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 “knows” the kW capacity of the generator sets available on line can automatically compensate for the difference in sizes by using different sequencing when different combinations of equipment are available.
How much difference in size is “practical”? That is a matter of considerable discussion in the industry. We know that it’s technically possible to parallel anything with anything, but where it’s practical is less clear. In general, you probably will have a manageable system when the smallest generator set is no less than 30% of the capacity of the largest generator set in the system. If there is a larger disparity in equipment sizes, it mightbe easier to make the larger machine bigger, and forget about paralleling altogether. It’s useful to note that there are often large kW size differences in prime power applications, such as are used in remote, isolated cities and towns. In those applications, loads during the daylight hours are often much higher than in the evenings. A pair of 350 kW generator sets may be running during the day, and only a 50 kW set running at night. This is less of an issue in prime power applications because the load profile does not quickly change, and the systems generally have no automatic load management systems.
As a technical matter, the smallest power circuit breaker that we can provide is an 800 amp frame. So it can be costly to parallel smaller generator sets. In those cases it’s possible to source 5-cycle operating molded case breakers for use in paralleling the generator.
These breakers are smaller, available in fixed frame configurations so less space is needed for the equipment, and less expensive, but they are available only from a limited number of suppliers. In some cases motor starting contactors might be used for paralleling applications where smaller generator sets are used, but the contactors must generally be protected by current limiting fuses for a safe design.
در شرایط غیر مشابه بودن ژنراتورها ملاحظات مهمتری نسبت به ریتینگ آنها به شرح زیر موجود است که باید مد نظر قرار داده شوند.
When paralleling dissimilar generator sets, a designer should:
- Verify that the existing generator set can carry full load with good voltage and frequency regulation.
- Review the transient performance of all the generator sets in the system and verify that the load steps (particularly load rejection) of all the gensets will not result in objectionable levels of reverse power.
- Verify that the pitch of all alternators is the same. Specify 2/3 pitch on all generator sets to avoid problems with this. Take mitigating steps if dissimilar alternators cannot be avoided.
- For emergency/standby applications, load sharing controls (both kW and kVAR) should all be of the same manufacturer and model. Droop control may be suitable for some prime power applications and isolated bus kVAR load sharing.
- Attention should be given to the specific control interfaces to system master controls and facility management systems monitoring the generator bus. In general, these can be made to work in nearly every case, but it cannot be assumed that they will work without checking and verifying that the proper equipment is in place.