Effective selection and application of modern gensets require a strong knowledge of generator basics as well as an in-depth awareness of fuel sources and NEC requirements.
Engine-generator sets now have countless applications, often with multiple uses within the same facility. Most engine-generator systems provide for human safety and protection of property in applications such as office buildings, hotels, places of assembly, and government facilities. Hospitals and nursing homes have special needs vital to life, and these emergency power systems reflect these requirements.
Other applications
At the same time, engine-generator systems can be used to supply power to a local utility in a peak power or interruptible power program when the utility needs additional capacity. This application frequently justifies the cost of an expensive auxiliary power system because of the excellent rebate or payback programs available from most utilities.
Cogeneration is another energy-saving, cost-cutting technique utilizing waste heat from one or more engine-generator sets to perform useful functions.
Utility power may be derived from a traditional power company, or from an independent power producer (IPP). The law permits an IPP to "wheel" power via the utility lines to any user as desired. As a result, traditional design guidelines must be adjusted to accommodate these changes.
When such energy-conservation installations are implemented, grants or rebates from local utilities amounting to hundreds of thousands of dollars may be available from the local utility or governmental body. As a result, not only are energy costs reduced, but also capital costs are frequently slashed, permitting a fast payback of initial investment.
Newer technologies, such as computer installations, communications networks, and modern research and development laboratories, are demanding an increasing amount of auxiliary power. Some such facilities require highly dependable power requiring multiple engine-generator sets and incorporating a wide variety of support equipment and systems.
Synchronous generator
Most applications today use the synchronous generator because of its versatility, reliability, and capability of operating independently. Most modern synchronous generators are of the revolving field alternator design. Essentially, this means that the armature windings are held stationary, and the field is rotated. Therefore, generated power can be taken directly from the stationary armature windings. On the other hand, revolving armature alternators are less popular because generated output power must be derived via slip rings and brushes.
The exact value of the AC voltage generated by a synchronous machine is controlled by varying the current in the DC field windings, while frequency is controlled by the speed of rotation.
Power output is controlled by the torque applied to the generator shaft by the driving engine. In this manner, the synchronous generator offers precise control over the power it can generate.
Generator exciters
Almost all modern synchronous generators use a brushless exciter, which essentially is a small AC generator on the main shaft. The AC voltage generated is rectified by a 3-phase rotating rectifier assembly also on the shaft. The DC voltage thus obtained is applied to the main generator field, which is also on the main shaft. A voltage regulator is provided to control the exciter field current and, in this manner, the field voltage can be precisely controlled, resulting in a stable, well-controlled generator output voltage.
Because of recent problems with nonlinear loads, which cause unwanted harmonics in a power system, some generator manufacturers are providing an additional generator furnished with a permanent magnet exciter (PME) to provide field magnetism to the brushless exciter. This approach eliminates or greatly reduces harmonic distortion because exciter voltage is derived from magnets instead of the line. In some designs, the PME serves as the main exciter. However, it's not actually an exciter in the true sense of the term. When selecting a generator, be sure to investigate the need or availability of the permanent magnet exciter.
Voltage regulation
The output voltage of a synchronous generator is controlled by the excitation in the field windings. To control this, the generator's voltage regulator measures the output voltage, compares it with a standard reference voltage obtained from a Zener diode that continuously samples output voltage, and adjusts the excitation current up or down as needed to maintain the output voltage at its rated value. If the load varies, the excitation is continuously adjusted to keep the voltage constant.
Frequency
Frequency of the AC current produced is dependent on two factors: The number of poles (n) built into the machine and the speed of rotation (rpm). Thus, frequency (f) is calculated by using the follow equation:
f = (rpm x n) / 120
Therefore, rpm = (f x 120) / n
As a result, a two-pole generator must have an rpm of 3600 to provide 60 Hz while a four-pole generator must rotate at 1800 rpm to provide 60 Hz. An eight-pole generator must operate at 900 rpm to provide 60 Hz. To obtain a 50 Hz output, the generator speeds must be slightly slower as calculations would show.
