CN104345224B

CN104345224B - Electric energy quality monitoring method and device

Info

Publication number: CN104345224B
Application number: CN201310323182.1A
Authority: CN
Inventors: 仲刚
Original assignee: Siemens Power Automation Ltd
Current assignee: Siemens Power Automation Ltd
Priority date: 2013-07-29
Filing date: 2013-07-29
Publication date: 2017-09-29
Anticipated expiration: 2033-07-29
Also published as: CN104345224A

Abstract

The present invention proposes a kind of electric energy quality monitoring method and device.The electric energy quality monitoring method includes：Obtain the voltage or current sampling data of at least one cycle；Based on acquired voltage or current sampling data, the voltage or RMS current of current period are calculated；Cache the voltage or RMS current of each half week wavefront, only record also records the voltage or RMS current beyond the voltage of predetermined tolerance limit scope or the half week wavefront before of RMS current beyond the voltage or RMS current of predetermined tolerance limit scope.Thus, the data record of power quality monitoring device measures to be greatly reduced.

Description

Power quality monitoring method and device

Technical Field

The present invention relates generally to a power quality monitoring method and apparatus, and more particularly, to a power quality monitoring method and apparatus capable of reducing data storage capacity.

Background

The quality of electric energy refers to the quality of alternating current electric energy supplied to a customer terminal through a power grid (e.g., a public power grid). An ideal utility grid should supply consumers with a constant frequency, sinusoidal waveform, and standard voltage. Meanwhile, in a three-phase alternating current system, the amplitudes of the phase voltages and the current of each phase are equal in magnitude and symmetrical in phase, and the phase difference is 120 degrees. However, the ideal state does not exist due to the nonlinearity or asymmetry of the equipment such as the generator, the transformer and the line in the system, the variable load property, the imperfect regulation and control means, the operation, the external interference, various faults and the like. Therefore, various problems in the links of power grid operation, power equipment and power supply and utilization are caused, and the concept of power quality is generated.

Viewed from different angles, the meaning of power quality may include, for example:

(1) voltage quality: the concept that whether the electric energy supplied to the user by a power supply enterprise is qualified or not is reflected by the deviation of the actual voltage and the ideal voltage. This definition can include most power quality issues, but cannot include frequency-induced power quality issues, nor impact and pollution of the consumer on the power quality of the grid.

(2) Current quality: the method reflects the change of current closely related to the voltage quality, and is characterized in that power consumers not only have the requirements on constant frequency and sine waveform of an alternating current power supply, but also need the current waveform and the supply voltage to be in the same phase to ensure the operation of high power factors. This definition contributes to the improvement of the power quality of the grid and the reduction of line loss, but does not summarize most of the power quality problems due to voltage reasons.

In order to analyze the voltage quality or the current quality, it is generally necessary to sample the voltage and the current in the power supply network, and simultaneously, to store the monitored voltage and current data for the quality analysis of the power supply. In order to ensure the reliability and accuracy of power quality analysis, an important step in power quality monitoring is to continuously record the monitored voltage or current value at a fixed time period. For example, voltage or current sample data is recorded every cycle, or every 1 minute, every hour. Assuming a single phase of a 50Hz power line is sampled at 625 microseconds, 32 samples are obtained per cycle. If data of 10 consecutive seconds is recorded, 16K pieces of sampling point data, that is, for example, data of 16K × 4 bytes to 64K bytes, are recorded. Thus, if 24 hours of data a day is recorded, the amount of data will reach 552 Mbytes. This requires that the power quality monitoring device have a considerable storage capacity, which is difficult to achieve with most power quality monitoring devices.

Meanwhile, if data of each sampling point is recorded, once the recorded data exceeds a predetermined recording range due to, for example, voltage pauses or fluctuations in voltage (current), the recorded data will fail from then on, or inaccurate recorded data is obtained. To address this problem, one existing approach is to further narrow the data recording interval to obtain more or more accurate data. However, this solution puts higher demands on the storage capacity of the power quality monitoring device. Another solution is to record this event when out of range invalid data occurs and mark the invalid data in the data record. However, this method has a disadvantage that continuous sampling of data is interrupted due to the presence of invalid data, and is not advantageous for analyzing the quality of electric power.

