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Differences Between Medium and Low Voltage Filter Design Abstract—The use of harmonic filters at both the low and medium voltage level is becoming quite prevalent due to the recent advances in solid-state power converters. The market structure and the real world application of passive harmonic filters is still relatively new, and as such, the real world aspects of the present harmonic filter market is not well understood by many consulting and facility engineers. The purpose of this paper is to introduce the present harmonic filter market and how harmonic filters are applied, designed, and manufactured in the US. There are many papers written about the subject of harmonics, but there are few, if any, that address the practical aspects of this relatively new market. I. Introduction Wide spread use of harmonic filters on low and medium voltage industrial systems did not occur until the early 80’s, when the use of variable frequency drives became quite prevalent and the harmonic problem became widely understood. The development of cost-effective, high-powered, semi-conductor technology is what drove this market. Before this time, harmonic problems were rare and were isolated to a select number of industries using devices such as the Mercury Arc Rectifier. As such, the application and design of harmonic filters were limited, and was done on a job-by-job basis by specialized engineering firms and manufacturers. The harmonic filter market, being conceived in the early 80’s, is relatively young and its market dynamics are not well understood by many engineers. The low, and medium voltage filter markets are different from a technical, marketing, and application standpoint. The aspects of medium voltage (above 600 volts) filters are quite different than low voltage (600 volts and below) filters, while medium voltage (2.4 kV and 4.16 kV filters) filters seem to share commonalities to both voltage levels. There are many papers written about the subject of harmonics, but there are few, if any, that address the practical aspects of this relatively new market. This paper addresses the present harmonic filter market, with an emphasis on “real world” harmonic filters.
Fig.
1 and Fig. 2 show the typical layout of low and medium voltage harmonic
filters. In many cases a filter bank may Fig. 1. Typical Medium Voltage Filter Layout consist
of two or more separate stages (smaller banks), connected to a main
switch. This is typically done to provide more precise control
of voltage and/or power factor, or to limit the size of the contactors
or capacitor switches. Except for the reactor(s), the components
in a harmonic filter are similar to that of a shunt capacitor bank.
In fact, a harmonic filter behaves like a capacitor bank at 60 Hz
(it provides leading kvar, and can be used to control power factor
and voltage). It is at frequencies above 60 hertz that the
two behave differently. Although there are many types of filters,
nearly all harmonic filters used in industrial facilities are either
single-tuned or high-pass filters as shown in Fig. 3. The more complex
filter arrangements, such as the C-form filter are more costly,
less effective, (at equal fundamental var ratings), and are rarely
furnished. These filters also require much more engineering. Fig. 3 - Types of Harmonic Filters Harmonic filters function by providing a low impedance path for harmonic currents generated by non-linear loads, as shown in Fig. 5 for the system shown in Fig. 4. The filter impedance at the tuned frequency is much lower than the system/source impedance. The net effect (parallel combination of the harmonic filter impedance and the source impedance at the 5th harmonic) is much less impedance at the fifth harmonic. This effect is illustrated in Fig. 6 for all frequencies up to the 25th harmonic. The plot shows three harmonic impedance scans (looking from the drive) of a typical industrial facility. As shown, the impedance for the system with a harmonic filter, is much lower at its tuned frequency (5th harmonic in the plot). This is particularly true at the 5th and 7th harmonic where a significant amount of harmonic current would be present for a 6-pulse rectifier. A low net system impedance is desirable at all frequencies where harmonic currents are present since many non-linear loads act as ideal current sources, and a lower impedance would result in a lower voltage distortion.
Fig. 4- Simplified System Diagram
Fig.
5 - Equivalent Circuit of System Shown in Fig. 4 Other important points that should be noted in Fig. 6 are as follows:
Table 1 summarizes the major application and construction features of low and medium voltage harmonic filters. Only the more pertinent aspects are discussed in this paper. A. Capacitor Design Most
(if not all) capacitors used in harmonic filters at the low voltage
level make use of a “metalized” electrode. A metalized electrode
has a “self-healing” characteristic, and offers the advantage that
a dielectric fault inside the capacitor does not translate into
a faulted capacitor, but rather a small loss of capacitance.
