el propietario para una partida en frío para cada

unidad y de la documentación técnica del fabricante que también fue exhibida por el propietario.

No obstante lo anterior, es relevante destacar lo siguiente:

Existe una diferencia entre la definición que actualmente tiene el CDEC SIC respecto del

tiempo de partida y la definición aplicada por el propietario para calcular el valor

informado. En efecto, el CDEC SIC considera como tiempo de partida el período que se

extiende desde la orden de partida hasta que la unidad alcanza su mínimo técnico. Por el

contrario, los valores informados por el propietario consideran el período entre el

encendido de la caldera y la sincronización de la unidad.

Para efectos de comparación, y dada la diferencia explicada en el punto anterior, se

contrastaron los valores informados con los valores observados entre el encendido de la

caldera y la sincronización.

En el caso del escenario de partida en frío entregado por el propietario para la unidad 1,

se detectó un tiempo de 2 horas de “espera” entre el momento en que la turbina alcanzó

plena velocidad y la sincronización. Dicho lapso no fue computado para efectos de calcular

el tiempo observado.


En el caso del escenario de partida en frío entregado por el propietario para la unidad 3,

se produjeron dos “trips” en el proceso. Para tener un tiempo estimado realista de lo que

debería haber durado el proceso en circunstancias normales, se hizo una estimación del

tiempo extra producido por los trips y se descontó para efectos de calcular el tiempo

observado.

La experiencia del Consultor en instalaciones de similares características señala que para partidas frías (con tiempos de detención de 24 horas o más) el tiempo de partida puede ir entre 6:00 horas y 6:30 horas. Las causas probables para la diferencia que se observa entre los tiempos de partida en frío informados y los tiempos que podrían considerarse estándares tienen relación con que las unidades no cuentan con automatización en el proceso de partida y el hecho que el incremento de temperatura y presión es más bien conservador durante el proceso.

(d) Tiempo de Partida en Caliente

Los Tiempos de Partida en Caliente informados por el propietario a la DO son consistentes con

los tiempos observados para las unidades 1 y 3. En tanto que los tiempos de las unidades 2 y 4

mostraron Tiempos de Partida en Caliente muy superiores a los informados y que se explican

por los tiempos transcurridos desde que las unidades salieron de servicio y por las fallas

ocurridas durante el proceso de arranque.

Los tiempos de partida en caliente de las unidades fueron estimados, al igual que en el caso

anterior, a partir del análisis de los datos del DCS entregados por el propietario ante una partida

en caliente para cada unidad y de la documentación técnica del fabricante.

Al respecto, se puede destacar lo siguiente:

Aplica también a este caso la diferencia señalada para los tiempos de partida en frío,

respecto de la definición empleada por el propietario y la definición del CDEC SIC. Debido

a ello, para efectos de comparación se contrastaron los tiempos desde encendido de la

caldera hasta la sincronización.

Se evidencia una diferencia en las condiciones iniciales de cada una de los proceso de

partida observados.

La partida de la unidad 1 ocurre casi inmediatamente después de un trip (15 minutos), por

lo que las condiciones de la unidad son óptimas, resultando en un tiempo observado

incluso mejor que el tiempo informado.

La partida de la unidad 2 registra un tiempo de espera de 8 horas antes de girar/empujar

la turbina. Dicho tiempo fue descontado.


Valores de este parámetro que se pueden considerar como estándar están alrededor de 1:30 a 2:00 horas para detenciones menores que 8 horas, y entre 3:30 a 4:00 para detenciones entre 8 y 24 horas. Las desviaciones de los Tiempos de Partida en Caliente observados en la auditoría se explican por los tiempos transcurridos desde que las unidades salieron de servicio y por las fallas ocurridas durante el proceso de arranque.

(e) Tiempo Mínimo en Servicio

Del Proceso de Auditoría no se obtuvo información suficiente para sostener que el Tiempo

Mínimo en Servicio de las unidades generadoras sea consistente con lo informado a la DO.

A partir de la información recogida en el Proceso de Auditoría se deduce que las unidades

generadoras de Guacolda no requieren de un Tiempo Mínimo en Servicio mayor que el Tiempo de

Estabilización. Es decir, a juicio de la DO las unidades generadoras de Guacolda deberían ser

capaces de iniciar su detención una vez transcurrido el Tiempo de Estabilización. Por esta razón, el

Tiempo Mínimo en Servicio debiera ser igual al Tiempo de Estabilización.

(f) Tiempo Mínimo Fuera de Servicio

Los parámetros de Tiempo Mínimo Fuera de Servicio informados a la DO por el propietario son

consistentes con los observados en el Proceso de Auditoría.

Tal como se señala en 2.2.- Valores de los Parámetros Informados por el Propietario, de la

información proporcionada por el Coordinado en su comunicación AES Gener N° 55/2015, de

fecha 10 de noviembre de 2015 (Anexo C), se desprende que este parámetro es nulo. Es decir las

unidades generadoras de Guacolda no tienen restricciones para atender una orden de partida

inmediatamente después de una detención. En consecuencia, en caso que una unidad generadora

se encuentre fuera de servicio, el único tiempo que es necesario para que la unidad generadora

vuelva a estar en servicio es el Tiempo de Partida correspondiente.

(g) Costo de Partida en Frío

Del análisis de los valores observados se puede indicar que dichos valores son consistentes con

los valores previamente informados por el propietario a la DO.


Los costos de partida en frío fueron estimados a partir de los tiempos de partida en frío indicados

en la letra c) anterior, los consumos de combustible de cada partida obtenidos del DCS y los costos

de combustibles informados por el propietario.

La diferencia en el caso del valor de costo de partida en frío de la unidad 3 está condicionada

porque su tiempo de partida resultó excepcional a consecuencia de los dos trips que se

presentaron durante el arranque de la unidad.

(h) Número Máximo de Partidas al Año

Los valores de número máximo de partidas por año de cada unidad que se pueden deducir de

las observaciones realizadas durante la auditoría son consistentes con los valores informados

por el propietario a la DO.

Dichos valores observados fueron deducidos a partir de evidencia documental presentada por el

propietario.

Al respecto es relevante tener en cuenta los siguientes comentarios aportados por el Consultor

que apoyó a la DO en esta auditoría:

El número máximo de partidas al año se refiere a partidas en frío (más de 24 horas

fuera de servicio).

Cada partida en frío causa estrés térmico en ciertas componentes de la unidad

generadora (como por ejemplo en la turbina) deteriorando su vida útil remanente.

No debería haber una limitación en el número de partidas por año. En su lugar se

debiera establecer un límite en el número total de partidas a lo largo de la vida útil.

