Pursuant to the supplement to the Medium Voltage Guideline in 2013, the certification directives for co-generation plants have been adopted and the obligation to furnish evidence of compliance with the BDEW guidelines of 2009 is now also binding for co-generation plants. As a result, certificates have to be furnished for CHPs connected to the medium voltage grid with reference to their behavior in the event of a failure.
This primarily means that the components are capable of withstanding power demand changes in the case of a voltage dip. The plant must be in the position to cope with torque variations due to the loss of grid voltage without suffering any stability loss. This is confirmed during the testing and certification process, and attested with a certificate for the individual power generation unit or generation plant.
"Unit certificates for each machine type or series are mainly issued upon completion of tests that are performed on their own test facilities," says Gunnar Kaestle, member of the board of the Bundesverband Kraft-Wärme-Kopplung e.V. (German CHP Association).
"For co-generation plants, ‘model families’ are often created, i.e. the certificate is valid for identical designs of various sizes."
A plant certificate is required for each co-generation plant with a power above 1 MW – even if this power is attained by several smaller units. “These certificates are necessary if several units are combined in one plant, for example, if a transformer is added,” explains Gunnar Kaestle. For this reason, the plant certification may also be relevant for manufacturers of smaller CHPs with a power of less than 1 MW, or for suppliers of components.
Step by step towards certification
Units and plants are first tested on the test facilities of the manufacturer. This presents the first challenge, as Kaestle explains:
"A test stand of the required size and capacity is needed, and not all grid operators allow for all of the necessary tests to be performed."
The reason is that the grid operator has to give his explicit approval for parallel operation when the units are tested under real conditions. It is in particular the short circuit test which can adversely affect neighboring grid connections. Once this hurdle has been cleared, the actual test process takes place. “A test institute is charged to perform the conformity test on site or at a suitable test facility”, says Kaestle. “The final step is certification which includes written documentation and verification as to whether the results fulfill the requirements. In Germany, this certification work is stipulated in the Technical Guidelines of the FGW e.V.
An additional supplement to the Medium Voltage Guideline will follow in 2016
Another amendment to the Medium Voltage Guideline is planned with an intended publication at the end of 2016. The former Medium Voltage Guideline was developed under the umbrella of the "Bundesverbandes der Energie- und Wasserwirtschaft e.V. (BDEW) (German association of energy and water industries)," explains Kaestle. "Currently, however, the Forum Netztechnik und Netzbetrieb (FNN) (Forum on Network Technology and Network Economy) within the VDE is working on the application guideline 4110, which is to replace the Medium Voltage Guideline of the BDEW", Kaestle continues.
"One of the most important changes will cover the expansion of the Fault-Ride-Through. In the event of a short circuit, it will not only apply to three-phase short circuits, but to two-phase short circuits as well. In the future, these will have to be tested and certified, too."