HARMONIC LOADING-RELATED PERFORMANCE OF POWER LINE FILTERS FOR SHIELDED ENCLOSURES

Arshad Mansoor EPRI Power Electronics Applications Center

10521 Research Drive, Suite 400 Knoxville, TN 37932

Abstract - Power line filters (PLFs) are used to maintain integrity of shielded enclosures by preventing unwanted signals from either entering or leaving the enclosure via the power supply conductors. These filters are intended to provide a high insertion loss-typically about 100 dB-above 14 kHz without affecting the quality of the power frequency voltage waveform that is fed to equipment inside the enclosure. Connecting nonlinear loads to these filters will distort the filter input and output voltage waveform, cause increased filter heating, and diminish filter attenuation performance. Most performance specifications require PLFs to have less than 1% fundamental voltage drop under rated load conditions and require a voltage-drop test at rated load. This paper shows that this test is inadequate if the load consists of typical electronic equipment, and proposes a new waveform quality test to augment the test described in military standards commonly used by industry. The new test will identify any degradation in voltage quality caused by the connection of a PLF between a power source and equipment in a shielded enclosure.

Whenever a power line or any other electrical conductor passes through the wall of a shielded enclosure, the isolating function of the enclosure may be compromised. If the conductor penetrating the enclosure is fitted with an appropriate filter, and the filter is carefully bonded to the inner and outer walls of the enclosure, then the isolation performance can be preserved. Figure 1 shows the configuration of a typical five-element (double -7~) power line filter (PLF) used to maintain the integrity of a shielded enclosure.

-I- -L- -I-

Return conductor or metal case Figure 1. Typical Five-Element (Double-n) PLF Used for Shielded

Enclosures.

Under full load, PLFs provide low insertion loss (co.5 dB) at 50 or 60 Hz. The internal series inductors offer low impedance and the shunt capacitors provide high impedance at these frequencies. At frequencies above 14 kHz, PLFs provide a high insertion loss (typically about 100 dB ). The high leakage currents of PLFs at the power frequency, which are due to the shunt capacitors, are considered a side effect of isolation. The internal inductors present a

Thomas S. Key EPRI Power Electronics Applications Center

10521 Research Drive, Suite 400 Knoxville, TN 37932

high impedance and the capacitors a low impedance to high frequencies. Typical values of inductors and capacitors used in PLFs are in the range of 100 to 500 pH and 1 to 50 uF. Several applicable standards, including MIL-STD-220A and CISPR-17, describe methods for measuring the insertion loss of a PLF. The objective of this paper is not to compare and contrast the usefulness of the different test methods for insertion loss [ 1,2,3], but to analyze the power delivery performance of PLFs under nonlinear loading conditions with respect to voltage requirements at the loads inside a shielded enclosure.

FUNDAMENTALVOLTACEDROPTESTFOR PLFs

PLFs are designed to maintain voltage quality at the load inside a shielded enclosure while providing the intended insertion loss. Most performance specifications require PLFs to have less than a 1% fundamental voltage drop under rated full-load conditions. The following is the voltage drop test for PLFs as described in section 4.6.8 of MIL-F-15733(3 (Filters & Capacitors, Radio Frequency Interference, General Specification):

For filters rated AC only. The voltage drop is the difference between the input voltage to the filter and the output voltage of the filter when the filter is carrying rated current at rated voltage with a resistive load at the maximum rated power frequency. The method of voltage measurement is shown in Figure 2. Measurements shall be made by using expanded scale-type meters, which will enable voltage differences of less than 1 volt to be read.

++

V, - V, = Voltage Drop Figure 2. Measurements of Voltage Drop (MIL-F-15733G).

The purpose of this test is to determine the effect of the filter on the voltage applied to the loads inside a shielded enclosure. Figure 3 shows the input and output voltage of a filter rated at 60 amps and connected to resistive load of 50 amps. This is a typical tive-element filter as shown in Figure 1. The values of the main components are: C =22uFandL=390pH.

