Wind power goes baseload
Automation standards and rapid control systems can make wind turbines intelligent enough to provide baseload power, say our two guest authors today from Siemens and Freqcon.
Freqcon has developed a complete standard automation system for future wind turbines with gearboxes. One crucial goal is to make the turbine more efficient by facilitating constant controls. Another goal is to add energy storage in the form of lithium batteries to make wind power production dispatchable – so much so that it can provide baseload reliability.
The automated system is to work with Profinet, which connects central computer controls to the converter via data cables. The converter records all data about the main drivetrain components. Frecqon's system records standard logs and visualizes curves for power production, temperature, and oscillation.
Inverters are integrated via fast control technology. The inverters installed in the wind turbine have switching units (IGBTs) that can react to Siemens' Microbox PC, a control computer, within 250 µs – equivalent to a fourth of 1,000th of a second. The result is real-time controls for wind turbines – a novelty in German wind farms. The benefits are tremendous when wind turbines have homogenous control systems. All of the components are better coordinated to an extent often not possible in the wind sector today. This coordination requires a fast, high-performance data cable – a bus.
Real-time controls are key
Freqcon planned all of the control technology for the development of a new 2.2 MW wind turbine. The system is based on Profinit as an open, high-performance bus system for which all of the required standard components are available.
A program similar to Windows is used (Soft SPS with WinAC RTX). It allows real-time events to be programmed for the turbines. The software accesses more than 50 virtual reference works for the wind turbine's various technical tasks. This Wind Library provides, for instance, constant data about how the turbine reacts to the wind, how to control temperatures within the generator, how blade pitch can be changed to accelerate or slow down the rotor, and how the power curve can be balanced.
The problem with conventional wind turbine control systems is that each individual function has its own controls. Developers of control systems, turbines, and components simply add new functions to the control system for their equipment. The lack of standardization in the control architecture can slow things down, and if a developer wants to change something in the controls, the downstream impact first has to be studied. For instance, if you want to remove a control instrument, you have to make sure it is not used by other programs.
Profitnet RT (real-time) handles communication, and programming is done in structured text – a user-friendly, high-register software language that allows system maintenance to make small changes to loops. For instance, service technicians can easily optimize systems themselves to speed up initiation and maintenance.
Pitch systems and yaw drives work with and especially intelligent frequency converter (Sinamics G120); the converter carefully adjusts the frequency of power supply for pitch adjustments and the alignment of the nacelle to face the wind. The goal is to have the rotors and nacelle change orientation without jolting and causing vibrations throughout the system. This frequency converter spreads loads evenly across the four motors of the yaw drive.
WinCC, a Windows-based system, visualizes the data flexibly. 24-volt power packs are used. Scalance switches in the communication network make sure that all control signals reach components and functions. The communication network even helps stabilize the grid.
Siemens calls the system Totally Integrated Automation (TIA). In the new turbine, TIA allows for a plug-and-play approach – you simply plug in and switch on the hardware and software you need to change the controls.
Saving energy for service
This one-stop-shop automation opens up new options. To begin with, there is energy storage. In markets like China, wind turbines are often built only to stand around for months waiting on a grid connection. Service technicians need power, of course, for repairs and maintenance. In such cases, and in case of grid outages, wind turbines could provide that power themselves by storing it.
In our system, the water-cooled inverter for the synchronous generator is connected to a lithium battery system. Intern, they are both connected to the turbine's central control system via Profinet. The lithium cells have a voltage of 3.2 and around 1,000 amp-hours. The batteries are housed within the turbine and store energy when the grid does not need it. The total storage capacity is around 200 kilowatt-hours – enough for the turbine's own demand at all times.
At present, these batteries are just an idea, but we are expanding the concept to include this storage option. The battery systems will be connected to the turbine's inverter's power circuit so that no additional transformers will be needed.
Giant battery block for base load
The efficiency of such battery systems is around 90-95%. During charging and discharging, losses are only around 1%, so that the batteries can be densely packed without overheating.
This combination makes the wind turbines capable of providing baseload power. Modern wind turbines have towers up to 149 m tall, which provides enough space for up to 10 MW-hours across 80 m³ of space. In fact, with a base diameter of 8 m, you could put those batteries in the tower's basement.
Freqcon has developed a system to this effect, in which each lithium battery has sensors that measure temperature, voltage, and balancing.
By reducing the maximum charge and discharge levels by 10%, we have found that cycles can not only be sped up, but their frequency can even be doubled; if we assume one complete cycle per day and 7,000 cycles, we are talking about a lifespan of around 20 years.
China already requires 10% of a wind turbine's installed capacity to be able to be stored for two hours, and Germany is considering such a requirement.
The best thing is that the new control system does not cost more than the ones currently used. New, innovative components cost more, but other redundant components are no longer needed. At present, the redundancy is needed because each separate control system needs its own bus system and data inputs. (Craig Morris)
Leiter Vertrieb und Promotion Wind
Siemens Industry Automation