ENERGY AUDIT AT R-ENERGY AUDIT AT R-INFRA DAHANUINFRA DAHANU

THERMAL POWER STATION THERMAL POWER STATION (250 X 2 MW UNIT)(250 X 2 MW UNIT)

CENTRAL POWER RESEARCH CENTRAL POWER RESEARCH INSTITUTE, BANGALORE-560080INSTITUTE, BANGALORE-560080

DESIGN CAPACITY/ DESIGN CAPACITY/ RATING OF DTPSRATING OF DTPS

INSTALLED DURING BSES PERIOD 1995INSTALLED DURING BSES PERIOD 1995 TAKEN OVER BY R-INFRA IN 2003TAKEN OVER BY R-INFRA IN 2003 NO MAJOR CHANGE IN HARDWARE SINCENO MAJOR CHANGE IN HARDWARE SINCE IDENTICAL TO OVER 25 250 MW UNITS INSTALLED ALL IDENTICAL TO OVER 25 250 MW UNITS INSTALLED ALL

OVER INDIA INCLUDING PARLI, PARAS & TATA OVER INDIA INCLUDING PARLI, PARAS & TATA TROMBAY. TROMBAY.

SAME DESIGN REPLICATED IN ALL UNITS SAME DESIGN REPLICATED IN ALL UNITS PG TEST, INSTALLATION MANUALS, NAME PLATES, PG TEST, INSTALLATION MANUALS, NAME PLATES,

CAPACITY TESTS OF EQUIPMENT AND C & I INDICATE CAPACITY TESTS OF EQUIPMENT AND C & I INDICATE UNITS ARE OF 250 MW CAPACITY.UNITS ARE OF 250 MW CAPACITY.

Rating TerminologyRating Terminology

‘‘Maximum Continuous Rating’ (MCR)Maximum Continuous Rating’ (MCR) of a of a generating unit means the normal rated full generating unit means the normal rated full load MW output capacity of a Generating Unit load MW output capacity of a Generating Unit which can be sustained on a continuous basis which can be sustained on a continuous basis at specified conditions.at specified conditions.

Ref: [CENTRAL ELECTRICITY REGULATORY Ref: [CENTRAL ELECTRICITY REGULATORY COMMISSION NOTIFICATIONCOMMISSION NOTIFICATION

No. L/68(84)/2006-CERC New Delhi, the 14th No. L/68(84)/2006-CERC New Delhi, the 14th March, 2006]March, 2006]

Hence, 100 % MCR which refers to full load unit Hence, 100 % MCR which refers to full load unit capacity is 100 % Unit MCR or 100 % UMCRcapacity is 100 % Unit MCR or 100 % UMCR..

Unit MCR (UMCR) Unit MCR (UMCR) refers to 100 % MCR of the refers to 100 % MCR of the unit. For the units under study UMCR is 250 unit. For the units under study UMCR is 250 MW. MW.

Boiler MCR (BMCRBoiler MCR (BMCR) refers to maximum rating ) refers to maximum rating of the boiler. The boiler rating corresponding to of the boiler. The boiler rating corresponding to 100 % UMCR (i.e., 250 MW) is called as NCR 100 % UMCR (i.e., 250 MW) is called as NCR (normal continuous rating). BMCR is higher (normal continuous rating). BMCR is higher than NCR by 8-10 % usually.than NCR by 8-10 % usually.

Turbine MCR (TMCR) Turbine MCR (TMCR) refers to rating of the refers to rating of the turbine corresponding to 100 % UMCR (i.e., 250 turbine corresponding to 100 % UMCR (i.e., 250 MW). VWO condition refers to turbine rating MW). VWO condition refers to turbine rating under valve wide open conditions which is under valve wide open conditions which is higher than the 100 % UMCR by 5 % usually.higher than the 100 % UMCR by 5 % usually.

Generator ratings Generator ratings are given by apparent are given by apparent power (MVA) [vector sum of active power + power (MVA) [vector sum of active power + reactive power] at a given power factor and reactive power] at a given power factor and not by not by active power (MW) (power capable active power (MW) (power capable of doing work). of doing work). This is because This is because reactive reactive power (power to overcome inductance power (power to overcome inductance or electrical inertia in the system)or electrical inertia in the system) in the in the system also generates heat.system also generates heat.

Generator transformers Generator transformers are rated by are rated by apparent power (MVA) and not by active apparent power (MVA) and not by active power as reactive power (power to overcome power as reactive power (power to overcome inductance) also generates heat. inductance) also generates heat.

Overload capability of generating Overload capability of generating units: units: Each Generating Unit shall be Each Generating Unit shall be capable of instantaneously increasing capable of instantaneously increasing output by 5% when the frequency falls output by 5% when the frequency falls limited to 105% MCR. Ramping back to limited to 105% MCR. Ramping back to the previous MW level (in case the the previous MW level (in case the increased output level can not be increased output level can not be sustained) shall not be faster than 1% per sustained) shall not be faster than 1% per minute.minute.

Ref: [CEA Indian Electricity Grid Code, Ref: [CEA Indian Electricity Grid Code, 2005]2005]

All Generating Units, operating at or up to 100% All Generating Units, operating at or up to 100% of their Maximum Continuous Rating (MCR) of their Maximum Continuous Rating (MCR) shall normally be capable of (and shall not in shall normally be capable of (and shall not in any way be prevented from) instantaneously any way be prevented from) instantaneously picking up five per cent (5%) extra load when picking up five per cent (5%) extra load when frequency falls due to a system contingency. frequency falls due to a system contingency. The generating units operating at above 100% The generating units operating at above 100% of their MCR shall be capable of (and shall not of their MCR shall be capable of (and shall not be prevented from) going at least up to 105% be prevented from) going at least up to 105% of their MCR when frequency falls suddenly. of their MCR when frequency falls suddenly. After an increase in generation as above, a After an increase in generation as above, a generating unit may ramp back to the original generating unit may ramp back to the original level at a rate of about one percent (1%) per level at a rate of about one percent (1%) per minute, in case continued operation at the minute, in case continued operation at the increased level is not sustainable.increased level is not sustainable.

