1, Combined cycle power plant output has many impact factor
Power output of a CT is directly proportional to and limited by the mass flow rate of compressed air available to it from the air compressor that provides high-pressure air to the combustion chamber of the CT system. An air compressor has a fixed capacity for handling a volumetric flow rate of air for a given rotational speed of the compressor. Even though the volumetric capacity of a compressor is fixed, the mass flow rate of air it delivers to the CT changes with fluctuations in ambient air temperature. This mass flow rate of air decreases with an increase in ambient temperature because the air density decreases when air temperature increases. Therefore, the power output of a combustion turbine decreases below its rated capacity when the ambient temperature increased.
Figure 1. Effect of Ambient Temperature on the Output of CTs
- summer time
Gas turbine inlet filter house install evaporation cooling, a centrifugal pump start to pump the deminerlation water to the evaporation cooling media, atomized water absorb heat from air to be vapor to decrease the inlet air temperature increase air density, hence increase the gas turbine output, the degree of inlet air decrease largely depends the humidity, for example in the 20 °C, if the humidity less than 20 %, inlet air temperature can decrease 10 °C down to 10 °C, and if in 90% humidity, the temperature only can decrease 1 °C.
1,2 winter time
Gas turbines that are installed in areas where icing conditions could exist shall be equipped with an anti-icing system. In general, ambient temperature dropping below 4ºC (39 ºF) with high humidity could be subject to icing problems. Icing could induce very significant pressure drop in the inlet air filter house which leads to gas turbine power output deterioration.
In the winter time when the ambient temperature is below 4 c and humidity is high and also the IGV is not fully open, inlet heating need start to operation, glycol pump cycles the glycol through the heater to pass though the inlet heating coil to increase the temperature and decrease the humidity to prevent the icing.
1.3 gas turbine degradation
Several mechanisms cause the degradation of engines: Fouling is caused by the adherence of particles to airfoils and annulus surfaces. The adherence is caused by oil or water mists. The result is a build-up of material that causes increased surface roughness and to some degree changes the shape of the airfoil (if the material build up forms thicker layers of deposits). Many of the contaminants are smaller than 2 μm. Water wash can restore portion of this kid of degradation.
1.4 Duct burner factor
Duct burner is another factor to impact plant output, duct burner impactor is come from duct burner reliability and sometime IP steam temperature high will limit the duct burner gas input .
1.5 steam turbine output
Steam turbine output impact by the weather and humidity, the humidity is impact for the cooling tower performance, the humidity is lower, cooling tower has can take more heat to atmosphere through evaporation of water, then the condenser vacuum lower and steam turbine output will increase.
1.6 the challenge for the for the plant out forecast in Calgary
The temperature change is big in the location of power plant, in summer time the ambient temperature can be 32 °C, and in the winter time the down to -32 °C, and humidity change hour by hourly, for the 2X1 MHI 501G combine cycle with duct burner the plant out can be 870MW in winter and summer can down to 720MW if not use evaporation cooling, the accuracy to forecast the plant out for is key factor to economic operation power plant
2, Power Grid dispatch requirements
Plant operator need submit the next 24 hours output forecast plus 7 days average output in the morning time before 6 am, which is important for the trader to calculation the bid strategy for next day, and operator will submit another declaration reports before 7:30pm, which will be enter to the grid dispatch computer system for the dispatch, start next day mid night, the dispatch center will issue dispatch per hours, the plant must dispatch output within 10 MW difference with the submitted results. If out of the ditch the potential of the out of compliance and cause economic penalty, so the operator needs to issue a restatement reports for correct the dispatch results
3. Output forecast tool
3.1 dispatch tool requirements
Due to the extremely strictly dispatch requirements and the large range of the change of the weather, and a lot of the factor will impact the plant, so it is require to build a good tool to support operator to issue a reports and the output forecast tool need accuracy to provide results and, the user friendly, to minimize the operator job, reports need highly automatically and just need to several click, then reports can be send out and, the reports need a log for history trace
3.2 Ambient temperature source
Ambient temperature source is critical for build a forecast tool, due to the gas turbine heavily impacted by ambient temperature and humidity, to get the relay able source for the ambient temperature is key factor to build a successfully operator tool
There is two source to used for the data, the first source used thought he Oracle data base connect to accuracy weather website, but accuracy weather not provide the humidity information, another source is from a free website which only provide minimum usage for each day, less than 2000 points, if the over 2000 points, it is requirements to pay of the money
3.3 Excel, VBA program
3.3.1 Excel is basic platform to establish the tool, excel is general Microsoft software which can provide powerful calculation function and open data source to connect to excel spreadsheets.