Thousands of such dielectric faults can occur before there will
be any measurable change in capacitance. The main concern
with the use of a self-healing capacitor in low voltage harmonic
filters is the change in tuning frequency, and the shift in the
anti-resonant frequency from the unpredictable loss of capacitance
over time. This is illustrated in Fig. 7. Low voltage
manufacturers compensate for this problem by tuning their filter
banks away from the 5th harmonic, usually to a frequency
near the 4.7th, but some tune down to 4.1. Tuning down to
4.1 can cause the anti-resonant frequency to shift down near the
3rd, and can cause problems with 3rd harmonics generated by the
magnetizing reactance of motors and transformers. The one
draw back in tuning away from the fifth harmonic is that it causes
unnecessarily high filter impedance at the 5th harmonic (less filtering
capability).
Fig. 7 - Shift in Tuning Frequency Due to Loss of Capacitance in Low Voltage Capacitors At the medium voltage level, the capacitor usually consists of a separate film/foil design, in which the electrode is totally separate from the capacitor dielectric. These capacitors are not self-healing and therefore, their capacitance changes very little with time. This offers a very stable tuning point, but does not provide protection from dielectric faults. There is no industry data, however, that suggests medium voltage capacitors are less reliable than low voltage, metalized film capacitors. In addition to electrode design, some low voltage manufacturers have begun to manufacture dry-type capacitors. The use of a dry dielectric in harmonic filter capacitors is relatively new, and the technology is somewhat unproven in electric power applications. B. Capacitor Connection Due to economics, low voltage capacitors are generally three phase, and are internally connected as ungrounded wye or delta. The internal connection makes for a more compact design, but it conceals the neutral and removes the possibility of utilizing neutral unbalance voltage detection schemes and the choice of putting the reactor on the load side of the capacitor bank. The neutral detection scheme, commonly used on medium voltage banks, protects capacitors from elevated voltages due to blown fuses on parallel, ungrounded-wye connected (split-wye) capacitors banks. Elevated voltage on unbalanced capacitor banks is not a problem if the capacitors are delta connected. This is not the case for all low voltage manufacturers as some capacitors are connected internally in a wye configuration to alleviate the voltage stress associated with harmonic filters.
Since medium voltage capacitors are typically single phase and may have two bushings, they can be connected in any manner. The most common is the ungrounded-wye connection due to the simplicity in bus work. This permits the detection of blown fuses by a neutral voltage relay. C. Capacitor Voltage Rating Depending upon the manufacturer, low voltage filter manufacturers may use 480-volt, 600-volt, or “harmonic-hardened” capacitors. The harmonic-hardened capacitors are rated to carry excess harmonic currents and voltages. The problem with the harmonic-hardened capacitor concept is that a comparison between the capacitors of one manufacturer and that of another are difficult to evaluate, and are based on claims. To make the comparison even more difficult, the ANSI/IEEE shunt capacitor standard does not really apply to low voltage capacitors, and therefore, there is not really a standard for shunt capacitors at the low voltage level. Underwriters Laboratories provides a standard, but this standard is for safety only. At the medium voltage level, harmonic filters make use of standard, “off-the-shelf” capacitors. The capacitors are usually rated at about 115% of the systems nominal voltage rating to account for the fundamental voltage rise and harmonic voltages. The actual voltage rating used is determined by the design engineer, and varies depending upon the tuned frequency, philosophies of the engineer, harmonic current levels, and the interpretation of the standards. This does not present a problem from an evaluation standpoint because medium voltage capacitors are more likely to adhere to the ANSI/IEEE shunt capacitor standard and therefore, can be evaluated based on nameplate data. D. Capacitor Protection Capacitor protection at the low voltage level is simple and straightforward. Low voltage capacitors, unlike medium voltage capacitors, fail open and contain pressure interrupters to prevent case rupture. A common rule of thumb used by most manufacturers would be to fuse the capacitor at 250% to 300% of the capacitor’s rated current. At the medium voltage level, capacitors are more difficult to fuse because they fail shorted, and do not contain pressure interrupters. Therefore, there is a higher risk for case rupture. The fuse must be sized low enough to detect low level, evolving faults, but high enough to prevent spurious fuse blowings. In addition, the application of fuses at the medium voltage level is more complex than at the low voltage level. There is no simple rule of thumb. For example, a medium voltage current limiting fuse can be classified as either “general purpose” or as “back up current limiting”. They can also be classified as either “limited expulsion” or “expulsion” and can have slant voltage ratings. The application and operating characteristics of these fuses are considerably different. E. Engineering Design Almost all low voltage filter banks are standard, “off-the-shelf” designs. Cut-sheets, catalogs, and list prices are available from most of the major manufacturers. Since the designs are standard, filters can be offered with lead times as little as one week. The disadvantage of the standard design concept is that unusual conditions may not be accounted for. For example, a facility with an unusually high harmonic current level may get the same reactor and capacitor as a facility with an abnormally low current level. This may be true even if a written specification is offered. Some manufacturers may rather assume the risk of filter bank failure instead of offering a custom more costly design. Rating, rating criteria, and rating standards on low voltage harmonic filters are not readily available and therefore, it is difficult to compare filter ratings between manufacturers. Most if not all medium voltage filters are built and designed on a job-by-job basis. In many cases the manufacturer follows a specification that was either written by a consultant or A&E firm. Since medium voltage filters are not standard in design, designs can vary considerably from job to job, and lead times are usually on the order of 10 to 15 weeks. F. Filter Tuning Almost all filters at the low voltage level are tuned near the 5th harmonic (actually around the 4.7th harmonic). A couple of manufacturers tune their banks lower, but still refer to their filters as 5th harmonic filters. Tuning the bank to a harmonic near the 4th lowers the harmonic current and voltage stress on the filter components. This reduces costs but also reduces filtering capability. This concept is advantageous to a facility looking to improve power factor and avoid resonance rather than improving voltage distortion. Facilities that require filtering should use filters that are tuned near the 5th harmonic. In either case, the end-user should request tuning frequency from the manufacturer. G. Reactor Filter reactors can have either an iron-core or air-core. In general, low voltage filters are usually iron-core, while medium and medium voltage filters can be either iron- or air-core. Air core reactors take up more space and are difficult to put into enclosures. They are typically more economical on large filter banks, and are usually applied in conjunction with outdoor rack-mounted filters. Air-core reactors do not saturate and therefore, contribute to a more stable tuning point even for high currents under unusual conditions. Low voltage iron-core reactors typically make use of a three-phase core as shown in Fig. 8. Reactors built on these cores weigh less, take up less space, have lower losses, and cost less than three single-phase reactors of equal capability. Fig. 8 - Typical Iron Core Designs Medium voltage iron-core reactors can be either three phase or single phase and are primarily dependent upon the manufacturer and their capabilities. The
primary draw back to iron-core reactors is that they saturate.
The saturation level is dependent upon the fundamental current and
the harmonic currents that the reactor will see. There is
not an ANSI or NEMA standard for rating harmonic filter reactors
and therefore, it is difficult to evaluate reactors from different
manufacturers. For example, some reactor manufacturers base
their core designs (cross sectional area of core) on RMS flux, while
other will based it on peak flux (with the harmonic flux directly
adding). There is a significant difference between these two
design criteria. For evaluation purposes, reactor weight and
temperature rise are a primary indication of the amount of iron
that is used. IV. Conclusion This paper has illustrated the present harmonic filter market and how filters are designed, manufactured, and applied in the US. It should be quite clear from the paper that the design and manufacturing of medium and low voltage filters are quite different. It should also be clear that there is a lack of standards in the area of harmonic filters, which makes the consistency, and evaluation of harmonic filters difficult. It is hoped that this paper will stimulate an interest in the actual design, application, and manufacturing of passive harmonic filters, and increase the number of papers and standards devoted to harmonic filters.
Paul B. Steciuk - Mr. Steciuk received his BS Degree in Electric Power in 1988 from Rensselaer Polytechnic Institute in Troy, New York and his MS degree from the same institution in 1996. His experience is in the areas of product applications and power system analysis. He has been involved in the design, specification, development and fabrication of harmonic filters and power factor correction equipment. His duties have also included harmonic analysis, the development of harmonic measuring techniques, problem solving in the areas of power factor correction capacitors, performing load flow studies on power systems to establish thermal transfer limits and reactive power flow requirements, and other power system analyses.
Northeast
Power Systems, Inc. Website: www.nepsi.com
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