Following on from our recent blog post about hybridization, we would like to introduce you to the topic of grid connectivity.
All equipment that is connected to an electric grid has an effect on it, either directly or indirectly. And AC drives are no exception.
Aside from solar-pump-style applications, where energy is fed directly from an off-grid power source to an electrical machine, any, and all, AC drives are connected to one or more grids. In some cases, keeping the grid in question as stable as possible can be solely up to the AC drive. Alternatively, it could be that the AC drive is just an actuator on the grid, but it may still have responsibility to actively operate in a way that causes minimal disturbances on the grid. On the other hand, it can still be an actuator, but with responsibility to support the grid through its actions, or actively negate the disturbances of other equipment on the grid.
The grids can vary from national to island grids with peak powers ranging from tens of kilowatts to megawatts, and from 50/60 Hz to 400 Hz. They can be stable or less reliable, and, depending on the requirements set by the use case, the grid can even be generated in different ways by the AC drives or just supported by them. All these different types of grids, the roles the AC drives play inside them, and the requirements from the grid owners bring new requirements for them.
As the AC drives are not simple linear loads, they distort the input currents’ waveforms. In short, they generate harmonics. Depending on the size of the AC drive in question, these harmonics can have rather nasty effects on the grid. In this series, we are going to concentrate on the functional actions that AC drives can be programmed to do to support the grid.
Let’s take the simplest possible use case: a frequency converter running a motor while connected to the national grid. For frequency converters, the grid-supporting functions are limited and they always have an effect on the process itself. As such, it depends on the process if grid-supporting actions are allowed. Without any special options, the visibility a frequency converter has on the grid is usually limited to the input voltage, either from direct measurement of it or from a DC-voltage level. To support the grid by utilizing only the voltage information, the frequency converter can ease up the load if the AC voltage level deviates lower than what is defined as nominal AC voltage. If the AC voltage is getting higher than nominal, then the load could be increased. In certain processes, these load adjustments can be practically unnoticeable, like in HVAC, but the effect on the grid can be a big one. While the contribution of, for example, a single 1.5 kW HVAC drive may not be much, imagine 1000 of them supporting the grid in a skyscraper or large mall. This operation is done well inside the limits of the AC drive’s own under- and over-voltage regulators, which are considered the drive’s own protection functions.
The next installments in this blog post series concentrate on more advanced and demanding uses of inverters in grid support and actual maintenance, and not forgetting hybridization. These uses vary from simple AFE-based systems aimed at simple supporting actions to Grid Converter-based grid creation and maintenance combined with energy storage.
Author: Timo Alho, Product Manager, Applications, Danfoss Drives