Grid operations on a very high level can be divided into two distinctly different operations: either the grid tied inverter is supporting the grid or actively maintaining it.
Grid support refers to traditional Active-Front-End (AFE) operation. This means the inverter is working in current control mode following the grid’s voltage and frequency while being capable of performing grid supporting actions, for example, acting according to Grid Code standards.
AFE operation is used everywhere. In heavy industry use-cases, AFE operation is used to power the other inverters connected to the DC-link controlling the motors, while at the same time minimizing the harmonic impact on the factory grid. In power generation, AFE operation is responsible for converting the power generated by the generator side inverters to good quality AC for the grid.
VACON® NXP Grid Converter can utilize AFE operation. It can also actively control the grid’s voltage and frequency and perform expected actions from µGrid generating equipment. For instance, it can isolate a failed component by feeding short-circuit current injection and, as a result, help the island grid recover.
µGrid operation mode is used where our equipment is required to actually create or at least actively maintain an island grid, such as in large cruise ships. They are integral parts of, for example, shaft generator systems, where they work as the grid-side inverter to keep the ship’s grid voltage and frequency stable. At the same time, the generator-side inverter produces energy from the rotor shaft with changing speed rotated by the ship’s diesel engine.
AFE and µGrid operation modes are based on the same bi-directional hardware solution, inverter + LCL-filter. Both can be utilized in hybridization solutions, either connected directly to energy storage or using a DC/DC converter. The exact needs of the customer have to be mapped out together with Danfoss experts so that the optimal solution can be built.
Then it is possible in special cases to use normal motor-use-style software to create small island grids (as long as the necessary parameters are available). This requires also a sine filter and the operation does not have any functionalities normally required from grid connected equipment. It can be used in small islands with very high frequency requirements where there is no other power generating equipment connected. Operating in this way is not very common due to limited functionalities. For example, there is no possibility for short circuit current injection, and the use cases are limited to special cases such as in the aviation industry.
This was just a very short peek into the different ways Danfoss Drives’ grid-side inverter products can support, maintain and create electric grids and what is actually meant by ‘support’ and ‘maintain’ in this context. This post only concentrated on the individual inverters interacting with the grids. Next time we will dive a little deeper into how larger systems built from different components can interact with the grids in tandem.
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Author: Timo Alho, Product Manager, Applications, Danfoss Drives