0-7803-5015-4/98/$10.00 0 1998 IEEE

Using the voltage drop test method described by MILF-15733G leads to a voltage drop calculated as follows:

Input voltage (RMS) = 266 volts; Output voltage (RMS) = 265 volts; Voltage drop (%) = (266-265)/266 *100 % = 0.38%

Filter Input

\ / /

I

Filter Output

Figure 3. Input and Output V&age Waveform of a Filter Rated at 60 Amps and Connected to a 50-Amp Resistive

Load.

However, subsequent results will show that this method does not provide a true indication of the voltage delivery performance of the filter under nonlinear loading conditions. The problem here is that an Rh4S (root-mean-square) value is not a sufficient measure of voltage quality. Total harmonic distortion (THD), crest factor (the peak of the voltage divided by the RMS values), slope, and number of zero crossings are all important factors that may affect the performance of the load. Meeting only the RMS voltage requirement does not guarantee that the other quality factors will be maintained.

CHARACTERISTICSOFNONLINEARLOADS

The number and significance of nonlinear, harmonic-producing loads has increased substantially over the past several years. The power supplies for DC-circuits used in most single-phase power electronic devices are the main source of harmonic currents. Personal computers, printers, faxes, and other communication devices, television receivers, and many other loads used in residential and commercial environments are equipped with a capacitor-filtered diode bridge rectifier to convert the incoming utility voltage to the DC level required by these devices.

Figure 4 shows a basic model of a switch-mode power supply (SMPS) and its power source. A typical switch-mode power supply consists of a full-wave diode bridge rectifier, DC-smoothing

Ll RI

Figure 4. Capacitor-Filtered Diode Bridge Rectifier Circuit in a Switch-Mode Power Supply.

capacitor, and a DC voltage regulator. The model in Figure 4 includes:

V,: R,,L,:

R,,L, : c: R: c4

Thevenin-equivalent system voltage, Thevenin-equivalent system impedance parameters, Local line impedance parameters, Smoothing capacitor, Equivalent load resistance.

As shown in Figure 5, the input current drawn by these SMPSs is characterized by a pulse-shaped waveform formed by current flowing during the charging period of the smoothing capacitor. This type of current is rich in odd harmonics, and the total harmonic distortion

100% 90% 80%

p 70%

H 60%

P 50% i r; 40%

* 30%

20%

10%

0% 1 3 5 7 9 11 13 15

Harmonic Number

g I- I \ E

4 z 0 B 1 0.004 012 OS 16 B -1 -

-3 ’ I Time (Seconds)

Figure 5. Harmonic Spectrum and Current Waveform of a PC Power Supply.

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(THD) is typically in the range of 80 to 120%. The main harmonic components in the load current are the 3rd, 5” and 7’h harmonic, as shown in the harmonic spectrum in Figure 5. Because of the symmetry in the waveform for the positive and the negative half cycle, there are no even harmonic components (2”d, 4’h, and so on). The current waveform is also characterized by a high crest factor in the order of 1.8 to 2.5 as compared to a 1.414 crest factor for a sinusoidal wave. Because RMS current is computed using all the frequency components-including the fundamental-these loads draw a much higher RMS current as compared to the fundamental (60-Hz) component.

VOLTAGE DISTORTION RESULTING FROM NONLINEAR LOAD CURRENT

The nonlinear load current injected into the power system interacts with the system impedance, resulting in voltage distortion. Figure 6 shows the resulting voltage at the common point of a nonlinear load connection. The high pulse of load current causes a higher voltage drop, resulting in a flat-top output voltage. The amount of voltage distortion depends on bdth the magnitudes of the system impedance and the nonlinear load current. For low voltage public power systems in the U.S., the system impedance typically ranges from 0.01 to 0.3 ohms resistive and from 0.004 to 0.4 ohms inductive reactance at 60 Hz (L = 10 to 1000 pH). The standard impedances specified by International Electrotechnical Commission (IEC) for 50-Hz system is 0.4 ohms resistive and 0.25’ohms inductive reactance at 50 Hz (L = 796 pH).

ril Bus VoltageV

Figure 6. Voltage Distortion Resulting from Nonlinear Current

The addition of a PLF in a circuit changes the total system impedance and increases the voltage distortion at the load. Figure 7 shows the voltage per amp of current at different frequencies for two cases: system impedance only (0.4 ohms resistance and 600 pH inductance) and system impedance plus impedance of a typical double-element X- filter (L = 100 uH and C = 50 pF). This curve, commonly known as a “frequency scan,” clearly shows the effect of the additional impedance introduced by the filter inductor and capacitor elements. One would expect that for the same amount of harmonic current, the presence of the filter would cause an increased voltage distortion at the load terminal and thereby degrade the quality of the voltage fed to the load.

