SE1650776A1 - Device and method for loading a voltage source - Google Patents

Abstract

70546 ABSTRACT A device is connectable to a DC voltage source for loading the DC voltage sourceThe device comprises a plurality of legs (10a, 10b, 10c) connected in parallelbetween a first terminal and a second terminal, wherein each leg comprises, a firstswitch element (11a, 11b, 11c), a second switch element (13a, 13b, 13c) aresistive element (12a, 12b, 12c) connected and a rectifier element (14a, 14b, 14c)having an anode and a cathode. The cathode of each of the rectifier elements isconnected to a first node of a subsequent leg, wherein the cathode of a last leg isconnected to the first node of the first leg. A device for loading a voltage sourcewherein the load current is controlled is thereby provided. A corresponding methodis also disclosed. (Fig. 1)

Description

DEVICE AND METHOD FOR LOADING A VOLTAGE SOURCE Technical field id="p-1"

[0001] The present invention relates generally to loading of voltage sources andmore particularly to a device and a method for loading a voltage source whereinthe load current is controlled.

Background art id="p-2"

[0002] Batteries are used to ensure that critical electrical equipment is alwayson. There are many places where batteries are used and some of the applicationsfor batteries include electric generating stations and substations for protection andcontrol of switches and relays, telephone systems to support phone service,industrial applications for protection and control and back up of computers. id="p-3"

[0003] There are some main reasons to test battery systems: to insure thesupported equipment is adequately backed-up, to prevent unexpected failures bytracking the battery's health, and to forewarn/predict permanent failure there arethree basic questions that battery users ask: What is the capacity and thecondition of the battery now? When will it need to be replaced? What can be done to improve/ not reduce its life? id="p-4"

[0004] Batteries are complex chemical mechanisms. They have numerouscomponents from grids, active material, posts, jar and cover, etc. - any one ofwhich can fail. As with all manufacturing processes, no matter how well they aremade, there is still some amount of uncertainty related to batteries. id="p-5"

[0005] A battery is two dissimilar metallic materials in an electrolyte.Nonetheless, to work the way it is supposed to work a battery must be maintainedproperly. A good battery maintenance program may prevent, or at least, reducethe costs and damage to critical equipment due to an AC mains outage, for example. id="p-6"

[0006] Batteries come in various configurations themselves. Add to that themany ways that they can be arranged and in is realized that the number of 70546 possible configurations is endless. Voltage plays the biggest part in a batteryconfiguration. Batteries have multiple posts for higher current draws. The morecurrent needed from a battery, the bigger the connections must be. That includesposts, intercell connectors and bus bars and cables. The many configurations ofbatteries mean that testing equipment must be adaptable to different voltages and currents etc. id="p-7"

[0007] There are a number of standards and company practices for batterytesting. Usually they comprise inspections (observations, actions andmeasurements done under normal float condition) and capacity tests. Most well-known are the IEEE standards: IEEE 450 for flooded lead-acid, IEEE 1188 for sealed lead-acid, and IEEE 1106 for nickel-cadmium. id="p-8"

[0008] So-called capacity tests or discharge tests are performed at the time ofinstallation of batteries and then periodically, depending on the capacity of thebattery. Capacity test is the only way to get an accurate value on the actualcapacity of the battery. While used regularly it can be used for tracking thebattery's health and actual capacity and estimating remaining life of the battery.When the battery is new its capacity might be slightly lower than specified. This is normal. id="p-9"

[0009] During the test it is measured how much capacity (current times timeexpressed in Ah) the battery can deliver before the terminal voltage drops to theend of discharge voltage times number of cells. During testing, the current shall bemaintained at a constant value. lt is recommended to select a test time that is approximately the same as the battery's duty cycle. id="p-10"

[0010] Batteries can also be tested at a shorter time than their duty cycle, forinstance at 1 hour. Then the current rate has to be increased. An advantage is thatless capacity is drained from the battery (valid for lead-acid) and it requires less time to recharge it. Also less man-hour is needed for the test. 70546 id="p-11"

[0011] Tests are thus usually conducted at constant current, but constant power,constant resistance or a pre-selected load profile can also be used. This implies that the discharge current must be controlled in a flexible and still reliable way.

