Patent Publication Number: US-2022227258-A1

Title: Power line system with ripple generator for electric vehicles

Description:
BACKGROUND 
     The present inventions relate generally to electric trams, metros, trains and the like, and more particularly, to a power line system for supplying electric power to electric vehicles connected to the power line. 
     Electric trams, metros, trains and the like use a power line that extends along a travel length and provides power to the electric vehicles as they move along the power line. That is, the power line remains electrically charged along the length of the power line, and the electric vehicles draw power from the power line as the electric vehicles travel by staying connected to the power line along the travel length. The power line may be an overhead line (also referred to as a catenary line) that the electric vehicle follows or may run along the ground parallel to the rails upon which the vehicle travels. Typically, such power systems supply DC power to the electric vehicles. In such an arrangement, the power line may have a DC positive line and a DC negative line. Commonly, the overhead power line or separate line along the ground will be the DC positive line, while the rails upon which the vehicle&#39;s wheels travel may be the DC negative line. 
     In electric vehicle systems described above, it is preferable for the electric vehicles to be able to recover energy which would otherwise be lost during braking events. In traditional vehicle systems, mechanical brakes may be used to slow the speed of an electric vehicle, but these systems are inefficient due to the lost energy that occurs during braking events. Mechanical brakes also require time consuming maintenance and create undesirable pollution during use. Although it is known in some vehicle systems that regenerative braking may be used to increase efficiency by slowing the vehicle with an electric generator that produces electricity during braking events, it has been difficult to incorporate regenerative braking into power line systems like those described above. 
     SUMMARY 
     A power line system is described for electric vehicles, such as electric trams, metros, trains and the like. The system includes a power line extending along a length that provides electric power to the vehicle as it travels along the length. A substation provides power to the power line with first voltage ripples that are detectable by the vehicle. A ripple generator also generates second voltage ripples on the power line. An energy storage system is also connected to the power line. The vehicles are able to operate in an energy recuperation mode when either the first or second voltage ripples are detected by the vehicle. During the energy recuperation mode, excess electricity produced by the vehicle is provided to the power line which may be stored by the energy storage system. The invention may also include any other aspect described below in the written description or in the attached drawings and any combinations thereof. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       The invention may be more fully understood by reading the following description in conjunction with the drawings, in which: 
         FIG. 1  is a schematic of a power line system for electric vehicles; 
         FIG. 2  is a chart of voltages on the power line of the power line system; 
         FIG. 3  is schematic of a standby state of the power line system; 
         FIG. 4  is schematic of a power draw state of the power line system; 
         FIG. 5  is schematic of a power sharing state of the power line system; 
         FIG. 6  is schematic of a generation mode of the power line system with energy being dissipated by the vehicles; 
         FIG. 7  is schematic of an energy recuperation state of the power line system with energy being absorbed by an energy storage system; 
         FIG. 8  is schematic of a ripple generator; and 
         FIG. 9  is a simplified schematic of the energy storage system and the ripple generator. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the figures, and particularly  FIG. 1 , a power line system for electric vehicles  10  is shown. In the system, the electric vehicles  10  are electrically connected to the power line  12  with a connector  14  that slides or runs along the power line  12  as the vehicle  10  travels. Each vehicle  10  has a controller  16  that controls the motors  18  that drive the wheels of the vehicle  10 . The controller  16  allows the motors  18  to operate in a drive mode where the vehicle  10  draws power from the power line  12  to drive the electric vehicle  10 . Alternatively, the controller  16  allows the motors  18  to operate in a generation mode in which the motors  18  produce electricity during a braking event. A power line detector  20 , or dead-line detector  20 , is also provided on the electric vehicle  10  to sense electric properties of the power line  12 . As described further below, the detector  20  may control whether electricity produced by the motors  18  during the generation mode (e.g., during a braking event) is supplied to the power line  12  in an energy recuperation mode or whether it is dissipated on the electric vehicle  10  by a rheostat  22 , or electric energy dissipater  22 . An automatic receptivity unit (ARU)  24  may also be connected to the power line  12 . That ARU  24  functions like the rheostats  22  on the vehicles  10  but has significantly greater capability to dissipate electric energy from the power line  12 . 
     A substation  26  is also connected to the power line  12  to supply electric power to the line  12 . Typically, the power supplied to the substation  26  will be AC power provided by an AC power grid  28 , and the power supplied from the substation  26  to the power line  12  will be DC power. Preferably, the substation  26  has an input control switch  30  and an output control switch  32 , which may be circuit breakers  30 ,  32  used for electrical protection and maintenance. Typically, the substation  26  includes a diode rectifier  34  that converts the AC power supply from the AC grid  28  to DC power to be supplied to the power line  12 . A filter  36  may also be provided after the diode rectifier  34 . 
