Generators Sizing Calculations – Part One

Today, we will start the explanation of Generator Sizing Calculations and as usual we start by the glossary of all terms and expressions that we will deal with through our explanation of Generator Sizing Calculations.

This glossary will help you to understand the technical meaning for any term or expression which will facilitate your understanding for the Generator Sizing Calculations.

Please read this glossary carefully because you will need to come back to it while our explanation for Generator Sizing Calculations.


Glossary of Generators – Part One

Alternating Current (AC)
Alternating Current (AC) is electric current that alternates between a positive maximum value and a negative maximum value at a characteristic frequency, usually 50 or 60 cycles per second (Hertz).
American National Standards Institute.
Acoustic Material
Acoustic material is any material considered in terms of its acoustic properties, especially its properties of absorbing or deadening sound.
Active Power
Active power is the real power (kW) supplied by the generator set to the electrical load. Active power creates a load on the generator set's engine and is limited by the horsepower of the engine. Active power does the work of heating, turning motor shafts, etc.
Air Circuit Breaker
An air circuit breaker automatically interrupts the current flowing through it when the current exceeds the trip rating of the breaker. Air is the medium of electrical insulation between electrically live parts and grounded (earthed) metal parts.
Alternator is another term for AC generator.

Amortisseur Windings
The amortisseur windings of a synchronous AC generator are the conductors embedded in the pole faces of the rotor. They are connected together at both ends of the poles by end rings or end plates. Their function is to dampen waveform distortion during load changes.
Ampacity is the safe current-carrying capacity of an electrical conductor in amperes as defined by code.
The ampere is a unit of electric current flow. One ampere of current will flow when a potential of one volt is applied across a resistance of one ohm.
An annunciator is an accessory device used to give remote indication of the status of an operating component in a system. Annunciators are typically used in applications where the equipment monitored is not located in a portion of the facility that is normally attended. The NFPA has specific requirements for remote annunciators used in some applications, such as hospitals.
Apparent Power
Apparent power is the product of current and voltage, expressed as kVA. It is real power (kW) divided by the power factor (PF).
The armature of an AC generator is the assembly of windings and metal core laminations in which the output voltage is induced. It is the stationary part (stator) in a revolving-field generator.
Authority Having Jurisdiction
The authority having jurisdiction is the individual with the legal responsibility for inspecting a facility and approving the equipment in the facility as meeting applicable codes and standards.
Automatic (Exciter) Paralleling
Automatic (Exciter) Paralleling describes a system where two or more generator sets can be started and paralleled while coming up to rated frequency and voltage. Because the generator excitation system is not turned on until the generator set is started (thus the term "dead field"), the generator sets automatically synchronize as they come to rated speed and voltage.
Backup Protection
Backup protection consists of protective devices, which are intended to operate only after other protective devices have failed to operate or detect a fault.
The amount of data that can be transmitted in a fixed amount of time. For digital devices, it is expressed as bits per second, or bytes per second. For analog devices, it is usually expressed as cycles per second, or Hertz.
Base Load
Base load is that portion of a building load demand which is constant. It is the "base" of the building demand curve.
Baud Rate
The speed of data transmission in serial data communications approximately equal to the number of code elements (bits) per second (BPS). Bits per second are also termed BPS, with the prefix (k) denoting thousands.
The process of making the logical connections to the network (also called connecting). This involves connecting network variable outputs to network variable inputs using LonWorks software.
Binary Digit.
Black Start
Black Start refers to the starting of a power system with its own power sources, without the assistance from external power supplies.
A logical system used to express one of two states, such as on or off (yes or no, 1 or 0, etc.)
A network communication technique whereby a node automatically receives a network variable from a sender node whenever the sender node sends it out. Whenever this condition exists, the node is said to be "bound".
Bumpless Transition
Bumpless transition is make-before-break transfer of an electrical load from one source to another where voltage and frequency transients are kept to a minimum.
Bus can refer to the current-carrying copper bars that connect the AC generators and loads in a paralleling system, to the paralleled output of the AC generators in a system or to a feeder in an electrical distribution system.
