the continent. Thus both magnitudes and phase angles of AC currents and voltages can be measured and stored today, although the number of measurements taken that incorporate phase (phasor measurements) in the first place is still very low. However, the availability of digitized phasor measuring at high sampling rates raises the possibility of many new and fast control applications not previously available.

The modern transmission system should have all of its high-voltage sub stations equipped with measurement systems that will be sampling critical data at rates of 30 to 120 times per second (and even faster for localized applications) with an absolute GPS time reference, allowing a more complete picture to be created of the current real-time state stability of the system. Although the hardware costs of these measurement units themselves are modest, they have to be retrofitted into the thousands of existing substations at significant cost. In this regard, developing countries such as China have an advantage over more established industrialized countries. They are able to leapfrog directly to the latest technologies for substation automation as they expand their electric grids.

On the distribution side, there are about four times as many lower-voltage substations as there are transmission substations. Distribution systems can use measurement instrumentation with slower sampling rates than those needed for transmission systems, but the flood of data requires high-bandwidth communication to use these data for control. Also, synchronizing measurements by using GPS at the low-voltage substations is not yet considered cost-effective. With regard to end users, there is a move toward replacing the existing kilowatt-hour meters for billing with intelligent (i.e., microprocessor-based) meters that can provide the customer with new buying options, such as time-of-day pricing. These meters can also bring control signals from the power company directly into appliances and other equipment on the customer side.

The ubiquity of more and faster measurements throughout the T&D system raises the issue of how to handle this proliferation of measurement data. Certainly they can be stored at the substations where they are collected and then used for various local engineering analyses as needed. However, a higher value of these real-time measurements is in helping to monitor and control the overall T&D system more efficiently and reliably. Such applications require the development of real-time data-handling software that can collect and move these data where they are needed.



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