AU634722B2 - Digital measurement of A.C. active and reactive values using half-cycle integration technique - Google Patents

Description

Form PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Ci:;% Application Number: Lodged: Ceompleto Specification-Lodged: Accepted: Lapsed: Published: see: Priority: 0 R~elated Art:, M o. 21i57 070O989 TO BE COMPLETED BY APPLICANT Name of Applicant:, GEORGEY KATHIB Address of Applicant: 5 tPoden street, West Brunswick, Victoria, 3055 Actual Inventor: GEORGEY KATRII3 Address for Service:.

As above Complete Specification for the Invention entitledt DIGITAL MEIFASUREMENT OF~ A, Ac-rivE AND HEACTIV1 VAnFs USING IHALF-CYCLE INTEGRATION nrcH-NicluE 'the followlng statement Is a full description of this invention, Including the best method of performing It known to me:-* Note: The desctiption lsate be typed In double spacing, Pica type face, In an area net exceeding 260 min In depth and 160 mm In width, on tough white Paper of good quality and it Is to be Inserted inide this form.

1 4tI78- L Printed by C. 3. TitommoN, Commonwealth Government Printer, Caniberra This invention relates to the digital measurement of A.C.

active and reactive values (active current, watt, watthour, power factor, reactive current, var, and varhour) using Half-Cycle Integration Technique.

The present invention attempts to provide a simple technique for accurate, fast, and low cost digital measurement of A.C. active and reactive values.

According to the present ivnention there is provided a metering apparatus for measuring active and reactive values in an AC electric circuit, comprising: load current sensing means for providing a current representing signal that is proportional to an AC current through a load; voltage sensing means for providing a voltage representing signal that is proportional to the voltage across the load; integration half-cycle determining means for 20 determining integration intervals of the current representing signal, an output of the integration halfcycle determining means being connected to an input of a control means; nullifying voltage producing means for providing 25 one or more DC nullifying voltages used to produce currents required to balance charges of capacitors of an integrating means; switching means for connecting the current representing signal and the nullifying voltages, as required by the control means, the switching means having outputs connected to inputs of the integrating means; the integrating means integrating the outputs of .u ".She switching means, the integrating means including one or more group of integrators, each group of integrators

Q

6 3 x includes one or more integrator stages in cascade and a means for selecting use of an odd or even number of integrating stages; level detecting means for determining levels of an output of the integrating means, an output of the level detecting means being connected to an input of the control means; measuring pulses generating means for providing a continuous train of measuring pulses, the pulse rate of the measuring pulses being selectable such that for measuring watt, watt-hour, var and varhour, the pulse rate is proportional to the voltage across the load, and for measuring active and reactive currents and power factor, the pulse rate is constant, the measuring pulses being supplied to an input of the control means; counter and display means for counting the measuring pulses during a integration period of one of the nullifying voltages to determine the active and reactive values, thereby producing an indication of the active and 20 reactive values; and the control means controlling the operation of the switching means, the counter and display means and the nullifying voltage producing means.

25 The invention uses a method where the load current is a integrated for a half cycle to obtain a voltage proportional to the active or reactive current. Herein this method is called the Half-Cycle Integration Technique

(HCIT).

The simplicity of the technique allows low cost digital meters to be produced, and in the case of the digital watthour meter a highly sophisticated load management if 4\ system can be included which makes it far superior and cost compatible with the traditional rotating disc watthour meter widely used for measuring electricity consumption.

The HCIT will now be described: The instantaneous value of the supply voltage is given by the formula: v -2 V sin wt (1) where V is the rms value of the supply voltage, and w is the angular frequency; and the instantaneous load current can be represented by the formula: i 4 I sin (wt 0 (2) where X is the rms value of the load current and is the phase angle between the load current and the supply voltage.

The HCIT calls for multiple integration of the load current for a half-cycle. The output of the last integration es..

0*~ S S

S.

"C

Or, 2 stage (at the end of the integration half-cycle) is given by the formula: A 2 I cos V 1 (3) SK w n 2 where K is the product of the integrators time constants, n is the number of the integration stages,and o( is the integration starting angle which is the angle between the integration starting point and the zero-crossing point of the supply voltage to the positive values.

Formula shows that when the integration is performed 'f during the positive half-cycle of the supply voltage or Sduring the negative half-cycle of the supply voltage =TT then will be proportional to 'I cos b /Kw for odd numbers *000 of integration stages,and proportional to 'I sin /Kwn* for even numbers of integration stages;f-rmula further shows that if the integration is performed when the supply voltage's slope is positive (X E) or when the supply voltage's slope is negative +'),then will be proportional to '1 cos /Kwn' for even numbers of integration stages,and proportional to 'I sin /Kw n for odd numbers of integration stages.