Definir valores estándares para este parámetro depende del criterio de diseño de la unidad

generadora respecto de su régimen de operación. Para unidades generadoras diseñadas para

operar regularmente en base, se puede estimar el número de partidas en frío a lo largo de su vida

útil en 60, lo que equivale a un promedio de 2 partidas por año. Para unidades generadoras

diseñadas para operar bajo ciclos de operación (“cycling power plant”), el número total de

partidas durante la vida útil es aproximadamente de 180, es decir en promedio 6 partidas por año.

Se puede observar que este parámetro, además de ser consistente con lo informado a la DO,

también lo es con los estándares para plantas similares.


3 Posibilidades de Mejora

El Consultor que apoyó a la DO en esta auditoría indicó una serie de áreas en las cuales se pueden

implementar mejoras tendientes a aumentar la flexibilidad en la operación de las unidades de

Guacolda. Es importante destacar que estas mejoras no han sido evaluadas económicamente y

que los valores de inversión han sido estimados a partir de la experiencia de STEAG en este

campo.

(a) Optimización del proceso de partida

Automatización o semi-automatización del proceso de partida, permitiendo adaptar el proceso a

las condiciones reales, así como el apagado automático de los quemadores y el giro de la turbina

tan pronto las condiciones necesarias sean alcanzadas. Instalación y uso de un calculador

predictivo de estrés térmico, lo que permite controlar de manera óptima el flujo de combustible

en la partida de modo que el estrés térmico permanece dentro de los límites permitidos.

Actualmente el proceso es manual, por lo tanto el incremento de presión y de temperatura se

hace de manera conservadora.

La implementación de la optimización del proceso de partida, con los elementos indicados en el

párrafo anterior, tiene impacto sobre los tiempos de partida, en frío y en caliente, sobre los costos

de partida (por reducción de tiempos) y en menor medida sobre el número máximo de partidas

por año.

El costo de inversión estimado es de USD 400.000 por unidad, asumiendo que no se requieren

cambios de equipamiento adicionales. Se debe considerar que para comisionar correctamente el

sistema se requieren alrededor de 6 partidas por unidad.

(b) Instalación de bypass de alta presión y condensador auxiliar de descarga

Instalación de un bypass para pasar el vapor desde la caldera directamente al condensador auxiliar

sin pasar por la turbina.

La implementación de un bypass de alta presión y condensador auxiliar de descarga tiene impacto

sobre los tiempos de partida en frío y en caliente, sobre los costos de partida (por reducción de

tiempos) y en menor medida sobre el número máximo de partidas por año.

El costo de inversión de una estación bypass de 60% de alta presión, con condensador auxiliar

dump, incluyendo piping, aislación, obras civiles y montaje, asciendo a USD 2.500.000 por unidad.


(c) Optimización y sintonización de los lazos de control

La recomendación en este caso incluye un análisis y sintonización de los lazos de control, tanto en su estructura como en los parámetros. Es probable que en el proceso de realizar esta optimización, sea necesario el cambio de algunos actuadores o medidores. Si el mínimo técnico de las unidades se reduce, esta revisión y adaptación del control se hace completamente necesaria, ya que las unidades no han sido comisionadas para mínimos técnicos más bajos que los actuales. Esta optimización tiene impacto directo en el tiempo de estabilización de la unidad, junto con tener impacto en la reducción del estrés causado por los cambios de carga y la mejora en las tasas de cambio de carga. El costo de inversión estimado para esta mejora es de USD 500.000 por unidad.

(d) Sistema de monitoreo de vida útil

Un sistema de monitoreo de vida útil, en conjunto con un calculador de estrés térmico, puede

acumular la información del estrés al que están siendo sometidas las paredes de los componentes,

en particular en los procesos de partida, y de esta manera administrar de mejor forma la

operación con mayor flexibilidad de las unidades generadoras.

La instalación de este sistema, aunque no tiene un efecto directo, podría incidir en el

mejoramiento del número máximo de partidas por año, del tiempo de partida en frío y del costo

de dichas partidas.

El costo de inversión estimado es de USD 100.000 por unidad.

Adicionalmente, y en relación con la posibilidad de reducir el mínimo técnico de las unidades

generadoras, el Consultor que apoyó a la DO en esta auditoría sostiene que el mínimo técnico

actual de la caldera no es la principal restricción y que la limitación para operar bajo el 40% MCR

estaría en la turbina. Para determinar la causa precisa y poder implementar acciones tendientes a

reducir los actuales mínimos técnicos de las unidades generadoras, el Consultor sugiere investigar

en detalle las posibles limitaciones de las turbinas tipo Westinghouse de la central Guacolda.


4 Conclusiones Finales

En general, la información entregada por el propietario de la central a la DO es consistente con la

información que se pudo observar a partir de la evidencia documental, la evidencia física en

terreno y la información adicional recabada en las entrevistas con personal técnico de la planta.

El parámetro auditado que no cumple con lo indicado anteriormente es el Tiempo Mínimo en

Servicio, pues el proceso de auditoría no encontró evidencia suficiente que respalde lo informado

por el Coordinado a la DO. Al respecto, y de no mediar antecedentes suficientes por parte del

Coordinado, la DO considerará para el cumplimiento de sus funciones que el Tiempo Mínimo en

Servicio corresponderá al tiempo de estabilización.

Finalmente, dentro del Proceso de Auditoría se identificaron áreas de mejora al comparar los

valores de los parámetros de las unidades generadoras con valores de instalaciones de similares

características, en particular habría espacio para reducir el Tiempo de Estabilización y el Tiempo

de Partida en Frío, sujetos a las correspondientes inversiones. Con respecto a los mínimos

técnicos, se necesitan análisis adicionales para explicar las limitaciones que imponen las turbinas

de las unidades generadoras de Guacolda y que podrían estar impidiendo reducir estos valores. Al

respecto, la DO solicitará al Coordinado mayores antecedentes del fabricante de las unidades.


