Each type of drive impacts power quality, power factor, and harmonic content as a function of the line (source) inductances.
AC variable speed drives (VSDs) are finding their way into all types of industrial and commercial power system applications. Their application to the load (motor) generally is reasonably well selected, at least within heavy industry. However, there are many line (source) design parameters learned in the 1950s and 1960s that are now being frequently overlooked. In this article, which represents many years of experience in conducting power quality investigations, we'll use this experience and combine it with recent technology to discuss some areas of concern that frequently result in serious performance or power quality problems. Finally, we'll provide basic design guidelines to greatly minimize system problems, especially those problems stemming from the intermixing of newer and older technologies.
First, let's begin with the basic theory of VSDs.
VSDs are modern day replacements for motor-generator (MG) sets (an AC motor driving a DC generator, which in turn drives a DC motor and its variable speed load) and eddy-current drives (a fixed 60-Hz motor driving a variable speed load via a clutch). VSDs provide much better control and efficiency and are more economical than those mechanical drives they are replacing.
Fig. 1 is a block diagram showing their basic components (both AC and DC drives shown). The converter section, which converts AC to DC, dominates the interaction of the drive with its source system. The basic components generally used in the converter module are either diodes or thyristors; the inverter module typically uses thyristors, transistors, or GTOs (Gate Turn Off Thyristors). Transistors have found applications in smaller sized drives with GTOs more commonly applied in large, specialized medium voltage (2.4kV to 6.9kV) applications. This leaves conventional thyristors as the most commonly applied, especially within the existing installed base.
Fig. 2, on page 71, shows a basic 3-phase, 6-pulse thyristor drive, which is the most common. Note the current path for phase A current. The thyristors shown will conduct during the positive 1/2 cycle and after a gate pulse (steep voltage spike) has been received. As the current waveform crosses zero and the current tries to reverse itself through the phase A thyristor, the negative bias causes the thyristor to commutate (turn off). At this same point in time, phase B thyristor is beginning to conduct, causing a brief but severe phase-to-phase short circuit. This results in commutating notches in the voltage waveform.
Fig. 3, on page 71, shows a basic waveform of source line-to-line voltage, input current, and fundamental current over one cycle. This would apply at the input thyristors of a drive. Note the input current and voltage waveforms are no longer clean sinusoidal waveforms; the input current approximates a discontinuous square wave rich in harmonic content. The steepness, width, and area of the voltage commutating notches can result in much higher frequency harmonic content as well as voltage spikes, both of which can result in drive control or stability problems. The inductance of the primary system (commutating reactance) plays a significant role in the notching, commutation, and harmonic generation of a drive/drive system. If the ASD is a current source inverter (no DC link capacitor), the drive is dependent on the load inductance for proper commutation.
Types of drives
The inverter section of a drive can also impact the application. Fig. 4, on page 74, shows a voltage source inverter (VSI) type ASD along with its waveform, Fig. 5, on page 74, shows a current source inverter type, and Fig. 6, on page 76, shows a pulse width modulated (PWM) type. Each of these type drives impacts power quality, power factor, and harmonic content as a function of the line (source) inductances.
VSI drive. The VSI drive controls the DC voltage by using converter thyristors, which in turn control the inverter output voltage. The load draws whatever current it needs. The inverter does not use high frequency switching.
CSI drive. The CSI drive controls the current to the motor to maintain the required voltage and frequency with a more complex inverter section.
PWM drive. The PWM drive uses a simple diode bridge converter, which minimizes the lower order harmonics generated and does not generate commutation notches. However, this type of drive has a complex inverter section that must control both the output voltage and frequency. It uses higher frequency switching devices on the output than both VSI and CSI drives.
Commutation is the process by which one set of thyristors (or diodes) turns OFF and the next set turns ON. It occurs when thyristors are used.
