Grid management in the age of renewables
By Timo Holopainen, Jari Jappinen, Juhani Mantere, John Shibutani, Jan Westerlund, Mats Ostman and Joonas Helander
Tuesday, 13 June, 2017
Driven by legislation and the underlying climate concerns, renewable energy penetration is on the rise. The increased usage of renewable power generation creates new challenges for the electrical grid, system control and the existing power generation facilities.
More and more renewable power generation sources are connecting to the power grid. The power output of many of these sources can be highly variable and their fluctuations have to be compensated for by flexible grid-support plants. In contrast to traditional power generators, grid-support plants are subject to frequent starts and stops, as well as rapid load cycling. As is confirmed by studies of real-life loading cycles in grid-support duty, the key factor that must be taken into account in the design is the increased number of thermal and speed loading cycles. Improving the design of the alternator, so it can withstand additional stresses, is fundamental to the reliability (see Figure 1).
Power plant load cycles
Traditionally, alternators are operated at rated conditions and constant speed over long, uninterrupted periods. This has determined the design principles and dimensioning of alternator structural parts. Grid-balancing operation entails rapid alternation of operation and standstill periods — resulting in a much higher number of starts and stops (as shown in Figure 2). In principle, the difference between the traditional and grid-balancing generator is the number of loading cycles and the steepness of the load change.
Modern generating sets can get from zero to full speed in 30 s and to full load in 5 min; stopping time from full load to standstill is 1 min. The plant shown in Figure 3 had nine starts and stops over a six-day period, averaging to 500 annually. In practice, the number of cycles can be even much higher.
Loading profiles
In general, the warming and cooling of alternator parts is not even and their thermal time constants differ. This transient anisotropy is the main contributor to thermal stress and makes the analysis of thermal cycles demanding.
To analyse and simulate thermal behaviour, two different load profiles — derived from the real site described above — were selected (as shown in Figure 4). These examples provide a maximum number of load/standstill cycles, which also gives a maximum number of thermal loading cycles for evaluation (Figure 4a), as well as a temperature gradient between the winding and core that is close to its maximum value (Figure 4b).
Analysis of the thermal cycles
It is expected that the thermal stresses are mainly generated in the windings and the core region of the alternator. The prediction of thermal stresses requires that the temperature distribution can be simulated. The thermal conductivity of copper is excellent and that of steel is good. Thus, the largest temperature gradients are in the electrical insulation layers between the copper–copper and copper–steel joint surfaces. The temperature difference between these parts defines thermal stress in an alternator.
A thermal network method was applied to predict the transient thermal behaviour of the active parts of an alternator, such as the stator. In the case with several consecutive short loading/idle cycles, the temperature difference between the winding and core can vary by as much as 10 to 25 K during the load cycles (Figure 5a). Where there is a longer full-load period reaching close to maximum operating temperatures, the temperature difference between the winding and core can reach 30 K (Figure 5b).
As the stator coils are bonded to the slot walls due to the impregnation treatment and cannot move freely, internal stresses are generated in the insulation layers, which can lead to cracking if appropriate measures are not taken.
Analysis of the speed cycles
Usually, the origins of alternator vibrations are the reciprocating forces of the combustion engine. A four-stroke internal combustion engine creates excitation forces on full and half harmonics of the rotational speed. The generating unit is so complex that only numerical simulations can predict the vibration behaviour with the required accuracy. The only way to reliably investigate the fatigue strength of the structural design is to perform a response analysis for the whole generating set. The vibration design of continuously operating alternators is based on the avoidance of main resonances. Due to the high number of starts and stops, fatigue design of grid-balancing applications requires analysis also for start and stop cases.
Implications for alternator design
Based on the thermal and speed-cycle analysis, as well as experience from other high cyclic generator and motor applications, there are several parts in the alternator that must be carefully considered when designing reliable alternators for grid-balancing applications.
Insulation and winding system
As discussed above, winding and insulation are detrimentally affected by thermal cycling. Experience has shown — and analysis has confirmed — that global vacuum pressure impregnation (VPI) gives outstanding characteristics to the whole stator and rotor (laminated steel core and windings).
In the development process, the verification of the system by testing is always important. In a typical thermal cycling test procedure, several sets of test bars are heated in an oven to different temperatures and cycle times. The test bars are then exposed to mechanical stress on a vibration bench, to humidity and finally to voltage testing of conductor insulation and main insulation.
Test cycles are repeated until a certain number of test bars in each set fails voltage testing. The lifetime is then calculated from the test results of each set using the so-called Arrhenius rule (See Figure 6). Successful tests have been recently performed for the impregnation system in use.
End windings
End windings, along with their support construction and connections, are exposed to thermal cycling and vibrations caused by acceleration, deceleration and frequent grid switching.
The vibration of stator end windings is of major concern in large electric machines. Particularly in two-pole machines, the natural frequencies of winding ends tend to decrease to close to the twice-line frequency (100/120 Hz). Thus, in these machines special support structures are needed in order to increase the winding end stiffness and natural frequencies. However, in multipole alternators the winding ends are inherently short and the natural frequencies sufficiently high without any additional support structures.
In the development and design of the end winding construction a set of modern methods is used, including 3D finite element analysis (FEA). This method is used for the calculation of forces together with static and dynamic response.
The construction and design of the end winding support system with global VPI gives very good characteristics given existing forces and stresses. This means that the end winding design of medium-speed grid-balancing alternators is robust and resilient against vibrations.
Operation at underexcitation (consuming reactive power) causes thermal stresses in the core-end region. In the case of medium-speed alternators (high pole number), this effect is less severe thanks to the smaller coil width and more favourable flux distribution at the end region.
Frame
The frame of the alternator is mounted on the common base frame together with the combustion engine. The design of the alternator frame is determined significantly by the vibration excitations of the engine transmitted to it by the base frame. This leads to a slightly more robust frame design compared to alternators mounted on a concrete foundation.
The alternator frame design is determined by fatigue resistance. The ability to design reliable alternators, and still have a cost-efficient frame structure, requires thorough knowledge of the dynamics of the whole generating set. A response analysis (numerical simulation) of the whole generating unit is the key to success here.
The fatigue stresses can be simulated during the start-up and shutdown periods. Based on the calculated stress histories, the fatigue life can be evaluated by conventional methods and the critical structural details can then be modified to resist the fatigue loads. Ultimately, this approach ensures that the alternator frame reaches the desired lifetime without any fatigue failures.
Rotor and bearings
Regarding rotor design, medium-speed alternators are always subcritical. This means that the first flexural critical speed of the rotor is above the rated rotational speed of the alternator. The rotor does not cross any flexural critical speeds during the cycling loading, thus giving freedom to rotor and bearing design. This is a clear advantage over higher speed alternators (eg, two-pole design).
The thermal cycles have effects on the rotor similar to those on the stator. The prevailing principle of rotor design is to retain the contact between the components over the temperature cycles — thus avoiding the resin mechanical fatigue. Moreover, the bearings are equipped with a jack-up system, enabling a very large number of starts without any wear.
Good design ensures long life
The age of variable renewable generation means that grid-balancing generators must endure a much larger number of thermal and speed cycles than traditional generating units. The design of the grid-balancing alternator requires particular attention for reliable operation. However, with an optimal design, alternators will be able to withstand these new, greater stresses and deliver high reliability over very long lifetimes.
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