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Island operation tests in a SCC-800 CHP Plant - an example from an operating
plant in Sweden.
Presented by Göran Tjellander; Product Manager Industrial Plants,
Siemens Industrial Turbomachinery AB, Finspong, Sweden.
Abstract
The 261-MWe Rya Combined Heat and Power (CHP) plant supplies power and heat
to the city of Gothenburg, Sweden. This combined-cycle CHP comprises three
Siemens SGT-800 gas turbines, each connected to a supplementary-fired Heat
Recovery Steam Generator (HRSG) supplying steam to a Siemens SST-900 steam
turbine. High flexibility with regard to load range, high efficiency, operational reliability
and security of power and heat supply were dominant factors in the selection of the
Siemens multi-shaft combined-cycle solution.
The plant is normally connected to the national grid, but can also operate on the local
grid, which is divided into 25 sections. This allows the local grid to be restored after a
power cut. Hence reliable house-load operation after loss of grid – or a black start
capability to house-load operation using the plant’s own black-start diesel generator were essential requirements. Subsequent energizing of the local grid and loading the
sections one-by-one, demands an advanced step-loading capability for the plant from
zero to 100% plant power-generation capacity.
Extensive testing to verify these properties was carried out during the commissioning
period, autumn 2006. Switchover from normal grid operation to house-load operation
was tested by opening the plant breakers during normal operation. A black-plant
incident followed by a black start-up to house-load operation was tested with minimal
disturbance to the grid.
The subsequent sequences, i.e. energizing and loading the local grid, were simulated
in a computer and fed to the plant controller, enabling verification of the requested
plant island-operation behavior whilst being connected to the large national grid.
This paper identifies the main advantages of combined-cycle solutions based on
multiple medium-sized industrial gas turbines in situations where reliability and
secure power and heat generation are important. It gives island-operation test results
and findings and compares them with the required plant behavior.
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---------------------------------------------------------------------------------------------------------------Rya Combined Heat and Power (CHP) Plant – a brief overview
Figure 1 - Rya CHP
Rya combined-cycle CHP plant (Figure 1), delivered by Siemens Industrial
Turbomachinery AB under EPC contract, supplies power and heat to the city of
Gothenburg in Sweden. It has been in commercial operation since mid-December
2006.
The plant comprises three SGT-800 gas turbines (GT) , each connected to a
supplementary-fired HRSG supplying steam to a SST-900 steam turbine (ST). 261
MWe power and 295 MJ/s district heat (DH) are generated at max. continuous rating
(MCR) which is enough to cover the heat and power demand for 30 to 35% of the
inhabitants of Gothenburg. An installation overview is shown below (Figure 2).
Air
Flue gas
Natural gas
Gas turbine
HRSG
Power
District heat
Steam turbine
Heat condensers
Figure 2 - 3D-view of the installation
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---------------------------------------------------------------------------------------------------------------A sea-water-cooled DH auxiliary cooler is installed in parallel with the DH consumers.
See the simplified flow diagram in figure 3 below.
Auxiliary
cooler
Figure 3 - Plant simplified flow diagram, SCC-800 3x1 DH
Access to the auxiliary cooler, a direct parallel to having access to a steamturbine condensing tail, allows power generation up to 80% of MCR (limited by the
auxiliary cooler itself) without any DH connection - or in a combined mode both
supplying heat to the DH net and the auxiliary cooler; power generation can be
maximized up to 100% MCR.
Furthermore, it allows power generation even in an extraordinary situation
when the DH-net is not available, for example in a situation where island operation is
required due to an external grid cut-off. In this case, heat-condenser cooling is
achieved by the sea-water-cooled auxiliary cooler and thus power generation is
controlled entirely within the fence of the plant – a situation that is similar to a normal
condensing power-generation unit.