In a full 6-pulse converter, the thyristors operate in pairs to convert AC to DC by switching the load current among the six thyristor pairs six times per AC cycle. During this process, the current begins to transfer from one phase to the next, creating a momentary phase-to-phase short circuit. Source inductive reactance prevents instantaneous transfer (commutation) resulting in a commutating notch. The duration of this short circuit is a function of the total system inductance and the DC output current. Referring to Fig. 3 again, we see a typical AC input to a 6-pulse full thyristor converter. Fig. 7, on page 76, shows just the phase-to-phase input voltage with line notches identified. It also defines notch width and depth. The notch depth and area will differ depending where in the system they are measured. Let's refer to Fig. 8, on page 76, for a better understanding. Here, the notch depth at Point A will be 100%. At point B, it is calculated by using the following equation:
Notch depth (%) = [([L.sub.1] + [L.sub.2])/([L.sub.1] + [L.sub.2] + [L.sub.3])] x 100 (eq. 1)
Since a line notch is a sudden change in voltage, resistor/capacitor (snubber) networks will begin to discharge/charge during commutation notches. Where multiple drives are applied on a common bus, the composite commutation notching can overwork these snubber circuits and, in severe cases, cause them to fail catastrophically. These networks are applied to absorb transient voltages occurring across the thyristors due to high-speed electronic switching. If these networks are out of tolerance or nonfunctional, a transient voltage overshoot may occur, with a high incidence of thyristor fuse blowing and/or drive component failures. Also, other sensitive equipment fed from the common system can exhibit power quality/control problems. IEEE 597-1983, Standard Practice and Requirements for General Purpose Thyristor DC Drives states that "the repetitive peak deviations of the fundamental line voltage from the instantaneous value of the line voltage may not exceed 25% of the crest working line voltage." This gives a practical upper limit.
These line notches are also rich in high frequency harmonics and are propagated throughout the power system. Sensitive electronic equipment, such as computers, programmable logic controllers (PLCs), and instrumentation are especially sensitive to the high frequency content due to line notching. For this reason, it's always a good idea to keep these sensitive-type loads electrically isolated from these drives.
Line notching can also cause thyristor misfiring. This can happen when notch width exceeds gate pulse width or with excessive notch depth. Per IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, ASDs should be selected/designed to operate with notch widths of 250 microseconds and a depth of 70% of rated peak line current or less.
Table 1, on page 74, excerpted from IEEE 519-1992, defines commutation notching limits for low voltage systems. For those systems having notching out of limits, you should add commutating reactance ([L.sub.1] '[L.sub.2] '[L.sub.3] as shown in Fig. 8); this is the most practical solution. Generally, this is best accomplished by making the sum of [L.sub.2] plus [L.sub.3] greater than [L.sub.1]. The addition of a series commutating reactor in the drive itself or a drive isolation transformer easily serves this purpose.
Another benefit of adding commutating reactance is that the current harmonics generated can be significantly reduced. However, a dilemma can occur when the addition of inductance to decrease notch depth results in too large of a notch area. Reducing [L.sub.1] (larger stepdown transformer) or the addition of power factor capacitors at Point C in Fig. 8 may be alternatives; however, they affect other system design parameters such as short circuit levels and harmonics.
Higher frequency harmonics caused by commutation notching, thyristor-induced transients, high frequency (radiated or conducted) components caused by high frequency switching in the inverter section (primarily CSI drives), inadequate or improper signal grounding, static discharge from insulation to ground (common mode noise), as well as use of walkie-talkies in close proximity to drives have all been known to cause drive malfunctions.
The higher frequency sources and thyristor-induced transients are sometimes called crosstalk or electrical noise. If this electrical noise is coupled into controls or signal paths (such as thyristor gate leads), thyristor misfiring, unstable speed control, erratic behavior and/or control board failures can and will occur. This is especially a concern where multiple drives are fed from a common bus without individual commutating reactors or isolation transformers applied. The application of newer digital ASDs to existing analog drive systems may result in similar problems.
Solutions could consist of adding commutating reactance, isolation transformers, and carefully solving grounding installation problems. Where digital and analog drives are intermixed, adding metal oxide varistors (MOVs) and/or changing out thyristors so that they are more compatible with each other are additional considerations.
