ENERGY AUDIT AT RINFRA DAHANU
THERMAL POWER STATION
(250 X 2 MW UNIT)
CENTRAL POWER RESEARCH
INSTITUTE, BANGALORE-560080
DESIGN CAPACITY/
RATING OF DTPS






INSTALLED DURING BSES PERIOD 1995
TAKEN OVER BY R-INFRA IN 2003
NO MAJOR CHANGE IN HARDWARE SINCE
IDENTICAL TO OVER 25 250 MW UNITS INSTALLED
ALL OVER INDIA INCLUDING PARLI, PARAS & TATA
TROMBAY.
SAME DESIGN REPLICATED IN ALL UNITS
PG TEST, INSTALLATION MANUALS, NAME PLATES,
CAPACITY TESTS OF EQUIPMENT AND C & I
INDICATE UNITS ARE OF 250 MW CAPACITY.
Rating Terminology
‘Maximum Continuous Rating’ (MCR) of a
generating unit means the normal rated full
load MW output capacity of a Generating Unit
which can be sustained on a continuous basis
at specified conditions.
Ref: [CENTRAL ELECTRICITY REGULATORY
COMMISSION NOTIFICATION
No. L/68(84)/2006-CERC New Delhi, the 14th
March, 2006]

Hence, 100 % MCR which refers to full load unit
capacity is 100 % Unit MCR or 100 % UMCR.
Unit MCR (UMCR) refers to 100 % MCR of the
unit. For the units under study UMCR is 250
MW.
 Boiler MCR (BMCR) refers to maximum
rating of the boiler. The boiler rating
corresponding to 100 % UMCR (i.e., 250 MW)
is called as NCR (normal continuous rating).
BMCR is higher than NCR by 8-10 % usually.
 Turbine MCR (TMCR) refers to rating of the
turbine corresponding to 100 % UMCR (i.e.,
250 MW). VWO condition refers to turbine
rating under valve wide open conditions which
is higher than the 100 % UMCR by 5 %
usually.



Generator ratings are given by apparent
power (MVA) [vector sum of active power +
reactive power] at a given power factor and
not by active power (MW) (power
capable of doing work). This is because
reactive power (power to overcome
inductance or electrical inertia in the
system) in the system also generates heat.
Generator transformers are rated by
apparent power (MVA) and not by active
power as reactive power (power to overcome
inductance) also generates heat.
 Overload
capability of generating
units: Each Generating Unit shall be
capable of instantaneously increasing
output by 5% when the frequency falls
limited to 105% MCR. Ramping back to
the previous MW level (in case the
increased output level can not be
sustained) shall not be faster than 1%
per minute.
Ref: [CEA Indian Electricity Grid Code,
2005]
All Generating Units, operating at or up to
100% of their Maximum Continuous Rating
(MCR) shall normally be capable of (and shall
not in any way be prevented from)
instantaneously picking up five per cent (5%)
extra load when frequency falls due to a
system contingency. The generating units
operating at above 100% of their MCR shall be
capable of (and shall not be prevented from)
going at least up to 105% of their MCR when
frequency falls suddenly. After an increase in
generation as above, a generating unit may
ramp back to the original level at a rate of
about one percent (1%) per minute, in case
continued operation at the increased level is
not sustainable.
Ref: [CEA Indian Electricity Grid Code, 2005]






All machines are provided with peak plant load
capabilities which are configured by the OEM
(original equipment manufacturer) as
continuous peak load (without impairing
equipment life) and peak load for limited
periods (with the effect of reducing the
operating life of the equipment due to its effect
on other quality parameters). When operating on
continuous peak load duty OEM has ensured
that all quality parameters are within safe limits
and there is no acceleration of ageing/life
reducing effect to the equipment for indefinite
duration. Continuous peak plant load duty is
denoted as follows:
Boilers: BMCR rating
Turbines: VWO rating
Generators: MVA rating
Generating transformers: MVA rating


Plant load (active power or MW)
dependent and plant load independent
parameters: In all coal fired thermal power
plants the majority of the quality parameters
like temperature, pressure (except for variable
pressure operation), voltage, etc., are designed
by the OEM to be nearly constant and first
order load independent for the load range of 60
% UMCR through maximum load and changes
are only second order.
However, the quantity parameters like flow,
current, etc. are directly proportional to active
power (MW) or plant load or machine loading.
As energy efficiency increases these quantities
decrease in magnitude for a given output.

