Meter for accurately measuring integrated electric power

• US 4,992,725 A
• Filed: 12/21/1989
• Issued: 02/12/1991
• Est. Priority Date: 12/26/1988
• Status: Expired due to Fees

• Current Assignee: Korea Electric Power Corporation (Government of South Korea), NEC Corporation
• Original Assignee: NEC CORPORATION AND THE TOKYO ELECTRIC POWER COMPANY, INCORPORATED
• Inventors: Kobayashi, Tadashi, Takagi, Toshio, Nakamura, Hidetake, Komatsu, Masatoshi, Fushimi, Shigeo, Wada, Shigenori
• Primary Examiner(s): Karlsen, Ernest F.
• Application Number:
  US07/454,510
• Time in Patent Office: 418 Days
• Field of Search: 324/142, 324/107, 324/111, 324/103 R, 328/160, 364/483
• US Class Current: 324/142
• CPC Class Codes: G01R 21/1331 Measuring real or reactive component, measuring apparent energy

Abstract

In a watt-hour meter in which first and second input signals representing an analog electric current and an analog voltage of an alternating current are converted into first and second digital signals, a first adder produces a sum signal which represents a sum of the first input signal and an additional analog signal of a frequency different from that of the first input signal and which is substituted for the first input signal. The meter may further comprise a second adder which adds another additional analog signal to the second input signal. Alternatively, the meter comprises low-pass filters and oversample-and-convert circuits to produce the first and the second input digital signals. As a further alternative, the meter comprises a first inverter for inverting a selected one of the first and the second input signals into a first inverted signal. In this event, a first selector alternatingly selects one of the first inverted signal and the selected one of the first and the second input signals and the other of the first inverted signal and the selected one of the first and the second input signals at a predetermined time interval as a first selected signal. A second inverter for inverting a product signal representing a product of the first and the second digital signals into a second inverted signal. Ganged with the first selector, a second selector alternatingly selects one of the second inverted signal and the product signal at the predetermined time interval as a second selected signal so that the second inverted signal is selected when the first selector selects the first inverted signal.

Claims

1. An electric power meter comprising:

a first low-pass filter having a first predetermined cutoff frequency for filtering a first input signal representative of an analog electric current of an alternating current to produce a first filtered signal, said first predetermined cutoff frequency being higher than a first Nyquist rate which is substantially equal to twice a first frequency of said first input signal;

first oversample-and-convert means connected to said first analog low-pass filter for oversampling said first filtered signal at a first sampling frequency higher than said first Nyquist rate into a first sampled signal and for converting said first sampled signal into a first digital signal;

a second low-pass filter having a second predetermined cutoff frequency for filtering a second input signal representative of an analog voltage of said alternating current to produce a second filtered signal, said second predetermined cutoff frequency being higher than a second Nyquist rate which is substantially equal to twice a second frequency of said second input signal;

second oversample-and-convert means connected to said second low-pass filter for oversampling said second filtered signal at a second sampling frequency higher than said second Nyquist rate into a second sampled signal and for converting said second sampled signal into a second digital signal;

multiplying means connected to said first and said second oversample-and-convert means for calculating a product of said first and said second digital signals to produce a product signal representative of said product; and

integrating means connected to said multiplying means for integrating said product signal to produce a meter output signal representative of an integrated electric power of said alternating current.

2. An electric power meter as claimed in claim 11, further comprising:

a first digital filter connected to said first oversample-and-convert means for filtering said first digital signal into a first filtered digital signal so as to remove first quantization noise from said first digital signal, said first quantization noise being generated in said first oversample-and-convert means, said first filtered digital signal being supplied to said multiplying means as said first digital signal; and

a second digital filter connected to said second oversample-and-convert means for filtering said second digital signal into a second filtered digital signal so as to remove second quantization noise from said second digital signal, said second quantization noise being generated in said second oversample-and-convert means, said second filtered digital signal being supplied to said multiplying means as said second digital signal.