Ref: [CEA Indian Electricity Grid Code, 2005]Ref: [CEA Indian Electricity Grid Code, 2005]

All machines are provided with peak plant load All machines are provided with peak plant load capabilities which are configured by the OEM capabilities which are configured by the OEM (original equipment manufacturer) as (original equipment manufacturer) as continuous peak loadcontinuous peak load (without impairing (without impairing equipment life) and equipment life) and peak load for limited peak load for limited periodsperiods (with the effect of reducing the (with the effect of reducing the operating life of the equipment due to its effect operating life of the equipment due to its effect on other quality parameters). When operating on on other quality parameters). When operating on continuous peak load dutycontinuous peak load duty OEM has ensured OEM has ensured that all quality parameters are within safe limits that all quality parameters are within safe limits and there is no acceleration of ageing/life and there is no acceleration of ageing/life reducing effect to the equipment for indefinite reducing effect to the equipment for indefinite duration. Continuous peak plant load duty is duration. Continuous peak plant load duty is denoted as follows:denoted as follows:

Boilers: BMCR ratingBoilers: BMCR rating Turbines: VWO ratingTurbines: VWO rating Generators: MVA ratingGenerators: MVA rating Generating transformers: MVA rating Generating transformers: MVA rating

Plant load (active power or MW) Plant load (active power or MW) dependent and plant load independent dependent and plant load independent parameters: parameters: In all coal fired thermal power In all coal fired thermal power plants the majority of the quality parameters plants the majority of the quality parameters like temperature, pressure (except for variable like temperature, pressure (except for variable pressure operation), voltage, etc., are designed pressure operation), voltage, etc., are designed by the OEM to be nearly constant and first by the OEM to be nearly constant and first order load independent for the load range of 60 order load independent for the load range of 60 % UMCR through maximum load and changes % UMCR through maximum load and changes are only second order. are only second order.

However, the quantity parameters like flow, However, the quantity parameters like flow, current, etc. are directly proportional to active current, etc. are directly proportional to active power (MW) or plant load or machine loading. power (MW) or plant load or machine loading. As energy efficiency increases these quantities As energy efficiency increases these quantities decrease in magnitude for a given output.decrease in magnitude for a given output.

Hence, the plant load limiting parameters Hence, the plant load limiting parameters are primarily the quantity parameters like are primarily the quantity parameters like flow and current. As the energy efficiency flow and current. As the energy efficiency of the equipment decreases these quantity of the equipment decreases these quantity parameters for a given output will increase parameters for a given output will increase thereby limiting their maximum values thereby limiting their maximum values and posing a limitation on the maximum and posing a limitation on the maximum loadability of the unit. The other quality loadability of the unit. The other quality parameters like temperatures, pressures, parameters like temperatures, pressures, voltages, etc., are designed to be load voltages, etc., are designed to be load independent.independent.

Peak parameters for limited periods are defined in Peak parameters for limited periods are defined in terms of permissible peak loading of certain identified terms of permissible peak loading of certain identified parameters such as currents, voltages, temperatures, parameters such as currents, voltages, temperatures, pressures, flows, etc. and the time limits in seconds, pressures, flows, etc. and the time limits in seconds, minutes or hours in one excursion as well as total time minutes or hours in one excursion as well as total time in the lifetime of the equipment. Hence OEM has in the lifetime of the equipment. Hence OEM has defined the parameters which constitute peak defined the parameters which constitute peak parameter loading along with the time for single parameter loading along with the time for single excursion as well as the operating duration in the total excursion as well as the operating duration in the total lifetime of the machine. lifetime of the machine.

Continuous peak parameter loadingContinuous peak parameter loading is done is done purposefully for achieving the maximum performance purposefully for achieving the maximum performance or output from the machine whereas the or output from the machine whereas the limited limited period peak parameter loadingperiod peak parameter loading occurs because of occurs because of system operational transients or constraints or system operational transients or constraints or limitations or system mismatch. When parameters go limitations or system mismatch. When parameters go out of operating range or out of control, then a out of operating range or out of control, then a transient results which amounts to limited period peak transient results which amounts to limited period peak parameter loading.parameter loading.

TESTS ON UNITS TESTS ON UNITS

Maximum load- 268 MWMaximum load- 268 MW 100 % UMCR- 250 MW100 % UMCR- 250 MW F-GRADE LOAD- MAXIMUM LOAD F-GRADE LOAD- MAXIMUM LOAD

REACHED WAS 240 MWREACHED WAS 240 MW

BOILERSBOILERS NCR (normal continuous rating) refers NCR (normal continuous rating) refers

to steaming requirements which to steaming requirements which correspond to 100 % UMCR (250 MW). correspond to 100 % UMCR (250 MW).

Boiler MCR or BMCR refers to Boiler MCR or BMCR refers to steaming requirements for valve wide steaming requirements for valve wide open condition of the turbine + open condition of the turbine + auxiliary steam + operating margin. auxiliary steam + operating margin. This will normally be around 8-12 % This will normally be around 8-12 % higher than the 100 % UMCR capacity. higher than the 100 % UMCR capacity. Hence, boilers have operating Hence, boilers have operating margins of 8-12 % above the 100 % margins of 8-12 % above the 100 % UMCR capacity (i.e., steam UMCR capacity (i.e., steam requirement at 250 MW).requirement at 250 MW).

BOILERSBOILERSBMCR capacity consists of: 102 % of steam flow at HP turbine

throttle inlet under turbine valve wide open (VWO) condition, 7.18 kPa (69 mm Hg) condenser pressure

3% cycle make-up (to compensate for steam lost through the system)

20 t/h steam for meeting normal auxiliary steam requirements of the unit (steam which is used for non-motive purposes).

BOILERSBOILERS

In other words, the boiler maximum steaming rates (BMCR) are designed (continuous rating) at:

7.1 % additional steam flow over and above the 100 % NCR flow (2 % over 105 % for VWO condition of turbine =1.071)

3 % make up which amounts to =0.03. The boiler is also capable of supplying

auxiliary steam (20 t/h) of 2.67 % of the NCR flow. If the auxiliary steam is drawn from another boiler or another source or is minimized by best practices this will provide an additional margin up to 2.67 %.

