Jump to section:
- Understanding Power Factor
- Generator Set Pitch Pointers
- Understanding Generator Set Transient Reactance
- SCRs and How They Affect Gen Set Load Profile
- Generator Sets and Uninterruptible Power Systems
Understanding Power Factor
Question: “If an AC generator is rated 480 volts and 900 amperes at 0.8 power factor, why can’t the generator produce 480 volts and 900 amperes at 1.0 power factor?”
Answer: “It can, or can not, depending on how the generator set has been set up and rated. “How’s that for an answer!
In reality, the generator is capable of producing 480 volts at 9000 amperes, but the engine side of the unit is not sized to be able to deliver the horsepower (kW) to permit the generator to carry the load at the rated speed. Here’s why:
Physics at work
Today’s generators can produce electricity at 93.5 percent efficiency; the rest is lost in windage, bearing friction and heat losses. Further, 1 hp is equal to 0.746 kW of power which is equal to kVA times the power factor. These two considerations give us enough ammunition to figure horse power needed to produce a given kW.
For example a CAT 3412 gen set is rated at 600kW at 1800 rpm. The following engine horsepower is needed to deliver that power from a 93.5 percent efficient generator.
This equation indicates that an 880 hp engine must drive the generator. (This gives no consideration to overload capability.)
The 3412 gen set engine is factory set to provide 894 hp. So based on the formula presented, the CAT 3412 engine meets the horsepower needs to produce slightly over 600 kW. The generator nameplate would read:
kVA 750 Amperes 1,804-902 rpm 1,800
kW 600 PF 0.8 Phase 3
Volts 240-480 Cycles 60
The kVA is equal to the rated voltage and amperage multiplied by 1,.732 divided by 1,000.
Therefore, the kVA for the above generator is:
Because kW equals kVA times the power factory (0.8 lagging power factor is the NEMA standard) so the true output of this generator is 750kVA times 0.8 which equals 600kW.
Leading or lagging power factor?
Power factor can be leading or lagging, or in some cases, at unity.
A leading power factory can be caused by capacitor-intense load, a lightly loaded synchronous motor or an induction motor this is being driven by its load. Lagging power factor is caused mainly by induction motors.
Unity power factor can be found in loads dominated by electronic devices or resistance loads such as lights and heaters.
Average industrial loads include many motors, so the recognized standard is 0.8 lagging power factor. Leading power factor is practically unattainable with today’s loads.
Cross referencing the graph and the table shows how power factor affects generator and engine loading. In essence, it shows that:
1. At any PF in excess of rated (greater tan 0.8), the gen set output is limited by engine horsepower.
2. At any PF less than rated (0.8) output is limited by generator amperage.
Knowing the exact site requirements assures proper equipment selection. A complete audit of the load profile will identify the load power factor, helping you and the gen set supplier to size the best unit for the application.
Electric Power SpecSizer, an electronic gen set sizing program available through your CAT dealer can greatly simplify this process.
|Engine HP||KW Output||Amps||Graph Line||Comment|
|1.0||1095||750||902||O-A||Rated amperes, 215 horsepower overload|
|1.0||880||600||722||O-B||Less than rated amperes. Rated horsepower.|
|.9||988||675||902||O-C||Rated amperes. 108 horsepower overload.|
|.9||880||600||802||O-D||Less than rated amperes. Rated horsepower.|
|.8||880||600||902||O-E||Rated amperes. Rated horsepower.|
|.7||880||600||1031||O-F||Ampere overload. Rated horsepower.|
|.7||753||525||902||O-G||Rated amperes. Less than rated horsepower.|
Generator Set Pitch Pointers
As non-linear loads make up and increasing portion of total electrical load profile, more thought is being given to dealing with the phenomenon of their harmonics and its effect on gen sets, loads, cabling buses, protective relays and circuit breakers. The problem becomes even more complex if the gen set is paralleled with the utility when harmonics on the utility line may cause some hard-to-trace problems.
Harmonics are multiples of the sine waveform produced by the generator. For example, 60 Hz is the fundamental waveform, then 180 Hz (60 Hz x 3) is third harmonics, 300 Hz is fifth harmonics, 420 Hz is seventh harmonics, etc. Only odd-numbered harmonics are important in this discussion.
All harmonics affect current waveform. If the waveform change is great enough (most notably in the third harmonic), loads that use the 60 Hz waveform to trigger switching are affected. It can also deceive the gen set voltage regulator so it continually “searches” for the correct excitation level to meet the needed voltage. It also can create excessive heat in transformers, UPS and computers as well as throw off instrument readings.
