Patent Publication Number: US-2016233757-A1

Title: Low level harmonics control system for groups of impedances connected in parallel in a three-phase system

Description:
TECHNICAL FIELD 
     The invention relates to a control system for control of supply of electrical power from a network to various groups of impedances. The invention also relates to the use of such a control system in a non-contact drying system. 
     BACKGROUND ART 
     A control system for supply of electrical power from a network to electrical resistors is known from U.S. Pat. No. 5,053,604. 
     This prior art control system discloses a process and a device for the control of heating of a furnace by means of electrical resistors. Switching devices such as thyristors are being used. The switching devices are controlled in a syncopated mode by an industrial computer, a logic controller or some equivalent device. A control table with digital values “1” and “0” is being generated by the industrial computer or the like and is being used in a synchronised way as logical control (on-off) of the thyristors. 
     The prior art control system of U.S. Pat. No. 5,053,604, however, is designed for controlling industrial furnaces, and, more particularly, glassmaking furnaces. The heating of glass sheets for tempering and bending needs to be controlled within very narrow ranges. As a result, U.S. Pat. No. 5,053,604 discloses measures to adapt for fluctuations in the electrical power as provided by the supply net. 
     Industrial furnaces with electrical resistors, such as in U.S. Pat. No. 5,053,604, have a great time constant or a high thermal inertia, so that the need for a quickly reacting control system is not present. 
     In addition, the electrical connection used in U.S. Pat. No. 5,053,604 is an open delta (triangle) connection, which does not generate a high level of harmonics so that the need for avoiding harmonics is not present. 
     Moreover, the electrical scheme of U.S. Pat. No. 5,053,604 only uses two phases and not the three phases. 
     EP-A1-0 040 017 discloses a control system for control of supply of electrical power from a network to various groups of impedances. The control system comprises a programmable control unit. The impedances are connected in star and the groups are connected in parallel. 
     DE-A1-33 04 322 discloses an electrical water heater. The electrical power from the network is proportional to the volume flow of water. 
     DISCLOSURE OF INVENTION 
     The primary object of the invention is to provide a control system for control of supply of electrical power to a wider variety of impedances. 
     It is a particular object of the invention to control the electrical power to a group of impedances with a small time constant or low thermal inertia. 
     It is another object of the invention to provide a control system which allows to reduce harmonics. 
     It is yet another object of the invention to provide a control system which makes full and optimal use of a three phase electrical supply network. 
     According a first aspect of the invention there is provided a control system for control of supply of electrical power from a network to various groups of impedances. The control system comprises a synchronisation system for synchronising with the cycles, i.e. the frequency, of a supply network. The control system further comprises a programmable control unit. The programmable control unit has control tables. Each control table corresponds with a particular level of maximum electrical power and each control table has a sequence of bits (0 or 1) where the percentage of 1 values corresponds to the level of maximum power. The control programmable unit provides digital outputs in the form of one of these control tables for the command of static power switches which control electrical power to the various groups of impedances. The impedances within at least one group are connected in star, and the various groups of impedances are connected in parallel. 
     The feature of having various groups of impedances connected in parallel allows reducing harmonics. 
     The connection in star of the impedances within one group allows optimizing the electrical power. 
     According to a preferable embodiment, the impedances within at least one group are connected in star without connection to a neutral lead or neutral wire. This avoids a double cabling or double wiring. 
     According to another preferable embodiment, the synchronisation system generates a signal having cycles. The control unit further comprises pointers, one pointer per group of impedances, and each pointer is positioned on a control table and proceeds with one bit per cycle of said signal. A pointer for a following adjacent next group of impedances is on a position in the control table which is different than the pointer of the previous adjacent group. This difference in position is e.g. one bit. 
     This difference of position of pointers in different control tables avoids peaks in electrical currents and further reduces the level of harmonics. 
     In a most preferable embodiment, the signal of the synchronisation system has a phase difference with the supply network. 
     As will be explained hereinafter with reference to the drawings, this phase difference results in an equal power spreading over the three phases. 
     According to a practical embodiment, the signal of the synchronisation system is a square wave with a mounting side and a descending side and the above-mentioned pointers proceed with one bit per mounting side of said square wave. 
     According to another preferable embodiment, the 1 values in each control table are uniformly spread over the control table. 
     The advantage of this uniform spreading is apparent in case the impedances have a small time constant, which is the case for infrared lamps. The uniform spreading limits the so-called flickering of the lamps. 
     According to still another preferable embodiment, static power switches are only used in two of the three phases. This saves static power switches in one phase. 
     According to a second aspect of the invention, there is provided a non-contact drying system comprising a control system according to the first aspect of the invention. In the non-contact drying system the impedances are infrared lamps. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS 
         FIG. 1 a    illustrates the synchronisation system of the control system according to the invention and  FIG. 1 b    illustrates a phase difference between the cycles of a synchronisation signal and a supply network; 
         FIG. 2  illustrates the control system according to the invention; 
         FIG. 3  shows curves of voltages and currents over a star connection of three infrared lamps. 
         FIG. 4  shows a preferred embodiment of a power supply towards one group of impedances. 
         FIG. 5 a   ,  FIG. 5 b    and  FIG. 5 c    show curves of voltages and currents over the embodiment of  FIG. 4 . 
     