The DH auxiliary cooler has another advantage. It allows high-load testing during
commissioning independently of the prevailing DH conditions. This was especially
important during autumn 2006 when the weather was extremely warm in Gothenburg and consequently no full-load demand arose due to low DH-net consumption.
Customer priorities
The plant is intended to operate as a base-load unit during wintertime with maximum
DH generation and provide the highest possible power generation with regard to the
given heat duty, i.e. maximizing the power-to-heat-ratio. During spring and autumn,
1500 h part-load operation and DH-net load following is foreseen.
High flexibility with regard to load range, excellent fuel efficiency, operational
reliability and secure power and heat distribution were also important factors when
Gothenburg Energy AB selected the Siemens multi-shaft combined-cycle solution.
The optional sea-water-cooled DH auxiliary cooler was added to the scope as
a further measure to secure island operation and power generation independent of
the DH-system heat demand.
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---------------------------------------------------------------------------------------------------------------This is, of course, of importance for the total national grid owner and operator,
not only during a longer cut-off situation but also in a potential power imbalance
situation. Hence this option was financed by the national Swedish grid owner
Svenska Kraftnät.
Siemens Gas Turbine SGT-800 (x3) and Steam Turbine SST-900 DH (x1)
SGT-800 is a reliable single-shaft 45 MWe (ISO) industrial gas turbine launched 1997
with high efficiency and high temperature exhaust suitable for combined-cycle
application. It was recently upgraded to 47 MWe.
Fuelled with natural gas from a 28-bar supply system during normal operation
and diesel oil stored at the site as a backup fuel, the possibilities of forced outages
due to fuel disturbances are minimized both during normal operation and in island
operation. Switchover from natural gas to diesel oil is performed automatically at
reduced load if and when the supply-gas pressure has decreased to a defined level.
Configuring the plant solution as a 3x1 multishaft solution, i.e. 3 gas turbines
and HRSGs supplying steam to one steam turbine, ensures exceptional generation
availability.
Air intake
Cold
end
drive
Combustors
Compressor
Turbine
Turbine outlet diffuser
Figure 4 – Siemens gas turbine SGT-800
The installed non-geared single-shaft steam turbine SST-900 (figure 5 below) has
steam data 100 bara/540ºC upstream of the emergency stop valve. The relatively
high steam temperature is attainable and maintainable in the major part of the load
range due to supplementary firing (SF). SF increases the steam generation from 46
to 100% in each HRSG.
The ST is connected to two heat condensers heating the return DH water in
two steps. Downstream of the heat condensers, a dump condenser (DC) with
individual dump-steam lines from each HRSG outlet is installed, designed for
receiving 70% of the total steam generation. This is enough to maintain heat
generation at 100% in case the ST is not available. However - as only the GTs
generate power, the total power generation decreases to 50%.
The DC allows starting the HRSG individually and synchronizing steam data
to the main steam line while minimizing steam loss to the ambient environment. The
ST can thus be started and stopped at any combination of GTs in operation provided
steam data is sufficient and steam generation is high enough.
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----------------------------------------------------------------------------------------------------------------
Blading
Exhaust
Turbine Shaft
Generator
Steam Admission
Turbine Casing
Figure 5 – Siemens steam turbine SST-900 DH
Heat Recovery Steam Generators (x 3)
The HRSGs are single-pressure water-tube boilers with comprehensive SF
(up to 1000ºC), equipped with a Selective Catalytic NOx Reactor (SCR) and a laststage DH water-cooled economizer that minimizes the exhaust losses. The HRSG(s)
are delivered by Austrian Energy and Environment CZ. The main internals of the
HRSG are shown in figure 6.
SF and the low stack temperature (70ºC) allow full-load plant net efficiency
close to 94%.
