By Michael Lowenstein, Harmonics Limited and John Hibbard, TransCoil, Inc.
The most effective way to meet harmonic distortion limits i to filter the harmonics at each individual load and measure them at the point of common coupling
Despite efforts by its authors to set forth guidelines for limits on harmonic currents within industrial distribution systems, IEEE 519-1992, ¡°Recommended Practices and Requirements for Harmonic Control in Electric Power Systems¡± and its recommendations are still a source of confusion for some in the electrical industry (Sidebar below). Section 10 of this standard, ¡°Recommended Practices for Individual Consumers,¡± describes the recommended current distortion limits that apply within an industrial plant. Consulting and applications engineers appear to be uncertain as to the proper use of Table 10.3, which outlines the limits of harmonic distortion as a function of the nature of the electrical distribution system.
To help clear the confusion and uncertainty, we'll explain, with examples, the proper use and interpretation of this table.
Explaining Table 10.3. To appreciate the effect of IEEE-519-1992, you must understand the meaning of the terms used in Table 10.3 of the document (Table).
Point of common coupling (PCC). This is probably the most important and most controversial term in the entire document. The standard defines it as ¡°the electrical connecting point or interface between the utility distribution system and the customer's or user's electrical distribution system.¡± While simple in concept, identification of this point is sometimes misunderstood, which leads to confusion and misapplication of the specifications in the table.
The single-line diagram shown in Fig. 1 represents a small distribution system. The electric utility distributes power at 69kV, which in turn feeds a 13.8kV distribution system via a 13.8kV, 3-phase, 60-Hz, 20MVA distribution transformer with an impedance of 8.5%. In this example, the industrial facility uses a 1,000kVA, 6.7% impedance service transformer to step the voltage down from 13.8kV to 480V, which is bused throughout the plant.
The columns of Table 10.3 that you should use to determine harmonic limits will depend on the location of the PCC. PCC-1 in Fig. 1 is the primary of the service transformer. When the utility's customer owns the service transformer, the utility will meter the medium voltage ¡ª in this case, 13.8kV ¡ª at this point. If the utility meters the 480V bus, PCC-2 is the interface. As we shall see shortly, the allowable harmonic distortion depends on the defined PCC.
Engineers are more frequently applying the limits of Table 10.3 to an individual load, as represented by Point A in Fig. 1. You must remember that any distortion noted at this point in the system is produced by the drive (when it's operating) located inside the customer facility, and won't affect the drive's functions. Furthermore, high distortion at Point A doesn't necessarily result in out-of-limit distortion on the distribution system. If you attempt to meet limits for each individual load, you'll discover that either currently available technology is incapable of doing the job, or you'll need very high-cost equipment to do it. However, if you remember that IEEE 519-1992 is meant to apply to system harmonic distortion rather than to individual load distortion, you'll avoid unnecessary expense. The most effective way to meet harmonic distortion limits is to filter the harmonics at each individual load and measure them at the PCC.
The electric utility often selects the PCC within the system. However, you should be aware of the effect the location of the PCC has on harmonic limits, and you should work with your electric utility to ensure that both parties meet the spirit of IEEE 519-1992 without excessive expense.
Short circuit current at PCC (ISC). As its name implies, this is the available short circuit current at this point. The size, impedance, and voltage of the service feeding the PCC determines the amount of this current.
Maximum demand load current at PCC (IL). This current, at the fundamental frequency, is measured at the PCC. In existing facilities, you should measure this over a period of time and average it. For new designs, calculate the IL using the facility's anticipated peak operation.
Ratio of available short circuit current at PCC to maximum demand load current at same point (ISC/IL). This is actually a measure of the ¡°stiffness¡± of the electrical system relative to the load. For example, if Niagara Falls is available to feed a small load, the ratio is larger (>1,000). If a small transformer with just enough capacity for the load is the only available power source, the ratio is small (<20).
Total demand distortion (TDD). This is based on the maximum demand load current (fundamental frequency component) and is a measure of the total harmonic current distortion at the PCC for the total connected load. TDD isn't intended to be the limit for any individual load within a distribution system.
Harmonic order (h<11, 11¡Üh<17, etc.). These columns indicate the limits for any individual harmonic current at the PCC, expressed as a percentage of the fundamental frequency portion of the maximum demand load current.
Calculations using Table 10.3. Fig. 2 ( click here to see Fig. 2) shows several simple calculations based on the distribution system shown in Fig. 1. If PCC-1 in Fig. 1 is the measuring point, the data from Table 10.3 show that the TDD permitted for an I SC/I L ratio of 283 is 15%. The 5th and 7th harmonics are each permitted to be 12%. The values measured (10% TDD, 9% 5th, and 4.4% 7th) are within the permitted limits so you don't need to take any further action. Since a relatively stiff (20MVA) system transformer feeds a relatively small 1,000A (830kVA) load, you should expect this.If PCC-2 is the measuring point, the data from Table 10.3 show that the TDD permitted for an ISC/IL ratio of 18 is 5%. The table permits the 5th and 7th harmonics to be 4%. But the values measured are greater than the permitted limits. So you would need to install harmonic mitigation to meet the recommendations of IEEE 519-1992. This is an example of a small load on a system that's only adequate for one of that size.
Lowenstein, president, Harmonics Limited, Brookfield, Conn., co-authored this article with the late John Hibbard, vice president of engineering, while working at TransCoil, Inc., Milwaukee.
IEEE 519-1981 established levels of voltage distortion acceptable to typical distribution systems. But with the increase in industrial usage of adjustable speed drives, rectifiers, and other nonlinear loads, it became very apparent that a rewrite of IEEE 519 was necessary, with a directed focus on the relationship of harmonic voltages to the harmonic currents flowing within industrial plants.
IEEE 519-1992, the result of this rewrite effort, sets forth limits for harmonic voltages on the utility transmission and distribution system as well as for harmonic currents within industrial distribution systems. Since the passage of harmonic currents through a distribution system's impedances generates harmonic voltages, you can control harmonic voltages on your utility's distribution by controlling the currents or system impedances within your facility.