HARMONIC CURRENTS ISOLATING NETWORK

Abstract: A network for connection between an AC power line and a DC power supply to isolate harmonic currents generated by the DC power supply from the AC power supply, the network employing three similiar portions each connected between a respective conductor of the (three phase) AC power line and a respective conductor of the DC power supply. Each of the portions includes the series combination of three "reflectors" each having the parallel resonant combination of an inductor and a capacitor. Further, three series resonant inductor-capacitor combinations are employed each connected between a unique pair of the DC power supply conductors.

Description: The present invention relates to a network for connection between a line conducting AC power of predetermined voltage and frequency and the input of a DC power supply according to claim 1. The power factor that a load presents to an AC power line has long been of concern. At one time, the primary concern was the inductive component of some loads. Consider, for example, the section taken from the text entitled "American Electricians' Handbook", 8th edition (1961) which was edited by Terrell Croft, revised by Clifford C. Carr, and published by McGraw Hill Inc.: "143. Correction of Low Power Factor. In industrial plants, excessively low power factor is usually due to underloaded induction motors because the power factor of motors is much less at partial loads than at full load. Where motors are underloaded new motors of small capacity should be substituted. Power factor can be corrected (1) by installing synchronous motors (2) by connecting static capacitors across the line" (end of section). Of late, the "power factor" presented by DC power supplies has become a concern. Typically, DC power supplies employ a bridge rectifier, a filter capacitor, and, sometimes, a filter choke. The input of the rectifier is coupled (by a fuse, switch, etc.) across an AC power line. The output of the rectifier is either coupled by the choke across the capacitor (choke input filter) or, absent the choke, directly connected across the capacitor (capacitor input filter) to develop a DC (output) potential across the capacitor. With the choke (input filter), DC power supplies draw from the AC power line a current the waveform of which approximates a square wave (when the inductance of the choke is much greater than what is commonly referred to as the "critical" inductance). Absent the choke, the waveform more approximates a series of pulses each of which is synchronized with a corresponding peak of the AC power-line potential. In either case, the current drawn from the AC power line includes harmonic components (currents), one for each of the odd harmonics of the AC power-line frequency. DC power supplies do not conform to all of the old power factor conventions. (For example, it makes little sense to define the power factor a DC power supply presents to an AC power line as the cosine of the phase angle between the voltage developed across the input of the DC power supply and the current flowing into it.) However, they (DC power supplies) do present many of the same problems. DC power supplies, like other loads having a relatively low power factor, draw from the AC power line a current the rms level of which is disproportionately high in relation to the current that should be drawn for the power consumed. (In other words, they do conform to the definition which states that the power factor of a load (in this case, a DC power supply) is given by the ratio of the actual power consumed (in this case by a load connected to the output of the DC power supply) (as indicated by a wattmeter) to the apparent power (as indicated by the combination of a (true rms, iron-vane or thermocouple-type) ammeter and a voltmeter) (connected to the input of the DC power supply). A relatively high AC power-line rms current is of concern in that the AC power-generating facilities and AC power-transmission facilities (lines and transformers) must be sized to accommodate the current. Further, generation and transmission losses are primarily resistive losses which, therefore, increase as the square of the level of the rms AC power-line current. It is important to note that even relatively small loads (DC power supplies) may be of concern. Although a small personal computer, for example, may not draw the level of the current drawn by a large smoke stack scrubber, if the DC power supply of the computer has a relatively low power factor, the current drawn by the DC power supply may be of such a level as to limit what may also be plugged into a single AC power-line wall outlet. In addition, DC power supplies present many special problems, particulary for the military. As the number of airborne and shipborne systems employing DC power supplies has increased, so has the level of harmonic currents generated on the various AC power lines. With the increased levels of harmonic currents has come an increase in the above mentioned generation and transmission problems. In addition, and of potentially much more serious consequence, the high levels of harmonic currents are causing problems of