![]() ![]() This turbine is equipped with a DFIG hence, it has two back-to-back converters with 12 IGBTs. Thus, the operation is not automatic and must be controlled by precise timing for the signals to the gates of the thyristors (IGBTs).įigure 4 shows some of the IGBTs and the DC link capacitors for a 1.5 MW turbine. Recall that the function of each converter could be defined by the firing angle of its thyristors. Then, by controlling the thyristors $T_$, which feeds the DC link and then a three-phase alternating current can be obtained at A, B, and C.Īs can be seen, the switching actions and the voltages available at A, B, and C terminals and A′, B′, and C′ terminals (which one is larger) determine in what direction the power can transfer. In the circuit of Figure 3 if A, B, and C are connected to the mains with a given voltage and frequency the diodes D 1 to D 6 work as a rectifier to provide a DC voltage keeping the DC link capacitor charged. The schematic of such a converter is shown in Figure 3. We consider only the voltage source inverters in this case because that is the type used in many of the modern wind turbines. Compared to the unidirectional converter, power can be controlled to transfer from either end to the other.įor this arrangement, it is necessary to use switchable devices for both converters. In many applications, including the wind turbines equipped with wound rotor induction machine (the same as DFIG), a bi-directional back-to-back converter is employed. Also, the inclusion of series diodes in series with the thyristors is to protect them (like freewheel diodes) and prevent the unwanted discharge or higher voltage at the thyristor terminals during switching. The three capacitors in the output provide a path for the currents even if there is no load. ![]() if a capacitor or an inductor is used in the DC link).įigure 2 illustrates the arrangement for a current source inverter. The two terms voltage source inverter and current source inverter, used in industry, stem from whether a voltage source or a current source is used for an inverter (i.e. In such a case the inductor behaves as a short-term current source. It is possible to use an inductor instead of the capacitor. Thus, there is a limit for the maximum value that V o can reach.įigure 1 Unidirectional diode-based AC-to-AC converter.įigure 2 Diode-based unidirectional AC-to-AC converter with current source inverter. Whereas f o can be greater or smaller than f i, in general, V o and V i are bound through the DC link voltage. The DC voltage in the DC link then is converted to AC with the required frequency f o and the desired voltage V o by using pulse width modulation or other technique. This alternative electricity is rectified and keeps the capacitor at a voltage determined by V i. The input can be connected to a three-phase system with a frequency f i and voltage V i. The link between the two parts of the system, containing the capacitor, is referred to as the DC link. The capacitor behaves as a short-term storing device.Īlternatively, the capacitor can be regarded as a filter to smooth the output of the rectifier. In other words, there is no need for a battery to store the rectifier output. The capacitor serves as a voltage source. The rectifier and the inverter are coupled together by a capacitor. Voltage control can be done through the rectifier if it is thyristor-based otherwise, both voltage control and frequency control is done through the inverter. Then, the DC electricity is converted to AC with the required frequency and voltage. AC is first converted to DC through a rectifier that can be based on diodes or thyristors. The first case is simpler and is discussed in this section.įigure 1 illustrates a unidirectional AC-to-AC converter. In the latter case, the input and output can switch functions, and power transfer can be either way. In the former case, one side is always the input and the other side is always the output, and power always is transferred from the input to the output. Then, there is the question of if the device is required to work in one direction only, all the time, or if it must work both ways. The key issue in AC-to-AC converters is to convert AC to DC first and then DC to AC by putting together a rectifier and an inverter. ![]()
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