Why Do Power Plants Need Reactive Power Generation?
A common analogy likens reactive power to the foam on a beer mug—seemingly unnecessary but actually essential. In alternating current systems, reactive power is crucial for voltage regulation. Without it, devices like induction motors—the most common electrical consumers—would fail to start.
According to Fingrid’s System Technical Requirements (VJV2018), power plants must have the capacity to produce and consume reactive power. Specifically, a plant should be able to generate at least one-third of its rated active power in reactive power across all operating conditions, provided the voltage remains within normal limits. For inverter-based plants, this requirement is assessed at the connection point or at the high-voltage terminals of the main transformer.
Where Does Reactive Power Come From?
In inverter-based plants—such as solar, wind, and battery storage systems—reactive power can be supplied by the inverters themselves or by an additional compensation unit, like a capacitor. While capacitors are traditionally used to compensate for reactive power consumption in industrial settings, in power plants, they supplement the reactive power capacity.
For solar power plants, it’s generally more cost-effective to generate reactive power using inverters, as their capacity is relatively inexpensive compared to additional compensation units. However, it’s important to note that inverter capacity is typically rated in apparent power (kVA), so reserving capacity for reactive power reduces the available active power output.
Determining Reactive Power Capacity: An Iterative Process
When designing a power plant, it’s essential to ensure that the reactive power capacity requirement can be met to obtain grid connection approval. In solar power plants, this determination is often an iterative process, balancing component specifications and quantities against costs.
Key factors influencing reactive power capacity include:
Main transformer reactive power losses
Unit transformer reactive power losses
Number and sizing of inverters
Cabling
Transformer reactive power losses are directly proportional to their short-circuit impedance. The plant’s cabling is significant, especially considering active power losses from DC cabling and potential voltage rise from long DC cable runs. Inverter capacity typically limits when voltage exceeds 10% of the nominal value.
It’s beneficial to calculate the plant’s reactive power capacity early in the project and update it as data becomes more precise, ensuring correct equipment selection and avoiding surprises later on.
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Heidi Pösö is responsible for Owner’s Engineering services at Despro. Get in touch!