Generator Sizing Calculations A generator set consists of an engine driver and an electri- cal dynamo known as an alternator or generator. The prime mover delivers the mechanical power to generate the elec- tricity, and the dynamo creates the voltamperes of electrical apparent power. Ignoring inefficiencies or losses, the kilo- watt output of the dynamo is equal to the kilowatt power capability of the prime mover, the engine. On the other hand, the power factor of the load may cause the kilo- voltampere flow to greatly exceed the numeric value of kilo- watts. Accordingly, the kilowatt rating of a generator set is limited by the output of the engine, whereas the kilo- voltampere rating of the generator set is limited by the elec- tric current output of the dynamo connected to the engine. Engine horsepower output is determined by the burning of fuel and air and is almost always power limited by air. The larger the number of pounds of air the engine can take in over a certain time interval, the more oxygen is available to burn the fuel, and the more power the engine can produce. Air is heavier and contains more oxygen molecules when it is cold and when its pressure is high. Therefore, an engine operating on a cold day at sea level can produce more horsepower than Chapter 6 195 v Copyright 2001 by The McGraw-Hill Companies, Inc. Click here for Terms of Use. an engine operating on a warm day on a mountaintop. It is little wonder that engine manufacturers go to great lengths to provide the largest “charges” of air possible for their engines. In gas-turbine engines this can take the form of a zero-stage turbine compressor section or an inlet air chiller, and in reciprocating engines it can take the form of an exhaust-driven turbocharger set with intercoolers. In unusual cases, engine horsepower is limited by the heat content of the fuel. For example, in dual-fuel recipro- cating engines, the engine can produce about 40 percent more power from diesel fuel than it can from methane fuel, and this is true strictly because of the limited amount of heat energy contained in the methane fuel for each intake stroke charge of fuel-air mixture compared with the amount of heat energy contained in an equivalent volume of diesel fuel–air intake stroke charge. Sizing a Gas-Turbine Generator Set for a Known Kilowatt Load The gas-turbine generator set consists of two distinct parts, and each must be sized with a separate calculation. The engine must be rated with sufficient horsepower (this can be measured in kilowatts) to drive the watt load, and the elec- trical dynamo must be sized to allow selected motors to start while other selected electrical loads are in operation. Except in very unusual electrical systems where there is one very large motor load, the manufacturer of the generator set matches an electrical dynamo with the engine that is kilo- watt rated at or above the maximum engine kilowatts and is rated in kilovoltamperes at 125 percent of its kilowatt rat- ing. (This is usually stated by saying that the dynamo is rat- ed at 80 percent power factor.) The method of calculating the generator size is as follows: A1. Sum all running loads, including approximately an 11 percent motor inefficiency factor for motor windage and friction. A2. Add the largest nonsimultaneous load that must be started while its counterpart motor is still running. For 196 Chapter Six example, if there is a pump A and a backup pump B, then only pump A is used in the A1 calculation. In this A2 calcu- lation, pump B is added into the load sum because both pumps A and B can be running simultaneously. A3. Call the load sum determined in step A2 load A. B1. Select the engine-generator set whose International Standards Organization (ISO) kilowatt rating is slightly greater than load A. Note that the ISO rating of a gas-turbine engine is at 59°F at mean sea level. B2. Derate the ISO rating of the engine-generator set for the maximum historical ambient temperature recorded for the site. Do not use the maximum design temperature stat- ed for the facility, since there will be days that are hotter than the design temperature, and it is unacceptable for the engine-generator set to trip off on underfrequency (the result of the engine not being able to deliver enough power for the load) just because a day occurs that is hotter than the design temperature. Call this resulting temperature rating rating B. This value is determined either from the engine manufacturer’s performance curves for the machine or from derating formulas for other than ISO conditions. Either of these normally requires specific performance information for each machine. In the absence of specific performance data on the gas-turbine engine used, an approximation of the reduction in power can be made by assuming that engine shaft horsepower will be reduced by 1 percent for every 2.7°F above the ISO rating of 59°F. B3. Derate rating B by 4 percent for dirt buildup on the turbine blades, and call the resulting rating rating C. B4. Derate rating C by 3 percent for blade erosion losses after 25,000 hours of operation (this is immediately before a scheduled hot-section overhaul). Call the resulting rating rating D. C1. Make certain that rating D is greater than load A. If it is not, then incorporate additional generator sets into the system or select a machine with a greater kilowatt rating. The amount by which rating D is greater than load A is the percent spare capacity for future loads. Generally, the amount of this spare capacity is within a 15 to 30 percent Generator Sizing Calculations 197 window, depending