Although 2/3-pitch generators produce little third harmonics current, they do produce much higher fifth and seventh harmonics when compared with 4/5- and 5/6-pitch generators. This increases heating in motors which can shorten life.
The best way to deal with harmonics concern is at specification. Present and future load profiles offer some insight into non-linear loads. If it is harmonic-rich, specifications should compensate for it.
If inductive loads make up the majority of the load, 4/5- or 5/6-pitch generators can be used with correct sizing. These generators also result in phase-to-neutral faults much lower than 2/3-pitch unit.
Many questions must be answered about an installation before it can be decided whether a 2/3-pitch or 4/5-pitch generator is best. We stand ready to recommend the best course of action.
There has been much written and even more speculated about the pros and cons of 2/3-pitch generators vs. 4/5- (and 5/6- ) pitch machines. Because the effect of third harmonics on electrical systems is installation-specific, few hard and fast rules apply. But, in general, the following points are consistent across all generator and electrical systems:
- Third harmonics current is generated almost totally by connected load – computer systems, UPS, variable-speed and fluorescent lighting. Only a negligible amount is produced by the generator, no matter what its winding pitch.
- Third harmonic currents in identical paralleled gen sets are no problem if gen sets are carrying equal load. However, it may be a problem if two generators of different pitches are paralleled.
- While 2/3-pitch generators have very little third harmonic current compared to other pitches, the fifth and seventh harmonics are nearly maximum at 2/3 pitch. Further, if a phase-to-neutral fault (the cause of 65 percent of all faults) occurs on a 2/3-pitch machine, there will be higher fault currents, with the potential of more system damage and the need for higher interrupting capability circuit breakers – adding cost to the installation.
Understanding Generator Set Transient Reactance
Listed reactance per unit values (transient, subtransient, synchronous, negative sequence and zero sequence) are used extensively for comparison in gen set specification, but are also a source of confusion.
Identifying a generator’s transient reactance helps the specifier approximate voltage dip when large motors are started. It also helps the specifier approximate the current in a three-phase short-circuit condition to specify correct circuit breaker protection.
Transient reactance is usually expressed by the symbol . Reactance figures are always used with a related kVA rating (base kVA), the ampere rating (base amperes), and the related voltage, (base voltage).
You’ll find reactance stated as a per unit (P.U. or p.u.) value, and can be expressed as a percentage of some whole value. Because it is a pure number, it has no label (e.g. volt, ampere or Ohms) until it is applied against the line-to-neutral value at the kVA and ampere or volt condition identified by the generator’s reference number and/or rating.
The various ratings a gen set may have do not change its inherent reactance.
It’s important to always convert to line-to-neutral voltage for correct reactance comparisons. To convert line-to-line voltage to line-to-neutral voltage, divide the line-to-line voltage by square root 3. This is your base voltage.
To convert, use Ohm’s Law: Divide the line-to-neutral rated voltage by the rated line amperes, then multiply by the P.U. value to get Ohms reactance.
For example: If transient reactance is 0.2490 per unit, base voltage is 480 and base amperage is 263:
277 / 263 x 0.2490 = 0.262 Ohms
How generator sizing affects reactance
Most engineers prefer data presented in the following format: line-to-line voltage, prime power kVA, line amperes (at the listed line-to-line voltage), and the line-to-neutral reactances in the per-unit value.
The various ratings a gen set may have do not change its inherent reactance. However, the per-unit value reactances do change directly with the generator kW rating.
Per-unit reactance changes inversely (voltage down, reactance up) with the square of the voltage ratio if the kVA rating stays the same. For example: If at 480 volts, the listed transient reactance is 0.2490, and the base voltage will be reduced to 416 volts, the per-unit transient reactance at the lower voltage is:
(480 / 416) 2 x 0.2490 = 0.3310 P.U.
Other reactance terms
Caterpillar regularly quotes direct axis reactance per unit values. Occasionally, quad axis figures are requested and can be supplies.
Reactances are generally quoted at magnetic saturation. However, in some cases, specifiers request and unsaturated value of reactance, which can be supplied by special request.
When considering reactances in your specification process, call us. We are here to help you obtain the proper specifications so you or your client can make the best gen set selection decision.
SCRs and How They Affect Gen Set Load Profile
The load profile found in today’s hospitals, offices and public buildings requires very high-quality power. However, non-linear loads caused by variable-speed drives or uninterruptible power supplies (UPS) serving data processing systems can greatly affect line current and voltage. While utility power can often accommodate these changes, standby generating systems can be greatly affected if they are undersized or designed incorrectly.
