Phase angle drift method for loss of mains/grid protection
View Patent ↗A phase angle drift method for loss of mains/grid protection is disclosed. According to one aspect, an accumulated electrical phase angle drift derived from the difference between the current measured local frequency and the estimated frequency using historical data is compared to an angle threshold. An estimated grid frequency may be calculated based on the historical delay, and the window, over which the estimated frequency is calculated. An addition/subtraction of a phase angle offset value is calculated for a half cycle is performed when the frequency difference between the estimated frequency f n est and the measured frequency f n is greater or equal to a first determined value.
1. A phase angle drift detection method for loss of mains/grid protection, wherein the accumulated electrical phase angle drift derived from the difference between a current measured local frequency from a tracking algorithm and an estimated frequency calculated using historical data is compared to an angle threshold with the phase angle drift α n being such that:
α n =α n-12 +2π( f n est −f n ) T 12samples
where:
n: Sample index
α n : Updated phase angle
α n-12 : Previous phase angle
Δα: 2π(f n est −f n )T 12samples
f n est : Estimated frequency
f n : Measured frequency
T 12samples : Time interval between two algorithm executions
the estimated grid frequency being calculated using the following equation based on the historical delay, D cycles, and the window, W cycles, over which the estimated frequency is calculated:
f
n
est
=
f
n
-
D
-
W
+
(
T
W
+
T
D
)
T
W
(
f
n
-
D
-
f
n
-
D
-
W
)
where:
f n-D-w : Oldest frequency value
f n-D : Newest frequency value
T D : Historical time delay
T W : Estimation window
wherein the addition/subtraction of the phase angle increase/decrease Δα calculated for the half cycle in the phase angle drift equation is not carried out unless the frequency difference between the estimated frequency f n est and the measured frequency f n is greater or equal to a first determined value, and in that the phase angle is reset to zero once changes Δα in the phase angle have fallen below a second determined value over a moving window equal to the size of the estimation window T W .
2. The method of claim 1 , wherein, if the difference between a new estimated frequency and a previous estimated frequency is greater or equal to a third determined value, then the new estimated frequency is discarded and replaced with the previous estimated frequency plus the third determined value.
3. The method of claim 1 , wherein, if a three phase fault is detected with a retained positive sequence voltage magnitude of less than a fourth determined value and negative sequence voltage magnitude less than a fifth determined value, then the angle threshold is increased to twice its set value for the duration of the fault.
4. The method of claim 3 , wherein once the fault has been cleared, the angle threshold is linearly reduced to its set value over a period of a sixth determined value.
5. The method of claim 1 , wherein the first determined value is 0.05 Hz.
6. The method of claim 1 , wherein the second determined value is 0.08°.
7. The method of claim 2 , wherein the third determined value is 0.002 Hz.
8. The method of claim 3 , wherein the fourth determined value is 60%.
9. The method of claim 3 , wherein the fifth determined value is 40%.
10. The method of claim 4 , wherein the sixth determined value is 0.25 second.
11. A phase angle drift detection apparatus for loss of mains/grid protection, the apparatus comprising circuitry configured to:
derive an accumulated electrical phase angle drift from the difference between a current measured local frequency from a tracking algorithm and an estimated frequency calculated using historical data by comparing an angle threshold with the phase angle drift such than an updated phase angle value α n is calculated according to a phase angle drift equation as:
α n =α n-12 +2π( f n est −f n ) T 12samples
where:
n: Sample index
α n-12 : Previous phase angle
Δα: 2π(f n est −f n )T 12samples
f n est : Estimated frequency
f n : Measured frequency
T 12samples : Time interval between two algorithm executions
the estimated grid frequency f n est being calculated as:
f
n
est
=
f
n
-
D
-
W
+
(
T
W
+
T
D
)
T
W
(
f
n
-
D
-
f
n
-
D
-
W
)
based on the historical delay, D cycles, and the window, W cycles, over which the estimated frequency is calculated, where:
f n-D-w : Oldest frequency value
f n-D : Newest frequency value
T D : Historical time delay
T W : Estimation window
wherein the addition/subtraction of a phase angle offset value Au calculated for the half cycle based on the phase angle drift equation is not carried out unless the frequency difference between the estimated frequency f n est and the measured frequency f n is greater or equal to a first determined value, and in that the phase angle is reset to zero once the phase angle offset value Δα is below a second determined value over a moving window equal to the size of the estimation window T W .
12. The apparatus of claim 11 , wherein, if the difference between the new estimated frequency and the previous estimated frequency is greater or equal to a third determined value, then the new estimated frequency is discarded and replaced with the previous estimated frequency plus the third determined value.
13. The apparatus of claim 11 , wherein, if a three phase fault is detected with a retained positive sequence voltage magnitude of less than a fourth determined value and negative sequence voltage magnitude less than a fifth determined value, then the angle threshold is increased to twice its set value for the duration of the fault.
14. A phase angle drift detection method for loss of mains/grid protection, the method comprising:
calculating an estimated grid frequency f n est over a historical delay period and within an estimation window T W , the estimated grid frequency f n est being calculated based on an oldest frequency value, a newest frequency value, a historical time delay, and the size of the estimation window;
calculating a phase angle drift every half cycle of a fundamental waveform based on the estimated frequency f n est , a measured frequency f n , and a time interval between two executions of an estimated grid frequency f n est being calculated, wherein n corresponds to the sample index;
calculating a phase angle offset value Δα based on the phase angle drift;
if the frequency difference between the estimated frequency f n est and the measured frequency f n is greater or equal to a first determined value, applying the phase angle offset value phase angle offset value Δα to derive an updated phase angle α n ; and
resetting the phase angle α n to zero if the phase angle offset value Δα is below a second determined value over a moving window equal to the size of the estimation window T W .
15. The method of claim 14 , wherein, if the difference between a new estimated frequency and a previous estimated frequency is greater or equal to a third determined value, then the new estimated frequency is discarded and replaced with the previous estimated frequency plus the third determined value.