Generators Sizing Calculations – Part Two

Today, we will continue the explanation of Glossary of Generators to start explaining Generators Sizing Calculations later. In the previous article “Generators Sizing Calculations – Part One” I indicate the First Part of Glossary of Generators and now we will indicate the second part.

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 Two


Mains is a term used extensively outside of the United States to describe the normal power service (utility).
Master Control
A control section in a typical paralleling system that provides total system metering and the interface point between the paralleling system and the facility.
The main network bus defined by two characteristics: 1) The electrical signal level and 2) the characteristics of the wiring they will travel over. Typically, our standard PowerCommand Network uses 22 AWG Unshielded Twisted-Pair (UTP) wire operating at 78 KBPS.
Medium Voltage
AC system operating voltages from 601 to 15000 VAC.
An industrial networking system that uses RS-232 serial master-slave communications at data transfer rate of up to 19.2 KBPS.
Modbus II
An industrial networking system that uses token-passing peer-to-peer communications at data transfer rates of five megabits per second (MBPS). The network medium is coaxial cable.
Modbus Plus
An industrial networking system that uses token-passing peer-to-peer communications at data transfer rates of one megabit per second (MBPS). The network media is shielded twisted-pair cable.

Modules are also called nodes or devices. These are devices such as Genset Communication Modules (GCMs), Control Communication Modules (CCMs), and Digital Input/Output Modules (DIMs).
Molded Case Circuit Breaker
A molded case circuit breaker automatically interrupts the current flowing through it when the current exceeds the trip rating of the breaker. Molded case refers to the use of molded plastic as the medium of electrical insulation for enclosing the mechanisms, and for separating conducting surfaces from one another and from grounded (earthed) metal parts. Molded case circuit breakers usually contain thermal-magnetic trip units, although larger sizes can be equipped with solid state trip sensors.
In paralleling applications, unless a generator set is disconnected from the bus when its engine fails (usually as a result of a fuel system problem), the generator will drive (motor) the engine, drawing power from the bus. Reverse power protection which automatically disconnects a failed set from the bus is essential for paralleling systems. Also, in certain applications such as elevators, the load can motor the generator set if insufficient additional load is present.
Multi-drop Bus Topology
The wiring arrangement used for the network data. The bus starts at one point and ends at another. Both the start and end of a network must be terminated through the use of a terminate switch. The maximum stub length (See Definition of Local Loop) must not exceed 10ft. and must be included in the total length of the main network bus.
NEC (National Electrical Code)
This document is the most commonly referenced general electrical standard in the United States.
National Electrical Manufacturers Association.
NEMA 1 Enclosure
This enclosure designation is for indoor use only-where dirt, dust, and water are not a consideration. Personnel protection is the primary purpose of this type of enclosure.
National Fire Protection Association.
NFPA 110
National Fire Protection Agency Section 110 (NFPA 110) deals with the regulations concerning Emergency Power Systems (EPS). This section deals with regulations on installation, operation, and monitoring of EPS.
A collection of Nodes that communicate with one another over a common medium.
Network Annunciator Module (NAM)
A device providing LED indication in the event of an alarm condition on a PowerCommand Network device. For example, we can provide NFPA 110-alarm annunciation for gensets with the use of a NAM.
Network Bus
The main "backbone" of the network data wire. It must be terminated at both the start and end of the network. Stubs off the main bus wire cannot exceed 10ft. (3m.). The wire is "daisy-chained" from one node to the next. The bus cannot exceed 4,600ft. without the use of a router. Bus can also refer to the devices that connect the generators and loads to a system.
Network Data
A signal that carries messages between nodes. PowerCommand Networks use Manchester Encoding that makes the signal insensitive to polarity. The signal is transformer-coupled to the network data wire at a rate of 78 KBPS.
Network Data Wire
Unshielded-Twisted Pair (UTP) cable that carries the network data over the main network bus. The maximum network length is 4,600 ft. without the use of routers.
Network Gateway Module
A device acting as an interface between a modem or PC and the network wire. The Gateway takes the UTP wire and then provides an RS-232 port for connection to either a modem or PC.
Neutral refers to the common point of a Y-connected AC generator, a conductor connected to that point or to the mid-winding point of a single-phase AC generator.