Disclosure of Invention

An object of the present invention is to provide a power quality monitoring method capable of recording data of continuous reaction current or voltage change for power quality analysis.

According to one embodiment of the invention, the method for monitoring the power quality provided by the invention comprises the steps of initializing a cache for storing half cycle wave front data and a cache for storing a previous recording data point; acquiring a voltage or current sampling value of at least one period, and calculating the root mean square value of the voltage or current of the current period based on the acquired voltage or current sampling value; judging whether the difference between the RMS value of the voltage or current of the current period and the previously recorded data point exceeds a predetermined first tolerance; if the current period voltage or current is not exceeded, updating the half-period front data according to the root mean square value of the current period voltage or current and the relevant time mark, and continuing to execute the step of acquiring the voltage or current sampling value of the subsequent period; if the half cycle wave front data is a valid value, recording the half cycle wave front data as a data point, and if the half cycle wave front data is a valid value, recording the voltage or current root mean square value of the current cycle and the related time mark as a data point, and updating the previously recorded data point by the voltage or current root mean square value of the current cycle.

Preferably, the initializing step includes: emptying the buffer for storing the half cycle front data. Moreover, the case where the half-cycle wavefront data is a valid value further includes: if the half cycle-wave front data is not empty, indicating that the half cycle-wave front data is a valid value; and the step of recording the half cycle wavefront data as a data point further comprises; and after the half cycle wave front data are recorded, emptying the buffer for storing the half cycle wave front data.

More preferably, the case that the half cycle wavefront data is a valid value further includes: if the half cycle front data is not empty, further judging whether the difference between the voltage or current root mean square value of the current cycle and the half cycle front data exceeds a second tolerance (Tc); if the half cycle wave front data exceeds the effective value, the half cycle wave front data is indicated to be the effective value.

In another embodiment of the present invention, it is further preferable that the predetermined first margin is at least one of a first sub margin and a second sub margin, and the first sub margin is larger than the second sub margin, and the step of recording the rms value of the voltage or current further includes: judging whether the difference between the current calculated voltage or current root mean square value and the rated voltage or current value exceeds a preset tolerance threshold value; the predetermined first tolerance is a first sub tolerance if the difference between the currently calculated rms value of the voltage or current and the nominal voltage or current value does not exceed the predetermined tolerance threshold, otherwise the predetermined first tolerance is a second sub tolerance, wherein the predetermined tolerance threshold is greater than either of the first and second sub tolerances. Preferably, the predetermined tolerance threshold is 8% to 12% of the rated voltage or current mean square value. More preferably, the first tolerance is twice the second tolerance. More preferably, the predetermined tolerance is 1% to 6% of the rated voltage or current mean square value.

According to another embodiment of the present invention, the present invention further provides an apparatus for power quality monitoring, comprising: the initialization module is used for initializing data before a half cycle and a previous recording data point; the sampling module is used for acquiring a voltage or current sampling value of at least one cycle; the calculation module is used for calculating the root mean square value of the voltage or the current in the current period based on the acquired voltage or current sampling value; the judging module is used for judging whether the difference between the voltage or current root mean square value of the current period and the previously recorded data point exceeds a preset first tolerance; the first updating module updates the half cycle front data according to the voltage or current root mean square value of the current cycle and the relevant time mark if the judgment result of the judging module is that the first tolerance is not exceeded; the first recording module is used for recording the half cycle front data as a data point under the condition that the half cycle front data is an effective value if the judgment result of the judging module exceeds a first tolerance, and the second recording module is used for recording the voltage or current root mean square value of the current cycle and the related time mark thereof as a data point if the judgment result of the judging module exceeds the first tolerance; and the second updating module updates the prior recorded data point according to the root mean square value of the voltage or the current in the current period after the second recording module finishes recording.

The foregoing aspects and advantages of the invention will become more apparent by referring to the following detailed description of various embodiments of the invention, taken in conjunction with the accompanying drawings.