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---------------------------------------------------------------------------------------------------------------Economizer
Drum and evaporator
DH-cooled economizer
SCR
Superheater
Membrane walls
Flue gas to stack
Supplementary-firing burners
Distribution plate
Cold DH water
Hot DH Water
Condensate
Superheated steam
to steam turbine
Hot flue gas
from GT
Figure 6 - HRSG
The multi-shaft SCC-800 combined-cycle CHP solution
The plant concept, using 3 GTs and 1 ST (3x1), implies excellent load range and
part-load efficiency as reducing the number of GT´s in operation, when the DH load
is reduced, means higher load and efficiencies for the GTs in operation.
Compared with a single-GT concept, the continuous load range is expanded
from 60% to 90% of MCR approximately, while also maintaining high part-load
efficiencies and low NOx-emissions. The minimum continuous plant load is therefore
decided by the ST minimum load rather than by the minimum continuous GT load.
See figure 7 below showing power-to-heat ratio versus DH load. Here the
optimum GT switch on/off DH loads and the effect of SF can be clearly extracted.
120,0
Pwer to Heat ratio (%)
100,0
80,0
3 GT
100% GT load
100% SF
60,0
2 GT
1 GT
40,0
100% GT load
No SF
Switch-off load
(2 to 1 GT)
20,0
0,0
0
50
100
150
200
250
300
DH Heat Duty (MJ/s)
Figure 7 - Power to heat ratio versus DH generation
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Reliability and generation security aspects
Loss of generation probability
4 shafts for power generation secures power and heat generation. For reference, see
figure 8 below.
Figure 8 - Simplified single line diagram
¾ One GT failure from full load causes an output drop of 1/3 MCR. However, if
occurring in the part load range, enough and instant capacity may be available
by increasing SF of the HRSG in operation or by putting a shut-off gas turbine
into operation again.
¾ Loss of the total power and heat generation requires that 3 out of 3 GT(s) in
operation fail which has a low probability and is realistic only in a commonmode failure incident.
¾ If ST is lost, DH generation will continue as steam will bypass the ST directly
to the DC.
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---------------------------------------------------------------------------------------------------------------¾ Loss of power generation due to faults in the internal HV power transmission
is not very likely since there are two redundant three-winding step-up
transformers 1 supplied from the gas turbines GT1/GT2 and GT3 respectively.
¾ Loss of power generation due to failures in the MV and LV distribution
systems are also unlikely since there are two redundant internal auxiliary
power distribution systems 6/0.4 kV. There is also a 10.5 kV external spare
supply connection to the MV/LV systems.
One of these common-mode failures could be an externally caused complete loss of
power incident, i.e. losing the 142 kV grid.
If the plant is in operation at the instant of the power cut, it should be
automatically switched over to “House load operation” and continue to operate - now
isolated from the external power grid and operating with a load corresponding to its
own auxiliary power consumption.
If the plant is not in operation when the external power cut occurs or if houseload operation fails for some reason, a black start is foreseen by using the black-start
diesel generator sized for 2.8 MVA or 2.5 MW active power. This power is primarily
required for motoring one of the 3 GT´s during HRSG purging and until flame is
established and rotor accelerates. The starting motor requires peak 1.5 MWe and
remains high during a 6-minute period.
Black start-up to house-load operation can be carried out by any of the three
GT(s) on natural gas or diesel oil, whichever is available. However, start of GT1 or
GT3 is prioritized as each of them serves one independent auxiliary grid (see Figure
8). Having GT1 and GT3 in operation, these power redundant auxiliary objects, such
as pumps, from two independent power sources.
House-load operation capability
By “House- load operation” we understand the situation where the 142 kV plant
breaker is opened due to external grid faults or failures such as unacceptable voltage
or frequency deviations.
The turbine load is suddenly lost and it turns from load to frequency control
with the plant’s own internal auxiliary power as the only load. The auxiliary power at
MCR is around 3% but decreases to around 1% of MCR during House-load operation
as the main auxiliary objects like large speed-controlled pumps gradually decrease
their speed.
