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Saturday, 19 November 2011

Active Power Management of Electric Power System Using Emerging Power Electronics Technology


Operation of today's very large scale and interconnected electric power systems depends critically on the devices that facilitate management of power flow on the grid. These devices can be based on passive components such as capacitor and inductor, or rely on the solid state power electronics technology to achieve much faster control bandwidth. Widely known as FACTS (Flexible AC Transmission system) devices, these power electronics controllers can regulate voltage and improve stability, hence increasing power flow capability, or they can be used to directly control the power flow, or they can be used to separate regional grid from each other to reduce the interactions between these grids. More advanced applications include the incorporation of energy storage to shape the peak power requirement and to smooth the power output of large wind farms. While the benefit of an actively managed power grid is well understood, widespread use of power electronics controllers in electric power grid is still limited. The main reason behind this is the higher cost and perceived lower reliability. In this paper, the authors will discuss emerging power electronics controllers that are under development at Semiconductor Power Electronics Center of NC State University, and explain how these developments will facilitate wider and broader applications.

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maximum power point tracker (or MPPT)

A maximum power point tracker (or MPPT) is a high efficiency DC to DC converter which functions as an optimal electrical load for a photovoltaic (PV) cell, most commonly for a solar panel or array, and converts the power to a voltage or current level which is more suitable to whatever load the system is designed to drive. PV cells have a single operating point where the values of the current (I) and Voltage (V) of the cell result in a maximum power output. These values correspond to a particular resistance, which is equal to V/I as specified by Ohm's Law. A PV cell has an exponential relationship between current and voltage, and the maximum power point (MPP) occurs at the knee of the curve, where the resistance is equal to the negative of the differential resistance (V/I = -dV/dI). Maximum power point trackers utilize some type of control circuit or logic to search for this point and thus to allow the converter circuit to extract the maximum power available from a cell. Traditional Solar Inverters perform MPPT for an entire array as a whole. In such systems the same current, dictated by the inverter, flows though all panels in the string. But because different panels have different IV curves, i.e. different MPPs (due to manufacturing tolerance, partial shading, etc.) this architecture means some panels will be performing below their MPP, resulting in the loss of energy.
The Power point tracker is a high frequency DC to DC converter. They take the DC input from the solar panels, change it to high frequency AC, and convert it back down to a different DC voltage and current to exactly match the panels to the batteries. MPPT's operate at very high audio frequencies, usually in the 20-80 kHz range. The advantage of high frequency circuits is that they can be designed with very high efficiency transformers and small components. The design of high frequency circuits can be very tricky because the problems with portions of the circuit "broadcasting" just like a radio transmitter and causing radio and TV interference. Noise isolation and suppression becomes very important. There are a few non-digital (that is, linear) MPPT's charge controls around. These are much easier and cheaper to build and design than the digital ones. They do improve efficiency somewhat, but overall the efficiency can vary a lot - and we have seen a few lose their "tracking point" and actually get worse. That can happen occasionally if a cloud passed over the panel - the linear circuit searches for the next best point, but then gets too far out on the deep end to find it again when the sun comes out. Thankfully, not many of these around any more. The power point tracker (and all DC to DC converters) operates by taking the DC input current, changing it to AC, running through a transformer (usually a toroid, a doughnut looking transformer), and then rectifying it back to DC, followed by the output regulator. In most DC to DC converters, this is strictly an electronic process - no real smarts are involved except for some regulation of the output voltage. Charge controllers for solar panels need a lot more smarts as light and temperature conditions vary continuously all day long, and battery voltage changes.

grid tied inverter,grid interactive inverter

grid-tie inverter (GTI) is a special type of inverter that converts direct current(DC) electricity into alternating current(AC) electricity and feeds it into an existing electrical grid. GTIs are often used to convert direct current produced by many renewable energy sources, such as solar panels or small wind turbines, into the alternating current used to power homes and businesses. The technical name for a grid-tie inverter is "grid-interactive inverter". They may also be called synchronous inverters. Grid-interactive inverters typically cannot be used in standalone applications where utility power is not available.

A recent development in renewable energy technology is 'grid-interactive' or two way grid interconnection. These systems use sophisticated control equipment so that when your renewable energy system produces more power than you need, the excess power is fed back into the grid i.e. power is exported to the grid. When your system doesn't produce or have enough power, then you draw power from the grid. Some electricity retailers offer 'net billing' arrangements, so that they buy the electricity you produce at the same price as they sell their electricity to you. The renewable electricity is produced as Direct Current (DC). The DC electricity from the panels passes through a grid-interactive inverter, which converts the DC electricity into Alternating Current (AC), which is the type of electricity supplied by the grid

This AC electricity is then used by any appliances operating in the house. If more electricity is produced than the house needs then the excess will be fed into the main electricity grid. Conversely, when the renewable system is not generating enough electricity to power the house, the house will draw power from the grid. Grid interactive systems eliminate the need for a battery backup for when the sun doesn't shine (if it’s a solar system) or the wind doesn't blow (if it’s a wind turbine). In effect, the grid serves as your battery. This means that maintenance costs for your system will be less. It should be noted that without battery storage, a grid connected system will shut down when there is no power on the grid.

  • Grid interactive systems
  • Low Maintenance Costs
  • Eco-Friendly
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