Critical data for sizing gen-sets

The complexities of modern loads, such as PWM power supplies, variable frequency drives, and other electronic equipment, place a whole new light on sizing generator sets.Engine-generator sets supply power to a very wide range of facilities - from police stations, fire houses, radio and TV stations to apartment buildings, hotels, and hospitals. In such applications, gen-sets function in a wide variety

The complexities of modern loads, such as PWM power supplies, variable frequency drives, and other electronic equipment, place a whole new light on sizing generator sets.

Engine-generator sets supply power to a very wide range of facilities - from police stations, fire houses, radio and TV stations to apartment buildings, hotels, and hospitals. In such applications, gen-sets function in a wide variety of ways - from providing standby and emergency power to "backing-up" the local utility during peak power demand periods. As utility deregulation spreads, the demand for engine-generator sets in various applications will inevitably grow.

Many loads demand a better quality of supplied power, while at the same time, some loads are corrupting power with harmonics, surges, voltage dips, and other power quality problems. As a result, engine generator selection and sizing must be done with greater care than ever before.

To assist in proper selection, many gen-set manufacturers provide software sizing tools; however, this software may not tell you everything you need to make the right choice. Most software simplifies the sizing process by collecting commonly needed data and minimizing calculations that you need to pick the right generator set size for the job at hand. By using software, for the repetitious and difficult computations, and good judgment (along with the information in this article), you can make better sizing decisions for on-site power system equipment.

Here are some important facts and hints to help you select the best gen-set size for the job.

A generator set doesn't perform like a utility service

Engine-generator sets used in auxiliary power systems, even those supplying large on-site systems, are limited power sources compared with a utility service. As a result, these generators will not provide power with the same characteristics as the utility. In general, you'll find the following performance characteristics.

Voltage output. Engine-generator set voltage output will be constant as long as the system load is constant. However, as loads are turned ON and OFF, the voltage momentarily will rise and fall as the generator corrects its output voltage for the appropriate load level. This is very similar to the situation when you start your electric clothes dryer and notice the lights dim for a second. The lights dim because of the drop in voltage as extra load is being added to your home's electric circuits. The momentary voltage drop when load is added will be greater on a generator set system than when the same load is started from a utility service.

Non-linear loads. These type of loads distort the voltage waveform in a building powered by a utility service, and will have even greater impact on the output of a generator set. Non-linear loads include computers, UPS equipment, variable-speed drives, electronic ballasts, solid-state processing equipment, and similar solid-state devices or systems.

Frequency. Power frequency is not as constant as utility power and will change significantly with load changes. The frequency of utility-supplied power on the other hand, will be constant under almost all conditions. This is the most significant difference between engine-generator set power and utility power, and it means that frequency-sensitive loads like UPS equipment are likely to initiate alarms if variations occur when operating on generator set power. In addition, some loads may not function properly due to quickly changing frequency of gen-set power, especially on load changes.

All loads are not alike in their power quality needs

A simple incandescent light bulb is not a very demanding device in terms of power quality. The amount of light produced will drop as the voltage drops, but the lamp doesn't care what the frequency is, or how "clean" (free of harmonic distortion) the voltage waveform feeding it is.

Most other loads are pickier than that. Across-the-line starting motors demand that the voltage not drop lower than a minimum value (typically 65%). In addition, voltage must be maintained at a high enough level to provide enough torque to accelerate the load on the motor. Electronic motor drives often include internal protective devices that switch off the drive when voltage drops below 90% of nominal to protect the motor served.

UPS equipment frequently has varying power quality requirements depending on its operating mode. When the rectifier is ramping up, relatively broad frequency and voltage swings can occur without disrupting equipment operation. However, when the bypass is enabled, both frequency and voltage must be constant, or an alarm condition will occur.

Most loads are relatively tolerant of voltage waveform distortion, in the sense that they will continue to operate with a total harmonic distortion (THD) in excess of 15% to 20%. However, with voltage distortion of that magnitude, problems can develop in sensitive electronic equipment. Also, serious attention must be given to the load-carrying conductors, panelboards, etc., to be sure they are operating within their design ratings.

Most loads vary in magnitude with respect to time

About the only loads you can find that will be constant from the time you apply them to the generator set to the time you shut them OFF are incandescent lighting and resistive load banks. All other loads, whether they're motors, motor drives, rectifiers, or lighting equipment, etc., apply a varying load to the generator set as they are being energized. [ILLUSTRATION FOR FIGURE 1 OMITTED].

There will be considerable differences in the load applied as the manufacturer or type of load changes. For example, a typical 50-hp NEMA Code G motor started across-the-line will demand almost 300kVA on initial energization. If a 50-hp high-efficiency motor is applied, you can expect a much higher starting demand on the generator set. Thus, a generator set will successfully operate loads for many years; then a change to a different type of load with the same capacity can result in a generator set that is too small to do its job.

As such, when sizing a generator set for an application, you need to look ar not only the steady-state load, but also the maximum transient load applied. So, for each load, you need to know the maximum kVA, maximum kW, and running kW and kVA. Also, you need an appreciation for the time required for the load to stabilize at its steady-state value.