    
    
     MODES(S) FOR CARRYING OUT THE INVENTION 
       FIG. 1 a    illustrates the synchronisation system of the control system of the invention. A three-phase supply network  10  of electrical power (400 V, AC) is connected to a delta-star transformer  11  with neutral connection. The transformer voltage ratio is 1/10 so that the outcome voltage of the transformer is 40 V, AC. The voltage  12  between the neutral and a phase is 40/√{square root over (3)}=23.1 V AC or about 24 V. This voltage  12  is fed to a static command switch  13  which commands solid static relays  15 . These solid static relays  15  are being fed by a 24 V direct current, and this generates a square wave  17  with amplitude 24 V and a frequency which is equal to the frequency of the supply network  10 . Due to the fact that the AC voltage  12  is taken between the neutral and a phase, there is a phase difference φ of 30° between the voltage  12  and the supply network  10 . This is illustrated in  FIG. 1 b   . This phase difference φ is also existing between the synchronisation square wave  17  and the supply network  10 . The advantage of this phase difference will be explained hereinafter. The synchronisation square wave signal  17  has a mounting side and a descending side. 
       18  is the synchronised PLC input. 
       FIG. 2  illustrates (part of) the control system according to the invention. A three-phase supply network  200 , which is usually the same supply network as in  FIG. 1 , supplies electrical power to a non-contact dryer and profiler. Non-contact drying is done by means of infrared lamps. The infrared lamps are divided in various groups  210 ,  220 , . . . For example, there can be three infrared lamps  212  in group  210 , three infrared lamps  222  in group  220 , and so on. The number of infrared lamps per group can also be a multiple of three: six, nine . . . 
     Each group  210 ,  220  of infrared lamps  212 ,  222  is connected in parallel to the supply network  200 . 
     Inside each group  210 ,  220 , the infrared lamps  212 ,  222  are connected in star without a connection to a neutral lead or neutral wire. 
     Static power switches such as power thyristors  214 ,  224  on phases A and C determine the electrical currents to the infrared lamps  212 ,  222 . 
     The static power switches  214 ,  224  receive a digital input signal  216 ,  226 . The signal is digital, which means that a “0” stands for off and a “1” for on. The value of the digital input signal  216 ,  226  is determined by the position of a pointer  218 ,  228  on a control table  250 . Pointer  218  is associated with the first group of infrared lamps  212 . Pointer  228  is associated with the second group of infrared lamps  222 . Pointer  228  is shifted with one bit with respect to the position of pointer  218 . Every pointer proceeds with one bit per each mounting side of the synchronisation square wave  17 . The shifting of each pointer per group of infrared lamps limits the peaks in electrical currents. 
     There are  101  control tables  250 . Each control table corresponds with a power level between 0% and 100%: 0%, 1%, 2%, . . . , 99%, 100%. Each table has e.g. 256 bits. The percentage of 1 values corresponds to the percentage of maximum power level. The table of 0% has only 0 values: 00000000 . . . 
     The table of 50% has 50% 0 values and 50% 1 values: 11001001100 . . . 
     The table of 75% has 75% 1 values: 1110111011101110 . . . 
     The table of 100% has only 1 values: 111111111111111 . . . 
     Most preferably the 1 values are uniformly spread in a control table, since this reduces the flickering effect of the infrared lamps. 
       FIG. 3  shows curves of voltages and currents over a star connection of three infrared lamps over four periods. 
     The curves in the upper half are the curves of the voltages, which are all sinusoidal. 
     Curves U AB , U BC  and U CA  are resp. the voltages between the points A-B, B-C and C-A on  FIG. 2 . 
     Curves V A , V B  and V C  are resp. the voltages between the middle of the start and A, the middle of the star and B and the middle of the star and C. 
     Looking at the curves in the lower half, curve  30  is the command signal. The power thyristors  214  only conduct in case the corresponding voltage V A , V B  or V C  cross the zero Volt and the command signal  30  has a positive value. The electrical currents thus generated are represented by curves I A , I B  and I C  resp. 
     These electrical currents are not sinusoidal and each electrical current, taken alone, generates harmonics. However, due to the fact that various groups, e.g. ten groups, of infrared lamps are connected in parallel the global effect of harmonics is reduced. 
       FIG. 4  shows a preferred embodiment of a power supply towards one group of impedances. As is the case with the embodiment of  FIG. 2 , the embodiment of  FIG. 4  has the advantage that it has only two static relays  40  and  42 . 
       FIG. 5 a    shows curves of voltages and currents over the embodiment of  FIG. 4 . 
     The power supply network is a three phase network with 400 VAC and 50 Hz. Phases A and C are supplied over a static relay, phase B is directly supplied. The three infrared lamps to be supplied have impedances R A , R B  and R C . These infrared lamps have a nominal power of 3000 W under 235 VAC. In this example the command signal  50  corresponds to 50% of the maximum power and has as control table 110011001100 . . . The joint command signal  50  for both static relays  40  and  42  is a square wave  50  with a positive signal during 40 msec followed by a zero signal during 40 msec. 
     V RA , V RB  and V RC  are the voltages over resp. the impedances R A , R B  and R C  of the three infrared lamps. The power obtained for each infrared lamp is: 
       P A =1501 W 
       P B =1490 W 
       P C =1501 W 
     So the power for each infrared lamp is more or less equal. In order to obtain this uniform distribution of power, it is important to avoid that the mounting sides of the command signal  50  coincide with the passage through zero of the voltages U AB  and U BC . As explained with reference to  FIG. 1 a    and  FIG. 1 b   , this may be obtained by cabling the static command switch of the synchronisation system at the secondary site of a delta star transformer between one phase and a neutral. In this way a phase difference φ is created between the square wave  17  of the synchronisation system and the voltages U AB , U BC  and U CA . This phase difference avoids the simultaneous starting of the mounting side of the square wave  17  and the passage through zero of U AB  and U BC . Suppose the phase difference φ is not present, the minor delay of the square wave  17  may result in a static relay or power switch staying off instead of switching on, or vice versa. 
       FIG. 5 b    shows in more detail when the 1 st  static relay  40  and the 2 nd  static relay  42  and the three impedances R A , R B  and R C  start to conduct. 
     The 1 st  static relay  40  starts to conduct from t 1  on at a time when the voltage U AB  passes through zero and the command signal  50  is positive. The voltage over R A  is then equal to U AB /2 and over R B  equal to −U AB /2. 
     From t 2  on, at a time when V C  passes through zero, the 2 nd  static relay  42  starts to conduct. From t 2  on the three impedances R A , R B  and R C  are fed in all three phases. 
     Referring to both  FIG. 5 b    and  FIG. 5 c   , the three impedances R A , R B  and R C  are fed in all three phases between t 2  and t 3 . 
       FIG. 5 c    shows in more detail when the 1 st  static relay  40  and the 2 nd  static relay  42  and the three impedances R A , R B  and R C  stop to conduct. 
     At t 3  V A  passes through zero at a moment when the command signal  50  has already turned to zero. The 1 st  static relay  40  stops to conduct. The 2 nd  static relay  42  continues to conduct until t 4 . The voltage over R C  is equal to −U BC /2 and the voltage over R B  is equal to U BC /2. 
     At t 4 , at a time when U BC  passes through zero and the command signal  50  is zero, the 2 nd  static relay also stops to conduct. 
     As may be derived from  FIG. 5 a   , apart from the start period t 1 -t 2  and the stop period t 3 -t 4 , the signals are very close to a sinusoidal form so that the level of harmonics is limited. 
     In addition, and as already mentioned, since various groups of impedances are cabled in parallel, the effect of harmonics is further minimized. 
     TABLE WITH REFERENCE NUMBERS AND SYMBOLS 
     