on the project and the certainty of the known loads at the time of the generator sizing calculation. Additional spare capacity occasionally can be provided through load shedding. C2. So that electrical power can still be provided while one generator set is down for maintenance, common practice is to add a second generator that is of the same kilowatt rat- ing as the first generator and normally to operate both gen- erator sets simultaneously at approximately 50 percent load. Another commonly used possibility is to provide three generator sets, two of which can operate the load, while the third is the running spare. An example of sizing the gas-turbine generators for an industrial plant is given in Fig. 6-1. Sizing a Reciprocating Engine-Driven Generator Set for a Known Kilowatt Load Sizing a reciprocating engine-driven generator set for a known kilowatt load is similar to sizing a gas-turbine generator, as described earlier, except that the derating of the engine due to blade-tip burning, dirt buildup, and turbine blade fouling need not be considered. In addition, responses to the altitude of operation are different for the two types of engines, as are their responses to ambient intake air temperature. This is so because most reciprocating engine-driven generator sets incor- porate turbocharging and intercooling to effectively negate much of the density altitude characteristics of low-pressure “thin” air and high-temperature “thin” air. Accordingly, as long as the largest motor that must be started by the reciprocating engine-driven generator set is less than 10 percent of the over- all rating of the generator set, the problem simply becomes one of determining the kilowatt value of the load, selecting the next larger commercially available reciprocating engine-driven gen- erator set, and making certain that the kilovoltampere rating of the electrical dynamo bolted to the engine is large enough to provide current to the running and starting loads without exceeding the allowable voltage dip for the system. Normally, the allowable voltage dip is around 80 percent of normal sys- 198 Chapter Six tem voltage at the terminals of the motor being started and around 90 percent of the normal system voltage at the genera- tor switchgear bus. A good rule of thumb to achieve this is to assume that the generator has a subtransient reactance of approximately 15 percent (normally this is a valid assumption) and then to select a generator set whose kilowatt rating exceeds the total kilowattage of the summed loads [setting 1 kilowatt (kW) equal to 1 horsepower (hp) for this calculation] and whose kilovoltampere rating is equal to or greater than 50 Generator Sizing Calculations 199 Figure 6-1 Solve for generator site rating given load, temperature, and altitude. percent of the total of the running kilowattage plus six times the kilowatt rating of the largest motor to be started while the other loads are running. For example, if the running load is 1000 kW and a 150-hp motor is to be started meanwhile, the minimum kilowatt rating of the generator would be 1000 kW ϩ 150 kW, or 1150 kW, and the minimum kilovoltampere rat- ing of the generator would be 0.5 ϫ [1000 kW ϩ 6(150)], or 950 kilovoltamperes (kVA). Select a generator set that meets or exceeds both the 1150-kW and the 950-kVA requirements and that has a voltage regulator that can offset approximately a 15 percent internal voltage dip (15 percent regulation) while the large motor is starting. In this case, the usual offering of a gen- erator set manufacturer would be a set rated at 1150 kW that carries a kilovoltampere rating of 1150/0.8, or 1437 kVA. If a larger motor is to be started, then the kilovoltampere calcula- tion would have more largely determined the size of the gen- erator set than would the running kilowatt load. Sizing of Generator Feeder Conductors As is shown in Fig. 6-2, solving for the required ampacity of generator power conductors and generator overcurrent pro- 200 Chapter Six Gas-turbine generators arrive at the installation site prepackaged on skids. Figure 6-2 Solve for generator overcurrent protection and the wire ampacity of generator conductors. 201 tection devices requires unique methodologies. The genera- tor kilowatt rating is set by the engine power, but the kilo- voltampere and current-carrying capabilities of the electrical dynamo bolted to the engine depend on the size of the dynamo and its reactance values. Almost universally, the generator set kilovoltampere rating is designed to be 125 percent of the generator kilowatt rating; and this is 202 Chapter Six Diesel engine generator set. done to accommodate the lagging power factor of the loads, since quadrature-component currents do not require engine power because they do no work. Figure 6-1 shows how to protect the electrical dynamo in the generator set from over- current and how to size and protect the conductors that extend from the generator set toward the loads. Generator Sizing Calculations 203 . sizing the gas-turbine generators for an industrial plant is given in Fig. 6- 1 . Sizing a Reciprocating Engine-Driven Generator Set for a Known Kilowatt Load Sizing a reciprocating engine-driven generator. power conductors and generator overcurrent pro- 200 Chapter Six Gas-turbine generators arrive at the installation site prepackaged on skids. Figure 6- 2 Solve for generator overcurrent protection. reciprocating engine-driven generator sets incor- porate turbocharging and intercooling to effectively negate much of the density altitude characteristics of low-pressure “thin” air and high-temperature