Improvement of power quality is one attempt to increase the efficiency of electrical energy. One effort is by monitoring the amount of voltage and current RMS grid. The use of measuring devices such as voltmeters and ammeters used to sometimes do not give accurate results. Some measuring devices (including some digital measuring instrument) only measure the average value of which is calibrated to indicate RMS value. Determination of the true RMS value for a variety of waveforms to be done by using a true-RMS digital measuring instrument.
The use of digital measuring instrument gives the consequences of rising prices of measuring instruments must be purchased. So for the purposes of measuring RMS voltage value with a certain specification and accuracy should be used in microcontroller-based measuring instrument that we can make. For the purposes of the laboratory and further development, the instruments should be able to determine the RMS value for the wave of contaminated several harmonic components.
Implementation of voltage and current measurement based on microcontroller is done in order to the effort of monitoring and improving the quality of electric power. Thus the results of application of this research should have a great opportunity to develop a system of monitoring and improving the quality of electric power.
How it works
To be able to measure the RMS value we must:
1. determine the frequency signal
2. determine the sampling count per signal period
3. sampling the signal
4. calculate the RMS value
5. RMS value of the display update periodically (the period can be improved with precision algorithm for precision frequency generator).
System Block Diagram
System for measuring RMS voltage and current values are made based block diagram in Figure 1. Input signal for this system are the voltage and current on a grid. Detection of voltage and current do not use voltage and current sensors. Most applications using current and voltage sensors. However, the use of both sensors will result in the current and voltage signals can not be analyzed again for other purposes.
The use of digital measuring instrument gives the consequences of rising prices of measuring instruments must be purchased. So for the purposes of measuring RMS voltage value with a certain specification and accuracy should be used in microcontroller-based measuring instrument that we can make. For the purposes of the laboratory and further development, the instruments should be able to determine the RMS value for the wave of contaminated several harmonic components.
Implementation of voltage and current measurement based on microcontroller is done in order to the effort of monitoring and improving the quality of electric power. Thus the results of application of this research should have a great opportunity to develop a system of monitoring and improving the quality of electric power.
How it works
To be able to measure the RMS value we must:
1. determine the frequency signal
2. determine the sampling count per signal period
3. sampling the signal
4. calculate the RMS value
5. RMS value of the display update periodically (the period can be improved with precision algorithm for precision frequency generator).
System Block Diagram
System for measuring RMS voltage and current values are made based block diagram in Figure 1. Input signal for this system are the voltage and current on a grid. Detection of voltage and current do not use voltage and current sensors. Most applications using current and voltage sensors. However, the use of both sensors will result in the current and voltage signals can not be analyzed again for other purposes.
Figure 1
Sampling Voltage and Current
One period of the voltage sampled 64 times, so for a period of electric current. So here it is necessary an algorithm to determine the period and frequency of electric voltage. Voltages and currents have the same frequency, but the determination is made on the frequency ofthe voltage signal .
Sampling the voltage and current signals performed alternately as Figure 4. Thus the frequency of samples to be f = 128 × f_grid. Figure 4 is an example of sampling the grid voltage of 255 volts and the current with 500 mA amplitude of the fundamental components that contaminated by 3rd and 5th harmonic and electric current lag 11.25 degrees. Signal fed to pin ADC0 voltage and current signals fed to the ADC1 pin.
One period of the voltage sampled 64 times, so for a period of electric current. So here it is necessary an algorithm to determine the period and frequency of electric voltage. Voltages and currents have the same frequency, but the determination is made on the frequency ofthe voltage signal .
Sampling the voltage and current signals performed alternately as Figure 4. Thus the frequency of samples to be f = 128 × f_grid. Figure 4 is an example of sampling the grid voltage of 255 volts and the current with 500 mA amplitude of the fundamental components that contaminated by 3rd and 5th harmonic and electric current lag 11.25 degrees. Signal fed to pin ADC0 voltage and current signals fed to the ADC1 pin.
Figure 2
Selection bits ADC Data Used
Although the ADC has a maximum resolution of 10 bits. In this study, we use the ADC with a resolution of 8 bits. Use of 10-bit resolution will certainly provide a higher accuracy. Instantaneous voltage and current represented by the 8 bit register ADCH of 10 bit ADC data samples that have been arranged left-adjust as Figure 3.
Although the ADC has a maximum resolution of 10 bits. In this study, we use the ADC with a resolution of 8 bits. Use of 10-bit resolution will certainly provide a higher accuracy. Instantaneous voltage and current represented by the 8 bit register ADCH of 10 bit ADC data samples that have been arranged left-adjust as Figure 3.
Figure 3
The system uses 8-bit ADC and sample frequency of 64 times the fundamental frequency of grid voltage. Measurement error caused by quantization error is reduced by using the oversampling and 2.5 VDC offset addition method.
The Result
We have been able to measure the true-RMS of voltage from 10 to 250 volts and true-RMS of currents of 50mA to 1 A. Absolute error value of RMS voltage measurement below 0.5% for the range of voltage from 160 volts to 250 volts. And absolute error value of RMS current measurement below 4% for the range of currents from 200 mA to 1 A for currents slightly contaminated harmonic components. With a few suggestions for improvement, this system can be developed into the next research of the power quality measurement based on microcontroller.
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The Result
We have been able to measure the true-RMS of voltage from 10 to 250 volts and true-RMS of currents of 50mA to 1 A. Absolute error value of RMS voltage measurement below 0.5% for the range of voltage from 160 volts to 250 volts. And absolute error value of RMS current measurement below 4% for the range of currents from 200 mA to 1 A for currents slightly contaminated harmonic components. With a few suggestions for improvement, this system can be developed into the next research of the power quality measurement based on microcontroller.
click here for more information