Since has been obtained by integrating the load current for a half-cycloethe effect of any even harmonics present in the load current will be eliminated,and the effect of any odd harmonics component of an order will be divided by the value any D.C. component in the load current should be filtered out. The number of the integration stages therefore depends on the required odd harmonics attenuation.

Having obtained ,active and reactive values measurement can be achieved according to the following methods: 1. The first method uses either analog multipliers or analog to digital converters with digital multiplication means to multiply with a multiplication factor determined by the measurement required; 1.1 when the supply frequency can be considerc~d constant then, -for active and reactive current measurements,no multiplication is needed; -for power factor and sin measurements,the multiplication factor is inversely proportional to 111; -for watt,watthour,var,and varhour measurements,,the 000: multiplication f actor is proportional to IV Iand in the cases 0 6 of watthoiir and varhour meters,tho results of multiplication obtained every cycle are added during the time of measurement.

1 .2 when the supply frequency cannot be considered constant then, 9* 00-for active and reactive current measurements,the 0O 0 -fo power factor or sin measurements, the multiplication factor is proportional -to ,Kw n/11; 0 -for watt arnd var measuremenit, the multiplication factor is proportional to IWnk -for watthour anid varhour, measurements ,the multiplication factor is proportional to 'Kw n-lVI and the resultS, of multiplication obtained every cycle are added during the time of measurement.

2. TChe second method: after* obtaining as previously doeibed,ii D.C. voltage IV 0 is; integrated at -the end of the integration hal f-cycle by the last integration s tage, the polarity of this DXC. voltage is chosen to cause,1 a linear 4 reduction of the last integration stage's output toward zero.

Hereir',his D.C. voltage will be called -the nullifying voltage.

The time needed for the last integration stage's output to reach zero will be called the measuring time; the pulses from a pulse generating means (oscillating at frequency It c are counted during this measuring time and the number of the counted pulses IN' is given by the formula: N 1-f CA/V 0(4) F*ormula shows that there are different ways for Qq obtaining active and reactive values measurements. 'rhose are 0 described below: 81 when 'the supply frequency can be considercd cornstont then, -for active and reactive current measurements, If 0 and IV are constants; -for power factor anid sin 4Measurettents"I C I is; constant and IV&0 is; proportional to or If e is; proportLional 'to *500 -f~or watt, wa tthour ,var I and varhour meatsurements, It CI 00.V: proportional to I' and IV 0 I is constant; or 'f I isj eonstant anid I V 0 1 it proportional to I 1/Vl In the castv5 of wattholnr and varhour metrsi IN inN added very cycle dutrinq( the timeft of Pea Ure"Men.L 212 whenl 1w, cnobecon'Mdered vol.'attol -for active and reactiveyecur rent maueeta,.I i pr oportional to IWO, (where 0011p4 u and 'Vo 6 proport loud to I l/Kw nI ~-for pow'er factor% or" sin 41I eesrmns Ic it; prorUUtional to lwtv' and IV 0 in proportiona~l to l t,,RW 11P 1 e I isrprop ortional to~ 1w 1 and IV WV is pr oport lomna to '1'I -J -for wat~t andl var measurements, 'Ef is proportional to lP and 'Vol is proportional to ,l/Kw n-p,or lfc is proportional 'to I wPI and I VOI is proportional to I I/Kwn-pV I -for watthour and varhour measurements, I Ec is propor-~ tional to IwpV' and 'Vol is proportional to 'l/Kw-p-1,or If c is proportional 'to 'wy" and 'Vol is proportional to 'I./Kw n-P-VI and IN' is added every cycle during the time of measurement.

3. The 'third method: the nullifying voltage IV 0 described in met~hod 2 is integrated by the last integration stage durin~g :t.Q the inte gration halt cycle with the right polarity to keep the 0P* Ouput of t:he lost inteqration stage nlear zerothe difforenco too$ between the total1 integration time oA the positive nullifyin~g *:voltage and the total integration -time of the negative nullify,- .J ing vol tzage is proportional to The pulses from Clhe polre generating meanst (oscillafting at frequency It 01) are used to Increment a couter when Mhe posiitive nul1lii yinq vol tage In AW ';ated andt to (decremenit the~ counter when the negative :.nullifying voltage it; integrated (or vice versa) The counter ntent at tWe end of tho integ r"Uti half -cycle will1 be 110 chanijed by the value 'NI tat; given by formula Thit; counlter' tore'r"t to belore the start of the integration half -cycle vxcvLt in the cases; of watthour and varhour moLtr where the couiainq rersul tt are added durinq the time of measflurement. Thle values of IfI and I V 0 are asj detailed in paragraph A particular embodiment of the invention and the bet met ho I of ierformi iii It known to tue will1 now be dotscribed with r elerence t o thv accot parly wq draviltillt in which: P'i jurv 1 Ws tho exp ceted waveformy whown top thve arne of an induetive load CantI a 1jilile "Ate inte1rtion.

