Low Cost Digital Measurement System for 2-9 kHz Harmonic Emissions
Sujith Kumar Neelam 1*, Kaushal Patel 2, Paramasivam S 3, Thiruvenkadam M 4
1 Engineer, Danfoss Drives, Oragadam, Chennai-602105, India
2 Lead Engineer, Danfoss Drives, Oragadam, Chennai-602105, India
3 Senior Manager, Danfoss Drives, Oragadam, Chennai-602105, India
4 Senior Engineer, Danfoss Drives, Oragadam, Chennai-602105, India
Abstract: This paper presents a fast and a simple approach to estimate harmonics emissions of a low voltage within the range of 2-9 kHz frequency to address IEC 61000 standards. Voltage waveform can be analysed with 200ms window and 200Hz grouping bands by using DFT. Several tests have been carried out to measurement harmonics voltages within the range of 2-9 kHz frequency an active power filter in a low-voltage network. The paper presents the simple method for voltage distortion measurement in range of 2-9 kHz frequency in compliance to IEC 61000-2-4 standard.
Harmonics emissions in range of 2-9 kHz frequency needs in depth study of latest power networks with high penetration of grid connected systems. The harmonic distortion in the range of 2-9 kHz frequency is mainly due to different type of power electronic equipment with high-frequency switching. [1-3].
Bollen et al. in  have discussed about the voltage harmonics distortion characteristics in the range of 2-9 kHz frequency. The authors studied the effect of emitting distortion caused by equipment in the rage of 2-9 kHz frequency, 0.5% of the fundamental voltage in less than 2 kHz per a band of 200Hz has obtained from extrapolation.
The purpose of this paper is to propose a low cost digital measurement system for 2-9 kHz harmonic emissions in the low voltage network. Generally, to measure the high frequency voltage or current distortions, a high end measuring equipment is required. Presently in the market, there are high quality and expensive power quality equipment with high cost.
To perform a pre-estimation of harmonics emissions, it is not necessary to invest huge amount of money on equipment, while a simple measurement can be performed based on data analysis and grouping techniques. The analysis complies with IEC 61000-2-4 standard.
Harmonics Emissions for the Frequency Range of 2-9 kHz
In relation with the grid short circuit power at PCC, harmonics pollutions generated by high rated power electronic converters are the key issues to integrate them, compatibly with power grids.
Power electronic converters operates with different switching frequencies – from 3 kHz to 50 kHz – depends on the power levels and topologies. Power quality regulation normalizes the low frequency emissions less than 2 kHz and low frequency EMC regulation normalizes the emissions above 9 kHz frequency. Hence there is a characteristic gap exists in terms of regulations availability in the rage of 2-9 kHz.
Harmonic distortions frequency ranges are extremely wide. The maximum frequency is defined as 30 MHz by CISPR standard. There are two sub bands are available between 9kHz and 30MHz. up to 150 kHz is defined by CISPRA and above 150 kHz is defined by CISPRB (Fig. 1). 
Fig.1. Sub bands of frequency harmonic distortions
Total Harmonic Distortion is calculated by considering the low frequency components less than 2 kHz for analysing and solving PQ problems. Integer multiples of fundamental harmonic are known as harmonics, are considered up to 40th harmonic order.
In the last years, harmonic distortions were being considered in the frequency bands up to 2 kHz and (9 kHz - 30MHz). The PWM modulation carrier frequency of power converters are often within the range of 2-9 kHz frequency, which increases the harmonic emissions in this range .
Harmonic Distortion Measurement
Measurement method of harmonic distortion for the range of 2-9 kHz frequency is defined by IEC 61000-4-7 in Annex B .
According to IEC 61000-4-7, a DFT should be performed on the input signal using 200ms rectangular data windows. Asa result, two consecutive components frequency resolution by DFT analysis is 5 Hz. It is not required to synchronize the sampling of input signal with the fundamental frequency. As these frequency components are low level, lower order harmonics and above 9 kHz harmonics should be attenuated. Grouping of thirty-five 200Hz bands covers the range of 2-9 kHz.