Summarv of the invention id="p-12"

[0012] An object of the present invention is therefore to provide a device and amethod for loading a voltage source wherein the load current is controlled. id="p-13"

[0013] According to a first aspect of the invention there is provided a deviceconnectable to a DC voltage source for loading the DC voltage source, the DCvoltage source having a positive terminal and a negative terminal, the devicecomprising: a first terminal connectable to the positive terminal of the DC voltagesource, a second terminal connectable to the negative terminal of the DC voltagesource, a plurality of legs comprising a first leg, a second leg and further legs untila last leg, wherein the plurality of legs are connected in parallel between the firstterminal and the second terminal, wherein each leg comprises, a first switchelement having a first end and a second end, wherein the first end of the firstswitch element is connected to the first terminal, a second switch element having afirst end and a second end, wherein the second end of the second switch elementis connected to the second terminal, a resistive element connected, at a first node,to the second end of the first switch element, and a second node, to the first end ofthe second switch element, and a rectifier element having an anode and acathode, wherein the anode of the rectifier element is connected to the secondnode, wherein the cathode of each of the rectifier elements is connected to the firstnode of a subsequent leg, wherein the cathode of a last leg is connected to thefirst node of the first leg. id="p-14"

[0014] ln a preferred embodiment, the number of legs is three.[0015] ln a preferred embodiment, the number of legs is eight. id="p-16"

[0016] ln a preferred embodiment, each of the switch elements comprises asemiconductor switch, such as a transistor with an anti-parallel diode. 70546 id="p-17"

[0017] ln a preferred embodiment, each of the resistive elements comprises aresistor connected in series with a parasite inductor. id="p-18"

[0018] According to a second aspect of the invention there is provided a methodfor loading a voltage source by means of the device, the method comprising thefollowing steps: a) determining operating parameters for discharging the DCvoltage source, b) measuring the voltage of the DC voltage source, c) determininga control scheme based on the operating parameters and the measured voltage ofthe DC voltage source, d) measuring physical entities, e) adjusting the controlscheme based on the measured physical entities, and repeating steps d) and e)until an end condition is fulfilled. id="p-19"

[0019] ln a preferred embodiment, wherein the operating parameters arechosen from the following: voltage, discharge current, discharge time. id="p-20"

[0020] ln a preferred embodiment, an effective current in each leg is controlledby adjusting the duty cycle of the switch elements. id="p-21"

[0021] ln a preferred embodiment, an effective current is conducted in two ormore legs at a time, with a cyclical commutation between the different legs. id="p-22"

[0022] ln a preferred embodiment, a switching frequency per leg is less than 20kHz.

Brief description of drawinqs id="p-23"

[0023] The invention is now described, by way of example, with reference to the accompanying drawings, in which: Fig. 1a is a circuit diagram of a first embodiment of a device according to theinvention in a first mode of operation, the device having tree legs.

Fig. 1b is the same circuit diagram as in Fig. 1a but showing a second mode ofoperation, Fig. 1c is the same circuit diagram as in Fig. 1a but showing a third mode ofoperation, 70546 Fig. 2a is a curve diagram showing the currents flowing in the device of Figs. 1a-cin the first mode of operation, Fig. 2b is a curve diagram showing the combined currents flowing in the device of Figs. 1a-c in the first mode of operation, Figs. 3a-c are diagrams showing different timings of the switches shown in Figs.1a-c, Fig 4 is a circuit diagram of a second embodiment of a device according to theinvention, having eight legs, Fig. 5 shows an example of serially connecting two devices according to the invenüon,and Fig. 6 shows a device adapted to provide a variable voltage output.

