Abstract:
A subsea power distribution system includes a three-phase AC power source; multiple variable frequency drives having inputs and outputs, with their inputs connected to the AC power source; an electric motor connected in series to the output of each variable frequency drive; and a passive harmonic filter system having its input connected, in parallel with the variable frequency drives, to the AC power source. The filter system includes multiple harmonic filters, each harmonic filter tuned to a specific harmonic frequency. Each harmonic filter includes a plurality of sub-filters. Each sub-filter includes a reactor connected in series to a main capacitor and one or more detuning capacitors. Each of the multiple harmonic filters is tuned to a different specific harmonic, and includes sub-filters also tuned to the same respective harmonic, and each sub-filter is sized to equally share the kVAR load of its respective harmonic filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to filters for filtering out unwanted harmonics in power distribution systems that use variable-frequency drives (“VFDs”) for controlling the rotational speed of three-phase alternating current (AC) electric motors by controlling the frequency of the electrical power supplied to the motor. 
     2. Description of Related Art 
     Large harmonic distortions cause malfunctions of meters and relays, nuisance tripping of circuit breakers, and equipment overheating. Typical prior art single-tuned passive harmonic filters are illustrated in FIG. 3 of U.S. Pat. No. 5,444,609, which patent is incorporated herein in its entirety by reference. Such filters are tuned to a series of specific harmonic frequencies. A simplified version of FIG. 3 of U.S. Pat. No. 5,444,609 is depicted in  FIG. 1  as a passive harmonic filter system  101 , which includes harmonic filters  103   a ,  103   b , and  103   c , respectively tuned at harmonic frequencies of the 5th, 7th, and 11th harmonics. The passive harmonic filter system  101  also includes circuit breakers or switches  105   a ,  105   b ,  105   c , and  105   d  which serve to switch the harmonic filters in or out of the electrical power systems. 
     The passive harmonic filter system  101  usually has an interlocking control (not shown) among the filters during filter operation. For example, when the filter  103   a  fails, the interlocking control logic turns off filters  103   b  and  103   c  in order to prevent them from overloading, and to prevent a serious resonance condition in the power distribution system. However, this leaves the power distribution system that was being protected by the passive harmonic filter system  101  without any harmonic filtering. 
     Although there are many designs for passive harmonic filters that are well known in the art, considerable shortcomings remain. What is needed is a passive harmonic filter system that will not fail when interlocking controls shut off individual parts of the filter system. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a passive harmonic filter system having its input connected, in parallel with variable frequency drives, to an AC power source, the filter system comprising multiple harmonic filters, each harmonic filter tuned to a specific harmonic frequency, each harmonic filter comprising at least three sub-filters, each sub-filter comprising: a circuit breaker or switch connected to the AC power source; an inductor or reactor connected to the circuit breaker/switch; and a capacitor connected in series to the inductor/reactor. 
     In another aspect of the invention, a method for constructing a subsea power distribution system is provided, comprising the steps of: (a) connecting subsea cables to a three-phase AC power source; (b) connecting the inputs of multiple variable frequency drives to the subsea cable; (c) connecting an electric motor in series to the output of each variable frequency drive; and (d) connecting the input of a passive harmonic filter system, in parallel with the variable frequency drives, to the AC power source, the filter system comprising multiple harmonic filters, each harmonic filter tuned to a specific harmonic frequency, each harmonic filter comprising at least three sub-filters, each sub-filter comprising: a circuit breaker/switch connected to the AC power source; an inductor/reactor connected to the circuit breaker/switch; and a capacitor connected in series to the inductor/reactor. 
     Additional objectives, features, and advantages will be apparent in the written description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features characteristic of the invention are set forth in the appended claims. However, the invention itself, as well as a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings in which the left-most significant digit in the reference numerals denotes the first figure in which the respective reference numerals appear, wherein: 
         FIG. 1  is a schematic diagram of a prior art passive harmonic filter system which includes a grouping of single-tuned passive harmonic filters; 