Since frequency is normally a constant (60 Hz or 50 Hz), control of the generator speed is essential. This is accomplished by providing precise rpm control of the prime mover, which is done by a governor. (Engine speed governors are discussed later on.)
Specifications
When specifying, sizing, or selecting a generator, you must take many factors into consideration. As previously mentioned, factors that relate to the prime mover must receive equal attention simultaneously.
Important generator considerations include:
* Type of generator;
* RPM and frequency;
* Location and any required enclosures;
* Kilowatt rating and efficiency;
* Number of phases and power factor;
* Controls and related switchgear;
* Transfer switching; and
* Duty, starting conditions, etc.
Be sure to contact the manufacturer if the generator will have to supply a large motor load. Some motors draw vary large starting currents that will place heavy demands on the generator.
Harmonics
If the facility loads include non-linear loads (computer power supplies, variable frequency drives, electronic ballasts or other similar electronic equipment, particularly those furnished with switch-mode power supplies), it's essential that you advise the generator supplier of such so that proper steps can be taken to avoid equipment overheating or other problems due to harmonics. Some generator manufacturers recommend low-impedance generators and have developed winding design techniques to reduce the effects of the harmonic currents generated. In some instances, the generator may have to be derated and neutral size increased to safely supply complex nonlinear loads.
Induction generator
The construction of an induction generator is essentially the same as that of an induction motor: Both have a squirrel-cage rotor and wound stator. When this machine is driven above its designed synchronous speed, it becomes a generator; at less than synchronous speed, it functions as a motor. Because the induction generator does not have an exciter, it must operate in parallel with the utility. This outside power source provides the reactive power for generator operation. Also, its frequency is automatically locked in with the utility.
An induction generator is a popular choice for use when designing cogeneration systems, where it will operate in parallel with the utility. This type of generator offers certain advantages over a synchronous generator. For example, voltage and frequency are controlled by the utility; thus voltage and frequency regulators are not required. In addition, the generator construction offers high reliability and little maintenance. Also, a minimum of protective relays and controls are required. Its major disadvantage is that it normally cannot operate alone as a standby/emergency generator in such an application.
Type of driving engine
Gasoline engines are economical up to about 100kW. Initial costs are comparatively low, and they have reliable starting ability. The engine can be two or four stroke.
Diesel engines are popular because of their reliability, ruggedness, low maintenance, economical operation, and low initial cost for larger capacities. For auxiliary power applications, diesel engines are built in sizes from a few kW up to about 2000kW. For prime power applications, they may be sized up to 20,000 hp or more. Diesel engines can be two- or four-stroke, turbocharged, and air- or liquid-cooled.
Initial costs for diesel engines are higher than those for gasoline or gaseous-fuel engines. In some instances, these engines are built to operate on either diesel or gaseous fuel. Installation for large auxiliary-power units (1000 to 2000kW) is usually more costly because of requirements for foundation, vibration damping, sound reduction, water cooling, exhaust stack, and starting equipment. In addition, special considerations are usually required for use in ambient temperatures below 40 [degrees] F.
Gaseous fuel engines are comparable to diesel engines in many ways, except that the normal gas supply is subject to interruption in the event the supply line is broken or cutoff. However, an on-site propane gas tank can be installed to provide an alternate supply of fuel in the event the normal supply is lost.
Gas turbine sets have had considerable success as on-site power sources for heavy loads ranging from about 500kW up. Chief advantages include small size, light weight, and lack of vibration. They are well suited for installation on upper floors or on roofs. Water cooling is not required; however, the high volume of exhaust may necessitate large stacks. Also, sound levels are high, but soundproofing is comparatively easy to accomplish. They are relatively slow-starting unless provided with special starting aids, such as a compressed-air-driven pneumatic motor.
Some turbine-driven sets have the capability of burning a variety of fuels, either liquid or gas. They are also available as dual-fuel sets, such as natural gas and diesel, or natural gas and liquid petroleum gas.
