Efficiency tests
The main elements of the installation, whose efficiency must be determined accurate from the commissioning, are the turbine, pump or turbine-pump, the generator and the hydraulic circuit.
Efficiency tests should preferably be carried out by an independent firm, with professionals with accredited experience in this kind of tests, with the optimum instrumentation and with the method that offers the minimum measurement uncertainty.
Efficiency tests should preferably be carried out by an independent firm, with professionals with accredited experience in this kind of tests, with the optimum instrumentation and with the method that offers the minimum measurement uncertainty.
Why carry out efficiency tests?
Verify that contractual manufactured warranties (hydraulic circuit, turbine and generator) are met.
Operate the plant at maximum efficiency or maximum power, as appropriate at any given time.
Quantify the increase in energy production by runner change, stator rewind, etc.
To control unit wear and its long-term economic loss.
Assess changes in efficiency as a result of overhauls, repairs or modifications.
Adjust the openings correlation cam in double regulation turbines and the optimal sequence of operation of injectors in Pelton turbines.
Calculate the hydraulic losses of the different parts of the hydraulic circuit.
Determine the appropriate operating range of the unit.
Thermodynamic (direct variant)
The energy lost in a hydraulic machine due to the internal friction of the water and of the water with the bucket-blades of the runner and walls is transformed into heat that passes to the fluid; therefore, at the outlet of the machine, an increase in the temperature of the water is observed, which, by measuring it, makes possible to calculate the amount of energy lost and, consequently, the efficiency of the turbine.
The direct or no-decompression thermodynamic method, described by M. Poirson in 1914, is based on measuring the absolute efficiency of the turbine by knowing the difference in temperature between two sections as close as possible, one upstream and the other downstream of the runner; it consists in the extraction, by means of a sampling probe, of a small flow that is taken directly to a measuring vessel, thermally insulated from the outside, in an isoenthalpic process in which a small decompression is produced that slightly raises the temperature of the water.
Of all the methods included in the IEC 60041, thermodynamic is the one with the minimum measurement uncertainty, which makes it the preferred method in commissioning tests and repowering studies for plants with heads of more than 100 metres, particularly in Pelton.
The direct or no-decompression thermodynamic method, described by M. Poirson in 1914, is based on measuring the absolute efficiency of the turbine by knowing the difference in temperature between two sections as close as possible, one upstream and the other downstream of the runner; it consists in the extraction, by means of a sampling probe, of a small flow that is taken directly to a measuring vessel, thermally insulated from the outside, in an isoenthalpic process in which a small decompression is produced that slightly raises the temperature of the water.
Of all the methods included in the IEC 60041, thermodynamic is the one with the minimum measurement uncertainty, which makes it the preferred method in commissioning tests and repowering studies for plants with heads of more than 100 metres, particularly in Pelton.
Standards
- IEC 60041
- IEC 62006
Requeriments
- The flow is conservative in mass.
- The characteristic quantities are uniform in the input and output sections.
- The regime is permanent.
- The water does not experience any change of state or any chemical reaction.
Main parameters
- Specific mechanical energy
- Specific hydraulic energy
- Corrective terms
- Temperature variations
- Heat exchange through walls
- Influence of condensation
- Direct heat exchange with ambient air
- Heat exchange with still water areas
Main measuring equipment
- Thermometers (accuracy 0,001 K).
- Pressure transducers (accuracy 0.01 % f.s.).
Advantages and limitations
- Uncertainty less than ± 0,8 % .
- Applicable to heads in excess of 100 meters of water column.
Gibson (pressure-time)
The method of absolute measurement of the efficiency of the pressure-time diagram, known as Gibson's method, was developed in the 1920s by engineer N. R. Gibson; the method is based on Newton's law and the derived laws of fluid mechanics, providing the relationship between the force due to the change in differential pressure between two measurement sections and the acceleration or deceleration of the mass of water between those sections as result of the closing movement of the distributor or valve.
The use of data acquisition systems and calculation software, together with the improvement in the accuracy and response time of the pressure transducers, have significantly improved the results obtained with this method.
The use of data acquisition systems and calculation software, together with the improvement in the accuracy and response time of the pressure transducers, have significantly improved the results obtained with this method.
Standards
- IEC 60041
- IEC 62006
- ASME PTC 18
Requeriments
- Conduction pipe greater than 1 metre.
- Absence of free surfaces between the two pressure measurement sections.
- Distance between sections of measure greater than or equal to 10 meters.
- Closing time of the flow regulator element greater than 5 seconds.
- Leakage flow less than 5 % of the flow to be measured, and with a relative uncertainty of measurement less than 0,2 % of the flow to be measured.
- Straight measuring section, with constant cross-section and without singularities.
Main parameters
- Pressure, differential pressure
- Closing element position
- Rotation speed
Main measuring equipment
- Pressure transducers (accuracy 0.01 % f.s.).
- Data acquisition system.
- Position transducers
Advantages and limitations
- Speed of execution.
- Reduced economic cost.
- Information of the dynamic behavior in the transients of the machine.