Anexo A Información del Propietario de Valores de Parámetros


Anexo B Reporte de STEAG Energy Services

STEAG Energy Services

Final Report on the Audit of Guacolda Power Plant

for CDEC-SIC, Chile

17.11.2015, rev. 4

Final Audit Report

No. Revision Date Page Guacolda Audit Final Report 4 17-11-2015 2 of 62

Authors Department Phone Hendrik Lens, Manfred Schesack, Osvaldo Santos SES-ET-PRC +49 201 801 2891

Guacolda Audit Report 2015-11-17 rev 4.docx

Distribution

V. Lopez (CDEC-SIC)

D. González (CDEC-SIC)

D. Lehmann (STEAG)

Title

Final Report on the Audit of Guacolda Power Plant 4x150 MW

Content

LIST OF ABBREVIATIONS ................................................................................................ 4

REMARKS ON NOTATION ................................................................................................. 4

1 EXECUTIVE SUMMARY .................................................................................... 5 1.1 Objective of the audit .................................................................................................................. 5

1.2 Main findings ............................................................................................................................... 5

1.3 Conclusions and recommendations ....................................................................................... 10

2 INTRODUCTION .............................................................................................. 11

3 AUDIT PROCEDURE ....................................................................................... 12 3.1 Audit process ............................................................................................................................ 12

3.2 Definition of the parameters .................................................................................................... 12

4 RESULTS OF PLANT WALK-THROUGH, DOCUMENT ANALYSIS AND INTERVIEWS .................................................................................................... 15

4.1 Boiler and mechanical .............................................................................................................. 15

4.2 Control and Instrumentation .................................................................................................... 16

5 ANALYSIS OF DCS DATA .............................................................................. 17 5.1 General ....................................................................................................................................... 17

5.2 Cold start-up .............................................................................................................................. 17

5.3 Hot start-up ................................................................................................................................ 21

5.4 Load changes ............................................................................................................................ 24

5.5 Conclusions ............................................................................................................................... 26

6 AUDIT RESULTS WITH RESPECT TO THE PARAMETER VALUES ............ 28

Final Audit Report




6.1 Original values .......................................................................................................................... 28

6.2 Minimum Technical Load ......................................................................................................... 28

6.3 Cold and hot start-ups .............................................................................................................. 30

6.4 Minimum Operating time .......................................................................................................... 36

6.5 Minimum Downtime .................................................................................................................. 36

6.6 Stabilization time ....................................................................................................................... 37

6.7 Cost to maintain unit in operating temperature (“banking”) ................................................ 38

6.8 Cost of operation for powers between 0 MW and Minimum Technical ............................... 39

6.9 Parameters as determined in the audit ................................................................................... 41

7 RECOMMENDATIONS .................................................................................... 42 7.1 Minimum load of the turbine .................................................................................................... 42

7.2 Minimum load of the boiler ...................................................................................................... 42

7.3 Start-up process optimization ................................................................................................. 43

7.4 Installation of a HP bypass station and a dump (auxiliary) condenser ............................... 43

7.5 Control optimization and tuning .............................................................................................. 44

7.6 Lifetime monitoring system ..................................................................................................... 44

7.7 Verification of start-up time definition .................................................................................... 44

8 STEAG ENERGY SERVICES .......................................................................... 45

9 FIGURES .......................................................................................................... 45

Final Audit Report




List of Abbreviations

Abbreviation Meaning

barg Bar gauge (unit of pressure relative to atmosphere)

°C Degrees Celcius (unit of temperature)

DCS Distributed Control System

CDEC-SIC CDEC Sistema Interconectado Central

FGD Flue Gas Desulfurization

HFO Heavy Fuel Oil

HP High Pressure

IP Intermediate Pressure

K Kelvin (unit of absolute temperature and of temperature difference)

kUSD 1000 US Dollar

LP Low Pressure

MCR Maximum Continuous Rate

MSV Main Stop Valve

MW Megawatt (unit of electrical power)

N/A Not applicable

PI(D) algorithm “Proportional integral (derivative)”, a class of standard algorithms for au-tomatic control

RH Reheater

SCR Selective Catalytic Reaction

TMCR Turbine Maximum Continuous Rate

USD US Dollar

Remarks on notation 04:50 h denotes a time duration of 4 hours and 50 minutes

04:50 denotes a time of day (50 minutes past 4)

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1 Executive Summary 1.1 Objective of the audit To obtain an optimal economic dispatch that respects the technical restrictions of power plants, CDEC-SIC uses different parameters to describe those restrictions. Each power plant is parameterized according to its capabilities. To set the parameter values correctly, CDEC-SIC has to rely on the information provided by the power plants. In cooperation with STEAG Energy Services GmbH, CDEC-SIC has conducted an audit in the Guacolda power plant units 1-4 to verify the parameters.

The objectives of the audit were:

• Comparison of parameters informed by the owner (Guacolda) against information obtained from documents or gathered during the site visit (interviews or data)

• Evaluate parameters informed by the owner (Guacolda) with respect to values that could be consid-ered as realistically achievable based on STEAGs experience for similar facilities

• Provide recommendations if there are significant differences between the parameter values informed and the values that are considered to be achievable.

1.2 Main findings The following tables summarize the main findings of the audit and compare these with the values informed by the owner of the power plant as well as state of the art values. The definition of the parameters is provid-ed in Section 3.2.

In each table, the source of the values observed is indicated by one or more of the following:

a) analysis of DCS data provided by owner b) manufacturer document provided by owner (operation manual, for example) c) other document provided by owner (for example Docto 11, Parte 1, describing own experience with

stability of unit operation) d) document provided by owner with question by owner and corresponding answer from manufacturer e) answer from Guacolda staff in interviews

Remark: The state of the art values are based on experience of STEAG in numerous hard coal fired power plants. Specific technical restrictions that go beyond the scope of this audit may apply to the Guacolda units.

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Minimum Technical Load (% of full load)

Value informed

Value observed

Sources Consistent State of the art

Comments

Unit 1 40 % 40 % a, d yes 10-15 %

Unit 2 40 % 40 % a, d yes 10-15 %

Unit 3 50 % 50 % a, c yes - No state of the art as pet coke is not typically used in Germa-ny. Tests and optimization work are needed to evaluate how far the minimum load could be lowered.

Unit 4 40 % 40 % a, d yes 10-15 %

Details: Section 6.2, p. 28

Recommendations: Sections 7.1 and 7.2, p. 42

Number of start-ups

Value in-formed

Value ob-served

Consistent Source State of the art

Comments

Unit 1 10 / year 10 / year yes d > 2 per week

State of the art refers to warm or hot start-ups.




Details: Section 6.3.3, p. 35

Recommendations: Sections 7.3, 7.4 and 7.6, p. 43

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Stabilization time Value informed

Value observed


Comments

Unit 1 02:00 h 02:00 h yes a < 15 min

Unit 2 02:00 h 02:00 h yes a < 15 min

Value refers to the time after retrofit of the material of superheater 4.

Unit 3 02:00 h 02:00 h yes a < 15 min

Unit 4 02:00 h 02:00 h yes a < 15 min


Recommendations: Section 7.5, p. 44

Cold start-up time

Value informed

Value ob-served

Con-sistent

State of the art

Source Comments

Unit 1 13:00 h 15:03 h yes 06:00 h - 06:30 h

a, b Values ob-served are based on the definition “fire on until synchroni-zation”, see Section 6.3.1., Remark 1.

The values are considered to be consistent tak-ing normal devi-ations into ac-count.