The cause of many non-linear load problems result from silicon-controlled rectifiers (SCR) used in systems to convert AC to DC Power. These devices inherently affect the system’s sine wave form. If the distortion is great enough, the gen set cannot adequately hold voltage and current output. The gen set engine speed will vary, attempting to seek the correct rpm to meet the load requirements. This can set up further line disturbances that can affect connected loads. It also creates excessive heat in the generator and may create heating in SCRs.
Solutions to SCR concerns
The most common way to handle non-linear load concerns today in data processing and computer centers is to use a static uninterruptible power supply. This device uses an AC to DC converter, a battery system and an AC to DC inverter. These units can tolerate wide swings in voltage and current, yet still provide high-quality power to the connected loads. These systems also can affect waveform and should be filtered to reduce their harmonic output. However, these systems are designed to allow input power to bypass the critical load directly, so it’s important that the power source (utility or gen set) can provide good quality power. For gen sets, that means sizing is critical.
Sizing criteria used in the past for gen sets and transformers are not adequate for the non-linear loads found in today’s load profiles. Following is a method to better size for these loads:
- Establish UPS and/or non-linear load input kW. The UPS input kW is equal to its output, divided by the UPS and/or non-linear load input kW. The UPS input kW is equal to its output, divided by the ups efficiency, plus any battery recharging that takes place while the gen set is operating.
- Multiply the kW by the following k-factors if:
- The gen set will ONLY power the UPS or non-linear load system:Pulse Rectifier System: 6 k-factor: 1.6Pulse Rectifier System: 12 k-factor: 1.4
- The gen set will power UPS/non-linear loads AND other loads:Pulse Rectifier System: 6 k-factor: 1.15Pulse Rectifier System: 12 k-factor: 1.10
- Add the resulting kW rating based on the k-factor to the kW needed for additional loads (if applicable) to determine the minimum kW rating for a gen set.
Don’t forget motor starting
Because of high motor skVA, it’s imperative to include these in your sizing exercise or develop a strategy to minimize its effect on the total motor load.
We stand ready to help you combine these factors to help you select the best gen set for the application. Please call us.
Generator Sets and Uninterruptible Power Systems
An uninterruptible power supply (UPS) provides power free of voltage frequency variations, transient pulses, line noise and interruption.
Most UPS systems can only sustain power for a system shutdown, so a reliable, right-sized standby EPG system is needed to maintain data processing. The standby EPG system can also power the computer’s support systems such as the HVAC, lighting and emergency lighting systems.
Combining a UPS system with a gen set presents some special considerations to ensure compatibility. We use the following four-step procedure to size Cat gen sets that have static UPS systems as part of all of their load:
- Establish UPS input kWYou can use the input kW from the UPS supplier data sheets. If it’s not available from the supplier, we recommend the following formula on the equation to the right.UPS output for computer loads frequently is expressed in kVA. For approximating, multiply kVA by a 0.9 power factor to identify UPS output kW.Battery recharge kW generally ranges from 0 to 25 percent of input kW (15 percent is typical). If it’s unknown, use 25 percent of output kW for an approximation.If UPS efficiency is unknown, we recommend the following guidelines:
- Use 0.85 if UPS is less than 100 kW
- Use 0.875 if UPS is greater than or equal to 100 kW and less than 500 kW
- Use 0.90 if UPS is greater than or equal to 500 kW
- Set minimum-size gen setIn this step, establish the gen set size needed to contain waveform distortion. If your UPS system has a six-pulse rectifier, minimum standby get set equals UPS input kW x 1.6. For a 12-pulse rectifier, the formula is UPS input kW x 1.4.
- Consider other loadsBe sure to size the gen set to accommodate other loads in the application. Establish the kW of the other loads, then add it to the UPS input kW x 1.15 for a 12-pulse.
- Gen set sizingFor your final selection, choose the larger gen set rating of part 2 or 3. Round up to the nearest larger size standby gen set.
Our experience shows that most system compatibility problems involving gen sets and UPS systems arise because equipment selection and system design did not consider any power source other than a stiff utility system. It’s important to note that loads drawing harmonic currents cause distortion from the source; the source does not produce distortion.
If you have questions about sizing a gen set for a UPS system, call us. We’ll help you make sure the fit is right for your application.
Sizing a Cat gen set
Here’s an example of the Caterpillar gen set sizing procedure. The UPS system is rated 200 kVA/180 kW; other loads connected to the gen set total 100 kW.
- From supplier data, the UPS input is 255 kW, including battery recharge.
- 255 kW x 1.6 (six-pulse rectifier) = 408 kW minimum standby rated gen set.
- (255 kW x 1.15) + 100 kW = 393 kW minimum standby rated gen set with other loads.
- 408 kW is larger than 393 kW, therefore a standby gen set of at least 408 kW is recommended.