3.3.2 Connect to weather information method
Oracle database: use Excel database through the query connection, the data provide 15 days weather forecast, hours by hours, provide temperature but not includes the humidity information which is largely impact the evaporation performance and cooling tower performance impact
Weather website download:
Free download weather information form Dark Sky API, which only allow 2000 points, it is good enough to do 10 reports per days, we never have chance to send 10 reports to dispatch center per day, so free version is good enough for one power plant. then JsonConverter to converter Json data to excel and take the temperature and humidity. Make sure the API link coordination is set up the site location
3.3.3 set up the report page
Use the gas turbine manufacture provide the performance degradation formula and dispatch center requested the reports form to set up automatic calculation, excel build in calculation formula can be used to set up the calculation, combine the humidity and temperature, the barometric pressure can be set up constant, due to the elevation of the plant not change.
For MHI 501G correction curve;
3.3.4 set up the macro to automatic calculation inlet heating and inlet cooling
Find the relationship the evaporation cooling results with temperature and humidity change is key for the success forecast the MW, through the years inlet filter evaporation cooling statistic, the tool use the below formula:
The ambient temperature between 12 to 18 °C:
∆t = -T * F* (1 – H)
The ambient temperature greater than 18 °C:
∆t = -T * F * (1 – H) + (T – 18) / 2.2
T: forecast ambient temperature °C
H: Humidity %
F : humidity correct factor, (1-H) when 1-F less than 0.55 must use o.55
∆t: the inlet air temperature drops after evaporation cooler
For the inlet heating, set up 6 °C increase the inlet air temperature
Plant DCS create a curve to show the icing show, operator will start the inlet heating operation, in general, any time the temperature blew 4 °C, and humidity above 70%, inlet heating needs to be in service, the tool set up the automatic calculation to add 6 °C bias to add the forecast temperature.
Due to weather forecast always is forecast, some time it is not accuracy, so the tool provide the bias import to easy adjust the temperature, use the PI get the data for the past hours to compare current time forecast temperature, operation can use bias to adjust temperature also can manual input to adjust the temperature go obtain the actually plat output to meet the dispatch requirements.
3.3.5 Use VBA to build automatic calculation and generator reports
Use VBA macro transfer calculation results to separate spreadsheets for report, name the report file and prepare reports with data and reporter. And with the separate spreadsheet includes a macro to send email to dispatch company excel file as attachment with receiver name and copy lists. No need to write any words anything is set up, just send out.
3.3.6 Set up automatic compensation for the afternoon
When the sunshine to pass noon, the inlet filter house will expose to the sunshine and it gradually accumulate the heat in the filter house, the temperature is 2 degree higher than outside of the filter house, it is last until 10:00 pm. the plant output will decrease out of the ditch, operator will request to send the restatement, so macro needs automatically to set up the 2 degree compensation for this kind of situation when forecast weather forecast temperature is bigger than 18 degrees
4, Operation results
The forecast tool operation is good, the forecast become more and more accuracy and it is used almost over 6 years, each report, most time the temperature and humidity are accuracy so operator can finish the reports within 5 clicks, it can be sent out to the provincial grid dispatch center. The reports need to be update if the gas turbine and steam turbine update, for example ESEC did CT2 Kai upgrade the gas turbine increase 12MW after Gas turbine output upgrade.