BPA HQ Power Electronic Load Estimation Namrata Kogalur, Bernie Lesieutre Jan 2018

Data BPA Portable Power System Monitor (PPSM) technology, based on National Instruments hardware and software is used to measure data. Data is sampled at 5000 samples per second. 21 rows of data are present in every file. First 3 rows are that of voltage waveform. The remaining 18 rows are 3 phase current waveforms.

Current waveforms from data Figures below show a small portion of current waveform from feeder 1 on 2nd August and its FFT. We observe the presence of 5th and 7th harmonics in the FFT plot which appears as rabbit ears in the current waveform.

Typical loads Power electronic converters in general draw current from the grid in bursts. The current voltage relationship of these loads, looking from the grid side, is nonlinear and the loads are called nonlinear loads. Typical nonlinear loads: Water pumps Chillers, cooling towers Air conditioners Fans Elevators Lights like CFL Data center

Variable frequency drive Variable speed drives are used to control motor speeds. It can be achieved using power electronic converters. DC voltage is converted to AC voltage of desired magnitude and frequency using 3 phase inverter. The required DC voltage is produced from 3 phase rectifier as shown in the figure.

V/Hz operation In the constant V/Hz operation, we can shift the torque-speed curve left or right without changing the shape of the curve. This enables us to get the desired motor speed and torque as per the load line.

Simulink model

Parameters 220 Vrms, 60 Hz 3 phase source PWM carrier frequency = 18*60 = 1080 Hz Modulation index is calculated using the below equation. m=0.9, freq=60 Load: Torque is a second order function of speed such as fan or pump.

Source side current waveforms Simulink model source side current waveforms obtained from 3 phase measurement block before rectifier.

Effect of varying load on harmonics The simulink model was evaluated for 2 cases. Case 1: m=0.9, f=60Hz Case 2: m=0.45, f=30Hz We notice that harmonic ratios change with varying loads.

Rough estimate of fundamental current The table below gives the relation between reactor size and percentage of the harmonics. 23% and 7.5% are the closest to the ratios obtained from the Simulink model. Using this, a rough estimate of the fundamental current is obtained. “Harmonic Distortion from Variable Frequency Drives” Eaton Electrical

Feeder Current Sample Analysis August 14, 2018, 1:11 pm 20 Minute time frame Examine currents on 5 feeders, same phase Examine Harmonics, calculated with 3-cycle moving average

Feeder Current Sample Analysis: 1 Very small amount of harmonics present.

Feeder Current Sample Analysis: 2 26% 12% Substantial Harmonics Present. Power Electronic Loads dominate this feeder.

Feeder Current Sample Analysis: 3 24% 16% Substantial Harmonics Present. Power Electronic Loads dominate this feeder.

Feeder Current Sample Analysis: 4 13% 4% Some Harmonics Present. Power Electronic Loads could account for approximately 50% of load current.

Feeder Current Sample Analysis: 1 Very small amount of harmonics present.

Future work In the studies that has been done, three phase loads are restricted to VFDs and assumed to be the only drive in the system. This has to be changed to multiple drive system and also the effect of line reactance and variable motor load on harmonic content has to be studied more in detail. We need to consider single phase loads like battery charging, CFLs etc and obtain a relation between the harmonics from such loads and their respective fundamental currents. Using this, we can better fit the estimate to the data that is present.

References Book: D.W.Novotny, T.A. Lipo and T.M. Jahns, “Introduction to Electric Machines and Drives”, Wempec Harmonic Distortion from Variable Frequency Drives by Eaton Electrical Miona Stosovic, Slobodan Bojanic, “Characterization of non linear loads in Power distribution grid” Nptel lectures