Waiting time of 2:00 h with turbine at full speed has been dis-counted

Unit 2 13:00 h 13:51 h yes 06:00 h - 06:30 h

a, b

Unit 3 10:30 h 11:33 h yes 06:00 h - 06:30 h

a, b Additional time due to trips after first syn-chronization has been dis-counted

Unit 4 10:30 h 13:09 h yes 06:00 h - 06:30 h

a, b


Recommendations: Section 7.3, 7.4 and 7.6, p. 43

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Cold start-up costs

Value in-formed

Value observed

Con-sistent

Source State of the art Comments

Unit 1 46.8 kUSD 49,4 kUSD

yes a (consumption), c (prices)

Diesel consum-ption < 75% of current consumption

The values of Units 1, 2 ad 4 are con-sidered to be con-sistent taking nor-mal deviations into account.

The value of Unit 3 is considered to be consistent taking into account that additional fuel con-sumption has been caused by two trips


yes a, c


(yes) a, c


yes a, c


Recommendations: Section 7.3 and 7.4, p. 43

Hot start-up time

Value informed

Value observed


Comments

Unit 1 02:30 h 01:00 h (yes) a, b 01:30 – 02:00 h

Values observed are based on the definition “fire on until syn-chronization”, see Section 6.3.1., Remark 1.

Inconsistencies and variations between units are due to the unclear definition of “hot start-up” and different initial condi-tions of the units. See Sections 3.2.2 and 6.3.2.

The state of the art value refers to a hot start-up after 8 hours, which is why this value is higher than the value informed by Guacolda.

Unit 2 02:30 h 07:36 h (no) a, b 01:30 – 02:00 h

Unit 3 01:10 h 02:06 h (yes) a, b 01:30 – 02:00 h

Unit 4 01:10 h 05:06 h (no) a, b 01:30 – 02:00 h


Recommendations: Section 7.3, 7.4 and 7.7, p. 43

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Minimum operation time

Value informed

Value observed


Comments

Unit 1 08:00 h 02:00 h no a < 15 min

The values ob-served are solely due to the stabili-zation time. A re-duction of stabiliza-tion time reduces the minimum oper-ation time.

Unit 2 08:00 h 02:00 h no a < 15 min

Value refers to the time after retrofit of the material of superheater 4.

Unit 3 04:00 h 02:00 h no a < 15 min

Unit 4 04:00 h 02:00 h no a < 15 min

Details: Section 6.4, p.36

Recommendations: Section 7.5, p. 44

Minimum down-time

Value in-formed

Value ob-served


Comments

Unit 1 02:00 h 00:00 h no e 00:00 h

Unit 2 02:00 h 00:00 h no e 00:00 h

Unit 3 02:00 h 00:00 h no e 00:00 h

Unit 4 02:00 h 00:00 h no e 00:00 h


Cost to main-tain operation conditions (banking)

Value in-formed

Value observed


Comments

Unit 1 1.1 kUSD N/A no N/A N/A Technically not possible, the background of the cost informed is not clear. Unit 2 1.1 kUSD N/A no N/A N/A

Unit 3 1.1 kUSD N/A no N/A N/A

Unit 4 1.1 kUSD N/A no N/A N/A


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1.3 Conclusions and recommendations Most values could be confirmed based on the current condition of the Units. Taking the state of the art known to STEAG into account, there is significant potential for improvement of some of the parameters.

The following recommendations have been derived from the observations in the audit. The invest is an esti-mation. For more precise figures, more detailed technical investigations are required.

Section, page

Area Action required Effect Estimated invest (kUSD) per Unit

7.1, 42 Minimum load of the turbine

Expert consulting with respect to particular turbine design

Reduction of minimum load

Unknown

7.2, 42 Minimum load of the boiler

Optimization of controls Reduction of minimum load

See 7.5 for con-trol optimization and tuning

7.3, 43 Start-up process optimization

Automate and optimize start-up process

Reduction of start-up time and costs

400

7.4, 43 Installation of HP (high pressure) by-pass station and dump condenser

Detailed analysis of benefits / planning & engineering

Reduction of start-up time, costs and enabling „bank-ing“ operation (at high cost, however)

2 500

7.5, 44 Control optimization and tuning (also for minimum load)

Expert consulting w.r.t. boiler dynamics / DCS programming works

Reduction of stabilization time

500

7.6, 44 Lifetime monitoring Installation of such a lifetime monitoring sys-tem

Monitoring of increase of start-up numbers

100

7.7, 44 Verification of start-up time definition

Refine start-up time def-inition (recommendation to CDEC-SIC)

Correct parametrization of start-up time and cost in the model of CDEC-SIC

N/A

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2 Introduction CDEC-SIC is responsible for the load dispatch in the largest interconnection system in Chile, called SIC (Sistema Interconectado Central). In the north of this system, transportation restrictions lead to a need for flexible operation of the Guacolda thermal power plant, taking into account the infeed of renewable sources.

The installed capacity of renewable sources (both solar and wind) has been increasing over the last years. As a consequence, situations may occur in which available renewable energy cannot be integrated into the grid because this would require operation at very low load or even a shut-down of one or several units of the Guacolda thermal power plant. If the renewable energy is available only for a limited time, the missing power cannot be provided by the grid due to the transportation limitation of 320 MW. On the other hand, the Gua-colda power plant units would not be able to restart fast enough.

The dispatch of CDEC-SIC is based on the technical and commercial parameters that are provided by the power plants. CDEC-SIC can audit power plants in order to verify these parameters. This audit report is the result of such an audit, performed by CDEC-SIC in cooperation with STEAG Energy Services GmbH. It is focused on those parameters that are related to flexibility.

The Guacolda Power Plant is located close to Huasco, Atacama Region, Chile. It consists of four units (cur-rently, Unit 5 is under construction) with a gross generation capacity of 152 MW each.

The power plant is operated by Empresa Eléctrica Guacolda S. A. (in short called “Guacolda”), a subsidiary of AES Gener. The time of the start of operation, the date of the last overhaul and the service hours of each unit are shown in Table 1.

The supplier of all boilers and turbines is Mitsubishi, Japan. The boilers are coal fired drum type boilers with natural circulation. Different hard coal qualities can be burnt, including pet coke, which is mainly burnt in unit 3.

All units are equipped with Netmation, a DCS (Distributed Control System) of Mitsubishi with a graphical user interface. Units 1 and 2 have been migrated to this system.

The units are quite similar, but not identical. More detailed information on specific design issues can be found in Section 4.

Table 1: Basic information on the operation of the Guacolda Units (Source: Guacolda)

Guacolda Unit Put into service Last overhaul Service hours by September 2015

1 1995 2011 161,101.30

2 1998 2012 153,553.30

3 2008 Scheduled Fall 2015 52,521.60

4 2009 Scheduled June 2016 46,893.90

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3 Audit procedure 3.1 Audit process The scope and purpose of the audit have been described in Section 1.1. To this end, the audit consisted of the following steps:

• Analysis of documentation before site visit • Inspection of the plant on site • Interviews on site • Analysis of data obtained from/after site visit

In the framework of the audit, STEAG verified that the understanding and definition of those parameters is identical for all parties involved. Moreover, STEAG has performed a technical analysis of the plant and dis-cussions with the staff of Guacolda to determine the technical reasons for limitations of the technical parame-ters. Where applicable, STEAG has identified possible areas of improvement with respect to the technical parameters.