In converting AC power to DC power in a DC drive application, the converter effectively breaks or chops the AC current waveform by only allowing the current to flow during a portion of the cycle. The example given previously in Fig. 2 for a 3-phase, 6-pulse converter shows a square AC current waveform. The distorted waveform can be separated into components using Fourier analysis techniques, which will not be discussed here. However, DC drives are typically operated with the thyristors phased back, and the 5th harmonic current may be as high as 30%.
Harmonics produced by line-commutated converters are related to the pulse number of the device. This is expressed mathematically by the following equation.
h = (n x p) [+ or -] 1 (eq. 2) where h = the harmonic order, n = any integer p = pulse number of converter
For example, for a 6-pulse converter, the harmonics present will be 5th, 7th, 11th, 13th, 17th, 19th, etc. Fig. 9, on page 79, shows typical characteristic harmonic magnitudes, both numerically and graphically, of the most common pulse-number static power converters. However, inaccuracies in thyristor firing, differences in thyristor characteristics, and system unbalances would cause the production of other "noncharacteristic" harmonic orders such as 3rd, 4th, and 6th.
Six- and 12-pulse converters are the most widely used. Three-pulse devices are used in small power applications, such as extruders, while 18-pulse devices, are used in much larger applications. The choice of pulse number is a matter of economics versus harmonic control.
Larger AC drives, 100 hp and greater, typically use thyristor converters to control bus voltage or line current to maintain desired speed. Unlike the diodes used in DC drive converters that provide constant DC bus voltage, controlling the switching of a thyristor with respect to the incoming voltage waveform determines the bus voltage output. The disadvantage of the thyristor converter is a greater AC current distortion during slower motor speeds. Fifth harmonic current distortions greater than 50% have been recorded at AC drive installations. The increase of harmonic current distortion is a function of the type of AC drive and its speed.
Whether drives are AC or DC, a common means of reducing harmonics generation while in the design process is by phase-multiplication or harmonic cancellation. For example, if another converter were supplied by a voltage 30 [degrees] out of phase, the 5th, 7th, 17th, 19, etc. harmonic components would be 180 [degrees] out of phase with those of the first converter. Thus, if two equally loaded and identical drives were supplied by identical delta-delta and delta-wye transformers respectively, the overall harmonic production would resemble a 12-pulse device (greatly reduced 5th and 7th harmonics). Of course, the less identical the drives supplied by phase-shifted voltages, the less cancellation would occur.
However, significant harmonic cancellation can occur within a specific process or throughout a plant by the judicious choice of transformer winding connections. Fig. 10, on page 80, shows a 24-pulse application and Table 2, on page 74, excerpted from IEEE 519-1992, lists the recommended harmonic voltage distortion levels for various voltage ranges. Even though the limits are for the utility point of common coupling (PCC), these values can be used as a design guide in controlling harmonics within an industrial facility. If economics preclude phase multiplication techniques, larger sized harmonic filters or multiple single-tuned filters can be installed to reduce harmonic magnitudes to acceptable levels.
Operating variable frequency AC drives with thyristor converters at slow speeds generally results in poor power factor. If the number of AC drives is small compared with overall plant load, then the total power factor at the utility metering point may not be cause for concern. However, large systems of variable frequency drives would require reactive compensation. One characteristic of PWM-type ASDs is a good power factor (due to diode front end). Where lots of smaller drives are applied with expected low-speed operation, PWM-type drives will significantly improve power factor and could reduce the cost of harmonic filtered capacitor banks.
The practice of installing and switching a capacitor with a motor is not recommended with AC drives. If the supply transformer and capacitor create a resonance frequency that is the same as a harmonic produced by the drive, high harmonic voltage and current magnitudes can occur at the capacitor, which may cause fuse melting or component failure.
A safer application, from a harmonic viewpoint, is to install the properly rated shunt capacitor with a series tuning reactor at the high voltage circuit as shown in Fig. 11, on page 80.
