ESTIMATING AN ELECTRICITY SUPPLY'S FUNDAMENTAL FREQUENCY
This disclosure relates to an apparatus, computer readable medium and a method for estimating a frequency of an electrical signal. The method comprises converting a first signal having three components into a second signal having two components in accordance with a first transformation, wherein each component of the first signal corresponds to a phase component of a three-phase electrical signal and the two components of the second signal are representative of characteristics of the three-phase electrical signal. The method further comprises filtering the second signal in accordance with a previous frequency estimation. Then the method comprises converting the filtered second signal into a third signal having a single component in accordance with a second transformation, wherein the single component of the third signal is representative of characteristics of the three-phase electrical signal. Finally, the method comprises estimating a frequency of the third signal, wherein the estimated frequency of the third signal is indicative of a frequency of the three-phase electrical signal.
1 . A method for estimating a frequency of an electrical signal, the method comprising:
converting a first signal having three components into a second signal having two components in accordance with a first transformation, wherein each component of the first signal corresponds to a phase component of a three-phase electrical signal and the two components of the second signal are representative of characteristics of the three-phase electrical signal;
filtering the second signal in accordance with a previous frequency estimation;
converting the filtered second signal into a third signal having a single component in accordance with a second transformation, wherein the single component of the third signal is representative of characteristics of the three-phase electrical signal; and
estimating a frequency of the third signal, wherein the estimated frequency of the third signal is indicative of a frequency of the three-phase electrical signal.
2 . The method according to claim 1 , wherein the filtering comprises applying a band pass filter to the second signal, the band pass filter being centered on the frequency of a previous frequency estimation.
3 . The method according to claim 2 , wherein the band pass filter is arranged to apply unity gain and zero phase shift at the frequency of the previous frequency estimation.
4 . The method according to claim 1 , further comprising feeding back the current frequency estimation for filtering the next frequency estimation.
5 . The method according to claim 1 , wherein the first transformation is a Concordia transformation, as defined by:
[
V
α
V
β
]
=
2
3
[
1
-
1
2
-
1
2
0
3
2
-
3
2
]
[
V
a
V
b
V
c
]
wherein V a , V b , V c are the three components of the first signal and V α and V β are the two components of the second signal.
6 . The method according to claim 1 , wherein the second transformation utilizes a previous estimation of a phase of the three-phase electrical signal.
7 . The method according to claim 1 , wherein the second transformation is a Park transformation, as defined by:
[
V
d
V
q
]
=
[
cos
(
θ
^
)
sin
(
θ
^
)
-
sin
(
θ
^
)
cos
(
θ
^
)
]
[
V
α
V
β
]
wherein V α and V β are the two components of the second signal after being filtered, {circumflex over (θ)} is a previous estimation of a phase of the three-phase electrical signal, and V d and V q are outputs of the second transformation, wherein V d is forced to zero and V q therefore corresponds to the third signal.
8 . The method according to claim 1 , wherein the frequency is estimated by:
estimating a phase of the third signal; and
estimating the frequency of the third signal in accordance with the estimated phase.
9 . The method according to claim 8 , wherein the phase is estimated using a phase locked loop.
10 . The method according to claim 9 , wherein the phase locked loop comprises a loop filter arranged to estimate an angular frequency of the signal having a single component and a voltage controlled oscillator arranged to estimate the phase of the estimated angular frequency.
11 . The method according to claim 8 , further comprising feeding back the estimated phase to the second transformation for estimating a next phase.
12 . The method according to claim 1 , further comprising calculating the average frequency over a period of time, wherein a plurality of frequency estimations are made during the period of time.
13 . The method according to claim 12 , further comprising determining an error level in the frequency estimation and excluding any frequency estimations from the average frequency calculation having an error level greater than an error threshold.
14 . The method according to claim 12 , further comprising determining a rate of change of frequency from the average frequency.
15 . The method according to claim 1 , further comprising measuring the three components of the three-phase electrical signal.
16 . The method according to claim 15 , wherein the measurement comprises measuring voltages of the three-phase electrical signal.
17 . The method according to claim 1 , further comprising converting a single-phase electrical signal into the three-phase electrical signal in accordance with a third transformation before converting the three-phase signal in accordance with the first transformation.
18 . An apparatus for estimating a frequency of an electrical signal, the apparatus arranged to implement the method of claim 1 .
19 . The apparatus according to claim 18 , wherein the apparatus is an electricity meter.
20 . The apparatus according to claim 19 , wherein the electricity meter further comprises:
a sensor arranged to measure one or more characteristics of an electrical signal;
a memory arranged to store the one or more characteristics of the electrical signal and store a computer program for implementing the method;
a processor arranged to perform the method in accordance with the stored characteristics of the electrical signal and the computer program; and
a communications unit arranged to transmit information relating to the frequency estimation to a central server.
21 . A computer readable medium comprising computer readable code operable, in use, to instruct a computer to perform the method of claim 1 .