Factors in fuel selection
Fuel availability frequently determines the choice of the engine-generator set. If a given fuel is already in use at the site, a genset using the same fuel may be most desirable.
If the genset is to be located in an isolated area where public utilities are not available, LPG or diesel fuel are logical choices. If the unit is to be located in a seismic risk area, an on-site fuel supply must be considered. A dual-fuel engine, such as a natural gas/propane unit is often selected to satisfy this need. (Refer to NEC Sec. 700-12b, and NFPA 110:3-1.2.)
Gaseous fuels. Air pollution due to engine exhaust is becoming a serious consideration, in particular because of recent clean-air legislation. Often, a dual-fuel unit capable of burning natural gas or propane can be advantageous because its exhaust is less toxic than that of a diesel engine. If a dual-fuel unit is selected, be sure to check the output rating for each fuel. The kW capability of a given genset is lower on natural gas than on propane.
Gaseous fuels, which are all clean-burning, offer minimum carbon buildup, cleaner crankcase oil, no fuel storage problems, and less engine maintenance than either diesel or gasoline fuels. However, LPG presents the greatest hazard of any of the fuels because any vapors leaked or released, being heavier than air, will fill low areas such as basements, and create an explosion hazard.
Any fuel system must be installed in accordance with applicable codes, and this is especially important with an LPG system. Reputable manufacturers provide guidance in the design of fuel systems. An excellent source of fuel system design for LPG is NFPA 54, National Fuel Gas Code.
Recent EPA rulings require stricter standards for installations of fuel tanks. An approved LPG fuel tank must be used to store liquid propane. All appropriate shut-off valves and pressure gauges must be included. An LPG tank should never be installed inside a structure; it should be installed outside, some distance away from any structures and away from open flames, sparks, or electrical connections.
Gasoline. Most smaller engine-generator sets operate on gasoline for a number of reasons. The smaller gensets (to about 100kW) are used in portable applications, such as on construction sites, for mobile emergency power, in motor homes or recreation vehicles, etc. In these applications, ready fuel availability is important. Also, gasoline engines start easier than diesels in cold temperatures. You should take note that most of these applications use fuel regularly and that the fuel supply must be replenished. Therefore, they usually do not stand unused for long periods of time.
Gasoline is rarely used in alternate power-source applications because storage of this fuel can be a fire hazard, and long-term storage of gasoline can be detrimental to the engine.
Diesel fuel. In many applications, diesel fuel is chosen because it offers easy on-site storage, has fewer problems with long-term storage, has reduced fire hazards, and is capable of more operating hours between overhauls. Also, modern diesel engines offer many advantages over other types of prime movers.
A disadvantage of diesel fuel is its low volatility at low ambient temperatures. This problem is minimized by providing diesels with thermostatically controlled coolant heaters to maintain the water jacket temperature at 90 [degrees] F or higher.
Combustion air
The output power of any type of engine is proportional to the lbs of fuel burned per unit of time. For diesels, each lb of fuel requires 17 lb of air for complete combustion. When more air is forced into the cylinder, more fuel is burned and more power is developed. Several methods are used to increase engine power. The most common are turbocharging (supercharging), after-cooling, and intake tuning.
Turbochargers use the pressure of the exhaust gas to drive a turbine/compressor in the combustion air intake system. This forces additional air into the combustion chamber for more power production.
Aftercoolers employ heat exchangers in the combustion air system to reduce air temperature, thereby making the air more dense and providing more oxygen for combustion.
Governors
In nearly all instances, a governor applied to an engine-generator set controls engine speed to assure the driven AC generator provides the proper frequency of the AC power output. There are many types of governors; however, for auxiliary power applications, an isochronous governor is normally selected. It controls the speed of the engine so it remains constant from no-load to full load, assuring a constant AC power output frequency from the generator.
A modern system consists of two primary components: An electronic speed control and an actuator that adjusts the speed of the engine. The electronic speed control senses actual engine speed and provides a feedback signal to the mechanical/hydraulic actuator, which, in turn, positions the engine throttle or fuel control to maintain a very accurate engine speed.