There are two areas of grounding concerns for drives or drives systems: power system grounding and signal/equipment grounding.
Power system grounding. The majority of the smaller hp drives are applied at 480V. Most of these 480V systems have a solidly grounded, wye-connected source transformer. In many cases, the solid grounding is an NEC requirement, whether or not ASDs are applied. Consider that if a ground fault occurs internal to the drive itself, the very high magnitude ground fault current available on a solidly grounded system could cause catastrophic failure. If this is a concern, you should use a drive isolation transformer to derive a new ground system for the drive. This type of transformer also adds commutating reactance (reduces harmonics; commutating notching, etc.) as well as a special ground shield between the primary and secondary windings. This shield also gives extra isolation for noise.
Larger dedicated drives or drive systems traditionally are supplied by an ungrounded power system (that includes isolation transformers to individual drives). These are actually capacitively grounded through the ground insulation (cables, transformers, drives, etc.). This tradition has been the industry practice based on the perception that an ungrounded system provides maximum service continuity and that a phase-to-ground fault causes very little ground fault current flow. (There may also be some internal drive reasons.) Here, we are assuming, and accepting, the 173% voltage on the unfaulted phases, a true perception providing only solid ground faults occur.
But most ground faults begin as arcing faults that can result in 600% to 700% (or greater) voltage buildup with respect to ground. If allowed to persist, system-wide insulation failure can occur (multiple failures on a common bus), with snubber circuits, MOVs to ground, etc., all seriously overstressed. Also, significantly increased insulation deterioration over time can result.
The solution is to apply high resistance grounding (almost ungrounded). This eliminates the transient overvoltages from occurring while still providing the service continuity perceived to be provided by an ungrounded system. However, the system must be engineered specific to a given installation. This is especially true for isolation transformers with single drives. High resistance grounding has been successfully applied for these purposes over the last 20 years in many other industries. The most notable are drive systems within the paper industry. It's just now gaining acceptance by some drive manufacturers. Special note: High resistance grounding is not practical for most systems over 5kV.
Signal/equipment grounding. Signal and equipment grounding are probably the most misunderstood aspects of grounding. The equipment grounding function is a mandated (NEC green wire) requirement for equipment and personnel safety. The signal ground is necessary to prevent unwanted avenues of electrical noise from being fed into the drive control system. Problems can exist when installing the power system ground, the equipment grounding conductor (green wire), and the signal ground.
Historically, drive systems, distributed control systems, and large computer facilities have specified isolated grounds; this is not only unsafe, but it violates the NEC on at least two counts (Secs. 250-26 and 250-81). In many instances, it does not solve signal grounding requirements as expected. The long insulated ground conductor to the isolated ground mat frequently acts as an "antenna," feeding noise directly into the signal boards. The correct method is to use a "single point ground."
The following standards give detailed examples and methods on how to install the equipment grounding conductor, the single-point ground, and the signal ground into one common, safe, and clean signal system.
* IEEE 142-1991, Recommended Practice for Grounding of Industrial and Commercial Power Systems (the Green Book).
* IEEE 1100-1992, Recommended Practice for Power and Grounding Sensitive Electronic Equipment (the Emerald Book).
* Federal Information Processing Standards Publication 94, Guideline on Electrical Power for ADP Installations (FIPS 94).
Power quality is a term used to combine all the system disturbances that affect the performance of drives, computers, and other sensitive electronic equipment. There are three major divisions: Harmonics, grounding, and sags/swells/transients. We've discussed the first two; now onto the last division.
There are certain normal conditions that occur periodically (usually in the utility system external to the facilities) that can have adverse effects on ASDs. The Electric Power Research Institute (EPRI) technical brief "Power Quality Considerations for Adjustable Speed Drives," explains one common adverse condition.