Neutral Current
Neutral current is the current that flows in the neutral leg of a paralleling system. Often, this term is used in reference to circulating currents or cross currents.
A module that can communicate over the network data to other modules. A module contains a Neuron Chip. Certain devices are nodes such as Genset Communication Modules (GCMs) and Control Communication Modules (CCMs). Other devices are not nodes, as they cannot communicate with other devices, but only receive messages. An example is the Network Annunciator Module (NAM).
Nonlinear Load
A nonlinear load is a load for which the relationship between voltage and current is not a linear function. Some common nonlinear loads are fluorescent lighting, SCR motor starters and UPS systems. Nonlinear loads cause abnormal conductor heating and voltage distortion.
Normal Standby Mode
In the normal standby mode, power to the load is supplied by the utility. The paralleling system is ready to provide power to the load in the event of utility failure.
Octave Band
In sound pressure measurements (using an octave band analyzer), octave bands are the eight divisions of the measured sound frequency spectrum, where the highest frequency of each band is twice that of its lowest frequency. The octave bands are specified by their center frequencies, typically: 63, 125, 250, 500, 1,000, 2,000, 4,000 and 8,000 Hz (cycles per second).
The ohm is a unit of electrical resistance. One volt will cause a current of one ampere to flow through a resistance of one ohm.
On-Set Paralleling
On-set paralleling is a manual paralleling system that is built onto the generator set, no additional switchboards are required.
One-Line Diagram
A one-line diagram is a schematic diagram of a three-phase power distribution system which uses one line to show all three phases. It is understood when using this easy to read drawing that one line represents three.
Operating Source
An operating source is a source of electrical power that is delivering power to a load. The operating source can be either a generator set or a commercial (utility bus) power line.
Out-Of-Phase refers to alternating currents or voltages of the same frequency which are not passing through their zero points at the same time.
Overcrank is an alarm function provided with most generator sets that indicate that the generator set has failed to start.
Overload Rating
The overload rating of a device is the load in excess of the nominal rating the device can carry for a specified length of time without being damaged.
Overshoot refers to the amount by which voltage or frequency exceeds the nominal value as the voltage regulator or governor responds to changes in load.
Parallel Operation
Parallel Operation is the operation of two or more sources of AC electrical power whose output leads are connected to a common load. Connection of the power sources is made so that the sources electrically function as a single source of power. Parallel Operation requires that the two sources of electrical power must match in voltage, frequency, and number of phases.
Paralleling Breaker
A paralleling breaker is the circuit breaker that connects the generator set to the emergency bus, and across which all the individual generator synchronizing functions occur.
Paralleling Control
A paralleling control contains the electrical equipment provided in a paralleling system for control of a single generator set.
Paralleling Suppressers
Paralleling suppressors are semiconductor devices that protect the silicon diodes on a brushless excitation system from damaging overvoltages. Overvoltages, usually of short duration, occur when a generator is paralleled out of phase with the energized bus.
In error detecting schemes, a Bit (even or odd) that represents the binary sum of the data transmitted. Primarily used when transmitting data over a long distance. For example, when transmitting information using modems.
Pass Thru
Refers to a junction box connection where the network bus comes to a connector and then continues straight on through. In most Pass Thru connections, very little input and output is done. An example of this connection is the Junction Box/Terminator (JBT).
Peak Load
Peak load is the highest point in the kilowatt demand curve of a facility. This is used as the basis for the utility company's demand charge.
A network operating system where any device on the main network bus can initiate communication.
Phase refers to the windings of an AC generator. In a three-phase generator there are three windings, typically designated as A-B-C, R-S-T or U-V-W. The phases are 120 degrees out of phase with each other. That is, the instants at which the three phase voltages pass through zero or reach their maximums are 120 degrees apart, where one complete cycle is considered 360 degrees. A single-phase generator has only one winding.
Phase Angle
Phase angle refers to the relation between two sine waves which do not pass through zero at the same time. Considering one full cycle to be 360 degrees, the phase angle expresses how far apart the two waves are in relation to each other in degrees.
Phase Rotation
Phase rotation (or phase sequence) describes the order (A-B-C, R-S-T, or U-V-W) of the phase voltages at the output terminals of a three-phase generator. The generator phase rotation must match the facility phase rotation. This must be checked prior to operation of the electrical loads in a facility with an on-site generator.