         
           10  electric power supply network 
           11  transformer 
           12  voltage between neutral and one phase 
           13  static command switch 
           15  solid static relays 
           16  24 DC voltage 
           17  synchronisation square wave 
           18  synchronisation input PLC 
         Y star connection 
         Δ delta connection 
         φ phase difference  200  electric supply network 
           210  first group of infrared lamps 
           212  infrared lamp 
           214  power thyristors 
           216  digital input signal 
           218  pointer on control table 
           220  second group of infrared lamps 
           222  infrared lamp 
           224  power thyristors 
           226  digital input signal 
           228  pointer on control table 
           250  control table 
           30  command signal 
           40  1 st  static relay 
           42  2 nd  static relay 
           50  command signal 
         U AB  voltage between A and B 
         U BC  voltage between B and C 
         U CA  voltage between C and A 
         V A  voltage on A 
         V B  voltage on B 
         V C  voltage on C 
         I A  current through infrared lamp of phase A 
         I B  current through infrared lamp of phase B 
         I C  current through infrared lamp of phase C 
         R A  impedance of infrared lamp 
         R B  impedance of infrared lamp 
         R C  impedance of infrared lamp 
         V RA  voltage over impedance R A    
         V RB  voltage over impedance R B    
         V RC  voltage over impedance R C