         FIG. 2  is a schematic diagram of an offshore power distribution system; 
         FIG. 3  is a schematic diagram of an illustrative embodiment of the present invention; and 
         FIG. 4  is a schematic diagram of detuning capacitors which are included in an embodiment of the present invention. 
         FIG. 5  is a schematic diagram of a portion of FIG. 3 of U.S. Pat. No. 5,444,609 and a schematic diagram of detuning capacitors which are included in an embodiment of the present invention. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     Referring now to  FIG. 2 , an offshore power distribution system  201  includes a 3.5 MW, 11 kV, 229.6 A power generator  202 , and various cable links  203   a ,  203   b ,  203   c , and  203   d . Cable link  203   d  is a 15 km subsea cable. 
     The dominant loads on the power distribution system  201  are nine VFDs  205   a ,  205   b ,  205   c ,  205   d ,  205   e ,  205   f ,  205   g ,  205   h , and  205   i  that are driving  300  horsepower electric motors  207   a ,  207   b ,  207   c ,  207   d ,  207   e ,  207   f ,  207   g ,  207   h , and  207   i , which may each drive an electrical submersible pump (ESP). Between each VFD  205  and a 380 V motor switchgear  208  are circuit breakers  209   a ,  209   b ,  209   c ,  209   d ,  209   e ,  209   f ,  209   g ,  209   h , and  209   i , which are used for switching the VFD/ESP system in and out of the circuit, and also provide system protection. 
     Without harmonic filters, the voltage total harmonic distortion (VTHD) is 21.59 percent and the current total harmonic distortion (ITHD) is 46.13 percent at the 11 kV generator switchgear  210  in the power distribution system  201 . Such high harmonic distortions are caused by parallel resonance due to the cable link  203   d  interacting with harmonic currents injected from the input of VFDs  205  into the power distribution system  201 . Based on IEEE Standard 519-1992, meters (not shown) in the system  201  could develop significant errors when the harmonic distortions are larger than 20 percent. Also, harmonic distortions of 10-20 percent could cause problems in relay operation. Therefore, in order to operate the power distribution system  201 , harmonic filters  211  must be installed and remain in operation at all times. As shown in  FIG. 2 , the best location to install the harmonic filters  211  in the power distribution system  201  is at the 380 V motor switchgear  208 . 
     The power distribution system  201  includes 380 V utility switchgear  215  for 0.5 MVA utility loads  217 , and 22 kV buses  219   a  and  219   b  for connecting subsea cable link  203   d  and supplying power to nine ESP served wells. Other loads connected to the subsea cable link  203   d  branch are lumped motors  221  and lumped static loads  223 . The power distribution system  201  includes three transformers  225   a ,  225   b , and  225   c . The transformer  225   a  is rated at 1 MVA and 11/0.38 kV with 5% impedance Z %. The transformer  225   b  is rated at 3 MVA and 11/22 kV with 7% impedance Z %. The transformer  225   c  is rated at 3 MVA and 22/0.38 kV with 7% impedance Z %. As can be seen in  FIG. 2 , at various places in the power distribution system  201  are circuit breakers (CB 1 , CB 2 , etc.)  227 . The circuit breakers  227   a ,  227   b ,  227   c ,  227   d ,  227   e ,  227   f ,  227   g , and  227   h  serve to switch the loads in and out of the circuit and also provide protection. 
     Referring back to the prior art passive harmonic filter system  101  of  FIG. 1 , suitable sizes for the filters  103   a ,  103   b , and  103   c  are 300 kVAR, 180 kVAR, and 105 kVAR, respectively. If the passive harmonic filter system  101  is installed in the power distribution system  201  as harmonic filters  203 , then when all three filters  103   a ,  103   b , and  103   c  are operating, the prior art passive harmonic filter system  101  can effectively mitigate harmonics in the power distribution system  201 . The VTHD is 1.14 percent, and the ITHD is 1.74 percent at the switchgear  208  in the power distribution system with the passive harmonic filter system  101  installed as harmonic filters  211 , compared to VTHD of 21.59 percent and ITHD of 46.13 percent at the switchgear  208  without the passive harmonic filter system  101 . 
     However, when the 5th-harmonic filter  103   a  fails, the interlocking control will switch the 7th-harmonic filter  103   b  and the 11th-harmonic filter  103   c  out to protect the system and to prevent the 7th-harmonic filter  103   b  and the 11th-harmonic filter  103   c  from overloading, with the result that the power distribution system  201  will have no operating harmonic filters. The usual interlocking control strategy for the prior art passive harmonic filter system  101  is shown in Table 1. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Interlocking Control for Traditional Passive Harmonic Filters 
               
