A STATIC, ELECTRIC APPARATUS FOR MEASURING POWER AND ENERGY DRAWN FROM A POWER SUPPLY NETWORK

Abstract: A static electricity meter has for each phase a signal conditioning circuit (10, 11 and 12) which detects the voltage and current and applies voltage pulses representing these quantities to a multiplexer (13), from which they are transferred sequentially through a multiplier (14) to a V/F converter (15), whose output signal is a pulse train having a frequency which is an expression of the power consumption on the phase detected at any time. This pulse train is fed to a microprocessor (16) which counts the pulses and calculates the energy consumption on the basis of an incorporated time factor and transfers it to a random access store (18). The microprocessor also controls the signal transfer to the multiplexer so that the detection of the phases takes place in a arbitrarily varying order and with a frequency independent of the network frequence. This prevents a consumer from avoiding registration of consumption by synchronized connection of the load to the network with the metering system. To prevent inaccurate measurements because of ageing and temperature drift of the component of the electricity meter, the meter is designed to automatically perform self-calibration/adjustment on the basis of an accurate reference voltage source having long-term stability and very low temperature drift.

Description: A static, electric apparatus for measuring power and energy drawn from a power supply network The invention concerns an apparatus of the type defined in the introductory portion of claim 1. The GB Patent Application 2 061 641 discloses such an appa¬ ratus which serves to determine the peak power of each of the phases in a three-phase network and to switch in and out the consumers on each of the phases so that a predeter¬ mined maximum load per phase is not exeeded. The GB Patent Application 2 046 925 discloses a metering system which serves as an additional unit for a conven¬ tional Ferrari meter, and this unit enables registration of the consumption according to three orMmore different tariffs so that different billing rates can be applied for different times of consumption. The object of the invention is to provide an apparatus of the present type which ensures correct metering of the actual consumption, even if periodic loads are switched in on the individual phases, and which is convenient in the sense that, in contrast to the conventional Ferrari meter, it requires no regular and thorough service and maintenance, no repair or replacement of worn components and no adjustment. This object is achieved as stated in the characterizing portion of claim 1 in that the detecting frequency is in¬ dependent of the network frequency, and that the detection order of the phases is randomized, whereby these features prevent synchorization of a load, e.g. a heating element, with the meter to avoid registration of the actual con- sumption. To avoid inaccurate measurements because of ageing or temperature drift or the components, the apparatus may be designed for self-calibration, as stated in claim 2. An expedient embodiment of the self-calibration means is sta- ted in claim 3. An embodiment of the apparatus of the invention is sbhema- tically shown in the drawing and will be described more ' fully below with reference to the drawing. In the drawing, a power supply network is represented by three phase conductors R, S and T and a neutral conductor N. A signal conditioning circuit 10, 11 and 12 is inserted in each phase conductor, and each of these circuits more¬ over connects with the neutral conductor and senses the phase voltage through ohmic voltage dividers and the phase currents through measuring transformers and converts these quantities to voltage signals having an acceptable level for the subsequent circuits. The signals thus conditioned are fed in pairs to a multiplexer 13 which transfers the signals per phase with time delay with respect to each other to a multiplier 14. This multiplier produces the pro¬ duct of current and voltage for each phase. These pro¬ ducts are fed to an RMS converter which is contained in the multiplier 14 and applies a voltage whose size is an expression of the instantaneous power in the phase concerned. This voltage is fed to a voltage/frequency converter 15, called V/F converter in the following, whose output signal is a pulse train having a frequency which is pro¬ portional to the voltage signal applied to the input. The pulse train is fed as an input signal to a microprocessor 16, in which the number of pulses per second from the V/F converter is counted. This count is an expression of the instantaneous power. The microprocessor 16 multiplies the power expression with a time factor, and the result of this multiplication is an expression of the consumed amount of energy. This expression of the consumed amount of energy is stored in the store 18 and added here to the previous¬ ly calculated amounts of energy consumed. Thus, the store 18 contains the total registered consumption. The micro¬ processor 16 also controls the signal transfer from the signal conditioning circuits 10, 11 and 12 to the multi¬ plexer 13 through an address bus 17. This transfer takes place sequentially in an arbitrarily varying order and with a frequency which is independent of the network frequency. This makes it impossible for a consumer to avoid registra- tion of the consumption by synchronized switching in of the load to the network with the metering system. The sequential signal transfer causes registration of the power consumption on all three phases. The total consump¬ tion of energy is calculated as the sum of the consumption of the individual phases. To ensure that a registrered consumption of power is main¬ tained even if the voltage supply is interrupted, these data are fed to a random access store 18, which is so de¬ signed as to maintain the stored data even in case of failure in its supply voltage. The stored consumption of energy can be read by actuation of a switch 19, which causes a pulse to be fed to the microprocessor 16, which is caused by the pulse to transfer the energy consumption stored in the store to a read-out unit 20, which corre- sponds to the counter in a conventional electricity meter. The analog components incorporated in the measuring loop, i.e. the multiplexer 13, the multiplier 14 with its RMS converter and the V/F converter 15 are subject to ageing and temperature drift, since electricity meters are often placed in surroundings in which temperature and air humi¬ dity vary greatly. Accordingly, it is important to take measures to prevent these phenomena from affecting the metering accuracy. With this end in view, a reference vol¬ tage source 21 is provided, said source being based on a semiconductor reference diode having a very low temperature drift and very great long-term stability. The apparatus is self-calibrated by means of this voltage reference upon start of the apparatus and then at intervals of e.g. 30 minutes. This is done in that the microprocessor applies a control signal to the multiplexer 13, said signal causing all inputs to the multiplexer to be disconnected, and then the signal from the V/F converter 15 is measured. This sig¬ nal is then an expression of the zeropointerror of the system. The microprocessor now applies a new control signal to the multiplexer, said signal causing the reference vol¬ tage source to be connected to the multiplexer, and the signal from the V/F converter is measured again. On the basis of these measurements and a reference power incor- porated in the store at the manufacture of the meter, it is now possible to calculate the number of pulses from the V/F converter which correspond to e.g. 0.1 kilowatt hour, and this value is stored in the store IB. This calibration procedure, which relies on the accuracy of the reference voltage and the knowledge of the corre¬ sponding reference power, serves to currently and automati¬ cally recalibrate/adjust the electricity meter, so that ageing and temperature drift have no influence of the long- term accuracy of the metering system.