Hence, the plant load limiting parameters
are primarily the quantity parameters like
flow and current. As the energy efficiency
of the equipment decreases these quantity
parameters for a given output will
increase thereby limiting their maximum
values and posing a limitation on the
maximum loadability of the unit. The other
quality parameters like temperatures,
pressures, voltages, etc., are designed to
be load independent.


Peak parameters for limited periods are defined in
terms of permissible peak loading of certain identified
parameters such as currents, voltages, temperatures,
pressures, flows, etc. and the time limits in seconds,
minutes or hours in one excursion as well as total time
in the lifetime of the equipment. Hence OEM has
defined the parameters which constitute peak
parameter loading along with the time for single
excursion as well as the operating duration in the total
lifetime of the machine.
Continuous peak parameter loading is done
purposefully for achieving the maximum performance
or output from the machine whereas the limited
period peak parameter loading occurs because of
system operational transients or constraints or
limitations or system mismatch. When parameters go
out of operating range or out of control, then a
transient results which amounts to limited period peak
parameter loading.
TESTS ON UNITS
Maximum load- 268 MW
 100 % UMCR- 250 MW
 F-GRADE LOAD- MAXIMUM LOAD
REACHED WAS 240 MW

BOILERS


NCR (normal continuous rating) refers
to steaming requirements which
correspond to 100 % UMCR (250 MW).
Boiler MCR or BMCR refers to
steaming requirements for valve wide
open condition of the turbine +
auxiliary steam + operating margin.
This will normally be around 8-12 %
higher than the 100 % UMCR capacity.
Hence, boilers have operating
margins of 8-12 % above the 100 %
UMCR capacity (i.e., steam
requirement at 250 MW).
BOILERS
BMCR 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 nonmotive purposes).
BOILERS
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
%.
BOILERS
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
Dahanu 250 MW
Units 1 & 2
UNITS
NCR
BMCR
MARGIN
Design
t/h
746.60
805
1.08
CE/BHEL
t/h
736.20
810
1.10
CE/BHEL
t/h
738.21
810
1.10
CE/BHEL
t/h
736.20
810
1.10
CE/BHEL
t/h
652
700
1.07
CE/BHEL
6
Chandrapur 210 MW
Unit 3
t/h
652
700
1.07
CE/BHEL
7
Bhusawal 210 MW
Unit 3
t/h
654
700
1.07
CE/BHEL
t/h
652
700
1.07
CE/BHEL
Khaperkheda 210 MW
Unit No 4
t/h
624.23
690
1.11
CE/BHEL
Chandrapur 500 MW
Unit 7
t/h
1540
1670
1.08
CE/BHEL
1
2
3
4
5
8
9
10
Paras 250 MW Unit 3
Parli 250 MW Unit 6
Tata Trombay Unit 8
Nasik 210 MW Unit 5
Koradi 210 MW Unit 7
BOILERS
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.
1
2
3
4
5
Partic
ulars
at 100
%
BMCR
condit
ions
Units
Dahan
u 250
MW
Paras
250 MW
Parli 250
MW Unit
6
Nasik
210 MW
Unit 5
Chandr
apur
210
MW
Unit 3
Bhusawa
l 210 MW
Unit 3
Koradi
210
MW
Unit 7
Khaper
kheda
210
MW
Unit No
4
Chandra
pur 500
MW Unit
7
Desig
n coal
GCV
kcal/
kg
3700
3400
3400
5000
4445
5100
5000
3500
3500
Annua
l
avera
ge
GCV
kcal/
kg
3966
3652
3608
3422
3170
3235
3642
3354
3170
Total
heat
to
boiler
Mcal
/h
599.
4
611.7
527.34
536
533.4
531.42
527.5
515.5
5
1215.9
Total
fuel
quanti
ty
t/h
162
179.9
155.1
107.2
120
104.2
105.5
147.3
347.4
Restri
ction
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)]
Variation of Steaming rate with boiler efficiency at
constant coal consumption and constant GCV
Steaming rate, t/h
810
800
790
780
770
760
750
740
80
82
84
86
Boiler efficiency, %
88
90
Coal consumption, t/h
Variation of coal consumption with boiler efficiency
at a constant boiler steaming rate and constant
GCV
164
162
160
158
156
154
152
150
80
82
84
86
Boiler efficiency, %
88
90
Dependence of unit loading (MW) on the coal quality
280
y = -4E-05x 2 + 0.351x - 465.57
R2 = 0.7627
Unit load (MW)
270
260
250
240
230
3300
3500
3700
3900
Coal GCV (kcal/kg)
4100
4300
4500
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
Heat loading rate (106 kcal/h)
640
620
600
design
580
Operating
560
Linear (design)
540
Linear (Operating)
520
500
480
235
240
245
250
255
260
Plant load (MW)
265
270
275
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
TURBINES
The 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. Unit particularsNo. Steam turbine
UNITS
TMC
R
Dahanu 250 MW
Units 1 & 2
MW
MARGIN
Design
250 262.82
1.05
Seimens
MW
250 264.78
1.06
Seimens
MW
250 264.78
1.06
Seimens
MW
250 264.78
1.06
Seimens
MW
210 213.30
1.02
Russian
6
Chandrapur 210 MW
Unit 3
MW
210 215.80
1.03
Russian
7
Bhusawal 210 MW
Unit 3
MW
210 215.00
1.02
Russian
MW
210 215.80
1.03
Russian
Khaperkheda 210
MW Unit No 4
MW
210 221.70
1.06
Seimens
Chandrapur 500 MW
Unit 7
MW
500
1.05
Siemens
1
2
3
4
5
8
9
10
Paras 250 MW Unit 3
Parli 250 MW Unit 6
Tata Trombay Unit 8
Nasik 210 MW Unit 5
Koradi 210 MW Unit 7
VWO
524.40
TURBINES
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
TMC
R
VWO
MARGIN
Hitachi
1
MW 210
222.00
1.06
MW 210
222.00
1.06
MW 210
221.70
1.06
MW 210
215.60
1.03
Mitsubishi
2
Siemens
3
Russian
4
TURBINES