House-load operation as such is a requirement in Sweden for powergenerating plants larger than 50 MWe. The old regulation required 1 h continuous
house-load operation, while the new one stipulates 12 h house-load operation time
after an external grid disturbance (frequency or voltage disturbance that allows the
plant to be isolated from the external grid) or a total external power cut.
The new requirements reflect the consequences of deregulations of the
electricity market in 1996. Since then, an intensified focus has been aimed at
securing island operation areas around the larger cities in Sweden in case of national
power cuts.
1
The three-winding transformer connected to GT3 is prepared for a fourth GT/HRSG (option)
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---------------------------------------------------------------------------------------------------------------There are two auxiliary power sections within the plant, supplying power to redundant
objects like feed-water pumps etc. One is fed by GT1 and GT2 and the other by GT3
(please refer to figure 8 – cross-feeding is also possible). Thus if all three GTs are in
operation, two of them, GT1 and GT3, will remain in house-load operation supplying
power to one section each, while GT2 and the ST will trip..
Consequently, if two or more gas turbines are in operation (which is the case
during the load-intensive part of the year) on the occasion of the disturbance, the
high availability continues to be maintained during house-load operation and
subsequent resynchronization to the external grid.
From house-load operation to island operation
In case a longer power cut occurs – for example longer than the earlier required 1h
but much shorter than the currently required 12 h house-load operation - a decision
may be taken to energize and load the local grid within the operability of Gothenburg
Energy.
Energizing and loading the sections one by one demands both careful
preparation and an advanced plant step-loading capability. As starting and loading
the plant in steps are defined by the grid section to be loaded and not by the plant
operator or the plant load control, the sequence of preparations has the character of
preparing the operational ground that will not jeopardize the subsequent loading and
collapsing the island operation. Hence there has to be an intensive communication
between the plant operators and the island-grid operators.
The preparation involves the following main steps seen from the view of the plant
operators.
¾ The gas turbine(s) are initially in house-load operation. This initial condition
may be obtained either from a successful switchover to house-load operation
directly after the initiating external grid disturbance or by a black-start
sequence using the black-start diesel generator as a means to bring GT(s) to
house-load operation.
¾ Selection of island operation on the operator display which switches the GTs
to frequency control. Steam turbine frequency control is not affected. This
means that the gas turbines and steam turbine will share load; however the
gas turbines only will control frequency.
¾ Switch-over to sea-water-cooled auxiliary cooler operation instead of DH
generation in case the DH system is not available. This will take up to half an
hour, as the auxiliary cooler has to be evacuated from air before starting the
cooling pumps.
¾ Planned or automatic switch-over from natural gas fuel to reserve fuel in case
the gas supply pressure tends to decrease. Fuel change during ongoing island
operation is not suitable as the change may cause an intolerable frequency
disturbance during island operation.
¾ After energizing the first and nearest island section and before loading, the
steam pressure is put to fixed full steam pressure. This measure has two
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---------------------------------------------------------------------------------------------------------------purposes. Fixed steam pressure will reduce the boiler temperature transients
during subsequent step-loading and will also shorten the response time for the
ST.
¾ Loading the individual sections step by step where each step has to have a
known or predefined load level. This is so because the plant has a limited
ability to handle step loads that depends on its operational status, such as
how many of the GTs are in operation and if SF or ST is in operation or not.
Step-load capability and frequency disturbance requirements during island operation
When powering the island grid sections, the following main requirements with
reference to the plant’s step load capability were set up with reference to figure 9
below:
¾ Step-load capability at load increase: 10% of nominal load 2 from zero to 80%
of nominal (available maximum load)
¾ Step-load capability at load decrease: 10% of nominal load from 100% to
20% of nominal (available maximum load)
¾ Momentary overspeed/frequency variation: < 4% / 2 Hz
¾ Frequency recovery time constant: < 30 sec
¾ Speed/frequency band ± 0.2% / 0.1 Hz
Mått
på reglerprestanda
Mått på
reglerprestanda
- lastfrånslag
1.08
1.07
Relative
speed or
frequency
Varvtal
[pu]
rad storhet
1.06
1.05
Momentary overspeed, So
1.04
1 03
Recovery time, Tr
Speedband, Ss
Time (sec)
Figure 9 - Definition of momentary overspeed, recovery time and speed band (illustrated during
a step load decrease transient)
2
Nominal load is equal to available maximum load that is dependent on ambient conditions
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Verification tests during commissioning
Extensive testing to verify the plant’s characteristics with regard to stationary and
dynamic properties in general and in particular its capabilities with respect to island
operation was carried out during the plant commissioning period, autumn 2006.