Claims: P a t e n t C l a i m s 1. A static, electric apparatus for measuring and register¬ ing power and energy drawn by a consumer from a power supply network, comprising means for periodic detection of the current and voltage of each phase, as well as means for converting the power expressions thus established to vol¬ tage pulses fed to a voltage/frequency converter whose out¬ put frequency at any time represents the power drawn, c h a r a c t e r i z e d in that the detecting frequency is independent of the network frequency, and that the de¬ tection order of the phases is randomized. 2. An apparatus according to claim 1, c h a r a c t e r ¬ i z e d by comprising means for storing software for self-calibration on the basis of an incorporated reference voltage source. 3. An apparatus according to claim 2, c h a r a c t e r ¬ i z e d in that the self-calibration means are designed to periodically disconnect the phase conductors from the apparatus and to measure the output signal of the voltage/ frequency converter and then to connect the voltage refe¬ rence source to the detection means and to repeat the measurement, as well as to calculate the actual metering system calibration factor at any time on the basis of the known power equivalent of the reference voltage source.

Inventors: LAURITZEN JOERGEN, PEDERSEN FRANK, SERUP KURT
Applicants: TEE AS TOTAL ELECTR ENTERPRISE
Application year: 1985
Main classification: G01R21
IPC codes: G01R21, G01R22