3.2 Definition of the parameters The technical capabilities and restrictions of the operation of a thermal power plant that are relevant for the dispatch by CDEC-SIC are described by the following parameters.

3.2.1 Definitions with common understanding

Parameter Definition

Minimum Technical (Load) Minimum amount of gross power output that the unit can gen-erate in a stable condition with coal (main fuel), measured in MW.

Minimum Operating time Minimum time in hours that the unit must keep in service, be-fore giving the stop signal.

Minimum Downtime Minimum time in hours that the unit must keep out of service, before receiving the order to restart.”

Stabilization time If the unit has increased its generation, the minimum time in hours, that the unit must remain in the new level of generation or ramping up, and cannot reduce the generation. If the unit has reduced its generation, the minimum time in hours, that the unit must remain in the new level of generation or ramping down, and cannot increase the generation.”

Cost to maintain unit in operating tempera-ture (“banking”)

Cost in USD per hour to keep the unit at operation conditions (pressure and temperature) but without synchronization.”

Cost of operation for powers between 0 MW and Minimum Technical

Cost of operation in USD per MWh below minimum technical if this operation is feasible, for example with auxiliary fuel.

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3.2.2 Definitions that require comments or adaptations

Start-up time

CDEC-SIC definitions:

• Cold start-up: “Time in hours to go from ‘Order to Start’ to ‘Minimum Technical’ ”. • Hot start-up: “Time in hours to go from “banking” to “Minimum Technical” (banking is a stage where

the unit is in temperature to operate but still not synchronized)”

In contrast to this definition, Guacolda has stated in the opening meeting that they consider the cold start-up time as the time between “Fire On” and “Synchronization of the turbine”. For this reason, STEAG has deter-mined the time of the following phases of the start-up procedure:

1. “Purging”: Start of purging (air flow) until Fire On. In case of a hot start-up after a trip, the air flow may remain active and not go to zero. In such a case, the purging time is defined as the time be-tween the trip and the Fire On.

2. “Boiler start-up”: Fire On until Turbine rolling 3. “Turbine start-up”: Turbine rolling until Synchronization 4. “Load increase”: Synchronization until Minimum Technical Load (60 MW)

The knowledge of the length of these phases enables a comparison with the values of Guacolda and, addi-tionally, an assessment of the duration according to the definition of CDEC-SIC. However, the time between “Order to start” and “Fire On” cannot be determined, because the time of the “Order to start” is not visible in the data. Instead, the earliest signal indicating the beginning of the start-up that can be evaluated is the air flow, which indicates the start of purging.

The purpose of purging is to make sure that any combustible gases are removed from the boiler before igni-tion. The minimum purging time is constant (9.6 minutes according to the manual). However, significant longer purging times have been observed in the data.

Any time needed before the purging is related to preparation steps. These include (but are not limited to) the following items:

• Boiler filling • Check of boiler condition • Check of condenser • Check of cooling water • Check of chemical condition of feedwater • Check of bearings of main equipment • …

The time needed for preparations depends on the state of the unit. Of course, after a retrofit quite some preparation work is necessary before the start-up.

In the case of hot start-ups, the time needed for preparations usually is negligible because all systems have been in operation.

Start-up costs

CDEC-SIC definition: “Cost in dollars to go from “Order to Start” to “Minimum Technical” “

The cost of a start-up is related to the consumption of ancillary and main fuel. In the case of Guacolda, the consumption of Diesel, Heavy Fuel Oil (HFO) and coal during the start-up determines the cost.

Strictly speaking, the cost for electricity obtained from the grid is a start-up cost component too. However, in the case of Guacolda, at least one other unit is running during almost every start-up, so that the cost of elec-

Final Audit Report




tricity is negligible. Note that the electricity costs of a start-up have not been considered by Guacolda in their cost calculation either.

Differentiation between cold and hot start-ups

According to the current definition used by CDEC-SIC and by Guacolda, STEAG has analyzed the length and cost for both cold and hot start-ups by using data of such start-ups provided by Guacolda.

The understanding of a cold start-up is that the unit has had a long standstill. Both boiler and turbine have cooled down and have (almost) ambient temperature.

The understanding of a hot start-up is that the unit has tripped or has been shut-down only a few hours ago. After a trip or if it is known that a restart will follow shortly, usually the pressure and temperature are kept as high as possible by closing all corresponding valves in the boiler and the turbine. The turbine is still hot.

These two categories are typical for base load units, because such units typically will only shut down for maintenance (leading to a cold start-up afterwards) or due to technical problems (leading to a hot start-up if the problem can be solved fast enough).

Due to inevitable heat losses in the boiler, the temperature and pressure in the boiler continuously decrease over time. There currently is no possibility to actively keep the boiler at operation temperature (see Section 6.7 for details).

The turbine cools down over time too, albeit more slowly than the boiler due to its compact form and better insulation. As long as temperatures of boiler and turbine are close to their values at normal operation, a hot start-up is possible during which there is no long warm-up phase of either boiler or turbine. Usually, after a trip, a restart is performed as fast as possible (~ 30 min – 1h). As a rule of thumb, a hot start-up is possible for 6 to 8 hours after shutdown. However, even in this comparatively short time period, the boiler continuous-ly loses temperature and pressure, and the start-up length increases over time. The hot start-up of Unit 2 is a good example for this; see Section 5.3.2 on page 22.

In the case of flexible operation, more start-ups with intermediate conditions will be observed, for example after a standstill of one or two days. As mentioned above, it should be considered that the boiler cools down more quickly than the turbine. Hence, it may be possible that the boiler is in almost cold condition while the turbine is still quite hot. This is the case in particular if the boiler is forced to cool down in order to do repair works. The boiler start-up then would be a cold start-up, while the turbine does not need a long warm-up phase.

Consequently, in the future, a further differentiation of the start-up types may become necessary. In fact, the length of a start-up depends on the conditions of the boiler and turbine at the moment the start-up begins in a continuous fashion. We recommend considering this in the framework of an optimization of the start-up process. More information on this issue is provided in section 7.1.

As a conclusion, for CDEC-SIC this means that the parameter definition of the start-up time and costs may need a redefinition in the future. In terms of the dispatch optimization, it may be worthwhile to consider start-up costs that depend on the length of the shut-down in a continuous manner, or at least on an in-creased number of start-up types (e.g. “warm”, “very hot”, etc.). Refer to Section 6.3.1 for a common exam-ple for such a definition.