BOILERSBOILERS

Thus, most boilers have a margin of around 7 % with additional margin of around 3-4 % if DM water and auxiliary steam are prudently utilized and minimized with reference to the design value.

Sl. No. Unit particulars-boiler UNITS NCR BMCR MARGIN Design

1Dahanu 250 MW Units 1 & 2 t/h

746.60 805 1.08 CE/BHEL

2Paras 250 MW Unit 3 t/h

736.20 810 1.10 CE/BHEL

3Parli 250 MW Unit 6

t/h738.2

1 810 1.10 CE/BHEL

4Tata Trombay Unit 8 t/h

736.20 810 1.10 CE/BHEL

5Nasik 210 MW Unit 5 t/h 652 700 1.07 CE/BHEL

6Chandrapur 210 MW Unit 3 t/h 652 700 1.07 CE/BHEL

7Bhusawal 210 MW Unit 3 t/h 654 700 1.07 CE/BHEL

8Koradi 210 MW Unit 7 t/h 652 700 1.07 CE/BHEL

9Khaperkheda 210 MW Unit No 4 t/h

624.23 690 1.11 CE/BHEL

10Chandrapur 500 MW Unit 7 t/h 1540 1670 1.08 CE/BHEL

BOILERSBOILERS

Another important factor (besides high energy efficiency) which governs the capacity is the design coal GCV of the boiler and the operating GCV. The operating GCV must always be higher than the design GCV if the boiler is to be used effectively. If the operating GCV of the boiler is lower than the design GCV then the firing rate will have to be increased

Sl. No.

Particulars at 100 %

BMCR

conditions

Units

Dahanu 250 MW

Paras 250 MW

Parli 250 MW Unit 6

Nasik 210 MW Unit 5

Chandrapur 210 MW Unit 3

Bhusawal 210 MW Unit 3

Koradi 210 MW Unit 7

Khaperkheda 210 MW Unit No 4

Chandrapur 500 MW Unit 7

1 Design

coal GCV

kcal/kg

3700

3400 3400 5000 4445 5100 5000 3500 3500

2 Annual

average

GCV

kcal/kg

3966

3652 3608 3422 3170 3235 3642 3354 3170

3 Total heat to

boiler

Mcal/h

599.4

611.7 527.34 536533.

4531.42

527.5

515.55

1215.9

4 Total fuel quantity

t/h 162 179.9 155.1 107.2 120 104.2105.

5147.

3347.4

5 Restriction

on BMCR to GCV

% less No No No Yes No Yes Yes No No

Effect of GCV on loading rate or steaming rate of the boiler:

Coal firing rate (t/h) = 317.38 – 0.0421[(GCV) (kcal/kg)]

Effect of GCV on specific fuel consumption (SFC):

SFC (kg/kWh) = 1.1869 – 0.00021[(GCV) (kcal/kg)]

Effect of GCV on boiler efficiency:

Boiler efficiency (%) = 84.046 +0.001[(GCV) (kcal/kg)] (valid up to 5000 kcal/kg)

Effect of GCV on UHR: The UHR decreases with GCV and the sensitivity index is -0.0968 kcal/kWh per kcal/kg:

UHR (kcal/kWh) = 2643.7-0.0968 [(GCV) (kcal/kg)] (valid up to 5000 kcal/kg)

Variation of Steaming rate with boiler efficiency at constant coal consumption and constant GCV

740

750

760

770

780

790

800

810

80 82 84 86 88 90

Boiler efficiency, %

Ste

amin

g ra

te, t

/h

Variation of coal consumption with boiler efficiency at a constant boiler steaming rate and constant

GCV

150

152

154

156

158

160

162

164

80 82 84 86 88 90

Boiler efficiency, %

Co

al co

nsu

mp

tio

n, t/h

Dependence of unit loading (MW) on the coal quality

y = -4E-05x2 + 0.351x - 465.57

R2 = 0.7627

230

240

250

260

270

280

3300 3500 3700 3900 4100 4300 4500

Coal GCV (kcal/kg)

Uni

t loa

d (M

W)

Study of operational parameters The operational parameters of the boiler be

classified into two types: Continuous normal and continuous overload

parameters with no time restriction on them. Overload parameters with restriction on a

single excursion as well as total time in the life of the unit.

There are two types of parameters in a boiler:– Quantity parameters (like flow, current, etc.)

which are directly plant load dependent– Quality parameters (like temperature, voltage,

pressure, etc.) which are designed to be plant load independent for the load range 60 % UMCR through maximum load.

Study of operational parameters On scrutiny of the data it is seen that the DTPS has not exceeded any parameter beyond the values set by the OEM and they have set alarms and trippings for parameters well within the limits set by OEM for asset management and asset preservation. It is ensured that restricted time overload parameters are never reached control action in the form of alarms and tripping is designed to be activated well before they are reached. All control loops are in action and continuous recording of all data is available including water chemistry data.

Heat loading rate (Million kcal/h) in the boiler

480

500

520

540

560

580

600

620

640

235 240 245 250 255 260 265 270 275

Plant load (MW)

Hea

t loa

ding

rat

e (1

06 k

cal/h

)

design

Operating

Linear (design)

Linear (Operating)

DTPS boilers are designed for BMCR of 805 t/h & design steam flow at 250 MW is 746 t/h. As per the study it is seen, DTPS has ensured that boilers are operated well within BMCR design limit. Steam flow from the boiler is being monitored on real time basis through DCS and the limit of < 775 t/h- priority 1 Alarms are configured in HMI Also daily/monthly/yearly basis deviation report is reviewed and all parameters are being recorded and ensured to be within limits. Better quality of coal (Blend Indian washed coal with imported coal) which is higher than the design GCV by 600-800 kcal/kg also contributes in maintaining high steaming rate of the boiler besides the high boiler efficiency.

