Inverters

The inverters are used to convert the DC power generated from the PV array or supplied by the batteries to AC power needed by the electrical loads within buildings. The capacity of inverters expresses the maximum electrical power level that can safely convert and ranges from few hundred watts (W) for small systems to Megawatts (MW) for large-scale utility grid applications. There are three inverter types depending on the PV system applications:

  • Stand-Alone Inverters. These types of inverters are specified for standalone PV systems, and their source of DC power is the battery. Thus, their capacity depends on the battery capacity.
  • Grid-Connected Inverters. These inverters are specific to grid-connected PV systems, and their source of DC power is the PV array. Thus, their capacity can be limited to the maximum power rating of the PV array. Several types of grid-connected inverters are available, including the following:

a. Module inverters are generally integrated within the PV modules and are suitable for small systems ranging from 200 W to 300 W.

b. String inverters with 1-10 kW capacities are applicable for residential and small commercial buildings that operate using single-phase loads.

c. Central inverters with capacities up to 500 kW are specified for commercial buildings and are capable of meeting three-phase loads.

d. Utility-scale inverters applicable for solar farms with capacities up to 1 MW and can include transformers and switchgears for easy integration with the grid distribution systems. Special controls are typically required to ensure seamless integration of these inverters to the grid.

Multimode Inverters. These types are battery-based interactive inverters that can generate AC power to regulate the PV array charging process of the battery. During grid power outages, these inverters act like stand-alone units by serving the loads through the battery and/or the PV array.

Inverters are made of high-speed switching transistors that convert DC to AC power. Specifically, metal-oxide semiconductor field-effect transistors (MOSFETs) are considered for low-voltage inverters, while insulated gate bipolar transistors (IGBTs) are common for high-voltage inverters. Typically, MOSFET-based inverters are suitable for small to medium systems ranging from 1 kW to 10 kW capacities. The IGBT-based inverters are specified for large systems of at least 100 kW power rating. For grid-connected inverters, high-quality conversion using sine wave are typically required. For stand-alone converters, square wave conversion is often adequate. In addition to input and output voltages, currents, and power ratings, the inverters are characterized by their power quality and conversion efficiency. It is desirable that the inverter efficiency, defined as the ratio of AC power output and the DC power input, remains high generally above 95% at different operating conditions.

Conductors and Protective Devices

PV systems configured as stand-alone or grid-connected have several wires that need to be sized adequately to ensure safe operation. For the case of grid-connected PV systems, the w'ires can include the following:

  • • Wires that serve DC pow'er subsystems, including conductors that connect various PV modules, guide the entire PV array output current and serve the batteries as well as the inverter input.
  • • Wires that feed AC power subsystems, including conductors that serve the inverter output and connect to the building’s main panel.

The size of these wires follows the general guidelines specified by the local electrical codes. In the United States, NEC set the sizing methods and procedures for determining the required wire sizes depending on various criteria, including the electrical demand load, voltage drop limit, and short-circuit current tolerance (NEC, 2017; Krarti, 2017). Specifically, the following currents are used to size the conduction of most PV installations:

  • Maximum Current of the PV Source Circuit. This current is the sum of the short-circuit currents of all the modules connected in parallel within the PV array.
  • Maximum Current of the PV Output Circuit. This current is the sum of all the maximum currents of the parallel module circuits that make up the PV array.
  • Maximum Current from the Inverter Output. This current is associated w'ith the AC power converted by the inverter from the DC pow'er input.
  • Rated Input and Output Currents of the Battery System. The input current is specific to the DC power entering the inverter, w'hile the output current is a function of the AC power exiting the inverter.

For safety, conductors as well as the overcurrent protection devices (i.e., OCPD) are sized using the demand factor of 125% of the maximum currents that flow' continuously (i.e., more than 3 hours) in the AC or DC circuits. The required ampacity of the conductors depends on the demand load but also on the OCPD rating, the fill factor (i.e., the number of the conductors placed in the same conductor or raceway), and the ambient conditions (i.e., the actual temperature levels). In particular, it is important to ensure that conductors are sized so their ampacity under operating conditions is larger than that of the OCPD rating. Moreover, the conductors have to be selected to withstand various outdoor conditions, including moisture (i.e., with wet or W rating) and heat (i.e., with temperature rating of 90°C). For outdoor circuits, USE-2 conductors are often used for rooftop PV installations. For indoor circuits, XHHW-2 are recommended even though USE-2 can still be specified if they are rated to be used indoors.

Grounding Protection

The PV installations need to have both DC and AC equipment grounding conductors. These conductors need to be sized using local code guidelines such as those set by the NEC requirements. Moreover, the PV systems specific to residential and commercial buildings need to have ground-fault current protection systems.

 
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