Abstract:
In a method, system and apparatus of measuring electrical power to consumers ( 16   a–d ) in a power distribution network ( 12 ), an electromagnetic field is sensed around at least one electrical conductor ( 12 ) in a power meter ( 30   a–d ) located at a consumer. The current flowing through the conductor is then derived from the sensed electromagnetic field. Instantaneous current values are stored in an electronic memory powered by the electromagnetic field. These instantaneous values or values derived from the instantaneous values are transmitted as digital information on the network to a receiver ( 20 ) provided at a distance from the power meter.

Description:
FIELD OF INVENTION 
   The present invention relates generally to a method, a system and an apparatus for measurement of electrical power and more specifically to a method, a system and an apparatus wherein electrical power meters connected to an electrical distribution network are remotely controlled by a remote server. 
   BACKGROUND 
   In the field of electrical power measurement, many kinds of electrical power meters are known. However, prior art apparatus have been dimensioned with regard to electromechanical measuring devices. This has led to bulky devices, which are difficult to install. 
   The UK patent document GB-2 321 305 discloses a remote meter reading apparatus provided for retrofitting to an existing meter. This reading apparatus relies on an already installed meter having a Ferraris disc. The apparatus is provided with a wireless transmitter for transmitting data derived from a sensor to a remote location. However, this solution provides for a bulky device limited to its application in existing networks. 
   An electronic meter or measuring electrical power fed from an electrical power distribution network to an electrical power consumer itself requires electrical power in order to function. A convenient way of providing this power is to furnish the electronic meter with means for drawing electrical power from the power network used to supply the consumer. However, the current which an electronic meter is permitted to draw from an electrical power distribution network is limited by statutory regulation. 
   The UK patent document GB-2 301 903 discloses an electrical power supply meter provided with an opto-transmitter arranged for communicating data appertaining to a meter reading and at the same time not to disturb the power network connected to the meter. 
   Trip units for tripping an electronic circuit are known for example through the European patent document EP 0 949 734-A2. In the device disclosed therein processors are arranged to trip an electric circuit on detection of a fault condition. However, there are other instances where tripping of an electric circuit is desired. 
   OBJECTS OF THE INVENTION 
   An object of the present invention is to provide an electrical power consumption measuring system wherein electrical power meters are remotely read and controlled in an efficient way. 
   Another object is to provide an electrical power meter, which is inexpensive, easy to install and is adapted for communication through the electrical power network to which it is connected. 
   Another object is to provide a method of remotely controlling an electrical power meter. 
   SUMMARY OF THE INVENTION 
   The invention is based on the realization that the electromagnetic field around an electric conductor can be used to measure the power flowing through the conductor by means of sensors without any movable parts and to drive an electronic circuit. This is used together with digital communication through the electric conductor to provide for remote measuring and control of the electrical power consumed by a consumer connected to an electric power network. 
   The invention provides a measuring system wherein the measuring device can be installed without skilled personnel, i.e., the subscriber caters for the installation. 
   A method is also provided by means of which it is possible to remotely collect information regarding electrical power consumption of customers in an efficient way. 
   Another advantage is the possibility to remotely disconnect a subscriber by means of a circuit breaker provided in the sensing apparatus. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention is now described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is an overview of a measuring system according to the invention, 
       FIG. 2  is a schematic overview of an electrical power meter used in the system shown in  FIG. 1   
       FIGS. 3   a  and  3   b  are cross-sectional views of a master and a slave module, respectively, to be incorporated in electrical power meter shown in  FIG. 2 , and 
       FIG. 4  is a schematic block diagram of the measuring apparatus shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following, a detailed description of the invention will be given. Some of the elements described herein are identical to their shape and function and are then given the same reference numeral followed by an accompanying letter. When identical elements are referred to collectively the accompanying letter can be omitted. 
   Reference is first made to  FIG. 1 , wherein part of an electrical power network system, generally designated  10 , is shown. The system shown in the figure comprises electrical three phase power lines  12  running between a connection point, such as a power station  14 , and a number of electrical power consumers  16   a–d.    
