NZ233194A

NZ233194A - Detecting dc in ac waveforms using mean square

Info

Publication number: NZ233194A
Application number: NZ23319490A
Authority: NZ
Inventors: Lloyd Stanley Thomas; Evan John Stanbury
Original assignee: Alcatel Australia
Priority date: 1990-04-04
Filing date: 1990-04-04
Publication date: 1992-06-25

Description

<div class="application article clearfix" id="description"> 2 3 3 19' p fiority Co.T.r'—• ^ ,(f.r..U'30. cu,i: u; 6.0(^.1 Q. }az, iu.,3 ■ Public*" P.O. t5 JUN 1992 7S5'f NEW ZEALAND PATENT OFFICE "4 APR 1990 RECEIVED rc.<- NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "DIRECT CURRENT METER CIRCUIT" WE, _SiEANBA-R-B—TE-tiE-PH0N-E-S-A-ND-e-A-BLE-S—P£-Y-v—L-m£SED, A Company of the State of New South Wales, of 252-280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly dsscribed in and by the following statement! 1 This invention relates to a method and apparatus for detecting (and measuring) a DC component In an AC current. One area where such a need arises is in the supply of electricity by a generating authority. 5 It is common for AC kWh meters to use current transfor mers to measure the current flowing into a load, from which the power may be derived by known means in conjunction with the measured voltage across the load. Certain types of load containing half-wave rectifiers 10 or triac-controlled devices may introduce a DC component to the current consumed, which may saturate the current transformer and reduce measurement accuracy. Some types of current transducer or processing circuitry may not pass DC levels. Thus some electricity consumers may obtain free 15 power by this method. DC currents may cause saturation in • distribution transformers, causing loss of efficiency, higher temperatures and reduced lifetime, as well as causing corrosion of water or gas pipes and ground rods, with consequent safety hazards, and maintenance costs. 20 This invention describes a method of detecting and W measuring DC current flowing through the primary of a cur rent transformer by measurements taken on the secondary. A hardware method is described, but the method may equally be applied using analogue or digital hardware or software. In 25 particular, the software version may be implemented as a byproduct of the A h calculation. The method can be adapted to other transducers which isolate DC. \ 2 233 The method according to the invention comprises the steps of separating the positive and negative half-cycles, squaring the half-cycles and subtracting the squared negative pulses from the squared positive pulses or vice versa, accumulating (summing/integrating) the results, and detecting whether the accumulated result exceeds an upper (positive) or a lower (negative) threshold. The number of times a threshold is exceeded is counted and the accumulation process is restarted each time a threshold is exceeded. The count of the number of times a threshold is exceeded Is a measure of the DC. This value may be multiplied by a scaling factor and the square root taken to provide a value for the DC. The method may be implemented in analog (or digital) hardware or in a microprocessor controlled system incorporating digital hardware. The invention will be more fully described with reference to the accompanying drawings in which: Pig. 1 is a block diagram of an embodiment of the invention. Pig. 2 is a block diagram of the embodiment of Fig. 1 with no-load offset correction. Pig. 3 is a current/time diagram showing the effect of squaring the current. With reference to Figure 1, the current transformer 1 produces a current on its secondary proportional to the AC component of its primary current. The average current on 23 3 1 the secondary is zero, when averaged over a large number of cycles. Rectifier 2 separates the positive and negative half-cycles, which are then squared at 3 and 4 and accumulated in integrator 5, one half-cycle causing the output of integrator 3 to increase, the other half-cycle causing the output of integrator 5 to decrease. If the load current (primary CT current) consists of pure AC, the output of integrator 5 will increase and decrease by the same amount every cycle, causing no net effect . If, however, a half-wave rectified load is attached, one half-cycle will dominate, and integrator 5 will move steadily up or down, depending on the DC current polarity. Comparators 6 and 7 detect when the output of integrator 5 exceeds upper limit 8 or lower limit 9S respectively, and then reset integrator 5, as well as incrementing counter 10 which measures the magnitude of DC current. The contents of register 10 may be read periodically (at the regular meter-reading time) to detect if DC is being consumed, upon which the electricity supplier may initiate appropriate measures. Register 10 may then optionally be manually reset by reset means 11 to start another measurement period. Alternatively, when the comparators show that DC is being consumed, an alarm may be raised at the electricity supply company, if the measuring device is connected to a suitable communication system. 23 3 1 9 The thresholds selected at 8 and 9 should be large enough so that DC Is not indicated by the.occasional switch-ing on or off of heavy loads, which may result in a heavy current for half a cycle, effectively an instantaneous DC 5 current. The attached graph (Figure 3) shows the current trans-O former output current for one cycle of a 50Hz waveform, where the load is half-wave rectified. The integration output shows an increase over the period of the AC cycle due to 10 the DC component in the current. This method may be implemented in a system including a microprocessor programmed as follows (I is the transducer output, Sum is the output of integrator 5, Limit represents boxes 8 and 9 and DC is register 10). 15 I = Current Sample IF I > 0 THEN Sum = Sum + 1*1 w' ELSE Sum = Sum - 1*1 IF Sum > +Limit THEN Sum = Sum -Limit 20 DC = DC+1 ■-"K ELSE IF Sum < -Limit / THEN Sum= Sum +Limit DC = DC+1 Figure 2 is the same as Figure 1 with the addition of 25 functions to eliminate a false DC current indication due to DC offsets in the circuitry, which could indicate DC consumption even under no-load conditions. A digital hardware 5 23 3 1 n implementaton is assumed here, where the DC offset occurs only in the A/D converter 13. 1 Low-pass filter 14 measures the average output of the current transformer with a time constant of many cycles. 5 This calculates the average DC offset of A/D converter 13, which is subtracted from the input signal by adder 15 to o eliminate DC offset. Monostable function 16 is re-triggered when a comparator (6 or 7) threshold is exceeded. If the 10 monostable delay expires without a DC level exceeding the threshold, the integrator 5 will be reset. This will prevent a DC 'creep'. These methods using the low pass filter or the monostable may be used individually or separately. If it is desirable to measure both the magnitude and 15 polarity of DC consumed, the single register 10 may be replaced by two separate registers 10 and 12, one of which is Incremented for positive DC, the other for negative DC con-sumption. Because current is squared, the output will be propor-20 tional to DC current squared, and thus penalise occasional heavy DC current consumption more than slight continuous consumption. An electricity-measuring device which calculates Amps-squared hours (A h, a measure relating to transmission 25 losses) is required to measure the square of instantaneous current, and the DC detection of the present invention may be implemented as a by-product of this calculation. 6 233 While the present invention has been described with regard to many particulars, it is to be understood that equivalents may be readily substituted without departing from the scope of the invention. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 3-^1^4 What we claim is:-