FILTER OUTPUT VOLTAGE WAVEFORM WITH NONLINEAR LOAD

Figures 8 and 9 show the filter output voltage waveform gathered from a laboratory test with one diode bridge rectifier as the test load and a laboratory test with several personal computer power supplies.

The filter output voltage waveforms for most of the cases show a similar pattern of high distortion, low crest factor (flat-topping), multiple zero crossings, and ringing. This type of voltage waveform

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Harmonic Number (hndamental = 60 Hz)

Figure 7. Voltage per Amp of Current at Different Frequencies.

’ O-

\ pj

Figure 8. Filter Output Voltage Waveform for Diode Bridge Rectifier Load.

-100 - \ I I

-150 - I 1

Figure 9. Filter Output Voltage Waveform for PC Power Supply Load.

is expected in any shielded enclosure where nonlinear loads such as personal computers or other forms of SMPSs are used. The voltage flat-topping is primarily caused by the interaction of the filter inductance with the nonlinear load current. The multiple-zero crossings and the ringing are, on the other hand, mainly caused by the interaction with the nonlinear load current and the filter capacitors.

In order to assess the applicability of the fundamental voltage drop test in determining voltage quality at the filter output, the same test filter operated with a resistive load in Figure 3 was connected to a diode bridge rectifier load. Filter input and output waveforms for a

15amp nonlinear load current (25% of the filter full-load rated current) were measured. The waveform and measurements are shown in Figure 10 and Table 1. The filter under test easily passes the voltage drop test as the RMS voltage at the input and at the output differs by only 0.3%. However, the filter’s output voltage distortion and crest factor are considerably different than its input voltage. There are also multiple zero crossings in the output. This result points out the inadequacy of the voltage drop test to ensure quality of the voltage waveform fed to the loads inside a shielded enclosure.

-Filter Output Voltage -Filter Input Voltage

-200 -

Figure 10. Filter input and Output Voltage Waveform for 25% Loading (Diode Bridge Rectifier Load).

Table 1. Measured Characteristics of Filter Input and Output Waveform

Voltage Characteristics RMS (volts)

Input output

120.3 120.7

Percent Difference

0.3% THD (%) I 3.6 1 13.9 1 286% Crest Factor 1 1.39 1 1.33 1 -4.3% Zero Crossings Per Cycle

Nominal (Two)

Two Extra per Cycle 100%

EFFECTOFFILTEROUTPUTVOLTAGEONLOADOPERATION

There has been anecdotal evidence that power line filters have caused misoperation of various equipment inside shielded enclosures, such as electronic processing equipment, clock circuits, and power converters. Even uninterruptible power supplies (UPSs), which may be installed to protect critical loads inside a shielded enclosure, have themselves been affected by the voltage waveform at the output of a PLF. In some instances, the filters have been removed, thereby compromising the integrity of the enclosure in order to maintain the proper operation of loads inside it. Some reasons for these problems may be seen in the voltage waveforms at the filter output shown in Figures 8 and 9. These waveforms are expected at the filter output when the loads in the shielded enclosure are nonlinear, most likely PCs and computer peripherals. Some of the common affected equipment and the negative interactions are:

The voltage-sensing circuit in a UPS that determines the transfer from utility line to battery during an undervoltage condition can be susceptible to the distorted voltage waveform present at the filter output. One type of voltage-sensing circuit is based on peak detection. For a sinusoidal voltage waveform, the peak is 1.414 (square root of 2.0) times the RMS voltage. However, this condition does not hold true for a distorted voltage waveform. The ratio of peak to RMS voltage for a flat-topped voltage waveform is lower than

1.414. Therefore, the UPS voltage-detection circuitry may initiate a transfer to battery because the incoming peak voltage is outside the UPS voltage tolerance.

