Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R21/00—Arrangements for measuring electric power or power factor
  • G01R21/133—Arrangements for measuring electric power or power factor by using digital technique

Definitions

• This Invention relates to techniques and equipment for sampling two or more signals and will be described in the context of sampling voltage and current amplitudes in an AC kWh meter.
• the S/H amplifiers or track/hold circuits thus ensure a fairly constant input to the A/D converter while the conversion takes place, otherwise a random phase error is Introduced in the measurement due to the measurement being taken at an indeterminate time in the measurement period.
• This specification discloses a technique for sampling two or more signals without the use of a S/H amplifier by measuring the signals sequentially.
• the random phase error introduced by this procedure is cancelled when averaged over a large number of samples by reversing the order of successive samples.
• the technique produces a residual amplitude error related to the sine of the phase angle between the samples.
• This residual error can be eliminated by the standard calibration procedure used to eliminate gain errors due to component variation.
• Fig. 1 shows a block diagram of a single-phase Ac kWh meter embodying the invention
• Fig. 2 shows a polyphase embodiment
• transducers 1 and 2 respectively. These signals may be selected by analog multiplexer 3 under control of processing means 5 (which may in turn be controlled by software). The signal selected by 3 is converted to digital format by A/D converter 4, which is started by processing means 5 at intervals dictated by timing means 7.
• the processing means 5 first selects the desired analog input and then starts A/D converter 4. On completion of the conversion, the value measured by 4 is read by processing means 5 which may then initiate further conversions by controlling multiplexer 3 and A/D converter 4. Processor 5 performs calculations on the values measured by 4 to derive useful quantities (such as kWh, etc.) from the measured quantities by known means.
• processing means 5 Since the meter does not contain Sample/Hold amplifiers (S/H) to measure the transducer outputs as a simultaneous pair, the processing means 5 must sequentially read the input signals, introducing a phase error into the measurements. However, processing means 5 may cancel this phase error by taking half of the samples with the voltage followed by the current, and the other half by sampling first the current, then the voltage. In a preferred embodiment, the order is reversed on successive samples.
• S/H Sample/Hold amplifiers
• the calculations performed by processing means 5 typically include a factory-settable calibration constant to account for component variations. This invention utilises the same calibration means to correct for the ratio error introduced by non-simultaneous sampling of current and voltage.
• the calibration constant may be entered via a serial calibration port 8.
• a successive-approximation A/D converter measuring lowfrequency (50 or 60Hz) signals takes 150us to measure a voltage transducer, with a further 50us delay before starting measurement of the current transducer. This results in a phase error of 3.6° at 50Hz, which would produce a power measurement error of 11 per cent at a phase angle of 60° (and greater errors at larger phase angles or higher frequencies).
• phase errors in the transducers may be corrected by altering the proportion of samples taken in each order from an exact 50%. In a further embodiment, this proportion may change based on the current being measured, to correct phase errors which are worse at low current, for example.
• Fig. 2 shows a polyphase application of the invention.
• a multiplicity of current and voltage transducers labc and 2abc are sequentially sa ⁇ pled using miltiplexer 3.
• the order of sampling current and voltage in each phase may be alternated to correct for phase errors in sampling.
• Such an embodiment may be used for a polyphase industrial meter capable of calculating further parameters such as V, I, VAR, VA, harmonic power and phase angle. Sets of measurements may be taken over several cycles with alternating order on successive sets and the necessary calculations are performed on the averaged readings.
• the invention has been described in relation to AC power measurements, it can also be used in other embodiments such as DC power measurements and AC phase meters.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Current Or Voltage (AREA)
• Stabilization Of Oscillater, Synchronisation, Frequency Synthesizers (AREA)
• Analogue/Digital Conversion (AREA)

Applications Claiming Priority (3)

Publications (2)

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Family Applications (1)

Country Status (3)

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• 1990
  • 1990-02-22 WO PCT/AU1990/000067 patent/WO1990012325A1/en not_active Ceased
  • 1990-02-22 AU AU51574/90A patent/AU5157490A/en not_active Abandoned
  • 1990-02-22 EP EP19900903581 patent/EP0465476A4/en not_active Withdrawn

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