Drawings

The drawings are only for purposes of illustrating and explaining the present invention and are not to be construed as limiting the scope of the present invention. Wherein,

FIG. 1 is a schematic diagram showing the manner in which the root mean square voltage is calculated;

FIG. 2 is a schematic diagram showing the manner in which the root mean square of the voltage per half cycle (instantaneous half cycle) is calculated;

FIG. 3 is a schematic illustration of recorded data obtained using a power quality monitoring method;

FIG. 4 illustrates a flow diagram of a power quality monitoring method according to one embodiment of the invention;

FIG. 5 illustrates a power quality monitoring data plot obtained according to the power quality monitoring method of FIG. 4;

FIG. 6 illustrates a flow diagram of a power quality monitoring method according to yet another embodiment of the invention;

fig. 7 is a schematic diagram of recorded data obtained by the power quality monitoring method proposed by the present invention;

fig. 8 is a block diagram of a power quality monitoring apparatus according to an embodiment of the present invention.

Detailed Description

In order to more clearly understand the technical features, objects, and effects of the present invention, embodiments of the present invention will now be described with reference to the accompanying drawings.

Fig. 1 schematically shows a voltage waveform of a single phase in a power supply line, and a calculation method of a voltage root mean square value (Urms). In fig. 1, the ordinate is the sampled voltage value, which is represented by x (i); the abscissa is the time axis in seconds (S). As shown in fig. 1, the rms value of the voltage Urms (i.e., y (T) in fig. 1) is the square root of the square of the voltage sample x (i) integrated over the entire period T divided by the period duration T. The root mean square value of the voltage Urms is also referred to as the effective voltage. The change in magnitude of the Urms is sufficient to reflect the change in voltage waveform, i.e., the change in state of the voltage. A voltage state change as referred to herein is a voltage value transition from a stable magnitude state to a new stable magnitude state. The state here may also refer to a changing state that increases or decreases at a constant rate in amplitude.

In an embodiment of the invention, for power quality analysis, the root mean square value (Urms) of the voltage is recorded for a half cycle every second error in time. Fig. 2 shows a sinusoidal voltage waveform of at least two cycles. Also, FIG. 2 shows each data point, denoted by an "x" in the figure, corresponding to this voltage waveform. Data points indicated by the third to fifth "x" from the left in the figure may be represented by sample data ranges corresponding thereto, respectively. In fig. 2, the range indicated by the nth 1/2 th cycle is the range of sampling data used for calculating the corresponding Urms. For example, the first x point (1)^st) Is in the first cycle (i.e. 1 in the figure)^st1/2 cycles) according to the formula shown in fig. 1. Second x point (2)^nd) The value of (c) is calculated according to the formula shown in FIG. 1 during the period indicated by the second Cycle (i.e., 2nd1/2Cycle in the figure). 2^nd1/2Cycle ratio 1^st1The/2 Cycle is staggered by a half Cycle. In other words, if 2^nd1/2Cycle indicates a range of Cur Cycle, 1^st1/2Cycle is the Urms of half a Cycle front (Pre-1/2). In this way, the Urms, point "x" in fig. 2, can be calculated in turn according to the range shown in fig. 2. The curve formed by the points x describes the trend of the voltage.

FIG. 3 shows a further developmentThe recording mode of the data recording quantity is simplified. Similarly, the recorded Urms is denoted by "x" in fig. 3. As shown in fig. 3, only the changed Urms is recorded, so the amount of data recorded is greatly reduced. For example, the power quality monitoring device determines whether the difference between the calculated Urms for the current cycle (Cur) and the Urms for a previously recorded data point exceeds a predetermined tolerance T_L. For example, assume that the currently calculated Urms is 2 in FIG. 2^ndUrms, then the power quality monitoring equipment calculates 2^ndThe absolute value of the difference between the Urms and the Urms of the previous recorded data point, Δ 1/2 Urms. If Δ 1/2Urms is greater than a predetermined tolerance T_L(e.g., 1% to 6% of the nominal voltage mean square value) then the current calculation (Cur) (e.g., 2)^ndUrms) are recorded as data recording points. Otherwise, no recording is performed, and a new sampling value is obtained to calculate the rms value of the voltage in the period staggered backward by half a cycle, for example, 3 in fig. 2^rdUrms. Thus, a voltage variation tendency as shown in fig. 3 can be obtained.

The amount of data recorded can be greatly reduced in the manner shown in fig. 3. For example, if the voltage is stable, the tolerance T is not exceeded by the changes in the Urms, for example within one hour, one day or one week_LThen only one Urms value may be recorded. Also, as the voltage amplitude increases or decreases, as shown in the middle of the waveform of FIG. 3, the change in voltage can also be captured in time. Thus, the approach shown in FIG. 3 relies on changes in Urms to record events such as fluctuations, pauses in voltage waveform, so that the recorded data is always continuous and authentic without being disturbed by events such as voltage pauses.