In conclusion, it can be said that the DTPS has been able to maintain steaming rates below the BMCR levels prescribed by OEM while simultaneously not overloading any parameter beyond OEM limits through:

– High boiler efficiency (85.35 %)– High GCV of coal (600-800 kcal/kg above the design value)– Minimizing auxiliary steam requirements

TURBINESTURBINESThe terminology to designate capacity of turbines is as

follows: TMCR refers to steam demand for 100 % UMCR VWO (valve wide open) condition of the turbine

refers to steam demand when the turbine valves are fully opened. These are normally 5 % over and above the 100 % UMCR capacity. Hence turbines have operating margins of 5 % above the 100 % UMCR capacity.

The turbines are sized such that they shall be capable of operating continuously with valves wide open (VWO) at rated main steam and reheat steam parameters. The total steam flow to turbine is the steam flow at HP turbine stop valve inlet plus external steam supplied to the turbine cycle such as gland steam, stack steam, etc.

Sl. No.

Unit particulars-Steam turbine UNITS

TMCR VWO

MARGIN Design

1

Dahanu 250 MW Units 1 & 2

MW 250262.8

2 1.05 Seimens

2

Paras 250 MW Unit 3

MW 250264.7

8 1.06 Seimens

3

Parli 250 MW Unit 6

MW 250264.7

8 1.06 Seimens

4

Tata Trombay Unit 8

MW 250264.7

8 1.06 Seimens

5

Nasik 210 MW Unit 5

MW 210213.3

0 1.02 Russian

6

Chandrapur 210 MW Unit 3

MW 210215.8

0 1.03 Russian

7

Bhusawal 210 MW Unit 3

MW 210215.0

0 1.02 Russian

8

Koradi 210 MW Unit 7

MW 210215.8

0 1.03 Russian

9

Khaperkheda 210 MW Unit No 4

MW 210221.7

0 1.06 Seimens

10Chandrapur 500 MW Unit 7 MW 500 524.40 1.05 Siemens

TURBINESTURBINES However, it is common experience that most Siemens

turbines have continuous overload margins up to 8 %, that is, 210 MW operate at up to 227 MW. Siemens turbines normally have a built in margin of 15 % in torque. It has been clarified from turbine specialists that turbines have margins up to 15 % in power output without any harmful effect provided efficiency and steam cleanliness is maintained. Since turbines are constant speed machines with load directly proportional to the transmitted torque, the heat generation in the journal bearings is second order dependent on load while the heat generation in thrust bearings is directly proportional to load. If the turbine efficiency is maintained very near the design value, then the heat generation in the thrust bearings can be kept well within OEM limits even with high load ability.

Sl. No.

Unit particulars - Steam turbine UNITS

TMCR VWO MARGIN

1

Hitachi

MW210 222.00 1.06

2

Mitsubishi

MW210 222.00 1.06

3

Siemens

MW210 221.70 1.06

4

Russian

MW210 215.60 1.03

TURBINESTURBINES Apart from maintaining the load ability of turbines, DTPS

has made provisions to supply the required steam through:

minimized auxiliary steam flow (by cutting down on tracing steam for HFO and introducing zero steam leak policy)

minimized DM water consumption (by cutting down steam lost to the atmosphere)

minimized auxiliary steam consumption in the turbine itself (for gland sealing steam, stack steam and vent steam) to ensure that most steam goes into the turbine and turbine load ability further improves.

DTPS has ensured turbine load ability of more than 100% TMCR by some additional measures such as the following: Good condenser vacuum due to open cycle operation of condenser cooling & by use of sea water for condenser cooling. Rated parameters of equipments are never violated & regular monitoring of same through deviation report thereby preventing parameter excursions and loss of value of the asset. Maintaining very good quality of process chemistry parameters of steam and water Program of routine, preventive, predictive maintenance and planned AOH. Continuous monitoring of process cycle efficiency on line & off line. Simulator training for operators using the customized 250 MW simulator.

Variation of steam input with turbine heat rate at constant power output

735

740

745

750

755

760

765

1920 1940 1960 1980 2000

Turbine heat rate, kcal/kWh

Ste

am

inp

ut t

o tu

rbin

e,

t/h

Variation of power output with turbine heat rate at constant steam input to turbine

254

256

258

260

262

264

1920 1940 1960 1980 2000

Turbine heat rate, kcal/kWh

Po

we

r o

utp

ut, M

W

TURBINESTURBINES In conclusion it can be said that the DTPS

has be able to maintain good load ability on the machine within the OEM margins and without exceeding any OEM parameter limits by:

Maintaining high turbine efficiency (turbine heat rate deviates from design by only 4.4 kcal/kWh)

Strictly maintaining water quality parameters as per OEM guidelines

Minimizing auxiliary stack, vent and gland sealing steam requirements in the turbine.

TURBINESTURBINES All turbine related flow, pressure (except for variable

pressure operation), temperature, vibration and position parameters are archived for past two years in EXCEL files. In additional the water chemistry parameters such as electrical conductivity, pH, silica and chlorides have been archived for the last 2 years on daily average as well as hourly basis for 2009 and 2010. Figures 77-91 give the turbine steam, oil and metal parameters including mechanical parameters such as vibration, axial shift, etc., over the past one year on a daily average basis for Unit 1. Similar data is also studied for Unit 2. The hourly average is also archived and studied for both Units 1 & 2 for 2 years (2009 & 2010). On scrutiny of the data it is seen that the DTPS has not exceeded any parameter beyond the values set by the OEM and they have set alarms and trippings for parameters well within the limits set by OEM for asset management and asset preservation. Restricted time overload parameters are never reached to preserve the value of the asset. Very good water chemistry parameters are being maintained.

The parameters which have most critical effect on the life of the turbine are:

Main steam and reheat temperature Main steam pressure

Sl. No.

