6.6. Switching devices
Switching function in inverters is needed to alternate the direction of the DC current in order to produce AC power. Usually, electronic semiconductor devices are used to perform switching, such as transistors and thyristors.
Thyristors are used in basic models of inverters. They have three leads and usually "switch on" in response to current applied to one of the leads. Thyristor have only two modes: ON and OFF, the same as mechanical switches. More details on thyristors can be found on this Thyristor Wikipedia Page.
Transistors are similar in switching capability to thyristors, but they instead respond to voltage applied rather than current. That allows to smoothly vary the transistor's internal resistance. So in addition to ON and OFF functions, transistors also allow dimmer capability. More details on transistors can be found on this Transistor Wikipedia Page.
There are two main types of transistors used in solar inverters:
- Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)
- Insulated Gate Bipolar Transistors (IGBTs)
The MOSFET type is suitable for very high switching speeds (up to 800 kHz), but operate at relatively low voltage. The IGBT type switch at lower speeds (below 20 kHz), but withstand higher voltage and high current (Dunlop, 2010).
Switching devices, such as thyristors and transistors, need to be controlled by an external signal. In the basic inverter designs, switching is controlled by the utility power line. Such switching devices are referred to as line-commutated. They are turned on and off by alternating half-cycles of the utility voltage, thus synchronizing the inverter output with grid line current. Although efficient, the line-commutated inverters have one disadvantage: they cannot operate independently of the grid.
Some inverters may contain an internal device that controls switching. Such a device is usually a microprocessor that provides precise timing. Such inverters are called self-commutated. Self-commutated inverters have additional capabilities of shaping the AC output and suppressing harmonics. And they can operate independent of utility power. There are two varieties of self-commutated inverters: voltage-source and current-source. They take the DC input as voltage source or current source, respectively, for conversion of the power to the AC output. Most of present day inverters involved in solar plants are self-commutated (Dunlop, 2010).