Fig. 4 shows a flow diagram of a power quality monitoring method according to an embodiment of the invention. In the flow shown in fig. 4, in particular, a buffer Buf is used for buffering the Urms and their associated time stamps (time stamps) before half a cycle, which may also be referred to as half cycle front data, and is shown as Pre-1/2 Urms. When the current root mean square value Cur _ Urms needs to be recorded in the manner shown in fig. 3, the buffered half-cycle front data Pre-1/2Urms is also recorded. Therefore, the recorded curve data continuity can be better through adding a small buffer, and the change situation of the actual voltage can be reflected more truly.

Specifically, the flow shown in fig. 4 starts from the initialization step S410. In step S411, the margin T as described above is set_L. Optionally said tolerance T_LOr may be a predetermined fixed value. The cache Buf for storing Pre-1/2Urms is initialized in step S413. The initialization here is, for example, to empty the buffer. Alternatively, the initialization step S413 may also store the first cycle of the Urms in the buffer Buf as initial data. In step S414, a cache for storing prior Recorded data points (Recorded points) is initialized, e.g., the first calculated Urms and its associated timestamp are stored as the initial prior Recorded data.

After the initialization step is completed, the process enters a normal operation process. During normal operation, the power quality detection device samples the voltage on the line in real time. In the example shown in fig. 4, only voltage sampling is taken as an example, and in actual use, a current value may be sampled. In step S420, the voltage sampling value of the current cycle is obtained, and the Urms, i.e., Cur _ Urms, of the current cycle is calculated accordingly. Further, in step S430, it is determined whether or not the Cur _ Urms needs to be recorded. Specifically, in step S435, the difference Δ 1/2Urms between the calculated Cur _ Urms and the previously recorded data point (i.e., the previously recorded data point) and the determined first tolerance T are compared_LA comparison is made. If Δ 1/2Urms exceeds tolerance T_LThen it indicates that the recording of Cur _ Urms is required and the flow proceeds to the recording operation sections S440 and S450. If Δ 1/2Urms does not exceed tolerance T_LThen there is no need to record Cur _ Urms and the flow proceeds to step S437. In step S437, the Cur _ Urms and its associated timestamp are stored in the buffer Buf as the updated Pre-1/2Urms, and step S420 is repeated to obtain the Urms in the cycle half the cycle later.

If Δ 1/2Urms exceeds tolerance T_LThen the flow proceeds to step S440 to determine and record half cycle front data, and then proceedProceeding to step S450, the current data Cur _ Urms is recorded. Specifically, in step S441, it is first determined whether the buffer Buf is empty (NULL). If it is empty, it indicates that Pre-1/2Urms is invalid, otherwise the flow proceeds to step S445, where Pre-1/2Urms and its associated time stamp stored in the buffer Buf are recorded as one recording data point. After the recording is completed, the buffer Buf is emptied in step S447. Finally, in step S450, the currently calculated current data Cur _ Urms and its associated timestamp are recorded as a record data point. After step S450, the flow returns to step S420 to continue to acquire data of the next half-wave.

Fig. 5 is an exemplary graph of power quality monitoring data obtained according to the method shown in fig. 4. At least two recorded data points P1 and P2 are added to the curve shown in fig. 5 as compared to the recorded data curve shown in fig. 3. Both P1 and P2 exceed the tolerance T in the difference between the current data and the previously recorded data_LThe half cycle front data in the buffer Buf is recorded. The data curve thus obtained is more continuous than the curve shown in fig. 3 and reflects the actual voltage variation trend.

