Maximizing VFD and UPS Performance

If you're installing a variable-frequency drive (VFD) or uninterruptible power supply (UPS) — or if you already have this equipment in place — you should be monitoring certain power quality parameters to maximize their performance. To determine what to measure and when, you can start by understanding the power quality issues associated with this equipment as it draws current from incoming power in short pulses.

Photo. To measure the effects of the harmonics generated
by the VFD or UPS, set up your logger/analyzer at the
point of common coupling (PCC).

Here's what to do

VFDs and UPS systems are susceptible to power quality issues from the line power that supplies them, and they also produce harmonic currents that are reflected back into the distribution system. The best practice is to monitor power quality before installation to verify the supplied power to the equipment meets manufacturers' specifications. Here, you would gather specific data for equipment manufacturers so they can analyze any present harmonics and design filters to limit the amount of harmonic currents reflected back into the distribution system. Finally, you can monitor power quality during system operation to ensure the VFD or UPS does not exceed harmonic distortion limitations on the distribution system. You can also use your monitoring data to ensure your power capacity is adequate before installation.

Prior to installing a VFD or UPS, you should measure power quality parameters on the feeder or branch circuit that will supply these devices. Then, compare your data to manufacturers' specifications to ensure it's within spec. Also remember to save any recorded data to establish baseline data for future use. As an example, here are the input power requirements for one typical manufacturer:

• Voltage input between +10% to -10%.
• Frequency of 60 Hz (±5 Hz).
• Maximum sag ride-through of 0% voltage for one cycle and up to 60% voltage sag for 10 cycles.
• Minimum 0.92 lagging power factor at full load with nominal input voltage.

Fig. 1. Screen shot of analyzer showing voltage,
current, and frequency readings.

Swells, or too high of an incoming voltage, can be one cause of an overvoltage fault. Sags or dips can cause an undervoltage fault on the equipment, shutting down the VFD or UPS.

To ensure the system will meet this manufacturer's specifications, use a power quality analyzer or recorder to monitor and record data. Recording data over a period of time will show what you can expect during an entire plant cycle. Then, you can download the data to a PC for analysis. If you use a combined logger/analyzer, you can also take immediate checks on the incoming power. While you're logging:

Fig. 2. Screen shot of analyzer showing power
and energy readings.

• Observe that V_rms and Hz are within the ±10% specification for voltage and within ±5 Hz for frequency (Fig. 1).
• Make sure Displacement Power Factor (DPF) meets specifications. DPF is the power factor for the fundamental frequency while PF takes into account harmonics. DPF typically applies to manufacturer's specifications (Fig. 2).
• Though perhaps not required, it's a good idea to monitor incoming power for any harmonic distortions already created on the system from other sources. In many cases, you may have to make a determination to isolate the VFD or UPS on its own power source to minimize incoming disturbances. It's wise to have harmonic data to make such decisions before installation and, as always, save this data for future comparison.
• After logging is complete, verify voltage sags do not exceed manufacturers' specifications (Fig. 3).

Be aware of situations unique to your equipment

Fig. 3. Screen shot of analyzer showing dip and swell events.

For example, a static UPS system may have additional requirements on incoming power. While the acceptable voltage range may vary by manufacturer and allow voltage as low as 30% of nominal, some units may stop charging batteries at 15% below nominal voltage. Be sure to know your equipment's limitations by consulting the manufacturer's specifications.

Once the VFD or UPS is operating, it may present another set of power quality issues you must be prepared to correct, including the effects of harmonics created by the VFD or UPS and the resulting Total Harmonic Distortion (THD) on the power distribution system. Basically, you must understand how the voltage is distorted; determine the point at which to measure THD; and understand that limits are set based on distortion of the distribution voltage sine wave (see SIDEBAR: How VFDs and UPSs Produce Variable Voltage and Frequency below).

