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 (Figure 8.1). 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
Fig. 8.1: Grid-Connected Inverter. This inverter by Magnetek is designed for batteryless grid-connected PV systems.
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 pre-programmed 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.
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. To learn more about them, you may want to check out Dan’s book, Power from the Wind.
Multifunction Inverters
Grid-connected systems with battery backup require multifunction inverters. They’re also sometimes referred to as multifunction or, less commonly, multimode inverters (Figure 8.2).
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
Fig. 8.2: This multifunction inverter from Xantrex is designed for grid-connected systems with battery backup.
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. (And, as a rule, remember that older batteries lose charge faster than new batteries.)
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.
