Protecting Generators From Overload

Pay close attention to how you size overload protection for generators. Now there's new technology that lets you better coordinate that protection with downstream protective devices.Many installers don't think about sizing generator protection nearly as much as they think about the protective arrangements for the conductors the generator supplies. In terms of electrical design, this is a classic example

Pay close attention to how you size overload protection for generators. Now there's new technology that lets you better coordinate that protection with downstream protective devices.

Many installers don't think about sizing generator protection nearly as much as they think about the protective arrangements for the conductors the generator supplies. In terms of electrical design, this is a classic example of ignoring the forest for the trees. Although we certainly don't want to damage the conductors from overloading, the Code also requires overload protection for generators. In addition, a large generator represents a major capital expenditure.

While the NEC requires overload protection of generators, it isn't as straightforward as using a main output molded case thermomagnetic circuit breaker or set of fuses based on the wire size. This is very important when either local design constraints or other standards require selective coordination with downstream protective devices. Selective coordination means that, even under the worst-case available fault, a fault in a smaller feeder originating below the level of the main generator protection will do two things. First, the overcurrent device next upstream will open and clear that fault safely. Second, all other overcurrent devices further upstream will stay closed, limiting the extent of the outage as much as possible.

The NEC requires this kind of selective coordination on most elevator feeders in Sec. 620-62. In addition, Sec. 4-5.1 in NFPA 110, Emergency and Standby Power Systems, requires designers to "optimize selective tripping of the circuit overcurrent protective devices when a short circuit occurs." The word "optimize" was chosen deliberately in recognition that full selectivity is very expensive, difficult, and in some cases impossible to achieve. However, although the word isn't ironclad, you still need to address the issue as well as you can.

If you're not careful, you'll have overload protection for the generator that is fully selective with downstream devices but doesn't really protect the generator. You might also end up with the reverse as well-great overload protection for the generator, but so sensitive that any major fault further down ends up clearing it as well. Here are some ideas on how to strike a balance.

First, how should you figure overload protection for the generator? The NEC doesn't provide an exact percentage in Sec. 445-4(a), merely requiring that it be present. Engineers usually specify generator protection between 100% and 125% of the rated generator current at rated kW and power factor (typically 0.8 lagging). Rated in amperes, overcurrent devicesneed to be able to handle the actual current. This means taking the current based on true power (kW) and dividing it by the power factor.

In some cases, the overcurrent protection goes at the end of the generator conductors. In this case, you need to be sure the conductor ampacity is at least 115% of the generator nameplate current, per Sec. 445-5. Although there isn't any distance restriction between a generator and the first overcurrent device [see Sec. 240-21(i)], locating the OCD closer to the generator is usually preferable to minimize the length of unprotected conductor. We commonly see specifications for a single set-mounted thermal-magnetic circuit breaker as an integral part of the generator set assembly. In this case, Sec. 445-5 Ex. 2 applies, and you can size the generator conductors by their ampacity without including the additional 15%.

But is that enough? Does that circuit breaker (or set of fuses) provide effective generator protection? We need to know the thermal overload capability of the generator. For this purpose, the best available tool is a generator damage curve. This is, in effect, the manufacturer's engineering judgment as to the level of overload current versus time that the generator can sustain without significant damage.

Fig. 1 (on page 46 of the original, print version of the article) shows a good example of the difficulties involved in allowing for both selective coordination and overload protection for the generator. The figure assumes the generator is rated at 500kW, 0.8 PF, 480 VAC, and 752A, and the overcurrent device is an inverse-time 800A circuit breaker. In this case, for example, 2100A for 10 sec would damage the generator but it wouldn't clear the 800A breaker, even if the breaker were set with the instantaneous trip adjustment all the way down to three times the rating. As shown, the breaker would need about 1 min to open under these conditions, well beyond the point of damage to the generator.

Furthermore, the drawing is actually based on very conservative assumptions. If the circuit breaker were sized based on 125% of the generator rated amperes, the generator protection would only get worse. In general, for a typical (non-electronic) thermal magnetic circuit breaker to provide effective generator overload protection, it may need to be sized as low as 50% of the generator FLA. No one would do that, of course, because you'd sacrifice much of the full generator capacity.

Modern technology is a big help at this point. Circuit breakers with electronic tripping devices include adjustable tripping characteristics that can be shaped to the generator damage curve and achieve protection for most levels of overcurrent. One possible problem with electronic breakers, however, is the possibility of nuisance tripping for slight overloads that persist for more than 15 min. Another problem with any circuit breaker that includes instantaneous tripping is selective coordination with downstream circuit breakers.

Another solution is inherent generator protection as permitted by Sec. 445-4(a). There are several means available for providing inherent generator protection. One is to provide a generator with a collapsing field excitation system. This type of excitation system provides overload protection for high power factor overloads, but not necessarily protection for low power factor overloads or unbalanced faults. Thus, inherent protection using collapsing field machines is usually limited to small single-phase generators. With generators capable of sustaining overloads, another type of inherent protection means uses current transformers on the output and a special circuit breaker in the CT secondary that will deenergize the generator field under an overload.

Recently, some manufacturers have introduced microprocessor-based generator controls that provide inherent generator protection and a solution to the dilemma of providing both generator overload protection and selective coordination. The microprocessor comes programmed with both the generator's thermal damage curve and an overcurrentprotection curve, as shown in Fig. 2. Instrument-grade CTs feed an accurate overload current signal to the control. A summing algorithm programmed in the microprocessor then compares the sensed RMS current to the stored protection curve.

If the current exceeds the protection curve, the generator control: 1) shuts down the engine fuel; 2) removes field excitation; and 3) operates a dry contact for shunt tripping external devices, if required. The microprocessor control also alarms an overload signal when current exceeds 110% of rated current for more than 60 sec as permitted by Sec. 445-4 (a) through (e) Ex.

This type of generator protection allows generators with sustaining short-circuit current capability to provide as much current as possible for as long as possible to allow downstream devices to clear the overload or short- circuit condition. Since the protection characteristic has an inherent delay of approximately 0.4 sec, it also allows selective coordination with instantaneous trip elements in downstream overcurrent devices.

This is yet another example of how modern technology and digital electronics help increase safety and reliability at the same time. Much of this just wouldn't have been possible in the recent past.

EC&M article: "Using Overcurrent Protection For Generator Conductors," December, 1996 issue. For copies, call (913)-967-1946. There is a fee of $10 for the first article ordered and $5 for every subsequent article. Standards: NFPA 110, Emergency and Standby Power Systems, National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. For ordering information, call 800-344-3555. IEEE Std. 446-1987, IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications (IEEE Orange Book). For ordering information, call 800-678-IEEE.

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