Inverters for Wind Energy System
The inverter is an indispensable component of virtually all electric-generating renewable energy systems. In this article, we’ll discuss the types of inverters and the functions they provide in a wind energy system.
Inverters come in three basic types:
- grid-connected,
- off-grid and
- grid-connected systems with battery backup.
Grid-Connected Inverters
Today, the vast majority of renewable energy systems — both wind and solar electric — are grid-connected. These systems require inverters that operate in sync with the utility grid and produce electricity that’s identical to grid power. Grid-connected inverters are also known as utility-tie inverters. They convert DC electricity from the controller in a wind system into AC electricity.
Electricity then flows from the inverter to the breaker box and is then fed into active circuits, powering refrigerators, computers and the like. Surplus electricity is backfed onto the grid, running the electrical meter backward.
Grid-tied inverters produce electricity that matches the grid both in frequency and voltage. To do this, these inverters continuously monitor the voltage and frequency of electricity on the utility lines. They adjust their output so it matches grid power. That way, electricity backfed from a wind-electric system onto utility lines is identical to the electricity that utilities are transmitting to their customers.
Grid-compatible inverters are equipped with anti-islanding protection — a feature that automatically disconnects the inverter from the grid in case of loss of grid power. That is, grid-connected inverters are programmed to shut down if the grid goes down. The inverter stays off until service is restored. This feature protects utility workers from electrical shock.
Grid-compatible inverters also shut down if there’s an increase or decrease in either the frequency or voltage of grid power outside the inverter’s acceptable limits (established by the utility companies). If either varies from the preprogrammed settings, the inverter turns off.
Grid-connected inverters also come with a fault condition reset — a sensor and a switch that turns the inverter on when the grid is back up or the inverter senses the proper voltage and/or frequency.
The inverter shuts down, in part, because it requires grid connection to determine the frequency and voltage of the AC electricity it produces. Without the connection, the inverter can’t operate. In most systems, the electrical output of the wind turbine is diverted to a dump load. In others, the controller shuts down the turbine.
In a grid-connected system with battery backup the inverters disconnect from the utility during outages, but continue to operate and can draw electricity from the battery bank to supply active circuits. Such systems, however, are typically designed to provide electricity only to essential circuits in a home or business, supplying the most critical loads.
Grid-connected inverters also frequently contain LCD displays that provide information on the input voltage (the voltage of the electricity from the turbine) and the output voltage (the voltage of the AC electricity the inverter produces and delivers to a home and the grid). They also display the current (amps) of the AC output.
Grid-connected inverters for wind systems are frequently sold with the wind turbine. Manufacturers specify the grid-tied inverters for their wind turbine because every turbine has a different output voltage range. One turbine may produce AC that ranges from 0 to 300 volts. Another may produce wild AC from 0 to 200 volts. Manufacturers select inverters with an input range that corresponds to the output voltage of the turbine.
Off-Grid Inverters
Rather than receiving electricity directly from the wind turbine, off-grid inverters typically receive their input from the battery bank. They convert the DC electricity from the battery bank into AC and boost the voltage to 120 or 240 volts.
Off-grid inverters and inverters installed in grid-connected systems with battery backup also perform a number of other functions, described below. (We’ll refer to these collectively as battery-based inverters.) If you’re installing an off-grid system, be sure to read this carefully.
Battery-based inverters contain battery chargers. Battery chargers charge batteries from an external source — usually a gen-set in an off-grid system or the utility in a grid-connected system with battery backup. The battery charger in the inverter converts AC from the gen-set into DC electricity. It then feeds the DC electricity to the batteries.
In off-grid systems, battery charging gen-sets are used to restore battery charge after periods of deep discharge — if there’s not enough wind or solar and wind energy. This prolongs battery life and prevents irreparable damage to the plates.
High-quality battery-based inverters also contain high- and low-voltage disconnects. These features protect various components of a system, such as the batteries, appliances and electronics in a home or business. They also protect the inverters.
