همانطور که بدرستی فرموده اند به لحاظ تئوری محدودیت جدی برای کار موازی واحدهای مختلف با ظرفیتهای متفاوت وجود ندارد ، با این حال آنجا که قابلیت اطمینان و در دسترس بودن اهمیت می یابد برخی نکات طراحی به شرح زیر( مرجع یکی از مقالات شرکت سازنده کومینز- سازنده دیزل ژنراتور) می باید مد نظر واقع شود. If generator sets are of dissimilar sizes, there can be impacts on the system design. An emergency system with generator sets that have matching kW ratings can support a higher first priority load than a system that has generator sets with dissimilar kW ratings. For example, an emergency system with two 500 kW generator sets will handle a first priority load as large as 500 kW. A system with one 750 kW generator set and one 250 kW generator set, while having the same total power rating, should be limited to a 250 kW first priority load, because if the first generator set closing to the bus is the 250 kW machine, any load greater than 250 kW could cause it to be overloaded. When faced with the use of dissimilar-sized generator sets, some designers will insert bus isolation ties between generator sets in order to prevent overloading on startup. This, however, sets up another series of issues because of the added complexity in the sequence of operation and usually the need for more synchronizers in the system. Manual operation and failure mode effects are also more difficult to deal with as tiebreakers are inserted into a system. Changes in sequence of operation may be necessary due to use of dissimiliar-sized machines. It is not advisable to design a system sequence of operation contingent on synchronizing generator sets within 10 seconds. Many manufacturers cannot provide equipment that is certain to be paralleled in 10 seconds, so in situations where local codes require service to emergency loads within 10 or 15 seconds, the system designer must choose between preventing smaller machines from closing to the bus first (minimizing the advantage of redundancy offered by many systems), or making sure that first priority loads can always be served by the smallest machine in the system. From a load shed perspective, the problem is less difficult, but still needs a bit of thought. If there is an under-frequency condition on the bus, the system will need to drop load. With dissimilar-sized machines, it is desirable to drop load in large enough steps to relieve the 250 kW set in the event that the 500 kW unit becomes the unit that is not available. Cascading type load shedding systems continue to drop load until the system recovers, so they automatically deal with the problem via that mechanism. A system design that
