POWER QUALITY Q&A

Our Expert Offers Answers to Frequently Asked Questions About Power Quality

Question:
We are purchasing a Variable Frequency Drive (VFD) for a 100 HP load in our plant. Do I have to be concerned with harmonics or other system problems?

Answer:
Yes and No. First of all, you want to be sure that you specified the correct drive. This isn't as easy as it used to be. To make the drives smaller and less costly, many items have been reduced or eliminated from the standard drive. This means that in specifying the drive, the vendor must be made aware of the exact application and take the necessary precautions to achieve a successful installation. Alternatively, an engineer experienced in drives applications should be engaged. The new drives, however, tend to reduce the amount of harmonic content reflected back into the power system resulting in less noticeable effects. Some drives do this better than others. Therefore, initial drive specification is important.

The next step is correct installation practices. Correct grounding (as always) is key to the installation. Insure that the manufacturer's installation recommendations are followed. Try to isolate the drive on a separate bus and always use an isolation transformer or line reactor to prevent the passage of transients and higher level harmonics. Keep the VFD away from buses with sensitive electronic loads. It is always wise to monitor the harmonic levels prior to installation to ensure that you don't already have harmonic problems.

Ensure the drive is installed on a stiff bus. This means one with plenty of capacity and adequately sized feeders. Drive effects will be more noticeable if the feeding transformer or cables are too small or there is poor regulation on the feeding circuit.

Avoid putting VFD's on circuits where power factor correction capacitors are installed. This could lead to some unexpected and undesirable interaction between the capacitors and the drive.

Try to limit the amount of drive load on a single transformer. Typically, if you have more than 50 per cent of the load on a transformer from VFD's, you will experience some problems.

In severe cases, drives can be ordered with an AFE (or active front end) which will modify the waveform through active filtering techniques. However, this is usually done for larger drives, as it generally doubles the cost of the drive.

Attention to the above guidelines will reduce the effects of any potential harmonic or transient problems.

Question:
I need an uninterruptable power system (UPS) for a computer system that we are installing. How big does it need to be and are there any installation issues of which I should be aware?

Answer:
The primary purpose for a UPS system is to ride through short power interruptions and protect the memory and data located within a computer. It can also be used to provide a continuous power source for critical equipment or provide a clean power source for sensitive electronic equipment.

The size of the UPS must be large enough to handle the equipment load and yet be capable of supplying the extra current necessary for equipment starting inrushes. This is generally specified in VA or kVA. Use equipment nameplates or specification data to determine the necessary size. Check with the manufacturer for any inrush requirements. Allow more capacity than you think you need.

When it comes to specifying the battery capacity, consider the length of time it is necessary to keep the equipment running. If life critical equipment is involved, the battery capacity must last until the source is restored or an alternate source is switched on. Normally with a computer, we are looking at keeping the equipment running only long enough to save the necessary data and perform an orderly shutdown. In many cases, major plant or facility equipment will also go down, debilitating normal operation anyway.

Battery sets are generally specified in ampere-hours. This is a nominal term and actual performance will depend on the actual load conditions. If in doubt allow extra. This is an important decision as not only the length of time the equipment runs is affected, but as the amount of battery capacity goes up, the cost of the system significantly increases.

One of the installation issues which is not always understood is how the UPS should be connected to the power system. As the UPS involves an AC/DC conversion, it will inherently generate noise and harmonics in its supply. Therefore, we do not want to connect it to an existing clean supply, as it will corrupt it. The best place to install it is in a relatively stable and reliable bus where there are no sensitive electronic devices connected.