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3.9 Three-Phase Power Measurement and Data Logging

Three-phase power calculation techniques are well established. However, a number of factors have precipitated the need for flexible and reliable measurement and data logging systems for use in power system applications.

Power monitoring may be needed to analyze the power demand. Furthermore, observing the quality of the power helps us to analyze the source of disturbance that may cause important stability problems in power systems, or may seriously affect the operation of a device, or may cause catastrophic failure of a device.

In this section, a simple and cost-effective measurement tool that is based on LabVIEW is presented. This tool measures three-phase voltages and currents in real time using voltage and current transducers, and calculates all aspects of power (such as active, reactive, apparent power and power factor, and harmonic distortion). In addition, it records various parameters to view the trend over a period of time (from minutes to months).

Measurement Equations

In this study, three-phase measurement is done assuming that the load is star-connected and the star point is accessible. The three-phase system setup is illustrated in Fig. 3-27, which also illustrates the correct channel assignment for the VI.

03fig27.gifFigure 3-27. Three-phase measurement block diagram and channel assignments for four-wire system (with neutral).

It should be emphasized here that the voltage and current interfaces should be selected (or designed) to operate the system safely and accurately. In addition, note that in high voltage measurements, insulating via differential amplifiers or via differential input channels of the data acquisition card should not be considered since such methods do not provide real isolation, and thus are not safe.

To achieve complete isolation, I suggest that Hall-effect or clamp-based current transducers for current measurement and high-performance isolation amplifiers (or voltage transformers at low frequency) for voltage measurement should be employed (as provided in the Appendix).

The phase angle (or power factor) calculation is achieved using a single line-to-neutral voltage and a line current of the same phase (Ch1 and Ch4 in Fig. 3-27).

The following power calculations per phase were employed in the VI. Instantaneous, average, and rms powers per phase are estimated by

Equation 3.60

03equ60.gif


Equation 3.61

03equ61.gif


Equation 3.62

03equ62.gif


Hence, the total apparent, active, and reactive powers in this four-wire three-phase system are calculated as

Equation 3.63

03equ63.gif


Equation 3.64

03equ64.gif


Equation 3.65

03equ65.gif


In the VI, the phase angle θ is calculated measuring the time instances of the first peaks of the phase voltage and the phase current. Then the time difference is calculated as a reference to the period of the waveform and converted to degrees.

The interval between the two consecutive peaks of the same waveform is used to calculate the period T of a selected waveform, and then the frequency of the waveform is calculated, f = 1/T.

If the voltage and current waveforms of an ac system are measured in real time, it is much easier to calculate the power factor simply by using the definitions of average and rms values, which were given earlier in equation 3.3 for nonsinusoidal systems (distorted waveforms).

Note that since the majority of three-phase power systems are not balanced, the VI developed here does not consider a balanced system. Hence, all of the three-phase line-to-neutral voltages and three line currents are measured for correct estimation of the power.

3.9.1 Virtual Instrument Panel

Open the VI named Data Logging.vi. Fig. 3-28 and Fig. 3-29 illustrate the front panel and the brief user guide for this VI. The descriptions of the front panel items are all available in the VI's help menu. The features of the VI are summarized below.

03fig28a.jpgFigure 3-28. The front panel of Data Logging.vi (LabVIEW 4.0).

03fig29.jpgFigure 3-29. The brief user guide for Fig. , Power and Data Logging.vi.

The following information can be displayed on the graphs:

  • Voltages (rms and instantaneous) and their harmonics

  • Line currents (rms and instantaneous) and their harmonics

  • Period, frequency, phase angle, and power factor (leading/lagging)

  • Instantaneous power and calculated values of active (W), reactive (VAR), and apparent (VA) powers

  • Harmonics, total harmonic distortion (THD), THD + noise, peak power, peak frequency, and the amplitude and frequency of the selected number of harmonics

The following settings can be made:

  • Per-unit values and dc off-set for voltages and currents

  • Gains of the measured signals

  • Triggering based on the reference parameters (rms or instantaneous), triggering channel selection, and triggering bandwidth selection for data logging

  • Logging period (from one minute to a day/a month/a year)

  • Number of samples to be saved (from seconds to hours)

3.9.2 Some Features of the VI and Operating Scenarios

The details of the measurement program and some of the principal calculations are explained in the following paragraphs.

Channel number in the VI starts from zero, “0”. Hence, for six analogue inputs use the numbers 0, 1, 2, 3, 4, and 5 to set the channels to Ch1, Ch2, Ch3, Ch4, Ch5, or Ch6, respectively.

The power factor calculation is based on the phase specified on the front panel, Phase No.

The data is saved into a spreadsheet file with an extension of .dat and can be read by Microsoft Excel. Only the rms values of the voltages, currents, power factor, and frequency are saved in a column format. However, a path named c:\labview\datafile\ has to be created first.