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4 Results of plant walk-through, document analysis and interviews 4.1 Boiler and mechanical Judging from the visual inspection of the plants, the components generally appear to be in a good condition.

Some particular items regarding the plant design are discussed in the following:

• The final superheater has bends that are not drainable. This is a limiting factor, in particular with re-spect to the cold start-up time.

• The first reheater heating surface is located in the high loaded radiation chamber directly down-stream the furnace. Furthermore, the first reheater heating surface is assembled in front of the membrane wall. It is uncooled during start-up, due to the absence of a HP (high pressure) bypass. This causes long boiler start-up times.

• Due to the absence of a HP bypass and LP (low pressure) bypass, only drains and vents are used for the start-up procedures.

• The installed material of the last heating surface bundles of the Units 1 and 2 originally was T91. This material has caused several problems, in particular a high number of tube ruptures in this area. The material has been substituted by the more reliable material HFG 347 in Unit 1 in June 2015. In Unit 2, the substitution will take place in the near future. The use of T91 inside the flue gas path of coal fired boilers is not permitted in STEAG power plants. The reason is an outside scaling of the heating surface tubes, caused by high temperatures and the flue gas ambient of coal firing. Along with the unavoidable internal oxidation, the life time of the heating surface tubes inside the flue gas path of coal fired boilers is shortened significantly, especially in cycling power plants. Furthermore, the stress values of T91 have been degraded by more than 30% since being established on the market. The tube material HFG 347 has been developed in parallel to T91. Long term use in practice has shown that HFG 347 is more reliable than T91. Due to the future Guacolda operation require-ments, the use of HFG 347 instead of T91 in all boilers is the most reliable installation. With HFG 347, the unit will be more robust with respect to flexible operation.

• The boilers are equipped with tilting burners. These can be used to support steam temperatures at low loads and to support reheater outlet temperature control. This should be positive for flexibility, but apparently the tilting ability is not used to its full potential. Furthermore, the hot reheat outlet tem-perature could be held constant during lower load operations.

• Diesel fuel oil flow is controlled manually during start-up and only used for start-up. This manual op-eration may pose a limitation for start-up automation.

• Unit 3 is equipped with a wet FGD (flue gas desulfurization). This is the reason why pet coke is burnt in Unit 3. It may be that the use of pet coke will become impossible in the future due to environmen-tal reasons. Units 1, 2 and 4 will be equipped with a bag filter and a dry FGD (U1 in 10/2015, U2 in 11/2015, U4 in 1/2016). From our experience, the FGDs do not pose a restriction with respect to flexibility.

• Unit 1 is equipped with an SCR (Selective Catalytic Reaction) catalyst for the removal of nitrogen ox-ides from flue gases, but without an economizer bypass. This may be relevant for future minimum load reduction, because the SCR catalyst needs a minimum flue gas temperature of about 280 °C. At very low loads, this temperature may not be reached if no economizer bypass is available. Unit 4 is equipped with an SCR catalyst and with an economizer bypass. Alstom has performed windbox tuning and succeeded in reducing the NOx (nitrogen oxides) values.

• In contrast to Units 1 & 2, Units 3 & 4 have dynamic classifiers, low-NOx-burners and overfire air for NOx reduction.

• The steam turbines of Units 1 to 4 are outdoor installations and located on a turbine table.

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• For cooling purposes, cooling water is taken from the sea by main cooling water pumps for the steam condensers. The closed cooling water system will be served by Auxiliary cooling water pumps. Service water for the units is generated from seawater by means of desalination plants.

The applied technology of the units from the Guacolda power plant is generally in line with other power plant installations of the same size that are known to STEAG. From our experience, a more flexible operation of such plants is feasible without damaging the plant. Therefore, the described durations of start-ups, stabiliza-tion times, ramping etc. should be optimized with respect to a more flexible operation in future.

4.2 Control and Instrumentation All units are equipped with a DCS called Netmation (by Mitsubishi) with a graphical user interface that allows to show trends and to export data.

The control philosophy, the architecture of the control loops and the used control methods are focused on steady state operation. The controllers are based on classical PI(D) algorithms, and there is no model based feedforward channel in order to obtain smooth load changes. Moreover, according to Guacolda, the PID-parameters (controller parameters) have never been tuned for optimal control performance. The effect of this is that the physical variables (pressure, temperatures, etc.) are disturbed by the load change and oscillations can be observed (see, for example, Figure 21).

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5 Analysis of DCS data 5.1 General After observation of load changes on site, the trends needed for the audit have been specified in more detail. Guacolda has provided numerical data for the following situations:

• Cold start-up • Hot start-up • Load changes

However, the archived data of time periods that lie further in the past have a limited time resolution of one sample per three minutes. For this reason, the start-ups are available with a time resolution (sample rate) of 3 minutes, which means that the evaluation of time duration of start-up phases has an inaccuracy of at least ±3 min. Moreover, certain short term phenomena may be invisible or biased due to the limited time resolu-tion. All load changes have been performed in the near past and therefore are available with a time resolu-tion of 1 minute.

The figures of the start-ups can be found at the end of this report (Section 8, page 45). The following remarks may be helpful for the interpretation of the Figures:

• The Curtis stage is the first stage in the turbine. The measurement of its metal temperature provides an indication on the thermal condition of the turbine during the phase of cooling down after shut-down and of warming up after rolling the turbine.

• Some of the temperature measurements have a lower limit of the measurement range that is at about 280 °C. Hence, if the curve shows this value, the true temperature is unknown.

• Not all values have been provided for all start-ups. In particular, the Curtis stage metal temperature of Unit 3 is missing, and Unit 4 is the only unit for which the boiler load demand has been provided.

• The turbine is rolled in the range of 60 bar (cold start-up) up to 80 bar drum pressure. The live steam temperature should exceed the saturation temperature by at least 40 K.

• The saturation temperature has been calculated from the pressure signal. • The steam temperatures may show a fast jump to the saturation temperature. This is due to the first

steam flow and provides an indication on the start of steam production.

In the following, the data will be analyzed in more detail. The comparison with the parameters provided by Guacolda is done in Section 6.

5.2 Cold start-up The start-ups are divided in four phases as follows: Table 2: Start-up phases. Note that for the sake of comparability, for Unit 3 the load increase time has also been based on reaching 40% load, even though its current minimum load is at 50%.

Phase Start End

Purging time Start of air flow Fire on (first Diesel)

Boiler start-up time Fire on Turbine rolling (turbine speed > 0)

Turbine start-up time Turbine rolling Synchronization (generation > 0)

Load increase time Synchronization Minimum load (generation > 40%)

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5.2.1 Unit 1

A cold start-up of Unit 1 is shown in Figure 9. The corresponding characteristic values are shown in the table below.

