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Power topic #7005 | Page 3

In addition to determining the mathematical sum of all the electrical loads, the following factors also affect the sizing of gensets.

  • Power factor (PF): Capacitive loads, overexcited synchronous motors, etc. cause leading power factor, where current leads voltage. Generators have a very limited capacity for supplying leading power factor loads. If not controlled, these can lead to loss of voltage control and damage to the generator. Lagging power factor, where current lags voltage, is more generally the case and is a result of the total inductance of the circuit. Power Factor is the ratio of kW to kVA and is expressed as a decimal figure (0.8) or as a percentage (80%). Three-phase gensets are rated for 0.8 lagging PF loads and single-phase gensets for unity power factor loads. Lower power factor loads require larger alternators or gensets to properly serve the load.

  • Single-phase loads and load imbalance: Single phase loads should be distributed as evenly as possible between the three phases of a three-phase genset in order to fully utilize genset capacity and limit voltage imbalance.

  • Peak power surges caused by loads that cycle on and off: Such as welding equipment, medical imaging equipment, or motors controlled by some process device such as a thermostat, level control, etc. Taking cyclic loads into account can significantly increase the size of the recommended genset despite painstaking efforts to place loads in a step starting sequence.

  • Motor loads over 50 HP: Calculating specific motor loads is complex and is best handled by sizing software such as GenSize® available from Cummins Power Generation. When starting large motors across the line with a generator set, the motor represents a low-impedance load while at locked rotor or in a stalled condition, causing a high sustained inrush current—typically six times rated motor running current until the motor reaches rated running speed. This high current demand causes voltage dips that can destabilize the generator. Sizing software typically selects an alternator sized to provide the sustained locked rotor kVA of the motor load that will recover to a minimum of 90 percent of rated voltage. Also, 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.

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  • Nonlinear variable frequency drives (VFD): VFD’s employ rectifiers on the input to covert AC to DC and an output inverter to produce variable voltage and frequency. Although newer technology drives (such as PWM types) produce lower current harmonics, older technology drives induce significant distortion in generator output voltage. Larger alternators are required to prevent overheating due to the harmonic currents and reduce system voltage distortion by lowering alternator reactance.

  • Nonlinear uninterruptible power supplies (UPS): UPS’s use silicon controlled rectifiers or other static devices to convert AC voltage to DC voltage for charging storage batteries. Use the full nameplate rating of the UPS for determining load to allow sufficient capacity for genset battery charging and accommodating full UPS load capacity.

  • Medical imaging loads: CAT scan, MRI, and X-ray equipment require that the genset be sized to limit the voltage dip to 10 percent in order to protect image quality.

  • Lighting loads: In addition to lamp wattages, ballast wattages and starting and running power factors should be considered.

  • Regenerative loads: Elevators, cranes and hoists, require that the power source be able to absorb power during braking. Generally, the regeneration problem can be solved by making sure there are other connected loads or dedicated braking resistors which absorb the regenerative power. Excessive regenerative load can cause a genset to over-speed and shut down.

Multiple-gene ato systems

A single, large diesel standby generator may have suffi- cient capacity to supply all your critical loads; however, it is often advisable to divide the load among smaller, multiple standby generators to maximize reliability and operational flexibility. In the unlikely event that one standby generator does not start when needed, the others will start and supply the load by drawing on their built-in reserve capacities. Also, with multiple genera- tor systems, one unit can be offline for maintenance without affecting the availability of standby power for emergencies.

Another factor in the general sizing of a standby power system has to do with the amount of physical space that is available to house the system. Standby systems

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