In some situations, you can specify equipment with load characteristics to minimize the transient load on a generator set, thus improving performance. For example, in many UPS control systems, you can control the rectifier ramp rate and the maximum load drawn by the UPS. By maximizing the ramp time and minimizing the maximum load applied, the generator set frequency will be more constant as the UPS ramps on to the generator set.

Another example of this is in elevator applications. If the elevator system is powered by an electronic drive, the maximum load applied to the generator set is a function of the rate of acceleration and deceleration in the elevator. By changing the performance requirements of the elevator, you can minimize the impact of elevator loads on the generator set.

The load application sequence makes a big difference

Because most loads don't draw power from their supply source in a steady-state fashion, the sequence of when they are called to operate, and what else is running when they are called, is very important in generator set selection. Furthermore, you may have only a limited amount of flexibility when setting load sequence, because codes and standards require specific loads to be fed first in many situations. For example, in a legally required emergency system, the designated emergency loads (such as egress lighting or smoke ventilation equipment) must be fed first before optional loads are fed.

When you do have flexibility in determining the sequence of operation of loads, these "rules of thumb" can be helpful if the smallest, least expensive but most effective on-site power system is to be designed.

* When starting motors with "across-the-line" motor starting provisions, always start the largest motor first; smaller motors can start sequentially after that.

* When starting motors that use electronic drives, the "largest motor first" rule may not apply. The use of electronic drives for starting and running motors allows you to better control the actual load applied to the generator set by controlling the maximum current load, rate of load application, etc. The thing to remember about these loads is they are more sensitive to voltage variation than motors that are started "across-the-line."

* When applying UPS equipment on a generator set, put the UPS system on last. The generator set is most stable and least affected by the non-linear characteristics of a UPS when other loads are running on the system before the UPS load is applied.

* Loads that include power factor correction or filters for power quality improvement should not be applied to a generator set operating at light load levels. The capacitive elements of these loads can cause the voltage of a generator set to rise uncontrollably at light load levels.

In some applications, the generator set must absorb power, as well as produce it

Sometimes, the limiting factor in the size of the gen-set is not the amount of power it can produce, but rather the amount of energy it can absorb from the load. Loads like some elevators and conveyors sometimes depend on their power supply to absorb power during some operation modes, such as when the load is decelerating. The term "regeneration" is sometimes used to describe the power produced by these loads.

If the power generated by the load exceeds the capacity of the generator set, the generator set can fail due to an overspeed condition. Applications that are most susceptible to this type of problem are small buildings where an elevator is the major load on a generator set.

Generally, the regeneration problem can be resolved simply by making sure there are other adequate loads in the system that are large enough to absorb the power generated. So, for example, in the building with the elevator, you would put all the facility lighting loads on the generator set, and then add the elevator load.

Occasionally, an auxiliary load bank and load-bank controls are needed to be sure the generator set is not affected by regeneration from load devices.

You can pick a generator set that is too big for the application

Periodically, someone believes that if the facility needs a 500kW gen-set, the system will be twice as good and twice as reliable with a 1000kW gen-set. That not only results in major wasted cost for the initial purchase of the generator set, but also higher operating cost, inadequate facility space, wrong conductor sizing, etc. To make matters worse, the oversized generator set may malfunction due to inefficient fuel combustion in the engine. Operating at light load levels is of concern in diesel engines because operating at less than 30% of standby rating can result in plugged injectors. This, in turn, causes poor transient performance and may even affect the system's ability to carry loads when a real emergency occurs.

While it may sound easy, determining the maximum load that might be applied to a generator set takes a good deal of judgment and experience. Sometimes, the generator set must be oversized to deal with expected increases in loads in the future. Other times, there are a large number of motor loads, or other loads that demand oversizing of the generator set. When a generator set is to be operating at a light load, it's helpful to at least design the facility so that the generator set can be conveniently tested under heavier loads.

Load testing can be done simply by connecting portable load banks to the system. If there aren't provisions for testing the generator under a high load in the initial facility design, temporary connections to the system can be installed.

Some loads make special demands on generator sets

If the only loads in a facility are emergency lighting and a few across-the-line-starting motors, the generator set will not need any special controls to power the loads successfully and reliably. However, in modem buildings, it's rare to find a situation where you don't have electronic motor drives, UPS equipment, and other electronic and solid-state devices.

Many of these loads put special demands on generator sets, particularly with regard to frequency control. Remember, every piece of electrical equipment in a building probably has been designed to work on utility power, so generator sets that don't work much like a utility may cause problems. Look carefully at the frequency requirements of the loads being served. If they require frequency to be constant at 60 (or 50) Hz, you'll need to provide an electronic isochronous governor for the generator set. For example, most UPS systems will operate over a broad range of frequency conditions to charge the UPS batteries, but will require electronic governing on the generator set to enable synchronizing output to the gen-set input power.

