Grid-connected photovoltaic power systems are power systems energised by photovoltaic panels which are connected to the utility grid. Grid-connected photovoltaic power systems comprise of Photovoltaic panels, MPPT, solar inverters, power conditioning units and grid connection equipments. Unlike Stand-alone photovoltaic power systems these systems do not have batteries. When conditions are right, the grid-connected PV system supplies the excess power, beyond consumption by the connected load, to the utility grid.[1]
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Residential grid-connected photovoltaic power systems which have a capacity less than 10 kilowatts can meet the load of most consumers. It can feed excess power to the grid, which in this case acts as a battery for the system. The feedback is done through a meter to monitor power transferred. A typical case is when a consumer moves out of the house on vacation, the power produced by the panels will be much in excess of the demand. In this case, the excess power can yield revenue by selling it to the grid. The consumer only needs to pay the cost of electricity generated deducted from the cost of electricity consumed.
Connection of the photovoltaic power system can be done only through an interconnection agreement between the consumer and the utility company. The agreement details the various safety standards to be followed during the connection.[2]
In most cases, the grid-connected photovoltaic system will require a transformer to step up the voltage from the solar inverter to be connected to the grid. In some systems, a dc-dc converter is utilized to step up the voltage from the inverter, so that a step-up transformer is not required.[3]
Islanding refers to the condition in which a distributed generator continues to power a location even though power from the electric utility is no longer present. Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered, even though there's no power from the electrical grid. For that reason, distributed generators must detect islanding and immediately stop producing power; this is referred to as anti-islanding.
In the case of a utility blackout in a grid-connected PV system, the solar panels will continue to deliver power as long as the sun is shining. In this case, the supply line becomes an "island" with power surrounded by a "sea" of unpowered lines. For this reason, solar inverters that are designed to supply power to the grid are generally required to have automatic anti-islanding circuitry in them.
In intentional islanding, the generator disconnects from the grid, and forces the distributed generator to power the local circuit. This is often used as a power backup system for buildings that normally sell their power to the grid.
There are two types of anti-islanding control techniques:
The voltage and/or the frequency change during the grid failure is measured and a positive feedback loop is employed to push the voltage and /or the frequency further away from its nominal value. Frequency or voltage may not change if the load matches very well with the inverter output or the load has a very high quality factor (reactive to real power ratio). So there exists some Non Detection Zone (NDZ).
This method employs injecting some error in frequency or voltage. When grid fails, the error accumulates and pushes the voltage and/or frequency beyond the acceptable range.[4]