Inverters and batteries
Introduction
A grid inverter converts direct current (DC) into alternating current (AC) suitable for injection into a power grid, typically 120 V RMS at 60 Hz or 240 V RMS at 50 Hz. Grid tie inverters are used between local generators of electrical power: solar panel, wind turbine, hydroelectric and the grid.[1].
To efficiently and safely inject electrical power into the grid, grid-connected inverters must precisely match the voltage and phase of the grid's AC sinusoidal waveform.
Some electricity companies pay for the electrical energy that is injected into the grid.
Payment for injected power
Electricity companies, in some countries, pay for the electrical energy that is injected into the electrical grid. Payment is made in several ways.
With the net balance, the electric company pays for the net power injected into the network, recorded by a meter at the customer's facilities. For example, a customer can consume 400 kW·h during a month and can return 500 kW·h to the grid in the same month. In this case, the electricity company would pay the balance of 100 kW·h of energy that is returned to the grid. In the US, net balance policies vary by jurisdiction.
Net metering, based on a contract with a distribution company or other energy authority, is where the customer is paid for the electrical energy injected into the grid.
In the United States, grid-interactive power systems are specified in the National Electrical Code, which also has requirements for grid-interactive inverters.
Operation
Grid tie inverters convert DC electrical power into AC power suitable for injection into the power company's grid. The Grid Tie Inverter (GTI) must adjust to the grid phase and maintain the output voltage slightly higher than the grid at any time. A modern, high-quality grid-tie inverter has a fixed unity power factor, meaning its output voltage and current are perfectly aligned, and its phase angle is within 1° of the AC mains. The inverter has an on-board computer that detects the current AC grid waveform and outputs a voltage that corresponds to the grid. However, it may be necessary to supply reactive power to the grid to keep the voltage on the local grid within permissible limits. Otherwise, in a segment of the grid with considerable power from renewable sources, voltage levels could rise too high at peak production times, for example around midday with solar panels.