If the facility has a high percentage of non-linear loads, it's desirable to use 3-phase sensing voltage-regulation equipment and a generator set with an independent power supply for the generator set excitation system. Common examples are permanent magnet generator (PMG) and series boost type excitation systems. These make the generator set excitation system less sensitive to voltage waveform distortion. Ask for these features so the generator set can provide a constant voltage, even if voltage waveform quality is poor due to high amounts of non-linear loads.

Utility support applications can put increased demands on generator sets

Most engine generator sets sold in North America are used for emergency/standby applications. However, with changes in utility rate structures and reductions in the availability of rolling electrical power reserves, it's becoming increasingly common to use generator sets for duties other than their primary emergency/standby service.

Most generator sets installed in North America are rated based on their use in standby applications. The generator standby rating means it is designed to carry its nameplate rating for the duration of a utility outage, which is typically limited to 2 hrs out of a 12-hr period. The generator set supplier assumes that because the generator set is not paralleled to a utility source, the actual load on the generator set will vary somewhat over the time that it's running. Sustained operation at the standby rating level shortens the life of the engine and may even cause a gen-set malfunction.

If a gen-set is expected to be used in an utility plan (such as in an on-site interruptible application), then it's important to size the generator set based on its prime rating, rather than standby. Generally, the prime rating of a generator set is approximately 10% less than its standby rating. Operating at the prime rating allows the engine life to therefore be significantly improved.

If the gen-set is expected to operate for extended periods of time in parallel with a utility service, its operating load may need to be further reduced to optimize engine life. Different engine manufacturers have different recommendations for generator set ratings. Consult your supplier for the best product for a specific application. Keep in mind that this doesn't have much to do with whether or not all the loads will start and run in an emergency situation. The object here is simply to operate the generator set at a load level that will minimize the long-term operational cost of the engine.

All generator sets of the same size are not alike

It's difficult to compare equipment from various suppliers and then make a quality decision regarding the best equipment to purchase. Many things might be important in that decision-making process, including past experience, service availability, space requirements, etc. From a technical perspective, there are a couple of performance characteristics that can be reviewed as manufacturers are compared.

Alternator subtransient reactance. This parameter describes the instantaneous response of an alternator to a large overload. It's important in defining the performance of the generator set when motors are started, or in minimizing the effects of non-linear loads. In general, a lower index value is better when comparing subtransient reactance values. A good objective is to get a generator set with subtransient reactance of 0.12 per unit, or lower, based on the steady-state rating of the unit.

Alternator temperature rise. As temperature rise increases, the life of an alternator decreases, so it makes sense to pick an alternator that operates at well within its rated insulation class. For example, you may specify an alternator with Class H insulation, which is suitable for operation at a temperature rise of up to 155 [degrees] C, but then only operate the machine at a load level that results in a 105 [degrees] C temperature rise. If you want a lower-temperature-rise alternator, you usually can get it without increasing engine size, so the impact on the total cost of the project is minimal.

Alternator voltage waveform quality. Many loads in a modem building setting are non-linear in their characteristic, and these loads will distort the engine gen-set voltage waveform. These distortions are a fact of life, and the voltage waveform distortion typically will be greater than when the loads are operating on utility power. Therefore, it makes sense to minimize the magnitude of voltage waveform that's inherent in the alternator. The distortion level of alternators should be compared at full load. A typical specification is that the alternator should produce a voltage waveform with not more than 5% total harmonic distortion (measured line-to-neutral), with a maximum of 3% distortion in any one harmonic order. Harmonic distortion values generally increase with increasing load, so they should be compared based on alternator operating at full load.

Generator set transient response. Engine generator sets, by nature of the design of the engine, governing, and voltage regulation control systems, all perform a little bit differently when loads are applied. The issue isn't always getting the fastest response possible. Some loads, like some UPS equipment, actually "like" slower responding generator sets. You should always compare the transient response characteristics of the equipment proposed for use at a site, and be certain that it will be suitable for the loads applied. Finally, the system should be designed so momentary overloads don't cause a generator set shutdown.


Auxiliary power system. An auxiliary power system is composed of a generator set and the equipment and controls necessary to transfer loads from the primary source of power in the facility to the generator set.

Ramprate. A controlled change in speed or voltage. A gradual increase or decrease in power of a generator set.

Regeneration. The creation of power by a load device as a result of a normal operation mode.

Steady-state. Steady-state describes operation of a device, like a generator set, with a non-varying load so that voltage and frequency are as constant as possible.

Total harmonic distortion (THD) (voltage). The total harmonic distortion (voltage) is a term that allows a comparison of the amount of variation in a sinusoidal voltage waveform due to the effects of harmonic currents drawn by load devices or produced by a power source.

Transient response. The transient response of a generator set is a description of the maximum voltage and frequency change on application of a load and the time to recover to nominal voltage and frequency conditions.

Subtransient reactance. The subtransient reactance of an alternator is a factor that describes the maximum current available from the alternator for very short time periods. The value provided is generally a per-unit (p.u.) value based on the electrical output of the generator set. For example, if a generator set has a rated output of 1000A and a subtransient reactance of 0.10 p.u., the symmetrical short-circuit current available from the alternator is 1000, 0.10, or 10,000A.

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