With the aim to provide electricity to all, the Indian government is focusing on building new power plants in India. It is expected that in the XII Plan period alone, Indian electricity generation capacity would be enhanced by at least 75,000 mw. All these new power plants needs to be designed for to provide best efficiencies and utilizing newer technologies such as the use of supercritical steam generator or advanced class gas turbines. While power plant engineers will design optimum solutions, the plant once built has to be tested for the "Guaranteed performance".

Performance testing is not a new trend but a necessity. CEA of India has brought out its report on existing power plants, which suggest that many existing power plants require to be renovated and modernized (R& M) and before R&M, the performance test needs to be done.

Globally, this concept is already popular. At Tractebel Engineering, we have given our consultancy services in this area in various projects worldwide - seven nuclear power plants( >5,000 mw), over 50 hydropower plants (>20,000 mw), over 20 coal-fired power plants ( >28,000 mw), 30-plus combined cycle power plants (>20,000 mw), 30 cogeneration plants (> 1,000 mw), 30 wind farms (>500 mw) and other renewables like solar ( both PV and CSP), biomass plants (> 1,000 mw), etc.

Two of our projects have been recently commissioned. They are the 685-mw CCPP Al-Dur project in Bahrain and the 2,750-mw CCPP Marrafiq project in South Arabia. Hence we bring on table the international experience. During our long journey of providing consultancy services, we have learnt that Performance tests are an integral part of a quality engineering service program.

DESIGN STAGE
During the preparation of specifications, performance guarantee parameters are always specified. If not, it is always agreed during the contract signing stage. The ASME PTCs are often referenced in equipment purchase and/or engineering, procurement, and construction (EPC) contracts to provide a standard means of determining compliance with performance guarantees. Specifications also include instructions for designing and executing performance tests at both the overall plant level and the component level. After the design of the plant is fixed, a plan for conducting the test and analyzing its results is prepared and documented. A well-written plan envisages provisions for both expected and unexpected test conditions.

Test procedure is established between all stakeholders before the commencement of the test. This covers all aspects of health, safety, environment and performance.

APPLICATION OF CORRECTION CURVES
All performance parameters are guaranteed at reference conditions. Since the reference conditions are rarely met during the test conditions, correction curves (for atmospheric pressure, humidity, CW temp, etc) are normally agreed in the contract.

For example, a plant may be guaranteed to have a capacity of 220 mw at a heat rate of 1,700 Kcal/kWh-under typical reference conditions with an ambient temperature of 30ºC and relative humidity of 60 per cent. The purpose of testing is to confirm whether the plant performs as guaranteed under those specific conditions. But how do you verify that a plant has met its guarantees when the test must be done on a dry winter day, with a temperature of 15ºC and 30 per cent relative humidity? The most challenging part of the performance testing is correction of results for differences in atmospheric conditions. Good Engineering consultants ensure that OEMs and contractors provide ambient correction factors as a set of correction curves or formulas for their individual components. Also, as engineering consultants, we have to integrate the component information into the overall performance correction curves for the facility.

KNOWLEDGE OF SPECIFIC APPLICATION
The reference conditions for performance guarantees are unique to every site. For a gas turbine, the condition of the inlet air has the biggest impact on gas turbine based plants because the mass flow of air through the turbines (and consequently the power they can produce) is a function of pressure, temperature, and humidity. Performance guarantees for steam plants depends more on the cooling water temperature. The effect of humidity, however, is more pronounced on cooling towers. A very humid ambient air reduces the rate at which evaporation takes place in the tower, lowering its cooling capacity. Downstream effects are an increase in steam turbine backpressure and a reduction in the turbine generator's gross capacity

Hence the most important correction for gas turbine plant performance tests involves compressor inlet air temperature. Although a site's barometric pressure typically varies by no more than 10% over a year, its temperatures may range from sub Zero to almost 50ºC over the period. Because air temperature has a direct effect on air density, temperature variation changes a unit's available power output. For a typical heavy-duty frame gas turbine, a 3ºC change in temperature can affect its capacity by 1 per cent. A temperature swing of 30ºC could raise or lower power output by as much as 10 per cent.

Beyond site ambient conditions, the PTCs defines the numerous other correction factors that the test designer must consider and it is important to include the specific requirements in the testing procedure to be agreed between the contractor and the owner/owner's engineer.

PREPARATION OF THE TEST
Before the test begins, an important aspect is the preparation of the test. Necessary isolation of some systems is made to satisfy guarantee conditions. Some modifications (temporary arrangements) are necessary to allow measurements which are not installed in the system like the flow meters. The measurement of generator output at the designated points and the requirements of additional metering may arise for specific schematics. Instrumentation used to record test measurements should be selected carefully to have an accuracy class as specified in the test procedure with calibration certificates. The data acquisition system along with measuring devices needs to be calibrated to PTC-46. Wiring up of the measuring instruments to a data acquisition system connected to laptops for performing real time calculations is done.

PERFORMING THE TEST
First step is to make a visual inspection of the existing installation. The schematics which is approved for the test, is the reference document. If site installations are not matching with the reference documents then deviations should be discussed and documented to make sure their impact is understood by all parties. It is verified that all systems are operating properly. This is done through planned preliminary tests and such tests shall be done in steady state conditions with constant load.

The actual test is started at a predetermined time. All the steady state values are recorded as per the plan in DCS/DAS.

If all the preliminary checks are done properly, the actual performance tests are actually done with more easiness. This is an area that requires expertise and good consultants such as Tractebel Engineering performs a major role. Having done all the "real" work beforehand, a consulting firm ensures that nothing changes during the test that may affect the stability of the data.

During the analysis phase, the calculations for the performance test are done as per the test codes. Necessary corrections are applied using the contractual correction curves and the final results compared with the guaranteed parameters. If the obtained results are found to be within contractually agreed limits, the performance test is declared successfully accomplished.

THE FUTURE
In an era of increased participation by private players in power sector, it is evident that performance testing is given prime importance. Demonstrating the performance of the power plant can be critical to its financial success of the plant and also an endeavor for greener future, performance testing is a way out to attain maximum efficiency and least operation costs. There will always be a need to standardize and avoid contractual litigations. This requires an active participation of all stakeholders— developers, contractors and consultants.

(Bhupendra Jain is Addl. General Manager - Projects, Tractebel Engineering Pvt Ltd)

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