Of all known power quality problems, voltage sags provide the greatest cause for concern. Sags occur more frequently than outages and therefore, tend to be more costly for the high-technology industry as modern technical equipments need a good quality voltage. Voltage sag can affect both the magnitude and phase of the voltage. Although the voltage sag is only for a short duration, sensitive equipments may malfunction. Solutions for voltage sag can be divided into utility based solutions and customer based solutions. Utility based solutions encompass prevention of fault and improving fault clearing practices. Customer based solutions include power conditioning for sensitive loads.
I researched the present method used in customer based solution, which is the six step inverter. It has some disadvantages. It is connected to the power line using transformer, which in turn reduces the efficiency and increases the cost. It is less redundant and thus less reliable. The harmonics increase as the magnitude of voltage compensation increases. The unavailability of the solid state switching devices with required ratings reduces the design flexibility.To get around the limitations of the inverter, I have connected full bridge inverters together in ways which reduce their harmonic content and combine their output power with less complex control circuit . The MLI structure can overcome the shortcomings in solid state switching device ratings, limitations in operation at high frequency mainly due to switching losses and the problems associated with series-parallel combination of devices. There is also a significant decrease in the harmonics as the magnitude of voltage compensation increases. Then I tested the working of my new arrangement.
I have developed a sample single phase circuit with an AC supply, load, back-to-back connected switches, cascaded multilevel inverter, carrier based Pulse Width Modulation switching angle generator and a Neuro controller. The DC supply for the MLI can be provided by renewable energy sources like fuel cells and solar power. Sine Pulse Width Modulation strategy is used to produce the gate pulses for the devices in Full Bridge Inverter. When a sag or swell in the voltage is detected, the Neuro controller and the sine pulse width modulator together generate the required width of the gate signal pulses. My experimental setup is initially developed using MATLAB_SIMULINK blockset. Strategies developed are then implemented in real time using dSPACE. dSPACE has a high end computer based add-on card that interfaces between the matlab simulation environment and the hardware. I have developed this sample power line conditioner for single phase and low voltage, as cost was a major constraint to implement in real time.
When I lived in a small town in India, I experienced these voltage fluctuations frequently. The lack of good quality voltage supply affected the lighting and the other appliances, often. This problem, I saw, was more pronounced in the rural areas, where the children could not read after sunset and the farmers could not irrigate their fields using motors. Also there are many remote areas in the world, where even hospitals don’t get good quality voltage. Since the inverter can be powered by locally available energy sources, like biomass and solar energy, it can be used to provide reliable voltage to such places.
Harine 15-16 Presentation