Fault magnitude calculation during current transformer saturation

1. A line-mounted device for an electric power delivery system, configured to determine a fault magnitude under conditions of a saturated current transformer (CT), comprising:

a current transformer in electrical communication with a conductor of the electric power delivery system, comprising a core and a winding including secondary leads;

sensor circuitry in electrical communication with the secondary leads, configured to:

calculate a frequency of a current signal on the electric power delivery system using a secondary signal from the secondary leads;

compare a current magnitude of the secondary signal against a predetermined fault current threshold;

when the current magnitude exceeds the predetermined fault current threshold, signal a controller of a fault condition;

the controller in communication with the sensor circuitry, comprising:

a processor;

a non-transitory computer-readable storage in communication with the processor, comprising instructions that when operated cause the processor to:

record samples of the secondary signal from the sensor circuitry to form a sampled secondary signal;

determine first and second peaks of the secondary signal;

calculate a first unsaturated region of the first peak of the secondary signal and a second unsaturated region of the second peak of the secondary signal;

determine valid pairs of samples from the sampled secondary signal within the first and second unsaturated regions;

calculate a DC component of the sampled secondary signal from the valid pairs of samples;

remove the DC component from the sampled secondary signal;

calculate a fault magnitude from the sampled secondary signal with the DC component removed; and

transmit the calculated fault magnitude to a consuming device for effecting a protective action on the electric power delivery system.

2. The line-mounted device of claim 1 , further comprising communication circuitry in communication with the processor for transmitting the calculated fault magnitude to the consuming device.

3. The line-mounted device of claim 1 , wherein each of the valid pairs includes a first sample in the first unsaturated region and a second sample in the second unsaturated region.

4. The line-mounted device of claim 3 , wherein the first sample and the second sample are separated by one power system cycle.

5. The line-mounted device of claim 1 , wherein the secondary signal is distorted due to saturation of the CT.

6. The line-mounted device of claim 1 , wherein the calculation of the fault magnitude comprises calculation of a fault current magnitude matrix as a function of the calculated frequency of the current signal.

7. The line-mounted device of claim 1 , wherein the calculation of the fault magnitude comprises use of a pre-populated fault current magnitude matrix associated with the calculated frequency of the current signal.

8. The line-mounted device of claim 1 , wherein the instructions further cause the processor to:

determine a third peak of the secondary signal;

calculate a third unsaturated region of the third peak of the secondary signal;

determine valid pairs of samples from the sampled secondary signal within the second and third unsaturated regions; and

wherein the fault magnitude is calculated as an average of a first fault magnitude calculated using valid pairs from the first and second unsaturated regions and a second fault magnitude calculated using valid pairs from the second and third unsaturated regions.

9. A system for protecting an electric power delivery system, comprising:

a line-mounted device comprising a current transformer (CT) in electrical communication with a conductor of the electric power delivery system, and comprising a controller configured to:

during a pre-event mode, record times of zero crossings of a secondary current signal from the CT;

upon detection of a fault condition:

record a time of the detection of the fault condition;

calculate a frequency of a current on the electric power delivery system using the times of zero crossings of the current signal;

record samples of the secondary current signal from the CT to form a sampled secondary signal;

determine first and second peaks of the secondary signal;

calculate a first unsaturated region of the first peak of the secondary signal and a second unsaturated region of the second peak of the secondary signal;

determine valid pairs of samples from the sampled secondary signal within the first and second unsaturated regions;

calculate a DC component of the sampled secondary signal from the valid pairs of samples;

remove the DC component of the sampled secondary signal using the calculated DC component;

calculate a fault magnitude from the sampled secondary signal with the DC component removed; and

transmit the calculated fault magnitude to a consuming intelligent electronic device (IED) for effecting a protective action on the electric power delivery system; and,

the IED in communication with a portion of the electric power delivery system configured to receive the calculated fault magnitude; determine the protective action based on information from the electric power delivery system; and, effect the protective action on the electric power delivery system by signaling a circuit breaker to open in response to the determined protective action.

