Utility Intertie Systems Mick Sagrillo c.1992 Mick Sagrillo There are many people currently connected to the utility grid who are interested in renewable energy systems. For them the prospect of tearing out their utility line and installing batteries seems silly. The question arisesÄhow does one use renewable energy to produce electricity without necessarily cutting loose from the utility's umbilical cord? PURPA The answer is a federal law known as PURPA, the Public Utility Regulatory Policies Act. PURPA was passed as part of the National Energy Act of 1978 under the Carter Administration. Its purpose was threefold. In response to the second OPEC oil embargo, PURPA would decrease United States dependence on foreign oil and gas. Second, PURPA would encourage the development of new sources of energy and electricity and therefore help to ensure the energy security of the US through diversification. Finally, because the target technologies were renewable sources of energy (solar, wind, hydro, and biomass), PURPA would help foster the use of environmentally benign energy technologies. The key to PURPA is that independent electricity producers, if they meet the criteria of their local utility, can generate electricity and backfeed their excess into the utility's grid. The PURPA regulations as they apply to the 800 utilities in the United States are quite complex. However, for small independent power producers like the homeowner interested in backfeeding excess electricity to the utility, PURPA can be simplified into a few rules. These same rules apply whether you generate electricity from a wind generator, photovoltaic modules, a hydro site, or a biomass generator. In exchange for meeting the rules set forth by the utility, the utility must buy the excess electricity you produce. Intertie Equipment Utility power is all alternating current. In order to feed your renewable energy generated electricity into the grid, you need equipment that can make electricity exactly in step with grid power. If your renewable energy system produces DC electricity, it must be converted to ac in order to be usable by a utility. The equipment used to change DC to utility grade ac is called a synchronous inverter. Basically, a synchronous inverter takes its signal from the utility's grid, then converts DC to ac, perfectly matching the grid's voltage and frequency. Excess power not used by the homeowner is backfed into the grid by the synchronous inverter. RE grid-intertie systems which harness spinning mechanical motion can usually be built without a synchronous inverter. These systems use induction generators. Induction generators are very similar to the induction motors that drive appliances such as your washing machine. If an induction generator or motor is spun faster that its rated speed of 1750 rpm, it will backfeed ac electricity into the grid, matching the utility's voltage and frequency. Utility Requirements For the most part, utilities are concerned that the electricity you produce is "good enough" for their grid. In virtually all instances, this is not a problem today. Both synchronous inverters and induction generators are capable of producing grid-quality power. As a matter of fact, many of these devices produce "cleaner" power than what is delivered to your house by the utility! Utilities are also concerned that, in the case of a power outage, your independent generator doesn't keep producing electricity. They argue that this would be a safety hazard to any lineman working on their power lines with the assumption that the line is dead. This is a very valid concern. All synchronous inverters and induction generator controls that are available today for wind generator applications contain something called a "line activated contactor". Wind is what I'm most familiar with; I'd assume this is true for other generating devices. This is a relay device that connects the synchronous inverter or induction generator to the utility. As the name implies, it works only when the ac line is present. When the line is down, the contactor disconnects the generating device from the utility grid. When the utility line is "hot" again, the contactor reconnects the generating device to the grid. The use of a line-activated contactor should assure the utility that the system cannot continue producing when there is a power outage. (This type of device is used quite commonly in industrial applications for similar reasons.) Finally, utilities are concerned about lawsuits. Your utility will most likely require you to carry a liability insurance policy on your generating system. In today's litigious society, this is also a valid concern. For people with homeowner's insurance coverage on their house and property, the additional coverage is not a problem. The generating system is simply added to your homeowner's insurance, just as you would add a garage, barn, silo, or radio antenna tower. Advantages and Disadvantages What's in it for you? Why would you want to connect your renewable energy system to the utility in the first place? The primary advantage of a utility-tie-in system is that batteries are not needed in the system. Remember, you backfeed excess power into the utility's grid. Therefore, the grid is actually your storage system. All of the other equipment used in the typical renewable system is still required, whether you have a stand-alone system or utility-tie-in system. The equipment includes the generating device, wire runs, inverter (note that synchronous inverters and stationary inverters used in battery systems are quite different animals), and various different control and metering systems. Because you do not need a bank of batteries, you save the considerable expense of purchasing the batteries as well as the time required to maintain the bank. The primary disadvantage of a utility-tie-in system is that when there is a power outage, your system will not operate. Even though you are producing your own electricity, you are dependent on the utility for your electricity, just like everyone else an the grid. Remember, synchronous inverters and induction generators get their "operating orders" from the grid. The other advantage to a utility-tie-in system is that, if you meet their requirements for connection to the grid, the utility must pay you for electricity that you backfeed into the grid. This can get rather sticky. Utilities are granted monopolies in their service territories. Therefore, the utility may view you as competition and try very hard to dissuade you from operating as a co-generator. Avoided Cost As defined by PURPA, utilities are required to pay you their "full avoided cost" for the electricity that you sell to them. "Full Avoided cost" as defined by most utility regulatory commissions includes the fuel cost associated with the production of electricity, the cost of the power plant itself, the transmission line cost, and operation and maintenance cost. It can even include the interest owed on the utility debt for capital construction costs of that power plant, and depreciation on the equipment. Obviously, this can become a considerable sum of money. Many utilities have chosen to drop the word "full" and pay only their avoided cost. This is usually their cost for the generating fuel only. Most homeowners don't have the resources to fight a utility on this point and therefore accept whatever the utility offers them. Dollars and Sense What's this come down to? In best case scenario, you are paid the retail cost of the electricity. This is the cost, (that is, cents per kilowatt hour) you actually pay the utility for the electricity you consume. In the worse case scenario they pay you nothing, stating that they are doing you a favor by allowing you to interconnect to their grid. This is actually illegal on their part. But unless you push the issue, your regulatory agency will not know what's going on and the utility can get away with this policy. Many utilities opt to pay the wholesale rate for the electricity they purchase from small co-generators. This will obviously be only a fraction of the retail rate. But all is not lost! You merely adjust the size of your system so that your generating device meets your need, with little to spare. In other words, you want to defer using their expensive kilowatts by producing your own, but not enough that they are able to buy your kilowatts cheaply. This is easier to do with hydro systems, where you can cut down your production by limiting water flow, or with a biomass generator. It is a bit more difficult with something like a wind generator, because you have little control over your fuel, the wind. One very good way of controlling output is to dump excess electricity in a resistive load, like a water heater or a space heater. While this is not an ideal situation, it allows you to produce hot water or heat your living space with kilowatts that you will not be making a profit on. Thereby deferring the use of a possibly more expensive fuel like fuel oil, propane, or natural gas. To Be Continued... Next time, we'll take a look at buy-back rates and options, metering options, and how to deal with your local utility to your best advantage. Access Author: Mick Sagrillo sells electricity to his utility at Lake Michigan Wind & Sun, E3971 Bluebird Rd., Forestville, WI 54213 ù 414-837-2267