Fig. 6 shows a flow diagram of a power quality monitoring method according to another embodiment of the invention. In fig. 6, the same reference numerals are used for the same steps as those in fig. 4, and the details of these steps are referred to above and will not be repeated here. Unlike fig. 4, the initialization step S511 in the flow shown in fig. 6 is performed except that the first margin T is set_LIn addition, a second margin Tc is set. The second margin Tc is used to determine whether or not recording of Pre-1/2Urms is required. For example, the second margin Tc is 1% to 6%. Further, in fig. 6, step S543 is added after step S441. In S543, it is further judged whether or not the difference between the currently calculated Cur _ Urms and the Pre-1/2Urms already stored in Buf exceeds the second margin Tc. If so, the flow proceeds to steps S445 and S447, recording the cached Pre-1/2Urms in turn and caching Buf after recording. If the difference between Cur _ Urms and the cached Pre-1/2Urms is less than the second margin Tc, then proceed directly to step without recording Pre-1/2Urms in BufSteps S447 and S450. With the method shown in FIG. 6, the Pre-1/2Urms is recorded only when the difference between Cur _ Urms and the cached Pre-1/2Urms is large enough. In contrast, if Cur _ Urms is very close to the cached Pre-1/2Urms, the Pre-1/2Urms need not be recorded, but instead Cur _ Urms, which can further reduce the amount of recorded data without affecting the continuity of the recorded data.

Fig. 7 shows a flow diagram of a power quality monitoring method according to yet another embodiment of the invention. Only the portions different from fig. 4 and 6 are emphasized in fig. 7, and the other portions are omitted and not shown. In addition, the same reference numerals are used for the same steps in fig. 7 as those in fig. 4 or fig. 6, and the specific contents of these steps are referred to above and will not be repeated here. Unlike fig. 4 and 6, steps S731 to S733 are added to fig. 7 before step S435. The new step is used for adopting different first tolerances T for voltages in different variation sections or different variation ranges_LTo record the relevant data with different sensitivities. For example, for a voltage variation within a range of interest to a user, the tolerance T_LA smaller value may be selected to record details of the voltage change. In contrast, for the case where the voltage variation is in a range that is not of interest to the user, the tolerance T_LA larger value may be chosen in order to record only a large trend of the voltage change.

Specifically, as shown in fig. 7, in step S731, the power quality monitoring apparatus determines whether the difference between the currently calculated Cur _ Urms and a rated voltage root mean square value Unom exceeds a predetermined tolerance threshold Th. Here, the setting of the margin threshold Th is used to distinguish the voltage variation range in which the user is interested from the voltage variation range in which the user is not interested. For example, if the voltage variation does not exceed the threshold Th (Th being, for example, 8% to 12%, preferably 10% of the root mean square of the nominal voltage values), the data falls within a range that is not of interest to the user, and high-resolution data recording need not be provided. Conversely, a change in voltage beyond this threshold requires a higher resolution data record for detailed analysis. In the present embodiment, assuming that the absolute value of the difference between Cur _ Urms and Unom currently calculated is greater than the threshold Th, the row is executedProceeding to step S732, a smaller tolerance T2 is selected as the tolerance T_L. In contrast, if the absolute value of the difference between Cur _ utrs and Unom calculated at present is smaller than the threshold Th, the process proceeds to step S733, where a larger margin T1 is selected as the margin T_L. Preferably, T1 may be twice as large as T2. For example, if the tolerance T2 is 2.5%, then T1 may be, for example, approximately 5%. Further, in step S435, according to the determined tolerance T_LWhether the current half-wave data needs to be recorded is determined in the same manner as the corresponding steps in fig. 4 or fig. 6. If necessary, steps S440 and S450 are performed in a similar manner to fig. 4 or fig. 6, otherwise, it returns to step S420.

Fig. 5 also shows the result of monitoring and data recording the voltage waveform in which fluctuation occurs by the method shown in fig. 7. As shown in fig. 5, when the voltage is within a range of interest to the user with respect to the rated voltage variation, data recording is performed with higher resolution as shown by recording points P3 and P4. Thus, using the method illustrated in FIG. 7, more data records may be provided over a range of variations of user interest for further power quality analysis.

The methods shown in fig. 4, 6 and 7 may be implemented by software, hardware or embedded programming, or a combination of software and hardware. Fig. 8 schematically shows a block diagram of an apparatus for power quality monitoring (or power quality monitoring device).

As shown in fig. 8, the apparatus for monitoring power quality includes an initialization module 810, a sampling module 820, a calculation module 830, a judgment module 840, a first update module 850, a second update module 860, a first recording module 870, and a second recording module 880.