   Somewhere close to the connection point there is a host server  20  connected to the power lines  12 . The host server  20  is an ordinary computer provided with a power-net modem supporting TCP/IP. The server  20  is running administration software etc. and will be further described below. 
   At each consumer there is provided a respective electrical power meter  30   a–d , an overview of which will now be given with reference to  FIG. 2 . The three-phase meter is made up of three parts shaped and configured as conventional fuses or circuit breakers. The three parts comprise two identical slave modules  50   a ,  50   b  connected to a master module  40 . The three modules are arranged to be mounted as conventional plug fuses in a fuse box with the master module  40  arranged in the center position and the slave modules  50   a ,  50   b  arranged on either side thereof. The slave modules are connected to the master module by means of a respective strip  60   a ,  60   b  made of polyester or another fragile or brittle material. The reason for this choice of material is that it should be difficult to remove the strip once it is installed, thereby preventing fraudulent manipulation of the arrangement. 
   The strip  60  is permanently attached to the master module  40  and electrically connected thereto by means of a pattern (not shown) of three rather wide printed copper paths  62  running in parallel between the master module  40  and the slave module  50  when connected thereto. The procedure of connecting the slave modules to the master module follows the following steps. First, the master module  40  and the slave modules  50   a ,  50   b  are screwed into a respective socket with the master module  40  positioned between the slave modules  50   a ,  50   b . Special care must be taken to ensure that the strips  60   a ,  60   b  are not damaged during this process. The strips  60   a ,  60   b  are then inserted into a respective slot  52   a ,  52   b  arranged in the slave modules. Once inserted into the slots  52 , the strips  60  cannot be withdrawn from the slave modules because of a one-way retaining means provided in the slot. 
   With the strips  60  attached, it is not possible to unscrew the modules  40 ,  50  because the strips  60  would then break, destroying the arrangement by breaking the electrical connection between the master module and the slave modules. 
   When mounted in the fuse box, the modules  40 ,  50  function as ordinary fuses, normally of the 10 or 16 Amps size. 
   The structure of the master module  40  will now be described with reference to  FIG. 3   a , which shows a cross-section through the center of the master module. The module has a general outline similar to a conventional fuse, with a bottom connector  41  adapted for connection to the bottom surface of the fuse socket (not shown) and thus functioning as a first connector of the module. The bottom connector  41  is electrically connected to a thread  42  by means of a conductive wire  43 . The thread is shaped so as to fit with the internal thread (not shown) provided in the fuse socket and thus functions as a second connector of the module. A major portion of the conductive wire  43  runs essentially parallel to the longitudinal axis of the fuse  40 . 
   With the module  40  mounted in the socket, the wire  43  forms a part of the wire  12  supplying the consumer  16  with electrical power, see  FIG. 1 . Thus, all power consumed passes through the wire  43 . 
   A coil  44  is provided around a portion of the conductive wire  43 , preferably made of copper. The two ends of the coil are connected to inputs of an electronic circuitry  70  provided in the fuse. By means of the coil  44 , the electromagnetic field generated by current flowing in the wire  43  is detected. More specifically, the generated field in turn generates a current in the coil  44 , which is read and interpreted by the circuitry  70 , thus generating instantaneous values of the current flowing in the wire  43 . The number of turns of the coil is adapted to the expected induced field so as to give a suitable measuring value. In the preferred embodiment, the number of turns in the coil  44  is about 500, giving an input voltage of about 3 Volts to the circuitry  70 . However, too many turns lead to too much metal, giving an inductance that will decrease the maximum practical frequency induced in the coil. 
   As stated above, the two strips  60   a ,  60   b  are fixedly connected to the master module  40 . The conductive paths  62  on the strips are connected to respective inputs of the electronic circuitry  70  of the master module  40 . 
