An on-site Power Management System (PMS) will provide you with security and peace of mind by having a generator standing guard, ready to provide reliable electricity during a power outage with your normal utility power supplier.
A generator insures that during utility outages, there will be enough stand-by power for such things as life-safety, security, emergency lighting, alarm systems, business operations, production schedules, elevators and UPS systems as desired by the facility manager.
Today, more and more on-site PMS operations are becoming a business decision, driven by economic risk assessment and a growing dependence on a backup source of electric power. The question is: "How much will a utility outage cost per hour in terms of lost product, lost revenue, lost data or customer dissatisfaction?" In order to reduce these risks, facilities managers are continuously evaluating their options for providing on-site PMS operations.
Our pre-packaged on-site Power Management System offers the benefit of having a stand-by and emergency source of reliable electricity. However, PMS parameters must first be established for system sizing and identifying loads. The following are a brief description of such considerations.
Minimum load capacity - PMS should be sized to provide a 20% reserve margin of power for better stability and to accommodate future load growth. However, PMS operations less then 30% rated load can lead to engine damage and reduced reliability.
Altitude and temperature - As altitude and ambient temperatures rise based on geographic locations the PMS sizing must increase for a given level of performance.
Fuel - For extended hours of operation natural gas and liquid propane (LP) models are best. However diesel generators are most common.
Voltage, phase and frequency - generator voltage, the number of phases and generator frequencies should all be specified before selecting a specific on-site power system.
Operating load characteristics - PMS powers a variety of loads having different operating characteristics. There are linear and nonlinear loads, loads that are extremely sensitive to voltage disturbances, loads that have very high starting current requirements such as motors, loads that cycle on and off, stepped sequence loading and loads that have high peak load demand when operating. The PMS must accommodate the highest anticipated starting, peak, and running load expected.
Load current harmonics - load current harmonics are created by electronic loads that consume power in bites. These harmonics cause additional heating in the alternator and generator output voltage distortion. To prevent overheating and on-site PMS is normally oversized.
Power Factor (PF) - Leading power factor (where current leads voltage) is caused by capacitive loads, oversized synchronous motors, etc. Generators have a very limited power factor load and must be controlled or it can cause a loss of voltage control and damage the generator. Lagging power factor, where current lags voltage, is more a result of the total inductance of the current. Power factor is the ratio of kilowatts to kilovolt-amperes (kva), expressed as a decimal figure (0.8) or as a percentage (80%). Three-phase generators are rated for (80%) lagging PF loads and single-phase generators for unity power factor loads. Lower power factor loads require larger alternators or generators to properly serve the loads.
Load imbalance - Single phase loads should be distributed as evenly as possible between the three-phases of a three-phase gen set in order to fully utilize the generator capacity and limit voltage imbalance.
Peak power surges - Peak power surges caused by loads that cycle on and off (i.e. medical imaging equipment, etc.) can significantly increase the size of the PMS even if the loads are placed in a step starting sequence.
Large motor loads - Calculating large motor loads over 50 horsepower is complex and when starting large motors across the line with a generator, the motor represents a low-impedance load while at locked rotor or in a stalled condition. This causes a high sustained inrush current, typically six times rated motor running current until the motor reaches rated running speed. The high current demand causes voltage dips that can destabilize the generator.
Variable Frequency Drives (VFDs) - These loads are nonlinear having rectifiers on the input to convert alternating current (AC) to direct current (DC) and an output inverter to produce variable voltage and frequency. Older technology VFDs induces significant distortion in generator output voltage and larger alternators are required to prevent overheating due to the harmonic currents. The larger alternators also reduce system voltage distortion by lowering alternator reactance.
Uninterruptible Power Supplies (UPS) - UPS systems are nonlinear and use silicon-controlled rectifiers or other static devices to convert AC to DC voltage for charging storage batteries. Larger alternators are required to prevent overheating due to the harmonic currents induced by the rectifiers and to limit system voltage distortion by lowering alternator reactance. Using the full nameplate rating of the UPS for determining loads allows sufficient capacity for generator battery charging and accommodating full UPS load capacity.
Battery Chargers - Battery chargers are nonlinear loads and requires an oversized alternator based on the number of rectifiers (pulses). Three-pulse require up to 2.5 times the steady-state running load and twelve-pulse require up to 1.15 times the steady-state running load.
Medical imaging loads - These include CAT Scan, MRI and X-Ray equipment that require that the generator be sized to limit voltage dip of 10% to protect image quality.
Lighting loads - lighting loads require lamp wattage, ballast wattage, as well as starting and running power factors to be considered when sizing the PMS.
Regeneration loads - Loads that require the power source be able to absorb power during braking (i.e. elevators) can be resolved by making sure there are other connected loads or dedicated braking resistors that absorb the regenerated power. Excessive regeneration load can cause a generator to over-speed and shut down.
On-site Power Management Systems can also help in peak demand situations to help control overall energy cost. PMS operation can be setup to run in parallel with the local utility provider in order to take advantage of utility incentive programs. Utility "interruptible" programs give businesses a credit on their electric bill for allowing the utility provider to determine when the on-site standby PMS should be operated to provide all or part of the business' load during peak demand.
From Design to complete installation, our PMS team can help evaluate the economic risk associated with a utility outage and recommend the desired level of response to that risk.
