Pacify the power: GPS harness for large-area electrical grid

GPS World, April, 2005 by Dennis Erickson, Carson Taylor

Wide-area power blackouts triggered by uncommonly high usage, natural causes, human design or operating errors, or terrorism can create social and economic system collapse. A control system now in testing at Bonneville Power Administration uses GPS equipment to timetag with microsecond accuracy voltage and current measurements throughout the large-area power grid stretching from the Pacific through the Rockies. Once implemented, such an infrastructure could minimize catastrophic failures and keep the energy supply stable and available.

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Few products are more critical to a complex modern society than electrical energy. Wide-area blackouts can create catastrophe, but the proper means to pacify or calm the system grid could reduce such incidents to minor inconvenience.

Disturbances on a large power grid propagate through the system in milliseconds to fractions of a second. Major blackouts occur when an unconfined system disturbance cascades throughout the power grid. Precise measurements of the voltages and currents in the grid, including both the magnitudes and the phase angles between voltages and currents, can initiate the process of preventing or at least minimizing such events.

The phase angle between voltages and currents is very important as it relates to the power factor. If this angle is zero degrees, the load is purely resistive; if the angle is not zero, there is some reactive loading, or additional storage in the system. Also when voltage angles between regions change rapidly, something can be wrong.

With that data, pacifying the grid requires that the disturbance be diagnosed, and counteractive actions taken. One of the most promising ways to stabilize the grid is to selectively shed load (or trip generators) and/or selectively switch reactive power compensation devices (capacitor/reactor banks). Operating and stabilizing such a system over a wide area requires an infrastructure in place to take these voltage and current measurements at selected generation and transmission sites and time-tag them to within microseconds, using GPS equipment.

The transmission or latency times from these measurement sources to the control facility must also be deterministic (time-stable and predictable) and as short as possible, to ensure that measurements are made and analyzed quickly to enable a timely response to the problem. Finally, this requires installation of suitable reactive power compensation equipment; Bonneville Power Administration (BPA) has such an infrastructure in place.

The Wide Area stability and voltage Control System (WACS), a BPA demonstration project, with assistance from Ciber, Inc and Washington State University, uses phase angle data to provide grid pacification. This system is designed to prevent or minimize outages, and also to increase power delivery capability. Once testing is complete, BPA plans to implement it over a wide-area grid such as the western United States and Canada interconnected high voltage transmission system (Figure 1).

Pacifying Large-Area Grids

Ordinarily, slight variations can occur between phase angles in wide areas, but when one or more regions swing suddenly and dramatically one way or the other, this indicates a possible disturbance. Dealing with local disturbances traditionally involves relaying out faulty equipment and isolating the problem before it cascades. With the complexity of high-voltage transmission networks, however, failures or very severe disturbances may cause other sections to overreact, creating instability. The most promising solution treats the problem as a whole and regulates the grid's frequency and voltages within manageable limits, by generator or load shedding, and/or inserting reactive compensation devices. These tactics can modify the magnitude and phase relationship between voltages and currents in the power system network.

The overall WACS system consists of several key components:

* A variety of GPS Universal Coordinated Time (UTC)-synchronized phasor measurement units (PMUs) read the voltage and current at selected sites within the interconnected transmission grid and convert these quantities into phasors (see later discussion), using a local GPS-synchronized time and frequency source as a reference. This records not only the local phase relationship between the voltage and current (to obtain watt and var readings) but also the absolute phase and time relationship with other stations in the system.

* An extensive fiberoptic network ensures that transmission latency times remain very low, on the order of 1-3 electrical cycles, or 17 to 50 milliseconds (ms), a critical factor, and this ensures that transmission will be as deterministic as possible.

* A phasor data concentrator (PDC) at the central command site collects all the data packets from the PMUs, time correlates them, and forms a message packet used by the WACS controller. BPA developed and implemented a system currently operating at 30 packets per second, that could be upgraded to 60 packets per second.