             
          
           
               
                 Conditions triggering 
                   
               
               
                 interlocking control 
                 Interlocking Control 
               
               
                   
               
               
                 5th harmonic filter 103a fails 
                 7th and 11th harmonic filters 103b 
               
               
                   
                 and 103c OFF (No filters in the 
               
               
                   
                 system) 
               
               
                 5th and 7th harmonic filters 
                 11th harmonic filter 103c OFF (No 
               
               
                 103a, 103b fail 
                 filters in the system) 
               
               
                 7th harmonic filter 103b fails 
                 5th harmonic filter 103a ON, 11th 
               
               
                   
                 harmonic filter 103c OFF 
               
               
                   
               
             
          
         
       
     
     Referring now to  FIG. 3 , one preferred embodiment of the present invention is depicted. A passive harmonic filter system  301  includes multiple sub-filters for each harmonic. Thus, for the 5th-harmonic filter, instead of using a 300 kVAR 5th-harmonic filter  103   a  as shown in  FIG. 1 , the inventive passive harmonic filter system  301  uses three 100 kVAR 5th-harmonic sub-filters  305   a ,  305   b , and  305   c . Similarly, there are three 60 kVAR 7th-harmonic sub-filters  307   a ,  307   b , and  307   c , and three 35 kVAR 11th-harmonic sub-filters  311   a ,  311   b , and  311   c . Each 100 kVAR 5th harmonic sub-filter  305  has its own reactor and capacitor, and is tuned at the 5th-harmonic frequency. Each 60 kVAR 7th-harmonic sub-filter  307  has its own reactor and capacitor and is tuned at the 7th-harmonic frequency. Each 35 kVAR 11th-harmonic sub-filter  311  has its own reactor and capacitor and is tuned at the 11th-harmonic frequency. The circuit breakers  302 ,  303 , and  304  connect each set of sub-filters  305 ,  307 , and  311  to a 380 V filter bus  306 .  FIG. 3  also shows breakers/switches  302   a ,  302   b ,  302   c ,  303   a ,  303   b ,  303   c ,  304   a ,  304   b  and  304   c.    
     Referring back to  FIG. 2 , when the passive harmonic filter system  301  is used in the power distribution system  201  as the harmonic filters  211 , if one of the sub-filters  305  fails, the other two sub-filters  305  will still be able to work with the sub-filters  307  and  311  to provide effective harmonic mitigation. For the case when one sub-filter  305  fails, and two sub-filters  305 , three sub-filters  307 , and three sub-filters  311  remain in operation, the VTHD value for the power distribution system  201  is 1.31 percent, and the ITHD value is 2.38 percent, at the switchgear  208 . The simulated harmonic distortions at the switchgear  208 , for different scenarios with various sub-filters in operation status, are summarized in Table 2, although Table 2 shows only some of the scenarios. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Summary of Simulated Harmonic Distortions 
               
             
          
           
               
                   
                 VTHD, 
                 ITHD, 
               
               
                 Sub-filters operating status 
                 % 
                 % 
               
               
                   
               
             
          
           
               
                 All sub-filters out of service 
                 21.59 
                 46.13 
               
               
                 All sub-filters in service 
                 1.14 
                 1.74 
               
               
                 One sub-filter 305 out of service; two sub-filters 
                 1.31 
                 2.38 
               
               
                 305, and all sub-filters, 307 and 311, in service 
               
               
                 One sub-filter 307 out of service; three sub-filters 
                 1.24 
                 1.96 
               
               
                 305, two sub-filters 307, and three sub-filters 311 in 
               
               
                 service 
               
               
                 One sub-filter 311 out of service; three sub-filters 
                 1.26 
                 1.86 
               
               
                 305, three sub-filters 307, and two sub-filters 311 in 
               
               
                 service 
               
               
                 One sub-filter 305, and one sub-filter 307, out of 
                 1.39 
                 2.48 
               