Bus Bars
Bus Bars are rectangular copper or aluminum bars that connect the output of the generator set circuit breakers to the transfer switches, circuit breakers, or fusible switches that transfer power to the load. The bus bars are sized and assembled in multiples according to the current they must carry under load. A typical sizing criteria for copper bus bars rated from 500-5,000 amps is to maintain a current density of 1,000 amps per square inch of cross-sectional area. This results in a bus temperature rise at full load that is within acceptable limits.
Bus Capacity
Bus capacity is the maximum load that can be carried on a system without causing degradation of the generator frequency to less than a prescribed level (usually 59 Hz in a 60 Hz system).
CT (Current Transformer)
Current transformers are instrument transformers used in conjunction with ammeters, control circuits and protective relaying. They usually have 5 ampere secondaries.
Refers to a communication system that divides geographic regions into sections called cells. The purpose of this division is to make the most use of the limited number of transmission frequencies.
A Channel is the physical communications media that connects the devices and the properties of these media (such as transmission speed). Most PowerCommand network installations will have only one channel (UTP cable and 78 KBPS transmission speed). In a large network, there may be multiple channels and each channel may or may not be of the same media type. Typically, channels are linked together using routers.
Channel Terminator
This is used to terminate networks on devices that do not have terminate switches. These are devices such as Gateways, RCI's, Routers, etc. that do not terminate circuits built into their design.
A circuit is a path for an electric current across a potential (voltage).
Circuit Breaker
A circuit breaker is a protective device that automatically interrupts the current flowing through it when that current exceeds a certain value for a specified period of time. See Air Circuit Breaker, Main Breaker, Molded Case Circuit Breaker and Power Circuit Breaker.
Circulating Harmonic Currents
Circulating Harmonic Currents are currents that flow because of differences in voltage waveforms between paralleled power sources, or induced by operation of non-linear loads.
Comma Separated Value (CSV)
A record layout that separates data fields with a comma and usually surrounds character data with quotes. PowerCommand for windows uses the CSV record format.
Connecting Devices
Connecting to refers to the process of assigning connections--linking an output variable of one device to an input variable of another device. This process is also called "binding".
A contactor is a device for opening and closing an electric power circuit.
Continuous Load
A continuous load is a load where the maximum current is expected to continue for three hours or more (as defined by the NEC for design calculations).
Cross Current
Cross currents are currents that circulate between paralleled generator sets when the internal (excitation) voltage of one genset is different from the other genset(s). The genset with the higher internal voltage supplies reactive power (kVAR) to the other genset(s). The amount of cross current that flows is a measure of this reactive power. Cross currents are 90 degrees out of phase (lagging) compared to the current that the generator would supply at 1.0 (unity) power factor.
Cross Current Compensation
Cross current compensation is a method of controlling the reactive power supplied by AC generators in a paralleling system so that they share equally the total reactive load on the bus without significant voltage droop.
Cross Current Transformer (CCT)
Cross Current Transformers are used to step down the higher line current to a lower current that the control system was designed for.
Current is the flow of electric charge. Its unit of measure is the ampere.
Current Limiting Fuse
A current limiting fuse is a fast-acting device that, when interrupting currents in its current-limiting range, will substantially reduce the magnitude of current, typically within one-half cycle, that would otherwise flow.
A cycle is one complete reversal of an alternating current or voltage from zero to a positive maximum to zero again and then from zero to a negative maximum to zero again. The number of cycles per second is the frequency.
Dead Bus
Dead Bus refers to the de-energized state of the power connections between outputs of paralleled generator sets. The term bus in this usage can either be rigid solid bus bars or insulated flexible cables.
Dead Field Paralleling
Automatic (Exciter) Paralleling
Delta Connection
Delta connection refers to a three phase connection in which the start of each phase is connected to the end of the next phase, forming the triangle-shaped Greek letter Delta. The load lines are connected to the corners of the triangle.
Demand Mode Standby Unit(s) (DMSU)
Demand Mode Standby Units are generator sets that can be shut down by the system when there is a low load level on the system.
Deviation Factor
The deviation factor is the maximum instantaneous deviation, in percent, of the generator voltage from a true sine wave of the same RMS value and frequency.
Dielectric Strength
Dielectric strength is the ability of insulation to withstand voltage without breaking down.
Differential Relay
A differential relay is a protective device that is fed by current transformers located at two different series points in the electrical system. The differential relay compares the currents and picks up when there is a difference in the two, which signifies a fault in the zone of protection. These devices are typically used to protect windings in generators or transformers.