Pitch is a mechanical design characteristic of a generator that indicates the ratio of the number of winding slots per generator pole to the number of slots enclosed by each coil. The generator designer may use the pitch of a machine to optimize the generator cost versus the quality of the voltage waveform generated.
Pole is used in reference to magnets, which are bipolar. The poles of a magnet are designated North and South. Because magnets are bipolar, all generators have an even number of poles. The number of poles determines how fast the generator will have to be turned to obtain the specified frequency . For example, a generator with a 4-pole field would have to be run at 1800 rpm to obtain a frequency of 60 Hz (1500 rpm for 50 Hz). Pole can also refer to the electrodes of a battery or to the number of phases served by a switch or breaker.
The external connector on a device at which the network cable or medium is attached.
Power refers to the rate of performing work or of expending energy. Typically, mechanical power is expressed in terms of horsepower and electrical power in terms of kilowatts. One kW equals 1.34 hp.
Power Circuit Breaker
A power circuit breaker is a circuit breaker whose contacts are forced closed via a spring-charged, over-center mechanism to achieve fast closing (5-cycle) and high withstand and interrupting ratings. A power circuit breaker can be an insulated case or power air circuit breaker.
Power Factor
Power factor is the cosine of the angle between the active power (kW) and apparent power (kVA) in a circuit.
Prime Power
Prime Power describes an application where the generator set(s) must supply power on a continuous basis and for long periods of time between shutdowns. No utility service is present in typical prime power applications.
Priority Control
Priority control is the process by which the total loads on the bus is limited to the most critical loads in the system until adequate generation capacity is available to serve all loads.
A set of rules used mutually by two or more devices to communicate. Also, known as the "language" used in a network.
Pulse Alarms
Pulse alarms are alarm logic systems that allow all alarms to be annunciated, even if a previous alarm has been silenced but is still present in the system.
RMS(Root Mean Square)
The RMS values of a measured quantity such as AC voltage, current and power are considered the "effective" values of the quantities. See Watt.
Revolutions Per Minute.
Radio Frequency (RF)
Any frequency within the electromagnetic spectrum associated with radio wave propagation.
Radio Interference
Radio interference refers to the interference with radio reception caused by a generator set.
Radio Interference Suppression
Radio interference suppression refers to the methods employed to minimize radio interference.
Random Access Paralleling
Random access paralleling is a paralleling operation where any generator may be the first unit to close to the bus on startup of the system. Random access systems use active synchronizing to force the second and all subsequent generator sets to close to the bus as fast as possible.
Reactance is the opposition to the flow of current in AC circuits caused by inductances and capacitances. It is expressed in terms of ohms and its symbol is X.
Reactive Differential Compensation
Reactive differential compensation (also called cross current compensation)is a method of controlling the reactive power supplied by generators in a paralleling system so that they equally share the total reactive load on the bus, without inducing significant voltage droop in the system.
Reactive Droop Compensation
Reactive droop compensation is one method used in paralleled generator sets to enable them to share reactive power supplied to a load. This system causes a drop in the internal voltage of a set when reactive currents flow from that generator. Typically, at full load, 0.8 PF, the output voltage of a set is reduced by 4% from that at no load when reactive droop compensation is used.
Reactive Power
Reactive power is power that flows back and forth between the inductive windings of the generator and the inductive windings of motors, transformers, etc., which are part of the electrical load. This power does no useful work in the electrical load nor does it present load to the engine. It does apply load to the generator and limits the capacity of the generator.
A reactor is an electrical device that applies only reactive load to a system.
Real Power
Real power is the product of current, voltage and power factor (the cosine of the angle by which current leads or lags voltage) and is expressed as W (watts).
Resistance is the opposition to the flow of current in DC and AC circuits. It is expressed in ohms and its symbol is R.
Reverse Power Relay
A reverse power relay is a relay with a wattmeter movement that senses the direction of power flow. In paralleled sets, a flow of reverse power (i.e., power flow into set) will actuate the reverse power relay and disconnect the set from the system. If one set stops and reverse power protection is not provided, the set still running will drive the set that has stopped. The generator on the set that has stopped will act as a motor.
Risers are rectangular copper or aluminum bars that connect circuit breakers, fusible switches, and transfer switches with the main system bus. As with bus bars, they are sized and assembled in multiples according to the current they must carry.