undesired system interaction. As a consequence, standards have been promulgated including those in the Department of Defense document which is identified as DOD-STD-1399(NAVY) SECTION 300 1 AUGUST 1978 and which is entitled MILITARY STANDARD INTERFACE STANDARD FOR SHIPBOARD SYSTEMS SECTION 300 ELECTRIC POWER. ALTERNATING CURRENT . Of particular relevance is the following section. "6.2.7 Harmonic current . The operation of equipment shall be designed to have minimum harmonic distortion effect on the electrical system. The operation of such equipment with the following specified ratings shall not cause harmonic line currents to be generated that are greater than 3 percent of the unit's full load fundamental current between the 2nd and 32nd harmonic. ..." (For a power-source frequency of 60 Hz, a 1kVA or more unit rating is specified.) "Additionally, currents with frequencies from the 32nd harmonic through 20 kilohertz (kHz) shall not exceed 100/n percent ..." In Table I. (on page 5) the total harmonic distortion is limited to 5 percent; and, in section 6.2.2 the power factor is limited to the range of 0.8 lagging to 0.95 leading. Further, in Table I. (on page 5) the worst case frequency excursion from nominal frequency is listed as 5-1/2 percent. Heretofore, great difficulty has been had in meeting the above-mentioned standard. A prior-art-type circuit for improving the power factor a DC power supply presents to an AC power line is shown in the German patent number DE 3012747 and the Japanese patent number 58-163271. Both circuits include an inductor and a capacitor which is connected in parallel with the inductor. The inductor-capacitor combination is connected between the AC power line and the input of the bridge rectifier of a (capacitor-input-filter-type) DC power supply to couple the DC power supply to the AC power line. In the German patent it is indicated that the inductor-capacitor combination is usually tuned to the fifth but maybe the ninth or thirteenth harmonic of the AC power-line frequency. And, in the Japanese patent it is indicated that the purpose of the inductor-capacitor combination is to reduce the level of the third and fifth harmonic waves (currents?). Although of some value in reducing the level of some of the harmonic currents, it is important to note that the reductions afforded by the above-mentioned inductor-capacitor combination does not approach that required to meet the above mentioned standard. Further, while reducing the level of certain harmonic currents, the above-mentioned inductor-capacitor combination may increase the level of other harmonic currents. Finally, The use of the above mentioned inductor-capacitor combination may reduce the level of the DC output potential developed by the associated DC power supply. Disclosed in the British patent 1,472,411 of T. Kennedy is a filter network which is for use with a load having a non-linear control device (saturable reactor) and which is for absorbing unwanted harmonic currents. The filter network employs a plurality of filters each including an inductor and a capacitor which is connected in series with the (associated) inductor. Each of the filters (inductor-capacitor combinations) is connected in parallel with the load. An additional inductor is employed connected between the AC power line and the load to couple the load to the AC power line. It is indicated (on page 2 in lines 113-115 of the British patent) that each of the filters (inductor-capacitor combinations) is tuned to a frequency less than the harmonic frequency which it is to filter. Further, it is indicated (on page 3 in lines 115-128) that "... the existance of harmonics in the line makes it not only impractical, but also impossible to filter out all of the harmonic current. An attempt to do so may result in overloading the filters. Accordingly, it has been determined that the better approach is to remove only a percentage of the harmonic currents generated as a result of the load and control device. Thus, a certain amount of harmonic current is reflected back into the supply line. The amount of harmonic current so reflected can be adjusted depending upon the requirements of the utility system supplying power." In an example in GB-A-1,472,411 it is indicated (page 4, lines 109-112) that 70 percent of the harmonic currents are removed and 30 percents of the harmonic currents are reflected into the AC power line. Further, it is indicated (page 4, lines 112-115) that "(i)f the utility requirements on harmonics are more severe, it is possible to remove an even higher percentage such as 75%". Again, although of some value in reducing the level of some of the harmonic currents, it is important to note that the reductions in the levels of harmonic currents afforded by the network disclosed in GB-A-1,472,411 does not approach that required to meet the above-mentioned standard. In US-A-4,222,096 and US-A-4,369,490 a circuit is disclosed which includes a capacitor connected in parallel with the input of the bridge rectifier of a (capacitor-input-type) DC power supply and an inductor connected between the AC power line and the input of the rectifier to couple the DC power supply to the AC