Digital Master Control (DMC)
This device is designed to control the power systems in a facility. It is offered as an option on Cummins switchgear.
Direct Current (DC)
Direct current is current with no reversals in polarity.
Distributed Control System
A collection of nodes that interact to control a system whose components are spread out over some distance. Each node has intelligence for operating its own particular component of the system. Different parts of the system communicate status and control information with one another to form a distributed control system. Typically, they communicate on a peer-to-peer level. This is different from a type of system where all control and interaction between components is dictated by one central control. This is a common master/slave arrangement.
Distribution Circuit Breaker
A distribution circuit breaker is a device used for overload and short current protection of loads connected to a main distribution device.
Distribution Switchgear
Distribution switchgear may include automatic transfer switches, drawout air frame circuit breakers, fusible switches, or molded case breakers.
A domain is a network concept that allows independently functioning networks to share resources such as transmission media. A domain designation provides an ID number to identify the devices that can communicate within that domain. A network must have at least one domain. PowerCommand Network installations will usually have only one specified domain.
Draw Out Unit
A draw out unit is a structure that holds a circuit breaker in an enclosure. It has a movable carriage and contact structures that permit the breaker to be removed from the enclosure without manually disconnecting power cables and control wires.
Droop Load Sharing
Droop load sharing is a method of making two or more parallel generator sets share a system kW load. This is accomplished by having each governor control adjusted so that the sets have the same droop (reduction of speed). Typical droop is two cycles in frequency from no load to full load.
Earth Fault Protection
A grounding bar is a copper bar that electrically joins all the metal sections of the switchgear. This bar is connected to the earth or ground connection when the system is installed. The grounding or earthing protects personnel from stray currents that could leak to the metallic enclosures.
Efficiency (EFF)
Efficiency is the ratio of energy output to energy input, such as the ratio between the electrical energy input to a motor and the mechanical energy output at the shaft of the motor.
Electrical Operator
An electrical operator is the electric motor driven closing and tripping (opening) devices that permit remote control of a circuit breaker.
Emergency Bus
An emergency bus is the silver-plated copper bus bars or flexible cable used to connect the paralleling breakers to the emergency system feeder breakers, and ultimately to automatic transfer switches or other distribution devices.
Emergency System
An emergency system is independent power generation equipment that is legally required to feed equipment or systems whose failure may present a life safety hazard to persons or property.
Energy is manifest in forms such as electricity, heat, light and the capacity to do work. It is convertible from one form to another, such as in a generator set, which converts rotating mechanical energy into electrical energy. Typical units of energy are kW/h, Btu (British thermal unit), Hp/h, ft/lbf, joule and calorie.
An exciter is a device that supplies direct current (DC) to the field coils of a synchronous generator, producing the magnetic flux required for inducing output voltage in the armature coils (stator). See Field.
Exciter Paralleling Control
An exciter paralleling control initiates the start of generator excitation in generator sets used in automatic paralleling systems.
A fault is any unintended flow of current outside its intended circuit path in an electrical system.
Feeder Circuit Breaker
See Distribution Circuit Breaker.
Fiber Optic Cable
A technology using glass or plastic threads (fibers) to transmit data. A fiber optic cable is a bundle of either glass or plastic threads capable of transmitting messages modulated into light waves. Typically, fiber optic cable has greater bandwidth allowing them to carry more data than metal wires. Fiber optic cable is lighter and less susceptible to interference than metal wires. Also, data can be transmitted digitally rather being transformed into analog data for transmission as is the case with metal wires when used for computer data transmission. Fiber optics are becoming increasingly more common for use with Local-Area Networks (LANs).
The generator field (rotor) consists of a multi-pole electromagnet which induces output voltage in the armature coils (stator) of the generator when it is rotated by the engine. The field is energized by DC supplied by the exciter.
Field Breaker with Auxiliary Switch
This is the circuit breaker (usually mounted in the generator control panel) that monitors the alternating current input to the automatic voltage regulator. If a malfunction occurs in the excitation system, the circuit breaker trips on overcurrent-closing the auxiliary switch, shutting down the generator set, and energizing the alarm circuit.
First Start Sensor
A first start sensor is an electronic device within some paralleling equipment that senses generator set and bus voltage and frequency, and determines whether or not a generator set is the first unit ready to close to the bus following a call to start under "black start" conditions.