Fig.2. Frequency bands for measurement in the range of 2-9 kHz frequency 
As per IEC 61000-2-4 standard, distortion of the voltage wave form in the frequency band above the 50th harmonic and less than 9 kHz is also represented by sinusoidal components, which can occur both at discrete frequencies and in relatively broad bands of frequencies. In the case of such higher frequency voltages, it is generally not significant whether they are harmonics or inter harmonics [8, 9].
For a discrete frequency above the 50th harmonic and up to 9 kHz, levels are expressed as the ratio U of the rms voltage at that frequency to the rms value of the fundamental component of the voltage.
For a band of frequencies above the 50th harmonic and up to 9 kHz, the levels are related to any 200 Hz bandwidth centered at frequency F, and are expressed as follows:
Y_b= 1/Y_1N *√(1/(200 Hz) ∫_(F-100)^(F+100)▒〖Y^2 (f)*df〗) (1)
Y1N is rated rms value of fundamental component of voltage/current;
Y(f) is rms harmonic voltage/current value at frequency of f;
F is center frequency of the band (the band is above the 50th harmonic).
IEC 61000-2-4 voltage distortion limits for class 2 and class 3 IPCs are as given below:
U = 0.2 % for class 2 IPCs; U = 1 % for class 3 IPCs;
Ub = 0.3 % for class 2 IPCs; Ub = 1.5 % for class 3 IPCs;
where, U = rms voltage at that frequency to the rms value of the fundamental component of the voltage
Results and Discussions
To calculate input voltage distortion in 2-9 kHz range, input voltage waveform of 10 cycles (0.2 sec) was stored in .csv format at sampling interval of 2 µsec with 100k data points using Tektronix MSO 3034.
Fig.3 shows the input voltage waveform of 10 cycles (0.2 sec) stored in .csv format at sampling interval of 2 µsec with 100k data points using Tektronix MSO 3034 when the Active Filter connected with 315kW VFD at full load in the Laboratory.
Fig.3. Input voltage waveform of 10 cycles
Below flow chart explains the MATLAB coding procedure to plot 2-9 kHz frequency spectrum using 200 Hz grouping method according to IEC 61000-2-4.
Fig.4. Flow chart of MATLAB coding procedure
IEC 61000-2-4 defines different voltage distortion limits for class 2 and class 3 IPCs for 200 Hz grouping and discrete methods. Going forward all the active filters manufacturers should meet these standards in 2-9 kHz frequency range [15-18].
Voltage distortion of input voltage according to IEC 61000-2-4 with 200 Hz grouping method using equation (1) is as shown in Fig.5.
Fig.5. Voltage distortion in 2-9 kHz with 200 Hz grouping method
Voltage distortion of input voltage according to IEC 6100-2-4 with discrete method is as shown in Fig.6.
Fig.6. Voltage distortion in 2-9 kHz with discrete method
From Fig.5 and Fig.6, it is observed that with the Active Filter, higher order voltage harmonics are increased which should result in reduction in lower order current harmonics. Fig.7 shows the comparison chart of input current harmonics without and with Active Filters. Red colour corresponds to the input current harmonics without Active Filter and blue colour corresponds to with Active Filter. From Fig.7, it is observed that lower order current harmonics are reduced with Active Filter because of increase in the higher order voltage harmonics with Active Filter.
Fig.7. Input current harmonics without and with Active Filter
The grid impedance is not considered for measurement and the measurement is based on the background noise from the grid.
To measure the 2-9 kHz high frequency harmonic voltage distortions, a simple method is proposed. To measure these high frequency harmonic voltage distortions, it is not necessary to have high end measuring equipment rather an MSO with high sampling rate is sufficient. Acquired data in MSO should be processed in MATLAB for the measurement of voltage distortions in 2-9 kHz frequency range in compliance with IEC 61000-2-4 standard. The proposed method is low cost voltage distortion method for 2-9 kHz frequency range in power networks in compliance with IEC 61000-2-4 with discrete and 200 Hz grouping methods.
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