1. A method of dctccting a DC component in an alternating current waveform comprising the steps of separating the positive and negative half-cycles, squaring the half-cyclcs and subtracting the squared negative half cycles from the squared positive pulses or vice versa, accumulating the results of the subtraction step to obtain an accumulated result, and detecting whether the accumulated result exceeds an upper or a lower threshold.

2. A method of measuring a DC component in an alternating current waveform comprising performing the method of claim 1 and counting each occurrcnce of the accumulated result exceeding the upper or lower threshold to give a count representative of value for the DC, and restarting the accumulation process each time a threshold is exceeded. 3. A method as claimed in claim 2, including the step of multiplying the count by a scaling factor and taking the square root to obtain a weighted average DC current. 4. A method as claimed in any one of claims 1 to 3, wherein one or more of the steps of the method arc carricd out using electronic circuits, the method including the step of calibrating the electronic circuits to substantially eliminate DC offset in the electronic circuits. 5. A method of detecting a DC component in an alternating current waveform as herein described with reference to the accompanying drawings. 6. A method of measuring a DC component in an alternating current waveform as herein described with reference to the accompanying drawings. 7. Apparatus for detecting a DC component in an alternating current waveform, the apparatus including waveform discriminating means to separate the positive and negative portions of the waveform, multiplier means to square.the positive and nega- i, " / ■ O a ' J -7 APR 1992 " 8

3-2>a> |0| live portions; subtraction means to obtain the diffcrcncc between the squared positive and negative portions; accumulation means to accumulate the output of the subtraction means;, comparator means to detect whether the output of the accumulation 5 8. Apparatus for measuring a DC component in an alternating current waveform including the apparatus claimed in claim 7 and counting means to count cach occur-rencc of the comparator detecting whether the upper or lower threshold has been exceeded, the apparatus including a reset circuit to restart the accumulation process cach time a threshold is exceeded. 10 9. Apparatus as claimcd in claim 8, including a scaling circuit to multiply the contents of the counter by a scaling factor. 10. Equipment for measuring or detecting a DC component in an alternating current waveform as herein described with reference to the accompanying drawings. n means exceeds an upper or a lower threshold. 15 ALCATEL AUSTRALIA LIMITED 20 O P.M. Conrick Authorized Agent P5/1/1703 25 -7 APR 1992 r,. 9 </div>