   An electronic trip circuit  45  is provided in serial connection with the wire  43 . The trip circuit  45  is controlled by the circuitry  70 , which thereby can break the current path between the connectors  41  and  42 . 
   All components included in the modules  40  and  50  are preferably embedded in a mold made of a suitable polymer. 
   In  FIG. 3   b , a slave module  50  is shown in cross-section. The slave module is similar to the master module with the exception of the electronic circuitry  70 , which is omitted in the slave modules. Thus, a slave module comprises a first connector  51 , a second connector  57 , a conductive wire  53  there between, a coil  54  and a trip circuit  55 . The coil  54  and the trip circuit  55  are connected to a contact means  56  arranged to connect to the conductive paths  62  of a strip  60  inserted into the above mentioned slot  52  of the slave module  50 . In that way, both the coil  54  and the trip circuit  55  of a slave module are connected to the electronic circuitry  70  of the master module  40 . 
   The electronic circuitry  70  of the master module  40  will now be described, partly with reference to  FIG. 4 , which is a schematic block diagram of the electronic function of the master-slave arrangement shown in  FIG. 2 . The main part of the circuitry  70  is a processor  72 . This is preferably a low voltage version of the type 8751 processor, operating at 2.7 Volts or lower, and provided with an internal EEPROM. The low power consumption of this device, less than 150 mamps, makes this circuit ideal for this kind of application. 
   An oscillator  74  is provided as a reference clock for the circuitry  70 . A preferred frequency of the oscillator is 100 kHz. 
   There is also provided an internal reference  76 . Preferred values for this reference is 1 Volt and 1 Ohm. 
   Filters  78  functioning as stabilizers are also provided between the coil  44  and processor  72 . 
   Finally, there is provided a capacitor  80  with a preferred value of 40 μF. The function of this component will be described below. 
   The function of the measuring system will now be described. As already mentioned, the currents in the wires  12  leading to the consumers  16 , see  FIG. 1 , are detected by means of the coils  44 ,  54   a ,  54   b  provided in the modules  40 ,  50   a , and  50   b , respectively. The measuring values are directed to inputs of the microprocessor  72 . The actual currents flow ng to the consumer  16  is there derived from the measured values by means of mathematical functions known to the person skilled in the art. Samples of measured values are taken with a frequency of 1000 Hz, i.e., 1000 samples are taken per second. The samples comprise both current and voltage values. This is necessary as the current and voltage in a power line are mutually displaced. Due to this, in order to get a correct power measurement, both current and voltage values are required. 
   Superposed on the basic electric power frequency in the lines  12 , normally 50 or 60 Hz, is a Frequency Shift Keying (FSK) signal on a certain undefined frequency band. This FSK signal is used for communication via one of the power lines  12  between the server  20  and the different electrical power meters  30 . In the preferred embodiment, this communication uses the TCP/IP protocol. This protocol is suitable for this kind of application, wherein it sometimes is necessary to retransmit a message several times before it is received successfully. 
   Each electrical power meter  30 , i.e., master module  40 , has its own IP address. In the microprocessor  72 , there is a software application listening for messages intended for this particular address. Thus, the FSK signal is extracted from the current induced in the coil  44  by means of the filter  78  and is interpreted and if the address given in the header of the message is correct, the rest of the message is also interpreted. Simultaneously, measuring values are taken and stored in the memory  73 . 
   Two different types of measuring values are stored: an instantaneous value stored as a 16-bit value and a cumulative value stored as a 64-bit value. The cumulative value is effectively an odometer keeping track of the total consumed power. This value can be used for billing purposes, as will be described below. 
   The communication between the server  20  and the master modules  40  follows any suitable command structure adapted to this application. Thus, there are commands for the various tasks for the master modules  40 . An example thereof is the GET — ACCOUNT command. The server  20  sends this command together with an IP address for the electrical power meter  30  to be read. When the meter  30  in question reads the command, it retrieves the cumulative value from the memory  73  and sends it together with its IP address onto the power lines  12 . This message is then read by the server, which uses the value as a basis for billing. 
