As a manufacturer of both generator sets and transfer switches, the “Inside PQ” article in the March 2007 edition of EC&M titled “Ground Fault Current: Problems and Solutions” was of great interest and some concern to Cummins Power Generation. As a manufacturer of transfer switches with a simultaneously switched neutral pole (as shown in Fig. 3 on page 28 of the said article), we are interested in the “abnormal voltage” issue raised in the article.
With today's fast operating (approximating 6-cycle contact-to-contact transfer time) transfer switches and symmetrical mechanical switching of the load, transient overvoltages are of minimal magnitude, regardless of the load current balance, load voltage balance, or power factor.
In the event of the neutral pole switching in near perfect synchronism with the phase poles, abnormal voltage magnitudes are negligible. The duration of the switching action is short, relative to the period of the 60Hz waveform. The duration of any voltage unbalance is also inconsequential until complete separation and opening of the switch contacts. These voltage transients are not unusual on commercial power or during any routine switching action.
Transient voltages often randomly occur to commercial power equipment, which is protected by either inherent immunity or external devices. Sensitive electronic equipment is exposed to — and should be protected from — potentially damaging transient overvoltages. We do not see an “abnormal voltage” problem in the field with our customers' equipment having a simultaneously switch neutral pole. Today, in fact, all major transfer switch manufacturers offer a simultaneously switched neutral pole.
Another concern with the article involves safety. Figure 2 on page 28 shows that unbalanced current will return to the source on the equipment grounding conductor(s) (metallic boxes, conduits, etc.), which have multiple neutral-to-ground connections. This occurs under normal conditions and without a ground fault. The unbalanced current flow on this path is a safety concern that would be eliminated by removing the neutral-to-ground connection at the generator. The system depicted in Fig. 2 is called out in the caption as a separately derived system, but actually it is not. The generator neutral should not be grounded at the generator as shown.
The discussion of overlapping neutral contacts (shown as Solution 3) misses an important point. The GFP time delay is required in order for the GFP sensor to ignore the parallel path on the neutral during overlapping. The duration of the overlap may vary with the size of the transfer switch, with larger ampere switches having longer overlap duration.
The GFP time delay must be coordinated with and set for a longer duration than the duration of the overlapping contact. Because overlapping contacts require a GFP time delay, zone selective interlocking that can operate instantaneously would be preclusive. Zone selective interlocking may be desirable for the increased protection for ground faults and for coordination purposes.
Our last concern is in regard to the recommendation to retrofit an existing transfer switch with overlapping neutral contacts. This recommendation should be approached very carefully; as in general, field modification to UL-listed equipment may affect the listing. To quote from the UL Web site, “In addition, minor modifications performed in the field are permissible on equipment or products provided that the modifications are fully in accordance with the UL procedure. Such modifications shall be made by the manufacturer authorized in the UL procedure that covers the equipment or product.”
— Beth Kerr, Power Electronics, Cummins Power Generation, Minneapolis
Author's Response: Thank you for your detailed letter regarding our March 2007 article on ground-fault current. Let me address your comments, as you have listed them.
My discussion of abnormal voltage on switched neutral transfer switches during transfer is based on coverage of this specific item in IEEE 446 (The Orange Book). The standard states “unbalanced loads may cause abnormal voltages for as long as 10 to 15 milliseconds when the neutral conductor is momentarily opened during transfer of the load.”
The article (and the standard) warns of the possibility of this problem. You are correct in that the fast transfer times of today's transfer switches have substantially reduced the magnitude of this problem. Nevertheless, it could remain a potential problem in installations with older transfer equipment.
Regarding the article's Fig. 2, you are correct in that the neutral of the generator should not be grounded. The point of showing it this way is to point out how multiple neutral-ground connections can cause unbalanced neutrals from unbalanced loads to flow through ground-fault protection, causing nuisance tripping. You are also correct in that the generator, as shown here, is not a separately derived system because the generator neutral is solidly interconnected to the service equipment neutral.
Regarding the article's citing of overlapping neutrals as Solution 3, your comment regarding coordination of ground-fault protection with the duration of overlap is correct. Also, your comments on the factors affecting this coordination are right on. The article touched upon this, but did not provide enough detailed information on those various factors.
Finally, your comment on the article's point of adding overlapping neutrals as a field modification to existing transfer equipment is correct in that such modification must be made according to UL procedure. The point of this discussion is to inform the reader of the possibility of this modification as an option, if space allows in the existing equipment. Nevertheless, the article should have listed the caveat of the possible negative effect of the modification on the UL listing.
— John DeDad, senior director, editorial and EC&M development