While on-grid inverters in Germany, Europe's largest PV market, nearly all come equipped with one anti-islanding method
– the so-called ENS – this technology has not found acceptance in all European countries for several reasons. With an upgraded device, Klaus Köln, the inventor of the ENS method, hopes to open the door to new markets. He presented his new ENS to the PV industry at the Staffelstein
Teamwork: In order to decrease mutual interference, in the future, several ENS devices will co-produce their test signals. Moreover, developer Klaus Köln - shown here at his booth at the Staffelstein conference - hopes that by adapting the switching thresholds to the individual grid conditions, he can increase acceptance of inverters operating under the ENS anti-islanding method outside Germany.
Klaus Köln has reacted. His ENS, an
electro-mechanical anti-islanding device using two switches connected
in a series, has not caught on in Europe. Although it satisfies the
German Association of Electrical, Electronic, and Information
Technology's (VDE) authorized draft of a standard, which prescribes
the disconnection of PV systems from the grid in Germany in the case
of a 0.5 ohm impedance jump, outside of the country other regulations
apply. One reason is the presence of different grid conditions.
Another is that many non-German inverter manufacturers and scientists
working on national standard committees consider ENS a »unilateral German
Klaus Köln, owner of the company UfE GmbH in Rostock, has faced the technical barrier and has developed an ENS
»suited for Europe« that automatically adjusts its switching thresholds to meet the individual grid conditions. This is made possible by a permanent self-testing of the impedance measurement: if the switching threshold is set at too sensitive a level, the device gradually increases the setting automatically until it eliminates the faulty disconnection. If the switching threshold is set at a level considerably higher than the grid disturbances, the device slowly decreases the setting to allow for higher sensitivity.
Köln wants to realize an adjustable range for the impedance of between 0.1 and 1 ohm
– a window that is close to what is required in Austria, for
instance. »This will convince more people to accept the device, says Köln. Since this new method differs from the one stipulated in the current German draft standard VDE 0126, he must acquire permission from the Accident Prevention and Insurance Association of employers in the private sector before the product's release in Germany, which is planned for May.
Another problem with his old ENS (the ENS25 as single-phase and the ENS30 as three-phase device) is that it blocks the inverter under less than ideal impedance circumstances
– for instance at the end of long grid branches. That means that when the impedance threshold is exceeded under volatile grid conditions, the PV system disconnects from the grid, although actually no uncontrolled islanding effect is taking place. With his new process, Köln can differentiate between an overreaction on the part of the ENS and an uncontrolled islanding effect, since the grid voltage still exists after the ENS accidentally shuts off. The shutoff threshold is thus increased to a value somewhat above the one at which the system had triggered a disconnection. If, on the other hand, the impedance fluctuations are significantly below the present switching threshold, the threshold is adapted to a lower value above that level.
Teamwork means more stability
The limitation of the test signal's strength is an additional optimization of the new ENS (the ENS26 as single-phase and the ENS31 as triple-phase device). So far, impedance measurements have negatively influenced each other and the grid if the density of PV systems with ENS is too high. By limiting signal strength, the new device overcomes this problem. The concept, called Teamwork, has the advantage that several ENS devices can co-produce a test signal. This reduces the grid disturbances, and the signal is more stable when measured at the point of grid connection.
At the Staffelstein PV conference in March, Klaus Köln presented the new ENS26 to the public for the first time (the device shown at the European PV Conference at the end of 2001 in Munich was just a prototype). And the ENS26 is already being employed by German inverter manufacturer Sun Power Solartechnik GmbH, which is based near Frankfurt am Main. Sun Power's manager Georg Scharl is satisfied with the
device. »The new ENS works great,"« he says. Köln now wants to test the new devices at problematic grid points in order to demonstrate the product's advantages and to further optimize its performance. By the end of the year, he intends to equip all his customer's inverters with the new ENS
– among them inverters from Fronius International GmbH, Sunways AG, Sun Power, Aixcon Elektrotechnik GmbH, Sputnik Engineering AG, and the Dutch companies Philips and Mastervolt BV.
Klaus Köln isn't the only one pushing for the Europe-wide introduction of an ENS. Market leader SMA, which uses its own ENS with their inverters, also has set its sights outside of Germany. Since June 2002, the company has offered the network monitoring system called Grid Guard, through which the inverter can be adapted to the grid conditions in any country (see PI 10/2002, p. 34). And this coming June, Fürth-based Siemens AG plans to follow SMA's lead with a similar system, that later will be included in all of the company's inverters for low-power