1. Field of the Invention
The present invention relates generally to power distribution systems and more particularly to an improved power management system for reducing peak power consumption and improving consumption efficiency.
2. Description of the Prior Art
Power management is the supervising and controlling of a number of power consuming devices in such a manner as to produce energy cost savings. Control of various electric devices, commonly called "loads", is done to achieve savings in two basic ways -- demand limiting and consumption limiting. To further understand what demand and consumption limiting are, the terms "demand" and "consumption" must be defined.
Demand is a term which relates to one aspect of the measurement of power for most commercial users of electricity. As more and more electrical devices in a building are turned on, a greater and greater load on the power company equipment occurs. This load is measured in kilowatts and is referred to as demand. The power meter in the building monitors this demand and records the highest occurrence of demand for the month. The power company bill will reflect this demand "peak" and charge for it -- even if a portion of the demand value was due only to some extra heavy usage which lasted for just a few minutes on only one day of the billing period.
Consumption is a more widely understood aspect of power measurement and relates to the units of energy used, regardless of how fast or slowly they are used. These units of energy are "kilowatt-hours". The power compamny bill will reflect charges for the total number of kilowatt-hours used during the billing period.
The control of demand and consumption requires two different approaches. First, demand control necessitates the monitoring of the total incoming power to the facility. When the demand value is known at all times, loads can be turned off whenever an increase or "peak" begins to occur. In this way, a power management system will "orchestrate" the operation of a portion of the loads in a building such that power usage becomes smooth and continuous -- demand peaks are prevented. This type of control will result in a lower demand reading on the power meter, and thus, a lower charge for demand on the monthly bill.
Consumption control is much different from demand control because it deals directly with operation efficiency not simply operating orchestration. Consumption savings cannot be achieved unless something can be turned off which is running wastefully or needlessly. The key to all consumption savings if the avoidance of energy waste. Some devices, such as refrigeration equipment, have significant inherent operating waste -- inefficiency. It has not been until recent years that manufacturers have been pressed to design and produce electric motors and other equipment which would do more work for each kilowatt-hour used. This does not mean, however, that otherwise inefficient devices cannot be goverened in such a manner that they will operate more efficiently. This is the job of a power management system.
Another area in which power management is useful is in startup, following a shutdown or a power failure. If all energy consuming devices are allowed to start simultaneously, each requiring a large starting current, the combination can result in a large power surge which can exceed the wiring capacity of many older buildings. As a result, in the past it has been necessary to manually turn off all of the circuit breakers and then turn them on sequentially to avoid the surge. A well-designed power management system can accomplish this automatically.
Finally, power management can extend equipment life by eliminating short cycling. Short cycling is a condition where a compressor or other load cycles on and of rapidly. This of course causes increased contact erosions and wear as well as fatiguing of the mechanical components of the system. By controlled cycling of the power to such equipment for relatively long periods of time, short cycling can be elimiated and equipment life can be extended.
Many prior art devices periodically switch groups of loads off for a preset time. The period of the sequence is often keyed to the period used by the utility companies in computing peak demand. Typically such periods are 32 minutes long. Since the off time can usually be varied, the duty cycle can be set, but it must be based on a 32-minute base irrespective of the most efficient period for each compressor. Because a fixed base period is used, it is necesssary to group several loads together in order to achieve significant off times. Since the grouping is at least in part dictated by the power consumed by each load, it is difficult to optimize the off time of a particular load.
Period art management systems are normally unable to sense the needs of each compressor. Rather than turning off an operating compressor to reduce the peak demand, the system instead insures that a load will not come on by removing power to it. Thus, power may be available to a compressor which is not in need, while power is unavailable to one which has been off for some time and is in need of power to restore the temperature to within preset limits, such as in a critical meat case.
Many prior art devices also require timing signals from the utility company's meter in order that both the period and rate of power consumption can be determined. This typically requires special equipment and connections by the power company and thus increases the cost to the consumer. Additionally, many of these systems operate only during peak demand periods to reduce the peak demand, ignoring reductions in consumption which may result from cycling the loads during lower demand times such as at night.