power line. In US-A-4,369,490, column 5, lines 23-29, it is indicated that without the above-mentioned circuit, the DC power supply was found to present a power factor of 65 percent to the AC power line. Also, it was found that without the above-mentioned circuit, the level of the third harmonic current was 88 percent, the level of the fifth harmonic current was 65 percent, and the level of the seventh harmonic current was 38 percent of the level of the fundamental current. In one example, with the above-mentioned circuit, the DC power supply was found to present a power factor of 94 percent to the AC power line. Also, with the above-mentioned circuit, the level of the third harmonic current was 20 percent, the level of the fifth harmonic current was 6 percent, and the level of the seventh harmonic current was 2 percent of the level of the fundamental current. A prior art arrangement for a high voltage direct current transmission is described by the DE-A-1,927,415. A harmonic filter is connected between an alternate current network, and a rectifier feeding a direct current transmission line. The known arrangement is capable of limiting to a sufficient degree the noticability of harmonics, caused by the frequency changer, in the alternating current network. Therefore, the harmonic filter consists of a π part, out of a longside part being the rejector circuit and two cross parts both switched at the end of the a longside part being the parallel filters. Although the above-mentioned circuit greatly increases the power factor a DC power supply presents to an AC power line and greatly reduces the levels of the harmonic currents, it is important to note that the DC power supply (and circuit combination) still does not even come close to meeting the above-mentioned military standard. It is an object of the present invention to provide a means by which the levels of harmonic currents generated on an AC power line by a DC power supply may be reduced so that the DOD-STD-1399 (NAVY) SECTION 300 1 AUGUST 1978 standard may be met, and by which the power factor a DC power supply presents to an AC power line may be improved. The object of the present invention is solved by the features as claimed in the characterizing part of claim 1. Preferred embodiments are set forth in the subclaims. The ability to provide a means by which DC power supplies may be made to meet the DOD-STD-1399 (NAVY) SECTION 300 1 AUGUST 1978 standard is the principal advantage of the present invention. Another advantage of the present invention is the ability it affords to provide a means by which DC power supplies may be made to meet commercial/industrial standards. Another advantage of the present invention is the ability it affords to provide a means by which the levels of harmonic currents generated on an AC power line by a DC power supply may be reduced. Still another advantage of the present invention is the ability it affords to provide a means by which the power factor a DC power supply presents to an AC power line may be improved. These and other objects and advantages of the present invention will no doubt be obvious to those skilled in the art after having read the following detailed description of the presently preferred embodiments which are illustrated in the several figures of the drawing. FIGURE 1 is a schematic diagram of the presently preferred embodiment in accordance with the present invention of a harmonic currents isolating network of military applications; FIGURE 2 is a schematic diagram of the presently preferred embodiment in accordance with the present invention of a harmonic currents isolating network for commercial/industrial applications; and, FIGURE 3 is a schematic diagram of a harmonic currents isolating network for more stringent commercial/industrial applications. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the present invention, illustrated in FIGURE 1 of the drawing generally designated by the number 10 is the presently preferred embodiment of a military grade network for isolating from an AC power line the harmonic currents generated by a DC power supply. Also illustrated with network 10 is a (three phase) AC power line, generally designated 12, and a (capacitor-input-type) DC power supply, generally designated 14. Network 10 is shown to employ three terminals (nodes), respectively designated 20, 20′, and 20″, each for connection to a respective conductor of AC power line 12 and three terminals (nodes), respectively designated 22, 22′, and 22″, each for connection to DC power supply 14. Network 10 also employs three similiar portions, respectively designated 30, 30′, and 30″, each connected between a respective one of terminals 20, 20′, and 20″ and a respective one of terminals 22, 22′, and 22″. (Herein, similiar portions/components are similiarly numbered.) Additionally, network 10 employs three similiar portions, respectively designated 32, 32′, and 32″, each connected between a respective pair of terminals 20 and 20′, 20′ and 20″, and 20″ and 20. Portion 30 includes an inductor (choke) 40 which is connected between terminal 20 and a node 42. Additionally, portion 30 includes a resistor 44 and a capacitor 46 which