Free Field (Noise Measurements)
In noise measurements, a free field is a field in a homogeneous, isotropic medium (a medium having the quality of transmitting sound equally in all directions) which is free of boundaries. In practice, it is a field in which the effects of the boundaries are negligible in the region of interest. In the free field, the sound pressure level decreases 6 dB for each doubling of the distance from a point source.
Frequency is the number of complete cycles per unit of time of any periodically varying quantity, such as alternating voltage or current. It is usually expressed as (Hz) Hertz or CPS (cycles per second).
Frequency Adjust Potentiometer
A frequency adjust potentiometer is used to manually bring the frequency (speed) of the incoming set to that of the bus for synchronizing purposes. When the generator set is paralleled, operation of this potentiometer will adjust the kW load assumed by the generator set.
Frequency Regulation
Frequency regulation is a measure that states the difference between no-load and full-load frequency as a percentage of full-load frequency.
Fusible Switch
A fusible switch is an isolating switch and overcurrent protective device used for feeder or transfer switch isolation and protection. It is typically a manually operated, stored energy opening and closing, bolted compression blade switch, with provisions for installing current limited fuses.
A device that acts as an interface between two different communication protocols. The Network Gateway Module (NGM) provides a communication protocol that a PC can understand. Other gateway devices may be used to interface between our Lontalk protocol and other systems such as a SCADA or Building Automation System. Typically, a gateway becomes necessary when a SCADA or BAS does not have a driver developed for Lontalk.
A generator is a machine which converts rotating mechanical energy into electrical energy.
Genset Communication Module (GCM)
The GCM provides a communication gateway between the Model 3100 PowerCommand Control (PCCI) and the network. The GCM communicates with the PCCI control over a serial data link. The GCM gets data from the PCCI controls such as voltage, current, engine speed, oil temperature, etc. and then sends it out on the network if another network node is bound to it or requesting data.
A governor is a device on the engine which controls fuel to maintain a constant engine speed under various load conditions. The governor must have provision for adjusting speed (generator frequency) and speed droop (no load to full load).
The utility-owned power distribution system.
A ground is a connection, either intentional or accidental, between an electrical circuit and the earth or some conducting body serving in place of the earth.
Ground Fault Protection
This function trips (opens) a circuit breaker or sounds an alarm in the event that there is an electrical fault between one or more of the phase conductors and ground (earth). This ground fault protection function may be incorporated into a circuit breaker.
Ground Return
Ground return is a method of ground fault detection that employs a single sensor (CT) encircling the main bonding jumper between the power system neutral and ground. This device in itself is not capable of locating the faulted circuit but when used in conjunction with ground fault sensors on all feeders and source connections, can provide bus fault protection when properly coordinated (delayed).
Grounded Neutral
A grounded neutral is the intentionally grounded center point of a Y-connected, four-wire generator, or the mid-winding point of a single phase generator.
Harmonic Distortion (Total Harmonic Distortion)
Total harmonic distortion is an expression of the total harmonic content of a voltage waveform. The harmonic distortion (or harmonic content) of a waveform is usually expressed as the square root of the sum of the squares of each of the harmonic amplitudes (with amplitudes as a percent of the fundamental voltage amplitude).
Harmonics are voltage or current components which operate at integral multiples of the fundamental frequency of a power system (50 or 60 Hertz). Harmonic currents have the effect of distorting the shape of the voltage wave form from that of a pure sine wave.
Hertz (Hz)
The term Hertz is the preferred designation for cycles per second (CPS) and is used to describe frequency.
A common connection point for devices or nodes in a network or sub-network. Hubs are commonly used to connect segments of a LAN and contain multiple ports.
Hunting is a phenomenon that can occur upon load changes in which the frequency or the voltage continues to rise above and fall below the desired value without reaching a steady-state value. It is caused by insufficient damping.
Incoming Set
This is the generator set that is about to be connected to (paralleled with) the energized bus.
Insulated Case Circuit Breaker
An insulated case circuit breaker is a power circuit breaker that is provided in a preformed case, similar to a molded case breaker.
Insulation is non-conductive material used to prevent leakage of electric current from a conductor. There are several classes of insulation in use for generator construction, each recognized for a maximum continuous-duty temperature.
