Electricity meters using current transformers

• US 5,563,506 A
• Filed: 03/24/1995
• Issued: 10/08/1996
• Est. Priority Date: 07/10/1990
• Status: Expired due to Term

• Current Assignee: Polymeters Response International Ltd.
• Original Assignee: Polymeters Response International Ltd.
• Inventors: James, Andrew L., McCabe, Barnaby J., Fielden, John S.
• Primary Examiner(s): Nguyen, Vinh P.
• Application Number:
  US08/426,402
• Time in Patent Office: 564 Days
• Field of Search: 324/126, 324/127, 324/142, 324/132, 324/140 R, 324/141, 324/76 R, 324/74, 364/481, 364/483, 364/571.07, 364/571.01, 336/82, 336/111, 327/356
• US Class Current: 324/142
• CPC Class Codes: G01R 21/133 by using digital technique

Abstract

Electronic electricity meter, in which the electrical energy consumed by a load is evaluated using a signal indicative of voltage supplied to the load and a signal indicative of load current. The load current signal is derived from an estimated load current value produced by a current transformer (4). The meter includes a memory (9) storing one or more constants from equations indicative of how current transformer phase and ratio errors vary as primary current varies. An adjusted value for the load current is produced by calculating the phase and/or ratio error using the stored constants and the estimated load current value and correcting the estimated load current value to eliminate the error. Alternatively, the meter includes a memory (9) storing a plurality of values of phase error and/or ratio error for a respective plurality of primary current values of the current transformer of predetermined configuration. The estimated load current value is used to select from the storing means (9) a value of phase error and/or ratio error for one of said plurality of primary current values corresponding to the estimated value of primary current. The load current value is corrected using the selected value of phase and/or ratio error.

Claims

1. An electronic electricity meter comprising:

means for determining electrical energy consumed by a load including a current transformer sensing load current, the transformer comprising a core having a primary winding having N₁ turns carrying actual load current I and a secondary winding having N₂ turns carrying a secondary current I₂ ;

means connected to said transformer secondary winding for developing an estimated load current value I₁ from the measured secondary current according to the equation

space="preserve" listing-type="equation">I.sub.1 =N.sub.2 /N.sub.1 ×

I.sub.2,

the estimated value I₁ differing from load current I due to transformer phase error and ratio error, each such error varying as primary current I varies;

means for storing information representing transformer phase error or ratio error; and

means operatively associated with said developing means and said storing means for calculating a corrected estimated load current value using the stored information for at least one of said phase error or ratio error and the estimated value I₁.

2. The electronic electricity meter of claim 1 wherein said calculating means calculates a value of phase error P for correcting estimated load current using the equation

3. The electronic electricity meter of claim 1 wherein said calculating means calculates a value of ratio error RE for correcting estimated load current using the equation

4. The electronic electricity meter of claim 3 wherein said calculating means calculates a value of phase error P for correcting estimated load current using the equation

5. The electronic electricity meter of claim 1 wherein said current transformer is internal to the meter.

6. The electronic electricity meter of claim 1 wherein said current transformer is external to the meter.

7. An electronic electricity meter comprising:

means for determining electrical energy consumed by a load including a current transformer sensing load current, the transformer comprising a core having a primary winding having N₁ turns carrying actual load current I and a secondary winding having N₂ turns carrying a secondary current I₂ ;

means connected to said transformer secondary winding for developing an estimated load current value I₁ from the measured secondary current according to the equation

space="preserve" listing-type="equation">I.sub.1 =N.sub.2 /N.sub.1 ×

I.sub.2,

the estimated value I₁ differing from load current I due to transformer phase error which varies as primary current I varies;

means for storing data representing relationship between phase error and load current I; and

means operatively associated with said developing means and said storing means for calculating a corrected estimated load current value using the stored information for said phase error and the estimated load current value I₁.

8. The electronic electricity meter of claim 7 wherein said calculating means calculates a value of phase error P for correcting estimated load current using the equation

9. The electronic electricity meter of claim 7 wherein the data stored in said storing means comprises a plurality of pairs of values, each pair comprising a primary current value and an associated predetermined phase error value and wherein said calculating means is adapted to select a value of phase error for use in correcting estimated load current that forms a pair with the estimated load current value.

10. The electronic electricity meter of claim 7 wherein the estimated value I₁ also differs from load current I due to transformer ratio error which varies as primary current I varies and wherein said calculating means calculates a value of ratio error RE for correcting estimated load current using the equation

11. The electronic electricity meter of claim 10 wherein said calculating means calculates a value of phase error P for correcting estimated load current using the equation

12. The electronic electricity meter of claim 7 wherein said current transformer is internal to the meter.

13. The electronic electricity meter of claim 7 wherein said current transformer is external to the meter.