Unit particulars-Generator

MVA rating

MW at pf = 0.85

MW at pf=0.99

MARGIN pf=0.85 to 0.99

1

Dahanu 250 MW Units 1 & 2

294.0 250

291.18 1.16

2

Paras 250 MW Unit 3

294.1 250

291.18 1.16

3

Parli 250 MW Unit 6 294.1 250

291.18 1.16

4

Tata Trombay Unit 8

294.1 250

291.18 1.16

5

Nasik 210 MW Unit 5

247.0 210

244.59 1.16

6

Chandrapur 210 MW Unit 3

247.0 210

244.59 1.16

7

Bhusawal 210 MW Unit 3

247.0 210

244.59 1.16

8

Koradi 210 MW Unit 7

247.0 210

244.59 1.16

9

Khaperkheda 210 MW Unit No 4

247.0 210

244.59 1.16

10Chandrapur 500 MW Unit 7

588.0 500

582.35 1.16

Sl. No. Particulars of continuous parameters

Designed value

01 Maximum temperature of stator core

105 0 C

02 Maximum temperature of stator windings

105 0 C

03 Maximum temperature of stator windings

115 0C

04 Maximum temperature of cold gas

45 0C

05 Maximum temperature of hot gas

75 0C

06 Maximum temperature of inlet water

38 0C

07 H2 gas pressure 3 bar08 Voltage variation 5 %09 Unbalanced load 8 %10 Unsymmetric short circuit

current (I2t)10

11 Frequency 5 %

The schedule of tolerances in the generator must be as per IS The schedule of tolerances in the generator must be as per IS 4722: 2001 and the operating specifications including 4722: 2001 and the operating specifications including

combinations of parameters at any given output must be combinations of parameters at any given output must be according to IS 5422: 1996 (reconfirmed 2002).according to IS 5422: 1996 (reconfirmed 2002).

Sl. No.

Factors Description

01 Short term operation or individual occurrence of the event

1.1 Reverse power flow

20 s/event

1.2 Over current of 150 %

30 s

1.3 Short circuit at 100 % MVA and 105 % voltage

3 s

Sl. No.

Factors Description

02 Long term operation or cumulative occurrence in an year

Steam temperatures

2.1 Type 1 80 h/year (single event 15 min/occurrence)

2.2 Type 2 400 h/year (single event 15 min/occurrence)

Power frequency

1 % frequency drop (-0.5 Hz)

No effect

2 % drop (-1.0 Hz)

< 90 min/year

3 % drop (-1.5 Hz)

< 10 min/year

Power ramp rates

2 %/min 80 % of nominal power

5 %/min 50 % of nominal power

10 %/min 20 % of nominal power

GENERATORSGENERATORS In conclusion it can be said that

the operation of the generator at a load of 268.7 MW (108 % of the UMCR) without exceeding the OEM margin and without exceeding any parameter from OEM limits is possible because of:

Power factor improvement from 0.85 to 0.99.

High generator efficiency (as good as design efficiency) thereby reducing the current and heat generation.

Figure : Variation of permissible active power output (% of MCR)

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

1.04

1.06

0.84 0.89 0.94 0.99 1.04 1.09 1.14

Terminal voltage of generator (p.u.)

Gen

erat

or a

ctiv

e po

wer

out

put (

p.u.

)

Figure :Variation of permissible stator current (% of MCR)y = -14.713x2 + 28.461x - 12.707

R2 = 0.9971

0.8

0.84

0.88

0.92

0.96

1

1.04

0.84 0.89 0.94 0.99 1.04 1.09 1.14

Terminal voltage of generator (p.u.)

Gen

erat

or s

tato

r cur

rent

(p.u

.)

Figure :Variation of permissible stator current occurance time (min)

y = 745704e-8.6624x

R2 = 0.6521

0

10

20

30

40

50

60

1 1.2 1.4 1.6 1.8 2

Stator current of generator (p.u.)

Per

mis

sibl

e sa

fe ti

me

(min

)

Figure :Variation of permissible rotor current occurance time (min)

y = 63.296e-1.7162x

R2 = 0.8686

0

2

4

6

8

10

1 1.2 1.4 1.6 1.8 2

Rotor current of generator (p.u.)

Per

mis

sibl

e sa

fe ti

me

(min

)

GENERATOR GENERATOR TRANSFORMERSTRANSFORMERS

Generator transformer are rated by apparent power (MVA) and not by real power (MW). Generator transformers are normally rated at 7.1-10 % higher than the generator 100 % MCR MVA rating at the designed power factor. Generator transformers are designed for continuous operation at any tap at rated 315 MVA with voltage variation of ±10 % of rated tap voltage; and capable of delivering rated current at a voltage equal to 105 % of rated voltage without exceeding specified temperature rise giving a continuous overload capacity of 5 %.

If the power factor is improved from 0.85 to near 1.0 then a 15 % margins will be available to the station in the form of active power (MW) capacity. Thus, generator transformers have a margin of around 7-10 % with an additional margin if the power factor is increased from 0.85 to 0.99 as the capacity is controlled by the MVA rating and not the MW rating.

Sl. No.

Unit particulars-Generator transformer Units

MVA of Gen

MVA of GT MARGIN

1

Dahanu 250 MW Units 1 & 2 MVA

294.1 315 1.07

2

Paras 250 MW Unit 3

MVA294.

1 315 1.07

3

Parli 250 MW Unit 6

MVA294.

1 315 1.07

4

Tata Trombay Unit 8

MVA294.

1 315 1.07

5

Nasik 210 MW Unit 5

MVA247.

0 250 1.01

6

Chandrapur 210 MW Unit 3 MVA

247.0 250 1.01

7

Bhusawal 210 MW Unit 3 MVA

247.0 250 1.01

8

Koradi 210 MW Unit 7

MVA247.

0 250 1.01

9

Khaperkheda 210 MW Unit No 4 MVA

247.0 250 1.01

10Chandrapur 500 MW Unit 7 MVA

588.0 600 1.02

Thus the generator transformers have a margin of 7 % over the generator apparent power.