The same situation could occur for voltage-sensing circuits based on slope detection or rectified average-value measurement. In general, any sensing circuit that expects the voltage waveform to be sinusoidal will not work properly with high distortion. Some UPS systems are also equipped with over/under frequency relays, which may respond incorrectly to the distorted voltage waveform or multiple zero crossings.

The switch-mode power supplies used in PCs can operate as long as the RMS voltage remains within the steady-state tolerances. In fact, a typical PC operates satisfactorily with a rectangular voltage waveform so long as the peak voltage is sufficiently high. However, flat topping of the voltage waveform results in a reduced DC-bus voltage in the power supply. Also, the additional line impedance caused by the series-connected PLF may cause an undervoltage condition during intermittent high-current demand of loads such as laser printer or copier heaters. The combination of these two factors-voltage flat-topping and inrush current-may cause PCs to reset.

Extra zero crossings of the filter output voltage will negatively interact with any load that requires a timing signal synchronized with the power line voltage. One such device is a digital clock; another is a power converter using a line-synchronized control. For example, silicon-controlled rectifiers (SCRs) used in a rectifier or converter must be triggered to begin conduction and must see a zero crossing to end conduction. Voltage is sometimes regulated by varying the “on” time to “off’ time using phase angle control. When the voltage crosses zero more than two times per cycle, an SCR may fire erratically from one cycle to the next. Ideally, timing circuits should respond only to the 50- or 60-Hz fundamental voltage waveforms.

The long-term effect of distorted waveforms on electromechanical loads such as motors depends not only on the level of distortion but also on the particular shape of the voltage waveform. Dr. Ewald Fuchs has conducted extensive testing of end-use devices under distorted voltage conditions at University of Colorado and published the results in several technical papers [4, 51. In general, the flat- topping of voltage waveforms results in additional losses and temperature rise in electromagnetic devices. Based upon the decrease in lifetime with temperature for different insulating materials, reduced life can be correlated with the harmonic distortion of applied voltage.

EFFECTOFNONLINEARLOADINGONFILTERCOMPONENTS

Filter components such as series inductors and shunt capacitors are typically designed for sinusoidal voltage. The pulse-shaped current waveform of nonlinear loads and the resulting voltage distortion will affect these filter components and may cause premature failure. Distorted current primarily affects the inductive component of the filter. Numerous articles on this issue have been published documenting the negative impact of pulsed current waveforms on inductor saturation, increased I’R, and eddy current losses with associated temperature rise [6,7]. These effects have been recognized for a long time on power transformers, and effective standards such as [8] are in place to help determine the proper derating of inductors for harmonic current-carrying capability.

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In contrast, the effect on filter capacitors is not well understood. Plastic-film capacitors have been typically used in PLF applications with foil and more commonly metallized film. The voltage iating used by a capacitor manufacturer is usually based on DC voltage. The rating is the specified operating DC voltage, which the capacitor can withstand continuously at a given temperature. In the case of applied AC voltage, the capacitor dielectric is exposed to the peak voltage for a brief period during each cycle. However, as was shown in Figures 8 through 10, the resulting voltage waveform at the filter output terminal has a lower peak voltage than a normal sinusoidal waveform. Therefore, the likelihood of a dielectric breakdown because of voltage stress is minimal.

Nevertheless, current through the capacitor with the expected filter output voltage waveform could be a critical factor. Metallized-film capacitors have a limit to the amount of peak current they can withstand. In the case of a sinusoidal AC waveform, the current stress on a metallized-film capacitor is normally below the limitation of the capacitor. In this case, the shunt capacitor on the load side of the filter will be subjected to the distorted voltage waveform, which has a sharp slope at the edges and a relatively flat characteristic in the middle.

Realizing that the current through a capacitor is proportional to the rate of change of voltage (I = C dv/dt), one would expect a high current through a capacitor during the high rate of change in voltage. Figures 11 and 12 show the current through a 50-J.IF capacitor subjected to a sinusoidal voltage and a distorted voltage that would be expected with PLFs and nonlinear loads. The figures are based upon simple numerical calculations using the digitized voltage data. These figures clearly show that the peak current through the capacitor under the distorted-voltage conditions can be several times higher than what would be expected under sinusoidal voltage conditions.