are connected in series between terminal 20 and node 42. Further, portion 30 includes an inductor 50 and a capacitor 52, the combination (of inductor 50 and capacitor 52) being connected in parallel (parallel resonant) between node 42 and a node 54. Finally, portion 30 includes an inductor 58 and a capacitor 60, the combination (of inductor 58 and capacitor 60) being connected in parallel (parallel resonant) between node 54 and terminal 22. Portion 32 includes an inductor 70 and a capacitor 72, the combination (of inductor 70 and capacitor 72) being connected in series (series resonant) between terminals nodes 22 and 22′. DC power supply 14 employs a bridge rectifier 80 and a filter capacitor 82. The input of rectifier 80 is connected to terminals 22, 22′, and 22″; and, the output of the rectifier is (in this case) directly connected across capacitor 82 (between a pair of terminals 84 and 86) to develop a DC (output) potential (between the terminals) across the capacitor. A load (for DC power supply 14) is represented by a resistor 90 connected between terminals 84 and 86. Bridge rectifier 80 includes three rectifier diodes (not shown) each having a cathode coupled to terminal 84 and an anode coupled to a respective one of terminals 22, 22′, and 22″. Further, bridge rectifier 80 includes three more diodes (also not shown) each having an anode coupled to terminal 86 and a cathode coupled to a respective one of terminals 22, 22′, and 22″. The inductor (70, 70′, or 70″) and capacitor (72, 72′, or 72″) combination of each of portions 32, 32′, and 32″ form (what is referred to herein as) a "resonator" for (it is believed) (in conjunction with power supply 14) generating harmonic currents for the power supply. In the presently preferred embodiment, the inductance of the inductor (70, 70′, or 70″) and the capacitance of the capacitor (72, 72′, or 72″) are chosen such that each inductor-capacitor combination resonates at a frequency near the sixth harmonic frequency of the power-line frequency and has a Q of approximately one to generate currents at both the fifth and the seventh harmonic frequencies. The inductor-capacitor combinations of portion 30 (40 and 46, 50 and 52, and 58 and 60) (as well as the similiar combinations of portions 30′ and 30″) form (what is referred to herein as) "reflectors" for (it is believed) reflecting harmonic energy back toward power supply 14. In the presently preferred embodiment, inductor 58 and capacitor 60 resonate near the seventh harmonic frequency; inductor 50 and capacitor 52 resonate near the fifth harmonic frequency; and, inductor 40 and capacitor 46 resonate near the thirteenth harmonic frequency. Each of the inductor-capacitor combinations of the "reflectors" have a Q which is in the range of 0.5 to 3. (A low "reflector" Q is employed to, among other things, avoid excessively attenuating the level of the fundamental current.) It is important to note that to attenuate the level of higher order harmonic currents, a resistor (44) is included in series with the capacitor (46) in the highest frequency "resonator". The resistor (44) resistance is approximately equal to the characteristic impedance of each of the "resonators" (each of portions 32, 32′, and 32″). Finally, it is important to note that no inductor-capacitor combination resonates at an exact multiple of the resonate frequency of another inductor-capacitor combination; because, it has been observed that certain combinations of frequency and Q have resulted in excessive levels of harmonic currents in the line. In the presently preferred embodiment, the following component values are employed: inductor 40 .... 2.41 mH, resistor 44 .... 12 ohms, capacitor 46 ... 23.2 mfd, inductor 50 .... 2.84 mH, capacitor 52 ... 97.4 mfd, inductor 58 .... 2.43 mH, capacitor 60 ... 46.9 mfd, inductor 70 .... 3.55 mH, capacitor 72 ... 10.95 mfd, capacitor 82 ... 3000 mfd, and resistor 90 .... a constant 2000 watt load (such as is presented by a switching power supply and its load). The following performance characteristics were calculated by means of the computer program entitled Micro-Cap II by Spectrum Software: Turning now to FIGURE 2, the presently preferred embodiment of a commercial/industrial grade harmonic currents isolating network is illustrated generally designated 110. With network 110 is illustrated a (single phase) AC power line, generally designated 112, and another (capacitor-input-type) DC power supply, generally designated 114. Network 110 is shown to employ two terminals (nodes), respectively designated 120 and 120′, each for connection to a respective conductor of AC power line 112 and two terminals (nodes), respectively designated 122 and 122′, each for connection to DC power supply 114. Network 110 employs two "reflectors" and one " resonator". The first "reflector" includes an inductor 140, which is connected between terminal 120 and a node 142. Additionally, the first "reflector" includes a resistor 144 and a capacitor 146 which is connected in series with resistor 144 between terminal 120 and node 142. The second "reflector" includes another inductor 150 and another capacitor 152 which are connected in parallel between node 