Specifically, the initialization module 810 is used to initialize two buffer areas, one for storing a half cycle front data Pre-1/2Urms and the other for storing a previous Recorded data Point Recorded _ Point. The sampling module 820 obtains voltage (or current) samples for the current cycle. The calculation module 830 is based on a sampling module820, calculating the root mean square value (such as Cur _ Urms) of the voltage (or current) of the current period. The determining module 840 determines whether the difference between the calculated current data Cur _ Urms) and the previous Recorded data Point (Recorded _ Point) exceeds a predetermined first tolerance T_L. The first updating module 850 determines that the first margin T is not exceeded according to the determination result of the determining module 840_LTime updated with the current data Cur _ Urms) and its associated time stamp, the half cycle front data Pre-1/2 Urms. The first recording module 870 determines that the first margin T is exceeded at the determining module 840_LIn the case where the Pre-1/2Urms is a valid value, the half cycle wavefront data Pre-1/2Urms is recorded as one data point. The judgment result of the second recording module 880 exceeding the first tolerance T in the judging module 840_LThe current data Cur _ Urms and its associated time stamp are also recorded as a data point. The second updating module 860 updates the previous Recorded data Point Recorded with the current data Cur _ Urms after the second recording module finishes recording.

More preferably, the initialization module 810 empties the cache Buf used to store Pre-1/2Urms upon initialization. The first recording module 850 determines Pre-1/2Urms to be a valid value when it determines that the buffer Buf is not empty. Also, the first recording module 870 empties the cache Buf after recording Pre-1/2 Urms. More preferably, the first recording module 870 further determines whether the difference between Cur _ Urms and Pre-1/2Urms exceeds a second tolerance Tc if the buffer Buf is not empty. If so, it indicates that the Pre-1/2Urms is a valid value and can be recorded.

It should be understood that although the present description has been described in terms of various embodiments, not every embodiment includes only a single embodiment, and such description is for clarity purposes only, and those skilled in the art will recognize that the embodiments described herein may be combined as suitable to form other embodiments, as will be appreciated by those skilled in the art.

The above description is only an exemplary embodiment of the present invention, and is not intended to limit the scope of the present invention. Any equivalent alterations, modifications and combinations can be made by those skilled in the art without departing from the spirit and principles of the invention.

Claims

1. A method for power quality monitoring, comprising:

initializing a buffer for storing half cycle wave front data (Pre-1/2 Urms) and a buffer for storing a previously Recorded data Point (Recorded Point);

acquiring voltage or current sample values for at least one cycle,

calculating the root mean square value (Cur _ Urms) of the voltage or the current in the current period based on the acquired voltage or current sampling value;

judging the voltage or current of the current periodWhether the difference between the root mean square value (Cur _ Urms) and the previously Recorded data Point (Recorded _ Point) exceeds a predetermined first tolerance (T)_L)；

If not, updating the half cycle front data (Pre-1/2 Urms) with the current cycle voltage or current root mean square value (Cur _ Urms) and the relevant time mark, and continuing to execute the step of acquiring the voltage or current sampling value of the subsequent cycle;

if so, recording the half cycle front data (Pre-1/2 Urms) as a data point in the case where the half cycle front data (Pre-1/2 Urms) is a valid value, and

if the current period voltage or current root mean square value (Cur _ Urms) exceeds the current period voltage or current root mean square value (Cur _ Urms), and the relevant time mark is Recorded as a data Point, and the previous Recorded data Point (Recorded _ Point) is updated according to the current period voltage or current root mean square value (Cur _ Urms).

2. The method of claim 1, wherein the initializing step comprises: emptying the buffer for storing the half cycle front data (Pre-1/2 Urms);

moreover, the case where the half cycle wavefront data (Pre-1/2 Urms) is a valid value further includes: if the half cycle front data (Pre-1/2 Urms) is not empty, indicating that the half cycle front data (Pre-1/2 Urms) is a valid value; and

recording the half cycle wavefront data (Pre-1/2 Urms) as a data point; after recording the half cycle front data (Pre-1/2 Urms), emptying the buffer for storing the half cycle front data (Pre-1/2 Urms).

3. The method of claim 2, wherein the case where the half cycle wavefront data (Pre-1/2 Urms) is a valid value further comprises:

if the half cycle front data (Pre-1/2 Urms) is not empty, further determining whether the difference between the RMS value (Cur _ Urms) of the current cycle voltage or current and the half cycle front data (Pre-1/2 Urms) exceeds a second tolerance (Tc);

if the half cycle wave front data (Pre-1/2 Urms) is exceeded, the half cycle wave front data is indicated to be a valid value.

