Sizing solar inverters (Part 7)

The Role of Inverters in the Design and Sizing of Solar Photovoltaic Systems

Sizing solar inverters – Inverters play a vital role in the design and function of solar PV systems. They not only convert electricity but also ensure that the energy generated matches the power needs of the users. Choosing the right inverter involves assessing various factors, including the system size, site conditions, and the energy consumption patterns of the user. Properly sized inverters can improve system efficiency and optimize energy output.

1. Types Of Inverters

Inverters, which are also known as Power conditioning units, convert direct current (DC) electricity (from batteries or solar arrays) into alternating current (AC) electricity.

They may be classified into 3 types:

1. Stand-alone Inverters are used in isolated systems not connected to the grid. The inverter draws its DC energy from batteries charged by photovoltaic arrays and supplies AC energy to the facility’s use. Many stand-alone inverters also incorporate integral battery chargers to replenish the battery from an AC source, when available. Normally, these do not interface in any way with the utility grid, and as such, are not required to have anti-islanding protection.
2. Grid-tie inverters do not provide backup power during utility outages. They are designed to shut down automatically upon loss of grid supply, for safety reasons. They need to match the phase with a utility-supplied sine wave.
3. Battery Backup Inverters: These are special inverters that are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid. These inverters are capable of supplying AC energy to selected loads during a utility outage and are required to have anti-islanding protection.

When specifying an inverter, it is necessary to consider the requirements of both the DC input and the AC output.

1. For a grid-connected PV system, the DC input power rating of the inverter should be selected to match the PV panel or array.
2. For standalone systems, the power inverters are selected based on the input battery voltage, maximum load, the maximum surge required, variations in voltage, and any optional features needed.

1.1 Standalone Inverters

Stand-alone inverters typically operate at 12, 24, 48, or 110-volts DC input and create 110- or 208-volts AC at 60 Hertz. The selection of the inverter input voltage is an important decision. Let’s understand some inverter terminology used in the manufacturers’ datasheets:

1. Power Conversion Efficiency
   This value gives the ratio of output power to input power of the inverter. Some power is lost in the conversion process. Modern inverters commonly used in PV power systems have peak efficiencies of 92-94%, but these again are measured under well-controlled factory conditions. Actual field conditions usually result in overall DC – to – AC conversion efficiencies of about 88-
   92%.
2. Duty Rating
   This rating gives the amount of time the inverter can supply its rated power. Some inverters can operate at their rated power for only a short time without overheating. Exceeding this time may cause hardware failure.
3. Input Voltage
   This is determined by the total power required by the AC loads and the voltage of any DC loads. Generally, the larger the load, the higher the inverter input voltage. This keeps the current at levels where switches and other components are readily available.
4. Surge Capacity
   Most inverters can exceed their rated power for limited periods of time (seconds). Surge requirements of specific loads should be determined or measured. Some transformers and AC motors require starting currents several times their operating level for several seconds.
5. Standby Current
   This is the amount of current (power) used by the inverter when no load is active (power loss). This is an important parameter if the inverter will be left on for long periods of time to supply small loads. The inverter efficiency is lowest when the load demand is low.
6. Voltage Regulation
   This indicates the variability in the output voltage. Better units will produce a nearly constant root-mean-square (RMS) output voltage for a wide range of loads.
7. Voltage Protection
   The inverter can be damaged if DC input voltage levels are exceeded. Remember, battery voltage can far exceed nominal if the battery is overcharged. A 12-volt battery may reach 16 volts or more, and this could damage some inverters. Many inverters have sensing circuits that will disconnect the unit from the battery if specified voltage limits are exceeded.
8. Frequency
   Most loads in the US require 60 Hz. High-quality equipment requires precise frequency regulation; variations can cause poor performance of clocks and electronic timers.
9. Modularity
   In some systems, it is advantageous to use multiple inverters. These can be connected in parallel to serve different loads. Manual load switching is sometimes provided to allow one inverter to meet critical loads in case of failure. This added redundancy increases system reliability.
10. Power Factor
    The cosine of the angle between the current and voltage waveforms produced by the inverter is the power factor. For resistive loads, the power factor will be 1.0, but for inductive loads, the power factor will drop. Power factor is determined by the load, not the inverter.

1.2 Grid Connected Inverter

For grid-connection, the inverter must have the words “Utility-Interactive” printed directly on the listing label. Here are some guidelines:

1. 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.
2. 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 are automatically drawn from the grid.
3. Surge Capacity
   The starting surge of equipment, such as a motor, 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.
4. Frequency and Voltage Regulation
   Better quality inverters will produce a near constant output voltage and frequency.
5. Efficiency
   Modern inverters 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.
6. 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.
7. Inverter Chargers
   For battery-based systems, inverters are available with a factory-integrated 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.”
8. 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.
9. Disconnects
   Automatic and manual safety disconnects protect the wiring and components from power surges and other equipment malfunctions. They also ensure the system can be safely shut down and system components can be removed for maintenance and repair. For grid-connected systems, safety disconnects ensure that the generating equipment is isolated from the grid, which is important for the safety of utility personnel. In general, a disconnect is needed for each source of power or energy storage device in the system.For each of the functions listed below, it is not always necessary to provide a separate disconnect. For example, if an inverter is located outdoors, a single DC disconnect can serve the function of both the array DC disconnect and the inverter DC disconnect. Before omitting a separate disconnect, however, consider if this will ever result in an unsafe condition when performing maintenance on any component. Also consider the convenience of the disconnect’s location. An inconveniently located disconnect may lead to the tendency to leave the power on during maintenance, resulting in a safety hazard.
10. Array DC Disconnect
    The array DC disconnect, also called the PV disconnect, is used to safely interrupt the flow of electricity from the PV array for maintenance or troubleshooting. The array DC disconnect may also have integrated circuit breakers or fuses to protect against power surges.
11. Inverter DC Disconnect
    Along with the inverter AC disconnect, the inverter DC disconnect is used to safely disconnect the inverter from the rest of the system. In many cases, the inverter DC disconnect will also serve as the array DC disconnect.
12. Inverter AC Disconnect
    The inverter AC disconnects the PV system from both the building’s electrical wiring and the grid. Frequently, the AC disconnect is installed inside the building’s main electrical panel. However, if the inverter is not located near the electrical panel, an additional AC disconnect should be installed near the inverter.
13. Exterior AC Disconnect
    Utilities commonly require an exterior AC disconnect that is lockable, has visible blades, and is mounted next to the utility meter so that it is accessible to utility personnel. An AC disconnect located inside the electrical panel or integral to the inverter would not satisfy these requirements. One alternative that is as acceptable to some utilities as an accessible AC disconnect is the removal of the meter itself, but this is not the norm. Prior to purchasing equipment, consult the electric utility to determine its requirements for interconnection.
14. Battery DC Disconnect
    In a battery-based system, the battery DC disconnect is used to safely disconnect the battery bank from the rest of the system.

1.3 Installation

An inverter should be installed in a controlled environment because high temperatures and excessive dust will reduce its lifetime and may cause failure. The inverter should not be installed in the same enclosure as the batteries because the corrosive gassing of the batteries can damage the electronics, and the switching in the inverter might cause an explosion. However,
the inverter should be installed near the batteries to keep resistive losses in the wires to a minimum. After conversion to AC power, the wire size can be reduced because the AC voltage is usually higher than the DC voltage. This means the AC current is lower than the DC current for an equivalent power load.

References:

A. Bhatia, Course No: R08-002, https://www.cedengineering.com

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