531 Power Electronics Technology

Modern power electronics is primarily based on the technique of switchmode energy conversion: high power transistors are rapidly switched on and off in order to control the flow of electric power (this is in contrast to a transistor operating in the linear region). At any instant of time, the transistor is either fully on (the voltage across the device is zero), or it is fully off (the current passing through the transistor is zero); therefore, the losses in the transistor are minimized. In reality, it is impossible to create a perfect transistor that has no losses, but significant improvements continue to be made. These high power transistors can be configured into various topologies to form the desired power conversion function (e.g., DC-to-DC, AC-to-DC, or DC-to-AC). Pulse width modulation (PWM) is then normally used to vary the on and off times of the transistors in order to synthesize the desired output voltage and frequency.

For power levels up to approximately 500 kW, the insulated gate bipolar transistor (IGBT) is the most common power semiconductor device used today. Switching frequencies up to 20 kHz can be obtained using these devices. Because the transistors are controlled to be only on or off , the waveforms produced by the converters consist of pulses with very high harmonic content. High power filters are necessary to remove the high frequency harmonics and allow only the useful fundamental component of energy to pass. In order to minimize the losses in these filters, reactive components are used (inductors and capacitors). Filter design is a very complex and challenging aspect of power electronic systems.

Modern power electronic converters can achieve efficiencies greater than 96% for a single power conversion stage (including the filter losses). Even with these very high efficiencies, a significant amount of power is dissipated in the converters. For this reason, thermal management is a critical aspect of power electronics design. Thermal management systems can use both liquid and forced air-cooling techniques, with air-cooling being the most desirable from a cost and reliability standpoint.

Figure 5.11 shows an oscillogram from an MT system showing the current produced by the high-speed generator (Channel 1), along with the resultant current simultaneously supplied to the load (Channel 2), in this case an electric utility. This figure indicates the importance of the power electronics function in an advanced DG power system.

r T i



{j I ff 1if 11|| 1(1!


-1) [TekTHS720P].REF1 50 A 2 HIS -2) [Tek THS720R].REF0 50 A 2 HIS

Oscillogram from an operating microturbine system. (Courtesy of Capstone Turbine Corporation, with permission.)


Oscillogram from an operating microturbine system. (Courtesy of Capstone Turbine Corporation, with permission.)

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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