Multifunction Inverters
Grid-connected systems with battery backup require multifunction inverters. They’re also sometimes referred to as multifunction or, less commonly, multimode inverters.
Multifunction inverters contain features of grid-connected and off-grid inverters. Like a grid-connected inverter, they contain an anti-islanding feature that automatically disconnects the inverter from the grid in case of loss of grid power, over/under voltage or over/under frequency.
They also contain fault condition reset — to power up an inverter when a problem with the utility grid is fixed. Like off-grid inverters, multifunction inverters contain battery chargers and high- and low- voltage disconnects.
If you are installing an off-grid system, you may want to consider installing a multifunction inverter in case you decide to connect to the grid in the future.
Although multifunction inverters allow system flexibility, they are not always the most efficient inverters. That’s because some portion of the electricity generated in such a system must be used to keep the batteries topped off. This may only require a few percent but over time, but a few percent add up.
In systems with large battery banks, the electricity required to maintain them (to counter self-discharge) can be quite substantial. It is also worth noting that as batteries age, they become less efficient; more electricity is consumed to maintain the charge, which reduces the efficiency of the system.
If you want the security of battery backup in a grid-connected system, isolate and power only your most critical loads from the battery bank. This minimizes the size of the battery bank and reduces system losses and the cost of the system. Unless you suffer frequent or sustained utility outages, a batteryless grid-connected system usually makes more sense from economic and environmental perspectives.
Buying an Inverter
Most homes and small businesses require inverters in the 2,500 to 5,500-watt range. Which inverter should you select?
If you are installing a grid-connected wind system, the decision will be made for you by the manufacturer as noted earlier. If you are installing a battery-based system, you’ll need a battery-charging wind turbine and an inverter that’s compatible with batteries.
Most installers carry inverters they have a high degree of confidence in. Consequently, they will make a recommendation that fits your needs from their product line.
System Voltage: When shopping for a battery-based inverter, you’ll need to select one with an input voltage that corresponds to the battery voltage of your system. System voltage is the voltage of the electricity produced by the wind turbine. That is, the generators in these turbines are typically wired to produce 12-, 24- or 48-volt electricity. The batteries are wired similarly.
Because all components of an off-grid renewable energy system must operate at the same voltage, the inverter must match the source (wind turbine) and the batteries.
If you are installing a 48-volt you’ll need a 48-volt battery-based inverter, and you must wire your battery bank for 48 volts. It is a good idea to talk with the wind turbine manufacturer to obtain their input on the best inverter.
Modified Square Wave vs. Sine Wave: The next inverter selection criterion is the output waveform. Battery-based inverters are available in modified square wave (often called modified sine wave) and sine wave. Grid-connected inverters are all sine wave so their output matches utility power. What does all this mean?
Waveform refers to the voltage of AC electricity as it changes over time (alternates). Modified square wave electricity is a crude approximation of the grid power voltage pattern. It works fairly well in many appliances and electrical devices in our homes.
Although most all office and household electronic equipment and appliances can function on modified square wave electricity, they run less efficiently, producing less of what you want — i.e., light, water pumped, etc. — and more waste heat for a given energy input. When operated on modified square wave electricity, microwave ovens cook slower. Equipment and appliances that run warmer might last fewer years.
Computers and other digital devices operate with more errors and crashes. Digital clocks don’t maintain their settings as well. Modified square wave electricity may cause an annoying high-pitched buzz or a hum on TVs and stereos and may also produce annoying lines on TV sets.
It can even damage sensitive electronic equipment. Some equipment, like modern washing machines, may not operate at all on modified sine wave electricity. The computer that controls these units won’t run on it. Unless money is tight, get a sine wave battery-based inverter for an off-grid system.
Output Power, Surge Capacity and Efficiency: When selecting an inverter, even a grid-tied inverter, be sure to pay attention to continuous output, surge capacity and efficiency.