               
                 service; two sub-filters 305, two sub-filters 307, 
               
               
                 and three sub-filters 311 in service 
               
               
                 One sub-filter 305, and one sub-filter 311, out of 
                 1.41 
                 2.43 
               
               
                 service; two sub-filters 305, three sub-filters 307, 
               
               
                 and two sub-filters 311 in service 
               
               
                 One sub-filter 307, and one sub-filter 311, out of 
                 1.35 
                 2.02 
               
               
                 service; three sub-filters 305, two sub-filters 307, 
               
               
                 and two sub-filters 311 in service 
               
               
                 One sub-filter 305, one sub-filter 307, and one sub- 
                 1.49 
                 2.52 
               
               
                 filter 311 out of service; two sub-filters 305, two 
               
               
                 sub-filters 307, and two sub-filters 311 in service 
               
               
                 Two sub-filters 305 out of service; one sub-filter 
                 2.01 
                 4.54 
               
               
                 305, three sub-filters 307, and three sub-filters 311 
               
               
                 in service (If the one remaining sub-filter 305 is not 
               
               
                 overloading) 
               
               
                 Two sub-filters 307 out of service; three sub- 
                 1.63 
                 2.73 
               
               
                 filters 305, one sub-filter 307, and three sub-filters 
               
               
                 311 in service (If the one remaining sub-filter 307 
               
               
                 is not overloading) 
               
               
                 Two sub-filters 311 out of service; three sub-filters 
                 1.50 
                 2.07 
               
               
                 305, three sub-filters 307, and one sub-filter 311 in 
               
               
                 service (If the one remaining sub-filter 311 is not 
               
               
                 overloading) 
               
               
                   
               
             
          
         
       
     
     Depending on the harmonic content in the power distribution system  201 , when one or two sub-filters fail for each specific filter tuning frequency, the remaining sub-filters are very likely to be overloading. In order to avoid the overloading problem, the conductor size of the reactors  315   a - c ,  317   a - c , and  321   a - c  for the sub-filters must be over-sized. How much over-sizing for the conductors depends on the chosen number of sub-filters at each tuned harmonic frequency by the design. On the other hand, the rms current flowing through the sub-filters is also restricted by the capacitors. Based on IEEE Standard 18-2002, the maximum continuous operating voltage, current, and kVAR for the capacitors are 110 percent of rated rms voltage and 120 percent of rated peak voltage, 135 percent of nominal rms current based on rated kVAR and rated voltage, and 135 percent of rated kVAR, respectively. For example, for the 100 kVAR 5th sub-filter  305   a  (rated at 380 V), the nominal rms current is I=Q/(√{square root over (3)}U)=100/(1.732*0.38)=151.93 A, the maximum continuous operating current for the capacitor is equal to 135 percent of the nominal rms current, i.e. 205.11 A. Therefore, the maximum rms current allowed to flow through each sub-filter is determined by the conductor size of the reactor and the current capability of the capacitors. 
     Referring now to  FIG. 4 , another step to improve the overloading capability for the sub-filters  305 ,  307 , and  311 , is to provide taps  401  on the reactors  315 ,  317 , and  321 , and add detuning capacitors  403  to the main capacitor  405  to detune the sub-filters. Table 3 shows the influence on the current flow on the sub-filters  305   a ,  305   b , and  305   c  by adding detuning capacitors  403   a  and  403   b , and by adding taps  401   a ,  401   b ,  401   c ,  401   d , and  401   e  ( 401   c  in the nominal 0 percent tap) on the reactors  315   a - c ,  317   a - c , and  321   a - c . By adding the taps  401   a - e  and the detuning capacitors  403   a  and  403   b  on each of the 100 kVAR sub-filters  305   a ,  305   b , and  305   c , the current flowing through the sub-filter  305  is reduced by up to 32.6 percent. Further, by properly choosing over-sized conductors for the reactors and combining with the taps  401   a - e  on the reactors and the detuning capacitors  403   a  and  403   b , the sub-filters for each specific tuning frequency have significantly increased overloading capabilities. 
       FIG. 5  shows a portion  500  of the circuit of FIG. 3 of U.S. Pat. No. 5,444,609 and an example of a portion of a filter. As stated with respect to  FIG. 4 , by adding the taps  401   a - e  and the detuning capacitors  403   a  and  403   b  on each of the 100 kVAR sub-filters  305   a ,  305   b , and  305   c , the current flowing through the sub-filter  305  is reduced by up to 32.6 percent.  FIG. 5  shows taps  501 , a main capacitor  505  and detuning capacitors  503   a  and  503   b . As shown the capacitors  505 ,  503   a  and  503   b  are in series with the tapped reactor. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Improvements Using Detuning Capacitors and Taps 
               