Description of embodiments id="p-24"

[0024] ln the following, a detailed description of a device and a method according to the invention will now be given. id="p-25"

[0025] Referring first to Fig. 1a, the layout of a device for loading a DC voltagesource, generally designated 1, is shown. The device is connectable to a DCvoltage source, referenced DC, having a positive terminal and a negative terminal.The device has a first terminal P connectable to the positive terminal of the DCvoltage source and a second terminal N connectable to the negative terminal ofthe DC voltage source. id="p-26"

[0026] A plurality of legs 10a, 10b, 10c, in the embodiment shown in Figs. 1a-cthree legs, enclosed by dashed lines, are connected in parallel between the firstterminal P and the second terminal N. Each leg comprises a first switch element11a, 11b, and 11c, respectively, having a first end and a second end, wherein thefirst end of the first switch element is connected to the first terminal P. Each leg also comprises a second switch element 13a, 13b, and 13c, respectively, having a 70546 first end and a second end, wherein the second end of the second switch elementis connected to the second terminal P. id="p-27"

[0027] ln each leg a resistive element 12a, 12b, and 12c, respectively, isconnected, at a first node Pa, Pb, and Pc, respectively, to the second end of thefirst switch element, and a second node Na, Nb, and Nc, respectively, to the firstend of the second switch element. A rectifier element 14a, 14b, 14c having ananode and a cathode, is connected to the respective second node by means of itsanode. Also, the cathode of each of the rectifier elements is connected to the firstnode of a subsequent leg, wherein the cathode of a last leg is connected to thefirst node of the first leg. Thus, in this case the rectifier element 14c is connectedbetween the third and the first legs. id="p-28"

[0028] Each of the switch elements 11a-c and 13a-c comprises in the preferredembodiment a semiconductor switch, such as a transistor with an anti-paralleldiode. Each switch element is controlled by control electronics (not shown in the figures. id="p-29"

[0029] Each of the resistive elements 12a, 12b, and 12c comprises in thepreferred embodiment a resistor connected in series with a parasite inductor 16a,16b and 16c. This results in a time constant for the current changes. Also, it ispreferred to provide flywheel diodes 15a, 15b and 15c for handling reverse currents. id="p-30"

[0030] The device 1 operates as follows. First, it is connected by means of theterminals P, N to a battery source DC to be tested, i.e., to be discharged in acontrolled way. Different parameters are entered into the control electronics, suchas voltage, discharge current, discharge time etc. Based on these parameters, anoperating mode is selected to obtain the best possible discharge scheme. id="p-31"

[0031] According to Ohm's law, the discharge current is determined by thevoltage V across the DC voltage source and the resistance of the resistiveelements 12a, 12b, 12c. The effective resistance of these resistive elements willbe determined by the operation mode, as will be described in the following. 70546 id="p-32"

[0032] The resistive elements can be connected in different configurationsdepending on the state of the different switches 11a-c and 13a-c, i.e. if they areON (closed, conducting current) or OFF (open, interrupting current). This meansthat the resistive elements can be connected in parallel, in series, or a combination of in parallel and in series. id="p-33"

[0033] ln Fig. 1a an operation mode is shown wherein all switches are on,conducting current through each leg 10a-c from the positive terminal P to thenegative terminal N, shown by arrows in the figure. This means that the threeresistive elements 12a-c are connected in parallel. Assuming that each resistiveelement has the resistance value R, the resulting current I will be the following: l=3V/R id="p-34"

[0034] ln Fig 1b an operation mode is shown wherein the switches 11a, 13b,11c, and 13c are ON and the remaining switches 13a and 11b are OFF. ln thismode, the current is conducted from the positive terminal, through the firstresistive element 12a, via the rectifying element 14a, and through the secondresistive element 12b to the negative terminal N. ln other words, the first andsecond resistive elements 12a, 12b are connected in series. The third resistiveelement 12c in the third leg 10c conducts current from the positive terminal P tothe negative terminal N. Thus, two resistive elements connected in series areconnected in parallel with one single resistive element. Assuming that eachresistive element has the resistance value R, the resulting current I will be the following:l=3V/2R id="p-35"

[0035] ln Fig 1c an operation mode is shown wherein the switches 11a and 13care ON and the remaining switches 13a, 11b, 13b, and 11c are OFF. ln this mode,the current is conducted from the positive terminal, through the first resistiveelement 12a, via the first rectifying element 14a, through the second resistiveelement 12b, via the second rectifying element 14b, and via the third resistiveelement 13c to the negative terminal N. ln other words, the first, second, and third 70546 resistive elements 12a, 12b, 12c are connected in series. Assuming that eachresistive element has the resistance value R, the resulting current I will be the following:l=V/3R id="p-36"