House-load operation tests
Switch-over from normal grid operation to house-load operation was tested by
opening the 142 kV plant breaker with all GTs in operation at 90% load (40 MWe), SF
not in operation and ST in operation at 56 MWe.
The test was successful as ST was disconnected while GT1 and GT3 stayed in
house-load operation for 1.5 hours, after which the test was interrupted. The
overspeed of each of the gas turbines was below 5.3% (compare overspeed trip
levels: 10%). This was expected as the SGT-800 has full load-rejection capability.
Figure 10 below shows speed and power generation for the three GT´s in operation
for 30 minutes after test initiation. The power generation for the two GT´s that remain
in house-load operation stabilized at 1.5 and 0.9 MWe respectively after 50 minutes
of house-load operation, mainly achieved by the DH pumps and feed water pumps
reaching their minimum speed.
GT1/2/3
load
GT1/3 speed in house
load operation
GT1/3 load in house
load operation
Parameter data
at time 15.07
immediately
before initiation
of house load
test
GT2 speed
Figure 10 – GT data before and after switch-over to house load operation
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---------------------------------------------------------------------------------------------------------------Island operation tests
A black plant incident followed by a black start-up to house-load operation could also
be tested without disturbing the grid as carried out by disconnecting the plant from
the external grid immediately after having stopped GT1 and ST.in operation and then
starting GT1 to house load operation by using the black-start diesel generator.
The intended subsequent sequences, i.e. energizing and loading the local
grid, could for natural reasons not be carried out. However, simulating the response
from that grid in a computer and feeding it to the plant controller, gives an opportunity
to verify the requested plant island operation behavior while simultaneously being
connected to a large national grid.
Solvina AB and the Swedish national-grid operator Svenska Kraftnät have
together designed a hardware-in-the-loop simulator called SolvSim Power Station
SSPS. SSPS is a combined real-time simulator, data acquisition system and
controller used for island-operation testing in Rya CHP.
The test principles are shown in figure 11 below. The load transient and the islandfrequency response from the island grid are generated and calculated by the
computer replacing the real power and frequency as input to the plant load/frequency
controller. Hence the plant reacts as if being exposed to the load transient and island
frequency although the real grid frequency is stable and the power transient does not
affect the real large grid to any noticeable extent.
Figure 11 - Principle of SSPS
Testing was carried out with combinations of 1, 2 and 3 GT´s in operation, with or
without ST and SF in operation, intended to mirror a real situation when the plant is in
island operation from zero to full load. The island load was dependent on the number
of shafts in operation but typically exposed to 1,2,3,4,5,6,7,8,10,12,14, 15, 20 and 25
MWe unit loads from 10 MWe (1 GT only in operation) up to 150 MWe (2 GT and ST
in operation).
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Figure 12 below shows a typical response when two gas turbines (GT2 and GT3)
only are in operation and exposed to a +10 MWe island operation step-load change
(green curve). The gas turbines share load quite nicely (red and black curve) and the
frequency dip is around 1.25 Hz (blue curve).
GT2 and GT3 power generation
Power
Frequency
Frequency
Time (sec)
Figure 12 – Power and frequency response in island operation mode for a +10 MW step load
when two gas turbines only are in operation
Figure 13 shows results of a 20 MWe step-load reduction at the 100 MWe load level
when two gas turbines and the steam turbine are in operation. Here the frequency
control of the gas turbine is assisted by the steam turbine loss-of-load controller that
quickly reduces the steam turbine load.