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.
Steam input to turbine,
t/h
Variation of steam input with turbine heat rate
at constant power output
765
760
755
750
745
740
735
1920
1940
1960
1980
Turbine heat rate, kcal/kWh
2000
Power output, MW
Variation of power output with turbine heat
rate at constant steam input to turbine
264
262
260
258
256
254
1920
1940
1960
1980
Turbine heat rate, kcal/kWh
2000
TURBINES
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.
TURBINES



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.
MW at
pf =
0.85
MARGIN
MW at
pf=0.85
pf=0.99 to 0.99
Unit particularsGenerator
MVA
rating
Dahanu 250 MW
Units 1 & 2
294.0
250
291.18
1.16
294.1
250
291.18
1.16
294.1
250
291.18
1.16
294.1
250
291.18
1.16
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
247.0
210
244.59
1.16
Khaperkheda 210 MW
Unit No 4
247.0
210
244.59
1.16
Chandrapur 500 MW
Unit 7
588.0
500
582.35
1.16
1
2
3
4
5
8
9
10
Paras 250 MW Unit 3
Parli 250 MW Unit 6
Tata Trombay Unit 8
Nasik 210 MW Unit 5
Koradi 210 MW Unit 7
The schedule of tolerances in the generator must be as per IS
4722: 2001 and the operating specifications including
combinations of parameters at any given output must be
according to IS 5422: 1996 (reconfirmed 2002).
of
continuous
Designed value
Sl. No.
Particulars
parameters
01
Maximum temperature of stator core
105 0 C
02
Maximum
windings
temperature
of
stator
105 0 C
03
Maximum
windings
temperature
of
stator
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 bar
08
Voltage variation
5%
09
Unbalanced load
8%
10
Unsymmetric short circuit current
(I2t)
11
Frequency
10
5%
Sl.
No.
01
Factors
Description
Short term operation or
individual occurrence of the
event
1.1
Reverse
flow
power
1.2
Over current
150 %
1.3
Short circuit at 100
% MVA and 105 %
voltage
of
20 s/event
30 s
3s
Sl. No.
02
Factors
Long
term
operation
cumulative occurrence in
year
Description
or
an
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
GENERATORS



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)
Generator active power output (p.u.)
1.06
1.04
1.02
1
0.98
0.96
0.94
0.92
0.9
0.88
0.86
0.84
0.89
0.94
0.99
1.04
Terminal voltage of generator (p.u.)
1.09
1.14
Figure
:Variation of permissible stator current (% of MCR)
y = -14.713x 2 + 28.461x - 12.707
R2 = 0.9971
Generator stator current (p.u.)
1.04
1
0.96
0.92
0.88
0.84
0.8
0.84
0.89
0.94
0.99
1.04
Terminal voltage of generator (p.u.)
1.09
1.14
Figure
:Variation of permissible stator current occurance time
(min)
y = 745704e-8.6624x
R2 = 0.6521
Permissible safe time (min)
60
50
40
30
20
10
0
1
1.2
1.4
1.6
Stator current of generator (p.u.)
1.8
2
Figure
:Variation of permissible rotor current occurance time
(min)
y = 63.296e-1.7162x
R2 = 0.8686
Permissible safe time (min)
10
8
6
4
2
0
1
1.2
1.4
1.6
Rotor current of generator (p.u.)
1.8
2
GENERATOR
TRANSFORMERS