What does all this really mean to you? Basically, you must monitor harmonic distortion during VFD and UPS startup and make necessary corrections if limits are exceeded. As mentioned earlier, if you provide the equipment manufacturer with the information, it can conduct harmonic studies and design filters to limit the harmonic distortion created by the VFD or UPS during operation.

Making the most of monitoring

Once you've installed any harmonic filters and the systems are operating, you should monitor and record harmonic distortion created by the VFD or UPS (see Photo). Because the recommendations noted in IEEE 519, “Recommended Practices and Requirements For Harmonic Control In Electrical Power Systems,” are based on the point of common coupling (PCC), you should set up and monitor at this point in the system. Typically, this PCC is the point where the VFD or UPS load feeder leaves a bus energized by a power source.

A harmonic graph displays the magnitude of each harmonic current in relation to the fundamental frequency of 60 Hz. Based on the type of drive or rectifier circuit in the VFD or UPS, you can expect higher magnitudes at certain harmonic frequencies. For example:

• A 6-pulse drive can be expected to generate greater harmonics at the 5^th, 7^th, 11^th, 13^th, etc., harmonic frequencies.
• An 18-pulse drive can be expected to generate greater harmonics at the 17^th, 19^th, 35^th, 37^th, etc., harmonic frequencies.
• A switch-mode power supply can be expected to generate greater harmonics at the 3^rd, 9^th, 15^th, etc., harmonic frequencies.

In all cases, you should observe a decrease in the magnitude of harmonics as the harmonic order increases. However, be especially aware of any abnormal magnitudes at any harmonic frequency as observed on the harmonic graph. This could be an indication of the VFD or UPS harmonics creating a resonance situation with power factor correction capacitors in the system. You must take corrective action here to avert this dangerous situation.

After viewing the harmonic graph for expected harmonic frequencies and making sure no abnormalities exist, you should view the THD for voltage. This should not be more than 5% (Fig. 4).

Fig. 4. Screen shot of analyzer showing harmonic
current content and THD levels.

If the measured THD is greater than this, you should determine the best solution to bring THD within acceptable limits. Possible solutions could involve harmonic filters, isolation transformers, or moving loads to other feeders or branch circuits.

Keep in mind that monitoring, especially at the PCC, is a long-term concept. In other words, don't stop monitoring once you've established installation conditions. You should return for periodic monitoring and broaden your view to the entire power system.

You should also evaluate your total maximum demand, because it will change significantly over time — especially as new devices and loads are introduced. Any resulting changes may impact what was once a clean supply to your VFD or UPS.

The bottom line

When installing, operating, and maintaining VFDs and UPS systems, make sure you provide good, clean, reliable power to this equipment and minimize the harmonic distortions reflected back into the distribution system. Use power quality analyzers to monitor and record key power quality parameters prior to installation, during startup, and during normal operation. By working together with manufacturers, you can use the observed power quality data to meet the ultimate goal: maximized VFD and UPS performance.

Barnett is training director for American Trainco, Inc., Englewood, Colo., and technical author for Fluke Corp., Everett, Wash. He can be reached at RandyB@americantrainco.com.

SIDEBAR:How VFDs and UPS Systems Produce Variable Voltage and Frequency

VFDs and static UPS systems operate by converting incoming AC power into DC. Then, by precisely switching this DC off and on, they produce a variable voltage and variable frequency waveform. UPS systems switch the DC to provide “clean” power at the fundamental frequency to its critical loads. VFDs will vary frequency and voltage to adjust the speed of an AC motor.

The conversion of the AC into DC in most VFDs and UPS systems is accomplished by a rectifier circuit. A capacitor is located across the output of the rectifier circuit. Power is drawn for the DC switching from this capacitor. The capacitor draws current from the line (through the rectifier circuit), to charge itself during the peaks of the positive and negative half cycles.

This draw of short current pulses results in a voltage drop. It also produces a “flat-topping” of the incoming sine wave. In addition, harmonic currents are created by the rectifier circuit. These harmonic currents flow back into the distribution system, causing voltage and current distortion of sine waves in the distribution system.