GT1 and GT3 power
generation
Frequency
Power
Frequency
Steam turbine power generation
Time (sec)
Figure 13 - Power and frequency response in island operation mode for a - 20 MW step-load
when two gas turbines and steam turbines are in operation
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---------------------------------------------------------------------------------------------------------------Testing was carried out for three days; however some problems with one of the gas
turbines made it appropriate for the tests to be split into two occasions.
The problems encountered concerned GT1. At step-loads above 5 MWe,
undamped oscillations occurred and revealed too large clearances in the main gas
valve with motor control. The valve and servo were immediately replaced by Siemens
and tests were repeated a few days later with consistent results comparable with
GT2 and GT3.
Test results are summarized in the table below. As seen, a sufficient
frequency margin is received for step loads of 7 MWe for each gas turbine in
operation. Operation cases with two gas turbines in operation thus can take a 14step MWe load and so forth. If the steam turbine is in operation a further step-load
capability is possible. Based on the test results it was estimated that a step-load
capability around 40 MWe is obtainable within the frequency requirements when all
shafts are in operation. These results were well above the initial step-load capability
requirements.
Shafts in
operation
GT1
Max
3
Load
43 MWe
Step
load
± 7 MWe
Frequency
transient
1,8 Hz
Remark
Estimated
GT2
43 MWe
± 7 MWe
1,8 Hz
Measured
GT3
43 MWe
± 7 MWe
1,7 Hz
Measured
2xGT
86 MWe
± 14 MWe
1,7 Hz
Measured
3xGT
129 MWe
± 20 MWe
1,8 Hz
Measured
2xGT+ST
130 MWe
± 20 MWe
1,3 Hz
Measured
3xGT+ST
170 MWe
±40 MWe
<2,0 Hz
Estimated
3xGT+ST+SF
265 MWe
Limited by GT max
load / SF max/min load
Figure 14 - Results in summary
SF complicates the picture somewhat because attention has to be given to the
maximum power generation limits of the gas turbine during load increase and the
(automatic) shut-off limit of the burners during load decrease. Hence, before a
planned load increase, the degree of SF has to be adjusted so that GT-load is
around 70% (80% during load decrease) to allow for the subsequent highest possible
gas-turbine step-load capability (based on automatic shutdown of SF at 65% GTload). Reducing the gas turbine load to 70% from a higher load in a preceding step
means increasing the SF and thus increasing the steam turbine load (decreasing the
SF in case of load decrease).
3
Max load refers to max gross power generation load at the current ambient conditions
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---------------------------------------------------------------------------------------------------------------Concluding remarks
Combined-cycle solutions based on multiple gas turbines, like the Rya CHP, possess
the built-in flexibility that support high part-load efficiency, extended load range and
secure heat and power generation during normal operation. The high operational
reliability has been verified during commercial operation up to now.
Certainly, when discussing the security of heat and power supply in the event
of external grid disturbances, the same arguments apply. Multiple gas turbines
increase the possibility of keeping the plant in operation after an external power cut.
In the same way, multiple shafts may increase the probability of successfully
building up an island grid if the plant is rightly prepared for it and the operational
limits are well controlled, especially if supplementary firing is in operation.
Tests performed during commissioning confirmed both the expected house-load
operation capability and the step-load capability in island operation.
The latter was verified as being higher than 16% of the current load with GTs only in
operation and being estimated up to 23% with all shafts in operation, which fulfilled
the requirements by a comfortable margin. This capability was practically
independent of GT load level as long as it did not approach the output limits of the
gas turbines or the load limits of SF. The test method used proved also to be an
effective way to discover any malfunctions in the affected equipment, in this case
unacceptable clearances in the main gas valve actuator.