Internal Voltage
The internal voltage is the voltage a generator would develop at no load if it were not connected in a parallel operation. Excitation of the generator field controls internal voltage.
Design to allow one product to work with another product without modification.
This refers to the practice of operating on-site power systems, at the request of a utility, to reduce electrical demand on the utility grid during periods of high consumption. Interruptible facilities may also be disconnected from all electrical service in the event of high demand on the utility grid, even if no on site power system is available.
Interrupting Capacity
Interrupting capacity is the magnitude of electrical current that a device can safely interrupt (open against), without failure of the component.
kW Load Sensor
The kW load sensor is an electronic device provided to sense kW level at various points in a system, for use in control functions within the system, such as kW load alarms, or load demand.
kVA (kilo-Volt-Amperes)
kVA is a term for rating electrical devices. A device's kVA rating is equal to its rated output in amperes multiplied by its rated operating voltage. In the case of three-phase generator sets, kVA is the kW ouput rating divided by 0.8, the rated power factor. kVA is the vector sum of the active power (kW) and the reactive power (kVAR) flowing in a circuit.
kVAR (kilo-Volt-Amperes Reactive) is the product of the voltage and the amperage required to excite inductive circuits. It is associated with the reactive power which flows between paralleled generator windings and between generators and load windings that supply the magnetizing currents necessary in the operation of transformers, motors and other electromagnetic loads. Reactive power does not load the generator set's engine but does limit the generator thermally.
This is an abbreviation for kilowatt, an alternate term for rating electrical devices. Generator sets in the United States are usually rated in kW. Sometimes called active power, kW loads the generator set engine.
This is a unit of electric energy. It is equivalent to one kW of electric power supplied for one hour.
Lagging Power Factor
Lagging power factor in AC circuits (a power factor of less than 1.0) is caused by inductive loads, such as motors and transformers, which cause the current to lag behind the voltage. See Power Factor.
Lead Unit
In a paralleling system that has a load demand feature, the lead unit is the last unit to be shut down in the event that load demand mode is in operation.
Leading Power Factor
Leading power factor in AC circuits (0.0 to -1.0) is caused by capacitive loads or overexcited synchronous motors which cause the current to lead the voltage. See Power Factor.
A leg is a phase winding of a generator, or a phase conductor of a distribution system.
Line-To-Line Voltage
Line-to-line voltage is the voltage between any two phases of an AC generator.
Line-To-Neutral Voltage
In a 3-phase, 4-wire, Y-connected generator, line-to-neutral voltage is the voltage between a phase and the common neutral where the three phases are tied together.
Load Demand
Load Demand is a paralleling system operating mode in which the system monitors the total kW output of the generator sets, and controls the number of operating sets as a function of the total load on the system. The purpose of load demand controls is to reduce fuel consumption and limit problems caused by light load operation of reciprocating diesel generator sets.
Load Factor
The load factor is the ratio of the average load to the generator set power rating.
Load Management
Load management is the overall control of load connected to match available generator capacity. Priority control and load shedding are the two features required for load management.
Load Shedding
Load shedding is the process by which the total load on a paralleling system is reduced, on overload of the system bus, so that the most critical loads continue to be provided with reliable electrical service.
Local Loop
A method of branching out or creating a stub on the network. The maximum distance this stub can be is 10ft. (3m) from the main network bus. Effectively the node is "daisy-chained" into the network. This involves two wires, one that goes to the node and another that returns to the main network bus. The total local loop distance must be added to the total network length. This becomes important when the main network bus nears the 4,600 ft. length and requires the use of Routers.
Local-Area Network (LAN)
A computer network that spans a relatively small area. Most LANs are confined to a single building or group of buildings.
Locations are subdivisions of a network that can be selected for easier organization. Locations may designate physical places, but are not required to do so. For example, network devices in one location may communicate with network devices in another location when requested to do so.
Low Voltage
AC system operating voltages from 120 to 600 VAC.
Main Breaker
A main breaker is a circuit breaker at the input or output of the bus, through which all of the bus power must flow. The generator main breaker is the device, usually mounted on the generator set, that interrupts the genset's power output. Main breakers provide overcurrent protection and a single disconnect point for all power in a switchboard or device.

In the next article, we will continue explaining the Glossary of Generators – Part Two to start explaining Generators Sizing Calculations later. So, please keep following.


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