Technical investigation of power fluctuations at power plants

Client: International turnkey power plant provider
Asset type: Power plant
Location: South America

Coupled mechanical-electrical interaction calculations to understand power fluctuations at power plants

Business challenge for the client

With the ever rising demand for energy consumption, power plant operators experience increased pressure to maximize the availability and operate more flexibly with short windows of operation to cope with peak loads.

Additional complexity arises when installing power plants in remote locations where the electrical grid may be weak, especially if other large consumers such as heavy machinery and furnaces, cause power fluctuations on the grid.

In this case, Lloyd’s Register Consulting was contracted by a leading provider of turnkey power plant solutions, to investigate observed power fluctuations of two generator sets at a fossil fired power plant in South America.

The power fluctuation levels were below the allowable limits but the operator nevertheless wanted to pursue an investigation to get further insight into the matter.

Key issue

Power fluctuations on two recently commissioned generator sets at a power plant

How we helped the client

A comprehensive measurement program was set up to identify causes of the power fluctuations.

A high-resolution and versatile measurement system developed by Lloyd’s Register Consulting was used to record various signals including rotating speed, dynamic torque, main currents, governor input and vibration levels among other things.

Measurements were taken at various operating conditions including loading, full load, and during coast-down (see image).

The measurements clearly showed that the power fluctuations occurred at the half-order frequency of the drive motors.

An experimental verification of the location of the natural frequencies indicated that the measured torsional natural frequencies were slightly different to the predicted frequencies which may explain some of the power fluctuations. In order to investigate this further, a number of torsional calculations were performed 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.

Results and benefits

The key benefit from the study is a better understanding of important mechanisms relevant for the observed power fluctuations, in this case the mechanical-electrical interaction and the influence of the electrical grid stiffness.

With this knowledge, appropriate mitigation measures can be taken if deemed necessary. Further, the information can be used early in the design process if it desired to make the design less sensitive to different properties of the electrical grid in different locations where a power plant may be installed in the future.

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