     An energy storage system  38  may also be connected to the power line  12 . The energy storage system  38  may include a DC battery  40  for electrical energy storage. The energy storage  40  may be used to absorb excess energy from the power line  12  to charge the battery  40  and may supply electric power to the power line  12  when needed or desirable. The energy storage system  38  may also include a DC/DC converter  42  connected to the battery  40  and a filter  44  connected to the converter  42 . A control switch  46 , such as a circuit breaker  46  for electric protection and maintenance, may also be provided at the connection between energy storage system  38  and the power line  12 . 
     A ripple generator  48  may also be connected to the power line  12 . In the embodiment of  FIG. 1 , the ripple generator  48  may be a part of the energy storage system  38  and may be connected between energy storage  40  and the power line  12 , and more preferably between the filter  44  and the circuit breaker  46 . As described further below, the ripple generator  48  generates electrical ripples on the power line  12  which can be sensed by the power line detectors  20  on the electric vehicles  10 . 
     Although the length of the power line  12  may vary greatly, the travel length of the power line  12  will typically be within a range of ½ km to 30 km. This does not mean that the electric vehicles  10  are limited to this length for overall travel purposes. As shown in  FIG. 3 , it may be possible to connect multiple power line systems together with section insulators  50  that allow vehicles  10  to cross over to other power line systems and potentially travel unlimited lengths with such a system. In some embodiments, it may be desirable for the substation  26  to be located in the middle 40% of the power line  12  length. This allows the power line  12  to extend as far as possible in each direction while keeping the voltage levels at the extremities above the minimum voltage necessary for proper operation. It may also be desirable in some embodiments to separate the substation  26  from the energy storage system  38  by a distance to allow the energy storage system  38  to provide voltage stabilization at distances away from the substation  26 . For example, it may be desirable to separate the substation  26  and the energy storage system  38  from each other by a distance of at least 30% of the travel length of the power line  12 . 
       FIG. 2  illustrates voltage ranges that the power line  12  may experience during operation. Preferably, the voltages on the power line  12  satisfy standards set for railway applications, such as EN 50163 set by the European Committee for Standards. Although various standards or non-standard embodiments may be used, the described embodiment may be particularly suitable for 750 V DC traction systems, e.g., according to EN 50163. In the Figures, U max2  represents the highest non-permanent (e.g., max of 5 min.) voltage (e.g., 1,000 V). U max1  represents the highest permanent voltage (e.g., 900 V). If the voltage exceeds U max1 , the ARU  24  and/or vehicle rheostats  22  may dissipate the excess energy. U n1  represents the regeneration mode threshold voltage. Although U n1  ideally would be equal to U n , U n1  will typically be above U n  by a small amount to account for impedances and inefficiencies in the system. U n  represents the specified nominal voltage at no load (e.g., 750 V). U min1  represents the lowest permanent voltage (e.g., 500 V). On the right side of the chart, one or more loads (i.e., vehicles  10 ) are drawing power from the power line  12  which causes a voltage drop. In response, the substation  26  supplies power to the power line  12 . On the other hand, on the left side of the chart, the voltage is above the nominal voltage U n  (and U n1 ) and the substation  26  quits supplying power to the power line  12 . This may be caused by one or more vehicles  10  operating in the generation mode during a braking event and supplying the produced electricity to the power line  12  in an energy recuperation mode. It is noted that in the preferred embodiment, the substation  26  is a unidirectional source of electric power, in that the substation provides electric power to the power line  12  as needed but cannot absorb excess power from the power line  12 . 
     Turning to  FIGS. 3-7 , various possible operating states are shown. In  FIG. 3 , two vehicles  10  are connected to the power line  12  but are both operating in standby mode, meaning that the vehicles  10  are not drawing power from the power line  12  or supplying power to the power line  12 . This may occur when the vehicles  10  are standing still or coasting, etc. As shown in the voltage chart for  FIG. 3 , the voltage on the power line  12  remains at Uni. Notably, the voltage is characterized by ripples (first ripples). This is caused by the diode rectifier  34  of the substation  26  which is supplying power to the power line  12  in this state. It is understood that the frequency of the ripples is higher than the frequency of the AC grid frequency supplying power to the substation  26 . In particular, it is standard for the frequency of the ripples generated by the diode rectifier  34  to be 6 times or 12 times the AC grid frequency, which means the ripples will have a frequency of 360 Hz or 720 Hz where the grid frequency is 60 Hz. 
     In  FIG. 4 , the two vehicles  10  are both drawing power from the power line  12 . This may occur, for example, when the vehicles  10  are accelerating. In this case, the voltage on the power line  12  drops, and the diode rectifier  34  supplies power to the power line  12 . Thus, as shown, the ripples generated by the diode rectifier  34  are still present. 