142 and terminal 122. Finally, the "resonator" includes still another inductor 170 and still another capacitor 172 connected in series between terminals 122 and 122′. Terminal 120′ is directly coupled (connected) to terminal 122′. DC power supply 114 employs a bridge rectifier 180 and a filter capacitor 182. The input of rectifier 180 is connected to terminals 122 and 122′; and, the output of the rectifier is directly connected across capacitor 182 (between a pair of terminals 184 and 186) to develop a DC (output) potential (between the terminals) across the capacitor. A load (for DC power supply 104) is represented by a resistor 190 connected between terminals 184 and 186. In the presently preferred embodiment, the inductance of the inductor (170) and the capacitance of the capacitor (172) of the "resonator" are chosen such that they resonate at a frequency near the sixth harmonic frequency of the power-line frequency and have a Q of approximately one to generate currents at both the fifth and the seventh harmonic frequencies. Additionally, the (inductor 170 and capacitor 172) "resonator" generates an appreciable amount of third harmonic current without the high no load voltage characteristic of a "resonator" resonate at a lower frequency. The "reflector" which includes inductor 150 and capacitor 152 resonates at a frequency near the fifth harmonic frequency and has a Q of approximately one. The other "reflector", which includes inductor 140 and capacitor 146, resonates near the seventh harmonic frequency and has a Q of approximately 3/4. The resistor (144) resistance is approximately equal to the characteristic impedance of the "resonator". In the presently preferred embodiment, the following component values are employed: inductor 140 ... 36 mH, resistor 144 .... 115 ohms, capacitor 146 ... 4.84 mfd, inductor 150 .... 67.2 mH, capacitor 152 ... 5.08 mfd, inductor 170 .... 56.5 mH, capacitor 172 ... 4,27 mfd, capacitor 182 ... 600 mfd, and resistor 190 .... a constant 200 ohm resistive load consuming approximately 100 watts. The following performance characteristics were calculated: For more stringent applications, a commercial/industrial grade harmonic currents isolating network is illustrated in FIGURE 3 generally designated 210. Network 210 employs two terminals (nodes), respectively designated 220 and 220′, each for connection to a respective conductor of an AC power line and two terminals (nodes), respectively designated 222 and 222′, each for connection to a DC power supply. Additionally, network 210 employs five "reflectors" connected in series between terminals 220 and 222 and two "resonators". The first three "reflectors" include the parallel connection of an inductor 240 and a resistor 244 in the first; the parallel connection of an inductor 250 and a capacitor 252 in the second; and an inductor 258 and a capacitor 260 in the third. Connected between the third and fourth "reflector", the first "resonator" includes an inductor 270 and a capacitor 272 connected in series between a node 274 and terminal 222′. The last two "reflectors" include the parallel combination of an inductor 340 and a capacitor 346 in the fourth and the parallel connection of an inductor 350 and a capacitor 352 in the fifth. Finally, the last (second) "resonator" includes an inductor 370 and a capacitor 372 connected in series between terminals 222 and 222′. Terminal 220′ is directly coupled (connected) to terminal 222′. In the presently preferred embodiment, the following component values are employed: inductor 240 .... 30 mH, resistor 244 .... 115 ohms, inductor 250 .... 30.5 mH, capacitor 252 ... 9.23 mfd, .. freq. ... 5th harmonic, Q ... 1/2, inductor 258 .... 21.8 mH, capacitor 260 ... 6.59 mfd, .. freq. ... 7th harmonic, Q ... 1/2, inductor 270 .... 102 mH, capacitor 272 ... 7.66 mfd, .. freq. ... 3rd harmonic, Q ... 1, inductor 340 .... 16.9 mH, capacitor 346 ... 5.12 mfd, .. freq. ... 9th harmonic, Q ... 1/2, inductor 350 .... 13.9 mH, capacitor 352 ... 4.19 mfd, .. freq. ... 11th harmonic, Q .. 1/2, inductor 370 .... 51 mH, and capacitor 372 ... 3.84 mfd, .. freq. ... 6th harmonic, Q ... 1, a DC power supply capacitor capacitance of 600 mfd, and a constant 200 ohm resistive load consuming approximately 100 watts. The following performance characteristics were calculated: (Of course, since the components employed are all passive, they may be scaled up or down. First, the ratio of the desired power rating to the given power rating is obtained. Then, the inductance of each of the inductors is divided by the ratio; the capacitance of each of the capacitors is multiplied by the ratio; and, the resistance of each of the resistors is divided by the ratio.) In another embodiment, inductor 270 and capacitor 272 (of the first "resonator") are connected in series between terminal 222′ and a node 400 at the juncture of inductors 340 and 350. It is contemplated that after having read the preceeding disclosure, certain alterations and modifications of the present invention will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted to cover all such alterations and modifications as fall within the true spirit and scope of the invention.