[0036] The total current I loading the voltage source is the sum of the currents inthe three legs 10a-c, i.e., Ia + Ib + Ic _ By switching the different switches in anintelligent way, the total current I can be controlled to a desired value which isshown on Fig. 3a. The effective current in each leg can also be controlled byadjusting the duty cycle of the switches. Referring again to Fig. 1a, instead ofconducting current in all three legs simultaneously, current can be conducted firstin the first leg 10a, then in the second leg 10b, then in the third leg 10c, then againin the first leg 10a and so on. Alternatively, the effective current can be conductedin two or more legs at a time, with a cyclical commutation between the differentlegs. For example, current can be conducted first in the first and second legs 10a,10b, then in the second and third legs 10b, 10c, then in the third and first legs 10c,10a, then again in the first and second legs 10a, 10b and so on. ln this cyclicswitching way, all semiconductor switches are equally used, dissipating samepower, no matter of chosen topology, which results in uniform heat spreading andreduced heat needed to be dissipated from each of the components included inthe device. id="p-37"

[0037] In more general terms, if the maximum total current, in when currentflows in all three legs, is designated Iiviax, and the duty cycle is designated D=(0.. 1), then the current I loading the voltage source isl = D X 'Max id="p-38"

[0038] An example of the currents is illustrated in Figs. 2a and 2b. Referring tothe examples described with reference to Fig. 1a, Fig. 2a show the differentcurrents la, lb, and lc, conducted through the first, second, and third leg 10a, 10b,10c, respectively. In Fig. 2b, the total current I = Ia + Ib + Ic is shown by a thickline. By switching the switches with a high enough frequency, i.e., commuting the 70546 current between the different legs with high frequency, the current ripple can bekept low enough to satisfy the requirements of the discharge operation, i.e,. withan essentially constant current l. For example, as switching frequency of 20 kHzper leg will be sufficient, resulting in 60kHz of total current ripple and minimizingfilter components needed for filtering the same. id="p-39"

[0039] Different cyclic switching schemes are illustrated in Figs. 3a-c. Transistorsteering in Fig. 3a is used for highest discharging current, resulting in highestfrequency of current ripple. For medium discharging current, transistors can beoperated as on Fig. 3b and for lowest discharging current and highest batteryvoltage, transistor switches are operated as on Fig. 3c. According to maximumcurrent ripple frequency, adequate current filter can be dimensioned andimplemented, such as filter 17 in Fig. 4. id="p-40"

[0040] A method of loading a DC voltage source by means of a deviceaccording to the invention will now be described. lnitially, operating parameterssuch as discharge duration, for example 1 h, discharge current etc. aredetermined. These parameters are input into the control electronics controlling theoperation of the switches of the device. The voltage of the voltage source is thenmeasured by means of a voltage meter. This voltage meter can be a separatemeasuring device but is preferably integrated in the device 1 for loading the voltage source. id="p-41"

[0041] A control scheme is then determined based on the measured voltage andthe operating parameters. Switching of the switches according to the controlscheme is then initiated. Different physical entities, such as the voltage V acrossthe voltage source, the total current l, the current in the different legs, thetemperature at different locations, such as the switches and the resistive elements,etc. are continuously monitored and the control scheme is adjusted in dependenceof the values of the different physical entities. For example, when the voltage Vacross the voltage source decreases over time as it is discharged, in order tomaintain a constant current l, the effective resistance of the resistive elementsmust be decreased. This can be effected by adjusting the duty cycle of the 70546 different switches and/or changing the operation mode. For example, instead of anoperation mode wherein the different resistive elements are connected in seriesbetween the positive terminal P and the negative terminal N, resulting in a aneffective resistance 3 x R, they can be connected in parallel, resulting in aneffective resistance R/3, assuming 100 % duty cycle. id="p-42"

[0042] The physical entities are measured either continuously or with regularintervals and the control scheme is adjusted based on the measured physicalentities until an end condition is fulfilled. This can be that a predetermined time haslapsed from the start of the discharge, the voltage of the DC source has droppedbelow a threshold value, an alarm condition etc. id="p-43"

