POWER ELECTRONICS in Smart Grid
The biggest technological revolution in the last decade is “Smart Grid”. As compared to the conventional grid, smart grid is automated, highly integrated, technology driven and modernised grid. In coming years smart grid will have a key role in transforming the electrical networks, its topology and power system operation. Energy efficiency, electricity supply and sustainability are the foundation pillars of smart grid technology.
The reliability of electric supply has become the utmost priority of consumers in developed as well as developing countries throughout the world. Through smart grid implementation, monitoring, control and real time measurement of generation, transmission and distribution of electrical energy has become possible and hence reliability of electric supply is improved. Smart grid has the potential to reduce the carbon footprints by integration of renewable energy sources, energy storage and plug-in hybrid electric vehicles with the main grid.
Power electronic devices such as Metal Oxide Silicon Field Effect Transistor (MOSFET), Insulated Gate bipolar Junction Transistor (IGBT), Integrated Gate Commutated Thyristors (IGCT), Gate turn off Thyristor (GTO), Triode as an AC Switch (TRIAC) etc. has high current carrying capacity and high voltage handling capacity also. They have higher switching frequencies which is useful characteristics for voltage magnitude conversion and frequency control. These devices are used in converters. Depending upon the converter topology these converters are able to control the power flow also. Power electronics therefore plays a vital role in smart grid implementation and its development.
Applications of power electronic devices in Smart Grid
Volt-Var Optimization
Power electronic voltage regulators using TRIAC are used to regulate the voltage on the distribution feeders. The capacitor banks are used to boost the voltage of the line by generating Vars.
For integration of renewable energy sources
Exponential growth of renewable energy has been enabled in recent years, this is only because of technological advances in ‘Power Electronics’ devices and their ability to control power flow. Power electronics based Flexible AC Transmission (FACTS) technologies and automation technologies are necessary for smooth integration of renewable energy sources with the main grid.
Different energy sources are integrated with power electronic interfacing technologies as follows:
- Large wind farms have been connected increasingly with the grid using technologies such as Power Electronic Voltage Source Converters (VSC), HVDC systems consisting of Dual Converters, FACTS and Static VAR compensators (SVC) with energy storage system. Now a days, Full scale converters are used as power electronic interface which is placed between the wind turbine generator and the main power Grid as shown in Figure 1. This interface satisfies the generator and grid side requirements. These converters always ensure that the turbine speed is adjusted so that maximum power can be generated. Also, on the grid side, regardless of the speed of wind, this power electronic interface, controls frequency, active, reactive power as well as voltage. The wind turbine generators, whether it is Double-Fed Induction Generator (DFIG) or variable speed Permanent Magnet Synchronous Machine (PMSM) rotate at asynchronous speed with respect to the frequency of the grid. DFIG uses Partial scale converters which are two-level Pulse width modulation Voltage source converters (VSC) and which have 30 per cent capacity of the wind turbine. These converters work at optimum operating points of the machine to produce electrical energy at 50/60Hz.Technically it shows full power controllability with a simple structure which is reliable and cost effective. For off-shore applications, Wind turbines with Permanent Magnet machines always require full scale converters. These converters are mostly three-level Neutral point diode clamped back-to-back converters. These converters respond to frequency changes on both sides of DC link. The output power of the converters can be adjusted to maintain the system frequency. These types of converters give one more output voltage level and less dV/dt stress as compared to two-level converters. Therefore, it is possible to convert power at medium voltage level and lower current and use smaller filter size. These power electronic converters are simple modular structures with compact designs based on high power semiconductors, Integrated Gate Commutated Thyristors (IGCT) or Insulated Gate Bipolar Junction Thyristors (IGBT). Due to their compact design, these converters can fit inside the turbine tower along with the grid harmonic filters and generator harmonic filters.
Figure 1: Basic structural diagram of full scale, three-level wind-power converter. - In case of photovoltaic (PV) system, its output power is DC and therefore a power electronic converter (DC-AC inverters) are required to energize the AC load. The power electronic interface for PV systems has two main functions i.e to convert the generated DC voltage into a suitable AC current for the grid (Inverters DC/AC); the other (DC/DC Converter) is to control the terminal conditions of the PV module(s) so as to track the Maximum Power Point (MPP) for maximising the energy capture. The PV systems composed of a storage device with mode of operation as stand-alone or grid connected is as shown in Figure 2. In stand-alone mode, if the available power from the PV panel is more than the required power, the PV panel should supply the load power and the excess power should be used to charge the storage device. The storage device with the controller should provide the power difference when the available power from the PV panel is smaller than the required power at the load bus. In grid connected mode also, the load draw power from the grid when PV panel system cannot produce energy. When PV panel system produces surplus energy, it can be used to charge the battery banks or injected into the grid.
Figure 2: A simple Power Electronic devices for interfacing of PV system in stand-alone or grid connected mode.In Microgrid
Microgrid is becoming more popular technological advancement nowadays. Microgrid can reduce the cost of grid extension, provide a very reliable power supply as well as reduce the carbon emissions. In this microgrid architecture Power Electronic converters play a very vital role. They are used in the Energy management system for voltage control by “droop control” method. The output power from the Distributed Generators can be controlled by these converters. They are used for interfacing the loads with the Sources and also control the active loads in stand-alone mode and grid connected mode. Similarly, different energy resources like fuel Cell, Hydro-storage pumps, Bio-gas energy source, Combined Heat and Power (CHP), Combined Cooling Heat and Power (CCHP) systems require the Power Electronics based conversion (Rectifiers/Inverters) system as per the requirements of DC and AC load and the distribution system as shown in Figure 3.
For Electric Mobility in Smart Grid Environment
The main Purpose of Electric Vehicles is to fulfil all mobility needs at the costs equivalent to those of the conventional vehicles taking into consideration green-house gas emission reduction. The power train system of Electric vehicle consists of power electronic building blocks as shown in Figure 4. such as voltage regulators, Choppers (DC-DC converters), Traction Inverters (DC-AC converters), on- board charger etc. Alternators require Voltage regulators are required to produce constant voltage at the battery terminals by modulation of field current. Choppers (DC-DC Converters) are used for soft-switching where the switches are subjected to low stress and therefore give longer -life. These covert 400 V to 12V in electric vehicle. As AC motors have high efficiency instead of DC motors, AC motors are used in Electric vehicles. Traction Inverters (DC-AC inverters) are used for to supply power, to AC motors which is stored in batteries of the Electric vehicle. On -board chargers are power electronic converters in rectification mode used to convert AC to DC in order to charge the batteries in the electric vehicle. All other components also like ignition switch, control module, vehicle speed sensor, steering sensor etc are power electronics devices.
Figure 4: Basic power electronic components of Electric Vehicle
Low voltage DC Grid for LED lighting system
The low voltage DC grid is proposed to overcome the disadvantages of DC-AC conversion and AC-DC Conversion. As compared to conventional AC power grid the LV DC grid is a more efficient way to provide DC power for the electrical LED lighting system. Not only LED lighting but also other power electronic loads such as computers, printers, cell phone chargers etc. working on 3V,5V,9V,12V or 24 V can be benefited from this LV DC grid of 24 Volts/48 Volts. A single power electronic conversion system between AC power source and LV DC grid is sufficient and can bring in reduction in conversion losses and can be very cost effective.
Thus, for implementing all the important features of smart grid, power electronic interfacing devices are necessary. As discussed, whenever control of power flow and conversion of power from AC/DC or DC/AC is required there is no other efficient alternative than power electronic devices.
- T S ARUN M.E.,
LECTURER / EEE
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