Laboratory tests were conducted with a .50+F capacitor under normal and distorted voltage created with an arbitrary waveform generator and a power amplifier. The results, shown in Figure 13, confirm the increase in peak current through the capacitor for distorted voltage. If the peak current is too large, it is possible that the interconnection between metallized-electrode layer and the end spray could gradually be eroded. Under these conditions, the capacitor dissipates more heat and may lose capacitance. In the worst case it is possible that the capacitor could fail open.

MITIGATING NEGATIVE INTERACTION BETWEEN PLF AND NONLINEAR LOAD CURRENT

We have shown that interaction between a PLF and distorted load current will usually adversely affect the quality of the voltage waveform fed to loads inside a shielded enclosure. This same distortion stresses the filter components and can lead to premature filter failure. Ensuring that the filter components are rated to handle the harmonic loading will prevent filter failures without affecting the quality of the voltage waveform. To ensure that PLFs do not adversely affect the voltage quality, two possible modifications are considered. The first potential modification is changing the design of the filter so that it meets the insertion loss requirement while handling the distorted currents and not adversely affecting the quality of the voltage waveform. Considering that the amount of inductance in the PLF is the main reason for the degradation of voltage waveform, one possible design change could be to decrease the inductance [9]. However, to meet the insertion loss requirement at a

Figure 11. Capacitor Current under Sinusoidal Voltage Waveform (Calculated).

Figure 12. Capacitor Current under Distorted Voltage Waveform Expected at PLF Output (Calculated).

50 , 7 40 - 30 -

-40 ’ i

Figure 13. Measured Capacitor Current for Sinusoidal and Distorted Voltage.

lower inductance, the capacitor values may have to be increased considerably, thereby increasing leakage current, which is already high for typical PLFs. Whether such a filter can be designed or not is left best to filter manufacturers and designers.

The second potential modification is adding a harmonic-elimination circuit to switch-mode power supplies, which transforms electronic loads to linear and effectively eliminates the problem. The widespread use of linear power supplies will require a global change in attitude from manufacturers and purchasers. However, current work in the IEC and relevant standards for limiting harmonic currents for small power electronic loads such as PCs may ultimately lead to universal requirements for power-factor-corrected power supplies, which will eliminate harmonics from these loads [lo]. Technology already exists to manufacture harmonic-free power supplies by using active switch-mode compensation methods [ 1 I]. However, the

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relative cost of harmonic elimination is the main barrier for manufacturers.

NEW VOLTAGE DROP TEST FOR PLFs

Given that many nonlinear loads are used in shielded enclosures, the authors propose a change in the voltage drop test as required by MIL- F-15733G and subsequently used by most end users. The rationale behind changing this test is that nonlinear loads are increasing rapidly. In fact, it will be difficult, if not impossible, to find enclosures with no nonlinear loads in the near future. Understanding this fundamental change in the load type, it is only natural that ensuring quality voltage should depend not only on the RMS of the voltage but also on the waveform characteristics, such as crest factor, THD, and zero crossings. Also, understanding that nonlinear loads will affect the voltage waveform even without the presence of a PLF, the emphasis of the test is to ensure that the incremental effect of a PLF on the voltage waveform should be limited.

The proposed waveform quality test will require a nonlinear load, and the authors propose a diode bridge rectifier as standard nonlinear load. The full-wave bridge rectifier should be capable of handling the rated full-load current of the filter under test. The DC bus capacitor should be sized to ensure that the crest factor of the load current is greater than 2.2 when connected to the power supply source without the PLF. The source should be stiff enough to deliver full-load current for the filter under test.

Two PLFs rated at 100 amps were tested using this method at 50% loading. The load current, input and output voltage waveforms, and the measured electrical parameters are shown in Figure 14, Figure 15, Figure 16, and Table 2. Clearly, the output voltage waveform of the two filters are quite different for the same nonlinear loading. However, there is not much of a difference in the RMS voltage magnitude.

Table 2. Input and Output Voltage Waveform Quality Parameters for Filters A and B

Filter A Filter B

Voltage Input 1 Output 1 % Diff Input Output % Diff Vrms 120.2 I 118 I -2% 120.2 121.1 -1%

_ 150 J.-..-----...- -,__ ___..__- -“_- -... -.._--- _..I..-.-. ---._ -_.. I

Time (Seconds)

Figure 14. Diode Bridge Rectifier Load Current Waveform for Filter Under Test.

