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High frequency SMPS based Solar Power Inverter


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High frequency SMPS based Solar Power Inverter

  1. 1. Quantum Power Conversions (P) Ltd, Bangalore, India | | HIGH FREQUENCY SMPS based POWER INVERTER Sine wave Power Inverters are used extensively in UPS, Motor drives, Solar Inverters, High power audio amplifiers, servo drives and so on. These inverters convert DC typically from Battery to 230V single phase or 415V, 3phase ac. Traditionally such inverters have been designed based around low frequency (50/ 60 Hz) transformers. In this technique, firstly the low voltage DC is switched at high frequency by MOSFET or IGBT bridge which generates an alternating voltage whose duty cycle follows a sinusoidal function. The high frequency carrier will be about 10 to 20 kHz and the modulating signal will be of the fundamental frequency of 50 or 60 Hz. This is then filtered through a Low Pass filter, to get the low voltage sinusoidal output of the required fundamental frequency. This output being at low voltage is further stepped up using a low frequency (50 or 60Hz) transformer to get the required ac output. Engineering challenge comes in the optimum design of this LP filter and the transformer. In an LP filter, we have an Inductance and a capacitance as its basic elements. Circuit theory will tell us that energy storage in an Inductor is proportional to the square of the current flowing through it and for a capacitor it is proportional to the square of the voltage across it. The inductor is located in series with the low voltage primary of the transformer to reduce the effect of the high frequency on the transformer. Here the current is the highest, making the inductor quite bulky and lossey. The capacitor is placed at the transformer secondary where the voltage is at its highest. In effect, the inductor, the transformer and the capacitor together forms the LP filter plus the voltage stepping up device all combined into one. Some clever designers modify the construction of the transformer itself to get high leakage inductance in the primary. This leakage inductance is in series with the primary and hence can either eliminate or reduce the size of the filtering Inductor. Though this gives a certain degree of economy in design, it is also the root cause of energy loss and poor efficiency in such designs. The low frequency transformer is designed using silicon steel which whose loss is optimizer for 50/ 60 Hz operation. Since the transformer is also part of the filtering process, they are subjected to high frequency from the switching. Silicone steel is not meant for high frequency operation. They exhibit extremely high hysteresis loss and eddy current loss which increases exponentially with frequency. Also there is high loss due to skin effect in the copper winding which increases with frequency. Skin effect is the tendency of the current to flow in the outer skin of the wires due to magnetic field interaction caused by the current itself. This effectively reduces the available area of the copper thereby increasing copper losses. Moreover, it is against basic engineering principles to design a transformer with high leakage inductance as they are also a cause of power loss.
  2. 2. Quantum Power Conversions (P) Ltd, Bangalore, India | | This makes the transformer based inverter extremely inefficient. The only way to overcome this is to overdesign the transformer & inductor. They are designed at very low operating flux and current density. This makes it very bulky and also costly as it uses much more iron and copper. When market & economy consideration takes over, it is easy to compromise on efficiency rather than on cost. Therefore these designs end at 60 to 70% efficiency. No matter what the claims, if the price is low, then efficiency is low because the Low Pass filter including the transformer is inefficient. Otherwise the size has to be very high to get a reasonable efficiency. Pure physics of the material, there is no escape! What is then the solution? New design solutions are coming out from advanced countries including India, which are based on switched mode converters (SMPS). In these designs, first the low voltage DC is converted into high voltage DC using a high frequency DC-DC SMPS converter. The DC to ac inversion is carried out in the high voltage side. The transformer of the dc-dc SMPS converter is made out of ferrite material. Ferrite material is meant for low loss operation at high frequency. Also the size of the transformer reduces with increasing frequency. Hence the mass of ferrite transformer is very low as compared to that of an iron core transformer. Therefore the core loss which is a function of its mass is very low. The transformer does not have to step up the fundamental frequency since the DC to ac inversion is done at the high voltage side where the power has already passed through the HF transformer. The HF Transformer is designed with low leakage inductance and not the other way making it closer to ideal. Finally the LC filtering is done at the high voltage side. At high voltages, the currents are lower, hence the inductor is smaller. Again losses are low on account of the small mass required for the inductor and also on account of the special low loss material used in the inductor. Overall efficiencies of 90 to 95% have been achieved. SMPS based inverter are a most suitable for Solar power generation. The Solar Photo Voltaic panel and the storage battery is one of the highest cost factors in a solar installation. A poor inverter can force the system integrator to oversize both the panel and battery for the same performance, increasing the cost of ownership. Moreover advanced techniques like resonant switching, phase shifted bridge and so on can further improve the efficiency in high frequency conversion. Modern powerful microcontrollers can make complex design of the control system extremely reliable and repeatable. With more and more awareness on energy efficiency and the need to minimizing material usage for various climate policies, the trend is surely going towards high frequency SMPS based power conversion.