Grid-Connected Photovoltaic System

27.6.1 Grid-Connected Photovoltaic System

Grid-connected photovoltaic systems are composed of PV arrays connected to the grid through a power conditioning unit and are designed to operate in parallel with the electric utility grid as shown in Fig. 27.13. The power conditioning unit may include the MPPT, the inverter, the grid interface as well as the control system needed for efficient system performance [29] There are two general types of electrical designs for PV power systems: systems that interact with the utility power grid as shown in Fig. 27.13a and have no battery backup capability, and systems that interact and include battery backup as well as shown in Fig. 27.13b. The latter type of system incorpo- rates energy storage in the form of a battery to keep “critical load” circuits operating during utility outage. When an outage occurs, the unit disconnects from the utility and powers spe- cific circuits of the load. If the outage occurs in daylight, the PV array is able to assist the load in supplying the loads.

The major component in both systems is the DC-AC inverter or also called the power conditioning unit (PCU). The inverter is the key to the successful operation of the system, but it is also the most complex hardware. The inverter requirements include operation over a wide range of voltages and currents and regulated output voltage and frequency while providing AC power with good power quality which includes low total harmonic distortion and high power factor, in addition to highest possible efficiency for all solar irradiance levels. Sev- eral interconnection circuits have been described in [30, 31]. Inverters can be used in a centralized connection (Fig 27.14a for the whole array of PV or each PV module string is connected to a single inverter (Fig. 27.14b [29]. The second proposed pro- cedure is more efficient since it minimizes the losses due to voltage/current mismatching as well as it enhances it modu- larity capability. Moreover, the inverter may contain protective devices that monitor the grid and islands the grid from the PV system in case of fault occurrence [32].

For the last twenty years, researchers have been work- ing on developing different inverter topologies that satisfy the above listed requirements. The evolution of solid state devices such as Metal Oxide semiconductor Field Effect

718 Lana El Chaar

PV array

Controller protection

Power conditioning unit (a) Without battery back-up

PV array

MPPT

Charge controller

Battery storage

Power conditioning unit (b) With battery storage

FIGURE 27.13 Grid-Connected PV system.

Transistors (MOSFETs), Insulated Gate Bipolar Transistors These batteries cause losses in the PV system due to limited (IGBTs), microprocessors, PWM integrated circuits have all- availability of time and energy to recharge the battery in addi- owed improvements on the inverter. However, more research tion to the insufficient battery maintenance. Hence, a charge is being carried to ensure quality control, reliability and lower controller is then used to control the system and prevent the cost since inverters are the key for a sustainable photovoltaic battery from overcharging and overdischarging. Overcharging market.

shortens the battery life and may cause gassing while under- The main advantage of PV systems is their flexibility to be charging may lead to sulphation and stratification, which result implemented in remote locations where grid connection is in the reduction in battery effectiveness and lifetime [34–37]. either impossible or very expensive to execute. Such systems

Batteries are often used in PV systems for storing energy pro- are called stand-alone PV systems and are described in the duced by the PV array during daytime and supplying it to elec- following section.

trical loads as needed (during nighttime or cloudy weather). Moreover, batteries are also needed in the tracker systems to operate at MPP in order to provide electrical loads with sta-