6 Figure 2 is a block diagram of the active current,watt, watthour,power factor,reactive current,var,and varhour meters.

The Supply Voltage Sensing Means provides a suitable low A.C. voltage proportional to the supply voltage and in phase with it; the Integration Half-Cycle Determining Means (2) determines the half-cycles in which the load current is to be integrated,it determines when the supply voltage is positive (or negative) or when the supply voltage's slope is positive 4. (or negative); these informations are sent to the control means; 0 49 the Load Current Sensing Means provides a suitable low A.C. voltage proportional to the load current and in phase with 0 it,and filters out any D.C. component which is present in the load current; the Nullifying Voltage Producing Means supplies the D.C. voltage (with the appropriate polarity as determined by the control means) required to reduce the output of the S* integration means linearly to zero; this voltage is constant in the cases of watthour and varhour meters;proportional to '1/w' in the cases of active current,reactive current,watt,and var meters; and proportional to in the case of power factor meter; the Integration Means integrates the load current for a half-cycle (as determined by the control means),followed by the nullifying voltage until the output reaches zero; from this moment on,the output will be held at zero until the next integration cycle starts; the Zero Detecting Means detects the point where the outpuat of the integration means roaches zero voltage,and determines whether it was positive or negative just before reaching that point (in order to determinc polarity of'V coonting up or down,and -the sign of the meter reading). Those informations are sent to the control means; -the Pulse ProducinU Means generates the measuring pulses which are counted during the measuring -time in or~der to obtiAtn the required measurement; -the frequency of these pulses is cons~tant in -the cases of -the active current, reactive current, and power factor moters; and proportional. to the supply voltage :in the cases of watt, watthour, iar, and varhour meters; the Counter and1 isplay Means (8 counts thne meotri.ng Spulses ,divides the count by a sui table scale ,and displays the meter reading; the counts are added every cycle in the cases of watthour and varhour meters ,uid the counting means resets to .e:zero every cycle in the cases of active current, watt, power.

S. e*e £ctorareactive currerit,and var meters; the Control Means controls the following: 1- the switching of; the loead current sens~ing mem uptand IV01 (with Lhe right polarity) to the input o1 the ivtvqration 2~the .witelhing of the meanurt' q pulnoes to the countotr means, 3- -the solection of the counting direction (up/down) and the reading Ai, of the counter, kind dinplay mea01NO

Claims (4)

1. A metering apparatus for measuring active and reactive values in an AC electric circuit, comprising: load current sensing means for providing a current representing signal that is proportional to an AC current through a load; voltage sensing means for providing a voltage representing signal that is proportional to the voltage across thie load; integration half-cycle determining means for determining integration interals of the current representing signal, an output of the integration half- cycle determining means being connected to an input of a is control means; nullifying voltage producing means for providing one or mnore DC nullifying voltages used to produce currents t.. required to balance charges of capacitors of an integrating means; switching means for connectinig the current representino signal and the nullifying voltages, as required by the control means, the switching means having, :4...outputs connected to inputs of the integrating means; the integrating means integrating the outputs of 6 .4:6 0 25 tho switching meana, the integrating means including one or *Soso: (A more group of integrators# each group of integrators includes one or more integrator staves in cascade and a .X.A means for selecting use of an odd or even number of integrating stages; level 4,aecting means for determining levels of an output of the integrating moans, an output of the level detecting matin being comnected to an input of the control means; measuring pulses generating means for providing a continuous train of measuring pulses, the pulse rate of the measuring pulses being selectable such that for measuring watt, watt-hour, var and varhour, the pulse rate is proportional to the voltage across the load, and for measuring active and reactive currents and power factor, the pulse rate is constant, the measuring pulses being supplied to an input of the control means; counter and display means for counting the measuring pulses during a integration period of one of the nullifying voltages to determine the active and reactive values, thereby producing an indication of the active and reactive values; and the control means controlling the operation of the switching means, the counter and display means and the nullifying voltage producing means.

2. A metering apparatus according to claim 1, wherein the load current sensing means includes a harmonics reduction means. 0

3. A metering apparatus according to claim 1 or 2, wherein the voltage sensing means include a harmonics reduction means.

4. A metering apparatus according to any one of claims 1 to 3, wherein the counter and display means is capable of scaling the counted measuring pulses and storing and displaying counted measuring pulses. A metering apparatus according to claim 1 ard substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings. Dated this 11th day of January 1993 KATRIB, Georgey By his Patent Attorneys: GRIFFTH HACK CO. Fellows Institute of Patent Attorneys of Australia. foot 6 0 6