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Smart Grid: The future of energy

Wed, 06/12/2013 - 11:23am
M. Simon, Technical Contributor

The Smart Grid is coming. It is needed in order to raise or lower prices for consumers depending on the time of day. It is needed to control consumer loads to better integrate intermittent alternative energy into the grid (industrial loads are already demand controlled by the grid operator depending on the type of service contract they have). Doing this will make the grid reliable at a reasonable cost even with the addition of solar, wind, and other intermittent sources. And it is required in order to maintain grid reliability as these alternative energy sources become a bigger fraction of grid power.

Grid frequency and power control
The electrical grid without storage is an interesting device. At all times the power delivered has to match the power demanded within a few tenths of a percent in order to keep the grid frequency within tolerance. As currently designed small loads - like households are uncontrolled so grid operators monitor grid frequency in order to determine if the generation capacity matches the load. If the grid frequency is rising there is excess generating capacity and if it is falling there is not enough. So grid frequency wanders a bit during the day as loads and generating capacity are added or subtracted from the grid. Motors have a fairly tight frequency tolerance requirement of plus or minus five percent. At 60 Hertz that is an instantaneous tolerance of plus or minus .15 Hertz. Since there are still clocks (on your microwave for instance) that depend on grid frequency for timing, the grid is adjusted once a day to keep grid timed clocks accurate on a day to day basis.

Smart Grid meters - overview
What about Smart Grid Meters? How can they help? In a number of ways. But there are some requirements they have to meet to do their job. They have to accurately measure power for billing purposes. For home electrical power the unit of measure of power is the kilowatt (kW) and the unit of energy is the kilowatt hour (kWh). The power and energy accuracy is ultimately traceable back to National Institute of Standards and Technology (NIST). The meters have to know what time it is for time of day billing. And because the meters are networked they have to have security to not only prevent physical tampering with the meters but they also have to have data security in order to prevent electronic tampering with their functioning since they are networked.

Meter accuracy and traceability
The American National Standards Institute (ANSI) has a standard for power meters that are used to bill customers. The latest one is ANSI C12.20. However it often refers back to an earlier standard ANSI C12.1. There are two classes of meters one class requires .5% accuracy for power and the other requires .2%. Electronic meters rarely use shunts for current measurements because in order to get sufficient accuracy at low currents they have to dissipate a lot of power at high currents. Instead AC power meters use current transformers (CTs). The ANSI standards require CTs that meet the IEEE standard C57.13 for accuracy. The meters need calibration that is traceable back to NIST to insure that over their lifetime they can meet the promised accuracy requirement. They must be able to meet the accuracy standard over variations in current versus voltage phase, and with various amounts of harmonic current and voltage. The phase and harmonic requirements are generically referred to as Power Factor (PF). Some of the more advanced meters can report power direction (in cases where the "load" also generates power - such as a home with solar panels installed or motors with regenerative braking), peak load (required for sizing power distribution), reactive load (a form of power factor reporting), line voltage, line current, and even the harmonic content of the loads in some cases (such harmonic content can be generated by DC rectifier power supply loads with capacitor inputs - low cost DC arc welders for home use fit into this category - and DC loads with switching power supplies which are not power factor corrected). Because the standard requires 38 tests it is generally not feasible in general literature to describe all the tests and provide manufacturing control charts so manufacturers need only say that the meters conform to the ANSI C12.20 standard.

The Smart Meter and The Internet of Things
So far the discussion of meters could apply (except for a few points) to the traditional home electrical energy meters found all over America for the last one hundred or so years. How do Smart Meters differ from those traditional meters besides being electronic rather than electromechanical? First off they can report back to the grid operator parameters previously unavailable on a house by house basis. They can also perform time of day metering which was in the past confined to industrial sites. In addition they can report continuously back to the grid operators the individual load requirements of each home. And the grid operator can perform a disconnect of individual homes remotely. In the past this type of disconnect was only used for interruptable industrial loads and was done over a dedicated telephone line. This kind of set up was not perfectly secure of course but it was very secure since easy access to the control signal was only possible at the source and destination end points.

But now the meters are connected to the internet. Your Smart Meter is part of the Internet of Things (IoT). But it is not necessarily the internet that connects you to your favorite blogger although it could be. Signals can be sent over power lines, they could be aggregated from local wireless networks and sent to a satellite and then down linked to the grid operator. There is no one size fits all solution. An urban network has different requirements and possibilities compared to a rural network. Mountainous terrain requirements will be different from the requirements of plains. What they all have in common is that the signals are no longer as physically secure as they were when the communication was carried over dedicated lines. Even in the case of  power line communications all you have to do to get access to the signals is to become a customer of the utility. And if there are wireless links in the system, that represents another point of general vulnerability.

The Smart Grid is coming
The smart grid is coming. It provides many advantages for a grid operator and there is some promise of lower utility costs if time of day pricing affects customer behavior. Reduced peak loads can delay the addition of new generating plants and transmission services. But development is not by any means completed and customers for the most part are not yet ready for remote control of their electric power. I'd say that now is the time for utilities to be trying out the technology but it is premature to be making a full scale commitment.

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