Acoustic
The absolute acoustic method is based on the measurement of the transit time of an acoustic pulse along the measurement paths. The acoustic probes, with transmitter-receiver pairs, are placed in specific positions in the conduction and connected by signal cables. The speed on each of the measurement paths, thus defined, is determined using the transit time difference method, which uses the fact that an acoustic pulse moves in water downstream direction faster than one that moves upstream.
The use of two planes with eight cross-measurement paths minimises the usual errors of transit time measurement systems to a minimum. The use of external clamp-on sensors is not permitted.
The use of two planes with eight cross-measurement paths minimises the usual errors of transit time measurement systems to a minimum. The use of external clamp-on sensors is not permitted.
Standards
- IEC 60041
- IEC 62006
- ASME PTC 18
Requeriments
- Straight conduction section of at least 13 diameters.
- Pipe diameter greater than 0.8 m.
- Water velocity greater than 1.5 m/s.
Main parameters
- Transit time
- Pipe geometry (L,D)
Main measuring equipment
- Acoustic probes and integration equipment.
- Data acquisition system.
Advantages and limitations
- Only equipments based on transit time is allowed as absolute method, preferably using eight measuring paths with sixteen transducers and internal assembly.
- Equipment based on Doppler or on moving particles is not valid.
- Equipment based on external transducers are not valid even if they are by transit time.
- If the above limitations are met, this measurement method is allowed as absolute, only by mutual agreement or if another primary method is used simultaneously, which will be the valid one.
Relatives (index type)
The relative measurement methods or index type of evaluation of the flow that is passing through the turbine, in its different variants, are applicable to practically all types of turbines, without limitation of head or flow values. The most commonly used are:
- Winter-Kennedy: By measuring the differential pressure between two pressure taps located in the spiral chamber or enclosure of reaction turbines (Francis, Kaplan, Bulb, Deriaz).
- Acoustic with external pipe mounting: Generally used in action turbines (Pelton, Turgo).
Standards
- IEC 60041
- IEC 62006
- ASME PTC 18
Main parameters
- Differential pressure
- Transit time
Main measuring equipment
- Differential pressure transducers.
- External acoustic probes.
Relative methods allows
- To control the test carried out with the selected absolute method and to facilitate its accomplishment combining measures.
- Assess changes in efficiency as a result of repairs or modifications (unless the flow in the spiral chamber is altered, as in the case of Winter-Kennedy).
- Extend the efficiency curve obtained by an absolute method to higher heads and powers.
- Verify the evolution of efficiency over the years.
- Check the flow through the turbine.
- To obtain the optimum cam correlation between the runner blades and guide vanes for double regulation turbines.
Calorimetric
To obtain the efficiency of large electrical generators with air-air and air-water cooling systems from the heat generated by them due to their different types of losses (radiation, conduction and convection).
The test basically considers heat transfers through the air-water or air-air heat exchangers of the main generator cooling system, heat exchange in the oil-water cooling system of the bearings, convection and radiation losses on the radiant surfaces of the generator housing and bearings, conduction losses through the generator shaft and conduction through the concrete structure, the energy required for the operation of the generator excitation system and losses in the external elements (circulation pumps, lubrication, etc.).
It allows the determination of both total losses and segregated losses (bearing friction, friction and turbulence with air, iron, rotor copper, stator copper and stray).
The test basically considers heat transfers through the air-water or air-air heat exchangers of the main generator cooling system, heat exchange in the oil-water cooling system of the bearings, convection and radiation losses on the radiant surfaces of the generator housing and bearings, conduction losses through the generator shaft and conduction through the concrete structure, the energy required for the operation of the generator excitation system and losses in the external elements (circulation pumps, lubrication, etc.).
It allows the determination of both total losses and segregated losses (bearing friction, friction and turbulence with air, iron, rotor copper, stator copper and stray).
Standards
- IEC 60034-2-2
- IEEE 115
Test runs
- No load and no excitation.
- Without load and with excitation.
- Permanent short circuit.
- Loading at different percentages.
Main parameters
- Flow.
- Temperature.
- Differential pressure.
- Current.
- Voltage.
- Ohmic resistance.
Factors to consider
- Thermal equilibrium.
- Voltage variation.
- Current variation.
- Rotational speed variation.
- Air density variation.
- Ohmic resistance variation.
Head losses
The hydraulic circuit is the main transport element of the fluid, the determination of its global efficiency, defined as the relationship between net (H) and gross (Hb) head, as well as that of each of its elements, makes it possible to control the wear degree of the conduction and verify compliance with contractual guarantees. Pressure losses are due to two factors, water friction (with the pipe and with itself) and non-uniform flow (obstructions, changes in shape and/or size of the pipe and flow direction). In the most general case, the following head losses are determined: total, head race tunnel, main penstock, penstock branches and guard valves (butterfly type).
Standards
- IEC 60041
- IEC 62006
Main parameters
- Static pressure.
- Flow rate.
- Water conduction geometry.
Main measuring equipment
- Absolute pressure transducers.
- Differential pressure transducers.
- Water level sensors.
- Data acquisition system.