A rotor is the rotating element of a motor or generator.
A device for passing network messages over another media and sometimes protocol. Our network router is programmed as a "repeater" to create another channel on the main network bus. Each channel can have a 4,600 ft. network bus and is capable of having 44 nodes. The PowerCommand Network can have up to twenty (20) channels.
Silicon Controlled Rectifier -- a three-electrode solid-state device which permits current to flow in one direction only, and does this only when a suitable potential is applied to the third electrode, called the gate.
Selective Coordination
Selective coordination is the selective application of overcurrent devices such that short circuit faults are cleared by the device immediately on the line side of the fault, and only by that device.
Separately Derived
A separately derived on-site power system has no direct neutral connection with the neutral of the normal electrical service.
Sequential Paralleling
Sequential paralleling is a type of automatic paralleling system where the generators in a system close to the bus in a prescribed order, typically by use of a single synchronizer.
Service Entrance
The service entrance is the point where the utility service enters the facility. In low voltage systems the neutral is grounded at the service entrance.
Short Circuit
A short circuit is generally an unintended electrical connection between current carrying parts.
Shunt Trip
Shunt trip is a feature added to a circuit breaker or fusible switch to permit the remote opening of the breaker or switch by an electrical signal.
Sine Wave
A sine wave is a graphical representation of a sine function, where the sine values (usually the y axis) are plotted against the angles (x axis) to which they correspond. AC voltage and current wave shapes approximate such a curve.
A single instance where a network has been installed.
A networked device that is controlled by another device. Slave devices do not initiate data transmission. They respond to commands or requests initiated by a master device.
Soft Loading
Soft loading refers to the ramping of load onto or off of a generator in a gradual fashion for the purpose of minimizing voltage and frequency transients on the system.
Sound is considered both in terms of the sound pressure waves travelling in air (pressures superimposed on the atmospheric pressure) and the corresponding aural sensation. Sound can be "structure-borne", that is, transmitted through any solid elastic medium, but is audible only at points where the solid medium "radiates" the pressure waves into the air.
Sound Level Meter
A sound level meter measures sound pressure level. It has several frequency-weighted decibel (dB) scales (A, B, C) to cover different portions of the range of measured loudness. Sound level meters indicate RMS sound, unless the measurements are qualified as instantaneous or peak sound level.
Sound Pressure Level (SPL)
Sound pressure level is a measurement of the pressure fluctuations of a sound wave as it propagates through the air. Because of the wide range of pressures to which the ear responds, a logarithmic scale is used and is expressed as a ratio of the measured pressure referenced to a pressure of 2x10-5 N/m2 (20 m Pa) which is the threshold of human hearing at 1000 Hz. The measure is expressed in decibels (dB). The Bel unit is named after Alexander Graham Bell.
Standby System
A standby system is an independent power system that allows operation of a facility in the event of normal power failure.
Star Connection
See Wye Connection.
Star Topology
A topology where all the devices must connect to a central hub. Star topologies are relatively easy to install and manage, but can have bottlenecks occur as all the information must pass through the hub.
Starting Current
The initial value of current drawn by a motor when it is started from standstill.
The stator is the stationary part of a generator or motor. See Armature.
Steady State Rating
Steady state rating is the maximum load that a generator set or paralleling system can carry, on a continuous basis, for the duration of a utility power outage.
Surge is the sudden rise in voltage in a system, usually caused by load disconnect.
Surge Rating
Surge rating is the rating of a machine, usually in excess of its normal operating level, for which it can provide power for a very short time.
Surge Suppressor
Surge suppressors are devices capable of conducting high transient voltages. They are used for protecting other devices that could be destroyed by the transient voltages.
Switching Hub
Short for port-switching hub, a special type of hub that actually forwards information to the appropriate port based on the IP address assigned. Conventional hubs simply rebroadcast information to every port. Switching hubs forward information only to the required port.
Sync Check Relay
A sync check relay is an electrical device that monitors the phase relationship between two voltage sources and provides a signal when the voltage sources are within specific preset parameters.
In a paralleling application, synchronization is obtained when an incoming generator set is matched with and in step to the same frequency, voltage, and phase sequence as the operating power source.