Standard temperature limits for power transformers

Average winding temperature rise: 65 ºC Above ambient

Hot-spot temperature rise: 80 ºC Above ambient

Top liquid temperature rise: 65 ºC Above ambient

Maximum temperature limit: 110 ºC AbsoluteThe generator transformer rating is 315 MVA. During the performance test the maximum load on generator was computed as 251.6 MVA (load factor: 79.87 %). The computed current was 667 A and is lower than the design value of 773.9 A.The GT winding temperature was in the range of 64 – 71 oC at power output of 265.5 MW during Test 1 and was lower than the design value of 55 oC above ambient temp. (during test

ambient temp. was 31.25 oC). Similarly the GT oil temperature was in the range of 43 – 48 oC at power output of 265.5 MW during Test 1 and was lower than the design value of 50 oC above ambient temp. (during test ambient temp. was 31.25 oC).

LIFE LIMITING FACTORSLIFE LIMITING FACTORS

The power plant assets (boiler, turbine, generators, major auxiliaries, etc.) are designed for an operational life of 3,00,000 (3 lakh) operating hours or around 35 years of service under normal operating regime. If the operating regime is deviated, the acceleration of ageing takes place and the operational life gets reduced. In other words, the equipments get due for replacements much sooner than expectations.

The factors which affect the operational life are both the physical running hours as well as cyclic (on/off) operations. Each on/off or start/stop operation can be taken as an expenditure of 20 h of steady operational life. The allowable starts of base load units are 10 hot starts/year, 5 warm starts/year and 3 cold starts/year. For peaking units the starts are much higher. For all units, starts and stops are factored into the life expenditure @ 20 h/start on an average

Sl. No.

Type of Starts Designed number of Starts in life time of the unit

01 Hot start (within 8 hours of unit shut down)

4550

02 Warm start (within 36 hours of unit shut down)

910

03 Cold start (after 72 hours of unit shut down)

455Sl. No.

Particulars of transients Designed value(MINIMUM)

01 Step load change + 15%

02 Ramp Rate under variable pressure

+ 3%

03 Ramp Rate under constant pressure

+ 5%

LIFE LIMITING FACTORSLIFE LIMITING FACTORSThe plant and equipment besides normal ageing is

affected by:Severity of:

operating duty cyclic operationsexcursions in the operating regime.

Frequency and duration of:Cyclic operationsParameter excursions

Translating these factors into engineering factors, the mechanical life of equipment is finally controlled by:

CreepFatigueThermal stresses

LIFE LIMITING FACTORSLIFE LIMITING FACTORS

The deterioration process starts with microstructure degradation, crack formation and finally results in failure of the component or equipment.

The remaining life of an in-service equipment/component is taken as,

Nremaining = 1 – (fc + ff + ft)

Where fc , ff , ft are expended life fractions of the fractions due to deterioration effects of creep, fatigue and thermal stresses.

Translating the life limiting factors into engineering parameters, the electrical life of equipment is controlled by:

Rate of heat accumulation in the equipment (heat generation minus the heat withdrawal)

Voltage stresses and their patterns (cyclic, steady deviation, periodic, etc.)

RemainingRemainingLife Life (h, years)(h, years)

Real time years Real time years

We are hereWe are here

ӨӨ <45<45˚̊

GoodGoodӨӨ > 45˚ > 45˚poorpoor

ӨӨ=45˚=45˚NormalNormal

MTBF: days (Av R-Infra: 122 days)Average outage period: h/outage (Av R-Infra: 74 hours)

Average time from Boiler light up to 100 % Load (h)

Unit no. Hot start

Warm start

Cold start

Unit 1 3.61 7.06 21.23

Unit 2 3.53 8.31 18.30

Sl. No.

Type of Starts

Number of Starts in life time of Unit 1

Number of Starts in life time of Unit 2

Number of starts/year for Unit 1

Number of starts/year for Unit 2

01 Hot start

110 120 7.3 8.0

02 Warm start

26 27 1.7 1.8

03 Cold start

13 14 0.8 0.9

MAXIMUM LOADMAXIMUM LOAD The maximum plant load clocked under Indian

conditions by few stations in the regime of continuous overload parameters and without reaching limited time overload parameters [either quality (temperature, pressure, voltage) or quantity (flow, current)] are as follows:

Raichur TPS 210 MW: 228-230 MW : 9.5 % > 100 % UMCR

Vijayawada TPS 210 MW: 228 MW: 8.5 % > 100 % UMCR

Parli 210 MW Unit 5: 228 MW: 8.5 % > 100 % UMCR

The DTPS Units have clocked a maximum average plant load value of 7.0 % > 100 % UMCR rating which is in conformation with best performance of other units.

The loadability is coupled with energy efficiency. Decrease in energy efficiency causes decrease in loadability because the flows per unit will increase. By maintaining low heat rates, the DTPS have been able to load the plant to 268.7 MW but within the OEM margins and OEM parameter limits.

Max load in 2008-2009

Maximum load in 2009-2010

Unit 1 268.65 268.80

Unit 2 265.98 266.37

Study of current Study of current operating levelsoperating levels

The capacity exceeds the UMCR rating in the following circumstances:

Physically overrated plant with under rated name plate.Normally rated plant with overloading within the prescribed

limits of OEM.Overloading a normal plant indiscriminately with parameters

exceeding limits. In the present case of DTPS of R-Infra after detailed study

and analysis we are of the opinion that the plant is rated at 250 MW and it is indeed a 250 MW. It is identical to the units at Paras Unit 3, Parli Unit 6 and Tata Trombay Unit 8. There is no physical over rating or under rating of any equipment.

Around 25 units of 250 MW (identical in design to DTPS as OEM BHEL has frozen the design) have been installed in India and they have clocked a PLF of 89 % as per CEA in 2008-09 as compared to 83 % for 210 MW sets and 88 % for 500 MW sets. 250 MW units as a class have out performed 210 MW and 500 MW units.

