Don't place a variable frequency drive (VFD) on a circuit until you determine the nature and magnitude of harmonics the drive may pump back into the feeding circuit. Here's where a computerized harmonic distortion study by the drive manufacturer comes into play.
For the manufacturer to do the study, it has to know the location of any points of common coupling (PCCs). You are responsible for identifying these locations, not the VFD manufacturer. As such, you must carefully choose the position of PCCs because the results of such a study will vary greatly, depending on where in the electrical system you select these points.
ANSI/IEEE 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control In Electric Power Systems, lists recommended limits for total current and voltage harmonic distortions in an electrical distribution system at the PCC. Within a facility, the PCC is the point between the non-linear load and its connection to a power source, either the serving utility or an on-site generator. The PCC is usually the point where the non-linear load feeder leaves a bus energized by a power source. You must use good judgment in choosing the location of the PCC. If it's too close to an infinite source (this is based on the power available compared to the load served), the effect of harmonics injected into the electrical system will not be significant when performing a harmonic analysis. We don't recommend such a PCC location.
Choosing the location of PCCs To allow a VFD manufacturer to perform a harmonic analysis, you must provide electrical system information such as service size, service transformer impedance data and loading, system short-circuit equivalent impedance, characteristics of the system's conductors, ratings of power factor correction capacitors, characteristics of all loads, and presence of emergency on-site generators. Armed with this information, the manufacturer can make calculations and decide on harmonic filter requirements. Let's take a look at a recent VFD installation project.
The Figure on page 38 shows the simplified one-line of a hospital's electrical system that our firm was involved with. If the PCC is chosen at the 4.16kV collector bus, the results of the study may be very satisfactory and reveal that VFDs will not require special filters to comply with IEEE 519 requirements. However, this PCC is physically too far away from those hospital loads that may be affected by the VFDs.
In this case, we decided the PCCs should be located at the 480V level; in other words, at the local building substations feeding motor control centers with VFDs and at local 480/120/208V transformers distributing power to patient beds. A study by the manufacturer at this voltage level confirmed that filters would be required on the feeders serving the VFDs to minimize harmonic distortion at selected PCCs. We noted that generators in the distribution system provided further restrictions on the calculations, based on Table 10.3 of IEEE 519 (shown in the Table).
PCC No. 1. For the harmonic analysis, we designated the output of the three 1400kW generators as PCC No. 1. This was the most restrictive condition per IEEE 519. We, serving as the electrical consultant, determined the voltage and current distortion should be no higher than 5% to assure the total harmonic distortion (THD) would not have a negative affect on sensitive hospital equipment. The manufacturer did the harmonic analysis based on the assumption all VFDs would be running simultaneously, with 5% line reactors on each VFD.
Using a computer-generated program to determine the THD current at the motor control centers, and then using a Fourier analysis to sum these currents, the VFD supplier arrived at the following distortion levels at the generators (PCC No. 1): 4.8% current distortion and 3.07% voltage distortion. These levels satisfied our maximum 5% THD level and were less than the 5% limit established by Table 10.3 of IEEE 519.
PCC No. 2. We designated PCC No. 2 to be the load terminals of each of the six 4.16kV/480/277V transformers feeding the building on normal operation, with power supplied by the utility company. Also we stipulated as before that the THD be no more than 5% for both voltage and current.
The VFD manufacturer analyzed the THD at the secondary of each transformer, with each VFD having a 5% line reactor. The analysis showed distortion on all transformers (except one) meeting the 5% limitation. The transformer feeding motor control centers EMCC-9N1 and -9N2 had a current distortion of 6.5% and a voltage distortion of 1.89%. To reduce current distortion to the required level, the VFD supplier added harmonic filters to all 40-hp, 50-hp and 60-hp VFDs fed from the above MCCs. The final distortion levels were 4.64% current distortion and 1.75% voltage distortion. As before, we satisfied our 5% requirement.
A final requirement of our VFD specification called for the VFD manufacturer to conduct on-site measurements of harmonic contributions due to the new VFDs. The supplier made measurement studies both before and after the start-up and gave results of these measurements to us one month after start-up.
This project is almost completed, with all electrical equipment installed and connected. The formal test results on THD levels are not yet submitted by the electrical contractor. However, while testing the engine-generator sets (which have extensive monitoring equipment), we managed to get preliminary THD readings as a by-product of this testing program. Because the generator's synchronous panel-board includes a digital electronic metering package and can read THD levels, we measured the THD placed on the emergency distribution system during generator testing (when all the VFDs transferred to the emergency generators). The digital readout indicated constantly fluctuating values, the approximate average being slightly above the specified 5% requirement. Such variances between calculated data and field test results are not unusual.
Specifying the VFDs In writing the project's VFD specifications, we felt the VFDs should have the following characteristics: * Ease of programming for motor operation and problem identification. * A high degree of trip-free operation. * A low magnitude of harmonic injection into the distribution system. * Compliance with ANSI/IEEE 519-1992. This guide applies to all types of static power converters used in industrial and commercial power systems and offers recommended limits of disturbances (harmonics) placed on AC power distribution systems.
After a careful review of the VFDs available, we specified units having a 40-character alphanumeric display, with all readouts in English. Operating data, such as output frequency, motor speed, motor current, motor torque, etc., were to be easily accessed and displayed. Any faults were displayed in English, for fast diagnosis and response.
In addition, we required an insulated gated bipolar transistor (IGBT) design in the power switching unit. This fast-acting device enables the VFD to adjust instantaneously to an overload condition, reduce current momentarily, and then accelerate the motor to the required speed set point if the overload condition is no longer present. With this feature, the VFD can tolerate temporary overloads and trip instantaneously only on a dead short or ground fault.
The VFD design incorporates a DC reactor built into the drive's DC bus. This reduces the harmonics at the VFD line terminals by approximately 50%, and greatly assists in meeting IEEE 519 requirements limiting THD.
Suggested Reading EC&M article: "Mating New Variable Frequency Drives To Existing Motors," March 1996 issue.
For photocopies, call (913) 967-1946. There is a fee of $10 for the first article ordered and $5 for every subsequent article.
ANSI/IEEE standards: ANSI/IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control In Electric Power Systems.
For ordering information on IEEE publications, call 1-800-678-4333