Frequency converter controller configuration

The input to both the rectifier and inverter will be assumed to be a DC voltage source, as was already assumed in Section 6.1. Such rectifiers and inverters are in literature referred to as voltage-source converters (VSC). The VSC makes it possible to connect each of the three generator phases to the positive or negative side of the DC link voltage Udc. Although the shown VSC consists of six IGBT's with 26 = 64 different switching states, it is important to realize that the VSC must be operated so that none of its phases is short-circuiting the link voltage Udc [159]. Hence only one of the two switches in each phase can be conductive (e.g. when Si is turned on, S4 must be turned off).



r '

Figure 6.2: Schematic of a VSC consisting of six IGBT's and its 2-way switch representation with the appropriate switch positions.

S,= ON


s. = 0

s,= OFF

St= ON

S. = l

s,= ON

S6 = OFF


S, = OFF

Ss= ON


S,= ON

S2 = OFF

S„= 0


Sj= ON

Figure 6.2: Schematic of a VSC consisting of six IGBT's and its 2-way switch representation with the appropriate switch positions.

It is common practice to represent each stator phase as an ideal 2-way switch as shown in Fig. 6.2. This implies that there are 23 = 8 different switching states possible. Six of these states yield active vectors (i.e. [Sa Sb Sc] = [100], [110], [010], [011], [001], and [101]) for the terminal stator voltages, and two states yield zero vectors (i.e. [111], and [000]). In the latter two cases the generator is short-circuited because each of the three-phases is connected to either the positive ([Sa Sb Sc] = [111]) or negative ([Sa Sb Sc] = [00 0]) side of the DC link.

The required switching states can be determined in various ways using either the measured three-phase stator currents or the measured three-phase stator voltages. When the current is used to determine the state of the switches, the VSC is referred to as a current-controlled VSC, otherwise as a voltage controlled VSC. The current-controlled VSC, however, is preferred to the voltage-controlled VSC [26]. The main reason for the selection of the current as the controlled variable is the same as for the DC machine: the stator dynamics (stator resistance, stator inductance, and induced stator voltages or EMF) are eliminated. Consequently, the complexity of the controller can be significantly reduced.

There are two ways of implementing current controllers: as AC or as DC current controllers. From a control perspective, DC current controllers are preferred since the steady-state currents represented in the rotating dq reference frame are DC currents [19]. This implies that any current controller with integral action (e.g. a Pi-controller) will result in zero steady-state error.

Current controllers for AC machines are, however, more complex than for DC machines because an AC current controller must control both the amplitude and phase of the three-phase stator currents.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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