Does efficiency come at the expense of reliability?

Based on a discussion webinar, Friday 15th February 2008

Two major concerns exist about our electrical power system. The first is the reliability and quality of the electrical power, summarised under the term ‘Power Quality’ (PQ). Poor PQ affects the profitability and productivity of our economy. The second concern is the energy efficiency (EE) of the electrical system. Poor EE affects both the environment and the profitability of the economy.

But how do measures to improve PQ influence the efficiency, and vice versa? Are PQ and EE measures synergetic, or rather counter-productive?

Leonardo ENERGY addressed this subject in a discussion webinar on 15th February 2008. The following are a few of the major points arising from that discussion.

EE increases the reliability of the network

EE measures decrease the energy demand in general and consequently also peak demand. This increases the operational margin of the system, leading to a higher reliability. However, some attending the seminar doubted if this effect is significant. Moreover, it could also have the reverse effect, since an under-loaded system can also be unstable.

What applies to the public grid is also true for private networks: energy efficiency measures create a higher operational margin and in this way enhance the reliability of the system. In general, one can state that an integrated view of the energy systems combined with Life Cycle Costing will simultaneously improve the energy efficiency, the power quality, the productivity, and the maintenance efficiency of an industrial site.

A reliable power infrastructure affects EE

Most devices that improve reliability and power quality, such as a storage device, an Uninterruptable Power Supply (UPS), or a Dynamic Voltage Restorer (DVR), come with a certain price in energy efficiency.

One could, however, also defend the point of view that those devices deliver a necessary energy service. So, the question is not whether those devices consume energy, but whether they deliver their service in the most energy efficient way, just as you don’t question whether a house needs windows, but rather try to minimise the energy losses through the windows.

EE solutions have undesirable PQ impacts

EE might have a positive effect on the reliability of the system (see point 1), but most probably not on the Power Quality in the strictest sense. Many devices that have positive effect on the EE also affect the Power Quality in the network. A typical example is a variable speed drive for a motor, which also induces harmonics in the network. Compact fluorescent lamps also create harmonic distortions as well as a high neutral current.

Another important question is whether devices used to save energy have enough immunity against power quality problems. Inverter drives, for instance, are known to be sensitive to voltage dips.

The energy savings potential of PQ measures

Some PQ solutions have energy savings as a positive side effect. Examples are harmonic filters and capacitor batteries for correcting the power factor. Both reduce network losses. Likewise, upgrading cable diameter has a positive effect on both PQ and EE.

However, one seminar participant noted that some of the energy saving claims of PQ solutions are questionable. They mainly eliminate losses that would not have been there had there been a proper system design in the first place. Moreover, harmonic losses in cables (in contrast to transformers and motors) are limited; they rarely surpass 1%.

Beware of black-box technologies

One should beware of black-box EE solutions such as voltage reducers. They are said to save energy by simply plugging them into a network. This might be true, but the boxes might not be as green as they claim. Their impact on energy efficiency may not be as high as stated, and they may negatively affect the power quality of your network.

For a deeper analysis of the PQ impact of EE measures, download the paper Power Quality Guidelines for Energy-Efficient Device Application by the Electric Power Research Institute.