Controls and monitoring
A wide variety of instruments, safety devices, and controls are available for special needs. These include a load-test switch, engine-start relays, time-clock or engine-start circuit, and engine-time running meters. Alarm relays include overload, over/under voltage, reverse-current relays, and ground-fault protection. Engine protection includes alarms for low fuel, low oil, low cooling water, overtemperature, etc. Be sure to check applicable articles in the NEC for required controls and signaling devices, such as automatic starting and derangement signals.
Synchronizing controls permit automatic synchronizing of two or more generators. Usually, visual indication of synchronization (two pilot lights) is also provided at the generator control and paralleling switchgear. In addition, a programmable logic controller (PLC) with keyboard may be included to provide for programming of voltage/phase synchronization, speed control, and frequency adjust. Generators may be programmed to operate in certain sequences to obtain maximum efficiency and reliability of operation.
NEC-defined applications
The National Electrical Code provides guidance for safe and proper installation of on-site engine-generator systems. Local codes may vary and must be reviewed during early design stages.
Whatever the application, a great many components are involved in the installation of an on-site power system. The system selected and components involved depend on the type of occupancy, type of process or activity, and specific needs of the facility. Additional important considerations include budget considerations, degree of safety or reliability desired, codes and standards, and the particular set of special functions and technologies required for the application.
Applicable Code rules depend on the purpose of the system under consideration. In addition, the requirements of these rules serve as an excellent guideline that helps effective design, selection, and installation.
Article 700 (Emergency Systems) covers systems that are legally required to be installed and that supply loads essential to safety and life, such as emergency lighting, essential refrigeration and ventilation, and signaling systems. Typically, emergency systems are installed in places of assembly, such as theaters, schools, stadiums, or wherever large numbers of people may gather. The purpose of the system is to assure safe evacuation by providing electric power for adequate emergency lighting, fire detection, operation of fire pumps, alarm signals, and communication.
Article 701 (Legally Required Standby Systems) covers those systems required and classified as legally required standby by municipal, state, federal, and/or other codes, or by any governmental agency having jurisdiction. These systems are intended to supply power automatically to important selected loads (other than those classed as emergency systems) in the event of failure of the normal power source. Legally required standby power systems are typically installed to serve such loads as heating and refrigeration systems, communication systems, ventilation and smoke-removal systems, sewage disposal, lighting, and industrial processes which, when stopped during any power outage, could create hazards or hamper rescue or fire fighting operations.
Article 702 (Optional Standby Systems) says that optional standby systems are intended to protect private business or property where life safety does not depend on the performance of the system. Optional standby systems (other than those classed as emergency or legally required standby systems) typically serve as an alternate power source for industrial and commercial buildings, farms, and residences by supplying such loads as heating and refrigeration systems, data processing and communications systems, and industrial processes which, when stopped during any power outage, could cause discomfort, serious interruption of the process, or damage to the product or process.
Article 705 (Interconnected Electric Power Production Sources) provides rules that apply to engine-generator sets that operate in parallel with the utility (or other sources). Regulations cover the generator connection point, characteristics, and protection as well as rules concerning synchronous and induction generators.
Article 517 (Health Care Facilities) includes numerous regulations that apply to on-site power systems installed in hospitals, nursing homes, and other healthcare facilities. Especially important are Secs. 517-31, 34, 35, 44, and 50. Requirements are similar to those of Article 700, with a few variations.
Article 455 (Generators) provides general guidelines for proper and safe installation of generators. Regulations cover location, marking, overcurrent protection, and others. Of particular significance is an exception to Sec. 445-4, which permits the generator overload devices to be connected to an annunciator or alarm instead of where they would interrupt the generator circuit. This is permitted only where deemed by the authority having jurisdiction that the generator should operate to failure to prevent a greater hazard to persons.
The IEEE Orange Book, Recommended Practice for Emergency and Standby Power Systems (IEEE 446) is another valuable aid.