Tchuidjan Roger, PhD Electrical Engineer 1 Connection of disturbing loads in micro-grids National Advanced Polytechnique Institute of Yaounde - Cameroon

Tchuidjan Roger, PhD Electrical Engineer 2 Objectifs du projet Many papers may be found in the literature studying the impact of dispersed generation on power quality, where the generators are considered as disturbing installations (sources of flicker, harmonics, or higher- frequency components). We report on a study with another point of view : what about the connection of "classical" disturbing loads in a micro-grid fed by renewable energy generators ?

Tchuidjan Roger, PhD Electrical Engineer 3 Description The system which was modeled is shown in Fig.1. It simply consists of a DC Generator connected to load by DC/AC converter. Fig. 1 : Electrical System to study

Tchuidjan Roger, PhD Electrical Engineer 4 The analysis of dynamic behavior of this system Knowing that load power can be explained as a function of the short circuit power and the relation between positive and negative sequence voltages, we can write that: (1) The determination of the load power can be done using (2)

Tchuidjan Roger, PhD Electrical Engineer 5 After the calculation of the short circuit power we can get the short circuit impedance through the relation (3) The objective of this calculation is to compare the Z cc obtainedby simulation (DC Source- Converter- Unbalanced Load) with the Z cc of a similar system where the DC Source-Converter can be replaced by an AC Source.

Tchuidjan Roger, PhD Electrical Engineer 6 Results of simulations Fig. 2 : Voltages obtained at the phases (a, b, c) of the unbalanced load

Tchuidjan Roger, PhD Electrical Engineer 7 Fig. 3: Current at the DC side of the converter when AC connected to unbalanced load

Tchuidjan Roger, PhD Electrical Engineer 8 What we were looking for, is the equivalent short circuit impedance (given by S cc = S load x U d /U i ). The value obtained is around Ω. The short circuit impedance obtained from the result of the simulation (1.083 Ω) is not very greater than the calculated value from the filter ( Ω). Thus, with the simple model which was used for the PWM AC/DC converter (no active filtering function), the "short-circuit" impedance (Thevenin equivalent) of the generator is practically equal to the impedance of the filter. The question is then : how to choose the parameters L and C ? (the influence of C being low for 50 Hz phenomena, but important at harmonic frequencies).

Tchuidjan Roger, PhD Electrical Engineer 9 Choice of filter parameters It can be shown that the filter parameters L and C result from the choice of: -z cc (pu) : the series 50 Hz impedance of the generator, and -u m = U m /U (pu) : the accepted relative magnitude of the modulation frequency component (H) (F)

Tchuidjan Roger, PhD Electrical Engineer 10 Where: -U : phase-to-phase 50 Hz voltage of the generator (V). -  = 2  f = 2  50 (rad/s). -Sn =  3UIn : nominal power of the generator (VA). -n = fm/f : ratio between modulation frequency and power frequency. If we want to have zcc as low as possible, we have to accept if and um as high as possible. If we accept 0.5 % for the magnitude of the HF voltage component and 10 % for the magnitude of the no-load current in the filter then:

Tchuidjan Roger, PhD Electrical Engineer 11 zcc = 2.2 % at 15 kHz zcc = 5 % at 10 kHz zcc = 20 % at 5 kHz. It appears that the achievable short-circuit power strongly depends on the modulation frequency. We know that a classical value for the impedance of a MV/LV transformer is 4 %. Such a low impedance may be obtained with the PWM generator if the modulation frequency is higher than 10 kHz.