   The master module  40  is also used for transmitting data to the server  20 . However, the energy received from the power supply is not sufficient for superposing a FSK signal on the lines  12 . Therefore, the capacitor  80  is provided for storing the energy needed for transmitting the FSK signal. In the preferred embodiment, the capacitor has a value of about 40 μF, giving a voltage of 10 Volts for 400 μs. Thus, the microprocessor  72  buffers the message in a high voltage buffer and then transmits the signal. 
   An application implemented in the master module  40  is the remote trip function. In the case a particular consumer is to be excluded from the power network, e.g. due to failing to pay an earlier invoice, the fuses of the electrical power meter  30  can be tripped from the server  20 . This is carried out in the following way. A message is sent to the master module  40  belonging to the electrical power meter  30  to be tripped, telling it to open the trip circuits  45 . The electronic circuitry then issues a command to the trio circuits  45 ,  55   a ,  55   b  to open, thereby cutting the current path through the modules. 
   In case the connection between the master module  40  and at least one of the slave modules is broken, this is detected by the microprocessor  70  because there is no current flowing through the loop comprising the slave coils  54 . It that case, a message is sent to the server  20 , telling that the electrical power meter has failed. 
   It is also possible to calibrate the master module  40  by means of a predetermined FSK pattern superposed on the normal current of the wires  12 . As an example, a signal comprising solely “1”:s is transmitted from the server  20 . This signal represents a certain current level, which is then detected and interpreted by the master module  40 . By means of this detected current shift, the module  40  can then be self-calibrated. Thus, a signal having a predetermined level is input and the output level is determined. The module then calibrates until the output level is equal to the input level. With the described system, billing of the customers is effected in the following way. The server  20  collects and compiles the odometer readings from the meters  30 . A software application then connects a number of measurements from a certain device to a predefined tariff and adds information of the account customer  16 . This gives the full information to create a record and, hence, a bill to be sent to the customer in any convenient way, such as through the Internet or by ordinary mail. 
   As the invoice contains all information about the customer and the content of the invoice to be paid, in an electronic format, it is very well suited to be sent directly to a billing service. The billing service lets the customer view the account and the bill over the Internet and also lets him/her select a convenient way to settle it. 
   The billing and payment service may let the customer get access to the status of the bill and the ways to pay it over the Internet, possibly by means of a so-called Set Top Box (STB). With a STB or other device provided with a smartcard interface, it is possible to view the reception and the status of the electricity consumption on the display of a SWATSCard®. This complete package will make the need for any mailings to the customer obsolete. It also gives the provider of electrical power the possibility to have a diversified tariff, for example on a daily or hourly basis. 
   A preferred embodiment of the invention has been described. However, the person skilled in the art realizes that this can be varied within the scope of the appended claims without departing from the inventive idea. Thus, an electrical three-phase system has been shown. It is realized that the invention is applicable to single phase systems as well, in which case the slave modules are omitted. 
   Furthermore, the connection between master and slave modules has been shown in the form of strips fixedly attached to the master module. However, any kind of connecting means is possible as long as it is impossible to remove it once attached. Thus, in an alternative embodiment, the strips  60  are separate parts, being inserted into a slot in both the master and the slave module to be interconnected. Alternatively, the strips  60  are fixedly attached to the slave modules instead. 
   Although frequency shift keying has been described as the preferred communication method, other communication methods are also possible, such as Phase Shift Keying (PSK). 
   The electrical power meter has been described as having no display. From a technical point of view, this is entirely feasible. However, in order to comply with regulations and also for the sake of convenience, the master module  40  may comprise a display means, such as a LCD, on which stored current values are displayed. 
   Although current has been described above as the measured quantity, also voltage is measured in order to calculate the electrical power. 
   The coils  44 ,  54  have been described as connected by their respective end portions to the circuitry  70 . However, also a portion essentially at the middle of the coils can be connected to the circuitry  70 . In that way, more signals are obtained for subsequent interpretation.