Inverters, Controlled Rectifiers, and the SCR
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 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 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.