10. The system of claim 9 , wherein each of the valid pairs includes a first sample in the first unsaturated region and a second sample in the second unsaturated region.

11. The system of claim 10 , wherein the first sample and the second sample are separated by one power system cycle.

12. The system of claim 9 , wherein the secondary current signal is distorted due to saturation of the CT.

13. The system of claim 9 , wherein the calculation of the fault magnitude comprises calculation of a fault current magnitude matrix as a function of the calculated frequency of the current signal.

14. The system of claim 9 , wherein the calculation of the fault magnitude comprises use of a pre-populated fault current magnitude matrix associated with the calculated frequency of the current signal.

15. The system of claim 9 , wherein upon detection of the fault condition, the controller is configured to:

determine a third peak of the secondary signal;

calculate a third unsaturated region of the third peak of the secondary signal;

determine valid pairs of samples from the sampled secondary signal within the second and third unsaturated regions; and

wherein the fault magnitude is calculated as an average of a first fault magnitude calculated using valid pairs from the first and second unsaturated regions and a second fault magnitude calculated using valid pairs from the second and third unsaturated regions.

16. A method of improving functioning of a line-mounted device in determining fault magnitude in presence of current transformer (CT) saturation, comprising the steps of:

before detection of a fault, the line-mounted device receiving a secondary current signal using the CT in electrical communication with an electric power delivery system, determining zero crossings of the secondary current signal, time stamping and recording the zero crossings, and comparing a magnitude of the secondary current signal with a predetermined fault current threshold;

calculate a power system frequency using the zero crossings;

upon the magnitude of the secondary current signal exceeding the predetermined fault current threshold, the line-mounted device:

sampling the secondary current signal to form a sampled secondary current signal;

determining first and second peaks of the secondary current signal using the sampled secondary current signal;

calculating a first unsaturated region of the first peak of the secondary signal and a second unsaturated region of the second peak of the secondary signal;

determining valid pairs of samples from the sampled secondary signal within the first and second unsaturated regions;

calculating a DC component of the sampled secondary signal from the valid pairs of samples;

removing the DC component of the sampled secondary signal; and

calculating a fault magnitude from the sampled secondary signal with the DC component removed; and

effecting a protection action on the electric power delivery system using the calculated fault magnitude.

17. The method of claim 16 , wherein each of the valid pairs includes a first sample in the first unsaturated region and a second sample in the second unsaturated region.

18. The method of claim 16 , wherein the fault magnitude is calculated using a pre-populated fault current magnitude matrix associated with the calculated power system frequency.

19. The method of claim 16 , further comprising the steps of:

determining a third peak of the secondary current signal;

calculating a third unsaturated region of the third peak of the secondary current signal;

determining valid pairs of samples from the sampled secondary signal within the second and third unsaturated regions; and

wherein the fault magnitude is calculated as an average of a first fault magnitude calculated using valid pairs from the first and second unsaturated regions and a second fault magnitude calculated using valid pairs from the second and third unsaturated regions.

20. A system for protecting an electric power delivery system, comprising:

a line-mounted device comprising a current transformer (CT) in electrical communication with a conductor of the electric power delivery system, and comprising a controller configured to:

during a pre-event mode, record times of zero crossings of a secondary current signal from the CT;

upon detection of an overcurrent condition using the secondary current:

record a time of the overcurrent condition;

calculate a frequency of a current on the electric power delivery system using the times of zero crossings of the current signal;

record samples of the secondary current signal from the CT to form a sampled secondary signal;

determine a first peak and a second peak of the secondary signal;

calculate a first unsaturated region of the first peak and a second unsaturated region of the second peak;

calculate a DC component of the sampled secondary signal;

remove the DC component from the sampled secondary signal;

calculate a fault magnitude from the sampled secondary signal with the DC component removed; and

transmit the calculated fault magnitude to a consuming intelligent electronic device (IED) for effecting a protective action on the electric power delivery system.