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 MVA
Gen
of GT
MARGIN
1
Dahanu 250 MW Units 1 &
2
MVA
294.1 315
1.07
2
Paras 250 MW Unit 3
MVA
294.1 315
1.07
3
Parli 250 MW Unit 6
MVA
294.1 315
1.07
4
Tata Trombay Unit 8
MVA
294.1 315
1.07
5
Nasik 210 MW Unit 5
MVA
247.0 250
1.01
MVA
247.0 250
1.01
6
Chandrapur 210 MW Unit 3
7
Bhusawal 210 MW Unit 3
MVA
247.0 250
1.01
8
Koradi 210 MW Unit 7
MVA
247.0 250
1.01
9
Khaperkheda 210 MW Unit
No 4
MVA
247.0 250
1.01
10
Chandrapur 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 Absolute
The 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 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
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)
455
Sl.
No.
Particulars of transients
01
Step load change
02
Ramp Rate under variable
pressure
+ 3%
03
Ramp Rate under constant
pressure
+ 5%
Designed
number
of
Starts in life
time of the unit
Designed value
(MINIMUM)
+ 15%
LIFE LIMITING FACTORS
The plant and equipment besides normal ageing is
affected by:
Severity of:
operating duty
cyclic operations
excursions in the operating regime.
Frequency and duration of:
Cyclic operations
Parameter excursions
Translating these factors into engineering factors,
the mechanical life of equipment is finally
controlled by:
Creep
Fatigue
Thermal stresses
LIFE 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.)
We are here
Remaining
Life
(h, years)
Ө
Ө > 45˚
poor
Ө=45˚
Normal
Real time years
<45
˚
Good
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
MTBF: days (Av R-Infra: 122 days)
Average outage period: h/outage (Av RInfra: 74 hours)
Sl. No.
Type of Number Number
Starts
of
of Starts
Starts
in
life
in
life time of
time of Unit 2
Unit 1
01
Hot
start
02
03
Number of
starts/yea
r for Unit
1
Number
of
starts/ye
ar
for
Unit 2
110
120
7.3
8.0
Warm
start
26
27
1.7
1.8
Cold
start
13
14
0.8
0.9
MAXIMUM 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- Maximum
2009
2010
load
in
Unit 1
268.65
268.80
Unit 2
265.98
266.37
2009-
Study of current
operating 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
0.016000
Normal Distribution
0.014000
0.012000
0.010000
0.008000
0.006000
0.004000
0.002000
0.000000
0
20
40
60
Plant Load Factor, %
80
100
120
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 200708 of CEA]
Study of current
operating 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
operating 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
operating 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
320
300
Peak load of DTPS: 268.5 MW
290
280
270
260
250
er
ns
fo
rm
or
er
at
G
en
er
at
or
tra
G
en
rb
in
e
Tu
oi
le
r
240
B
Plant load (MW)
310



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-overloadregime 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 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 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
107
y = -0.0003x 2 + 1.2457x - 1292.4
R2 = 0.6528
Loading rate (% of MCR)
106
105
104
103
102
101
100
2220
2240
2260
2280
2300
2320
Unit Heat rate (kcal/kWh)
2340
2360
2380
Annual Loading rate ( % of MCR) for the period 1995-2010
y = -0.2221x + 609.83
R2 = 0.7339
Annual Loading rate (% of MCR)
110
105
100
95
90
85
80
2240
2260
2280
2300
2320
Unit Heat rate (kcal/kWh)
2340
2360
2380
Study of current
operating 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 rate
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 HR) is 2275.34 kcal/kWh at the test load of 265.5 as
compared to the design heat rate of 2230.60 kcal/kWh.
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 side it is 4.4 kcal/kWh. On the generator side the
deviation is 0 kcal/kWh.
The annual unit heat rate (UHR) of the unit considering all factors is
2292.7 kcal/kWh for Unit 1.
The average degradation of SHR corresponds to a degradation
rate of 0.18 % of design HR/year. The degradation of
SHR has averaged 0.18 %/year over the past few years.
The CPRI test TG HR is showing a degradation rate of
0.13 %/year. The degradation is 2.0 % of the DHR.