     In  FIG. 5 , the vehicle  10  on the left is in a generation mode where the vehicle  10  is producing electricity (e.g., braking). In contrast, the vehicle  10  on the right is drawing power from the power line  12  (e.g., accelerating). Since the power required by the right vehicle  10  is more than the power that the left vehicle  10  can provide, the substation  26  supplies power to the power line  12  (and to the right vehicle  10 ) to make up the difference between the power provided by the left vehicle  10  and the power required by the right vehicle  10 . Notably, because the substation  26  is providing some power to the power line  12  in this state, the ripples are present on the power line  12 . As further explained below, the power line detector  20  on the left vehicle  10  senses the ripples and because the ripples are present, the controller  16  uses the energy recuperation mode to provide the produced electricity to the power line  12 . 
     In  FIG. 6 , both vehicles  10  are operating in the generation mode (e.g., both vehicles  10  are braking). As a result, the voltage on the power line  12  may exceed U n . Because the voltage is above U n , the substation  26  stops supplying power to the power line  12 . As a result, the diode rectifier  34  is no longer operating to supply power, and therefore, there are no ripples on the power line  12 . This could create a potentially unsafe condition if the vehicles  10  were allowed to supply all of their produced electricity to the power line  12  and if the voltage on the power line  12  were allowed to exceed U max2 . Because of this, the controllers  16  on the vehicles  10  only operate in the energy recuperation mode (i.e., supplying energy to the power line  12 ) when the power line detectors  20  sense ripples on the power line  12 . Thus, in  FIG. 6 , the vehicles  10  may initially operate in the energy recuperation mode when a braking event begins, but this extra energy recuperation supplied to the power line  12  will quickly boost the voltage on the power line  12  above U n . As a result, the substation  26  stops supplying power and the ripples are no longer present. Then, because the power line detectors  20  do not sense any ripples, the controllers  16  stop operating in the energy recuperation mode (even though the vehicles  10  are operating in the generation mode). This means that the excess electrical energy produced by the vehicles  10  must be dissipated onboard each vehicle  10 , which is done by the rheostats  22 . However, in this situation, the electrical energy produced by the vehicles  10  is inefficiently wasted. 
     In  FIG. 7 , the power line system is shown in conjunction with the energy storage system  38 . In this case, the vehicle  10  is operating in the generation mode due to a braking event. As a result, the voltage on the power line  12  is boosted above U n , which stops the substation  26  from supplying power and ordinarily would stop any ripples from being present on the power line  12 . However, because a separate ripple generator  48  is provided, ripples (second ripples) may still be generated on the power line  12 . Since the power line detector  20  on the vehicle  10  senses ripples on the power line  12 , the controller  16  operates in the energy recuperation mode by providing excess produced electrical energy to the power line  12 . This excess energy is then absorbed by the energy storage system  38  to charge the energy storage  40 . Thus, energy produced by a vehicle  10  during a generation mode may be efficiently stored even when the voltage on the power line  12  is above U n . 
     The ripple generator  48  may be controlled in various ways. For example, it may be desirable to deactivate the ripple generator  48  when the battery  40  is at full capacity. Thus, the ripple generator  48  may only generate ripples when the energy storage system  38  is below the maximum capacity. The ripple generator  48  may also be deactivated when the voltage on the power line  12  is above U max1  Thus, the ripple generator  48  may not generate ripples when the voltage is above a maximum. It may also be desirable to deactivate the ripple generator  48  when the voltage on the power line  12  is below U n . Thus, the ripple generator  48  may only generate ripples when the voltage is above a threshold. For example, it is possible for the ripple generator  48  to be only activated within a range of voltages on the power line  12 , e.g., between U n  and U max1 . 
     The power line detectors  20  on the vehicles  10  are preferably designed to sense voltage ripples of particular frequencies of at least a minimum amplitude. In particular, the detectors  20  preferably sense ripple frequencies greater than the grid frequency, and most preferably, 6 or 12 times the grid frequency. Thus, it is preferred that the ripples generated by the diode rectifier  34  and the ripple generator  48  be at the same frequency which is the frequency sensed by the detector  20 . Thus, it is preferable for the frequencies of the ripples of the diode rectifier  34  and the ripple generator  48  to both be greater than the grid frequency and to both be 6 or 12 times the grid frequency. As a result, the detector  20  does not need to distinguish between the ripples from the diode rectifier  34  and the ripple generator  48 . Therefore, in the preferred embodiment, the vehicles  10  do not operate in the energy recuperation mode if the detectors  20  do not sense ripples from either the diode rectifier  34  or the ripple generator  48 . That is, the vehicles  10  can only operate in the energy recuperation mode if the detectors  20  sense ripples from the diode rectifier  34  or the ripple generator  48  or from both. Although the power line system may be controlled so that ripples are only generated from the diode rectifier  34  and the ripple generator  48  at different times to prevent overlapping of the ripples (e.g., with the diode rectifier  34  and ripple generator  48  being controlled by mutually exclusive power line voltages or by communication between the substation  26  and the energy storage system  38 ), it is also possible for the power line system to allow ripples from the diode rectifier  34  and the ripple generator  48  to be generated and present on the power line  12  at the same time. In such a case, the ripples of the diode rectifier  34  and the ripple generator  48  may amplify each other above the minimum ripple amplitude sensed by the power line detectors  20 . Thus, the detectors  20  may not distinguish whether the sensed ripples are from the diode rectifier  34 , ripple generator  48  or both. 
