inverters controlled rectifiers and the scr
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Inverters, Controlled Rectifiers, and the SCR | Power Circuits
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Inverters, Controlled Rectifiers, and the SCR
January 30, 2015 admin Leave a comment
In many cases, the distinction between a rectifier and an inverter is artificial; In a rectifier, energy flows from an ac source to a dc load. In
an inverter, the flow is from a dc source to an ac load. An inverter thus has much the same function as a rectifier, except for the direction
of energy flow. A ‘generic conversion circuit,’ connecting an ac circuit to a dc circuit, could support bidirectional energy flow. Such a
circuit provides dual rectifier and inverter operation. The image below is an example using parallel and series elements to handle
thousands of megawatts.
Although a dual use rectifier and inverter circuit is possible
in principle, the rectifier diode does not support such a
circuit. A diode, as a true two-terminal element, is a
passive device. This means that its behavior is determined
solely by terminal conditions, and there is no direct
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Inverters, Controlled Rectifiers, and the SCR | Power Circuits
A rectifier-inverter control set for the NorNed high-voltage DC line between
Feda in Norway and the seaport of Eemshaven in the Netherlands.
opportunity for adjustment or other control. A rectifier built
with diodes cannot be ‘told’ to begin working as an
inverter, since there is no circuit input to alter the function
of the devices.
One of the most important power electronic devices, the silicon-controlled rectifier or SCR, addresses this need for control. The SCR,
introduced in 1957, provides the function of a diode with the addition of a third terminal for control. The conventional SCR will not
conduct unless a signal is applied to this control terminal, or gate. Once a gate signal is present, the device operates more or less as a
conventional diode. In this way, the gate permits adjustment of the conduction behavior, and leads to the concept of an adjustable diode.
The SCR was not the first technology to provide controlled rectifier function. By the 1920s, passive circuit methods were combined with
vacuum diodes to create similar functions. Grid control was used with mercury arc tubes to provide controlled rectification by the 1930s.
The cycloconverter – a complicated controlled rectifier adapted for ac-ac conversion – was introduced in about 1931. The SCR provides
the function of a grid-controlled arc tube, but with much better speed and efficiency. The device brought about a revolution in electronic
power conversion. Through the use of the SCR, electronic converters for electrochemical processing, transportation systems, industrial
dc motor controls, and electric heating and welding became practical during the 1960s. Such familiar applications as variable speed
kitchen appliances and lamp dimmers rely on the SCR and its relatives for control. It is sometimes said that power electronics began
when the SCR was introduced.
Once a controlled rectifier can be built, the step to inverters is a small one. Inverters are the critical conversion method for most
alternative energy resources. Sources as diverse as wind energy, solar panels, battery banks, and superconducting magnetic energy
storage (SMES) rely on inverter circuits to transform their energy to an ac power grid. The SCR remains crucial for these kinds of
systems.
Very high power levels have always been an important application for inverters and controlled rectifiers. This is because dc power is the
most economical form for transmission of energy over very long distances. Beyond about 1000 km, wavelength effects begin to bring
trouble to ac power networks. Resonances and reflections can affect behavior or crate failures. Dc power avoids these fundamental
problems, and high-voltage dc (HVDC) power transmission remains an important application. The power levels can be extreme: A major
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Inverters, Controlled Rectifiers, and the SCR | Power Circuits
link from the Columbia River to Southern California on the U.S Pacific coast is rated at up to 600 KV and 6000 MW. These levels are far
beyond the capacity of any individual device, and large series and parallel combinations of devices must be used to provide diode or
controlled rectifier functions. In this particular case, each line terminal can act either as a rectifier or inverter ( there are two sets of
devices at each end) so that the line power can be adjusted for seasonal changes in energy flow. Some installations elsewhere in the
world support only unidirectional power flow.
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