Solar Energy Systems - Solar Inverter - Solar Panel Inverter
Inverters take care of four basic tasks of power conditioning:
• Converting the DC power coming from the PV modules or battery bank to AC power
• Ensuring that the frequency of the AC cycles is 60 cycles per second
• Reducing voltage fluctuations
• Ensuring that the shape of the AC wave is appropriate for the application, i.e. a pure sine wave for grid-connected systems
Criteria for Selecting a Grid-Connected Inverter – The following factors should be considered for a grid-connected inverter:
• A UL1741 listing of the inverter for use in a grid-interactive application
• The voltage of the incoming DC current from the solar array or battery bank.
• The DC power window of the PV array
• Characteristics indicating the quality of the inverter, such as high efficiency and good frequency and voltage regulation
• Additional inverter features such as meters, indicator lights, and integral safety disconnects
• Manufacturer warranty, which is typically 5-10 years
• Maximum Power Point Tracking (MPPT) capability, which maximizes power output
Most grid-connected inverters can be installed outdoors, while most off-grid inverters are not weatherproof. There are essentially two types of grid-interactive inverters: those designed for use with batteries and those designed for a system without batteries.
Power Quality – Inverters for grid-connected systems produce better than utility-quality power. For grid-connection, the inverter must have the words “Utility-Interactive” printed directly on the listing label.
Voltage Input – The inverter’s DC voltage input window must match the nominal voltage of the solar array, usually 235V to 600V for systems without batteries and 12, 24 or 48 volts for battery-based systems.
AC Power Output – Grid-connected systems are sized according to the power output of the PV array, rather than the load requirements of the building. This is because any power requirements above what a grid-connected PV system can provide is automatically drawn from the grid.
Surge Capacity – The starting surge of equipment such as motors is not a consideration in sizing grid-connected inverters. When starting, a motor may draw as much as seven times its rated wattage. For grid-connected systems, this start-up surge is automatically drawn from the grid.
Frequency and Voltage Regulation – Better quality inverters will produce near constant output voltage and frequency.
Efficiency – Modern inverters commonly used in residential and small commercial systems have peak efficiencies of 92 percent to 94 percent, as rated by their manufacturers. Actual field conditions usually result in overall efficiencies of about 88 percent to 92 percent. Inverters for battery-based systems have slightly lower efficiencies.
Integral Safety Disconnects – The AC disconnect in most inverter models may not meet requirements of the electric utility (see section “Disconnects”). Therefore, a separate exterior AC disconnect may be required even if one is included in the inverter. All inverters that are UL listed for grid-connection include both DC disconnects (PV input) and AC disconnects (inverter output). In better inverters, the inverter section can be removed separately from the DC and AC disconnects, facilitating repair.
Maximum Power Point Tracking (MPPT) – Modern non-battery based inverters include maximum power point tracking. MPPT automatically adjusts system voltage such that the PV array operates at its maximum power point. For battery-based systems, this feature has recently been incorporated into better charge controllers.
Inverter-Chargers – For battery-based systems, inverters are available with a factoryintegrated charge controller, referred to as inverter-chargers. Be sure to select an inverter-charger that is rated for grid-connection, however. In the event of a grid power outage, use of an inverter-charger that is not set up for grid-connection would result in overcharging and damaging the batteries, known as “cooking the batteries.”
Automatic Load Shedding – For battery-based systems, the inverter can automatically shed any unnecessary loads in the event of a utility power outage. Solar loads, i.e. the loads that will be kept powered up during the outage, are connected to a separate electrical sub-panel. A battery-based system must be designed to power these critical loads.
Warranty – Inverters typically carry warranties of 5 years, although the industry is moving toward a 10-year warranty. The transformer and solid state components of an inverter are both susceptible to overheating and damage from power spikes, reducing its life. Transformerless inverters, long available in Europe, are beginning to move into the U.S. market.
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