[0043] An alternative embodiment of the device according to the invention isshown in Fig. 4. The general structure is similar to the one described above withreference to Figs. 1a-c, but in this case the device is provided with eight legs 10a-h instead of three. This number: 2 X 2 X 2, allows a number of suitable switching schemes. id="p-44"

[0044] More than one device can be connected in series or parallel, allowinghandling of higher voltages or higher current. One example thereof is shown inFig. 5, wherein the positive terminal of a master device 1 is connected to thepositive pole of the voltage source. The negative pole of the master device isconnected to the positive terminal P of a slave device 1". The negative terminal Nof the slave device is connected to the negative pole of the voltage source. ln thisway, twice the voltage level can be handled as compared to the voltage levelhandled by a single device, assuming that the master and slave devices have the same voltage ratings. id="p-45"

[0045] lt will be appreciated that more than two devices can be connected inseries, such as one master device and two slave devices. id="p-46"

[0046] A different application can be obtained by replacing the restive elements with transformers or DC bridges, allowing power to be input into the device 705461 1 resulting in a variable DC output voltage between the positive terminal P and thenegative terminal N. An example of such an embodiment is shown in Fig. 6. id="p-47"

[0047] Preferred embodiments of a device and a method according to theinvention have been described. lt will be appreciated that these can be variedwithin the scope of the appended claims without departing form the inventive idea.For example, the inventive idea is applicable on a device comprising as few as two legs.

Claims (10)

1. A device connectable to a DC voltage source for loading the DC voltagesource, the DC voltage source having a positive terminal and a negative terminal, the device comprising:a first terminal (P) connectable to the positive terminal of the DC voltage source, a second terminal (N) connectable to the negative terminal of the DC voltagesource, a plurality of legs (10a, 10b, 10c) comprising a first leg (10a), a second leg (10b)and further legs until a last leg, wherein the plurality of legs are connected inparallel between the first terminal and the second terminal, wherein each leg comprises, a first switch element (11a, 11b, 11c) having a first end and a secondend, wherein the first end of the first switch element is connected to thefirst terminal (P), a second switch element (13a, 13b, 13c) having a first end and asecond end, wherein the second end of the second switch element isconnected to the second terminal (N), a resistive element (12a, 12b, 12c) connected, at a first node (Pa, Pb,Pc), to the second end of the first switch element, and a second node (Na, Nb, Nc), to the first end of the second switch element, and a rectifier element (14a, 14b, 14c) having an anode and a cathode,wherein the anode of the rectifier element is connected to the secondnode (Na, Nb, Nc), wherein the cathode of each of the rectifier elements is connected to the first nodeof a subsequent leg, wherein the cathode of a last leg is connected to the firstnode of the first leg. 7054613

2. The device according to claim 1, wherein the number of legs (10a, 10b,10c) is three.

3. The device according to claim 1, wherein the number of legs (10a, 10b,10c) is eight.

4. The device according to any one of claims 1-3, wherein each of theswitch elements (11a-c, 13a-c) comprises a semiconductor switch, such as atransistor with an anti-parallel diode.

5. The device according to any one of claims 1-4, wherein each of theresistive elements (12a-c) comprises a resistor connected in series with a parasiteinductor (16a-c).

6. A method for loading a DC voltage source by means of a deviceaccording to claim 1, the method comprising the following steps: a) determining operating parameters for discharging the DC voltage source (DC),b) measuring the voltage of the DC voltage source (DC), c) determining a control scheme based on the operating parameters and themeasured voltage of the DC voltage source, d) measuring physical entities,e) adjusting the control scheme based on the measured physical entities, andf) repeating steps d) and e) until an end condition is fulfilled.

7. The method according to claim 6, wherein the operating parameters arechosen from the following: voltage, discharge current, discharge time.

8. The method according to claim 6 or 7, wherein an effective current in each leg is controlled by adjusting the duty cycle of the switch elements. 7054614

9. The method according to any one of claims 6-8, wherein an effectivecurrent is conducted in two or more legs at a time, with a cyclical commutation between the different legs.

10. The method according to any one of claims 6-9, wherein a switchingfrequency per leg is less than 20 kHz.