Continuous output is the power an inverter can produce on a continuous basis. It is measured in watts, although some inverter spec sheets also list continuous output in amps (to convert, use the formula watts = amps x volts).
To determine the continuous output you’ll need, add up the wattages of the common appliances you think will be operating at once. Be reasonable, though. Typically, only two or three large loads operate simultaneously.
Electrical devices with motors, such as vacuum cleaners, washing machines and power tools, require a surge of power to start up. It typically lasts only a fraction of a second. Even though the power surge is brief, if an inverter can’t provide the power, the motor won’t start.
Moreover, the stalled motor will draw excessive current and could overheat, unless it is protected by a thermal cutout. If not, it may burn out.
When shopping for an inverter, be sure to check out the surge capacity. All quality inverters are designed to permit a large surge of power over a short period, usually about five seconds. Surge power is listed on spec sheets in watts and/or amps.
Converting one form of energy to another results in a loss of energy. Efficiency is calculated by dividing the energy coming out by the energy going in. Fortunately, efficiency losses in inverters are quite low — usually only 5 to, at the most, 15 percent. It should be noted, however, that inverter efficiency varies with load.
Generally, an inverter doesn’t achieve its highest efficiency until output reaches 20 to 30 percent of its rated capacity. A 3,000-watt inverter, for instance, will be most efficient at outputs above 600 to 900 watts. At lower outputs, efficiency is dramatically reduced.
Noise and Other Considerations: Battery-based inverters are typically installed inside, close to the batteries to reduce line loss. Grid-tied inverters are almost always installed near the service entrance — where the utility service enters the house, which is near the breaker box. (Most inverter manufacturers like their equipment to be housed at room temperature.)
If you are planning on installing an inverter inside your home or office, be sure to check out the sound it produces. Inquire about this upfront. Ask to listen to the model you are considering in operation.
Some folks are concerned about the potential health effects of extremely low frequency electromagnetic waves emitted by inverters, electronic equipment and electrical wires. If you are concerned about this, install your inverter away from people. Avoid locations in which people will be spending a lot of time — for example, don’t install the inverter on the other side of a wall from your bedroom or office.
Be sure to add ease of programming to the checklist of features to consider when purchasing an inverter. Find out in advance how easy it is to change settings. Spend some time with the manual.
Stackability: Finally, when buying a battery-based inverter, you may want to select one that can be “stacked” — connected to a second inverter of the same kind. Stacking permits homeowners to produce more electricity if demands increase over time. Two inverters can be wired in parallel, for example, to double the output (amps) of a battery-based wind system.
Stacking may also be needed to supply 240-volt AC electricity to operate appliances such as electric clothes dryers, electric stoves or central air conditioning. We recommend that you avoid such appliances, especially when installing an off-grid system.
That’s not because a wind or hybrid wind and solar system can’t meet those needs, but rather because these appliances use lots of electricity and you’ll need a very large and costly system to power them.
Well-designed, energy-efficient homes can usually avoid using 240 VAC. An exception is a deep well pump, which may require 240-volt electricity. In most cases, high efficiency 120-volt AC pumps, or even DC pumps, perform admirably.
If you must have 240-volt AC electricity, purchase an inverter that can be wired in series to produce 240 VAC. Or you can purchase an inverter that produces 120- and 240-volt electricity. Or you can install a step-up transformer that converts 120- volt AC electricity from your inverter to 240-volt AC. Or you can simply install a dedicated 240-volt output inverter for that load.
Conclusion
A good inverter is key to the success of a wind system, so shop carefully. Size it appropriately. Be sure to consider future electrical needs. But don’t forget that you can trim electrical consumption through conservation and efficiency. Efficiency is always cheaper than adding more capacity!
When shopping, select the features you want and buy the best inverter you can afford. Although modified square wave inverters work for most applications, you will most likely be happier with a sine wave inverter. If you are installing a grid-connected system, you must install a sine wave inverter.
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