200 , 1

Figure 15. Input and Output Voltage Waveform for Filter A.

V-THD 4.9% 4.2% -14% 4.8% 19.1% 298% V-Crest 1.37 1.37 0% 1.37 1.43 4%

Figure 16. Input and Output Voltage Waveform for Filter B.

1 Factor 1 I I I I I Table 3. Proposed Limits for Waveform Quality Test % Difference between Input

The following quantities should be measured at both the input and Voltage and Output Terminals output terminal of the filter: RMS voltage, voltage THD, and voltage Voltage RMS f 1% crest factor. The difference between input and output measurements for these quantities should be within a specified limit to ensure that Voltage THD rt 10% the incremental effect of a PLF in degrading the quality of the load Voltage Crest Factor f 10% voltage is limited. As a starting point, the authors propose the limits in Table 3 for the various voltage quality factors when a filter is CONCLUSION

tested with a diode bridge rectifier load. New electronic loading used in many shielded enclosures affect the

In essence, this proposed test is very similar to the voltage drop test; performance of power line filters, stress filter components, and lead

the only difference is in the change of load type from linear to to overheating or require filter derating. PLFs can be redesigned to

nonlinear and the additional measurement of several waveform handle nonlinear loads, much like power transformers are specially

characteristics. The manufacturer can decide to perform this test for a designed and built for heavy electronic loading. The loading problem

rated current less than the filter rating for power-frequency current. In can also be solved by modifying the electronic loads to be more

that case, filters should have a linear and a nonlinear current rating linear. However, given the widespread use of electronic equipment, new filters should be tested to determine their performance under

and be properly marked to indicate that. typical nonlinear loading.

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ACKNOWLEDGEMENTS

The authors thank Kermit Phipps and Tom Geist for completing the laboratory analysis required for this paper. Also, the Power Line Filter Manufacturers, Romesh Rajput of Fil-Coil, Terry Forsythe of Lindgren, Robert Hassett of RFI Corp., and Gerald Womer of LCR Electronics provided invaluable insights and assistance in completing this work.

REFERENCES

[l] Method of Insertion-Loss Measurement, Miiitary Standard (MIL- STD)-220-A, Government Printing Office, March 1978.

[2] Comitt International Special Des Perturbations Radioelectriques C.I.S.P.R., Publication 17.

[3] Hassett, R. E., “EMI Filter Characteristics and Measurement Techniques,” ITEM, 1996, pp. 18-20, 23-28,252-258.

[4] Fuchs, E. F., “Investigation on the Impact of Voltage and Current Harmonics on End-Use Devices and Their Protection,” EPRI Power Quality for End-Use Applications, San Francisco, California, March 21-23, 1990.

[5] Fuchs, E. F., K. P. Kovacs, and D. J. Roesler, “Reduction of Lifetime of Induction Motors Due to Harmonics of Terminal Voltage,” Proceedings of the International Conference on Electrical Machines, Munich, West Germany, September 1986.

[6] Briggs, S. J, et al, ‘The Effects of Nonlinear Loads on EMI/RFl Filters,” Proceedings of the IEEE/IAS Industrial and Commercial Power Systems Technical Conference, May I-5, 1994

[7] The Effects of Power System Harmonics on Power Loads and Equipment, IEEE Report, PAS-104, No. 9, September 1985.

[ 81 ANSI/IEEE C57.1 IO- 1986, Recommended Practice for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents.

[9] Hawthorne, W. T., “Solving Interaction Problems of a TEMPEST AC Power Filter and High Current Equipment,” ITEM, 1994, pp. 174-178,264.

[lo] IEC 1000-3-2, Electromagnetic Compatibility, Part 3: Limits- Section 2: Limits for Harmonic Current Emissions.

[l l] Lai, J. S., and T. Key, “Switch-Mode Power Supply Power Factor Improvement via Harmonic Elimination Methods,” Proceedings of the IEEE Applied Power Electronics Conference, Dallas, Texas, March, 199 1.

TH2B

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