Sizing Matters: Generator-set sizing requires analysis of parameters and loads
Written by Jim Iverson
It is important to understand the factors that affect the operation of your generator set so you can be confident you have the right equipment for your application. Get what you need with these steps.
PROJECT PARAMETERS The ļ¬rst step is to establish project parameters.
ā¢ Minimum generator-set load/capacity: Running a generator set under light load can lead to engine damage, reducing reliability. Load banks should supplement the regular loads when loading falls below the recommended value.
ā¢ Maximum allowable step voltage dip: As you reduce the maximum allowable step voltage dip during initial startup, when loads cycle under automatic controls or when high peak loads operate, you need to increase the size of the generator set speciļ¬ed. Choosing lower allowable voltage dip requires a larger generator set.
ā¢ Maximum allowable step frequency dip: As you reduce the maximum allowable frequency dip, you increase the size of the generator set needed.
ā¢ Altitude and temperature: Based on the site location, the size of the generator set must increase for a given level of performance as altitude and ambient temperature rise.
ā¢ Duty cycle: Generator-set size is also inļ¬uenced by whether the application is for standby power, prime power or utility paralleling. Standby power systems generally have no overload capability. Prime power systems generally have a minimum of 10 percent overload capacity. Generator sets that are intended to operate extended hours at steady constant load should not be operated in excess of the continuous rating.
ā¢ Fuel: The preference for gas, diesel or LP will affect generator-set choices. Often, generator sets running on gas or LP must be oversized due to derating. Emergency systems typically must be supplied by fuel stored locally.
ā¢ Phase: Select either single or three-phase. The three-phase selection permits single-phase loads but the assumption is that the single-phase loads will be balanced across the three phases.
ā¢ Frequency: Either 50 Hz or 60 Hz.
ā¢ Voltage: Voltage choices are usually a function of selected frequency.
LOADS The next step in sizing a generator set is to identify every type and size of load the generator set will power. In general, when non-linear loads are present, it may be necessary to oversize the alternator. Following is a general discussion of how various loads and electrical factors affect the sizing of generator sets.
ā¢ Power factor (PF): This is the ratio of kW to kVA and is expressed as a decimal ļ¬gure (0.8) or as a percentage. Three-phase generator sets are rated for 0.8 PF loads and single-phase generator sets for 1.0 PF loads. Lower PFs require larger alternators or generator sets to properly serve the load. Caution should be used whenever applying generator sets to leading power factor loads. Only slightly leading power factor can cause generator sets to lose voltage control.
ā¢ Single-phase loads and load imbalance: Single-phase loads should be distributed as evenly as possible between the three phases of a three-phase generator set in order to fully utilize generator set capacity and limit voltage imbalance.
ā¢ Peak loads: Peak loads are caused by loads that cycle on and off, such as welding equipment or motors. Taking cyclic loads into account can significantly increase the size of the recommended generator set despite painstaking efforts to place loads in a step starting sequence.
ā¢ Motor loads: Calculating speciļ¬c motor loads is best handled by sizing software which will convert types of motors into load starting and running requirements. For this discussion, however, it is sufficient to broadly characterize loads as high-inertia or as low-inertia loads for the purpose of determining engine power needed to start and accelerate motor loads.
ā¢ Low-inertia loads include fans and centrifugal blowers, rotary compressors, rotary and centrifugal pumps.
ā¢ High-inertia loads include elevators, single and multi-cylinder pumps, single and multi-cylinder compressors, rock crushers and conveyors.
ā¢ Motors over 50 HP: A large motor started across the line with a generator set represents a low-impedance load while at locked rotor or initial stalled condition. The result is a high inrush current, typically six times the motor rated (running) current. The high inrush current causes generator voltage dip, which can affect other systems. The manner in which generator voltage recovers from this dip is a function of the relative sizes of the generator, the motor, engine power and generator excitation forcing capability. Depending on the severity of the load, the generator should be sized to recover to the rated voltage within a few seconds, if not cycles. Various types of reduced-voltage motor starters are available to reduce the starting kVA of a motor in applications where reduced motor torque is acceptable.
ā¢ Variable frequency drive (VFD) motors: VFDs are non-linear loads used to control the speed of induction motors, induce distortion in generator output voltage. Larger alternators are required to prevent overheating due to the harmonic currents induced by the VFD and to lower system voltage distortion by lowering alternator reactance.
There are also other types of loads to consider, including uninterruptible power supplies, battery chargers and regenerative loads (for applications such as elevators, cranes and hoists where the power source is often relied upon for absorbing power during braking).
LOAD STEP SEQUENCING In many applications, the generator set is sized to pick up all loads in one step. In some applications it is advantageous to start up the loads that cause the largest starting surge ļ¬rst and then the rest in multiple steps - the largest-motor-ļ¬rst rule. Codes may require sequenced load starting to start emergency and life safety loads within as little as ten seconds, while allowing other loads longer periods of time. In general, sequenced startup allows the smallest generator set in relation to the steady state load. When cycling motor loads exist, it will still be necessary to size the generator set to start the largest cycling motor last, with all other loads connected.
FUTURE NEEDS Power use is not ļ¬xed and tends to grow over time. Therefore, any generator-set sizing exercise needs to take system expansion into consideration. Even with sophisticated software solutions, the ļ¬nal decision on generator-set size needs to be tempered with judgment. And, the more you know about the parameters that affect sizing, the better that judgment will be.
Jim Iverson is a senior applications engineer for Cummins Power Generation. For more information, visit www.cumminspower.com.
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