Protecting against wind turbine grid disturbances

Paul Boughton

Faced with new and emerging standards for increased grid reliability, Iberdrola Renovables of Spain—the world’s largest operator of wind farms—has selected GE Energy grid integration technology to upgrade its GE 1.5 MW turbines at wind farms in the Castilla La Mancha, Murcia and Rioja regions of Spain.

GE Energy will equip 290 GE 1.5-megawatt wind turbines in the three regions with new GE frequency converters incorporating the company’s proprietary low voltage ride-thru (LVRT) technology, which allows uninterrupted wind turbine operation through many types of grid disturbances.

“Spanish utilities have increased their requirements for wind energy,” said Victor Abate, vice president-renewables for GE Energy. “Wind turbines without capabilities such as those provided by LVRT no longer qualify for special feed-in tariffs.”

The project with Iberdrola Renovables will mark GE’s first LVRT upgrade of wind turbines in Europe and GE’s Spanish field operations team will do the installations.

“More and more utilities worldwide are adding wind to their power generation mix. With this increased level of wind power penetration into electric power networks, turbine manufacturers face more stringent transmission standards,” said Abate. “GE’s LVRT capability is one in a series of continuing technology advancements helping to move the global wind industry forward. It enables the turbines to meet transmission reliability standards similar to those demanded of thermal generators.”

In the past, wind turbines were designed to trip off-line in the event of major system disturbances such as lightning strikes, equipment failures or downed power lines. This loss of generation impacted grid system stability and could lead to cascaded tripping and loss of revenue. Today, grid operators are requesting that wind farms ride through grid disturbances, remaining on-line to continue supporting the grid.

GE’s LVRT technology is designed to deliver ride-through capability for up to 15 per cent grid voltage for at least 500 milliseconds. The technology is part of the company’s WindRIDE-THRU family of products, which also includes zero voltage ride-thru. Both packages are designed to meet present and emerging grid reliability requirements worldwide.

Iberdrola Renovables’ wind farms generate 7608 MW of wind power worldwide and the company is building an additional 1686 MW of wind power capacity. GE Energy is a leading supplier of wind turbines to Iberdrola Renovables. Earlier this year, Iberdrola Renovables announced it is purchasing an additional 200 GE 1.5 MW wind turbines capable of generating 300 MW of power, or enough to power 75 000 homes. Those turbines will be delivered in 2010.

Meanwhile, also in Spain, an engineer at the Public University of Navarre has developed two new methods to protect wind generators during voltage dips.

In his PhD, Jesús López Taberna describes a rotor model that anticipates how a wind power unit will behave in these situations. López has patented two protection techniques, one of which has already been transferred to a manufacturer who will exploit it at international level.

This system allows the generator turbine to remain in operation during these voltage dips and thus prevent the wind energy converter from ceasing to function.

The title of the PhD is: “The behaviour of wind-powered generators with double-fed asynchronous motor during voltage dips”.

Over the past few years, the growth and development of wind energy converters has been slowed by problems that have arisen from the increase in the number of these connected to the electric grid. One of the most important problems is precisely the manner in which the wind generators behave during these voltage dips.

A voltage dip is a sudden reduction in potential in the electric grid, followed by a rapid return to its normal value. This, at times, can be caused by lightening or a tree falling on power cables but also due to a large company consuming a lot of energy in one go.

This drop in voltage happens in a matter of milliseconds. “We are aware of it because the lights begin to flicker or because they go off and on momentarily – but, for a machine, this can be an eternity,” explained López. In fact, an interruption of half a second in a productive process can cause the whole process to block and it may have to be reinitiated.

With wind generators, in the case of a voltage dip, the electronic part of the unit can burn out or otherwise be destroyed, unless a protection system is installed

“The current system of protection, known as Crowbar, has the advantage of being able to protect the machine, but the disadvantage that the machine comes to a halt. For example, if a large company suddenly consumes a lot of current, the voltage drops. This causes the wind power units at El Perdon in Navarre to disconnect and cease producing electricity. As a result, the power dip is even more accentuated and, consequently, it is even more difficult to bring the voltage up to its normal operating value.”

