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Synchronizing on West Point

Could local generators be used either to regulate voltage or control the power factor on distribution systems in New York?

Fortnightly Magazine - February 2006

the payback time for reactive power supply would be a very long time.

There are alternate possibilities, however. West Point may have some large motors used for pumps or fans that could be used to supply reactive power if they were equipped with adjustable speed drives.

Use of Adjustable Speed Drives

Adjustable speed drives (ASDs) change the voltage magnitude and frequency at the motor terminals. Adjustable speed drives are tremendous energy savers because motors that drive pumps or fans can be easily controlled to supply just the amount of water or air that is needed, with no wasted energy. When a pump or fan is used in an application where the flow requirement varies, as they often are, controlling the pump flow with an adjustable speed drive instead of a throttle valve can save energy equal to one half the horsepower rating of the motor. Adjustable speed drives often have payback periods of less than one year.

Today’s adjustable speed drives can be used to change power factor. The option to control power factor is called an active front end. With the use of adjustable speed drives at West Point, it is easy to imagine that the net power factor could be corrected to near 1.0 for the entire year. We can roughly approximate the savings as follows.

Motor-driven equipment accounts for 64 percent of the electricity consumed in the U.S. industrial sector. 2 Let’s assume that just 30 percent is large motor load. To simplify, let’s assume that 10 motors draw 200 kW at 0.8 power factor, slightly less than 30 percent of the 7-MW average load discussed above. If we equipped these motors with adjustable speed drives, we could supply about 1.6 MVAR of reactive power from these adjustable speed drives. Importantly, we could supply this reactive power at the motor terminals, where it does the most good in reducing losses.

How much energy goes into heating in the cable and transformer system that feeds the 480-V motors? Average distribution system losses account for 2 percent of plant annual energy use. 3 If we use this 2 percent to calculate a system resistance for the circuits feeding the motors, the extra current flow associated with the 0.8 power factor accounts for roughly 15 kW. Installing adjustable speed drives to correct the power factor to 1.0 would save roughly this amount of power. The 15 kW at an average energy cost of 0.062 $/kWh gives a total dollar savings of about $8,000 per year.

The cost of installing adjustable speed drives usually is amortized by the energy savings realized by the reduction of losses in the air or water flow. Drives often are paid back in six months or less. In addition, some utilities offer rebates for the installation of adjustable speed drives. For this reason, we will not consider the cost of installing the drives. If they are warranted by the conventional energy savings analysis, their cost will be quickly amortized. The savings of $8,000 per year simply will be an additional incentive.

Savings in Source Measured Energy

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