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Power Quality Issues Residue of Electronic Motor Drives

EMA Inc.The application of electronic variable speed drives to electric motors has grown dramatically. Since transistorized inverters (AC Drives) became readily available in the early 80’s, the expansion of this technology has been remarkable. No longer an esoteric discipline, reserved for a few specialists, Drives have become commonplace. The expansion of usage hasn’t been without problems; earlier AC Drives were notoriously unreliable, expensive, and difficult to tune. There was additionally, little understanding of their function and theory outside of select groups.

The recent growth of electronic variable speed drive applications within modern industrial facilities has been accompanied by increased disturbances of the plant power systems. Drives, like computers, are much less expensive today, and the interfacing software is available to a variety of applications.

Perfect Harmony VFDDrives communicate very easily with most PLCs and process systems, and the reliability of modern drives is excellent. The trade literature is full of information about new and better ways to use them, and it seems that every year drives get smaller, more efficient, and more featured. Its now a very common practice, that the same drive unit can be an open loop vector drive (meaning it needs no speed feedback), a closed loop vector drive (meaning it needs feedback), an open loop volts/Hz drive (no feedback), or a closed loop volts/Hz drive (needs feedback). The internal drive software tells it how to operate, and with the self tuning algorithms now standard on most units, tuning to an individual motor or application is quite easy. This is a major change from the past, where tuning  an AC drive could be very difficult.

Vector technology allows the use of drives in high torque applications, such as positive displacement pumps, that spelled trouble for earlier drive generations. Don’t really understand Vector drives? You don’t need to. It’s in the software and all you do is input the parameters asked for, and the drive does all the calculations internally. 

current pulseDrives do however, come with a caveat, they disturb power systems. The reason for this is straight forward, drives draw non-sinusoidal (or non-linear) current, and this can distort the power system. This residue of the process improvement allowed by electronic drives, has proven troublesome for a number of plants, and in extreme cases, the costs associated with resolving this problem can cost much more than the drives themselves. This fact is often overlooked when deciding to install state of the art drives, available at low initial costs. It should however, be a consideration. 

Harmonic problems often appear as overheating supply transformers, inexplicable circuit breaker tripping, nuisance fuse blowing, or perhaps even a nasty call from the Power Company.How will this harmonic distortion affect your system? To use an example, let’s say that you have a supply transformer rated at 500 amps, running a drive system which is rated at 400 amps. Despite this seemingly conservative sizing, the transformer continually overheats, and perhaps even fails, so what’s up? Probably harmonic distortion, that’s what!

IEEE recognized this as a problem years ago, and issued IEEE Standard 519, which for the most part, is the benchmark against which harmonic distortion is measured. Looking once again at our supply transformer, if we’re drawing 400 amps non-linear load at 60 Hz, and we have significant distortion, we could be drawing 200 amps at the fifth harmonic (300 Hz) for a total amp draw of 600 amps. That’s what’s nasty about harmonic distortion. Power must be supplied not only for the 60 Hz fundamental, but for the harmonic energy as well.

In addition to the single problem with this transformer, this distortion can be reflected back into the power system and cause trouble elsewhere in the plant.

How much non-linear loading can a given system handle? Standard 519 contains specific guidelines which state how much non-linear load is allowable, given transformer effectiveness and how much inductive reactance is present. Transformer effectiveness is in general, a statement of the short circuit rating, over the load rating. Typical, is a ratio of 20:1 . A link reactor within the drive, or line reactors installed between the transformer and drive can substantially increase the amount of non-linear load a given system can handle, and still meet Standard 519 guidelines.

How does one combat harmonic distortion? Perhaps the most common method is to install a line reactor between the drive and power system. If purchasing new drives, consider buying one with a reactor built in. As a general rule, I normally recommend reactors anytime that multiple drives are supplied from a common source, if insufficient impedance is present between the drive and power line, or if the power line is especially stiff.

Reactors however, can be a problem if the power line is too soft. There will be a voltage drop across the reactor, determined by the impedance of both the reactor itself, and the associated system impedance. This could cause undervoltage trips on some drives, during heavy loading.

Reactors are inexpensive, and very easy to install. With smaller horsepower units, they can be installed right in the cabinet, between the circuit breaker and the drive. For larger units, it’s usually best to install them outside the cabinet. The reactors will generate some heat, so if cabinet cooling is marginal, an external installation is best.

For more severe problems, a harmonic trap or filter may be required. Rather than simply placing inductive reactance in the line, these are actually tuned traps which capture the harmonic energy and filter it from the power line. While these are more costly than simple reactors, they’re much more effective. These units tend to be larger than reactors, but again can be installed within some cabinets. They also produce heat however, and therefore consideration should be given to an external installation. Like reactors, installation is quite simple. These units simply go between the breaker and the drive.

Other cases may require more drastic action. Supply transformers may need to be replaced with more efficient ones, and perhaps even increased in size. High horsepower drive applications can utilize 12 pulse in lieu of six pulse drives, which offers significantly less distortion, albeit at a considerable cost. Breakers and even power cables may need to be changed.

Harmonic distortion can form a resonance with other power circuits, and create oscillations. This is sometimes accompanied by breakers tripping all over the plant for no discernable reason.

Since the advent of IGBT drives, a new problem has been standing waves which can develop on long cable runs between the drive and motors. Almost all AC drives now use IGBTs as the primary switching devices, replacing the older Bi-Polar transistors. IGBTs offer distinct advantages, being more reliable and efficient, but their steep switching slopes (50 to 200 ns) contain a high harmonic content. Power cables contain parasitic inductance and capacitance, and this begins to act like a series resonant tank circuit. In long cable runs, this can have a natural resonance, which is excited by the high frequencies present within the steep IGBT switching slopes. The result can be high voltage spikes in the vicinity of 1400 to 1500 volts on a 460 volt system. This can punch through motor insulation and destroy the motor windings. Many motors have failed as a result of this.

Inverter Grade motors attempt to resolve this problem by having better insulation, but for very long runs, some filtering is usually necessary. Output filters are available for drives that will allow significant cable lengths between the drive and motor. These filters which are relatively small in size, are usually three wire in, three wire out units, placed between the drive and motor. These are quite effective.

Variable frequency drives are attractive. Modern drives are very reliable, considerable energy savings are possible when compared to fixed speed motors, and precise control of process parameters can be achieved. They are relatively easy to install. The old adage is “keep them cool, keep them clean, (and as we jokingly tell our customers), keep our competition away from them! “ They are a very viable technology, and should be considered in nearly every industrial process system.

There is no escaping the fact however, that electronic motor drives can be disruptive to a power system, and power quality considerations should be taken into account when deciding to apply them. Many, if not most applications will experience little or no power problems with a drive installation. Those that do have problems however, can have very expensive and disruptive consequences from failing to adequately consider the interaction of the power system and drive. Failure to do so can add unanticipated costs and problems to an otherwise simple project.


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