Since the internal DC link capacitor is essentially connected alternately across each of the three phases, drives of this type can be extremely sensitive to overvoltages on the AC power side. One event of particular concern is capacitor switching on the utility system. As an arc is drawn switching the capacitor bank, it excites an LC ring wave at the natural frequency of the inductance of the system and the bank capacitance. These utility-generated voltage switching transients result in a surge of current into the DC link capacitor at a relatively low frequency (300-800 Hz). This current surge charges the DC link capacitor and causes an overvoltage to occur (through Ohm's law). The overvoltage (not necessarily magnified) exceeds the voltage tolerance thresholds associated with the overvoltage protection, properly tripping the ASD out of service. This is called nuisance tripping because the situation can occur day after day, often at the same time of the day. Several methods are available to resolve such tripping, some of which are simple and some costly.
The use of a harmonic filter to reduce overvoltages (a relatively expensive alternative) is very effective in protecting drives from component failure but may not completely eliminate nuisance tripping of small drives.
In at least two other instances known by the authors, simply switching utility company 69kV circuit breakers several miles away resulted in a relatively high frequency ring wave that caused internal drive under/over voltage tripping. The built-in protective functions from the drive manufacturer were not rms sensing but peak sensing. Thus, the protection sensed an instantaneous under/over voltage condition, assumed it was an rms condition and erroneously tripped the ASD off-line.
The most effective (and relatively inexpensive) way to eliminate nuisance tripping of small drives is to isolate them from the power system by adding commutating reactance in the primary of the drive. Sometimes, the addition of an appropriate UL 1449 Transient Voltage Surge Suppressor (TVSS) is also warranted.
While we've discussed most of the issues of power quality and line considerations in the application of ASDs, the following guidelines can help ensure assuring trouble-free ASD reliability.
* Select the ASD primarily based on application. A common error is to buy strictly on price, without proper application considerations.
* Address the issue of applying the correct commutating reactance. Note that commutating notch depth, area, electrical noise, harmonic generation, and response to switching transient ring waves are all influenced.
* Review and evaluate each vendor's drive for surge protection (snubbers, transient overshoot); harmonic signature (of particular concern are the higher order voltage harmonics such as 20th through 50th; this is especially a concern for CSI drives); peak deviations greater than or equal to 125% of crest working voltage (inverter output also); and internal "power system protection" [peak sensing or rms? If peak sensing (or DC link), any time delay?].
* Address electrical noise, especially for numerous drives on a common bus. If you have a noisy electrical environment, use isolation transformers in lieu of just commutating reactors. Also, keep other sensitive electronic equipment electrically isolated if possible. Exercise care when intermixing new digital drives with analog drives.
* Where a significant portion of the load is comprised of ASDs, perform a power factor and harmonic study. Try not to switch power factor capacitors with individual drives. Installation must meet IEEE 519 requirements at the utility point of common coupling. Consider using a 12-pulse or 18-pulse type for large drives. Use phase cancellation where practical by intermixing delta-delta and delta-wye transformers (stepdown or isolation). Selecting all PWM-type drives will improve power factor but may aggravate harmonic condition.
* Address the following grounding issues: Always apply the NEC "equipment grounding conductor" (green wire) since the use of conduit for this function will tend to be "noisy;" use high resistance grounding where practical; and use a single-point signal ground design.
* Conduct an in-depth power quality investigation if your facility has a history of unexplained nuisance tripping or drive failures. The new drive system should be designed and applied to function reliably in its environment. Note that some of the solutions may be independent of the drive itself.
Evaluate the manufacturer's system for internal protection. Cheaper designs having no commutating reactors, minimal snubber circuits, no input MOVs, and extremely sensitive peak sensing under and overvoltage protection should be ruled out. If external switching transients (ring waves, etc.) are present and adding commutating reactance is not practical, consider active power line conditioners, "sine-wave" trackers (a new type of TVSS that "tracks the sine wave"), and/or MOVs. You also can have the utility identify and resolve the source of any switching transients in their system.
David D. Shipp, P.E. and W.S. Vilcheck David D. Shipp, P.E., is Senior Consulting Engineer, and W. S. Vilcheck is Electrical Engineer at Westinghouse Electric Corp., Oakdale, Pa.