Last week, I gave an introduction to power networks training session for a mixed customer audience. One of the topics we discussed was reactive power and its role in present AC power systems, as well as the changes caused by Smart Grid. Reactive power, as a reminder, is the power that flows back and forth between inductive windings of generators, transformers, and similar electrical load-bearing devices.
Non-power engineering professionals sometimes wonder why reactive power is talked about so much, and why it is so important for a stable and reliable operation of the power grid. For most people, reactive power has some mystical properties:
- You cannot light a lamp or run a motor with reactive power—you need active power (Watts) for this, and that is what you pay for (kWh’s).
- You cannot convert reactive power into active power (or vice versa).
- You not only have to keep the active power balance at all times (load = generation), but you must also keep the reactive power balanced.
- Unlike active power, reactive power cannot be transmitted over long distances; it has to be generated locally.
- The amount of reactive power increases fast with higher voltages; underground cables need more reactive power than overhead lines.
- It is needed for voltage control.
- It permits extra losses in cables and wires of the feeders.
- It reduces the transmission capacity of cables and wires.
- It is needed to energize overhead lines and cables, and creates the electromagnetic field that is needed to enable active power transmission.
- It limits the distance over which power can be transmitted in AC systems.
- A serious shortage of reactive power at the transmission level may cause large black outs.
Without explaining in detail, it is clear from the list above that an AC transmission & distribution electric grid will not function without reactive power. In the table below provides a qualitative indication of the importance of reactive power for different voltage levels.
For Smart Grid, reactive power control becomes even more important when we change from overhead lines to underground cables, realize bulk power transmission over longer distances, and start transmitting locally generated renewable power through the distribution grid.
In my next few blogs I will discuss some of the above mentioned reactive power aspects, and how these aspects effects Smart Grid.
By: Peter Vaessen, principal consultant, DNV KEMA Energy & Sustainability