A synchronizer is an electronic device that monitors the phase relationship between two voltage sources and provides a connection signal to an engine governor, to force the generator set to synchronize to the system bus.
Synchronizing Lights
Synchronizing lights are lamps connected across the line contactor of the incoming generator set. The lights indicate when the voltage waveforms of the incoming and operating power sources coincide and paralleling can be completed.
Synchronous Generator
A synchronous generator is an AC generator having a DC exciter. Synchronous generators are used as stand-alone generators for emergency power and can also be paralleled with other synchronous generators and the utility system.
A synchroscope is a meter that indicates the relative phase angle between an incoming set voltage and the bus voltage. The synchroscope pointer indicates whether the set is faster or slower than the bus and allows the operator to adjust the frequency (speed) accordingly before manually paralleling to the bus.
Telephone Influence Factor (TIF)
The higher harmonics in the voltage wave shape of a generator can cause undesirable effects on telephone communications when power lines parallel telephone lines. The telephone influence factor is calculated by squaring the weighted RMS values of the fundamental and the non-triple series of harmonics, adding them together and then taking the square root of the sum. The ratio of this value to the RMS value of the no-load voltage wave is called the Balanced TIF. The ratio of this value to three times the RMS value of the no-load phase-to-neutral voltage is called the Residual Component RIF.
Both ends of the main network bus must be terminated to avoid transmission reflections. The effective network data bus may be made up of several different physical buses. The Terminator is a RC circuit that matches the impedance of the physical media.
A resistive load placed at the end of a cable to prevent data signals from reflecting back into the data path.
In data transmission, a frame passed on a network that gives a networked device the current authority to transmit.
Token-Ring Topology
All of the devices or nodes are connected to one another in the shape of a closed loop. Ring topologies are relatively expensive to install, but they offer high bandwidth and can span larger distances.
The physical shape of a network. There are three principal topologies: multi-drop bus, token-ring, and star.
Transfer Switch
A transfer switch is an electrical device for switching loads between alternate power sources. An automatic transfer switch monitors the condition of the sources and connects the load to the alternate source if the preferred source fails.
Undershoot refers to the amount by which voltage or frequency drops below the nominal value as the voltage regulator or governor responds to changes in load.
The utility is a commercial power source that supplies electrical power to specific facilities from a large central power plant.
The volt is a unit of electrical potential. A potential of one volt will cause a current of one ampere to flow through a resistance of one ohm.
Voltage Control
The voltage control is a rheostat that sets the operating point of the voltage regulator and therefore controls the output voltage of the generator set, within its design limits.
Voltage Dip
Voltage dip is the dip in voltage that results when a load is added, occurring before the regulator can correct it, or resulting from the functioning of the voltage regulator to unload an overloaded engine-generator.
Voltage Regulation
Voltage regulation is a measure that states the difference between maximum and minimum steady-state voltage as a percentage of nominal voltage.
The watt is a unit of electric power. In direct current (DC) circuits, wattage equals voltage times amperage. In alternating current (AC) circuits, wattage equals effective (RMS) voltage times effective (RMS) amperage times power factor times a constant dependent on the number of phases. 1,000 watts equal one kW.
Watt-Hour Demand Meter
A watt-hour demand meter is similar to a watt-hour meter except that it also provides an indication of the highest kW load level achieved during operation.
Watt-Hour Meter
A watt-hour meter records the total power output at a specific point in a system. Typical recording increment is in kW-hours.

A wattmeter records power being delivered from a source to the load. Wattmeters for paralleling systems are calibrated in kilowatts (kW).
Wide-Area Network
A system of LANs connected over a large distance via a fiber optic line, telephone line, or radio wave.
Wye Connections
A Wye connection is the same as a star connection. It is a method of interconnecting the phases of a three-phase system to form a configuration resembling the letter Y. A fourth (neutral) wire can be connected at the center point.
Zero Sequence
Zero sequence is a method of ground fault detection that utilizes a sensor (CT) that encircles all the phase conductors as well as the neutral conductors. The sensor will produce an output proportional to the imbalance of ground fault current in the circuit. This output is then measured by a relay to initiate circuit breaker tripping or ground fault alarm.
Zones of Protection
Zones of protection are defined areas within a distribution system that are protected by specific groups.

In the next article, we will start explaining Generators Sizing Calculations. So, please keep following.



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