In a January 25, 2018, T&D World article, National Grid describes its aggressive program to facilitate expansion of Distributed Generation (DG). This program, as the article depicts, has been effective in reducing the cost of DG integration. One of its elements is the introduction of recloser blocking as an anti-islanding (AI) alternative to more costly direct transfer trip (DTT). While this has the advantage of preventing out-of-phase reclosing, it is not applicable to all situations because it relies on each DG’s local AI logic to detect an island and trip. Sandia National Laboratories has identified a number of situations in which this approach cannot be counted on:
- “Cases with very large numbers of inverters. The literature indicates that the speed with which inverters detect an island decreases as the number of inverters in the island increases, and that the amount by which the effectiveness degrades depends on both the specific anti-islanding method used and on the configuration of the potential island.”
- “Cases with inverters from several different manufacturers. Some studies have found that mixing different types of LOMD (Loss of Mains Detection), or even mixing inverters with the same type of LOMD but different implementations, leads to a degradation of islanding detection effectiveness in the multi-inverter case. This situation could represent a case in which a multi-inverter installation uses units from several different manufacturers. “
- “Cases including both inverters and rotating generators”
With the rapid growth of DG penetration, such complex DG combinations will become increasingly common.
In these situations, DTT has been the only available solution in the past. But today there is a proven alternative that is more reliable and less costly. That alternative is Distributed Generation Permissive (DGP) and it has been deployed in dozens of applications ranging from 12 kV to 34 kV. It is a much simpler approach than DTT, because it monitors the continuity of the DG-grid connection by continuously transmitting a digital message over the same path. Loss of this message means a break in the path (i.e. island formation) and the need to trip. A signal loss can be due to any break in the line, not just an open recloser. A circuit equipped with DGP can be considered DG Ready because new DG connections can simply add a DGP terminal and be ready to connect with little time lost.
As National Grid and other utilities continue to refine their approach to DER integration, the application of DGP may represent a preferred approach to Recloser Blocking and DTT.