4. A method according to any one of claims 1-3, wherein said predetermined first tolerance (T) is_L) Is at least a first sub-margin (T)₁) And a second sub-margin (T)₂) And said first sub-margin (T)₁) Greater than the second sub-tolerance (T)₂) And an

Determining whether the difference between the root mean square value (Cur _ Urms) of the current period and the previously Recorded data Point (Recorded _ Point) exceeds a predetermined first tolerance (T)_L) Further comprising:

judging whether the difference between the root mean square value (Cur _ Urms) of the voltage or the current in the current period and a rated mean square value (Unom) of the voltage or the current exceeds a preset tolerance threshold value (Th);

if the predetermined tolerance threshold (Th) is not exceeded, the predetermined first tolerance (T)_L) Is a first sub-margin (T)₁) Otherwise, said predetermined first margin (T)_L) Is a second margin (T)₂)，

Wherein the predetermined tolerance threshold (Th) is greater than the first sub-tolerance (T)₁) And a second sub-margin (T)₂) Any of them.

5. The method of claim 4, wherein the predetermined tolerance threshold is 8% to 12% of the rated voltage or current mean square value.

6. The method of claim 4, wherein the first sub-tolerance is twice the second sub-tolerance.

7. Method according to claim 1, wherein said predetermined first tolerance (T)_L) Is 1-6% of the mean square value of rated voltage or current.

8. A method as claimed in claim 3, wherein the second margin (Tc) is 1% to 6% of the mean square value of the rated voltage or current.

9. An apparatus for power quality monitoring, comprising:

an initialization module (810) for initializing a half cycle wavefront data (Pre-1/2 Urms) and a preceding Recorded data Point (Recorded Point);

a sampling module (820) for obtaining voltage or current samples of at least one cycle,

a calculating module (830) for calculating the root mean square value (Cur _ Urms) of the voltage or the current in the current period based on the acquired voltage or current sampling value;

a judging module (840) for judging whether the difference between the RMS value (Cur _ Urms) of the current period and the previously Recorded data Point (Recorded _ Point) exceeds a predetermined first tolerance (T)_L)；

A first updating module (850) for updating the first tolerance (T) if the first tolerance (T) is not exceeded as a result of the determination by the determining module_L) Updating the half cycle front data (Pre-1/2 Urms) with the current cycle voltage or current root mean square value (Cur _ Urms) and its associated time stamp;

a first recording module (870) for determining whether the first margin (T) is exceeded if the determination result of the determining module is_L) Recording the half cycle front data (Pre-1/2 Urms) as a data point in the case that the half cycle front data (Pre-1/2 Urms) is a valid value, and

a second recording module (880) for determining whether the first margin (T) is exceeded or not if the determination result of the determining module is_L) Recording the root mean square value (Cur _ Urms) of the voltage or the current in the current period and the related time mark thereof as a data point;

a second updating module (860) for updating the previously Recorded data Point (Recorded _ Point) with the current period root mean square value (Cur _ Urms) after the second recording module finishes recording.

10. The apparatus of claim 9, wherein,

-said initialization module (810) empties a buffer for storing said half-cycle front data (Pre-1/2 Urms);

the first recording module (870) determines that the half cycle front data (Pre-1/2 Urms) is valid when judging that the half cycle front data (Pre-1/2 Urms) is not empty; and

the first recording module (870) empties the buffer for storing half cycle wave front data (Pre-1/2 Urms) after recording the half cycle wave front data (Pre-1/2 Urms).

11. The apparatus of claim 10, wherein said first recording module (870) further determines whether a difference between a root mean square value (Cur _ Urms) of a voltage or current of said current cycle and said half cycle front data (Pre-1/2 Urms) exceeds a second tolerance (Tc) if said half cycle front data (Pre-1/2 Urms) is not empty; if the half cycle wave front data (Pre-1/2 Urms) is exceeded, the half cycle wave front data is indicated to be a valid value.

12. The device according to claim 9, wherein said predetermined first tolerance (T)_L) Is 1-6% of the mean square value of rated voltage or current.

13. The apparatus of claim 11, wherein the second margin (Tc) is 1% to 6% of the rated voltage or current mean square value.