This invention relates to AC power meters and more particularly to the determination of the internal impedance of a distribution transformer and the sensing of DC current flowing in such a meter.
Electronic metering of electrical energy is a mature technology and today""s meters must minimize hardware cost to be competitive. Typically, one of the major cost elements in a meter is the current transformer that accurately reproduces the waveforms of the current to be measured.
Normally, the current to be measured is the power frequency current, 60 Hz for systems in the United States and 50 Hz for many international systems. In addition to the power frequency currents there are abnormal currents in the form of harmonic and DC currents that can be present in power systems.
Some of the harmonic and DC currents can be created by normal loads found in industry as well as in the home. Others of the abnormal currents are generated by customers that intentionally try to alter the accuracy of their power meter. This is known in the power industry as tampering. The currents resulting from tampering can adversely affect the performance of current transformers at the power frequency where extremely accurate billing information is required. In addition, it is estimated that tampering results in billions of dollars of unpaid for electric power usage in the United States. Thus it is desirable to detect DC current so that customer tampering can be known and reported.
Previous metering technologies have used a variety of current sensing techniques to detect the abnormal currents flowing in an AC power meter. Generally, these sensing techniques compromise the accuracy for the power frequency current. The DC current tends to saturate magnetic materials and consequently it adversely affects accuracy. A typical current sensing material like Supermalloy tends to be expensive and is priced proportional to weight (volume). Because the DC current requires more magnetic material to obtain the same AC accuracy there is a significant cost benefit associated with sizing the current transformer for AC current only and finding a technique for compensating the result when there is DC current present.
Industry has developed other current sensing techniques to solve the problems associated with current transformers and DC current. Devices such as Hall effect sensors have been used to detect AC current in the presence of DC current to prevent tampering. These devices typically do not achieve the accuracy that a high quality current transformer does.
Therefore it is desirable to accurately detect in a cost effective manner the flow of DC current through an AC power meter in order to determine if the customer is tampering with the meter. The present invention meets these requirements. Once the level of DC current flow through the meter has been determined the reporting of tampering can be visually displayed on the meter or brought back through a communications system to a computer for display or warning and the meter billing parameters can be modified.
A method for determining the DC current flowing through an AC power meter comprising:
(a) measuring the internal impedance of a distribution transformer associated with the meter;
(b) determining the amplitude of the DC voltage component of the AC voltage in the meter; and
(c) calculating the DC current from the measured distribution transformer internal impedance and the determined DC voltage amplitude.
In a power distribution network comprising:
(a) a distribution transformer; and
(b) one or more AC power meters associated with the distribution transformer;
a method for determining the DC current flowing through at least one of the one or more AC power meters comprising:
(i) measuring the internal impedance of the distribution transformer;
(ii) determining the amplitude of the DC voltage component of the AC voltage in the at least one meter; and
(iii) calculating the DC current from the measured distribution transformer internal impedance and the determined DC voltage amplitude.
A method for determining the internal impedance of a distribution transformer comprising:
(a) measuring for a first load condition the current flowing through the distribution transformer internal impedance;
(b) measuring for a second load condition the current flowing through the distribution transformer internal impedance;
(c) matching each of the current measurements for the first and the second load conditions with a corresponding input voltage reading of a power meter connected to the secondary of the distribution transformer;
(d) dividing the difference between the corresponding power meter input voltage readings by the difference between the current measurements for the first and the second load conditions; and
(e) repeating steps (a) to (d) for a predetermined number of iterations performed over a predetermined period of time.
A method comprising:
(a) measuring for a first load condition the current flowing through a distribution transformer internal impedance;
(b) measuring for a second load condition the current flowing through the distribution transformer internal impedance;
(c) matching each of the current measurements for the first and the second load conditions with a corresponding input voltage reading of a power meter connected to the secondary of the distribution transformer;
(d) dividing the difference between. the corresponding power meter input voltage readings by the difference between the current measurements for the first and the second load conditions; and
(e) repeating steps (a) to (d) for a predetermined number of iterations performed over a predetermined period of time to determine the internal impedance of the distribution transformer.