     An example of the ripple generator  48  is shown in  FIG. 8 . As shown, the ripple generator  48  may have a resonant capacitor  52  and an AC transformer  54  in series between the DC positive line  12 A and the DC negative line  12 B of the power line  12 . The resonant capacitor  52  may perform two functions. First, the resonant capacitor  52  may block the DC current flow through the AC transformer  54 , which is potentially dangerous for the AC transformer  54 . Second, the resonant capacitor  52  may tune the resonant circuit. Thus, the resonant capacitor  52  may have a capacitance that is selected to tune the frequency of the resonant circuit (i.e., filter inductor  60 , filter capacitor  58 , the AC transformer  54  and the resonant capacitor  52 ) close to the frequency of the desired ripples provided to the power line  12 . The AC transformer  54  may also be useful to provide galvanic separation for safety and simplification of the DC/AC power converter  56 . A power converter  56  which generates an AC frequency may be connected to the AC transformer  54 . The power converter  56  preferably generates an AC voltage with a frequency and magnitude that emulates the ripples generated by the diode rectifier  34  of the substation  26 . The power converter  56  may be supplied with power from a DC power source (e.g., an auxiliary DC power supply or the battery  40 ), in which case the converter may be a DC to AC converter  56 , or the power may be provided from the AC grid  28 , in which case the power converter  56  may be an AC to AC frequency converter. Although the transformer  54  may be desirable, it may be possible to omit the transformer  54  and connect the resonant capacitor  52  and power converter  56  directly in series between the positive  12 A and negative  12 B lines. A filter capacitor  58  (e.g., from the filter  44 ) may also be connected between the DC positive line  12 A and the DC negative line  12 B of the power line  12  in parallel with the resonant capacitor  52 . A filter inductor  60  (e.g., from the filter  44 ) may also be connected on the DC positive line  12 A between the filter capacitor  58  and the resonant capacitor  52 . 
       FIG. 9  illustrates a simplified electric circuit for the energy storage system  38  and the ripple generator  48 . In the schematic, U DC  represents the mean voltage of a DC/DC converter (e.g., a battery  40 ). Power can be supplied by or absorbed by the U DC  power source  40 . Typically, the U DC  power source  40  will utilize a PWM square wave voltage which necessitates the use of a filter inductor  60  and filter capacitor  58 . U AC  represents the voltage of the AC transformer  54  and power converter  56 . U AC  power source  56  provides energy for the loop including the resonant capacitor  52 , filter inductor  60  and filter capacitor  58 . The loop may be a serial resonant circuit, where w res =1/sqrt{(L f +L Tr )*((C f *C r )/(C f +C r ))}. The values of L f  and C f  are preferably defined by the DC/DC converter  40  filtering requirements. The value of L Tr  is preferably the transformer  54  leakage inductance which is typically much smaller than L f  (and could be neglected). The resonant frequency of the loop (i.e., C r , L f , C f , U AC ) may be set to a value close to desired ripples (e.g., 6×60 Hz). The loop may also be tuned by the selection of the capacitance of the resonant capacitor  52 . It is also preferable for the capacitance of the resonant capacitor  52  to be much less than the capacitance of the filter capacitor  58 . The resonant circuit gathers and keeps energy in reactive components (i.e., the inductor  60  and capacitors  52 ,  58 ) that oscillates in the loop. In the present example of the loop, parasitic resistances of the components and connections may be neglected. 
     While preferred embodiments of the inventions have been described, it should be understood that the inventions are not so limited, and modifications may be made without departing from the inventions herein. While each embodiment described herein may refer only to certain features and may not specifically refer to every feature described with respect to other embodiments, it should be recognized that the features described herein are interchangeable unless described otherwise, even where no reference is made to a specific feature. It should also be understood that the advantages described above are not necessarily the only advantages of the inventions, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the inventions. The scope of the inventions is defined by the appended claims, and all devices and methods that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.