In a growing number of electricity markets around the world, the renewable energy industry and its regulators are coming head to head with the problems of success; namely, the effects on the stability of grids caused by the large-scale penetration of intermittent renewable energy capacity.
The simple fact is that the world’s electricity grids were not designed to handle renewable energy capacity — and, as penetration rates rise above 10-15%, grid operators from the U.S. to China are responding by mandating higher standards from renewable energy operators in terms of the quality of the power they supply into the grid.
But as operators and regulators move to raise standards and reinforce grid stability, cooperation with the renewable energy industry is vital — and it is worth examining the Spanish example, as a jurisdiction that faced early problems with high levels of renewables penetration, to see how such cooperation can benefit all sides.
A crucial issue for grid stability is what is known as “low-voltage ride through” (LVRT) — the ability of generating plant to cope with sudden short-term drops in grid voltage, as can be caused by short-circuits.
While most thermal generating capacity can easily respond to voltage drops (in simple terms by increasing the amount of reactive power they produce), many older wind turbines are designed instead to shut down, to prevent damage to their power electronics. In grids with a few percentage points of renewable energy penetration, this is not an issue. When levels rise into double figures, however, so do the problems.
This was an issue foreseen by the Spanish grid operator, TSO. In 2005, wind power capacity in Spain went above 10,000 MW, and accounted for 8.3% of demand coverage. By 2007, those figures had risen to 15,000 MW and 10%, respectively.
By that year, the TSO was finding that the grid was falling victim to capacity drops of up to 1,000 MW, as large proportions of its wind energy capacity shut down in response to voltage drops. Clearly, the grid operator faced a major problem.
However, TSO was ahead of the game. As early as 2004, it had begun working with universities, technological centres and wind turbine manufacturers to develop technical requirements to improve the LVRT capability of wind generators.
In 2006, Spain introduced its P.O 12.3 regulation. Among other things, this defined an envelope — in terms of time period and voltage drop — within which renewable energy generators must be able to remain connected to the grid, in the event of a voltage dip. The regulation also dictates levels of reactive power that must also be generated by the facility in the event of such a voltage drop.
It gave a deadline of 1 January 2010 for all facilities to be able to meet this standard, which they were required to do via a certification process. Newer turbine models from market leading producers are, in most cases, able to meet higher LVRT requirements. For example, ‘full convertor’ (FC) turbines are well able to meet the more stringent grid codes, as are those models using the doubly-fed induction generator (DFIG) concept.
Where there were greater problems were with existing capacity. Given the consolidation seen in the emerging wind turbine market over recent years, larger manufacturers are now supporting a range of legacy products, making the implementation of retrofit solutions difficult for many operators.
In its regulations, TSO gave manufacturers a pass, and said that, where there was no technology solution available, operators would not be required to meet tighter standards. However, a number of independent technology providers stepped up, developing stand-alone solutions to improve LVRT capacity of existing turbines.
For example, w2pS, a Spanish company bringing together experts from academia with engineers and renewable energy specialists, developed its Coverdip solution for the Spanish market. The product — which captured some 50% of the Spanish market – is technology agnostic and is installed at the base of the turbine tower, avoiding any intervention with the turbine’s power electronics (which often invalidates the manufacturer’s warranty).
Certainly, the regulation has had the desired effect. In 2007, the number of wind power losses of more than 100 MW peaked at 87, with almost 14,000 MW of wind capacity not certified to meet the standards set out in P.O 12.3. However, the number of incidents had fallen to 30 last year, with less than 5,000 MW of capacity remaining uncertified.
The Spanish experience with LVRT shows that, in cooperation with industry and academia, cost-effective solutions can be found to the problems posed by high levels of renewable energy generation. Understandably, the default position of a transmission grid operator is to ensure grid stability over environmental considerations – and this poses a threat to increased renewable energy penetration. However, the experience in Spain – and that with other regulators, such as in Germany – shows that grid stability can be maintained cost-effectively, ensuring that environmental and energy security objectives can also be met.
Jose Luis Rodrigues Amenedo is the founder of w2pS.