Normal distribution of plant load factor

0.000000

0.002000

0.004000

0.006000

0.008000

0.010000

0.012000

0.014000

0.016000

0 20 40 60 80 100 120

Plant Load Factor, %

No

rmal

Dis

trib

uti

on

There are 66 units in India which have achieved average plant loads of 100-110 % and there are 14 stations which have achieved annual station loads in excess of 100 % during 2007-2008. The maximum average plant load is 110.69 % which is in conformation with the OEM design margins. [Ref: Performance review of 2007-08 of CEA]

There are 9 units and 3 stations in India which have clocked a PLF of over 100 % with the maximum being 104.14 % in 2007-2008. [Ref: Performance review of 2007-08 of CEA]

Sector wise private sector units have clocked a PLF of 91 % as compared to Central sector of 87 %.[Ref: Performance review of 2007-08 of CEA]

Study of current Study of current operating levelsoperating levels

In the case of boilers the R-Infra have utilized the margin of BMCR of up to 8 %. They have used the margin steam meant for auxiliary steam use and for DM make up for generation by minimizing/optimizing the margin steam usage. Further, they have maintained a coal GCV of around 4000 to 4200 kcal/kg (20-23 % higher than the design GCV) thereby proving advantage of optimization of the boiler capacity which has a design GCV of 3400 kcal/kg. The additional 600 kcal/kg has been taken advantage to load the boiler to obtain steaming rates near but below the BMCR ratings. Scrutiny of records (hourly and daily data for the past 2 years) indicates that BMCR ratings have not been exceeded. This has been possible by maintaining a high boiler efficiency which ensure that coal, air, flue gas flow rates are reduced and heat release rates in the boiler are minimized. If the boiler efficiency is decreased additional quantity of fuel would have to be fired causing a higher heat release rate in the boiler which would lead to parameters exceeding the design values and therefore imposing restriction on loadability. Thus, it can be said that the high boiler efficiency is a major factor responsible for the high boiler loadability.

Study of current Study of current operating levelsoperating levels

On the turbine side the capability of the turbine has been well utilized. By periodically overhauling the turbines and utilizing the margin steam for the turbine (the auxiliary steam for turbine gland sealing steam, stack steam, vent steam, etc.), they have been able to generate almost 268.7 MW without exceeding the parameters at any point of time and without crossing the OEM VWO margins. In other words, the high turbine efficiency is responsible for maintaining a high turbine loadability well within the VWO margins and parameters below their OEM limits. In addition to this maintaining of very good water chemistry has contributed to the loadability. It is generally accepted that the Siemens turbines can easily be loaded continuously up to 10 % without any adverse impact on the life provided their efficiency very near the design value. If the efficiency of the turbine drops then the heat losses in the turbine decrease and their loadability would come down because the parameters (like main steam flow, HRH flow, CRH flow and condensate flow) would exceed their OEM prescribed levels.

Study of current Study of current operating levelsoperating levels

On the generator side the margin power factor (from the design value of 0.85 to unity) through implementing a high power factor of 0.99 on the 33 kV distribution side of the energy supply has been made full use of to get maximum active power while minimizing the heat generation from the reactive power component. The loading is within the OEM margins and the parameters are within OEM limits. The same is the case of the generating transformer. Besides the high efficiency of the generator and generator transformer have contributed to low heat generation. If the generator efficiency had decreased, then the present level of loadability would not have been possible.

Figure : Maximum continous overload rating of the units

240

250

260

270

280

290

300

310

320

Pla

nt

loa

d (

MW

)

Peak load of DTPS: 268.5 MW

Units 1 & 2 have been commissioned in Jan and March 1995 and have completed 15 years of service and nearly 1,20,000 operating hours.

Acceleration of life expenditure takes place due to parameter excursions into the limited-time-overload-regime for long periods, due to frequent cyclic loading, due to frequent transients with high ramping rates. Scrutiny of operation and parameters indicates that the DTPS has avoided operation in these life limiting regimes thereby preserving the longevity of their assets. The cyclic operations are far lower than their design values.

RLA reports do not indicate any deterioration of the hardware especially the generator and generator transformers and the unit according to the RLA can be continued safely for a further period. The Unit is designed for an engineering life of 35 years or 3,00,000 operating hours. The expended operating life is almost the same as the expended physical operating hours indicating that acceleration of deterioration or damage has not been observed.

Considering a total engineering life of 35 years of service or 3,00,000 operating hours the physical (actual) life expenditure of both units is expected to be 40 %. Except for generator, where the life expenditure is 50 % (remaining life= 50 % or 14 years), the remaining life of other components matches roughly with the physical life expenditure. This indicates that acceleration of life has not taken place. The low degradation rate coupled with the high loading on the plant also leads to the conclusion that the equipment are healthy and factors in the nature of non-repairable damage are not present.

Study of current operating levelsStudy of current operating levels Two of the most common life limiting

factors are parameter overloading and cyclic (on/off) operations. A scrutiny of daily and hourly data from the DCS for the past two years (2008-09 and 2009 till date) did not indicate any evidence of parameter overloading beyond the continuous overload limit. Cyclic operations are also not present. In no case the restricted time overload parameter limits have been reached in terms of parameters overshooting their permissible continuous limits as a step for preserving their assets. The controls have been designed to trigger alarms well before the restricted time overload parameters are reached and tripping settings are also well designed.

Study of current operating levelsStudy of current operating levels A major factor contributing to high level of

machine loading in the case of boiler, turbine and generator is the energy efficiency or unit heat rate. Low difference of the unit heat rate from the design heat rate implies that the heat accumulation or generation is minimum in all the equipment enabling their very high loadability. Further another major life limiting factor, i.e., cyclic operations caused by forced outages has been minimized to a very large extent. As a result of this the life of the assets are being preserved while maintaining the load and loadability.

