Electricity metering with a current transformer

• US 20060095219A1
• Filed: 08/11/2005
• Published: 05/04/2006
• Est. Priority Date: 09/28/2004
• Status: Active Grant

• Current Assignee: Veris Industries LLC (Schneider Electric SE)
• Original Assignee: Veris Industries LLC (Schneider Electric SE)
• Inventors: Bruno, David A.
• Granted Patent: US 7,359,809 B2
• Time in Patent Office: Days
• US Class Current: 702/64
• CPC Class Codes: G01R 11/20 Compensating for phase errors in induction meters
• CPC Class Codes: G01R 15/183 using transformers with a magnetic core
• CPC Class Codes: G01R 19/02 Measuring effective values, i.e. root-mean-square values
• CPC Class Codes: G01R 21/06 by measuring current and voltage G01R21/08 - G01R21/133 take precedence
• CPC Class Codes: G01R 22/10 using digital techniques

Abstract

The data storage and processing resources required for an electric meter are reduced by basing the selection and application of phase error correction on a root mean square load current flowing through the meter.

Claims

1. A method of a load current in a conductor, the method comprising the steps of:

(a) determining a phase error correction factor corresponding to a root mean square of a load current from a first plurality of sampled currents;

(b) determining a root mean square of a load current from a second plurality of sampled currents; and

(c) computing a phase adjusted root mean square load current from said root mean square load current for said second plurality of sample currents and said phase error correction factor.

2. A method of measuring load current in a conductor, the method comprising the steps of:

(a) determining a first root mean square load current from a first plurality of sample currents;

(b) determining a second root mean square load current from another plurality of sample currents;

(c) determining a phase correction factor corresponding to said first root mean square load current; and

(d) determining a phase corrected root mean square load current from said second root mean square load current and said phase correction factor.

3. The method of measuring current of claim 2 wherein the step of determining a root mean square load current from a plurality of sample currents comprises the steps of:

(a) sampling a secondary current induced in a winding of a current transformer by said load current;

(b) computing an assumed load current corresponding to said secondary current of said sample;

(c) squaring said assumed load current;

(d) repeating steps (a)-(c); and

(e) computing a square root of an average a plurality of squared assumed load currents.

4. The method of measuring a load current of claim 3 wherein the step of computing an assumed load current corresponding to a sampled secondary current comprises the step of computing the product of said sample secondary current and a turns ratio of said current transformer.

5. The method of measuring current of claim 2 wherein the step of determining a phase correction factor corresponding to said first root mean square current comprises the step of selecting a phase correction factor from a table comprising a plurality of phase correction factors and a plurality of respectively corresponding root mean square currents.

6. The method of measuring current of claim 2 wherein the step of determining a phase correction factor corresponding to said first root mean square load current comprises the step of calculating a phase correction factor from an equation expressing a mathematical relationship of said phase correction factor and a root mean square current.

7. A method of measuring current in an electrical distribution system, said method comprising the steps of:

(a) periodically sampling a secondary current in a winding of a current transformer,

(b) calculating an assumed load current from said sample secondary current;

(c) repeating steps (a) and (b) for a first plurality of samples of said secondary current;

(d) calculating a first root mean square of said assumed load currents from said first plurality of samples;

(e) repeating steps (a) and (b) for a second plurality of samples of said secondary current;

(f) calculating a later root mean square of said assumed load currents from said second plurality of samples of secondary current;

(g) determining a phase correction factor corresponding to said first root mean square assumed load current, and

(h) calculating an adjusted load current from said later root mean square assumed load current and said phase correction factor.

8. The method of measuring current of claim 7 wherein the step of determining a phase correction factor corresponding to said first root mean square current comprises the step of selecting a phase correction factor from a table comprising a plurality of phase correction factors and a respectively corresponding plurality of root mean square currents.

9. The method of measuring current of claim 7 wherein the step of determining a phase correction factor corresponding to said first root mean square current comprises the step of calculating a phase correction factor from an equation expressing a mathematical relationship of said phase correction factor and said root mean square current.

10. A meter for measuring a load current in a conductor, the meter comprising:

(a) a current transformer including a primary winding and a secondary winding, said load current inducing a secondary current in said secondary winding; and

(b) a data processing system that;

(i) periodically samples said secondary current;

(ii) calculates a first root mean square of assumed load currents from a first plurality of samples of said secondary current;

(iii) determines a phase correction factor corresponding to said first root mean square assumed load current; and

(iv) calculates a second root mean square of assumed load currents from a temporally later second plurality of samples of said secondary current; and

(v) calculates a phase adjusted load current from said second root mean square assumed load current and said phase adjustment factor.

11. The meter of claim 10 further comprising a memory for storing a plurality of phase adjustment factors and a plurality of respectively corresponding root mean square current values.

12. The meter of claim 10 further comprising a memory accessible by said data processing system, said memory storing an equation relating said phase adjustment factor to said root mean square current, said equation used by said data processing system to calculate said phase adjustment factor from said first root mean square load current.

13. A meter for calculating power, the meter comprising:

(a) a current transformer including a primary winding and a secondary winding, said load current inducing a secondary current in said secondary winding;

(b) a data processing system that;

(i) periodically samples said secondary current;

(ii) calculates a correction factor representative of the phase shift been the secondary current and the load current;

(v) calculates a phase adjusted load current from said secondary current and said correction factor;

(c) said data processing system periodically samples a corresponding voltage associated with said load current at a temporal offset from said sampling of said secondary current, and determines said power based upon said phase adjusted load current and said voltage.