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Power Electronics:Thyristor Devices

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Power Electronics Introduction The modern age of power electronics began with the introduction of thyristors in the late 1950s [1]. There are now a number of power devices available for high-power and high-frequency applications. The most notable include: • Gat e turn-off (GTO) thyristor • Power Darlington transistor • Power MOSFET • I nsulated-gate bipolar transistor (IGBP) These power devices are used primarily as switches to convert energy from one form to another. They are used in motor control systems, uninterruptible power supplies, high-voltage dc transmission, ac-to-dc power supplies, induction heating, and other power-conversion applications. Thyristor Devices The thyristor, also called a silicon-controlled rectifier (SCR), is basically a four-layer three-junction pnpn device [1]. It has three terminals: anode, cathode, and gate. The device is turned on by applying a short pulse across the gate and cathode. After the device turns on, the gate loses its control t...
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Grounding Tower Elements:Grounding on Bare Rock and Rock-Based Radial Elements.

Grounding on Bare Rock A bare rock mountaintop location provides special challenges to the facility design engineer [1]. There is no soil, thus there are no ground rods. Radials are the only means to develop a ground system. Install a large number of radials, laid straight, but not too taut. The portions not in contact with the rock are in air and form an inductance that will choke the surge current. Because rock is not conductive when it is dry, keep the radials short. Only a test measurement will determine how short the radials should be. A con- ventional earth-resistance tester will tell only half the story (besides, ground rods cannot be placed in rock for such a measurement). A dynamic ground tester offers the only way to obtain the true surge impedance of the system. Rock-Based Radial Elements On bare rock, a radial counterpoise will conduct and spread the surge charge over a large area. In essence, it forms a leaky capacitor with the more conductive earth on or under the moun...
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Power Factor:PF Correction Techniques

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PF Correction Techniques The term compensation is used to describe the intentional insertion of reactive power devices, either capacitive or inductive, to achieve one or more desired effects in an electric power system. These effects include improved voltage profiles, enhanced stability, and increased transmission capacity. The devices are either in series or in shunt (parallel) with the load(s) at one or more points in the power circuit. To keep the power factor as close as possible to 100%, utility companies place capacitor banks in par- allel with the load at various locations in the distribution system, offsetting the inductive loading (lagging power factor) of most user equipment. The goal is to create an equal amount of leading PF in the system to match the lagging PF of the load. When balanced, the power factor is 100%. In practice, this is seldom attainable because loads are switched on and off at random times, but utilities routinely maintain an overall power factor of appr...
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