International turnkey power plant provider
Fossil-fired power plant, South America
Two generator sets
Our client, a leading provider of turnkey power plant equipment, asked us to investigate observed power fluctuations of two recently commissioned generator sets. While the levels were below the allowable limits, the plant operator wanted to know why this was happening.
Insight would help in a sector facing increased pressure to maximise plant availability and operate more flexibly with short windows of operation to cope with peak loads. The operator faced additional complexity; the power plant was in a remote location where the electrical grid could be weak. This was especially true if other large consumers, such as heavy machinery and furnaces, caused power fluctuations on the grid.
Our client was also keen to gain new knowledge that would help enhance future design.
How we helped
We developed a comprehensive measurement programme to identify causes of the power fluctuations. This included:
- developing a high-resolution, versatile measurement system
- recording various signals, including rotating speed, dynamic torque, main currents, governor input and vibration levels
- taking measurements at various operating conditions, including loading, full load and during the time between rotor system switch off to rest
The measurements identified that power fluctuations occurred at the half-order frequency of the drive motors.
An experimental verification of the location of the natural frequencies showed that the measured torsional natural frequencies were slightly different to the predicted frequencies, potentially explaining some of the power fluctuations. To investigate this further, we performed several torsional calculations, using various values for the contribution of the electrical grid stiffness.
The analysis revealed that even small variations of the electric grid stiffness can change the torsional natural frequencies such that one will coincide with the half order excitation and consequently cause power fluctuations as torque variation in a generator will generate current variation.
The study provided our client with a better understanding of the important mechanisms behind the power fluctuations: the mechanical-electrical interaction and the influence of the electrical grid stiffness.
This understanding enables appropriate mitigation measures to be taken, if considered necessary. The information will also help shape future design. Tomorrow, it may be possible to develop generators that are less sensitive to different properties of the electrical grid across a range of power plant locations.