             
          
           
               
                 Parameters 
                 Tuning 
                 Current of sub- 
               
             
          
           
               
                 Rated voltage, V 
                 380 
                 frequency in 
                 filter varied from 
               
               
                 Rated frequency, Hz 
                 50 
                 harmonic order 
                 nominal, percent 
               
               
                   
               
             
          
           
               
                 Nominal main capacitors, 
                 100  
                 4.8 
                 0 
               
               
                 kVAR 
               
               
                 Detuning capacitor, kVAR 
                 15 
                 4.48 
                 −16.6 
               
               
                   
                 30 
                 4.21 
                 −24.6 
               
               
                 Taps on reactor 
                 +8% 
                 4.62 
                 −15.5 
               
               
                   
                 +4% 
                 4.71 
                 −8.5 
               
               
                   
                  0% 
                 4.80 
                 0 
               
               
                   
                 −4% 
                 4.90 
                 +10.3 
               
               
                   
                 −8% 
                 5.00 
                 +23.0 
               
               
                 15 kVAR detuning capacitor 
                 +8% 
                 4.31 
                 −27.1 
               
               
                 and taps on reactor 
                 +4% 
                 4.39 
                 −22.3 
               
               
                   
                  0% 
                 4.48 
                 −16.6 
               
               
                   
                 −4% 
                 4.57 
                 −9.8 
               
               
                   
                 −8% 
                 4.67 
                 −1.6 
               
               
                 30 kVAR detuning capacitor 
                 +8% 
                 4.05 
                 −32.6 
               
               
                 and taps on reactor 
                 +4% 
                 4.13 
                 −28.9 
               
               
                   
                  0% 
                 4.21 
                 −24.6 
               
               
                   
                 −4% 
                 4.30 
                 −19.6 
               
               
                   
                 −8% 
                 4.39 
                 −13.5 
               
               
                   
               
             
          
         
       
     
     The interlocking control method for the passive harmonic filter system  301  can be flexible, depending on how many sub-filters are chosen for each tuning frequency. As long as one sub-filter  305 , one sub-filter  307 , and one sub-filter  311  remain in operation, and these sub-filters have no overloading issue in the power distribution system  201 , then the remaining sub-filters will operate without triggering the interlocking control, as shown in Table 4. However, if the power distribution system  201  has a large amount harmonic content which could heavily overload the remaining sub-filters, the interlocking control strategy can be adjusted accordingly. 
     Although the example given in Table 4 is for six-pulse VFDs, the method of designing harmonic filters, shown in Table 4, can also be used for high-pulse VFD applications such as twelve-pulse VFDs. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Interlocking Control Strategy 
               
             
          
           
               
                   
                 Conditions Trigging 
                   
               
               
                   
                 Interlocking Control 
                 Interlocking Control 
               
               
                   
                   
               
               
                   
                 All sub-filters 305 
                 All sub-filters 307 and 311 OFF 
               
               
                   
                 fail 
                 (no filters in the system) 
               
               
                   
                 All sub-filters 305, 
                 All sub-filters 311 OFF (no 
               
               
                   
                 and all sub-filters 307 
                 filters in the system) 
               
               
                   
                 fail 
               
               
                   
                 All sub-filters 307 
                 All sub-filters 305 ON, and 
               
               
                   
                 fail 
                 all sub-filters 311 OFF 
               
               
                   
                   
               
             
          
         
       
     
     During operation, if one or more sub-filters fail, the remaining sub-filters for a specific tuning frequency will still work and continue to provide harmonic filtering. The chances that all sub-filters must be switched off due to the interlocking control are significantly reduced. The invention offers the flexibility of power factor control, which is particularly helpful for systems with only generators as the power supply. 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. Accordingly, the protection sought herein is as set forth in the claims below. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications.