POWER QUALITY Q&A

Our Expert Offers Answers to Frequently Asked Questions About Power Quality

Question:
I recently sustained a lot of damage to my computer from lightning. What precautions should be taken to prevent this from happening again?

Answer:
Because of the immense energy involved and the many ways lightning can affect electronic equipment, it is difficult to quantify the exact failure mode.

However, there are a number of strategies that can be used to minimize the significant damage that can occur. Grounding is always an essential issue. The computer ground should be very low impedance (less than 1 ohm). This usually entails a generously sized ground conductor going back to the main service. Of course, the main service must be well grounded to the building ground nest.

Some consideration should be given to an isolated ground receptacle. Again, the computer is grounded but an isolated ground (insulated from the safety ground) is run back to the main service separately from the safety ground. As the ground pin is open-ended at the computer, there are no ground loops to create noise on the computer ground. To meet Electrical Safety Code requirements, the isolated ground must be connected to the safety ground at the main service. If the problem of short outages or sags is also experienced, the best solution may be an UPS system to provide a reliable source of power. The UPS units designed for single computer use are quite reasonable in cost. They usually come with three battery backed up receptacles and three surge protected receptacles. Only the CPU and (possibly the monitor) should be connected to the battery backed-up receptacles. Peripherals should only be powered from the surge protected receptacles.

Question:
How are harmonics generated and how do they flow through a power system?

Answer:
Harmonics are generated by non-linear loads. These are loads whose impedance varies non-linearly with the voltage applied. It also could be a load where the current flow is discontinuous during portions of the voltage sine wave. Examples of this are switch mode power supplies (computers), Variable Freq-uency Drives, and Uninterruptible Power Supplies. An example of a VFD input waveform is shown in the diagram.

Harmonic currents generated by the non-linear load flow in the power system towards the lowest impedance points. For the lower order harmonics, this tends to be towards the utility power source.

If the harmonic levels are significant, the current will flow back into the utility system creating a distorted voltage waveform. The higher order harmonics tend to flow towards capacitors connected to the power system.

These could be power factor correction capacitors, motor start capacitors, or even capacitors within electronic devices. Significant harmonics will create blown capacitors, fuses, and heating effects on transformers, motors and conductors.

Significant damage can occur if the combination of inductance and capacitance create a resonance point at a harmonic frequency. In this case, the harmonic levels increase well above what would normally be expected, amplifying the normal harmonic effects.

Under normal conditions, loads and equipment not directly in the harmonic flow are not affected unless the voltage waveform has been distorted.

For further information on the generation and effects of harmonics, refer to IEEE Standard 519.

David is the President of Wintek Engineering. You can forward your questions or comments to him at wintek@wintek-eng.com.
Some of these questions will be addressed in future issues of Electricity Today's Power Quality Question & Answer Column. ET