Claims: A network for connection between a line conducting AC power (112) of predetermined voltage and frequency and the input of a DC power supply (114) which generates on the AC power line currents at harmonic frequencies of the AC power line frequency when directly connected thereto, the network comprising in combination: a first terminal (120); a second terminal (122); a third terminal (120') for connecting with said first (120) terminal to the AC power line (112); a fourth terminal (122') for connecting with said second terminal (122) to the DC power supply input; first means (140, 144, 146; 150, 152) including a parallel resonance circuit connected between said first terminal (120) and said second terminal (122); second means including a series resonance circuit (170, 172) being connected across the DC power supply input (122, 122'); said DC power supply (114) being connected with its input to said second and fourth terminals (122, 122'); characterized in that said first means include two series connected parallel resonance circuits (140, 144, 146; 150, 152), the first parallel resonance circuit (150, 152) resonates near the fifth harmonic of the AC power line frequency, and the second parallel resonance circuit (140, 144, 146) resonates near the seventh harmonic of the AC power line frequency, and said series resonance circuit (170, 172) resonates near the sixth harmonic of the AC power line frequency. A network as recited in claim 1 wherein said series resonance circuit (170, 172) resonates at a frequency higher than the fifth and lower than the seventh harmonic of said AC power line frequency. A network as recited in claim 1 or 2 wherein said series resonance circuit (170, 172) has a Q which is greater than 0.5 and less than 3.0. A network as recited in any one of claims 1 to 3 wherein said second parallel resonance circuit (140, 144, 146) further includes a resistor (144). A network as recited in any one of claims 1 to 4 wherein there is connected between the first terminal (220) and the second terminal a third parallel resonance circuit (258, 260) and further there is connected between the second terminal (274) and one input terminal (222) of the DC power supply a series connection of a fourth and fifth parallel resonance circuits (340, 346; 350, 352) resonating near the nineth and eleventh harmonic, respectively, of the AC power line frequency, and a further series resonance circuit (370, 372) resonating near the sixth harmonic of the AC power line frequency being connected across the input (222, 222') of the DC power supply. A network as recited in any one of claims 1 to 4 characterized in that for a three phase AC power supply (14) of predetermined voltage and frequency having three power lines and a DC power supply (14) having three inputs (22, 22', 22"), each power line includes said two series connected parallel resonance circuits (2, 3; 2', 3'; 2", 3") and that there are connected between each different pair of inputs (22, 22'; 22', 22"; 22, 22") of the DC power supply (14) said series resonance circuit. A network according to claim 6 wherein each power line includes a third parallel resonance circuit (40, 44, 46; 40', 44', 46'; 40", 44", 46") connected in series with the first and second parallel resonance circuits (50, 52, 58, 60; 50', 52', 58', 60'; 50", 52", 58", 60"). A network according to claim 7 wherein third parallel resonance circuits (40, 44, 46; 40', 44', 46'; 40", 44", 46") resonate near the thirteenth harmonic of the AC power line frequency.

Inventors: SMITH, STEVE
Applicants: SMITH, STEVE
Application year: 1987
Main classification: H02M1
IPC codes: H02M1, H03H7