Taking into account that, in Spain, there are days that wind-powered energy can account for one third of electricity production, the problem can prove to be a serious one. The idea being worked on currently is focused on the generator behaving more as a conventional power station and not disconnecting during a voltage dip/power failure but helping to bring the grid voltage back up. This is why wind generator manufacturers are currently working on finding a new system of protection that is efficient and efficacious.

Before looking for a solution, the problem had to be studied from a theoretical perspective: why does this machine behave as it does when there is a voltage dip? And why, if a protection system is not installed, does the machine start to burn out?

The research produced a rotor model “sufficiently simple to be able to deal with without having to carry out simulations. A model in which we can see what role each parameter of the machine plays, how they interact, how the current drops if we increase the leak inductances and so on”.

Once this model was developed, it was more or less easy to propose solutions. “The most important thing is that we have achieved solutions that enhance the behaviour of the machine without any need to change anything, except the control. It’s like changing the version of a text treatment programme on the computer, without needing to change the PC. There a number of computers inside a wind energy converter and one of these – that which controls the electrical machinery – is the one the control of which we have proposed to modify in order to enhance the behaviour of the machine,” he concluded.

López ‘s second system of protection is also patented and continues to be developed for applications in new generations of wind generators.

Dotcom bubble warning for wind farms

Concerns about the security of energy supply and climate change, rising demand for power as well as record energy commodity prices are all driving the development of renewable energy generation. While the total power output of these energies remains very small compared with traditional power generation, the growth of global wind energy generation has outpaced that of total global energy generation ten-fold over the past 15 years. With the majority of new capacity now outside Europe, wind power has become one of the broadest-based renewables technologies with installations in more than 70 countries.

Wind is often considered an unreliable generation technology, yet research from Datamonitor shows that while wind turbine load factors might vary from site to site and country to country, they need not be unpredictable. Turbine load factors are typically very consistent, albeit low, says Datamonitor energy and utilities senior analyst Alex Desbarres. “Over the past eight years, European wind turbines have returned consistent load factors of 18-20 per cent. This consistency and predictability have played a large role in underpinning recent record investment levels in wind assets.”

Despite recent escalating wind power capital costs, wind is still very competitive against coal and gas. When the cost of carbon is ignored, gas and coal are cheaper than wind. However, in markets where fossil fuels do carry a carbon penalty, wind power beats thermal power generation.

Therefore, as primary energy costs soar the attraction of wind power as a generation technology with no fuel price risk has never been greater, Desbarres says. “In the current context of soaring generation costs and until true demand-pull can be created, it is however worth remembering that the wind industry is increasingly driven by public policy trends and is at the mercy of government programmes that drive artificially stimulated demand.”

With demand for wind power generation at an all time high and record dependence on a burgeoning array of tariff and fiscal support initiatives, escalating valuation multiples in the wind sector are now driving dotcom comparisons. This raises the question as to whether current onshore and offshore wind market entry strategies are still profitable, as talk of a renewables bubble spreads across the industry, he says. 

"Whilst the speculative IT bubble was driven by investors focusing on multiples of forecast profits, wind farms differ in that they generate actual profits and more predictable long-term returns.” 

However, the recent decline in the share price of Iberdrola Renovables and EDF Energies Nouvelles - the renewables divisions of major European utility companies - might suggest enthusiasm for renewables is on the decline. This could echo market doubts as to whether these companies will be able to deliver on their ambitions to strategically grow renewable generating capacity, or growing concerns over recent record acquisition and development prices.

Applying conservative financing standards and typical turbine load factor characteristics and subsidy prices, Datamonitor’s cost and profitability model shows that onshore wind farm projects can be profitable and competitive, provided capital costs are kept below the US$1.75m/MW threshold, whereas offshore has become a high risk, high margin business. The model also shows that acquiring onshore wind can deliver more value for money than new build development, provided the cost is kept below US$2.35m/MW. However, the high premium paid for the acquisition of offshore wind farms often makes the overall investment less attractive than new build, according to Desbarres.

“Historically wind generation technology has been seen as a chair with solid feet, yet it is now on the tipping point of having to undergo some share price correction, particularly if national and state-based support policies disappear or fail to keep up with price increases, both of which are admittedly unlikely scenarios,” he concluded.

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