Specific observations:

• The firing rate is increased very slowly over one hour and almost constant • The Curtis stage metal temperature increases to about 100°C – the saturation temperature at ambi-

ent pressure – and then increases during turbine warm-up. It reaches a steady state at the end of the turbine warming phase.

• There is a long waiting time of ca. 2 hours after the turbine has reached full speed until synchroniza-tion. This is the reason for the long turbine start-up time. The reason behind the waiting time is un-known. If the waiting time is discounted, the turbine start-up time is in a normal range.

Table 3: Characteristics of cold start-up of Unit 1

Purging time 03:30 h

Boiler start-up time 10:30 h

Turbine start-up time 04:33 h 1

Load increase time 03:42 h

Fire On until Synchronization time 15:03 h 1

Total start-up time 22:15 h 1

Start-up Diesel consumption (t) 54.1

Start-up HFO consumption (t) 10.5

Start-up Coal consumption (t) 10.7

5.2.2 Unit 2 A cold start-up of Unit 2 is shown in Figure 10. The corresponding characteristic values are shown in the table below.


• The remarks of Unit 1 apply here as well. However, the waiting time before synchronization is short-er, leading to a shorter turbine start-up time.

• This start-up corresponds quite well to the time and cost information provided by Guacolda

1 after discounting waiting time of ca. 2h with turbine at full speed

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Turbine start-up time 05:36 h


Fire On until Synchronization time 13:51 h

Total start-up time 19:45 h






• As can be seen in the Figure, obviously there has been a trip twice during this start-up, leading to a non-representative consumption of fuel.

• Assuming that none of the trips occurred, the synchronization took place at 22:47 (the day before) instead of 05:59 This corresponds to a time of 11:36 h between Fire On and Synchronization.

• Different to Unit 1 and 2, all the drain valves of both MSV and of the last superheater header are used. The drain valves of the MSVs are operated in parallel.

• The load increase time is rather short, because the turbine has been warmed during the time periods between the trips. As a result, the turbine probably already was quite hot (due to the missing Curtis stage metal temperature measurement data, it is not possible to verify this) so that after the third synchronization, the turbine could increase its load quite quickly.

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Turbine start-up time 03:18 h 2



Total start-up time 17:09 h 3






• The pressure increases more slowly than in the other start-ups (see also Figure 23). One possible reason is that the MSV drain valves are fully open during the whole start-up.

• This start-up has a very long load increase time. This is the reason for the high coal consumption. It is not clear why the load has not been increased faster.

2 Due to two trips (both after synchronization), the turbine start-up time (time between rolling of the turbine and final synchronization) is exceptionally long. The final (third) synchronization took place 07:12 h after the first synchronization. The turbine start-up time therefore has been discounted by 07:12 h. 3 This time takes into account the discounted turbine start-up time of 03:18 h until the first synchronization and the load increase time after the last synchronization of 01:27 h. Due to the fact that the load increase time is shorter than usual at a cold start-up, the total start-up time indicated is shorter than we would expect for a start-up without trips.

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5.3 Hot start-up

5.3.1 Unit 1

A hot start-up of Unit 1 is shown in Figure 13. The corresponding characteristic values are shown in the table below.

This start-up is a good example of a fast start-up nearly from operation conditions. Fire On is only 15 minutes after the trip. As the boiler and the turbine are still hot, both can be started very quickly. Table 7: Characteristics of hot start-up of Unit 1










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5.3.2 Unit 2



• Due to a longer standstill, the boiler was already quite cold. The remaining temperature at HP turbine inlet was 255 °C at the time of Fire On and the remaining pressure was 0 barg. We would rather classify this as a warm start. This start-up is an example for the different types of start-ups dis-cussed in Section 3.2.2.

• There was a very long waiting time (ca. 8h) before rolling the turbine while the boiler was at almost constant temperature and pressure. This is the reason why this start-up has high start-up fuel con-sumption (mainly Diesel) in comparison to the hot start-ups in the other units. The reason for the waiting time is unknown. Most probably, it was due to technical problems (that caused the trip or that were a consequence of the trip) that could not be solved fast enough during the start-up.

The trip preceding this start-up is a good example that in “banking” mode, the pressure and temperature cannot be kept constant at operation conditions. Figure 22 illustrates the following:

• Boiler pressure is lost quite quickly, reaching ambient pressure after about 7 hours. • Temperatures in the boiler go down at a similar pace. The constant high pressure boiler outlet tem-

perature at the end is due to the lower limit of the measurement range, the real temperature is lower. • The turbine temperature even slightly increases in the beginning, because the turbine is still turning

but there is no cooling steam flow. The kinetic energy stored in the turning turbine is partly trans-formed to heat when the turbine slows down. After that, the turbine temperature goes down too, but much slower than the boiler temperatures. This leads to the situation described above, that the boiler is quite cold and the turbine still warm (but not hot). This leads to a long boiler start-up phase and a comparatively short turbine start-up phase.

Table 8: Characteristics of hot start-up of Unit 2


Boiler start-up time 06:00 h 4




Total start-up time 10:064




4 after discounting waiting time of ca. 8:00 h

Final Audit Report




5.3.3 Unit 3



• This also is a hot start-up, but the time passed after the trip is longer than it was for the start-up of Unit 1. Hence, the boiler already lost some pressure and the boiler start-up time is longer.

• The turbine is still hot, so its start-up time is short.











5.3.4 Unit 4



• The remaining temperature at HP turbine inlet was ca. 370 °C at the time of Fire On, so the start-up is not really hot, resulting in a slightly longer boiler start-up time.

• The turbine speed increases in an apparently uncontrolled way before opening the turbine control valve. This may be a sign of unwanted steam leakage. As this turbine speed increase was assigned to the turbine start-up, this phase also is longer than for Unit 1 and 3, even though the turbine is still hot.

• Apart from this, the start-up itself does not show any obvious abnormalities. The drain valves are opened step by step and support a smooth pressure increase.

• The operation after the start-up, see Figure 17, shows oscillations and stability issues. See the dis-cussion of the load changes in Section 5.4 for more details.

Final Audit Report














5.4 Load changes

5.4.1 Unit 1

The behavior of Unit 1 at load changes is shown in Figure 18.


• There may be an issue with the differential expansion measurement in Unit 1, as it shows discontinu-ities.

• The load follows the set-point well, which is as expected since the boiler operates at fixed pressure. • Significant temperature deviations can be observed, even at full load and in steady state. In steady

state, for example at about 5:30, the HP temperature drops to 525 °C and the RH (reheater) temper-ature to 516 °C, while the set point (nominal temperature) is 535°C in both cases. During the load change, at about 9:10, the drop in temperature is even worse and reaches 485°C RH temperature. This may be significant for the turbine!