To distinguish the data files in future use, a unique name is given to each data file that is created automatically by the VI. All data files created the same day are saved into a single folder.

For example, the name of a file on 17 December 1997 will be located in a folder named 97D17M12 (year+D+day+M+month). If this file is created at 12:33:12 AM, then the data file will have the following name:

12AM3312.dat (hour+AM(or PM)+minute+second)

The data format used to set DATE/TIME TRIGGERING SAVING mode is shown below.

DATE FORMAT: 17/09/96 or 7/09/96
TIME FORMAT: 3:56:12 AM or 11:5:12 AM

It should be noted here that sampling should be synchronous with the system frequency. Therefore, the settings for Scan Rate and No. of Scan are important in the harmonic analysis.

When Pretrigger scans are not zero, data is acquired continuously while waiting for the start trigger. When the trigger occurs, the specified pretrigger scans are kept, and the rest of the scans are acquired after the trigger.

The LED of Trigger Timeout, which is in the harmonic analysis section, turns yellow if a trigger timeout condition occurs, with an error message of “Trigger did not occur.” Therefore acquisition timed out. Under this condition, if the Save button is pressed, the software keeps trying.

Choose either a Software Analog Trigger or a hardware Analog Trigger for the acquisition. With a Software Analog Trigger, for example, the acquisition starts immediately but the data is not retrieved by the program until the level and slope conditions are met. With a hardware Analog Trigger, the acquisition does not start until the level and slope conditions are met. Furthermore, remember that the hardware Analog Trigger is not available on all data acquisition devices.

Two autosaving modes are possible in the VI:

  • by using a pre-set rms value

  • by using a specified Date and Time

To start autosaving, enable the autosaving function by selecting the mode AUTOSAVING ENABLED. Remember that all the settings in this part of the front panel have to be done before running the software.

Select either RMS VOLTAGE TRIGGERING SAVING SELECTED or DATE/TIME TRIGGERING SAVING SELECTED mode. In both modes of operation, the number of samples can be selected using the settings that are located just below the triggering selection.

In the RMS mode of operation, Reference Voltage and Hysteresis % should also be specified.

Let us assume that Reference Voltage is 240 V and Hysteresis % is 10. The operation of the RMS triggering is illustrated in Fig. 3-30. As illustrated, the saving starts if the measured rms value of any one of the phase voltages is outside the specified hysteresis bandwidth.

03fig30.gifFigure 3-30. An illustration of RMS VOLTAGE TRIGGERING.

The selection of DATE/TIME TRIGGERING SAVING is similar to the one just described for RMS VALUE TRIGGERING.

When DATE/TIME TRIGGERING SAVING is selected, Start Time, Start Date, Stop Time, and Stop Date should also be specified. In this mode of operation, autosaving starts when the specified date and time match with the actual date and time of the computer, and saving stops when Stop Date and Stop Time are reached.

Example Setting 1

To save the rms values into a spreadsheet file, make the following settings on the front panel and run the VI:

  • Autosaving Enabled

  • RMS Voltage Triggering Saving Selected

  • Trigger Channel, Triggering Channel Reference Value, and Hysteresis %

Saving will start when the measured rms voltage is outside the hysteresis bandwidth that was determined earlier. Then the data will be saved into a spreadsheet file that is named and placed in a folder created automatically.

A new directory is created if the data logging continues next day. This automatic file generation eases the retrieving of data for further processing.

It should be noted here that percentage of the reference value is used to determine the hysteresis bandwidth, which also determines the triggering condition in the auto-saving mode.

Example Setting 2

Alternatively, if DATE/TIME TRIGGERING SAVING is selected, triggering will start when the current date and the current time match the starting date and start time. The saving will stop when the current time and the date match the settings STOP DATE and STOP TIME.

The settings for this example are

  • Autosaving Enabled

  • Date/Time Triggering Saving Selected

  • Start Date, Start Time, Stop Date, Stop Time

Example Setting 3

If Analog Trigger is selected, the triggering channel and the triggering signal level have to be specified. The level in the Triggering Level box indicates the input value of the actual analog signal before it is multiplied by the gain. Therefore, the level setting should be scaled accordingly. In addition, the other triggering features (Rising edge, positive or Falling edge, negative) have to be specified; these can be accessed from the Edge/Slope menu.

Let's assume that Rising slope is selected, and the triggering level is set to 0.4. If the gain is 100, the triggering will occur at a voltage level of +40 V.

Example Setting 4

Let's assume that the option Save 5 samples within 5 second(s) is selected. Under this condition, a maximum of one sample per second will be saved into a spreadsheet file after the rms triggering condition is met. In this mode of operation, the program waits 1 second between the two consecutive time intervals, regardless of the speed of the execution. However, remember that the minimum sampling interval is limited by the specifications of the DAQ card and the software.

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