Monthly Loading rate (% of MCR) for the period 2006-2009

y = -0.0003x2 + 1.2457x - 1292.4

R2 = 0.6528

100

101

102

103

104

105

106

107

2220 2240 2260 2280 2300 2320 2340 2360 2380

Unit Heat rate (kcal/kWh)

Load

ing

rate

(% o

f MC

R)

Annual Loading rate ( % of MCR) for the period 1995-2010 y = -0.2221x + 609.83

R2 = 0.7339

80

85

90

95

100

105

110

2240 2260 2280 2300 2320 2340 2360 2380

Unit Heat rate (kcal/kWh)

An

nu

al L

oad

ing

rat

e (%

of

MC

R)

Study of current Study of current operating levelsoperating levels

We are also of the conclusion that the operation regime of quality parameters (temperatures, pressures, voltages, etc.) as well as quantity parameters (flows & currents) have been restricted to the limits imposed by the OEM for all parameters and overshooting of critical parameters is not seen. The DTPS has introduced continuous monitoring of parameters through DCS as well as condition monitoring of selected parameter. The continuous safe overloading margins provided by the OEM have been made use of to operate the plant up to a load of 268 MW without violating either the OEM margins or the OEM limits of parameters.

Thus after a thorough scrutiny of the data sheets, daily and hourly readings for the past two years, physical inspection of equipment, study of name plate ratings and efficiency tests on equipment we have come to the following conclusions:

The parameter limits set by the OEM for all equipment have been maintained by setting the alarms and trip setting below these values. Parameter monitoring and condition monitoring have been installed. This has led to asset preservation and management.

The margins provided by the OEM for the boiler, turbine and generator have not been crossed but been made use of by measures to ensure that loadability is high.

On the boiler side DTPS have minimized DM water make up and auxiliary steam besides firing coal with around 600 kcal/kg higher than the design coal thereby maintaining good boiler loadabiliy within the BMCR limits. A high boiler efficiency is being maintained. If the boiler efficiency drops down then the heat load in the boiler will increase and it will not be possible to sustain the required steaming rate. High boiler efficiency thus enables loading of the unit to 268.7 MW without crossing the 100 % BMCR limit or any parameters exceeding.

On the turbine side they have maintained very good water chemistry regime, regularly overhauled turbines and minimized auxiliary steam to the turbine (gland steam, stack steam, etc.) thereby maintaining good turbine loadability well within the 100 % VWO limits given by OEM. If the turbine efficiency decreases, the present loading rate will not be possible to be maintained in the turbine and the load will have to be decreased.

On the generator side they have taken advantage of the high power factor of 0.99 against the generator design power factor of 0.85, and the high generator efficiency to get an increase in loadability by around 8 % but within the OEM limit of 291 MVA without any parameters crossing the OEM limits.

Combining all these factors R-Infra has been able to load the unit to 268.7 MW against the design value of 250 MW. Maintaining this load is not harming the life of the unit as the DTPS has ensured boiler operation is within 100 % BMCR limits, turbine operation is within 100 % VWO limits and generator operation is within the capability curve. Further, all parameters are kept within OEM recommended limits.

The plant capacity is 250 MW and the present continuous load limit of 268 MW is well within the frame work of the OEM margins and parameter limits. There is no evidence to show that the parameters have been exceeded in any equipment.

The present unit loadability of 268 MW achieved because of the high boiler, turbine and generator efficiencies (low unit heat rate); coal quality of around 4,000 kcal/kg, prudent use of margin steam in the boiler and turbine (through zero leak policy), proactive control of water chemistry regime and margin power factor of near unity. All design margins provided by the OEM have been utilized without parameters shooting beyond the safe levels and minimizing cycling excursions. If coal GCV, boiler efficiency, turbine efficiency, generator efficiency and power factor decrease, then it will not be possible to maintain the loadability of the units to 268 MW. There is an inverse relationship between unit heat rate and unit loadability as explained earlier in the text.

The issue of continuous operation of the unit 7 % higher than the 100 % UMCR (at 260 to 268 MW) was discussed with OEM and other units. They have opinioned that the sustainability of the 265-268 MW may not be of a permanent nature and both availability and loadability are bound to drop in due course of a few years. A large number of factors are responsible for this loadability and any let up in any side may affect loading. Loadability is linked to heat rate and if heat rate increases implying that losses in the boiler, turbine and generator increase then the loadability will not be sustainable.

Also, the PG tests indicate the OEM assurance is for 250 MW which is identical to several units installed in India by the OEM and no capacity upgrades have been added since inception. Moreover, the margins are not uniform in boiler, turbine and generator and limitations may be created in any one equipment. Further, the original design considers a margin as envisaged by the CEA grid code 2005 and up-rating of the unit could affect this margin. Considering all the above factors we are of the opinion that the rating can be maintained at 250 MW.

Heat rateHeat rateThe gross overall efficiency of the Unit 1 TG set is 37.8 % against the The gross overall efficiency of the Unit 1 TG set is 37.8 % against the

design efficiency of 38.5 %. The gross overall TG heat rate (TG design efficiency of 38.5 %. The gross overall TG heat rate (TG HR) is HR) is 2275.34 kcal/kWh2275.34 kcal/kWh at the test load of 265.5 as at the test load of 265.5 as compared to the design heat rate of compared to the design heat rate of 2230.60 kcal/kWh 2230.60 kcal/kWh..

The deviation in the test TG heat rate is 44.2 kcal/kWh which can be The deviation in the test TG heat rate is 44.2 kcal/kWh which can be attributed to boiler side as 39.8 kcal/kWh and due to the turbine attributed to boiler side as 39.8 kcal/kWh and due to the turbine side it is 4.4 kcal/kWh. On the generator side the deviation is 0 side it is 4.4 kcal/kWh. On the generator side the deviation is 0 kcal/kWh. kcal/kWh.

The annual unit heat rate (UHR) of the unit considering all factors is The annual unit heat rate (UHR) of the unit considering all factors is 2292.7 kcal/kWh2292.7 kcal/kWh for Unit 1. for Unit 1.

The average degradation of SHR corresponds to a The average degradation of SHR corresponds to a degradation rate of 0.18 % of design HR/year. The degradation rate of 0.18 % of design HR/year. The degradation of SHR has averaged 0.18 %/year over the degradation of SHR has averaged 0.18 %/year over the past few years. The CPRI test TG HR is showing a past few years. The CPRI test TG HR is showing a degradation rate of 0.13 %/year. The degradation is 2.0 degradation rate of 0.13 %/year. The degradation is 2.0 % of the DHR. % of the DHR.