• Significant pressure deviations can be observed, in particular during and after load changes. Oscilla-tions are visible, even for small load changes without mill switching (see, for example, Figure 18 at 15:20).

• The turbine expansion difference remains between 5.2 and 6.4 mm. The exact warning levels are not known, but from the DCS printout shown here, it can be seen that they are similar to those in Unit 4 (warning levels at -3.5 and 11.2 mm).

Final Audit Report




From our point of view, the turbine expansion does not require a stabilization time of two hours, even though the value of the turbine expansion needs about 2h to reach its steady state. Moreover, in part the expansion reaction may also be due to temperature differences at load changes. For example, it is not clear why the temperature increases by 15 K during a load change from 60 to 70 MW (see Figure 18 at 15:20). A detailed analysis of the temperature control is necessary to find the reason.

5.4.2 Unit 2



• Significant temperature deviations can be observed, even at full load and in steady state. • Significant pressure deviations can be observed, in particular during and after load changes. Oscilla-

tions are visible, even for small load changes without mill switching. • The turbine expansion difference remains between 4.8 and 6.5 mm, the warning limits are the same

as for Unit 1. • At 17:13, the coal flow measurement is reduced from about 32.5 to 26 t/h and remains at that level

for over 2 hours. However, there is no visible effect on the unit pressure. Most probably, there has been a problem with one of the coal flow measurements.

• As a whole, the behavior is similar to Unit 1.

5.4.3 Unit 3



• There are issues with respect to following the load set-point. This may be a unit control issue and needs more detailed investigation.

• Significant temperature deviations can be observed, even at full load and in steady state. • Significant pressure deviations can be observed, in particular during and after load changes. Oscilla-

tions are visible, even for small load changes without mill switching. • The turbine expansion difference remains between 5.5 and 6.6 mm. Warning limits for turbine ex-

pansion are at -3.5 and +11.2 mm

5.4.4 Unit 4



• There are obvious stability problems at load changes. All main variables oscillate (pressure, coal flow, power, …), which is an indication for a problem with some actuators (coal feeder for example) or with the control tuning.

• The printout of the DCS of a load change during the audit shown in Figure 1 has a higher time reso-lution than the archived data. This printout shows both fast oscillations (in the order of seconds) with respect to power output and boiler outlet pressure and slower oscillations with higher amplitude. Both are a further indication for a need of control tuning.

• The HP temperature hardly drops at the load change (showing some oscillations as well, however). • The turbine expansion difference remains between 6.1 and 6.9 mm. Warning limits for turbine ex-

pansion are at -3.5 and +11.2 mm.

Final Audit Report




Figure 1: Observed oscillations during and after a load change. Note that the time resolution in this printout is higher than in the figures below created from archived data.

5.5 Conclusions The following common observations can be made:

5.5.1 Start-ups

• In all units, the firing rate is increased very slowly. The pressure and temperature increase is also slow.

• Pressure fluctuations can be observed due to a lack of a possibility to control the pressure during start-up.

• The waiting times observed in each start-up are different and some are much longer than the times in the manual (for example, the purging time). Without knowing possible technical reason, most probably an important factor is that the start-ups are operator dependent.

• Currently, the operators are probably not focused on performing a fast start-up but a start-up that is on “the safe side”.

• It is not possible to see from the data if there are technical reasons behind these waiting times or if they are the result of, for example, beginning the start-up too early in order to be sure to synchronize on time.

• In Units 1 and 2, the drain valve of the main steam valve 2 (M-HS-104) is operated manually. The position information of the other drain valves (M-HS-103 and M-HS-159) that are mentioned in the start-up procedure is zero during the complete start-up (both cold and hot). The manual operation results in pressure fluctuations, it is not possible to control the pressure. Moreover, the operation is

Final Audit Report




different for each unit and hence, the pressure increase and the time needed are different for each start-up, as Figure 23 shows. Moreover, the pressure increase is much slower than the curve indi-cated in the manual.

• In Units 3 and 4, all three drain valves (2x MSV and superheater 4) are used during start-up.

5.5.2 Load changes

• All units have – to a different extent – issues with control performance during load changes and – in part – during steady-state.

• After a load change, the main parameters (pressure, temperature, …) reach their steady state after about 1-2 hours. From our experience, this is a very long time.

• The turbine expansion difference is the difference between the expansion of the rotor and of the cas-ing of the turbine. It should not exceed certain limits, because otherwise parts of the rotor and of the casing may rub. The turbine expansion difference seems not to be a restriction with respect to the stabilization time, as there always is sufficient distance to the warning levels. In particular, if the tem-perature control performance is better – meaning that the steam temperature at the turbine inlet is constant – the turbine expansion difference amplitudes will be reduced.

Final Audit Report




6 Audit results with respect to the parameter values 6.1 Original values At the time of the audit, the following parameters had been provided to CDEC-SIC:

Table 11: Values for technical parameters as provided by Guacolda to CDEC-SIC as of September 2015

Unit

Gross power output [MW]

Cold start-up time [h]

Cold start-up costs

[kUSD]

Hot start-

up time [h]

Cost to maintain operation conditions [USD/h]

Minimum operating time [h]

Minimum downtime

[h]

Stabilization time [h]

Maximum number of start-ups

per month Minimum Maximum

1 60 152 13:00 h 46.8 2:30 h 1067 8:00 h 4:00 h 2:00 h 1 2 60 152 13:00 h 46.8 2:30 h 1067 8:00 h 4:00 h 8:00 h 1

3 75 152 10:30 h 41.2 1:10 h 1067 4:00 h 4:00 h 2:00 h 1

4 60 152 10:30 h 41.2 1:10 h 1067 4:00 h 4:00 h 2:00 h 1

6.2 Minimum Technical Load Currently, the minimum technical load of the turbine is at 40% load, according to the supplier of the turbine (Mitsubishi) who stated (Source: Document Docto 11 Parte 6):

Question by Guacolda Answer by Mitsubishi

“What is the safe technical minimum load to operate this machines?”

“The safe technical minimum load for U1-U5 is 40% TMCR.”

(TMCR: Turbine Maximum Continuous Rate)

The technical reason behind this restriction is not mentioned and, therefore, unknown to STEAG. Also, it is not clear what exactly is meant by “safe”. Without detailed knowledge of the turbine design, it is not possible to gain a clear understanding of the limitation of the minimum technical load imposed by the supplier. Hence, in the particular case of the Guacolda power plants, STEAG cannot assess free of doubt whether this re-striction is justified and/or if there is a possibility to relax it.

However, from our point of view, this turbine minimum technical load is unusually high. STEAG has project experience with several turbines of

informe de auditoría entral guacolda 2015 · 2015. 11. 25. · informe de auditoría central...

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